Ligand Design & Trisamidophosphine Complexes
Trisamidoamines (tren-type systems or [NN3]-ligands) are privileged trisanionic ligand scaffolds, which are well-known to coordinate to numerous main group, rare earth and transition metals, usually via all four nitrogen donors. Variation of the amido functionalities within these pivotal ligand systems led to more than 50 [NN3]-derivatives with the common structural motif readily identified as the central amino linkage. Most of these ligand scaffold have been employed in the synthesis of transition metal complexes, which often exhibit exciting reactivities, for example in the activation of small molecules. Formal substitution of the central amine for a phosphine gives rise to a new class of ligands, namely the trisamidophosphine ligand family. These trisamidophosphines combine hard and soft donor functionalities and are thus expected to stabilize not only high-valent but also low-valent metal ions. A tetradentate coordination of these ligands is desired, but only found for [PN3]-ligands comprising a three-atomic linkage between the central phosphine and the three amido donors (see Scheme). Such [PN3]-coordinate half-cage complexes of the early transition metals and their reactivity are currently explored in detail.
Catalysis & Small Molecule Activation
Based on the observation that trisamidophosphine complexes tend to undergo intramolecular cyclometalations, application of these systems in catalysis comes into reach. In this context, it is noteworthy that similar cyclometalated species have been used previously for dehydrocoupling reactions of phosphines, silanes and aminoboranes. Therefore, current efforts focus on the use of our systems as novel dehydrocoupling catalysts.
A second focus is set on the activation of element-element triple bonds in robust small molecules, such as molecular dinitrogen (N≡N). These efforts aim towards the development of coordinatively unsaturated low-valent trisamidophosphine metal complexes that are suitable to reductively activate such small molecules in a substantial manner. In the case of molecular dinitrogen, the intended reductive activation is expected to result in the formation of diazenido, hydrazinido and nitrido species. Thus, pathways for subsequent functionalization by reaction with silanes, dihydrogen, cumulenes, alkynes or other reductants are opened up. Cycloadditions with alkynes are particularity fascinating, as such reactions might lead to new and atom-efficient N-C coupling protocols. In long term, we envision to perform these conversions not only in a stoichiometric, but also in a catalytic fashion.