Thermodynamics of complexation of manganese(II) and zinc(II) ions with pyridine, 3-methyl- and 4-methyl-pyridine in cyanomethane

Abstract

Complexation of manganese(II) and zinc(II) ions with pyridine (py), 3-methyl-(3Me-py) and 4-methyl-pyridine (4Me-py) has been investigated by titration calorimetry in cyanomethane (MeCN) containing 0.1 mol dm^–3 NEt₄ClO₄ as constant ionic medium at 25 °C. The calorimetrie titration data are well explained in terms of the formation of [MnL]²⁺, [MnL₂]²⁺ and [MnL₃]²⁺, and [ZnL]²⁺, [ZnL₂]²⁺, [ZnL₃]²⁺ and [ZnL₄]²(L = py, 3Me-py or 4Me-py), and their formation constants, reaction enthalpies and entropies were determined. In the zinc(II)–py and –4Me-py systems the ΔH₃^⊖ and ΔS₃^⊖ values are large relative to those for other steps, suggesting that an extensive liberation of solvent molecules occurs at the third step. A structural change frpm octahedral to tetrahedral takes place and octahedral [ZnL(MeCN)₅]²⁺ and [ZnL₂(MeCN)₄]²⁺ and tetrahedral [ZnL₃(MeCN)]²⁺ and [ZnL₄]²⁺ are formed in MeCN. Although an apparent discontinuity in the stepwise thermodynamic parameters is not observed in the zinc(II)–3Me-py system, the overall formation entropy ΔS_β4^⊖ indicates a similar structural change. On the other hand, the stepwise thermodynamic quantities for the manganese(II) complexes changed regularly, suggesting an octahedral structure for [MnL_n(MeCN)_6–n]²⁺(n= 1–3). The formation of the manganese(II) and zinc(II) complexes is more favourable in MeCN than in dimethylformamide (dmf), mainly due to the more negative reaction enthalpies in MeCN, resulting from the weaker solvation of the metal ions.