~0r~hedron vd. 6, NO. 3, pp. 535438, 1987 printed zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA in Chat Britain 0277~5387/87 ~3.oof.00 0 1987 P~WO~ zyxwvutsrqponm h~rnais ~td zyxwvutsr SPECTROPHOTOMETRIC STUDY OF SOME ALKALINE- EARTH AND OF SILVER COMPLEXES WITH DIBENZO- 30-CROWN-10 IN METHANOL, DIMETHYLFORMAMIDE AND DIMETHYLSULFOXIDE SOLUTIONS MOHAMMAD BAGHER GHOLIVAND, SOHEILA KASHANIAN and MOJTABA SHAMSIPUR* Department of Chemistry, Razi University, Bakhtaran, Iran zyxwvutsrqponmlkjihgfedc (Received 21 M ay 1986 ; accepted after revision 31 July 1986) Abstract-The interaction between Ag+, Ca2+ , Sr2+ and Ba2+ ions and the cyclic polyether dibenzo-30-crown-10 (DB30ClO) in methanol, dimethylformamide and dimethylsulfoxide solutions has been studied by a visible spectroscopic technique, using murexide as a metal ion indicator. The stabilities of the resulting complexes were determined. Strontium ion was found to form the most stable complex with the ligand in all three solvents used. There is an inverse relationship between the stabilities of the complexes and the Gutmann donicity of the solvents. The effect of the solvent on the stability of the alkaline-earth complexes is most notable with DB30C10-Ca2+ and the least with DB30ClO-Ba2+. The data obtained in this study support the existence of a ‘wrap around’ structure for the above complexes in solutions. Among the macrocyclic crown ethers first syn- thesized by Pedersen, ’ large ligands, such as diben- zo-13-crown-10 (DB30ClO), have some inter- esting properties. These are very flexible molecules, with enough oxygen atoms in the ring to enable them to twist around a metal ion of suitable size to envelope it completely and form a three-dimen- sional ‘wrap around’ complex. 2-5 Some alkali complexes of large crown ethers (i.e. larger than 18-crown-6) have been studied in different solvents.s6,‘4 Negative enthalpies and entropies of complexation of cesium ion complexes of large crowns in different non-aqueous solvents, which are definitely solvent dependent, have been reported.4’5 It is assumed that the decrease in the conformational entropy of the large crown ethers, from a rather flexible free molecule to a rigid ‘wrap around’ complex, would be the main reason for the large negative values of entropy changes. Despite the interesting properties of large crowns, not much attention has been focused on the study of metal ion complexes of these ligands, so that the information for understanding their behavior in solution is quite sparse. *Author to whom correspondence should he addressed. It was of interest to us to study the interactions of alkaline earth and of silver ions with a large crown ether, capable of forming three-dimensional complexes with these cations. In this paper, we report the study of Ag+, Ca2+, Sr2+ and Ba2+ com- plexes with DB30ClO in methanol, dimethyl- formamide and dimethylsulfoxide solutions, by a previously described visible spectroscopic tech- nique, using murexide as a metal ion indicator. 7*a EXPERIMENTAL DBlOClO was synthesized by a slight modi- fication of Pedersen’s method. ’ The product was recrystallized from reagent grade acetone and vacu- um dried. Methanol (Baker, MeOH), dimethyl- formamide (Fisher, DMF) and dimethylsulfoxide (Fisher, DMSO) were purified and dried by the previously described method.’ Reagent grade murexide (Merck), calcium chloride (Merck), stron- tium nitrate (Merck), barium chloride (Fluka), and silver nitrate (Merck), were dried over P205 under vacuum for 72 h. All spectra were obtained with a model 34 Beckman W-Vis spectrometer at 25 f. 1°C. The formation constants for the metal ion- 535