Heteroatom-Containing Porphyrin Analogues - Chemical Reviews

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Heteroatom-Containing Porphyrin Analogues Tamal Chatterjee,† Vijayendra S. Shetti,‡ Ritambhara Sharma,† and Mangalampalli Ravikanth*,† †

Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India Department of Chemistry, BMS College of Engineering, Bull Temple Road, Bengaluru 560019, India

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‡

ABSTRACT: The heteroatom-containing porphyrin analogues or core-modified porphyrins that resulted from the replacement of one or two pyrrole rings with other five-membered heterocycles such as furan, thiophene, selenophene, tellurophene, indene, phosphole, and silole are highly promising macrocycles and exhibit quite different physicochemical properties compared to regular azaporphyrins. The properties of heteroporphyrins depend on the nature and number of different heterocycle(s) present in place of pyrrole ring(s). The heteroporphyrins provide unique and unprecedented coordination environments for metals. Unlike regular porphyrins, the monoheteroporphyrins are known to stabilize metals in unusual oxidation states such as Cu and Ni in +1 oxidation states. The diheteroporphyrins, which are neutral macrocycles without ionizable protons, also showed interesting coordination chemistry. Thus, significant progress has been made in last few decades on core-modified porphyrins in terms of their synthesis, their use in building multiporphyrin arrays for lightharvesting applications, their use as ligands to form interesting metal complexes, and also their use for several other studies. The synthetic methods available in the literature allow one to prepare mono- and diheteroporphyrins and their functionalized derivatives, which were used extensively to prepare several covalent and noncovalent heteroporphyrin-based multiporphyrin arrays. The methods are also developed to synthesize different hetero analogues of porphyrin derivatives such as heterocorroles, heterochlorins, heterocarbaporphyrinoids, heteroatom-substituted confused porphyrins, and so on. This Review summarizes the key developments that have occurred in heteroporphyrin chemistry over the last four decades.

CONTENTS 1. Introduction 2. Heteroporphyrins and Functionalized Heteroporphyrins 2.1. Monoheteroporphyrins 2.2. Diheteroporphyrins 2.3. General Properties of Heteroporphyrins 3. Covalently and Noncovalently Linked Heteroporphyrin-Based Multiporphyrin Arrays 3.1. Covalently Linked Heteroporphyrin-Based Arrays 3.1.1. Porphyrin Dyads 3.1.2. Porphyrin Triads 3.1.3. Porphyrin Tetrad 3.1.4. Porphyrin Pentads 3.1.5. Porphyrin Hexads 3.1.6. Porphyrin Octads 3.2. Noncovalently Linked Porphyrin Arrays 4. Metal Complexes of Heteroporphyrins 4.1. Metal Complexes of 21-Thiaporphyrin (N3S Core) 4.2. Metal Complexes of 21-Oxaporphyrin (N3O Core) 4.3. Metal Complexes of 21-Selenaporphyrin (N3Se Core) 4.4. Metal Complexes of 21,23-Dithiaporphyrin (N2S2 Core) 4.5. Metal Complexes of 21,23-Dioxaporphyrin (N2O2 Core) © 2016 American Chemical Society

4.6. Metal Complexes of 21,23-Diselenaporphyrin (N2Se2 Core) 4.7. Metal Complexes of Phosphoheteroporphyrin (N2PS Core) 5. Heterocorroles 5.1. Oxacorroles 5.2. Thiacorroles 6. Heterocarbaporphyrinoids 7. Heteroatom-Substituted Confused Porphyrins 7.1. N-Confused Heteroporphyrins 7.2. Heteroatom-Confused Heteroporphyrins 8. Heterochlorins 9. Heterotetrabenzoporphyrins 10. Heterocalixphyrins 11. Metallocene-Incorporated Heteroporphyrinoids 12. Conclusions Author Information Corresponding Author Notes Biographies Acknowledgments References

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Special Issue: Expanded, Contracted, and Isomeric Porphyrins

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Received: July 28, 2016 Published: November 4, 2016 3254

DOI: 10.1021/acs.chemrev.6b00496 Chem. Rev. 2017, 117, 3254−3328

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1. INTRODUCTION Porphyrins are tetrapyrrolic 18-π electron aromatic conjugated macrocycles present at the active sites of several biomolecules owing to their ability to bind metals in various oxidation states and their flexibility to adopt any desired conformation necessary to perform specific biological functions, and also for their attractive physicochemical properties.1−5 Besides their biological importance, the porphyrins have found applications in almost every research field ranging from materials to medicine5−8 as their electronic properties can be fine-tuned by suitable structural modifications. One of the attractive strategies is modification of porphyrin core by replacing one or two pyrrole N’s with other hetero atoms such as O, S, Se, Te, Si, P, and C; the resulting class of porphyrinoids are known as heteroatom-containing porphyrins or core-modified porphyrins9−13 and possess very interesting physicochemical properties that are quite different from regular N4-porphyrins (Chart 1). For example, core-modified porphyr-

porphyrins, which is very unique and interesting, has also been continuously exploited.9 Several covalently and noncovalently linked porphyrin arrays containing heteroporphyrins as one of the components were synthesized, and their energy/electrontransfer properties have been studied.14−16 Thus, core-modified porphyrins are now comparatively better studied porphyrin analogues and can be used as substitutes for regular porphyrins and their derivatives for various applications. There are few reviews on the chemistry of core-modified porphyrins in which Latos-Grażyński et al.,9,11,12 we,10,14,15 and others13 contributed by discussing the synthesis, metalation, and properties of heteroatom-substituted porphyrins. In this Review, we describe the developments that occurred in core-modified porphyrin chemistry over the years by covering various important aspects including syntheses, structure, coordination chemistry, and properties of core-modified porphyrins and their derivatives. The discussion is limited to core-modified porphyrins in which one or two pyrrole rings were substituted with other fivemembered heterocycles such as thiophene, furan, selenophene, tellurophene, indene, phosphole, silole, and also heteroatomsubstituted carbaporphyrinoids but does not include the heteroexpanded porphyrinoids containing more than four heterocycles in macrocyclic frame.28,29

Chart 1. Different Types of Heteroatom-Substituted Porphyrins

2. HETEROPORPHYRINS AND FUNCTIONALIZED HETEROPORPHYRINS 2.1. Monoheteroporphyrins

The monoheteroporphyrins that resulted from the replacement of one pyrrole with a range of other five-membered heterocycles can be synthesized by different synthetic strategies. The most common synthetic route is condensation of 1 equiv of appropriate 2,5-bis(arylhydroxymethyl)heterocyclopentadiene 1 (symmetrical dicarbinol) with 2 equiv of benzaldehyde and 3 equiv of pyrrole under mild acid-catalyzed porphyrin-forming conditions to afford monoheteroporphyrins30,31 (N3X type) (Scheme 1). This method is widely used to prepare monoheteroporphyrins such as monothia- (2), monooxa- (3), monoselena- (4), and monotellura- (5) porphyrins. The method also gives access to prepare the functionalized monoheteroporphyrins32 where one or more functional groups were introduced at the meso-aryl group(s). The monofunctionalized monoheteroporphyrins were synthesized by using the functionalized unsymmetrical dicarbinol32 as a key precursor, which was synthesized over two steps starting from heterocycle. In the first step, the heterocycle monocarbinol 6 was synthesized by treating heterocycle with aryl aldehyde under lithiated conditions, and in the second step, the heterocycle monocarbinol was treated with appropriate functionalized aryl aldehyde under the same lithiated conditions to obtain the functionalized unsymmetrical heterocycle dicarbinol 7 (Scheme 2). The appropriate functionalized unsymmetrical heterocycle dicarbinol 7 was condensed with aryl aldehyde and pyrrole under mild acid-catalyzed conditions followed by simple column chromatographic purification, which afforded monofunctionalized 21-heteroporphyrins32 8/9 in 10− 12% yields (Scheme 2). A series of monofunctionalized 21-thia(8) and 21-oxaporphyrins (9) containing one functional group such as −I, −Br, −OH, −CCTMS, or −NO2 at one of the mesoaryl groups or pyridyl group at the meso-position were synthesized by following a functionalized unsymmetrical heterocycle dicarbinol approach (Scheme 2). The unsymmetrical dicarbinol approach was also used to prepare mono-mesounsubstituted 21-thiaporphyrins 11 and 21-oxaporphyrins33 12

ins such as 21-thiaporphyrins have an ability to stabilize metals in oxidation states such as copper and nickel in +1 oxidation state, which is unusual for regular porphyrins.9 The core-modified porphyrins absorb and emit light at lower energy depending on number of heteroatoms substituted and thus can act as good energy acceptors upon covalent or noncovalent linking with regular porphyrins, which act as energy donors.14−16 Although the first heteroatom(s)-substituted porphyrins were synthesized in the 1970s,17−19 the chemistry of core-modified porphyrins evolved over the years and many new heteroatom-substituted porphyrinoids such as heterocorroles, heterochlorins, confused heteroporphyrins, and heterocarbaporphyrins were synthesized and their properties have been explored.10 Furthermore, the properties of core-modified porphyrins were also fine-tuned by introducing different substituents at the periphery, both at β- and meso-positions.20−27 The coordination chemistry of hetero3255


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Scheme 1. Synthesis of Symmetrical 21-Monoheteroatom-Substituted Porphyrins 2−5

Scheme 2. Synthesis of Monofunctionalized 21-Heteroporphyrins 8 and 9 by Unsymmetrical Dicarbinol Method ((a) K2CO3 in THF−CH3OH; (b) KOH, Benzene, Reflux; (c) SnCl2−HCl; (d) tBuONO, TMSN3)

succinimide (NBS) at room temperature to afford meso-bromo 21-heteroporphyrins34 13a/14a in ∼65% yield, which were further converted to meso-ethynyl heteroporphyrins in two steps. In the first step, meso-trimethylsilylethynyl 21-heteroporphyrins 13b/14b were prepared in 75−78% yields by treating meso-

by condensing appropriate unsymmetrical dicarbinol 10 with aldehyde and pyrrole under similar porphyrin-forming conditions as shown in Scheme 3. The free meso-position was functionalized with bromo group by treating mono-mesounsubstituted 21-thia-/21-oxaporphyrin 11/12 with N-bromo3256


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Scheme 3. Synthesis of meso-Unsubstituted 21-Heteroporphyrins 11−12 and Derivatives 13−14

Scheme 4. Synthesis of Monofunctionalized 21-Heteroporphyrins 8 and 9 by Monocarbinol Method ((a) K2CO3 in THF− CH3OH; (b) KOH, Benzene, Reflux)

deprotected with K2CO3 in THF−CH3OH (THF = tetrahydrofuran) to afford meso-ethynyl 21-heteroporphyrins 13c/14c in 78−80% yields.

bromo 21-heteroporphyrin 13a/14a with trimethylsilylacetylene (TMSA) under Pd(0)-catalyzed coupling reaction conditions. In the second step, the trimethylsilyl group of 13b/14b was 3257


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Scheme 5. Synthesis of Monofunctionalized 21-Heteroporphyrins Using Unsymmetrical Tripyrrane As Precursor

Scheme 6. Synthesis of 21-Monoheteroatom-Substituted Porphyrins 2−3

Scheme 7. Approach of Lee and Co-workers for the Synthesis of 21-Heteroporphyrins

mono-ols can be prepared in multigram quantities easily in one step by treating the appropriate heterocycle with functionalized aryl aldehyde under n-BuLi conditions. Lee and co-workers39 prepared 21-heteroporphyrins 2a/3a by condensing the symmetrical dicarbinol 1 with symmetrical azatripyrrane 16a in CH3CN under mild acidic conditions (Scheme 5). The researchers have used this approach to prepare monofunctionalized 21-thia-/21-oxaporphyrins 8a/9a by condensing the symmetrical thiophene/furan diol 1a/1b with functionalized unsymmetrical azatripyrrane 16a under Lindsey’s38 conditions. The condensation resulted in the formation of a mixture of porphyrins that were separated by column chromatography.

Instead of 2,5-disubstituted heterocycle dicarbinol, the N3X porphyrins 8/9 were also synthesized via “monocarbinol” method35,36 by condensing 2 equiv of 2-(arylhydroxymethyl)heterocyclopentadiene 15 with 2 equiv of aryl aldehyde and 3 equiv of pyrrole under the conditions of Adler et al.37 or Lindsey et al.38 (Scheme 4). Although the condensation resulted in the formation of regular N4 porphyrin as side product, the N4 porphyrin was separated easily from 21-heteroporphyrin 8/9 by simple column chromatography. The monocarbinol method36 is used effectively to prepare the monofunctionalized 21-thia-/21-oxaporphyrins 8/9 with a wide range of functional groups because the desired functionalized thiophene/furan 3258


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Scheme 8. Synthesis of Symmetrical cis-Difunctionalized 21-Heteroporphyrins 19 and 20 ((a) K2CO3 in THF−CH3OH)

ins 4 3 − 4 5 19/20 by condensing the 2,5-bis(α-aryl-αhydroxymethyl)thiophene 1a or furan 1b with 2 equiv of functionalized aryl aldehyde and 3 equiv of pyrrole under standard acid-catalyzed porphyrin-forming conditions (Scheme 8). This condensation resulted in the formation of a mixture of three porphyrins with three different modified porphyrin cores: the desired difunctionalized cis-21-thia- or 21-oxaporphyrin (N3X porphyrin) along with 21,23-diheteroporphyrin (N2X2 porphyrin) and N4 porphyrin, which were separated by column chromatography. The method was used extensively to prepare difunctionalized cis-21-thia-/21-oxaporphyrins43−45 19/20 containing functional groups such as iodophenyl, ethynylphenyl, 3pyridyl, and 4-pyridyl groups at meso-positions (Scheme 8). These cis-difunctionalized 21-heteroporphyrin building blocks were used extensively to synthesize complex covalent and noncovalent heteroporphyrin-based systems43−45 (vide infra). The cis-difunctionalized 21-thiaporphyrins containing two different types of functional groups46 at meso-positions 22 were synthesized in a sequence of steps as shown in Scheme 9. In the first step, the thiophene was treated with one functionalized aldehyde under n-BuLi conditions to obtain thiophene monocarbinol 15f, which was treated in the second step with a second functionalized aldehyde under the same n-BuLi conditions to afford functionalized unsymmetrical thiophene dicarbinol 21 with two different functional groups. The

Chandrashekar and co-workers40,41 prepared 21-thia/21-oxaporphyrins 2c/3c by condensing meso-mesityl dipyrromethane 17a with appropriate symmetrical thiophene/furan diol 1a/1b under mild acid-catalyzed conditions (Scheme 6). Lee and coworkers42 synthesized regular and monofunctionalized 21-thia-/ 21-oxaporphyrins by condensing thienylpyrromethane diol/ furylpyrromethane diol 18a/18b with meso-aryl functionalized dipyrromethane 17b/17c under mild acid-catalyzed conditions (Scheme 7). By following this synthetic route, 21-thia- and 21oxaporphyrins containing a trimethylsilylethynylphenyl functional group at the meso-position adjacent to the pyrrole ring 8n/ 9d were synthesized. However, this method involves more number of steps and tedious column chromatographic purifications to prepare a wide variety of monofunctionalized 21-thia-/21-oxaporphyrins. Thus, sufficient synthetic methods are now available to prepare the desired monofunctionalized 21heteroporphyrins that can be used to synthesize a wide range of 21-heteroporphyrin-based fluorescent systems and multiporphyrin arrays. The 21-heteroporphyrins containing two different types of meso-aryl groups were synthesized by condensing symmetrical heterocycle diol 1a/1b containing one type of meso-aryl group with a different type of aryl aldehyde and pyrrole under porphyrin-forming conditions. The method was adopted to prepare a series cis-difunctionalized 21-heteroporphyr3259


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yielded trifunctionalized 21-heteroporphyrins 25/26 containing three of the same functional groups, such as iodophenyl, ethynylphenyl, and pyridyl groups at meso-positions. The tetrafunctionalized 21-thiaporphyrin47 28a containing four of the same functional groups were synthesized by two different routes (Scheme 12(i)). The symmetrical tetrafunctionalized 21thiaporphyrins47 were prepared by condensing the functionalized monocarbinol with the substituted benzaldehyde containing the same functional group and pyrrole or by condensing the functionalized thiophene dicarbinol 27 with pyrrole under mild acid-catalyzed conditions. The condensations resulted in the formation of a mixture of porphyrins that were separated by column chromatography. The unsymmetrical tetrafunctionalized 21-thiaporphyrin 29 containing two different types of functional groups47 was also synthesized by condensing functionalized thiophene monocarbinol 15a with different functional-groupsubstituted benzaldehyde and pyrrole under Adler’s37 propionic acid conditions (Scheme 12(ii)).

Scheme 9. Synthesis of Unsymmetrical cis-Difunctionalized 21-Heteroporphyrins 22

2.2. Diheteroporphyrins

The diheteroporphyrins that resulted from the replacement of two pyrrole “N”s with two of the same (N2X2 type) or two (N2XY type) different heteroatoms possess different properties compared to monoheteroporphyrins. The diheteroporphyrins are of two types based on heteroatom(s) location inside the porphyrin core: 21,23-diheteroporphyrins and 21,22-diheteroporphyrins (Chart 1). The 21,23-diheteroporphyrins of N2X2 type 30−33 containing two of the same type of heteroatoms48−52 were prepared by condensing appropriate 2,5-bis(aryl-αhydroxymethyl)heterocycle 1 with pyrrole under mild acidcatalyzed porphyrin-forming conditions (Scheme 13(i)). This methodology was used to prepare several meso-tetraaryl 21,23diheteroporphyrins of N2X2 type such as N2S2 (30), N2O2 (31), N2Se2 (32), and N2Te2 (33) porphyrins. The mono-mesounsubstituted N2X2 porphyrins33 (X = S and O) 34/35 were prepared by condensing appropriate unsymmetrical diol 10a/ 10b with symmetric tripyrrane 16b/16c under mild acidcatalyzed conditions (Scheme 13(ii)). The mixed 21,23diheteroporphyrins53 of N2XY type 36−38 were prepared by condensing 1 equiv of the corresponding heterocyclopentadiene diol 1a/1b with modified appropriate tripyrrane 16c/16d/16e under two-step, one-flask, room-temperature conditions (Scheme 13(iii)). Similarly, the mono-meso-unsubstituted mixed 21,23-diheteroporphyrins33 39/40 were synthesized by acid-catalyzed condensation of the appropriate unsymmetric diol 10a/10b with symmetric tripyrrane 16b/16c (Scheme 13(iv)). Alternately, the mono-meso-unsubstituted 21-thia-23-oxaporphyrin 39 and 21-oxa-23-thiaporphyrin 40 were prepared by

unsymmetrical dicarbinol 21 was then condensed with aryl aldehyde and pyrrole under porphyrin-forming conditions to afford unsymmetrical cis-difunctionalized 21-thiaporphyrins46 22 containing two different functional groups at meso-positions. The unsymmetrical cis-difunctionalized 21-thiaporphyrins containing two different functional groups 22 are very useful building blocks to prepare very complex fluorescent triads with interesting physicochemical properties. The trans-difunctionalized 21-heteroporphyrin building blocks containing two different types of functional groups were very elegantly synthesized by Lee, Lindsey, and co-workers42 over a sequence of steps as shown in Scheme 10. Condensation of the functionalized thienylpyrromethane dicarbinol 18c or furylpyrromethane dicarbinol 18d with meso-aryl-functionalized dipyrromethane 17c under acid-catalyzed conditions followed by column chromatographic purification yielded difunctionalized 21-heteroporphyrins 23/24 having iodophenyl and trimethylsilylethynylphenyl groups, respectively, at the meso-positions in trans fashion.42 These building blocks 23/24 were used to synthesize linear covalently/noncovalently linked triads and tetrads (vide infra). The trifunctionalized 21-heteroporphyrins 25/26 were prepared by mono-ol method36 (Scheme 11). The thiophene/furan mono-ol 6a/6b was condensed with functionalized aryl aldehyde and pyrrole under porphyrin-forming conditions followed by column chromatographic purification

Scheme 10. Synthesis of trans-Difunctionalized 21-Heteroporphyrins 23 and 24

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Scheme 11. Synthesis of Trifunctionalized 21-Heteroporphyrins 25 and 26

Scheme 12. Synthesis of Tetrafunctionalized 21-Heteroporphyrins 28 and 29 ((a) K2CO3 in THF−CH3OH)

condensing the corresponding unsymmetrical tripyrrane 16f/ 16g with an appropriate symmetrical diol 1a/1b (Scheme 13(v)). Interestingly, there are very few reports on 21,22-diheteroporphyrins52,54 because these porphyrins are not very stable and decompose rapidly. Lee and Cho54 developed a novel synthetic strategy to prepare 21,22-diheteroporphyrins containing sulfur and oxygen atoms. The acid-catalyzed condensation of

1 equiv of appropriate 1,9-bis(phenylhydroxymethyl)-5-phenyldifuryl or dithienyl or thienylfuryl diol 18c/18d/18e with 1 equiv of 5-phenylpyrromethane 17b followed by oxidation with 2,3dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and column chromatographic purification resulted in the formation of 21,22-diheteroporphyrins 41−43 (Scheme 13(vi)). The chemistry of 21,22-diheteroporphyrins have not been explored to a greater extent because of their inherent unstable nature. 3261


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Scheme 13. Synthesis of Diheteroatom-Substituted Porphyrins 30−43

metrical thiophene diol 7 as key precursor.32 Condensation of the functionalized unsymmetrical thiophene diol 7 with 16-

The monofunctionalized 21,23-N2X2 44 and N2XY 45 porphyrins were prepared by using the functionalized unsym3262


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Scheme 14. Synthesis of Monofunctionalized Diheteroporphyrins ((a) K2CO3 in THF−CH3OH; (b) SnCl2−HCl; (c) tBuONO, TMSN3)

thiatripyrrane 16b under mild acid-catalyzed conditions gave the monofunctionalized 21,23-dithiaporphyrins 44, whereas the condensation of unsymmetrical thiophene diol 7 with 16oxatripyrrane 16c under the same reaction conditions gave the

monofunctionalized 21-thia-23-oxaporphyrins 45 in 10−12% yields (Scheme 14(i)). Another interesting 21,23-dithiaporphyrin building block containing an alkyne functional group 48 at the end of a long alkoxy chain attached to the thiophene ring of the 3263


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Scheme 15. Synthesis of Symmetrical and Unsymmetrical cis-Difunctionalized 21,23-Dithiaporphyrin Building Blocks 49 and 50 ((a) KOH, Benzene, Reflux)

porphyrin55 is shown in Scheme 14(ii). The desired thiophene dicarbinol 7 was prepared over a sequence of steps and condensed with 16-thiatripyrrane 16b in the presence of a catalytic amount of BF3·OEt2 in CH2Cl2 followed by oxidation with DDQ to afford 21,23-dithiaporphyrin 44 in 8% yield. The 21,23-dithiaporphyrin building block55 containing an alkyne functional group 48 was prepared in 52% yield by reacting 47 with propargyl tosylate in the presence of NaH−DABCO (DABCO = 1,4-diazabicyclo[2.2.2]octane) in THF at room temperature. The difunctionalized 21,23-N2X2 porphyrins 49 (X = S) containing two of the same type of functional groups such as 4pyridyl group at meso-positions in cis-fashion45 were synthesized by condensing the functionalized thiophene dicarbinol 27b with symmetrical tripyrrane 16b under standard acid-catalyzed conditions (Scheme 15(i)). The difunctionalized unsymmetrical cis-21,23-dithiaporphyrins containing two different types of functional groups46 50 were synthesized by reacting the unsymmetrical difunctionalized thiophene dicarbinol 21 with symmetrical 16-thiatripyrrane 16b under standard reaction conditions (Scheme 15(ii)). Although, in all these condensations, the other porphyrins were also formed along with desired unsymmetrical difunctionalized 21,23-dithiaporphyrin, but these were easily removed by column chromatographic purification

and afforded cis-difunctionalized 21,23-dithiaporphyrin building blocks 50 in decent yields. The symmetrical tetrafunctionalized 21,23-dithiaporphyrins56,57 51 were synthesized by condensing the functionalized thiaphene dicarbinol 27 with pyrrole under mild acid-catalyzed porphyrin-forming conditions (Scheme 16(i)). The 21,23dithiaporphyrin with four ethynyl groups at the para-position of meso-phenyls 51d was prepared by deprotecting the trimethylsilyl group of 21,23-dithiaporphyrin 51c using K2CO3 in THF/CH3OH at room temperature. The unsymmetrical tetrafunctionalized 21,23-dithiaporphyrin 52 containing two different types of functional groups,47 such as three mesoiodophenyl groups and one protected ethynylphenyl group, was prepared by condensing 1 equiv of functionalized unsymmetrical dicarbinol 21f with 1 equiv of difunctionalized symmetrical 16thiatripyrrane 16b under acid-catalyzed conditions (Scheme 16(ii)). Matano and co-workers58−60 reported the first examples of very interesting phosphorus-containing heteroporphyrins. To incorporate a phosphole ring in porphyrin, the key precursor phosphotripyrrane 55 was prepared from 2,5-difunctionalized phosphole 53, which in turn was readily accessible from commercially available reagents. The reduction of 53 with diisobutylaluminum hydride (DIBAL-H) produced the corre3264


