Syntheses, Electronic Structures, and EPR/UV−Vis−NIR Spectroelectrochemistry of Nickel(II), Copper(II), and Zinc(II) Complexes with a Tetradentate Ligand Based on S-Methylisothiosemicarbazide

Abstract

Template condensation of 3,5-di-tert-butyl-2-hydroxybenzaldehyde S-methylisothiosemicarbazone with pentane-2,4-dione and triethyl orthoformate at elevated temperature resulted in metal complexes of the type MIIL, where M = Ni, Cu and H2L = novel tetradentate organic ligand. These complexes are relevant to the active site of the copper enzymes galactose oxidase and glyoxal oxidase. Demetallation of NiIIL with gaseous hydrogen chloride in chloroform afforded the metal-free ligand H2L. Then by the reaction of H2L with Zn(CH3COO)2·2H2O in 1:1 molar ratio in chloroform/methanol 1:2 the complex ZnIIL(CH3OH) was prepared. The three metal complexes and the prepared ligand were characterized by spectroscopic methods (IR, UV–vis and NMR spectroscopy), X-ray crystallography and DFT calculations. Electrochemically generated one-electron oxidized metal complexes [NiL]+, [CuL]+ and [ZnL(CH3OH)]+ and the metal-free ligand cation radical [H2L]+• were studied by EPR/UV–vis–NIR and DFT calculations. These studies demonstrated the interaction between the metal ion and the phenoxyl radical.

Charge and Spin States in Schiff Base Metal Complexes with a Disiloxane Unit Exhibiting a Strong Noninnocent Ligand Character: Synthesis, Structure, Spectroelectrochemistry, and Theoretical Calculations

“…This signal becomes only slightly anisotropic with a rhombic pattern at low temperatures (see red trace 4 in Figure a) indicating a small contribution of Ni­(II) with its more than half-filled 3d 8 configuration. On the other hand, for the one-electron oxidized Ni­(II) complex with a single phenolic moiety, suitably protected by bulky tert -butyl groups in the 3,5-positions of the parent phenol, we observed anisotropic EPR spectra from two different species with different g values at low temperatures (see red trace 3 in Figure a), in contrast to the single isotropic narrow EPR signal ( S = 1 / 2 ) measured at room temperature (see black trace 3 in Figure a) . At room temperature, the g value of 2.014 is slightly higher than that for the complex with a thiophenyl group in position 3 of phenolic moiety, and this EPR signal is more anisotropic at 110 K. This indicates the formation of a ligand-centered radical upon one-electron oxidation but with slightly more pronounced delocalization of the unpaired spin onto orbitals of the nickel ion.…”

“…For the Ni­(II) complex with a tetradentate ligand based on S -methylisothiosemicarbazide, we confirmed the temperature-dependent valence tautomerism between nickel­(III)-phenolate species and its nickel­(II)-phenoxyl radical counterpart. At room temperature, the [Ni II (L •– )] + species dominates, while at 77 K both [Ni II (L •– )] + and [Ni III (L 2– )] + are present in an approximately 1:1 molar ratio …”

5,5′-Bis-vanillin derivatives as discriminating sensors for trivalent cations

“…5 On the other hand, the semicarbazone group also has been used as a complexing moiety. 6 This group presents an additional carbonyl group that can be involved in complexation giving rise to more stable complexes. Among the different dialdehydes that could be used in the synthesis of dihydrazones or disemicarbazones we decided to explore the utility of 5,5 0 -bis-vanillin.…”

In Situ and Operando Techniques for Investigating Electron Transfer in Biological Systems

“…The first approach is more precise, allowing the corresponding changes to be evaluated with greater control without the perturbation induced during the transfer process. In the latter case, however, the intermediates are EPR‐active only at low temperatures, and redox measurements must be carried out in the frozen state, usually at liquid nitrogen temperature (77 K), which makes this approach more difficult to implement and suitable only for few molecules [16] . EPR spectroelectrochemical experiments are particularly useful for determining the half‐wave potential of redox couples when the corresponding cyclic voltammograms are very broad because of the slow ET at the electrode surface.…”