Preparation of Organorhodium Complexes for Water Activation

Abstract

Solar-driven water-splitting has the potential to revolutionize the global energy landscape and could usher in an unprecedented era of socio-economic equality currently unattainable by disproportionate distribution of fossil fuels worldwide. To drive hydrogen and oxygen formation from water, a suitable catalyst must be realized. To this end, we proposed a water-splitting cycle driven by an organorhodium catalyst that invoked a metal-hydroxy-hydride as the active intermediate. This thesis looked to expand upon prior learnings regarding this complex cycle by screening several ancillary ligands with the goal of identifying scaffolds capable of facilitating the desired reactivity. In contrast to the wealth of bond activation chemistry developed for iridium, there are fewer reports describing well-defined rhodium complexes capable of analogous transformations. Rhodium’s propensity to form weaker bonds, which should favour catalytically relevant processes, prompted us to explore Rh(I)/Rh(III) redox couples to observe Rh hydroxy-hydrides. Initial attempts with the monoanionic bidentate, β-diketiminato (nacnac) framework led to rapid degradation both in protic media and in the presence of in situ generated Rh-H moieties. Thus, tridentate 2,6-bis(imino)benzene (NCN) pincers were explored. The resultant NCN-rhodium(III) complexes were treated with various hydride and hydroxide sources to yield the desired hydroxy-hydride species. However, the unsaturated and electrophilic imine arms were susceptible to nucleophilic attack, ultimately affording a number of inseparable and unidentifiable products. Thus, rhodium complexes supported by 2,6-bis(di-tert-butylphosphinomethyl)benzene (PCP) pincer were examined. Generation of an active 14-electron PCP-Rh precursor, did show activity towards O-H bonds of alcohols and water, but the organometallic framework lacked sufficient electron density to stabilize Rh(III) oxidative addition products relative to square planar Rh(I) coordination complexes. Transient interactions between cationic PCP-Rh complexes and water were observed, though ligand lability precluded isolation and characterization. To encourage the formation of Rh(III) species, modification of the PCP architecture was made through inclusion of a para-methoxy substitution (MeO-PCP). It was rationalized that the increased electron density should aid oxidative addition of this unique substrate. Indeed, water O-H bond activation by MeO-PCP-Rh(I) complexes was observed to be facile at under mild conditions, representing a rare example of this reactivity. The MeO-PCP-Rh(OH)(H) was characterized by IR and multinuclear NMR spectroscopy.