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)X₂L type complexes, using inverse two-dimensional (³¹P, ¹⁸⁷Os){¹H} and (¹H, ¹⁸⁷Os) NMR spectroscopy. In the series Os(p-cymene)X₂PMe₃, shielding increases in the order Cl < Br < I < Me < H. Systematic variation of the phosphite ligand of Os(p-cymene)Cl₂P(OR)₃ (R = Me, Et, ⁿBu, Ph, ⁱPr) 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)X₂PR₃ types (X = Cl, I; PR₃ = PMe₃, PMe₂Ph, PⁿBu₃, PPh₂Me, P(CH₂Ph)₃, PPh₃, P(m-tolyl)₃, PⁱPr₃, PCy₃) 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(C₆H₅X)Cl₂PMe₃ (X = H, Me, Et, ⁱPr, ^tBu) 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)HClPR₃ (PR₃ = PMe₃, PMe₂Ph, PⁿBu₃, PPh₃, PⁱPr₃, PCy₃), has it been possible to separate both steric and electronic effects of the phosphorus ligand on the observed ¹⁸⁷Os chemical shifts. There are some correlations between ¹J(¹⁸⁷Os,³¹P) and the acceptor character of the phosphorus ligand, and a relationship between Os−P bond lengths and coupling constants was found. Osmium relaxation rates T₁^-1 and their B₀ field dependence are reported for selected complexes. The crystal structures of four Os(p-cymene)Cl₂L complexes (L = PMe₃ (1), P(OMe)₃ (6), PBz₃ (13), and PPh₃ (15)) have been determined.