Date of Award

Winter 2021

Project Type

Dissertation

Program or Major

Chemistry

Degree Name

Doctor of Philosophy

First Advisor

Christine A Caputo

Second Advisor

Gonghi Li

Third Advisor

Richard P Johnson

Abstract

Global climate change is one of the most significant risks to both the planet and all life on it. The largest contributor to this ever-growing threat is CO2 and its continued emission into the atmosphere. As a result, many scientists are researching CO2 reduction processes to help slow and potentially reverse the climate change by reducing the atmospheric levels of CO2. Ideally, these reduction processes convert CO2 into useful materials such as hydrocarbon-based fuels such as methanol or ethanol, which could also help remove our dependence on finite natural energy resources. CO2 reduction is challenging to do efficiently however, due to the inherent stability of the molecule which requires a significant input of energy to overcome. Catalysis offers scientists a powerful tool and the most alluring form of catalysis for this task is photocatalysis because it can utilize the greatest untapped energy resource available, the sun.Photocatalytic systems have been used extensively for CO2 reduction and other environmentally relevant processes. Hybrid photocatalytic systems in particular possess several advantages over both homogeneous and heterogeneous systems including the high efficiency and selectivity of using a molecular catalyst, the stability of heterogeneous photosensitizers, and increased stability of molecular catalysts resulting from anchoring to the surface of a heterogeneous photosensitizer. The linkage between the molecular catalyst and surface in hybrid systems has not been extensively explored however, and the effect it has on interfacial electron transfer and catalysis is not well understood. We hypothesize that certain structural features of the linker such as length, flexibility or specific geometry will result in more efficient electron transfer from the surface to the catalyst and increase photocatalytic activity. To investigate the effects of these linkages, a series of hybrid photocatalytic systems which use a well-studied molecular catalyst and heterogeneous photosensitizer is needed. Modification of this molecular catalyst with linkers which vary in length, flexibility and geometry will allow for the identification of beneficial linker features during hybrid photocatalysis. One such catalyst which has been studied for photocatalytic CO2 reduction is the cobalt cyclam complex. Synthetic modification of the cyclam ligand is a challenging task that should require C-functionalization, because N-functionalization is expected to disrupt the ligand inner coordination sphere thus altering catalytic activity. This work describes our efforts to generate modified cobalt cyclam molecular catalysts so that a series of hybrid photocatalytic systems may be tested to investigate the effects of different linkers in hybrid photocatalytic CO2 reduction. Previous attempts to generate cyclam ligands modified with alcohols and carboxylic acids were unsuccessful, as these ligands were unsuitable for organic coupling to amine functionalized linkers. Incorporation of larger modifications like p-nitro and p-bromo benzyl groups into the cyclam ligand synthesis was also unsuccessful due to synthetic challenges encountered during substitution of the modified biselectrophiles with protected 2,3,2-tetraamine. Successful C-functionalization of cyclams employed a metal templated synthesis with cobalt(II) acetate and 2,3,2-tetraamine followed by condensation with acetyl acetone which yielded the cyclam like complex [CoIII(Me2[14]-1,4-diene-N4)Cl2]BF4. This ligand framework was readily C-functionalized after ligand exchange with NaSCN, deprotonation, and nucleophilic substitution with the resulting CoIII(-1)dieno-(NCS)2 onto both benzyl bromide and 1-bromomethyl pyrene which function as anchors to carbon nitride. Reduction to the ligand imines with NaBH4 proved challenging but yielded [CoIII(Me2[14]-N4-6-benzyl)C2]Cl and a mixture of [CoIII(Me2[14]N4-3-(2-methylpyrenyl)Cl2]Cl and [CoIII(Me2[14]4-ene-N4-3-(2-methylpyrenyl)Cl2]Cl. Photocatalytic activity of [CoIII(Me2[14]-N4-6-benzyl)C2]Cl and the mixture of [CoIII(Me2[14]N4-3-(2-methylpyrenyl)Cl2]Cl and [CoIII(Me2[14]4-ene-N4-3-(2-methylpyrenyl)Cl2]Cl was investigated homogeneously with p-terphenyl and heterogeneously anchoring to carbon nitride. Activity of both the benzyl-modified catalyst and pyrenyl modified catalyst mixture with p-terphenyl was similar with a TONCO of 27.5 and 28.9 for the pyrenyl and benzyl-modified catalysts, respectively. On carbon nitride, these catalysts produced a TONCO < 1. The difference in activity between the two catalysts was clear however, and the pyrenyl modified catalyst produced up to 6 times more CO than the benzyl-modified catalyst. This increased production of CO by the pyrenyl modified catalyst is consistent with a higher anchoring affinity of the pyrene moiety to the aromatic carbon nitride surface by π – π interactions. Measurable differences between these hybrid photocatalytic systems which differed only in linker/ anchor demonstrated the success of this synthetic strategy for the investigation of linker design with modified cobalt cyclam molecular catalysts.

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