Metalation chemistry of meso-aryl-substituted expanded porphyrins

MICROREVIEW

DOI: 10.1002/ejic.200501097

Metalation Chemistry of meso-Aryl-Substituted Expanded Porphyrins

[a]

Soji Shimizu and Atsuhiro Osuka*

Keywords: Aromaticity / Conjugation / Macrocyclic ligands / Porphyrinoids

meso-Aryl-substituted expanded porphyrins are structural

analogues of meso-aryl-substituted porphyrins. They exhibit

rich and novel coordination behaviors, including multi-metal

chelation due to the large cavities, spin–spin interaction aris-

ing from the proximity of metal centers, and large changes

in their structural and electrochemical properties upon met-

alation. The recent discovery of a facile, one-pot synthesis of

a series of meso-aryl-substituted expanded porphyrins en-

ables us to investigate their metal-complexation chemistry.

The aim of this Microreview is to highlight recent progress

in the synthesis and characterization of metal complexes of

meso-aryl-substituted expanded porphyrins.

Germany, 2006)

Introduction

their intriguing properties, which are very difficult or im-

possible to achieve in porphyrins. Among these, expanded

porphyrins bearing more than four pyrrole rings have been

demonstrated to be quite promising due to their larger con-

jugation and diversity that allow remarkable absorption

spectra ranging from the UV/visible to the near-IR region,

variable oxidation states that are interconvertible among

aromatic, antiaromatic, and nonaromatic systems, and

Porphyrins, which play a key role in biological as well as

artificial processes, are undoubtedly one of the most widely

studied macrocycles and are characterized as 18π-electron

aromatic ligands that can accommodate most metal ions

to form 1:1 metal complexes.^[In recent years, porphyrin

analogues that include modified, confused, contracted, and

expanded ones have attracted increasing attention owing to

1]

[

2–7]

multi-metal coordinating behaviors.

On account of these

promising properties, they are in the spotlight in various

University, and Core Research for Evolutional Science and fields, including receptors for anions, ligands for several

[

a] Department of Chemistry, Graduate School of Science, Kyoto

[

8]

Technology (CREST), Japan Science and Technology Agency,

transition and lanthanoid metal ions, sensitizers for photo-

Sakyo-ku, Kyoto 606-8502, Japan

[

5,9]

dynamic therapy (PDT),

aging (MRI) contrast agents.

and magnetic resonance im-

Fax: +81-75-753-3970

[

10]

E-mail: osuka@kuchem.kyoto-u.ac.jp

In addition, they have re-

Soji Shimizu was born in 1979 in Kyoto, Japan. He received his BSc and MSc degrees in 2002 and 2004, respectively,

from Kyoto University. He is currently a PhD student with Professor Osuka at the same university, focusing on the

synthesis and exploration of expanded porphyrins. During spring 2005, as a visiting fellow, he stayed at Professor Jonathan

L. Sessler’s laboratory in the Department of Chemistry and Biochemistry, the University of Texas at Austin. He was

selected as a Research Fellow of the Japan Society for the Promotion of Science (JSPS) in 2004.

Atsuhiro Osuka was born in Gamagori, Aichi, in 1954. He received his PhD degree from Kyoto University in 1982 on

the photochemistry of epoxyquinones. In 1979, he started his academic career at the Department of Chemistry, Ehime

University as an assistant professor. In 1984, he moved to the Department of Chemistry, Kyoto University, where he

became a professor of chemistry in 1996. He was awarded the CSJS Award for Young Chemists in 1988 and the Japanese

Photochemistry Association Award in 1999. His research interests cover many aspects of synthetic approaches toward the

artificial photosynthesis and development of porphyrin-related compounds with novel structures and functions. He was

selected as a project leader of Core Research for Evolutional Science and Technology (CREST) of JST.

MICROREVIEWS: This feature introduces the readers to the authors’ research through a concise overview of the

selected topic. Reference to important work from others in the field is included.

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1319

S. Shimizu, A. Osuka

MICROREVIEW

cently been shown to have potential as nonlinear optical the porphyrin synthesis. A series of fully fluorinated ex-

materials owing to their large two-photon absorption (TPA) panded porphyrins was prepared in a similar manner from

cross-sections, which will lead to a broad range of applica- the reaction of 3,4-difluoropyrrole and pentafluorobenzal-

[

23]

tions such as three-dimensional microfabrication, optical dehyde (Scheme 2).

