Computational Studies on Bimetallic Catalysis and X-ray Absorption Spectroscopy
Computational studies are very important to gain an insight into reaction mechanisms and in interpreting and understanding complicated experimental observations. This report contains a discussion on computational studies performed on bimetallic catalysis and on X-ray absorption spectroscopy of insulators. The viability of a bimetallic rhodium and cobalt catalysts for industrially important hydroformylation and aldehyde-water shift catalysis (AWS) is discussed. Density functional theory (DFT) studies were used for bimetallic catalysis and time-dependent DFT studies were used for excited state dynamics. These studies were performed using Gaussian 09 package and NWChem. Hydroformylation is experimentally performed in acetone and 30% water/acetone systems and results in dicationic dirhodium complexes and monocationic dirhodium complexes respectively. DFT studies were used to determine the active catalyst and possible intermediates. Computational studies support the mechanism proposed by Prof. Stanley for hydrofomylation in acetone, but DFT studies demonstrate a different mechanism for hydroformylation in water/acetone which contains mono-bridging complexes. A detailed discussion on this is given in chapter 2. DFT studies were used to study the AWS catalysis with the bimetallic [rac-Rh2(μ-CO)2(CO)2(et,ph-P4)]2+ complex. These studies were performed in both vacuum and using explicit water molecules, and lower energies were obtained when explicit water molecules were used. The computations support an alternate mechanism with protonated acid intermediates different from the originally proposed mechanism. This mechanism is discussed in great detail in chapter 3. DFT studies are also performed to study the suitability of dicobalt analogs for hydroformylation and AWS catalysis. The most suitable active catalyst and possible mechanism for hydroformylation using Co2(μ-H)(μ-CO)(CO)3(H)(et,ph-P4)]2+ (Co_2), [rac- and [rac-Co2(μ-H)2(CO)4(et,ph-P4)]2+ (Co_2*) and [rac-Co2(μ-CO)2(CO)2(H)2(et,ph-P4)]2+ (Co_2**) are discussed in chapter 4. The capability of [rac-Co2(CO)4(et,ph-P4)]2+ (Co_4*) catalyst for AWS catalysis is also discussed in chapter 4. Chapter 5 discusses a method to generate X-ray absorption spectra of insulators using time-dependent DFT. α-Quartz was used as a model for insulators. Bulk-mimicking embedded finite cluster models, atom-centered basis sets, tuned range-separated functionals and molecular orbital-based absorbing boundary conditions were utilized to model near and above ionization spectral features without experimental parameterization. The calculated spectra match well with the experimental results over the range of approximately 105 – 130 eV.
molecular orbital, DFT, Time-dependent DFT, Hydroformylation, Aldehyde-water shift, organometallic
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