Preparation and characterization of asymmetric α-alkoxy dipyrrin ligands and their metal complexes

Article abstract of DOI:10.1039/b615801c

Asymmetric α-substituted dipyrrins have been synthesized and characterized. The compounds were formed by a metal mediated reaction involving a single alkoxy group substituted into the α-position of an α,β-unsubstituted dipyrrin. An α-methoxy dipyrrin, 5-(

Full text of DOI:10.1039/b615801c

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PAPER
www.rsc.org/dalton | Dalton Transactions
Preparation and characterization of asymmetric a-alkoxy dipyrrin ligands
and their metal complexes†
Sara R. Halper, Jay R. Stork and Seth M. Cohen*
Received 31st October 2006, Accepted 25th January 2007
First published as an Advance Article on the web 8th February 2007
DOI: 10.1039/b615801c
Asymmetric a-substituted dipyrrins have been synthesized and characterized. The compounds were
formed by a metal mediated reaction involving a single alkoxy group substituted into the a-position of
an a,b-unsubstituted dipyrrin. An a-methoxy dipyrrin, 5-(4-cyanophenyl)-1-methoxydipyrrin
(a-OMe-4-cydpm), was prepared from 5-(4-cyanophenyl)-4,6-dipyrromethane. Methoxy, ethoxy, and
propoxy derivatives (a-OMe-4-mecdpm, a-OEt-4-mecdpm, a-OPr-4-mecdpm) of 5-(4-methoxy-
carbonylphenyl)-4,6-dipyrromethane have also been prepared. A homoleptic, bis(1-methoxy)-
dipyrrinato zinc(II) complex, [Zn(a-OMe-4-mecdpm)2], has been synthesized, as has a heteroleptic
cobalt(III) complex with one a-OMe-4-cydpm ligand and two unsubstituted 5-(4-cyanophenyl)dipyrrin
(4-cydpm) ligands ([Co(a-OMe-4-cydpm)(4-cydpm)2]). The rotational barrier of the meso-aryl
substituent of [Zn(a-OMe-4-mecdpm)2] was found to be 17.3 kcal mol−1 by variable-temperature NMR
spectroscopy. The compounds a-OMe-4-cydpm and [Zn(a-OMe-4-mecdpm)2] have also been
characterized by X-ray diffraction. The formation of the new dipyrrin derivatives is shown to be general
and can be performed on dipyrrins with various meso-aryl substitutents, with a variety of alcohols, and
can be promoted by several metal salts.
known to be rather reactive, undergoing transformations such
as bromine-substitution,31 Diels–Alder reactions,32 and ring
Introduction
fusion.31,33 Polypyrrolic compounds with oxygen atoms in the a-
position have been identified in several natural products and natu-
ral product derivatives34 including, phycobilins,35 phytochromes,36
and bile pigments.37,38
Dipyrrins are an important class of conjugated, bipyrrolic chela-
tors that have received increasing attention in recent years. Asym-
metrically substituted dipyrromethanes and dipyrrins have been
used extensively in the synthesis of porphyrins,1–3 boron dipyrrin
(BODIPY) dyes,4,5 and supramolecular dimers and trimers.6–9
meso-Substituted dipyrrins form a variety of discrete supramolec-
ular structures,10–13 coordination polymers,11,12,14,15 and metal–
organic frameworks.16,17 Asymmetrically a-substituted, meso-aryl
dipyrrin compounds (Fig. 1), and their corresponding metal
complexes, have been prepared as precursors to porphyrins, and
porphyrin derivatives, or have been isolated as side products during
porphyrin syntheses.18–22 Substitution by alkoxy groups at the a-
position of N-confused porphyrins has also been reported.23–30
The a-positions of dipyrrins and N-confused porphyrins are
Herein, a route to asymmetric a-substituted meso-dipyrrins has
been discovered and investigated. The compounds are formed
by the substitution of an alkoxy group into the a-position of a
dipyrromethane during oxidation to the corresponding dipyrrin
(Fig. 1). These compounds were initially detected as side products
from the synthesis of various metal dipyrrinato complexes. Further
tests revealed more favorable conditions to obtain the alkoxy
substituted dipyrrin, without the formation of metal complexes.
A series of these asymmetric a-alkoxy dipyrrins, as well as metal
complexes of this ligand with cobalt(III) and zinc(II), have been
prepared and characterized. The synthetic route to asymmetric
a-alkoxy dipyrrins described here may prove useful, upon further
optimization, for the synthesis of interesting new ligands, chro-
mophores, or natural products containing this functional motif.
Results and discussion
Fig. 1 The a-, b-, and meso-positions in dipyrrins (left). Generic structure
of the asymmetric a-alkoxy meso-substituted dipyrrin described in this
report (right).
In ongoing synthetic efforts in our laboratory, meso-sub-
stituted dipyrromethanes were being prepared using standard
methods,39–42 followed by oxidation with 2,3-dichloro-5,6-dicyano-
1,4-benzoquinone (DDQ) or p-chloranil to form the correspond-
ing dipyrrins in situ (Scheme 1) for use in the synthesis of
dipyrrinato coordination complexes.14,16,43 During some of these
preparations, we observed the formation of a new compound via
TLC (as a mobile yellow band), an apparent side product during
metal complex formation.
