Date of Award
Doctor of Philosophy (PhD)
James R. Gardinier
James R. Kincaid
Chae S. Yi
There is current interest in the coordination chemistry of simple AE4 pentadentate ligands that occupy one axial (A) and four equatorial (E) positions about a given transition metal center considering that systems capable of mediating spectacular organic transformations such as alkane oxidation have been identified. The discovery of new systems will improve understanding of C-H bond activation processes which is critical to help make more efficient use of the currently diminishing fossil fuel supplies. As such, a new pentadentate ligand α,α,α',α'-tetra(pyrazolyl)lutidine (pz4lut) and its derivatives pz4,4lut, (pz4, = 4-methylpyrazole); pz*4lut, (pz* = 3,5-dimethylpyrazole); pzDIP4lut, (pzDIP = 3,5-diisopropylpyrazole) have been synthesized and their coordination chemistry towards a host of main group and transition metals has been investigated.
Divalent first row transition metals were used to establish the binding mode(s) (both κ5 and κ2-μ are observed) of the newly synthesized, unsubstituted pz4lut ligand. Substitution along the pyrzolyl periphery with various alklyl groups (4-methyl, 3,5-dimethyl, 3,5-diisopropyl) provided a way to examine the effects of substitution on binding behavior with transition metals. Cobalt(II) complexes tend to be thermo and solvatochromic giving both pink κ5 octahedral complexes as well as blue κ2 tetrahedral complexes for all ligands except pzDIP4lut owing to the steric demands of the isopropyl group. Substitution along the pyrazolyl periphery also allowed for investigation into supramolecular studies of self assembly using silver(I) salts of the newly synthesized ligands. It was found that the silver(I) complexes display unique solution behavior in which the solid state structure is not retained.
Finally, given the importance of oxyferryl species in mediating C-H bond oxidation reactions; the chemistry of iron(II) complexes of the newly synthesized ligands was explored. The reaction between 2 equivalents of m-chloroperoxybenzoic acid and the iron(II) complex [Fe(Cl)(pz4lut)](BAR4f) at -30 oC gives a solution with the characteristic green color of iron(IV) oxo species. Due to the highly reactive nature and short lifetime of these species it has not been further characterized. Further efforts have been directed at the synthesis of ligands capable of stabilizing such reactive species, such as the pzEt4lut ligand; which is substituted at the methyne carbon blocking a potential decomposition pathway.