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 p<i>K</i>_a^MeCN = 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 <i>E</i>_1/2 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: <i>k</i>_dimerization ≈ 2.5 × 10⁴ M^–1 s^–1, <i>k</i>_homolysis ≈ 0.5 s^–1 (i.e., <i>K</i>_dim ≈ 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-<i>tert</i>-butyl-1,4-benzoquinone, chloroform, and dihydrogen