Pincer ligand complexes for the study of nickel oxygen species

Abstract

The catalytic oxidation of organic molecules in an environmentally friendly and cost effective manner represents a substantial challenge. One approach employed to address this challenge has been to draw inspiration from biological systems, which utilize base metal ions in a wide variety of catalytic reactions. The recent discovery of a nickel containing dioxygenase enzyme has triggered an interest in nickel based oxidation catalysis. This thesis addresses the synthesis of a variety of pincer ligand supported nickel complexes and their reactivity with dioxygen. Two pincer ligand systems were investigated, namely PCP and NCN pincer ligands. The synthesis and characterization of [PCPCy2Ni(OH2)]BF4, PCPCy2Ni(BF4), [PCPCy2Ni(CN-t-Bu)]BF4, and [PCPCy2Ni(CO)]BF4, are detailed, as well as attempts to reduce these complexes to give a NiI species. The strongly pi-accepting ligands were introduced in the hope that they would stabilize a NiI species and prevent disproportionation. The only discernible products of reduction attempts were colloidal nickel and starting material, suggesting that disproportionation occurs even in the presence of strongly pi-accepting ligands. Nickel complexes supported by the NCN pincer ligand system were also examined. The synthesis and characterization of [NCN(iPr)2Ph]NiBr is discussed. Reduction of [NCN(iPr)2Ph]NiBr with KC8 gives a bimetallic complex featuring a NiI-NiI bond. This bimetallic core is supported by two NCN ligands. While each ligand is bound in a mu-eta(2):eta(1) fashion, the overall structure is asymmetric, with the two Ni ions displaying tau(4) values of 0.46 and 0.65. This species, [NCN(iPr)2PhNi]2, readily reacts with dioxygen to give the terminal hydroxo complex, [NCN(iPr)2Ph]NiOH, which has been independently synthesized. No intermediate species were detected in the activation of dioxygen by [NCN(iPr)2PhNi]2 at -78 °C by UV-Vis. In an attempt to further understand the effect of ligand substituents on the structure of [NCNRNi]2 complexes, a new, general synthetic route for [NCNR]NiX complexes was investigated. This new route focused on metalation of a protoligand, followed by installation of the imine arms. Trans-(4-tert-butyl-2,6-diformylphenyl)chloro-bis(triphenylphosphine)nickel was successfully synthesized. However, attempts to install the imine moiety were unsuccessful.