Comproportionation of Cationic and Anionic Tungsten Complexes Having an N-Heterocyclic Carbene Ligand To Give the Isolable 17-Electron Tungsten Radical CpW(CO)2(IMes)•

A series consisting of a tungsten anion, radical, and cation, supported by the N-heterocyclic carbene 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) and spanning formal oxidation states W(0), W(I), and W(II), has been synthesized, isolated, and characterized. Reaction of the hydride CpW(CO)2(IMes)H with KH and 18-crown-6 gives the tungsten anion [CpW(CO)2(IMes)]−[K(18-crown-6)]+. Electrochemical oxidation of [CpW(CO)2(IMes)]− in MeCN (0.2 M nBu4N+PF6–) is fully reversible (E1/2 = −1.65 V vs Cp2Fe+•/0) at all scan rates, indicating that CpW(CO)2(IMes)• is a persistent radical. Hydride transfer from CpW(CO)2(IMes)H to Ph3C+PF6– in MeCN affords [cis-CpW(CO)2(IMes)(MeCN)]+PF6–. Comproportionation of [CpW(CO)2(IMes)]− with [CpW(CO)2(IMes)(MeCN)]+ gives the 17-electron tungsten radical CpW(CO)2(IMes)•. This complex shows paramagnetically shifted resonances in the 1H NMR spectrum and has been characterized by IR spectroscopy, low-temperature EPR spectroscopy, and X-ray diffraction. CpW(CO)2(IMes)• is stable with respect to disproportionation and dimerization. NMR studies of degenerate electron transfer between CpW(CO)2(IMes)• and [CpW(CO)2(IMes)]− are reported. DFT calculations were carried out on CpW(CO)2(IMes)H, as well as on related complexes bearing NHC ligands with N,N′ substituents Me (CpW(CO)2(IMe)H) or H (CpW(CO)2(IH)H) to compare to the experimentally studied IMes complexes with mesityl substituents. These calculations reveal that W–H homolytic bond dissociation energies (BDEs) decrease with increasing steric bulk of the NHC ligand, from 67 to 64 to 63 kcal mol–1 for CpW(CO)2(IH)H, CpW(CO)2(IMe)H, and CpW(CO)2(IMes)H, respectively. The calculated spin density at W for CpW(CO)2(IMes)• is 0.63. The W radicals CpW(CO)2(IMe)• and CpW(CO)2(IH)• are calculated to form weak W–W bonds. The weakly bonded complexes [CpW(CO)2(IMe)]2 and [CpW(CO)2(IH)]2 are predicted to have W–W BDEs of 6 and 18 kcal mol–1, respectively, and to dissociate readily to the W-centered radicals CpW(CO)2(IMe)• and CpW(CO)2(IH)•.