The ring-size-selective synthesis of

These promising at- expanded porphyrins has been achieved by using dipyrro-

tributes have encouraged diverse synthetic efforts towards methane and tripyrromethane as starting substrates under

data storage, and optical limiting.^[

11,12]

[

24]

various expanded porphyrins that differ in ring size, ring milder reaction conditions (Scheme 3). This ring-size-se-

connectivity, peripheral substituents, and core modifica- lective synthesis has also allowed the preparation of hexa-

[

5,13–21]

tion.

themund–Lindsey porphyrin synthesis leads to the forma- substituents.

tion of a series of meso-aryl-substituted expanded porphy- The metalation chemistry of expanded porphyrins can

Recently, we found that a modified Ro- phyrins and octaphyrins bearing electron-rich meso-aryl

[

25]

rins that are all real porphyrin homologues in terms of a be traced back to the 1980s. Representative examples are

regularly alternating pyrrole and methine carbon arrange- summarized in Figure 1. In 1983, Gossauer et al. reported

[

22]

ment (Scheme 1).

This synthesis requires aryl aldehyde the preparation of β-dodecaalkyl-substituted [26]hexaphyr-

II

substrates bearing electron-deficient and bulky substituents ins, which were converted into their bis-Zn and bis-Pd

both at the 2- and 6-positions and 10 times higher concen- complexes.

[

26]

Vogel and co-workers have synthesized a

trations of substrates (ca. 67 m) than those optimized for series of β-alkyl-substituted octaphyrins bearing twisted fig-

Scheme 1. Synthesis of meso-aryl-substituted expanded porphyrins.

Scheme 2. Synthesis of perfluorinated expanded porphyrins.

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Metalation Chemistry of meso-Aryl-substituted Expanded Porphyrins

MICROREVIEW

Scheme 3. Ring-size selective synthesis.

Figure 1. Representative examples of metal complexes of expanded porphyrins.

ure-eight conformations and revealed their facile complex- rolic subunits and thus in cavity size, conformational flexi-

[

27]

ation behaviors with several transition metal ions.

figure-eight conformations were fixed upon complexation,

which enabled optical separation of the metalated octaphyr- butions in the synthesis, characterization, and properties of

The bility, and stable conformation.

In this Microreview we summarize our own contri-

ins for the first time.^[

27a]

Sessler et al. have investigated lan- metal complexes of meso-aryl-substituted expanded por-

thanoid metal complexes of texaphyrins and medicinal ap- phyrins, which are presented in order of their ring size. The

plications of gadolinium texaphyrins as sensitizers for pho- compound names appearing in this review are partly based

[

5,9]

II

todynamic therapy.

They synthesized the bis-Cu com- on a nomenclature put forward by Franck and Nonn for

[

30]

plex of amethyrin and revealed the proximity of the metal porphyrinic system. The number in square brackets cor-

centers and their antiferromagnetic behavior.^[Recently, responds to the number of π-electrons in the shortest conju-

they have investigated the usage of expanded porphyrins as gation pathway and the core name represents the number

a colorimetric actinoid sensor.^[Thus, expanded porphy- of pyrroles.

28]

29]

rins are becoming increasingly important platforms for se-

veral metal ions and as promising materials in various fi-

elds. From this point of view, the meso-aryl-substituted ex-

^N^-^F^u^s^e^d^P^eⁿ^t^a^p^h^y^rⁱⁿ⁽^N^F^P5⁾

panded porphyrins can be regarded as another promising

candidate to explore novel functional metal complexes since

Structures and Properties of the Free Bases

this facile, one-pot reaction affords a series of expanded

meso-Pentafluorophenyl-substituted pentaphyrin was ob-

porphyrins that differ in the number of the constituting pyr- tained as its N-fused form, whicht was found to exist as a

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S. Shimizu, A. Osuka

MICROREVIEW

mixture of two oxidation states {[22]N-fused pentaphyrin entire structure of 2a is planar, while that of 2b is severely

[22]NFP , 2a) and [24]NFP (2b); Figure 2} from the reac- distorted.

(

5

[

22,31]

tion shown in Scheme 1.

The yield of each compound

varied depending on the oxidation time. Thus, 2a was ob-

tained after oxidation with an excess amount of DDQ for

a long time (Ͼ15 h) in 18% yield as the sole pentapyrrolic

product (Scheme 1).^[

31b]

Alternatively, 2a was prepared in

28% yield by an acid-catalyzed [3+2] condensation of tri-

pyrrane and dipyrromethane (Scheme 4). Compound 2a is

easily reduced to 2b with NaBH , which is quantitatively

4

oxidized back to the original 2a upon treatment with DDQ

(

2

Figure 2). An aromatic ring current effect arising from the

2π-electron conjugation is observed in the H NMR spec-

1

5

Figure 2. Redox behavior of NFP .