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Scheme 16. Synthesis of Symmetrical and Unsymmetrical Tetrafunctionalized 21,23-Dithiaporphyrin Building Blocks 51 and 52 ((a) K2CO3 in THF−CH3OH)

porphyrin macrocycle. The spectroscopic properties support their aromaticity despite core modification. The alteration in electronic properties of heteroporphyrins clearly reflects in their spectral and electrochemical properties as outlined here. (1) The alteration in π-delocalization pathway due to the presence of heteroatoms results in considerable downfield shifts in 1H NMR resonances of pyrrole and heterocyclopentadiene moieties. The diheteroporphyrins exhibit less downfield-shifted resonances compared to monoheteroporphyrins because the diheteroporphyrins are relatively more distorted due to the presence of two large heteroatoms, and the distortion decreases the ring-current effect arising from the π-delocalization. (2) The heteroporphyrins exhibit large red-shifted Soret and Q-bands in their absorption spectra, and the magnitude of the red-shift depends on the size and number of heteroatoms present in the core.61 The red-shifts in absorption bands exhibited by diheteroatomsubstituted porphyrins are greater than those of the monoheteroatom-substituted porphyrins. (3) The emission bands of the heteroporphyrins are also red-shifted with the reduction in quantum yields and singlet-state lifetimes except for oxaporphyrins, whose singlet-state lifetimes are almost comparable to those of N4 porphyrins.61 The red-shifted absorption and emission bands thus reflect on lower HOMO−LUMO (HOMO = highest occupied molecular orbital; LUMO = lowest unoccupied

sponding diol 54, which, on further reaction with pyrrole in the presence of BF3·OEt2, afforded phosphotripyrrane 55. The phosphotripyrrane 55 on condensation with 2,5-bis[hydroxy(phenyl)methyl]pyrrole 56 in the presence of BF3·OEt2 gave σ4phosphaporphyrinogen 57a as a mixture of diastereomers. Desulfurization of 57a with excess P(NMe2)3 in refluxing toluene afforded σ3-phosphaporphyrinogen 57b, which was then treated with DDQ at room temperature followed by column chromatography and afforded PN3 18π−σ3-phosphaporphyrin 58 (Scheme 17(i)). The σ3-P,N2,S-hybrid porphyrin 60 was prepared similarly as shown in Scheme 17(ii). The BF3promoted dehydrative condensation of phosphotripyrrane 55 with thiophene dicarbinol 1a in CH2Cl2 gave σ4-P,N2,Sporphyrinogen 59a as a mixture of diastereomers. Desulfurization of 59a in refluxing toluene afforded σ3-P,N2,S-porphyrinogen 59b, which was subsequently oxidized by DDQ to afford σ3-P,N2,S-hybrid porphyrin58−60 60. Both these phospholecontaining porphyrins 58 and 60 exhibit aromaticity as 18πelectron systems in terms of both geometric and magnetic criteria. 2.3. General Properties of Heteroporphyrins

The introduction of one or two heteroatoms in place of pyrrolic nitrogen(s) significantly alters the electron delocalization pathway, thereby affecting the electronic properties of the 3265


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Scheme 17. Synthesis of (i) Monophosphaporphyrin 58 and (ii) σ3-P,N2,S-Hybrid Porphyrin 60

Chart 2. Singlet-State Energy Levels of meso-Tetraarylporphyrins with Various Cores

donor or energy acceptor depending on the other porphyrin(s) to which it is connected covalently or noncovalently. Because the singlet-state energy levels of heteroporphyrins and N4 porphyrin are arranged in a cascade manner,62 several unsymmetrical covalent and noncovalent multiporphyrin arrays containing two or more different types of porphyrin units were synthesized, and their photoinduced energy-transfer properties were studied as described in the next section.

molecular orbital) energy gap of heteroporphyrins compared to N4 porphyrins. (4) The heteroporphyrins are easier to reduce but difficult to oxidize compared to N4 porphyrins, which is attributed to the inductive or electron-withdrawing effect that the heteroatom has on the frontier orbitals of the porphyrin. (5) The heteroporphyrins have a strong ability to stabilize metals in unusual oxidation states that are not possible with N4 porphyrins, and (6) the crystal structures of heteroporphyrins and metalloheteroporphyrins have also revealed that the presence of large heteroatoms shrinks the core size and also distorts the macrocycle. Furthermore, the singlet-state energy levels of heteroporphyrins can be fine-tuned by varying the number of heteroatoms inside the porphyrin core,62 as shown in Chart 2. Accordingly, the heteroporphyrin can be used as either energy

3. COVALENTLY AND NONCOVALENTLY LINKED HETEROPORPHYRIN-BASED MULTIPORPHYRIN ARRAYS Because a wide variety of mono- to tetrafunctionalized heteroporphyrins are readily accessible, the heteroporphyrins 3266


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Scheme 18. Synthesis of Covalently Linked Heteroporphyrin Dyad 62 and Its Zn(II) Derivative Zn62

alkoxy units to rigid ethyne units, and in some porphyrin arrays, the porphyrin units were directly linked with each other through meso-carbons. 3.1.1. Porphyrin Dyads. The first examples of unsymmetrical porphyrin dyads63 where two different porphyrins were linked by flexible alkoxy units were synthesized by using difunctionalized thiaporphyrins under very mild, controlled reaction conditions because there were no methods known in the literature to synthesize monofunctionalized heteroporphyrin building blocks during that point in time. A cis-difunctionalized thiaporphyrin building block, 5,20-bis(p-tolyl)-10,15-(p-hydroxyphenyl)-21-thiaporphyrin 19i, was reacted with 5-[4-(5-bromo1-pentoxy)phenyl]10,15,20-tri(p-tolyl)porphyrin 61 in dimethylformamide (DMF) in the presence of K2CO3 for 2 weeks at

were used as building blocks to prepare a series of covalently/ noncovalently linked symmetrical and unsymmetrical multiporphyrin arrays and their ground- and excited-state dynamics were studied. 3.1. Covalently Linked Heteroporphyrin-Based Arrays

Several covalently linked symmetrical and unsymmetrical multiporphyrin arrays including porphyrin dyads, triads, tetrads, pentads, hexads, and octads were readily synthesized using appropriate heteroporphyrin building blocks under mild reaction conditions.14,15 The aim was to synthesize unsymmetrical porphyrin arrays containing two or more different porphyrin units, so that the energy gradient is created to transfer energy at the singlet state from one porphyrin unit to another. The linkers between the two porphyrin units were also varied from flexible 3267


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Scheme 19. Synthesis of Diphenylethyne-Bridged Covalently Linked Heterodyads 64

emit light at different regions, a sufficient energy gradient exists between the two porphyrin subunits. Thus, in these dyads, one of the porphyrin units acts as an energy donor and the other porphyrin unit acts as an energy acceptor, facilitating the possibility of singlet-state energy transfer from donor porphyrin unit to acceptor porphyrin unit at singlet state on selective excitation of donor porphyrin unit. This was best illustrated by taking the example of dyad 64d containing Zn(II) porphyrin unit and N2S2 porphyrin unit; on selective excitation at 550 nm, where the Zn(II) porphyrin unit absorbs strongly, the emission of Zn(II) porphyrin unit at 600 and 650 nm was significantly quenched (93%) and strong emission was noted at 700 nm corresponding to the 21,23-dithiaporphyrin unit supporting a possibility of energy transfer at singlet state from the Zn(II) porphyrin unit to the 21,23-dithiaporphyrin unit in dyad 64d. The time-resolved fluorescence studies indicated that the rate of excitation energy transfer, KENT, was 152 ps and the yield of energy transfer, ΦENT, was 93% for dyad 64d. However, the rate of energy transfer was noted to be much slower in dyad 64d compared to the diphenylethyne-bridged dyad containing ZnN4 and N4 porphyrin subunits66 (24 ps). This difference was attributed to the presence of two sulfur atoms in one of the porphyrin units of dyad 64d, which enhances the contribution of nonradiative decay channels. Similarly, the other dyads 64a−g also showed singlet−singlet energy transfer from donor porphyrin unit to acceptor porphyrin unit as confirmed by preliminary photophysical studies. The diphenylethyne-bridged porphyrin dyads 67a−f containing meso-tolyl and meso-furyl porphyrin subunits67 were synthesized by coupling the appropriate meso-tolyl phenylethynyl porphyrin building block with N3S (8d), N2S2 (44d), and N4 (65) cores with meso-furyl iodophenyl porphyrin building block with N3S (8l) and N2S2 (66) cores under mild Pd(0)catalyzed conditions followed by column chromatographic

room temperature followed by column chromatographic purification and afforded alkoxy-bridged unsymmetrical N4− N3S porphyrin dyad 62 in 27% yield (Scheme 18). The Zn(II) was inserted into N4 porphyrin of dyad 62 under standard metalation conditions to afford dyad Zn62. The spectral and electrochemical studies indicated that porphyrin/Zn(II)porphyrin and 21-thiaporphyrin units in dyads 62/Zn62 interact weakly and retain their individual characteristic features. The preliminary photophysical studies64 indicated the efficient energy transfer from donor porphyrin/Zn(II) porphyrin unit to acceptor 21-thiaporphyrin unit in dyads 62/Zn62. After accessibility of monofunctionalized heteroporphyrin building blocks, a series of unsymmetrical porphyrin dyads containing two different porphyrin cores were synthesized. The diphenyl ethyne-bridged unsymmetrical porphyrin dyads containing a heteroporphyrin unit as one of the subunits linked to the porphyrin/metalloporphyrin/heteroporphyrin were synthesized by coupling the appropriate porphyrin building block having an iodophenyl group at the meso-position with the porphyrin building block having a phenylethne group at the meso-position under mild copper-free Pd(0)-coupling conditions32,36 (Scheme 19). To illustrate, the diphenylethyne-bridged dyad36 64d containing ZnN4 porphyrin and N2S2 porphyrin units was synthesized by coupling of 5,10,15-tri(p-tolyl)-20-(4iodophenyl)porphyrinato zinc(II)65 63a with 5-(4-ethynylphenyl)-10,15,20-tri(p-tolyl)-21,23-dithiaporphyrin32 44d in the presence of Pd2(dba)3/AsPh3 at 35 °C in toluene/triethylamine for 4 h followed by column chromatographic purification. Similar reaction conditions were employed to synthesize the other diphenylethyne-bridged unsymmetrical porphyrin dyads32,36 64a−g. In all these dyads 64a−g, the spectral studies indicated that the two constituted porphyrin units interact weakly with each other and retain their individual characteristic features. Furthermore, because both the porphyrin units in dyads absorb/ 3268


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Scheme 20. Synthesis of Diphenylethyne-Bridged Covalently Linked Heterodyads 67 Containing meso-Tolylporphyrin and mesoFurylporphyrin Subunits

Scheme 21. (i) Synthesis of Rigid Diphenyl Triazole-Bridged Porphyrin Dyads 68; (ii) Synthesis of Flexible Triazole-Bridged Porphyrin Dyads 70

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purification (Scheme 20). In these dyads 67a−f, the singlet-state energy of porphyrin having six-membered meso-aryl groups was high and acts as an energy donor, whereas the singlet-state energy of porphyrin having five-membered meso-furyl groups was low and acts as an energy acceptor. Thus, the photophysical studies68 showed a clear possibility of energy transfer from meso-tolyl porphyrin subunit to meso-furyl porphyrin subunit in all symmetrical and unsymmetrical porphyrin dyads 67a−f. The “click reaction”69,70 conditions were employed to synthesize the triazole-bridged unsymmetrical porphyrin dyads. The rigid diphenyltriazole bridged unsymmetrical dyads 68a−c were synthesized in 46−50% yields by coupling appropriate azido 21-thia-8h/21,23-dithiaporphyrin building block 44i with appropriate ethynyl 21-thiaporphyrin 8d or 5-(4-ethynylphenyl)-10,15,20-tri(p-tolyl) porphyrin 65 and its Zn(II) derivative Zn65, respectively, in the presence of CuI−DIPEA (DIPEA = N,N-diisopropylethylamine) in THF−acetonitrile solvent mixture (Scheme 21(i)).71 The flexible triazole-bridged β-mesolinked dyads55 70 and Zn70 were synthesized in 46−48% yields by treating the 21,23-dithiaporphyrin building block 48 with 5(4-azidophenyl)-10,15,20-tri(p-tolyl)porphyrin72 69 or its Zn(II) derivative Zn69, respectively, in the presence of CuSO4− sodium ascorbate in water−acetone mixture (Scheme 21(ii)). In all these triazole-bridged dyads 68a−c/70/Zn70, the steadystate fluorescence studies indicated a possibility of energy transfer from high-energy-absorbing donor porphyrin unit to low-energy-absorbing acceptor porphyrin unit. The steady-state fluorescence spectra recorded for diphenylethyne-bridged dyad32 64e and triazole-bridged dyad55 68c containing donor N4 porphyrin unit and acceptor N2S2 porphyrin units at 420 nm, where donor N4 porphyrin unit absorbs more strongly, showed that, in diphenylethyne-bridged dyad 64e, the emission was noted from both porphyrin and 21,23-dithiaporphyrin units, whereas in triazole-bridged dyad 68c the major emission was noted from the 21,23-dithiaporphyrin unit. These observations indicated that the triazole was a better linker for efficient energy transfer from porphyrin to 21,23-dithiaporphyrin unit compared to diphenyl ethyne linker in covalently linked N4−N 2S2 porphyrin dyads.55 However, the detailed photophysical studies have not been carried out to estimate the energy-transfer efficiencies and rates. The two different porphyrin units in dyads73 were also connected by phenyl ethyne bridges, which help in strong interaction between the two porphyrin units unlike diphenylethyne-bridged porphyrin dyads, where the interaction between the two porphyrin units is minimal. Especially in phenylethynebridged porphyrin dyads Zn71a/71a−c, the electronic properties of the porphyrin unit having an ethyne group at the mesoposition were significantly altered, whereas the properties did not change much for the porphyrin having a meso-phenyl group. The phenylethyne-bridged porphyrin dyads73 Zn71a/71a−c were synthesized by coupling of meso-bromo 21-thiaporphyrin34 13a with appropriate meso-ethynylporphyrins Zn65/65/44d/9c in the presence of a catalytic amount of Pd2(dba)3/AsPh3 at 35 °C for 15 h followed by column chromatographic purification (Scheme 22). The photophysical studies supported the energy transfer at singlet state from one porphyrin unit to another in phenylethyne-bridged dyads Zn71a/71a−c. Furthermore, the rate of energy transfer in phenylethyne-bridged dyad such as ZnN4−N3S porphyrin dyad Zn71a was much faster (26 ps−1) than that in diphenylethyne-bridged ZnN4−N3S porphyrin dyad 64a (59 ps−1), which was attributed to the short distance between energy donor and energy acceptor in phenylethyne-

Scheme 22. Synthesis of Phenylethyne-Bridged Dyads 71

bridged dyad Zn71a compared to diphenylethyne-bridged dyad73 64a. Two phenyl-bridged unsymmetrical porphyrin dyads,74 such as phenyl-bridged meso−meso-linked N3S−N2S2 porphyrin dyad 73 and phenyl-bridged β-meso-linked ZnN 4 −N 2 S 2 porphyrin dyad 74, were prepared using (5,5-dimethyl-1,3dioxaborinan-2-yl)-5,10,15-tri(p-tolyl)-21,23-dithiaporphyrin 44e as key precursor. The coupling of 44e with meso-bromo 21thiaporphyrin34 13a in toluene/triethylamine in the presence of catalytic amounts of Pd(PPh3)4/CsCO3 at 80 °C for 4 h afforded phenyl-bridged meso−meso-linked N3S−N2S2 porphyrin dyad 73, whereas coupling with [2-bromo-5,10,15,20-tetra(p-tolyl)porphyrinato]Zn(II)75 72 under the same conditions afforded phenyl-bridged β-meso-linked ZnN4−N2S2 porphyrin dyad 74 (Scheme 23). The excitonic interactions between the porphyrin units in dyads 72/74 were evident in the splitting of the Soret band, and the dyads74 also showed an efficient singlet−singlet energy transfer as confirmed by photophysical studies. A series of ethyne-bridged unsymmetrical porphyrin dyads76 76−77 containing two different porphyrin units were synthesized as shown in Scheme 24. The β-meso-ethyne-bridged unsymmetrical ZnN4−N2XY porphyrin dyads76 76a−d were synthesized by coupling of β-ethynyl ZnN4 porphyrin 75 with appropriate meso-bromo heteroporphyrin building block34 13a/ 14a/34a/40a in the presence of Pd2(dba)3/AsPh3 in toluene/ triethylamine at 35 °C for 3 h. The ethyne-bridged unsymmetrical free base N4−N2XY porphyrin dyads 77a−c were synthesized by demetalation of appropriate ethyne-bridged ZnN4−N2XY porphyrin dyads 76a−d with trifluoroacetic acid (TFA) in CHCl3 followed by simple column chromatographic 3270


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Scheme 23. Synthesis of meso−meso- and β-meso-Linked Phenyl-Bridged Heteroporphyrin Dyads 73 and 74

Scheme 24. Synthesis of β-meso-Ethynyl-Bridged Heteroporphyrin Dyads 76 and 77

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purification. The fluorescence studies indicated a possibility of energy transfer at the singlet state from donor porphyrin unit to acceptor porphyrin unit in all ethyne-bridged unsymmetrical porphyrin dyads. The direct meso−meso-linked ZnN4−N3S porphyrin dyad34 79 was synthesized by coupling of (4,4,5,5′tetramethyl-1,3-dioxaborolan-2-yl)-10,20-bis(p-tolyl)porphyrinatozinc(II)77 78 with meso-bromo 21-thiaporphyrin34 13a in toluene/dimethyl dimethyl sulfoxide (DMSO) at 85 °C in the presence of PPh3, Cs2CO3, and Pd2(dba)3 for 16 h (Scheme 25). As can be seen from the steady-state fluorescence spectra recorded at 550 nm where the Zn(II) porphyrin unit absorbs strongly, the emission of Zn(II) porphyrin was quenched significantly and a strong emission was noted from the N3S porphyrin unit, indicating the possibility of energy transfer at

singlet state from ZnN4 porphyrin unit to N3S porphyrin unit in direct meso−meso-linked porphyrin dyad34 79. 3.1.2. Porphyrin Triads. The difunctionalized cis- and transheteroporphyrin building blocks containing two different types of functional groups and monofunctionalized porphyrin building blocks were used to synthesize covalently linked diphenylethynebridged unsymmetrical triads containing three types of porphyrin subunits. The L-shaped covalently linked diphenylethyne-bridged triad46,78 80 containing ZnN4, N3S, and N2S2 porphyrin subunits was synthesized over a sequence of coupling reaction steps as shown in Scheme 26. In the first step, the ethynylphenyl ZnN4 porphyrin building block Zn65 was coupled with cis-difunctionalized N3S porphyrin building block46 22b containing iodo and protected ethynylphenyl functional groups on meso-phenyl rings in the presence of Pd2(dba)3/AsPh3 at 35

Scheme 25. Synthesis of meso−meso-Linked Heteroporphyrin Dyad 79

Scheme 26. Synthesis of L-Shaped Porphyrin Triad 80

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Scheme 27. Synthesis of Covalently Linked Linear Porphyrin Triad 82

Scheme 28. Synthesis of Covalently Linked meso−mesoLinked Porphyrin Triad 85

°C for 4 h; column chromatographic purification afforded ZnN4−N3S dyad 81a containing the protected ethynylphenyl group at the meso-position of N3S porphyrin. In the second step, the deprotection of the ethyne group of the N3S porphyrin unit of dyad 81a was carried out by reacting dyad 81a with KOH in C6H6/CH3OH at 80 °C to afford ZnN4−N3S porphyrin dyad 81b containing a free phenylethynyl functional group at the meso-position of the N3S porphyrin unit. Dyad 81b, on subsequent Pd(0)-catalyzed coupling with iodophenyl 21,23dithiaporphyrin building block 44a, afforded L-shaped covalently linked unsymmetrical ZnN4−N3S−N2S2 porphyrin triad 80. The absorption studies indicated that the three porphyrin subunits retain their features in triad 80. The steady-state fluorescence spectrum recorded for triad 80 at 550 nm, where the ZnN4 porphyrin unit absorbs strongly, showed emission at 705 nm corresponding to N2S2 porphyrin unit, suggesting that the energy is transferred from the donor Zn(II) porphyrin unit to the acceptor N2S2 porphyrin unit via N3S porphyrin unit in triad 80. However, the detailed studies were not done to estimate the energy-transfer dynamics in triad 80.

