Recent Advances in the Design and Syntheses of Porphyrinoids by Embedding Higher Analogues of Arene and Pyridine Units.

Due to enthralling applications in various fields and augmenting fundamental wisdom, π-conjugated macrocycles in general and porphyrin systems in particular are constantly explored. Subtle modifications of porphyrin structure can amend the rudimentary properties. Pursuing innovative properties provides impetus to underpin arene or pyridine moiety embedded porphyrin derivatives. There have been several reviews related to arene incorporated carbaporphyrinoids; however, recent developments of porphyrin analogues by introducing higher analogues of arenes and pyridine units are not adequately inspected. This mini-review mainly focuses on biphenyl, bipyridine, terphenyl, and mixed arene pyridine embedded porphyrin analogues and their coordination chemistry.

Introduction

Porphyrin is a highly conjugated extensively studied 18π Hückel aromatic tetrapyrrolic macrocycle.[1] Its analogues are ubiquitous in several biological processes as a pivotal part of numerous prosthetic groups. Due to these higher abundances, they are even termed as “pigments of life”.[1] Innumerable studies revealed that the Life on Earth is essentially controlled by a number of porphyrin entities, which includes oxygen carrier and storage, electron transport, photosynthesis in green plants, etc. Diversified utility impelled chemists to synthesize and decipher the novel porphyrin analogues with cogent studies. The advancement of porphyrin chemistry augments the inquisitiveness of human beings who thrive to embark upon innovative avenues by doing subtle modifications of different parts of tetrapyrrolic skeletons, especially such amendments encompass (i) peripheral modifications, (ii) porphyrin isomers, (iii) contracted porphyrins, (iv) core-modified porphyrins, and (v) expanded porphyrins.[2] All these porphyrinoids act as distinctive functional models in various fields.

Carbaporphyrinoids

Carbaporphyrinoids are porphyrin analogues which include at least one carbon atom within the macrocyclic cavity[3] (Figure ) and have been widely inspected over the last two decades. Since the discovery of carbaporphyrinoids, several methodologies have been applied to accommodate carbon atom(s) in the coordination sphere; these include (i) confusion/inversion of rings in the porphyrin frame (1)[4] and (ii) incorporation of the carbocyclic rings as part of the macrocyclic framework (2)[3] which have become trivial. Very recently synthesis of an NHC-containing porphyrinoid, known as carbenaporphyrin (3),[5] has opened a new gateway to procure carbaporphyrinoids. Essentially, the evolution of carbaporphyrinoids underpins a unique platform towards (i) comprehensive study of the nature of aromaticity; (ii) intriguing spectral properties; (iii) metal–arene interactions; (iv) stabilization of the higher oxidation state organometallic complexes; (v) miscellaneous modes of binding upon metal coordination; and (vi) unusual chemical reactivities.

Figure 1

Structures of different carbaporphyrinoids.

Benziporphyrins

One of the important class of carbaporphyrinoids is benziporphyrin. Basically, these are porphyrin analogues in which one of the pyrrole rings is replaced by a benzene unit. These analogues can be categorized according to the bonding motif of benzene in the macrocyclic core where it can be 1,3- or 1,4-connected and denoted as meta- or para-benziporphyrins, respectively (Figure ). The properties are subjected to the nature of the bonding motif (m-/p-). In 1994, the first meta-benziporphyrin 4 was reported by Berlin and Breitmaier.[6a] They obtained 4 by acid-catalyzed [3 + 2] MacDonald type condensation between isophthaladehyde and tripyrrane followed by oxidation. Subsequently, Latos-Grażyński and co-workers described a modified synthetic methodology to afford meso-substituted m-benziporphyrin 5 by the condensation of pyrrole, benzaldehyde, and 1,3-bis(phenylhydroxymethyl)benzene.[6b] The presence of a meta connection in the macrocyclic core preserves its local aromaticity and remains isolated from the overall conjugation in the system. Therefore, 5 demonstrates nonaromatic behavior. The presence of C–H in the macrocyclic core is further exploited for stabilizing organometallic complexes and weak metal C–H bond interactions. Thus, the nonaromatic 5 affords a series of organometallic complexes with PdII, PtII, NiII, and RhIII and weak metal-arene interactions with ZnII, CdII, HgII, NiII, and FeIII ions.[3]

Figure 2

Structures of benziporphyrins.