Department of Chemistry and Biochemistry, University of California, San
Diego, 9500 Gilman Drive, La Jolla, Calfornia, 92093-0358, USA. E-mail:
scohen@ucsd.edu; Fax: +1 858-822-5598; Tel: +1 858-822-5596
† Electronic supplementary information (ESI) available: Crystallographic
details, Fig. S1–S15 (includes NMR spectral data of all new compounds).
See DOI: 10.1039/b615801c
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Scheme 2 Synthesis of a-OMe-4-cydpm, [Co(4-cydpm)3], and [Co(a-
OMe-4-cydpm)(4-cydpm)2] from 5-(4-cyanocarbonylphenyl)-4,6-dipyrro-
methane.
Scheme 1 Synthesis of dipyrromethanes, dipyrromethenes, and a-alkoxy
dipyrromethenes.
did not require the presence of a base but did require a metal salt.
The use of bases such as triethylamine promoted the formation
of the dipyrrinato metal complexes over the formation of the new
alkoxy-substituted dipyrrins. In the absence of base formation of
the dipyrrinato metal complex was minimized with no change in
the yield of the alkoxy substituted species. Reducing the quantity
of metal salts to catalytic amounts (5 mol% of Na3Co(NO2)6
versus the dipyrromethane) resulted in very slow reactions, which
required several days at room temperature. Therefore, 0.3–0.4
equivalents of the metal source was used, as employed in our initial
discovery, and the reaction was heated to reflux. These reaction
conditions constituted a general approach to asymmetric a-
alkoxy dipyrrins, consisting of the oxidation of a dipyrromethane
followed by in situ reaction of the resulting dipyrrin with a metal
salt in the presence of an alcohol. While these conditions did
not improve the yield of the asymmetric a-alkoxy dipyrrin per se
(based on our original observations), it did reduce the number of
products detected as mobile bands by TLC. This method greatly
facilitated purification, as the asymmetric a-alkoxy dipyrrin was
the only major product formed that was readily eluted from silica
gel column chromatography.
Our initial observation specifically involved a reaction with 5-(4-
cyanophenyl)-4,6-dipyrromethene (4-cydpm), Na3Co(NO2)6, and
triethylamine in a 1 : 5 mixture of methanol and chloroform
(Scheme 2). This reaction was designed to form the complex [Co(4-
cydpm)3], but yielded three products each in modest yields (∼25–
35% each). The components of this mixture were separated by
silica column chromatography for identification. The first product
to elute from the column was isolated as a red solid and was
identified as the expected [Co(4-cydpm)3] complex.17 The second
product to elute was isolated as a yellow solid and after extensive
characterization determined to be an a-methoxy dipyrrin (a-OMe-
4-cydpm). The last product to elute from the column was found to
be a dark red solid, suggestive of a cobalt(III) dipyrrinato complex.
NMR spectroscopy and mass spectrometry indicated that this was
indeed a cobalt(III) complex, containing an unusual ligand set of
two unaltered dipyrrinato and one a-methoxy dipyrrinato ligand
([Co(a-OMe-4-cydpm)(4-cydpm)2]).
Exploring the reaction conditions
Further experiments were also performed to probe the reaction
conditions and to attempt to improve the yield. These efforts
explored alternative solvents, alcohols, oxidants, metal salts, and
dipyrrin precursors. The results are summarized in Table 1.
Various reaction conditions were used to examine the formation
and isolation of the new asymmetric dipyrrin compounds. It was
found that the formation of the alkoxy-substituted compounds
Table 1 Reaction conditions evaluated for the formation of a-alkoxy dipyrrins. Reactions deemed successful but not fully characterized were evaluated
by TLC and electronic absorption spectroscopya
Dipyrromethane
Oxidant
Solvent
Alcohol
Metal salt
Isolated yield or conversion
(4-mecdp)methane
(4-mecdp)methane
(4-cydp)methane
(4-dp)methane
DDQ
p-Chloranil
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CH2Cl2
THF
Benzene
EtOAc
CH3CN
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
EtOH
Na3Co(NO2)6
Na3Co(NO2)6
Na3Co(NO2)6
Na3Co(NO2)6
Na3Co(NO2)6
Na3Co(NO2)6
Na3Co(NO2)6
Na3Co(NO2)6
Na3Co(NO2)6
Na3Co(NO2)6
Na3Co(NO2)6
Na3Co(NO2)6
Na3Co(NO2)6
Na3Co(NO2)6
Na3Co(NO2)6
Mn(OAc)2·4H2O
FeCl3·6H2O
19% yield
35% yield
15% yield
Product detected by TLC
Product detected by TLC
No product detected
Product detected by TLC
Product detected by TLC
Product detected by TLC
No product detected
No product detected
35% yield
(4-mtdp)methane
(4-ntdp)methane
(4-mecdp)methane
(4-mecdp)methane
(4-mecdp)methane
(4-mecdp)methane
(4-mecdp)methane
(4-mecdp)methane
(4-mecdp)methane
(4-mecdp)methane
(4-mecdp)methane
(4-mecdp)methane
(4-mecdp)methane
(4-mecdp)methane
(4-mecdp)methane
PrOH
38% yield
iPrOH
BnOH
MeOH
MeOH
MeOH
MeOH
Product detected by TLC
Product detected by TLC
13% yield
15% yield
Product detected by TLC
No product detected
[Co(pyr)4Cl2]Cl
Cu(acac)2
a Select abbreviations: DDQ, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone; (4-mecdp)methane, 5-(4-methoxycarbonylphenyl)-4,6-dipyrromethane; (4-
cydp)methane, 5-(4-cyanophenyl)-4,6-dipyrromethane; (4-dp)methane, 5-(4-phenyl)-4,6-dipyrromethane; (4-mtdp)methane, 5-(4-methylthiophenyl)-4,6-
dipyrromethane; (4-ntdp)methane, 5-(4-nitrophenyl)-4,6-dipyrromethane.
