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

Journal Article · Mon Mar 12 04:00:00 EDT 2012 · Organometallics

DOI:https://doi.org/10.1021/om201162g· OSTI ID:1036918

van der Eide, Edwin F.; Liu, Tianbiao; Camaioni, Donald M.; Walter, Eric D.; Bullock, R. Morris

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 compared 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 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 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 x 104 M-1 s-1, khomolysis ~ 0.5 s-1 (i.e., Kdim ~ 5 x 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 rotomers. 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. This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Biosciences and Geosciences. Pacific Northwest National Laboratory is a multiprogram national laboratory operated for DOE by Battelle. The EPR studies were performed at EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research located at Pacific Northwest National Laboratory.

• Research Organization: Pacific Northwest National Laboratory (PNNL), Richland, WA (US), Environmental Molecular Sciences Laboratory (EMSL)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 1036918
• Report Number(s): PNNL-SA-84309; 44681; 39981; KC0302010
• Journal Information: Organometallics, Journal Name: Organometallics Journal Issue: 5 Vol. 31; ISSN 0276-7333
• Publisher: American Chemical Society
• Country of Publication: United States
• Language: English

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ISOMERS
KINETICS
MIXTURES
OXIDATION
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