Zirconium amidoboranes and aqueous carbene complexes

Abstract

The target of the research described in this thesis covered two areas: the investigation of the ligand properties of amidoboranes, and the synthesis of water soluble N-heterocyclic carbene (NHCs) complexes. Ammonia borane (AB) has shown great potential as a hydrogen storage medium as it contains 19.6 wt% hydrogen, which can be easily released. Extensive studies of metal catalyzed dehydrogenation of AB have been conducted, however, transition metal complexes containing unsubstituted AB and its derivatives have not been isolated. To better understand the nature of the transition metal catalyzed decomposition of AB, I sought to synthesize metal complexes featuring the amidoborane ligand, NH2BH3-. Several zirconocene amidoboranes complexes were isolated featuring an amidoborane ligand coordinated to a metal center in a bidentate fashion through a dative nitrogen zirconium bond and a ??-B-agostic B-H-Zr interaction. This gave rise to a pair of agostic isomers, which differed with respect to the orientation of the amidoborane ligand. Investigation into the reactivity of these complexes showed that they behaved much differently than their isoelectronic ethyl analogue. NHCs metal complexes are a highly successful class of compounds which have been applied to a very diverse array of catalytic processes. Most of these processes require organic solvents and produce large quantities of waste. The appendage of ionic moieties to the NHC framework to increase its solubility in aqueous media has been explored. The synthesis of metal complexes feature repo11ed NHCs with N-aryl substituted substituents was investigated m order to better understand their catalytic properties in an aqueous environment. Silver NHC complexes are well known transmetallation reagents and thus silver complexes of sulfonated NH Cs were the first compounds synthesized. These novel silver complexes were found to be extraordinarily stable, in tum making them poor transmetallation reagents. An alternate route to metal complexes of sulfonated NHCs was pursued through the synthesis of the free carbene, which, while not isolable, could be produced and was stable in wet dimethylsulfoxide. Solutions containing the free carbene were used to synthesize copper, gold, and rhodium complexes of sulfonated NH Cs.