Synthesis and physicochemical properties of metallobacteriochlorins

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Synthesis and Physicochemical Properties of Metallobacteriochlorins

Chih-Yuan Chen,^†Erjun Sun,^†,∥Dazhong Fan,^†Masahiko Taniguchi,^†Brian E. McDowell,^†

Eunkyung Yang,^‡James R. Diers,^§David F. Bocian,*^,§Dewey Holten,*^,‡and Jonathan S. Lindsey*^,†

^†Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, United States

^‡Department of Chemistry, Washington University, St. Louis, Missouri 63130-4889, United States

^§Department of Chemistry, University of California, Riverside, California 92521-0403, United States

S

* Supporting Information

ABSTRACT: Access to metallobacteriochlorins is essential for investigation of a wide variety of

fundamental photochemical processes, yet relatively few synthetic metallobacteriochlorins have been

prepared. Members of a set of synthetic bacteriochlorins bearing 0−4 carbonyl groups (1, 2, or 4

carboethoxy substituents, or an annulated imide moiety) were examined under two conditions: (i)

standard conditions for zincation of porphyrins [Zn(OAc)₂·2H₂O in N,N-dimethylformamide (DMF)

at 60−80 °C], and (ii) treatment in tetrahydrofuran (THF) with a strong base [e.g., NaH or lithium

diisopropylamide (LDA)] followed by a metal reagent MX_n. Zincation of bacteriochlorins that bear 2−4

carbonyl groups proceeded under the former method whereas those with 0−2 carbonyl groups

proceeded with NaH or LDA/THF followed by Zn(OTf)₂. The scope of metalation (via NaH or LDA

in THF) is as follows: (a) for bacteriochlorins that bear two electron-releasing aryl groups, M = Cu, Zn,

Pd, and InCl (but not Mg, Al, Ni, Sn, or Au); (b) for bacteriochlorins that bear two carboethoxy groups, M = Ni, Cu, Zn, Pd, Cd,

InCl, and Sn (but not Mg, Al, or Au); and (c) a bacteriochlorin with four carboethoxy groups was metalated with Mg (other

metals were not examined). Altogether, 15 metallobacteriochlorins were isolated and characterized. Single-crystal X-ray analysis

of 8,8,18,18-tetramethylbacteriochlorin reveals the core geometry provided by the four nitrogen atoms is rectangular; the

diﬀerence in length of the two sides is ∼0.08 Å. Electronic characteristics of (metal-free) bacteriochlorins were probed through

electrochemical measurements along with density functional theory calculation of the energies of the frontier molecular orbitals.

The photophysical properties (ﬂuorescence yields, triplet yields, singlet and triplet excited-state lifetimes) of the zinc

bacteriochlorins are generally similar to those of the metal-free analogues, and to those of the native chromophores

bacteriochlorophyll a and bacteriopheophytin a. The availability of diverse metallobacteriochlorins should prove useful in a

variety of fundamental photochemical studies and applications.

A further distinction caused by metals concerns the change in

optical properties. The introduction of a metal in a porphyrin

typically increases the symmetry (e.g., D_2hto D_4h) and causes the

spectral features in the visible region (500−650 nm) to collapse

from primarily four bands (due to partially overlapping x and y

transitions) to a two-banded spectrum (wherein the x and y

transitions are degenerate).⁷The two visible bands are the Q(0,0)

and Q(1,0) transitions. (Weaker additional vibronic overtone

bands also contribute to the spectra with or without a metal ion.)

The resulting absorption of the metalloporphyrin occurs at shorter

wavelength than for that of the free base porphyrin. For a chlorin,

insertion of a metal does not alter the symmetry but does typically

cause a hypsochromic shift in the position of the long-wavelength

absorption band. An example is provided by chloro-

phyll a and pheophytin a (the free base of chlorophyll a) which

absorb at 662 and 667 nm, respectively.¹For a bacteriochlorin,

insertion of a metal also does not alter the symmetry but typically

causes a bathochromic shift in the position of the long-wave-

length absorption band. An example is provided by bacterio-

chlorophyll a (Bchl a) and bacteriopheophytin a (Bphe a), which

I. INTRODUCTION

Naturally occurring chlorophylls and bacteriochlorophylls are

essential constituents in plant and bacterial photosynthesis. Both

types of hydroporphyrins contain magnesium as the central

metal.¹The introduction of diﬀerent metals in tetrapyrrole

macrocycles can alter the electronic,²axial-ligation,³and photo-

physical⁴⁻⁶properties of the coordination complex. The eﬀect of

metals can be seen by comparing the properties of metal-

loporphyrins containing magnesium, zinc, copper, or palladium,

each of which is a divalent metal. Magnesium is ﬁve or six

coordinate, and gives a reasonable yield of ﬂuorescence (Φ_f∼ 0.1),

a long-lived excited singlet state (τ ∼10 ns), and a good yield

of intersystem crossing to the triplet state.⁴Zinc is four or ﬁve

coordinate, and gives a lower yield of ﬂuorescence (Φ_f∼ 0.03), a

shorter excited singlet state (τ ∼2 ns), and a higher yield of

intersystem crossing to the triplet state.⁴Copper is four coordi-

nate and gives essentially no detectable ﬂuorescence, a very

short-lived nominal excited singlet state, and highly temperature-

dependent properties of two excited-states borne from the

coupling of the porphyrin triplet with the unpaired metal elec-

tron.⁵Palladium is four coordinate and gives no detectable

ﬂuorescence, a near unity yield of intersystem crossing, and a

short-lived excited triplet state.⁶

Received: June 13, 2012

Published: August 23, 2012

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absorb at 772 and 749 nm, respectively.¹The ability to shift the

absorption to longer wavelength upon metalation is quite

attractive given the multiple motivations for access to chro-

mophores with strong absorption in the near-infrared (NIR)

spectral region. The relatively low energy of photons in the NIR

region (1.76−1.23 eV, 700−1000 nm) enables photochemical

studies in an energy regime that has been comparatively

unexplored versus studies of organic photochemistry in the

ultraviolet (6.17−3.09 eV, 200−400 nm) or visible regions

(400−700 nm, 3.09−1.76 eV). Applications of NIR-active

bacteriochlorins include artiﬁcial photosynthetic light-harvest-

ing,⁸optical imaging^9,10photodynamic therapy¹¹of soft tissues,

and ﬂuorescent markers in clinical diagnostics.¹²In addition,

selected photosynthetic organisms are now known to employ

zinc-containing analogues of bacteriochlorophylls (rather than

the expected magnesium).¹³For all of these reasons,

fundamental studies of diverse metallobacteriochlorins are

warranted.

Despite the range of physical behavior that can be elicited with

metalloporphyrins, relatively few metallobacteriochlorins have

been prepared, and most that have been prepared are derived

from Bchl a.^14,15While data from the naturally derived

macrocycles are quite valuable, lack of access to diverse synthetic

metallobacteriochlorins has precluded wide-ranging studies of

eﬀects of peripheral substituents on spectral and photophysical

properties, an approach that has been extensively pursued with

porphyrins and chlorins. We have been working to develop a

rational, de novo synthesis of bacteriochlorins.¹⁶⁻¹⁹The result-

ing bacteriochlorins bear a geminal dimethyl group in each

reduced, pyrroline ring to resist adventitious oxidants that

otherwise could result in dehydrogenation. We recently char-

acterized the photophysical properties of a large set of free base

bacteriochlorins²⁰derived from this synthetic approach, and also

examined several indium(III) chelates thereof,²¹but rela-

tively few metal chelates of the synthetic bacteriochlorins have

heretofore been prepared.

The metalation of bacteriochlorinsan ostensibly simple

reactionhas proved more diﬃcult than for porphyrins and

chlorins. As one illustration, treatment of a chlorin−bacteriochlorin

dyad with zinc acetate in CHCl₃/methanol at room tem-

perature for 4 h aﬀorded selective metalation of the chlorin; the

resulting zinc chlorin-free base bacteriochlorin dyad was isolated

in nearly quantitative yield.⁹As a second illustration, conditions

that aﬀord smooth zincation of the chlorin pheophytin a

(Zn(OTf)₂in methanol or acetonitrile at room temperature)

upon application to Bphe a resulted in decomposition rather

than metalation.²²The origin of the diﬃculty of metalation of

bacteriochlorins remains unclear, but has been attributed to a

number of factors. The factors include (1) nucleophilicity, which

decreases with increased saturation of the macrocycle (porphyrin

> chlorin > bacteriochlorin),²²and (2) acidity of the N−H

protons, which decreases with increasing electron-richness of the

ligand (porphyrin > chlorin > bacteriochlorin).²³A factor that

complicates interpretation is that many bacteriochlorins

examined in metalation studies to date are derived from natural

ligands of somewhat limited stability. In short, the dearth of

bacteriochlorins that withstand a broad range of reaction

conditions has impeded a thorough investigation of these issues.

In this paper, we ﬁrst summarize methods that have been used

to date for metalation of bacteriochlorins, and identify corre-

lations between methods and structural features of the

bacteriochlorins. We then describe the development and applica-

tion of a new method for metalation of synthetic bacteriochlorins.

Finally, we report the spectral and photophysical features of a set

of metallobacteriochlorins. While no metalation procedure

has yet been developed that is generically applicable to all

bacteriochlorins, the present work should expand the availability

of a variety of metallobacteriochlorins that have heretofore been

inaccessible.

II. RESULTS AND DISCUSSION

A. Bacteriochlorin Synthesis. The metalation studies were

carried out on a series of synthetic macrocycles that spanned a

range from electron-deﬁcient to electron-rich bacteriochlorins.

The most electron-deﬁcient bacteriochlorin (BC4-MeO)

contains four carboethoxy groups (denoted by “4”) and a

methoxy group at the 5-position (denoted by “MeO”), which is

followed by the bacteriochlorin-imide BC3-2E with three

carbonyl groups and two ethyl groups. Bacteriochlorins with

two carboethoxy and two alkyl groups BC2-2E, BC2-2H, and

BC2-2H-MeO (E = ethyl, H = heptyl) are in the middle of the

range, followed by BC2-2M-MeO with two carboethoxy and

two mesityl groups. The most electron-rich bacteriochlorin

(BC0-2T) bears electron-donating, p-tolyl groups at the 2- and

12-positions. This feature makes BC0-2T an appropriate bench-

mark to gauge the scope of the metalation for electron-rich

bacteriochlorins. Additionally, the unsubstituted bacteriochlorin

BC0 lacking any β-pyrrole substituents provides a benchmark for

comparison with diverse substituted bacteriochlorins (Chart 1).

The term carbonyl here denotes 1, 2, or 4 carboethoxy sub-

stituents and/or the 2 substituted acyl moieties of the annulated

imide ring.

The bacteriochlorins thus comprise a far broader range of

substituents than has heretofore been examined, and all members

of the set diﬀer from the naturally occurring bacteriopheophytins

in the following ways: (i) absence of an isocyclic ring (ring E),

(ii) presence of a geminal dimethyl group rather than trans-

dialkyl substituents in each pyrroline ring, and (iii) absence of

alkyl (sp³-hybridized) groups on adjacent β-pyrrole carbons,

which could induce nonplanarity. On the other hand, within the

set of bacteriochlorins in Chart 1, some are sparsely substituted

(i.e., lack 2 or 4 β-pyrrole substituents); those with mesityl

substitutuents are sterically hindered; and the set encompasses

molecules with 0−4 carbonyl groups. Collectively, the

bacteriochlorins present a rich test of the generality of metalation

methods.

The syntheses of bacteriochlorins BC4-MeO,¹⁸BC3-2E,²⁴

BC2-2E,¹⁸BC0-2T,¹⁶and BC0¹⁸have been reported. The

syntheses of BC2-2H, BC2-2H-MeO, and BC2-2M-MeO are

shown in Scheme 1. The general approach relies on installation

of the desired β-pyrrole substituents at the earliest stage of

the synthesis.¹⁸Thus, treatment of α,β-unsaturated ester 1H

(R = heptyl) or 1M (R = mesityl)²⁵with p-toluenesulfonyl

methylisocyanide (TosMIC) via the van Leusen method²⁶

aﬀorded the corresponding pyrrole 2H or 2M. Formylation²⁷

gave pyrrole-2-carboxaldehyde 3H or 3M wherein the formyl

group is positioned adjacent to the heptyl or mesityl moiety,

respectively. Treatment of 3H or 3M to sequential nitroaldol

(Henry) condensation^17,28and reduction²⁹gave the nitro-

vinylpyrrole 4M (4H was not isolated) and 2-(2-nitroethyl)-

pyrrole 5H or 5M, respectively. Michael addition³⁰with the α,β-

unsaturated ketone−acetal 6^16,19gave the nitrohexanone−pyrrole

7H or 7M, which upon McMurry-type reductive cycliza-

tion¹⁷aﬀorded the dihydrodipyrrin-acetal 8H or 8M. Macrocycle

formation was carried out by self-condensation at room temperature

via two catalytic conditions:¹⁸TMSOTf/2,6-di-tert-butylpyridine

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ionization mass spectrometry (ESI-MS); compounds 2H,

3H, 5H, 7H, and 8M also were veriﬁed by elemental analysis.

Bacteriochlorins BC2-2H, BC2-2H-MeO, and BC2-2M-MeO

Chart 1. Synthetic Bacteriochlorins Examined Herein

1

were characterized by H NMR spectroscopy, ¹³C NMR

spectroscopy, absorption spectroscopy, MALDI-MS and

ESI-MS.

Bacteriochlorin BC2-2M-MeO contains mesityl groups at the

2- and 12-positions. The location of the β-pyrrole substituents in

the macrocycle is set at the stage of formylation of the pyrrole

(2M → 3M). It is noteworthy that Vilsmeier formylation²⁷of

3-mesitylpyrrole (S1), obtained by decarboxylation³¹of pyrrole

2M, aﬀords the 2-formyl-4-mesitylpyrrole (S2) and the isomeric

2-formyl-3-mesitylpyrrole in 4:1 ratio. The availability of

S2 enabled synthesis of a bacteriochlorin that contains mesityl

groups at the 3- and 13-positions (BC0-2M; see Supporting

Information).

B. Bacteriochlorin Metalation. 1. Literature Methods for

Metal Insertion. A classic method for metalation of porphyrins

entails treatment of the free base macrocycle with a metal acetate

(or metal acac) in a somewhat polar solvent at elevated

temperature.^32,33For porphyrins, the use of high temperatures

often is acceptable because most porphyrins are stable at high

temperature and in solution exposed to air. However, many

bacteriochlorins do not survive at high temperatures even if the

reactions are run anaerobically.³⁴There are two general methods

for metalation of bacteriochlorins: (1) direct metalation of a free

base bacteriochlorin with a metal salt (MX₂) in a solvent at

temperatures ranging from room temperature to >100 °C to

obtain the Zn(II), Cu(II), Pd(II), Ni(II), or Cd(II) bacterio-

chlorin (Table 1, entries 1−5);^23,35−48and (2) transmetalation

wherein a Cd(II) chelate of a bacteriochlorin is formed in situ in

acetone and then treated with a metal chloride at room tem-

perature to obtain the target metallobacteriochlorin (entry 6).^23,39−41

Strell and Urumow ﬁrst prepared a variety of metallochlorins via the

transmetalation method,⁴⁹which was subsequently applied by Scheer

and co-workers²³to bacteriochlorophylls.

The bacteriochlorins that have been prepared via these

methods are displayed in Charts 2 and 3. Examination of

structural features of the naturally derived bacteriochlorins

(I−VII) subjected to metalation reveals the presence of at least one

if not two carbonyl (ketone, aldehyde, amide, or imide) groups,

whereas some of the synthetic bacteriochlorins (VIII−XII) lack

such electron-withdrawing groups. The electron-richness of the

macrocycle is believed to be an important feature that aﬀects the

rate of metalation and the propensity of the metallobacterio-

chlorin toward protolytic demetalation.

