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).

Transition metals are known to be the essential part in most of the catalysts, the heterogeneous and the homogenous catalysts; however, the ligands that attached to the metal centers can also alter the reactivity of the catalyst, and that is widely observed in nature. In our project, we are interested in the metal-ligand cooperation of a special type of ligand called the pincer ligand. Our focus is mainly on the tridentate Pincer Ligands with a pyridine backbone. Also, it contains a spacer that could be deprotonated and protonated during the aromatization and dearomatization process. Aromatization and dearomatization of the pincer ligand are responsible for the unique reactivity of the pincer complexes, especially in the hydrogenation and dehydrogenation reactions. Recently, huge developments have been made in the dehydrogenative coupling of aniline and benzyl alcohol via manganese pincer complexes. The most recent papers on that subject have been done by Beller in 2016[1], Kempe 2018 [2], and Hultzsch 2019 [3]. However, rhenium complexes have not been studied enough even though it is in the same seventh row of the transition metal. Therefore, the rhenium was studied as a possible alternative. Then, the synthesis of a well-defined PN3 rhenium complex was performed from the bipy-tBu ligand and the metal precursor Re(CO)5Cl. The ligand has a unique deformity as the phosphine sidearm is not attached to the metal center. Further investigation of the aniline and benzyl alcohol dehydrogenative coupling via PN3 rhenium pincer complex has been done. An optimal reaction condition was achieved, and the substrate scope was further examined with various alcohols and amines, and the result shows good to moderate conversion with decent selectivity towards the imine. Except for the secondary alcohols.

Rhenium oxides as well as other rhenium compounds are known to display a broad spectrum of catalytic activity,^25 being resistant to attack by acids under nonoxidizing conditions, and showing a remarkable ability to resist poisoning which is a serious limiting factor in many commonly used catalysts. Rhenium catalysts are cheaper than the platinum metals with the exception of palladium. One of the most remarkable properties of the rhenium catalyst is the ability to catalyze the hydrogenation of carboxylic acids and amides to the corresponding alcohols and amines respectively. The carboxyl group (-COOH) is notoriously difficult to reduce catalytically and most of the commonly used catalysts are ineffective in catalyzing the hydrogenation of carboxylic acids. Either hydrogenolysis of the carbon-oxygen bond occurs or the carboxylic acid remains inert. It is for this reason that low molecular weight carboxylic acids are often used as inert solvents in catalytic hydrogenations. This investigation was undertaken to study the effects of various parameters upon product formation in the hydrogenation of carboxylic acids using the catalyst formed from the reduction of rhenium heptoxide i situ. Acetic acid was used throughout the investigation as the representative carboxylic acid. The parameters studied were pressures ranging from 2000 to 3000 psig, temperatures ranging from 115° to 175°, reaction time varied over the range of 0 to 24 hours, the effects on product formation from repeated use of the catalyst, and agitation of reactants during the reaction with emphasis on the optimization of these parameters. In several series of reactions repeated reuse of the catalyst showed that rhenium catalyst can be reused at least 8 times while still achieving greater than 50 per cent reduction. Relative decrease in catalyst activity appears to be greater when temperature and pressure of the hydrogenation are high compared to the range. Experimental data shows that the concentration of ethyl acetate in the product mixture as a function of time passes through a maximum between 1.0 and 1.5 hours. Beyond this maximum there is a relatively rapid decrease in ester concentration. A change in initial hydrogen pressure from 2000 to 3000 psig does not appreciably change the product composition in the reaction mixture while a modest increase, i.e., a temperature increase of 25°, was observed to improve yields of the alcohol by a very significant amount. Moreover, the investigation shows that for a reaction time of 5 hours the "optimum" conditions for reaction are a temperature of 175°, a catalyst to substrate ratio of 1.0 g Re_2O_7/50 g AcOH, and an initial hydrogen pressure of 3000 psig while reactions carried out for only 1.5 hours require a catalyst to substrate ratio of from 1.0 to 2.5 g Re_2O_7/50 g AcOH to effect a quantitative reduction optimally. Whether or not the reaction system was agitated during the warm-up period seemed to have little effect on the composition of the product mixture, however, it was observed that partial reduction takes place during the initial heating period before agitation of the reactor begins. The results of this work have shown that "optimum" conditions for quantitative reduction of acetic acid to the corresponding alcohol are the mildest yet reported and clearly confirm the superiority of the rhenium catalyst over other catalysts in the hydrogenation of carboxylic acids.

