Author ORCID Identifier
Year of Publication
Doctor of Philosophy (PhD)
Arts and Sciences
Dr. John P. Selegue
Polyheterocycles such as polythiophene and its derivatives comprise an important class of conducting polymers used for electronic applications. They have been of great interest for use in electronic materials due to their increased environmental stability as well as novel electronic properties in their polymer states. We have been interested in exploring the electronic properties of organometallic analogues of the low-band-gap polymer poly(benzo[3,4-c]thiophene) (polyisothianaphthene) that incorporates η5-cyclopenta[c]thienyl monomers such as ferroceno[c]thiophene. First chapter of this dissertation involved synthetic attempts to ferroceno[c]thiophene. Exploring a shorter synthetic route to starting material, 1,2-di(hydroxymethyl)ferrocene was the first task. This was followed by attempts to synthesize an important precursor, 1,3-dihydroferroceno[c]thiophene to our target molecule, ferroceno[c]thiophene. In order to achieve our target precursor molecule, 1,3-dihydroferroceno[c]thiophene, we reacted 1,2-di(hydroxymethyl)ferrocene with H2S/H2SO4 and Na2S/HBF4 respectively. Reaction of 1,2-di(hydroxymethyl)ferrocene with either H2S/H2SO4 or Na2S/HBF4 results in 2,16-dithia[3.3](1,2)ferrocenophane instead of monomeric 1,3-dihydroferroceno[c]thiophene. Dehydration of 1,2-di(hydroxymethyl)ferrocene with dilute H2SO4 resulted in 2,16-dioxa[3.3](1,2)ferrocenophane. Formation of the five-membered tetrahydrothiophene or tetrahydrofuran rings is probably disfavored compared to formation of the ten-membered ferrocenophane rings because of greater strain in the five-membered rings. Thus, in order to achieve our target molecule ferroceno[c]thiophene, we took an alternate route. We decided to pursue the route with 1,4-dihydro-2,3-ferrocenodithiin being the precursor to our final target molecule. This was successfully accomplished. 1,2-Di(hydroxymethyl)ferrocene reacts with thiourea in the presence of catalytic trifluoroacetic acid to give a water-soluble thiouronium salt, which reacts with aqueous potassium hydroxide in air to give 1,4-dihydro-2,3-ferrocenodithiin, via oxidation of the intermediate 1,2 di(mercaptomethyl)ferrocene. 1,4-dihydro-2,3-ferrocenodithiin, an important precursor to our desired heterocyclic chemistry was synthesized.
The increased emission of CO2, a greenhouse gas, to the atmosphere is a matter of serious worldwide concern. Every year a few gigatons of CO2 are added to the atmosphere by various anthropogenic activities like burning of fuel for electricity, running industry and transportation. Thus, developing ways to reduce the emission of CO2 to the atmosphere is of major importance. Although the amine-based absorption method is considered the most reliable, it is an expensive alternative. The catalyzed enhancement of CO2 absorption is a critical component to reduce the capital cost of CO2 capture. Specifically, an effective catalyst will increase the CO2 hydration rate, thereby decreasing the size of the absorber tower needed. In biological systems, CO2 hydration is catalyzed by the enzyme carbonic anhydrase, which contains ZnII in its active site. Carbonic anhydrase typically is not stable enough to be used in an industrial process, therefore, there is a need to synthesize robust, inexpensive CO2 hydration catalysts.
Majority work of this dissertation focuses on designing catalysts that show high CO2 hydration rate similar to carbonic anhydrase while showing superiority towards temperature, pH and inhibitors. We focused our efforts on complexes of Zn, Cu and Co with ligands such as 1,4,7,10-tetraazacyclododecane (cyclen), 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane (teta and tetb), tris(benzimidazolylmethyl)amine (BIMA) and anionic tris(pyrazolylborate)s that mimic the enzyme, carbonic anhydrase. Several of these complexes have been reported for their interesting CO2 capture properties but they contain hazardous perchlorate ion. We desired to replace them with benign, non-coordinating counterions like PF6-, BF4-, Cl-, CH3COO-, NO3-, CF3SO3-, SiF62- that avoid the potentially explosive perchlorate salts. In order to test the activity of synthesized catalysts under industrial capture conditions, we designed a quick experimental screening pH drop method. [[Zn(cyclen)(H2O)][SiF6]•2H2O as well as a number of other catalysts have been synthesized and tested for their post-combustion CO2 capture enhancement capabilities in aqueous solvent mixtures under both pH-drop screening and stopped-flow conditions.
[Zn(cyclen)(H2O)][SiF6]•2H2O, which has an unreactive counteranion, is found to catalyze CO2 hydration in aqueous solvent mixtures under both pH-drop screening and stopped-flow conditions. However, under pH-drop which has conditions similar to industrial post combustion capture, activity of Zn(cyclen)(H2O)][SiF6]•2H2O drops as compared to observed in stopped-flow conditions probably because of bicarbonate coordination to Zn active site in these systems. The Zn center is highly electron deficient and therefore easily coordinates anions, inhibiting the ability to reform hydroxyl species on the metal. Thus, we decided to test the catalysis of benchmark enzyme carbonic anhydrase under similar conditions to determine the threshold value. Carbonic anhydrases catalyze the hydration of carbondioxide at ambient temperatures and physiological pH with the highest known rate constant= 106 M–1 s–1, but in our system (CAER pH drop screening) came out to be 438797 M–1 s–1. The lower catalytic rate constant for carbonic anhydrase in 0.1000 M K2CO3, similar to Zn-cyclen, strengthens the conjecture that at high bicarbonate concentrations, HCO3– binding to the Zn(II) active site slows catalysis by inhibiting bicarbonate displacement with water to regenerate the active species.
The complexes containing anionic ligands that donate electron density into the metal center may serve to remove anionic bicarbonates/carbamates from the secondary coordination sphere and away from the metal center, thereby facilitating bicarbonate/anion dissociation and increasing CO2 hydration rates. We studied catalysis of trispyrazolylborate molecule in 30% MEA and found the molecule to be catalytically active. We also developed an NMR-based method to see if the coordination of solvents to CO2 capture solvents can be studied.
Digital Object Identifier (DOI)
Gupta, Deepshikha, "NEW APPROACHES TO CYCLOPENTADIENYL-FUSED THIOPHENE COMPLEXES OF IRON and SYNTHESIS AND CHARACTERIZATION OF CARBONIC ANHYDRASE ACTIVE-SITE MIMICS FOR CO2 HYDRATION" (2018). Theses and Dissertations--Chemistry. 92.