Exploring Structure-Function Relationship in Small-Molecular Catalysts Using Computational and Experimental Methodologies

Abstract

Molecular modeling is a useful tool in the field of catalyst design for various processes. The use of Density Functional Theory (DFT) is routine in almost every discipline of chemistry. This allows for a deeper understanding of a molecular system even in situations where implementation of an experimental technique is unfeasible. However, without the right choice of theory and insufficient description, the model becomes susceptible to produce ambiguous results. This often leads to poor correlation with experimental findings hence an incomplete understanding of the system under study. Hence, to acquire a thorough knowledge of the intricacies involved in a system, a judicious survey of the molecular model is necessary. Explored herein are embodiments of four catalytic systems, combining computational and experimental techniques, to better understand the structure-function relationship. The systems of choice include twelve homoleptic, and two heteroleptic Ni(II) tris-pyridinethiolate water splitting catalysts, an organo-photocatalyst for aerobic oxidation of benzylic alcohols, and finally a series of eighteen diarylhalonium salts and diarylchalcogenides. The first chapter describes a detailed study on homoleptic water splitting catalysis that demonstrates the impact of intramolecular hydrogen bonding (H-bonding) on the pKa of octahedral tris-(pyridinethiolato)nickel (II), [Ni(PyS)3]-, commonly referred to as Ni(II) tris-pyridinethiolate. Protonation is a key step in catalytic proton reduction to produce hydrogen gas, and thus optimizing the catalyst's pKa is critical for catalyst design. DFT calculations on a Ni(PyS)3]- catalyst, and eleven derivatives, demonstrate geometric isomer formation in the protonation step of the catalytic cycle. Through Quantum Theory of Atoms in Molecules (QTAIM), we show that the pKa of each isomer is driven by intramolecular H-bonding of the proton on the pyridyl N to a S on a neighboring thiopyridyl (PyS-) ligand. Experimental measurements used to determine the pKa and reduction potential (E0) of the catalysts support the formation of the geometric isomers upon protonation, although the isomers complicate understanding the experimental results. This work demonstrates that ligand modification via the placement of electron-donating (D) or electron-withdrawing (W) groups may have unexpected effects on the catalyst's pKa due to intramolecular H bonding. This work suggests the possibility that modification of substituent placement on the ligands to manipulate H bonding in homogeneous metal catalysts could be explored as a tool to simultaneously target both desired pKa and E0 values in small molecular catalysts. In the subsequent chapter a strategy to fine-tune the efficiency of a water splitting Ni(PyS)3]- catalyst through heteroleptic ligand design was explored