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

Abstract

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- N 2O 2) 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 ( K 1 and Ni(salen-SEt) 2·2py ( K 2) were found to be K 1 = 1.76 ± 0.40 M -1 and K 2 = 145 ± 34 M -1. The kinetics of the reaction Ni(salen-SEt) 2 + H 2salen → Ni(salen) + 2Hsalen-SEt as studied in acetone by stopped-flow spectrophotometry follow the rate law, rate = ( k S = k H 2 salen H 2salen]) × [Ni(salen-SEt) 2] with k S = 0.038 ± 0.013 s -1 and k H 2 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