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), IMesCl2 (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‐d7, 4c‐d7, and 5c‐d7, were established in solution. The compounds exchanged with HD (2 bar) at 60 °C in deuterated THF to produce statistically distributed isotopomers. This exchange pointed to the intermediacy of H2 complexes and dynamic systems with fast equilibration processes of all hydride positions. T1min measurements were run on 2b, 4b, 5b and 1 in deuterated methylcyclohexane: 2b T1min = 144 ms, 5b T1min = 127 ms, both at 203 K. For 4b two T1min values were obtained corresponding to the 1H‐NMR signals of the hydrides: T1min = 119 ms and T1min = 128 ms both at 203 K. The T1min values determined are typical of terminal rhenium polyhydrides and for 1 was reported a comparably short value of T1min (70 ms, [D8]toluene, at –73 °C), typical of complexes that contain dihydrogen ligands. D‐NMR T1min measurements were also run for the fully deuterated compounds 2c‐d7, 4c‐d7, 5c‐d7 and of 1c‐d7 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‐d7: T1min = 15 ms, i = 61 %; 4c‐d7: (A) T1min = 20 ms, i = 67 %, (B) T1min = 41 ms, i = 77 %; 5c‐d7: T1min = 24 ms, i= 70 %; and 1c‐d7: T1min = 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.