Nickel(II) complex with a tridentate ONS-Schiff base ligand: X-ray structure,VIS and NIR solution spectra and kinetics of ligand substitution

The ligand N-(3-thia-n-pentyl)salicylaldimine (Hsalen-SEt) and its green nickel(II) complex Ni(salen-SEt)₂ was prepared. The complex crystallizes in the orthorhombic space group Pbca with a a = 2538.3(4) pm, b = 1490.0(3) pm, c = 1163.5(2) pm and Z = 8. The coordination sphere of the nickel is a distorted octahedron with two oxygen atoms in a cis-position, two nitrogen atoms in a trans-position and two sulfur atoms in a cis-position. The two NiO distances were 197.8 and 198.1 pm, the two NiN distances 201.8 and 200.6 pm, whereas the two NiS distances are 272.0 and 266.3 pm. The magnetic susceptibility of Ni(salen-SEt)₂ was measured in the temperature range 2.6-281 K, the magnetic moment being μ_eff=3.02μ_B.The VIS and NIR solution spectra of the complex in different solvents indicate that the two tridentate ONS-ligands are coordinated as bidentate ON-ligands, the coordination geometry being square-planar (trans- N₂O₂) in non-coordinating solvents (e.g., toluene) and octahedral in coordinating solvents (e.g., pyridine), due to addition of two solvent molecules. From spectrophotometric titration the individual complex formation constants for the species Ni(salen-SEt)₂·py (K₁ and Ni(salen-SEt)₂·2py (K₂) were found to be K₁ = 1.76 ± 0.40 M^-1 and K₂ = 145 ± 34 M^-1. The kinetics of the reaction Ni(salen-SEt)₂ + H₂salen → Ni(salen) + 2Hsalen-SEt as studied in acetone by stopped-flow spectrophotometry follow the rate law, rate = (k^S = k_H2^salen H₂salen]) × [Ni(salen-SEt)₂] with k_S = 0.038 ± 0.013 s^-1 and k_H2^salen = 17.2 ± 0.4 M^-1 s^-1 at 25°C. The spectroscopic and kinetic properties of Ni(salen-SEt)₂ are compared with those of bis(N-alkyl-salicylaldiminato)nickel(II) complexes