Copper-Sulfur, Copper-Oxygen, and Copper-Alkyl Complexes Relevant to Copper Protein Active Site Intermediates and Catalysis

Abstract

The report of the NNN-pincer ligands, <italic>N,N'</italic>-bis(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamide and<italic>N,N'</italic>-bis(2,6-dimethylphenyl)-2,6-pyridinedicarboxamide, are presented as supporting ligands for a variety of copper(II) and nickel(II) complexes. These dianionic ligands were used due to their ability to enforce a square planar geometry in order to stabilize both the metal(II) and metal(III) complexes. The stabilization of metal(II) and metal(III) oxidation states was proposed to be a key component for obtaining mechanistic details about the oxidation pathways of these molecules, which may provide insight into the mechanistic pathways of metallo-enzymes. Specifically, efforts were aimed at the creation of copper complexes capable of C-H bond activation. This work includes the synthesis of two novel moncopper(II)-sulfur complexes supported by the sterically hindered <italic>N,N'</italic>-bis(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamide ligand: [copper(II)-SH]<super>1-</super> and [copper(II)-SSiPh₃]<super>1-</super>. These complexes have potential to act as building blocks for selectively building more complex copper-sulfur structures, such as those observed in the active sites of enzymes. Efforts to model copper proteins that active oxygen, led to an investigation of the effects of the substituent on the NNN-pincer ligand (isopropyl versus methyl substitution). The effect of steric protection on the stability and reactivity of copper(III)-OH complexes towards weak C-H bonds and their self-decay behavior is presented. In addition, this work reports the first [copper(II)-CH₂CN]<super>1-</super> complex, which was obtained from the reaction of a [copper(II)-OH]<super>1-</super> complex with acetonitrile. The [copper(II)-CH₂CN]<super>1-</super> complex can be oxidized to what is believed to be a copper(III)-CH₂CN complex based on spectroscopic data and theoretical calculations. The analogous [nickel(II)-CH₂CN]<super>1-</super> complex is also reported.