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Non- or weakly-coordinating solvents such as CH2Cl2, THF, and
benzene were tolerated, while more polar, coordinating solvents
such as ethyl acetate and CH3CN inhibited formation of the
desired product, leaving the unsubstituted dipyrrin. The use of
alcohol substrates other than methanol, such as ethanol, propanol,
isopropanol, and benzyl alcohol, was successful; however, the
yield from the substitution of isopropanol and benzyl alcohol
was extremely low. No attempts to further optimize the reaction
conditions for these substrates were explored. Several metal salts
were tested with methanol and found to be competent to perform
the desired reaction (albeit in lower isolated yields), including
Mn(OAc)2·4H2O, FeCl3·6H2O, and [Co(pyr)4Cl2]Cl. In contrast,
Cu(acac)2 failed to promote the reaction, which was expected
as dipyrrinato ligands rapidly form copper(II) complexes in the
absence of base.14,43 From these experiments it appears that the
reaction scope is limited to metal ions, such as manganese(II),
iron(III), and cobalt(III), that do not readily form dipyrrinato
complexes under the reaction conditions. While an extensive study
of the different reaction conditions has been performed without
an improvement in the yield, a more comprehensive investigation
may also reveal higher yielding preparations.
Testing of additional reaction conditions revealed another
essential component involved in the formation of the alkoxy
dipyrrins. Several important observations were made: (1) When
purified 4-mecdpm was reacted with Na3Co(NO2)6 in a methanol
solution, the a-substitution did not occur. (2) The alkoxy substitu-
tion also did not occur upon addition of DDQ to a solution of pu-
rified 4-mecdpm. (3) When 2,3,5,6-tetrachlorohydroquinone (the
reduced form of p-chloranil) was added to a reaction mixture of
pure 4-mecdpm, Na3Co(NO2)6, and methanol, a-OMe-4-mecdpm
was formed. Thus, it appears that the hydroquinone is required
for the alkoxy-substitution to occur. It should be noted that
2,3-dichloro-5,6-dicyanohydroquinone would have been present
under the original reaction conditions as the product of the
reduction of DDQ.
dicyanohydroquinone was involved. However, in our system, use
of 2,3,5,6-tetrachlorohydroquinone, which does not contain a
cyano moiety, gave the desired products. Osuka, Furuta et al.
have also isolated various N-confused porphyrins that have a
methoxy or ethoxy group substituted in the a-position of the
confused pyrrole ring (Fig. 3).24–30 These reactions utilize NaOMe
or BF3OEt and have been reported both with and without the
presence of 2,3-dichloro-5,6-dicyano-hydroquinone. Overall, the
essential components of the present reaction have been elucidated,
but the mechanism of the reaction reported here will require
further investigation.
Spectroscopic and structural features of a-alkoxy dipyrrins
Several of the a-alkoxy dipyrrin compounds have been examined
by mass spectrometry, electronic, infrared, and NMR spec-
troscopy. The electronic spectroscopy of the a-alkoxy dipyrrin moi-
ety has an absorbance maximum at 407 nm (e ∼20000 M−1 cm−1,
attributed to the p–p* transition, Fig. S15†), slightly shifted to
higher energy when compared to that of the corresponding unsub-
stituted dipyrrins (∼435 nm).43 The NMR spectra of the a-alkoxy
dipyrrins also have features distinct from the parent dipyrrins. For
1
example, the H NMR spectrum of a-OMe-4-cydpm shows five
unique resonances (Fig. S5†), one for each of the pyrrolic protons
(compared with three resonances for the symmetric dipyrrin). The
hydrogen bound to the pyrrolic nitrogen atom, which is generally
not observed in the NMR spectra of the dipyrrin precursors,43 is
readily located at 12.1 ppm for a-OMe-4-cydpm.
In addition to complete spectroscopic characterization, the
compound a-OMe-4-cydpm was crystallized and the structure was
determined by single-crystal X-ray diffraction (Table 2). The crys-
tal structure reveals several interesting structural features, which
again distinguish the compound from the parent unsubstituted
dipyrrin (Fig. 2). First, the structure unambiguously reveals the
constitution of the molecules, showing the methoxy substituent
in the a-position of one of the pyrrolic subunits of the dipyrrin.
Second, unlike the fully delocalized, aromatic structure of unsub-
stituted dipyrrins the crystal structure of a-OMe-4-cydpm reveals
a distinct asymmetry in the bond lengths of the bipyrrolic chro-
mophore. The structure reveals alternating short and long C–C
The contribution of the hydroquinone in the formation of the
a-substituted dipyrrins is consistent with a mechanism proposed
by Dolphin and coworkers involving alkoxy substitution of an N-
confused porphyrin (Scheme 3).23 The reported reaction involves
the addition of NaOCH3 and DDQ to an N-confused porphyrin
in a mixture of CH2Cl2/MeOH. The methoxy a-substituted
porphyrin was only obtained in 9% yield. As reported in reference
23, we were able to isolate the re-oxidized hydroquinone (p-
chloranil) from the reaction mixture. In the same report it was
also proposed that the cyano group of the 2,3-dichloro-5,6-
˚
and C–N bond distances in the a-substituted pyrrolic ring: 1.30 A
˚
˚
˚
(N1–C1), 1.45 A (C1–C2), 1.35 A (C2–C3), and 1.46 A (C3–C4).
