Smaragdyrins and Sapphyrins Analogues

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 Huckel and Mobius). 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.

Synthesis of Expanded Hetero 2,6‐Pyrihexaphyrins

“…Expanded porphyrins [1][2][3][4][5][6][7][8][9][10] containing more than four pyrroles have received tremendous attention over the years because of their excellent optical, electrochemical and coordination properties arising from their larger π-conjugation frameworks than those of tetrapyrrole porphyrins. [11][12][13][14] Because of the large cavity, the expanded porphyrins are highly promising as multimetal coordinating ligands, allowing the formation of oligo nuclear complexes with notable metal-metal interaction or catalytic activity.…”

“…[11][12][13][14] Because of the large cavity, the expanded porphyrins are highly promising as multimetal coordinating ligands, allowing the formation of oligo nuclear complexes with notable metal-metal interaction or catalytic activity. [7,[11][12][13] Furthermore, expanded porphyrins are emerging functional molecules with potential utilities in a range of applications, such as anion receptors, [3,7,15] transition or lanthanide ion chelates, [16] photodynamic therapy sensitizers, [4,16,17] magnetic resonance imaging (MRI) contrast agents, [16] optical data storage and optical limiting agents. [18] Since the expanded porphyrins are fascinating macrocycles with interesting properties and wide range applications, [4,7,[11][12][13]16,17] many synthetic efforts towards a variety of expanded porphyrins differing in size, ring connectivity, peripheral substitution and core-modification have been carried out in recent years.…”

Rhodium Complexes of a New‐Generation Sapphyrin: Unique Structures, Axial Chirality, and Catalysis

“…Insertion of rhodium into the new 5,10,15,20‐tetra(trifluoromethyl)sapphyrin ( H 3 tfs ), appeared to be much more facile (1 h, RT, 90 % yield) than for 5,10,15,20‐tetra(pentafluorophenyl)sapphyrin ( H 3 tpfs ), which is reminiscent of the mild reaction conditions for metallation of heterosapphyrins . The decrease in symmetry from C 2 v to C 1 on metalation resulted in four instead of two CF 3 signals in the 19 F NMR spectrum (Figure S1 in the Supporting Information) and doubling of the number of 1 H NMR resonances of both the normal and inverted β‐pyrrole H atoms (Figure a) . The molecular structure of the product was deduced to be Rh(H 2 tfs)(CO) 2 , composed of a d 8 low‐spin rhodium(I) centre coordinated by two N atoms of the monoanionic macrocycle and two carbonyl ligands.…”

“…Sapphyrin chemistry is by large focused on the free‐base compound with impressive achievements in their utilisation as anion and cation receptors, photodynamic therapy sensitisers, and nonlinear optical materials . Meanwhile, their coordination chemistry has remained limited; and only rhodium complexes have been reported for meso ‐aryl‐substituted sapphyrins . The two reasons for the restricted progress are: a) the N 5 core of sapphyrins is a much less suitable coordination sphere than the N 4 cores of porphyrins and corroles, and b) rotation of pyrrole ring A (Scheme ) dramatically alters the chelating properties of the sapphyrin core .…”

One‐Pot Synthesis of Contracted and Expanded Porphyrins with meso‐CF3 Groups

“…[2b] Large efforts on developing modified and new synthetic methods led to the current stage:c orroles with almost any meso-aryl substituents are nowadays accessible in quite straightforward fashions. [3,5] Hemes present in numerous enzymes do not have any meso-C-substituents and the cobalt complexes of porphine, and 5,10,15,20-tetramethylporphyrin are among the most efficient electrocatalysts. [3,5] Hemes present in numerous enzymes do not have any meso-C-substituents and the cobalt complexes of porphine, and 5,10,15,20-tetramethylporphyrin are among the most efficient electrocatalysts.…”