Smaragdyrins and Sapphyrins Analogues - Chemical Reviews (ACS

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Smaragdyrins and Sapphyrins Analogues Tamal Chatterjee,† A. Srinivasan,‡ Mangalampalli Ravikanth,*,† and Tavarakere K. Chandrashekar*,‡ †

Department of Chemistry, Indian Institute of Technology, Powai, Mumbai 400076, India School of Chemical Sciences, National Institute of Science Education and Research, Bhubaneswar 752050, Odisha, India

‡

ABSTRACT: Porphyrins and expanded porphyrins have attracted the attention of chemists for a long time in view of their diverse applications in catalysis; as anion, cation, and neutral substrate receptors; as ligands to coordinate large metal ions; as nonlinear optical materials, MRI contrasting agents, and sensitizers for photodynamic therapy (PDT); and more recently as models for aromaticity (both Hückel and Möbius). A diverse range of synthetic expanded porphyrins containing up to 96π electrons have been reported, and their properties have been exploited for various applications. The present Review is only confined to 22π electron expanded porphyrins containing five pyrrole/ heterocyclic rings such as sapphyrins and smaragdyrins. Even though these two macrocycles contain 22π electrons and five pyrrole/heterocyclic rings, they are structurally different. In sapphyrins, the five pyrrole/heterocyclic rings are connected through four meso-carbon bridges and one direct pyrrole−pyrrole bond, whereas in smaragdyrins, the five pyrrole/heterocyclic rings are connected through three mesocarbon bridges and two direct pyrrole−pyrrole bonds. The chemistry of sapphyrins has been well-established in recent years due to the availability of easy and efficient synthetic methods. On the other hand, smaragdyrins are not explored significantly because of their unstable nature. However, recently it was shown that smaragdyrins can be stabilized if one of the pyrrole rings is replaced with a furan ring to afford stable oxasmaragdyrin. The availability of oxasmaragdyrin allowed the exploration of smaragdyrin in recent years. Thus, an attempt has been made in this Review to describe the chemistry of both sapphyrins and smaragdyrins in terms of their synthesis, characterization, metal ion coordination, and anion-recognition properties.

CONTENTS 1. Introduction 2. Sapphyrins 2.1. Syntheses 2.1.1. β-Alkyl Sapphyrins 2.1.2. meso-Aryl Sapphyrins 2.1.3. Carbasapphyrins 2.1.4. Core-Modified Sapphyrins 2.1.5. N-Confused Sapphyrins 2.1.6. Fused Sapphyrins 2.1.7. Nonaromatic Sapphyrins 2.1.8. Covalently Linked Conjugates 2.2. Anion-Binding Interactions 2.2.1. Halide Ions 2.2.2. Carboxylate Ions 2.2.3. Phosphate Ions 2.2.4. DNA Binding and Photocleavage 2.2.5. Binding Studies with Fused Sapphyrins 2.3. Applications 2.3.1. Sapphyrin−Nanotube Assemblies 2.3.2. Sapphyrin−Fullerene Conjugates 2.3.3. Photodynamic Therapeutic Agents 2.4. Coordination Chemistry of Sapphyrins 2.4.1. Complexes with β-Alkyl Sapphyrins 2.4.2. Complexes with Core-Modified Sapphyrins

© XXXX American Chemical Society

2.4.3. Complex with Normal and N-Confused meso-Aryl Sapphyrins 3. Smaragdyrins 3.1. Smaragdyrins (1970−1998)/Earlier Attempts of Synthesis of Smaragdyrin 3.2. Smaragdyrins (1998−Present) 3.3. Oxasmaragdyrin-Based Conjugates 3.4. Coordination Chemistry of 25-Oxasmaragdyrins 3.5. BF2−Smaragdyrin-Based Conjugates 3.6. PO2 Complexes of meso-Triaryl-25-Oxasmaragdyrins 3.7. Mixed B(III) and P(V) Complexes of 25Oxasmaragdyrins 3.8. Calixsmaragdyrins 4. Conclusions Author Information Corresponding Authors Notes Biographies Acknowledgments Abbreviations

B C C C C D D H H J K M N O R U V V V W W W X

Z Z Z AA AD AG AK AM AO AP AQ AR AR AR AR AR AR

Special Issue: Expanded, Contracted, and Isomeric Porphyrins

Z

Received: August 4, 2016

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DOI: 10.1021/acs.chemrev.6b00507 Chem. Rev. XXXX, XXX, XXX−XXX

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research fields including as sensitizers for PDT,11 MRI contrasting agents,12 multimetallic chelates for catalysis,13 receptors for anions and neutral substrates,14 and models for aromaticity.15 This led to a flurry of research activity on synthesis, characterization, structure, and spectroscopic and electrochemical properties of various expanded porphyrins. Because of their large size and higher number of donor atoms, the expanded porphyrins provide a suitable coordination environment to form a variety of coordination complexes. Even though the pioneering work in the 1960s by the Woodward16 and Johnson17 research groups established the existence of expanded porphyrins, it is only in the late 1980s and early 1990s that the chemistry of expanded porphyrins has been exploited due to the availability of dipyrromethanes and tripyrromethanes, which are the key precursors for the synthesis of expanded porphyrins.17 A variety of expanded porphyrins with varying degrees of ring sizes such as sapphyrins,18 smaragdyrins,19 pentaphyrin,20 hexaphyrin,21 rosarian,22 rubyrin,23 heptaphyrins,24 octaphyrins,25 nonaphyrins,26 turcasarin,27 and dodecaphyrins,28 including the largest expanded porphyrin to date,29 have been synthesized and their anion-binding, metal-chelation, ground-state, and excited-state properties of some of them have been evaluated.13,14 The present Review is focused only on macrocycles containing five pyrrole/heterocyclic rings that are linked to each other in a cyclic fashion via meso-carbon bridges and direct bonds such as sapphyrins and smaragdyrins. Sapphyrins16,17 are one class of expanded porphyrins with five pyrrole/heterocycle rings joined with four meso-carbon bridges and one direct pyrrole/ heterocycle bond, and these macrocycles are one of the most extensively studied expanded porphyrinoids. On the other hand, smaragdyrins19 are macrocycles with five pyrrolic/ heterocyclic rings containing three meso-carbon bridges and two direct pyrrole/heterocycle links that are relatively underdeveloped because of synthetic difficulties and their inherent instability (Chart 2). In the recent past, several reviews

AR

1. INTRODUCTION Porphyrins, the tetrapyrrolic macrocycles with 18π electrons, are the most versatile ligands present in nature because of their unique attractive properties as ligands and photoactive materials.1 Nature has selected the porphyrin skeleton, generally in metalated form, for a surprising number of functions that include oxygen transportation in mammals (hemoglobin), energy production from oxygen reduction as a part of respiratory chain (cytochrome oxidase), electron transport/redox (cytochromes), peroxide breakdown (catalase, peroxidases), photosynthesis (chlorophylls), and so on.2 The synthetic porphyrins also have a wide range of applications in various fields that include material science,3 medicine,4 and catalysis.5 There are many structural variants of porphyrin (A) macrocycles that are generated by modifying either the porphyrin periphery and/or the core, and some of the resulting macrocycles are shown in Chart 1. The different porphyrinoids Chart 1. Molecular Structures of Porphyrinoids

Chart 2. Molecular Structures of Sapphyrin (I) and Smaragdyrin (II)

appeared on expanded porphyrinoids describing the various aspects of different expanded porphyrinoids including sapphyrins and smaragdyrins, but the chemistry of sapphyrins and smaragdyrins was not discussed to a greater extent in any one particular review. Sessler and Davis18 described sapphyrins as versatile anion-binding agents in their accounts published in 2001. In 2012, we made an attempt to bring the attention of researchers on the less-explored but promising smaragdyrin macrocycles by writing accounts on smaragdyrins.19 However, several interesting articles appeared on the chemistry of both sapphyrins and smaragdyrins in the recent past. In this Review, we made an attempt to describe the synthesis, characterization, metal coordination, anion recognition, and applications of sapphyrins and smaragdyrins reported to date.

are created (a) by changing the substituents at the periphery of the porphyrin ring (B),6 (b) by removing one meso-carbon from the porphyrin to generate a corrole skeleton, which has one direct pyrrole−pyrrole link (C),7 (c) by inverting one of the pyrrole rings, which leads to the formation of “N-confused porphyrins” (D),8 (d) by substituting one or more pyrrole nitrogen atoms with chalcogens such as O, S, Se, Te, P, Si, etc. to form “core-modified porphyrins” (E),9 and (e) by enhancing the π electron conjugation by increasing the number of heterocyclic rings (F)10 or by introducing additional mesocarbons in the porphyrin skeleton by keeping the number of heterocyclic rings the same as in the parent porphyrinoid to form a class of macrocycles called “expanded porphyrins”. The expanded porphyrins have diverse applications in various B


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Scheme 3. Synthesis of β-Alkyl Sapphyrin 9

2. SAPPHYRINS 2.1. Syntheses

2.1.1. β-Alkyl Sapphyrins. Sapphyrin is the simplest possible expanded porphyrin with 22π electrons. The existence of sapphyrins was noted serendipitously by Woodward and coworkers16 during their efforts to synthesize vitamin B12, although the synthesis of azasapphyrins was reported much later.30 Woodword named these pentapyrrolic compounds as sapphyrins because of their intense blue-colored crystals. Johnson and co-workers17 were the first to report the synthesis of the dioxa analogue of sapphyrin in 1972, which we discuss in the Core-Modified Sapphyrins section. The all-aza sapphyrin30 3 was obtained initially in 71% yield by [4 + 1] condensation between the linear tetrapyrrolic precursor 1 and 2,5-diformyl3,4-dimethylpyrrole 2 under acidic conditions followed by aerial oxidation (Scheme 1).

synthetic methodology was an alternate route for the [3 + 2] approach and gave better overall yield and also reduced the number of synthetic steps to prepare β-alkyl azasapphyrin. Smith and co-workers33 have also reported the synthesis of β-alkyl-substituted sapphyrin 12 by condensation of 1,19diunsubstituted a,c-biladiene salt 11 and pyrrole carboxaldehyde 10 under acidic conditions (Scheme 4). In this case, trace



Later, Sessler and co-workers31 modified the methodology by adopting the MacDonald-type [3 + 2] condensation approach to prepare β-alkyl azasapphyrin 6. The acid-catalyzed condensation of diformylbipyrrole 4 and tripyrromethane diacid 5 followed by oxidation resulted in the formation of 6 (Scheme 2). In Sessler’s approach, the precursor with the direct

amounts of porphyrin and corrole were also formed, which may be due to the partial acidolysis of a,c-biladiene under the reaction conditions. Recently, Panda and co-workers reported the synthesis of highly electron-rich decamethoxysapphyrin by [3 + 2] MacDonald-type acid-catalyzed condensation of methoxy-substituted bipyrrole dialdehyde and methoxy-substituted tripyrrane diacid.34 The pyrrole ring inversion of sapphyrins upon protonation was commonly observed for mesoaryl sapphyrins but was not observed in β-substituted sapphyrins. However, Panda and co-workers34 noted that the diprotonated N-benzyl-substituted decamethoxysapphyrin showed counteranion-induced out-of-plane distortion of the N-benzylpyrrole moiety, although complete inversion did not occur. 2.1.2. meso-Aryl Sapphyrins. Latos-Grażyński and coworkers35 were the first to isolate all-aza meso-aryl sapphyrin 15 by BF3·OEt2-catalyzed condensation of pyrrole 13 and benzaldehyde 14. The condensation resulted in the formation of sapphyrin 15 along with N-confused porphyrin and tetraarylporphyrin, which were separated by column chromatography and afforded meso-aryl sapphyrin in 1% yield (Scheme 5). The 1H NMR spectrum of 15 showed that the pyrrolic βCH proton resonated at −1.50 ppm, while the NH proton was observed at 12.24 ppm, indicating that the pyrrole ring opposite to the bipyrrole moiety was inverted. The chemical shift difference between the most shielded and deshielded CH and NH protons in 1H NMR and splitted Soret-like band in absorption spectrum strongly support the aromatic character of the meso-aryl sapphyrin 15. Upon protonation, the inverted pyrrole ring underwent 180° (Scheme 5) ring flipping and


pyrrole−pyrrole link was used to prepare β-alkyl azasapphyrin. However, the preparation of such a substituted bipyrrolic precursor requires multistep synthesis. To avoid such a precursor, which requires multistep synthesis, the same research group introduced the [3 + 1 + 1] methodology to synthesize βalkyl azasapphyrin 9, in which the direct pyrrole−pyrrole bond formation takes place in the final step.32 The acid-catalyzed condensation reaction of bisformyl tripyrromethane 8 with 2 equiv of β-substituted pyrrole 7 followed by oxidation afforded the β-substituted azasapphyrin 9 in 34% yield (Scheme 3). This C


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Scheme 5. Synthesis of meso-Aryl Sapphyrin 15 and Its Protonated Sapphyrin 15.2H+

isolated yields were dependent on the nature of the acid catalyst and the concentration of the acid used (Scheme 6). Imahori and co-workers41 reported a simple acid-catalyzed condensation reaction of pyrrole and 3,5-bis(trifluoromethyl)benzaldehyde followed by p-chloranil oxidation, which afforded a series of normal and expanded porphyrins up to heptaphyrin, and the meso-aryl sapphyrin was isolated as one of the products in 2% yield. Sessler and co-workers42 reported the synthesis of meso-diaryl sapphyrin 26 (Scheme 7) by Lewis acid-catalyzed condensation of 3 equiv of pyrrole 13, 2 equiv of benzaldehyde 14, and 1 equiv of β-alkyl-substituted diformyl bipyrrole 4 followed by oxidation, which afforded the meso-aryl sapphyrin in the form of its chloride salt in 10% yield. 2.1.3. Carbasapphyrins. The first carbasapphyrin was reported by Richter and Lash43 via [4 + 1] MacDonald acidcatalyzed condensation of tetrapyrrole dicarboxylic acid 27 and 1,3-diformyl indene 28 (Scheme 8) followed by oxidation with DDQ and afforded the benzocarbasapphyrin 29 in 38% yield. The [4 + 1] synthetic methodology was extended further with a series of diformyl derivatives inluding pyrrole, furan, thiophene, fused phenanthrene, acenaphthylene, azulene, 3-hydroxypyridine, and triformyl cyclopentadiene and afforded the corresponding carbasapphyrins 30−37 in ∼70% yields. The authors44 have also mentioned that varying the oxidizing agent from DDQ to dilute aqueous FeCl3 under similar reaction conditions improved the yields of 29−37 to ∼90%. 2.1.4. Core-Modified Sapphyrins. Replacing one or more pyrrolic units by other heterocyclic rings such as furan, thiophene, selenophene, tellurophene, and N-methyl pyrrole leads to a new class of sapphyrins, called core-modified sapphyrins/heterosapphyrins. The core modification leads to a change in the cavity size and altered electronic structure with changes in optical, electrochemical and excited state properties that are different from their pyrrolic counterparts. The first core-modified sapphyrin was reported by Johnson and coworkers17 by [3 + 2] MacDonald-type condensation reaction, where tripyrromethane diacid 39 was condensed with diformyl bifuran 38 followed by oxidation and afforded the dioxasapphyrin 40 in 10% yield (Scheme 9). The core-modified monothiasapphyrin 43 and monoselenasapphyrin 44 were reported by Sessler and co-workers.45 The required starting materials such as thiatripyrromethane 41 or selenatripyrromethane 42 were synthesized from oxatripyrromethane by treating it with H2S or H2Se under anaerobic acidic conditions. The condensation of appropriate core-modified tripyrromethane 41/42 with β-alkyl-substituted diformyl bipyrrole 4 in the presence of p-TSA as acid catalyst followed by oxidation in the presence of molecular oxygen afforded

adopted the normal form, where all the pyrrolic nitrogen atoms are inside the macrocyclic framework.36 This observation was clearly evident in the shifts of β-CH and NH protons in 1H NMR and the intense Soret band in the absorption spectrum, which in turn supported the structural diversity in meso-aryl sapphyrin. Similar observations were made by Dolphin and coworkers37 in their 5,10-diphenyl sapphyrin and meso-phenyl sapphyrins, which were synthesized by [3 + 2] methodology. Lindsey and co-workers38 also reported the synthesis of mesoaryl sapphyrins in 21 ppm, and the intense Soret-like band at 535 nm reflects the typical aromatic character of the expanded sapphyrin (Scheme 27). Like diprotonated 6, two TFA ions

