Syntheses and Structures of Iron(II) and Cobalt(II) Complexes of Substituted Tetra(pyrazolyl)lutidine Derivatives
Pentadentate ligands provide stability to metal complexes as well as a binding site for substrates that make them exceptionally useful for studying reactions. They have been used in biomimetic studies, water splitting, and small molecule activation studies. A series of six pentadentate ligands derived from a,a,a',a'-tetra(pyrazolyl)lutidine, pz4lut, with methyl substituents decorating the ligand periphery has been synthesized. These ligands were coordinated to FeIICl, and the electronic and structural properties of the resulting complexes were studied in order to deduce the effects of methyl substitution at the 3-, 4-, and 5-positions of the pyrazolyl groups and at the methine positions. When analyzed via cyclic voltammetry, the resulting complexes exhibited oxidation waves between 0.95 V and 0.75 V. It was determined that the 4-pyrazole position controls electronic effects, and the 3-pyrazolyl position controls steric effects. Substitution at the methine positions favors a low-spin FeII complex. It is thought that adding bulkier groups to the methine positions may change the coordination environment of the resulting complex. An [FeIII(OH)(pz4depy)] complex (pz4depy = a,a,a',a'-tetra(pyrazolyl)diethylpyridine) could be capable of oxidizing hydrocarbons with BDE (bond dissociation enthalpy) values less than 93 kcal/mol. A [CoII(H2O)(pz4depy)]2+ complex was shown to be capable of water oxidation via a concerted proton-electron transfer (PCET) mechanism. This complex was compared to a similar WOC, [CoII(H2O)(PY5)]2+, and found to react with water via a more favorable pathway. The BDFE(OH) (bond dissociation free energy of the OH bond) was also calculated via DFT calculations to be 76.7 kcal/mol, which is higher than that of [CoII(H2O)(PY5)]2+ by 5 kcal/mol. Further studies are needed to make sure that the catalytically active species is not CoOx nanoparticles that may have formed in solution. The [CoII(H2O)(pz4depy)]2+ complex is also thought to be capable of CH-activation reactions. This thesis demonstrates the versatility of a new pentadentate ligand family for controlling the electronic and structural properties of transition metal complexes. The data reported herein may be used to select the appropriate complex to participate in CH-activation reactions.