The covalently linked diphenylethyne-bridged linear ZnN4− N3S−N2S2 porphyrin triad79 82 containing three different porphyrin subunits was synthesized similarly, as shown in Scheme 27. The ZnN4−N4 porphyrin dyad 84a containing a protected ethynylphenyl group at the meso-position of the N4 porphyrin unit was prepared under Pd(0)-coupling reaction conditions by coupling of ethynylphenyl ZnN4 porphyrin building block Zn65 with trans-N4 porphyrin building block80 83 containing iodophenyl and protected ethynylphenyl functional groups. The protected ethynyl group of 84a was deprotected by treating it with K2CO3 in CHCl3/CH3OH to afford ZnN4−N4 porphyrin dyad 84b containing a free mesoethynylphenyl group, which was subsequently reacted with mesoiodophenyl N2S2 porphyrin building block 44a under the same Pd(0)-coupling conditions to afford linear covalently linked ZnN4−N3S−N2S2 porphyrin triad79 82. Because all the singletstate energy levels of all three porphyrin subunits in linear triad 82 are arranged in a cascade manner, the photophysical studies clearly indicated a possibility of energy transfer from the ZnN4 porphyrin subunit to the N2S2 porphyrin via the N4 porphyrin subunit on selective excitation of the ZnN4 porphyrin subunit at 550 nm. The covalently linked meso−meso linear N3S−ZnN4− N3S porphyrin triad34 85 was synthesized by coupling of 2 equiv of meso-bromo N3S porphyrin building block 13a with 1 equiv of bis[5-(4,4,5,5-tetramethyl-1,3-dioxaborolan-2yl)]-10,20-bis(ptolyl)porphyrinato zinc(II) 85 in toluene/DMSO under Pd(0) conditions (Scheme 28). The authors invoked the possibility of energy transfer from the central ZnN4 porphyrin to the peripheral N3S porphyrin in triad 85 by recording its steadystate emission spectrum at 550 nm and excitation spectrum at 750 nm. 3.1.3. Porphyrin Tetrad. The covalently linked unsymmetrical ZnN4−N4−N3S−N2S2 tetrad79 87 was prepared as 3273


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Scheme 29. Synthesis of Covalently Linked Linear Porphyrin Tetrad 87

shown in Scheme 29. The ZnN4−N4 porphyrin dyad 84b containing a free ethynylphenyl group at the meso-position of the N4 porphyrin unit was reacted with trans-N3S porphyrin building block42 23 under mild Pd(0)-coupling conditions to afford ZnN4−N4−N3S porphyrin triad 88a containing the protected ethynylphenyl group at the meso-position of the N3S porphyrin unit. The deprotection of the ethynyl group of triad 88a with K2CO3 in THF/CH3OH at 60 °C gave the ZnN4−N4−

N3S porphyrin triad 88b containing a free ethynylphenyl group at the meso-position of N3S porphyrin. The ZnN4−N4−N3S porphyrin traid 88b, on further reaction with iodophenyl N2S2 porphyrin building block 44a under the same mild Pd(0)coupling conditions, gave the unsymmetrical ZnN4−N4−N3S− N2S2 porphyrin tetrad 87. All four porphyrin units in tetrad79 87 retain their features, as evident from their absorption study. The steady-state fluorescence spectral studies were carried out at 550 3274


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unit, and thus the porphyrin pentads 89a−e act as lightharvesting porphyrin assemblies. The pentads 90a/90b containing three different porphyrin units47 were synthesized using the unsymmetrical tetrafunctionalized (A3B type) thiaporphyrin building blocks as shown in Scheme 31. The unsymmetrical tetrafunctionalized thiaporphyrin47 29/52 was reacted with 3 equiv of Zn65 under mild Pd(0)catalyzed coupling conditions to afford unsymmetrical porphyrin tetrad 91a/91b containing the protected phenylethynyl group at the meso-position of the thiaporphyrin unit. The deprotection of protected ethynylphenyl porphyrin tetrad 91a/91b with KOH in C6H6/CH3OH at 80 °C to afford the porphyrin tetrad 91c/91d containing a free ethynyl group at the meso-position of the thiaporphyrin unit. The porphyrin tetrad 91c/91d was reacted with the iodophenyl N2S2 porphyrin unit 44a/66 under Pd(0)coupling conditions and afforded the pentads 90a/90b in decent yields. The singlet-state energy levels of the three different types of porphyrin units are arranged in a cascade manner in pentads 90a/90b. The three peripheral Zn(II) porphyrin units absorb at higher energy and transfer energy to the low-absorbing central thiaporphyrin unit, which then transfers further to the muchlower-absorbing dithiaporphyrin unit. Thus, the fluorescence study of pentads47 90a/90b indicated that, on selective excitation of peripheral Zn(II) porphyrin units at 550 nm, the emission from Zn(II) porphyrin was quenched significantly and no emission was noted from the central thiaporphyrin unit, but the major emission was noted mainly from the peripheral 21,23dithiaporphyrin unit, supporting efficient energy transfer from peripheral Zn(II) porphyrin units to another peripheral 21,23dithiaporphyrin unit mediated via the central thiaporphyrin unit. 3.1.5. Porphyrin Hexads. The cyclotriphosphazenes were used as scaffolds to synthesize symmetrical heteroporphyrin hexads 92a/92b as shown in Scheme 32. The N3P3Cl6 was reacted with 6 equiv of meso-hydroxyphenyl thiaporphyrin building block 8i/44i in THF in the presence of Cs2CO3 under simple reaction conditions and afforded thiaporphyrin hexads 81−83 92a/92b. NMR study indicated that the six thiaporphyrin units were arranged in symmetrical fashion around the robust cyclophosphazene rings and the protons of the thiaporphyrin unit in hexads 92a/92b experienced upfield and downfield shifts compared to monomeric thiaporphyrin because of the ring-current effects of neighboring porphyrin units. The absorption and emission properties were enhanced in thiaporphyrin hexads 92a/92b compared to their corresponding monomeric thiaporphyrins. The electrochemical studies indicated that all six thiaporphyrin units get oxidized and reduced at the same potentials, indicating that the hexads 92a/92b are symmetric in nature and the six thiaporphyrin units around the cyclophosphazene ring are identical.81−83 3.1.6. Porphyrin Octads. The cyclotetraphosphazene was used to synthesize octaporphyrin assemblies83 93a/93b under the same reaction conditions used for the hexaporphyrin assemblies 92a/92b. The N4P4Cl8 was treated with 8 equiv of appropriate hydroxyphenyl thiaporphyrin building block 8i/44j in THF/CH3OH in the presence of Cs2CO3 at mild reaction conditions followed by column chromatographic purification and afforded porphyrin octads 93a/93b in high yields (Scheme 33). The compounds were thoroughly characterized by 1D and 2D NMR spectroscopy, and all the observed resonance signals were unambiguously identified. The NMR spectral studies83 indicated that the porphyrin units were arranged symmetrically around the cyclotetraphosphazene scaffold. The absorption, electrochemical, and fluorescence properties of porphyrin octads

Scheme 30. Synthesis of Covalently Linked Star-Shaped Porphyrin Pentads 89

nm, where the Zn(II) porphyrin unit absorbs strongly. The systematic fluorescence studies79 clearly showed that the energy transfer was not efficient up to ZnN4−N4−N3S porphyrin triad 88b, because the triad 88b showed emission bands corresponding to the N4 porphyrin and N3S porphyrin units, indicating that the energy was not completely transferred from the N4 porphyrin unit to the N3S porphyrin unit but some energy was leaked from the N4 porphyrin unit. However, in ZnN4−N4−N3S−N2S2 porphyrin tetrad 87, the energy was efficiently transferred from the ZnN4 porphyrin unit to the N2S2 porphyrin unit mediated via N4 and N3S porphyrin units. Thus, the N2S2 porphyrin unit in tetrad 87 acts as an energy sink and helps in efficient energy transfer across the tetrad from the donor ZnN4 porphyrin unit. 3.1.4. Porphyrin Pentads. The symmetrical and unsymmetrical tetrafunctionalized heteroporphyrin building blocks were used to synthesize unsymmetrical porphyrin pentads. The symmetrical pentads56,57 89a−e were prepared in one step by coupling the tetraethynyl-functionalized thiaporphyrin54 building block 28b/51d with appropriate iodophenyl Zn(II) porphyrin building block 63a or its free base derivative 63b or meso-iodophenyl N3S porphyrin building block32 8a under mild Pd(0) coupling conditions to afford star-shaped light-harvesting pentads 89a−e containing the low-energy-absorbing porphyrin in the center and four peripheral high-energy-absorbing porphyrin units (Scheme 30). The fluorescence studies indicated that, on excitation of high-energy-absorbing peripheral porphyrins, the energy was efficiently transferred to the central porphyrin 3275


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Scheme 31. Synthesis of Covalently Linked Porphyrin Pentads 90 Containing Three Different Porphyrin Units

3.2. Noncovalently Linked Porphyrin Arrays

93a/93b also followed the same trend as noted for the porphyrin hexads 92a/92b. Thus, the cyclophosphazenes are robust scaffolds to construct multiporphyrin assemblies, and recently a dodecaporphyrin assembly84 was constructed on cyclotriphosphazene scaffold using covalent and noncovalent interactions.

In recent times, the noncovalent strategies such as metal−ligand coordination, hydrogen bonding, and electrostatic interactions have been used extensively to obtain multiporphyrin arrays easily without involvement of long, tedious chromatographic purifications that are required for covalent synthesis of multiporphyrin arrays. The noncovalent synthetic strategies have been used 3276


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extensively to synthesize several multiporphyrin assemblies based on regular N4 porphyrins and their metal derivatives.85−87 Specially, the coordination strategy called “axial-bonding strategy” has been used for the construction of multiporphyrin arrays when the porphyrin cavity possesses a metal ion that has at least one axial site available for coordination. It is well-established in the literature85 that porphyrins containing nitrogen donor groups such as pyridyl, imidazole, pyrazole, and amine functionality on their periphery prefer to coordinate with Zn(II), Ru(II), Os(II), Mg(II), and Rh(III) porphyrins via M−“N” interaction, whereas the porphyrins with oxygen donors such as carboxylates and aryloxides on their periphery prefer to coordinate with Fe(III), Mn(II), and Sn(IV) porphyrins via M−“O” interaction to form axial-bonding-type multiporphyrin arrays.86 Several heteroporphyrins14,15 containing pyridyls, carboxyphenyl, and hydroxyphenyl groups at the meso-position are now available because of recent synthetic developments in heteroporphyrins, which have been used to synthesize heteroporphyrin-based axial-bonding-type unsymmetrical multiporphyrin arrays. The meso-pyridyl and meso-hydroxyphenyl heteroporphyrin building blocks were used to construct unsymmetrical porphyrin dyads, triads, and tetrads by reacting with metalloporphyrins such as Ru(II), Al(III), and Sn(IV) porphyrins. A series of unsymmetrical heteroporphyrin-based dyads assembled via M−“N” and M−“O” interactions is presented in Chart 3. The dyads containing Ru(II) porphyrin and N3S porphyrin88 94a/ 95a or N2S2 porphyrin89 94b/95b subunits were synthesized by reacting the appropriate mono-meso-pyridyl thiaporphyrin

building block 8j/44k/8k/44l with RuTPP(CO)(EtOH) 96 in toluene at refluxing temperature for overnight as shown in Scheme 34(i) for dyad 94a. Dyads 94/95 were stable for column chromatographic purification conditions, and their formation was established by significant upfield shifts of meso-pyridyl protons of the thiaporphyrin unit because of the ring-current effect of the RuTPP(CO) unit. For example, the 2,6- and 3,5protons of the meso-pyridyl group of monomeric 21thiaporphyrin showed two sets of signals at 7.63 and 8.96 ppm, which experiences a large ring-current effect of the RuTPP(CO) unit upon coordination via Ru-pyridyl “N” interaction in N3S− RuN4 porphyrin dyad 94a and appears at 5.52 and 1.68 ppm, respectively, supporting the formation of dyad 94a. In dyads 94b/95a/95b also, the meso-pyridyl protons of axial heteroporphyrins shifted significantly due to the ring-current effect of the RuTPP(CO) unit, confirming their formation. The 2,6- and 3,5-protons of the meso-pyridyl group in dyads 94/95 were easily identified by 1H−1H COSY NMR. The photophysical studies of dyads 94/95 showed that quantum yields and singlet-state lifetimes of heteroporphyrin units were reduced compared to their corresponding monomeric heteroporphyrins because of their coordination with the heavy RuTPP(CO) unit, which enhances nonradiative decay channels. These kinds of dyads would be expected to show interesting photophysical properties at the triplet state, but such studies were not investigated. LatosGrażyński and co-workers90 used the cis-pyridyl-21-thiaporphyrin 19e and reacted it with the bistriflate of 1,3-bis(diphenylphosphino)platinum(II) in CHCl3/CH3OH, affording a self-assembled cyclic rhomboid homo dyad 97 containing two

Scheme 32. Synthesis of Hexathiaporphyrin Arrays 92

Scheme 33. Synthesis of Octathiaporphyrin Arrays 93

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Chart 3. Molecular Structures of Noncovalent Porphyrin Dyads 94−100

19e with 1 equiv of cis-pyridyl-21-oxaporphyrin 20e in the presence of 2 equiv of Re(CO)5Cl in THF at reflux temperature for overnight (Scheme 35). The reaction resulted in the formation of two Re(I)-bridged homo dyads containing two of the same type of heteroporphyrin units such as N3S−Re(I)−N3S dyad 100a and N3O−Re(I)−N3O porphyrin dyad 100c and one Re(I)-bridged hetero dyad containing two different types of porphyrin units such as N3S−Re(I)−N3O porphyrin dyad 100b. All three dyads 100a−c were separated by column chromatography and confirmed their identities by mass and NMR studies.The 1H NMR of dyads revealed that the 2,6- and 3,5pyridyl proton resonances of the heteroporphyrin unit in the dyad experienced downfield shifts compared to their corresponding monomeric heteroporphyrins. Also, the 2,6-pyridyl protons were relatively more downfield shifted compared to 3,5-pyridyl proton resonances because of strong coordination of mesopyridyl “N” with Re(I) in dyads. The absorption and electrochemical studies supported weak interaction between the porphyrin units in homo and hetero dyads. The homo dyads 100a/c are sufficiently fluorescent, and the fluorescence study on hetero dyad 100b indicated a possibility of energy transfer from the N3O porphyrin unit to the N3S porphyrin unit at singlet state.

21-thiaporphyrin units (Scheme 34(ii)). Furthermore, the heteroporphyrin−Al(III) porphyrin dyads91 98a/98b were assembled via Al−“O” interaction. Dyads 98a/98b were synthesized by refluxing 1 equiv of AlTPP(OH) 99 with 1 equiv of corresponding meso-hydroxyphenyl thiaporphyrin 9d in benzene at refluxing temperature (Scheme 34(iii)). The formation of dyads 98a/98b were confirmed by upfield shifts of phenoxo-bridged protons and also the pyrrole and thiophene protons of axial thiaporphyrin unit because these protons experience a strong ring-current effect of the basal Al(III) porphyrin unit. The photophysical studies indicated that the quantum yield of the Al(III) porphyrin unit in dyads 98a/98b was decreased compared to the free monomeric Al(III) porphyrin, which invoked a possibility of singlet−singlet energy transfer from the Al(III) porphyrin unit to the thiaporphyrin units in dyads 98a and 98b. Furthermore, the cis-dipyridyl-21thiaporphyrin 19e and 21-oxaporphyrin 20e building blocks were used to construct the Re(I)-bridged homo dyads 100a/ 100c containing two of the same type of heteroporphyrins and hetero dyad 100b containing two different types of heteroporphyrins in one-pot condensation reaction.92 Dyads 100a−c were prepared by reacting 1 equiv of cis-pyridyl-21-thiaporphyrin 3278


Chemical Reviews

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Scheme 34. (i) Synthesis of Noncovalent Porphyrin Dyad 94a; (ii) Synthesis of Cyclic Rhomboid Homo Dyad 97; (iii) Synthesis of Al(III) Porphyrin-Based Axial-Bonded Dyad 98a

A series of unsymmetrical heteroporphyrin-based triads45,88 and tetrads36 assembled via M−“N” and M−“O” interactions are presented in Chart 4. The cis-dipyridyl heteroporphyrin building blocks46 were used to construct unsymmetrical noncovalent porphyrin triads 101a/101b using Ru(II)−“N” interaction by reacting 1 equiv of appropriate cis-pyridyl heteroporphyrin building block 19e/19f with 2 equiv of RuTPP(CO)(EtOH) 96 in toluene at refluxing temperature for overnight (Scheme 36). Similarly, the noncovalent unsymmetrical porphyrin tetrads36 102a/102b were synthesized by treating 1 equiv of appropriate meso-tripyridyl heteroporphyrin 25a/25b with 3 equiv of RuTPP(CO)(EtOH) 96 under the same reaction conditions (Scheme 36). All these arrays were stable for column chromatographic purification and confirmed their identities by NMR and mass spectral studies. The porphyrin units in triads 101a/101b and tetrads 102a/102b interact weakly, and the arrays were weakly fluorescent due to the presence of heavy Ru(II) ion.

The unsymmetrical porphyrin triad78 103 containing RuN4, N2S2, and N3S porphyrin subunits assembled by using both covalent and noncovalent interactions was synthesized as shown in Scheme 37. The unsymmetrical cis-difunctionalized N2S2 porphyrin building block 50e containing meso-pyridyl and meso-ethynylphenyl functional groups was coupled with the meso-iodophenyl N3S porphyrin building block 8a under Pd(0)coupling conditions to afford diphenyl ethyne bridged N2S2− N3S porphyrin dyad 104 containing meso-pyridyl group at the N2S2 porphyrin unit. In the next step, the N2S2−N3S porphyrin dyad 104 was reacted with RuTPP(CO)(EtOH) 96 in toluene at refluxing temperature followed by column chromatographic purification and afforded unsymmetrical RuN4−N2S2−N3S porphyrin triad 103 assembled via using both covalent and noncovalent interactions. The three porphyrin units in triad 103 retain their independent features, and the fluorescence study indicated that the fluorescence yields of both N3S and N2S2 porphyrin subunits in triad were decreased compared to their 3279


Chemical Reviews

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Scheme 35. Synthesis of Re(I)-Bridged Thia-Oxa Porphyrin Dyad 100b

corresponding monomers due to the heavy ruthenium ion that was coordinated to the meso-pyridyl N2S2 porphyrin subunit. The meso-hydroxyphenyl thiaporphyrin building blocks were used as axial ligands to construct Sn(IV) porphyrin-based axialbonded-type unsymmetrical multiporphyrin arrays93 by employing complementary and noninterfering Sn(IV)−“O” and Ru(II)−“N” interactions (Chart 5). The axial-bonding type of unsymmetrical Sn(IV) porphyrin triads94 105a/105b containing thiaporphyrins as axial units were synthesized by refluxing 1 equiv of SnTTP(OH)295 106a with appropriate 2 equiv of mesohydroxyphenyl thiaporphyrin building block 8i/44j in benzene at refluxing temperature for overnight followed by column chromatographic purification (Scheme 38). The formation of triads 105a/105b was confirmed by significant upfield shifts of 2,6- and 3,5-phenoxide protons, which were identified by using 2D NMR techniques. The fluorescence studies indicated the possibility of energy transfer at the singlet state from basal Sn(IV) porphyrin unit to axial thiaporphyrin units in triads 105a/105b. The oxophilic nature of the Sn(IV) ion was further exploited by using Sn(IV) porphyrins containing 1−4 mesopyridyl groups that have preferential bonding with Ru(II) ion to synthesize porphyrin tetrad 107, pentad 108, hexad 109, and

heptad 110 by invoking their complementary and noninterfering interactions.96 The porphyrin tetrad to porphyrin heptad96 107− 110 were prepared by following two simple strategies as shown in Scheme 39 for tetrad 107. The porphyrin tetrad 107, containing Sn(IV) porphyrin, N3S porphyrins, and Ru(II) porphyrin, was prepared by treating the porphyrin triad 111 having one mesopyridyl group with Ru(II) porphyrin 96 in CHCl3 at room temperature for 2 days followed by column chromatographic purification (Scheme 39). Alternately, the porphyrin tetrad 107 was also prepared in one-pot reaction by treating 1 equiv of Sn(IV) porphyrin 106b, 2 equiv of meso-hydroxyphenyl N2S2 porphyrin 44j, and 1 equiv of RuTPP(CO)(EtOH) 96 in benzene at reflux for 1 day followed by column chromatographic purification. The formation of tetrad to heptad 107−110 was confirmed by detailed 1D and 2D NMR studies. The absorption and electrochemical studies indicated weak interaction among different porphyrin units in tetrad to heptad.96 The tetrad to heptad 107−110 were weakly fluorescent due to the presence of heavy Ru(II) porphyrin unit(s), which quench the fluorescence of the Sn(IV) porphyrin and 21,23-dithiaporphyrin units. 3280


Chemical Reviews

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Chart 4. Molecular Structures of Noncovalent Porphyrin Triads 101 and Tetrads 102

4. METAL COMPLEXES OF HETEROPORPHYRINS

4.1. Metal Complexes of 21-Thiaporphyrin (N3S Core)

Compared to tetrapyrrolic porphyrins, the core-modified porphyrins are less reactive chemically and geometrically to bind to the metal ions and have not shown rich coordination chemistry. One of the main reasons is due to its nonplanar orientation of the core heteroatoms (XNNN/XYNN) as compared to the normal (NNNN) set of donor atoms in porphyrin rings. Furthermore, incorporation of relatively bigger heteroatom(s) (S, O, Se, and Te) inside the porphyrin core resulted in the change in the cavity size, which in turn is responsible for the peculiar metal-binding behavior of the heteroporphyrins.9 However, in some cases, the core-modified porphyrins showed the ability to form stable metal complexes in which metal can be stabilized in a rare oxidation state. As the number of heteroatoms increased, the porphyrin ring became more nonplanar and showed very poor metal binding affinity. The monoheteroporphyrins (21-thia/oxa/selenaporphyrins (N3S/N3O/N3Se)) contain one ionizable inner NH atom and act as monodentate ligands. Thus, monoheteroporphyrin metal complexes required axial ligands to stabilize metal ion in its +II/ +III oxidation state, whereas the diheteroporphyrins (21,23-thia/ oxa/selena (N2S2/N2O2/N2Se2)) have no ionizable inner NH atoms and act as neutral ligands. Broadhurst et al.16−18 first reported the synthesis of several β-alkylated mono-oxa/ thiaporphyrin and their Zn(II) and Ni(II) complexes, but the characterization of metal complexes was incomplete. However, the pioneering efforts of Latos-Grażyński and co-workers9 shed a light on the potential ability of heteroporphyrins in forming metal complexes.

Latos-Grażyński, Balch, and co-workers97 reported the first synthesis of Cu(II), Fe(II), and Ni(II) complexes of 21thiaporphyrin Cu(II)2a/Fe(II)2a/Ni(II)2a by treating free base 21-thiaporphyrin (N3STPP) 2a with corresponding anhydrous metal chloride salts under reflux conditions (Scheme 40). In the solid state, all three complexes were stable, but in solution, Fe(II)2a underwent slow demetalation upon standing. All three metal complexes have chloride as axial ligands and were paramagnetic in nature. Later, Latos-Grażyński and co-workers98−101 carried out chemical and electrochemical reduction of Ni(II)2a to prepare its one-electron reduced Ni(I) complex of 21-thiaporphyrin Ni(I)2a and characterized the complex Ni(I)2a thoroughly (Scheme 40). The same group100 also investigated the reactivity of the axial chloride (Cl) ligand by reacting Ni(II)2a with Grignard reagent (PhMgBr) to afford Ni(II)2a(Ph), in which the axial chloride ligand was replaced by phenyl group (Scheme 40). In 1H NMR of compound Ni(II)2a(Ph), the protons of the axial phenyl group exhibited strong downfield shifts due to porphyrin ring-current effect. The palladium(II) complex of thiaporphyrin102 Pd(II)2a was synthesized by treating STPPH 2a with palladium chloride in acetonitrile solution under reflux conditions (Scheme 40). The Pd(II) complex Pd(II)2a was stable in solid and solution state. The one-electron reduced product, Pd(I)2a, was generated by treating Pd(II)2a with moderate reducing agent like aqueous sodium dithionite or zinc amalgam. In the cyclic voltammogram of Pd(II)2a, the reversible reduction waves also supported the formation of [Pd(II)2a]+ and Pd(II) → Pd(I) reduced product Pd(I)2a. The Pd(I)2a was paramagnetic but it did not show axial coordination of solvent molecule at the metal center unlike 3281


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Scheme 36. Synthesis of Noncovalent Porphyrin Triad 101a and Tetrad 102a

Ni(I)2a. In 2000, Arnold and co-workers103 reported the Li(I) complex of thiaporphyrin Li(I)2a by reacting 21-thiaporphyrin with lithium bis(trimethylsilyl)amide in THF at room temperature (Scheme 40). The green-colored complex was soluble in common organic solvents but very much sensitive toward moisture. The presence of a trace amount of water resulted in the decomplexation of lithium ion from thiaporphyrin core. The diamagnetic Li(I)2a complex showed a single peak at −10.15 ppm in 7Li NMR spectrum, which confirmed the presence of the lithium cation in the macrocycle. Chen and co-workers104 synthesized the diamagnetic Hg(II) complex of 21-thiaporphyrin Hg(II)2a by reacting 21-thiaporphyrin 2a with Hg(OAc)2 in dichloromethane at room temperature (Scheme 40). LatosGrażyński and co-workers105 reported the Rh(III) complex of 21-thiaporphyrin Rh(III)2a by reacting 21-thiaporphyrin 2a with RhCl3 in the presence of zinc dust in acetonitrile and afforded Rh(III)2a. The presence of zinc dust proved to be essential for the complexation because it helps to generate more reactive Rh(I) species, which was the intermediate species. The Rh(III)2a complex was stable in solution as well as in solid state and showed splitted Soret band. In 2011, Hung and coworkers106 reported the synthesis of ruthenium-21-thiaporphyrin complex Ru(III)2a by treating 21-thiaporphyrin with Ru3(CO)12 in o-dichlorobenzene under reflux conditions. The compound Ru(III)2a was stable and has one carbonyl (CO) and one chloride (Cl) as axial ligands. However, the axial chloride Ru(III)2a was found to be easily replaced by nitrate, nitrite, and azide ligands by treating with their corresponding silver(I) or sodium salts and afforded [Ru(III)2a(X)] (X = NO3, NO2, N3)

in 55−72% yields (Scheme 40). However, nitro- and nitratosubstituted complexes were less stable and underwent slow demetalation in air. Similarly, the stable axial dichloridesubstituted Ru(III) complex of 21-thiaporphyrin was prepared by reacting 21-thiaporphyrin with Ru(COD)Cl2 under the same conditions. The axial dichloride-substituted Ru(III) thiaporphyrin complex showed metal-centered reduction (Ru3+/Ru2+) under electrochemical conditions. In 2012, Ghosh and Ravikanth107 reported the first stable Re(I) complex of 21-thiaporphyrin Re(I)2a, where the Re(I) ion was coordinated with two of the three inner nitrogens and one sulfur, and the remaining three axial positions were occupied with three carbonyl (CO) ligands (Scheme 40). The hexacoordinated Re(I)2a complex was synthesized by reacting Re2(CO)10 with free base 21thiaporphyrin 2a in o-dichlorobenzene at reflux followed by simple column chromatographic purification. Complex Re(I)2a showed fluxional behavior as verified by variable-temperature 1H NMR and 13C NMR spectra. Re(I)2a exhibited a very broad Soret band and ill-defined Q-bands. The X-ray structures of all known metal complexes of 21thiaporphyrin are presented in Figure 1. Out of all the metal complexes of 21-thiaporphyrin, Cu(II)/Fe(II)/Ni(II)/Hg(II)2a are five-coordinated complexes having axial chloride (Cl) ligand and adopt square-pyramidal or distorted trigonalbipyramid geometry. The Ni(I)99 and Pd(I)102 complexes of 21-thiaporphyrin Ni(I)/Pd(I)2a are tetracoordinated and adopt near square-planar geometry. The Li(I) complex of thiaporphyrin103 Li(I)2a also formed five-coordinated square-pyramidal complex with THF occupied as axial ligand. The Re(I)2a, 3282


Chemical Reviews

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complexes, whereas in other complexes, the M−S bond distances were in the range of 2.143−2.336 Å. However, the smallest M−S bond distance (M−S = 2.143 Å) was observed in the case of square-planar Ni(I)2a99 (Table 1). The variation in M−S bond distances clearly reflects the position of corresponding metal ion with respect to the macrocyclic plane. The Ni(I) ion in Ni(I)2a99 was placed at a distance of 0.090 Å with respect to the 24-atom mean plane, whereas Hg(II) in Hg(II)2a104 and Re(I) ion in Re(I)2a107 were placed above the macrocyclic plane, at distances of 1.464 and 1.666 Å, respectively. Both the M−S distance and the metal ion distance from the mean plane of the macrocycle support the maximum deviation of Hg(II) in Hg(II)2a and Re(I) in Re(I)2a and the minimum deviation of Ni(I) in Ni(I)2a from the mean plane of the macrocycle. As the thiophene sulfur “S” atom was bent remarkably from the macrocyclic plane, the metal ion deviation from the 4-atom mean plane (NNNS) also turned out to be maximum in the case of Hg(II)2a (1.453 Å) and Re(I)2a (1.491 Å) complexes and minimum in the case of Ni(I)2a (0.101 Å) (Table 1). This observation again confirmed that the metal ion deviations in metallo-21-thiaporphyrin complexes were mainly attributed to the deviation of the coordinating sulfur atom. Because the sulfur atom showed deviation, the thiophene ring also remained no longer planar with the macrocycle and showed significant upward or downward bending with respect to the macrocycle (Figure 1). The average M−N distances remain almost the same in all the complexes and lie in the range of 1.980−2.457 Å, whereas the deviation of the coordinating core nitrogen atoms lies in the range 0.020−0.254 Å. This observation indicated that core nitrogen atoms remained almost in the plane of the macrocycle and play an insignificant role in the macrocyclic distortion in metallo-21-thiaporphyrin complexes.

