Abstract
Steam-methane reforming (SMR) reaction and the water-gas shift (WGS) reaction are the major processes for the dihydrogen production in the U.S., which cost enormous amount of energy. As a result, utilization of the products for SMR and WGS processes (H2, CO and CO2) more efficiently could create economic benefits. In the past few decades, many homogenous catalytic processes that use H2, CO or CO2 to produce higher-value chemicals with transition metal catalysts have been developed.
My primary focus of this Dissertation was to study the redox chemistry of a series of capping arene-ligated rhodium complexes during catalysis. Olefin hydrogenation was selected as the model for this study because Rh-catalyzed olefin hydrogenation has been well-developed, and the proposed mechanisms for Rh-catalyzed olefin hydrogenation included the formal oxidation state change on the Rh catalysts. The ligand influence on olefin hydrogenation using four capping arene ligated Rh(I) catalyst precursors (FP)Rh(η2-C2H4)Cl {FP = capping arene ligands, including 6-FP (8,8′-(1,2-phenylene)diquinoline), 6-NPFP (8,8′-(2,3-naphthalene)diquinoline), 5-FP (1,2-bis(N-7-azaindolyl)benzene) and 5-NPFP [2,3-bis(N-7-azaindolyl)naphthalene]} has been studied. Our studies indicate that relative observed rates of catalytic olefin hydrogenation follow the trend (6-FP)Rh(η2-C2H4)Cl > (5-FP)Rh(η2-C2H4)Cl. Based on combined experimental and density functional theory modeling studies, we propose that the observed differences in the rate of (6-FP)Rh(η2-C2H4)Cl and (5-FP)Rh(η2-C2H4)Cl-catalyzed olefin hydrogenation are most likely attributed to the difference in the activation energies for the dihydrogen oxidative addition step. We are unable to directly compare the rates of olefin hydrogenation using (6-NPFP)Rh(η2-C2H4)Cl and (5-NPFP)Rh(η2-C2H4)Cl as the catalyst precursor since (5-NPFP)Rh(η2-C2H4)Cl undergoes relatively rapid formation of an active catalyst that does not coordinate 5-NPFP.
The cobalt-catalyzed styrene hydrogenation has also been studied. A capping arene-ligated cobalt complex (5-FP)CoCl2 was successfully synthesized. The hydrogenation of styrene was observed when using (5-FP)CoCl2 as the catalyst and Zn(0) as an additive. The reaction conditions are further optimized, and the styrene hydrogenation reached 5.5 ± 2.4 turnovers with the best conditions.
Moreover, the capping arene ligated Rh and Ir complexes have also been studied for their catalytic activities on MeI promoted methanol carbonylation. A series of capping arene ligated Rh or Ir carbonyl complexes with the general formula [(FP)MI(CO)2]BF4 (FP = 5-FP or 6-FP, M = Rh or Ir) were synthesized. The performances of these complexes on MeI promoted methanol carbonylation were examined. The turnovers using [(FP)MI(CO)2]BF4 (FP = 5-FP or 6-FP, M = Rh or Ir) as the catalyst were not improved when compared to catalysis using [Rh(CO)2(µ-Cl)]2 or [Ir(COE)2(µ-Cl)]2. Mechanistic studies revealed that the [(5-FP)Rh(CO)2]BF4, [(5-FP)Ir(CO)2]BF4 and [(6-FP)Rh(CO)2]BF4 are unstable at 135 °C without MeI. When MeI is present, all four [(FP)MI(CO)2]BF4 complexes as well as the 5-FP and 6-FP ligands show reactivity with MeI at 135 °C. The decomposition products for [(5-FP)Rh(CO)2]BF4 or [(5-FP)Ir(CO)2]BF4 with MeI at 135 °C are identical to the decomposition product of 5-FP ligand reacting with MeI. Similarly, the decomposition products for [(6-FP)Rh(CO)2]BF4or [(6-FP)Ir(CO)2]BF4 with MeI at 135 °C are identical to the decomposition product of 6-FP ligand reacting with MeI. The identical decomposition products could suggest that the capping arene ligand falls off from the Rh or Ir complexes. Thus, the active catalyst for the methanol carbonylation using [(FP)MI(CO)2]BF4 and MeI is likely a metal complex without the capping arene ligand, which can explain why no ligand effect is observed for the methanol carbonylation with MeI promoter.
Additionally, carbon dioxide (CO2) offers the possibility of a C1 synthon because of the low expensive, low toxicity, and high abundance. Thus, the direct carboxylation of hydrocarbons would have many advantages compared to current processes for the carboxylic acids production (e.g. hydroformylation of ethene followed by oxidation). A mechanism for direct carboxylation of benzene under basic condition catalyzed by PCP- ligated {(PCP = 2,6-(R2PCH2)2C6H3; R = isopropyl, tertbutyl, or phenyl} palladium complexes has been proposed and analyzed by DFT calculations in order to predict the feasibility of the reaction. A series of PCP Pd complexes have been prepared to study the direct carboxylation of benzene with CO2. As CO2 insertion into the Pd–Ph bond is an important step in the proposed catalytic cycle, the reactivity of the Pd pincer complex with CO2 is studied. (iPrPCP)PdPh ( iPr PCP = 2,6-(iPr2PCH2)2C6H3) has been prepared and the CO2 insertion product, (iPrPCP)PdOBz (OBz = benzoate), has been observed by 1H NMR spectroscopy. Under optimized conditions, the yield of the CO2 insertion product is approximately 70% after 2 h. The next step in the proposed catalytic cycle involves the C–H activation of benzene. The ability of (iPrPCP)PdOBz to activate a C–H bond of benzene has been studied.
