Synthesis and characterization of pyrrole-based group IV PNP pincer complexes

Abstract

The activation of small molecules such as N2, CO, H2 or CO2 is a fundamental subfield in organometallics chemistry. In the case of N2 activation group IV metals are known for their unique coordination modes of dinitrogen. One of the most important aspects in this research field is the choice of the ancillary ligand fine-tuning, the steric and electronic parameters of the complexes, which bind N2 in various modes. In the last decades it has been proven that amidophosphines are well suited for this task, due to the combination of the hard anionic nitrogen donor in combination with the soft phosphine moiety.This work focuses on the development, synthesis and characterization of group IV metal complexes as well as the functionalization of these for desired N2 activation. In this context a PNP pincer type ligand based on the pyrrole scaffold was chosen. The complexation was achieved upon utilization of MCl4∙2THF (M = Ti, Zr, Hf) as well as the less common M(NR2)4 (R = Me, Et) precursor. In course of the work, it became apparent that silyl reagent such as TMS-X (X = Cl, Br, I, N3) were suitable for targeted functionalization.Therefore, it was possible to isolate the corresponding chloride complexes by the utilization of MCl4∙2THF. In case of Titanium, mono nuclear complexes were achieved. Interestingly, zirconium- and hafnium-based organometallics form chloride bridged binuclear complexes. The functionalization with the silyl reagents proofed to be challenging for zirconium and hafnium congeners.In contrast, the obtained amido complexes, achieved by conversion of the ligand with M(NR2)4, showed a broader functionalization spectrum with silyl reagents. Among these transformations, it was possible to isolate rare examples of anionic zirconium and hafnium bromide complexes. In general, the heavier group IV metals proofed to be more suitable in functionalization with silyl reagents. Unfortunately, N2 activation mediated by the newly synthesized complexes could not be achieved by the utilization of different strong reducing agents (KC8, Na, NaHg). The rigidity of the utilized PNP ligand was signed to be the limiting factor for this transformation. Within this context, a novel PNP-based ligand was designed and potential synthetic routes towards the desired compound were investigated.