tert-Butylimido complexes of osmium-(VIII), -(VII) and -(VI). X-Ray crystal structures of [(ButN)2Os(µ-NBut)2Os(NBut)(µ-O)]2, [Os(NBut)2(µ-NBut)]2 and [Os(NBut)2(µ-NBut)]2[BF4]2

Abstract

The interaction of OsO₄ with neat NHBu^t(SiMe₃) gives the first homoleptic osmium(VIII) imido compound, Os(NBu^t)₄1, together with the tetranuclear osmium(VI) oxo-imido complex [(Bu^tN)₂Os(µ-NBu^t)₂Os(NBu^t)(µ-O)]₂2. Complex 1 can be reduced to the osmium(VI) dimer, [Os(NBu^t)₂(µ-NBu^t)]₂3, by triphenylphosphine or by sodium amalgam in tetrahydrofuran; with trimethyloxonium tetrafluoroborate an unusual reduction reaction leads to the osmium(VII) dimeric cationic complex [Os(NBu^t)₂(µ-NBu^t)]₂[BF₄]₂4. The syntheses of Os(NR)₃, R = 2,6-Me₂C₆H₃5, or 2,6-Prⁱ₂C₆H₃6, by interaction of OsO₄ with NHR(SiMe₃) are reported. The X-ray crystal structures of 2, 3 and 4 have been determined. Compound 2 has a chain tetrameric structure with two (Bu^tN)₂Os(µ-NBu^t)₂Os(NBu^t)(µ-O) units linked by a dioxo bridge. The outer Os atoms are four-co-ordinate, distorted tetrahedral, the inner Os atoms five-co-ordinate, square pyramidal. The bridging Os–N distances to the inner, five-co-ordinate osmiums are shorter than those to the outer osmiums; the terminal Os–N distances are essentially the same for both types of osmium. Compound 3 and the cation in 4 are dimers with imido bridges. The main difference between them is a significant shortening in the Os…Os distance in the osmium(VIII) compound as a result of Os–Os single bond formation. This also results in a slight shortening of the Os–N bridge distances in 4 as compared to the neutral molecule 3.