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)(PR 3 ) 2 ] (R = Cy 2a, i Pr 2b ) were prepared from [Re(Br) 2 (NO)(PR 3 ) 2 (η 2 -H 2 )] (R = Cy 1a, i Pr 1b ) via deprotonation of the η 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)(PR 3 ) 2 (L)] (L = O 2 3, CH 2 ═CH 2 4, acetylene 5, H 2 6, CO 7, CH 3 CN 8 ). The influence of the trans -coordinated ligand on the Re−H bond was examined. The 1 H NMR chemical shift of the hydride depends on L in the order O 2 > acetylene > CH 2 ═CH 2 > H 2 > CO > CH 3 CN. The reactions of 2a or 2b with the IMes or SIMes ligands afforded the five-coordinated complex [Re(Br)(H)(NO)(PR 3 )(NHC)] (NHC = IMes 9 (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), SIMes 10 (SIMes = 1,3-bis(2,4,6-trimethylphenyl)-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 dihydride complexes [Re(H) 2 (NO)(PR 3 ) 2 (η 2 - n -CH 2 ═CHC 2 H 5 )] (R = Cy 12a, i Pr 12b ). Species 1a and 1b reacted also with NaNMe 2 ·BH 3, affording the tetrahydride complexes [Re(H) 4 (NO)(PR 3 ) 2 ] (R = Cy 14a, i Pr 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-coordinated rhenium(I) hydride complexes 2a, 2b, 9a, and 9b catalyzed the dehydrocoupling of Me 2 NH·BH 3 and the transfer hydrogenation of olefins using Me 2 NH·BH 3 as a hydrogen donor, which showed high activities. Mechanistic studies were carried out indicating that these rhenium(I) hydride catalyses allowed formation of dihydrogen hydride complexes. A plausible catalytic cycle for both dehydrocoupling and transfer hydrogenation was proposed, which implies the ability of rhenium(I) complexes to activate B−H and N−H bonds by the facile redox interplay of Re(I) and Re(III) species.