Discrete Polynuclear Complexes: from Spin Crossover to Hydrogen Evolution

Abstract

Spin crossover and hydrogen evolution catalysis are two areas of chemical research that address modern society’s demand for nanoscale technologies and renewable energy supplies. One avenue to improving the desired properties of spin crossover materials and hydrogen evolution catalysts alike is the development of discrete polynuclear metal complexes, which provide built-in communication pathways that facilitate cooperativity between the metal centres. This thesis describes the design, synthesis, and characterisation of dinuclear and trinuclear complexes, and the progress made towards tetranuclear complexes for spin crossover and hydrogen evolution applications.

In Chapter 1 a basic overview of spin crossover is given, with particular attention given to the experimental techniques used in this thesis. Next, the potential advantages of discrete polynuclear complexes for spin crossover are discussed, and specific relevant classes from the literature are reviewed. Then, hydrogen evolution catalysis is introduced, with a focus on ligand design aspects of molecular cobalt-based systems. Finally, the ligand systems targeted in this thesis are introduced. In Chapter 2 the first spin crossover-active dinuclear Fe(II) complexes with N4S2 coordination environments are described. The design and synthesis of the PSRT ligands are discussed, along with the structurally characterised [Fe2(PSRT)2](BF4)4 complexes. Spin crossover behaviours are investigated, revealing opposing trends with respect to R group variation seen between the solid state and solution. In Chapter 3 the solution spin crossover behaviours of the [Fe2(PSRT)2](BF4)4 complexes are investigated further by applying high pressures. As no robust method for monitoring spin states in solution at variable pressures is known, an adaption to the Evans’ method is made, opening up its use in high pressure NMR. Application of the new method to the [Fe2(PSRT)2](BF4)4 complexes revealed spin crossover in solution induced by both pressure and temperature. In Chapter 4 the synthesis of new families of bis-bidentate and bis-terdentate 1,2,4-triazole ligands, designed for discrete polynuclear complex formation, is described. While tetranuclear architectures have proven elusive to date, a family of triply stranded dinuclear helicate complexes were prepared and structurally characterised. In Chapter 5 the attempted formation of a [M4L4] square, by self-assembly of a linear pyrazine linker ligand and octahedral metal ions, unexpectedly resulted in [M3L3] triangles. Structural and DFT analyses of the triangles are presented, and compared and contrasted with the anticipated square complexes. DFT correctly predicted the formation of triangles over squares, and also revealed that simple particle counting entropy arguments cannot always be reliably employed to predict the product of self-assembly reactions. In Chapter 6 three dinuclear cobalt complexes of the PSRT ligands, as well as 14 other previously synthesised cobalt complexes from our group, are screened for photocatalytic hydrogen evolution in DMF solution. All 17 complexes are active catalysts, with dinuclear triazole-bridged and mononuclear Schiff base macrocyclic complexes evolving hydrogen the most efficiently, and up to three times more efficiently than a literature cobaloxime standard. Two pyrimidine based complexes are also tested in water, and show encouraging photocatalytic activities.