Asian Journal of Chemistry Vol. 19, No. 2 (2007),

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1 Asian Jurnal f Chemistry Vl. 19, N. 2 (2007), Slvent Influence Upn Cmplex Frmatin Between Dibenz-18-Crwn-6 and Cd 2+, Cu 2+, Tl + and Zn 2+ Catins in Dimethylsulfxide-Methanl Binary Mixtures GHLAM HSSEIN RUNAGHI* and NRANI GAMIL GEREY Department f Chemistry, Faculty f Sciences, Ferdwsi University f Mashhad Mashhad, Iran ghrunaghi@yah.cm The cmplexatin reactins between Cd 2+, Cu 2+, Tl + and Zn 2+ catins with dibenz-18-crwn-6 (DB18C6) were studied in DMS-MeH binary mixtures at different temperatures using cnductmetric methd. In all cases, DB18C6 frms 1:1 cmplexes with Cd 2+, Cu 2+, Tl + and Zn 2+ catins. With the exceptin f Tl + catin in pure methanl, additin f DB18C6 t the catins slutins causes a cntinus increase in the mlar cnductivity which indicates that the mbility f the cmplexed catins is mre than the uncmplexed nes. The values f stability cnstants f the cmplexes which were determined frm cnductmetric plts using the GENPLT cmputer prgram, shw that the stability f the cmplexes is affected by the nature and cmpsitin f the mixed slvents. A nn-linear behaviur was bserved between the lg K f and the cmpsitin f the mixed slvents fr all f the cmplexes. The selectivity f DB18C6 fr Cu 2+, Cd 2+, Tl + and Zn 2+ is sensitive t the slvent cmpsitin and in sme cases the rder f selectivity is reversed in certain cmpsitins f the mixed slvent systems. The selectivity f DB18C6 fr the catins in pure MeH is : Tl + > Cd 2+ > Cu 2+ > Zn 2+. The values f thermdynamic parameters, H c ) fr frmatin f (DB18C6)Cd 2+, (DB18C6)Cu 2+, (DB18Cd)Tl + and (DB18C6)Zn 2+ cmplexes were btained frm temperature dependence f the frmatin cnstants f the cmplexes using the van't Hff plts. The btained results shw that in mst cases, the cmplexatin reactins between DB18C6 and these heavy metal catins are enthalpy destabilized but entrpy stabilized. ( S c Key Wrds: Dibenz-18-crwn-6, Cd 2+, Cu 2+, Tl +, Zn 2+, DMS-CH 3H binary mixtures, Cnductmetry. INTRDUCTIN The first macrcyclic plyethers were reprted by Kuttrinjhaus in Hwever, the imprtance f these cmpunds began frm 1967 with the synthesis f dibenz-18-crwn-6 (DB18C6) by Pedersen and the bservatin that this cmpund and its hmlgues frm very strng cmplexes

2 930 Runaghi et al. Asian J. Chem. with alkali and alkaline earth metal catins 1. The values f stability cnstants, as well as the values f enthalpy changes ( H c ), entrply changes ( S c ) and the heat capacity changes ( C p ) f varius crwn ether cmplexes are summarized in tw reviews by Christensen et.al 2,3. Naturally ccuring macrcycles were shwn t be capable f actively transprt the metal catins acrss the bilgical memberanes. Macrcyclic crwn ethers which are similar t antibitic ligands can be used as mdels fr investigatin f in transprt thrugh membranes in bilgical systems. Studies f crwn ether cmplexatin in different slvents shw that the thermdynamic and kinetic parameters are affected by the nature and cmpsitin f the slvent system 4,5. A number f physic-chemical techniques such as ptentimetry 6, calrimetry 7, cnductmetry 8-12, plargraphy 13-16, NMR spectrmetry 17-19, spectrphtmetry 20,21 and in exchange membrane 22 have been used t study the cmplexatin reactins between crwn ethers with different metal catins in slutins. Mst f these techniques still have prblems, i.e., measurements are carried ut in nn-ideal slutins in which the assciatin between the metal catins and anins is fairly large. Therefre, it is difficult t determine accurate cmplex frmatin cnstant under such cnditins.the advantage f cnductmetry is that measurements can be carried ut with high precisin at extremely lw slutin cncentratins, where the intractins between catins and anins are knwn t be very small. In additin, cnductmetry is a highly sensitive and inexpensive technique with a simple experimental arrangement. In this paper, the results f thermdynamic study fr cmplexatin reactins between DB18C6 (Fig. 1) with Cd 2+, Cu 2+, Tl + and Zn 2+ catins in DMS-MeH binary mixtures at different temperatures using cnductmetric methd is reprted. Dibenz-18-Crwn-6 Fig. 1. EXPERIMENTAL DB18C6 (Fluka), cpper nitrate, zinc nitrate, thalium nitrate and cadmium nitrate (all frm Merck) were used withut further purificatin.

