Dynamic processes of the first NHC-substituted rhenium heptahydrides [ReH7(NHC)2]

Abstract

In this paper are presented possible geometries and fluxional mechanisms of three [ReH₇(NHC)₂] complexes (NHC = IMes (1) (1,3-bis(2,4,6-trimethyl phenyl) imidazol-2-ylidene), IMesCl₂ (2) (4,5-dichloro-1,3-dihydro-1,3-bis(2,4,6-trimethylphenyl)-2H-imidazol-2-ylidene), BIMes (3) (1,3-bis(2,4,6-trimethylphenyl)-benzo imidazole ylidene)). Using DFT calculations, plausible fluxional processes were proposed, and a new process named “η²-quadratic twist” has been identified. For 2 the intermediate complex [Re(H)₅(η²-H₂)(IMesCl₂)₂] proved to be more energetically accessible at RT than those of the complexes 1 ([Re(H)₅(η²-H₂)(IMes)₂]) and 3 ([Re(H)₅(η²-H₂)(BIMes)₂]). Two different fluxional mechanisms have been proposed: the first one for 1 and 3, and the second one for 2. The mechanism for complex 2 implies the C–H activation of the hydrogens of the o-CH₃ group of the IMesCl₂ ligands.