Microcalorimetric study on host-guest complexation of naphtho-15-crown-5 with four ions of alkaline earth metal

Microcalorimetric study on host-guest complexation of naphtho-15-crown-5 with four ions of alkaline earth metal first-author †E-mail:sundezhisdz@163.com †E-mail:sundezhisdz@163.com sundezhisdz@163.com Department of Chemistry, Liaocheng University, Liaocheng 252059, China Department of Chemistry, Liaocheng University, Liaocheng 252059, China Thermodynamic parameters of complexation of naphto-15-crown-5 with four alkaline earth ions in aqueous media was determined using titration microcalorimetry at 298.15 K. The stability of the complexes, thermal effect and entropy effect of the complexation is discussed on the basis of the guest ions structure and the solvent effect. The stability constants tendency to vary with ion radius was interpreted. Complex of naphtha-15-crown-5 with calcium ion is very stable due to the synergism of static electric interaction and size selectivity between the host and the guest. Microcalorimetry Host-guest complexation Naphtho-15-crown-5 Alkaline earth metal Molecular recognition INTRODUCTION 1 As biologic model compounds, crown ethers can interact with many guests including inorganic ions and neutral organic molecules through weak non-covalent interactions (Yi et al., 1997 1997 ). Since the first series of crown ethers was synthesized by Pedersen ( 1967 1967 ) interest in this macro-cyclic compound has been growing steadily, due to its great importance in supramolecular chemistry (Lehn, 1988 1988 ), close relation to photochemistry (Krzysztof et al., 2002 2002 ), catalysis and a number of new scientific areas (Jensen et al., 2002 2002 ; Lu et al., 1994 1994 ; Li et al., 1998 1998 ). Complexes of potassium with cyclohexano-15-crown-5 and dicyclohexano-18-crown-6 formed in THF had been studied (Zbigniew et al., 2002 2002 ); and the decomposition mechanism had been explored; the bis-barium complex of a butterfly crown ether azobis (benzo-18-crown-6) had been found to be phototunable supramolecular catalyst in the basic ethanolysis of anilide derivatives (Roberta et al., 2003 2003 ); stability constants of complexes of Ag (I), Co (II), Cu (II), Mn (II), Ni (II), Zn (II), Pb (II), Cr (III) and Fe (III) with the macro-cyclic compounds, namely, 15-crown-5, benzo-15-crown-5, 18-crown-6, dibenzo-18-crown-6, dicyclohexano-18-crown-6, dibenzo-24-crown-8. Dicyclohexano-24-crown-8 had been determined with conductometric titrations (Vijaykumar and Ashwini, 2003 2003 ); syntheses and luminescence behavior of the diaquatris (thienoyltrifluoracetonate) europium (III) with dibenzo 18-crown-6 and 18-crown-6 in the solid state had been reported (Claudia et al., 2003 2003 ). Caesium complexation by tetrabenzo-24-crown-8 in 1,2-DCE had been synthesized and characterized (Levitskaia et al., 2000 2000 ; 2001 2001 ; Bryan et al., 2000 2000 ; Talanova et al., 1999 1999 ). It was found that improved solubility, strong caesium binding and weak ion-pairing tendency make 4,5′,4′′,5′′′-tetra-tert-butyltetrabenzo-24-crown-8 an attractive candidate for caesium-selective complexation and transport in biphasic liquid systems; water soluble crown-6 derivatives of calx[4] arene had been synthesized and their affinity toward Cs + , Rb + , K + and Na + cations in methanol and basic aqueous media had been studied (Pellet-Rostaing et al., 1998 1998 ; 2001 2001 ). Various aspects of the use of these new ligands in a nanofiltration-complexation process had been summarized; their use might be an efficient way to separate trace amount of Cs + from highly salted medium. Complexation of three cyclofructants in whose molecular centers crown ether 6, 7, 8 skeletons were respectively contained. Alkaline and alkaline earth metal cations had been reported (Hamada et al., 2003 2003 ), and the thermodynamic parameters of the host-guest complex processes in THF and acetone had been determined at 298 K. More fascinatingly, chiral bis-pyridino-18-crown-6(rare earth metal triflate complexes had been used as catalysts for asymmetric aldol reactions in aqueous media (Shiozuma et al., 2001 2001 ). It is well known that water is a solvent in living organisms. So studies on interactions between such hosts as crown ethers and calixarenes with inorganic ions are very helpful for gaining understanding of the important processes of ion carrying and transporting in living systems, in which some kind of peptides and natural ionophores such as monensin and valinomycin play crucial roles (Tanabe et al., 1987 1987 ). It is widely accepted that interaction between crown ethers and ions must in general be governed by a subtle balance of interactions