The initial reaction observed between the N-heterocyclic carbene IMes (IMes = l,3-bis(2,4,6trimethylphenyl)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)]. At longer 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(EvIeS)H to Ph3C+B(C6F 5)4- gives CpW(CO)2(IMeS) +B(C6F5)4-, which was crystallographically characterized using X-ray radiation from a synchrotron. 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 C=C bond of one mesityl ring. The weakly bound C=C ligand 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 to CpW(CO) 2(IMeS)+B(C6F5)4 - gives the alcohol complex [CpW(CO)2(IMes)(Et 2CH-OH)]+[B(C6F5)4] -, which was characterized by crystallography and exhibits no evidence for hydrogen bonding of the bound OH group. Addition of H2 to the ketone complex [CpW(CO)2(IMes)(Et2C=O)] +[B(C6F5)4]- produces an equilibrium with the dihydride [CpW(CO)2(IMeS)(H)2]+ (Keq = 1.1 × 103 at 25° C). The tungsten ketone complex [CpW(CO)2(IMeS)(Et2C=O)I+[B(C6F 5)4]- serves as a modest catalyst for hydrogénation of Et2C=O to Et2CH-OH in neat ketone solvent. Decomposition of the catalyst produces [H(IMes)]+B(C 6F5)4-, indicating that these catalysts with N-heterocyclic carbene ligands are vulnerable to decomposition by a reaction that produces a protonated imidazolium cation.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry