An NMR Study of Trans Ligand Influence in Rhodium Eta2 Triphenyltin Hydride Complexes

The complex [Rh(O(2)Cisoq)(H)(SnPh(3))(PPh(3))(2)] (O(2)Cisoq = isoquinoline-1-carboxylate), characterized by x-ray crystallography, was used as a precursor to three-center bonded complexes [Rh(O(2)Cisoq)(eta(2)-HSnPh(3))(PPh(3))(4-Rpy)] (R = carbomethoxy, acyl, bromo, aldehyde, hydrogen, methoxy, dimethylamino), which were prepared in situ from the bis(phosphine) complex in dichloromethane by addition of the pyridine. Of two isomeric forms of [Rh(O(2)Cisoq)(eta(2)-HSnPh(3))(PPh(3))(4-Rpy)], one, with 4-Rpy positioned trans to tin, is formed at 0 degrees C; the other, with isoquinoline positioned trans to tin, is formed by partial (25-50%) conversion of the first isomer (in solution) upon warming to 30-35 degrees C for a few minutes. The relative coordination geometries of the two isomers were established by using a (15)N-enriched pyridine. NMR parameters ((1)H, (31)P, (103)Rh, and (119)Sn) for the pyridine-containing complexes show trends consistent with a significantly greater SnH interaction in complexes having isoquinoline lying trans to tin and a threshold of sigma donor strength for the pyridine below which the influence on Rh(SnH) bonding is minimal and above which the influence is to enhance the SnH interaction. Possible reasons for these effects are discussed.