The aromatic counterpart of nonaromatic meta-benziporphyrin is para-benziporphyrin 6 where the p-phenylene unit is incorporated in the porphyrin ring. The compound 6 is achieved by modest modification of the synthesis described for the meta-benziporphyrin, where 1,3-disubstituted carbinol is replaced by 1,4-disubstituted carbinol.[7a] The spectral and structural features confirm continuous conjugation and 18π electronic aromatic property of the framework. The core of para-benziporphyrin is further employed to study the weak metal–arene interaction as well as coordination.[7b]

Expanded Benziporphyrins

Corriu and co-workers reported the first example of an expanded benziporphyrin, di-m-benzihexaphyrin(1.0.0.1.0.0) (7).[8] The spectral studies of 7 suggested the typical nonaromatic characteristics. Di-p-benzi[28]hexaphyrin (8) was synthesized by Latos-Grażyński and co-workers and demonstrated solvent polarity and temperature triggered Möbius aromaticity. A,C-di-p-benzi[24]pentaphyrin (9) was also reported by the same group and acts as an astute probe for aromaticity switching.[3] In 2018, Ravikanth and co-workers synthesized di-m-benzidecaphyrins(1.0.0.1.1.1.0.0.1.1) 10 and their bis-BF2 complexes.[9a] Unsymmetrical heterobenzisapphyrin 11 was also reported by same group by adopting a [4 + 1] acid-catalyzed condensation reaction.[9b] The spectral and structural analyses revealed that 11 is a planar molecule with an inverted furan ring and adopted nonaromatic characteristic (Figure ). Very recently, Song and co-workers reported the synthesis of an m-benzene ring incorporated octaphyrin and its PdII metal complex.[10]

Figure 3

Structures of expanded carbaporphyrinoids.

Polycyclic Aromatic Hydrocarbon Embedded Carbaporphyrinoids

It is pertinent to point out that the chemical compounds with aromatic hydrocarbons are considered as a major subset and are commonly recognized as polycyclic aromatic hydrocarbons (PAHs). Thus, these constituting units inevitably become the best pick to deepen the wisdom of different types of (anti/non) aromaticity and essentially aromaticity switching for extended π-systems of benzene moieties incorporated carbaporphyrinoids. The incorporation of naphthalene, anthracene, and phenanthrene moiety into a porphyrin framework leads to unification of the structural features of PAHs and porphyrins and such aceneporphyrinoids termed as naphthiporphyrin (12), anthriporphyrin (13), and phenanthriporphyrin (14), respectively.[3] The stabilization of organo-PdII in 12, PdII in 13, and organo-PV and organo-CuIII complexes in 14 were demonstrated by the research groups of Lash and Latos-Grażyński, respectively. In 2017, Sessler and co-workers reported dibenzo[g,p]chrysene-fused bis-dicarbacorrole (15) with two adj-CCNN subunits.[11] The bis-trianionic core of 15 was utilized to stabilize the bis-CuIII metal complex, alongside a hetero bis-metal complex of CuIII and PdII ions. Ravikanth and co-workers synthesized fluorene unit incorporated antiaromatic meso-fused heterobenziporphyrins (16) and nonaromatic fluorenophyrins (17) (Figure ).[12]

Figure 4

Structures of polycyclic aromatic hydrocarbons embedded carbaporphyrinoids.

