The goal of this research is to test the design constraints of active dpp-bridged RuII,RhIII (dpp = 2,3-bis(2-pyridyl)pyrazine)) supramolecular photocatalysts for water reduction to H2 and provide mechanistic insights into the catalytic cycle. Two member of a new RuII,RhIII motifs with only one Rh-'Cl bond, [(bpy)2Ru(dpp)RhCl(tpy)](PF6)4 ( bpy = 2,2'-bipyridine, tpy = 2,2':6,2"-terpyridine) and [(bpy)2Ru(dpp)RhCl(tpm)](PF6)4, (tpm = tris(1-pyrazolyl)methane), and a cis-RhCl2 model system, [(bpy)2Ru(dpp)RhCl2(bpy)](PF6)3, were prepared. This new motif was to test whether two Rh-'Cl bonds on RhIII are required for the photocatalytic water reduction. 1H NMR spectroscopic analysis of complexes prepared using deuterated ligands was used to characterize these three RuII,RhIII supramolecular complexes. Electrochemical studies suggested that replacing bpy with a tridentate ligand on RhIII shifts the RhIII/II and RhII/I reduction couples positively, which can modulate the orbital energetics of the RhIII LUMO (lowest-unoccupied molecular orbital). This substitute also changes the rate of ligand dissociation following the reduction of RhIII. In tpm and bpy systems, RhII intermediate is more stable than that in the tpy system. All three complexes were good light absorbers in the visible region and weak emitters from their emissive Ru(dπ)-'dpp(π*) 3MLCT (metal-to-ligand charge transfer) excited states at room temperature. The population of a low-lying 3MMCT (metal-to-metal charge transfer) ES (excited state) from the 3MLCT ES contributed to the weak emission, indicating an important intramolecular electron transfer process from dpp' to RhIII upon photoexcitation. The lower-lying 3MMCT excited state in the tpm and tpy systems relative to the bpy system result in a higher rate constant (ket = 2.6 x 10^7 vs 1.7 x 10^7 s-1) for intramolecular electron transfer. Spectrophotochemical analysis suggested that all three complexes were photoinitiated electron collectors capable of collecting two electrons on the RhIII center to generate the RuII,RhI species in the presence of DMA (N,N-dimethylaniline). The observed H2 production from water using [(bpy)2Ru(dpp)RhCl(tpm)](PF6)4 and [(bpy)2Ru(dpp)RhCl(tpy)](PF6)4 established that two halides on RhIII are not necessary in the dpp-bridge RuII,RhIII supramolecular photocatalytic-water-reduction system. This new discovery opens a new approach to the design of different RuII,RhIII motifs for photocatalysis. The active species for water reduction is proposed to be [(bpy)2Ru(dpp)RhICl(TL)]3+ from [(bpy)2Ru(dpp)RhCl(TL)](PF6)4 (TL (terminal ligand) = tpy or tpm) and [(bpy)2Ru(dpp)Rh(bpy)]3+ from [(bpy)2Ru(dpp)RhCl2(bpy)](PF6)3 respectively. Included here is the design and study of a RuII,RhI complex, [(bpy)2Ru(dpp)RhCl(COD)](PF6)3 (COD =1,5-cyclooctadiene) to provide more insights into the photophysical and photochemical properties of polypyridyl RuII,RhI species. Electrochemical and photophysical studies revealed a dpp-based LUMO in this RuII,RhI complex, suggesting dpp reduction upon photoexcitation. Photochemical study found that [(bpy)2Ru(dpp)RhCl(COD)](PF6)3 is an active photocatalyst for water reduction and that additional reduction(s) is (are) required after the generation of the RuII,RhI active species in the RuII,RhIII supramolecular photocatalytic H2 production system. This hypothesis was supported by the electrocatalytic behaviors of the RuII,RhIII supramolecular complexes for proton reduction. Cyclic voltammetry results in the presence of an acid suggested that the protonolysis of the RuII,RhIIH and RuII,RhIH species are electrocatalytic H2-evolution pathways. The mechanism is acid-dependent and influenced by terminal ligand. The studies of electrocatalytic proton reduction on these RuII,RhIII complexes suggested several possible intermediates involved in the photocatalytic water reduction cycle. The insights gained from this research can provide guidance in designing new type of RuII,RhIII and RuII,RhI complexes with better photocatalytic and/or electrocatalytic H2 production performance. / Ph. D.

Catalysis is a key area of chemistry. Through catalysis it is possible to achieve better synthetic routes, exploit molecules normally considered to be inactive and also attain novel chemical transformations. The development of new catalysts is crucial to furthering chemistry as a field. Computational chemistry, arising from applying the equations of quantum and classical mechanics to solving chemical problems, offers an essential route to investigating the underlying atomistic detail of catalysis. In this thesis calculations have been applied towards studying a number of different catalytic processes. The processing of renewable chemical sources via homogeneous reactions, specifically cardanol from cashew nuts, is discussed. All routes examined for monoreduction of a diene model by [Ru(H)(iPrOH)(Cl)(C₆H₆)] and [Ru(H)(iPrOH)(C₆H₆)]⁺ are energetically costly and would allow for total reduction of the diene if they were operating. While this accounts for the need of high temperatures, further work is required to elucidate the true mechanism of this small but surprisingly complex system. Gold-mediated protodecarboxylation was examined in tandem with experiment to find the subtle steric and electronic effects that dictate CO₂ extrusion from gold N-heterocyclic carbene activated benzene-derived carboxylic acids. The origin of a switch in the rate limiting step from decarboxylation to protodeauration with less activated substrates was also clearly demonstrated. Studies of gold systems are closed with examinations of 1,2-difluorobenzene C–H activation and CO₂ insertion by [Au(IPr)(OH)]. Calculations highlight that the proposed mechanism for oxazole-derived substrates cannot be extended to 1,2-difluorobenzene and instead a digold complex offers more congruent predicted kinetics. The lens of quantum chemistry was turned upon palladium-mediated methoxycarbonylation reactions. An extensive study was undertaken to attempt to understand the bidentate diphosphine ligand dependency on forming either methylpropanoate (MePro) or copolymers. Mechanisms currently suggested in literature are shown to be incongruous with the formation of MePro by Pd(OAc)₂ and bulky diphosphines. A possible alternative route is proposed in this thesis. Four mechanisms for methoxycarbonylation with Pd(2-PyPPh₂)ₙ are detailed. The most accessible route is found to be congruent with experimental reports of selectivity, acid dependency and slight steric modifications. A modification of 2-PyPPh₂ to 2-(4-NMe₂-6-Me)PyPPh₂ is shown to improve both selectivity and turnover, the latter by four orders of magnitude (highest transition state from 22.9 kcal/mol to 16.7 kcal/mol ∆G), and this new second generation in silico designed ligand is studied for its applicability to wider substrate scope and different solvents. The final chapter of this thesis is a mixed quantum mechanics and molecular mechanics (QM/MM) examination of an enzymatic reaction, discussing the need for certain conditions and the role of particular amino acid residues in an S[sub]N2 hydrolysis reaction.