Scheme 37. Synthesis of Porphyrin Triad 103 Assembled Using Covalent and Noncovalent Interactions

4.2. Metal Complexes of 21-Oxaporphyrin (N3O Core)

Broadhurst et al. reported17−19 the synthesis of Zn(II) and Ni(II) complexes of β-alkylated mono-oxaporphyrin and characterized them by mass and absorption spectra. In 1997, Latos-Grażyński and co-workers108 prepared Ni(II) complex of 21-oxaporphyrin Ni(II)3a by treating ethanolic solution of nickel(II) chloride with chloroform solution of 21-oxaporphyrin under reflux conditions (Scheme 41). The compound Ni(II)3a was fivecoordinated, paramagnetic square-pyramidal complex as confirmed by X-ray crystallography. Furthermore, they carried out chemical reduction of Ni(II)3a, with sodium dithionite, to prepare the paramagnetic four-coordinated Ni(I) derivatives of 21-oxaporphyrin Ni(I)3a (Scheme 41). The Ni(II) 21oxaporphyrin109 Ni(II)3a(Ph), in which the axial chloride ligand was substituted by a phenyl group, was prepared by treating Ni(II)3a with PhMgBr in diethyl ether at −10 °C (Scheme 41). In 2002, the same group110 also reported the formation of Fe(III) complex of 21-oxaporphyrin Fe(III)3a by treating 21-oxaporphyrin 3a with FeCl3 (Scheme 41). They also carried out detailed characterization of the high- and low-spin complexes of Fe(III)3a, one-electron-reduced compound Fe(II)3a, and two-electron-reduced compound Fe(I)3a by 1H NMR and electron paramagnetic resonance (EPR) spectroscopy. The axial chloride of Fe(II)3a was substituted with phenyl group by treating Fe(II)3a with phenylmagnesium bromide and afforded Fe(II)3a(Ph)111 (Scheme 41). However, the complex Fe(II)3a(Ph) was unstable and oxidized further with Br2 to afford stable Fe(II)3a(Br) complex with one bromide and one phenyl group as axial ligands.111 Hung and co-workers reported112 the Zn(II) complex of 21-oxaporphyrin Zn(II)3a

Ru(II)2a, and Rh(III)2a are hexacoordinated complexes. In Re(I)2a, the Re(I) ion was coordinated to sulfur, two pyrrolic nitrogens, and three axial carbonyl ligands,107 whereas in Ru(II)2a, the Ru(II) ion was coordinated with all four core atoms along with one chloride and carbonyl groups as axial ligands.106 However, in Rh(III)2a, the Rh(III) was coordinated with all four core atoms and two axial chlorides.105 The most common feature in all metallo-21-thiaporphyrin complexes is that the thiophene sulfur was sharply bent out of the macrocyclic plane and the macrocycle adopts a nonplanar structure. The thiophene ring coordinated in η1-bonding fashion through the sulfur atom and acquires square-pyramidal geometry. The maximum M−S bond distance was noticed in Hg(II)2a99 (M−S = 2.801 Å) and in Re(I)2a102 (M−S = 2.553 Å) 3283


Chemical Reviews

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Scheme 38. Synthesis of Sn(IV) Porphyrin-Based Axial-Bonded Porphyrin Triad 105a

Table 1 complex

coordination no.

geometry

crystal system

axial ligand

Cu(II)2a

5

square-pyramidal

triclinic Pi ̅

Cl

Fe(II)2a

5

square-pyramidal

triclinic Pi ̅

Cl

Ni(II)2a

5

square-pyramidal

triclinic Pi ̅

Cl

Ni(I)2a

4

square-planar

Li(I)2a Re(I)2a Ru(II)2a

5 6 6

square-pyramidal octahedral octahedral

Rh(III) 2a Pd(I)2a

6

octahedral

4

square-planar

Hg(II)2a

5

distorted trigonalbipyramid

monoclinic P21/c triclinic Pi ̅ triclinic Pi ̅ monoclinic C2/m monoclinic P21 monoclinic P21/c triclinic Pi ̅

THF CO CO, Cl Cl

Cl

color dark green dark brown dark blue dark brown green brown black dark green dark green green

metal O.S

M−Sa (Å)

av. M− Nb (Å)

Δ24Mc (Å)

Δ24Sd (Å)

Δ24Ne (Å)

Δ4Mf (Å)

S−M− Ng (deg)

II

2.336

2.024

0.284

0.798

0.020

0.450

83.98

II

2.294

2.127

0.557

0.853

0.083

0.708

83.1

II

2.296

2.046

0.279

0.762

0.026

0.445

83.96

I

2.143

1.980

0.090

0.602

0.114

0.101

86.71

I I II

2.329 2.553 2.245

2.121 2.222 2.065

0.701 1.666 0.126

0.558 0.159 0.799

0.103 0.254 0.152

0.725 1.491 0.078

76.02 70.94 88.76

III

2.250

2.091

0.114

0.877

0.111

0.256

90.77

I

2.209

2.047

0.109

0.723

0.085

0.002

87.9

II

2.801

2.457

1.464

0.495

0.087

1.453

61.15

a

Bond distances (Å) between metal ion and the sulfur (S) atom. bAverage bond distances between the metal ion and the pyrrolic nitrogen atoms. Displacement (Å) of corresponding metal ion from the 24-atom mean plane of the heteroporphyrin core. dDisplacement (Å) of heteroatom (S) from the 24-atom mean plane of the heteroporphyrin core. eAverage displacement (Å) of nitrogen atoms from the 24-atom mean plane of the heteroporphyrin core. fDisplacement (Å) of corresponding metal ion from the 4-atom mean plane of the heteroporphyrin core. gAverage bond angles of S−M−N (Å) bond(s). c

complexes of N3O porphyrins Mn(II)3a/Co(II)3a/Cu(II)3a by reacting free base N3O porphyrin 3a with the corresponding metal chloride salts in chloroform or chloroform/methanol mixture (Scheme 41). After routine column chromatographic purification, the desired metalloporphyrins Mn(II)3a/Co(II)3a/Cu(II)3a were obtained in quantitative yields. All the complexes showed spillted Soret bands and three Q-bands. The crystal structures of metallo-21-oxaporphyrin complexes are shown in Figure 2. The Ni(II),108 Zn(II),112 Mn(II),113 Co(II),113 and Cu(II)113 complexes of 21-oxaporphyrin Ni(II)-

by treating 21-oxaporphyrin 3a with ZnCl2 in the presence of 2,6lutidine (Scheme 41). The Zn(II)3a complex showed splitted Soret band. Recently, the Re(I) complex of 21-oxaporphyrin107 Re(I)3a was prepared by reacting free base 21-oxaporphyrin with Re3(CO)12 in o-dichlorobenzene at reflux temperature. In complex107 Re(I)3a, the Re(I) metal ion was coordinated with all three “N” atoms of the porphyrin core along with three axial carbonyl ligands and the furan oxygen atom was not involved in coordination with the Re(I) ion (Scheme 41). In 2013, Gloe and co-workers reported113 the Mn(II), Co(II), and Cu(II) 3284


Chemical Reviews

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Chart 5. Molecular Structures of Sn(IV) Porphyrin-Based Porphyrin Arrays 105−110

Fe(III)3a, Fe(III) ion and the furan ring were completely coplanar, and overall the macrocyle showed no deviation and exhibited perfect octahedral geometry.110 However, in the case of Re(I)3a complex, the Re(I) ion was coordinated only with the three nitrogen atoms of the oxaporphyrin core and the Re(I) metal ion was deviated above the mean plane of the macrocycle, at a distance of 1.544 Å.107 As the furan oxygen atom was not involved in bonding with the Re(I) ion, the furan ring was deviated to a very less extent (0.065 Å) from the 24-atom mean plane of the macrocycle. The Re−N average bond distance was found to be 2.289 Å, which is in good agreement with the M−N average bond distances in other metal complexes of 21oxaporphyrin (2.021−2.124 Å) (Table 2). However, the M−N average bond distance was minimum in the case of Ni(I)3a (1.989 Å), which was attributed to the square-planar geometry of the complex and also to the low oxidation state of the central metal ion. Thus, the important feature of the metal complexes of heteroporphyrins is their ability to stabilize Ni ion even in uncommon +1 oxidation state.

3a/Zn(II)3a/Mn(II)3a/Co(II)3a/Cu(II)3a adopt square-pyramidal geometry with an axial chloride ligand (Figure 2b, c, f−h). The Ni(I)3a adopt square-planar geometry with the Ni(I) placed almost at the center of the macrocyclic cavity (Figure 2d). On the other hand, Fe(III)3a111/Re(III)3a,107 were hexacoordinated octahedral complexes with either chloride or carbonyl groups as axial ligands (Figure 2a and e). All the core atoms (three nitrogens and one oxygen atom) were involved in the bonding with the central metal ion in all metal complexes of 21oxaporphyrin except in Re(I)3a complex. Unlike metal complexes of 21-thiaporphyrin, the furan ring in metallo-21oxaporphyrins was coordinated in η1-bonding fashion through the oxygen atom and acquires trigonal geometry. The furan ring in the metallo-21-oxaporphyrin complexes remained in the plane of the macrocycle unlike metallo-21-thiaporphyrin complexes where the thiophene ring was markedly deviated and bent out from the porphyrin plane. Thus, the porphyrin macrocycle showed very less deformation in metallo-21-oxaporphyrin complexes. As a consequence of the planarity of the oxaporphyrin ring, the M−O distances in these complexes were almost the same and lie in the range of 2.093−2.184 Å (Table 2). Following the similar trend, in all these metal complexes of 21-oxaporphyrin, the metal ion was also less deviated from the mean plane (0.000−0.614 Å). In complex

4.3. Metal Complexes of 21-Selenaporphyrin (N3Se Core)

To date, two reports are available on the metal complexes of 21selenaporphyrin. In 1996, Latos-Grażyński et al.114 reported the synthesis of Ni(II) 21-selenaporphyrin complex Ni(II)4a, by reacting 21-selenaporphyrin with NiCl2 under reflux conditions 3285


Chemical Reviews

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Scheme 39. Synthesis of Sn(IV) Porphyrin-Based Porphyrin Tetrad 107

Table 2 complex

coordination no.

Ni(II)3a

5

Ni(I)3a

5

Fe(III) 3a Zn(II)3a

6

Re(I)3a

6

squarepyramidal octahedral

Re(I)4a

6

octahedral

Re(I)30 Ru(II) 30 Ni(II)31 Re(I)32

6 6 6 6

5

geometry squarepyramidal squareplanar octahedral

crystal system monoclinic P21/n triclinic Pi ̅

axial ligand

color

metal O.S

M−Xa (Å)

av. M− Nb (Å)

Δ24Mc (Å)

Δ24Xd (Å)

Δ24Ne (Å)

Δ4Mf (Å)

X−M−Ng (deg)

Cl

black

II

2.184

2.021

0.350

0.048

0.042

0.329

86.33

-

black

I

2.120

1.989

0.166

0.173

0.084

0.061

88.32

tetragonal I4/m triclinic Pi ̅

Cl

red

III

2.077

2.077

0.000

0.000

0.000

0.000

90.00

Cl

brown

II

2.093

2.124

0.614

0.134

0.039

0.591

86.48

triclinic Pi ̅

CO

I

-

2.289

1.544

0.065

0.249

1.325

-

CO

I

2.592

2.196

1.571

0.406

0.291

1.486

72.80

octahedral octahedral

monoclinic C2/c triclinic Pi ̅ triclinic Pi ̅

red brown black

CO Cl

black black

I II

2.560 2.251

2.258 2.082

1.686 0.000

0.234 0.926

0.239 0.053

1.700 0.000

74.05 90.0

octahedral octahedral

triclinic Pi ̅ triclinic Pi ̅

Cl CO

black black

II I

2.133 2.605

2.010 2.215

0.008 1.670

0.012 0.353

0.068 0.250

0.002 1.734

90.00 73.84

a

Bond distances (Å) between metal ion and the heteroatom (O/Se/SS/OO/SeSe). bAverage bond distances between the metal ion and the pyrrolic nitrogen atoms. cDisplacement (Å) of corresponding metal ion from the 24-atom mean plane of the heteroporphyrin core. dDisplacement (Å) of heteroatom (O/Se/SS/OO/SeSe) from the 24-atom mean plane of the heteroporphyrin core. eAverage displacement (Å) of nitrogen atoms from the 24-atom mean plane of the heteroporphyrin core. fDisplacement (Å) of corresponding metal ion from the 4-atom mean plane of the heteroporphyrin core. gAverage bond angles of X−M−N (Å) bond(s).

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Scheme 40. Metal Complexes of 21-Thiaporphyrin 2a

(Scheme 42). The paramagnetic Ni(II)4a was a five-coordinated complex with axial Cl ligand. Furthermore, the six-coordinated Ni(II) complex Ni(II)4a(Im)2 was also generated by treating Ni(II)4a with imidazole and characterized by NMR spectroscopy. In their subsequent report, the authors also demonstrated that, upon treatment with phenyl magnesium bromide, the axial Cl ligand of Ni(II)4a was displaced by the phenyl group to afford Ni(II)4a(Ph) (Scheme 42). Recently, our group reported the synthesis of hexacoordinated rhenium(I) tricarbonyl complex of 21-selenaporphyrin,115 Re(I)4a, by refluxing 21-selenaporphyrin 4a with Re(CO)5Cl in 1,2-dichlorobenzene at reflux conditions (Scheme 42). The

Re(I)4a complex was stable in solution unlike Ni(II)4a, which undergoes slow decomplexation in solution. The Re(I)4a showed a single peak at 404.4 ppm in 77Se NMR. In Re(I)4a, the Re(I) ion was bound with selenium, two inner nitrogen atoms, and three axial carbonyl groups. The Re(I)4a showed typical metal porphyrin-type splitted Soret band in absorption spectrum, and the complex was stable under redox conditions. The crystal structure of Re(I)4a is shown in Figure 3a, and this is the only structure available in the literature on metal complexes of 21-selenaporphyrin.115 The Re(I) ion was in hexacoordination environment, and the Re(I) ion binds with the selenium, two nitrogen atoms of the core, and three axial carbonyl ligands. The 3287


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Figure 1. X-ray structures of various metal complexes of 21-thiaporphyrins 2a: (a) Cu(II)2a. Reproduced with permission from ref 97. Copyright 1989 American Chemical Society. (b) Li(I)2a. Reproduced with permission from ref 103. Copyright 2000 American Chemical Society. (c) Fe(II)2a. Reproduced with permission from ref 97. Copyright 1989 American Chemical Society. (d) Pd(I)2a. Reproduced with permission from ref 102. Copyright 1994 American Chemical Society. (e) Ni(II)2a. Reproduced with permission from ref 97. Copyright 1989 American Chemical Society. (f) Hg(II)2a. Reproduced with permission from ref 104. Copyright 2002 Elsevier. (g) Ni(I)2a. Reproduced with permission from ref 99. Copyright 1989 American Chemical Society. (h) Rh(III)2a. Reproduced with permission from ref 105. Copyright 1989 American Chemical Society. (i) Re(I)2a. Reproduced with permission from ref 107. Copyright 2012 American Chemical Society. (j) Ru(II)2a. Reproduced with permission from ref 106. Copyright 2011 American Chemical Society. 3288


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Scheme 41. Metal Complexes of 21-Oxaporphyrin 3a

Re(I) ion was displaced at a distance of 1.571 Å above the mean plane of the macrocycle (Table 2). In Re(I)4a, the selenophene ring was bent out of the plane and showed η1-bonding fashion with the Re(I) ion. It is noted that the distortion of the selenophene ring in complex Re(I)4a was found to be higher compared to the distortion of the thiophene ring in complex Re(I)2a.

studied the reactivity of the axial Cl ligand in Ru(II)30, by reacting Ru(II)30 with a large excess of AgNO3 and NaSePh, a ffo r d i ng [ Ru(S 2 T P P ) ( N O 3 ) 2 ] an d [ R u( S 2 TPP)(PhSeCH2SePh)2] complexes, respectively (Scheme 43). Most recently, Ravikanth and co-workers117,118 reported the Re(I) complex of dithiaporphyrin Re(I)30 by treating 21,23dithiaporphyrin 30 with Re(CO)5Cl in o-dichlorobenzene under reflux conditions (Scheme 43). The positive charge of Re(I)30 was neutralized by a large counteranion, the trichlorobridged dirhenium(I) complex [Re2(μ-Cl)3(CO)6]. In compound Re(I)30, the Re(I) ion was hexacoordinated and bound with two sulfur atoms, one nitrogen atom, and three axial carbonyl ligands. The crystal structures of Ru(II)116 and Re(I)117 complexes of 21,23-dithiaporphyrin, Ru(II)30 and Re(I)30, are shown in Figure 3b and c. Complex Ru(II)30 exhibited perfect octahedral

4.4. Metal Complexes of 21,23-Dithiaporphyrin (N2S2 Core)

In 2001, Hung et al.116 reported the first example of stable Ru(II) complex of dithiaporphyrin Ru(II)30 by treating 21,23dithiaporphyrin 30 with an excess of Ru(COD)Cl2 in odichlorobenzene at reflux temperature (Scheme 43). The Ru(II)30 was hexacoordinated with two Cl ligands occupied at the axial positions. The Ru(II)30 was stable in solid as well as in solution state and showed typical metalloporphyrin-type absorption features. Later in 2011, the same research group106 3289


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Figure 2. X-ray structures of various metal complexes of 21-oxaporphyrin 3a: (a) Fe(III)3a. Reproduced with permission from ref 111. Copyright 2004 American Chemical Society. (b) Zn(II)3a. Reproduced with permission from ref 112. Copyright 2011 Elsevier. (c) Ni(II)3a and (d) Ni(I)3a. Reproduced with permission from ref 108. Copyright 1997 Wiley-VCH. (e) Re(I)3a. Reproduced with permission from ref 107. Copyright 2012 American Chemical Society. (f) Cu(II)3a, (g) Co(II)3a, and (h) Mn(II)3a. Reproduced with permission from ref 172. Copyright 2005 American Chemical Society. 3290


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Scheme 42. Metal Complexes of 21-Selenaporphyrin 4a

geometry, and Ru(II) ion resides at the mean plane of the macrocyle. In Ru(II)30, to accommodate the large Ru(II) ion inside the small dithiaporphyrin core, both the thiophene rings were tilted in opposite directions with respect to the mean plane of the macrocycle.116 In contrast, in Re(I)30, the Re(I) ion resides on the top of the macrocycle plane at a distance of 1.686 Å from the mean plane of the macrocycle.117 The deviation of the Re(I) ion from the macrocyclic plane was attributed to the larger size of the Re(I) ion and the presence of the two large sulfur atoms in the dithiaporphyrin core. Both the thiophene rings coordinated with the Re(I) ion in η1-bonding fashion through sulfur atoms but tilted to the same side of the mean plane unlike complex Ru(II)30.117,118

report available in the literature on metal complexes of diselenaporphyrin. The free base 21,23-diselenaporphyrin (N2Se2TTP) was refluxed with Re(CO)5Cl in chlorobenzene, and complex Re(I)32 was isolated as a green-colored solid after chromatographic purification (Scheme 45). The Re(I)32 complex was crystallized with the counteranion Re2(μCl)3(CO)6. The crystal structure of Re(I)32 (Figure 3e) showed that it was a hexacoordinated complex, and the Re(I) ion was coordinated with the two inner selenium atoms, one pyrrole nitrogen atom, and three axial carbonyl groups.118 The Re(I) ion was placed at a distance of 1.670 Å above the mean plane of the macrocycle. Thus, the magnitude of deformation was more in Re(I)32 compared to Re(I)30.

4.5. Metal Complexes of 21,23-Dioxaporphyrin (N2O2 Core)

4.7. Metal Complexes of Phosphoheteroporphyrin (N2PS Core)

108

The Ni(II) complex of 21,23-dioxaporphyrin Ni(II)31 was prepared by refluxing the ethanolic solution of NiCl2 with chloroform solution 21,23-dioxaporphyrin 31 under reflux conditions (Scheme 44). The Ni(II)31 showed similar absorption features like free base 21,23-dioxaporphyrin 31 with minor shifts in the band positions and underwent demetalation upon acid treatment. The one-electron reduced Ni(I) complex of dioxaporphyrin Ni(I)31 was prepared by treating Ni(II)31 with reducing agents such as aqueous sodium dithionite or zinc amalgam.103 Complex Ni(I)31 was not isolated but characterized by electronic spectra. The chemical reduction was reversible, and complex Ni(I)31 readily reverts back to Ni(II)31 on addition of oxidant. The crystal structure of Ni(II) dioxaporphyrin108 Ni(II)31 is shown in Figure 3d. Ni(II)31 was hexacoordinated, and Ni(II) ion was coordinated to two inner oxygens, two inner nitrogens, and two axial chloride ligands. Both furan rings were planar and coordinated with the Ni(II) in η1-bonding fashion. The deviation of Ni(II) ion from the mean plane of the macrocycle was lower (0.008 Å), and overall the complex attains a more planar structure.

