Fostering Unsaturated Metal Centers with m-Terphenyl Isocyanide Ligands
- Neville, Michael
- Advisor(s): Figueroa, Joshua S
This dissertation describes the use of m-terphenyl isocyanides as a platform for isolating unsaturated metal centers in molecular and material systems. The first two chapters explore the electronic structure of homoleptic, unsaturated, metalates as well as their redox and reactivity profiles. Metalate complexes – those that possess metal centers in formally negative oxidation states – are typically coordinatively and electronically saturated organometallic species. While the chemistry of such complexes is extensive, there are very few examples were coordinative or electronic unsaturation accompanies highly reduced d-orbital manifolds as means to augment reactivity. Reported here is the isolation of the 16e–, d10 group 9 metalate anions, A[M(CNArDipp2)3], (A = Na, K; M = Co, Rh, Ir), where coordinative unsaturation is achieved through the use encumbering m-terphenyl isocyanide ligands. The cobalt metalate displays a high degree of Lewis-acidity, featuring an agostic interaction in the solid state, and binding N2 when exposed to an N2 atmosphere. The heavier rhodium and iridium metalates display an unpredicted distortion to a Y-shaped, roughly C2v symmetric geometry possible promoting electronic accessibility of the dx2-y2 as well as the dz2 orbital. Both the Rh and Ir derivatives are shown to support up to two, reverse dative bonding (Z-type) interactions and perform up to four electron swings in oxidation state with substrates like PCl3, forming complexes of the formulation MCl(PCl2)2(CNArDipp2)2 (M= Rh, Ir). While attempts to achieve similar chemistry with the cobalt metalates were unsuccessful, the electrochemistry of this series of group 9 metalates offer insights as to why. Cyclic voltammetry of the Co metalate reveals two reversible one electron features and one further irreversible oxidation. In contrast, both Rh and Ir congeners display one initial irreversible oxidation, highlighting the stark differences in their oxidative chemistry from formal M(1-) oxidation states. The final portion of this thesis takes lessons learned from molecular work with m-terphenyl isocyanides and applies them toward synthesizing MOFs with well-defined, coordinatively unsaturated metal nodes. Balancing the need for structural metal coordination and coordinative unsaturation for inner sphere substrate interactions in a difficult challenge in extended material systems such as metal organic frameworks (MOF). While methods to form unsaturated metal centers like defect engineering can generate some unsaturated sites, the lack of well-defined reactive metal centers makes these systems difficult to study and control. New multi-topic ligands are synthesized using the bulky, CNArDipp2 and are applied to form a new CuI based MOF and the first well-defined MOF with coordinatively unsaturated Pd0 metal nodes. Additionally, the redox properties of these large, conjugated linker ligands are described. The largest of these multitopic linkers show multiple reversible redox features that align well with DFT calculated LUMO energies.