The pincer ligand platform is generally recognized as a highly versatile concept in homogeneous catalysis, finding application in a wide range of chemical transformations such as, but not limited to, hydrogenations, nitrogen activation and carbon-heteroatom coupling reactions. The exceptional flexibility of this platform enables development of tailor-made catalysts to promote high catalytic activity and selectivity. However, the use of phosphines in these systems is a hurdle towards commercialization considering the associated synthetic difficulties. Therefore, development of a pincer ligand platform based on NHC donor groups presents an attractive alternative. These groups are stronger donors than phosphines and their synthesis and derivatization is straightforward and well-established. Here we report on the development of ruthenium lutidine (CNC)- and amine (CNHC) derived bis-NHC pincer catalysts (Figure 1) with unique chemical reactivity and catalytic properties. The Ru-CNHC catalysts promote the hydrogenation of a model ethyl heaxonate substrate with remarkable initial rates (TOF0 &gt; 283.000 h-1) and show very high stability under catalytic conditions (TON &gt; 100.000). To understand such an exceptional catalytic performance, we carried out a detailed analysis of this system by a combination of high-throughput catalytic experiments, kinetic studies and detailed mechansitic DFT analysis. This approach shed light on the mechanism and important parameters of Ru-CNHC catalyzed hydrogenation of esters. The mechansitic insights pointed to potentially active non-noble metal-based CNC catalyst capable of promoting the target reaction with a performance comparable to that of the ruthenium counterpart. The theoretical predictions were verified by complementary experimental tests.

Bis-N-heterocyclic carbene pincers as versatile ligand platform for uniquely active ester hydrogenation homogeneous catalysts

253rd National Meeting (2017)

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