Matano and co-workers119 successfully synthesized group 10 metal complexes of the 20π-P,S,N2-hybrid phosphoheteroporphyrin 60. Treatment of 1 equiv of Pd(dba)2 or Ni(cod)2 with P,S,N2-hybrid porphyrin 60 at room temperature afforded the metal complexes Pd60 and Ni60, respectively. However, Pt60 was prepared by reacting P,S,N2-hybrid porphyrin 60 with excess Pt(dba)2 in 1,2-dichlorobenzene under reflux conditions (Scheme 46). The 1H NMR of all three complexes suggested that there were no perturbation effects from the central metal ion on the π-electron circuit of the macrocycle. The X-ray structure of Pd60 revealed that the central Pd metal ion adopted a squareplanar geometry, but overall the metal complex showed a significantly distorted structure119 (Figure 3f). All the observed Pd−N and Pd−S bond distances suggested the +II oxidation state of the Pd ion. Both the coordinated phosphorus and sulfur atoms were deviated from the mean plane of the macrocyle.119 All the complexes showed a Soret-like band in the region of 375− 414 nm, but Q-bands were not observed. Thus, these observations indicated the ruffled and nonaromatic nature of the M60 macrocycle.

4.6. Metal Complexes of 21,23-Diselenaporphyrin (N2Se2 Core)

5. HETEROCORROLES Corroles are 18π aromatic tetrapyrrolic macrocyclic compounds, differ from porphyrins structurally in having one less meso-

Ravikanth and co-workers reported the first synthesis of Re(I) complex of 21,23-diselenaporphyrin Re(I)32, and this is the only 118

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Figure 3. (a) Re(I)4a. Reproduced with permission from ref 115. Copyright 2013 Royal Society of Chemistry. (b) Ru(II)30. Reproduced with permission from ref 116. Copyright 2001 American Chemical Society. (c) Re(I)30. Reproduced with permission from ref 117. Copyright 2014 American Chemical Society. (d) Ni(II)31. Reproduced with permission from ref 108. Copyright 1997 Wiley-VCH. (e) Re(I)32. Reproduced with permission from ref 118. Copyright 2016 American Chemical Society. (f) Pd60. Reproduced with permission from ref 119. Copyright 2008 American Chemical Society.

densation of bifuran dialdehyde 113 and β-substituted dipyrrolylmethane diacid 112a. The condensation resulted in the formation of the dioxacorrole 115a along with expanded porphyrin, the heterosapphyrin 116, which were separated by column chromatography (Scheme 47). Alternately, the dioxacorroles 115a−c were also obtained by acid-catalyzed condensation of diformyldifuryl sulfide 114 with appropriate dipyrrolylmethane diacids 112a−c under acid-catalyzed conditions (Scheme 47). Attempts to prepare the metal derivatives of 21,24-dioxacorroles 115a−c were unsuccessful. Chandrashekar and co-workers were the first to report the synthesis of meso-triaryl-22-oxacorroles128 118. The acidcatalyzed, oxidative coupling reaction between 16-oxatripyrrane 16c and appropriate meso-aryl-substituted dipyrromethanes 17

methine carbon, and also have reduced cavity because of direct α−α pyrrole linkage.120,121 Corroles are trianionic ligands and stabilize metals in higher oxidation states compared to dianionic porphyrins.122 Heterocorroles123 result from the replacement of one or two inner “N”s of normal corrole with heteroatoms such as “O” and “S”. The chemistry of tetrapyrrolic corroles has been extensively investigated over the years, and several review articles are available on these macrocycles.124,125 Interestingly, there are very few reports available on heterocorroles, and this section summarizes the syntheses, structure, metal coordination chemistry, and properties of heterocorroles. 5.1. Oxacorroles

Broadhurst et al.126,127 reported the synthesis of meso-free 21,24dioxacorrole 115a by acid-catalyzed, MacDonald-type con3292


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Scheme 43. Metal Complexes of 21,23-Dithiaporphyrin 30

Scheme 44. Metal Complexes of 21,23-Dioxaporphyrin 31

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Scheme 45. Re(I) Complex of 21,23-Diselenaporphyrin 32

Scheme 46. Synthesis of P,S,N2-Isophlorin-Metal Complexes M60

yielded meso-substituted 22-oxacorroles 118 as minor product (3−6% yield) along with major expanded porphyrin macrocycle, the meso-triaryl-25-oxasmaragdyrin 117, which were separated by column chromatography (Scheme 48). The formation and yields of 22-oxacorroles 118 were dependent on the meso-aryl group of dipyrromethane. Interestingly, no meso-triaryl-22thiacorrole 119a was observed when thiatripyrrane 16b was condensed with dipyrromethane 17b under similar reaction conditions. The X-ray analysis of meso-triaryl corrole 118a showed a nonplanar structure, and the heteroatom showed a small deviation from the mean plane (Figure 4a). The Cα−Cβ distances were found to be greater than the Cβ−Cβ distances, supporting the aromatic nature of the 22-oxacorrole 118a. The packing diagram also revealed a beautiful columnar structure because of π−π interactions between the macrocycles. The mesotriaryl-22-oxacorrole 118a showed tautomerism by rapid

Scheme 47. Synthesis of meso-Free Dioxacorroles 115

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Scheme 48. Synthesis of meso-Substituted 22-Oxacorrole 118 and Metal Derivatives of 118a

pyrrole of amine type. The other two coordination sites were occupied by carbonyl ligands.129 The 1H NMR spectrum of Ni(II) derivative Ni(II)118a did not show any signal in the negative region, indicating the coordination of all four core atoms to Ni(II) ion. The appearance of all the resonance signals in the range of 0−10 ppm indicated the diamagnetic nature of Ni(II)118a, which is in contrast to the paramagnetic nature exhibited by the nickel complex of monooxaporphyrin Ni(II)3a. The X-ray structural analysis of Ni(II) corrole Ni(II)118a revealed almost planar structure, and Ni(II) ion was coordinated to all four inner core atoms in distorted square-planar geometry (Figure 4c). The Ni(II) atom was placed only 0.008 Å above the mean plane of the corrole macrocycle with shorter Ni−N and Ni−O distances compared to corresponding Ni(II) oxaporphyrin Ni(II)3a, indicating the reduced core size of 22-oxacorrole.129 The absorption spectra of all these metal complexes Rh(I)118a, Cu(II)118a, Ni(II)118a, and Co(II)118a exhibited characteristic Soret type and Q-type bands in the 400−700 nm region. These metal complexes showed broad and splitted Soret band, indicating lower symmetry in solution.129 The extinction coefficient (ε) values of these metal complexes were reduced to ∼50% of 118a, presumably due to decreased π-electron conjugation upon metalation. The electrochemical studies revealed that the oxidation/reduction was only macrocyclebased and not metal-centered. Lee and co-workers130,131 reported a customized synthesis of meso-aryl monooxacorroles 123a/118c where an oxygen atom can be placed at a predesignated position. The key precursor for the corrole synthesis, p-tolyl-(furan-2-yl)-(pyrrol-2-yl)methane 120 was acylated at α-position of either pyrrole ring (121a) or

exchange of NH proton present on the bipyrrole unit, whereas the NH proton present on other pyrrole ring was static as inferred by NMR spectroscopy. The electronic absorption spectrum of free base oxacorrole 118a showed an intense Soret band at 411 nm and four Q-type bands in the range of 490−640 nm. These bands were hypsochromically shifted compared to normal tetrapyrrolic corroles. On protonation with TFA, the oxacorrole 118aH+ showed splitted Soret band due to diminished symmetry in solution. The meso-triaryl corrole 118a was strongly fluorescent with one strong emission band at 640 nm with a quantum yield of 0.88 and a singlet-state lifetime of 6.04 ns. The protonated compound 118aH+ showed bathochromically shifted emission band with a decrease in the quantum yield. The electrochemical study revealed that the 22oxacorroles 118a/118b were better electron acceptors but poor electron donors. The metal complexes of 22-oxacorrole129 such as Rh(I)118a, Cu(II)118a, Ni(II)118a, and Co(II)118a were prepared by reacting meso-triaryl-22-oxacorrole 118a with appropriate metal salt under standard conditions (Scheme 48). The 1H NMR spectrum of Rh(I)118a showed significant shielding experienced by certain pyrrole protons, one belonging to the bipyrrole unit and the second pyrrole, which is the one that is flanked by two meso-phenyl groups, indicating their involvement in coordination to Rh(I). The inner NH resonance from the uncoordinated pyrrole belonging to the bipyrrole unit was observed at −1.15 ppm. The crystal structure of Rh(I)118a (Figure 4b) showed that the Rh(I) ion was located above the corrole plane in a near square-planar geometry coordinated to only two nitrogen atoms, one from the bipyrrole unit (imine type) and the other from the 3295


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Figure 4. X-ray structures of compounds: (a) 118a. Reproduced with permission from ref 128. Copyright 1999 American Chemical Society. (b) Rh(I)118a. Reproduced with permission from ref 129. Copyright 2000 Wiley-VCH. (c) Ni(II)118a. Reproduced with permission from ref 129. Copyright 2000 Wiley-VCH. (d) 128. Reproduced with permission from ref 135. Copyright 2002 American Chemical Society. (e) 129H+. Reproduced with permission from ref 135. Copyright 2002 American Chemical Society. (f) 123b. Reproduced with permission from ref 137. Copyright 2013 American Chemical Society. (g) 133c. Reproduced with permission from ref 138. Copyright 2014 Wiley-VCH.

only one broad resonance signal for inner NH protons. However, the signal was upfield shifted in the case of 123a (−3.0 ppm) in comparison to 118c (−1.85 ppm). These spectroscopic observations led to a speculation that the corrole 123a containing “O−N” linkage must be less flexible and more resonancestabilized compared to the corrole 118c with “N−N” linkage.130,131 Chandrashekar and co-workers132 reported the synthesis of mono-meso-free 22-oxacorroles 124a−c by a [3 + 1] method. The acid-catalyzed MacDonald-type condensation of appropriate 16-oxatripyrranes 16c with pyrrole-2-carboxaldehyde

the furan ring (122a) by tuning the reaction conditions followed by reduction to obtain corresponding alcohols 121b and 122b, respectively. The condensation of 121b and 122b with p-tolyl dipyrromethane 17e under Lewis acid-catalyzed conditions followed by DDQ oxidation yielded 21-oxacorrole 123a and 22-oxacorrole 118c containing either “O−N” or “N−N” α−α direct linkages, respectively (Scheme 49). The 22-oxacorrole 118c showed bathochromically shifted Soret band compared to 21-oxacorrole 123a, and also the β-pyrrole proton resonances of 21-oxacorrole 123a were downfield shifted compared to 22oxacorrole 118c. Both the oxacorroles 123a and 118c exhibited 3296


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presumably due to the large distance separation (∼8.5 Å) between them. Dimer Cu126 also showed promising results in photolytic cleavage of DNA, where it selectively cleaved nucleic acids without affecting the proteins.133 The first hyperpolarizability (β) measurements carried out by hyper-Rayleigh scattering (HRS) method showed that the β values were doubled in the case of dimer 126 in comparison to monomer 124b, highlighting the meso−meso-linked porphyrin dyad’s importance as useful nonlinear optics (NLO) material. In a subsequent report, Chandrashekar and co-workers134 studied the effect of acid (protic and Lewis) and solvent on the formation of 22-oxacorroles 124a−c. Although 22-oxacorrole formation was observed with acids such as p-TsOH, chlorosulfonic acid, methanesulfonic acid, and TFA, but Lewis acids such as BF3·OEt2, SnCl4, and FeCl3 failed to yield 22oxacorroles. Furthermore, Chandrashekar and co-workers134 also showed two other methods to synthesize mono-meso-free 22-oxacorrole 124d containing two meso-t-butylphenyl groups (Scheme 51). In method 1, the 16-oxatripyrrane 16c was treated with pyrrole and paraformaldehyde in the presence of TFA, followed by oxidation with chloranil to obtain mono-meso-free 22-oxacorrole 124d. In method 2, condensation of 16c and 2(hydroxyphenyl methyl)pyrrole was carried out under identical reaction conditions to obtain 124d. Latos-Grażyński and co-workers135 reported the synthesis of 5-phenyl-10,15-bis(p-tolyl)-21,23-dioxacorrole 128 by condensation reaction of 2,5-bis(arylhydroxymethyl)furan 1b, 2-phenylhydroxymethylfuran 6b, and pyrrole in the presence of BF3·OEt2 in dichloromethane followed by oxidation with p-chloranil (Scheme 52). The aromatic character of this dioxacorrole 128 was evident from its NMR spectroscopic features such as downfield resonances for β-CH of furan and appearance of core NH resonance in upfield region. The inner NH showed rapid tautomerism by exchanging sites between two structurally unequivalent nitrogen atoms. The low-temperature NMR studies carried out on dioxacorrole 128 revealed that the dynamics of NH exchange was rapid even at 188 K. Upon protonation by TFA, the inner NH tautomerism was arrested, and two resonances at −1.46 and −1.98 ppm appeared for innercore NH protons in the resulting protonated derivative 128H+. The single-crystal X-ray analysis of the cation of dioxacorrole 128 and ZnCl42− showed a clamshell-like arrangement where the dihedral angle between the two dioxacorrole planes was found to be 61.5° (Figure 4d). The furan moieties displayed longer and shorter bond lengths for Cα−Cβ and Cβ−Cβ, respectively, when compared to free furan, indicating the alteration in π delocalization of furan rings in dioxacorrole 128. Dioxacorrole 128 exhibited markedly splitted Soret bands in the 395−425 nm region, reflecting reduced symmetry due to the presence of two oxygen atoms in trans-position along with a series of Q-bands in the 480−640 nm range. Further, when 3-phenylhydroxymethylfuran 127 was condensed with 1b and pyrrole instead of 2phenylhydroxymethylfuran 6b under identical reaction conditions, the formation of an unusual isomer of dioxacorrole135 containing protruded furan ring 129 was formed (Scheme 52). Relatively upfield-shifted β-H resonances with respect to 128 and markedly downfield-shifted resonance of NH (17.71 ppm) convincingly demonstrated the nonaromatic character of 129. The X-ray crystallographic analysis of protonated corrole 129H+ showed a puckered structure arising due to the contraction of the internal ring by one carbon atom compared to the regular corrole core (Figure 4e). The protonated species 129H+ acted as an anion receptor by exhibiting N−H···Cl hydrogen bonds by two

Scheme 49. Synthesis of meso-Substituted 21-Oxacorrole 123a and 22-Oxacorrole 118c

followed by oxidation with chloranil yielded mono-meso-free 22oxacorroles 124a−c, respectively (Scheme 50). The 1H NMR spectrum of 124c showed a sharp singlet for meso-hydrogen at 9.6 ppm and exhibited similar NH tautomerism as in the case of 118a, which was subsequently arrested upon addition of TFA at low temperature. Corrole 124a, when treated with n-butyllithium at 183 K followed by oxidation with DDQ at room temperature, yielded meso-butyl oxacorrole 125 (Scheme 50). The first examples of meso−meso-linked oxacorrole dimer133 126 were prepared by AgOTf- or FeCl3-catalyzed coupling of 22oxacorrole 124b (Scheme 50). The coupling reaction proceeded via free radical mechanism as revealed by EPR technique. The disappearance of meso-CH signal and retention of inner NH resonance in 1H NMR spectrum provided strong evidence for meso−meso-linked dimer 126. The absorption spectrum showed that dimer 126 exhibited red-shifted bands compared to monomer 124b, and the red-shift was more prominent in protonated dimer 126H+, indicating increased π-delocalization. The electrochemical studies showed that dimer 126 exhibited easier oxidation and decreased HOMO−LUMO gap compared to monomer 124b. Dimer 126 showed markedly red-shifted fluorescence and doubled quantum yield values compared to monomer 124b. The geometry-optimized structure for dimer 126 showed that the two corrole units were not coplanar but were tilted with respect to each other at an angle of ∼65°. The Cu and Ni derivatives of dimers Cu126 and Ni126 were synthesized by treating 126 with corresponding metal(II) acetates (Scheme 50). The EPR spectrum recorded for dimer Cu126 showed that the two Cu(II) centers were in slightly different environments and were not interacting with each other, 3297


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Scheme 50. Synthesis of Mono-meso-Free 22-Oxacorrole 124, meso−meso-Linked Oxacorrole Dimer 126, and Its Metal Derivatives

(21,23-O2Cor)NiIICl complex135 Ni128. The 1H NMR spectral pattern showed very much downfielded resonances for β-H of pyrrole and furan ring resonances of Ni128, indicating its paramagnetic behavior. A typical “corrole-like” absorption spectral pattern was displayed by Ni128. However, dioxacorrole 129 did not form Ni complex under similar reaction conditions. Chandrashekar and co-workers136 reported the synthesis of covalently linked ferrocene−oxacorrole conjugates 131a−c as minor products along with oxasmaragdyrin derivative 130 by acid-catalyzed reaction of 16-oxatripyrranes 16c with ferrocenyl dipyrromethane 17f followed by oxidation (Scheme 53). Because of the presence of covalently linked ferrocene moiety at the meso-position, the extinction coefficient (ε) value of the oxacorrole conjugates 131a−c was considerably reduced, supporting the electronic communication between ferrocene and macrocyclic π-electronic system. Also, both the Soret and Qbands of 131a−c were bathochromically shifted compared to those of oxacorrole 118a. The electrochemical studies revealed that the HOMO−LUMO gap in 131a−c was significantly reduced compared to that for oxacorrole 118a, which is in agreement with the absorption spectroscopy. The ferrocenyl oxidation in 131a−c was shifted to the more positive side as compared to free ferrocene, and the corrole ring-centered oxidations and reductions were shifted, respectively, in positive and negative directions, further supporting the electronic

Scheme 51. Two Different Routes for the Synthesis of Monomeso-Free 22-Oxacorrole 124d

NH groups. The Cl− anion was involved in two intramolecular (N)H···Cl and two intermolecular (C)H···Cl hydrogen bonds, forming a tetrafurcate system. The crystallographic data also revealed that the protruding furan preserved all the features of isolated furan, whereas the second furan moiety in the macrocycle 129H+ underwent perturbation in π-delocalization. The absorption spectroscopic features of 129 and its protonated form 129H+ were different from 128 and 128H+. The absorption spectrum of 129 exhibited a splitted Soret band at 375 and 409 nm along with a broad band at 753 nm, which was remarkably red-shifted to 962 nm in compound 129H+. The 21,23-dioxacorrole 128 was treated with nickel(II) chloride in boiling DMF to obtain five-coordinate high-spin 3298


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Scheme 52. Synthesis of meso-Substituted 21,23Dioxacorrole 128 and Its Isomer 129

Scheme 54. Unusual Synthesis of meso-Substituted 21Oxacorrole 123b from 21-Oxaporphyrin 3a

communication between the ferrocene unit and the macrocyclic π-system. Ravikanth and co-workers137 reported an unusual formation of 5,10,15-triphenyl-21-oxacorrole 123b containing a pyrrole− furan direct α−α linkage while performing phosphorus insertion reaction on 21-oxaporphyrin 3a (Scheme 54). Single-crystal Xray analysis showed that the 21-oxacorrole 123b crystallized in a monoclinic space group P21/c when compared to 22-oxacorrole 118a, which adopted an orthorhombic space group P212121 (Figure 4f). The 21-oxacorrole 123b attained a near-planar geometry with slight deviations of furan and three pyrrole rings from the mean plane, which were found to be 5.69°, 1.98°, 4.46°, and 5.50°, respectively. The electrochemical studies revealed that 123b exhibited one reversible and one irreversible peak each for oxidation and reduction. In general, the 21-oxacorrole 123b showed difficult redox behavior in comparison to 21oxaporphyrin 3a. The 21-oxacorrole 123b was brightly fluorescent with a quantum yield in the range of 0.3−0.4. The unsymmetrically substituted 22-oxacorrole138 132 possessing two aryl groups and one pyrrole at meso-position were synthesized by TFA-catalyzed condensation of meso-aryl dipyrromethane 17e with meso-free 16-oxatripyrrane 16g followed by DDQ oxidation (Scheme 55). To understand the

reactivity of the pyrrole moiety present on one of the mesopositions, the 22-oxacorrole 132 was subjected to bromination, formylation, and nitration reactions to obtain the corresponding functionalized 22-oxacorroles138 133a−c (Scheme 55). The Xray structural analysis of 133c showed that the meso-pyrrole group was perpendicular to the plane of the macrocycle (Figure 4g). The furan and pyrrole rings of the macrocycle 133c deviated slightly from the mean plane defined by three meso-carbon atoms, three pyrrole rings, and a furan ring to overcome the strain exhibited by two hydrogen atoms present in the core. The absorption spectra of corroles 132−133 exhibited a Soret band and four Q-bands in the range 410−650 nm. The presence of a meso-pyrrole group in these corroles did not alter the absorption peak maxima to a greater extent in comparison to meso-triphenyl 22-oxacorrole 118a, indicating the restricted extension of πdelocalization on the meso-pyrrole group, which has perpendicular alignment to the plane of the macrocycle. The electrochemical studies of 132−133 showed one or two reversible/ quasi-reversible reductions and three corrole ring-centered irreversible oxidation peaks. Most of these 22-oxacorroles 132−133 were fluorescent with decent quantum yields. The corrole 132 with quantum yield 0.46 was subjected to cellular internalization studies on HepG2 cells followed by confocal laser scanning microscope imaging to understand its biocompatibility. The cellular structure was not altered and had an intact nucleus even in the presence of a high concentration (10 μM) of 132, indicating its potential use as a nontoxic fluorescent probe for understanding intracellular trafficking.138 In the subsequent paper, Ravikanth and co-workers139 reported the synthesis of boron dipyrromethene (BODIPY)bridged 22-oxacorrole dyad 136 by using meso-pyrrolyl-22oxacorrole 134 as key synthon (Scheme 56). In a slightly modified synthetic strategy, the acid-catalyzed condensation of 16-oxatripyrrane 16g and meso-free dipyrromethane 17g followed by DDQ oxidation yielded 22-oxacorrole 134 containing one meso-free position. The reaction of 134 with ptolualdehyde in the presence of a catalytic amount of BF3·OEt2 resulted in the formation of dipyrromethane-bridged dyad 135.

Scheme 53. Synthesis of Mono-meso-Ferrocenyl Oxacorroles 131

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Scheme 55. Synthesis of Mono-meso-Pyrrolyl-Substituted 22-Oxacorrole 132 and Its Pyrrole-Functionalized Analogues 133

119b displayed a 1H NMR spectral pattern typical of aromatic macrocycles and exhibited resonance signals that were relatively upfield shifted compared to monothiaporphyrin 2a. A rapid tautomerism of NH protons present on the bipyrrolic unit of 119b was observed even at −40 °C on the NMR timescale. However, upon protonation at −40 °C, the tautomerism was arrested and three distinct signals were seen in the upfield region for inner-core NH protons. The quantum-chemical calculations performed by using density functional theory (DFT) showed that the structure of 22-thiacorrole 119b was considerably distorted. The absorption spectrum of 22-thiacorrole 119b was different from 22-oxacorrole 118a as it exhibited one broad Soret-like band at ∼435 nm and one broad Q-band-like peak at ∼620 nm. When compared to 22-oxacorrole 118a, the 22thiacorrole 119b was weakly florescent. The electrochemical studies indicated that 22-thiacorrole 119b was easy to reduce compared to thiaporphyrins such as 2a, indicating the electrondeficient nature of 22-thiacorrole140 119b.

