SOLVENT-DEPENDENT VARIATION OF THE SPIN STATE OF BIS(SALICYLALDOXIMATO) NICKEL(II) IN NONAQUEOUS SOLUTIONS

Abstract

Magnetic susceptibilities and chemical shifts of salicylaldoxime protons in CDCl3-DMSO-d6 solutions of bis(salicylaldoximato)Ni(II) were analysed according to the equilibrium between diamagnetic square planar Ni(Saldox)2 and paramagnetic solvates Ni(Saldox)2 · mDMSO. Magnetic susceptibility data yield thermodynamic parameters for inner sphere solvation (i.e., formation of the bis-solvate Ni(Saldox)2 · 2DMSO): δH = - 18.4 ± 2.9 kJ/mol, δS = - 102.1 ± 9.2 J/mol K and the magnetic moment of paramagnetic solvate μpara = 3.4 ± 0.2 μB. The dependences of chemical shifts of salicylaldoxime protons on solvent composition and temperature require more than two DMSO molecules solvating Ni(Saldox)2 for an adequate description. Analysis of these data according to the formation of Ni(Saldox)2 · 4DMSO yields thermodynamic parameters of both inner-sphere and outer-sphere solvation, namely δH = -56.5 ± 1.3 kJ/mol and δS = (-249.0 ± 5.0) J/mol K. This shows that outer sphere solvation is connected with higher enthalpies than inner-sphere solvation (38.1 and 18.4 kJ/mol, respectively) of this complex. This phenomenon is explained by the high contribution of electric dipole-dipole interactions of DMSO molecules in the Ni(Saldox)2 · 4DMSO solvate. Analysis of hyperfine coupling constants of the salicylaldoxime protons shows that the outer-sphere solvation of Ni(Saldox)2 proceeds via hydrogen bonding with oxime groups, which breaks intramolecular hydrogen bonds in Ni(Saldox)2. This model produces a good agreement between measured hyperfine coupling constants and ZINDO/1 computed spin-densities on protons. The observed small equilibrium constants of solvation were found to be due to strongly negative entropies of solvation (-102.1 and -145.2 J/mol K, respectively, for inner and outer sphere solvation).