Reactions of nitriles with binuclear rhodium hydrides. The stepwise reduction of a carbon–nitrogen triple bond at two metal centres

Abstract

The reaction of simple nitriles, R′CN (R′ = CH 3 , Ph, o-tol) with the electron-rich binuclear rhodium hydride derivatives [(R 2 PCH 2 CH 2 PR 2 )Rh] 2 (μ-H) 2 (R = Pr i : [(dippe) Rh] 2 (μ-H) 2 ; R = OPr i : [(dipope)Rh] 2 (μ-H) 2 ) results in the formation of alkylideneimido derivatives [(R 2 PCH 2 CH 2 PR 2 )Rh] 2 (μ-H)(μ-N=CHR′), apparently by insertion of the nitrile moiety into a bridging hydride bond; this was confirmed by the reaction of nitriles with the dideuteride [(dippe)Rh] 2 (μ-D) 2 , which resulted in the formation of [(dippe)Rh] 2 (μ-D)(μ-N=CHR′). Further reduction can take place by addition of H 2 to generate the corresponding amide hydride derivatives [(dippe)Rh] 2 (μ-H)(μ-NHCH 2 R′); this represents an overall stoichiometric reduction of a nitrile to a coordinated amide at a binuclear centre. These same amido-hydride complexes can be accessed by addition of amine to the starting binuclear rhodium hydride derivatives. The X-ray structure of [(Pr i 2 PCH 2 CH 2 PPr i 2 )Rh] 2 (μ-H)(μ-N=CHCH 3 ) was undertaken to confirm the structure of these particular intermediates. Crystals of this material are monoclinic, a = 19.036(2), b = 15.139(1), c = 13.604(1) Å, β = 104.119(7)°, Z = 4, space group P2 1 /c. The structure was solved by heavy atom methods and was refined by full-matrix least-squares procedures to R = 0.038 and R w = 0.041 for 5814 reflections with I ≥ 3σ(I).