Kinetic evidence of an arm-off mechanism in complexes of hemilabile hybrid ligands.: Oxidative addition of methyl iodide to the rhodium(I) complex [Rh(2,6-bis(benzylthiomethyl)pyridine)(CO)]PF6 via competitive pathways

Kinetic experiments of the oxidative addition of methyl iodide to the cationic rhodium(l) carbonyl complex [Rh(L)(CO)]PF6 (1; L = 2,6-bis(benzylthiomethyl)pyridine) forming the observable rhodium(III) methyl species [RhI(L)(CO)Me)1PF6 have been performed by FT-IR. The reaction yields the isolable rhodium acyl complex [RhI(L)(COMe)1PF6. Plots Of k(obs) versus concentration of methyl iodide indicate that the reaction proceeds by two parallel pathways, one which is first order in complex 1 and zero order in Mei (k(1)) and one which is first order in complex 1 and first order in MeI (k(2)). The first-order pathway displays scarce dependence on the nature of the solvent (k(1) (31 degreesC): 6.7 x 10(-6) s(-1), acetone; 1.1 x 10(-1) s(-1), dichloromethane; 1.7 x 10(-5) s(-1), acetonitrile), while a larger solvent effect is observed for the second-order pathway (k(2) (31 degreesC): 6.4 x 10(-6) M-1 s(-1), dichloromethane; 5.9 x 10(-5) M-1 s(-1), acetone; 1.4 x 10(-4) M-1 s(-1), acetonitrile), in agreement with the dissociative and associative character of the two routes, respectively. The k(obs) values for the reaction in methanol exhibit a saturation behavior when plotted against the concentration of methyl iodide. H-1 NMR spectra of complex I at different temperatures in the range 208-302 K display a fluxional behavior typical of tridentate complexes of S,N,S ligands, due to pyramidal inversion at the sulfur centers. The methyl iodide independent pathway and the saturation behavior in methanol are due to reversible decoordination of one thiobenzyl arm from rhodium, yielding a transient three-coordinate bidentate N,S species which reacts rapidly with MeI. The data are discussed in terms of the concept of kinetic detection of hemilability.