Redox-switching of ternary Ni(ii) and Cu(ii) complexes: synthesis, experimental and theoretical studies along with second-order nonlinear optical properties12. (17th June 2019) - Novoa, Néstor
Abstract : The redox-switching behavior of the title compounds has been probed for the Ni(ii ) derivatives using time-resolved spectroelectrochemistry under thin-layer conditions. Abstract : Four new ternary Ni(ii ) and Cu(ii ) complexes of ONO tridentate Schiff base and pyridylmethylenepyran (PyMP) ligands [(R-ONO)M(PyMP)] (R = anisyl = An: M = Ni1, Cu2 ; R = ferrocenyl = Fc: M = Ni3, Cu4 ) have been synthesized under facile reaction conditions starting from R-ONOH2, metal(ii ) nitrate salts, and PyMP; R-ONOH2 stands for the Schiff base ligand precursors obtained by condensation of either 1-anisyl- or 1-ferrocenyl-butan-1, 3-dione and 2-aminophenol. They have been thoroughly characterized with the help of various physicochemical tools, such as CHN analyses, IR and UV-vis spectra, 1 H NMR for diamagnetic Ni(ii ) derivatives1 and3, and HRMS for paramagnetic Cu(ii ) species2 and4 . The molecular structures of1–3 were authenticated by single-crystal X-ray diffraction methods, along with that of the doubly phenoxide-bridged dimer [Cu2 (μ-ONO-Fc)2 (PyMp)2 ] (4′ ), resulting from the recrystallization of4 . In1–3, the four-coordinate nickel and copper atoms adopt a square planar geometry, whereas in4′ the Cu II metal ion is five-coordinated in a square pyramidal environment with the pyridyl nitrogen occupying the apex. Electrochemical studies reveal two well-separated redox waves. The spin density distribution analyses reveal that the initial oxidation process is associated with a ligand-based level, with some ferrocenyl participation for the heterobimetallic compounds3 and4 . Reversible redox switching can be established for1 and3 by time-resolved spectroelectrochemistry under thin-layer conditions where electrochemical cycling is associated with a significant modification of the UV-vis spectra of the chromophores. The second-order nonlinear optical responses of1–4 along with those of assumed bispyrylium dimeric species5 and6, generated by chemical oxidation of1 and3, respectively, have been determined by harmonic light scattering measurements in dichloromethane solutions at 1.91 μm incident wavelength. Rather high β values ranging from 270–530 × 10 −30 esu were determined for1–4 . The β value of5 (800 × 10 −30 esu) was found to be almost twice that of its monomeric precursor1, whereas the β value of6 (160 × 10 −30 esu) is reduced by half with respect to that of3 . In5 the anisyl retains its donor ability whereas in6 the electron-donating character of the oxidized ferrocenyl moiety is cancelled. Optimized geometries of the four compounds1–4 as well as their electronic structures and that of their respective cations1 + –4 + have been analyzed through DFT calculations, while TD-DFT computation has been used to interpret the major features of the UV-vis spectra.