Effect of anions on the isocyanide insertion reaction in cationic alkyl complexes of iron(II): kinetic, thermodynamic and solution interionic structural studies

The equilibrium and kinetics of isocyanide insertion of complexes [Fe(PMe3)2(CO)2(CH3)(CNR)]+X− (1) [CNR=tert-butylisocyanide; X−=I− (1a), BPh4− (1b); BF4− (1c)] which afford complexes [Fe(PMe3)2(CO)2(η2-C(CH3)NC(CH3)3)]+X− (2) were investigated in dichloromethane, methanol, acetone and nitromethane. The results indicate that the reaction proceeds via an associative mechanism with the preliminary formation of contact ion pairs. The structure of the contact ion pair in solution was studied by 1H-NOESY and 19F{1H}-HOESY NMR spectroscopy. The results indicate that structure is independent of the nature of the solvent and of the counterion. In complexes 1b,c the counterion is located between the CO and the isocyanide ligands; in complexes 2b,c the counterion is located near the dihaptoiminoacyl ligand. The reaction rate increases with the charge density and the coordinating power of the anions. The effect of the solvent can be also explained on the basis of its coordinating power.