Journal of the American Chemical Society, Vol.133, No.37, 14593-14603, 2011
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)(center dot)
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 (Bu4N+PF6-)-Bu-n) is fully reversible (E-1/2 = -1,65 V vs Cp2Fe+center dot/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)(center dot). This complex shows paramagnetically shifted resonances in the H-1 NMR spectrum and has been characterized by IR spectroscopy, low-temperature EPR spectroscopy, and X-ray diffraction. CpW(CO)(2)(IMes)(center dot) is stable with respect to disproportionation and dimerization. NMR studies of degenerate electron transfer between CpW(CO)(2)(IMes)(center dot) 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)(center dot), and CpW(CO)(2)(IMes)H, respectively. The calculated spin density at W for CpW(CO)(2)(IMes)(center dot) is 0.63. The W radicals CpW(CO)(2)(IMe)(center dot). and CpW(CO)(2)(IH)(center dot) 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-WBDEs of 6 and 18 kcal mol(-1), respectively, and to dissociate readily to the W-centered radicals CpW(CO)(2)(IMe)(center dot) and CpW(CO)(2)(IH)(center dot).