of electron donors with the acceptors, guest ion with its counter ion, and the host or guest with solvent. These weak interactions will only be identified by the study of a wide range of combinations of crown ethers with ions, which are most easily generated by using different types of crown ethers and ions. The more detailed is the thermodynamic information on host-guest interaction, the better is our understanding of its mechanism. As a reliable method, microcalorimetric measurement had been used to identify the factors and driving forces governing the interactions in such systems (Alizadeh and Shamsipur, 1996 1996 ). In the present paper, we report a calorimetric study on the thermodynamics of the complexation of naphtho-15-crown-5 with alkaline earth cations, Mg 2+ , Ca 2+ , Sr 2+ , Ba 2+ , in aqueous solution at 298.15 K, and discuss the experimental results regarding crown ether molecular structure, size of the guest ion, and solvent effect. To the best of our knowledge, no thermodynamic research on complexation between naphtho-crown ether and metal ions based on data from microcalorimetry has been reported. EXPERIMENTAL DETAILS 1 Materials 2 Naphtho-15-crown-5 (Naph 15C5), namely, naphtha [2,3-b]-1,4,7,10,13-pentaoxacyclopentadecin or 2,3,5,6,8,9,11,12-octahydrocyclopentadecin (Fig. 1 1 ) was synthesized and purified to better than 99% in this laboratory with the method reported by Qin ( 1987 1987 ). Magnesium chloride, calcium chloride, strontium chloride and barium chloride were analytically pure reagents purchased from the Shanghai Reagent Company (Shanghai, China). The salts were baked at 598 K for 3 h before use. Water used in the experiments was obtained by repeated twice distillation in the presence of basic potassium permanganate. Microcalorimetric measurements 2 The calorimeter used was an isothermal titration microcalorimeter, Thermal Activity Monitor (TAM) Thermometric 2277 (Sweden), controlled by Digitam 4.1 software. This instrument has an electrical calibration precision of better than ±1% and the accuracy can be checked by measuring the dilution enthalpy of a concentrated sucrose solution (Bai et al., 2002 2002 ). 0.100 mol/dm 3 MCl 2 (M=Mg, Ca, Sr, Ba) solution, was injected in small aliquots (10.00 μl) into the 4 ml stainless steel ampoule containing 2.00 ml dilute Naph 15C5 solution (titrand, 5.00×10 −4 mol/dm 3 ), using a 500 μl Hamilton syringe controlled by a 612 Lund Syringe Pump. The interval between two injections was long enough for the signal to return to the baseline. The stirrer in the ampoule rotated at a constant speed of 50 rpm. All experiments were performed at temperature (298.15±0.01) K and carried out when the baseline was stable in the presence of constant stirring so heat of stirring was deducted automatically. To deduct the dilution heats of MCl 2 and the Naph 15C5 solutions, we experimented on titration of MCl 2 solution into pure water and pure water into the Naph 15C5 solutions, respectively. The dilution heat of the later one was found to be negligible. The experimental data of thermal effect of complexation with volume of titrant is shown in Table 1 1 . Note: Cation in titrant solution 2 V tt (μl) 4 Q (mJ) Ca 2+ Mg 2+ Sr 2+ Ba 2+ 10 −1.083 −0.937 −2.378 −1.383 20 −0.522 −0.296 −4.528 −4.02 30 −0.941 −1.387 −4.249 −4.253 40 −0.97 −3.559 −4.552 −6.072 50 −0.426 −3.666 −6.535 −6.111 60 −0.529 −3.02 −5.265 −8.59 70 −0.456 −3.401 −5.781 −9.463 80 0.172 −0.904 −6.365 −11.438 90 0.418 −1.58 −7.217 −12.564 100 0.275 −1.94 −8.62 −13.637 110 0.712 −2.891 −8.714 −15.391 120 0.807 −5.004 −9.605 −16.325 130 1.046 −4.813 −9.406 −17.76 140 1.155 −5.05 −9.129 −19.278 150 0.785 −6.118 −9.067 −20.495 160 0.595 −6.167 −10.534 −21.471 170 0.853 −7.373 −12.667 −22.24 180 0.809 −7.299 −15.23 −23.571 190 0.273 −7.868 −16.295 −23.995 200 0.89 −8.749 −15.611 −24.372 210 1.232 −9.116 −15.579 −25.628 220 1.436 −9.77 −15.346 −25.244 230 1.941 −10.402 −15.428 −25.974 RESULTS AND DISCUSSION 1 Data process 2 To rationalize the enthalpy of formation of complex of Naph 15C5 with the ions in aqueous solutions, apparent equilibrium constants for overall reactions have been defined as M+ i L=ML i β i , Δ H i º Where i =1, or 2, or 3, M and L represent the guest (M 2+ ) and host (Naph 15C5), respectively. The relationship between the overall equilibrium constants, β i , and the stepwise equilibrium constants, K i , is β i =Π K i Thermodynamic parameters, Δ H i º and β i or K i , can be obtained by the regression method (Stödeman and Wadsö, 1995 1995 ), which was performed with Ligand Binding Analysis process of Digitam 4.1 software. We obtained the optimum result by comparing the fitness of