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Synthesis, Characterisation and Kinetic Studies ofAcid-promoted Dissociation Reactions of the Nickel(II)Complex of a [Me4(14)-tetraene-N4]Macrocyclic Ligand

 

作者: Ashok K. Singh,  

 

期刊: Journal of Chemical Research, Synopses  (RSC Available online 1997)
卷期: Volume 0, issue 7  

页码: 227-227

 

ISSN:0308-2342

 

年代: 1997

 

DOI:10.1039/a608028f

 

出版商: RSC

 

数据来源: RSC

 

摘要:

Ni N N N N 11 1 8 4 J. CHEM. RESEARCH (S), 1997 227 J. Chem. Research (S), 1997, 227 J. Chem. Research (M), 1997, 1651–1662 Synthesis, Characterisation and Kinetic Studies of Acidpromoted Dissociation Reactions of the Nickel(II) Complex of a [Me4(14)-tetraene-N4] Macrocyclic Ligand Ashok K. Singh,* G. Bhattacharjee and Sudeshna Chandra Department of Chemistry, University of Roorkee, Roorkee-247 667, UP India The kinetics of acid-promoted dissociation reactions of NiII complex cations of an Me4-tetraene macrocyclic ligand follow good first-order reactions with acids in various solvents with the rate of reaction proceeding faster in media of higher dielectric constant.The chemistry of metal complexes of tetraaza macrocyclic ligands is of general interest because such compounds have been used as dyes and pigments1 as well as having been advanced as models for more complex biological systems.2 Much work has been carried out on 14-membered tetraaza macrocycles of transition metals (Ni, Cu, Cd)3–5 but not many detailed kinetic studies on acid-promoted dissociations of these complexes have been reported.6 A common feature of template reactions is the isolation of the macrocyclic ligand as a complex of the metal template ion, and such reactions are frequently the synthetic method of choice for the preparation of macrocyclic complexes.Here we report the synthesis, characterisation and acid-promoted dissociation of the NiII complex of a tetradentate tetraaza cyclotetradecane macrocyclic ligand.Ethylenediamine and acetylacetone react in the presence of divalent nickel to form a 14-membered N4-tetradentate macrocyclic complex. This complex, synthesised via a metal template method, is of the type [M[Me4(14)-tetraene-N4]] where the coordination takes place through the imine nitrogen atoms which are bridged by acetylacetone moieties. The complex was characterised by UV–VIS, IR, NMR and CHN analysis. The kinetics of acid-promoted dissociation reactions of complex cations of NiII were studied spectrophotometrically in HCl, HNO3 and H3PO4 at 25.0�0.1 °C in various organic solvents such as methanol, N,N-dimethylformamide, acetonitrile and 1,4-dioxane under pseudo-first-order conditions.Good first-order kinetics were observed in all cases. The rate-constant values show that the reaction proceeds faster in media of higher dielectric constant. The rate of reaction decreases with a decrease in the acid strength (H3PO4sHNO3sHCl).The general mechanism proposed for the acid-promoted dissociation of polyamine complexes6 may be adopted for this complex, as shown below: [M(L)]2+h[M(L)*]2+ [M(L)*]2++H+hproducts As shown above, a mechanism with the acid-dependent reaction of the ‘activated’ species [M(L)*]2+ has been proposed. Product analysis, done on the basis of a comparison of physical data such as boiling points, Rf values and spectral characteristics with those of the ligand, demonstrated that the product is the protonated ligand. Techniques used: UV–VIS, IR, 1H NMR, CHN analysis, TLC References: 14 Figs. 1–3: First-order rate coefficients for acid-promoted dissociation reactions of [Me4(14)-tetraene]Ni with HNO3, HCl and H3PO4 at 25 °C in various solvents Fig. 4: Arrhenius plots for the reactions of [Me4(14)-tetraene]Ni complex with HNO3 in 1,4-dioxane and DMF Figs. 5 and 6: Proposed stepwise mechanisms for the acid-promoted dissociations of [Me4(14)-tetraene]Ni with H3PO4, HCl and HNO3 Fig. 7: UV–VIS scan of the complex, ligand and product Tables 1–3: First- and second-order rate coefficients for acidpromoted dissociation reactions of [Me4(14)-tetraene]Ni with HNO3, HCl and H3PO4 at 25 °C in various solvents Table 4: Solvent parameter values for the solvents employed Table 5: Activation parameters for the reactions of [Me4(14)-tetraene]Ni with HNO3 in 1,4-dioxane and DMF Received, 27th November 1996; Accepted, 8th April 1997 Paper E/6/08028F References cited in this synopsis 1 R. Price, in The Chemistry of Synthetic Dyes, Academic Press, New York, 1970, p. 3. 2 D. H. Busch, Rec. Chem. Prog., 1964, 25, 107. 3 K. Hideg and O. H. Hankovsky, J. Inorg. Nucl. Chem., 1978, 40, 699. 4 R. Bembi, T. G. Roy, A. K. Jhanji and A. Maheshwari, J. Chem. Soc., Dalton Trans., 1990, 3531. 5 R. Bembi, Proc. Indian Natl. Sci. Acad., Part A., 1986, 52, 776. 6 N. F. Curtis and S. R. Osvath, Inorg. Chem., 1988, 27, 305. *To receive any correspondence (e-mail: chemt@rurkiu.ernet.in

 



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