Nickel(II) and copper(II) complexes of new unsymmetrically-substituted tetradentate Schiff base ligands: Spectral, structural, electrochemical and computational studies

Abstract

International audienceThe synthesis, spectroscopic and structural characterization, electrochemical properties and theoretical studies of a series of eight robust neutral Nickel(II) and Copper(II) complexes (4-11) supported by unsymmetrically-substituted N2O2-tetradentate Schiff base ligands are reported. The M(salophen)-type compounds are substituted by either a pair of donor (anisyl, ferrocenyl, methoxy) or acceptor (fluoro, nitro) groups, forming D-pi-D and A-pi-A systems, respectively. The compounds were prepared in good yields by condensation of the free amino group of the desired ONN-tridentate half-unit with the appropriate substituted salicylaldehyde in the presence of hydrated Nickel(II) or Copper(II) acetate salts. They were characterized by elemental analysis, FT-IR, UV-vis, and for diamagnetic species by multinuclear NMR spectroscopy, mass spectrometry and cyclic voltammetry. The crystal structures of one Ni(II) (4) and four Cu(II) complexes (5, 7, 9 and 11) revealed a four-coordinate square-planar environment for the nickel and copper metal ions, with two nitrogen and two oxygen atoms as donors. In 4, 5, 7 and 9, the crystallization solvent interacts through hydrogen bonding with the phenolato oxygen atoms of the Schiff base pocket, while 11 packs as centrosymmetric dimers with an apical CuAO short contact interaction (2.63 angstrom). the cyclovoltammograms of the nickel complexes present an irreversible monoelectronic Ni(II)/Ni(I) reduction wave while those of their copper counterparts exhibit a reversible or quasi-reversible one-electron Cu(II)/Cu(I) redox process. The electronic structures of the eight complexes were analyzed by DFT and TD-DFT calculations

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

“…This electrochemical behavior is reminiscent of that observed with [Fe(salen)Cl] derivatives possessing either symmetrically [28] or unsymmetrically substituted N 2 O 2 tetradentate Schiff-base ligands [52]. Here, the observed opposite redox behavior between 3 and 5 is in accordance with the electronic donor/acceptor ability of the two ligands as previously observed in the Ni(II) and Cu(II) series [29]. On the other hand, the CV of the homobimetallic complex 5 also presents a quasi-reversible redox process at 0.333 V (∆E p = 118 mV, i pc /i pa = 0.85) attributed to the monoelectronic oxidation of the ferrocenyl fragment that is anodically shifted by~160 mV with respect to free ferrocene under the same experimental conditions.…”

“…The acceptor unsymmetrically substituted Schiff base 2 was synthesized by the classical condensation reaction between half-unit (4-F-C6H4)-C(O)CH=C(CH3)-N(H)-C6H4-NH2 (1) [29] and 5-nitrosalicylaldehyde, in methanol at room temperature for 6 h (Scheme 1). Compound 2 was isolated as an air-and thermally stable yellow solid in good yield (79.7%).…”

“…As all attempts to synthesize and isolate the related electron-rich unsymmetrically substituted Schiff-base starting material were unsuccessful, the synthesis of the binuclear complex 5 was carried out via a previously used one-pot template methodology [29], that is, the condensation of the organometallic ONN-tridentate half-unit 4 [33] with 5-methoxy-2-hydroxibenzaldehyde in the presence of FeCl3 (Scheme 3). Complex 5 was isolated as black needles in 79.1% yield.…”

Synthesis, Characterization and Antibacterial Activities of New Bivalent Complexes of Transition Metals

“…These values depict the covalent character of the metal ligand bond. The values of the ν2/ν1 ratio at 1.504 suggest the octahedral structure of complexes [17][18][19].…”

Synthesis, structures, electrochemical and quantum chemical investigations of Ni(ii) and Cu(ii) complexes with a tetradentate Schiff base derived from 1-(2-thienyl)-1,3-butanedione

“…In the spectrum of the free Schiff base 2, the weak band observed at 3444 cm -1 is due to the ν(N-H) vibration, thus indicating that the enamine tautomer is the predominant form in the solidstate. 22 Three strong characteristic absorptions due to the [O … C … C … C … N] enaminone core are also observed at 1595, 1558 and 1529 cm -1 . In the spectra of complexes 3 and 4, those bands decrease in energy by 14/7, 28/31 and 21/23 cm -1 , respectively, in agreement with coordination of the dianionic Schiff base ligand 2 2to the Ni(II) and Cu(II) ions through the oxygen and nitrogen atoms, thus forming a six-membered heterometallacycle.…”