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)

John A.S. Roberts, James A. Franz, Edwin F. Van Der Eide, Eric D. Walter, Jeffrey L. Petersen, Daniel L. Dubois, R. Morris Bullock

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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+PF 6- 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).

Original languageEnglish
Pages (from-to)14593-14603
Number of pages11
JournalJournal of the American Chemical Society
Issue number37
Publication statusPublished - Sep 21 2011

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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