Biphenyl Embedded Contracted Carbaporphyrinoids

Since 2015, our group has contributed immensely in the field of carbaporphyrinoids. Initially, we selected one of the simple PAHs such as a biphenyl moiety to synthesize a series of contracted carbaporphyrinoids. The synthesis of biphenylcorrole 18 with adj-CCNN core was reported, where the bipyrrole unit in the corrole framework was substituted by a biphenyl moiety.[13a] The macrocycle 18 was achieved by the Lewis acid-catalyzed condensation of 3,3′-biphenyl-bis(dipyrromethane) 19 with pentafluorobenzaldehyde followed by oxidation with 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) (Scheme ).

Scheme 1

Synthesis of Biphenylcorrole 18 and its Copper(III) Complex 20

The existence of two inner core biphenyl CHs and pyrrolic NH were utilized for metal ion insertion. When 18 was treated with Cu(OAc)2 in a CH2Cl2/CH3OH solvent mixture, the organo-CuIII complex 20 was obtained (Scheme ). The aromatic biphenyl unit and the π-delocalized dipyrromethene units are connected together to produce overall nonaromatic character in 18 and retained as such upon metal ion insertion. Later, by reorganizing the bonding mode of biphenyl moiety with rest of the macrocycle, our group introduced another contracted derivative and demonstrated the synthesis of carbatriphyrin(3.1.1) (21). The CNN core is achieved by introducing the o- and m-phenylene units as part of the macrocyclic framework[13b] and adopted similar synthetic procedures as that of 18. The π-electron conjugation in the triphyrin(3.1.1) framework is prolonged up to the o-phenylene unit, however restricted in the m-phenylene unit, thus provide overall nonaromatic characteristics. The macrocyclic core of 21 was exploited for boron coordination and established a weak boron arene interaction (C–H···B) (22) and also stabilized organo-BIII complex 23 (Scheme ).

Scheme 2

Synthesis of Boron-Arene (C–H···B) 22 and Organoborane (C–B) 23 Complexes from Carbatriphyrin(3.1.1) 21

The peripheral transformation of carbatriphyrin(3.1.1) was done by a metal assisted strategy by using PtCl2 salt to obtain a meso-fused β–β′ dimer and its monomer analogue.[13c] Startlingly, (a) a fusion reaction between meso-mesityl-CH3 and pyrrolic β-CH of carbatriphyrin(3.1.1) produced a meso-fused monomer with 1H-benzo[f]indole units functionalized with a keto group 24 and (b) alteration of monomeric dianionic triphyrin core to trianionic dimer 25 were articulated (Scheme ).

Scheme 3

Synthesis of 24 and 25

Biphenyl Embedded Expanded Carbaporphyrinoids

Setsune and co-workers reported biphenyl units incorporated expanded porphyrins such as [24]rosarin(1.0.1.0.1.0) and dodecaphyrin.[14] These porphyrinoids core were utilized to coordinate with multiple metal units of Rh(CO)2. The single crystal analysis revealed that all the metal units were directly connected to dipyrrin unit of the macrocycles with substantial deviation of the metal atoms from the dipyrrin planes.

After prudent analyses of biphenyl unit incorporated contracted porphyrin derivatives, bis-biphenyl moiety embedded expanded porphyrinoids (27–29) were reported by our group (Figure ). By restructuring ortho with meta connectivity of the bis-biphenyl units, the hexaphyrin(3.1.1.3.1.1) 27 and its structural isomer octaphyrin(1.1.1.0.1.1.1.0) 28 were achieved.[15a] In the solid state, 27 adopts a bowl-like conformation in free-base form however, after protonation turns to an open conformation (27.2H), whereas 28 prevails figure-eight conformation. The bis-para–para biphenyl unit inserted octaphyrin 29 manifested three dissimilar conformational structures such as (a) a figure-eight conformation in free-base form (29); (b) open conformation in the protonated form (29.2H); and (c) singly twisted conformation upon RhI ion insertion (30) (Figure ).[15b]

Figure 5

Structures of biphenyl embedded expanded carbaporphyrinoids.

Figure 6

Molecular structures of 27, 27.2H, 28, 29, 29.2H, and 30.