Open-chain and cyclic covalently linked sapphyrin dimers 149−151 with chiral bridging units were also reported94 (Figure 15). Open-chain chiral sapphyrin dimers 149 and 150 were synthesized from the activated sapphyrin monoacid with chiral diamines such as (S)-2,2′-diamino-1,1′-binaphthalene and (1S,2S)-1,2-diaminocyclohexane by using 1,3-diisopropylcarbodiimide-mediated coupling agents and afforded respective dimers in good yield. The cyclic sapphyrin dimer 151 with stereogenic linker was synthesized by a stepwise synthetic approach, where 2 equiv of mono-t-BOC-protected diamine reacted with activated sapphyrin monoacid to obtain the tertbutyloxycarbonyl (BOC)-protected sapphyrin diamine unit. The BOC group was deprotected by using TFA, which further condensed with activated sapphyrin monoacid to afford the cyclic dimer in 60% yield. Dimers 149 and 150 were found to form complexes with N-carbobenzyloxy-protected aspartate and glutamate anions with a binding constant of 104−105 M−1, where greater binding affinity with glutamate was observed over that with aspartate anion. On the other hand, dimer 151 was bound with these anions less effectively but displayed excellent chiral discrimination between D- and L-antipodal N-carbobenzyloxy-protected glutamate anion.94 The sapphyrin−lasalocid conjugate95,96 152 was synthesized from sapphyrin monoacid with mono-tert-butyloxycarbonylethylenediamine-based lasalocid by using traditional coupling agents such as 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC) and HOBt (Figure 16). This chiral conjugate was designed as a zwitterionic ditopic receptor, capable of recognizing both carboxylate anions and ammonium

Figure 16. Sapphyrin−lasalocid conjugates 152. Q


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Figure 17. Noncovalently linked (a) sapphyrin−porphyrin conjugates 153 and (b) crystal structure of 154. Reproduced with permission from ref 99. Copyright 1996 Wiley-VCH.

derivative. The structure was found to be complicated because of phosphate aggregation in the crystal lattice. Monobasic phenylphosphate and diphenylphosphate form 2:1 complex with diprotonated sapphyrin derivatives, where the oxygen atoms are 1.22 and 1.60 Å above and below, respectively, the mean sapphyrin plane 159 (Figure 19b), and the oxygen atoms were held with three and two intermolecular hydrogen bonds as well as electrostatic interactions. In addition, the phenyl moiety of the phenylphosphate anion was present over a portion of the aromatic sapphyrin skeleton at distances similar to those expected for van der Waals contact. The binding constants were measured in both methanol and aqueous medium at pH 6.1 and were found to be 104 M−1 in methanol and 102 M−1 in aqueous solution.101 Sessler and co-workers102 have also shown that the protonated form of sapphyrins could serve as a carrier of nucleotide bases. This is important because the monophosphate derivatives of both 9-(β-D-arabinofuranosyl)adenine (Ara-AMP) and 9-(β-xylofuranosyl)guanine (Xylo-GMP) have high anti-HSV activity in cell free suspension. The transport of nucleotides was carried out in a U-tube type, AqI−CH2Cl2− AqII, model membrane system. A pH of 3.5 was determined to be the ideal condition for effective transport across the lipophilic barrier prior to the U-tube experiment. Mononucleo-

Scheme 27. Synthesis of Expanded Sapphyrin 157

were bound with the macrocyclic ring via three and two intermolecular hydrogen-bonding interactions with bond distances of 1.83 and 1.98 Å, respectively (Figure 18). 2.2.3. Phosphate Ions. Similar to the above-mentioned anions, the monobasic phosphoric acid also forms 1:1 complex 158 with diprotonated sapphyrin 6 derivatives,101 in which the single oxygen atom of the phosphate moiety was coordinated to the diprotonated derivative via five hydrogen bonds. The oxygen atom was placed 1.22 Å above the mean plane whereas the remaining three oxygen atoms remained noncoordinated (Figure 19a). Similarly, the neutral 1:1 complex formed between dibasic phosphoric acid and diprotonated sapphyrin

Figure 18. Crystal structure of 157: (a) top view and (b) side view. Reproduced with permission from ref 100. Copyright 2009 American Chemical Society. (The β-alkyl groups are omitted for clarity in the side view.) R


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Figure 19. (a) Crystal structure of 158. Reproduced with permission from ref 101. Copyright 1996 American Chemical Society. (b) Crystal structure of 159. Reproduced with permission from ref 101. Copyright 1996 American Chemical Society. (The β-alkyl groups are omitted for clarity.)

better than its congener. 161 binds selectively with 2′-GMP in the order of 2 × 104 M−1 as compared to 5′-GMP, where the association constant was found to be 8 × 103 M−1. Better transport efficiency was observed when the receiving AqII was highly basic. On the basis of the recognition, the possible mode of binding 165 was introduced where the complementary Watson−Crick base pairing and the phosphate chelation with the sapphyrin unit led to selective and effective transport of GMP104 (Figure 21). Overall, the monophosphorylated nucleotides and nucleotide analogues were transported by sapphyrin and nucleobase conjugates. For oligonucleotide binding and transport experiments, Sessler and co-workers105 have synthesized the covalently linked branched trimer and tetramer and linear trimer 166− 168 (Figure 22). The branched oligomers were synthesized under similar reaction conditions as adopted for the synthesis of dimer derivative, where 30% of the excess of sapphyrin in the activated form, like acid chloride, acylimidazole, mixed anhydride, or activated ester, was treated with tris(2aminoethyl)amine in the case of trimer 166 and N,N,N′,N′tetrakis(2-aminoethyl)ethylenediamine in the case of tetramer 167 and obtained in 50−80% yields. The linear trimer 168 was also synthesized from the activated form of sapphyrin monoacid with bisaminosapphyrin to afford 50−80% yields. All the derivatives were bound with ADP and ATP (adenosine di- and triphosphate, respectively) with the binding constant ranging from 1900 to 6800 M−1. Compounds 166 and 167 bound more effectively with ATP than with ADP, whereas 168 bound strongly with ADP as compared with ATP. At neutral pH, the membrane transport experiment was performed by using Pressman model system AqI−CH2Cl2−AqII, where 166 and 167 were found to be efficient carriers for the throughbulk-membrane transport of nucleotide diphosphates105 and 168 was found to be a suitable carrier for various phosphorylated species. The covalently linked molecular receptor and a solid support provide valuable understanding about the substrate−receptor interactions (Figure 23). The combinations of such systems were synthesized from aminopropyl silica gel with acid chloride-based sapphyrin in a starting sapphyrin-to-silica weight ratio of 1:10 (169). The anion-binding selectivities were tested by isochratic high-performance liquid chromatography (HPLC) conditions at neutral pH, where the sapphyrin-modified silica gel 169 was highly selective for sterically accessible oxyanions such as phosphates, phosphonates, and arsonates. The system

tides GMP, AMP, and Ara-AMP were efficiently transported across the CH2Cl2 membrane at this pH. A possible 1:1 complex formed between sapphyrin and GMP 160 is shown in Figure 20. The increase of pH or addition of competitive anions such as NaCl or NaF to Aq I leads to long induction periods for nucleotide transport.102

Figure 20. Possible mode of binding between sapphyrin and GMP 160.

In the above experiment, the addition of triisopropylsilylprotected cytidine (C-Tips), as a complementary cotransporting agent of GMP, led to effective sapphyrin-mediated transport of GMP. To enhance the transport of GMP further, Sessler and co-workers103 designed the sapphyrin with a nucleobase-recognition unit, where the cytosine unit was directly incorporated into the sapphyrin unit. Ditopic receptors 161 and 162 were designed for the phosphate interaction with the sapphyrin unit as well as simple base-pairing interaction with the cytosine/guanosine conjugates. Tritopic receptors 163 and 164 were designed for the formation of triple-helix complexation such as C-G-C and G-C-G motifs. These conjugates were synthesized from aminoethylcytosine and aminoethylguanosine coupled with sapphyrin mono- and dicarboxylic acids. The sapphyrin acids were activated by acid chlorides or via the use of carbodiimide or carbonyldiimidazole. The protecting groups such as trityl and benzoyl were removed by using TFA and ammonia. The transport experiments were performed by AqI−CH2Cl2−AqII liquid membrane cell. At neutral pH, the sapphyrin-appended monocytosine unit 161 was selectively transporting GMP through the cell membrane S


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Figure 21. Sapphyrin−nucleobase conjugates 161−164 and possible mode of binding 165.

Figure 22. Covalently linked branched trimer 166, tetramer 167, and linear trimer 168.

the mobile phase, where the elution time increases in the following order: arsenate > phosphate > chloride > sulfate > nitrate = bromide > iodide > acetate. The rate was consistent with the relative protonated sapphyrin−anion binding interaction as observed in the solution phase.107

was also capable of separating phosphorylated species such as AMP, ADP, and ATP and polyphosphorylated materials such as oligonucleotides, AMP, ADP, and ATP. The column also supports separation of 2−9 mers of polydeoxyadnylic acid and 3−5 mers of polydeoxycytidylic acid.106 Further, the rate of AMP elution in the column depends on the anion present in T


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in both monomer and single-stranded polymeric nucleotides and (ii) highly ordered aggregation of water-soluble sapphyrin derivatives on the surface of double-stranded, helical nucleic acids and templated by these nucleic acid polymers.109 These modes were identified by electronic absorption spectral analysis, where intense absorption is shown for the monomeric form at 450 nm, the dimeric form at 420 nm, and the highly ordered aggregation form at 410 nm. In continuation, sapphyrin−EDTA conjugates110 171 were designed to interact and influence the cleavage in the DNA backbone (Figure 24). The small-molecule conjugate was synthesized from activated sapphyrin monoacid with EDTA derivative in the presence of known amide-coupling agent such as EDC or HOBt followed by TFA deprotection of the tertbutyl group and afforded the conjugate in 72% yield. A supercoiled plasmid DNA was utilized to test the efficacy of conjugate as a DNA-binding and cleaving agent. Here, the iron chelate of the conjugate was prepared in situ from ferrous ammonium under aerobic conditions and reducing agent such as dithiothreitol (DTT) to affect the cleavage of the supercoiled plasmid pBR322. The efficient cleavage was observed by using 3 μM concentration of conjugate, which was as low as 25 μM concentration of Fe-EDTA alone required for cleavage. Sapphyrin−oligonucleotide conjugates 172 were reported by Sessler and co-workers111 (Figure 25). The monoprotected sapphyrin hydrogen phosphate was synthesized via monoprotection followed by phosphorylation. This compound was further activated by pivaloyl chloride and coupled with control pore glass beads supported the 5′-end of a (dT) 12 oligonucleotide. The 5′-end was deprotected by dichloroacetic acid and subsequently oxidized with basic aqueous iodine solution. In the final step, the phosphodiester linkages on the (dT)12 portion were deprotected and the CPG was cleaved by using concentrated aqueous ammonium hydroxide solution with an overall yield of 56%. Conjugate 172 was designed mainly for DNA photocleavage and nucleic acid-binding properties and was found to be suitable as a sequence-specific photomodification agent above 620 nm irradiation wavelength.111,112 The sapphyrin-based ditopic receptors were designed with the aim to cleave the oligonucleotides. The receptor113 was achieved by covalently linking both the diprotonated sapphyrin and various polyhydroxy subunits 173a−f, and bis(4nitrophenyl)phosphate (BNPP) was used as a model phosphodiester (Figure 26a). The enhanced hydrolysis rate constants were observed at pH 7.5 in the case of polyhydroxyl subunits, and rates are found to be 38 × 10−5 h−1 and 11 × 10−5 h−1 with a turnover of 1.2 over the course of 10 days. The crystal analysis of diprotonated dihydroxylated sapphyrin with BNPP was shown with 1:2 binding (Figure 26b), where oxygen in one of the BNPPs interacts with four imine NH groups and the second BNPP anion was coordinated with two oxygen

Figure 23. Covalently linked sapphyrin-modified silica gel conjugates 169.

2.2.4. DNA Binding and Photocleavage. The noncovalent interaction between the small molecule and DNA are usually addressed in the form of (i) intercalation, (ii) groove binding, and (iii) simple electrostatic interaction. The positively charged diprotonated sapphyrin 170 interacts with aqueous dsDNA at neutral pH113 (Figure 24). The interaction was

Figure 24. Molecular structure of 170 and sapphyrin−EDTA conjugates 171.

reflected from various spectral analyses: (i) circular dichroism (CD) spectral analysis shows the Soret-like transition at 408 nm of achiral sapphyrin; (ii) 31P NMR analysis reveals 3.6 ppm upfield shift; and (iii) electronic spectral analysis shows 11 nm bathochromic shift of the Soret band from 409 to 420 nm. On the basis of the solid-state evidence from monobasic phosphoric acid and diprotonated sapphyrin 158 (Figure 19a), it was concluded that the specific interaction such as “phosphate chelation” was observed between the diprotonated sapphyrin and phosphate oxyanions that were part of the DNA backbone via multiple NH-phosphate oxyanion hydrogen bonds. A similar trend was observed with ssDNA (singlestranded), and the binding constant was 25 000 M−1. However, the CD signal intensity was less as compared with dsDNA.108 In addition to phosphate chelation, other interactions were also observed such as (i) hydrophobic interaction with nucleobases

Figure 25. Sapphyrin−oligonucleotide conjugates 172. U


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Figure 26. (a) Molecular structures of sapphyrin−ditopic receptors 173a−f and BNPP. (b) Crystal structure of 173f−BNPP conjugate. Reproduced with permission from ref 113. Copyright 2006 American Chemical Society.

Figure 27. Crystal structure of 175: (a) top view and (b) side view. Reproduced with permission from ref 62. Copyright 2005 Wiley-VCH. (The βalkyl groups are omitted for clarity in the side view.)

Naphthosapphyrins66 108 bound with HCl as well as acetic acid were confirmed by single-crystal X-ray analysis, where two units of HCl and one unit of acetic acid interact with naphthosapphyrins. Two chloride ions were above and below the mean macrocyclic plane, each of which interacted with three pyrrolic NH groups with N···Cl distances from 3.08 to 3.26 Å, and the distances from the mean plane were 1.73 and 1.97 Å, which were higher as compared to dichloride salts of decaalkyl (6) (1.77 and 1.89 Å)88 and diphenyl sapphyrins (26) (1.82 and 1.85 Å).42 On the other hand, the oxygen atom of the acetate ion was bound to the three hydrogen atoms of the sapphyrin unit, where the N···O distances were in the range of 2.70−2.74 Å. In addition, the acetate ion was bound much closer to the mean plane with a distance of 1.04 Å, reflecting the stronger interaction of the acetate ion.

atoms. In addition, the preliminary studies also revealed that the incubation of a dA − dT mixed 10-mer with diprotonated sapphyrins (A−C) at pH 7.5 for 2 to 48 h afforded phosphorylated fragments. 2.2.5. Binding Studies with Fused Sapphyrins. Benzosapphyrin 101 was isolated as p-tosylate salt,62 where two tosyl units were bound above and below the mean macrocyclic plane. The above tosylate group was bound with three pyrrolic NH (N2, N3, and N5) groups, while the below tosylate group also interacts with three NH (N1, N2, and N4) groups with intermolecular hydrogen-bonding interactions (Figure 27). The structure of diprotonated bistosylate salt of dioxobenzosapphyrin63 102 was also elucidated, where the oxygen atom of one of the tosylate counteranions bound with three NH protons of the sapphyrin unit with intermolecular H-bonding as well as electrostatic interactions with average NH···OTs and O···OTs bond distances of 2.11 and 2.84 Å, respectively. The anion-binding experiment of 102 was further performed with various anions, and it was found that it binds effectively with F− ion as compared to other anions as well as binds effectively with neutral Ar−OH species.