the simulated (calculated) curve with the experimental points, i.e., after calculation of three reaction models (only i =1; i =1 and 2; i =1, 2 and 3) for M 2+ -Naph 15C5 complex system. It was found that mainly the 1:1 stoichiometry of the host-guest complex could be formed (so other stepwise reactions could be neglected), which might be the result of the very low concentration of Naph 15C5 whose low solubility in water prevented us from getting more concentrated solution of it. All of the thermodynamic parameters corresponding to the four M 2+ -Naph 15C5 complex systems are listed in Table 2 2 . The error attributable to enthalpy change is the deviation estimated by the Digitam program. Got from literature (Weast, 1974–1975 1974–1975 ) n aq =First order hydration number of ion, got from literature (Huang, 1983 1983 ) M 2+ Mg 2+ Ca 2+ Sr 2+ Ba 2+ 10 −3 β 1 (dm 3 /mol) 25.4 730 0.124 0.0813 −Δ H 1 º (kJ/mol) 0.286±0.035 0.835±0.036 22.41±0.52 66.02±0.56 −Δ G 1 º (kJ/mol) 25.1 33.5 11.9 10.9 T Δ S 1 º (kJ/mol) 24.8 33.6 −10.4 −55.1 * * r (nm) 0.066 0.099 0.112 0.134 ** ** n aq of M 2+ 13 10 2 1 or 2 Stability of the M 2+ -Naph 15C5 complex with the size of ions 2 Stability constants and the corresponding absolute value of formation Gibbs free energy of the four 1:1 complexes (Table 2 2 ) show that Ca 2+ -Naph 15C5 is the most stable one of the four complexes. This result accords quite well with that reported in literature (Rahman et al., 2001 2001 ), indicating that interaction between Ca 2+ and Naph 15C5 is stronger than that between any other baseline earth ion and the same crown ether. By comparing the values of the stability constants with the radii (Weast, 1974–1975 1974–1975 ) of the naked ions (Table 2 2 ), the size selectivity of the crown ether can be observed. The five oxygen atoms form a bowl-shaped space with negative electric charge, and only the cation with appropriate size can interact with the oxygen atoms efficiently. The stability of the complex will drop dramatically if the size of its guest ion deviates from the most appropriate radius, which indicates that Naph 15C5 possesses function of molecular recognition of alkaline earth metal. Earlier researches showed that 15-crown-5 do not have such function in aqueous media (Wu, 1992 1992 ), this can be explained by the flexibility of 15-crown-5 (Sun et al., 1999 1999 ). Thermal effect and the radius of cation 2 All formation enthalpies (Δ H 1 º ) of the four complexes were negative values, and the absolute value of Δ H 1 º obviously increased with the extension of the ion radius ( r ). This tendency is also related to hydration of the guest cation, because the shorter the radius is, the stronger is the hydration. According to Huang ( 1983 1983 ), the first order hydration number of the ion changes from 14 to 1 (or 2) with the cation alternation from Mg 2+ to Ba 2+ . A cation must be partly or completely dehydrated before its association with Naph 15C5, which is an endothermic process. Consequently, energy released by interaction between the crown ether and cation is partly consumed by the dehydration of the cation. The radius of Mg 2+ is the shortest one and its hydration is the strongest, so the formation of Mg 2+ (Naph 15C5) is the weakest exothermic process (Δ H 1 º =−0.286 kJ/mol). Conversely, the radius of Ba 2+ is the largest and its hydration is the weakest, and so the formation of Ba 2+ (Naph 15C5) is the strongest exothermic process (Δ H 1 º =−66.02 kJ/mol). If only the static electric attraction between the cation and the five oxygen atoms in Naph 15C5 molecules were considered, the formation of Ba 2+ (Naph 15C5) would be the weakest exothermic process. Entropy effect and the hydration of cation 2 Interaction between Naph 15C5 and a cation leads to entropy decrease, but dehydration of the cation increases entropy. The data on the entropy effect in Table 2 2 indicate that the formation of Ca 2+ (Naph 15C5) is accompanied by the strongest dehydration while the formation of Ba 2+ (Naph 15C5) is accompanied by the weakest dehydration. In fact, the first order hydration numbers of Sr 2+ and Ba 2+ are only 1 or 2, much less than those of Mg 2+ and Ca 2+ . 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Chem. , 2002 . 67 ( 22 ): 7807 - 7812 . . Preparation and decomposition of potassium alkalid-lipophilic crown ether complexes in tetrahydrofuran 10.1631/jzus.2005.B0069 Project (No. Y2003B01) supported by the Scientific Foundation of Shandong Province, China 2004-03-30 2004-04-27 2004-12-20 2022-01-05 Copyright © 2005, Journal of Zhejiang University Science 2005 Journal of Zhejiang University-SCIENCE B (Biomedicine and Biotechnology) 01 6B