Scheme 3 Reported reaction schemes for N-confused porphyrins by
Dolphin (top) and Osuka (bottom) that contain alkoxy group substitution
at the a-positions.
Fig. 2 Structural diagram of a-OMe-4-cydpm with partial atom number-
ing scheme (50% probability ellipsoids). Hydrogen atoms (except for the
pyrrolic hydrogen) have been omitted for clarity.
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However, this does not disrupt the delocalized nature of the unsub-
effect on the 1H NMR spectrum of the complex (Fig. S4†).
The protons associated with the dipyrrin core moieties of the
complex exhibit unique chemical shifts, as can be seen by the
numerous resonances from 6.0–7.0 ppm. These resonances appear
as a combination of singlets, associated with the a-protons,
and doublets associated with the b-protons. The phenyl protons
of all three ligands also exhibit unique chemical shifts as can
be seen by the multiplets at 7.4–7.8 ppm. The dipyrrinato,
phenyl, and methoxy protons integrate to 18, 12, and 3 protons,
respectively, indicating that the compound contains only one
methoxy substituted dipyrrin ligand per metal complex. Efforts
to recrystallize [Co(a-OMe-4-cydpm)(4-cydpm)2] did not generate
single crystals suitable for X-ray diffraction. Also, attempts to
prepare homoleptic tris-chelate cobalt(III) complexes with three a-
OMe-4-cydpm (or a-OMe-4-mecdpm) ligands were unsuccessful.
In an attempt to prepare a homoleptic metal complex with an
a-alkoxy dipyrrinato ligand, we anticipated that a 4-coordinate,
tetrahedral metal ion such as copper(II) or zinc(II) would better
accommodate the sterics of the a-alkoxy dipyrrinato ligand.
Indeed, zinc(II) complexes previously have been prepared from
asymmetric dipyrrins.20,22 The combination of a-OMe-4-mecdpm,
Zn(OAc)2·2H2O, and triethylamine in a chloroform/methanol
mixture at room temperature generated the desired complex,
which was crystallized upon slow evaporation from the reaction
mixture. Unlike other zinc(II) dipyrrinato complexes studied
in our laboratory, [Zn(a-OMe-4-mecdpm)2] was not stable to
purification by column chromatography,45,46 as exposure of [Zn(a-
OMe-4-mecdpm)2] to silica caused decomposition of the complex,
regenerating the free ligand. Electronic absorption spectroscopy
confirmed the formation of a zinc(II) complex with an absorption
maximum at 486 nm.46
[Zn(a-OMe-4-mecdpm)2] was characterized by a variety of
methods including mass spectrometry, NMR, infrared, and elec-
tronic spectroscopy. Single crystals were also obtained directly by
slow evaporation from the reaction mixture and X-ray structural
analysis confirmed the formation of the desired complex (Table 2).
Upon examination of the [Zn(a-OMe-4-mecdpm)2] structure,
several unusual features not seen in other zinc(II) dipyrrinato
complexes were found (Fig. 4).46 In other zinc(II) dipyrrinato
metal complexes the metal ion is approximately tetrahedral
with nearly orthogonal dipyrrin ligands. However, the [Zn(a-
OMe-4-mecdpm)2] complex adopts a very distorted tetrahedral
conformation with a mean interligand dihedral angle of 54.7◦.
By way of comparison, the interligand dihedral angles found
in bis(5-phenyldipyrrinato)zinc(II),46 which has two independent
complexes in the asymmetric unit, are 86.0◦ and 88.5◦. The
complex is also twisted so that the two methoxy groups are close
to each other, further perturbing the ligand set to give interligand
˚
stituted pyrrole ring, which has bond distances of N2–C9 (1.36 A),
˚
˚
˚
C9–C8 (1.38 A), C8–C7 (1.41 A), and C7–C6 (1.39 A). The bond
lengths agree well with an earlier crystal structure of a highly func-
tionalized asymmetric a-methoxy dipyrrin.44 Finally, the nitrogen-
bound pyrrolic hydrogen atom (H1N), expected to be bound
equally by either pyrrole ring in a dipyrrin, was identified in the
electron density difference map of a-OMe-4-cydpm localized on
the nitrogen atom of the unmodified pyrrole ring (Fig. 2). This may
1
explain why this proton was readily observed by H NMR, and
suggests that the alkoxy group decreases the basicity of the nitro-
gen of the a-substituted pyrrole, favoring a single tautomeric form.
Metal a-alkoxy dipyrrin complexes
The a-alkoxy dipyrrins demonstrated the capacity to bind metal
ions, as demonstrated by the cobalt(III) reaction mixture from
which the compound was first identified (vide supra). A complex
containing a single a-OMe-4-cydpm and two 4-cydpm ligands
([Co(a-OMe-4-cydpm)(4-cydpm)2]) was isolated from the mixture
of products. The cobalt(III) complex was characterized by high-
resolution mass spectrometry, 1H NMR, electronic, and IR spec-
troscopy. The a-methoxy substituent was found to significantly
perturb the ligand-to-metal charge transfer (LMCT) band (400–
620 nm) relative to the unsubstituted dipyrrinato complexes.
