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

The zinc(II) ion preferentially forms tetrahedral four-coordinate dihalogen o 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) io n also binds bidentate 1,10-phenanthroline to form octahedral six-coordinat e complexes preferentially. On the basis of these facts, interaction of the [ZnX2(DMF)(2)] complex with 1,10-phenanthroline has been investigated in D MF 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 pre vious 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 + 2ph en = ZnX2(phen)(2), is significantly enhanced in the order Cl < Br < I. The corresponding enthalpy values are negative and decrease in the order Delta H degrees (Cl) > DeltaH degrees (Br) > DeltaH degrees (I). This is unusual because the Zn(II)-DMF bond within [ZnX2(DMF)(2)] is expected to be stronge r in the order Cl < Br < I, suggesting that a specific interaction operates between 1,10-phenanthroline and the halide ion. Formation enthalpy and ent ropy values for reaction, ZnX2 + phen = ZnX2(phen), also show unusual behav ior, suggesting the iodide complex is five-coordinated as [ZnX2(phen)(DMF)] with one coordinating DMF molecule, while the chloro and bromo complexes a re four-coordinated without coordinating solvent molecule. Complexation of zinc(II) with thiocyanate ions and 1,10-phenanthroline has also been invest igated.