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This thesis focuses on the synthesis, characterization and reactivity of group VII transition metal complexes. It begins with exploring a new pincer geometry of Re(I) compounds and then examining both Re(I) and Mn(I) compound as homogenous catalysts for photocatalytic and electrocatalytic reduction of CO2. In the first chapter, I focus on some recently reported approaches to photocatalytic and electrocatalytic reduction of CO2 using homogenous catalysts of transition metal. The second chapter presents efforts to capture Re(I) in a neutral N,N,N pincer scaffold and the resulting enhanced absorption of visible light. Most of these results have appeared in a publication. In this thesis, I only present my work on rhenium compounds that are supported by the bis(imino)pyridine ligand and an examination of the differences in properties between the bidentate and tridentate ligand geometries. Later I examine both tridentate and bidentate complexes for the photocatalytic and electrocatalytic reduction of CO2 to CO. The failure of tridentate Re1 bis(imino)pyridine compounds to reduce CO2 to CO prompted a change in direction to rhenium compounds that are supported with diimine ligands. Thus, I choose 4,5-diazafluoren-9-one as supporting ligand for rhenium and manganese. This chapter explained the reasons behind choosing these particular ligand and metal combinations. ReI and Mn1 compounds of 4,5-diazafluoren-9-one have shown activity for the photocatalytic and electrocatalytic reduction of CO2 to CO. In the fourth chapter, as rhenium and manganese compounds of 4,5-diazafluoren-9-one have shown the great ability of CO2 reduction to CO, the focus here was to modify the ligand by attaching a photosensitizer to the ligand in order to prepare supramolecular complexes that may increase the efficiency and yield of reduction products. In this chapter, I examined two types of the photosensitizer; tris(bipyridine)ruthenium(II)chloride and osmium dichloro bis(4,​4'-​dimethyl-​2,​2'-​bipyridine).

Ph.D. (Chemistry) / Please refer to full text to view abstract

Computational techniques were employed to investigate pathways that would improve the properties and characteristics of transition metal (i.e., ruthenium) catalysts, and to explore their mechanisms. The studied catalytic pathways are particularly relevant to catalytic hydroarylation of olefins. These processes involved the +2 to +3 oxidation of ruthenium and its effect on ruthenium-carbon bond strengths, carbon-hydrogen bond activation by 1,2-addition/reductive elimination pathways appropriate to catalytic hydrogen/deuterium exchange, and the possible intermediacy of highly coordinatively unsaturated (e.g., 14-electron) ruthenium complexes in catalysis. The calculations indicate a significant decrease in the Ru-CH3 homolytic bond dissociation enthalpy for the oxidation of TpRu(CO)(NCMe)(Me) to its RuIII cation through both reactant destabilization and product stabilization. This oxidation can thus lead to the olefin polymerization observed by Gunnoe and coworkers, since weak RuIII-C bonds would afford quick access to alkyl radical species. Calculations support the experimental proposal of a mechanism for catalytic hydrogen/deuterium exchange by a RuII-OH catalyst. Furthermore, calculational investigations reveal a probable pathway for the activation of C-H bonds that involves phosphine loss, 1,2-addition to the Ru-OH bond and then reversal of these steps with deuterium to incorporate it into the substrate. The presented results offer the indication for the net addition of aromatic C-H bonds across a RuII-OH bond in a process that although thermodynamically unfavorable is kinetically accessible. Calculations support experimental proposals as to the possibility of binding of weakly coordinating ligands such as dinitrogen, methylene chloride and fluorobenzene to the "14-electron" complex [(PCP)Ru(CO)]+ in preference to the formation of agostic Ru-H-C interactions. Reactions of [(PCP)Ru(CO)(1-ClCH2Cl)][BAr'4] with N2CHPh or phenylacetylene yielded conversions that are exothermic to both terminal carbenes and vinylidenes, respectively, and then bridging isomers of these by C-C bond formation resulting from insertion into the Ru-Cipso bond of the phenyl ring of PCP. The QM/MM and DFT calculations on full complexes [(PCP)(CO)Ru=(C)0,1=CHPh]+ and on small models [(PCP')(CO)Ru=(C)0,1=CH2]+, respectively, offered data supportive of the thermodynamic feasibility of the suggested experimental mechanisms and their proposed intermediates.