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Abstract
The aqueous coordination chemistry of dihydrogen has potential applications in inorganic-biomimicry, regioselective hydrogenation catalysis, and green-chemistry. However, experimental and mechanistic studies that explore the aqueous behavior of these complexes are virtually unknown.
This dissertation outlines the general synthesis and reactivity of a new class of η2-H2 coordination complexes that are water-soluble and, perhaps more importantly, inert to substitution by water. Chapter I identifies the motivation for studying aqueous dihydrogen coordination complexes by providing a literature review of reactions that contain a dihydrogen ligand in highly polar solvents. Chapter II outlines the synthesis and reactivity of precursors to aqueous dihydrogen complexes that contain a water-solubilizing phosphine ligand in concert with a group 8 transition-metal.
Chapter III chronicles the preparation of group 8 complexes of the type trans[M(P2)2(H2)H]+ (P2 = a water-solubilizing, chelating phosphine ligand) in aqueous and non-aqueous solvents. The lability of the η2-H 2 ligand was modulated by ancillary phosphine ligands. The reactivity of these complexes toward ligand substitution reactions with olefins, nitriles, and N2 was investigated. Using H2 as the reductant, the trans-[Fe(DMeOPrPE)2(H2)H] + complex was exploited as a synthon in an N2 fixation scheme, producing modest yields (ca. 15%) of ammonia. Additionally, other ligand architectures such as piano-stool complexes containing Tp and Cp co-ligands were investigated as potential water-soluble η2-H 2 coordination complexes. While the participation of an η 2-H2 ligand in intermolecular hydrogen bonding in solution had long been an unresolved issue, our aqueous H2 complexes provided an ideal system to study their presence.
Chapter IV focuses on exploring the existence of dihydrogen hydrogen-bonding (DHHB). This was achieved by monitoring the chemical shift of H-bonded Ru-(H 2) complexes using NMR spectroscopy, by UV-visible spectroscopy, and by monitoring the rotational dynamics of an H-bonding probe molecule. The results of this study provide evidence that coordinated H2 can indeed participate in intermolecular hydrogen bonding to bulk solvent and other charge-neutral H-bond acceptors. Finally, Chapter V describes the rotational dynamics methodology developed for the detection of hydrogen-bonding interactions in solution using 2H NMR spectroscopy and Chapter VI provides a summary.
This dissertation includes previously published and unpublished co-authored material.