As shown in Fig. 3, the electronic absorbance spectra of the
unsubstituted and a-methoxy-substituted dipyrrinato cobalt(III)
complexes are easily distinguishable. The unsubstituted complex
[Co(4-cydpm)3] has absorbance maxima at 469 and 506 nm, while
the methoxy-substituted [Co(a-OMe-4-cydpm)(4-cydpm)2] has a
broad absorbance with a maximum at 490 nm. Although the
complex [Co(a-OMe-4-cydpm)(4-cydpm)2] contains only a single
modified ligand, it is still readily detected and distinguished by the
absorption spectrum.
Fig. 3 Electronic absorption spectra for [Co(4-cydpm)3] (—) and
[Co(a-OMe-4-cydpm)(4-cydpm)2] (---) in CH2Cl2.
Fig. 4 Structural diagram of [Zn(a-OMe-4-mecdpm)2] with partial atom
numbering scheme (ORTEP, 50% probability ellipsoids). Most hydrogen
atoms have been omitted for clarity.
The asymmetry afforded by the presence of the a-OMe-4-cydpm
ligand in [Co(a-OMe-4-cydpm)(4-cydpm)2] has a pronounced
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angles, N1–Zn–N3 and N2–Zn–N4, of 119◦ and 99◦, respectively
(Fig. 5). This distortion of the zinc coordination sphere may
be explained in part by the crystal packing, which places the
methyl ester moieties near the core dipyrrin ligands. Electronic
repulsion between the methyl ester and methoxy moieties could
cause the methoxy groups to gather onto one side of the zinc(II)
coordination sphere. This arrangement may also be stabilized by
a weak hydrogen bond involving the a-carbon of an unsubstituted
dipyrrin pyrrole ring and the carbonyl group of a nearby methyl
ester. Both influences are shown in Fig. S3.† The dipyrrin moieties
of each ligand in [Zn(a-OMe-4-mecdpm)2] are nearly planar with
torsion angles of 7.4◦ and 6.3◦, which is closer to planarity than the
free ligand, but more distorted than reported in other dipyrrinato
complexes.16,43,45,47,48
inequivalent. The chemical inequivalence of these protons was
used as a probe to measure the rotation barrier of the aryl group
using variable temperature NMR (Fig. 6). As the temperature
is increased the doublets of H12 and H16 coalesce at a Tc
≈
86 ◦C (359 K). This corresponds to a free energy of activation of
DG‡ = 17.3 kcal mol−1 for rapid rotation about the dipyrrin–aryl
bond.49 The phenyl protons H13 and H15, (Fig. 4) also undergo a
less pronounced splitting, which coalesce at elevated temperatures.
Similar phenyl proton coalescence has been reported when meso-
tetraphenylporphyrinato (TPP) complexes and para-substituted
TPP complexes have asymmetric axial ligation to the central metal
atom.50–53 For those compounds, the free energy of activation was
measured to be between 12.8 and 18.6 kcal mol−1, with electron-
withdrawing para substituents, such as trifluoromethyl groups,
leading to the higher barriers. The aryl rotation barrier for [Zn(a-
OMe-4-mecdpm)2] of 17.3 kcal mol−1 lies toward the higher end
of this range and might reflect the influence of the para-methyl
ester substituent. To our knowledge, this is the first time that such
aryl rotation has been experimentally observed in a meso-aryl
dipyrrinato complex.
Conclusions
ꢀ
While symmetrically-substituted a,a -thioalkyl dipyrrins54 and
asymmetrically-substituted a-methoxy dipyrrins55 recently have
been prepared from 2-thiocyanatopyrrole or pyrrolin-2-one
precursors, respectively, the present work represents the first
asymmetrically-substituted a-alkoxy dipyrrins that have been
synthesized directly from dipyrrins lacking substituents in
the a-positions. This methodology has now been applied to
reactions with a variety of alcohols and 5-(aryl)dipyrrins.
Several compounds, including Mn(OAc)2·4H2O, FeCl3·6H2O,
and Na3Co(NO2)6 were found to promote the reaction, and the
presence of a hydroquinone has proven vital for the formation
of the products under these reaction conditions. The synthesis of
both hetero- and homoleptic metal complexes has been described
and their structures have been probed both in solution and in
the solid state. The potential utility of this synthetic methodology
will be realized by further optimization of reaction conditions.
Fig. 5 [Zn(a-OMe-4-mecdpm)2] metal core with partial atom numbering
schemes (50% probability ellipsoids) viewed along two different off-axis
directions. The meso-phenyl ring and hydrogen atoms have been removed
for clarity. The coordination sphere shows an unusual cis geometry and
distortion away from an idealized tetrahedral geometry.
Solution dynamics of [Zn(a-OMe-4-mecdpm)2]
Upon close examination of the 1H NMR spectrum of [Zn(a-OMe-
4-mecdpm)2], the resonances associated with the ortho-protons,
H12 and H16, appear as a doublet of doublets (Fig. 6). This is in
contrast to the doublet of the same protons in the free ligand and
of other metal dipyrrinato complexes,16,45–47 and is indicative of
the asymmetry of the complex which renders the phenyl protons
Experimental
General methods
Unless otherwise noted, starting materials were obtained from
commercial suppliers and used without further purification.