2.3. Applications

2.3.1. Sapphyrin−Nanotube Assemblies. The single− wall carbon nanotubes (SWNTs) bind with functionalized sapphyrin114 diol by strong noncovalent interaction such as donor−acceptor stacking interactions (176), which affords water-suspendable nanotubes and also forms well-defined assemblies in ionic liquid such as BMIM-PF6 (1-butyl-3methylimidazolium hexafluorophosphate) (Figure 28). The V


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2.3.3. Photodynamic Therapeutic Agents. The biological properties of various water-soluble sapphyrins 179a−e were explored by Sessler and co-workers.116 Water-soluble sapphyrins 179a−e showed an absorption band at 675 nm in aqueous media and even at longer wavelength in the polar solvents; they generate a singlet oxygen with good quantum yield, which promotes them as suitable PDT agents (Figure 30). These derivatives were found to localize selectively pancreatic carcinoma tissue over surrounding normal tissues in a xenograft model. Chandrashekar and co-workers117 have also examined drug uptake into human erythrocyte tissues by using water-soluble core-modified sapphyrin derivative 180 (Figure 31) and found that the retention time was faster than that of the Photofrin. In addition, the in vivo experiment was performend on squamous cell carcinoma on mouse skin, where improved reduction in the tumor volume was observed from the photodynamic efficacy studies. In another set of experiments by Sessler and co-workers,118 under in vitro conditions, the newly designed water-soluble sapphyrins 181 (Figure 31) inhibit the proliferation of cancer cell lines such as human lung and prostate cancer at relatively low dose, which leads to cell death. This was associated with increased ROS (reactive oxygen species) and results in a decrease in the overall transcript level. The in vivo experiments revealed that the water-soluble derivatives (181) inhibit A549 tumor growth effectively in a xenograft model. Overall, the hydrophilic sapphyrin is an excellent PDT agent, which localizes to tumors, generates ROS, and restricts gene expression.

Figure 28. Sapphyrin−nanotube assemblies 176.

electronic absorption, emission, and time-resolved femtosecond transient spectral studies of noncovalent assemblies revealed that the photoexcited intramolecular electron transfer takes place upon photoexcitation, where the nanotube acts as electron donor and the sapphyrin acts as excited-state electron acceptor. Overall, the assemblies were suitable as donor− acceptor species for light-harvesting antenna. 2.3.2. Sapphyrin−Fullerene Conjugates. Noncovalent charge-transfer assemblies were designed by using suitable donor and acceptor units. Here, one of the components involved was the diprotonated sapphyrin, which acts as the receptor as well as donor upon irradiation in the presence of acceptor. The second component is a dendrimer, where two C60 fullerene cores are functionalized with multiple carboxylate anion groups, which acts as electron acceptor. The titration between these components formed 1:1 (177) and 1:2 (178) noncovalent assemblies115 (Figure 29). Electron transfer was observed from the protonated sapphyrin to polycarboxylate fullerene dendrimers, when irradiated with 387 nm light. The supramolecular assembly combined together to generate the charge-separated state with lifetimes of 470 and 600 ps, respectively.

2.4. Coordination Chemistry of Sapphyrins

The size of the internal core for the expanded porphyrins,31 and its apparent stability under strongly basic conditions, suggests that these macrocycles could display rich metal complexation chemistry. Because of the inward-pointing chelating nitrogen atoms, a variety of different metal coordination modes can be

Figure 29. Sapphyrin−fullerene conjugates 177 and 178. W


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Figure 30. Sapphyrin-based PDT-active 179.

Figure 31. Sapphyrin-based PDT-active 180 and 181.

Figure 32. Metal complexes of β-alkyl sapphyrins 182−187.

Scheme 28. Synthesis of Mono-, Bis-, and Heterobis-Rh(I) and Ir(I) Complexes 188−192

conceived.119 Expanded porphyrins containing heteroatoms in their core are also expected to bind transition metals. However, the coordination chemistry of sapphyrins is still in its infancy stage, as compared to the anion complexation, and only a handful of metal complexes of sapphyrins have been successfully isolated and characterized. 2.4.1. Complexes with β-Alkyl Sapphyrins. The coordination chemistry of sapphyrin was initially carried out by Woodward and co-workers.30 The CH3OH solution of sapphyrin 3 was treated with acetate salts of various transition metals such as Ni2+, Fe2+, Cd2+, Mn2+, Co2+, and Zn2+ in the

presence of sodium acetate. Electronic spectral analysis of complexes 182 and 183 revealed that these complexes showed a Soret band at 462 and 468 nm, respectively, which was 10−15 nm bathochromically shifted compared to free-base compound 3. The results were further confirmed by mass spectral analysis and proposed that only four out of five pyrrole nitrogen atoms were coordinated to the metal ion and two out of the three pyrrole NHs were involved in complexation (Figure 32).30 The results were later confirmed by Sessler and co-workers,91 and they observed two isomeric tetra ligated complexes that were in equilibrium, 184A−184B, with one another (Figure 32). X


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Figure 33. (a) Crystal structure of 189. Reproduced with permission from ref 121. Copyright 1991 Wiley-VCH. (b) Crystal structure of 190. Reproduced with permission from ref 121. Copyright 1991 Wiley-VCH.

carbonyl groups that were bound to the respective metal ions.121 Sessler and co-workers122 synthesized the stable uranyl complex of sapphyrin 193 by treating UO2Cl2 salt with sapphyrin in the methanol, pyridine, and trimethylamine media (Scheme 29). The spectral analysis indicated that the complex

However, the reaction was successfully executed with monothia or mono-oxa sapphyrin analogues. The CH2Cl2 solution of monothia 43 or mono-oxa 45 sapphyrin on treatment with appropriate Co(II) salt afforded the respective Co(II) complexes 185−187 where Co(II) was bound to sapphyrin ligand without deprotonation of the ligand. In the case of the Co(II) complex of monothiasapphyrin 185, the metal ion was placed 0.01 Å above the mean plane and adopted tetrahedral geometry with Co−N and Co−S bond distances of 2.01 and 2.23 Å, respectively, whereas in the case of the Co(II) complex of mono-oxasapphyrin 186, the Co(II) ion was placed 0.02 Å above the mean plane and the Co−N and Co−O distances were found to be 1.96−2.02 and 2.95 Å, respectively.120 The sapphyrin was further treated with second- and thirdrow transition metal salts such as HgCl2, CdCl2, PdCl2, RhCl3, IrCl3, and RuCl3, but the resulting metal complexes were decomposed. Treatment with PdCl2(CH3CN)2 afforded a 1:1 complex, but the complex was found to undergo gradual decomposition. The reactions were further performed with metal carbonyl complexes, where CH2Cl2 solution of sapphyrin was treated with 0.5 equiv of [RhCl(CO)2]2 or 1 equiv of [IrCl(CO)2Py] to afford the mono Rh(I) 188 and Ir(I) 189 complexes121 (Scheme 28). The crystal structure of Ir(I) complex 189 was shown in Figure 33. These complexes were further reacted with additional molar equivalents of respective metal carbonyl complexes and afforded the bimetallic Rh(I) 190 and Ir(I) 191 complexes (Figure 33). Alternatively, the 190 and 191 complexes were achieved by treating 6 with excess equivalents of respective carbonyl Rh and Ir salts. The reaction was also performed by using mono-Rh(I) or Ir(I) complex of sapphyrin with additional equivalents of Ir(I) or Rh(I) carbonyl complexes and yielded heterobis metallic complex 192, which was confirmed by mass and electronic spectral analysis. The crystal structure of 190 revealed that the Rh−N bond distances were in the range of 2.061−2.081 Å, and the geometry around the metal center was approximately square-planar. The metal ions in 189 and 190 were bound in a η2-fashion to the macrocyclic ligand, and metals were located above and below the macrocycle plane. In 190, the imine- and amine-type nitrogen atoms were alternate to one another, the rhodium atoms were bound between the dipyrromethane units, and the macrocyclic ligand was slightly ruffled. The Rh(I) and Ir(I) coordination spheres in 189 and 190 were filled by two

Scheme 29. Synthesis of Uranyl(VI) Complex 193

was nonaromatic, which was further confirmed by single-crystal X-ray analysis (Figure 34). The macrocycle was highly distorted from the planarity and adopted a bowl-shaped form with an average U−N bond distance of 2.5 Å. The structure also indicated that the nucleophilic attack of −OCH3 at the meso-

Figure 34. Crystal structure of 193. Reproduced with permission from ref 122. Copyright 1991 Royal Society of Chemistry. Y


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fused sapphyrin Re(I) complex 201 revealed that the Re center was connected by bipyrrolic nitrogen atoms and confused pyrrole nitrogen with Re−N bond distances of 2.29, 2.15, and 2.33 Å, respectively (Figure 36).

position resulted in the formation of nonaromatic complex, and the methanol solvent was essential for the formation of such a complex. 2.4.2. Complexes with Core-Modified Sapphyrins. The coordination chemistry of core-modified sapphyrins was also explored over the last 40 years. In 1971, Johnson and coworkers17 reported the first metal complexes of dioxasapphyrin with Ni(II) or Zn(II) salts. However, they failed to isolate the stable complex for detailed characterization. In the late 1990s, metal complexes of core-modified sapphyrin were extensively investigated by Sessler and co-workers.45 The bis-Rh(I) complex 194 was synthesized by treating monothiasapphyrin 43 with [Rh(CO)2Cl]2 in the presence of sodium acetate, and the structure was confirmed by single-crystal X-ray analysis. The Rh−N distance was found to be 2.08 Å, and the Rh−S contact distance was 3.16 Å. Similarly, monoselenasapphyrin 44 was reacted with [IrCl(CO)2Py] to form the bis-Ir(I) complex 195. The structure of complex 195 showed that the Ir−N distance was 2.09 Å and the Ir−Se distance was 3.10 Å. In the case of Rh(I) complexes of meso-aryl-substituted dithia- 196 and diselenasapphyrin 197, the Rh(I) ion was bound only to the bipyrrolic moiety as confirmed by spectral analysis (Figure 35).49

3. SMARAGDYRINS 3.1. Smaragdyrins (1970−1998)/Earlier Attempts of Synthesis of Smaragdyrin

Although Woodward and co-workers16 discovered the existence of a pentapyrrolic macrocycle with three meso-carbons and two direct pyrrole−pyrrole bonds that he named smaragdyrin (smaragdus: emerald), its synthesis and properties were not reported by them. In 1972, Broadhurst, Grigg, and Johnson17 reported the synthesis and spectral evidence of smaragdyrins, which they called norsapphyrins, as shown in Scheme 32. The substituted bipyrroles 202 were condensed with 3,4-dialkylpyrrole-2-carboxaldehyde 203 under HBr-catalyzed conditions and yielded the corresponding pyrrolyl dipyrromethane salts 204. The pyrrolyl dipyrromethane salts were then reduced with excess NaBH4 to the corresponding unstable pyrrolyl dipyrromethanes 205. Without isolation, the unstable pyrrolyl dipyrromethanes were then condensed with diformylbifuran 38 in the presence of HBr in CH3OH to afford dioxasmaragdyrins 206. The authors confirmed the formation of dioxasmaragdyrin by (M+2) ion peak in mass spectra and absorption spectra, which showed split Soret bands at 448 and 459 nm along with Q-bands in the 500−700 nm region. They have also attempted to prepare pentaazasmaragdyrin 208 by condensing 5,5′-diformyl bipyrroles 207 with pyrrolyl dipyrromethanes 205 under similar reaction conditions. Although absorption spectroscopy indicated the formation of pentaazasmaragdyrins, their attempts to isolate the macrocycles resulted in their decomposition. Thus, pentaazasmaragdyrins are too unstable to isolate for their characterization. After a long gap of 15 years, Sessler et al. synthesized a stable smaragdyrin124,125 isomer known as isosmaragdyrin (1.1.1.0.0) 217 by using α-free terpyrroles 213, which have direct pyrrole− pyrrole bonds and are condensed with dipyrromethanes 215 under acid-catalyzed conditions, as shown in Scheme 33. The required substituted terpyrrole 213 was synthesized over a sequence of steps. In the first step, the α-free pyrrole was reacted with propionyl chloride under SnCl4-activated coupling conditions to afford α-propionylpyrrole 209. Oxidative coupling of the lithium diisopropyl amide (LDA)-derived enolate of this α-propionylpyrrole with Cu(OTf)2 afforded the diketone 210 as a mixture of diastereomers. The standard ring closure of diketone gave a mixture of triaza- and diazamonoxa 211 and 212 products, which were separated by column chromatography. Saponification and decarboxylation in hot ethylene glycol afforded the bis-α-free terpyrroles 213 and 214 from 211 and 212, respectively. The terpyrroles 213 and 214 were then condensed with meso-free dipyrromethane 215 as

Figure 35. Rh(I) and Ir(I) complexes 194−197 of core-modified sapphyrins.

2.4.3. Complex with Normal and N-Confused mesoAryl Sapphyrins. The Rh(I) complex of 97 was isolated in high yield by Gross and co-workers. Pentapyrromethane 198 was treated with TFA in the presence of iodine/oxygen, which led to cyclization of the ring, followed by Rh(I) ion insertion by [Rh(CO)2Cl]2 to afford complex123 199, and the structure was characterized by spectral analysis (Scheme 30). Furuta and co-workers61 synthesized the Re(I) complexes of N-confused sapphyrin by treating 93 with [Re2(CO)10] in odichlorobenzene for 12 h and afforded two fused sapphyrins 200 and 201 in 7 and 55% yields, respectively (Scheme 31). Complex 200 was the same as that of N-fused porphyrin, while complex 201 was doubly fused sapphyrin metal complex and formed because of the formation of a bond between the βcarbon of the tricylic ring and the ortho-carbon of the mesopentafluorophenyl ring. The crystal structure of the doubly

Scheme 30. Synthesis of Rh(I) Complex 199 of meso-Aryl Sapphyrin

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Scheme 31. Synthesis of Re(I) Complexes 200 and 201 of N-Confused Sapphyrin

showed that the isosmaragdyrins are aromatic macrocycles. The crystal structures of three isosmaragdyrins were obtained, and the structures are presented in Figure 37. The X-ray studies revealed that the isosmaragdyrin macrocycles were almost planar; the only notable deviation from planarity is the central pyrrole of the terpyrrole moiety, which is slightly tilted out of the mean plane. The bound anion is placed above the macrocycle and interacts with four pyrrole NH’s of the macrocycle via hydrogen-bonding interactions (Figure 37). 3.2. Smaragdyrins (1998−Present)

One of the problems for unsuccessful attempts for the synthesis of smaragdyrin is the usage of precursors with direct pyrrole− pyrrole links, which are generally unstable. Alternately, the smaragdyrins can also be prepared by using precursors that do not have direct pyrrole−pyrrole links but can be introduced in the final step of condensation. This approach was adopted by Chandrashekar and coworkers126 to synthesize stable smaragdyrins (Scheme 34). Thus, [3 + 2] condensation of readily available precursors, meso-aryl dipyrromethane 16, 220−223 with 16-oxatripyrrane/ 16-thiatripyrrane 52 and 53, and 224 in CH2Cl2 by using 0.1 equiv of TFA followed by oxidation with chloranil, afforded the

Figure 36. Crystal structure of 201. Reproduced with permission from ref 61. Copyright 2008 Wiley-VCH.

well as meso-aryl dipyrromethane 216 under acid-catalyzed conditions and afforded the corresponding isosmaragdyrins 217−219. The NMR and absorption spectroscopic studies Scheme 32. Synthesis of Norsapphyrins 206 and 208

AA


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Scheme 33. Synthesis of Isosmaragdyrins 217 and 218

corresponding meso-triaryl 25-oxasmaragdyrins 225−228 in 51% yield and 25-thiasmaragdyrin 229 in 13% yield. The increase of acid concentration resulted in the decrease of smaragdyrin yields. Under the same reaction conditions, the condensation of meso-aryl dipyrromethane with azatripyrrane was expected to form pentaazasmaragdyrin, but the macrocycle was very unstable to isolate. Furthermore, later studies showed that the 25-thiasmaragdyrins 229 were also not very stable and decomposed readily. However, the 25-oxasmaragdyrins are very stable and easy to obtain in decent yields. Several different unsymmetrical functionalized 25-oxasmaragdyrins were synthesized by [3 + 2] oxidative coupling of functionalized meso-aryl dipyrromethane and 16-oxatripyrrane under similar reaction conditions. The 25-oxasmaragdyrins were aromatic and showed a typical intense Soret band at ∼443 nm and four Q-bands in the region of 500−750 nm. The 25-thiasmaragdyrins showed red-shifted absorption spectra compared to 25-oxasmaragdyrins, which was attributed to the presence of bulkier sulfur,

resulting in more distortion in the smaragdyrin macrocycle. In H NMR, the 25-oxasmaragdyrin showed five distinct sets of resonances in the region of 8.40−9.50 ppm for eight pyrrole and two furan protons. The three inner NH proton resonances were not observed as they are involved in rapid tautomerism at room temperature. However, upon protonation of smaragdyrin, the tautomerism was restricted and two kinds of NH signals were observed at −1.06 and −2.06 ppm. Protonated smaragdyrin also exhibited a splitted Soret band with a small red-shift of all bands. The free-base 25-oxasmaragdyrin and its monoprotonated salt were characterized by X-ray crystallography. The X-ray structure of free-base meso-triaryl 25oxasmaragdyrin 225 revealed that the macrocycle was nonplanar due to the strain imposed by the direct pyrrole−pyrrole bond and the steric repulsion operating between imino hydrogen atoms. Furthermore, the inner NH atoms of smaragdyrin were involved in strong hydrogen-bonding interactions with the heteroatoms. The crystal structure of 1

AB


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Figure 37. (a) Crystal structure of 217. Reproduced with permission from ref 124. Copyright 2001 Elsevier. (b) Crystal structure of 218. Reproduced with permission from ref 124. Copyright 2001 Elsevier. (c) Crystal structure of 219. Reproduced with permission from ref 125. Copyright 1998 American Chemical Society.