trum of 2a, while 2b shows antiaromatic character due to

its 24π-electron conjugation. According to X-ray diffraction

analyses, both 2a and 2b possess a fused tripentacyclic ring

Metal Complexes of NFP₅

(

pyrroles A and B in Figure 3), an outward-pointing pyrrole

Since both 2a and 2b possess a dipyrromethene moiety,

I

ring (pyrrole C), and an inward-oriented dipyrromethene we investigated complexation of Rh , which is known, in

moiety (pyrroles D and E). The inverted pyrrole C is signifi- the coordination chemistry of porphyrinoids, to bind to one

cantly tilted for each compound. Except for pyrrole C the dipyrromethene ligand and two carbonyl ligands in a

Scheme 4. Synthesis of 2a by a [3+2] condensation reaction.

Figure 3. X-ray crystal structures of a) 2a (preliminary) and b) 2b; top views (top) and side views (bottom). meso-Aryl substituents have

been omitted for clarity in the side views.

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Metalation Chemistry of meso-Aryl-substituted Expanded Porphyrins

MICROREVIEW

square-planar fashion.^[³²^–³⁴^]Two rhodium complexes 10 planar fashion (Figure 5). The entire structure of 10 is rela-

and 11 were isolated in 2.4% and 23% yields, respectively, tively planar except for the pyrrole C. On the other hand,

from the metalation of 2a with two equivalents of 11 has a severely distorted bent conformation despite the

[

RhCl(CO) ] in CH Cl in the presence of sodium acetate fully conjugated system. In accordance with these results,

2 2 2 2

[31b]

1

at room temperature for 4 h (Figure 4).

The product ra- the H NMR spectrum of 10 displays peaks for the inner

tio of 11 to 10 decreased as the reaction time increased. β-CH protons of pyrroles A and C as a singlet at δ =

Under refluxing conditions the yield of 10 (42%) was dis- 0.05 ppm and as a pair of doublets at δ = 1.62 and 1.16

tinctly higher than that of 11 (18%). These results suggest ppm, respectively, and the outer β-CH protons as three

that 10 is the thermodynamic product of this reaction. On pairs of doublets at δ = 9.23, 9.21, 8.61, 8.29, 8.21, and

the contrary, the complexation of 2b with [RhCl(CO)₂]₂8.13 ppm that indicate a diatropic ring-current effect arising

gave complex 11 as the sole product in 60% yield at room from the planar structure and 22π-electron conjugation. In

1

temperature after 1 h.

the H NMR spectrum of 11, however, resonances due to

In the crystal structures, both 10 and 11 preserve basic the inner and outer NH protons of pyrroles E and A are

NFP skeletons with a 22π- and 24π-electron conjugation, observed at δ = 0.68 and 9.44 ppm, respectively, indicating,

5

I

respectively, in which the Rh ion is bound above the center to our surprise, a substantial diatropic ring current for 11,

of the cavity by the dipyrromethene unit (pyrroles D and E which is also supported by the up- and downfield shifts of

for 10 and pyrroles C and D for 11), with N–Rh distances the peaks due to the inner and outer β-CH protons, respec-

of around 2.1 Å, and two carbonyl ligands in a square- tively.

I

Figure 4. Rh complexes of NFP

5

.

Figure 5. X-ray crystal structures of a) 10 and b) 11; top views (top) and side views (bottom). meso-Aryl substituents have been omitted

for clarity in the side views.

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S. Shimizu, A. Osuka

MICROREVIEW

1

Since complex 10 is the major product at high tempera- formation. On the other hand, the H NMR spectrum of

ture, thermal conversion of 11 to 10 was examined in re- 3b indicates aromatic rather than antiaromatic character.

fluxing CH Cl ; this reaction turned out to be unsuccessful. The explanation for this is uncertain at this stage.

2

Considering that the number of conjugated π electrons is

2 for 10 and 24 for 11, interconversion between these com-

2

plexes by two-electron oxidation and reduction should be

feasible. Thus, treatment of 11 with DDQ at room tempera-

ture quantitatively converted it into complex 10 by a novel

pivot-like-walk rearrangement of Rh. The details of this

1

process were monitored by H NMR spectroscopy, in which

an intermediate 12 (Figure 4) was detected before the final

formation of 10. In contrast, reduction of 10 with NaBH₄

caused demetalation to give 2b without producing 11.

