Structure and Reactivity Studies of Bis(pyridylimino)isoindolate Transition Metal Complexes.

Abstract

In this dissertation, transition metal complexes supported by bpi-type (bpi = 1,3-bis(2’-pyridylimino)isoindolate) ligands were designed and synthesized to study their activity, selectivity, and stability in hydrogenation and dehydrogenation reactions and to determine the role of the bpi ligand in these transformations. A new family of ruthenium bpi complexes capable of catalyzing promoterless and chemoselective dehydrogenation of alcohols and amines with liberation of dihydrogen were developed. In particular, the bis(phosphine) ruthenium bMepi hydride (bMepi = 1,3-bis(6’-methyl-2’-pyridylimino)isoindolate) system mediates dehydrogenation of secondary alcohols to ketones, dehydrogenative coupling of primary alcohols to esters, and double dehydrogenation of primary amines to nitriles with high conversion efficiency. An unusual feature of this catalyst system is the high selectivity for secondary alcohol dehydrogenation in the presence of primary alcohols. By avoiding the use of hazardous reagents and harsh oxidants, these dehydrogenative transformations provide environmentally benign methodologies for fine and commodity chemical synthesis with high atom economy. Furthermore, to understand the relationship between catalyst structure and reactivity, the catalytic mechanism of acceptorless alcohol dehydrogenation was elucidated by a series of kinetic and isotopic labeling studies, isolation of intermediates, and evaluation of new ligand variants. The new chemical knowledge acquired in the mechanistic investigation was applied to conceptualize and develop three new projects: (1) iron bMepi systems that feature control over catalytic alkene hydroboration activity and regioselectivity by remote site modifications, (2) ruthenium bpi complexes capable of upgrading ethanol to 1-butanol with state-of-the-art activity (53% conversion and 265 turnovers per hour), and (3) a new series of multifunctional ruthenium complexes with appended Lewis acidic borane sites for studying how Lewis acidity influences the reactivity of the ruthenium hydride moiety and biases the system for stereoselective semi-hydrogenation of alkynes. Collectively, the studies presented in this dissertation demonstrate the new development of highly active and chemoselective catalysts capable of promoting challenging dehydrogenation reactions and showcase how precise structural, electronic, and cooperative interactions in the secondary coordination environment can be used to regulate metal-based catalysis.