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.