Hexaphyrin

Structures and Properties of the Free Bases

Hexaphyrin 3, which was first synthesized by Cavaleiro

[

35]

and co-workers, was one of the major products from the

modified Rothemund–Lindsey reaction and was obtained

in 16–20% yield as an oxidized form with 26π electrons in

its conjugation (Scheme 1).^[The yield was improved by

using the ring-size selective synthesis.^[Upon treatment

22]

24]

with NaBH , [26]hexaphyrin (3a) was easily reduced to [28]-

hexaphyrin (3b), which could be quantitatively oxidized

4

Figure 6. Redox behavior of hexaphyrin.

back to the original [26]hexaphyrin (Figure 6). In the solid,

We have also investigated the synthesis of hexaphyrins

3

a exhibits a planar rectangular structure with two inverted bearing several substituents not only at the meso position

[35]

pyrrole rings (Figure 7, a). Although a structural analysis but also at the β position. Perfluorinated [28]hexaphyrin (7)

of 3b unfortunately has not yet been successful, the struc- was isolated in 2–5% yield from a mixture of other perfluo-

ture of an analogue of 3b bearing alkylamino substituents rinated expanded porphyrins obtained from the reaction of

at the p position of the pentafluorophenyl groups and exhi- 3,4-difluoropyrrole with pentafluorobenzaldehyde. In the

biting almost identical UV/Vis and NMR spectra to 3b was solid state, 7 takes a figure-eight conformation and hence

revealed crystallographically to possess a similar conforma- exhibits nonaromatic character (Figure 7, b). In addition to

[

23]

[

36]

tion to that of 3a. In solution, 3a exhibits aromatic char- the coordination properties of these hexaphyrins, they are

acter due to the 26π-electron conjugation and planar con- also drawing much attention as a near-IR absorbing dye.

Figure 7. X-ray crystal structures of a) 3a and b) 7; top views (top) and side views (bottom). meso-Aryl substituents have been omitted

for clarity in the side views.

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Metalation Chemistry of meso-Aryl-substituted Expanded Porphyrins

MICROREVIEW

–1

The absorption spectrum of 3a shows a Soret-like band at action values (J) of –8.27, –87.6, and –42.1 cm , respec-

68 nm and Q-like absorptions at 712, 898, and 1026 nm, tively. On the contrary, complex 16 has a temperature-inde-

5

–1

and is formally similar to that of porphyrin although with pendent χ T value (0.4229 emuKmol ) corresponding to

substantial red-shifts owing to the extension of the π-conju- an S = 1/2 state, in agreement with the Cu /Cu mixed-

gation.

at 1036 nm with a weak vibronic band at 1205 nm.

cently, molecules having large two-photon absorption

p

I

II

[

11,35]

Compound 3a exhibits fluorescence emission valence state.

[

11]

Re-

(2)

(TPA) cross-sections [σ ] have been of great interest due to

their potential applications in a variety of fields described

Doubly N-confused [26]Hexaphyrin from [26]Hexaphyrin

in the previous section. Aromatic 3a exhibits a large TPA

4

cross-section value of about 10 GM, which makes it quite

In the course of their extensive investigations on N-con-

attractive for further applications.^[

11]

[

38]

fused porphyrinoids, Furuta et al. have reported the syn-

thesis of a doubly N-confused hexaphyrin (DNCH, 17)

bearing two oxygenated confused pyrrole rings, which offer

efficient coordination sites for two transition metal ions

Copper Complexes of Hexaphyrins

II

[38c]

The complexation reaction of 3a with anhydrous CuCl₂such as Cu 18 and Ni 19 (Figure 10).

Compound 17

and NaOAc in CH Cl gave the dicopper complex 13 in and its bis-metal complexes are also attractive as near-IR-

2

[

37]

84% yield.

Interestingly, metalation in the presence of absorbing dyes due to their expanded π-electronic system

anhydrous Cu(OAc) and NaOAc led to isolation of a dif- but have been rather poorly explored, mainly due to the

2

ferent dicopper complex 14 in 90% yield (Figure 8). Both tedious, multi-step synthesis of the N-confused tripyrrane

[

38d]

structures were elucidated by X-ray single crystal analysis. precursor.

During our exploration of the metalation

The structure of 13 takes a gable structure with two semi- chemistry of hexaphyrin 3a, we serendipitously found that

I

planar tripyrrolic planes in which each copper ion is bound treatment with Cu under aerobic conditions in pyridine ef-

II

to the bridging chloride atom, the oxygen atom attached at ficiently provided the bis-Cu complex of DNCH (18;

the meso position, and three nitrogen atoms of the tripyr- 24.4% yield), whose structure was confirmed by X-ray

[

39]

rolic unit, in a square-pyramidal fashion (Figure 9, a). analysis (Figure 11, a).

