Four-coordinate nickel(ii) and copper(ii) complex based ONO tridentate Schiff base ligands: synthesis, molecular structure, electrochemical, linear and nonlinear properties, and computational study

Abstract

We report the synthesis, characterization, crystal structures, nonlinear-optical (NLO) properties, and density functional theory (DFT) calculations of nickel(ii) and copper(ii) complex based ONO tridentate Schiff base ligands: two mononuclear compounds, [Ni(An-ONO)(NC5H5)] (5) and [Cu(An-ONO)(4-NC5H4C(CH3)3)] (6), and two heterobimetallic species, [M(Fc-ONO)(NC5H5)] (M = Ni, 7; Cu, 8), where An-ONOH2 (3) and Fc-ONOH2 (4) are the 1 : 1 condensation products of 2-aminophenol and p-anisoylacetone and ferrocenoylacetone, respectively. These compounds were characterised by microanalysis, FT-IR and X-ray crystallography in the solid state and in solution by UV-vis and (1)H and (13)C NMR spectroscopy. The crystal structures of 3-5, 7 and 8 have been determined and show for Schiff base complexes 5, 7 and 8 a four-coordinated square-planar environment for nickel and copper ions. The electrochemical behavior of all derivatives 3-8 was investigated by cyclic voltammetry in dichloromethane, and discussed on the basis of DFT-computed electronic structures of the neutral and oxidized forms of the compounds. The second-order NLO responses of 3-8 have been determined by harmonic light scattering measurements using a 10(-2) M solution of dichloromethane and working with a 1.91 μm incident wavelength, giving rather high β1.91 values of 350 and 290 × 10(-30) esu for the mononuclear species 5 and 6, respectively. The assignment and the nature of the electronic transitions observed in the UV-vis spectra were analyzed using time-dependent (TD) DFT calculations. They are dominated by LMCT, MLCT and π-π* transitions.

Why is Ferrocene so Exceptional?

“…Ferrocenes such as the seminal example of cis ‐1‐ferrocenyl‐2‐(4‐nitrophenyl)ethylene (Figure , left) were found by Green's group, followed by other groups, to show nonlinear optical (NLO) properties, with applications in optical signal processing such as the frequency doubling of laser light.…”

“…It turned out that both the reduction reactions and the organic connectors had a significant influence on the 4,4′‐bipyridinium system . The groups of Hamon, Saillard, Carrillo, Manzur, and Ledoux‐Rak developed another remarkable series of molecular systems in which ferrocene is conjugated with Schiff bases; excellent NLO properties were disclosed . The largest quadratic hyperpolarizability value (970 × 10 –30 esu) was determined for palladium‐containing dimetallic ferrocene Schiff‐base species .…”

Pentacoordinated Chloro-Iron(III) Complexes with Unsymmetrically Substituted N2O2 Quadridentate Schiff-Base Ligands: Syntheses, Structures, Magnetic and Redox Properties

“…This proligand is soluble in the common organic solvents except diethyl ether and hydrocarbons. The neutral mononuclear pentacoordinate chloro-iron(III) complex 3 was synthesized following a known one-pot two-step procedure [32]. First, the diprotic derivative 2 was doubly deprotonated with potassium tert-butoxide in THF at ambient temperature, before addition of a THF solution of anhydrous iron(III) chloride (Scheme 2).…”

Redox-switching of ternary Ni(ii) and Cu(ii) complexes: synthesis, experimental and theoretical studies along with second-order nonlinear optical properties

“…The mononuclear (1, 2) and bimetallic (3, 4) complexes were readily prepared by reacting the known tridentate Schiff-base ligand precursors R-C(O)CH=C(CH3)NH-C6H4-2-OH (R = anisyl (L1), ferrocenyl(L2)) 18 with one equiv. of hydrated M(II) nitrate salt (M = Ni, Cu), and a slight excess of the electro-active pyridyl 2,6-diphenylmethylenepyran (PyMP), under basic conditions in THF for 12 h at room temperature (see Experimental for details and Scheme 1).…”

“…These four-coordinate Ni(II) and Cu (II) chromophores displayed good second-order NLO polarizabilities, with β values ranging from 170-350 x 10 -30 esu (HRS with 1.91 μm incident wavelength). 18 In a more recent paper, we showed that the [(R-ONO)M] units can be used as building blocks for the construction of expanded and more complex architectures such as dimers and coordination polymers, taking advantage of the lability of the pyridine ancillary ligand. 19 On the other hand, electrochemical studies have shown that the organic chromophores ferrocenyl-, [20][21][22][23] pyridyl-, 21 and phenylmethylenepyrans 24 can undergo fast dimerization through C-C σ-bond making upon oxidation, and that the resulting bispyrylium species are irreversibly reduced back to the initial methylenepyran by C-C bond breaking.…”