Solid state isostructural behavior and quantified limiting substitution kinetics in Schiff-base bidentate ligand complexes fac-[Re(O,N-Bid)(CO)3(MeOH)](n)

A range of N,O-donor atom salicylidene complexes of the type fac-[M(O,N-Bid)(CO)3(L)](n) (O,N-Bid = anionic N,O-bidentate ligands; L = neutral coordinated ligand) have been studied. The unique feature of the complexes which crystallize in a monoclinic isostructural space group for complexes containing methanol in the sixth position (L = MeOH) is highlighted. The reactivity and stability of the complexes were evaluated by rapid stopped-flow techniques, and the methanol substitution by a range of pyridine type ligands indicates significant activation by the N,O-salicylidene type of bidentate ligands as observed from the variation in the second-order rate constants. In particular, following the introduction of the sterically demanding and electron rich cyclohexyl salicylidene moiety on the bidentate ligand, novel limiting kinetic behavior is displayed by all entering ligands, thus enabling a systematic probe and manipulation of the limiting kinetic constants. Clear evidence of an interchange type of intimate mechanism for the methanol substitution is produced. The equilibrium and rate constants (25 °C) for the two steps in the dissociative interchange mechanism for methanol substitution in fac-[Re(Sal-Cy)(CO)3(MeOH)] (5) by the pyridine type ligands 3-chloropyridine, pyridine, 4-picoline, and DMAP are k3 (s(-1)), 40 ± 4, 13 ± 2, 10.4 ± 0.7, and 2.11 ± 0.09, and K2 (M(-1)), 0.13 ± 0.01, 0.21 ± 0.03, 0.26 ± 0.02, and 1.8 ± 0.1, respectively.