3 Vl. 19, N. 2 (2007) Cmplex Frmatin Between Crwn ether & metal ins 931 The slvents: dimethyl sulfxide and methanl (bth frm Merck) were used with the highest purity. The cnductance measurements were perfrmed n a digital AMEL cnductivity apparatus, mdel 60, in a water bath thermstated within ± 0.03ºC. The electrlytic cnductance was measured using a cell cnsisting f tw platinum electrdes t which an alternating ptential was applied. A cnductmetric cell with a cell cnstant f 0.79 cm -1 was used thrughut the studies. The experimental prcedure t btain the frmatin cnstants f cmplexes is as fllw: a slutin f the crrespnding metal salt ( M) was placed in a titratin cell and the cnductance f the slutin was measured, then a step-by-step increase in the crwn ether cncentratin was perfrmed by rapid transfer f crwn ether slutins prepared in the same slvent ( M) t the titratin cell using a micrburet and the cnductance f the slutin in the cell was measured after each additin at the desired temperature. RESULTS AND DISCUSSIN The variatins f mlar cnductance ( m ) vs. the ligand t the catin mlar ratis ([L]/[M]) fr cmplexatin f DB18C6 with Cd 2+, Cu 2+, Tl + and Zn 2+ in DMS-MeH binary systems were studied at different temperatures. The changes f Ω m vs. ligand/metal catin mle ratis ([L]/[M]) fr (DB18C6)Cd 2+ in DMS-MeH binary mixture and fr (DB18C6)Tl + in pure MeH are shwn in Figs. 2 and 3, respectively Ωm (hm -1 cm 2 ml -1 ) at 15ºC at 25ºC 250 at 35ºC at 45ºC at 55ºC Ωm (hm -1 cm 2 ml -1 ) [L]/[M] [L]/[M] Fig.2. Mlar cnductance-mle rati plts Fig. 3 Mlar cnductance-mle rati plts fr (DB18C6)Cd 2+ cmplex in DMS- fr (DB18C6)Tl + cmplex in pure MeH (ml % MeH = 75) binary MeH at different temperatures mixture at different temperatures at 15ºC at 25ºC at 35ºC at 45ºC at 55ºC The stability cnstants f DB18C6 crwn ether cmplexes at each temperature were calculated frm changes f mlar cnductance as a functin