1
Pyrrole was freshly distilled prior to use. H/13C NMR spectra
were recorded on a Varian FT-NMR spectrometer running at
400 MHz or a Unity FT-NMR spectrometer running at 500 MHz
at the Department of Chemistry and Biochemistry, University
of California, San Diego. Infrared spectra were collected on a
Mattson Research Series FT-IR instrument (using NaCl plates,
laboratory of Prof. Clifford P. Kubiak) at the Department of
Chemistry and Biochemistry, University of California, San Diego.
UV-Visible spectra were recorded using a Perkin-Elmer Lambda
25 spectrophotometer with the UVWinLab 4.2.0.0230 software
package. Mass spectra were acquired at the Small Molecule
Mass Spectrometry Facility located in the Department of Chem-
istry and Biochemistry, University of California, San Diego. A
Fig. 6 Variable temperature 1H NMR spectra of the ortho protons
H12 and H16 (see Fig. 4) of [Zn(a-OMe-4-mecdpm)2] in d6-DMSO. The
resonances coalesce upon heating, indicative of rapid rotation about the
dipyrrin–aryl bond.
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ThermoFinnigan LCQ-DECA mass spectrometer was used for
ESI and APCI analysis, and the data were analyzed using the
Xcalibur software suite. A ThermoFinnigan MAT 900XL mass
spectrometer was used to acquire the data for the high resolution
mass spectra (HRMS).
synthesis of a-OMe-4-cydpm, starting from 5-(4-methoxycar-
bonylphenyl)-4,6-dipyrromethane48 (0.20 g, 0.71 mmol) to afford
1
a yellow film. Yield: 19% (0.043 g). HNMR (CDCl3 400 MHz,
25 ◦C): d 3.96 (s, 3H), 4.13 (s, 3H), 6.11 (bd, 1H, J = 3.2 Hz), 6.17
(d, 1H, J = 4.8 Hz), 6.21 (bt, 1H, J = 3.2 Hz), 6.68 (d, 1H, J =
4.4 Hz), 7.15 (bs, 1H), 7.53 (d, 2H, J = 8.4 Hz), 8.10 (d, 2H, J =
8.4 Hz), 12.15 ppm (bs, 1H, NH). 13CNMR (CDCl3 100 MHz,
25 ◦C): d 52.2, 55.9, 110.3, 117.3, 117.7, 124.5, 128.8, 129.9, 130.8,
132.2, 132.4, 137.5, 142.0, 143.7, 166.8, 175.3 ppm. ESI-MS: m/z
309.15 [M + H]+. HR-FABMS: m/z 309.1239 [M + H]+ (calcd
m/z 309.1234). kmax (CH2Cl2 solution): 289, 407 nm. IR (thin film
CH2Cl2): v 3446, 2950, 1713, 1598, 1557, 1545, 1406, 1392, 1376,
1288, 1276, 1115, 1090, 1041, 764, 745, 718 cm−1.
Syntheses
[5-(4-Cyanophenyl)-1-methoxy-4,6-dipyrrinato]bis[5-(4-cyano-
phenyl)-4,6-dipyrrinato]cobalt(III),
cydpm)2 ].
[Co(a-OMe-4-cydpm)(4-
5-(4-Cyanocarbonylphenyl)-4,6-dipyrromethane39
(0.20 g, 0.81 mmol) was dissolved in 150 mL CHCl3 and stirred
in an ice bath. DDQ (0.20 g, mmol) was dissolved in benzene
(150 mL) and added slowly dropwise. The solvent was then
evaporated and the resulting dark residue was redissolved in a
1 : 1 mixture of CHCl3/MeOH (100 mL). Na3Co(NO2)6 (0.11 g,
0.27 mmol) dissolved in a 1 : 1 mixture of H2O/MeOH (6 mL)
was added. Triethylamine (1 mL) was added to the solution. The
resulting mixture was heated to 70 ◦C overnight. The solution
was evaporated to dryness and the product was purified by
column chromatography (SiO2; 3:2 to 9 : 1 CH2Cl2 : hexanes) to
afford three products: a-OMe-4-cydpm (yellow film, 0.064 g, 32%
5-(4-Methoxycarbonylphenyl)-1-ethoxy-4,6-dipyrromethene, a-
OEt-4-mecdpm. The same procedure was used as in the synthesis
of a-OMe-4-cydpm, starting from 5-(4-methoxycarbonylphenyl)-
4,6-dipyrromethane48 (0.20 g, 0.71 mmol) and etha◦nol (10 mL).