Remarkably, both the inverted

Compound 14 exhibits a similar gable structure with two pyrrole rings of 3a undergo simultaneous rearrangements

copper ions, which are each bound to the meso-attached to form N-confused pyrroles with concomitant oxygenation

oxygen atom and three pyrrolic nitrogen atoms in a square- at the internal pyrrolic α positions. Although the reaction

planar fashion (Figure 9, b). Contrary to complex 13, there mechanism has not been elucidated yet, we anticipate the

is no axial bridging in 14.

Similar copper metalation was tested for perfluorinated on the basis of several experimental facts.

28]hexaphyrin (7). Complex 15 was obtained as the sole The bound copper ions are easily demetalated upon

product (96% yield) from the reaction with CuCl , while treatment with TFA in CH Cl to give the free base 17,

involvement of CuCl–pyridine–O oxidation in this reaction

2

[

2

metalation with Cu(OAc) afforded a mixture of complex which is a good ligand system for two divalent or trivalent

2

II

III

1

5 and 16 in 26 and 56% yields, respectively (Figure 8). The metal ions (Cu 18, Ni 19, Co 20, Zn 21, Mn 22, and

III

1

structure of 15 is quite similar to that of 14, while 16 takes Fe 23; Figure 10). The H NMR spectra of 17 and 21

a different structure in which the two copper ions are not display strong diatropic ring currents in line with their 26π

directly bridged but are coordinated separately by the dif- aromatic conjugation. The absorption spectrum of 17 exhi-

ferent tripyrrolic ligand and meso-attached oxygen atom bits a Soret-like band at 566 nm with a shoulder at 586 nm

(Figures 9, c and d). The oxygen ligands in 16 are from a and Q-like bands at 722, 793, 911, and 1047 nm. Interest-

tertiary alcohol and an alkoxide. Importantly, this structure ingly, 17 shows fluorescence at 1058 nm, with a vibronic

I

II

leads to the assignment of 16 as a mixed-valence Cu /Cu

structure that has a very small Stokes shift. These observa-

II

complex. The proximity of the two Cu ions in 13, 14, and tions suggest a rigid structure supported by multiple intra-

15 allows antiferromagnetic couplings with exchange inter- molecular hydrogen-bonding interactions.

Figure 8. Bis-Cu complexes of hexaphyrins.

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S. Shimizu, A. Osuka

MICROREVIEW

Figure 9. X-ray crystal structures of a) 13, b) 14, c) 15, and d) 16; top views (top) and side views (bottom). meso-Aryl substituents have

been omitted for clarity.

Figure 10. Doubly N-confused hexaphyrin and its metal complexes.

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Metalation Chemistry of meso-Aryl-substituted Expanded Porphyrins

MICROREVIEW

Figure 11. X-ray crystal structures of a) 18 and b) 20; top views (top) and side views (bottom). meso-Aryl substituents have been omitted

for clarity in the side views. Water molecules each coordinate to the axial position of the cobalt ion in 20.

Group 10 Metal Complexes of Hexaphyrin

reaction of 3a with PtCl and NaOAc in refluxing CH Cl

2 2 2

under argon. Curiously, however, metalation with PdCl in

2

The copper complexes described above exhibit large

structural changes upon metalation. Next, we investigated

the complexation of hexaphyrin with group 10 metal ions

refluxing CH Cl in the presence of NaOAc provided a

2

II

mixture of mono-Pd (25) and bis-Pd (26) complexes,

which were isolated in 50 and 9% yields, respectively (Fig-

ure 12). Single crystal analyses revealed that complexes 24,

II

II [40]

such as Ni , Pd , and Pt .

When a toluene solution of

3

a and Ni(acac) was refluxed for one hour in the presence

2

5, and 27 possess a metal–carbon bond and have distorted

of sodium acetate under anaerobic conditions the mono-

1

structures (Figures 13, a–c). The H NMR spectra of 24,

5, and 27 show nonaromatic character due to the non-

II

Ni complex 24 was obtained in 50% yield. Similarly, the

2

II

mono-Pt complex 27 was obtained in 56% yield from the

II

planar structures. In the crystal structure of 26 the two Pd

ions are bridged by the chloride atom. One Pd ion is

II

bound to three pyrrolic nitrogen atoms and the other is

bound to two pyrrolic nitrogen atoms and a pyrrolic α-car-

bon, and an N-fusion reaction of the pyrrole E and the

adjacent pentafluorophenyl group has taken place to form

a planar pentacyclic ring (Figure 13, d). Thus, the internal

β-pyrrolic CH bond of 3a is activated to form a metal–

carbon bond in the metalation reactions with group 10 me-

tal ions.

Gold Complexes of Hexaphyrin

Metal complexes of hexaphyrin preserving the original

planar and rectangular shape are of great interest from the

viewpoint of regulation of aromaticity and antiaromaticity.

