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.