The dipyrromethene-bridged dyad 135 on oxidation with DDQ followed by complexation with BF3·OEt2 yielded BODIPYbridged dyad 136 (Scheme 56). The absorption spectrum of dyad 136 showed features of both BODIPY and oxacorrole units. The electrochemical studies revealed that dyad 136 was stable under redox conditions. Dyad 136 was weakly fluorescent, partly due to the presence of a low-lying charge-transfer state that facilitates nonradiative decay processes. When dyad 136 was excited at 505 nm, where the BODIPY unit absorbs predominantly, the major emission was observed at 639 nm, corresponding to the oxacorrole unit and indicating the possibility of energy transfer from the BODIPY unit to the oxacorrole units. 5.2. Thiacorroles

The first example of meso-substituted stable 22-thiacorrole140 119b was prepared in low yield by condensing thiophene monool 6a with 4-nitrobenzaldehyde and pyrrole in refluxing propionic acid followed by chromatographic purification (Scheme 57). The 22-thiacorrole formation was observed only when 4-nitrobenzaldehyde was used for condensation, but when other aromatic aldehydes such as p-tolualdehyde and phydroxybenzaldehyde were used, only the corresponding meso-substituted monothiaporphyrin formation was observed without even a trace of thiacorrole formation. The 22-thiacorrole

6. HETEROCARBAPORPHYRINOIDS Heterocarbaporphyrinoids141 resulted from the replacement of one or two pyrrole nitrogens of azacarbaporphyrinoid with other heteroatoms such as S, O, Se, etc. The carbaporphyrinoids chemistry142,143 is unique in terms of their metal coordination 3300


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Scheme 56. Synthesis of BODIPY-Bridged 22-Oxacorrole Dyad 136

Scheme 57. Synthesis of meso-Substituted 22-Thiacorrole 119b

established [3 + 1] strategy used for the synthesis of the azacarbaporphyrins. Lash and co-workers144 attempted to synthesize heteroazuliporphyrins such as thiaazuliporphyrin, selenaazuliporphyrin, and oxaazuliporphyrin by using azulitripyrrane in which the central azulene group was flanked by two pyrroles as key precursor.145 Azulitripyrrane 137a was condensed with 2,5-diformylthiophene 138a or 2,5-diformylseleneophene 138b, respectively, in the presence of TFA followed by oxidation with FeCl3 and purification by column chromatog-

chemistry because these macrocycles form organometallic complexes under mild reaction conditions. The azacarbaporphyrinoid chemistry142 was extensively developed over the years, and their unusual coordination properties with a range of metals and physicochemical properties were studied to a greater extent. On the other hand, the chemistry of heterocarbaporphyrinoids is still at the budding stage. The first examples of carbaporphyrins containing heteroatoms such as S, O, and Se in addition to pyrrole nitrogen atoms were synthesized by following the well3301


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raphy, affording opp-thiaazuliporphyrin 139 and opp-selenaazuliporphyrin 140 in which the heterocycle ring and carbacycle ring were opposite to each other (Scheme 58). However, azulitripyrrane 137a, upon condensation with 2,5-diformylfuran 138c under the same reaction conditions, gave the mixture of three fully aromatic oxacarbaporphyrins 143a−c in which the seven-membered ring was oxidatively contracted to the sixmembered ring. The conversion of thiaazuliporphyrin 139 and selenaazuliporphyrin 140 to corresponding heterocarbaporphyrinoids such as benzothiacarbaporphyrin 141a−b and benzoselenacarbaporphyrin 142a−b, respectively, by oxidative contraction of the seven-membered ring of heteroazuliporphyrin to the six-membered ring of heterocarbaporphyrinoid was also carried out under different reaction conditions.144 Thiaazuliporphyrin 139, upon treatment with tert-butyl hydroperoxide (t-BuOOH) in the presence of KOH, gave thiacarbaporphyrin 141a, whereas when it was treated with t-BuOOH in the presence of t-BuOK, an inseparable mixture of thiacarbaporphyrin 141a and aldehydesubstituted thiacarbaporphyrin 141b was formed (Scheme 58). Similarly, the selenaazuliporphyrin 140 with t-BuOK gave selenacarbaporphyrin 142a, whereas on treatment with KOH it gave a mixture of selenacarbaporphyrin 142a and aldehydesubstituted selenacarbaporphyrin 142b, which were separated by flash column chromatography (Scheme 58). Recently, Stateman and Lash reported146 an alternate synthesis for heterocarbaporphyrins 145−147 using the carbatripyrrane 144 as key precursor, as shown in Scheme 59. Carbatripyrrane 144, which was

Scheme 58. Synthesis of Azuliporphyrin Analogues 139−143

Scheme 59. (i) Synthesis of Diphenylheterocarbaporphyrins 145−147; (ii) Synthesis of Benzocarbaporphyrins 141−143 and Metal Derivatives M143

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Figure 5. X-ray structures of compounds: (a) Pd143a. Reproduced with permission from ref 147. Copyright 2004 American Chemical Society. (b) 151. Reproduced with permission from ref 148. Copyright 2002 Royal Society of Chemistry. (c) 164. Reproduced with permission from ref 153. Copyright 2005 American Chemical Society. (d) Pd167. Reproduced with permission from ref 154. Copyright 2010 American Chemical Society. (e) 170. Reproduced with permission from ref 155. Copyright 2014 Wiley-VCH.

Scheme 60. Synthesis of 10,15-meso-Aryl Azuliporphyrins 151−152

presence of BF3·OEt2 in CH2Cl2 for 30 min followed by DDQ oxidation, affording meso-diphenyl heterocarbaporphyrins 145−

synthesized over a sequence of steps, was condensed with appropriate heterocycle 2,5-dicarbinol 1a−c (Scheme 59i) in the 3303


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Scheme 61. Synthesis of Hetero Analogues of Pyri- and Benziporphyrins

Scheme 62. Synthesis of Heterobenziporphyrins 160−161

147 in 25% yield. Lash and co-workers147 synthesized heterocarbaporphyrins such as benzothiacarbaporphyrin 141a and benzooxacarbaporphyrin 143a also by condensing the appropriate β-substituted tripyrrane 148a/148b with indene1,3-dicarboxaldehyde 149 in CH2Cl2 in the presence of TFA followed by oxidation with DDQ (Scheme 59(ii)). Thiaazuliporphyrin 139 and selenaazuliporphyrin 140 exhibited borderline aromatic properties that were enhanced on protonation as confirmed by detailed UV−visible and NMR studies. However, the heterocarbaporphyrinoids 141a/143a/ 145−147 were completely aromatic like carbaporphyrinoids as judged from their spectral studies. Furthermore, the hetero-

carbaporphyrins such as oxacarbaporphyrin 143a showed potential to form novel organometallic complexes. The organometallic Ni(II), Pd(II), and Pt(II) complexes of oxabenzocarbaporphyrin Ni143a/Pd143a/Pt143a were prepared under standard metalation conditions, and the studies indicated that all three metal complexes of benzocarbaporphyrin retained their aromatic character, although the Pd(II) derivative Pd143a appeared to possess a slightly large diatropic ring current.147 The X-ray structure of Pd143a exhibited a planar macrocycle, and Pd(II) was bonded to two pyrrolic “N”s, furan “O”, and indene “C”. The dihedral angles of the component pyrrole, furan, and indene rings relative to the mean [18] annulene plane were >2.1° (Figure 5a). 3304


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Scheme 63. Synthesis of Tropone-Fused Carbaporphyrinoids

Scheme 64. Synthesis of Dithiadiazuliporphyrinogen 165 and Its Oxidized Forms

Chandrashekar and co-workers148 also synthesized thiaazuliporphyrin 151 and selenaazuliporphyrin 152 by [3 + 1] approach (Scheme 60). Condensation of appropriate heterotripyrrane 16b/16d with 1,3-azulene dicarboxaldehyde 150 under mild acid-catalyzed conditions in CH2Cl2 followed by DDQ oxidation afforded heteroazuliporphyrins 151/152. Their studies also

indicated that the heteroazuliporphyrins 151−152 exhibit borderline aromaticity, and the aromaticity was enhanced in their protonated derivatives. The X-ray structure obtained for thiaazulicarbaporphyrin148 151 indicated that the macrocycle was completely planar and the azulene moiety was in the plane defined by four meso-carbons. The aromatic nature was evident 3305


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Scheme 65. Synthesis of adj-Oxacarbaporphyrin 167 and Its Palladium Complex Pd167

Scheme 66. Synthesis of 21-Carba-23-thiaporphyrin 171 and Its Palladium Complex Pd171

from the smaller Cβ−Cβ distances than Cα−Cβ distances of pyrroles and thiophene in thiaazulicarbaporphyrin macrocycle 151 (Figure 5b). Lash and co-workers149 synthesized a series of β-substituted and meso-unsubstituted oxybenziheterocarbaporphyrins 155a/

156a and oxypyriheterocarbaporphyrins 155b/156b by condensing the appropriate modified β-substituted tripyrrane 148a/ 148b with 5-formylsalicylaldehyde 153 and 3-hydroxy-2,6pyridinedicarboxaldehyde 154, respectively, in CH2Cl2 in the presence of TFA followed by oxidation with DDQ (Scheme 61). 3306


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Scheme 67. (i) Synthesis of N-Confused 21-Thiaporphyrins 173 and 21-Oxaporphyrins 174; (ii) Different Synthetic Approaches for the Preparation of N-Confused Heteroporphyrins 178

159 (Scheme 62). The condensation of tripyrrane 159 with 2,5diformyl thiophene 138a or 2,5-diformyl furan 138c, respectively, in the presence of TFA followed by oxidation with FeCl3 gave correspondining carbaporphyrinoids 160a and 161a, respectively. The heterocarbaporphyrinoids 160a and 161a on further oxidation with PhI(OCOCF3)2 gave the aromatic heterocarbaporphyrinoids 160b and 161b, respectively. The metal derivates of these macrocycles were not reported. The tropone-fused thiacarbaporphyrin 162 was synthesized by Lash et al.152 as shown in Scheme 63. Condensation of tripyrrane 137b with 2,5-thiophenedicarboxaldehyde 138a in the presence of TFA followed by oxidation with FeCl3 resulted in the formation of tropone-fused thiacarbaporphyrin 162a along with methoxythiaazuliporphyrin 163. The tropone-fused thiacarbaporphyrin 162a exhibited strong diatropic characteristics, and

The absorption and NMR studies indicated that oxybenziheterocarbaporphyrins 155a/156a and oxypyriheterocarbaporphyrins 155b/156b are aromatic in nature. Chandrashekar and coworkers150 also synthesized the thia- and oxybenziporphyrins 157−158 by [3 + 1] acid-catalyzed condensation of 5-formyl salicylaldehyde 153 with corresponding thia- and oxatripyrrane 16b/16c under mild acid-catalyzed conditions. The oxybenziheteroporphyrins 157−158 are aromatic in nature as judged from absorption and NMR studies. The Pd(II) derivative of oxybenzioxaporphyrin Pd158 was prepared by reacting the free base 158 with Pd(OAc)2 in DMF, and the spectral studies of Pd158 indicated the retention of aromatic character in Pd158 (Scheme 61). Miyake and Lash151 reported the synthesis of a new class of heteroatom-substituted benziporphyrins using novel tripyrranes 3307


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delocalization pathway, and the macrocycle was aromatic in nature. Latos-Grażyński and co-workers155 recently reported the synthesis of true carbathiaporphyrin, 170/171, the analogue of 21-thiaporphyrin, which was produced by replacing one pyrrole unit of monothiaporphyrin by a cyclopentadienyl moiety. The 21-carba-23-thiaporphyrin 170 was prepared by condensing the carba analogue of an isomeric mixture of tripyrrane 168a−b with 2,5-bis(mesitylhydroxymethyl)thiophene 1a in CHCl 3 / C2H5OH in the presence of BF3·OEt2 followed by oxidation with p-chloranil. This resulted in the formation of thiacarbachlorin 170 instead of thiacarbaporphyrin 171 (Scheme 66). Thus, the initial condensation of an isomeric mixture of tripyrrane and thia dicarbinol resulted in the formation of an isomeric mixture of porphyrinogens 169, which, on further oxidation with p-chloranil, gave thiacarbachlorin155 170. Thiacarbachlorin 170, on further oxidation with 1 equiv of DDQ, afforded 21-carba-23-thiaporphyrin 171 in 25% yield (Scheme 66). The X-ray structure of thiacarbachlorin 170 revealed the aromatic nature of the macrocycle, and the cyclopentadienyl moiety was not involved in the macrocyclic delocalization (Figure 5e). The absorption spectra of thiacarbachlorin 170 and thiacarbaporphyrin 171 showed typical Soret and Q-bands like aromatic porphyrinoids. The NMR spectra of thiacarbachlorin 170 and thiacarbaporphyrin 171 displayed resonances at positions consistent with their aromatic structure. Thus, thiacarbachlorin 170 and thiacarbaporphyrin 171 macrocycles retain their macrocyclic aromaticity with an 18π-electron delocalization pathway and exhibited the basic features of aromatic carbaporphyrinoids. The palladium(II) complexes of thiacarbachlorin Pd170 and thiacarbaporphyrin Pd171 were prepared by treating the appropriate macrocycle with Pd(OAc)2 in toluene, and their formation was confirmed by changes in absorption spectra and high-resolution mass (Scheme 66). The DFT calculated structures of palladium complexes Pd170/ Pd171 revealed that the macrocycles were distorted to accommodate Pd(II) ion and the thiophene ring was tilted with respect to the macrocyclic plane to allow the pyramidal sideon coordination of the palladium(II) ion. Thus, thiacarbachlorin 170 and thiacarbaporphyrin 171 were potential ligands to form interesting organometallic complexes.155

the absorption spectrum showed multiple bands in the Soret region like thiaazuliporphyrin144 139. The tropone-fused thiacarbaporphyrin 162a was aromatic in nature, and the aromatic character was retained upon diprotonation. The protonation studies on 162a also indicated that initial protonation occurs on the carbonyl moiety to generate 162b rather than on the expected core pyrrolenine nitrogen atom (Scheme 63). Latos-Grażyński and co-workers153 reported the synthesis of hybrid of the azulene π-system and heteroporphyrin-like macrocyclic skeleton system called dithiadiazuliporphyrin 164 with C2S2 core. This is the first example of a non-nitrogenous heterocarbaporphyrinoid macrocycle.153 Condensation of azulene with thia dicarbinol 1a under BF3·OEt2-catalyzed conditions afforded C2S2 porphyrinogen 164 as a mixture of stereoisomers (Scheme 64). The crystal structure obtained for one of the isomers of 164 showed a chairlike conformation; the thiophene rings were coplanar with the dihedral angle between each of the azulene moieties, and the C4 plane defined by four meso-carbons was 71°. Furthermore, the bond lengths in the azulene and thiophene fragments of C2S2 porphyrinogen 164 were similar to those in independent moieties. The C2S2 porphyrinogen 164 was subjected to oxidation with various amounts of DDQ, which resulted in the formation of mixtures of the C2S2 porphyrin analogue 165, its radical cation 165.+, and dication 1652+ (Scheme 64). These three species constitute a multielectron system and form easily under simple chemical and redox conditions. The X-ray structure of 164 revealed that the macrocycle adopts a saddle conformation with azulene and the thiophene rings tilted in opposite directions. There was a significant alteration in bond lengths in thiophene fragment but no alteration in conjugation of azulene moieties, suggesting that the azulene moieties were not conjugated with the macrocycle in 164 (Figure 5c). Lash and co-workers154 reported the adj-heterocarbaporphyrinoid such as 22-oxa-21-carbaporphyrin 167 and its organometallic Pd(II) complex Pd167 as shown in Scheme 65. The required furan-derived fulvene dialdehyde 166 was prepared over a sequence of steps and condensed with dipyrrylmethane dicarboxylic acid 112b in the presence of TFA to give adjoxacarbaporphyrin 167 in high yield. Under the same reaction conditions, the thiophene-derived fulvene dialdehyde failed to form a macrocycle.154 Oxacarbaporphyrin 167 exists in two tautomeric forms 167a and 167b that cannot be distinguished by standard spectroscopic techniques. Oxacarbaporphyrin 167 showed typical Soret band and Q-bands in absorption spectrum and diatropic ring-current effect in NMR, supporting its aromatic nature. The X-ray structure of oxacarbaporphyrin 167 revealed that the macrocycle was highly planar and the NH was at position 23, indicating that tautomer 167a was relatively more stable than tautomer 167b. The various bond lengths of macrocycle were inconsistent with the presence of a significant 18π electron delocalization pathway, and the macrocycle was the perfect aromatic system. The oxacarbaporphyrin 167 readily converts into mono- and dicationic species; the NMR study showed that the second protonation occurs at the internal indene carbon, and it retains strongly diatropic characteristics. Oxacarbaporphyrin 167 reacted with Pd(OAc)2 in DMF to form the corresponding organometallic Pd(II) complex Pd167 (Scheme 65). The X-ray structure154 of Pd167 revealed that the Pd(II) complex has a nearly planar macrocyclic core and is coordinated to all four inner-core donor atoms (Figure 5d). The other structural data was also consistent with the presence of significant 18π electron

7. HETEROATOM-SUBSTITUTED CONFUSED PORPHYRINS In 1994, Latos-Grażyński and co-workers156 and Furuta et al.157 independently reported N-confused porphyrin 5,10,15,20tetraaryl-2-aza-21-carbaporphyrin, which is a porphyrin isomer containing the inner CH and external and internal nitrogen atoms. The presence of inner CH in N-confused porphyrin helped in the synthesis of several peculiar organometallic complexes by forming metal−carbon bonds easily.158 Furthermore, the presence of external and internal nitrogens along with inner CH also helped in the formation of inner and outer coordination complexes and supramolecular structures.159 Over the years, the chemistry of N-confused porphyrins has grown rapidly, and several new types of confused porphyrins were prepared and their metal-coordination properties were exploited.158,159 Core modification of N-confused porphyrin by replacing one or two pyrrole nitrogens by other heteroatoms such as O, S, and Se results in the formation of core-modified confused porphyrins. The interest in such molecules lies in the fact that they may form unusual metal complexes involving weak metal−heteroatom interactions, leading to the isolation of some 3308


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the existence of three isomers for N-confused 21-selenaporphyrin 176 based on detailed NMR studies. The X-ray structure163 obtained for N-confused 21-selenaporphyrin 176 exhibited saddle distortion (Figure 6a). The Se−Cα, Cα−Cβ, and Cβ−Cβ distances of N-confused 21-selenaporphyrin 176 were in the same range as those of tetraaryl-21-selenaporphyrin 32, supporting the perseverance of the aromatic character of the macrocycle on inversion of one of the pyrrole rings in Nconfused 21-selenaporphyrin 176. The same research group164 also obtained the N-confused 21,23-dithiaporphyrin 177 in 4.7% yield as one of the products by acid-catalyzed condensation of 2,5-thiadicarbinol with pyrrole (Scheme 67(ii)). Chandrashekar and co-workers165 prepared N-confused 21thiaporphyrin 178a, N-confused-21-oxaporphyrin 178b, and Nconfused 21-selenaporphyrin 178c having a confused pyrrole opposite to the respective heterocycle by using different precursors. Tripyrrane 175 containing the middle N-confused pyrrole ring was condensed with corresponding heterodicarbinol 1a/1b/1c under mild acid-catalyzed porphyrin-forming conditions to afford the desired N-confused 21-heteroporphyrin 178a−c in 19−32% yields (Scheme 67(ii)). This is a better strategy to prepare the N-confused 21-heteroporphyrins in high yields using the tripyrrane, which has a preformed confused pyrrole ring as the key precursor. They also observed the three tautomers for N-confused 21-heteroporphyrins 178a−c. The Xray structure of N-confused 21-thiaporphyrin165 178a showed the formation of a cyclophane dimer (Figure 6b) in the unit cell because of the presence of strong hydrogen-bonding interaction between the inner NH of one molecule and the outer nitrogen of the other molecule, which in turn supports that the N-confused 21-thiaporphyrin 178a prefers to exist in tautomer I form at room temperature (Chart 6).

unusual complexes with interesting properties. There are few confused heteroporphyrins reported in the literature. In heteroporphyrins, the confusion can occur with the pyrrole ring or with the other heterocycle, such as thiophene, furan, and selenophene. Thus, the confused heteroporphyrins were subclassified into N-confused heteroporphyrins and heteroatom-confused heteroporphyrins. 7.1. N-Confused Heteroporphyrins

The first N-confused 21-thiaporphyrins 173 and 21-oxaporphyrins 174 were prepared by Lee and co-workers160−162 by condensing 2,4-bis(α-hydroxy-α-phenylmethyl)pyrrole 172a or its N-alkyl derivative 172b−c with 16-thiatripyrrane 16b or 16oxatripyrrane 16c under standard BF3·OEt2-catalyzed conditions and afforded N-confused 21-thiaporphyrins 173 and N-confused 21-oxaporphyrins 174 (Scheme 67(i)). In these cases, the confused pyrrole was in trans position to the heterocycle. The NMR studies suggested the possible existence of three tautomers depending on the location of the inner NH proton for these Nconfused heteroporphyrins (Chart 6). In tautomers I and II, the hydrogen was located on either of the inner pyrrole nitrogen atoms, whereas in tautomer III, the proton was located on the nitrogen of the N-confused ring. The NMR studies indicated that tautomer I was the only stable form for 21-oxaporphyrin, whereas for 21-thiaporphyrin, tautomer III was the major isomer and tautomer I was the minor isomer at room temperature. However, the studies at 223 K suggested that 21-thiaporphyrin existed in all three (I, II, and III) tautomeric forms.162 Interestingly, the N-alkyl-substituted N-confused porphyrins existed only as tautomer III because the alkylation prevented it from alteration of tautomer form by freezing the amino-to-imino conversion. The crystal structure obtained for 173a indicated the nonplanarity of the macrocycle in a ruffled conformation where the meso-carbons were formed alternately above and below the mean plane defined by four meso-carbon atoms and the N-confused pyrrole ring, and one of the adjacent pyrrole rings showed the maximum deviation. The presence of a large sulfur atom inside the porphyrin core increased the repulsion between the inner NH and the sulfur atom, resulting in the deviation of the pyrrole away from the mean plane. The alteration of πdelocalization was evident in the significant changes in Cα−X, Cα−Cβ, and Cβ−Cβ distances relative to free thiophene and pyrrole units. Latos-Grażyński and co-workers163 synthesized N-confused 21-selenaporphyrin 176 in which the confused pyrrole ring was adjacent to the selenophene ring (Scheme 67(ii)). The Nconfused 21-selenaporphyrin 176 was obtained in 1% yield as a side product in a typical acid-catalyzed condensation of 2,5selenadicarbinol 1c with azatripyrrane 16a. They also proposed

7.2. Heteroatom-Confused Heteroporphyrins

The S-confused porphyrin, 5,10,15,20-tetraphenyl-2-thia-21carbaporphyrin 180, was synthesized for the first time by Latos-Graży ń s ki and co-workers 166,167 using 2,4-bis(phenylhydroxymethyl)thiophene 179a as key synthon. Condensation of 2,4-thiadicarbinol 179a with benzaldehyde and pyrrole via one-pot, two-step reaction or by condensing the 2,4thiacarbinol 179a with 5,10-diphenyltripyrrane 16a followed by oxidation with DDQ or p-chloranil gave S-confused porphyrin 180 (Scheme 68). In the presence of excess oxidant, a new compound 181 was formed. The S-confused porphyrin 180 showed borderline aromaticity as evident in the 1H NMR resonances of the inner NH proton, which appeared at 5.81 ppm, and the inner CH proton of inverted thiophene, which appeared at 4.76 ppm. However, the new S-confused porphyrin 181 derivative exhibited aromatic features with inner CH and two

Chart 6. Proposed Tautomeric Forms of 21-Heteroatom-Substituted N-Confused Porphyrin

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Figure 6. X-ray structures of compounds: (a) 176. Reproduced with permission from ref 163. Copyright 2000 American Chemical Society. (b) 178a. Reproduced with permission from ref 165. Copyright 2001 American Chemical Society. (c) Cd180. Reproduced with permission from ref 168. Copyright 2006 American Chemical Society. (d) Ag(III)183a. Reproduced with permission from ref 169. Copyright 2003 Wiley-VCH. (e) Ag(III)188. Reproduced with permission from ref 170. Copyright 2006 Wiley-VCH. (f) Ni183, Ag(III)183a. Reproduced with permission from ref 169. Copyright 2003 Wiley-VCH.

