Facile thermal W-W bond homolysis in the N-heterocyclic carbene containing tungsten dimer [CpW(CO) 2(IMe)] 2

Edwin F. Van Der Eide, Tianbiao Liu, Donald M. Camaioni, Eric D. Walter, R. Morris Bullock

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Abstract

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(PMe 3)] -, 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 pK 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 Ph 3C +PF 6 - in the presence of a coordinating ligand L (MeCN or THF) gives [CpW(CO) 2(IMe)(L)] +PF 6 -. Electrochemical measurements on the anion [CpW(CO) 2(IMe)] - in MeCN, together with digital simulations, give an E 1/2 value of -1.54(2) V vs Cp 2Fe +/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: k dimerization ≈ 2.5 × 10 4 M -1 s -1, k homolysis ≈ 0.5 s -1 (i.e., K dim ≈ 5 × 10 4 M -1). Reduction of [CpW(CO) 2(IMe)(MeCN)] +PF 6 - 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.

Original languageEnglish
Pages (from-to)1775-1789
Number of pages15
JournalOrganometallics
Volume31
Issue number5
DOIs
Publication statusPublished - Mar 12 2012

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Organic Chemistry
  • Inorganic Chemistry

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