Spectroelectrochemical, photochemical and theoretical study of octaazamacrocyclic nickel(II) complexes exhibiting unusual solvent-dependent deprotonation of methylene group

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

“…Our recent studies of 1-3 confirmed that the interaction of the CH 2 group in the macrocycle with dimethylformamide (DMF) results in the deprotonation of the macrocycle in 1-3. 23 The optical spectra of the dinickel(II) complex isomers 4a and 4s are very similar and together with 5s and 6a resemble the spectra of the corresponding mononuclear species 1-3 in CH 2 Cl 2 and CH 3 CN. The absorption intensity is roughly doubled (when comparing the spectra of dimers to monomers at the same concentration) as expected for complexes carrying two identical weakly interacting chromophores (Fig.…”

“…The starting complexes 1-3 showed quite interesting redox non-innocent electrochemical (EC) and spectroelectrochemical (SEC) behavior. 22,23 Therefore, it was of interest to study the electrochemical properties of dinickel(II) complexes 4-6 in comparison to the previously investigated 1-3.…”

“…Quite recently, we described the template synthesis of nickel(II) complexes with 15-membered octaazamacrocyclic ligands 1-3 (Chart 1) and their structures, as well as the spectroscopic properties of the 1e-oxidized and 1e-reduced species. 22,23 We also explored their ability to catalyze the microwave-assisted solvent-free oxidation of cyclohexane by tert-butyl hydroperoxide (TBHP) to the industrially significant KA oil (a cyclohexanone and cyclohexanol mixture). One issue that remained unaddressed in the reported works is whether these coordinated macrocycles can be joined into dimers via C-C bond formation between the γ-carbon atoms of the former Hacac moieties, even though these are now constituent atoms of a seven-membered chelate ring (Ni1NCCCNN1, see Chart 1).…”

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).…”