Experimental and computational study of the transformation of terminal alkynes to vinylidene ligands on trans-(chloro)bis(phosphine)Rh fragments and effects of phosphine substituents

Experimental and computational evidence points to unimolecular transformation of terminal alkynes on the title Rh(I) metal fragments. Lack of isotopic scrambling in double-crossover experiments is inconsistent with a previously proposed bimolecular pathway. Focusing on a unimolecular manifold, alkyne binding to the metal forms the Rh(I) alkyne pi-complex 2, which isomerizes to the Rh(III) hydrido(alkynyl) species 4, ultimately leading to Rh(I) vinylidene product 5. In making alkyne-free precursors, use of heterocyclic ligand (i-Pr)(2)PIm' (1b, Im' = 1-methyl-4-tert-butylimidazol-2-yl) led to species 8 with a labile P,N chelate, whereas a geometrically similar o-tolyl ligand suffered metalation at the methyl group and was unsuitable for alkyne transformation studies. Kinetic studies comparing 1b and (i-Pr)(2)PPh (1c) allowed determination of rate constants for the alkyne binding event and conversion of 2 to 5 (the latter, k(2-5), being 9.6 times faster for 1b). Based on a scan of the two-dimensional reaction surface, combined density functional/molecular mechanics calculations predict that eta(2)-(C,H) alkyne complex 3 is in a fast equilibrium with the lower energy hydrido(alkynyl) complex 4, and neither species is expected to be present at observable concentrations. Eyring model estimates of the rate constants from these computational data predict the available experimental values in this work to within a factor of 2 and the ratio of the rate constants k(2-5) for 1b and 1c to within 10%. The calculations also agree with the qualitative observation that reaction rates are faster for both ligands 1b and 1c than for (i-Pr)(3)P and predict that reactions using triphenylphosphine will be faster than those with (i-Pr)(3)P. NMR coupling constants, particularly (1)J(CC) values, were used to evaluate bonding and back-bonding in isotopomers of 2a-c and 5a-c derived from (HCCH)-C-13-C-13.