Redox and photoactive metal complexes with applications in catalysis, emissive materials, and sensing
Date
2017
Authors
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Publisher
University of Delaware
Abstract
In Chapter 1 a series of imidazolium based, pyridyl spaced dicarbene complexes were synthesized with varying N-substituents, or ligand wingtips, with increasing steric value (methyl, ethyl, isopropyl, cyclohexyl, mesityl, and diisopropylphenyl). The synthesis of this library of complexes revealed that metalation conditions with Pd(OAc)2 at elevated temperatures resulted in formation of impurities, which made the purification difficult and resulted in decreased yields. Altering the metalation procedure to formation of the silver carbene followed by transmetallation to palladium resulted in a much cleaner reaction, with increased yield and simple chromatography. Conversion to the acetonitrile bound solvato complex was easily accomplished in high yields and targeted due to a previous study which indicated a labile ancillary ligand enhanced electrocatalytic conversion of CO2. The entire solvato bound library, including many intermediates, were characterized through X-ray crystallography. Further, the solid-state structures of the solvato complexes were used to calculate the percent buried volume (%VBur) and ligand solid angles, which characterized the steric environment around the metal center. It was determined that despite an increase in molecular size, all alkyl groups showed a similar steric environment around the metal. The mesityl and diisopropylphenyl derivatives showed significant steric encumbrance when compared to the alkyl systems. Voltammetry studies were performed and indicated a current enhancement under CO2, albeit the observed current enhancement was slight. Current enhancement values followed the same trend observed as the steric environment calculations where Dipp > Mes > alkyl (alkyl systems gave similar values). Controlled potential electrolysis studies showed that applied potentials past the PdI/Pd0 couple resulted in passivation of the electrode. Applied potentials at or below the PdI/Pd0 couple resulted in stable currents and no passivation was observed, however, the complexes only produced trace amounts of CO. While the synthesized complexes were inadequate for the electrochemical conversion of CO2, the correlation between steric environment and current enhancement could be useful in future design of molecular systems for the electrochemical reduction of CO2. ☐ In Chapter 2, a rhenium-based molecular photocatalyst supported by a BODIPY appended bipyridine ligand was synthesized and studied for the photocatalytic conversion of CO2 to CO. The synthesis of the target complex was completed in two parts, an inorganic fragment, and the organic chromophore. The two fragments were then coupled using a Huisgen reaction to form the target photocatalyst. Similar synthetic strategies were used to generate a series of control compounds, a rhenium complex without the appended chromophore, and a chromophore without the appended inorganic fragment. The photophysical properties of all complexes were studied and revealed that the BODIPY bound rhenium complex was able to absorb visible light out to 600 nm with a molar absorptivity of 60,000 M–1•cm–1. Voltammetry studies showed that the designed photocatalyst maintained the activity of previously studied rhenium bipyridine complexes for the electrochemical conversion of CO2. Through a collaboration, we were able to characterize the photocatalytic properties and found that our designed photocatalyst was able to photochemically convert CO2 to CO with metrics of: TOF = 4 hr–1, and TON = 20. Unfortunately, our control complex displayed similar metrics. After obtaining the solid-state structure and through further probing of the photophysical properties, we found that the inactivity may be due to weak or absent electronic communication as evident through the luminescence quantum yield and lifetime of the photocatalyst and control compound. Further, the long distance (about 15.7 Å) between the chromophore and the metal center may also hinder electronic communication in the designed complex. ☐ In Chapter 3, while complexes containing imidazolium based, phenyl spaced dicarbene CCC pincer ligands, are prevalent in the literature for many transition metals, there is an absence in the primary literature for palladium systems incorporating these ligands. This may be due to the three C–H activations that need to take place for metalation. While the two imidazolium protons are easily deprotonated, the aryl C–H is not easily activated. Based on strategies used for analogous platinum compounds, we found success in using a transmetallation procedure in which zirconium is used to generate an intermediate followed by transmetallation to palladium. We have successfully synthesized a library of phenyl spaced dicarbene palladium complexes with varying ancillary ligands (Br, acetonitrile, pyridine, tertbutyl isonitrile, and triphenylphosphine), all complexes contain ethyl substituents on the ligand wingtips. Through analysis of the solid-state structures we have found that, when compared to the pyridyl spaced systems, we see a lengthening of the bond length of the ancillary ligand, due to the strong electron donation of the trans CPhenyl when compared to NPyridyl. Electrochemical analysis via cyclic voltammetry showed about a 1 V difference in PdII/PdI couples where the phenyl spaced system is at a more negative potential due to a more electron dense metal from the stronger donating ligand. Photophysical properties in the solid-state showed that the N-bound ancillary ligands (MeCN and pyridine) were luminescent materials with a deep blue color and International Commission on Illumination (CIE) coordinates of (0.16, 0.12) and (0.16, 0.13) respectively. We further discovered that the triphenylphosphine complex could act as a sensor through solid-state exchange of the ancillary ligand, yielding a luminescent material in a matter of minutes. This material was characterized through XPS and supported the solid-state exchange of the ancillary ligands. The synthetic strategy was extended to nickel analogs, another absent complex in the literature supported by this ligand system. While we were successful in synthesizing the nickel complexes, they were found to be non-luminescent, likely due to available non-radiative pathways.
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Keywords
Pure sciences, Catalysis, Emissive, Metal, Photoactive, Redox