Scheme 34. Synthesis of meso-Triaryl Oxasmaragdyrins 225−232: (i) Pd2(dba)3, AsPh3, HCCTMS, THF/TEA; (ii) K2CO3, THF/CH3OH

functionalization and further modification. The mono-mesofree 25-oxasmaragdyrin 234 containing meso-free carbon on the dipyrrin side was synthesized by adopting [3 + 2] condensation of meso-free dipyrromethane 233 with 16oxatripyrrane 224 under acid-catalyzed conditions followed by oxidation and chromatographic purification (Scheme 35a). However, the attempts to prepare meso-free 25-oxasmaragdyrin 237 containing meso-free carbon on the tripyrrin side by condensing meso-aryl dipyrromethane with 16-oxatripyrrane 235 containing one meso-unsubstituted carbon under similar acid-catalyzed conditions were not successful, but the condensation resulted in the formation of another interesting macrocycle, meso-pyrrolyl oxacorroles128 238 and 239 (Scheme 35b). The meso-free 25-oxasmaragdyrin showed slightly blueshifted absorption bands compared to those for meso-triaryl 25oxasmaragdyrin.

protonated smaragdyrin, which was generated by the addition of dilute HCl to the free-base smaragdyrin, revealed that the chloride ion was placed above the plane of the macrocycle and was involved in interactions with three pyrrole NH’s via hydrogen bonding. The three pyrroles that were involved in hydrogen-bonding interactions with the chloride ion tilted toward the chloride ion, whereas the pyrrole that was not involved in bonding with chloride remained in the plane defined by meso-carbons of smaragdyrin. Thus, the binding observed in protonated smaragdyrin was slightly different from that observed for the monoprotonated chloride salt of the smaragdyrin isomer where all four pyrrole nitrogens were involved in binding to the chloride anion (Figure 38). In addition to meso-triaryl 25-oxasmaragdyrins, attempts were also made to prepare mono-meso-free 25-oxasmaragdyrins127 because the meso-free carbon can be activated easily for AC


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Figure 38. (a) Crystal structure of 225. Reproduced with permission from ref 126. Copyright 1999 American Chemical Society. (b) Crystal structure of 225H+·Cl−. Reproduced with permission from ref 126. Copyright 1999 American Chemical Society.

Scheme 35. Synthesis of Meso-Free 25-Oxasmaragdyrins 234 and (b) meso-Pyrrolyl Oxacorroles 238 and 239

3.3. Oxasmaragdyrin-Based Conjugates

nenyl group was either directly attached to the meso-carbon of the smaragdyrin macrocycle or attached through different spacers. The X-ray structure of directly linked oxasmaragdyrin− ferrocenyl conjugate 247 revealed that the smaragdyrin macrocycle was planar with very small deviations of the mesocarbon atoms and the ferrocenyl group was more toward in plane with the macrocycle (dihedral angle of 38°), unlike the other two meso-aryl substituents, which were almost in perpendicular orientation with the macrocycle (Figure 39).

Because the meso-aryl dipyrromethanes containing a variety of substituents can be synthesized readily, different mesosubstituted smaragdyrins were synthesized by adopting the same [3 + 2] synthetic strategy. The ferrocenyl-substituted 25oxasmaragdyrin conjugates129,130 247−251 were synthesized by oxidative coupling of meso-ferrocenyl-substituted dipyrromethanes 240−244 and 16-oxatripyrranes 245 and 246 (Scheme 36). In these smaragdyrin−ferrocene conjugates, the ferroceAD


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Scheme 36. Synthesis of Ferrocenyl-Substituted 25-Oxasmaragdyrin Conjugates 247−251

Figure 39. Crystal structure of 247: (a) top view and (b) side view. Reproduced with permission from ref 129. Copyright 2004 Wiley-VCH. (The meso-tolyl groups are omitted for clarity in the side view.)

Scheme 37. Synthesis of Smaragdyrins 255−258

This indicates the presence of effective mixing of the molecular orbitals between the ferrocene unit and the macrocycle, which

in turn alters the electronic properties of the smaragdyrin macrocycle. The significant reduction of C−C bond length of AE


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Scheme 38. Synthesis of Smaragdyrin−Azobenzene Conjugates 262 and 263

Scheme 39. Synthesis of Porphyrin−Smaragdyrin Dyad 266

AF


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Scheme 40. Synthesis of Meso−Meso-Linked Dyad 267

was used for energy-transfer studies and as a fluorescent sensor for anions. The dyad was synthesized in 70% yield by coupling of iodosmaragdyrin 230 with 20-(4-ethynylphenyl)5,10,15tri(p-tolyl)porphyrin 264 in toluene/triethylamine at 50 °C in the presence of [Pd2(dba)3] and AsPh3 (Scheme 39). The Zn(II) ion was inserted into the porphyrin unit of the dyad under standard metalation conditions to afford Zn(II) porphyrin−25-oxasmaragdyrin dyad 266 in 90% yield. The spectral and electrochemical studies indicated that both porphyrin/Zn(II) porphyrin and 25-oxasmaragdyrin units in dyads interacted weakly and retained their independent characteristic features. Because smaragdyrin units in a dyad possess a low-energy singlet state compared to porphyrin/ Zn(II) porphyrin, the photophysical studies indicated a possibility of energy transfer from the porphyrin/Zn(II) porphyrin unit to the smaragdyrin unit in dyads. Furthermore, porphyrin−smaragdyrin dyads were also demonstrated as fluorescent sensors for anions because the smaragdyrin unit in the protonated state binds anions that were reflected in the fluorescence intensity changes of the porphyrin unit. Meso−meso-linked 25-oxasmaragdyrin dyad76 267 was synthesized by treating meso-free smaragdyrin monomers with either AgPF6 or n-BuLi, as shown in Scheme 40. The yields of the meso−meso-linked smaragdyrin dyad depend on the nature of the meso-substituents and the synthetic method, and better yields of dyad were obtained with the n-BuLi method. Electrochemical studies of the meso−meso-linked smaragdyrin dyad indicated the perturbation of the π-electron conjugation in the dyad as compared to that in monomers. The perturbation of π-electron delocalization in the smaragdyrin dyad was also reflected in its absorption spectra, which were broader and red-shifted both in Soret and Q-bands compared to those for the smaragdyrin monomer. The fluorescence spectrum of the dyad was also broader and red-shifted with a decrease in quantum yields compared to smaragdyrin monomers. The density functional theory (DFT) studies indicated a twist angle of 71°−64° between the smaragdyrin units in the dyad and support the enhanced π-conjugation in the dyad.

meso-carbon and the ferrocenyl carbon, which is connected to the meso-carbon of the dipyrromethane moiety, also supports the presence of electronic coupling between the ferrocenyl moiety and the smaragdyrin π-system. The X-ray structures of two oxasmaragdyrin−ferrocene conjugates with aryl spacers also supported the presence of moderate π-interaction between the π-system of the cyclopentadienyl ring and the smaragdyrin ring. The absorption and fluorescence data of smaragdyrin− ferrocene conjugates depend not only on the extent of electronic interaction between the two moieties but also on the nature and length of the spacer groups, and the maximum effects are observed where the ferrocene moiety is directly connected to the meso-carbon of the macrocycle. The electrochemical studies also reveal that the ferrocene unit is harder to oxidize in oxasmaragdyrin−ferrocene conjugates compared to free ferrocene, supporting the electron-donating nature of the ferrocene. Chandrashekar and co-workers131,132 extended their synthetic strategy to prepare smaragdyrin conjugates 256−259 bearing various extended phenylacetylenylphenyl substituents at one of the meso-positions by oxidative coupling of the mesophenylacetylenyl-substituted dipyrromethanes 252−255 with 16-oxatripyrrane 246 (Scheme 37). The spectral and electrochemical studies revealed that the properties were dependent on the conjugation length and the number of phenylacetylene units attached to the meso-phenyl of the smaragdyrin skeleton. The two-photon absorption (TPA) cross section values were measured for these phenylacetelenylphenyl-substituted smaragdyrins, and the cross section value increased with conjugation length. Para-substituted isomers showed larger cross section values compared to meta-isomers, which was attributed to the participation in conjugation for the para-isomer and the disruption in the conjugation for the meta-isomer. Smaragdyrin−azobenzene conjugates133 262 and 263 were synthesized by oxidative coupling of meso-aryldipyrromethanes 260 and 261 linked to azobenzene moiety with 16oxatripyrrane 224 (Scheme 38). The crystal structure revealed that the azobenzene unit is in the (E) conformation, and spectral studies indicated moderate electronic interaction between the azobenzene and smaragdyrin π-systems. The irradiation experiments revealed that the formation of Z-isomer from E-isomer led to the decomposition of the macrocycle. The electrochemical studies indicated that the smaragdyrin macrocycle was difficult to oxidize in the smaragdyrin−azobenzene conjugate compared to free smaragdyrin, which was attributed to the electron-withdrawing nature of the azobenzene moiety. The fluorescence studies indicated a possibility of energy transfer from the azobenzene moiety to the smaragdyrin macrocycle in the smaragdyrin−azobenzene conjugate. Rao and Ravikanth134 synthesized covalently linked diphenyl ethyne-bridged 25-oxasmaragdyrin−porphyrin dyad 265, which

3.4. Coordination Chemistry of 25-Oxasmaragdyrins

Because the meso-triaryl 25-oxasmaragdyrin contains four pyrrole nitrogens and one furan oxygen inside the macrocycle core, it is anticipated that the macrocycle forms complexes with metals/nonmetals. It has been demonstrated that the 25oxasmaragdyrins in their protonated state bind anions such as Cl−, F−, and AMP− via hydrogen-bonding interactions.135 However, the complexations of smaragdyrins toward various metals/nonmetals were explored over the years, and the 25oxasmaragdyrins form interesting complexes with certain metals/nonmetals. Chandrashekar and co-workers135 reported the synthesis of Rh(CO)2 complex 268 by treating 25AG


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Scheme 41. Synthesis of Rh(I) and Ni(II) Complexes of meso-Aryl Smaragdyrins 268−270

Figure 40. (a) Crystal structure of 268. Reproduced with permission from ref 135. Copyright 2000 American Chemical Society. (b) Crystal structure of 276. Reproduced with permission from ref 127. Copyright 2013 American Chemical Society.

workers also prepared Rh(I) complex of 25-thiasmaragdyrin 269 and proposed a similar coordination mode for Rh(I) based on various spectral studies. Interestingly, Chandrashekar and co-workers135 also prepared the Ni(II) complex of 25oxasmaragdyrin 270 and proposed that all four bipyrrole nitrogens of smaragdyrin were involved in coordination with Ni(II) ion (Scheme 41). The authors proposed that the Ni(II) coordination oxidizes the ligand and the complex was formulated as the π-cation radical of Ni(II) smaragdyrin based on broad signals in the 1H NMR spectrum and strong EPR signal with g = 2.0031. The metal derivatives of 25oxasmaragdyrins showed a splitted Soret band and 3−4 Qbands in the 400−800 nm region. Rao and Ravikanth 136 reported the synthesis of BF2 complexes of 25-oxasmaragdyrins 271−273 by taking advantage of the presence of the monoanionic dipyrromethene moiety in the smaragdyrin macrocycle. The BF2 complexes of monoanionic dipyrromethenes, popularly known as BODIPYs,

oxasmaragdyrin with di-μ-chlorobis[dicarbonylrhodium(I)] in CH2Cl2 in the presence of sodium acetate (Scheme 41). The Xray structure of the Rh(I) complex revealed that the Rh(I) ion was attached to the smaragdyrin skeleton in a μ2-fashion involving one amino and one imino nitrogen atom of the dipyrromethene unit and the other two coordination sites of Rh(I) were occupied by the carbonyl groups. Thus, Rh(I) formed an approximate out-of-plane square-planar complex with 25-oxasmaragdyrin macrocycle involving only the dipyrrin unit nitrogen atoms. Furthermore, the 1H NMR spectrum of Rh(I) complex showed a singlet at −1.71 ppm for two inner NH protons that belonged to the tripyrrane unit, indicating that these two pyrroles were not involved in binding Rh(I) ion. The X-ray structure also revealed that the two pyrrole nitrogens that were coordinated to Rh(I) ion were deviated more from the mean plane of the macrocycle compared to the other two pyrrole nitrogen and furan oxygen atoms, which were not involved in coordination (Figure 40a). Chandrashekar and coAH


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Scheme 42. Synthesis of BF2 Complexes 271−274 of 25-Oxasmaragdyrins and Functionalization of Meso-free BF2 Complexes of 25-Oxasmaragdyrins 275−278

atoms of dipyrrin moiety of the smaragdyrin macrocycle and two fluorine atoms in approximately tetrahedral fashion (Figure 40b). The BF2 unit was in the same plane defined by four pyrrole nitrogen atoms of smaragdyrin. Out of two fluorine atoms of the BF2 unit, one fluorine atom was placed above the plane of the macrocycle whereas the other fluorine atom was placed below the plane of the macrocycle. Furthermore, the fluorine atoms of the BF2 unit were involved in intramolecular hydrogen bonding with the inner pyrrole NH atoms of the macrocycle. This intramolecular hydrogen-bonding interactions in BF2−smaragdyrin complexes were also reflected in the NMR spectra. Usnig room-temperature 1H NMR, the resonances corresponding to the inner NH protons of free-base smaragdyrins were not observed because of their involvement in rapid tautomerism, but in BF2−smaragdyrins 271−273, the two inner NH protons were localized and appeared as an unresolved triplet in the upfield region at approximately −4.2 ppm because of strong hydrogen bonding with the two fluoride ions of the BF2 unit, which exposed the inner −NH protons to experience the strong ring-current effect of the macrocycle.

possess remarkable photophysical properties and have a wide variety of applications ranging from materials to biology and medicine. Because 25-oxasmaragdyrins possess a monoanionic dipyrromethene moiety, they were expected to bind the BF2 unit. The BF2−smaragdyrins 271−273 were prepared by treating free-base meso-triaryl 25-oxasmaragdyrins with 40−50 equiv of BF3·OEt2 in CH2Cl2 at room temperature (Scheme 42). Similarly, the mono-meso-free 25-oxasmaragdyrin was also converted to BF2 complex 274 under similar mild roomtemperature reaction conditions. The free meso-position of BF2-smaragdyrin 274 was functionalized with functional groups such as Br, −CHO, NO2, −CCTMS, etc. under standard functionalization reaction conditions to afford meso-functionalized BF2−oxasmaragdyrins 275−278 (Scheme 42).127 The crystal structure of meso-triaryl 25-oxasmaragdyrin was not obtained, but the authors have reported the crystal structure of the BF2 complex of meso-nitro-substituted 25-oxasmaragdyrin 276. The X-ray structure of BF2-meso-nitro 25-oxasmaragdyrin showed that the oxasmaragdyrin macrocycle ring was almost planar and the B(III) was coordinated to two pyrrole nitrogen AI


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Scheme 43. Synthesis of B(OR)2, B(OH)2, and B(OSiR3)2 Complexes of 25-Oxasmaragdyrins 279−290

Figure 41. (a) Crystal structure of 285. Reproduced with permission from ref 137. Copyright 2013 Royal Society of Chemistry. (b) Crystal structure of 288. Reproduced with permission from ref 140. Copyright 2015 Wiley-VCH.