In addition to the aforementioned general methods, several

more specialized procedures have been reported for the pre-

paration of metallobacteriochlorins. (1) Wasielewski employed a

hindered Grignard reagent and a non-nucleophilic base to

magnesiate Bphe a and thereby reconstitute Bchl a.⁵⁰(2)

Eschenmoser treated octaethylporphyrinogen with nickel acetate

in reﬂuxing xylene and obtained nickel octaethylbacteriochlorin;

the process entails metalation, tautomerization, and 2e⁻/2H⁺

oxidation (converting the hexahydroporphyrin to a tetrahy-

droporphyrin).^51,52(3) Stolzenberg applied the method of

Arnold (formation and isolation of the dilithium derivative of a

tetrapyrrole macrocycle followed by transmetalation with a metal

reagent^53,54) with tetra-p-tolylbacteriochlorin to prepare the

oxotitanyl chelate.⁵⁵(4) Chen caused a nickel tetrabromopor-

phyrin to undergo scission of the two β-pyrrole carbons on

opposing rings and thereby form a “bacteriophin,” which exhibits

an absorption spectrum comparable to that of a bacteriochlorin.⁵⁶

(DTBP) in CH₂Cl₂with 8H or 8M aﬀorded BC2-2H-MeO

or BC2-2M-MeO, whereas BF₃·OEt₂in CH₃CN with 8H

gave BC2-2H. The new compounds (2, 3, 5, 7, and 8 in the H

and M series) were characterized by melting point, ¹H NMR

spectroscopy, ¹³C NMR spectroscopy, and electrospray

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Scheme 1. Synthesis of Bacteriochlorins Bearing Two Carboethoxy Groups

base 3,13-dibromobacteriochlorin with Zn(CN)₂gave the

corresponding zinc(II)-3,13-dicyano-8,8,18,18-tetramethylbac-

teriochlorin (Chart 4),⁵⁸and (2) self-condensation of a

dihydrodipyrrin−acetal in CH₃CN containing InCl₃aﬀorded

the corresponding indium bacteriochlorin²¹(Scheme 2). The

formation of the zinc chelate in the former case might result from

direct metalation of the electron-deﬁcient 3,13-dicyanobacterio-

chlorin, and the latter case is thus far restricted to indium given

that the metal reagent must provide acid catalysis for the

condensation and also engender chelation during the course of

the reaction. In general, no broadly applicable method for metalating

synthetic bacteriochlorins has been developed to date.

Table 1. Preparative Methods for Metalation of

Bacteriochlorins

a

entry

1

metal salt

conditions

bacteriochlorin

Zn(OAc)₂or

Zn(OAc)₂·2H₂O

reﬂux in diverse

I,²³II,^35,36III,³⁷IV,³⁸

V,³⁸VI,³⁹VII,^40,41

VIII,⁴²IX⁴³

b

solutions

2

Cu(OAc)₂or CuO₂MeOH or AcOH at rt I,²³II,⁴⁴VI,³⁹IX⁴⁵

or reﬂux; CHCl₃/

MeOH at reﬂux

3

4

Pd(OAc)₂

MeOH at rt

I,⁴⁶VII⁴¹

X,⁴⁷XI,⁴⁸XII⁴⁸

c

NiCl₂·6H₂O or

NiCl₂

DMF at reﬂux

5

Cd(OAc)₂

DMF at reﬂux

I,²³VI,³⁹VII^40,41

d

2. Metal Insertion Studies. Survey of Methods for an

Electron-Rich Bacteriochlorin. We examined the metalation of

the di-p-tolylbacteriochlorin, BC0-2T, under four conditions.

(1) Treatment of BC0-2T with Zn(OAc)₂, Cu(OAc)₂·H₂O,

Ni(OAc)₂·4H₂O, Pd(OAc)₂, Pd(O₂CCF₃)₂, or Co(OAc)₂in

CHCl₃/MeOH at room temperature or reﬂux did not aﬀord the

metal chelate as determined by LD-MS and absorption spectro-

scopy. More forcing conditions employing elevated temperature

in two diﬀerent solvent systems (ClCH₂CH₂Cl/MeOH and

DMF) with Pd(O₂CCF₃)₂(used in porphyrin metalation)⁵⁹also

showed only starting material.

6

MnCl₂·2H₂O, CoCl₂, acetone at rt

NiCl₂, CuCl₂,

ZnCl₂, PdCl₂

a

The free base analogue of the structures shown in Charts 2 and 3

b

were used unless noted otherwise. 1,2-Dichloroethane/EtOH,

pyridine, AcOH, DMF, CHCl₃/MeOH, CH₂Cl₂/MeOH, or CHCl₃/

c

pyridine (6:1). Incomplete metalation and partial dehydrogenation

d

(to porphyrin) were observed. Transmetalation from the cadmium

complex.

An alternative approach to metallobacteriochlorins might be

envisaged as the simple hydrogenation of a metalloporphyrin.

Whereas tetrahydrogenation of a free base porphyrin aﬀords

the free base bacteriochlorin, tetrahydrogenation of a zinc

porphyrin aﬀords the zinc isobacteriochlorin rather than the zinc

bacteriochlorin.⁵⁷

Through our recent work concerning the de novo synthesis of

bacteriochlorins, two reactions were found unexpectedly to yield

metallobacteriochlorins: (1) Pd-catalyzed cyanation of a free

(2) The standard “acac” conditions³⁴with Zn(acac)₂in

reﬂuxing benzene did not aﬀord ZnBC0-2T (see Supporting

Information).

(3) Use of Cd(OAc)₂in DMF at 130 °C, the initial step in the

transmetalation method,²³aﬀorded a byproduct (M + 14; by LD-

MS analysis) that was consistent with an analogue of BC0-2T

wherein one of the pyrroline methylene units is oxidized to form

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Chart 2. Metal Chelates of Naturally Derived Bacteriochlorins

Chart 3. Metal Chelates of Synthetic Bacteriochlorins

a ketone (e.g., a putative oxobacteriochlorin). No bacteriochlorin

chelate was formed.

(4) A room-temperature method for magnesium insertion

into porphyrins employs MgI₂and a noncoordinating base

(e.g., diisopropylamine, DIEA) in a noncoordinating solvent

(e.g., CH₂Cl₂).⁶⁰An extension of this method, using ZnI₂and

DIEA in CH₂Cl₂at reﬂux (∼40 °C) for 12 h, aﬀorded ZnBC0-2T

in 30% yield as determined by absorption spectroscopy.

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also was found to be eﬀective. One key diﬀerence between NaH

and LDA is that typically the former aﬀords a heterogeneous

reaction whereas the latter aﬀords a homogeneous reaction. Use

of LDA aﬀorded ZnBC0-2T and CuBC0-2T from the same

reagents as with NaH (entries 4 and 5). On the other hand, InCl₃

gave no insertion with NaH but did aﬀord the ClIn(III) complex

(ClInBC0-2T) upon use of LDA (entry 6). For examination of a

variety of indium reagents, see the Supporting Information. No

metallobacteriochlorins were obtained under any condition

explored with metal reagents based on MgX₂(X = Cl, Br, I, OH,

OTf), Al₂O₃, AlX₃(X = acac, Cl, Br), NiX₂(X = acac, Cl, Br, I),

SnX₂(X = OH, OAc, acac, Br, I), or AuX₃(X = Cl, Br, I). In

summary, a few metals can be inserted into the electron-rich

bacteriochlorin BC0-2T with use of NaH or LDA in THF. It

warrants consideration that the alkali metal of the base (e.g., Na

or Li) is likely not a spectator but instead plays a role in

coordination of the deprotonated bacteriochlorin. In this regard,

the overall metalation reﬂects in part a competition between two

cations (acids) and two anionic ligands (bases). In-depth study of

the nature of reaction intermediates, the role of counterions, and

delineation of the kinetics and thermodynamics of reaction are

beyond the scope of the present paper.

Chart 4. Zinc Dicyanobacteriochlorin

Scheme 2. Metalation during Macrocycle Formation

Zincation of Bacteriochlorins Bearing 0−4 Carbonyl

Substituents. To better understand the scope of the metalation

method (MX_n/NaH or LDA), we examined the set of

bacteriochlorins shown in Chart 1 and isolated the correspond-

ing metal chelates. For comparison, the standard “porphyrin

metalation conditions” of Zn(OAc)₂·2H₂O in DMF were also

examined. The unsubstituted bacteriochlorin BC0 aﬀorded

ZnBC0 in 80% isolated yield upon treatment with NaH/THF

and Zn(OTf)₂(Table 3, entry 1) whereas no reaction was

observed with the standard porphyrin metalation conditions of

Zn(OAc)₂·2H₂O in DMF (entry 2). Treatment of diester-

bacteriochlorin BC2-2H with the NaH/THF method for 6 h at

60 °C gave the zinc chelate in 31% yield (entry 3) whereas

Zn(OAc)₂·2H₂O in DMF at 80 °C for 3 days gave some

metalation but extensive byproducts interfered with isolation

(entry 4). Essentially identical results were observed with the

5-methoxy analogue, namely, BC2-2H-MeO (entries 5 and 6).

Treatment of diester-bacteriochlorin BC2-2M-MeO with the

NaH/THF method for 5 h at 60 °C gave the zinc chelate in 54%

A base may be essential for metalation, to facilitate

deprotonation of the bacteriochlorin and/or remove the acid

liberated upon metalation (eq 1). We examined a wide variety of

bases and metal reagents at slightly elevated temperature.

Ultimately we found that treatment of BC0-2T with NaH in

tetrahydrofuran (THF) at room temperature for 1 h followed by

Zn(OTf)₂and heating to 60 °C for 12 h aﬀorded the desired

ZnBC0-2T (Table 2, entry 1). The development of this method

is described in the Supporting Information.

H₂BC + MX₂→ MBC + 2HX

(1)

The method was applied to other metal reagents. Thus, use of

PdBr₂or Cu(OAc)₂aﬀorded PdBC0-2T or CuBC0-2T,

respectively (entries 2 and 3). The base LDA, which has com-

parable strength to that of NaH (pK_aof conjugate acid ∼35),⁶¹

Table 2. Survey of Metalation (BC0-2T)

a

b

entry

base

metal reagent

time

product

yield

1

2

3

4

5

6

NaH

LDA

Zn(OTf)₂

PdBr₂

12 h

0.5 h

3 h

ZnBC0-2T

PdBC0-2T

CuBC0-2T

ZnBC0-2T

CuBC0-2T

ClInBC0-2T^c

80%

78%

80%

Cu(OAc)₂

Zn(OTf)₂

Cu(OAc)₂

InCl₃

2 h

quantitative

56%

0.5 h

1 h

85%

a

The reaction conditions entail (1) treatment of BC0-2T (1.1 mg, 4.0 mM) in THF at room temperature with NaH (100 equiv = 400 mM) for 1 h

or LDA (10 equiv = 40 mM) for 5 min, (2) addition of the metal reagent (30−80 mM, see Supporting Information), and (3) heating at 60 °C for

b

the indicated period. The crude mixtures were monitored by TLC and LD-MS. The yields were determined by absorption spectroscopy.

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Table 3. Zinc Metalation of Synthetic Bacteriochlorins

a

entry

substrate

BC0

conditions

temp./time

product(s)

isolated yield

1

NaH/THF, Zn(OTf)₂

DMF, Zn(OAc)₂·2H₂O

NaH/THF, Zn(OTf)₂

DMF, Zn(OAc)₂·2H₂O

NaH/THF, Zn(OTf)₂

DMF, Zn(OAc)₂·2H₂O

NaH/THF, Zn(OTf)₂

LDA/THF, Zn(OTf)₂

DMF, Zn(OAc)₂·2H₂O

60 °C/16 h

80 °C/24 h

60 °C/6 h

80 °C/3 days

60 °C/6 h

ZnBC0

80%

31%

50%

2

BC0

no reaction

ZnBC2-2H

3

BC2-2H

4

BC2-2H

ZnBC2-2H and byproduct

ZnBC2-2H-MeO

ZnBC2-2H-MeO and byproduct

ZnBC2-2M-MeO

ZnBC2-2E

5

BC2-2H-MeO

BC2-2M-MeO

BC2-2E

6

80 °C/3 days

60 °C/5 h

7

54%

b

8

60 °C/3 h

80 °C/24 h

80 °C/7 h

80 °C/3 h

quantitative

86%

c

9

c

10

11

BC3-2E

ZnBC3-2E

54%

c

BC4-MeO

ZnBC4-MeO

97%

a

The reaction conditions entail: (a) treatment of bacteriochlorin (4 mM) in THF at room temperature with NaH (150 equiv = 600 mM) for 1 h or

LDA (10 equiv = 40 mM) for 5 min, followed by Zn(OTf)₂(30 equiv) and heating as indicated; or (b) treatment of bacteriochlorin (4 mM) in

b

c

DMF with Zn(OAc)₂·2H₂O (30 equiv) and heating as indicated. Yield determined by absorption spectroscopy. After an initial period of 60 °C

for 16 h.

Table 4. Survey of Metalation of Bacteriochlorin BC2-2M-MeO

a

b

entry

base

metal reagent

time

product

yield

39%

1

2

3

4

5

6

7

8

9

NaH

LDA

NaH

Cu(OAc)₂

PdBr₂

8 h

2 h

CuBC2-2M-MeO

BC2-2M

c

29%

42%

59%

71%

78%

53%

31%

Cu(OAc)₂

InCl₃

20 h

28 h

3 h

CuBC2-2M-MeO

ClInBC2-2M-MeO

PdBC2-2M-MeO

NiBC2-2M-MeO

CdBC2-2M-MeO

Cl₂SnBC2-2M-MeO

PdBr₂

NiCl₂

3 h

CdCl₂

SnCl₂

3 h

d

e

1 h

SnCl₄

2 h

a

The reaction conditions (0.60 mg of BC2-2M-MeO) entail (1) treatment of BC2-2M-MeO (4.0 mM) in THF at room temperature with NaH

b

(600 mM) for 1 h or LDA (40 mM) for 5 min, followed by (2) addition of the metal reagent and heating at 60 °C for the indicated period. The

crude mixtures were monitored by TLC and MALDI-MS. The yields were determined by absorption spectroscopy (assuming equal molar

absorptivity of the respective free base and metallobacteriochlorins at the Q_y(0,0) band). Isolated yield based on 7.8 mg of BC2-2M-MeO. The

free base bacteriochlorin also was present (24% yield). The free base bacteriochlorin also was present (69% yield).

c

d

e

yield (entry 7) whereas use of LDA/THF for 3 h at 60 °C gave

ZnBC2-2M-MeO in quantitative yield (entry 8).

groups do not, and instead require use of a strong base (NaH or

LDA). Bacteriochlorins bearing two carboethoxy substituents

undergo metalation via both the NaH or LDA/THF method and

the DMF method, the cleanliness and ease of isolation depending

on the nature of the set of bacteriochlorin substituents.

Scope of Metalation of Diester-Bacteriochlorins. The

diester-bacteriochlorin BC2-2M-MeO was readily zincated

upon treatment with NaH or LDA in THF followed by Zn(OTf)₂

(Table 3, entries 7 and 8). We soughttoexamine the range of metal

chelates that could be prepared with this substrate versus that

lacking ester substituents (i.e., BC0-2T) as examined in Table 1.

Thus, metalation of BC2-2M-MeO was monitored by TLC,

absorption spectroscopy, and MALDI-MS. Metal reagents that

provided the best yield in the metalation of BC0-2T were

examined for BC2-2M-MeO and also gave reasonable yields

(Table 4, entries 1−5). In addition to insertion of Zn(II), Cu(II),

The remaining bacteriochlorins with 2−4 carbonyl groups

(BC2-2E, BC3-2E, BC4-MeO) were each treated to the

standard porphyrin zincation conditions [Zn(OAc)₂·2H₂O in

DMF] at 60 °C for 16 h, and examined for metalation (by

absorption spectroscopy and MALDI-MS). If metalation was less

than quantitative, the reaction mixture was then heated at

elevated temperature. Thus, BC2-2E, BC3-2E, and BC4-MeO

were successfully metalated upon subsequent heating at 80 °C

for 24, 7, and 3 h, respectively (entries 9−11). The diﬀerences in

yield are attributed to puriﬁcation procedures, given that each

reaction appeared to go to completion.

In summary, bacteriochlorins bearing three or four carbonyl

groups undergo zincation upon standard porphyrin metalation

conditions, whereas those with no such electron-withdrawing

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a

Pd(II), and In(III), treatment with NiCl₂, CdCl₂, and SnCl₂also

gave the corresponding metal chelates (Table 4, entries 6−8). For

insertion of SnCl₂as well as SnCl₄, only partial metalation was

observed despite supplemental reagents or prolonged reaction

time (entries 8 and 9). Here, the reaction failed with Mg(OTf)₂

Al(OTf)₂, and AuCl₃.

Surprisingly, when BC2-2M-MeO was treated with NaH and

PdBr₂, the reaction mixture showed a peak at [M − 30] in

comparison to the starting material by MALDI-MS (Table 4,

entry 2). The absorption spectrum showed a hypsochromically

shifted Q _xband and a bathochromically shifted Q _yband; the

positions of the resulting bands were typical for the absence of a

methoxy group.^16,20The reaction at the multimilligram scale

aﬀorded a product that upon isolation and characterization (by

¹H NMR spectroscopy, absorption spectroscopy, MALDI-MS

and ESI-MS) proved indeed to be the free base bacterio-

chlorin that lacks the 5-methoxy group (BC2-2M, Chart 5).

