Secondary Coordination Sphere Contributions to Primary Sphere Structure, Bonding and Reactivity.

Abstract

Metalloenzymes are unique in their ability to mediate the transformations of relatively inert substrates with impressive selectivity and speed. Secondary interactions such as hydrogen bonding are broadly appreciated to promote enhanced reactivity in metalloenzymes and are proposed to operate through one of several modes of action; including binding and orienting substrates and stabilizing transition states. In this thesis, transition metal constructs were designed and synthesized to study how secondary coordination sphere perturbations can broadly affect primary sphere structure, bonding and reactivity changes in synthetic systems. Metal complexes supported by a tripodal ligand (tris[quinolyl-2-methyl-7-(morpholinomethanone)]amine, TQA') featuring hydrogen bond acceptor amide groups far removed from the metal center provided a unique example of hydrogen bonding with CH groups. Moreover, these hydrogen bonding interactions to the non-traditional acceptor were shown to influence the primary coordination sphere geometry. In separate studies, a proton-responsive tripodal ligand containing pendent hydroxyl groups (tris(6-hydroxypyrid-2-ylmethyl)amine, H3thpa) was shown to alter coordination geometry and stability of copper halide complexes. Electronic perturbation at the metal center was shown to affect the hydrogen bonding manifold in the secondary coordination sphere of copper chloride complexes through combined crystallographic and computational studies. Copper fluoride complexes supported by H3thpa were investigated and the network of hydrogen bonding interactions presented by the ligand scaffold enabled the isolation of a rare copper(I) fluoride adduct featuring a tripodal ligand. The ability of metal-H3thpa constructs to deliver proton/electron equivalents to a reducible substrate was also demonstrated by exposing a nitrite reduction pathway that is reminiscent of the metalloenzyme copper nitrite reductase. In a final set of studies, transition metal complexes supported by a pincer ligand framework containing tautomerizable hydroxyl groups (6,6'-dihydroxy terpyridine, dhtp) were investigated for catalytic heterolysis reactivity. Ruthenium complexes were studied in detail for catalytic ketone transfer hydrogenation. The role of the pendent hydroxyl groups during catalysis was elucidated and a catalytic cycle is proposed in which alkali metal cations engage substrate to achieve reduction. Together, the studies presented in this thesis demonstrate how secondary coordination sphere interactions can be used to stabilize otherwise reactive products and/or transition states, in analogy to metalloenzymes.