Development of Rhenium Catalysts for Amine Borane Dehydrocoupling and Transfer Hydrogenation of Olefins

Five-coordinated rhenium(I) hydride complexes of the type [Re(Br)(H)(NO)(PR3)(2)] (R = Cy 2a, iPr 2b) were prepared from [Re(Br)(2)(NO)(PR3)(2)(eta(2)-H-2)] (R = Cy 1a, iPr 1b) via deprotonation of the eta(2)-H-2 ligands with various bases. Filling the vacant site of 2a or 2b by various less bulky two-electron donors produced the 18-electron complexes [Re(Br)(H)(NO)(PR3)(2)(L)] (L = O-2 3, CH2=CH2 4, acetylene 5, H-2 6, CO 7, CH3CN 8). The influence of the trans-coordinated ligand oil the Re-H bond was examined. The H-1 NMR chemical shift of the hydride depends oil L in the order O-2 > acetylene > CH2=CH2 > H-2 > CO > CH3CN. The reactions of 2a or 2b with the IMes or SIMes ligands afforded the five-coordinatcd complex [Re(Br)(H)(NO)(PR3)(NHC)] (NHC = IMes 9 (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), SIMes 10 (SIMes = 1,3-bis(2,4,6-trimethylpheiivl)-4,5-dihydroimidazol-2-ylidene)) via replacement of one phosphine. The reaction of 2a or 2b with n-BuLi leads to the formation of the n-butene-coordinated dillydride complexes [Re(H)(2)-(NO)(PR3)(2)(eta(2) -n-CH2=CHC2H5)] (R = Cy 12a, iPr 12b). Species la and 1b reacted also with NaNMe2 center dot BH3, affording the tetrahydride complexes [Re(H)(4)(NO)(PR3)(2)] (R = Cy 14a, iPr 14b) via the intermediacy of 2a and 2b. The molecular structures of complexes 8b, 10a, and 10b were established by single-crystal X-ray diffraction studies. The five-coordinatcd rhenium(l) hydride complexes 2a, 2b, 9a, and 9b catalyzed the dehydrocoupling of Me2NH center dot BH3 and the transfer hydrogenation of olefin using Me2NH center dot BH3 as a hydrogen donor, which showed high activities. Mechanistic studies were carried out indicating that these rhenium(l) hydride catalyses allowed formation of dillydrogen hydride complexes. A plausible catalytic cycle for both dehydrocoupling and transfer hydrogenation was proposed, which implies the ability of rhenium(l) complexes to activate B-H and N-H bonds by the facile redox interplay of Re(l) and Re(Ill) species.