Table 5. Survey of Magnesiation of Bacteriochlorins

metal

reagent

b

c

entry substrate condition

base

result

1

2

3

4

5

6

BC4-MeO 40 °C, [TEA] = 80 mM 40 mM no reaction

16 h

BC4-MeO 40 °C, [TEA] = 80 mM 120 mM decomposition

5 h

BC4-MeO 60 °C, [NaH] = 0.6 M 120 mM MgBC4-MeO and

24 h

byproducts

BC4-MeO 60 °C, [NaH] = 1.2 M 120 mM MgBC4-MeO

3 h

BC3-2E

BC2-2E

60 °C, [NaH] = 1.2 M 120 mM no reaction

40 h

60 °C, [NaH] = 0.6 M 120 mM no reaction

30 h

a

The reaction procedure entails treatment of substrate (4 mM) in

CH₂Cl₂(with TEA) or THF (with NaH) followed by MgI₂at the

indicated temperature for the indicated time. The quantity is given in

concentration units for ease of comparison even though not all

b

c

material may be dissolved. The crude mixture was checked by TLC,

absorption spectroscopy, and MALDI-MS.

Chart 5. Demethoxylated Byproduct upon Attempted

Palladiation

indicating formation of the magnesium chelate. However, the

crude mixture contained a signiﬁcant amount of unknown

impurities as examined by TLC and absorption spectroscopy.

Increasing the amount of NaH to 300 equivalents considerably

reduced the impurities and the reaction was completed within

3 h. The magnesium chelate was found to be quite unstable,

decomposing in CH₂Cl₂solution within 2 h at room temper-

ature, but could be handled by avoiding chlorinated solvents and

by performing chromatography on basic alumina. Although the

tetraester-bacteriochlorin BC4-MeO was successfully magnesi-

ated, BC3-2E and BC2-2E each gave no reaction under similar

conditions (entries 5 and 6).

A summary of our observations concerning metalation of

synthetic bacteriochlorins is shown in Figure 1. The chief results

are as follows: (1) Bacteriochlorins lacking any electron-

withdrawing groups, including unsubstituted BC0, aﬀord a

limited set of metal chelates upon treatment with a strong base

(NaH or LDA) in THF. (2) The same strong-base conditions

accommodate a broader scope of metal chelates upon application

to a bacteriochlorin bearing two carboethoxy substituents. (3)

Bacteriochlorins bearing 2−4 carbonyl (carboethoxy, imide)

substituents can be zincated with the standard “porphyrin

metalation conditions” of Zn(OAc)₂·2H₂O in hot DMF. (4)

Where direct comparisons were made for the bacteriochlorins

bearing two carboethoxy substituents, treatment with a strong

base (NaH or LDA) aﬀorded more rapid and cleaner metalation

than use of Zn(OAc)₂·2H₂O in hot DMF. (5) A bacteriochlorin

bearing 4 carboethoxy substituents could be magnesiated under

the strong base conditions, but the resulting magnesium chelate

exhibited limited stability. (6) Ortho-aryl substituents are known

to slow substantially the rate of metalation of meso-tetraarylpor-

phyrins,⁶⁴yet no adverse eﬀect was observed upon application of

the preparative procedures with the dimesitylbacteriochlorin

(BC2-2M-MeO).

A Ni-catalyzed process for reductive cleavage of aryl methyl

ethers has recently been reported.⁶²An in-depth scrutiny of the

Pd-mediated cleavage observed herein is beyond the scope of the

present paper.

Magnesiation of Ester-Bacteriochlorins. Magnesium tetra-

pyrroles (e.g., chlorophylls, bacteriochlorophylls) are ubiquitous,

a fact thrown into sharp relief given the historical diﬃculties that

have surrounded the chemical insertion of magnesium into the

free base macrocycles.⁶⁰The biosynthetic incorporation of

magnesium (into protoporphyrin IX) is endergonic.⁶³Magnes-

ium porphyrins are class IV metalloporphyrins³²and as such

readily demetalate upon exposure to weak acids such as silica gel

and acetic acid. Failure to magnesiate BC2-2M-MeO with

Mg(OTf)₂and NaH in THF, while not unexpected, also pointed

toward the use of other reaction conditions and/or more

electron-deﬁcient bacteriochlorins. As stated above, porphyrins

(and many chlorins) can be magnesiated upon use of MgX₂(X =

Br or I) under noncoordinating slightly basic conditions (e.g., in

CH₂Cl₂containing triethylamine (TEA)).⁶⁰

Application of the condition for magnesiation of porphyrins⁶⁰

(MgI₂in CH₂Cl₂containing TEA) to the tetraester-bacterio-

chlorin BC4-MeO did not yield any magnesium chelate after

16 h (Table 5, entry 1). A large excess of magnesium reagent only

resulted in the decomposition of BC4-MeO (Table 5, entry 2).

Returning to the conditions identiﬁed for zincation in Table 2,

BC4-MeO in THF was treated with NaH followed by MgI₂

(Table 5, entries 3 and 4). After heating at 60 °C for 24 h, the Q _x

band was found at 612 nm (versus 548 nm for BC4-MeO; all in

CH₂Cl₂) and a peak at [M + 22] was found upon MALDI-MS,

Isolation and Characterization of Metallobacteriochlorins.

A number of reactions were carried out at the multimilligram

scale to obtain suﬃcient metallobacteriochlorin for physicochem-

ical studies. Thus, eight zinc bacteriochlorins (Tables 2 and 3),

three copper chelates (CuBC2-2M-MeO, CuBC0-2T, CuBC0),

two palladium chelates (PdBC2-2M-MeO, PdBC0-2T), and

one indium chelate (ClInBC0-2T) were isolated and charac-

terized. Upon puriﬁcation, the metallobacteriochlorins were

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Figure 1. Summary of metalation of synthetic bacteriochlorins.

stable under dry conditions in the absence of light for an

extended period. Each metallobacteriochlorin was characterized

by H NMR spectroscopy (except for Cu bacteriochlorins),

absorption spectroscopy, MALDI-MS or LD-MS, and ESI-MS.

The magnesium chelate (MgBC4-MeO) was only partially

characterized owing to limited stability. The availability of these

various metallobacteriochlorins enabled the physicochemical

studies described in the following sections.

The core shape of the copper bacteriochlorin CuBC0-2T is

shown in the Supporting Information, Figure S1. Copper

bacteriochlorin CuBC0-2T is fairly planar, with the copper

atom located on the least-squares plane deﬁned by the four

nitrogen atoms. The average copper−centroid distance is 2.005 Å,

which is shorter than that of free base bacteriochlorins BC0 and

BC0-2M (∼2.095 Å).

1

2. Spectral Properties. The ground-state electronic absorp-

tion spectra of the metallobacteriochlorins and the free base

bacteriochlorins in toluene are shown in Figure 4 (Zn series) and

Figure 5 (BC0-2T series). The spectral data including the

position, intensity, and full-width at half-maximum (fwhm) of the

long-wavelength absorption band (Q _y); the shift (Δλ) in the

position of the Q _yband with respect to the free base bacterio-

chlorins; and intensity ratios of the Q _yto B_ybands (I_Qy/I_Byratio)

are listed in Table 6. Table 6 also gives spectral data for the na-

tive bacteriochlorins. In general, the absorption spectra of

the synthetic metallobacteriochlorins resemble that of Bchl a,

just as the spectra of the synthetic free base bacteriochlorins

resemble that of the native free-base (Mg-less) analogue Bphe a.

The spectrum of each bacteriochlorin exhibits four absorption

bands generally categorized as B_y(0,0), B_x(0,0), Q _x(0,0), and

Q _y(0,0) from short to long wavelength. (B_y(0,0) and B_x(0,0)

may reverse positions depending on the bacteriochlorin and have

mixed x and y polarizations.) In general, the B bands of all

bacteriochlorins examined herein fall in the region 330 to 419 nm.

The Q bands of the BC0-2T series including the free base

and all metal chelates lie at shorter wavelength (Q _x499 to

536 nm; Q _y737 to 763 nm) versus those of the BC2-2M-MeO

series (Q _x524 to 556 nm; Q _y758 to 779 nm). For the Zn series,

the Q bands are located at longer wavelength compared to

the corresponding free base bacteriochlorins. The shifts

in Q _xbands (19−34 nm) are generally more signiﬁcant

than those of Q _ybands (12−16 nm). The Q _ybands of the

C. Structural and Physicochemical Characteristics.

1. Structural Analysis. The single-crystal X-ray structures of

bacteriochlorins BC0, BC0-2M, and CuBC0-2T are shown in

Figure 2. Note that BC0-2M contains 3,13-dimesityl groups

whereas CuBC0-2T contains 2,12-di-p-tolyl groups. While a

sizable number of photosynthetic proteins containing bacterio-

chlorophylls have been examined by X-ray crystallography,

relatively few single-crystal X-ray studies have been carried out of

bacteriochlorins. These include synthetic free base bacterio-

chlorins,⁶⁵synthetic metallobacteriochlorins,^42,52,66and natu-

rally derived (free base) bacteriopheophorbides.⁶⁷

The core shape of porphyrin (porphine), chlorin (FbC), and

bacteriochlorin (BC0) macrocycles are shown in Figure 3. The

core shape of porphine is close to square,⁶⁸while that of chlorin

FbC is slightly kite-shaped because of the presence of one

pyrroline ring (D) and three pyrrole rings (A, B, and C).^69,70The

core shape of bacteriochlorin BC0 is slightly rectangular. The

two pyrrole rings and two pyrroline rings that constitute a

bacteriochlorin alternate upon circumambulating the macro-

cycle; thus, the two pyrroline rings occupy opposite corners, as

do the two pyrrole rings. The core size can be evaluated by the

comparison of the average distances between each of the

nitrogen atoms and their centroid.⁷¹The order of average

nitrogen-centroid distances is porphine (2.055 Å) < chlorin

(2.074 Å) < bacteriochlorin (2.096 Å).

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Figure 2. ORTEP drawing of (A) free base bacteriochlorin BC0, (B) free base bacteriochlorin BC0-2M, and (C) copper bacteriochlorin CuBC0-2T

(one molecule from the unit cell). Ellipsoids are displayed at the 50% probability level and hydrogen atoms are omitted for clarity. The large spherical

ellipsoids of BC0 result from the high R factor value due to the weakly diﬀracting crystal.

synthetic bacteriochlorins are quite intense. For BC0-2T, the

long-wavelength maximum (732 nm) has a molar absorptiv-

ity of ∼120,000 M⁻¹cm⁻¹.¹⁶

Within the same bacteriochlorin series, the extent of the Q _x

band shift increases in order of Pd < Cu < Zn (<ClIn) chelates,

while that of Q _yincreases in order of Pd < Zn < Cu (<ClIn).

Each bacteriochlorin features a sharp Q _yband with fwhm in the range

of 20−24 nm, except for the Cu chelates which exhibit broadened

Q _yband in the range of 29−40 nm. The intensity ratios of the Q _y

to B_ybands of the metallobacteriochlorins increase inversely with

the increase of the wavelength shift (Δλ) with respect to the free

base bacteriochlorins in order of (ClIn) < Cu < Zn < Pd. For the

Zn series, the intensity ratios of the Q _yto B_ybands fall in the

range of 1.3−1.9.

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Figure 5. Absorption spectra in toluene at room temperature of

bacteriochlorins (normalized at the Q_ybands). The labels in the graph

are as follows: (a) BC0-2T (black), (b) PdBC0-2T (blue), (c) ZnBC0-2T

(red), (d) CuBC0-2T (green), and (e) ClInBC0-2T (orange).

free base bacteriochorins, the ﬂuorescence intensities are much

weaker for the indium complexes and weaker still for the

palladium complexes as described in the following.

3. Photophysical Properties. Table 7 lists the photophysical

properities of the zinc and palladium bacteriochlorins, along with

representative data for the free base and indium analogues. The

table also gives data for the native chromophores Bchl a and

Bphe a in toluene. In comparing exact values of the photo-

physical characteristics of the zinc and free base bacteriochlorins,

one must take into account that some of the zinc chelates may be

axially ligated because they, like a few Fb complexes, were studied

in THF rather than toluene for greater solubility (as indicated in

Table 7 footnotes).

Figure 3. Comparison of core structural parameters across porphyrin,

chlorin, and bacteriochlorin macrocycles.

The zinc bacteriochlorins exhibit ﬂuorescence quantum yields

(Φ_f) generally in the range 0.08−0.20 with an average value of

0.13 that is comparable to that (0.15) for the free base analogues

studied here or previously.²⁰The exception is Φ_f= 0.033 for the

bacteriochlorin−imide ZnBC3-2E, which like that (0.040)²⁰for

the free base analogue is reduced because of the lower energy

(Q _y> 800 nm; Table 6) of the singlet excited state resulting in

more facile nonradiative internal conversion. The lifetimes (τ_S)

of the singlet excited state for the zinc bacteriochlorins are in the

range 2.2−4.4 ns, with an average value of 3.3 ns. These lifetimes

are also similar to those for the free base analogues (3.3−4.4 ns;

average 3.8 ns). The typical yield of intersystem crossing to the

triplet excited state (Φ_isc) for the zinc bacteriochlorins is ∼0.7,

which is somewhat greater than the average value of ∼0.5 for the

free base analogues because of a modest eﬀect of the metal ion on

spin−orbit coupling. The typical Φ_fand τ_Svalues for the indium

chelates (0.02 and ∼0.3 ns)²¹are reduced and the Φ_iscvalues

(∼0.9) increased from those for the zinc chelates because of

greater heavy metal enhancement of spin−orbit coupling.

The heavy metal eﬀect (and potential d-orbital contribution)

is greater still for the palladium bacteriochlorins, resulting in

essentially quantitative singlet-to-triplet intersystem crossing.

The consequence for PdBC2-2M-MeO is a very low ﬂuorescence

yield (Φ_f= 0.006) and singlet lifetime (τ_S= 15 ps). The two

values are somewhat greater for PdBC0-2T for reasons that

are not clear. Enhanced spin−orbit coupling also results in a

progressive shortening of the lifetime of the lowest triplet excited

state (τ_T) from a typical value of ∼100 μs for the zinc and free

base bacteriochlorins to ∼30 μs for the indium chelates and to

∼10 μs for the palladium chelates.

Figure 4. Absorption spectra in toluene at room temperature of

bacteriochlorins (normalized at the Q_ybands). The labels in the graph

are as follows: (a) ZnBC0 (black), (b) ZnBC0-2T (red), (c) ZnBC2-

2H-MeO (orange), (d) ZnBC2-2M-MeO (yellow), (e) ZnBC4-MeO

(green), (f) ZnBC2-2E (blue), (g) ZnBC2-2H (dark blue), and (h)

ZnBC3-2E (purple).

The ﬂuorescence spectrum of each zinc bacteriochlorin is

dominated by a Q _y(0,0) band that is only modestly (5−10 nm)

shifted to longer wavelength than the Q _y(0,0) absorption band

and has a comparable spectral width (Table 6). This behavior

is analogous to that observed for free base bacteriochlorins

(Table 6 and ref 20). Similar ﬂuorescence spectra are found for

the indium chelates, as we have reported previously,²¹and for the

palladium bacteriochlorins. However, compared to the zinc and

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a

Table 6. Spectral Properties of Bacteriochlorins

b

c

B_y(0,0) abs B_x(0,0) abs Q (0,0) abs Q (0,0) abs

Q (0,0) abs

Q (0,0) em

ΔQ

x

y

compound

Zn-BCs

λ (nm)

fwhm (nm)

λ (nm)

fwhm (nm)

Δλ (n^xm) Δλ (n^ym) I_{Q y}/I_By

ZnBC0-2T

ZnBC2-2M-MeO

ZnBC4-MeO

ZnBC3-2E

ZnBC2-2E

ZnBC2-2H

ZnBC2-2H-MeO

ZnBC0

344

353

354

356

347

353

336

384

389

385

419

391

389

375

521

565

581

564

546

547

548

514

749

773

774

830

773

775

750

723

23

25

22

27

24

23

26

14

756

780

782

835

778

780

758

725

26

27

23

25

24

26

18

22

27

31

20

25

26

25

13

15

12

13

10

1.3

1.6

2.0

1.7

1.6

1.2

1.3

1.7

Pd-BCs

PdBC0-2T

PdBC2-2M-MeO

Cu-BCs

330

337

379

382

499

538

739

758

21

20

745

765

25

23

0

3

0

1.7

2.8

CuBC0-2T

CuBC2-2M-MeO

CuBC0

337

348

332

383

390

378

512

556

507

755

780

728

29

37

19

13

18

22

15

1.2

1.5

1.7

FbBCs

BC0-2T

351

361

358

354

357

340

374

383

368

408

383

379

365

499

538

550

544

521

522

489

736

758

759

818

761

762

740

713

20

22

20

24

20

18

12

742

765

763

823

764

766

746

716

23

24

21

16

0

1.0

1.2

1.3

0.9

1.1

0.9

BC2-2M-MeO

d

BC4-MeO

d

BC3-2E

d

BC2-2E

BC2-2H

BC2-2H-MeO

d

BC0

In-ClBCs

e

ClInBC0-2T

350

360

388

380

539

599

763

776

23

31

769

780

31

33

40

49

27

17

1.1

0.9

MgBCs

MgBC4-MeO

Native BCs

BChl a

363

362

396

389

581

532

781

758

28

31

789

768

29

27

49

0

21

0

1.4

0.7

d

BPhe a

a

b

In toluene at room temperature. The nominal B_x(0,0) and B_y(0,0) absorption bands may alternate order with compound and have mixed x and y

c

d

e

polarization. The shift of the band relative to that of the free base analogue. Data from ref 20. Data from ref 21.

