Elsevier

Polyhedron

Volume 25, Issue 14, 23 October 2006, Pages 2698-2704
Polyhedron

Reactivity of neutral nitrogen donors in square-planar d8 metal complexes: The system chloro(2,2′:6′,2″-terpyridine)platinum(II) cation with five-membered N-donor heterocycles in methanol

https://doi.org/10.1016/j.poly.2006.03.017Get rights and content

Abstract

The kinetics of the forward and reverse steps of the reaction [Pt(terpy)Cl]+ + nu  [Pt(terpy)(nu)]2+ + Cl (terpy = 2,2′:6′,2″-terpyridine, nu = one of a number of thiazoles, oxazole, isoxazole, imidazole, pyrazole and 3,5-dimethylpyrazole, covering a wide range of basicities) have been studied in methanol at 25 °C. Both forward and reverse reactions obey the usual two-term rate law observed in square-planar substitution. The second-order rate constants for the forward reactions, k2f, show a slight dependence upon the basicity of the entering nu, while the steric hindrance due to the presence of one methyl group in the α position to the nitrogen markedly decreases the reactivity. The second-order rate constants for the reverse reactions, k2r, are very sensitive to the nature of the leaving group and a plot of logk2r against the pKa of the conjugate acids of the unhindered five-membered N-donors is linear with a slope of −0.51. The results are compared with data from the literature regarding a series of pyridines reacting with the [Pt(terpy)Cl]+ cation under the same experimental conditions. Both in the forward and in the reverse reaction, the reactivity depends not only upon the ligand basicity but also upon the nature of the nucleophile in the order: (thiazoles, oxazole, isoxazole, imidazole, pyrazoles) > pyridines for the entry of N-donors and on the contrary for the displacement by Cl. Steric retardation, due to the presence of a methyl group in the α position to the nitrogen, is remarkably lower for five-membered N-donors if compared to pyridines both in the forward and in the reverse reaction.

Graphical abstract

The kinetics of the reversible displacement of chloride from the square-planar [Pt(terpy)Cl]+ cation (terpy = 2,2′:6′,2″-terpyridine) by five-membered N-donor heterocycles (thiazoles, oxazole, isoxazole, imidazole and pyrazoles) have been studied spectrophotometrically in MeOH. The reactivity of the considered nucleophiles has been related to their electronic and steric characteristics.

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Introduction

As a part of a systematic study about the nucleophilicity and lability of neutral nitrogen donors towards square-planar four-coordinate complexes and the relationship between the reactivity and proton basicity, we have reported kinetic and equilibrium data concerning the reversible displacement of chloride by am (am = a number of pyridines and some heterocyclic and aromatic amines) from mono-cationic and bis-cationic complexes [1], [2], [3], [4]. In all the cases, the kinetic behaviour is in accord with a reaction profile involving a transition state in which the Pt–Cl bond is almost completely formed, whereas the Pt–N bond is significantly weakened. As a consequence, the reactivity of entering pyridines and the other amines depends linearly and only slightly upon their basicity according to a linear free energy relationship of the type log k2 = a(pKa) + b, where the parameters a and b take into account electronic and steric effects. On the other hand, the reverse reaction, the displacement by chloride of coordinated nitrogen donors from Pt(II) complexes, involves that am moves from a situation in which the Pt–N bond is completely formed to one in which it is greatly weakened. In this case, even if the lability of the amines is still linearly related to their basicity, the constant b for unhindered pyridines, that is their lability, is smaller than the one of other sp3 aliphatic N-donors bases and evidences have been presented for a significant contribution of a π interaction between the filled d orbitals of the metal and the empty antibonding orbitals of the pyridine ring, that contributes to the stabilization of the ground state of the substrates. This suggestion seems to be in agreement with further deviations observed for pyridines having a para-substituent able to contribute to the π system [1], [3]. In order to improve these experimental observations and the range of applicability of the proposed interpretation, we have now extended our study to five-membered nitrogen donors (thiazoles and isostructural species such as oxazole, isoxazole, imidazole and pyrazoles) by measuring the kinetics of the forward and reverse steps of the process (1).[Pt(terpy)Cl]++nu[Pt(terpy)(nu)]2++Cl-(nu = thiazole, 4-methylthiazole, 4,5-dimethylthiazole, 5-methylthiazole, oxazole, isoxazole, imidazole, 3,5-dimethylpyrazole, pyrazole and only for the reverse reaction 2-aminothiazole and benzothiazole). The kinetics were carried out in methanol at 25 °C in order to directly compare the results with those obtained with other nitrogen donors under the same experimental conditions [3] and to widen and complete our precedent study regarding only two unhindered five-membered N-donors [5]. The great importance of ligand substitution reactions on [Pt(terpy)L]n+ substrates is underlined by the recent publication of a number of papers regarding kinetics and mechanism of the reactions between biologically relevant ligands and the [PtCl(terpy)]+ or [Pt(H2O)(terpy)]2+ cations [6], [7], [8].

Section snippets

Materials

2,2′,6,2″-Terpyridine was purchased from Aldrich. Pure reagent-grade LiCl, CH3SO3Na, CH3SO3H, AgClO4 (Aldrich) were used without further purification. The thiazoles, oxazole, isoxazole, imidazole and pyrazoles were recrystallized or distilled before use when necessary. Anhydrous MeOH was obtained by distillation over Mg wires, but traces of water did not appear to have any appreciable effect upon the reactions.

Instruments

Electronic spectra and kinetic measurements were obtained on a Perkin–Elmer Lambda 15

Results and discussion

Thiazoles, oxazole, isoxazole, imidazole and pyrazoles are potentially ambidentate ligands and it is necessary to ascertain the site of binding to Pt(II) in its complexes, but there is considerable evidence to indicate bonding through nitrogen of which the following are probably the most important: (i) thiazoles and oxazoles bind to the analogous soft d8 Au(III) through nitrogen [11]; (ii) thiazole ligands were found to be bonded to platinum through the nitrogen atom from data in the literature

Acknowledgements

We thank the Italian Ministry of University for financial support.

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