187Os NMR Study of (η6-Arene)osmium(II) Complexes: Separation of Electronic and Steric Ligand Effects

Osmium-187 NMR data have been collected for 37 Os(arene)X 2 L type complexes, using inverse two-dimensional ( 31 P, 187 Os){ 1 H} and ( 1 H, 187 Os) NMR spectroscopy. In the series Os(p-cymene)X 2 PMe 3, shielding increases in the order Cl < Br < I < Me < H. Systematic variation of the phosphite ligand of Os(p-cymene)Cl 2 P(OR) 3 (R = Me, Et, n Bu, Ph, i Pr) reveals a linear dependence on Tolman's electronic parameter χ with metal shielding increasing as the acceptor character of the P(OR) 3 ligand increases. Conversely, the structurally related Os(p-cymene)X 2 PR 3 types (X = Cl, I; PR 3 = PMe 3, PMe 2 Ph, P n Bu 3, PPh 2 Me, P(CH 2 Ph) 3, PPh 3, P(m-tolyl) 3, P i Pr 3, PCy 3 ) exhibit a linear dependence on Tolman's cone angle θ, with shielding decreasing as larger phosphine ligands are introduced. Variation of the arene ligand of Os(C 6 H 5 X)Cl 2 PMe 3 (X = H, Me, Et, i Pr, t Bu) shows a linear dependence of chemical shift on Taft's steric parameter E s for group X. In only one case, for the monohydride complexes Os(p-cymene)HClPR 3 (PR 3 = PMe 3, PMe 2 Ph, P n Bu 3, PPh 3, P i Pr 3, PCy 3 ), has it been possible to separate both steric and electronic effects of the phosphorus ligand on the observed 187 Os chemical shifts. There are some correlations between 1 J ( 187 Os, 31 P) and the acceptor character of the phosphorus ligand, and a relationship between Os−P bond lengths and coupling constants was found. Osmium relaxation rates T 1 - 1 and their B 0 field dependence are reported for selected complexes. The crystal structures of four Os(p-cymene)Cl 2 L complexes (L = PMe 3 ( 1 ), P(OMe) 3 ( 6 ), PBz 3 ( 13 ), and PPh 3 ( 15 )) have been determined.