In the case of copper bacteriochlorins (CuBC0, CuBC0-2T,

CuBC2-2M-MeO), interactions involving the unpaired metal

electron associated with the d⁹conﬁguration of Cu(II) transform

the macrocycle singlet excited state into a “singdoublet” and split

the macrocycle triplet excited state into “tripdoublet” and

“quartet” excited states that are close in energy, in analogy to

copper porphyrins.⁷Normal ﬂuorescence is not expected (and

none is found in the case of CuBC2-2M-MeO). Transient

absorption studies of CuBC0, CuBC0-2T, and CuBC2-2M-

MeO indicate essentially complete decay to the ground state with

time constants of 0.3, 0.5, and 1.7 ns in THF. This time evolution

likely represents deactivation of the tripdoublet/quartet excited-

state manifold via a ring-to-metal charge-transfer state that has been

implicated in the excited-state dynamics of copper porphyrins,⁷²but

which now lies at lower energy in the corresponding bacterio-

chlorins because of the greater ease of macrocycle oxidation.

Zinc tetrapyrroles (generally porphyrins and chlorins until

the present) are often exploited in photophysical and photo-

chemical applications compared to the corresponding magnes-

ium complexes because of a reduced propensity for demetalation.

In the case of porphyrins, a sacriﬁce is a shorter singlet excited-

state lifetime (e.g., ∼2 versus ∼6 ns) and ﬂuorescence yield

(∼0.03 versus ∼0.13). Here we have found that the zinc bacterio-

chlorin ZnBC4-MeO has Φ_f, τ_S, Φ_isc, and τ_Tvalues comparable

to those of the corresponding magnesium bacteriochorin

MgBC4-MeO. In this regard, compared to the native magnesium

bacteriochlorin, Bchl a (Table 6),⁷³⁻⁷⁵the zinc bacteriochlorins

generally have similar Φ_f, comparable or greater τ_S, comparable

Φ_isc, and longer τ_Tvalues. This comparison is similar to that for

the free base bacteriochlorins relative to the native metal-free

bacteriochlorin Bphe a (Table 6).²⁰In summary, the synthetic

zinc bacteriochlorins (and the indium and palladium analogues),

like the free base bacteriochlorins, exhibit photophysical

characteristics suitable for a range of applications in solar-energy

conversion and photomedicine.

4. Electrochemical and Molecular Orbital Characteristics.

The redox properties (reduction potentials) and energies of the

frontier molecular orbitals (MOs) of the bacteriochlorins are

listed in Table 7. Only the potentials for the ﬁrst oxidation (E_ox)

and reduction (E_red) (which are both reversible) are presented in

the table, as these are most germane for the discussion below. It

should be noted, however, that the molecules also exhibit redox

processes corresponding to second oxidations and reductions.

Diﬀerences in the E_oxand E_redvalues among the diﬀerent

metallobacteriochlorins and free base analogues generally

parallel those for porphyrin systems.²In prior work on a large

number of chlorins,⁷⁶good correlations were found between the

E_oxand the highest occupied molecular orbital (HOMO) energy

and between the E_redand the lowest unoccupied molecular

orbital (LUMO) energy. Such a correlation is generally found in

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a

Table 7. Photophysical, Redox, and Molecular-Orbital Properties of Bacteriochlorins

c

b

compound

Zn-BCs

τ_S(ns)

Φ_f

Φ_isc

τ_T(μs)

E_ox(V)

E_red(V)

HOMO (eV)

LUMO (eV)

ZnBC0-2T

ZnBC2-2M-MeO

ZnBC4-MeO

ZnBC3-2E

ZnBC2-2E

ZnBC2-2H

ZnBC2-2H-MeO

ZnBC0

2.9

4.4

2.2

2.6

3.5

4.3

3.4

0.11

0.12

0.13

0.033

0.08

0.14

0.20

0.10

0.83

0.71

0.80

0.28

0.71

0.60

0.70

0.67

161

120

38

−0.04

+0.45

+0.16

+0.02

−0.12

0.00

−1.60

−1.38

−1.10

−1.12

−1.42

−1.47

−1.68

−4.26

−4.55

−4.87

−4.78

−4.48

−4.47

−4.48

−4.30

−2.20

−2.51

−2.92

−2.94

−2.53

−2.52

−2.46

−2.16

94

149

191

187

151

−0.14

−0.12

Pd-BCs

PdBC0-2T

PdBC2-2M-MeO

Cu-BCs

0.35

0.020

0.006

>0.99

12

+0.43

+0.29

−1.14

−1.29

−4.36

−4.63

−2.26

−2.54

0.015

5.8

c

CuBC0-2T

CuBC2-2M-MeO

CuBC0

0.5 ns

−0.04

+0.18

−0.04

−1.53

−1.32

−1.60

−4.25

−4.53

−4.27

−2.25

−2.55

−2.18

c

1.7 ns

c

0.3 ns

FbBCs

d

BC0-2T

3.3

3.9

4.3

1.9

3.3

4.4

3.9

0.18

0.15

0.16

0.04

0.14

0.10

0.17

0.14

0.55

0.35

0.24

0.51

0.55

0.45

0.49

0.24

163

52

+0.21

+0.38

+0.57

+0.45

+0.29

+0.28

+0.45

−1.49

−1.29

−1.05

−0.98

−1.32

−1.33

−1.43

−0.99

−4.40

−4.65

−5.00

−4.91

−4.68

−4.59

−4.60

−4.46

−2.22

−2.48

−2.95

−2.99

−2.58

−2.52

−2.45

−2.20

BC2-2M-MeO

d

BC4-MeO

46

d

BC3−2E

BC2-2E

85

d

110

86

BC2-2H

BC2-2H-MeO

d

BC0

169

In-ClBCs

e

ClInBC0-2T

0.21

5.4

0.016

0.16

0.9

44

90

+0.31

−1.25

−4.52

−4.86

−2.52

−2.94

MgBCs

MgBC4-MeO

Native BCs

BChl a

0.60

3.1

2.7

0.12

0.10

0.30

0.57

50

25

−4.75

−4.87

−2.86

−2.84

d

BPhe a

a

In toluene at room temperature except as follows: the τ_Tvalues for all compounds and the Φ_f, Φ_isc, and τ_Svalues for BC2-2H, BC2-2H-MeO,

b

ZnBC0, ZnBC2-2H, ZnBC2-2H-MeO, and MgBC4-MeO were determined in tetrahydrofuran. First oxidation (E_ox) and ﬁrst reduction (E_red

potentials measured in 0.1 M tetrabutylammonium hexaﬂuorophosphate in which the ferrocene couple has an E_1/2of 0.19 V. Decay of the

tripdoublet/quartet excited-state manifold in nanoseconds. Data from ref 20. Data from ref 21.

)

c

d

e

Figure 6. Eﬀect of the number of electron-withdrawing (carbonyl) groups on the redox potentials and frontier MO energies.

Table 7 and in Figure 6, which plots the redox potentials and MO

energies versus the number of electron-withdrawing groups on

the bacteriochlorin.

Comparison of HOMO and LUMO energies of compounds

BC0, BC0-2T, and BC3-2E listed in Table 7 with the values for

their counterparts studied previously²⁰that contain a 5-methoxy

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group shows that the 5-methoxy group shifts the MO energies by

a relatively small amount (≤0.08 eV). When there is a shift in the

MO energies, the shift is to slightly more negative values,

indicating that the compound should be slightly harder to oxidize

and easier to reduce. The data in Table 7 and Figure 6 further

show that an increasing number of electron-withdrawing groups

on the bacteriochlorin (aﬀording greater ease of metalation) is

reﬂected in a more positive E_ox(harder to oxidize) and a less

negative E_red(more diﬃcult to reduce). The one compound that

is an outlier is BC0. Along the same set of compounds, an

increasing number of electron-withdrawing groups is reﬂected in

shifts in the HOMO energy to more negative values (harder to

oxidize) and the LUMO energy to more negative values (easier

to reduce). Here, compound BC0 is not an outlier and has

essentially the same MO energies as compound BC0-2T. Thus,

the fact that BC0 is an outlier in the redox data may be in part a

solvation (electrolyte) eﬀect.

As expected, an increasing number of electron-withdrawing

groups shifts the redox potentials and MO energies so as to make

it harder to remove an electron (or electron density) and easier to

add an electron (or electron density). Because metalation

involves replacing two protons of the free base with a divalent

metal ion, and a pair of protons is typically more electropositive

than the metal ion, metalation eﬀectively involves a net addition

of electron density to the macrocycle. This property results in the

correlation between the ease of metalation and the redox and

MO energies.

The above comparisons are made for a set of bacteriochlorins

that diﬀer in the number and types of substituents at the same

macrocycle positions. These changes cause shifts in the energies

and electron densities of the HOMO and LUMO, but do not

alter the identities of these two orbitals. The ﬁnding of such

correlations, or even the interpretation if they are found, may be

more diﬃcult if the set of molecules diﬀer in the sites of

macrocycle substitution, and particularly if diﬀerent macrocycles

are involved. For example, depending on the substituent pattern,

in progressing from porphyrin to chlorin (and then to

bacteriochlorin), the HOMO may change from the a_2u(π)

orbital that has substantial electron density at the central

nitrogens (and metal ion once incorporated) to the a_1u(π)-like

orbital that has far less electron density or even nodes at these

positions. Such a switch would need to be taken into account in

assessing relationships between the ease (kinetics and

thermodynamics) of metalation versus the MO and redox

properties.

demetalation. The diﬃculty of metalation upon moving to

hydroporphyrins (porphyrin < chlorin < bacteriochlorin) has

been attributed to the diminution of ligand nucleophilicity that

accompanies saturation of the pyrrole rings.²²On the other hand,

a careful study by Saga et al. of identically substituted macro-

cycles revealed that the ease of zinc demetalation decreased along

the series porphyrin chlorin ≫ bacteriochlorin.⁷⁷In contrast to

>

∼

porphyrins, where the availability of collections of diverse

macrocycles in ample quantities have enabled systematic studies

of metalation and demetalation chemistry, comparable studies

with bacteriochlorins to assess kinetics and thermodynamics

have largely remained out of reach.

A de novo route to bacteriochlorins has provided a suite of

macrocycles that diﬀer in number and type of substituents. The

macrocycles provide the foundation for initiation of systematic

studies of metalation methods. While a full matrix deﬁned by

metalation conditions, metal types, metal ligands, and bacterio-

chlorin substrates has not been performed, attempts to metalate

the set of synthetic bacteriochlorins examined herein has led to

the following observations:

• The diﬃculty of metalation of tetrapyrrole macrocycles

decreases for bacteriochlorins with increasing number of

electron-withdrawing groups.

• Metalation of a bacteriochlorin occurs upon treatment with a

strong base (e.g., NaH or LDA) in THF followed by MX_n: (a) for

bacteriochlorins that bear electron-releasing groups, M = Cu, Zn,

Pd, and InCl; (b) for bacteriochlorins that bear two carboethoxy

(electron-withdrawing) groups, M = Ni, Cu, Zn, Pd, Cd, InCl,

and Sn (but not Al or Au); and (c) a bacteriochlorin with four

carboethoxy groups was metalated with Mg.

• Bacteriochlorins that bear ≥2 carbonyl groups typically can

be zincated by standard porphyrin metalation conditions

[Zn(OAc)₂·2H₂O in DMF at 60−80 °C].

Scheer has suggested that the rate-determining step of

bacteriochlorin metalation consists of deprotonation of the

pyrrole N−H protons.²³The use of a very strong base overcomes

this limitation, and resembles the method developed by Arnold

for preparing early transition metal chelates of porphyrins.

The Arnold method entails formation and isolation of the

dilithium derivative of the porphyrin as the reactive species for

transmetalation upon treatment with a metal reagent.^53,54Such

method has been applied by Stolzenberg with tetra-p-

tolylbacteriochlorin to prepare the oxotitanyl chelate.⁵⁵The

deprotonation of the N−H protons would be facilitated with

increasing number of electron-withdrawing groups located on

the pyrrole units, as observed here.

III. CONCLUSIONS AND OUTLOOK

In comparing the above results with other types of tetrapyrrole

macrocycles, it warrants emphasis that the (up to four) carbonyl

groups were located exclusively in the pyrrole (rings A and C)

and not in the pyrroline (rings B and D) units of the

bacteriochlorins. By contrast, studies of chlorins can incorporate

groups in the pyrrole (rings A and C), pyrrolenine (ring B), and

pyrroline (ring D) units. In porphyrins, both pyrrole and

pyrrolenine groups are present yet facile tautomerization

typically precludes localization of a substituent in a particular

heterocycle.

The studies reported herein concerning metalation of diverse

synthetic bacteriochlorinsan ostensibly simple reaction

provide access to a number of the corresponding metallo-

bacteriochlorins. One area of particular interest is the

examination of dyadic (and larger) arrays composed of free

base and metallobacteriochlorins. In this regard, a review of all

covalently linked arrays that contain one or more bacteriochlorins

The ability to prepare synthetic metallobacteriochlorins is

essential for biomimetic studies pertaining to the roles of

bacteriochlorophylls in bacterial photosynthesis and to probe the

electronic interplay of peripheral substituents and central metal

on photophysical properties. In this regard, the metalation of

bacteriochlorins over the years has in some cases proceeded

uneventfully and in other cases proved extremely diﬃcult. In

general, the reaction course for metalation of tetrapyrrole macro-

cycles has been interpreted in terms of a variety of parameters,

including macrocycle conformation, molecular rigidity (ability

to distort from a planar conformation to accommodate the

incoming metal ion), nucleophilicity of the nitrogens toward the

incoming metal ion, and solvent interactions that entail

deprotonation of the pyrrolic NH bonds as well as coordination

to the metal ion.²²Related to the ease of preparing a metal

chelate is the stability of the resulting metal chelate toward

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with longer retention times aﬀorded an increase in impurities. A

basiciﬁed eluant [hexanes/CH₂Cl₂(1:1 to 3:1) containing 1−2% of

TEA] and passage over a short column (ca. 10 cm length, 2 cm

diameter) diminished the decomposition but at the expense of resolu-

tion. The poor resolution impeded removal of hydrocarbons and

unreacted free base bacteriochlorin. Attempts to perform puriﬁcation on

alumina columns or the very traditional sugar columns (widely

employed for chlorophyll isolation)⁸¹also gave the same issue of

balance between retention time and purity.

Greater success at puriﬁcation was achieved by forcing the reaction to

completion with prolonged reaction time, thereby aﬀording a mixture

that contains only a small amount of the free base bacteriochlorin. The

crude mixture was then subjected to chromatography on silica gel

[hexanes/CH₂Cl₂(1:1) to CH₂Cl₂] to remove impurities. At small

scale, the chromatography could be performed in a Pasteur pipet. The

resulting product was concentrated to dryness. The method of

subsequent puriﬁcation of the resulting solid depended on the solubility

of the free base bacteriochlorin and metallobacteriochlorin.

reveals only ∼20 dyads prepared to date, and most of the

bacteriochlorins incorporated therein have been free base

species.⁸Thus, the study of heterometalated arrays, an approach

that has been widely used to probe photosynthetic-like mecha-

nisms in synthetic multipigment architectures,⁷⁸has largely

resided outside the scope of experimentation for bacteriochlorins

(but has been accessible via computational means⁷⁹). The

straightforward access described herein should open the door to

the study of fundamental properties, tuning NIR spectral

properties, and pursuit of a range of photochemical applications

of synthetic metallobacteriochlorins.

IV. EXPERIMENTAL SECTION

A. General Methods. ¹H NMR (400 MHz) spectra and ¹³C NMR

spectra (100 MHz) were collected at room temperature in CDCl₃unless

noted otherwise. Absorption spectra were collected in toluene at room

temperature. NaH (60% dispersion in mineral oil) and LDA (2.0 M

solution in heptanes/THF/ethylbenzene) were provided by Aldrich.

Bacteriochlorins were analyzed by laser desorption mass spectrometry

in the absence of a matrix (LD-MS) (e.g., the BC0-2T series) or in the

presence of the matrix POPOP (MALDI-MS).⁸⁰Silica gel (40 μm

average particle size) was used for column chromatography. All solvents

were reagent grade and were used as received unless noted otherwise.