quantum yield of ∼0.08. Thus, the absorption and emission bands in BF2−smaragdyrins experienced bathochromic shifts with significant enhancement in the extinction coefficients and quantum yields compared to free-base oxasmaragdyrins. The BF2−smaragdyrins are redox-stable and exhibit reversible oxidation and reduction waves. The BF2 complexation of smaragdyrins makes the macrocycles more electron-deficient compared to free-base smaragdyrins. Because the BF2−smaragdyrins are electron-deficient, a series of electron-rich alkoxy- and aryloxy-substituted B(OR)2− smaragdyrins 279−286 were prepared by treating BF2−

The BF2 complexation of 25-oxasmaragdyrins results in an enhancement of the stability of the macrocycle with significant alteration in their spectral and electrochemical properties compared to free-base oxasmaragdyrins. The BF2-smaragdyrin complexes show two well-resolved Soret-type bands in the 400−500 nm region and six well-defined Q-bands in the 520− 720 nm region in the absorption spectra. The most novel feature of BF2−smaragdyrins is their strong absorption band at ∼700 nm, which is 3−7 times more intense than the absorption band of free-base smaragdyrins.136 The BF2−smaragdyrins are decently fluorescent with an emission band at ∼710 nm and a AJ


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smaragdyrin 287 in ∼65% yield (Scheme 43).136 Although the crystal structure of the B(OH)2 complex of smaragdyrin was not obtained, the NMR studies supported the intramolecular hydrogen bonding between axial −OH groups and inner NH protons. Interestingly, the B(OH)2 complex of smaragdyrin was found to be an exclusive fluoride sensor as confirmed by spectral and electrochemical studies. In the presence of F− ion, the absorption and fluorescence bands of B(OH)2−smaragdyrin experienced a gradual red-shift with clear isosbestic points, slight downfield shifts in β-pyrrole and β-furan protons in 1H NMR, and a shift of oxidation potentials toward the less-positive side, altogether supporting the sensing of F− ion by the B(OH)2−smaragdyrin complex. The B(III) complexes of meso-triaryl 25-oxasmaragdyrin140 containing axial silyloxy groups 288−290 were synthesized in high yields by treating B(OH)2−smaragdyrin with different alkyl/aryl chlorosilanes in toluene in the presence of triethylamine at reflux temperature (Scheme 43). The X-ray structure of B(III)−smaragdyrin complex 288 containing trimethylsilyloxy groups revealed that the smaragdyrin macrocycle was almost planar and the −OSiMe3 groups were oriented in the up and down directions with respect to the macrocycle (Figure 41b). The absorption spectra of meso-triaryl 25-oxasmaragdyrin containing axial silyloxy groups was almost the same as that of the B(OH)2−smaragdyrin complex, but the reduction potential of B(OSiR3)−smaragdyrin was shifted more negative due to the electron-donating nature of axial silyloxy groups. The most interesting feature of B(OSiR3)−smaragdyrin complexes 288− 290 is their strong fluorescence, which is 5−6 times stronger compared to B(OH)2−smaragdyrin 287. B(OSiR3)−smaragdyrin complexes 288−290 exhibit high quantum yields in the range of 0.65−0.78 with longer singlet state lifetimes.

smaragdyrin complex with various aliphatic (ROH) and aromatic alcohols (ArOH) in the presence of AlCl3 in CH2Cl2 at room temperature (Scheme 43).137 The X-ray structure of B(OR)2−smaragdyrin complex 285 bearing the 3,5-dimethylphenoxide group as the axial ligand showed that the macrocyclic ring was almost planar and the boron atom adopted an approximate tetrahedral geometry, binding with the two pyrrole nitrogen atoms and two oxygen atoms of the 3,5dimethylphenoxide groups (Figure 41a). The boron atom was in the plane defined by three meso-carbon atoms of the macrocycle and the two axial 3,5-dimethyl phenoxide groups were out of the plane of the macrocycle and positioned above and below the plane, respectively. The oxygen atoms of the phenoxide groups were involved in intramolecular hydrogen bonding with the inner NH atoms along with the furan oxygen atom, but the intramolecular hydrogen bonding in B(OR)2− smaragdyrins was weaker compared to that in BF 2 − smaragdyrins, as supported by NMR studies. In 1H NMR, the two inner NH protons appeared at the downfield region (−0.6 ppm) in B(OR)2-smaragdyrins compared to BF2− smaragdyrins (−3.7 ppm), supporting the weaker intramolecular hydrogen-bonding interactions in B(OR)2−smaragdyrins compared to BF2−smaragdyrin. Hung and coworkers138,139 also prepared a series of B(III)−smaragdyrin complexes 292−294 containing axial alkoxy groups and carboxylate group on one of the meso-aryl rings (Figure 42).

3.5. BF2−Smaragdyrin-Based Conjugates

Because BF2−smaragdyrins absorb and emit in the visible region with decent quantum yields and singlet state lifetimes and these complexes are stable under redox conditions, several covalently linked BF2−smaragdyrin-chromophore/redox-active conjugates are synthesized to study the electron-/energytransfer processes. The monofunctionalized BF2−smaragdyrins containing functionalized meso-aryl groups such as iodophenyl, ethynylphenyl, and hydroxyphenyl groups were used as building blocks to synthesize covalently linked BF2−smaragdyrin-based conjugates,78,141,142 as shown in Scheme 44. The BF2− smaragdyrin 272 building block containing an iodophenyl group at the meso-position was used to couple with different ethynyl-functionalized chromophore/redox-active groups such as BODIPY, ferrocene, porphyrin, thiasapphyrin, and thiarubyrin under mild Pd(0) coupling conditions and afforded the corresponding covalently linked BF2−smaragdyrin−chromophore/redox-active conjugates 305−310. The NMR and absorption studies clearly indicated that the BF2−smaragdyrin and chromophore/redox-active moieties interact weakly and retain their independent features in conjugates. The conjugates exhibited interesting optical and redox properties. In BF2− smaragdyrin−BODIPY 306 and BF2−smaragdyrin−porphyrin conjugates 307, the fluorescence studies indicated a possibility of photoinduced singlet−singlet energy transfer from chromophore BODIPY/porphyrin unit to BF2 smaragdyrin unit. However, in BF2−smaragdyrin−ferrocene conjugate 310, the fluorescence studies invoked a possibility of photoinduced electron transfer from the ferrocene unit to the excited state of the BF2−smaragdyrin unit. Furthermore, BF2−smaragdyrin−

Figure 42. Structures of various boryl oxasmaragdyrin sensitizers 291−298 for DSSC studies.

Such complexes were used as photosensitizers for dyesensitized solar cell (DSSC) applications. Their studies showed that the device containing B(III)−smaragdyrin complex 291 achieves an energy-conversion efficiency of 5.7%. The same group has extended their strategy to prepare several BF2− smaragdyrin complexes 295−298 containing different donor and acceptor groups at the meso-positions and tested their efficiency for photovoltaic applications. Their extensive study revealed that BF2−smaragdyrin complex 295 containing two hexyloxyphenyl donors and one carboxylic acid anchor showed the best overall conversion efficiency of 4.36%. To prepare the B(OH)2 complex of smaragdyrin 287, BF2− smaragdyrin complex 271 was treated with water instead of alkyl/aryl alcohol in the presence of AlCl3 in CH2Cl2 solvent. The crude compound was purified by alumina column chromatography and afforded the B(OH)2 complex of AK


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Scheme 44. Synthesis of Various BF2−Smaragdyrin Conjugates 305−310: (a) Pd2(dba)3, AsPh3, Toluene/TEA, 40 °C

thiasapphyrin 309 and BF2−smaragdyrin−thiarubyrin 308 were used as fluorescent sensors for anions. In these conjugates, nonfluorescent thiasapphyrin and thiarubyrin units used to bind anions in their protonated form, and the BF2−smaragdyrin was used as a fluorescent signaling unit. The studies showed that the binding of the anion with the protonated thiasapphyrin and thiarubyrin units in their respective conjugates resulted in an enhancement of the fluorescence intensity of the BF2− smaragdyrin unit.78 The covalently linked triad 305 containing Zn(II) porphyrin, porphyrin, and BF2−smaragdyrin units was synthesized by

coupling covalently linked diphenyl ethyne bridged Zn(II) porphyrin−porphyrin dyad 299 containing an ethynyl-functionalized group on a porphyrin unit with the meso-iodophenyl BF2−smaragdyrin under mild Pd(0) coupling conditions.142 The three macrocycles in triad 305 interact weakly and almost retain their intrinsic features. The steady-state and timeresolved fluorescence studies supported an efficient energy transfer from the Zn(II) porphyrin unit to the BF2− smaragdyrin unit via the free-base porphyrin unit in the triad. Khan and Ravikanth143,144 reported the synthesis of another covalently linked trichromophoric system 314 containing a AL


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Scheme 45. Synthesis of Porphyrin−BODIPY−Smaragdyrin Triad 314

oxasmaragdyrins with POCl3 in toluene/triethylamine at refluxing temperature (Scheme 46a). The X-ray structure solved for one of the PO2 complexes of 25-oxasmaragdyrin showed that the phosphorus(V) ion was bound to two pyrrolic nitrogen atoms of the smaragdyrin macrocycle and two oxygen atoms in tetrahedral geometry. Thus, the PO2 and BF2 units were bound to smaragdyrin macrocycle in identical fashion. The X-ray structure of PO2-smaragdyrin macrocycle showed significant distortion upon insertion of a PO2 unit and the phosphorus atom situated at a distance of 1.339 Å above the mean plane defined by three meso-carbon atoms of the macrocycle (Figure 43a). Furthermore, the oxygen atoms of the PO2 unit were involved in strong hydrogen bonding with inner NH protons, which was reflected in downfield shifts of inner NH protons in 1H NMR. The PO2 complexes absorb strongly in the 400−720 nm region, and the absorption band at ∼708 nm was generally 3−4 times more intense than the absorption band of the free-base smaragdyrin in the same region. The PO2−smaragdyrin complexes are electrondeficient, and these complexes are easier to reduce but difficult to oxidize compared to the free-base smaragdyrins. Like BF2−

central BODIPY unit connected to a porphyrin unit through the 3-position and a BF2−smaragdyrin unit through the 5position over a sequence of two steps, as shown in Scheme 45. In the first step, the 3,5-dibromo-BODIPY 311 was reacted with hydroxyl porphyrin 312 in CHCl3/CH3CN in the presence of base, and the resulting 3-bromo-5-porphyrinyl BODIPY 313 was reacted in a second step with BF2− smaragdyrin containing hydroxyphenyl group 273 under similar reaction conditions to afford trichromophoric system 314 in decent yield (Scheme 45). The steady-state fluorescence studies showed that, upon excitation of the BODIPY unit at 488 nm or the porphyrin unit at 420 nm, the emission was observed mainly from the BF2−smaragdyrin unit, indicating that the singlet state energy level of theBF2−smaragdyrin unit was lower than those of the other two chromophores and the energy at the singlet state was transferred from BODIPY/porphyrin to BF2−smaragdyrin in trichromophoric system 314. 3.6. PO2 Complexes of meso-Triaryl-25-Oxasmaragdyrins

Ravikanth and co-workers145 also synthesized PO2 complexes of meso-triaryl smaragdyrins 315 by treating the free-base 25AM


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Scheme 46. Synthesis of (a) PO2 Complex of 25-Oxasmaragdyrin 315 and (b) PO2−Smaragdyrin−BODIPY Dyad 317

Figure 43. (a) Crystal structure of 315. Reproduced with permission from ref 145. Copyright 2014 American Chemical Society. (b) Crystal structure of 319. Reproduced with permission from ref 146. Copyright 2015 Wiley-VCH.

smaragdyrins, the PO2−smaragdyrins are also strongly fluorescent compared to free-base smaragdyrins. The authors also inserted the PO2 unit into covalently linked diphenyle-

thyne-bridged smaragdyrin−BODIPY dyad 316 under similar conditions to those used for smaragdyrin monomers (Scheme 46b). Their preliminary photophysical studies supported AN


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Scheme 47. Synthesis of Mixed B(III)/P(V) Complexes of 25-Oxasmaragdyrins 319 and 320

efficient energy transfer from the BODIPY unit to the PO2− smaragdyrin unit in smaragdyrin−BODIPY dyad 317.

mean plane bearing two chloro and oxo as axial groups. The other mixed B(III)/P(V) complex of 25-oxasmaragdyrin 320, which contains two seven-membered heterocycles, has no crystallographic evidence, although the 1D and 2D NMR studies supported the presence of two seven-membered heterocycles inside the smaragdyrin macrocycle and involvement of four pyrrolic nitrogens of the smaragdyrin macrocycle in coordinating with B(III)/P(V) ions.147 Furthermore, the structure also revealed that the pyrrole ring bonded to P(V) ion was more deviated compared to other heterocyclic rings in the smaragdyrin macrocycle. The presence of a seven-membered heterocycle inside the smaragdyrin macrocycle altered the electronic properties of the macrocycle, as reflected in their spectral and electrochemical properties. The absorption bands of one and two sevenmembered heterocycles containing smaragdyrins were shifted bathochromically compared to B(III) complexes of smaragdyrins, and the smaragdyrins containing two seven-membered heterocycles experienced more bathochromic shifts compared to smaragdyrin containing one heterocycle ring. All the B(III)/ P(V) complexes 319 and 320 of smaragdyrins generally show an intense absorption band at ∼700−750 nm, which highlights their potential applications in material science and medicine. The fluorescence band of mixed B(III)/P(V) complexes of smaragdyrins showed bathochromically shifted fluorescence band with reduction in quantum yields and singlet state lifetimes compared to free-base smaragdyrins. The electrochemical studies revealed that the mixed B(III)/P(V) complexes of smaragdyrins were difficult to oxidize but easier to reduce compared to free-base smaragdyrins. The DFT studies147 revealed that the mixed B(III)/P(V) smaragdyrin complex with two seven-membered heterocycles 320 was more stable than smaragdyrin containing one seven-membered heterocycle 319.

3.7. Mixed B(III) and P(V) Complexes of 25-Oxasmaragdyrins

Although 25-oxasmaragdyrin is a good macrocyclic ligand, most of the metal and nonmetal complexes described above clearly indicate that only nitrogen atoms of the dipyrromethene moiety of smaragdyrin are involved in coordination, whereas the other two donor nitrogen atoms, as well as the furan oxygen of the tripyrromethene moiety, remain uncoordinated. We made several attempts to involve the donor atoms of the tripyrrane moiety of 25-oxasmaragdyrin in coordination with metal/ nonmetal atoms, and we succeeded in preparing the first examples of mixed B(III)/P(V) complexes of meso-triaryl 25oxasmaragdyrins146,147 containing one or two seven-membered heterocycles that are composed of five different atoms such as B, C, N, O, and P inside the macrocycle. The mixed B(III)/ P(V) complexes of 25-oxasmaragdyrins 319 and 320 containing one and two seven-membered heterocycles were synthesized by reacting B(OH)(Ph)−smaragdyrin 318 and B(OH)2−smaragdyrin 287, respectively, with POCl3 in toluene at refluxing temperature (Scheme 47). The X-ray structure obtained for one of the mixed B(III)/P(V) complexes of smaragdyrin indicated that the macrocycle was significantly distorted (Figure 43b). In this complex, the B(III) was bound to two pyrrolic nitrogens of the dipyrromethene moiety and one axial phenyl group, whereas the P(V) ion was linked to B(III) ion through oxygen and also bound to one of the pyrrole nitrogen atoms of the macrocycle, resulting in a stable unusual seven-membered heterocyclic ring with five different atoms, such as B, C, N, O, and P, inside the smaragdyrin macrocycle.146 Overall, the P(V) ion in compound 319 was also tetracoordinated and was positioned at 1.64 Å above the AO


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Scheme 48. Synthesis of Calixsmaragdyrins 324−326 and Pd(II) Complex of Calixoxasmaragdyrin 327

Figure 44. (a) Crystal structure of 324. Reproduced with permission from ref 149. Copyright 2015 American Chemical Society. (b) Crystal structure of 326. Reproduced with permission from ref 148. Copyright 2014 Royal Society of Chemistry.

