[PCCP]- and [AsCCAs]-Pincer Complexes for the Activation and Functionalization of N2

• Applicant: Professor Dr. Joachim Ballmann
• Subject Area: Inorganic Molecular Chemistry - Synthesis and Characterisation
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 471911274

On the basis of our previous results of the first funding period, this project aims to continue and expand our fundamental research, which is directed towards the activation and subsequent functionalization of N2. Establishing energy-efficient alternatives for the synthesis of N-containing fine chemicals in order to contribute to the sustainable use of natural resources is defined as one of the most important long-term goals of our work. To stabilize the required metal complexes, [ECCE] pincer ligands featuring a central alkyne unit (CC) and two flanking donors € were found to be are particularly suited. The central alkyne within these pincer-type scaffolds is meant to flexibly adapt to the electronic needs of the coordinated metal ion (e.g. before and after N2 cleavage) and thus act as an electron relay (2e− vs. 4e− donor). By employing this concept, we recently succeeded to demonstrate that [AsCCAs]-coordinated tungsten complexes may be used to synthesize pyridine derivatives starting from N2, which was unprecedented previously and established over the course of BA 4859/3-1. Along these lines, we also realized that the use of a sterically demanding tBu2As-substituted [AsCCAs] ligand is of crucial importance for the desired N2 cleavage step. Hence, a comparable tBu2P-substituted [PCCP] scaffold was developed (during the preparation phase of this proposal), which is now to be used. In this proposal, it is suggested to employ both tBu2E-substituted [ECCE] ligands (E = P, As) to coordinate a number of 4d- and 5d-metals in order to subsequently explore their N2-chemsity. This exploratory work is going to include niobium and tantalum complexes, while very promising preliminary experiments suggested that N2 cleavage may be accomplished using [PCCP]-coordinated rhenium complexes. To continuously explore new frontiers, N2-activation using earth-abundant 3d-metals is also proposed and considered promising, although new ligands are needed for this purpose. Along these lines, a tailor-made [NCCN] ligand platform with a carefully adjusted bite angle has been developed, allowing for three analogous [ECCE] ligands (E = N, P, As) to be used for coordinating all the transition metals, which are of relevance for this project. In the case of tungsten, N2-splitting has been achieved previously, which prompted us develop new approaches for functionalizing the resulting nitrides. As presented in this proposal, it is highly likely that our tungsten nitrides are actually suited for nucleophilic aromatic substitution reactions, suggesting that aniline derivatives may be obtained directly from N2. Based on these N-functionalization studies, we aim to establish new synthetic (or even catalytic) cycles and thus utilize N2 in chemical synthesis.

• DFG Programme: Research Grants