THF was freshly distilled from sodium/benzophenone ketyl. Anhydrous

MeOH was reagent grade and was used as received. Electrospray

ionization mass spectrometry (ESI-MS) and fast atom bombardment

mass spectrometry (FAB-MS) data are reported for the molecular ion or

protonated molecular ion. The concentration of bases and metal

reagents is typically given in mM quantities for clarity although not all

material may be dissolved.

For the BC4-MeO, BC3-2E, BC2-2E, BC0-2T, and BC0 series, free

base bacteriochlorins were soluble in hexanes whereas the metal-

lobacteriochlorins derived therefrom were insoluble in hexanes. Accord-

ingly, the crude solid was treated with hexanes, sonicated in a benchtop

sonication bath, centrifuged, and the supernatant discarded. Repetition

once or twice resulted in a solid product that consisted of the

metallobacteriochlorin in pure form.

For the BC2-2E, BC2-2E-MeO, and BC2-2M-MeO series, both free

base bacteriochlorins and the metallobacteriochlorins derived therefrom

were soluble in hexanes. Accordingly, the crude solid was dissolved in

methanol and treated with hexanes. Two intensely colored phases

typically form, and can be separated with the aide of illumination to

identify the interface. Thus, the hexanes phase (upper layer) was

removed as this phase was highly enriched in free base bacteriochlorins

yet also contained some metallobacteriochlorin. The methanol phase

typically contained the desired metallobacteriochlorin in pure form.

The zinc bacteriochlorins, except those that contain ≥3 electron-

withdrawing substituents (ZnBC4-MeO and ZnBC3-2E) tend to

demetalate if exposed to prolonged chromatography on silica gel. In

most cases, the workup entailed a short column chromatography or

washing the solid metallobacteriochlorin product with hexanes to

remove hydrocarbons (derived from NaH) or other impurities. In the

case of palladium metalation with LDA, an isopropylamine-like

byproduct was always found in the crude mixture by ¹H NMR

spectroscopy; in the case of BC2-2M-MeO such impurity could be

removed by size-exclusion chromatography.

E. X-ray Crystallographic Data Collection and Processing. The

samples were mounted on a nylon loop with a small amount of NVH

immersion oil. All X-ray measurements were made on a Bruker-Nonius

X8 Apex2 CCD diﬀractometer at a temperature of 173 K (BC0 and

CuBC0-2T) or 110 K (BC0-2M). The frame integration was performed

using SAINT+.⁸²The resulting raw data were scaled and absorption-

corrected by multiscan averaging of symmetry equivalent data using

SADABS.⁸³The data are shown in Table 8.

F. X-ray Crystallographic Structure Solution and Reﬁnement.

The structures were solved by direct methods using SIR92.⁸⁴All non-

hydrogen atoms were obtained from the initial E-map. The hydrogen

atom positions were placed at idealized positions and were allowed to

ride on the parent carbon atom. The structural model was ﬁt to the data

using full matrix least-squares based on F². The model required 100

restraints to keep the anisotropic displacement parameters from going

nonpositive deﬁnite. The calculated structure factors included correc-

tions for anomalous dispersion from the usual tabulation. The structure

was reﬁned using the XL program from the SHELXTL package,⁸⁵and

graphic plots were produced using the version of ORTEP included in

the NRCVAX crystallographic program suite.⁸⁶

G. Optical and Photophysical Characterization. Static

absorption (Varian Cary 100 or Shimadzu UV-1800) and ﬂuorescence

(Spex Fluorolog Tau 2 or PTI Quantamaster 40) measurements were

performed at room temperature, as were all other studies. The ﬂuorescence

quantum yield (Φ_f), singlet excited-state lifetime (τ_S) and triplet yield (Φ_T)

measurements utilized dilute (μM) Ar-purged toluene and methanol

B. Survey of Metalation. Each reaction was carried out in a conical

microreaction vial equipped with a conical stir bar and ﬁtted with

a Teﬂon septum. A bulk solution of the bacteriochlorin was prepared

(∼6 mg of bacteriochlorin in ∼10 mL of THF) and divided without

dilution into the microreaction vials. The concentration of the bulk

solution was determined by 1000-fold dilution into an absorption cuvette,

relying on the known molar absorption coeﬃcient of representative

bacteriochlorins (BC0-2T has λ_{737 nm}= 130,000 M⁻¹cm⁻¹;¹⁶BC2-2M-

MeO has λ_{758 nm}= 120,000 M⁻¹cm⁻¹) in toluene.

The reaction mixtures were checked by TLC and absorption

spectroscopy, the latter again by 1000-fold dilution into an absorption

cuvette. The yield was determined spectroscopically assuming equal

absorptivity in the Q _yband for both the free base bacteriochlorin and

the metallobacteriochlorin. If the metalation was found to go to

completion, the reaction mixture was quenched by the addition of

saturated aqueous NaHCO₃solution and extraction with CH₂Cl₂or

ethyl acetate. The resulting mixture was concentrated and checked by

LD-MS or MALDI-MS. The results are reported in Tables 2, 4, and 5,

and in the Supporting Information.

C. Noncommercial Compounds. Compounds 1M,²⁵6,^16,19

BC0,¹⁸BC4-MeO,¹⁸BC3-2E,²⁴BC2-2E,¹⁸and BC0-2T¹⁶were

prepared as described in the literature.

D. Puriﬁcation. The NaH was received as a 60% dispersion in

mineral oil. The reaction could be carried out with the NaH as received,

without a prewash with hexanes to remove the mineral oil, in which case

the mineral oil would be removed from the metallobacteriochlorin upon

column chromatography or crystallization. In general, however, it was

preferable to remove the mineral oil from NaH, under argon, by washing

with hexanes, prior to treatment with the free base bacteriochlorin.

Typically the crude reaction mixture upon metalation is relatively

clean, the dominant impurities consisting of hydrocarbons (e.g., derived

from NaH) and any unwanted free base bacteriochlorin. Initially, we

attempted puriﬁcation by ﬂash chromatography on silica gel. During the

course of elution, an intense colored band gradually split into multiple

colored bands, which reﬂected the decomposition of the macrocycles.

Absorption spectroscopy analysis of each collected band indicated the

fastest eluting band was usually the desired metallobacteriochlorin

whereas the slower eluting bands consisted of impurities. Chromatography

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ammonium hexaﬂuorophosphate (Aldrich; recrystallized three times

from methanol and dried at 110 °C in vacuo). The electrochemical cell

was housed in a Vacuum Atmospheres glovebox (Model HE-93)

equipped with a Dri-Train (Model 493). The E_1/2values were obtained

with square wave voltammetry (frequency 10 Hz) under conditions

where the ferrocene couple has a potential of +0.19 V.

I. Density Functional Theory Calculations. Calculations were

performed with Spartan ’08 for Windows version 1.2.0 in parallel

mode⁹⁴on a PC equipped with an Intel i7-975 CPU, 24 Gb ram, and

three 300 Gb, 10k rpm hard drives. The calculations employed the

hybrid B3LYP functional and 6-31G* basis set. The equilibrium

geometries were fully optimized using the default parameters of the

Spartan program.

Table 8. Summary of Crystal Data for BC0, BC0-2M, and

CuBC0-2T

BC0

BC0-2M

C₄₂H₄₆N₄

CuBC0-2T

formula

C₂₄H₂₆N₄

370.49

C₃₈H₃₆CuN₄

612.25

formula weight

(g/mol)

606.85

crystal dimensions

(mm)

0.12 × 0.10 ×

0.02

0.30 × 0.26 ×

0.10

0.20 × 0.16 ×

0.10

crystal color and habit green prism

dark green

prism

red plate

crystal system

space group

temperature, K

a, Å

rhombohedral

triclinic

monoclinic

P2₁/c

R3

̅

173

P1

̅

110

J. Synthesis Procedures. 3-(Ethoxycarbonyl)-4-heptylpyrrole

(2H). Following the van Leusen method,²⁶a solution of α,β-unsaturated

ester 1H (12.3 g, 58.0 mmol) and TosMIC (12.6 g, 64.5 mmol) in

diethyl ether/DMSO (300 mL, 2:1) was slowly added via an addition

funnel to a suspension of NaH (5.0 g, 60% in oil suspension, 0.12 mol) in

100 mL of diethyl ether. The resulting exotherm caused the mixture to

reﬂux. The reaction mixture was stirred at room temperature for 16 h.

Water was added carefully, and the mixture was extracted with diethyl

ether. The organic layer was concentrated and dried (Na₂SO₄). Column

chromatography (silica, CH₂Cl₂) aﬀorded a light yellow solid (8.3 g,

55%): mp 54−56 °C; ¹H NMR (300 MHz) δ 0.88 (t, J = 6.6 Hz, 3H),

1.26−1.35 (m, 9H), 1.33 (t, J = 7.2 Hz, 2H), 1.58 (m, 2H), 2.71 (t, J =

7.2 Hz, 2H), 4.27 (q, J = 7.2 Hz, 2H), 6.52 (m, 1H), 7.37 (m, 1H), 8.78

(brs, 1H); ¹³C NMR (75 MHz) δ 14.1, 14.5, 22.7, 26.3, 29.3, 29.7, 30.6,

32.0, 59.4, 114.1, 116.8, 124.6, 126.4, 165.8; ESI-MS obsd 260.1614,

calcd 260.1621 [(M + Na)⁺, M = C₁₄H₂₃NO₂]; Anal. Calcd for

C₁₄H₂₃NO₂: C, 70.85; H, 9.77; N, 5.90. Found: C, 70.90; H, 9.81; N, 5.48.

4-(Ethoxycarbonyl)-2-formyl-3-heptylpyrrole (3H). Following a

general procedure,²⁷the Vilsmeier reagent was prepared by treatment

of dry DMF (30 mL) with POCl₃(4.6 mL, 49 mmol) at 0 °C and stirring

of the resulting mixture for 10 min. In a separate ﬂask, a solution of 2H

(10.7 g, 45.1 mmol) in DMF (150 mL) was treated with the freshly

prepared Vilsmeier reagent at 0 °C. The resulting mixture was stirred at

0 °C for 1 h and then 2 h at room temperature. The reaction mixture was

treated with a mixture of saturated aqueous sodium acetate/CH₂Cl₂

[400 mL, 1:1 (v/v)] and stirred for 1 h. The water phase was separated

and extracted with CH₂Cl₂. The combined organic phase was washed

with saturated NaCl, dried (Na₂SO₄), and concentrated. Column

chromatography (silica, CH₂Cl₂) aﬀorded a light brown solid (6.7 g,

56%): mp 45−47 °C; ¹H NMR (300 MHz) δ 0.88 (t, J = 6.4 Hz, 3H),

1.27−1.37 (m, 9H), 1.36 (t, J = 7.2 Hz, 2H), 1.66 (m, 2H), 3.04 (t, J =

7.2 Hz, 2H), 4.31 (q, J = 7.6 Hz, 2H), 7.63 (m, 1H), 9.60 (brs, 1H), 9.69

(s, 1H); ¹³C NMR δ 14.1, 14.4, 22.7, 24.3, 29.2, 29.6, 31.9, 32.3, 60.0,

116.5, 130.4, 131.5, 140.1, 164.1, 178.8; ESI-MS obsd 266.1746, calcd

266.1751 [(M + H)⁺, M = C₁₅H₂₃NO₃]; Anal. Calcd for C₁₅H₂₃NO₃: C,

67.90; H, 8.74; N, 5.28. Found: C, 67.86; H, 8.76; N, 5.17.

4-(Ethoxycarbonyl)-3-heptyl-2-(2-nitroethyl)pyrrole (5H). Follow-

ing a general procedure,¹⁷a stirred mixture of 3H (5.8 g, 22 mmol),

potassium acetate (1.7 g, 18 mmol), and methylamine hydrochloride

(1.2 g, 18 mmol) in absolute ethanol (8 mL) was treated with nitro-

methane (3.0 mL, 55 mmol). The mixture was stirred for 2 h,

whereupon water was added. The reaction mixture was ﬁltered, and the

ﬁltered material was washed with water and a small amount of cold

ethanol. The ﬁltered material was dried under high vacuum to aﬀord a

yellow solid, which was used directly in the next step. The crude solid

material was dissolved in CHCl₃/2-propanol (3:1, 250 mL). Silica

(24 g) and NaBH₄(1.5 g, 40 mmol) were added,²⁹and the mixture was

stirred at room temperature under argon for 2 h. The reaction mixture

was ﬁltered, and the ﬁltrate was concentrated. The resulting crude solid

was dissolved in CH₂Cl₂. The organic solution was washed (water,

brine), dried (Na₂SO₄), and concentrated to aﬀord a pale brown solid

(3.0 g, 44%): mp 93−95 °C; ¹H NMR (300 MHz) δ 0.88 (t, J = 6.4 Hz,

3H), 1.28−1.35 (m, 9H), 1.33 (t, J = 7.2 Hz, 2H), 1.49 (m, 2H), 2.63 (t,

J = 7.8 Hz, 2H), 3.25 (t, J = 6.3 Hz, 2H), 4.27 (q, J = 7.2 Hz, 2H), 4.54

(t, J = 6.3 Hz, 2H), 7.31 (d, J = 3.0 Hz, 1H), 8.39 (brs, 1H); ¹³C NMR δ

14.2, 14.5, 22.8, 23.4, 25.0, 29.4, 30.0, 31.9, 32.1, 59.6, 75.3, 114.8, 123.5,

123.7, 124.2, 165.5; ESI-MS obsd 311.1951, calcd 311.1965 [(M + H)⁺,

173

20.4174(6)

20.4174

12.4782(4)

90.00

7.2455(3)

10.4695(3)

12.0705(5)

71.0943(16)

85.586(2)

74.8557(17)

836.12(5)

1

17.3789(11)

35.432(2)

16.9880(12)

90.0

b, Å

c, Å

α, deg

β, deg

90.00

90.017(5)

90.0

γ, deg

V, Å³

120.00

4504.9(2)

9

10460.8(12)

14

Z

μ, (cm⁻¹

)

0.074

0.07

0.765

2Φmax (deg)

44.04

61.28

41.3

no. of reﬂections

measured

20677

31993

47168

unique reﬂections

1237

4362

10634

measured

a

R₁

0.1146

0.3003

0.1434

0.3304

0.065

0.056

0.067

0.066

0.0662

0.1533

0.1344

0.1930

b

wR₂

a

R₁(all data)

b

wR₂(all data)

GOF

0.095

1.96

1.126

a

b

2

4

R₁= ∑||F_o| − |F_c||/∑|F_o|. wR₂= [∑w(F_o− F_c²)²/∑w(F_o)]^1/2

.

solutions. The triplet lifetime (τ_T) measurements utilized Ar-purged 2-

methyltetrahydrofuran (2-MeTHF) solutions. Samples for Φ_fmeasure-

ments had an absorbance ≤0.1 at the excitation wavelength to minimize

front-face eﬀects and similarly low absorbance in the Q _y(0,0) band to

minimize inner-ﬁlter eﬀects.

Static emission measurements employed 2−4 nm excitation- and

detection-monochromator bandwidths and 0.2-nm data intervals.

Emission spectra were corrected for detection-system spectral response.

Fluorescence quantum yields were determined relative to several dif-

ferent standards. These standards are (i) chlorophyll a in deoxygenated

toluene (Φ_f= 0.325),⁸⁷which is the value measured in benzene;⁸⁸(ii)

free base meso-tetraphenylporphyrin (FbTPP) in nondegassed toluene,

for which Φ_f= 0.070 was established with respect to the zinc chelate

ZnTPP in nondegassed toluene (Φ_f= 0.030),⁸⁹consistent with prior

results on FbTPP;⁹⁰and (iii) 8,8,18,18-tetramethylbacteriochlorin⁹¹in

Ar-purged toluene, for which Φ_f= 0.14 was established with respect to

chlorophyll a in benzene and FbTPP in toluene.

Fluorescence lifetimes were obtained using time-correlated-single-

photon-counting detection on an apparatus with an approximately

Gaussian instrument response function with a full-width-at-half-

maximum of ∼1 ns (Photon Technology International LaserStrobe

TM-3). Samples were excited in the Soret or Q regions using excitation

pulses at 337 nm from a nitrogen laser or in the blue to green spectral

regions from a dye laser pumped by the nitrogen laser.

The Φ_iscvalues (triplet yields) were obtained using transient

absorption spectroscopy. The extent of bleaching of the ground-state

Q _xbands due to the formation of the lowest singlet excited state was

measured immediately following a 130 fs ﬂash in the Q _y(0,0) band and

compared with that due to the formation of the lowest triplet excited

state at the asymptote of the singlet excited-state decay.^20,92

H. Electrochemistry. The electrochemical studies were performed

in butyronitrile (Burdick and Jackson) using previously described

instrumentation.⁹³The supporting electrolyte was 0.1 M tetrabutyl-

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M = C₁₆H₂₆N₂O₄]; Anal. Calcd for C₁₆H₂₆N₂O₄: C, 61.91; H, 8.44; N,

9.03. Found: C, 62.52; H, 8.48; N, 8.81.

1.06 (t, J = 7.2 Hz, 3H), 2.00 (s, 6H), 2.30 (s, 3H), 4.14 (q, J = 7.2 Hz,

2H), 6.84−6.86 (m, 1H), 6.91 (s, 2H), 9.64−9.92 (br, 1H), 10.25

(s, 1H); ¹³C NMR δ 13.7, 20.7, 21.0, 60.3, 121.2, 123.8, 127.6, 130.4,

133.4, 136.8, 137.1, 163.5, 182.2; ESI-MS obsd 286.1439, calcd

286.1438 [(M + H)⁺, M = C₁₇H₁₉NO₃].

