Probing the influence of imidazolylidene- and triazolylidene-based carbenes on the catalytic potential of dioxomolybdenum and dioxotungsten complexes in deoxygenation catalysis†
Florian R. Neururer, Florian Heim, Marc Baltrun, Philipp Boos, Julia Beerhues, Michael Seidl and Stephan Hohloch
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引用次数: 0
Abstract
We report the synthesis of dianionic OCO-supported NHC and MIC complexes of molybdenum and tungsten 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 centre in this transformation. The results show that the molybdenum-based triazolylidene complex 2-Mo is by far the most active catalyst, and TOFs of up to 270 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 highly stable metal carbene bond, strongly exceeding the stability of 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-oxodimolybdenum(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 of the nitroarenes to the corresponding anilines with nitrosoarenes as intermediates. In summary, this work showcases the superior influence of MIC donors on the catalytic properties of early transition metal complexes.
本文报道了用通式(OCO)MO2 (OCO =双酚酸盐苯并咪唑聚乙基M = Mo (1-Mo),双酚酸盐三唑聚乙基M = Mo (2-Mo), M = W (2-W)和双酚酸盐咪唑聚乙基M = Mo (3-Mo), W (3-W))合成重VI族金属重阴离子OCO负载NHC和MIC配合物。对这些配合物在硝基芳烃的催化脱氧反应中进行了测试,以吡纳酚为牺牲氧原子受体/还原剂,考察了碳和金属中心对这一转化的影响。结果表明,钼基三氮偶氮配合物2-Mo是迄今为止最活跃的催化剂,tof高达263 h-1,而钨的类似物基本上是不活跃的。机理研究表明,与其他NHC配合物1-Mo和3-Mo相比,3-Mo基配合物2-Mo的优势是由于其金属碳键更稳定。这是由结构分离的三氮酰基吡咯酸酯配合物5-Mo证明,该配合物可以热转化为μ -氧化二钼(V)配合物7-Mo。后一种配合物是非常亲氧的,并在室温下化学计量脱氧硝基和亚硝基芳烃。相比之下,偶氮芳烃没有被7-Mo还原裂解,这表明硝基芳烃与相应的苯胺直接脱氧反应,而亚硝基芳烃是唯一的中间体。总之,这项工作展示了NHC/MIC供体对早期过渡金属配合物催化性能的主要影响。