Dioxomolybdenum(VI) complexes containing N-heterocyclic carbenes

The key role of R–NHC coupling (R = C, H, heteroatom) and M–NHC bond cleavage in the evolution of M/NHC complexes and formation of catalytically active species

“…Protonolysis reactions proceed very easily for Li/NHC 105 , 108 and Mg/NHC 112 complexes. Other complexes, including Ag I /NHC, 191 , 193 , 197 – 199 , 201 Al III /NHC, 119 In III /NHC, 119 Y III /NHC 125 and Ce III /NHC, 125 Mn I /NHC, Zn II /NHC, 213 Mo VI /NHC 130 and Ni II /NHC, 214 are also susceptible to protonolysis; the efficiency depends on the structure of NHC ligands, co-ligands and reaction conditions ( Table 1 column 3). For example, (NHC) 2 NiX 2 complexes (X = Cl, Br, I) suffer facile hydrolysis in aqueous MeCN or THF at 70 °C to give azolium salts and Ni(OH) 2 .…”

Dioxo-, Oxo-imido-, and Bis-imido-Molybdenum(VI) Complexes with a Bis-phenolate-NHC Ligand

“…22 The sensitivity of these complexes was further probed by Royo et al, showing that the substituents on the nitrogen atoms as well as on the NHC backbone have a major influence on the stability of the complexes toward hydrolysis. 23 According to the authors, the high sensitivity of molybdenum NHC complexes toward hydrolysis renders their application in chemical catalysis rather dim. However, in 2010, Hor and co-workers showed that in situ generated (NHC)CpMoO 2 complexes are indeed potent catalysts for the epoxidation of cyclooctene.…”

N-heterocyclic carbene complexes of Group 6 metals

“…Synthesis of M(VI)-NHC complexes [155]. Inspired by Herrmann's noble attempt, Royo et al reported in 2006 the synthesis of similar dioxo Mo(VI)-NHC complexes Mo-57 and Mo-58 (Scheme 3.5)[156]. Study on the backbone and N substituents modifications of NHCs helps understand the stereo-electronic effects on the stability of Mo-C carbene bond.…”

“…However, none of the complexes mentioned above are sufficiently stable for a definitive structural determination. The exceptionally moisture sensitive nature has precluded their catalytic potential to be fully exploited, although both Mo(VI) oxo complexes have been proven to be efficient catalysts in olefin epoxidation reactions[138, 140-57a , A = Me, R = Me Mo-57b , A = Me, R = iPr Mo-57c , A = Cl, R = nBu Synthesis of Mo(VI)-NHC complexes[156].In 2011, Spicer et al[162] described the synthesis of W(VI)-NHC complex W-62 using N-aryl imidazol-2-ylidenes as ligands. It is an extremely moisture sensitive complex, exposure of which Hydrolysis of W(VI)-NHC complex[162].In 2012, Hor and Zhao et al first reported a successful isolation and characterization of thermally stable Mo(VI)-NHC complexes Mo-59(a,b) from oxidative decarbonylation of Mo(II)-Mo-44(a,b) by TBHP (Scheme 3.7) [137].…”