Well‐defined Nickel P₃C Complexes as Hydrogenation Catalysts of N‐Heteroarenes Under Mild Conditions

Abstract

The reactivity of metal complexes can be primarily controlled by their coordinating ligands. Electronic effects and geometric restrictions imposed by ligands, prototypically modify their reactivity, but are not always straightforward accessible. In the course of the study, we explored new C3‐symmetric organometallic nickel (II) complexes ((P3C)Ni)) with the particularity of having a metalated apical carbon trans to a labile coordination site readily available as a catalytic site. The synthetic methodology allowed to tune the electronic properties by introducing substituents on the phosphines. We studied the nature of the C−Ni bond by X‐ray absorption spectroscopy (XAS) and DFT and applied them to the catalytic hydrogenation of quinolines with a good chemical scope and group tolerance. 1H‐NMR monitoring, isotopic labelling, and computational studies are consistent with a heterolytic activation of the dihydrogen molecule after forming a σ‐H2 adduct trans to the metalated carbon.

Ni(i)–TPA stabilization by hydrogen bond formation on the second coordination sphere: a DFT characterization

“…The Ni-N bond elongation observed for the Ni(I)-Br complex is not observed for [Ni(II)-Br] +1 , since no significant differences between the Ni-N bond lengths are detected, in line with results for P 3 C complexes. [101][102][103] Nevertheless, there is a clear elongation of one of the Br⋯H interactions since the H atom is not pointing towards the halide. This is clearly observed in Table 8, where the MBO for the corresponding Br⋯H interaction is null.…”

Balancing the Seesaw in Mn-Catalyzed N-Heteroarene Hydrogenation: Mechanism-Inspired Catalyst Design for Simultaneous Taming of Activation and Transfer of H2

“…Moreover, despite prominent advancement since then in the application of judiciously designed PNP- or NNP-pincer Mn catalysts toward hydrogenation of several challenging classes of substrates such as amides, ketones, carbamates, urea-derivatives, (cyclic) organic carbonates/polycarbonates, CO 2 , and CO 2 derivatives, − the breakthrough in the hydrogenation of N-heteroarenesanother important but formidable reactionby well-defined Mn complexes has been reported just recently in 2019, but further developments remained scarce. In terms of significance and usefulness, the catalytic hydrogenation of N-heteroarenes is a vital reaction in the production of pharmaceuticals and bioactive drug molecules. , Furthermore, the hydrogenated N-heteroarenes are prospective liquid organic hydrogen carriers and are recently advocated as regenerable alternative liquid fuels via releasing the stored H 2 as the terminal energy source. − However, their hydrogenation is challenging due to aromatic stabilization and the likelihood of catalyst poisoning by either substrate- or product-coordination via the nitrogen atom(s). , Mostly, noble metals such as Ir, Rh, and Ru complexes with N-heterocyclic carbene (NHC) ligands, − and N-donor ligands, , have been successfully utilized as catalysts in this reaction, while there are only a few reports with non-noble metal-based homogeneous catalysts. − Strong substrate-coordination-triggered catalyst poisoning is in fact expected to be facile when the metal center of the complex is normally not protected already by efficient, preferably chelating, ligands. For example, in 2020, Beller and co-workers, and Wang et al.…”

Cooperative H2 Activation at a Nickel(0)-Olefin Center

“…40 Additionally, the hydricity of the resulting Ni-H bond may be enhanced by the trans alkyl carbon. 41,42 Therefore, we set out to investigate the ability of Ni(0) complexes of PC=CP pincer ligands to activate and transfer H2. In this report, we show that the incorporation of a precoordinated olefin in a pincer ligand framework allows for the observation of both a non-classical Ni-(H2) intermediate and the corresponding alkyl(hydrido) activation product, both in rapid equilibrium with dissolved H2.…”