Development of urea-based pyrazolate -bridging dinucleating ligands containing hydrogen-bond donors and their mononucleating analogue: Synthesis, structure, and reactivity of the corresponding cobalt and iron complexes

Hydrogen bonds formed within a secondary coordination sphere can effectively control metal-mediated processes. This control is typified by their presence at the active sites of metalloproteins, in which hydrogen bonds are proposed to serve functions in catalytic cycles such as substrate orientation, delivery of reactants, and stabilization of reactive intermediates. Exploiting such non-covalent, intramolecular interactions in synthetic systems would be useful in establishing structure-function relationships that are necessary for the development of new chemical reagents.;In the interest of the effect of hydrogen bond interactions on bimetallic reactivity, this dissertation describes the design and preparation of new pyrazolate-based multidentate, dinucleating ligands (H 9PRbuam; 3,5-bis{bis[( N'-R-ureayl)-N-ethyl]-aminomethyl}-1H-pyrazole; R = tBu or iPr) containing urea-derived chelates that provide rigid frameworks with hydrogen bond donors. Upon insertion of two metal ions, the pyrazolate-bridging ligands enforce trigonal monopyramidal coordination geometry, allowing a fifth open coordination site per metal ion for binding external ligands. In addition, the urea-derived chelates position the hydrogen bond donors proximal to the metal centers, permitting formation of intramolecular hydrogen bonds with exogenous ligands. Also, the design and preparation of a mononucleating analogue (H9Pt Buuam; 3-(bis[(N'-tert-butylureayl)- N-ethyl]aminomethyl)-5-tert-butyl-1H-pyrazole) is discussed and is shown to produce mononuclear complexes with similar structural properties to its dinuclear analogues.;The synthesis, spectroscopy, and structure of cobalt(II) dimers utilizing H9PRbuam where chloride, hydroxide, acetamidate, and acetate are the exogenous bridging ligands are demonstrated. It is shown that the micro-hydroxo complexes are capable of hydrating unactivated nitriles and hydrolyzing unactivated esters at room temperature in a stoichiometric fashion. Comparing reactivity of the dinuclear micro-hydroxo compounds and their mononuclear analogue, which contains a terminal hydroxo ligand, aids in elucidation of a two-metal mediated hydration/hydrolysis mechanism.;In addition, the synthesis and structure of H9P Rbuam derived diferrous complexes with acetate and end-to-end azide secondary bridges are described. Preliminary results on the reactivity of Fe(II) dimers with H9PRbuam with dioxygen and O-atom transfer reagents indicate possible isolation of diferric products that contain terminal hydroxo and oxo ligands derived directly from the activation of dioxygen. Low-temperature oxygenation reactions were analyzed by UV-vis, EPR, and Mossbauer spectroscopies in the attempt to observe reactive intermediates in dioxygen activation.