Tautomerisation and hydrogen-bonding interactions in four-coordinate metal halide and azide complexes of N-donor-extended dipyrromethanes

Abstract

The synthesis and structures of Fe, Co, and Zn halide complexes [MX₂(H₂L)] (M = Fe, X = Br; M = Co, Zn, X = Cl) of the N-donor extended dipyrromethane ligand H₂L are described, from which it is clear that bond rearrangements from imine-pyrrole to amine-azafulvene tautomers occur on metal co-ordination, both in the solid state and in solution. In the structure of [FeBr₂(H₂L)], this H-migration results in a pendant amine that is involved in both inter- and intramolecular hydrogen bonds to the bromide ligands, so forming a dimer. As the tautomerisation renders the N–H protons less acidic, metal-based ligand substitution reactions can occur in favour of deprotonation. As such, the reaction between [MCl₂(H₂L)] (M = Co, Zn) and NaN₃ results in the formation of the bis(azide) complexes [M(N₃)₂(H₂L)] which for Co displays both inter- and intramolecular N–H⋯N₃–Co hydrogen bonds in the solid state. In contrast, reactions of the dihalides with the lithium bases LiNMe₂ or LiMe (M = Fe), or reduction reactions with C₈K (M = Fe, Co) result in the formation of the known dinuclear helicates [M₂(L)₂].