6-(4-(Ethoxycarbonyl)-3-heptylpyrrol-2-yl)-1,1-dimethoxy-4,4-di-

methyl-5-nitrohexan-2-one (7H). Following a general procedure,³⁰

a

mixture of 5H (2.8 g, 9.0 mmol) and 6 (4.2 g, 27 mmol, 3 equiv) was

treated with DBU (4.2 mL, 27 mmol). The reaction mixture was stirred

under argon at room temperature for 16 h. A saturated solution of cold

aqueous NH₄Cl was added. The mixture was extracted with ethyl

acetate, and the organic layer was washed with brine, dried (Na₂SO₄),

and concentrated. Column chromatography [silica, CH₂Cl₂/ethyl

acetate (9:1)] aﬀorded a pale brown solid (2.5 g, 59%): mp 78−82 °C;

¹H NMR (300 MHz) δ 0.88 (t, J = 6.8 Hz, 3H), 1.15 (s, 3H), 1.25 (s,

3H), 1.28−1.33 (m, 9H), 1.32 (t, J = 7.2 Hz, 2H), 1.51 (m, 2H), 2.61 (m,

2H), 2.60, 2.75 (AB, J = 18.7 Hz, 2H), 3.00 (ABX, ³J = 2.3 Hz, ²J = 15.5

Hz, 1H), 3.27 (ABX, ³J = 11.7 Hz, ²J = 15.5 Hz, 1H), 3.43 (s, 3H), 3.44

(s, 3H), 4.25 (q, J = 7.2 Hz, 2H), 4.36 (s, 1H), 5.11 (ABX, ³J = 2.3 Hz,

³J = 11.7 Hz, 1H), 7.26 (d, J = 8.8 Hz, 1H), 8.46 (brs, 1H); ¹³C NMR

(75 MHz) δ 14.2, 14.5, 22.8, 24.2, 24.3, 24.6, 25.0, 29.4, 30.0, 31.8, 32.1,

36.6, 45.0, 55.3, 59.4, 94.7, 104.9, 114.7, 123.5, 123.7, 124.1, 165.2,

203.7; ESI-MS obsd 469.2893, calcd 469.2908 [(M + H)⁺, M =

C₂₄H₄₀N₂O₇]; Anal. Calcd for C₂₄H₄₀N₂O₇: C, 61.52; H, 8.60; N, 5.98.

Found: C, 61.84; H, 8.68; N, 5.82.

4-(Ethoxycarbonyl)-3-mesityl-2-(2-nitrovinyl)pyrrole (4M). Fol-

lowing a general procedure,²⁸a stirred solution of acetic acid

(0.69 mL, 13 mmol) in methanol (1.75 mL) under argon at 0 °C was

treated dropwise with n-propylamine (0.95 mL, 12 mmol). The resulting

n-propylammonium acetate solution was stirred at 0 °C for 5 min, then

added dropwise to a stirred solution of 3M (5.90 g, 20.7 mmol) in

nitromethane (3.38 mL, 90.0 mmol) and freshly distilled THF (20 mL)

at 0 °C. The resulting mixture was stirred at 0 °C. After 15 min, the

cooling bath was removed, and stirring was continued at room

temperature. The color changed from yellow to dark red during the

course of reaction. After 3 h, CH₂Cl₂(100 mL) was added, and the

organic phase was washed with water and brine. The organic layer was

dried (Na₂SO₄) and concentrated to aﬀord a dark viscous mixture.

Filtration through a silica pad (ethyl acetate) aﬀorded a dark brown solid

(5.44 g, 80%): mp 68−70 °C; ¹H NMR δ 1.03 (t, J = 7.2 Hz, 3H), 2.01

(s, 6H), 2.30 (s, 3H), 4.11 (q, J = 7.2 Hz, 2H), 6.81 (d, J = 2.8 Hz, 1H),

6.89 (s, 2H), 7.56 (d, J = 14.0 Hz, 1H), 8.71 (d, J = 14.0 Hz, 1H), 8.91−

9.07 (br, 1H); ¹³C NMR δ 13.4, 20.7, 21.0, 60.5, 120.1, 123.2, 126.4,

127.6, 128.2, 128.4, 130.8, 134.2, 136.8, 136.9, 164.1; ESI-MS obsd

329.1501, calcd 329.1501 [(M + H)⁺, M = C₁₈H₂₀N₂O₄].

4-(Ethoxycarbonyl)-3-mesityl-2-(2-nitroethyl)pyrrole (5M). Fol-

lowing a general procedure,²⁹a solution of 4M (5.42 g, 16.5 mmol)

in CHCl₃(150 mL) and isopropanol (50 mL) was treated with silica

(19.8 g). The resulting suspension was treated in one portion with

NaBH₄(1.25 g, 33.0 mmol) under vigorous stirring. After 20 min, a

further portion of NaBH₄(355 mg, 9.38 mmol) was added in one batch.

After 20 min, TLC analysis showed complete consumption of the

vinylpyrrole. The mixture was ﬁltered, and the ﬁlter cake was washed

with CH₂Cl₂. The ﬁltrate was concentrated, and the resulting dark oil

was ﬁltered through a bed of silica (hexanes/ethyl acetate, 3:1) to aﬀord

a brown solid (3.48 g, 64%): mp 108−110 °C; ¹H NMR δ 0.91 (t, J =

7.2 Hz, 3H), 1.99 (s, 6H), 2.29 (s, 3H), 3.63 (t, J = 6.1 Hz, 2H), 3.98 (q,

J = 7.2 Hz, 2H), 4.77 (t, J = 6.1 Hz, 2H), 6.42 (d, J = 2.5 Hz, 1H), 6.86 (s,

2H), 8.52 (br, 1H); ¹³C NMR δ 13.6, 20.7, 21.0, 25.8, 59.2, 74.7, 111.5,

116.0, 124.7, 127.3, 132.4, 133.2, 136.0, 137.2, 165.2; ESI-MS obsd

331.1651, calcd 331.1652 [(M + H)⁺, M = C₁₈H₂₂N₂O₄].

8-(Ethoxycarbonyl)-2,3-dihydro-1-(1,1-dimethoxymethyl)-7-hep-

tyl-3,3-dimethyldipyrrin (8H). Following a general procedure,¹⁷in a

ﬁrst ﬂask a solution of 7H (2.2 g, 4.7 mmol) in freshly distilled THF

(13 mL) at 0 °C was treated with NaOMe (7.6 g, 24 mmol). The mixture

was stirred and degassed by bubbling argon through the solution for

45 min. In a second ﬂask purged with argon, TiCl₃(20 mL, 20 wt % in

3% HCl solution, 34 mmol), THF (70 mL), and NH₄OAc (20.0 g, 261

mmol) were combined under argon, and the mixture was degassed by

bubbling with argon for 45 min. Then, the ﬁrst ﬂask mixture was

transferred via cannula to the buﬀered TiCl₃mixture. The resulting

mixture was stirred under argon at room temperature for 16 h. The

mixture was extracted with ethyl acetate. The organic extract was washed

(saturated aqueous NaHCO₃), dried (Na₂SO₄) and concentrated.

Column chromatography (silica, CH₂Cl₂) aﬀorded a yellow oil (1.0 g,

1

51%): H NMR (300 MHz) δ 0.87 (t, J = 6.4 Hz, 3H), 1.23 (s, 6H),

1.23−1.36 (m, 11H), 1.50−1.58 (m, 2H), 2.63 (s, 2H), 2.77 (q, J =

7.2 Hz, 2H), 3.44 (s, 6H), 4.25 (q, J = 7.2 Hz, 2H), 5.03 (s, 1H), 5.86

(s, 1H), 7.42 (d, J = 3.0 Hz, 1H), 10.84 (brs, 1H); ¹³C NMR δ 14.1, 14.4,

22.6, 24.6, 29.14, 29.20, 29.5, 31.7, 31.9, 40.2, 48.2, 54.5, 59.1, 102.5,

104.6, 114.2, 124.6, 125.2, 128.6, 159.8, 165.4, 174.6; ESI-MS obsd

419.2884, calcd 419.2904 [(M + H)⁺, M = C₂₄H₃₈N₂O₄].

6-[4-(Ethoxycarbonyl)-3-mesitylpyrrol-2-yl]-1,1-dimethoxy-4,4-

dimethyl-5-nitro-2-hexanone (7M). Following a general procedure,³⁰a

mixture of 5M (3.48 g, 10.5 mmol) and 6 (4.99 g, 31.3 mmol) was

treated with DBU (4.49 mL, 30.0 mmol). CH₂Cl₂(5 mL) was added to

the reaction mixture to dissolve completely the nitroethylpyrrole

compound 5M. The reaction mixture was stirred at room temperature

for 7 h, diluted with ethyl acetate (100 mL), and washed with aqueous

NH₄Cl solution and brine. The organic layer was dried (Na₂SO₄) and

concentrated. The resulting oil was chromatographed [silica, ethyl

acetate/hexanes (1:2)] to aﬀord a brown solid (1.37 g, 27%): mp 131−

134 °C; ¹H NMR δ 0.90 (t, J = 7.2 Hz, 3H), 1.22 (s, 3H), 1.33 (s, 3H),

1.94 (s, 3H), 1.99 (s, 3H), 2.28 (s, 3H), 2.67, 2.76 (AB, ²J = 18.6 Hz,

2H), 3.34 (ABX, ³J = 11.8 Hz, ²J = 14.6 Hz, 1H), 3.83 (ABX, ³J = 2.5 Hz,

²J = 14.6 Hz, 1H), 3.42 (s, 3H), 3.43 (s, 3H), 3.94−4.07 (m, 2H), 4.41 (s,

3-(Ethoxycarbonyl)-4-mesitylpyrrole (2M). Following the van

Leusen method,²⁶a suspension of TosMIC (12.0 g, 61.5 mmol) and

the known α,β-unsaturated ester 1M²⁵(12.8 g, 58.6 mmol) in

anhydrous Et₂O/DMSO (2:1) (281 mL) was added dropwise under

argon into a stirred suspension of NaH (3.07 g, 60% dispersion in

mineral oil, 76.8 mmol) in anhydrous THF (118 mL). After stirring for

2.5 h, water (260 mL) was carefully added. The mixture was extracted

with diethyl ether and CH₂Cl₂. The combined extract was dried

(Na₂SO₄) and ﬁltered. The ﬁltrate was concentrated to aﬀord a yellow

oil. Chromatography [silica, ethyl acetate/hexanes (1:9 → 1:3)] gave a

white solid (9.13 g, 60%): mp 164−165 °C; ¹H NMR (300 MHz) δ 1.08

(t, J = 7.2 Hz, 3H), 2.03 (s, 6H), 2.30 (s, 3H), 4.07 (q, J = 7.2 Hz, 2H),

6.54−6.56 (m, 1H), 6.89 (s, 2H), 7.53−7.55 (m, 1H), 8.47 (br, 1H); ¹³C

NMR (75 MHz) δ 14.3, 21.0, 21.3, 59.5, 117.9, 124.6, 127.7, 129.7,

130.6, 132.3, 136.3, 137.7, 165.2; FAB-MS obsd 257.1414, calcd

257.1416 (C₁₆H₁₉NO₂).

4-(Ethoxycarbonyl)-2-formyl-3-mesitylpyrrole (3M). Following a

general procedure,²⁷a solution of 2M (19.8 g, 77.0 mmol) in DMF

(24.6 mL) and CH₂Cl₂(400 mL) at 0 °C under argon was treated

dropwise with freshly distilled POCl₃(8.50 mL, 92.7 mmol). After 1 h,

the ice bath was removed. The ﬂask was allowed to warm to room

temperature with stirring for 18 h. The reaction mixture was cooled to

0 °C, whereupon 2.5 M aqueous NaOH (350 mL) was added. The

mixture was extracted with CH₂Cl₂. The organic phase was washed

[10% (w/w) aqueous acetic acid and saturated brine], dried (Na₂SO₄),

and concentrated. The residue was triturated with hexanes and ﬁltered

to aﬀord a pale yellow solid (11.2 g, 56%): mp 214−216 °C; ¹H NMR δ

1H), 5.22 (ABX, ³J = 2.5 Hz, ³J = 11.8 Hz, 1H), 6.36 (d, J = 2.5 Hz, 1H),

6.85 (s, 2H), 8.23−8.30 (br, 1H); ¹³C NMR δ 13.9, 20.7, 20.9, 21.2, 23.8,

24.2, 27.1, 36.7, 44.7, 55.1, 59.2, 95.2, 104.6, 111.8, 116.2, 124.8, 127.27,

127.39, 132.8, 133.2, 136.0, 137.3, 137.6, 165.2, 203.3; ESI-MS obsd

489.2591, calcd 489.2595 [(M + H)⁺, M = C₂₆H₃₆N₂O₇].

8-(Ethoxycarbonyl)-1-(1,1-dimethoxymethyl)-3,3-dimethyl-7-me-

sityl-2,3-dihydrodipyrrin (8M). Following a general procedure,¹⁷

a

solution of 7M (1.37 g, 2.81 mmol) in anhydrous THF (12.0 mL) under

argon was treated with NaOMe (0.45 g, 8.3 mmol). The reaction

mixture was bubbled with argon for 15 min and then stirred for 1 h at

room temperature (ﬁrst ﬂask). In a second ﬂask, TiCl₃[8.6 wt % TiCl₃in

28 wt % HCl, 13.3 mL, 9.7 mmol] and THF (27 mL) were combined.

The mixture was bubbled with argon for 30 min. Then, NH₄OAc

(11.2 g, 145 mmol) was slowly added under argon bubbling to buﬀer the

mixture to pH 6.0 (pH paper). The mixture in the ﬁrst ﬂask containing

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Article

the nitronate anion of 7M was transferred via a cannula to the buﬀered

TiCl₃mixture in the second ﬂask. The resulting mixture was stirred

overnight at room temperature. Then the mixture was poured into a

vigorously stirred solution of saturated aqueous NaHCO₃(300 mL).

After 10 min, the mixture was extracted with ethyl acetate. The organic

layers were combined, washed with water, dried (Na₂SO₄), and

concentrated. Filtration through a alumina pad (alumina, hexanes/

ethyl acetate, 3:1) aﬀorded a dark brown solid (0.60 g, 49%): mp 148−

150 °C; ¹H NMR δ 0.93 (t, J = 7.2 Hz, 3H), 1.28 (s, 6H), 2.04 (s, 6H),

2.30 (s, 3H), 2.67 (s, 2H), 3.47 (s, 6H), 4.01 (q, J = 7.2 Hz, 2H), 5.05 (s,

1H), 6.56 (d, J = 2.5 Hz, 1H), 6.87 (s, 2H), 6.97 (s, 1H), 11.18−11.31

(br, 1H); ¹³C NMR δ 13.8, 20.9, 21.1, 29.1, 40.7, 48.4, 54.7, 59.0, 102.6,

106.3, 111.5, 117.8, 124.4, 127.3, 133.3, 135.5, 135.8, 137.4, 163.4, 165.6,

176.8; Anal. Calcd for C₂₆H₃₄N₂O₄: C, 71.21; H, 7.81; N, 6.39. Found:

C, 71.46; H, 7.98; N, 6.31.

3,13-Bis(ethoxycarbonyl)-2,12-diheptyl-8,8,18,18-tetramethyl-

bacteriochlorin (BC2-2H). Following a general procedure,¹⁸a solution

of 8H (340 mg, 0.81 mmol, 18 mM) in anhydrous CH₃CN (45 mL) was

treated with BF₃·O(Et)₂(0.80 mL, 6.5 mmol, 140 mM). The reaction

mixture was stirred at room temperature for 16 h. Excess TEA (1.2 mL)

was added to the reaction mixture. The reaction mixture was

concentrated, and the residue was chromatographed (silica, CH₂Cl₂).

