Borohydride reduction of a rhenium-bound acetonitrile: an example of a chelating iminoborane ligand at a low valent metal center

The reduction of acetonitrile by [Bu4N]BH4 at the rhenium(I) metal center in ReCl2(PMe3)2(CH3CN)NO to afford ReCl{(μ-H)B(H)2NC(H)Me}(PMe3)2NO (2) is reported. The coordinated iminoborane moiety is an intermediate in acetonitrile reduction. A mechanism is proposed for its formation. Further reaction with a hydride source (LiBH4, NaBH4, LiHBEt3) results in replacement of chloride with hydride to give ReH{(μ-H)B(H)2NC(H)Me}(PMe3)2NO (3), together with varying proportions of ReH2(PMe3)3NO (4). Compounds 2 and 3 have been characterized by NMR and infrared data. The structure of 3 has additionally been elucidated by a single crystal X-ray diffraction study. Crystal data: C8H26BN2OP2Re, triclinic, P, a=8.821(7) Å, b=12.4062(11) Å, c=15.5822(12) Å, α=70.004(9)°, β=82.726(9)°, Z=4, R=0.0535, Rw=0.1325. The Hb/Ht hydrogen exchange process in the iminoborane moiety of 2 and 3 has been studied by NMR spectroscopy and a negative value of ΔS# found for this process. The proposed mechanism for this exchange resembles that of β-hydrogen elimination in metal alkyls.