Osmium Phosphiniminato Complexes: Synthesis, Protonation, Structure, and Redox-Coupled Hydrolytic Scission of N-P Bonds

The osmium(VI) nitrido complex TpOs(N)Cl2 [1, Tp = hydrotris(1-pyrazolyl)borate] reacts with triarylphosphines to afford the OsIV phosphiniminato complexes TpOs(NPAr3)Cl2 [Ar = p-tolyl (tol) (2a), phenyl (2b), p-CF3C6H4 (2c)] in nearly quantitative yield. Protonation of 2a-c with 1 equiv of HOTf in MeCN occurs at the phosphiniminato nitrogen to give [TpOsIV(NHPAr3)Cl2]OTf (3a-c) in 68-80% yield. Solutions of 2a-c in CH2Cl2 react with excess H2O over 1 week to form the disproportionation products 1 (28%), TpOsIII(NHPAr3)Cl2 (4a-c) (60%), and OPAr3 (35%). Treatment of solutions of 3a-c with H2O also affords 1, 4a-c, and OPAr3. X-ray structures of 2b, 3b, and 4b are presented. Cyclic voltammograms of compounds 2a-c exhibit OsV/OsIV and OsIV/OsIII couples at ~0.3 and -1 V versus Cp2Fe+/0. Protonation to give 3 makes reduction easier by ~1.2 V, so that these compounds show OsIV/OsIII and OsIII/OsII couples. In the hydrolytic disproportionation of 2a-c, labeling studies using 18O-enriched O2 and H2O establish water as the source of the oxygen atom in the OPAr3 product. The conversions are accelerated by HOTf and inhibited by NaOD. The relative rates of hydrolytic disproportionation of 2a-c vary in the order tol > Ph > p-CF3C6H4. The data indicate that protonation of the phosphiniminato nitrogen is required for hydrolysis. The mechanism of the hydrolytic disproportionation is compared to that of the related reaction of the osmium(IV) acetonitrile complex [TpOs(NCMe)Cl2]+.