A single purple band was isolated and concentrated to aﬀord the title

compound as a purple solid (70 mg, 24%): ¹H NMR δ −1.41 (brs, 2H),

0.89−0.92 (m, 6H), 1.31−1.71 (m, 22H), 1.94 (s, 12H), 2.09−2.18 (m,

4H), 4.10 (t, J = 7.8 Hz, 4H), 4.42 (s, 4H), 4.78 (q, J = 7.2 Hz, 4H), 8.64

(s, 2H), 9.66 (s, 2H); ¹³C NMR δ 14.3, 14.8, 22.9, 27.5, 29.5, 30.4, 31.1,

32.1, 33.4, 46.0, 52.0, 60.9, 94.8, 98.7, 119.4, 134.0, 135.1, 140.5, 160.6,

166.7, 171.1; λ_abs(toluene) 353, 383, 520, 761 nm; λ_em(λ_exc522 nm)

768 nm; MALDI-MS obsd 710.5; ESI-MS obsd 711.4830, calcd

711.4844 [(M + H)⁺, M = C₄₄H₆₂N₄O₄].

3,13-Bis(ethoxycarbonyl)-2,12-diheptyl-5-methoxy-8,8,18,18-tet-

ramethylbacteriochlorin (BC2-2H-MeO). Following a general proced-

ure,¹⁸a solution of 8H (430 mg, 1.1 mmol, 18 mM) in anhydrous

CH₂Cl₂(60 mL) was treated ﬁrst with 2,6-DTBP (4.70 mL, 21.2 mmol,

360 mM) and second with TMSOTf (0.956 mL, 5.30 mmol, 90 mM).

The reaction mixture was stirred at room temperature for 16 h. The

reaction mixture was concentrated, and the residue was chromato-

graphed (silica, CH₂Cl₂). The second green band was isolated and

concentrated to aﬀord the title compound as a purple solid (160 mg,

44%): ¹H NMR δ −1.84 (brs, 1H), −1.56 (brs, 1H), 0.87−0.89 (m, 6H)

1.31−1.71 (m, 22H), 1.94 (d, J = 4.0 Hz, 12H), 2.15 (m, 4H), 3.78 (t, J =

7.6 Hz, 2H), 4.12 (t, J = 7.6 Hz, 2H), 4.22 (s, 3H), 4.36 (s, 2H), 4.40 (s,

2H), 4.78 (q, J = 6.8 Hz, 4H), 8.53 (s, 1H), 8.65 (s, 1H), 9.60 (s, 1H);

¹³C NMR δ 14.10, 14.11, 14.59, 14.68, 22.68, 22.70, 26.6, 27.3, 29.23,

29.37, 29.9, 30.2, 30.9, 31.1, 31.81, 31.91, 32.8, 33.3, 45.6, 45.9, 47.9,

51.7, 60.7, 61.7, 64.3, 93.8, 95.5, 97.6, 118.5, 124.6, 127.9, 132.5, 134.31,

134.36, 134.9, 135.2, 140.3, 155.7, 160.5, 166.6, 167.9, 168.9,171.3; λ_abs

(toluene) 358, 379, 522, 740 nm; λ_em(λ_exc521 nm) 744 nm; MALDI-

MS obsd 740.1; ESI-MS obsd 741.4938, calcd 741.4949 [(M + H)⁺,

M = C₄₅H₆₄N₄O₅].

3,13-Bis(ethoxycarbonyl)-2,12-dimesityl-8,8,18,18-tetramethyl-

bacteriochlorin (BC2-2M). Following Procedure A (see next section), a

solution of BC2-2M-MeO (7.8 mg, 10 μmol, 4 mM) in freshly distilled

THF (2.5 mL) under argon was treated with NaH (60 mg, 1.5 mmol,

60% dispersion in mineral oil) at room temperature for 30 min. PdBr₂

(80 mg, 0.30 mmol) was then added to the mixture, and the ﬂask was

heated at 60 °C for 2 h. The reaction was monitored by absorption

spectroscopy and TLC [silica, hexanes/ethyl acetate (3:1)]. The

reaction mixture was diluted with CH₂Cl₂and washed with saturated

aqueous NaHCO₃. The organic layer was dried (Na₂SO₄) and ﬁltered.

The ﬁltrate was concentrated and chromatographed [alumina, hexanes/

CH₂Cl₂(2:1) with 1% TEA] to yield a pink solid (2.2 mg, 29%):

¹H NMR (300 MHz) δ −1.22 (brs, 2H), 1.25 (t, J = 7.2 Hz, 6H), 1.95 (s,

12H), 1.99 (s, 12H), 2.54 (s, 6H), 4.25 (s, 4H), 4.46 (q, J = 7.2 Hz, 4H),

7.18 (s, 4H), 8.26 (s, 2H), 9.68 (s, 2H); λ_abs(toluene) 357, 383, 524, 765

nm; λ_em(λ_exc524 nm) 770 nm; MALDI-MS obsd 750.9; ESI-MS obsd

751.4218 calcd 751.4206 [(M + H)⁺, M = C₄₈H₅₄N₄O₄].

K. Metalation of Bacteriochorins. Procedure A (NaH/THF).

Zn(II)-8,8,18,18-Tetramethyl-2,12-di-p-tolylbacteriochlorin (ZnBC0-

2T). A solution of BC0-2T (16.5 mg, 30.0 μmol, 4 mM) in THF

(7.5 mL) under argon was treated with NaH (180 mg, 4.50 mmol) at

room temperature for 1 h. The color of the resulting heterogeneous

reaction mixture changed from light green to red. Then Zn(OTf)₂

(327 mg, 0.900 mmol) was added to the mixture, and the ﬂask was

heated to 60 °C for 12 h under argon. TLC analysis [silica, hexanes/

CH₂Cl₂(1:1)] showed the disappearance of BC0-2T and the presence

of only one spot. The reaction mixture was diluted with CH₂Cl₂and

washed with saturated aqueous NaHCO₃solution. The organic layer

was dried (Na₂SO₄) and ﬁltered. The ﬁltrate was concentrated, and the

residue was chromatographed on a short column [silica, hexanes/

CH₂Cl₂/TEA (49:49:2), v/v/v] to aﬀord a black-red solid (16.2 mg).

The crude solid was treated with hexanes, sonicated in a benchtop

sonication bath, centrifuged, and the supernatant discarded (as this

consisted of unreacted BC0-2T and hydrocarbon impurities). Repetition

twice aﬀorded a black-red powder (12.1 mg, 66%): ¹H NMR (THF-d₈) δ

1.93 (s, 12H), 2.55 (s, 6H), 4.45 (s, 4H), 7.51 (d, J = 8.0 Hz, 4H), 8.06

(d, J = 8.0 Hz, 4H), 8.62 (s, 2H), 8.63 (s, 2H), 8.76 (s, 2H); λ_abs(toluene)

344, 385, 523, 750 nm; λ_em(λ_exc523 nm) 760 nm; LD-MS obsd 612.3; FAB-

MS obsd 612.2233, calcd 612.2231 (C₃₈H₃₆N₄Zn).

Procedure B (LDA/THF). Zn(II)-8,8,18,18-Tetramethyl-2,12-di-p-

tolylbacteriochlorin (ZnBC0-2T). A solution of BC0-2T (11.0 mg, 20.0

μmol, 4 mM) in THF (5 mL) under argon was treated with a 2.0 M LDA

solution (100 μL, 200 μmol) at room temperature for 5 min. The color

of the resulting homogeneous reaction mixture rapidly changed from

light green to red. Then Zn(OTf)₂(14.1 mg, 40.0 μmol) was added to

the mixture, and the ﬂask was heated to 60 °C for 2−3 h under argon.

TLC analysis [silica, hexanes/CH₂Cl₂(1:1)] showed the disappearance

of BC0-2T and the presence of only one spot, and the absorption

spectrum did not show the Q _xband of BC0-2T. The reaction mixture

was diluted with CH₂Cl₂and washed with saturated aqueous NaHCO₃

solution. The organic layer was dried (Na₂SO₄) and ﬁltered. The ﬁltrate

was concentrated. The resulting solid was treated with hexanes, soni-

cated in a benchtop sonication bath, centrifuged, and the supernatant

discarded (as this consisted of unreacted BC0-2T and hydrocarbon

impurities). Repetition twice aﬀorded a black-red solid (9.7 mg, 79%)

with satisfactory characterization data (¹H NMR spectroscopy,

absorption spectroscopy, LD-MS and FAB-MS).

Procedure C (Zn(OAc)₂·2H₂O/DMF). Zn(II)-2,3,12,13-Tetrakis-

(ethoxycarbonyl)-5-methoxy-8,8,18,18-tetramethylbacteriochlorin

(ZnBC4-MeO). A solution of BC4-MeO (5.4 mg, 7.8 μmol, 4 mM) in

DMF (2 mL) was treated with Zn(OAc)₂·2H₂O (52 mg, 240 μmol,

30 equiv). The reaction mixture was heated to 60 °C for 16 h and then

80 °C for 3 h. TLC analysis [silica, hexanes/CH₂Cl₂(1:1)] showed the

disappearance of BC4-MeO and the presence of only one spot. The

reaction mixture was diluted with CH₂Cl₂and washed with saturated

aqueous NaHCO₃solution. The organic layer was dried (Na₂SO₄) and

ﬁltered. The ﬁltrate was concentrated. The crude solid was treated with

hexanes, sonicated in a benchtop sonication bath, centrifuged, and the

supernatant discarded. Repetition twice aﬀorded a dark blue solid (5.7

mg, 97%): ¹H NMR (300 MHz, THF-d₈) δ 1.51−1.61 (m, 12H), 1.92

3,13-Bis(ethoxycarbonyl)-2,12-dimesityl-5-methoxy-8,8,18,18-

tetramethylbacteriochlorin (BC2-2M-MeO). Following a general

procedure,¹⁸a solution of 8M (600 mg, 1.24 mmol) in anhydrous

CH₂Cl₂(69 mL) was treated ﬁrst with 2,6-di-tert-butylpyridine (4.75 mL,

24.8 mmol) and second with TMSOTf (1.21 mL, 6.21 mmol). The

resulting mixture was stirred at room temperature for 19 h. The reaction

mixture was concentrated and chromatographed (silica, CH₂Cl₂/ethyl

acetate, 1:1) to aﬀord a pink greenish solid (290 mg, 60%): ¹H NMR δ

−1.15 (brs, 1H), −0.90 (brs, 1H), 1.19 (t, J = 7.2 Hz, 3H), 1.25 (t, J =

7.2 Hz, 3H), 1.92 (s, 6H), 1.93 (s, 6H), 2.00 (s, 6H), 2.08 (s, 6H), 2.48

(s, 3H), 2.53 (s, 3H), 3.63 (s, 3H), 4.19 (s, 2H), 4.26 (s, 2H), 4.42 (q, J =

7.2 Hz, 2H), 4.47 (q, J = 7.2 Hz, 2H), 7.72 (s, 2H), 7.78 (s, 2H), 8.12 (s,

1H), 9.61 (s, 1H), 9.63 (s, 1H); ¹³C NMR δ 13.8, 13.9, 21.0, 21.24,

21.34, 21.38, 29.1, 30.9, 31.1, 45.8, 47.4, 51.4, 60.43, 60.49, 62.7, 96.9,

97.3, 97.5, 120.4, 121.5, 127.43, 127.8, 132.1, 134.2, 134.74, 134.80,

135.1, 135.8, 136.2, 136.7, 137.34, 137.47, 137.56, 139.5, 155.7, 162.0,

165.6, 166.2, 171.7; λ_abs(toluene) 361, 382, 538, 758 nm (λ_{758 nm}

=

120,000 M⁻¹cm⁻¹); λ_em(λ_exc538 nm) 763 nm; ESI-MS obsd 781.4322,

calcd 781.4323 [(M + H)⁺, M = C₄₉H₅₆N₄O₅].

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(s, 6H + 6H), 4.13 (s, 3H), 4.33 (s, 2H + 2H), 4.55−4.68 (m, 8H), 8.92

(s, 1H), 9.12 (s, 1H), 9.64 (s, 1H); λ_abs(toluene) 354, 384, 582, 774 nm;

λ_em(λ_exc582 nm) 781 nm; MALDI-MS obsd 750.9; ESI-MS obsd

773.2121, calcd 773.2135 [(M + Na)⁺, M = C₃₇H₄₂N₄O₉Zn].

aqueous NaHCO₃solution. The organic layer was dried (Na₂SO₄) and

ﬁltered. The ﬁltrate was concentrated, and the residue was chromato-

graphed on a short column [silica, hexanes/ethyl acetate/TEA

(74:25:1), v/v/v] to aﬀord a black-red solid (3.2 mg, 50%): ¹H NMR

(300 MHz, THF-d₈) δ 0.86−0.98 (m, 6H) 1.29−1.66 (m, 22H), 1.94 (d,

J = 4.0 Hz, 12H), 2.12 (m, 4H), 3.72 (t, J = 7.6 Hz, 2H), 4.10 (t, J = 7.6

Hz, 2H), 4.12 (s, 3H), 4.36 (s, 2H), 4.38 (s, 2H), 4.63 (q, J = 6.8 Hz,

4H), 8.43 (s, 1H), 8.58 (s, 1H), 9.58 (s, 1H); λ_abs(toluene) 349, 385,

542, 752 nm; λ_em(λ_exc542 nm) 757 nm; MALDI-MS obsd 802.1; ESI-

MS obsd 802.3995, calcd 802.4006 (C₄₅H₆₂N₄O₅Zn).

Zn(II)-3,13-Bis(ethoxycarbonyl)-2,12-diheptyl-8,8,18,18-tetrame-

thylbacteriochlorin (ZnBC2-2H). Following Procedure A, a solution of

BC2-2H (5.7 mg, 8.0 μmol, 4 mM) in THF (2.0 mL) was treated with

NaH (48 mg, 1.2 mmol) at room temperature for 1 h. The color of the

reaction mixture changed from light green to red. Then Zn(OTf)₂

(87 mg, 0.24 mmol) was added to the mixture, and the ﬂask was heated

to 60 °C for 6 h under argon. TLC analysis [silica, hexanes/ethyl acetate

(3:1)] showed the disappearance of the free base bacteriochlorin and the

presence of only one spot. The reaction mixture was diluted with

CH₂Cl₂and washed with saturated aqueous NaHCO₃solution. The

organic layer was dried (Na₂SO₄) and ﬁltered. The ﬁltrate was

concentrated, and the residue was chromatographed on a short column

[silica, hexanes/ethyl acetate/TEA (74:25:1), v/v/v] to aﬀord a black-

red solid (1.9 mg, 31%): ¹H NMR (300 MHz, THF-d₈) δ 0.88−0.92 (m,

6H), 1.31−1.66 (m, 22H), 1.94 (s, 12H), 2.12 (m, 4H), 4.12 (t, J = 7.8

Hz, 4H), 4.39 (s, 4H), 4.66 (q, J = 7.2 Hz, 4H), 8.55 (s, 2H), 9.60 (s,

2H); λ_abs(toluene) 347, 391, 547, 776 nm; λ_em(λ_exc547 nm) 781 nm;

MALDI-MS obsd 772.6; ESI-MS obsd 772.3879, calcd 772.3901

(C₄₄H₆₀N₄O₄Zn).

Zn(II)-3,13-Bis(ethoxycarbonyl)-2,12-diethyl-8,8,18,18-tetrame-

thylbacteriochlorin (ZnBC2-2E). Following Procedure C, a solution of

BC2-2E (5.0 mg, 8.8 μmol, 4 mM) in DMF (2.2 mL) was treated with

Zn(OAc)₂·2H₂O (58 mg, 260 μmol, 30 equiv). The reaction mixture

was heated to 60 °C for 16 h and then 80 °C for 24 h under argon. The

reaction was monitored by absorption spectroscopy and MALDI-MS.

The reaction mixture was diluted with CH₂Cl₂and washed with

saturated aqueous NaHCO₃solution. The organic layer was dried

(Na₂SO₄) and ﬁltered. The ﬁltrate was concentrated. The crude solid

was treated with hexanes, sonicated in a benchtop sonication bath,

centrifuged, and the supernatant discarded. Repetition twice aﬀorded a

dark blue solid (4.8 mg, 86%): ¹H NMR (THF-d₈) δ 1.64 (t, J = 6.8 Hz,

6H), 1.69 (t, J = 7.6 Hz, 6H), 1.95 (s, 12H), 4.10 (q, J = 7.6 Hz, 4H), 4.39

(s, 4H), 4.66 (q, J = 6.8 Hz, 4H), 8.55 (s, 2H), 9.60 (s, 2H); λ_abs

(toluene) 347, 391, 545, 774 nm; λ_em(λ_exc545 nm) 780 nm; MALDI-

MS obsd 631.7; ESI-MS obsd 632.2349, calcd 632.2341

(C₃₄H₄₀N₄O₄Zn).