inner NH resonances appearing at −5.31, −3.37, and −2.93 ppm, respectively. The coordination chemistry of S-confused porphyrin168 180 was explored by treating the S-confused porphyrin with metal chlorides such as CdCl2 and ZnCl2 and isolating the respective metal derivatives of S-confused porphyrin Cd180/Zn180 (Scheme 68). The Cd(II) and Zn(II) ions were bound to Sconfused porphyrin by three nitrogen atoms and one axial chloride ligand along with an interaction with the C(H) unit of the inverted thiophene of the macrocycle and, thus, the macrocycle acting as a monoanionic ligand. The proximity of the thiophene fragment to the metal ion induced direct scalar couplings between the spin-active nucleus of the metal (111/113Cd) and the adjacent 1H nucleus. The interaction

between the metal ion and the C(21)H unit was also reflected in significant changes in the C(21) chemical shifts in 13C NMR. The X-ray structure168 obtained for Cd(II) complex Cd180 showed that the Cd(II) ion was bounded by three pyrrolic donors with an apical chloride (Figure 6c). The Cd(II) ion was displaced from the N3 plane by 0.87 Å toward the chloride. The dihedral angle between the N3 and thiophene planes was 49.4°. Pawlicki and Latos-Grażyński169 also attempted to synthesize O-confused porphyrin 182 by following a similar strategy that they adopted for the preparation of S-confused porphyrin 180. Condensation of 2,4-bis(phenylhydroxymethyl)furan 179b with p-tolylaldehyde and pyrrole in a 1:2:3 ratio under BF3·OEt2catalyzed conditions followed by oxidation with DDQ did not yield the desired confused 21-oxaporphyrin 182 but yielded the 3310


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O-confused porphyrin 182 on treatment with sodium ethoxide afforded the 3-ethoxy-substituted O-confused porphyrin 185, and hence the process was reversible. Thus, the authors170 established that O-confused porphyrin 182 was the common intermediate in the formation of 3-substituted O-confused porphyrins 183 and 185. They further showed that the addition of pyrrole to diprotonated true O-confused porphyrin 182 afforded pyrrole-substituted O-confused porphyrin 185, whereas if water was added, it resulted in the formation of 3-hydroxysubstituted O-confused porphyrin171 184 (Scheme 69). Furthermore, if 3-pyrrole-substituted O-confused porphyrin 183 was reacted with sodium ethoxide, the α-hydrogen on the confused furan ring was substituted by an ethoxy group, leading to formation of an aromatic macrocycle 186. The ethoxy group of 186 was removed upon treatment with TFA to afford 187, which contains a pyrrole ring connected to the trigonally hybridized C(3) atom. The coordination chemistry of O-confused porphyrins169,172 183 was explored to a limited extent, but the complexes showed interesting reactivity and properties. The O-confused porphyrins were suitable for stabilizing higher oxidation states of coordinated metal ions. A dissociation of inner two N(H) and one C(H) proton produces trianionic aromatic macrocycle. However, the structural changes were easily triggered by the addition/elimination of a nucleophile at the C(3) position. The tetrahedral−trigonal rearrangements originating at the C(3) atom extend their consequences to the whole structure. The subtle interplay between their structural flexibility, perimeter substitution, coordination geometry, and aromaticity was detected for O-confused carbaporphyrinoids.169,172 The oxidation state of a central metal ion was a factor, which determines the ligand molecular structure; hence, the O-confused porhyrin acts as monoanionic, dianionic, and trianionic macrocyclic ligands to form complexes with various metal ions. Authors prepared Ag(III) complexes with different Oconfused porphyrins Ag(III)183/Ag(III)185 in which the Oconfused porphyrin acts as a trianionic ligand.169,170 The reaction of pyrrole-substituted O-confused porphyrin 183 with silver(I) acetate in ethanol afforded Ag(III) complex [(OEt,Py)OCP]Ag(III) Ag(III)183a substituted at the C-3 position by the ethoxy and pyrrole moieties. The X-ray structure of Ag(III)183a revealed that the Ag(III) ion was bound by three nitrogens and the trigonal carbon of the (CNNN) coordination core. The macrocycle was slightly distorted from planarity (Figure 6d). The tetrahedral geometry around the C(3) atom was clearly demonstrated. However, when macrocycle Ag(III)183a was treated with TFA, it resulted in the formation of a new aromatic Ag(III) complex Ag(III)183b via C(3)−O bond cleavage, followed by elimination of the ethoxy group. Compound Ag(III)183b was converted back to Ag(III)183a on treatment with sodium ethoxide in ethanol. In the course of this reversible process, the tetrahedral−trigonal rearrangement originated at the C(3) atom but extended its action on the whole structure. The O-confused porphyrin 185 containing ethoxy and hydrogen substituents was also converted to Ag(III) complex Ag(III)185a by treating it with silver(I) acetate. In both these Ag(III) complexes Ag(III)185a/Ag(III)183a, coordination through the nitrogen donors was reflected by the presence of 107/109Ag scalar splitting observed for the selected β-H signals.169,170 However, addition of TFA to Ag(III)185a produces a weakly aromatic silver(II) complex of the true O-confused carbaporphyrin Ag(III)185b. The O-confused carbaporphyrin Ag(III) complex Ag(III)185b was unstable and transforms to carbaporpholac-

3-pyrrole-substituted O-confused porphyrin 183 (Scheme 69). The 3-pyrrole-substituted O-confused porphyrin 183 showed 1H NMR spectral features such as downfield-shifted resonances for β-protons (8.2−8.4 ppm) and upfield-shifted resonances for inner C(H) (−5.11 ppm) and NH protons (−2.40, −2.79 ppm) typical of regular 18π-electron porphyrins. The absorption spectrum of 183 showed an intense Soret band at 437 nm and a set of Q-bands in the 500−700 nm regions. However, later authors170 realized that, by using the proper nucleophile, which can compete with the C-3 position of O-confused porphyrin, it was possible to avoid the formation of 3-pyrrole-substituted Oconfused porphyrin 183 because pyrrole was not a better leaving group to form true O-confused porphyrin170 182. Hence, ethanol was chosen as the perspective nucleophilic agent, and the same condensations were performed in the presence of various ethanol concentrations to afford 3-ethoxy-substituted Oconfused porphyrin 185 along with 3-pyrrole-substituted Oconfused porphyrin 183, which were subsequently separated by column chromatography. The 3-ethoxy-substituted O-confused porphyrin 185 showed a typical Soret band at 433 nm and a set of Q-bands in the 500−800 nm region, as well as strongly upfield positions of the internal CH(21) (−5.49 ppm) and inner NH (−2.82, −3.07 ppm) protons in the 1H NMR spectrum, supporting its aromatic nature. The treatment of 3-ethoxysubstituted O-confused porphyrin 185 with TFA yielded the true O-confused porphyrin 182 in its diprotonated form. The inner CH(21) resonance at +1.18 ppm and β-protons at 7.8−8.3 ppm in 1H NMR spectrum indicated diminished aromaticity of true O-confused porphyrin. However, the diprotonated form of true Scheme 68. Synthesis of S-Confused Heteroporphyrin 180 and Its Metal Derivatives

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Scheme 69. Synthesis of Pyrrole or Ethoxy Group Appended O-Confused Porphyrins and Their Ag Complexes

hybridized C21 atom of the inverted furan (Scheme 70). Complexes Ni183/Pd183 were also prepared by reacting the true O-confused carbaporphyrin 187 with appropriate MCl2 salts under standard conditions. The Ni(II) and Pd(II) insertion process, accompanied by hydrogen elimination, results in a reconstruction of the furan structure, restoring a trigonal geometry around C(3). The electronic spectra of Ni(II) and Pd(II) complexes Ni183/Pd183 showed several Soret-like bands and less-intense Q-bands with low extinction coefficients compared to free base O-confused porphyrin, indicating that the aromatic character was lowered in metal complexes compared to free base O-confused porphyrin 183. The NMR data also supports the lower aromatic character of the metal complexes. The crystal structure of Ni(II) complex169 Ni183 revealed that the macrocycle was only slightly distorted from planarity and the

tone 188, which contains a lactone functionality. Carbaporphalactone 188 was aromatic and showed β-H proton resonances in the region of 8.56−8.74 ppm and inner C(H) proton at −5.12 ppm in 1H NMR spectrum. Carbaporphyrinlactone 188 was treated with CH3COOAg to afford Ag(III)188. The X-ray structure of Ag(III) complex of carbaporpholactone Ag(III)188 revealed that the macrocycle was planar with a direct silver− carbon distance of 2.013 Å, which was similar to other silver(III) carbaporphyrinoids where the trigonal carbon atom coordinated to the metal ion (Figure 6e). The Ni(II) and Pd(II) complexes of O-confused porphyrin169,170 183 with pyrrole and hydrogen substituents under standard conditions resulted in the formation of organometallic complexes Ni183/Pd183, in which the metal ions were coordinated to three pyrrolic nitrogen atoms and the trigonally 3312


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Scheme 70. Metal Complexes of O-Confused Porphyrins 183 and 187

Scheme 71. Synthesis of Dihydroxydithiachlorins 189, Tetrahydroxydithiabacteriochlorins 190, Dithiaporpholactones 191, and Dithiamorpholinochlorins 192

furan, indicating that π-delocalization through the furan ring was altered. The zinc(II) and cadmium(II) complexes172 Zn187/Cd187 were also prepared similarly by reacting true O-confused porphyrin 187 with appropriate MCl2 salt (Scheme 70). In these complexes, the metal ions do not form a regular metal− carbon bond and the macrocycle acts as a monoanionic ligand.

dihedral angle between the macrocycle and the appended pyrrole planes reflects the biphenyl-like arrangement with the NH group pointing out toward the adjacent phenyl ring on the C5 position (Figure 6f). The Ni−N bond length was similar to other Ni(II) carbaporphyrinoids Ni143a where the trigonal carbon atom coordinated to the metal ions. The Cα−Cβ and Cβ−Cβ bond lengths were changed in Ni(II) complex Ni183 compared to free 3313


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Scheme 72. Synthesis of meso-Free Heterotetrabenzoporphyrins 198 and 199

and IO4− mediated diol-cleavage reaction of 189 resulted in pyrrole-modified derivatives dithiaporpholactone 191 and dithiamorpholinochlorin 192, respectively, by undergoing an oxidative ring-opening reaction (Scheme 71).

The metal ion was coordinated to three pyrrole nitrogen atoms, and the extra metal charge was compensated by axial chloride coordination. In 1H NMR, the inner C(H) proton resonance was observed at +0.51 ppm in Zn(II) complex Zn187 and at +0.15 ppm for Cd(II) complex Cd187. The proximity of the furan fragment to the metal ion induces direct scalar coupling between the spin-active nucleus 111/113Cd and the adjacent 1H nucleus. The interaction between the metal ion and C(21)−H was also evident by significant changes of C(21) chemical shifts in Zn(II) and Cd(II) complexes.172

9. HETEROTETRABENZOPORPHYRINS Ono and co-workers176 synthesized the first examples of mesounsubstituted core-modified tetrabenzoporphyrins (TBPs) 198 and 199 containing N3S, N3O, N2S2, N2SO, and N2SC atoms in the core. The acid-catalyzed [3 + 1] condensation of bicyclo[2.2.2]octadiene (BCOD)-fused tripyrrane 193 with BCOD-fused dialdehydes 194a−c yielded 196a−d, whereas the condensation of 193a with diformylindene 195 yielded BCOD-fused porphyrin 197. The core-modified tetrabenzoporphyrins 198a−d and 199 were obtained in quantitative yields when 196a−d and 197, respectively, were heated at 230 °C under vacuum (Scheme 72). The absorption spectral bands of 198a−d were similar to all-aza analogue of tetrabenzoporphyrin with minor shifts in the absorption bands, whereas 199 showed more pronounced changes. Due to the extended π-network, the longest wavelength absorption band of 198b (718 nm) was 19 nm bathochromically shifted compared to the absorption band of tetraphenyl-21,23-dithiaporphyrin (699 nm) with 4-fold enhancement in extinction coefficient value. The quantum yield values of core-modified tetrabenzoporphyrins 198a−c were

8. HETEROCHLORINS Lara et al.173,174 reported the reaction of 5,10,15,20-tetraaryl21,23-dithiaporphyrins 30 with osmium tetraoxide (OsO4) to obtain the first examples of meso-tetraaryl-7,8-dihydroxydithiachlorins 189 along with minor amounts of an isomeric mixture of tetrahydroxydithiabacteriochlorins 190 (Scheme 71). The 1H NMR of 189 showed a resonance signal corresponding to pyrrolidine protons coupled to OH, confirming the hydroxylation at the pyrrole ring. The absorption spectrum of 189 showed a slightly red-shifted absorption band compared to 30, but the extent of the red-shift was considerable (40 nm) when compared to all-aza dihydroxychlorin.175 A similar trend was observed in the absorption spectral pattern of 190 when compared to all-aza bacteriochlorin analogue.175 The MnO4− 3314


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Scheme 73. Synthesis of meso-Aryl-Substituted Heterobenzoporphyrins 202

(MCD) spectroscopy and time-dependent DFT (TD-DFT) studies and conceptualization of results using Michl’s perimeter model and Gouterman’s four-orbital model177 showed that orbital angular momentum (OAM) properties of four frontier πMOs (MO = molecular orbital) largely determine the optical properties even in the case of core-modified, nonplanar, and sterically crowded benzoporphyrins.

reduced considerably compared to all-aza tetrabenzoporphyrin due to heteroatom-induced internal heavy-atom effect and distortion of the chromophore. Ono, Kobayashi, Stillman, and co-workers177 further synthesized meso-aryl-substituted core-modified benzoporphyrins 202, 205, and 206 (Scheme 73 and 74) using different precursors for condensation to understand the relative effects of core modification, ligand folding, and partial benzo substitution at the ligand periphery on the electronic and structural properties. The 1H NMR studies of these macrocycles in general revealed a distinctive observation that the benzo proton resonances of isoindole units appeared at higher field than the corresponding isothianaphthene proton resonances. Also, the ortho-, meta-, and para-proton resonances of meso-aryl groups were anisotropically downfield shifted due to the enlarged macrocyclic π-system resulting from benzo substitution. The single-crystal X-ray analysis showed severe saddling in the case of 202b, 202d, and 206 due to peripheral expansion of the π-system by benzo substitution. The combination of magnetic circular dichroism

10. HETEROCALIXPHYRINS Calix(4)phyrins178 are a class of hybrid macrocycles containing a mixture of sp2- and sp3-hybridized meso-carbon bridges that lie at the structural crossroads between porphyrins, which contain only sp2-meso-carbons, and calixpyrroles, which contain only sp3-mesocarbons. Calixphyrins possess reasonably flexible frameworks as well as rather rigid conjugated networks, whose characteristics vary considerably depending on the number and position of the sp3-meso-carbon atoms. There are different classes of calixphyrins.178 These classes are porphomonomethenes I (1.1.1.1-, one sp2- and three sp3-hybridized meso-carbons atoms), porphodi3315


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Scheme 74. Synthesis of meso-Aryl-Substituted Heterobenzoporphyrins 205 and 206

Chart 7. Different Types of Calixphyrins

208/209 exhibit spectral properties that are quite different from those of thiaporphyrins 2a/30a because of the presence of sp3meso-carbon in thiaphlorins, which breaks the π-electron delocalization. The crystal structure180 obtained for 208d indicated that the macrocycle was not planar and showed a marked kink at the sp3-hybridized meso-carbon. The monofunctionalized thiaphlorins 208b−d/209b−d were used as building blocks to synthesize several novel porphyrin− thiaphlorin dyads180 210−212 under palladium(0)-coupling conditions (Chart 8). For example, the diphenylethyne-bridged dyad 210a containing 21,23-dithiaphlorin and Zn(II)porphyrin was synthesized by coupling iodophenyl 21,23-dithiaphlorin 208b building block with ethynylphenyl Zn(II) porphyrin Zn65 in toluene/triethylamine at 50 °C in the presence of catalytic amounts of AsPh3/Pd2(dba)3 under inert atmosphere followed by simple column chromatographic purification (Scheme 76). All other dyads 210−212 were prepared similarly under identical Pd(0)-coupling conditions by coupling appropriate ethynylphenyl and iodophenyl containing thiaphlorin and porphyrin/ Zn(II) porphyrin/thiaporphyrin building blocks. The spectroscopic studies indicated that the two macrocycles in thiaphlorin− porphyrin dyads 210−212 interact very weakly and retain their individual characteristic features. The preliminary anion-binding studies indicated that thiaphlorins 208a/209a in their protonated form bind anions as monitored by absorption spectroscopy, and thus, thiaphlorins can be used as optical sensors.180 The sensing behavior of thiaphlorins was used effectively in covalently linked thiaphlorin−porphyrin dyads 210−212, and certain dyads such as ZnN4− N2S2 dyad 210a and N3S−N2S2 dyad 212 were demonstrated as fluorescent anion sensors. In thiaphlorin−porphyrin dyads 210− 212, the flexible protonated thiaphlorin ring was used to bind an anion that was sensed by following the changes in the fluorescence of the porphyrin/Zn(II) porphyrin unit. For example, for the protonated dyad 212H+, which contained N2S2 thiaphlorin and protonated N3S porphyrin subunits when titrated with tetrabutylammonium iodide, the fluorescence band intensity of the N3S porphyrin unit was gradually increased, indicating that the iodide ion was bound at the N2S2 phlorin site and, thus, dyad 212 is a fluorescent anion sensor. Unusual 21,23-dithiaphlorins181 213a−c containing pyrrole and aryl groups at the sp3-meso-carbon that was present between the pyrrole and thiophene rings were obtained from the

methenes II and III (1.1.1.1- and 1.1.1.1-, two sp2- and two sp3hybridized meso-carbon atoms), and porphotrimethenes IV and V (1.1.1.1-, three sp2- and one sp3-hybridized meso-carbon atoms) (Chart 7). Calixphyrins have been shown to be good receptors for cations and, more importantly, for anions. Heterocalixphyrins resulted from the replacement of one or two pyrrole nitrogen atoms with other heteroatoms such as S, O, and P. Because of their novel features and applications, the chemistry of azacalixphyrins178 is relatively well-developed, but the reports on heterocalixphyrins are still very few. The thiaphlorins,179,180 the porphotrimethenes 208−209, were the first heterocalixphyrins synthesized under simple acidcatalyzed reaction conditions by using the unsymmetrically substituted thiophene dicarbinols 207 as key precursors. The thiophene monocarbinol 6b was reacted with different ketones such as acetone in the presence of n-BuLi in THF at 0 °C followed by column chromatographic purification, affording unsymmetrically substituted thiadicarbinol 207. The TFAcatalyzed condensation of appropriate thiadicarbinol 207 with 16-thiatripyrrane 16b in CH2Cl2 followed by oxidation with DDQ afforded 21,23-dithiaphlorins 208a (Scheme 75). Similarly, condensation of thiadicarbinol 207 with p-tolualdehyde and pyrrole in stoichiometric ratio in CH2Cl2 under acidcatalyzed conditions followed by oxidation and purification afforded 21-monothiaphlorin 209a. The monofunctionalized 21,23-dithiaphlorins 208b−208d and 21-monothiaphlorins 209b−209d were synthesized179 by using the functionalized unsymmetrical dicarbinols for condensation. The thiaphlorins 3316


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Scheme 75. Synthesis of 21,23-Dithiaphlorins 208 and 21Thiaphlorins 209

Scheme 76. Synthesis of Porphyrin−Thiaphlorin Dyad 210a

anions in their protonated state but did not show any selectivity.181 Matano et al.182 synthesized the thiophene-containing calixphyrins of the 5,10-porphodimethene type 215−217 by acid-promoted dehydrative condensation between a thiatripyrrane 214 and the appropriate dicarbinol 1a/1b/56 followed by DDQ oxidation. Thus, the condensation of thiatripyrrane 214 with 2,5-bis[hydroxy(phenyl)methyl]thiophene 1a produced a mixture including two types of S2,N2-hybrid calixphyrins 215a and 215b (Scheme 78). The column chromatographic purification afforded only 2e− oxidized 14-π hybrid compound 215a, whereas the 4e− oxidized 16-π compound 215b was decomposed. However, 215b was independently prepared by the DDQ oxidation of 215a. The condensation of thiatripyrrane 214

condensation of 2,5-bis(arylhydroxymethyl) thiophene 1a and pyrrole under mild acid-catalyzed conditions. The condensation resulted in the formation of meso-tetrarayl-21,23-dithiaporphyrin 30 as major product and the unusual 21,23-dithiaphlorins 213 as minor product in 4−5% yield (Scheme 77). The crystal structure of 21,23-dithiaphlorin 213b revealed that the macrocycle was significantly distorted because of the presence of the sp3hybridized meso-carbon, which disrupts the π-delocalization of the macrocycle.181 The dithiaphlorins 213 showed one broad featureless Q-type band at 690 nm and one strong Soret-type band at 390 nm along with a shoulder band at 411 nm, supporting their nonaromatic characteristic features. The preliminary studies indicated that thiacalixphyrins 213 bind

Chart 8. Molecular Structures of Porphyrin−Thiaphlorin Dyads 210−212

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Scheme 77. Synthesis of Dithiaphlorins 213

with 2,5-bis[hydroxy(phenyl)methyl]furan 1b under similar acid-catalyzed conditions afforded 14π-S,N2,O-hybrid 216 as a sole isolable product (Scheme 78). The condensation of thiatripyrrane 214 with 2,5-bis[hydroxy(phenyl)methyl]pyrrole 56 gave the 16π-S,N3-hybrid 217 (Scheme 78). Thus, the central heteroles intrinsically determine the oxidation states of the πconjugated N−X−N units in the S,N2,X-hybrid calixphyrins182 215−217. The X-ray structure of 215a possesses a slightly twisted N−S− N unit, and the sp3-carbon-bridged thiophene ring stands against a mean plane formed by the four meso-carbon atoms with dihedral angles of 72.7° (Figure 7a). The 14π-conjugated hybrid calixphyrins 215a/216 showed broad absorptions due to π−π* transitions in the 400−600 nm region whereas the 16πconjugated hybrids 215b/217 showed the transitions at longer wavelengths as compared to those of the 14π-conjugated hybrids. Furthermore, the S,N2,X-hybrids 215−217 showed no tendency to form stable metal complexes.182 Matano and co-workers183,184 also reported the synthesis, structure, and coordination properties of a series of phospholecontaining hybrid calixphyrins 222−224 of the 5,10-porphodimethene−P,(NH)2,X− and −P,N2,X− types (X = O, S, and NH). The phosphole-containing hybrid calixphyrins were synthesized as shown in Scheme 79. Treatment of a CH2Cl2 solution containing σ4-2,5-bis[(pyrrol-2-yl)methyl]phosphole 218 and 2,5-bis[hydroxy(phenyl)methyl]thiophene 1a with BF3·OEt2 at room temperature gave a mixture of condensation products, which was then oxidized with 2.2 equiv of DDQ to afford the 2e− oxidized σ4-P,(NH)2,S-hybrid calixphyrin 219a containing 14π-conjugated N−S−N unit and the 4e− oxidized σ4-P,N2,S-hybrid calixphyrin 219c containing 16π-conjugated N−S−N unit, which were separated by column chromatography. Furthermore, 14π-conjugated σ4-P,(NH)2,S-hybrid calixphyrin 219a was converted to 16π-conjugated σ4-P,N2,S-hybrid calixphyrin 219c by treating it with excess DDQ.183 Using thiophene dicarbinol 1a, where aryl groups were substituted with electron-withdrawing −CF3 groups or electron-donating −OCH3 groups, only either 14π-P,N2,S-hybrid 219b or 16πP,N2,S-calixphyrin 219d were isolated.184 Under similar reaction conditions, the condensation of 2,5-bis[hydroxy(phenyl)methyl] furan 1b with σ4-2,5-bis[(pyrrol-2-yl)methyl]phosphole 218 gave 2e− oxidized σ4-P,(NH)2,O-hybrid 220 containing 14πN−O−N unit, which was not possible to oxidize further with excess DDQ to obtain 4e− oxidized product. However, when 2,5-

Scheme 78. Synthesis of Thiophene-Containing Hybrid Calixphyrins 215−217

bis[hydroxy(phenyl)methyl]pyrrole 56 was condensed with σ42,5-bis[(pyrrol-2-yl)methyl]phosphole 218, only 4e− oxidized σ4-P,N2,NH hybrid calixphyrin 221 was obtained and 2e− oxidized product was not formed. Thus, the electronic nature of the central heterole (X = thiophene, furan, and pyrrole) strongly affects the conjugated structure of the N−X−N unit. The σ4-complexes 219−221 were heated strongly with P(NMe2)3 in refluxing toluene to convert to the corresponding σ3-complexes 222−224. Thus, the σ4-complexes 219c, 219d, and 221 were converted to their respective σ3-complexes 222c, 222d, and 224, respectively, by treating the corresponding σ4complexes 219c, 219d, and 221 with P(NMe2)3 in refluxing toluene.183,184 Similarly, the σ4 complexes 219a, 219b, and 220 were reacted with P(NMe2)3 in toluene at reflux conditions to afford σ3 complexes 222a, 222b, and 223, respectively. Matano and co-workers185 also prepared phosphacalixphyrins bearing a perfluorophenyl group at the core phosphorus atom by BF3promoted dehydrative condensation of phosphatripyrrane 225 with appropriate dicarbinol 1a or 56 followed by DDQ oxidation, affording P,X,N2 calixphyrins 226 and 227 in 43% and 17% yields, respectively (Scheme 80). The X-ray structures183 of 222c, 223, and 224 are shown in Figure 7. The 14π-σ3-P, 3318


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Figure 7. X-ray structures of compounds: (a) 215a, (b) 223, (c) 224, (d) Pd223, (e) Rh224a, (f) Rh224b, (g) Au222c, (h) 222c, and (i) Pd222a. Reproduced with permission from ref 172. Copyright 2005 American Chemical Society. (j) Ag232. Reproduced with permission from ref 186. Copyright 2008 Wiley-VCH.

