Solution and Solid-State Structure of the First NHC-Substituted Rhenium Heptahydrides

Abstract

The structure of three new bis(N-heterocyclic carbene) heptahydride rhenium complexes ([ReH7(NHC)(2)] with NHC= IMes (2b) (1,3-bis(2,4,6-trimethyl phenyl) imidazol-2-ylidene), IMesCl(2) (4b) (4,5-dichloro-1,3-dihydro-1,3-bis(2,4,6-trimethyl phenyl)-2H-imidazol-2-ylidene) BIMes (5b) (1,3-Bis(2,4,6-trimethylphenyl)-benzo imidazole ylidene), and of the known complex [ReH7(PPh3)(2)] (1) (bis(triphenyl phosphine)-rhenium heptahydride), and the one of their labelled counterparts 2c-d(7), 4c-d(7), and 5c-d(7), were established in solution. The compounds exchanged with HD (2 bar) at 60 degrees C in deuterated THF to produce statistically distributed isotopomers. This exchange pointed to the intermediacy of H-2 complexes and dynamic systems with fast equilibration processes of all hydride positions. T-1min measurements were run on 2b, 4b, 5b and 1 in deuterated methylcyclohexane: 2b T-1min = 144 ms, 5b T-1min = 127 ms, both at 203 K. For 4b two T-1min values were obtained corresponding to the H-1-NMR signals of the hydrides: T-1min = 119 ms and T-1min = 128 ms both at 203 K. The T-1min values determined are typical of terminal rhenium polyhydrides and for 1 was reported a comparably short value of T-1min (70 ms, [D-8]toluene, at -73 degrees C), typical of complexes that contain dihydrogen ligands. D-NMR T-1min measurements were also run for the fully deuterated compounds 2c-d(7), 4c-d(7), 5c-d(7) and of 1c-d(7) allowing to calculate the ionicities of the metal hydride bonds. The Re-D ionicity values were obtained from the Deuterium Quadrupole Coupling Constant (DQCC) determinations. They were in the range of 0.61-0.71 for the fully labelled complexes: 2c-d(7): T-1min = 15 ms, i = 61 %; 4c-d(7): (A) T-1min = 20 ms, i = 67 %, (B) T-1min = 41 ms, i = 77 %; 5c-d(7): T-1min = 24 ms, i= 70 %; and 1c-d(7): T-1min = 26 ms, i = 71 %. Single crystal X-ray diffraction studies on 4b enabled us to refine the hydride positions with only restraints in the bond lengths and thermal parameters. The X-ray solid structure resembles a pseudo-capped square antiprismatic geometry.