A Soft Chemistry Approach to Extended [Co6Se8] Materials

This dissertation describes advances toward creating programmable building blocks and assembling them into new, tailored materials. I describe extended materials formed of bonded sets of cobalt selenide clusters. Rationally designed chemical transformations that form these sets give us precise control over the extent and dimensionality of the material. The cobalt selenide clusters fundamental to this study are members of a larger class of clusters with the M6E8 core (M = metal, E = chalcogen). Chapter 1 introduces this family of clusters and reviews examples of previously made materials. Chapter 2 unveils a family of site-differentiated clusters, Co6Se8(CO)x(PEt3)6-x, their substitution reactions, and assembly into bridged dimers and trimers. Electrochemical methods were used to investigate electronic coupling between the cores, by comparing the electrochemical behavior of the dimer and trimer relative to their monomeric counterparts. We further performed magnetic susceptibility measurements of the monomers and assemblies. Chapter 3 introduces electrocrystallization as a method to synthesize extended, crystalline, solid state compounds from superatomic building blocks. By electrocrystallizing redox-active [Co6Se8] clusters with labile ligands in the presence of an ionic template, we created a crystalline polymeric material that exhibits weaving at the nanoscale. Chapter 4 presents a metal coordination approach to [Co6Se8] materials via reactive groups on the capping ligands. The redox-activity and multinuclearity of the superatom components creates a new level of complexity and synthetic sophistication to previously reported frameworks. In collaboration with Prof. Christopher Bejger (University of North Carolina at Charlotte), I installed carboxylate groups on the surface of the cluster. With this building block in hand and a simple metal salt, Zn(NO3)2, we discovered two sets of distinct solvothermal reaction conditions that yielded two different solids. Both are homogenous, crystalline, porous solids whose dimensionality is tuned by subtle changes in reaction conditions. I further showed that the dimensionality could be further reduced by chemical exfoliation to yield free-floating sheets of zinc-coordinated clusters in which the porosity and redox-activity of the bulk solid is preserved. Finally, Chapter 5 outlines a novel chemical transformation that dimerizes [Co6Se8] units to form a material with an expanded core, [Co12Se16], that exhibits electronic and optical properties distinct from the parent monomer. To accomplish this dimerization, I installed a reactive carbene on the [Co6Se8] core to create a latent fusion site. We show by cyclic voltammetry, infrared spectroscopy, single crystal X-ray diffraction, and density functional theory calculations that the resulting fused [Co12Se16] material exhibits strong electronic coupling and electron delocalization. These chapters present novel synthetic approaches toward creating [Co6Se8] materials with tuned dimensionality, size, and extensive charge delocalization.