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

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Chemical and Materials Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, K2-57, Richland, Washington 99352, United States
*E-mail: [email protected]
Cite this: Organometallics 2012, 31, 5, 1775–1789
Publication Date (Web):February 8, 2012
https://doi.org/10.1021/om201162g
Copyright © 2012 American Chemical Society
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    Abstract

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    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.

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    Text, tables, figures, and CIF files giving details of the synthesis of [IMeH]+[CpW(CO)3]−, discussion of C–H···O interactions in the crystal structure of [IMeH]+[CpW(CO)2(PMe3)]−, atomic coordinates and other details for calculations on CpW(CO)2(IMe)H, procedures, data, and plots relevant to the cis–trans equilibrium of CpW(CO)2(IMe)H, solid-state and solution IR spectra and 1H NMR spectra of [CpW(CO)2(IMe)]2, and crystallographic data. This material is available free of charge via the Internet at http://pubs.acs.org.

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