Synthesis, spectral, electrochemical, crystal structures and nonlinear optical properties of unsymmetrical Ni(II) and Cu(II) Schiff base complexes

Abstract

International audienceThis paper explores the synthesis, spectroscopic and structural characterization, linear and nonlinear optical properties, as well as the electrochemical behavior of two series of robust neutral Ni(II)- and Cu(II)-centered organometallic macroacyclic salen-type Schiff base complexes, namely, the 5-nitrosubstituted derivatives M[Fc-C(O)CH = C(CH3)N-o-C6H4-N = CH-(2-O,5-NO2-C6H3)] (M = Ni (2), Cu (3)), and their 3,5-difluorosubstituted analogues M[Fc-C(O)CH = C(CH3)N-o-C6H4-N = CH-(2-O-3,5-F2-C6H2)] (M = Ni (4), Cu (5); Fc = ferrocenyl = (η5-C5H5)Fe(η5-C5H4))). Compounds 2–5 were synthesized in good yields (75–87%) in refluxing ethanol for 4 h, via template reaction by condensation of the tridentate half-unit metalloligand Fc-C(O)CH = C(CH3)-N(H)-o-C6H4NH2 (1) with 5-nitrosalicylaldehyde and 3,5-difluorosalicylaldehyde, respectively, in the presence of hydrated nickel(II) and copper(II) acetate salts, respectively. Their composition and identity were inferred from elemental analysis, FT-IR, UV–Vis, and multinuclear NMR spectroscopy. Single crystal X-ray diffraction analysis of the two 3,5-difluorosubstituted compounds 4 and 5 indicates a slightly bowed structure of the unsymmetrical Schiff base scaffold. Both Ni2+ and Cu2+ ions are tetracoordinated in a distorted square planar environment, with two nitrogen atoms and two oxygen atoms as donors. The electrochemical behavior of the four binuclear complexes was investigated by cyclic voltammetry, showing greater anodic shifts of the reversible redox process ascribed to the FeII/FeIII couple of the ferrocenyl donor fragment of the 5-nitrosubstituted derivatives compared to their 3,5-difluorosubstituted counterparts. Harmonic Light Scattering measurements showed that compounds 2–5 exhibited rather high second-order nonlinear responses, found between 200 and 267 × 10−30 esu, establishing a good correlation between redox potentials and the hyperpolarizability β1.91 values

Side‐Chain Metallopolymers Containing Second‐Order NLO‐Active Bimetallic NiII and PdII Schiff‐Base Complexes: Syntheses, Structures, Electrochemical and Computational Studies

“…This is expected since complexation of the N 2 O 2 chelate by Ni II is accompanied by the formation of a geometrically constrained planar structure that increases conjugation and, hence, the nonlinear efficiency. Within the 10 % measurement uncertainty, the β 1.91 values obtained for 2 and 3 are very similar to those previously determined under the same experimental conditions for unsymmetric mono‐ and binuclear first row transition‐metal Schiff‐base chromophores, [26a], By contrast, bimetallic complex 4 containing the central Pd II metal ion exhibits a remarkably large quadratic hyperpolarizability β 1.91 of 970 × 10 –30 esu, that is almost four times greater than that of its first row Ni II analog (Table ). This very high NLO response of compound 4 can be attributed to higher electronic density and participation of the metal orbitals, favoring the electronic interaction between the electro‐donor and electro‐acceptor groups throughout the entire [Pd II (N 2 O 2 )] Werner coordination core.…”

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

“…This anodic shift is presumably due to the electron-withdrawing ability of the Schiff-base chloro-iron(III) unit. In fact, we previously found such a large anodic shift (168 mV) in the case of a Cu(II) complex with a ferrocenyl-containing Schiff-base ligand bearing a nitro substituent [36]. Lastly, the shoulders at 0.500 and 0.460 V observed at v = 0.1 V/s disappear at higher scan rates, suggesting some adsorption/desorption process at the vitreous carbon working electrode.…”

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 .…”

“…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 . Depending on their substitution patterns, these ferrocene Schiff‐base complexes can be polymerized to form either main‐chain oligomers or side‐chain metallopolymers, showing a strong increase in the second‐order NLO response.…”