Notes. Electronic effects of co-ordinated methoxide on the rates of entry of imidazole derivatives into [dimethyl-3,7,12,17-tetramethyl-8,13-divinylporphyrin-2,18-dipropanoato(2-)](methanol)methoxocobalt(III) in methanol

Abstract

The rates of replacement of MeOH by imidazole derivatives in [Co^IIIL(MeO)(MeOH)](L = dimethyl 3,7,12,17-tetramethyl-8,13-divinylporphyrin-2,18-dipropanoate) were measured. The most basic derivative enters most slowly, as was previously observed for pyridine derivatives. However the rates of entry of imidazoles are about 50 times smaller than those of pyridines. The energies and entropies of activation of the overall reactions with pyridine (py) and imidazole (Him) are E_a/kJ mol^–1= 40.0 ± 3.1 (py) and 76.4 ± 3.8 (Him), and ΔS^‡/J K^–1 mol^–1=–105 ± 10(py), and –13.2 ± 12.8(Him). It is inferred that the transition state in the reaction with py is stabilised by σ and partial metal-to-ligand π bonding, while that in the reaction with imidazole involves σ bonding only.