Facile synthetic access to Rhenium(II) complexes: activation of carbon-bromine bonds by single-electron transfer

Abstract

The five-coordinated ReI hydride complexes Re(Br)(H)(NO)(PR3)(2)] (R=Cy 1a, iPr 1b) were reacted with benzylbromide, thereby affording the 17-electron mononuclear Re-II 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 I afforded Re-II hydrides 3 in addition to Re-I carbene hydrides Re(=CHRI)(Br)(H)(NO)(PR3)(2)] (R-I=H 4, Br 5; R=Cy a, iPr b) in which the bydride ligand is positioned cis to the carbene ligand. For comparison, the dihydrogen Re-I dibromide complexes Re(Br)(2)(NO)(PR3)(2)(eta(2)-H-2)] (R = Cy 2a, iPr 2b) were reacted with allyl- or benzylbromide, thereby affording the monophosphine Re-II complex salts R3PCH2R']Re(Br)(4)(NO)(PR3)] (R'= -CH=CH2 6, Ph 7). The reduction of Re-II complexes has also been examined. Complex 3a or 3b can be reduced by zinc to afford 1a or 1b in high yield. Under catalytic conditions, this reaction enables homocoupling of benzylbromide (turnover frequency (TOF): 3a 150, 3b 434 h(-1)) or allylbromide (TOF: 3a 575, 3b 562 h(-1)). The reaction of 6a and 6b with zinc in acetonitrile affords in good yields the monophosphine Re-I complexes Re(Br)(2)(NO)(MeCN)(2)(PR3)] (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 3a, 6a, 6b, 7a, 7b, and 8a were established by single-crystal X-ray diffraction studies.