Yield: 35% (0.081 g). 1HNMR (CDCl3 400 MHz, 25 C): d 1.50 (t,
3H, J = 7.2 Hz), 3.96 (s, 3H), 4.53 (q, 2H, J = 7.6 Hz, J = 6.8 Hz),
6.11 (bd, 1H, J = 3.6 Hz), 6.16 (d, 1H, J = 4.4 Hz), 6.21 (bs, 1H),
6.67 (d, 1H, J = 4.8 Hz), 7.14 (bs, 1H), 7.53 (d, 2H, J = 8.8 Hz),
8.10 (d, 2H, J = 8.4 Hz), 12.14 ppm (bs, 1H, NH). 13CNMR
(CDCl3 100 MHz, 25 ◦C): d 14.5, 52.2, 64.6, 110.2, 117.5, 117.6,
124.3, 128.8, 129.9, 130.8, 132.1, 132.2, 137.3, 142.0, 143.9, 166.8,
174.7 ppm. ESI-MS: m/z 323.05 [M + H]+. HR-FABMS: m/z
323.1388 [M + H]+ (calcd m/z 323.1390). kmax (CH2Cl2 solution):
290, 407 nm. IR (thin film CH2Cl2): v 3446, 3301, 2976, 2950, 1724,
1597, 1549, 1531, 1406, 1392, 1378, 1346, 1286, 1257, 1112, 1090,
1039, 764, 741, 724 cm−1.
17
yield), Co(4-cydpm)3 (orange solid, 0.070 g, 33% yield), and
Co(a-OMe-4-cydpm)(4-cydpm)2 (red solid, 0.060 g, 27% yield).
Co(a-OMe-4-cydpm)(4-cydpm)2. 1HNMR (CDCl3 400 MHz,
25 ◦C): d 3.41 (s, 3H), 6.00 (d, 1H, J = 4.8 Hz), 6.20–6.44 (m, 7H),
6.50 (s, 1H), 6.55 (d, 1H, J = 4.8 Hz), 6.57–6.62 (m, 4H), 6.63 (d,
2H, J = 4.4 Hz), 6.75 (s, 1H), 6.94 (s, 1H), 7.43 (t, 2H, J = 8.8 Hz),
7.49–7.62 (m, 4H), 7.68 (d, 2H, J = 8.8 Hz), 7.73–7.77 ppm (m,
4H). APCI-MS: m/z 821.85 [M + H]+. HR-FABMS:m/z 822.2146
[M + H]+ (Calcd m/z 822.2135). kmax (CH2Cl2 solution): 266, 304,
408, 493 nm. IR (thin film CH2Cl2): v 2926, 2857, 2229, 1566, 1381,
1347, 1249, 1033, 998, 812 cm−1.
5-(4-Methoxycarbonylphenyl)-1-propoxy-4,6-dipyrromethene,
a-OPr-4-mecdpm. The same procedure was used as in the syn-
thesis of a-OMe-4-cydpm, starting from 5-(4-methoxycarbonyl-
phenyl)-4,6-dipyrromethane48 (0.20 g, 0.71 mmol) and propanol
(10 mL). Yield: 38% (0.091 g). 1HNMR (CDCl3 400 MHz, 25 ◦C):
d 1.09 (t, 3H, J = 7.6 Hz), 1.82–1.98 (m, 2H), 4.43 (t, 2H, J =
6.4 Hz), 6.11 (bd, 1H, J = 3.2 Hz), 6.18 (d, 1H, J = 4.8 Hz), 6.21
(bs, 1H), 6.68 (d, 1H, J = 4.8 Hz), 7.15 (bs, 1H), 7.53 (d, 2H,
J = 8.4 Hz), 8.10 (d, 2H, J = 8.0 Hz), 12.16 ppm (bs, 1H, NH).
13CNMR (CDCl3 100 MHz, 25 ◦C): d 10.5, 22.2, 52.1, 70.3, 110.1,
117.5, 117.6, 124.3, 128.8, 129.8, 130.8, 132.0, 132.2, 137.2, 142.0,
143.9, 166.7, 174.9 ppm. FAB-MS: m/z 336.2 [M]+. HR-FABMS:
m/z 336.1459 [M]+ (calcd m/z 336.1468). kmax (CH2Cl2 solution):
289, 407 nm. IR (thin film): v 3278, 2964, 2953, 2881, 1725, 1597,
1549, 1532, 1407, 1364, 1338, 1286, 1279, 1101, 1091, 1040, 960,
763, 742, 725 cm−1.
5-(4-Cyanophenyl)-1-methoxy-4,6-dipyrromethene, a-OMe-4-
cydpm. 5-(4-Cyanophenyl)-4,6-dipyrromethane39
(0.20
g,
0.81 mmol) was dissolved in 100 mL CHCl3 and stirred in an
ice bath. 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)
(0.18 g, 0.81 mmol) was dissolved in 100 mL benzene and added
dropwise over ∼1 h. After addition, the reaction mixture was
then evaporated to dryness and the resulting dark residue was
redissolved in 25 mL MeOH and 25 mL CHCl3. Na3Co(NO2)6
(0.11 g, 0.27 mmol) dissolved in 2 mL H2O and 1 mL MeOH
was added to the solution. The resulting mixture was heated to
reflux while stirring overnight. The solution was evaporated to
dryness and the product was purified by column chromatography
(SiO2; 4 : 1 CH2Cl2 : hexanes) to afford a yellow solid. Yield:
18% (0.041 g). 1HNMR (CDCl3 400 MHz, 25 ◦C): d 4.14 (s, 3H),
6.02–6.11 (m, 1H), 6.19 (d, 1H, J = 4.4 Hz), 6.20–6.27 (m, 1H),
6.62 (d, 1H, J = 4.4 Hz), 7.17 (bs, 1H), 7.56 (d, 2H, J = 8.0 Hz),
7.72 (d, 2H, J = 8.0 Hz), 12.11 ppm (bs, 1H, NH). 13CNMR
(CDCl3 100 MHz, 25 ◦C): d 56.0, 110.3, 112.0, 117.4, 117.8,
118.4, 124.6, 131.0, 131.3, 131.6, 136.9, 141.9, 143.6, 175.3 ppm.
