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 CpW­(CO)2(IMe)H as 61.3(6) kcal mol–1. In the electrochemical oxidation of [CpW­(CO)2(IMe)]−, reversible dimerization of the electrogenerated radical CpW­(CO)2(IMe)• occurs, and digital simulation provides kinetic and thermodynamic parameters for the monomer–dimer equilibrium: kdimerization ≈ 2.5 × 104 M–1 s–1, khomolysis ≈ 0.5 s–1 (i.e., Kdim ≈ 5 × 104 M–1). Reduction of [CpW­(CO)2(IMe)­(MeCN)]+PF6– with cobaltocene gives the dimer [CpW­(CO)2(IMe)]2, which in solution exists as a mixture of anti and gauche rotamers. As expected from the electrochemical experiments, the dimer is in equilibrium with detectable amounts of CpW­(CO)2(IMe)•. This species was observed by IR spectroscopy, and its presence in solution is also in accordance with the observed reactivity toward 2,6-di-tert-butyl-1,4-benzoquinone, chloroform, and dihydrogen.