Experimental and Theoretical Evaluation of Four NLO-Active Divalent Transition-Metal Complexes Supported by an Enantiomerically Pure Tetradentate Schiff Base Ligand

This paper reports on a series of five chiral Schiff base push-pull compounds derived from enantiomerically pure (1R,2R)-(−)-1,2-diaminocyclohexane. The Ni^II, Cu^II, Zn^II and Pd^II complexes supported by a ferrocene-containing unsymmetrically-substituted N₂O₂ quadridentate Schiff base ligand were prepared via template reactions and isolated in 74–87 % yields. A combination of EA, IR, UV/vis and ¹H/¹³C{¹H} NMR spectroscopy, HRMS spectrometry, cyclic voltammetry and single-crystal X-ray analysis (for the proligand 2 and its Ni^II derivative 3), together with computational methods (DFT and TD-DFT) was used to fully characterize and study the properties of all products. Both 2 and 3 crystallize in the orthorhombic non-centrosymmetric space group P2₁2₁2₁, with two (R,R)-(−)-chiral carbon atoms in their structure. Second-order nonlinear polarizabilities β have been measured by using Harmonic Light Scattering at 1.91 μm, with the Pd(II) species showing the higher NLO response of 460×10⁻³⁰ esu.