Download Abstract Coordination Chemistry of Tetra(pyrazolyl)

Abstract
Coordination Chemistry of Tetra(pyrazolyl)-lutidine Ligands
and Derivatives
Tyler J. Morin
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, 3,5dimethyl, 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. However, 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.