The effects of strong Lewis-base reagents on supramolecular hydrogen bonding of meso-tetra(carboxyphenyl)porphyrins

Abstract

Reactions of Pd-, Ru(CO)- and Cu-complexes of meso-tetra(carboxyphenyl)porphyrin with strong Lewis base reagents (as pyridine, 4-acetylpyridine or dimetylsulfoxide) avoided the formation of commonly observed 2-D or 3-D multiporphyrin arrays by direct hydrogen bonding. Structural analysis of the crystalline products indicates that this is due to preferential affinity of these reagents to associate to the carboxylic acid functions via H-bonds, in competition with the potential self-association of the porphyrin units. In the copper–porphyrin derivative all four carboxylic acid functions of a given porphyrin interact with the 4-acetylpyridine species, creating discrete 1 : 4 porphyrin : ligand pentameric assemblies. Then, in adducts of the ruthenium carbon monoxide porphyrins with pyridine or 4-acetylpyridine, two trans-related carboxylic acid groups of the porphyrin scaffold interact directly with neighboring porphyrin species via the common (COOH)₂ cyclic dimeric synthon to yield 1-D hydrogen bonded chains, while the other carboxylic functions donate their protons to molecules of the polar pyridine/acetylpyridine ligand that prevent further interporphyrin lateral binding along the perpendicular direction. The pyridyl-type moieties bear a single electron lone pair on the N-atom that can act as an effective hydrogen bond acceptor, allowing for a localized hydrogen bond with the carboxylic acid. On the other hand, the dimethylsulfoxide can involve readily through its O-site (bearing two lone pairs of electrons) in two hydrogen bonds in tetrahedral directions, and can thus serve as a bridging auxiliary between the carboxylic acid groups of neighboring porphyrins. Correspondingly, its reaction with the palladium complex leads to the formation of heterogeneous porphyrin–dimethylsulfoxide linear hydrogen bonded chains. Along these chains two ligand molecules are inserted between adjacent porphyrin units on both sides, and bridge between their cis-related carboxyphenyl arms. The different motifs of hydrogen-bonding effect different crystal packing features in three dimensions, some characterized also by the presence of solvent-accessible open channels that propagate through the lattice.