3.8. Calixsmaragdyrins

of calixaza- and calixoxasmaragdyrins were obtained, which revealed that the macrocycles were highly distorted due to the flexibility introduced by one sp3 meso-carbon (Figure 44). In calixazasmaragdyrin, the pyrrole rings are highly distorted to different magnitudes from the mean plane of the macrocycle composed of 28 atoms involving five pyrroles and three meso-carbons. The four pyrrole rings are deviated down to the mean plane, whereas one pyrrole, which is trans to the sp3 mesocarbon, is deviated significantly above the mean plane of the calixazasmaragdyrin macrocycle (Figure 44a). Similar macrocyclic distortion was also noted in calixoxasmaragdyrin. Thus, the presence of both sp2 and sp3 carbon atoms in the calixsmaragdyrin macrocycles make the macrocycle more flexible and distorted. The coordination chemistry of calixsmaragdyrins was also explored. The calixoxasmaragdyrin forms Pd(II) complex150 327 by treating the free-base calixoxasmaragdyrin 326 with PdCl2 in CH3CN at reflux temperature (Scheme 48). The crystal structure of Pd(II) calixoxasmaragdyrin revealed that the Pd(II) metal ion adopted approximately square-planar geometry and coordinated to the

Calix[n]phyrins are porphyrinoid macrocycles containing at least one sp3 meso-carbon along with sp2 meso-carbons, and the presence of sp3 meso-carbon induces sufficient flexibility in the macrocycle, which in turn helps the macrocycle to bind both cations and anions. Because the synthesis of stable pentaazasmaragdyrins remains a challenging problem to solve, our group attempted to synthesize stable pentaaza calixsmaragdyrin as well as calixthia- and calixoxasmaragdyrins. The stable aza-, thia-, and oxa-calixsmaragdyrins 324−326, containing two meso-sp2 carbons and one meso-sp3 carbon atom, were synthesized by adopting [3 + 2] strategy as shown in Scheme 48. The calixaza-, thia-, and oxasmaragdyrins148,149 were synthesized by condensation of 2,2′-(1-methylethylidene)bis(pyrrole) 321 with appropriate tripyrrane; azatripyrrane 322, thiatripyrrane 323, and oxatripyrrane 224, respectively, under mild acid-catalyzed conditions followed by oxidation with DDQ (Scheme 48). The calixsmaragdyrins are stable nonaromatic green solids and showed one strong absorption band at ∼420 nm and an ill-defined band at ∼685 nm. The crystal structures AP


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Figure 45. Crystal structure of 327: (a) top view and (b) side view. Reproduced with permission from ref 150. Copyright 2014 American Chemical Society. (The meso-tolyl groups are omitted for clarity in the side view.)

sapphyrins, fused sapphyrins, and core-modified sapphyrins, in addition to the traditional β-alkyl and meso-aryl sapphyrins. A wide range of applications of sapphyrins as anion and cation receptors, as sensitizers for photodynamic therapy, and as nonlinear optical materials and the use of sapphyrin conjugates for many biological functions reveal the rich chemistry of these compounds. We hope sapphyrin conjugates will be an attractive candidate to be used in optoelectronic and photonic devices in coming years. Research activity in this direction is being carried out in many laboratories across the world. On the other hand, earlier attempts to explore the chemistry of smaragdyrins were not successful owing to the inherent instability of these macrocycles, most probably due to the presence of two direct pyrrole−pyrrole links, which induces steric strain on the molecule. However, the 25-oxasmaragdyrins containing four pyrroles and one furan ring connected by three meso-carbons and two direct pyrrole−pyrrole bonds are found to be very stable for detail characterization and studies. The meso-triaryl 25-oxasmaragdyrins were prepared in decent yields by [3 + 2] condensation of 16-oxatripyrrane and meso-aryl dipyrromethane under acid-catalyzed porphyrin-forming conditions. The synthetic strategy gives access to introduce the functional groups as handles on meso-aryl groups. The free-base smaragdyrins absorb and emit in the 400−800 nm region and are stable under redox conditions. The meso-functionalized 25oxasmaragdyrins were used as building blocks to synthesize several covalently linked smaragdyrin-based conjugates and to study their properties. The smaragdyrins are fluorescent macrocycles and showed that these can be used as fluorescent sensors for anions. The macrocycles were found to form stable metal/nonmetal complexes, which were used for various applications. Thus, smaragdyrins are very good ligands to form novel and unprecedented complexes that possess very interesting spectral and redox properties. The calixsmaragdyrins were also synthesized under simple reaction conditions and were used for cation- and anion-sensing studies. Thus, we hope that the smaragdyrins will be exploited as a ligand for the preparation of a variety of metal/nonmetal complexes and that these macrocycles will be used for various applications in fields ranging from materials to medicine because of their attractive physicochemical properties.

four pyrrole nitrogen atoms; the furan oxygen atom was not involved in bonding (Figure 45a). The Pd(II) ion was placed slightly above the mean plane of the macrocycle (composed of 28 atoms), and the four Pd−N bond lengths were inequivalent. Furthermore, the Pd(II) was not positioned at the middle of the macrocycle but positioned slightly more toward the dipyrromethane moiety of the calixoxasmaragdyrin (Figure 45b). Overall the calixoxasmaragdyrin macrocycle in Pd(II)− calixoxasmaragdyrin was highly distorted and attained a boatshaped structure. The Pd(II)−smaragdyrin complex showed one broad absorption band at 427 nm and was not very stable under redox conditions. The Pd(II)−calixoxasmaragdyrin 327 showed good catalytic activity in the Suzuki−Miyaura crosscoupling reaction between bromoaryls and aryl boronic acids. The anion-sensing studies of calixsmaragdyrins 324−326 were also studied. The studies indicated that calixoxasmaragdyrins 326 have specificity toward the HSO4− ion in its neutral and protonated states, whereas calixthiasmaragdyrin 325 did not show any binding affinity toward anions.148 The calixazasmaragdyrins 324 were also tested for binding of both anions and cations. Because calixazasmaragdyrins have five nitrogen atoms with three ionizable hydrogen atoms, one can anticipate that calixazasmaragdyrins may bind anions and cations easily. The studies showed that calixazasmaragdyrins did not have any sensing behavior toward anions but showed selective sensing behavior toward Hg2+ ions, as verified by spectral and electrochemical studies.149 However, researchers have failed to isolate the complex for crystallographic characterization. The DFT and NMR studies indicated that the two imine nitrogen atoms were involved in binding with Hg2+ ion in an almost linear fashion.

4. CONCLUSIONS The pentapyrrolic macrocycles are considered to be the earliest member of the so-called “expanded porphyrin family.” Sapphyrins and smaragdyrins are the two members of this family that have been well-studied due to the availability of methods to synthesize them in gram quantities, and particularly a handful of literature reports are available on the chemistry of sapphyrins. The successful synthesis of stable sapphyrins first by Sessler’s group laid the foundation for the exploration of the fascinating chemistry of sapphyrins. Later on, several groups across the globe embarked on the syntheses of a diverse range of sapphyrins that include carbasapphyrins, N-confused AQ


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AUTHOR INFORMATION

ADP ATP ssDNA dsDNA EDTA CPG EDC HOBt BNPP SWNT TPA NLO

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Tamal Chatterjee received his B.Sc. and M.Sc. from The University of Burdwan, West Bengal. In 2011, he joined Indian Institute of Technology Bombay as a Ph.D. student under the supervision of Professor M. Ravikanth, where currently he is pursuing his research work on the synthesis and application of expanded porphyrins.

Adenosine diphosphate Adenosine triphosphate Single-stranded deoxyribonucleic acid souble-stranded deoxyribonucleic acid Ethylenediamine tetraacetic acid Control pore glass 1-Ethyl-3-(3-(dimethylamino)propyl)carbodiimide 1-Hydroxybenzotriazole Bis(4-nitrophenyl)phosphate Single-wall carbon nanotube Two-photon absorption Nonlinear optics

REFERENCES (1) Li, L.-L.; Diau, E. W.-G. Porphyrin-Sensitized Solar Cells. Chem. Soc. Rev. 2013, 42, 291−304. (2) Ryan, A. A.; Senge, M. O. How Green is Green Chemistry? Chlorophylls as a Bioresource from Biorefineries and their Commercial Potential in Medicine and Photovoltaics. Photochem. Photobiol. Sci. 2015, 14, 638−660. (3) Drain, C. M.; Varotto, A.; Radivojevic, I. Self-Organized Porphyrinic Materials. Chem. Rev. 2009, 109, 1630−1658. (4) Bryden, F.; Boyle, R. W. Metalloporphyrins for Medical Imaging Applications. Adv. Inorg. Chem. 2016, 68, 141−221. (5) Barona-Castaño, J. C.; Carmona-Vargas, C.; Brocksom, T. J.; de Oliveira, K. T. Porphyrins as Catalysts in Scalable Organic Reactions. Molecules 2016, 21, 310−346. (6) Senge, M. O. Stirring the Porphyrin Alphabet SoupFunctionalization Reactions for Porphyrins. Chem. Commun. 2011, 47, 1943−1960. (7) Aviv-Harel, I.; Gross, Z. Coordination Chemistry of Corroles with focus on Main Group Elements. Coord. Chem. Rev. 2011, 255, 717−736. (8) Toganoh, M.; Furuta, H. Blooming of Confused PorphyrinoidsFusion, Expansion, Contraction and More Confusion. Chem. Commun. 2012, 48, 937−954. (9) Szyszko, B.; Latos-Grażyński, L. Core Chemistry and Skeletal Rearrangements of Porphyrinoids and Metalloporphyrinoids. Chem. Soc. Rev. 2015, 44, 3588−3616. (10) Roznyatovskiy, V. V.; Lee, C.-H.; Sessler, J. L. π-Extended Isomer and Expanded Porphyrins. Chem. Soc. Rev. 2013, 42, 1921− 1933. (11) Tian, J.; Ding, L.; Xu, H.-J.; Shen, Z.; Ju, H.; Jia, L.; Bao, L.; Yu, J.-S. Cell-Specific and pH-Activatable Rubyrin-Loaded Nanoparticles for Highly Selective Near-Infrared Photodynamic Therapy against Cancer. J. Am. Chem. Soc. 2013, 135, 18850−18858. (12) Yoo, D.; Jeong, H.; Preihs, C.; Choi, J.-S.; Shin, T.-H.; Sessler, J. L.; Cheon, J. Double-Effector Nanoparticles: A Synergistic Approach to Apoptotic Hyperthermia. Angew. Chem., Int. Ed. 2012, 51, 12482− 12485. (13) Saito, S.; Osuka, A. Expanded Porphyrins: Intriguing Structures, Electronic Properties and Reactivities. Angew. Chem., Int. Ed. 2011, 50, 4342−4373. (14) Sessler, J. L.; Camiolo, S.; Gale, P. A. Pyrrolic and Polypyrrolic Anion Binding Agents. Coord. Chem. Rev. 2003, 240, 17−55. (15) Osuka, A.; Saito, S. Expanded Porphyrins and Aromaticity. Chem. Commun. 2011, 47, 4330−4339. (16) Woodward, R. B. Aromaticity: An International Symposium Sheffield, 1966; Special Publication no. 21; The Chemical Society: London, 1966. (17) Broadhurst, M. J.; Grigg, R.; Johnson, A. W. The synthesis of 22 π-Electron Macrocycles: Sapphyrins and Related Compounds. J. Chem. Soc., Perkin Trans. 1 1972, 1, 2111−2116. (18) Sessler, J. L.; Davis, J. M. Sapphyrins: Versatile Anion Binding Agents. Acc. Chem. Res. 2001, 34, 989−997. (19) Pareek, Y.; Ravikanth, M.; Chandrashekar, T. K. Smaragdyrins: Emeralds of Expanded Porphyrin Family. Acc. Chem. Res. 2012, 45, 1801−1816.

A. Srinivasan received his B.Sc. from Madurai Kamaraj University, his M.Sc. from Bharathidasan University, Trichy, and his Ph.D. from Indian Institute of Technology Kanpur in 2000. After his postdoctoral stay in Japan, he joined as a Scientist at National Institute for Interdisciplinary Science and Technology (NIIST-CSIR), Trivandrum, in 2005. In 2009, he moved to National Institute of Science Education and Research (NISER), Bhubaneswar, where he is currently working as a full professor. His current research interest includes pyrrole-based receptor macrocycles. M. Ravikanth received his B.Sc. and M.Sc. from Osmania University, Hyderabad, and his Ph.D. from Indian Institute of Technology Kanpur in 1994. After his postdoctoral stay in the United States and Japan, he joined Indian Institute of Technology Bombay as a faculty member, where he is currently working as a full professor. His current research interest includes porphyrins and related macrocycles. Tavarekere K. Chandrashekar received his Ph.D. from Indian Institute of Science, Bangalore, in 1982. As a postdoctoral researcher, he spent four years in the United States at the University of Massachusetts and Michigan State University, East Lansing, and in 1986 he joined the faculty of Indian Institute of Technology Kanpur. He was the Director of National Institute for Interdisciplinary Science and Technology (NIIST−CSIR), Trivandrum, India, in 2003, and founder Director of National Institute of Science Education and Research (NISER-DAE), Bhubaneswar, India, in 2008. He served as a founder Secretary of Science and Engineering Research Board (SERB-DST), New Delhi, India, until December 2015. Since January 2016, he has been working as a Senior Professor in NISER, Bhubaneswar. His research interest includes expanded porphyrins and their core-modified derivatives.

ACKNOWLEDGMENTS T.C. thanks CSIR for a SRF fellowship; A.S. thanks DAE, NISER; M.R. thanks DST; and T.K.C. thanks DST (J. C. Bose fellowship) for financial support. We also thank our co-workers whose names appear in the references. Special thanks to B. Adinarayana and Arindam Ghosh for their help in preparing this Review. ABBREVIATIONS p-TSA p-Toluenesulfonic acid DDQ 2,3-Dichloro-5,6-dicyano-p-benzoquinone TFA Trifluoroacetic acid MRI Magnetic resonance imaging PDT Photodynamic therapy BCOD Bicyclo[2.2.2]octadiene DTT Dithienothiophene GMP Guanosine monophosphate AMP Adenosine monophosphate AR


