Single-Crystal to Single-Crystal Transformations: Stepwise CO 2 Insertions into Bridging Hydrides of [(NHC)CuH] 2 Complexes

Mechanistic studies of substrate insertion into dimeric [(NHC)CuH] (NHC=N-heterocyclic carbene) complexes with two bridging hydrides have been shown to require dimer dissociation to generate transient, highly reactive (NHC)Cu-H monomers in solution. Using single-crystal to single-crystal (SC-SC) transformations, we discovered a new pathway of stepwise insertion of CO into [(NHC)CuH] without complete dissociation of the dimer. The first CO insertion into dimeric [(IPr*OMe)CuH] (IPr*OMe=N,N'-bis(2,6-bis(diphenylmethyl)-4-methoxy-phenyl)imidazole-2-ylidene) produced a dicopper formate hydride [(IPr*OMe)Cu] (μ-1,3-O CH)(μ-H). A second CO insertion produced a dicopper bis(formate), [(IPr*OMe)Cu] (μ-1,3-O CH)(μ-1,1-O CH), containing two different bonding modes of the bridging formate. These dicopper formate complexes are inaccessible from solution reactions since the dicopper core cleanly ruptures to monomeric complexes when dissolved in a solvent.