| Abstract
| - Hydride transfer from CpW(CO)2(IMes)H to Ph3C+B(C6F5)4- gives CpW(CO)2(IMes)+B(C6F5)4-, in which the IMes is bonded as a bidentate ligand, through the carbon of the carbene as well as forming a weak bond from the metal to a CC bond of one mesityl ring. When this complex is used in the catalytic hydrogenation of ketones, decomposition produces [H(IMes)]+B(C6F5)4-, which indicates a vulnerability of the N-heterocyclic carbene ligand.
- The initial reaction observed between the N-heterocyclic carbene IMes (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) and molybdenum and tungsten hydride complexes CpM(CO)2(PPh3)H(M = Mo, W) is deprotonation of the metal hydride by IMes, giving [(IMes)H]+[CpM(CO)2(PPh3)]-. Atlonger reaction times and higher temperatures, the reaction of IMes with CpM(CO)2(PR3)H (M = Mo,W; R = Me, Ph) produces CpM(CO)2(IMes)H. Hydride transfer from CpW(CO)2(IMes)H to Ph3C+B(C6F5)4-gives CpW(CO)2(IMes)+B(C6F5)4-, which was crystallographically characterized using X-ray radiationfrom a synchrotron. The IMes is bonded as a bidentate ligand, through the carbon of the carbene as wellas forming a weak bond from the metal to a CC bond of one mesityl ring. The weakly bound CCligand is hemilabile, being readily displaced by H2, THF, ketones, or alcohols. Reaction of CpW(CO)2(IMes)+ with H2 gives the dihydride complex [CpW(CO)2(IMes)(H)2]+. Addition of Et2CH−OH toCpW(CO)2(IMes)+B(C6F5)4- gives the alcohol complex [CpW(CO)2(IMes)(Et2CH−OH)]+[B(C6F5)4]-,which was characterized by crystallography and exhibits no evidence for hydrogen bonding of the boundOH group. Addition of H2 to the ketone complex [CpW(CO)2(IMes)(Et2CO)]+[B(C6F5)4]- producesan equilibrium with the dihydride [CpW(CO)2(IMes)(H)2]+ (Keq = 1.1 × 103 at 25 °C). The tungstenketone complex [CpW(CO)2(IMes)(Et2CO)]+[B(C6F5)4]- serves as a modest catalyst for hydrogenationof Et2CO to Et2CH−OH in neat ketone solvent. Decomposition of the catalyst produces[H(IMes)]+B(C6F5)4-, indicating that these catalysts with N-heterocyclic carbene ligands are vulnerableto decomposition by a reaction that produces a protonated imidazolium cation.
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