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

Abstract

The reduction of acetonitrile by [Bu 4N]BH 4 at the rhenium(I) metal center in ReCl 2(PMe 3) 2(CH 3CN)NO to afford ReCl{(μ-H)B(H) 2NC(H)Me}(PMe 3) 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 (LiBH 4, NaBH 4, LiHBEt 3) results in replacement of chloride with hydride to give ReH{(μ-H)B(H) 2NC(H)Me}(PMe 3) 2NO ( 3), together with varying proportions of ReH 2(PMe 3) 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: C 8H 26BN 2OP 2Re, triclinic, P 1 ̄ , 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, R w=0.1325. The H b/H t hydrogen exchange process in the iminoborane moiety of 2 and 3 has been studied by NMR spectroscopy and a negative value of Δ S # found for this process. The proposed mechanism for this exchange resembles that of β-hydrogen elimination in metal alkyls.