APCI-MS: m/z 276.117 [M + H]+. HR-FABMS: m/z 275.1054
[M]+ (calcd m/z 275.1053). kmax (CH2Cl2 solution): 264, 285,
407 nm. IR (thin film CH2Cl2): v 2936, 2229, 1598, 1557, 1532,
1406, 1377, 1290, 1122, 1089, 1041, 937, 804, 775, 746, 691 cm−1.
Bis[5-(4-methoxycarbonylphenyl)-1-methoxy-4,6-dipyrrinato]-
zinc(II), [Zn(a-OMe-4-mecdpm)2]. a-OMe-4-mecdpm (0.172 g,
0.558 mmol) was dissolved in 25 mL of CHCl3 and 25 mL of
MeOH. Zn(OAc)2·2H2O was dissolved in 5 mL of MeOH and
added to the solution. Triethylamine (0.5 mL) was added and the
reaction was stirred overnight at room temperature. The solution
was then slowly evaporated to yield bright orange crystals, which
were washed 3 times with 5 mL ◦of MeOH. Yield: 77% (0.146 g).
1HNMR (CDCl3 400 MHz, 25 C): d 3.76 (s, 6H), 3.98 (s, 6H),
6.00 (d, 2H, J = 4.4 Hz), 6.33 (d, 2H, J = 4.4 Hz), 6.41 (d, 2H,
J = 3.6 Hz), 6.65 (d, 2H, J = 4.4 Hz), 7.37 (s, 2H), 7.60 (q, 4H,
J = 5.2 Hz, J = 7.6 Hz), 8.04–8.20 (m, 4H). 13CNMR (CDCl3
5-(4-Methoxycarbonylphenyl)-1-methoxy-4,6-dipyrromethene,
a-OMe-4-mecdpm. The same procedure was used as in the
1072 | Dalton Trans., 2007, 1067–1074
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Table 2 Summary of X-ray structural parameters
a-OMe-4-cydpm
[Zn(a-OMe-4-mecdpm)2]
Empirical formula
Crystal system
Space group
C17H13N3O
Triclinic
P1
8.8386(9)
9.3207(9)
10.0246(10)
63.7840(10)
68.1580(10)
78.077(2)
C36H30N4O6Zn
Triclinic
P1
11.9083(7)
11.9234(7)
12.0192(7)
69.4180(10)
88.6430(10)
78.7560(10)
¯
¯
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
V/A , Z
686.82( 12), 2
0.30 × 0.17 × 0.15
100(2)
1565.77(16), 2
0.40 × 0.19 × 0.15
100(2)
Crystal size/mm
T/K
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices I > 2r(I)
R indices (all data)
5831
13652
2979, R(int) = 0.0182
6804, R(int) = 0.0155
2979/0/195
6804/0/428
1.066
1.025
R1 = 0.0455, wR2 = 0.1228
R1 = 0.0293, wR2 = 0.0777
R1 = 0.0507, wR2 = 0.1272
R1 = 0.0310, wR2 = 0.0791
◦
100 MHz, 25 C): d 52.3, 57.4, 103.9, 114.2, 127.4, 128.2, 128.3,
129.7, 130.7, 130.8, 134.1, 136.9, 138.1, 142.2, 143.5, 144.0, 166.7,
171.8. FAB-MS: m/z 678.4 [M]+. HR-FABMS: m/z 678.1437 [M]+
(calcd m/z 678.1451). kmax (CH2Cl2 solution): 306, 486 nm. IR (thin
film CH2Cl2): v 3012, 3050, 2951, 2926, 2846, 1723, 1556, 1508,
1497, 1404, 1379, 1346, 1318, 1276, 1269, 1194, 1113, 1048, 1005,
962, 829, 763, 725 cm−1.
4 M. Kollmannsberger, T. Gareis, S. Heinl, J. Breu and J. Daub, Angew.
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Crystallography
Single crystals of each compound suitable for X-ray diffraction
structural determination were mounted on quartz capillaries or
nylon loops by using Paratone oil and were cooled in a nitrogen
stream on the diffractometer. For further details see ESI† and
Table 2.
CCDC reference numbers 624663 and 624664.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b615801c
15 S. R. Halper and S. M. Cohen, Angew. Chem., Int. Ed., 2004, 43, 2385–
2388.
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Acknowledgements
18 G. R. Geier and J. S. Lindsey, J. Chem. Soc., Perkin Trans. 2, 2001,
677–686.
We thank Prof. Arnold L. Rheingold (U.C.S.D.) for help with
the X-ray structure determinations. We thank Loi Do and Dr
Drew Murphy for synthetic assistance and helpful discussions.
This work was supported by the University of California, San
Diego, the donors of the American Chemical Society Petroleum
Research Fund (42594-AC3), and the National Science Founda-
tion (0546531). This material is based upon work supported under
a National Science Foundation Graduate Research Fellowship
(S.R.H.). J.R.S was supported by an NIH training grant (T32
HL007444-25). S.M.C. is a Cottrell Scholar of the Research
Corporation.
19 J.-P. Strachan, D. F. O’Shea, T. Balasubramanian and J. S. Lindsey,
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