4 932 Runaghi et al. Asian J. Chem. f ligand/catin mle ratis using a GENPLT cmputer prgram 23. The details f calculatin f the stability cnstants f cmplexes by cnductmetric methd has been described elsewhere 13. The stability cnstants (lg K f ) fr DB18C6-M n+ (M n+ = Cd 2+, Cu 2+, Tl + r Zn 2+ ) cmplexes in varius slvent systems are listed in Table-1. Assuming that the activity cefficients f catin and cmplex have the same value, K f is a thermdynamic equilibrium cnstant n the mlar cncentratin scale. The changes f standard enthalpy ( H c ) fr the cmplexatin reactins were determined frm the slpe f the van t Hff plts and the changes in the standard entrpy ( S c ) were calculated frm the relatinship ( Gc ) = H c The thermdynamic data are summarized in Table-2. S c TABLE-1 lg K f VALUES F (DB18C6)Cd 2+, (DB18C6)Cu 2+, (DB18C6)Tl + AND (DB18C6)Zn 2+ CMPLEXES IN DMS-MeH BINARY MIXTURES AT DIFFERENT TEMPERATURES lg K f ± SD b (ºC) Medium a (DB18C6)Cu % DMS 25 % MeH-75 % DMS 50 % MeH-50 % DMS 75 % MeH-25 % DMS 100 % MeH (DB18C6)Cd % DMS 25 % MeH-75 % DMS 50 % MeH-50 % DMS 75 % MeH-25 % DMS 100 % MeH (DB18C6)Tl % DMS 25 % MeH-75 % DMS 50 % MeH-50 % DMS 75 % MeH-25 % DMS 100 % MeH (DB18C6)Zn % DMS 25 % MeH-75 % DMS 50 % MeH-50 % DMS 75 % MeH-25 % DMS 100 % MeH 2.93 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.04 a) The cmpsitin f the mixed slvents is expressed in mle % f each slvent b) SD = Standard deviatin 3.03 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.03 The changes f lg K f vs. the mle fractin f MeH fr (DB18C6)Cu 2+ cmplex in DMS-MeH binary systems at different temperatures are shwn in Fig.4. Smewhat similar behaviur was bserved fr (DB18C6)Cd 2+, (DB18C6)Tl + and (DB18C6)Zn 2+ cmplexes in these binary mixed slvents. The changes f lg K f fr these cmplexes vs. the catinic radii in varius DMS-MeH binary systems are shwn in Fig.5.

5 Vl. 19, N. 2 (2007) Cmplex Frmatin Between Crwn ether & metal ins 933 TABLE-2 THERMDYNAMIC PARAMETERS FR (DB18C6)Cd 2+, (DB18C6)Cu 2+, (DB18C6)Tl + AND (DB18C6)Zn 2+ CMPLEXES IN DMS-MeH BINARY MIXTURES Medium a lg K f ± SD b b b b G c ± SD G c ± SD G c ± SD (25ºC) (KJ ml 1 ) (KJ ml 1 ) (J ml 1 k -1 ) (DB18C6)Cu % DMS 25% MeH-75% DMS 50% MeH-50% DMS 75% MeH-25% DMS 100% MeH 3.00 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.6 (DB18C6)Cd % DMS 25% MeH-75% DMS 50% MeH-50% DMS 75% MeH-25% DMS 100% MeH (DB18C6)Tl + 100% DMS 25% MeH-75% DMS 50% MeH-50% DMS 75% MeH-25% DMS 100% MeH (DB18C6)Zn % DMS 25% MeH-75% DMS 50% MeH-50% DMS 75% MeH-25% DMS 100% MeH 2.92 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.3 a) The cmpsitin f the mixed slvents is expressed in mle % f each slvent b) SD=Standard deviatin 61.6 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.8 It is evident frm Fig. 2 that additin f DB18C6 t Cd 2+ slutins in DMS-MeH binary mixture at varius temperatures causes an increase in the mlar cnductivity which indicates that the (DB18C6)Cd 2+ cmplex is mre mbile than the crrespnding slvated catin. Similar behaviur was bserved fr Cu 2+, Tl + and Zn 2+ catins in varius slvent systems. Hwever, as seen frm Fig. 3, additin f DB18C6 t Tl + catin in pure MeH at different temperatures, results in a decrease in mlar cnductivity which shws that the (DB18C6)Tl + cmplex in pure methanl is less mbile than free slvated Tl + catin. As shwn in Fig. 4, a nn-linear behaviur is bserved between the stability cnstant (lg K f ) f (DB18C6)Cu 2+ cmplex vs. the cmpsitin f DMS-MeH binary mixture at different temperatures. Smewhat similar behaviur was bserved fr ther cmplexes. The nn-linear behaviur is bserved fr these studied systems may be related t the slvent-slvent interactins between DMS and MeH mlecules and therefre, changing the slvatin number f the catins, ligand, the resulting cmplexes