<p> A brief review of the rhenium-chlorine system is presented and a method to determine rhenium:chlorine atom ratios by neutron activation analysis is described. An infrared cell which is useful for highly re-active vapours at temperatures up to 400°C is also described. The compound claimed to be rhenium hexachloride is shown to be rhenium oxytetrachloride and a reliable preparation of β-rhenium tetrachloride is discovered. The hexachlororhenate(V)ion and several complexes containing rhenium(V), rhenium(VI) and rhenium(VII) are prepared. Chemical and physical evidence is used to predict structures of some of the above compounds.</p> / Thesis / Doctor of Philosophy (PhD)

<p> The crystal structures of three complexes of rhenium have been determined by single crystal x-ray diffraction methods. The structure and the probable position of the hydrogen atom in the complex tetracarbonyl-rhenium(I)-μ-oxo-μ-hydroxotetracarbonylrhenate(I) are discussed.</p> <p> The structural results for the other two complexes show the presence of carboxylato-bridged dinuclear rhenium core. The very short rhenium to rhenium distances and the eclipsed rotomeric configurations are similar to those found in octachlorodirhenate(III) anion. A reaction scheme for the formation of these complexes from rhenium(III) chloride is postulated and correlated with the structural results. The nature of bonding in the carboxylato complexes of rhenium is discussed.</p> / Thesis / Doctor of Philosophy (PhD)

The purpose of this study was to characterize the catalytic activity of rhenium trioxide and rhenium trichloride as hydrogenation catalysts. Work was also undertaken to further study the catalytic properties of rhenium heptoxide. The activity of this latter compound had received some study in a previous project. A complete review of the literature has been made on the organic and inorganic chemistry of rhenium trioxide and rhenium trichloride. A survey of the literature on catalytic hydrogenation and the chemistry of rhenium heptoxide has also been included. Rhenium trioxide was prepared by three similar methods. The first two involved the formation of a complex between rhenium heptoxide and anhydrous dioxane. The complex was isolated and subsequently decomposed by gentle heating to give the pure trioxide. The third method employed tetrahydropyran as a complexing agent. The complex was isolated and decomposed in the same manner as previously indicated. All three methods appeared to give the trioxide in high purity, and little difference was apparent in their catalytic activities. Rhenium heptoxide and rhenium trichloride were commercially available, and thus required no special preparation. Both were generally reduced in situ to the active catalyst. In three instances rhenium heptoxicid was reduced ex situ. It was found that the in situ derived catalysts were generally more active than those derived ex situ. All reductions were carried out in a high pressure hydrogenation vessel. The reduction products were analyzed in a gas chromatograph, by refractive indicies, distillation and/or chemical extraction. Catalysts were analyzed by dissolving in concentrated nitric acid or a 30% hydrogen peroxide and ammonia solution. The resulting perrhenate was precipitated from the solution with tetraphenylarsonium chloride. Generally the analytical data obtained was not of sufficient accuracy to determine the exact chemical structure of the catalyst. The activities of the catalysts were determined by performing a large variety of hydrogenations. Results indicated that the heptoxide and trioxide derived catalysts were generally very similar in their catalytic activities. The trichloride proved to be slightly lower in its activity than the oxides. The catalysts used in this study generally required slightly more drastic conditions for the reduction of a carbonyl group than was generally necessary for a rhenium derived catalyst. The reduction of cyclohexanone was catalyzed at 123° with rhenium tioxide, while the heptoxide and trichloride required temperatures of ca. 150°. The olefinic compounds such as 1-hexene were reduced at temperatures of 95-100°, while styrene required slightly higher conditions due to its conjugation with the benzene ring. Nitro compounds and benzenoid compounds were found the most difficult to reduce with the trioxide and trichloride derived catalysts. Nitrobenzene yielded aniline only under temperatures of 226-275° depending on the catalyst used. Benzene yielded slight reduction with the trichloride catalyst at 200°. The trioxide, heptoxide and trichloride catalysts were outstanding in their ability to reduce the carboxylic acids. In most cases reduction could be effected 150-160°. The catalysts were superior to any reported in the literature for the reduction of the cargoxyl group. Most rhenium catalysts exhibit this high activity toward the carboxyl group to some extent. The oxide catalysts were also found to possess an extremely high activity toward the hydrogenation of amides and anilides. The reduction generally took place at ca. 225° giving a good yield of the primary amine in most cases tried. These catalysts compared very favorably with the better catalysts reported in the literature.

The purpose of the present investigation was to examine the various analytical methods to ascertain the suitability of the methods for our use in determining the amount of rhenium used in our catalysts, and, if necessary devise a new method. Also, several catalysts were to be prepared and used in hydrogenation or organic compounds. Evaluation of the catalytic activity of each catalyst was to be made by using them with various selected organic substrates.

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Thesis (Ph.D.)--McMaster University, 2005. / Supervisor: John Fitzmaurice Valliant. Includes bibliographical references. Also available online.