Facile Thermal W–W Bond Homolysis in the N-Heterocyclic Carbene Containing Tungsten Dimer [CpW(CO)2(IMe)]2

The thermal W–W bond homolysis in [CpW(CO)2(IMe)]2 (IMe = 1,3-dimethylimidazol-2-ylidene) was investigated and was found to occur to a large extent in comparison to other tungsten dimers such as [CpW(CO)3]2. CpW(CO)2(IMe)H was prepared by heating a solution of [IMeH]+[CpW(CO)2(PMe3)]−, and it exists in solution as a mixture of interconverting cis and trans isomers. The carbene rotation in CpW(CO)2(IMe)H was explored by DFT calculations, and low enthalpic barriers (<3.5 kcal mol–1) are predicted. CpW(CO)2(IMe)H has pKaMeCN = 31.5(3), and deprotonation with KH gives K+[CpW(CO)2(IMe)]− (·MeCN). Hydride abstraction from CpW(CO)2(IMe)H with Ph3C+PF6– in the presence of a coordinating ligand L (MeCN or THF) gives [CpW(CO)2(IMe)(L)]+PF6–. Electrochemical measurements on the anion [CpW(CO)2(IMe)]− in MeCN, together with digital simulations, give an E1/2 value of −1.54(2) V vs Cp2Fe+/0 for the [CpW(CO)2(IMe)]•/– couple. A thermochemical cycle provides the solution bond dissociation free energy of the W–H bond of C...