Towards Trigonal Prismatic Hexanuclear Copper Complexes for Catalytic Water Oxidation

Our planet is running out of energy resources and traditional renewables are not easily transported nor will be sufficient to cover the void left by fossil fuels. Therefore, a new energy storage system needs to be adopted. The medium with highest energy density that is readily available is hydrogen from water. However, to access hydrogen, water needs to be oxidized and water oxidation catalysts (WOCs) will be required for widespread adoption. In pursuit of WOCs, several hexauclear copper complexes were synthesized and characterized, including the isolation and characterization of a mixed-valent Cu6 (formally, CuII5CuIII) complex. Furthermore, early in the research, several novel coordination polymers (CPs) were serendipitously synthesized. This dissertation is divided into three major parts: (i) A series of trigonal prismatic Cu6-pyrazolato complexes (Cu6) form a unique arrangement of two O2- ions in a favorable position to form an O-O bond without much external influence. The Cu6 complexes undergo two reversible one-electron oxidations and a structurally characterized one-electron oxidized mixed-valent complex displays an O···O distance ~0.3 Å closer than the homovalent compound, which highlights the propensity of the system towards O-O bond formation. Spectroscopic findings and DFT calculations on the electronic structure of the mixed-valent Cu6-complex are reported. The results support the hypothesis that a variation of a Cu6 motif could function as a water oxidation catalyst. (ii) Three new coordination polymers containing trinuclear Cu(II)-4-formyl pyrazolato units connected by formyl group coordination were prepared and the crystal structures are reported: a 1D polymer formed by linking secondary building units (SBUs) and two 3D polymers with novel topologies consist of 14-nodal 3842,54-c and 3-nodal 3,3,4-c nets, respectively. (iii) Two new motifs of Cu6-pyrazolato complexes were prepared. One motif has two pyrazolato bridges between the trinuclear copper subunits with a µ4-Cl situated inside the open cage, and the other has the subunits bridged by a single pyrazolato ligand. Variable low-temperature NMR and magnetic studies were conducted. And for the first time, far-infrared spectroscopy studies were used to determine the characteristic absorptions of various coordination bonds in copper pyrazolate complexes