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(20) Rexhausen, H.; Gossauer, A. The Synthesis of a new 22 πelectron Macrocycle: Pentaphyrin. J. Chem. Soc., Chem. Commun. 1983, 275. (21) Gossauer, A. Syntheses of some unusual Polypyrrole Macrocycles. Bull. Soc. Chim. Belg. 1983, 92, 793−795. (22) Sessler, J. L.; Weghorn, S. J.; Morishima, T.; Rosingana, M.; Lynch, V.; Lee, V. Rosarin: A new, easily prepared Hexapyrrolic Expanded Porphyrin. J. Am. Chem. Soc. 1992, 114, 8306−8307. (23) Sessler, J. L.; Morishima, T.; Lynch, V. Rubyrin: A new Hexapyrrolic Expanded Porphyrin. Angew. Chem., Int. Ed. Engl. 1991, 30, 977−980. (24) Sessler, J. L.; Seidel, D.; Lynch, V. Synthesis of [28]Heptaphyrin(1.0.0.1.0.0.0) and [32] Octaphyrin(1.0.0.0.1.0.0.0) via a Directed Oxidative Ring Closure: The first Expanded Porphyrins containing a Quaterpyrrole Subunit. J. Am. Chem. Soc. 1999, 121, 11257−11258. (25) Vogel, E.; Broring, M.; Fink, J.; Rosen, D.; Schmickler, H.; Lex, J.; Chan, K. W. K.; Wu, Y.-D.; Plattner, D. A.; Nendel, M.; Houk, K. N. From Porphyrin Isomers to Octapyrrolic “Figure Eight” Macrocycles. Angew. Chem., Int. Ed. Engl. 1995, 34, 2511−2514. (26) Shin, J.-Y.; Furuta, H.; Yoza, K.; Igarashi, S.; Osuka, A. mesoAryl-Substituted Expanded Porphyrins. J. Am. Chem. Soc. 2001, 123, 7190−7191. (27) Sessler, J. L.; Weghorn, S. J.; Lynch, V.; Johnson, M. R. Turcasarin, the Largest Expanded Porphyrin to date. Angew. Chem., Int. Ed. Engl. 1994, 33, 1509−1512. (28) Setsune, J.-I.; Katakami, Y.; Iizuna, N. [48]Dodecaphyrin(1.0.1.0.1.0.1.0.1.0.1.0) and [64]Hexadecaphyrin(1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0): The Largest Cyclic Polypyrroles. J. Am. Chem. Soc. 1999, 121, 8957−8958. (29) Setsune, J.-I.; Maeda, S. Bis(azafulvene) as a Versatile Building Block for Giant Cyclopolypyrroles: X-ray Crystal Structure of [64]Hexadecaphyrin (1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0). J. Am. Chem. Soc. 2000, 122, 12405−12406. (30) Bauer, V. J.; Clive, D. L. J.; Dolphin, D.; Paine, J. B., III; Harris, F. L.; King, M. M.; Loder, J.; Wang, S. W. C.; Woodward, R. B. Sapphyrins: Novel Aromatic Pentapyrrolic Macrocycles. J. Am. Chem. Soc. 1983, 105, 6429−6436. (31) Sessler, J. L.; Cyr, M. J.; Lynch, V.; McGhee, E.; Ibers, J. A. Synthetic and Structural Studies of Sapphyrin, a 22-π-Electron Pentapyrrolic “Expanded Porphyrin. J. Am. Chem. Soc. 1990, 112, 2810−2813. (32) Shevchuk, S. V.; Davis, J. M.; Sessler, J. L. Synthesis of Sapphyrins via a ‘3 + 1 + 1’ Procedure. Tetrahedron Lett. 2001, 42, 2447−2450. (33) Paolesse, R.; Licoccia, S.; Spagnoli, M.; Boschi, T.; Khoury, R. G.; Smith, K. M. A Novel Synthetic Route to Sapphyrin. J. Org. Chem. 1997, 62, 5133−5137. (34) Rana, A.; Kumar, B. S.; Panda, P. K. β-Decamethoxysapphyrin and Its N-Benzyl Analogue. Org. Lett. 2015, 17, 3030−3033. (35) Chmielewski, P. J.; Latos-Grażyński, L.; Rachlewicz, K. 5,10,15,20-Tetraphenylsapphyrin-Idenfication of a Pentapyrrolic Expanded Porphyrin in the Rothemund Synthesis. Chem. - Eur. J. 1995, 1, 68−73. (36) Rachlewicz, K.; Sprutta, N.; Latos-Grażyński, L.; Chmielewski, P. J.; Szterenberg, L. Protonation of 5,10,15,20-TetraphenylsapphyrinIdendification of Inverted and Planar dicationic forms. J. Chem. Soc., Perkin Trans. 2 1998, 2, 959−967. (37) Brückner, C.; Sternberg, E. D.; Boyle, R. W.; Dolphin, D. 5,10Diphenyltripyrrane, a useful building block for the synthesis of mesophenyl substituted Expanded macrocycles. Chem. Commun. 1997, 1689−1690. (38) Geier, G. R., III; Ciringh, Y.; Li, F.; Haynes, D. M.; Lindsey, J. S. A Survey of Acid Catalysts for use in Two-step, One-Flask Syntheses of Meso-Substituted Porphyrinic Macrocycles. Org. Lett. 2000, 2, 1745−1748. (39) Huang, W.; Ou, Z.; Chen, X.; Xue, Z.; Lu, G.; Wang, H.; Xiao, J.; Fang, Y.; Kadish, K. M. A facile synthetic route to meso-tetraaryl substituted N-5 sapphyrins and first single X-ray analysis confirming

the pyrrole inverted structure. J. Porphyrins Phthalocyanines 2015, 19, 794−802. (40) Narayanan, S. J.; Sridevi, B.; Srinivasan, A.; Chandrashekar, T. K.; Roy, R. One step Synthesis of Sapphyrin and N-confused Porphyrin using Dipyrromethane. Tetrahedron Lett. 1998, 39, 7389− 7392. (41) Kang, S.; Hayashi, H.; Umeyama, T.; Matano, Y.; Tkachenko, N. V.; Lemmetyinen, H.; Imahori, H. meso-3,5-Bis(trifluoromethyl)phenyl-substituted Expanded Porphyrins: Synthesis, Characterization and Optical, Electrochemical and Photophysical Properties. Chem. Asian J. 2008, 3, 2065−2074. (42) Sessler, J. L.; Lisowski, J.; Boudreaux, K. A.; Lynch, V.; Barry, J.; Kodadek, T. J. Synthesis and Characterization of Diaryl Sapphyrins Prepared under Lindsey-Type Conditions. J. Org. Chem. 1995, 60, 5975−5978. (43) Richter, D. T.; Lash, T. D. Synthesis of Sapphyrins, Heterosapphyrins and Carbasapphyrins by a “4 + 1” Approach. J. Org. Chem. 2004, 69, 8842−8850. (44) Richter, D. T.; Lash, T. D. Oxidation with dilute aqueous Ferric chloride Solutions greatly improves yields in the ‘4 + 1’ Synthesis of Sapphyrins. Tetrahedron Lett. 1999, 40, 6735−6738. (45) Lisowski, J.; Sessler, J. L.; Lynch, V. Synthesis and X-ray Structure of Selenasapphyrin. Inorg. Chem. 1995, 34, 3567−3572. (46) Sessler, J. L.; Hoehner, M. C.; Gebauer, A.; Andrievsky, A.; Lynch, V. Synthesis and Characterization of All-Alkyl-Substituted Mono-, Di-, and Trioxosapphyrins. J. Org. Chem. 1997, 62, 9251− 9260. (47) Srinivasan, A.; Anand, V. G.; Narayanan, S. J.; Pushpan, S. K.; Kumar, M. R.; Chandrashekar, T. K.; Sugiura, K.-i.; Sakata, Y. Structural Characterization of meso-Aryl Sapphyrins. J. Org. Chem. 1999, 64, 8693−8697. (48) Srinivasan, A.; Pushpan, S. K.; Kumar, M. R.; Mahajan, S.; Chandrashekar, T. K.; Roy, R.; Ramamurthy, P. meso-Aryl Sapphyrins with Heteroatoms: Synthesis, Characterization, Spectral and Electrochemical properties. J. Chem. Soc., Perkin Trans. 2 1999, 961−968. (49) Narayanan, S. J.; Sridevi, B.; Chandrashekar, T. K.; Vij, A.; Roy, R. Novel Core-Modified Expanded Porphyrins with meso-Aryl Substituents: Synthesis, Spectral and Structural Characterization. J. Am. Chem. Soc. 1999, 121, 9053−9068. (50) Narayanan, S. J.; Sridevi, B.; Chandrashekar, T. K.; Vij, A.; Roy, R. Sapphyrin Supramolecules through C-H···S and C-H···Se Hydrogen Bonds − First Structural Characterization of meso-Aryl Sapphyrins bearing Heteroatoms. Angew. Chem., Int. Ed. 1998, 37, 3394−3397. (51) Pushpan, S. K.; Narayanan, J. S.; Srinivasan, A.; Mahajan, S.; Chandrashekar, T. K.; Roy, R. One Pot Synthesis of Core-Modified Expanded Porphyrins. Tetrahedron Lett. 1998, 39, 9249−9252. (52) Sathyamoorthy, B.; Axelrod, A.; Farwell, V.; Bennett, S. M.; Calitree, B. D.; Benedict, J. B.; Sukumaran, D. K.; Detty, M. R. Novel 21,23-Ditelluraporphyrins and the First 26,28-Ditellurasapphyrin and 30,33-Ditellurarubyrin. Organometallics 2010, 29, 3431−3441. (53) Shin, K.; Lim, C.; Choi, C.; Kim, Y.; Lee, C.-H. Synthesis and Xray Crystal Structure of 27-Oxa-25,29-Dithiasapphyrin: BithiopheneContaining Sapphyrin Have an Inverted Structure. Chem. Lett. 1999, 28, 1331−1332. (54) Sprutta, N.; Latos-Grażyński, L. 25,27-Dithiasapphyrin and Pyrrole-Inverted Isomer of 21,23-Dithiaporphyrin from Condensation of Pyrrole and 2,5-Bis(p-tolylhydroxymethyl) thiophene. Org. Lett. 2001, 3, 1933−1936. (55) Nakabuchi, T.; Matano, Y.; Imahori, H. Remarkable Effects of PPerfluorophenyl Group on the Synthesis of Core-Modified Phosphaporphyrinoids and Phosphadithiasapphyrin. Org. Lett. 2010, 12, 1112− 1115. (56) Okujima, T.; Kikkawa, T.; Kawakami, S.; Shimizu, Y.; Yamada, H.; Ono, N.; Uno, H. Synthesis and Properties of BCOD-Fused Trithiasapphyrin and Trithiabenzosapphyrins. Tetrahedron 2010, 66, 7213−7218. (57) Ono, N.; Kuroki, K.; Watanabe, E.; Ochi, N.; Uno, H. A New Synthesis of Mono- and Dibenzosapphyrins. Heterocycles 2004, 62, 365−373. AS


Chemical Reviews

Review

Linked Expanded Porphyrin Dyads. Eur. J. Org. Chem. 2011, 2011, 5390−5399. (79) Shetti, V. S.; Ravikanth, M. Supramolecular Tetrads containing Sn(IV) Porphyrin, Ru(II) Porphyrin, and Expanded Porphyrins Assembled Using Complementary Meta-Ligand Interactions. Inorg. Chem. 2011, 50, 1713−1722. (80) Pareek, Y.; Lakshmi, V.; Ravikanth, M. Axially Bonded Pentads constructed on the Sn(IV) Porphyrin Scaffold. Dalton Trans. 2014, 43, 6870−6879. (81) Kaur, T.; Khan, T. K.; Ravikanth, M. Multi-Expanded Porphyrin Assemblies on Cyclophosphazene Scaffolds. Eur. J. Org. Chem. 2015, 2015, 3157−3163. (82) You, L.; Zha, D.; Anslyn, E. V. Recent Advances in Supramolecular Analytical Chemistry Using Optical Sensing. Chem. Rev. 2015, 115, 7840−7892. (83) Alibrandi, G.; Amendola, V.; Bergamaschi, G.; Fabbrizzi, L.; Licchelli, M. Bistren Cryptands and Cryptates: Versatile Receptors for Anion Inclusion and Recognition in Water. Org. Biomol. Chem. 2015, 13, 3510−3524. (84) Han, Y.; Meng, Z.; Ma, Y.-X.; Chen, C.-F. Iptycene-Derived Crown Ether Hosts for Molecular Recognition and Self-Assembly. Acc. Chem. Res. 2014, 47, 2026−2040. (85) Nimse, S. B.; Kim, T. Biological Applications of Functionalized Calixarenes. Chem. Soc. Rev. 2013, 42, 366−386. (86) Kim, D. S.; Sessler, J. L. Calix[4]pyrroles: Versatile Molecular Containers with Ion Transport, Recognition and Molecular Switching Functions. Chem. Soc. Rev. 2015, 44, 532−546. (87) Crini, G. Review: A History of Cyclodextrins. Chem. Rev. 2014, 114, 10940−10975. (88) Casero, R. A.; Marton, L. J. Targeting Polyamine Metabolism and Function in Cancer and Other Hyperproliferative Diseases. Nat. Rev. Drug Discovery 2007, 6, 373−390. (89) Shionoya, M.; Furuta, H.; Lynch, V.; Harriman, A.; Sessler, J. L. Diprotonated Sapphyrin: A Fluoride Selective Halide Anion Receptor. J. Am. Chem. Soc. 1992, 114, 5714−5722. (90) Sessler, J. L.; Cyr, M.; Furuta, H.; Král, V.; Mody, T.; Morishima, T.; Shionoya, M.; Weghorn, S. Anion-binding: A New Direction in Porphyrin-Related Research. Pure Appl. Chem. 1993, 65, 393−398. (91) Sessler, J. L.; Cyr, M. J.; Burrell, A. K. Sapphyrins: New Life for an Old “Expanded Porphyrin. Synlett 1991, 1991, 127−134. (92) Sessler, J. L.; Ford, D. A.; Cyr, M. J.; Furuta, H. Enhanced Transport of Fluoride Anion Effected using Protonated Sapphyrin as a Carrier. J. Chem. Soc., Chem. Commun. 1991, 1733−1735. (93) Král, V.; Andrievsky, A.; Sessler, J. L. A Covalently Linked Sapphyrin Dimer. A New Receptor for Dicarboxylate Anions. J. Am. Chem. Soc. 1995, 117, 2953−2954. (94) Sessler, J. L.; Andrievsky, A.; Král, V.; Lynch, V. Chiral Recognition of Dicarboxylate Anions by Sapphyrin-Based Receptors. J. Am. Chem. Soc. 1997, 119, 9385−9392. (95) Sessler, J. L.; Andrievsky, A. Sapphyrin-Lasalocid Conjugate: A Novel Carrier for Aromatic Amino Acid Transport. Chem. Commun. 1996, 1119−1120. (96) Sessler, J. L.; Andrievsky, A. Efficient Transport of Aromatic Amino Acids by Sapphyrin-Lasalocid Conjugates. Chem. - Eur. J. 1998, 4, 159−167. (97) Springs, S. L.; Gosztola, D.; Wasielewski, M. R.; Král, V.; Andrievsky, A.; Sessler, J. L. Picosecond Dynamics of Energy Transfer in Porphyrin-Sapphyrin Noncovalent Assemblies. J. Am. Chem. Soc. 1999, 121, 2281−2289. (98) Král, V.; Springs, S. L.; Sessler, J. L. A Noncovalent Assembly for Energy Transfer Based on Anion Chelation. J. Am. Chem. Soc. 1995, 117, 8881−8882. (99) Sessler, J. L.; Andrievsky, A.; Gale, P. A.; Lynch, V. Anion Binding: Self-Assembly of Polypyrrolic Macrocycles. Angew. Chem., Int. Ed. Engl. 1996, 35, 2782−2785. (100) Zhang, Z.; Ferrence, G. M.; Lash, T. D. MacDonald-Type Reactions Using Bis-acrylaldehydes: Synthesis of an Expanded

