A Tungsten Complex with a Bidentate, Hemilabile N-Heterocyclic Carbene Ligand, Facile Displacement of the Weakly Bound W−(CC) Bond, and the Vulnerability of the NHC Ligand toward Catalyst Deactivation during Ketone Hydrogenation

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)]-. 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(IMes)H to Ph3C+B(C6F5)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)(Et2CH−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)]+[B(C6F5)4]- serves as a modest catalyst for hydrogenation
of Et2CO 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 vulnerable
to decomposition by a reaction that produces a protonated imidazolium cation.