Synthesis, catalysis and Nitrosyl derivatives of Iron(ii) complexes bearing Pentadentate Polypyridyl ligands

Abstract

The work described in this thesis reports the synthesis, characterization, and solution chemistry of a series of new pentadentate polypyridyl ligands and their corresponding Fe(II) complexes. The ligand structure was based on the tripodal 1,1-di(2-pyridyl)-N,N-bis(2-pyridylmethyl)methanamine (N4Py) which was modified by replacing some of the pyridyl functions with quinolyl, isoquinolyl or phenyl pyridyl moieties on the amino function. The purpose of this modification was to vary the basicity and steric bulk of the ligand and thereafter study the effect of this structural variation on the properties of their iron(II) complexes. The properties of interest were: the coordination geometry of these complexes, ability to bind small molecules and catalytic performance in the oxidation of hydrocarbons. Five ligands were synthesized and these were: 1,1-di(pyridin-2-yl)-N,N-bis(quinolin-2-ylmethyl)methanamine (N2Py2Q), 1,1-di(pyridin-2-yl)-N-(pyridin-2-ylmethyl)-N-(quinolin-2-ylmethyl)methanamine (N3PyQ), N-((6-(4-chlorophenyl)pyridin-2-yl)methyl)-1,1-di(pyridin-2-yl)-N-(pyridin-2-ylmethyl)methanamine (N3Py(Py-PhCl), N,N-bis(isoquinolin-3-ylmethyl)-1,1-di(pyridin-2-yl)methanamine (N2Py2IQ) and N-((6-(4-chlorophenyl)pyridin-2-yl)methyl)-1,1-di(pyridin-2-yl)-N-(quinolin-2-ylmethyl)methanamine (N2PyQ(Py-PhCl)). The synthesis of the iron(II) complexes with the pentadentate ligands was accomplished in acetonitrile by reaction of the free ligand with an iron(II) salt of a weakly coordinating anion like tetrafluoroborate or triflate. X-ray diffraction, NMR and UV-Vis spectroscopy measurements showed that most of the ligands coordinated through their five N-donor groups exhibiting a square pyramidal coordination geometry around Fe(II). The acetonitrile solvent molecule completed the sixth coordination position in most of the complexes. The ligand N2PyQ(Py-PhCl) failed to achieve the pentadentate coordination due to steric hinderance factors. Instead, it lead to formation of a dinuclear complex in which the ligand coordinates to iron(II) in a tetradentate mode with the phenyl pyridine group being uncoordinated. The dimerization occurs through an oxo bridge (Fe-O-Fe) which occupies the fifth coordination site. The sixth site was occupied by a fluoride atom due to fluoride abstraction from the tetrafluoroborate anion. The ability of the complexes to coordinate a small molecule was illustrated using nitric oxide (NO) by attempting to synthesize nitrosyl derivatives of the above complexes. Complexes bearing the ligands N2Py2Q, N3Py(Py-PhCl) and N3PyQ were able to coordinate NO by successful replacement of acetonitrile with NO in the sixth coordination site. X-ray diffraction and vibrational infrared spectra showed that NO was bonded to Fe(II) in an end-on linear fashion. This therefore demonstrated the possibility of such complexes in coordinating similar molecules like oxygen which is an important step in the catalytic oxidation of hydrocarbons using oxygen or oxygen like species. The iron complex FeN3PyQ(OTf)2 was tested for catalytic oxidation of hydrocarbons using hydrogen peroxide as the primary oxidant. The complex catalysed the oxidation of selected alkanes to corresponding alcohols with moderate yields. This is a promising milestone in the attempt to utilize unreactive hydrocarbons.