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

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journal contribution
posted on 09.07.1996, 00:00 by Andrew G. Bell, Wiktor Koźmiński, Anthony Linden, Wolfgang von Philipsborn
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 Es 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.

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