Kinetics of ligand substitution in bis(N-alkylsalicylaldiminato)nickel(II) complexes: A search for kinetically-effective aromatic ring stacking |
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Authors: | B Bü sing H Elias I Eslick and KJ Wannowius |
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Institution: | aAnorganische Chemie III, Eduard-Zintle-Institut, Technische Hochschule Darmstadt, Hochschulstrasse 4, D-6100 Darmstadt, F.R.G. |
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Abstract: | 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. ComplexesII and III meet the steric requirements for intramolecular aromatic ring stacking. Stopped-flow spectrophotometry was used to study the kinetics of ligand substitution in complexesI–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 =kH2salen] 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 adductsIII·(py)2 and III·(MeOH)2 in acetone are reported. |
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