Terphenyl Embedded Expanded Carbaporphyrinoids

In 2017, our group reported the simplest expanded porphyrin such as meso-aryl [20]π aza-homoporphyrin 31 with the smallest Möbius topology in three different forms.[16a] The respective carba-analogue of the homoporphyrin was obtained by replacing the dipyrroethene moiety with higher arene unit such as meta-ortho-meta (m-o-m) and para-ortho-para (p-o-p) terphenyl moieties into the formal homoporphyrin framework.[16b] The spectral and structural analyses showed that the thwarted (m-o-m) (32) and permitted (p-o-p) (33) conjugation in the macrocyclic core provide overall nonaromatic characteristics which remain even after protonation and RhI coordination (Figure ).

Figure 7

Structures of aza- (31) and carba- (32 and 33) homoporphyrins.

The oligomeric decaphyrin(1.1.1.0.0.1.1.1.0.0) 34 with open framework was synthesized by the insertion of di-m-m-m-terphenyl moiety into the porphyrinoid core.[16c] The twin pocket of the ligand was exploited for bis-RhI metal ion coordination (35). In the presence of linear di-m-m-m-terphenyl units 34 and 35, the system adopts an almost rectangular-shaped conformation (Figure ).

Figure 8

Molecular structures of 34 and 35.

Pyriporphyrinoids

Substitution of pyrrolic units by pyridine moiety begets pyriporphyrinoids, renown for their intriguing metal coordination. Latos-Grażyński and co-workers reported first pyridine based contracted porphyrin, subpyriporphyrin 36. The boron ion coordination led nonaromatic to [14]π aromatic characteristics (Figure ).[17a] The pyridine based corrole analogue, pyricorrole was synthesized by Neya and co-workers and reported its NiII complex (37) (Figure ).[17b] The remarkable structural characteristic of 37 prevails intermediary character of porphyrin and corrole. The first attempt to synthesize pyriporphyrin was done by Berlin and Breitmaier through [3 + 1] MacDonald type condensation between pyridine-2,6-dicarbaldehyde with tripyrran to obtain pyriporphyrinone instead of pyriporphyrin.[18a] Thereafter, a plethora of pyridyl derivatives are reported which encompassed, true pyriporphyrin, confused pyriporphyrin, expanded pyriporphyrin, and its coordination complexes.[18b,19−23]

Figure 9

Structures of 36 and 37.

Latos-Grażyński and co-workers reported meso-tetraarylpyriporphyrin 38 by condensation reaction between in situ made diol of pyridine incorporated tripyrrane analogue and pyrrole followed by DDQ oxidation (Figure ).[18b] The spectral analysis of 38 reveals nonaromatic character. The N4 core of 38 was utilized to obtain ZnII and FeIII complexes.

Figure 10

Structures of 38, 39, and 40.

Depending on the position of pyridine nitrogen atom which is para (39) or meta (40) to inner core carbon atom, two different types of confused pyriporphyrins were independently reported by the Latos-Grażyński and Lash groups (Figure ).[19,20] The core of confused isomers are further exploited for metal ion insertion and attained FeII, FeIII, and PdII complexes.[19b]

Expanded Pyriporphyrinoids

Pyridine unit incorporated expanded porphyrinoids are principally contributed by the research groups of Corriu, Sessler, and Lee independently, such as dipyriamethyrin, cyclo[m]pyridine-[n]pyrroles, and meso-alkylidenyl (2,6-pyri)porphyrinoids.[8,22] In 2018, Latos-Grażyński and co-workers reported α,β′-pyridine moieties embedded rubyrin 41 and exploited its aromaticity and protonation triggered conformational flipping.[23] Recently, Song and co-workers reported pyridine incorporated octaphyrin(1.1.0.0.1.1.0.0) 42 and demonstrated its isomeric bis-PdII complexes (Figure ).[11] The immediate higher analogues of the pyridine unit such as bipyridyl unit embedded octaphyrin and its helical binuclear CoII complex were reported by Setsune and co-workers.[24]

Figure 11

Structure of 41 and 42.

