A specific interaction of dihalogeno complexes of zinc(II) with 1,10-phenanthroline in N,N-dimethylformamide

Abstract

The zinc(II) ion preferentially forms tetrahedral four-coordinate dihalogeno complexes [ZnX₂(DMF)₂] (X = Cl, Br, I) in N,N-dimethylformamide (DMF), while formation of the ZnX⁺ complex is strongly suppressed. The zinc(II) ion also binds bidentate 1,10-phenanthroline to form octahedral six-coordinate complexes preferentially. On the basis of these facts, interaction of the [ZnX₂(DMF)₂] complex with 1,10-phenanthroline has been investigated in DMF by potentiometry, titration calorimetry and spectrophotometry at 298 K. Thermodynamic parameters of the formation of binary zinc(II) halogeno and 1,10-phenanthroline complexes in DMF have been studied in advance in the previous and present works, respectively. It is found that 1,10-phenanthroline binds to the [ZnX₂(DMF)₂] complex to form ZnX₂(phen) and ZnX₂(phen)₂ in all the halide systems. Formation of ZnX₂(phen)₂, according to ZnX₂ + 2phen = ZnX₂(phen)₂, is significantly enhanced in the order Cl<Br<I. The corresponding enthalpy values are negative and decrease in the order ΔH°(Cl)>ΔH°(Br)>ΔH°(I). This is unusual because the Zn(II)–DMF bond within [ZnX₂(DMF)₂] is expected to be stronger in the order Cl<Br<I, suggesting that a specific interaction operates between 1,10-phenanthroline and the halide ion. Formation enthalpy and entropy values for reaction, ZnX₂ + phen = ZnX₂(phen), also show unusual behavior, suggesting the iodide complex is five-coordinated as [ZnX₂(phen)(DMF)] with one coordinating DMF molecule, while the chloro and bromo complexes are four-coordinated without coordinating solvent molecule. Complexation of zinc(II) with thiocyanate ions and 1,10-phenanthroline has also been investigated.