Probing the Influence of Imidazolylidene- and Triazolylidene-Based Carbenes on the Catalytic Potential of Dioxomolybdenum and Dioxotungsten Complexes in Dexoxygenation Catalysis

Abstract

We report the synthesis of dianionic OCO supported NHC and MIC complexes of heavy group VI metals with the general formula (OCO)MO2 (OCO = bis-phenolate benzimidazolylidene M = Mo (1-Mo), bis-phenolate triazolylidene M = Mo (2-Mo), M = W (2-W) and bis-phenolate imidazolylidene, M = Mo (3-Mo), W (3-W)). These complexes are tested in the catalytic deoxygenation of nitroarenes using pinacol as a sacrificial oxygen atom acceptor/reducing agent to examine the influence of the carbene and the metal center in this transformation. This reveals that molybdenum based triazolylidene complex 2-Mo is by far the most active catalyst and TOFs of up to 263 h-1 are observed, while the tungsten analogues are basically inactive. Mechanistic studies suggest, that the superiority of the triazolylidene-based complex 2-Mo is a result of a more stable metal carbene bond compared to the other NHC complexes 1-Mo and 3-Mo. This is proven by the structural isolation of a triazolylidene pinacolate complex 5-Mo, that can be thermally converted to a µ-oxodimolybdneum(V) complex 7-Mo. The latter complex is very oxophilic and stoichiometrically deoxygenates nitro- and nitrosoarenes at room temperature. In contrast, azoarenes are not reductively cleaved by 7-Mo, suggesting direct deoxygenation reaction of the nitroarenes to the corresponding anilines with nitrosoarenes as the only intermediates. In summary, this work showcases the major influence of the NHC/MIC donor on the catalytic properties of early transition metal complexes.