Bipyridine Embedded Contracted Porphyrinoids

The bipyridyl unit incorporated corrole analogue was reported by our group. The compound 6,11,16-triarylbipyricorrole (43) was obtained by BF3·OEt2 acid-catalyzed condensation of 2,2′-bipyridyldipyrromethane (44) with pentafluorobenzaldehyde followed by oxidation with DDQ (Scheme ).[25a] The introduction of the bipyridyl unit offers the stable tetra nitrogen (NNNN) core and this modification effectively changes the trivial corrole N4 coordination sphere from trianionic [(NH)3N] to a monoanionic [N3NH] core. The coordination chemistry was performed by treating the monoanionic core of 43 with Zn(OAc)2 in CH2Cl2/CH3OH at reflux condition, the ZnII complex (45) was obtained and showed chelation induced emission enhancement (CIEE). In addition, 43 can selectively sense ZnII ion over 100 equiv of other metal ions and the association constant was found to be 5.47 × 104 m–1. The macrocyclic π-conjugation is interrupted by bipyridyl unit and, thus, confirms the nonaromatic characteristics for both 43 and 45.

Scheme 4

Synthesis of Bipyricorrole 43 and its Zinc(II) Complex 45

The core of 43 was also utilized to stabilize IrIII (46) and RhIII (47) metal ions (Scheme ) and explored its photophysical and electrochemical properties.[25b] Hence, it is evinced that the monoanionic [N3NH] core of bipyricorrole (43) can stabilize various metal ions in different oxidation states. Nevertheless, it was found that the fluorescence quantum yield of 47 is 3-fold higher than 43, while 46 is nonemissive in nature.

Scheme 5

Syntheses of 46 and 47

The metal-templated condensation strategy for the synthesis of meso-free bipyricorrole complexes was also reported by our group.[25c] The reaction was performed by treating a CH2Cl2 solution of formylated bipyridyl dipyrromethane (48) with a CH3OH solution of Ni(OAc)2 and Pd(OAc)2 separately at room temperature to obtain meso free corrole complexes of NiII (49) and PdII (50) (Scheme ).

Scheme 6

Syntheses of 49 and 50

The meso-free CH in the monomeric complex of 49 and 50 was further reacted with FeCl3 to synthesis the meso–meso linked respective dimeric corrole homologue complexes (Scheme ). The spectral and structural study evaluates that the meso-free monomer and the respective dimeric NiII (51) and PdII (52) complexes are nonaromatic in nature.

Scheme 7

Syntheses of 51 and 52

Amalgamation of Pyridine and Benzene Embedded Porphyrinoids

A porphyrinoid cores with individual arene or pyridine units are well renown in the literature; however, concurrently both the units incorporated in the macrocyclic core were elusive. Recently, our group reported isosmaragdyrin 53 with an N3C2 core by embedding a 2,6-di-m-phenylpyridine unit in the porphyrin framework. Architecturally, the macrocycle is blended with arene and pyridine units and exhibits unprecedented properties as a rendezvous of both of the categories.[26] The core of the macrocycle is efficiently utilized to stabilize both RhI (54) and PtII (55) ions with distinctive bonding modes (Figure ). The RhI ion stabilizes through NN chelation and PtII ion through (N∧C)N coordination to form an organometallic complex. Especially, the insertion of PtII ion underpin a typical unsymmetrical pincer type complex in the macrocyclic core, and such results are hitherto unknown in porphyrin chemistry. In addition, the PtII complex (55) breeds weak metal–arene interaction with the ligand.

Figure 12

Structures of 53, 54, and 55.

Conclusion

In light of the selected literature reports, the budding curiosity about carba- and pyriporphyrinoids, along with their hybrid analogues, could lead to creating new pioneering molecular architectures with different π-conjugation pathways. We vividly believe that more specimens of such porphyrin analogues can be synthesized with due time and improvise fundamental wisdom about aromaticity, coordination, sensing, organometallic chemistry, and catalysis.