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C-H functionalisation of hydrocarbons using iron(II) complexes bearing pentadentate polypyridyl ligands and their nitrosyl derivatives Solomon Yiga* 1,2 , Henry Ssekaalo 1 and Ola F. Wendt 2* 1 Department of Chemistry, Makerere University, Kampala, Uganda 2 Centre for Analysis and Synthesis, Department of Chemistry, Lund University, Sweden The catalytic functionalization of hydrocarbons is one process where catalysts are desperately needed. This is because hydrocarbons are intrinsically inert in nature and present a formidable challenge during their transformation to other compounds. Yet, hydrocarbons are the most abundant and inexpensive organic chemicals in nature and typically make up 50% of petroleum. Functionalization of their ubiquitous C-H bond would enable an easy utilization of hydrocarbons in the synthesis of many organic compounds. However, such an abundant resource is presently so difficult to utilize. In comparison to noble metals, iron has achieved very little success in catalytic functionalization of hydrocarbons due to factors associated with its electronic environment. Yet, from the cost point of view, iron would be the metal of choice. However, the properties of iron complexes towards any chemical processes can be tuned through variation of their supporting ligands, thus making it applicable in a variety of catalytic conversions. It is against this background that we sought to design a special class of pentadentate N-donor chelate ligands which, when metalated with a cheap metal like iron would form a series of iron complexes of the type [(L)Fe(solv)] 2+ . The new Fe(II) complexes are based on the pentadentate ligand 1,1-di(2-pyridyl)- N,N-bis(2-pyridylmethyl)methanamine (N4Py) modified by quinolyl (Q) or phenyl pyridyl(Py-PhCl) moieties on the amino function. These complexes were characterized by X-ray crystallography, NMR, UV-Visible, infrared spectroscopy and cyclic voltammetry. One of the complexes, 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. The potential of these complexes to coordinate oxygen (O 2 ) was mimicked by employing nitric oxide (NO) to form [(L)Fe(NO)] 2+ 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. This work provides is a promising milestone in the attempt to utilize unreactive hydrocarbons.
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