Activation and transformation of small molecules at a bimetallic β-diketiminato nickel platform

Abstract

This dissertation focuses on the activation and transformation of small molecules by a dinuclear nickel platform. A series of nickel-complexes based on a binucleating bis(tridentate) ligand that consists of two ß-diketiminato compartments linked by a pyrazolate unit has been synthesized. Starting from a bromide-bridged complex, reduction with KC8 yields a highly reactive double “T”-shaped dinickel(I) complex which is capable of two-electron reductive binding of small molecules. The high reactivity of the Ni(I) metal centers was utilized to achieve the reductive splitting of water supported by metal-metal cooperativity and the mediating role of a alkali metal ion. Additionally, the suited proximity of the coordinated groups within the binding pocket resulted in the formation of an intramolecular dihydrogen bond, which further stabilized the reaction product. This attractive interaction was further investigated by NMR spectroscopy and DFT calculations. Eventually, the reaction mechanism was detailed investigated by computational studies to gain more understanding of the electronic structure and possible reaction pathways. The second part of this thesis deals with the complexation and transformation of the phosphaethynolate (PCO) anion. The salt metathesis reaction of bromide bridged dinickel complex LNi2(μ-Br) with sodium phosphaethynolate (Na[PCO]) leads to the surprisingly stable complex LNi2(μ-PCO) featuring an end-on bridging phosphaketene within the bimetallic pocket. Subsequent reactions with various nucleophiles illustrate a diverse reactivity of the metal-bridging PCO−, either via release of CO or via attack at the phosphaketene-C. The products include a diphospha-urea derivative trapped at the dinickel core. The bonding situation in these unusual compounds has been analyzed with the help of DFT calculations.