Synthesis and catalytic activity of N-heterocyclic carbene metal carbonyl complexes based on 1-[2-(pyrazol-1-yl)phenyl]imidazole

Abstract

A series of N-heterocyclic carbene transition metal (molybdenum, tungsten and ruthenium) carbonyl complexes derived from 1-[2-(pyrazol-1-yl)phenyl]-imidazole have been synthesized and characterized by physico-chemical and spectroscopic methods, and the structures of five complexes have been confirmed by X-ray crystal structure analyses. The azaaryl-functionalized N-heterocyclic carbenes act as chelating N,C-bidentate ligands in these five complexes. Preliminary catalytic tests of these NHC-molybdenum (tungsten) carbonyl complexes in the oxidation of styrene have been carried out, showing that benzaldehyde was the main oxidation product when hydrogen peroxide was used as the oxidant, while 2,5-diphenyl-1,4-dioxane was obtained as the exclusive product in the oxidation by tert-butyl hydroperoxide.

A Mononuclear Tungsten Photocatalyst for H2 Production

“…The two CO ligands trans to each other exhibit more linear displacement (172.7° and 174.4°) than the CO trans to the pyridyl (178.1°) or NHC ligand (178.6°). This is a phenomenon which has also been observed for W and Mo tetracarbonyl complexes, − The W–C NHC bond is shorter (2.185(4) Å) than the W–N bond (2.259(3) Å), and the W–C CO bond trans to the NHC is longer (1.973(4) Å) than the trans -pyridyl carbonyl (1.951(4) Å); these differences are presumably due to the strong σ-donor nature of the NHC ligand to W.…”

Recent progress on NHC-stabilized early transition metal (group 3–7) complexes: Synthesis and applications

“…100,101 Reaction of Ag2O with an unsymmetrical pyrazolylpyridyl bis-functionalized imidazolium salt followed by transmetallation with [Mo(CO)5(THF)] gave the corresponding bidentate complex 134 (Figure 13). 102 whereas the more flexible lutidine-based ligand adopted an uncommon fac-coordination in 136. 103 The activity of (mostly) monodentate NHC adducts of molybdenum(0) and tungsten (0) carbonyl complexes has been investigated in olefin epoxidation catalysis.…”