(58) Pushpan, S. K.; Srinivasan, A.; Anand, V. G.; Venkatraman, S.; Chandrashekar, T. K.; Joshi, B. S.; Roy, R.; Furuta, H. N-Confused Expanded Porphyrin: First Example of a Modified Sapphyrin with an Inverted N-confused Pyrrole Ring. J. Am. Chem. Soc. 2001, 123, 5138− 5139. (59) Sessler, J. L.; Cho, D.-G.; Stepień, M.; Lynch, V.; Waluk, J.; Yoon, Z. S.; Kim, D. Inverted Sapphyrin: A New Family of Doubly NConfused Expanded Porphyrins. J. Am. Chem. Soc. 2006, 128, 12640− 12641. (60) Yoon, Z. S.; Cho, D.-G.; Kim, K. S.; Sessler, J. L.; Kim, D. Nonlinear Optical Properties as a Guide to Aromaticity in Congeneric Pentapyrrolic Expanded Porphyrins: Pentaphyrin, Sapphyrin, Isosmaragdyrin and Orangarin. J. Am. Chem. Soc. 2008, 130, 6930−6931. (61) Gupta, I.; Srinivasan, A.; Morimoto, T.; Toganoh, M.; Furuta, H. N-Confused and N-Fused meso-Aryl Sapphyrins. Angew. Chem., Int. Ed. 2008, 47, 4563−4567. (62) Panda, P. K.; Kang, Y.-J.; Lee, C.-H. A Benzodipyrrole-Derived Sapphyrin. Angew. Chem., Int. Ed. 2005, 44, 4053−4055. (63) Cho, D.-G.; Plitt, P.; Kim, S. K.; Lynch, V.; Hong, S.-J.; Lee, C.H.; Sessler, J. L. Dioxabenzosapphyrin: A New Benzodifuran-Derived Sapphyrin Analogue. J. Am. Chem. Soc. 2008, 130, 10502−10503. (64) Kee, S.-Y.; Lim, J. M.; Kim, S.-J.; Yoo, J.; Park, J.-S.; Sarma, T.; Lynch, V. M.; Panda, P. K.; Sessler, J. L.; Kim, D.; Lee, C.-H. Conformational and Spectroscopic properties of π-extended, bipyrrole-fused Rubyrin and Sapphyrin Derivatives. Chem. Commun. 2011, 47, 6813−6815. (65) Shetti, V. S.; Kee, S.-Y.; Lee, C.-H. Core-Modified Analogues of Expanded Porphyrins containing Rigid β,β′-Linked o-Phenylene Bridges. Chem. - Asian J. 2014, 9, 734−738. (66) Sarma, T.; Anusha, P. T.; Pabbathi, A.; Venugopal Rao, S.; Panda, P. K. Naphthobipyrrole-Derived Sapphyrins: Rational Synthesis, Characterization, Nonlinear Optical Properties and ExcitedState Dynamics. Chem. - Eur. J. 2014, 20, 15561−15570. (67) Karthik, G.; Srinivasan, A.; Chandrashekar, T. K. meso-Aryl Core-Modified Fused Sapphyrins: Syntheses and Structural Diversity. Org. Lett. 2014, 16, 3472−3475. (68) Jeong, S.-D.; Sessler, J. L.; Lynch, V.; Lee, C.-H. Dithiabenzisapphyrin: A Core-Modified Sapphyrin Bearing Exocyclic Double Bonds at the meso-positions. J. Am. Chem. Soc. 2008, 130, 390−391. (69) Ishida, M.; Karasawa, S.; Uno, H.; Tani, F.; Naruta, Y. Protonation-Induced Formation of a Stable Singlet Biradicaloid Derived from a Modified Sapphyrin Analogue. Angew. Chem., Int. Ed. 2010, 49, 5906−5909. (70) Szyszko, B.; Małecki, M.; Berlicka, A.; Białek, M. J.; Białońska, A.; Kupietz, K.; Pacholska-Dudziak, E.; Latos-Grażyński, L. Incorporation of a Phenanthrene Subunit into a Sapphyrin Framework: Synthesis of Expanded Aceneporphyrinoids. Chem. - Eur. J. 2016, 22, 7602−7608. (71) Osuka, A. Towards meso-meso-linked Porphyrin Arrays and meso-Aryl Expanded Porphyrins. Chem. Rec. 2015, 15, 143−159. (72) Tanaka, T.; Osuka, A. Conjugated Porphyrin Arrays: Synthesis, Properties and Applications for Functional Materials. Chem. Soc. Rev. 2015, 44, 943−969. (73) Titi, H. M.; Tripuramallu, B. K.; Goldberg, I. Porphyrin-based Assemblies directed by non-covalent interactions: Highlights of Recent Investigations. CrystEngComm 2016, 18, 3318−3339. (74) Sessler, J. L.; Brucker, E. A.; Král, V.; Harriman, A. A Face-toFace Porphyrin-Sapphyrin Pseudo Dimer. Supramol. Chem. 1994, 4, 35−41. (75) Sessler, J. L.; Brucker, E. A. The first “Crowned” Expanded Porphyrin. Tetrahedron Lett. 1995, 36, 1175−1176. (76) Misra, R.; Kumar, R.; Chandrashekar, T. K.; Suresh, C. H. Meso-Meso Linked Core-Modified22π Smaragdyrins with Unusual Absorption Properties. Chem. Commun. 2006, 4584−4586. (77) Malakalapalli, R. R.; Mangalampalli, R. Synthesis of functionalized Core-Modified Sapphyrins and Covalently linked-Sapphyrin Dyads. Tetrahedron 2012, 68, 1306−1314. (78) Pareek, Y.; Ravikanth, M. Synthesis of Mono-Functionalized Core-Modified Expanded Porphyrin Building Blocks and Covalently AT


Chemical Reviews

Review

Sapphyrin and Vinylogous Hexaphyrins. Org. Lett. 2009, 11, 1249− 1252. (101) Král, V.; Furuta, H.; Shreder, K.; Lynch, V.; Sessler, J. L. Protonated Sapphyrins. Highly Effective Phosphate Receptors. J. Am. Chem. Soc. 1996, 118, 1595−1607. (102) Furuta, H.; Cyr, M. J.; Sessler, J. L. Phosphate Anion Binding: Enhanced Transport of Nucleotide Monophosphates Using a Sapphyrin Carrier. J. Am. Chem. Soc. 1991, 113, 6677−6678. (103) Král, V.; Sessler, J. L. Molecular Recognition via Base-Pairing and Phosphate Chelation. Ditopic and Tritopic Sapphyrin-Based Receptors for the Recognition and Transport of Nucleotide Monophosphates. Tetrahedron 1995, 51, 539−554. (104) Král, V.; Sessler, J. L.; Furuta, H. Synthetic Sapphyrin-Cytosine Conjugates: Carriers for Selective Nucleotide Transport at Neutral pH. J. Am. Chem. Soc. 1992, 114, 8704−8705. (105) Krái, V.; Andrievsky, A.; Sessler, J. L. Oligosapphyrins: Receptors for the Recognition and Transport of Nucleotide Di- and Tri-Phosphates. J. Chem. Soc., Chem. Commun. 1995, 0, 2349−2351. (106) Iverson, B. L.; Thomas, R. E.; Král, V.; Sessler, J. L. Molecular Recognition of Anionic Species by Silica Gel Bound Sapphyrin. J. Am. Chem. Soc. 1994, 116, 2663−2664. (107) Sessler, J. L.; Král, V.; Genge, J. W.; Thomas, R. E.; Iverson, B. L. Anion Selectivity of a Sapphyrin-Modified Silica Gel HPLC Support. Anal. Chem. 1998, 70, 2516−2522. (108) Iverson, B. L.; Shreder, K.; Král, V.; Sessler, J. L. Phosphate Recognition by Sapphyrin. A New Approach to DNA Binding. J. Am. Chem. Soc. 1993, 115, 11022−11023. (109) Iverson, B. L.; Shreder, K.; Král, V.; Sansom, P.; Lynch, V.; Sessler, J. L. Interaction of Sapphyrin with Phosphorylated Species of Biological Interest. J. Am. Chem. Soc. 1996, 118, 1608−1616. (110) Iverson, B. L.; Shreder, K.; Morishima, T.; Rosingana, M.; Sessler, J. L. Syntheses of a Sapphyrin-EDTA Conjugate and Preliminary Cleavage Results Using a Supercoiled Plasmid DNA Assay. J. Org. Chem. 1995, 60, 6616−6620. (111) Sessler, J. L.; Sansom, P. I.; Král, V.; O’Connor, D.; Iverson, B. L. Sapphyrin Oligonucleotide Conjugates. Novel Sequence-Specific DNA Photomodifying Agents with Increased Binding Affinity. J. Am. Chem. Soc. 1996, 118, 12322−12330. (112) Magda, D.; Wright, M.; Miller, R. A.; Sessler, J. L.; Sansom, P. I. Sequence-Specific Photocleavage of DNA by an Expanded Porphyrin with Irradiation above 700 nm. J. Am. Chem. Soc. 1995, 117, 3629−3630. (113) Král, V.; Lang, K.; Králová, J.; Dvořaḱ , M.; Martásek, P.; Chin, A. O.; Andrievsky, A.; Lynch, V.; Sessler, J. L. Polyhydroxylated Sapphyrins: Multisite Non-Metallic Catalysts for Activated Phosphodiester Hydrolysis. J. Am. Chem. Soc. 2006, 128, 432−437. (114) Boul, P. J.; Cho, D.-G.; Rahman, G. M. A.; Marquez, M.; Ou, Z.; Kadish, K. M.; Guldi, D. M.; Sessler, J. L. Sapphyrin-Nanotube Assemblies. J. Am. Chem. Soc. 2007, 129, 5683−5687. (115) Grimm, B.; Karnas, E.; Brettreich, M.; Ohta, K.; Hirsch, A.; Guldi, D. M.; Torres, T.; Sessler, J. L. Charge Transfer in SapphyrinFullerene Hybrids Employing Dendritic Ensembles. J. Phys. Chem. B 2010, 114, 14134−14139. (116) Král, V.; Davis, J.; Andrievsky, A.; Kralová, J.; Synytsya, A.; Poucková, P.; Sessler, J. L. Synthesis and Biolocalization of WaterSoluble Sapphyrins. J. Med. Chem. 2002, 45, 1073−1078. (117) Parmeswaran, D.; Pushpan, S. K.; Srinivasan, A.; Kumar, M. R.; Chandrashekar, T. K.; Ganesan, S. In Vitro and In Vivo Investigations on the Photodynamic Activity of Core-Modified Expanded Porphyrin − Ammonium Salt of 5,10,15,20-tetrakis-(meso-p-sulfonato phenyl)25,27,29-trithia Sapphyrin. Photochem. Photobiol. 2003, 78, 487−495. (118) Wang, Z.; Lecane, P. S.; Thiemann, P.; Fan, Q.; Cortez, C.; Ma, X.; Tonev, D.; Miles, D.; Naumovski, L.; Miller, R. A.; et al. Synthesis and Biologic Properties of Hydrophilic Sapphyrins, a New Class of Tumor-Selective Inhibitors of Gene Expression. Mol. Cancer 2007, 6, 9. (119) Sessler, J. L.; Tomat, E. Transition-Metal Complexes of Expanded Porphyrins. Acc. Chem. Res. 2007, 40, 371−379.

(120) Sessler, J. L.; Burrell, A. H.; Lisowski, J.; Gebauer, A.; Cyr, M. J.; Lynch, V. Cobalt(II) and Rhodium(I) Complexes of Thia- and Oxasapphyrins. Bull. Soc. Chem. Fr. 1996, 133, 725−734. (121) Burrell, A. K.; Sessler, J. L.; Cyr, M. J.; McGhee, E.; Ibers, J. A. Metal Carbonyl Complexes of Sapphyrins. Angew. Chem., Int. Ed. Engl. 1991, 30, 91−93. (122) Burrell, A. K.; Cyr, M. J.; Lynch, V.; Sessler, J. L. Nucleophilic Attach at the meso position of a Uranyl Sapphyrin Complex. J. Chem. Soc., Chem. Commun. 1991, 1710−1713. (123) Simkhovich, L.; Rosenberg, S.; Gross, Z. Facile Synthesis of a Novel Sapphyrin and its Rhodium(I) Complex. Tetrahedron Lett. 2001, 42, 4929−4931. (124) Sessler, J. L.; Seidel, D.; Bucher, C.; Lynch, V. Novel, Terpyrrole-containing, Aromatic Expanded Porphyrins. Tetrahedron 2001, 57, 3743−3752. (125) Sessler, J. L.; Davis, J. M.; Lynch, V. Synthesis and Characterization of a Stable Smaragdyrin Isomer. J. Org. Chem. 1998, 63, 7062−7065. (126) Narayanan, S. J.; Sridevi, B.; Chandrashekar, T. K.; Englich, U.; Ruhlandt-Senge, K. Core-Modified Smaragdyrins: First Examples of Stable Meso-Substituted Expanded Corrole. Org. Lett. 1999, 1, 587− 590. (127) Kalita, H.; Lee, W.-Z.; Ravikanth, M. Synthesis and Functionalization of BF2-Complexes of meso-Free 25- Oxasmaragdyrin. J. Org. Chem. 2013, 78, 6285−6290. (128) Kalita, H.; Kalita, D.; Lee, W.-Z.; Bellare, J.; Ravikanth, M. Synthesis of meso-Pyrrole-Substituted 22-Oxacorroles by a “3 + 2” Approach. Chem. - Eur. J. 2014, 20, 10404−10413. (129) Venkatraman, S.; Kumar, R.; Sankar, J.; Chandrashekar, T. K.; Sendhil, K.; Vijayan, C.; Kelling, A.; Senge, M. O. OxasmaragdyrinFerrocene and Oxacorrole-Ferrocene Conjugates: Synthesis, structure, and Nonlinear Optical Properties. Chem. - Eur. J. 2004, 10, 1423− 1432. (130) Kumar, R.; Misra, R.; Prabhuraja, V.; Chandrashekar, T. K. Modified Expanded Corrole-Ferrocene Conjugates: Syntheses, Structure and Properties. Chem. - Eur. J. 2005, 11, 5695−5707. (131) Misra, R.; Kumar, R.; Chandrashekar, T. K.; Nag, A.; Goswami, D. Modified (22π) Smaragdyrins with Large Two-Photon Absorption Cross Section: A Structure Function Correlation. Org. Lett. 2006, 8, 629−631. (132) Misra, R.; Kumar, R.; Chandrashekar, T. K.; Suresh, C. H.; Nag, A.; Goswami, D. 22π SmaragdyrinMolecular Conjugates with Aromatic Phenylacetylenes and Ferrocenes: Syntheses, Electrochemical, and Photonic Properties. J. Am. Chem. Soc. 2006, 128, 16083−16091. (133) Gokulnath, S.; Prabhuraja, V.; Sankar, J.; Chandrashekar, T. K. Smaragdyrin-Azobenzene Conjugates: Syntheses, Structure, and Spectral and Electrochemical Properties. Eur. J. Org. Chem. 2007, 2007, 191−200. (134) Rao, M. R.; Ravikanth, M. Synthesis and Studies of Covalently Linked Porphyrin-Expanded Heteroporphyrin Dyads. Eur. J. Org. Chem. 2011, 2011, 1335−1345. (135) Sridevi, B.; Narayanan, S. J.; Rao, R.; Chandrashekar, T. K.; Englich, U.; Ruhlandt-Senge, K. Meso-Aryl Smaragdyrin: Novel Anion and Metal Receptors. Inorg. Chem. 2000, 39, 3669−3677. (136) Rao, M. R.; Ravikanth, M. Boron Complexes of Oxasmaragdyrin, a Core-Modified Expanded Porphyrin. J. Org. Chem. 2011, 76, 3582−3587. (137) Kalita, H.; Lee, W.-Z.; Ravikanth, M. Synthesis, structure, spectral and electrochemical properties of B(OR)2- smaragdyrin complexes. Dalton Trans. 2013, 42, 14537−14544. (138) Mane, S. B.; Hu, J.-Y.; Chang, Y.-C.; Luo, L.; Diau, E. W-G.; Hung, C.-H. Novel expanded porphyrin sensitized solar cells using boryl oxasmaragdyrin as the sensitizer. Chem. Commun. 2013, 49, 6882−6884. (139) Mane, S. B.; Hung, C.-H. Molecular Engineering of Boryl Oxasmaragdyrins through Peripheral Modification: Structure-Efficiency Relationship. Chem. - Eur. J. 2015, 21, 4825−4841. AU


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(140) Ganapathi, E.; Chatterjee, T.; Ravikanth, M. Fluorescent Boron Complexes of 25-Oxa smaragdyrins Containing Axial Silyloxy Groups. Eur. J. Inorg. Chem. 2015, 2015, 4810−4818. (141) Pareek, Y.; Ravikanth, M. Synthesis and studies of covalently linked BF2-oxasmaragdyrin-BODIPY and BF2-oxasmaragdyrin-ferrocene dyads. J. Porphyrins Phthalocyanines 2013, 17, 157−164. (142) Pareek, Y.; Ravikanth, M. Synthesis and properties of covalently linked BF2-oxasmaragdyrin-porphyrin dyads and triad. Tetrahedron 2013, 69, 1590−1599. (143) Khan, T. K.; Ravikanth, M. Synthesis and Properties of Covalently Linked Trichromophore Systems. Eur. J. Org. Chem. 2011, 2011, 7011−7022. (144) Ganapathi, E.; Madhu, S.; Ravikanth, M. Synthesis and properties of triazole bridged BODIPY-conjugates. Tetrahedron 2014, 70, 664−671. (145) Kalita, H.; Lee, W.-Z.; Ravikanth, M. Phosphorus Complexes of meso-Triaryl-25-Oxasmaragdyrins. Inorg. Chem. 2014, 53, 9431− 9438. (146) Kalita, H.; Lee, W.-Z.; Theophall, G. G.; Lakshmi, K. V.; Ravikanth, M. A Stable Seven-Membered Heterocycle, Containing B, C, N, O, and P Atoms, inside a Smaragdyrin Macrocycle. Chem. - Eur. J. 2015, 21, 11315−11319. (147) Ganapathi, E.; Kalita, H.; Theophall, G. G.; Lakshmi, K. V.; Ravikanth, M. Mixed B(III) and P(V) Complexes of Meso-Triaryl 25Oxasmaragdyrins. Chem. - Eur. J. 2016, 22, 9699−9708. (148) Chatterjee, T.; Ghosh, A.; Madhu, S.; Ravikanth, M. Stable core-modified calixsmaragdyrins: synthesis, structure and specific sensing of the hydrogen sulphate ion. Dalton Trans. 2014, 43, 6050−6058. (149) Chatterjee, T.; Areti, S.; Ravikanth, M. Synthesis, Structure, and Hg2+-Ion-Sensing Properties of Stable Calixazasmaragdyrins. Inorg. Chem. 2015, 54, 2885−2892. (150) Chatterjee, T.; Ravikanth, M. Synthesis, Structure, and Catalytic Activity of Pd(II) Complex of Calixoxasmaragdyrin. Inorg. Chem. 2014, 53, 10520−10526.

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