Nickel(II) Complexes with Redox Noninnocent Octaazamacrocycles as Catalysts in Oxidation Reactions

Abstract

Nickel(II) complexes with 15-membered (<b>1</b>-<b>5</b>) and 14-membered (<b>6</b>) octaazamacrocyclic ligands derived from 1,2- and 1,3-diketones and <i>S</i>-methylisothiocarbohydrazide were prepared by template synthesis. The compounds were characterized by elemental analysis, electrospray ionization mass spectrometry, IR, UV-vis, ¹H NMR spectroscopies, and X-ray diffraction. The complexes contain a low-spin nickel(II) ion in a square-planar coordination environment. The electrochemical behavior of <b>1</b>-<b>6</b> was investigated in detail, and the electronic structure of 1e-oxidized and 1e-reduced species was studied by electron paramagnetic resonance, UV-vis-near-IR spectroelectrochemistry, and density functional theory calculations indicating redox noninnocent behavior of the ligands. Compounds <b>1</b>-<b>6</b> were tested in the microwave-assisted solvent-free oxidation of cyclohexane by <i>tert</i>-butyl hydroperoxide to produce the industrially significant mixture of cyclohexanol and cyclohexanone (i.e., A/K oil). The results showed that the catalytic activity was affected by several factors, namely, reaction time and temperature or amount and type of catalyst. The best values for A/K oil yield (23%, turnover number of 1.1 × 10²) were obtained with compound <b>6</b> after 2 h of microwave irradiation at 100 °C.