223 in its 1H NMR spectrum indicated that the N−O−N unit was exhibiting fluxionality in solution. On the contrary, the σ3P,N2,S-hybrid 222c and the σ3-P,N2,NH-hybrid 224 were

(NH)2,O hybrid 223 was largely twisted at the N−O−N unit with dihedral angles between the pyrrole and furan ring planes of 26.9−29.4°. The equivalent appearance of the pyrrole protons of 3319


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Scheme 79. Synthesis of σ3-P,(NH)2,X-Hybrid and σ3-P,N2,X-Hybrid Calixphyrins and Their Metal Complexes

meso distance in 223 was shorter by 0.16−0.21 Å than that of 224 because of the highly twisted conformation at the N−O−N unit in 223. The P-phenyl group in all hybrid calixphyrins was located outside the macrocycle due to steric reasons, and the lone pair of the phosphorus atom was oriented inside the core, which led to formation of different types of macrocyclic coordination sites consisting of P, N, O, and S donors.183 The absorption spectra of phosphole-containing hybrid calixphyrins showed broad absorptions bands in the region of 400−600 nm, and the 16π-conjugated hybrids 222c/224 showed absorptions at longer wavelengths as compared to those of 14π-conjugated hybrids 222a/223. Furthermore, the spectral shape of 224 was slightly

composed of almost-flat 16-N−X−N planes (X = S, N) with small dihedral angles of 1.9−19.1°. In the hybrids 222c and 224, the phosphole ring stands almost perpendicular to a mean plane formed by the four meso-carbon atoms with dihedral angles of 85.1° and 85.9°, respectively. The 14π-conjugated structure of the N−O−N unit in 223 and the 16π-conjugated N−S−N unit in 222c and N−N−N unit in 224 were clearly reflected in the carbon−carbon/nitrogen bond alteration at the heterole rings and the inter-ring bridges. The meso−meso distance at the N− X−N and the N···N distance of 222c are longer than the respective distances of 224, reflecting the difference in sizes between the thiophene and pyrrole rings. However, the meso− 3320


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The Pd−N and the Pd−P bond lengths of Pd222a were slightly longer than those of Pd223, reflecting the core size of their hybrid ligands.183,184 The crystal structures of Rh(III) complexes of 16π-P,N2,NH hybrid structures Rh224a/Rh224b were obtained. The rhodium center in each complex adopts a distorted octahedral geometry with the macrocyclic P,N3-platform that occupies the four equatorial sites, wherein the N−N−N units were not on the same plane to avoid steric congestion at the core.183,184 The two 2azafluvene rings were twisted from the central pyrrole ring. The axial chloromethyl group in Rh224b was located at the opposite side to the P-phenyl group. The Rh−Cl bond length of Rh224b was longer than that of Rh224a, reflecting the difference in trans influence between the chloromethyl group in Rh224b and the chlorine atom in Rh224a. The bond alteration at the N−N−N units of Rh224a/Rh224b was similar to its free base 224, implying that the oxidation state of the N−N−N unit was retained in the complex also. The structure of gold complex of 16π-P,N2,S-hybrid complex Au222c showed that the gold center possesses a linear geometry, coordinated by the phosphorus and chlorine atoms, and the phosphole ring was perpendicular to a mean plane formed by the four meso-carbons. The C−C bond length alteration at the N−X−N unit in Au(I) complex Au222c was comparable to that observed in its free base derivative 222c, indicating the nonvital role of the π-conjugated unit in the Pcoordination to the AuCl moiety. Thus, the observed structural features indicated that the phosphole-containing calixphyrins behave as neutral, monoanionic, or dianionic ligands, depending on the combination of heterole subunits.183,184 The authors also demonstrated that hybrid calixphyrin− palladium Pd222a/Pd223 and rhodium complexes Rh222c/ Rh224a/Rh224b catalyze the Heck and hydrosilylation reactions, respectively, which in turn supports that the metal center in the core was capable of activating the substrates under appropriate reaction conditions.183,184 Thus, the phospholecontaining hybrid calixphyrins are highly promising hemilabile macrocyclic ligands for the construction of efficient transition metal catalysts. Latos-Grażyński and co-workers,186 in their attempts to synthesize 21-silaporphyrin from acid-catalyzed condensation of 1,1-dimethyl-3,4-diphenyl-2,5-bis(p-tolylhydroxymethyl)silole 228, p-tolualdehyde, and pyrrole followed by DDQ oxidation, noticed unusual formation of dihydro-21-silaphlorin 229 and 21-silaphlorin 230 in minute yields (Scheme 81). The attempts to further oxidize 230 to obtain 21-silaporphyrin by DDQ led to the formation of nonaromatic macrocycle isocarbacorrole 232. The formation of 232 was attributed to the extrusion of silylene unit followed by intramolecular rearrangements from the intermediate 231 resulting from DDQ oxidation of 230. The 1H NMR spectroscopic studies and DFT optimized structures180 revealed the nonplanar nature of 230 and 232. The treatment of isocarbacorrole 232 with silver(I) tetrafluoroborate and copper(II) acetate in dichloromethane in the presence of triethylamine yielded Ag(III) and Cu(III) complexes186 Ag232 and Cu232, respectively (Scheme 82). These metal complexes possessed a M(III)−C(sp3) bond reminiscent of the binding mode of “true” carbacorroles. The absorption spectral patterns of Ag232 and Cu232 were markedly different from those of 232 and exhibited high extinction coefficient values, suggesting an aromatic nature of the complexes. The X-ray crystal analysis of Ag232 revealed distorted planar structure (Figure 7j), and Ag−N and Ag−C

Scheme 80. Synthesis of P,X,N2-Calixphyrins 226 and 227

different from 222c and 223, indicating that the character of the vibronic states of the N−X−N moiety is affected by the nature of the central heteroles.183 The coordination properties of the phosphole-containing hybrid calixphyrins183 such as 222a, 222c, 223, and 224 with metals such as palladium, rhodium, and gold were investigated as presented in Scheme 79. The σ3-P,(NH)2,S 222a and σ3-P,N2,Shybrids 222c upon reaction with Pd(OAc)2 and Pd2(dba)3, respectively, resulted in the formation of the same Pd(II)-P,N2,Shybrid complex Pd222a in which the calixphyrin platform was regarded as a dianionic ligand. The σ3-P,(NH)2,O-hybrid 223 was also reacted with Pd(OAc)2 at room temperature to form the P,N2,O-hybrid calixphyrin−palladium complex Pd223. The palladium complexes Pd222a and Pd223 showed broad absorptions in the 500−600 nm region. In the complexation with [RhCl(CO)2]2 in CH2Cl2, the σ3-P,N2,S hybrid 222c behaved as a neutral ligand and afforded an ionic Rh(I)-P,N2,Shybrid Rh222c, whereas the σ3-P,N2,NH-hybrid 224 behaves as an anionic ligand to produce Rh(III)-P,N3-hybrid complexes183 Rh224a/Rh224b. The Rh(I) complex Rh222c showed a broad absorption band at 539 nm, whereas Rh(III) complexes Rh224a/Rh224b showed broad absorptions at the 600−750 nm region. Thus, the absorption bands of Rh(III) complexes Rh224a/Rh224b were red-shifted by ∼150 nm compared to that observed for the Rh(I) complex Rh222c, which implies that the oxidation state of the rhodium center considerably affects the transitions of the N−X−N units in the calixphyrin platforms. The complexation of AuCl(SMe2) with the σ3-P,N2,X-hybrids (X = S, 222c; X = NH, 224) led to the formation of the corresponding Au(I) monophosphine complexes183,184 Au222c and Au224. The crystal structures of some of the metal complexes of phosphole-containing hybrid calixphyrins were obtained as shown in Figure 7. In palladium complexes Pd223 and Pd222a, the palladium was coordinated to the phosphorus, two nitrogens, and oxygen/sulfur atoms at the core to adopt a square-planar geometry.183,184 The N−O−N unit in Pd223 adopts twisted conformation like its free base derivative 223, and the dihedral angles between the pyrrole and furan rings as well as the meso−meso distances of Pd223 were almost comparable to that of free base derivative 223. Complex Pd222a adopts a similar conformation, where the N−S−N unit was not on the same plane and the pyrrole rings were significantly tilted from the thiophene mean plane. Furthermore, the meso−meso distances of Pd222a differ considerably from its free base derivative 222a. In both palladium complexes, Pd223 and Pd222a, the phosphorus ring bends toward the inside for binding the palladium. Hence, the dihedral angles between the phosphole ring and the mean plane formed by the four meso-carbon atoms become more acute in comparison with those of the corresponding free bases 223 and 222a. The sulfur atom in Pd222a was deviated from the thiophene mean plane by 0.13 Å to maintain the square-planar geometry at the palladium center. 3321


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11. METALLOCENE-INCORPORATED HETEROPORPHYRINOIDS Supramolecular assemblies with helical structure have attracted considerable attention in molecular-recognition study because of their screwlike motifs and novel structural features. Because flexible metallocenes such as ferrocene and ruthenocene units possess a higher degree of rotational freedom around the main axis, they were chosen to increase the degree of helicity of various porphyrinoid derivatives. Often ferrocene was covalently linked with numerous macrocyles as a “marker” and used as an electrochemical responsive marker.187 However, ansa-metallocene-based porphyrinoids where the metallocene unit is incorporated into the backbone of the fully or partially conjugated porphyrinoids were very few in the literature. In 2008, Latos-Grażyński and co-workers188 reported ansaferrocene-incorporated heterocalixphyrin 235 by condensing thiophene dicarbinol 1a with ferrocenenyl dipyrromethane 234 under BF3·OEt2-catalyzed conditions (Scheme 83). This was the first report where the metallocene unit was strapped together with a conjugated heterotripyrrin unit. Detailed 1D and 2D NMR studies on compound 235 were carried out to explore its helical conformations and dynamic behavior. Latos-Grażyński and co-workers189,190 also reported ferroceno/ruthenocenothiaporphyrinoids 238−239 by adopting [3 + 1] acid-catalyzed condensation between 1,1′-bis[phenyl(2pyrroyl)methyl]ferrocene/ruthenocene 236/237, respectively, and 2,5-bis-[hydroxyl(p-tolyl)methyl]thiophene 1a (Scheme 84). The studies showed that the metallomacrocyclic π-electron directly transmitted across the d-electron of metallocene unit in ansa-metallocene porphyrinoids 238−239. Furthermore, the systematic NMR studies were carried out by protonating porphyrinoids 238/239 to generate monoprotonated 238-H+/ 239-H+ and diprotonated species 238−2H2+/239−2H2+, and their aromatic features were discussed. They also generated dihydoferroceno/ruthenocenothiaporphyrins 238-H2/239-H2 by treating 238/239 with Zn amalgam and confirmed their aromatic nature. The X-ray structure solved for both the compounds 238−239 revealed that the π-systems in both compounds were uniformly distorted and two cyclopentadienyl rings in the metallocene unit adopt anticlinal eclipsed conformation.189,190

bond lengths were similar compared to those for other Ag(III) carbaporphyrinoids.186 Interestingly, the reaction of silver(III) carbacorrole Ag232 with dioxygen in the presence of aqueous HCl resulted in oxygenolysis where the benzylic C21-p-tolyl fragment and silver cation were extruded to yield 21-oxacorrole 233 (Scheme 82). Scheme 81. Synthesis of 21-Silaphlorins 229 and 230 and Isocarbacorrole 232

Scheme 82. Synthesis of Ag232 and Cu232 and Oxygenolysis of Ag232 To Form 21-Oxacorrole 233

12. CONCLUSIONS Modification of the porphyrin core by replacing one or two pyrroles by other five-membered heterocyclic rings such as furan, thiophene, selenophene, tellurophene, phosphole, silole, indenes, or azulenes or six-membered rings such as benzene, pyridine resulted in core-modified porphyrins or hetero analogues of porphyrins, which possess very interesting and different properties in terms of their coordinating, electronic, spectral, and redox properties compared to tetrapyrrolic porphyrins. As discussed in this Review, there may have been some developments in the field of core-modified porphyrins, but the growth is not significant. The synthetic methods are limited and allow preparation of only specific core-modified porphyrins. The synthetic methods available at present would allow preparation of monoheteroporphyrins, diheteroporphyrins, and their various functionalized derivatives. The functionalized coremodified porphyrins were used as building blocks to synthesize several covalent and noncovalent heteroporphyrin-based multiporphyrin arrays containing two or more porphyrins with different porphyrin cores. It is now understood that the energy 3322


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Scheme 83. Synthesis of Ferrocene-Incorporated Heterocalixphyrin 235

Scheme 84. Synthesis and Reactivity of Ferrocene/Ruthenocene-Incorporated Thiaporphyrinoids 238 and 239 (R1 = Phenyl, R2 = Tolyl)

levels of porphyrins can be fine-tuned by systematic alterations of the porphyrin core. With suitable arrangement of porphyrins and core-modified porphyrins in multiporphyrin arrays, one can achieve unidirectional flow of electron/energy transfer, and such multiporphyrin arrays can be used for light-harvesting and molecular electronics applications. Although the possibility of energy transfer was demonstrated in several heteroporphyrinbased arrays, the photophysical studies are too preliminary and detailed studies are required for better design of multiporphyrin arrays for suitable future applications. The advantage of heteroatom-substituted porphyrins is their unique but not so rich coordination chemistry. The coordination environment is very different from one core-modified porphyrin to another, and thus, the core-modified porphyrins provide unprecedented coordination environments in terms of the size, shape, and charge at the core. Interestingly, only handfuls of metal complexes of heteroporphyrins were reported in the literature because of the poor coordinating ability of heterocycles such as furan, thiophene, selenophene, phosphole, etc. However, it is established that core-modified porphyrins can stabilize metals in unusual oxidation states such as Ni and Cu in +1 oxidation states. It is of utmost interest that the neutral heteroporphyrins such as 21,23-dithiaporphyrins and 21,23-diselenaporphyrins with no ionizable protons inside the porphyrin core are also capable of forming metal complexes as reported in the recent literature.

Thus, it is important to explore the coordination chemistry of these exotic heteroporphyrins in the future. Several other hetero analogues of porphyrin derivatives such as heterocorroles, heterochlorins/bacteriochlorins, heterocarbaporphyrinoids, heterotetrabenzoporphyrins, confused porphyrins, and other modified derivatives were attempted to synthesize and study their properties. However, the developments in this direction are almost at the beginning stage. The very limited examples available on hetero analogues of corroles, chlorins, carbaporphyrinoids, and confused porphyrins indicate that the research in this area is still largely unrealized. Unlike their aza analogues, the hetero derivatives are not very stable and not easy to isolate to understand their potential for various studies. More research needs to be carried out in this direction to develop suitable synthetic methods to prepare stable hetero analogues of various porphyrin derivatives. In summary, considering the very different and unique properties of hetero analogues of porphyrins compared to regular porphyrins, it is indeed required to put efforts to develop newer synthetic methods to prepare different stable, new coremodified porphyrins and their derivatives. We hope that the chemistry of core-modified porphyrins and their derivatives will be further exploited in the future to understand their complete potential for various applications as substitutes for aza analogues. 3323


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(6) Zhou, Z.; Shen, Z. The development of artificial porphyrinoids embedded with functional building blocks. J. Mater. Chem. C 2015, 3, 3239−3251. (7) Higashino, T.; Imahori, H. Porphyrins as excellent dyes for dyesensitized solar cells; recent developments and insights. Dalton Trans. 2015, 44, 448−463. (8) Ding, Y.; Zhu, W.-H.; Xie, Y. Development of ion chemosensors based on porphyrin analogues. Chem. Rev. 2016; DOI: 10.1021/ acs.chemrev.6b00021. (9) Latos-Grażyński, L. Core modified Heteroanalogue of Porphyrins and Metalloporphyrins. The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: New York, 2000; pp 361−416. (10) Gupta, I.; Ravikanth, M. Recent Developments in Heteroporphyrins and Their Analogues. Coord. Chem. Rev. 2006, 250, 468− 518. (11) Szyszko, B.; Latos-Grażyński, L. Core Chemistry and Skeletal Rearrangements of Porphyrinoids and Metalloporphyrinoids. Chem. Soc. Rev. 2015, 44, 3588−3616. (12) Chmielewski, P. J.; Latos-Grażyński, L. Core Modified Porphyrins − a Macrocyclic Platform for Organometallic Chemistry. Coord. Chem. Rev. 2005, 249, 2510−2533. (13) Ravikanth, M.; Chandrashekar, T. K. Nonplanar porphyrins and their biological relevance: Ground and excited state dynamics. Struct. Bonding (Berlin) 1995, 82, 105−188. (14) Yedukondalu, M.; Ravikanth, M. Core-Modified Porphyrin Based Assemblies. Coord. Chem. Rev. 2011, 255, 547−573. (15) Pareek, Y.; Ravikanth, M. Thiaporphyrins: From Building Blocks to Multiporphyrin Arrays. RSC Adv. 2014, 4, 7851−7880. (16) Yedukondalu, M.; Ravikanth, M. Mono-functionalized Heteroporphyrin Building Blocks and Unsymmetrical Covalent and Noncovalent Porphyrin Dyads. J. Chin. Chem. Soc. 2011, 58, 1−14. (17) Broadhurst, M. J.; Grigg, R.; Johnson, A. W. 18- and 22-π-Electron Macrocycles Containing Furan, Pyrrole, and Thiophen Rings. J. Chem. Soc. D 1969, 0, 1480−1482. (18) Broadhurst, M. J.; Grigg, R.; Johnson, A. W. Preparation of Some Sulphur-Containing Polypyrrolic Macrocycles. Sulphur Extrusion from a Meso-Thiaphlorin. J. Chem. Soc. D 1970, 807−809. (19) Broadhurst, M. J.; Grigg, R.; Johnson, A. W. Synthesis of Porphin Analogues Containing Furan And/or Thiophen Rings. J. Chem. Soc. C 1971, 3681−3690. (20) Ravikanth, M. Synthesis of β-Substituted 21,23-Dithiaporphyrins. Chem. Lett. 2000, 29, 480−481. (21) Agarwal, N.; Mishra, S. P.; Kumar, A.; Hung, C.-H.; Ravikanth, M. Synthesis and Crystal Structure of 2,3,12,13-Tetraalkoxy-21, 23Dithiaporphyrins. Chem. Commun. 2002, 2642−2643. (22) Agarwal, N.; Mishra, S. P.; Kumar, A.; Ravikanth, M. Synthesis of β-Thiophene-Substituted 21,23-Dithiaporphyrins. Chem. Lett. 2003, 32, 744−745. (23) Agarwal, N.; Ravikanth, M. Synthesis of β-Pyrrole and βThiophene Substituted 21,23-Dithia and 21-Monothiaporphyrins. Tetrahedron 2004, 60, 4739−4747. (24) Agarwal, N.; Hung, C.-H.; Ravikanth, M. Synthesis and Crystal Structures of 2,3,12,13-Tetraalkoxy-21,23-Dithiaporphyrins and 2,3Dialkoxy-21-Monothiaporphyrins. Tetrahedron 2004, 60, 10671− 10680. (25) Gupta, I.; Ravikanth, M. Synthesis of Meso-Furyl Porphyrins. Tetrahedron Lett. 2002, 43, 9453−9455. (26) Gupta, I.; Ravikanth, M. Synthesis of Meso-Furyl Porphyrins with N4, N3S, N2S2 and N3O Porphyrin Cores. Tetrahedron 2003, 59, 6131− 6139. (27) Gupta, I.; Hung, C.-H.; Ravikanth, M. Synthesis and Structural Characterization Of meso-Thienyl Core-Modified Porphyrins. Eur. J. Org. Chem. 2003, 2003, 4392−4400. (28) Sessler, J. L.; Seidel, D. Synthetic Expanded Porphyrin Chemistry. Angew. Chem., Int. Ed. 2003, 42, 5134−5175. (29) Chandrashekar, T. K.; Venkatraman, S. Core-Modified Expanded Porphyrins: New Generation Organic Materials. Acc. Chem. Res. 2003, 36, 676−691.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Tamal Chatterjee was born in 1989 in West Bengal, India. He received his B.Sc. and M.Sc. degrees in 2009 and 2011, respectively, from University of Burdwan, West Bengal. Later he joined the research group of Prof. M. Ravikanth at the Indian Institute of Technology Bombay, where he is currently completing his Ph.D. degree. His research works mainly focus on the synthesis and exploration of expanded calixphyrins. Vijayendra S. Shetti was born in the state of Karnataka, India, in 1981. He obtained his Ph.D. from Indian Institute of Technology Bombay in 2010, working under the supervision of Prof. M. Ravikanth. After a couple of Postdoctoral assignments in Taiwan and South Korea, he joined Chemistry department, BMS College of Engineering, Bengaluru, in 2014 as an Assistant Professor. His current research is focused on synthesis of novel π-conjugated molecules for organic electronic applications. Ritambhara Sharma was born in India in 1989. She received her B.Sc. from Maharaja Ganga Singh University Bikaner in 2009 and M.Sc. from University of Rajasthan, Jaipur, in 2011. She joined Indian Institute of Technology Bombay as a Ph.D. student under the supervision of Professor M. Ravikanth in 2011. Her research topic is functionalized 3pyrrolyl boron dipyrromethenes and their applications, and recently, she submitted the thesis. M. Ravikanth was born in India in 1966. He received his B.Sc. and M.Sc. from Osmania University, Hyderabad, and Ph.D. from Indian Institute of Technology, Kanpur, in 1994. After his postdoctoral stay in U.S.A. and Japan, he joined as a faculty at Indian Institute of Technology Bombay, where he is currently a full professor. His current research interest includes porphyrin and related macrocycles and boron dipyrromethenes.

ACKNOWLEDGMENTS M.R. thanks Department of Science & Technology for financial support and all the co-workers whose names appear in the references for their hard work and dedication in developing this area of research in our laboratory. T.C. thanks CSIR and R.S. thanks UGC for fellowship. V.S.S. thanks Science and Engineering Research Board (SERB), New Delhi, for a Research Grant (File no. YSS/2015/001557/CS). REFERENCES (1) The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: New York, 2000; Vol. 1, Synthesis and Organic Chemistry. (2) The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: New York, 2000; Vol. 2, Heteroporphyrins, Expanded Porphyrins and Related Macrocycles. (3) The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: New York, 2000; Vol. 3, Inorganic, Organometallic and Coordination Chemistry. (4) The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: New York, 2000; Vol. 4, Biochemistry and Binding: Activation of Small Molecules. (5) The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: New York, 2000; Vol. 6, Applications: Past, Present and Future. 3324


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