Factors That Regulate the Conformation of m‐Benziporphodimethene Complexes: Agostic Metal–Arene Interaction, Hydrogen Bonding, and η²,π Coordination

Abstract

Group 12 and silver(I) tetramethyl-m-benziporphodimethene (TMBPDM) complexes with phenyl, methylbenzoate, or nitrophenyl groups as meso substituents were synthesized and fully characterized. The dimeric silver(I) complex displays an unusual η(2),π coordination from the β-pyrrolic C=C bond to the silver ion. All of the complexes displayed a close contact between the metal ion and the inner C(22)-H(22) on the m-phenylene ring. The downfield chemical shifts of H(22) and large coupling constants between Cd(II) and H(22) strongly support the presence of an agostic interaction between the metal ion and inner C(22)-H(22). Crystal structures revealed that the syn form is the predominant conformation for TMBPDM complexes. This is distinctively different from the exclusive anti conformation observed in m-benziporphyrin and tetraphenyl-m-benziporphodimethene (TPBPDM) complexes. Evidently, intramolecular hydrogen-bonding interactions between axial chloride and methyl groups stabilize syn conformations. Unlike the merely syn conformation observed in the solid-state structures of TMBPDM complexes that contain an axial chloride, in solution these complexes display highly solvent- and temperature-dependent syn/anti ratio changes. The observation of dynamic (1)H NMR spectroscopic scrambling between syn and anti conformations from the titration of chloride ion into the solution of the TMBPDM complex suggests that axial ligand exchange is a likely pathway for the conversion between syn and anti forms. Theoretical calculations revealed that intermolecular hydrogen-bonding interactions between the axial chloride and CHCl(3) stabilizes the anti conformation, which explains the increased ratio for the anti form when dichloromethane or chloroform was used as the solvent.

Synthesis and complexes of a constrained-cavity Schiff-base dipyrrin macrocycle

“…There is a noticeable difference in distance between the metal and the plane of the N 4 donor set; for zinc this is 0.593 Å above the plane while for iron it is 0.304 Å. Similarities can be found in previously reported structures of iron chloride octaethylpophyrin 37 and zinc chloride m-benziporphodimethene 28,38 in which the Fe and Zn are ca. 0.47 Å and 0.51 Å, respectively, out of plane.…”

“…26,27 Smaller cavity macrocycles such as the benziporphyrins have also been prepared by Latos-Grażyński and co-workers for which metalassisted C-H bond activation chemistry has been revealed. [28][29][30][31] The chemical and photochemical properties of the dipyrrin framework combined with the unique control of coordination and organometallic chemistry afforded by a macrocyclic environment led us to investigate the formation of macrocyclic analogues of 1. We report here the straightforward formation and isolation of a unique constrained-cavity [1 + 1] Schiff-base dipyrrin macrocycle HL that comprises an N 4 dipyrrin-imine donor set and describe the formation and properties of its lithium, iron, and zinc complexes.…”

Post‐Synthetic Modification of Hangman Porphyrins Synthesized on the Gram Scale

“…Whereas this interaction has been documented with aromatic compounds,6 its observation in the solid state with pyrrolic systems remains rather rare 5. 7 In solution, it has been investigated with tetrapyrrolic derivatives such as 3,3′‐bisdipyrrin helicates and Ce(IV) double‐decker porphyrins 8. 9 In the latter type of compounds, two pyrrolic rings of facing macrocycles form π‐clefts interacting with Ag I ions, as determined by NMR, Raman, and UV/Vis spectroscopies 9.…”

Metal Complexes of Carbaporphyrinoid Systems

“…[87] Dimeric silver(I) complexes of benziporphodimethenes have also been reported. [88] In addition, a related benziporphodimethene has shown promise as a selective zinc cation fluorescence switch-on sensor. [89] A zinc benziporphodimethene was also used as a building block to construct multidimensional nanostructure arrays.…”

“…Benziporphodimethenes such as 116 also reacted with NiCl 2 , ZnCl 2 , and CdCl 2 to give the corresponding metalated derivatives 117 (Scheme ) 87. Dimeric silver(I) complexes of benziporphodimethenes have also been reported 88. In addition, a related benziporphodimethene has shown promise as a selective zinc cation fluorescence switch‐on sensor 89.…”