David Carrillo

We have recently reported a series of neutral square planar tridentate Schiff base (L) complexes of the general formula [(L)M(py)], showing relatively high first-order hyperpolarizabilities and NLO redox switching behavior. In the present study, new members of this family of compounds have been prepared with the objective to investigate their potential as building blocks in the on-demand construction of D-π-A push–pull systems. Namely, ternary nickel(II) building blocks of general formula [(L^A/D)Ni(4-pyX)] (4–7), where L^A/D stands for an electron accepting or donating dianionic O,N,O-tridentate Schiff base ligand resulting from the monocondensation of 2-aminophenol or its 4-substituted nitro derivative and β-diketones R-C(=O)CH₂C(=O)CH₃ (R = methyl, anisyl, ferrocenyl), and 4-pyX is 4-iodopyridine or 4-ethynylpyridine, were synthesized and isolated in 60–78% yields. Unexpectedly, the Sonogashira cross-coupling reaction between the 4-iodopyridine derivative 6 and 4-ethynylpyridine led to the formation of the bis(4-pyridyl) acetylene bridged centrosymmetric dimer [{(L^D)Ni}₂(µ²-py-C≡C-py)] (8). Complexes 4–8 were characterized by elemental analysis, FT-IR and NMR spectroscopy, single crystal X-ray diffraction and computational methods. In each compound, the four-coordinate Ni(II) metal ion adopts a square planar geometry with two nitrogen and two oxygen atoms as donors occupying trans positions. In 8, the Ni…Ni separation is of 13.62(14) Å. Experimental results were proved and explained theoretically exploiting Density Functional Theory calculations.

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 = Ni 1, Cu 2; R = ferrocenyl = Fc: M = Ni 3, Cu 4) 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, ¹H NMR for diamagnetic Ni(II) derivatives 1 and 3, and HRMS for paramagnetic Cu(II) species 2 and 4. The molecular structures of 1–3 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 of 4. In 1–3, the four-coordinate nickel and copper atoms adopt a square planar geometry, whereas in 4′ 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 compounds 3 and 4. Reversible redox switching can be established for 1 and 3 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 of 1–4 along with those of assumed bispyrylium dimeric species 5 and 6, generated by chemical oxidation of 1 and 3, 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 for 1–4. The β value of 5 (800 × 10⁻³⁰ esu) was found to be almost twice that of its monomeric precursor 1, whereas the β value of 6 (160 × 10⁻³⁰ esu) is reduced by half with respect to that of 3. In 5 the anisyl retains its donor ability whereas in 6 the electron-donating character of the oxidized ferrocenyl moiety is cancelled. Optimized geometries of the four compounds 1–4 as well as their electronic structures and that of their respective cations 1⁺–4⁺ have been analyzed through DFT calculations, while TD-DFT computation has been used to interpret the major features of the UV-vis spectra.

Since their development in the 1930s, Schiff-base complexes have played an important role in the field of coordination chemistry. Here, we report the synthetic, spectral, structural, magnetic and electrochemical studies of two new pentacoordinated neutral chloro-iron(III) complexes (3,5) supported by dianionic [N₂O₂]²⁻ tetradentate Schiff-base ligands unsymmetrically substituted by either a pair of acceptor (F and NO₂) or donor (ferrocenyl and OCH₃) groups. The electron-withdrawing Schiff-base proligand 2 and the complexes 3 and 5 were prepared in good yields (79–86%). Complex 3 was readily obtained upon reaction of 2 with anhydrous iron chloride under basic conditions, while the bimetallic derivative 5 was synthesized by condensation of the free amino group of the ferrocenyl-containing O,N,N-tridentate half-unit 4 with 5-methoxysalicylaldehyde in the presence of FeCl₃. The three new compounds were characterized by elemental analysis, FT-IR, UV–Vis, mass spectrometry and in the case of 2 by multinuclear NMR spectroscopy. The crystal structures of 3 and 5 revealed that in the two five-coordinate monomers, the iron atom showed distorted square-pyramidal geometry, with the N and O atoms of the Schiff-base ligand occupying the basal sites and the chlorine atom at the apex of the pyramid. Magnetic measurements showed a high-spin configuration (S = 5/2) for the Fe(III) ion in 3 and 5. Reduction associated with the Fe(III)/Fe(II) redox couple occurred at −0.464 and −0.764 V vs. Ag/Ag⁺, and oxidation taking place at the Schiff-base ligand was observed at 1.300 and 0.942 V vs. Ag/Ag⁺ for 3 and 5, respectively. A high-electronic delocalization of the Schiff-base ligand substituted by fluoro and nitro groups stabilizes the Fe(II) oxidation state and shifts the redox potential anodically.

A series of symmetrical D–π–D type complexes based on ONO tridentate Schiff base nickel(II) and copper(II) building blocks connected by a bidentate conjugated spacer [(R-ONO)M(μ-4,4′-bipy)M(R-ONO)] (M = Ni^II, R = An = anisyl (1), R = Fc = ferrocenyl (2); M = Cu^II, R = An (3), R = Fc (4); 4,4′-bipy = 4,4′-bipyridine) have been prepared in 70–85% yield. The four new compounds were characterized by elemental analysis, FT-IR and UV-vis spectroscopy, and by ¹H-NMR spectroscopy in the case of the diamagnetic Ni^II derivatives 1 and 2. The molecular structures of complexes 1–3 were further confirmed by single-crystal X-ray diffraction methods, along with that of the doubly phenoxide-bridged derivative [Cu₂(μ-Fc-ONO)₂] 5, formed upon dissociation of the 4,4′-bipy spacer from 4. In 1, 2 and 5, the two four-coordinated nickel and copper atoms adopt a square planar geometry, whereas in 3 that forms a coordination polymer through apical Cu–O interactions (d_Cu–O = 2.411(5) Å) between dimers, the Cu^II metal ion is five-coordinated in a square pyramidal environment. The electrochemical behavior of compounds 1–4 has also been investigated by cyclic voltammetry. Variable temperature (2–300 K) magnetic susceptibility measurements reveal a weak antiferromagnetic coupling (J = −0.83 cm⁻¹) in 3, and that there is no interaction between the two Cu^II spins carriers in 4.

The crystal and molecular structure of the complex [CpFe(h6-C6H5)-N(Me) N=CH-C6H4-4-NMe2]+PF6-, [1]+PF6-, was solved by single crystal X-ray diffraction analysis. One of the most salient features observed in this structure is the depyramidalization of both the benzylic nitrogen and the dimethylamino nitrogen atoms which reveals the delocalization of the p-electron system along the entire hydrazone skeleton from the donor to the acceptor termini, through a quinonoidal deformation and a pseudo-cyclohexadienyl conformation (folding dihedral angle of ca. 7.4°) of the free and coordinated phenyl rings, respectively. Both Cipso-N bond lengths are virtually identical (1.37 Å), lying between pure single and double bonds. These and other peculiarities are described and compared with the structures of other organometallic hydrazones