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

Abstract

Nickel­(II) complexes with 15-membered (1–5) and 14-membered (6) octaazamacrocyclic ligands derived from 1,2- and 1,3-diketones and S-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 1–6 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 1–6 were tested in the microwave-assisted solvent-free oxidation of cyclohexane by tert-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 6 after 2 h of microwave irradiation at 100 °C.

Diastereomeric dinickel(ii) complexes with non-innocent bis(octaazamacrocyclic) ligands: isomerization, spectroelectrochemistry, DFT calculations and use in catalytic oxidation of cyclohexane

“…The obtained yields are slightly lower than those previously reported 22 for the corresponding monomeric Ni(II) complexes, that ranged from 7 to 23% for KA oil yield, under the same MWassisted reaction conditions. The experimental results allowed to confirm the formation of cyclohexyl hydroperoxide as a primary product, as observed in other systems, 22 following a method proposed by Shul'pin, 68,69,70,71 with a significant increase of the detected cyclohexanol amount after addition of triphenylphosphine to the reaction mixture (due to the reduction of CyOOH to CyOH by PPh 3 ) and a decrease in the amount of cyclohexanone. It is assumed that the reaction should proceed through a free radical mechanism, as previously discussed in ref.…”

“…66 The studied dinickel(II) complexes performed effectively as catalysts for the above reaction, leading to cyclohexanol and cyclohexanone (KA oil). The selected reaction conditions were in line with those reported in our previous study for the complexes 1-3 22 [100 °C and 3 h of low power (25 W) MW irradiation], in solvent-free conditions. For the dinickel(II) complexes, yields of KA oil in the range of 11-16% were obtained, using a low catalyst load (Figure 12 and Table 2), with moderate turnover number (TON) values, between 28 and a maximum of 40, for determined by GC analysis (upon treatment with PPh 3 ).…”

Ni Oxidation State and Ligand Saturation Impact on the Capability of Octaazamacrocyclic Complexes to Bind and Reduce CO2

“…Herein, we focus on the analysis of the relation between the electronic structure of 15-membered octaazamacrocyclic nickel complexes and their affinity for CO 2 . We have chosen the simplest nickel(II) complex from a previously reported series (originally denoted [ NiL 1 ], herein abbreviated [ NiL ], see Figure 1 a) [ 52 , 53 ], as our trial complex with an almost completely unsaturated (π-conjugated) macrocycle. The conjugation is interrupted by an sp 3 hybridized methylene group carbon in the 15-membered chelate ring.…”

“…The conjugation is interrupted by an sp 3 hybridized methylene group carbon in the 15-membered chelate ring. Interestingly, the hydrogens of the sp 3 -hybridized carbon in [ NiL ] were found to be acidic [ 52 , 53 ]. [ NiL ] does not show any affinity forwards CO 2 upon reduction (vide infra).…”

Fe(III) Complexes in Cyclohexane Oxidation: Comparison of Catalytic Activities under Different Energy Stimuli

“…In continuation of our work on the catalytic oxidation of alkanes and related species with metal-hydrazone catalysts [24][25][26][27][34][35][36][37][38][39][40][41][42][43][44], herein, we report the synthesis and characterization of the new mononuclear Fe(III) complex [Fe(HL)(NO 3 )(H 2 O) 2 ]NO 3 (1) obtained…”