Kinetics of ligand substitution in bis(N-alkylsalicylaldiminato)nickel(II) complexes: A search for kinetically-effective aromatic ring stacking

Article abstract of DOI:10.1016/S0020-1693(00)90603-0

The square-planar bis chelate complexes Ni(R-sal)₂ (= bis(N-alkyl)salicylaldiminato)nickel(II)) with R = -(CH₂)₂-Ph (I; Ph = phenyl), -(CH₂)₃-Ph (II), -(CH₂)₄-Ph (III) and -(CH₂)₂-(4-hydroxyphenyl) (IV) were prepared and characterized. Complexes II and III meet the steric requirements for intramolecular aromatic ring stacking. Stopped-flow spectrophotometry was used to study the kinetics of ligand substitution in complexes I-IV by H₂salen (= N,N′-disalicylidene-ethylenediamine) in acetone. For the substitution of the two bidentate ligands in Ni(R-sal)₂ only one step is kinetically observed which follows a second-order rate law, rate = k[H₂salen] [Ni(R-sal)₂], with k = 43.4 (I), 64.0 (II), 87.0 (III) and 49.5 (IV) M^-1 s^-1 at 298 K. It is found, therefore, that the size of k does not change significantly upon lengthening of the alkane chain in Ni(Ph-(CH₂)_n-sal)₂ from n = 2 to 4 and that there is no kinetic evidence for intramolecular stacking interactions. The equilibrium constants and thermodynamic parameters for the formation of the bis adducts III·(py)₂ and III·(MeOH)₂ in acetone are reported.