Towards this goal, metal ions with a strong C–H activation

ability would be promising. During the exploration of the

complexation chemistry of hexaphyrins, we fortunately

found that gold metalation of 3a accomplished this goal to

III

[41]

provide mono-Au and bis-Au complexes. A solution

of 3a and NaAuCl in a 4:1 mixture of CH Cl and meth-

4

2

anol was stirred in the presence of sodium acetate at room

temperature for three days. Separation on a silica gel col-

III

umn provided the mono-Au (28) and bis-Au (29) com-

Figure 12. Group 10 metal complexes of hexaphyrin.

plexes in 16 and 14% yield, respectively, along with 15% of

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S. Shimizu, A. Osuka

MICROREVIEW

Figure 13. X-ray crystal structures of a) 24, b) 25, c) 27 (preliminary), and d) 26; top views (top) and side views (bottom). meso-Aryl

substituents have been partially omitted for clarity.

recovered 3a (Figure 14). Curiously, in both crystal struc- 9.36, 9.35, and 9.03 ppm, and the inner β-CH and NH pro-

tures the inner β-CHs have been activated to form Au–C tons are seen at δ = –2.93 and –2.08 ppm, respectively, while

III

bonds and the Au ions are coordinated to two pyrrolic for 29 the outer β-CH protons appear at δ = 9.66 and 9.51

nitrogen atoms and two pyrrolic β-carbon atoms within a ppm (Figures 16, a and b). Rigidification by Au coordina-

flat [26]hexaphyrin core. The structure of 28 is planar in the tion has been revealed to play a very important role in en-

coordinated moiety and rather distorted in the cavity, while hancing aromatic character, which, in turn, suggests a pos-

29 has a much more planar but overall bent structure (Fig- sible attainment of antiaromatic [28]hexaphyrin in the

ure 15). These structural differences can be explained by the planar frame. Thus, complexes 28 and 29 were reduced with

steric congestion between the inner pyrrolic β-CHs. As an- NaBH to give complexes 30 and 31 without demetalation

4

ticipated from their crystal structures, aromaticity of these (Figure 14). The structures of 30 and 31 were both prelimi-

1

complexes was observed in their H NMR spectra. The narily revealed by X-ray analyses to keep the original rec-

outer β-CH protons of 28 appear downfield at δ = 9.51, tangular structures but to have two NH hydrogen atoms at

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Metalation Chemistry of meso-Aryl-substituted Expanded Porphyrins

MICROREVIEW

Figure 14. Gold complexes of hexaphyrin.

Figure 15. X-ray crystal structures of a) 28 and b) 29; top views (top) and side views (bottom). meso-Aryl substituents have been omitted

for clarity in the side views.

III

the inverted pyrroles. Very curiously, and consistent with ined by treating 28 with CF COOAg to give the Au /Ag

3

1

Hückel’s rule, the H NMR spectra of 30 and 31 show a hetero complex 32 in 58% yield with concomitant oxidation

I

III

paratropic ring-current effect (Figures 16, c and d). In the of Ag to Ag in order to fit into the core.

1

H NMR spectrum of 30, the inner β-CH and NH protons

are observed at δ = 19.39 and 24.57 ppm, respectively, while

Octaphyrin

the outer β-CH protons appear as four doublets at δ = 5.02,

.92, 4.32, and 4.07 ppm and the outer NH protons appear

4

Structures and Properties of the Free Bases

1

at δ = 4.10 ppm. Similarly, the H NMR spectrum of 31

exhibits the outer β-CH protons as a pair of doublets at δ

Due to its large conjugation, meso-aryl-substituted octa-

=

3.16 and 3.10 ppm and the outer NH protons as a broad phyrin is supposed to possess three different redox states

singlet at δ = 1.81 ppm. Considering the nonaromaticity of with 34π-, 36π-, and 38π-electrons (5b, 5a, and 5c, respec-

the group 10 metal complexes 24, 25, and 27, firm rigidifi- tively, in Figure 17). From the reaction shown in Scheme 1,

cation of the planar conformation causes the strong an- octaphyrin was obtained as [36]octaphyrin (5a) in 5–6%

tiaromatic character in [28]hexaphyrin.

yield. The yield of 5a was improved by using the ring-size

Complex 28 still has a vacant cavity that may allow coor- selective synthesis.^[²⁴^]In the crystal structure, 5a takes a

dination of another metal ion. This possibility was exam- twisted figure-eight conformation in which there are two

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S. Shimizu, A. Osuka

MICROREVIEW

1

Figure 16. H NMR spectra of a) 28, b) 29, c) 30, and d) 31 in CDCl

3

. * indicates solvent and solvent impurities.

