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Investigation of fast Lewis acid-base reactions between copper (II) bis (diethyldithiocarbamate) and heterocyclic bases using EPR spectrometry Tapping, Robert Laurence

Abstract

Fast Lewis acid-base reactions occur between square planar copper(II) complexes and weak organic bases in non-coordinating solvents. An example of such a reaction is that between copper(Il)bis-(diethyl-dithiocarbamate), CuDDC, and a series of pyridines: (1) CuDDC + pyridine ⇄ CuDDC•pyridine to form a five-coordinate adduct. Such an equilibrium can be treated as a two site exchange process using electron paramagnetic resonance (epr) spectrometry, with CuDDC the paramagnetic probe. The situation described by (1) is within the fast exchange approximation for epr. In order to analyse line position epr data for reaction (1) the stoichiometry of the adduct must be established. Using plots of epr line position shift it was shown that CuDDC forms only 1:1 adducts with pyridine and the methyl-substituted pyridines studied. Knowing this stoichiometry the line position data was analysed using a least squares procedure to obtain the equilibrium, or binding, constant, K. Linewidths were analysed as a function of base concentration to obtain the reverse rate constants, k[sub r] , which were related to the forward rate constants using k[sub f] = Kk[sub r]. Both equilibrium and rate constants were studied as a function of temperature using Arrhenius plots in order to obtain thermodynamic parameters. Temperature varitions in the epr parameters were included in the analysis of the linewidth and line position data. Reaction (1) was studied with pyridine as the base in benzene, toluene, and chloroform. Enthalpies of reaction in the reverse direction increased in the solvent order given, and in the forward direction negative activation energies were observed, also increasing in the solvent order shown. Studies of several methyl-substituted pyridines in benzene showed a similar behaviour. An isokinetic relationship between ΔH[sup ≠] and ΔS[sup ≠] was observed both for variation of solvent and of base. Such plots emphasise the dominant role solvent interactions play in reaction between neutral species in non-coordinating solvents. The results obtained can be consistently and qualitatively interpreted in terms of a dynamic solvent structure reorganisation model proposed by Bennetto and Caldin, and are not adequately interpreted in terms of more traditional ideas.

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