Electrochemical synthesis and crystal structure of zinc(II) complexes with N2N′2S2 amide–thioether hexadentate ligands

Correlation between Second Ionization Potential and Nonlinear Optical Properties of Bivalent Transition-Metal Complexes: A Quantum Chemical Study

Discovering new materials with excellent nonlinear optical responses has recently become a very interesting research topic in the different domains of materials science. Currently, density functional theory (DFT) has been shown to be a powerful tool in the explanation and prediction of the performance of novel nonlinear optical (NLO) materials. Quantum chemical calculations using DFT/TD-DFT with the B3LYP exchange-correlation functional are reported to study the NLO properties of 26 bivalent transition-metal (TM) complexed by six acyclic hexadentate ligands providing pyridyl/pyrazine-amide-thioether/ether coordination and differing by the nature of the methylene dichalcogenate spacer between the rings. However, the geometry parameters and the theoretically predicted UV-vis absorption spectra of the optimized compounds M(II)Lⁱ are in excellent agreement with the experiment, when available, the trends among the nature of the TM, the importance of the ligand spacer, and of the substituents of the pyridine/pyrazine amide ligand are discussed. To the best of our knowledge, our work evidences for the first time that the hyper-polarizability, second harmonic generation, and hyper-Rayleigh scattering response of TM coordination complexes can be correlated to the second ionization potential of metal and spin state of complexes.

Low-spin [MII(L)2] and [MIII(L)2]+ (M = Fe and Co) complexes of tridentate azo-containing pyridine/pyrazine amide ligands: structures, properties and redox potential correlations

Using deprotonated forms of tridentate azo-containing pyridine-2-/pyrazine-2-carboxamide 2-[N-(2-phenylazo)carbamoyl]-pyridine/pyrazine, seven bis-ligand complexes of Fe^II/Co^II and Fe^III/Co^III have been synthesized. Molecular structures of six of them reveal that these six-coordinate complexes utilize all available donor sites of the ligands and assume M^II/IIIN₂(pyridine/pyrazine)N'₂(amide)N''₂(azo) coordination. Complexes of Fe^II and Co^III are diamagnetic and those of Fe^III and Co^II are paramagnetic (S = 1/2; room-temperature magnetic data and EPR spectra). Cyclic voltammetry experiments in CH₂Cl₂ reveal facile metal-centred Fe^III/Fe^II and Co^III/Co^II redox responses, and all complexes display quasireversible-to-irreversible ligand(azo)-centred redox processes. The E_1/2 values of M^III/M^II redox processes for Fe, Co and Ni (reported earlier) complexes of the pyridine amide ligand linearly correlate with those for six-coordinate [M^III(bpy)₃]³⁺/[M^II(bpy)₃]²⁺, [M^III(terpy)₂]³⁺/[M^II(terpy)₂]²⁺, [M^III(L)]⁺/[M^II(L)]⁰ or [M^III(L')₂]⁺/[M^II(L')₂]⁰ (bpy = 2,2'-bipyridine, terpy = 2,2':6',2''-terpyridine, hexadentate L(2-) = 1,4-bis[o-(pyridine-2-carboxamidophenyl)]-1,4-dithiobutane and tridentate L'(-) = {2-[2-(arylimino)phenylazo]-pyridine}) couples. Density functional theory (DFT) at the B3LYP level and time-dependent (TD)-DFT calculations rationalize the electronic structure of the present complexes and throw light on the origin of observed electronic transitions.