Proton-Coupled Electron Transfer at Nickel Pincer Complexes

Abstract

The influence of metal coordination on carbon-centered proton-coupled electron transfer processes is an attractive field of research. Reported literature is limited to thermodynamic investigations and substrate oxidation using C-H bond formation at metal complexes is not reported. Comparison of organic HAT processes with related processes in coordination compounds suggests similar linear free energy relationships for N-H and O-H bond dissociation/formation. This thesis reports the synthesis of nickel pincer complexes containing a C-basic site and their redox and protonation reactivity. Oxidation to the formal NiIII oxidation state results in a pincer ligand centered oxidation and investigation of the electronic structure is performed by UV-vis, EPR, XANES and EXAFS spectroscopy, as well as cyclic voltammetry, crystallographic analysis and TD-DFT computation. Experimental measurement of the free energy of oxidation and protonation allows for the determination of C-H bond strength involving the NiIII/NiII and NiII/NiI redox couple. The formation of a strong C-H bond upon reduction from NiIII to NiII allows for benzylic C-H bond activation. Kinetic analysis of the oxidation of 9,10-dihydroanthracene reveals concerted proton-electron transfer at a comparably high rate considering a carbon centered C-H bond formation process. Measurement of the hydrogen atom transfer self-exchange rate reveals an unprecedentedly fast carbon centered self-exchange process and good agreement of the experimental rate for DHA oxidation with the value predicted by the Marcus cross relation. Reduction results in a severe weakening of the ligand centered C-H bond which results in H2 liberation. Changing the coordination geometry from square planar to T-shaped is shown to result in a significant strengthening of the C-H bond on this oxidation state. Carbon dioxide reduction at molecular complexes is a highly competitive field of research and CO selective CO2 reduction is of great interest due to the importance of carbon monoxide as intermediate towards further reduced products and as industrial C1 building block. While (photo-)electrochemical approaches using sacrificial electron and proton donors exist, the photochemically driven reverse water-gas shift (rWGS) reaction at molecular complexes is largely unexplored and presents an attractive alternative. Within this thesis a synthetic approach to the photodriven rWGS is presented using molecular nickel pincer complexes. The abnormal CO2 insertion into a metal hydrogen bond as selectivity determining step is a key challenge in this transformation. Using photochemically active PNP nickel hydride complexes, this previously unprecedented reactivity is observed. Mechanistic investigation of the photochemical CO2 activation is performed by transient and fluorescence spectroscopy, kinetic measurements, computational analysis, labeling and trapping experiments. Upon photoexcitation a redshifted nickel hydride stretching vibration is observed, suggesting weakening of the Ni-H bond due to population of a metal centered excited state. Prior to CO2 activation, a Ni0 photoproduct is identified as intermediate which is formed by intramolecular proton transfer of the former hydride to the pincer ligand introducing metal-ligand cooperativity as control tactic for excited-state reactivity. The photochemical reactivity can be transferred to olefins and aldehydes, allowing for photocatalytic hydrogenation using thermally inactive nickel hydride complexes.