Figure 17. Redox behaviors of octaphyrin.

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Metalation Chemistry of meso-Aryl-substituted Expanded Porphyrins

MICROREVIEW

Figure 18. X-ray structures of a) 5a and b) 9; top views (top), and side views (bottom). meso-Aryl substituents have been omitted for

clarity in the side views.

porphyrin-like teterapyrrolic hemimacrocycles; it thus has Copper Metalation of [36]Octaphyrin and Perfluorinated

the potential to coordinate one metal ion in each cavity [36]Octaphyrin

[

22]

(

Figure 18, a). Upon reduction with NaBH , 5a is quan-

4

titatively converted into [38]octaphyrin (5c), while oxidation

A solution of 5a in toluene was stirred at 50 °C in the

of 5a with MnO gives [34]octaphyrin 5b. Unfortunately, presence of ten equivalents of Cu(OAc) and five equiva-

2

however, the structures of [34]- and [38]octaphyrin have not lents of NaOAc and the metalation process was monitored

been determined yet, mainly due to the their instability un- by UV/Vis absorption spectroscopy. Compound 5a exhibits

der ambient conditions. On the other hand, perfluorinated absorption bands at 410 and 643 nm, which change to ab-

[

36]octaphyrin (9) was obtained in 4–7% yield from the re- sorption bands at 412 and 668 nm after 2 h and then to

action of 3,4-difluoropyrrole and pentafluorobenzalde- absorption bands at 351 and 522 nm after 4 h (Figure 19).

[

23]

hyde. Interestingly, 9 takes a different conformation: it is These spectral changes were ascribed to sequential forma-

II

not twisted figure-eight but has a round ellipsoidal struc- tion of mono-Cu (33) and bis-Cu (34) complexes by com-

ture (Figure 18, b). Mainly due to their nonplanar struc- parison with the absorption spectra of authentic samples

[

42]

tures, these octaphyrins do not exhibit any aromatic or an- (Figure 20).

On further heating the small, shoulder-like

tiaromatic character. absorption at 408 nm in the spectrum increased and finally

2

Figure 19. UV/Vis spectral changes recorded during metalation of 5a with Cu(OAc) at 50 °C in toluene.

Eur. J. Inorg. Chem. 2006, 1319–1335

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1331

S. Shimizu, A. Osuka

MICROREVIEW

Figure 20. Copper complexes of octaphyrins and porphyrins.

the spectrum showed only the absorption at 408 nm with

small bands at 535 and 570 nm after heating for two weeks

(

Figure 19). This final spectrum indicates the formation of

II

the Cu porphyrin 35 (Figure 20). The mono-Cu complex

3

II

3 and the bis-Cu complex 34 were prepared in 77 and

81% yields, respectively, under temperature-controlled con-

ditions in a CH Cl solution with a small amount of

2

MeOH. In the solid state, 34 takes a similar twisted figure-

eight conformation with two copper ions bound inside the

porphyrin-like tetrapyrrolic cores. The coordination of the

II

Cu ions is severely distorted from square-planar coordina-

tion (Figure 21). Considering that neither free base 5a nor

33 show splitting behavior, this structural feature suggests

that the relief of strain in 34 may be the main driving force

for this thermal splitting reaction.^[

42,43]

We thus estimated

the activation parameters of the splitting reaction by ab-

sorption spectroscopic analysis. The splitting reaction of 34

‡

was found to be a unimolecular process with ∆H

=

–

1

‡

–1

101 kJmol

and ∆S = –23.5 JK mol . A possible

mechanism of the splitting reaction may be a 2π + 2π cyclo-

addition process to give a spirocyclobutane intermediate, as

shown in Scheme 5.

Figure 21. X-ray crystal structure of 34; top view (top) and side-

view (bottom). meso-Aryl substituents have been omitted for clarity

This thermal splitting reaction was also seen in the solid

state. In the DSC experiment of 34, a sharp exothermic re- in the side view.

Scheme 5. A possible mechanism of the splitting reaction of 34.