6 934 Runaghi et al. Asian J. Chem. and even the preferential slvatin f the ligand in these binary mixed slvents. The MeH and DMS slvents are diplar liquids with large diple mments. Thus mixing f DMS with MeH will induce the mutual destructin f diplar structures f the cmpnent liquids and releasing the free diples. Therefre, the strng diplar interactins between MeH and DMS result in changing the structural and energetic prperties f the mixed slvent with the cmpsitin. lg K f at 15ºC at 25ºC at 35ºC at 45ºC at 55ºC Fig. 4. X(MeH) Variatin f stability cnstant (lg K f) f (DB18C6) Cu 2+ cmplex with the cmpsitin f the DMS-MeH binary systems at different temperatures 4 Tl lg K f Cu 2+ Cd 2+ Zn Fig. 5. r(å) Variatin f lg K f fr (DB18C6)Cu 2+, (DB18C6)Cd 2+, (DB18C6)Tl + and (DB18C6)Zn 2+ cmplexes vs. catinic radii in varius slvent systems at 25ºC

7 Vl. 19, N. 2 (2007) Cmplex Frmatin Between Crwn ether & metal ins 935 The slvatin f crwn ethers is f great imprtance during the cmplexatin prcess f these cmpunds with the metal catins. In DMS- MeH binary mixtures, the preferential slvatin f DB18C6 may depend n the structural and energetic prperties f the mixtures which varies with the slvent cmpsitin. The variatins f stability cnstants (lg K f ) f (DB18C6)Cd 2+, (DB18C6)Cu 2+, (DB18C6)Tl + and (DB18C6)Zn 2+ cmplexes vs. the inic radii in varius cmpsitins f DMS-MeH binary slutins are shwn in Fig. 5. As is evident frm this figure, in mst cases the selectivity f DB18C6 fr the studied metal catins is Tl + > Cd 2+ > Cu 2+ > Zn 2+. Sme reversals in selectivity are bserved in certain cmpsitins f the mixed slvents. The stability and selectivity f crwn ether cmplexes with metal catins depend n several factrs. These include the cavity size f the ligand, the character and the number f the heteratms in the plyether ring, the spatial distributin f the ring sites and the catin diameter 25. The fitness f the size f the catins int the crwn ether cavity is an imprtant factr t explain the selectivity f the crwn cmpunds twards the metal catins. In seems that the selectivity rder f DB18C6 which is bserved fr these heavy metal catins is due t the relative size f the metal catins and the cavity size f the plyether ring. The inic radii f the abve catins vary in the rder: Tl + > Cd 2+ > Zn 2+ > Cu 2+. The Tl + catin with a large inic size can attian a mre cnvenient fit cnditin than the ther three catins fr the ligand's cavity. Since the Cu 2+ and Zn 2+ catins are t small fr the ligand's hle, they frm the least stable cmplexes cmpared with the Cd 2+ and Tl + catins. The data in Table-2 shw that in mst cases, the cmplexatin reactins between DB18C6 and the studied heavy metal catins in DMS- MeH binary slutins are enthalpy destabilized but entrpy stabilized, and bth f these parameters are influenced by the slvent and vary with the slvent cmpsitin. It seems reasnable t assume that the psitive values f S c bserved fr cmplexatin reactins is due t additinal slvent mlecules which may be released frm the slvatin shell f the catins and the ligand upn cmplexatin prcess. REFERENCES 1. C.J. Pedersen, J. Am. Chem. Sc., 89, 2495 (1967). 2. J.J. Christensen, J.. Hill and R.M. Izatt, Science, 174, 459 (1971). 3. R.M. Izatt, D.J. Eatugh and J.J. Christensen, Struct. Bnding, 16, 113 (1973). 4. B.. Strasser and A.I. Ppv, J. Am. Chem. Sc., 107, 789 (1985). 5. V.M. Lyla, R. Pizer and R.G.W. Wikins, J. Am. Chem. Sc., 99, 7185 (1977). 6. A. Devilacqma, R.I. Gelb, W.B. Hebard and L. Zmpa, J. Inrg. Chem., 26, 2699 (1987). 7. R.M. Izatt, D.E. Terry, D.P. Nelsn, Y. Chan, D.J. Eatugh, J.S. Bradshw, C.D. Hansen and J.J. Christensen, J. Am. Chem. Sc., 53, 72 (1980).

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