Applications of Sterically Protected Hydrogen Bond Donors in the Secondary Coordination Sphere

Abstract

Hydrogen bonding interactions found within the secondary coordination sphere of metalloenzyme active sites are involved in regulating substrate binding, stabilizing reactive intermediates and facilitating proton and electron transfer reactions. Synthetic systems that mimic these interactions can be used to gain further insight into metalloenzyme mechanism and expand their utility. In this thesis, metal-ligand complexes incorporating secondary sphere hydrogen bond donors were prepared to study their influence on metal structure and reactivity in systems derived from metalloenzymes. The pincer ligand 6,6”-bis(2,4,6- trimethylphenylamino)terpyridine (H2TpyNMes) was prepared and incorporates sterically bulky H-bond donor groups in the secondary sphere. Inspired by [Fe]-hydrogenase, homogeneous ruthenium catalysts supported by H2TpyNMes show enhanced reactivity for transfer hydrogenation reactions and enhanced stability for the dehydrogenative oxidation of primary alcohols compared to unsubstituted catalysts. The planar binding and secondary hydrogen bond donors in H2TpyNMes were also used enforce an usual square planar geometry on copper(I). The geometric stabilization of complex Cu(H2TpyNMes)Cl by hydrogen bonding to the chloride ligand allows for fast electron transfer self-exchange rates reminiscent of blue copper proteins. In separate studies, a series of copper and zinc complexes bearing 6-substituted tris(2-pyridylmethyl)amine (LR) ligands appended with NH(p-R-C6H4) groups (R=H, CF3, OMe, NMe2) were prepared. The LR ligands are electronically tunable in addition to providing sterically protected H-bond donors in the secondary sphere. Hydrogen bonding to the chloride ligand affords C3-symmetric Cu(LR)Cl complexes that exhibit a reversible CuI/II redox event based on electronic character (ΔE1/2 = 160 mV) and CuI(LR)+ complexes react with oxygen to form hydrogen bonded (trans-1,2- peroxo)dicopper complexes. The additional stabilization provided by the H-bond donors allows for the stabilization of the first (trans-1,2-peroxo)dizinc complexes. Collectively, these studies demonstrate the role of secondary sphere hydrogen bonding interactions in the manipulation of metal complex structure, stability, and reactivity.