332

1

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Eur. J. Inorg. Chem. 2006, 1319–1335

Metalation Chemistry of meso-Aryl-substituted Expanded Porphyrins

MICROREVIEW

sponse was observed between 240 and 280 °C, and 35 was bis-Cu^I^Icomplex with a figure-eight structure similar to

obtained in high yield from this thermolysis experiment. that of 34. Thus, we tested the thermolysis reaction of 37

The present thermal splitting reaction may be attractive for both in CH Cl solution and in the solid state. Upon heat-

2

application as a recording device in light of the dramatic ing in solution it goes hydrolysis to give 36 with concomi-

spectral changes and the perfect material balance. Fortu- tant decomposition, whereas, interestingly, thermolysis in

nately, 34 was found to form a good transparent film when the solid state led to splitting into 38. During the copper

spin-coated onto quartz. A preliminary thermolysis experi- metalation of 9, one possible route to 36 may be initial for-

ment with this film indicated the more robust nature of 34 mation of 37 followed by subsequent hydrolytic ring open-

in the film up to 140 °C, but clean splitting into 35 at ing of one pyrrole ring facilitated by the strongly electron-

200 °C.

deficient nature of the perfluorinated macrocycle.

A similar copper metalation reaction of perfluorinated

II

[

36]octaphyrin 9 revealed the formation of two bis-Cu me-

Silver Metalation of [36]Octaphyrin and Perfluorinated

tal complexes 36 and 37 in 58 and 14% yields, respectively,

[36]Octaphyrin

when treated with Cu(OAc) and NaOAc in CH Cl solu-

2

[

44]

tion (Figure 20).

The crystal structure of 36 was deter-

Among the transition metal ions tested, we found the

mined by single-crystal X-ray analysis (Figure 22). It takes

a bent, U-shaped, ellipsoidal conformation in which one

pyrrole ring is hydrolytically cleaved to give a keto-imine

moiety. One of the two copper ions is bound to the keto-

carbonyl oxygen atom and the nitrogen atoms of the nearby

tripyrrolic subunit, while the other copper ion is bound to

the imine nitrogen atom and the nearby second tripyrrolic

subunit. The minor product 37 exhibits an absorption spec-

trum that is quite similar to that of 34, and the high-resolu-

tion ESI-TOF mass spectrum indicated the formation of

complexation of 5a and 9 with a salt of silver, a heavier

congener of copper, proceeded smoothly without any skel-

etal rearrangement of the macrocyclic ring, in sharp con-

trast to the metalations with copper, to provide similar bis-

silver complexes 39 and 40 almost quantitatively despite the

[44]

different structures of the free base (Figure 23).

The

I

bound silver ions are in the Ag oxidation state judging

from the coordination bond lengths and the diamagnetic

nature of the complex observed in the H NMR spectra,

1

which indicates that [36]octaphyrin is oxidized to [34]octa-

phyrin by an excess amount of silver salt. The oxidized form

II

the bis-Cu complex of perfluorinated [36]octaphyrin (37;

Figure 20). Although efforts to obtain suitable crystals of

I

acts as a divalent ligand to coordinate two Ag ions. Both

37 have been unsuccessful, these data suggest that 37 is a

structures exhibit almost C -symmetric figure-eight confor-

2

mations with two silver ions bound within porphyrin-like

tetrapyrrolic cavities with Ag–N bond lengths in the range

of 2.27–2.40 Å (Figure 24). Interestingly, pentafluorophenyl

substituents at the crossing meso position lie just above the

silver atoms in parallel with the tripyrrolic units, with in-

terplanar distances of approximately 3.2 Å, while the β-sub-

stituents (hydrogen and fluorine atoms) of pyrroles D and

H are placed below the silver ions with Ag–β-carbon dis-

tances of 2.89 and 2.87 Å for 39 and 3.14 and 3.16 Å for

40. This proximity between the Ag ions and the substituents

was supported by the additional through-space coupling

I

1

19

with Ag (I = 1/2) observed in the H and F NMR spec-

tra. Despite the figure-eight structures, which are similar to

that of 34, both 39 and 40 are thermally stable and do not

exhibit the splitting reactivity observed for 34. A major

reason for this may be the 34π-electron system of 39 and

40. Considering the instability of free base [34]octaphyrin

Figure 22. X-ray crystal structure of 36; top view (top) and side

view (bottom). meso-Aryl substituents have been omitted for clarity

in the side view.

I

Figure 23. Bis-Ag complexes of octaphyrins.

Eur. J. Inorg. Chem. 2006, 1319–1335

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1333

S. Shimizu, A. Osuka

MICROREVIEW

Figure 24. X-ray crystal structures of a) 39 and b) 40; top views (top) and side views (bottom). meso-Aryl substituents have been omitted

for clarity in the side views.

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[

39 and 40 upon heating, the Ag ions are easily removed

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Received: December 7, 2005

Published Online: February 9, 2006

kawa, H. Furuta, A. Osuka, Chem. Eur. J. 2005, 11, 2417–2425.

Eur. J. Inorg. Chem. 2006, 1319–1335

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1335

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Metalation chemistry of meso-aryl-substituted expanded porphyrins

DOI: 10.1002/ejic.200501097

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