Mg(II)-2,3,12,13-Tetrakis(ethoxycarbonyl)-5-methoxy-8,8,18,18-

tetramethylbacteriochlorin (MgBC4-MeO). Following a modiﬁcation

of Procedure A, a solution of BC4-MeO (5.0 mg, 7.3 μmol, 4 mM) in

freshly distilled THF (1.8 mL) was treated with NaH (52 mg, 2.1 mmol,

60% dispersion in mineral oil washed beforehand with hexanes) and

MgI₂(60 mg, 0.21 mmol). The reaction mixture was heated at 60 °C for

3 h under argon. The reaction was monitored by absorption spectro-

scopy and MALDI-MS. The reaction mixture was diluted with ethyl

acetate and washed with saturated aqueous NaHCO₃solution. The

organic layer was dried (Na₂SO₄) and ﬁltered. The ﬁltrate was con-

centrated. The crude solid was chromatographed in a Pasteur pipet

[basic alumina, ethyl acetate to ethyl acetate/MeOH (95:5)] to aﬀord a

blue solid (1.0 mg, 19%): λ_abs(CH₂Cl₂) 360, 612, 764 nm; MALDI-MS

obsd 711.2, calcd 710.3 (C₃₇H₄₂N₄O₉Mg). Limited stability precluded

further analysis.

Zn(II)-8,8,18,18-Tetramethylbacteriochlorin (ZnBC0). Following

Procedure A, a solution of BC0 (4.6 mg, 12 μmol, 4 mM) in freshly

distilled THF (3 mL) was treated with NaH (46 mg, 1.9 mmol, 60%

dispersion in mineral oil washed beforehand with hexanes) and

Zn(OTf)₂(131 mg, 0.360 mmol). The reaction mixture was heated at

60 °C for 16 h under argon. The reaction was monitored by absorption

spectroscopy and MALDI-MS. The reaction mixture was diluted with

CH₂Cl₂and washed with saturated aqueous NaHCO₃solution. The

organic layer was dried (Na₂SO₄) and ﬁltered. The ﬁltrate was con-

centrated. The crude solid was treated with hexanes, sonicated in a

benchtop sonication bath, centrifuged, and the supernatant discarded.

1

Repetition twice aﬀorded a pink solid (4.3 mg, 80%): H NMR (300

MHz, THF-d₈) δ 1.97 (s, 12H), 4.46 (s, 4H), 8.60−8.62 (m, 2H), 8.60

(s, 2H), 8.64−8.66 (m, 2H), 8.64 (s, 2H); λ_abs(toluene) 336, 376, 514,

723 nm; λ_em(λ_exc514 nm) 725 nm; MALDI-MS obsd 432.2; ESI-MS

obsd 432.1280, calcd 432.1292 (C₂₄H₂₄N₄Zn).

Zn(II)-3,13-Bis(ethoxycarbonyl)-2,12-dimesityl-5-methoxy-

8,8,18,18-tetramethylbacteriochlorin (ZnBC2-2M-MeO). Following

Procedure A, a solution of BC2-2M-MeO (9.4 mg, 12 μmol, 4 mM) in

freshly distilled THF (3 mL) was treated with NaH (72 mg, 1.8 mmol,

60% dispersion in mineral oil washed beforehand with hexanes) at room

temperature for 30 min. Zn(OTf)₂(131 mg, 0.360 mmol) was added,

and the ﬂask was heated at 60 °C for 5 h under argon. The reaction was

monitored by absorption spectroscopy and TLC [silica, hexanes/ethyl

acetate (3:1)]. The reaction mixture was diluted with ethyl acetate and

washed with saturated aqueous NaHCO₃. The organic layer was dried

(Na₂SO₄) and ﬁltered. The crude product was found to contain only

hydrocarbon impurities by ¹H NMR spectroscopy. The crude solid was

dissolved in methanol and treated with hexanes. Two intensely colored

phases formed. The hexanes phase was removed as this phase was highly

enriched in BC2-2M-MeO yet also contained some metallobacterio-

chlorin. The methanol phase, which contained the title metal-

lobacteriochlorin, was collected and concentrated to yield a pink solid

(5.5 mg, 54%): ¹H NMR (300 MHz, THF-d₈) δ 1.14 (t, J =

7.2 Hz, 3H), 1.20 (t, J = 7.2 Hz, 3H), 1.90 (s, 6H), 1.94 (s, 6H), 1.99

(s, 6H), 2.08 (s, 6H), 2.46 (s, 3H), 2.52 (s, 3H), 3.57 (s, 3H), 4.15 (s,

2H), 4.23 (s, 2H), 4.36 (q, J = 7.2 Hz, 2H), 4.40 (q, J = 7.2 Hz, 2H), 7.08

(s, 2H), 7.14 (s, 2H), 8.01 (s, 1H), 9.50 (s, 1H), 9.56 (s, 1H); λ_abs

(toluene) 353, 389, 565, 773 nm; λ_em(λ_exc565 nm) 779 nm; MALDI-

MS obsd 842.8; ESI-MS obsd 842.3393, calcd 842.3380

(C₄₉H₅₄N₄O₅Zn).

Zn(II)-15²-N-Benzyl-3-(ethoxycarbonyl)-2,12-diethyl-8,8,18,18-

tetramethylbacteriochlorin-13,15-dicarboximide (ZnBC3-2E). Fol-

lowing Procedure C, a solution of BC3-2E (6.9 mg, 11 μmol, 4 mM)

in DMF (2.6 mL) was treated with Zn(OAc)₂·2H₂O (69 mg, 320 μmol,

30 equiv). The reaction mixture was heated to 60 °C for 16 h and then

80 °C for 7 h. The reaction was monitored by absorption spectroscopy

and MALDI-MS. The reaction mixture was diluted with CH₂Cl₂and

washed with saturated aqueous NaHCO₃solution. The organic layer

was dried (Na₂SO₄) and ﬁltered. The ﬁltrate was concentrated, and the

residue was chromatographed on a short column [silica, CH₂Cl₂/

MeOH (98:2)] to aﬀord a purple solid (4.1 mg, 54%): ¹H NMR (THF-

d₈) δ 1.60−1.69 (m, 9H), 1.90 (s, 6H), 1.92 (s, 6H), 4.03 (q, J = 7.7 Hz,

2H), 4.15 (q, J = 7.3 Hz, 2H), 4.33 (s, 2H), 4.65 (q, J = 7.0 Hz, 2H), 4.74

(s, 2H), 5.55 (s, 2H), 7.18 (t, J = 7.3 Hz, 1H), 7.29 (t, J = 7.5 Hz, 2H),

7.76 (d, J = 7.0 Hz, 2H), 8.47 (s, 1H), 8.62 (s, 1H), 9.51 (s, 1H); λ_abs

(toluene) 356, 419, 563, 831 nm; λ_em(λ_exc563 nm) 834 nm; MALDI-

MS obsd 719.9; ESI-MS obsd 720.2501, calcd 720.2523 [(M + H)⁺, M =

C₄₀H₄₁N₅O₄Zn].

Zn(II)-3,13-Bis(ethoxycarbonyl)-2,12-diheptyl-5-methoxy-

8,8,18,18-tetramethylbacteriochlorin (ZnBC2-2H-MeO). Following

Procedure A, a solution of BC2-2H-MeO (5.9 mg, 8.0 μmol, 4 mM) in

THF (2.0 mL) was treated with NaH (48 mg, 1.2 mmol) at room

temperature for 1 h. The color of the reaction mixture changed from

light green to red. Then Zn(OTf)₂(87 mg, 0.24 mmol) was added to the

mixture, and the ﬂask was heated to 60 °C for 6 h under argon. TLC

analysis [silica, hexanes/ethyl acetate (3:1)] showed the disappearance

of the free base bacteriochlorin and the presence of only one spot. The

reaction mixture was diluted with CH₂Cl₂and washed with saturated

Cu(II)-8,8,18,18-Tetramethylbacteriochlorin (CuBC0). Following

Procedure A, a solution of BC0 (5.0 mg, 14 μmol, 4 mM) in freshly

distilled THF (3.4 mL) was treated with NaH (48 mg, 2.0 mmol, 60%

dispersion in mineral oil washed beforehand with hexanes) and

Cu(OAc)₂(74 mg, 0.41 mmol). The reaction mixture was heated at

60 °C for 16 h under argon. The reaction was monitored by absorption

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spectroscopy and MALDI-MS. The reaction mixture was diluted with

CH₂Cl₂and washed with saturated aqueous NaHCO₃solution. The

organic layer was dried (Na₂SO₄) and ﬁltered. The ﬁltrate was con-

centrated. The crude solid was treated with hexanes, sonicated in a

benchtop sonication bath, centrifuged, and the supernatant discarded.

Repetition twice aﬀorded a green solid (2.4 mg, 41%): λ_abs(toluene)

336, 376, 514, 723 nm; MALDI-MS obsd 431.1; ESI-MS obsd 431.1300,

calcd 431.1291 (C₂₄H₂₄N₄Cu).

Cu(II)-8,8,18,18-Tetramethyl-2,12-di-p-tolylbacteriochlorin

(CuBC0-2T). Following Procedure B, a solution of BC0-2T

(16.5 mg, 30.0 μmol, 4 mM) in THF (7.5 mL) was treated with LDA

(0.750 mL, 1.50 mmol, 200 mM) at room temperature for 5 min.

Cu(OAc)₂(54.5 mg, 300 μmol) was added, and the ﬂask was heated at

70 °C for 30 min under argon. TLC analysis [silica, hexanes/CH₂Cl₂

(1:1)] showed the disappearance of BC0-2T and the presence of only

one spot, and the absorption spectrum did not show the Q _xband of

BC0-2T. The reaction mixture was diluted with CH₂Cl₂and washed

with saturated brine. The organic layer was treated to the remaining

steps of the standard workup procedure to yield a dark powder (10.2 mg,

56%): λ_abs(toluene) 337, 383, 512, 755 nm; LD-MS obsd 611.3; FAB-

MS obsd 611.2238, calcd 611.2336 (C₃₈H₃₆N₄Cu).

Cu(II)-3,13-Bis(ethoxycarbonyl)-2,12-dimesityl-5-methoxy-

8,8,18,18-tetramethylbacteriochlorin (CuBC2-2M-MeO). Following

Procedure A, a solution of BC2-2M-MeO (6.2 mg, 8 μmol, 4 mM) in

freshly distilled THF (2 mL) was treated with NaH (48 mg, 1.2 mmol,

60% dispersion in mineral oil washed beforehand with hexanes) at room

temperature for 30 min under argon. Cu(OAc)₂(43 mg, 0.24 mmol)

was added, and the ﬂask was heated at 60 °C for 20 h under argon. The

reaction was monitored by absorption spectroscopy and TLC [silica,

hexanes/ethyl acetate (3:1)]. The standard workup procedure was

employed except for the chromatography procedure [silica, hexanes/

ethyl acetate (4:1) with 1% TEA], which yielded a pink solid (5.3 mg,

79%): λ_abs(toluene) 348, 390, 556, 779 nm; MALDI-MS obsd 841.3;

ESI-MS obsd 841.3381, calcd 841.3385 (C₄₉H₅₄N₄O₅Cu).

In(III)Cl-8,8,18,18-Tetramethyl-2,12-di-p-tolylbacteriochlorin

(InClBC0-2T). Following Procedure B, a solution of BC0-2T (11.0 mg,

20.0 μmol, 4 mM) in THF (4.5 mL) was treated with LDA in THF

(500 μL, 1.00 mmol, 200 mM) at room temperature for 5 min. InCl₃

(44.2 mg, 200 μmol) was added, and the ﬂask was heated at 60 °C for 1.5

h under argon. TLC analysis [silica, hexanes/THF (1:1)] showed the

disappearance of BC0-2T and the presence of only one spot, and the

absorption spectrum did not show the Q _xband of BC0-2T. The

reaction mixture was diluted with CH₂Cl₂and washed with saturated

brine. The organic layer was treated to the remaining steps of

the standard workup procedure to yield a black-red powder

1

(12.5 mg, 89%): H NMR (THF-d₈) δ 1.84 (s, 6H), 2.09 (s, 6H),

2.56 (s, 6H), 4.48, 4.71 (AB, ²J = 16.0 Hz, 4H), 7.55 (d, J =

8.0 Hz, 4H), 8.09 (d, J = 8.0 Hz, 4H), 8.78 (s, 2H), 8.82 (s, 2H), 8.87 (s,

2H); λ_abs(toluene) 350, 389, 539, 764 nm; λ_em(λ_exc539 nm) 772 nm; LD-

MS obsd 698.2; FAB-MS obsd 698.1688, calcd 698.1667 (C₃₈H₃₆ClInN₄).

ASSOCIATED CONTENT

■

S

* Supporting Information

Development of metalation conditions; further X-ray crystallo-

graphic information; and the synthesis and characterization of

dimesitylbacteriochlorin BC0-2M. This material is available free

of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

■

Corresponding Author

*E-mail: jlindsey@ncsu.edu (J.S.L.); david.bocian@ucr.edu

(D.F.B.); holten@wustl.edu (D.H.).

Notes

The authors declare no competing ﬁnancial interest.

^∥Visiting Scientist from Department of Chemistry, Changchun

Normal University, Changchun, Jilin, China 130032.

Pd(II)-8,8,18,18-Tetramethyl-2,12-di-p-tolylbacteriochlorin

(PdBC0-2T). Following Procedure A, a solution of BC0-2T

(16.5 mg, 30.0 μmol, 4 mM) in THF (7.5 mL) was treated with NaH

(120 mg, 3.00 mmol) at room temperature for 1 h. PdBr₂(240 mg, 90.0

mmol) was then added, and the ﬂask was heated at 60 °C for 0.5 h under

argon. TLC analysis [silica, hexanes/CH₂Cl₂(1:1)] showed the

disappearance of BC0-2T and the presence of only one spot. The

standard workup aﬀorded a black-red powder (9.3 mg, 48%): ¹H NMR δ

1.87 (s, 12H), 2.59 (s, 6H), 4.43 (s, 4H), 7.54 (d, J = 8.0 Hz, 4H), 8.01

(d, J = 8.0 Hz, 4H), 8.55 (s, 2H), 8.68 (s, 2H), 8.72 (s, 2H); λ_abs

(toluene) 329, 379, 499, 739 nm; λ_em(λ_exc499 nm) 745 nm; LD-MS

obsd 653.9; FAB-MS obsd 654.1958, calcd 654.1975 (C₃₈H₃₆N₄Pd).

Pd(II)-3,13-Bis(ethoxycarbonyl)-2,12-dimesityl-5-methoxy-

8,8,18,18-tetramethylbacteriochlorin (PdBC2-2M-MeO). Following

Procedure B, a solution of BC2-2M-MeO (7.8 mg, 10 μmol, 4 mM) in

freshly distilled THF (2.5 mL) was treated with LDA (50 μL, 0.1 mmol,

0.2 M) at room temperature for 10 min. PdBr₂(80 mg, 0.30 mmol) was

then added to the mixture, and the ﬂask was heated at 60 °C for 20 h

under argon. The reaction was monitored by absorption spectroscopy

and TLC [silica, hexanes/ethyl acetate (3:1)]. The reaction mixture was

diluted with ethyl acetate and washed with saturated aqueous NaHCO₃

solution. The organic layer was dried (Na₂SO₄) and ﬁltered. The

concentrated crude solid was chromatographed [silica, hexanes/ethyl

acetate (3:1)], and the only mobile band (pink) was collected. The

concentrated mixture was found to contain some amine-like impurity,

which was removed by size-exclusion chromatography (toluene, Bio-

Beads S-X1, 200−400 mesh). The collected band (pink) was then

chromatographed [silica, hexanes/ethyl acetate (3:1)] to aﬀord a purple

solid (3.1 mg, 35%): ¹H NMR (300 MHz) δ 1.12 (t, J = 7.2 Hz, 3H),

1.18 (t, J = 7.2 Hz, 3H), 1.87 (s, 6H), 1.90 (s, 6H), 2.00 (s, 6H), 2.09 (s,

6H), 2.46 (s, 3H), 2.50 (s, 3H), 3.46 (s, 3H), 4.18 (s, 2H), 4.26 (s, 2H),

4.34 (q, J = 7.2 Hz, 2H), 4.38 (q, J = 7.2 Hz, 2H), 7.07 (s, 2H), 7.13 (s,

2H), 8.04 (s, 1H), 9.55 (s, 1H), 9.61 (s, 1H); λ_abs(toluene) 337, 382,

538, 758 nm; MALDI-MS obsd 884.5; ESI-MS obsd 884.3228, calcd

884.3202 (C₄₉H₅₄N₄O₅Pd).

ACKNOWLEDGMENTS

■

This work was supported by grants from the Division of

Chemical Sciences, Geosciences, and Biosciences, Oﬃce of Basic

Energy Sciences of the U.S. Department of Energy to D.F.B.

(DE-FG02-05ER15660), D.H. (DE-FG02-05ER15661), and

J.S.L. (DE-FG02-96ER14632). Mass spectra were obtained at

the Mass Spectrometry Laboratory for Biotechnology at North

Carolina State University. Partial funding for the facility was

obtained from the North Carolina Biotechnology Center and the

National Science Foundation. We thank Dr. Paul Boyle for X-ray

crystallographic analyses.

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