Synthesis, crystal structures and electronic properties of Schiff base nickel (II) complexes: Towards solvatochromism induced by a photochromic solute

Two new mononuclear nickel (II) complexes incorporating Schiff base ligands, bis(N-R-1-phenylethyl- or N-2-propyl-3,5-dichlorosalicyclenaminato) nickel (II), have been synthesized and characterized. The former chiral brown paramagnetic complex adopts a compressed tetrahedral [NiN2O2] coordination geometry of Lambda(R,R) absolute configuration with the coordination bond angles of N-Ni-N = 116.3(2)degrees and O-Ni-O = 141.4(2)degrees. On the other hand, the latter green diamagnetic complex adopts a square planar trans-[NiN2O2] coordination geometry. It is revealed that flexible distortion of the [NiN2O2] coordination environment and the overall dipole moment of the complexes are affected by both electronic withdrawing Cl-substituents and steric factors of the ligands. A structural phase transition occurs by heating in the solid state for both complexes. Negative solvatochromism relating to d-d and pi-pi* transitions can be observed, depending on the polarity of the solvents in which these complexes are dissolved. Furthermore, we have attempted to design a supramolecular solvatochromism system induced by a photochromic solute, that is a CHCl3 solution of the chiral Ni(II) complex and polar 4-hydroxyazobenzene solute. After UV light irradiation, we have successfully observed slight spectral changes of the pi-pi* bands in absorption and CD spectra due to conformational change of the R-1-phenylethylamine moieties of the chiral ligands. (c) 2005 Elsevier Ltd. All rights reserved.