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Periodic Reporting for period 1 - Ni-CAT (Ni-CAT)

Reporting period: 2015-04-01 to 2017-03-31

The Ni-CAT project aims at boosting the catalytic activity of Ni for effectuating challenging oxidation and bond formation reactions. As a key concept to achieve this goal, the metal centers will be combined with mesoionic ligands, a class of ligands that is able to transiently store and release electrons and that is thus particularly suited for assisting a coordinated metal center in effectuating electron transfer processes. This metal-ligand synergy alleviates the limitations typically encountered for bond activation processes with nickel, which often undergoes one-electron redox changes only. This concept renders catalytic transformations with this less reactive metal center energetically feasible and eventually efficient and will introduce a new paradigm in using Earth-abundant metals in homogeneous catalysis. The exploitation of intrinsic metal ligand cooperativity for electron transfer provides a compelling strategy to develop a sustainable process towards efficient C–H bond activation that is not dependent on precious metals.
Hence, the key objective of the Ni-Cat project is to develop efficient catalysts for C-C coupling based on nickel complexes that contain donor-flexible mesoionic ligands, with the ultimate goal to activate C-H bonds for functionalization.
This overarching objective will encompass
i) successful application of synergistic metal/ligand effects for inducing catalytic activity.
ii) solid understanding of mechanistic key principles to tailor and improve performance in catalysis.
iii) a gradual shift to more challenging substrates for C–C cross coupling reactions
iv) expansion of the coupling reaction to halide-free substrates in order to induce nickel-catalyzed C–H bond activation, thus breaking into new grounds of catalysis using this cheap and Earth-abundant metal.
During the five months covered by the project, only the first work package (WP1) was implemented. This WP1 focused on the synthesis of mesoionic nickel complexes and the assessment of their intrinsic stability and redox behavior. The research approach therefore revolved around the synthesis of mesoionic MIC and PYA ligands in different combinations. In this sense, novel mesoionic chelating ligands that combine on one side a triazolylidene with a pyridylidene amide (Scheme 1a) and moreover two pyridylidene amides have been designed (Scheme 1b).

In these ligands both fragments, triazolylidene and pyridylidene, may adapt their donor properties and toggle between a mesoionic and a neutral state, a behaviour that could be ideal for inducing catalytic activity at redox-active metals centers. The coordination of ligands 1 and 2 to different transition metals such as Ru and Ir was achieved (Scheme 2), unfortunately the first attempts to coordinate both ligands to nickel precursors has not been successful yet.
The expertise of the hosting group in triazolylidene complexes with a vast know-how in synthetic methodologies and catalytic application has greatly spurred this portion of the work and the synthetic problems that have occurred will be overcome. By achieving the development of efficient nickel catalysts for C-C coupling that contain donor-flexible mesoionic ligands, the Ni-CAT project will disclose a new paradigm for boosting the catalytic activity of a typically less reactive first row transition metal to engage in the functionalization of C–H bonds, one of the most topical areas of contemporary catalysis. These preliminary data strongly enhance the success of this project, the credibility of the research, and the feasibility of the proposed methodology. The development of efficient oxidation processes by using robust and inexpensive first row transition metal complexes will have a major impact in synthetic methodology, providing an unprecedented approach to carbonyl groups and related functionalities, with commercial opportunities in follow-up projects.
The project provides an excellent training in one of the most topical areas of catalysis and its successful completion will greatly enhances the career profile and the research competence of the researcher, thus actively contributing to the fostering of young talents and for prospering scientific excellence in Europe.
Synthesis of the Ru and Ir complexes 3-5, bearing trz-pya ligand 1 and bis-pya ligand 2
Synthesis of the mesoionic chelating a) trz-pya ligand 1 and b) bis-pya ligand 2