Substituent effects of (transition metallo)methyl groups by fluorine-19 nuclear magnetic resonance

Abstract

A series of compounds of the general formula 3- and 4-FC₆H₄CH₂ML_mL′_n where M is a transition metal have been synthesised and characterised. Ground state substituent effects of the metallomethyl groups CH₂ML_mL′_n have been investigated by examination of the ¹⁹F substituent chemical shifts (SCS). The results indicate that whilst these metallomethyl groups produce net electron donation to the aromatic ring, the magnitude of the effect is strongly dependent on the metal and its attached ligands. The substituent effects in these ground states are markedly smaller than those previously detected in transition states and in studies of equilibria, but the order of electron-donating ability within the series of metallomethyl groups is the same for the ground states and for the transition states and equilibria. The results show that several neutral (carbonyl)metallomethyl substituents are as electron donating as the most effective main group metallomethyl substituents, but the electron-donating character of several neutral cobalt(III)-containing substituents is little different from that of the methyl group. The analogous rhodium(III) substituents are markedly more electron donating.