Facile synthetic access to rhenium(II) complexes: Activation of carbonbromine bonds by single-electron transfer

Article abstract of DOI:10.1002/chem.200902600

The five-coordinated Re¹ hydride complexes [Re(Br)(H)(NO) (PR³)₂] (R = Cy la, iPr lb) were reacted with benzylbromide, thereby affording the 17-electron mononuclear ReII hydride complexes [Re(Br)2(H)(NO)(PR₃)₂] (R = Cy 3a, iPr 3b), which were characterized by EPR, cyclic voltammetry, and magnetic susceptibility measurements. In the case of dibromomethane or bromoform, the reaction of 1 afforded Re^II hydrides 3 in addition to Re¹ carbene hydrides [Re(= CHR¹)(Br)(H)(NO)(PR₃)₂,] (R ¹ = H 4, Br 5; R = Cy a, iPr b) in which the hydride ligand is positioned cis to the carbene ligand. For comparison, the dihydrogen Re ¹ dibromide complexes [Re(Br)₂(NO)(PR₃MIf- H₂)] (R = Cy 2 a, iPr 2 b) were reacted with allyl- or benzylbromide, thereby affording the monophosphine Re^II complex salts [R ₃PCH₂R'][Re(Br)₄(NO)(PR₃)] (R' = -CH=CH₂ 6, Ph 7). The reduction of Re^II complexes has also been examined. Complex 3 a or 3 b can be reduced by zinc to afford la or lb in high yield. Under catalytic conditions, this reaction enables homocoupling of benzylbromide (turnover frequency (TOF): 3a 150, 3b 134 h^-1) or allylbromide (TOF: 3a 150, 3b 562 h^-1). The reaction of 6 a and 6 b with zinc in acetonitrile affords in good yields the monophosphine Re ¹ complexes [Re(Br)₂(NO)(MeCN)₂(PR ₃)] (R = Cy 8a, iPr 8b), which showed high catalytic activity toward highly selective dehydrogenative silylation of styrenes (maximum TOF of 61 h-1). Single-electron transfer (SET) mechanisms were proposed for all these transformations. The molecular structures of 3 a, 6 a, 6 b, 7 a, 7 b, and 8 a were established by single-crystal X-ray diffraction studies.