185 Acta Chim. Slov. 1999, 46(2), pp. 185-191 THE COMPLEXATION EQUILIBRIA OF CALCIUM ION IN ALL-FLUORINE ENVIRONMENT WITH OXYGEN-DONATING LIGANDS# Alojz Demšar, Franc Perdih, Andrej Petrič, Andrej Pevec and Saša Petriček Faculty of Chemistry and Chemical Technology, University of Ljubljana, Aškerčeva 5, P.O.B. 537, SI-1001 Ljubljana, Slovenia; e-mail: alojz.demsar@uni-lj.si (Received 28.4.1999) Abstract Calcium ion in [{(C5Me4Et)TiF3}4CaF2] 1 is capable of increasing its coordination number from eight to nine by binding an additional ligand. The complexation equilibria of 1 with ligands tetrahydrofurane (THF), dioxane (diox) and hexamethylphosphoramide (HMPA) have been studied by variable temperature 19F NMR spectroscopy. The following )H and )S values for complexation reactions were found: -22.8(5) kJ mol–1 and -70.4(3) J mol–1 K–1 (THF); -22.7(8) kJ mol–1 and -67.9(5) J mol–1 K–1 (diox); -25(1) kJ mol–1 and -56.3(6) J mol–1 K–1 (HMPA); respectively. Introduction The coordination chemistry of calcium is dominated by hard oxygen-donating ligands including small molecules as well as macrocyclic crown ethers and related ligands [1-3]. The most important small ligands are water, tetrahydrofurane and carboxylates. The exchange rate of complexation of aqua ligands on Ca2+(aq) in the range 6-9x108 s-1 is in contrast to 5x105 s-1 obtained for aqueous magnesium solutions [4]. The observed # Dedicated to the memory of Prof. Dr. Jože Šiftar 186 difference in lability of both ions is reflected in their biofunctions. Magnesium has largely a rigid structural role, the best known examples are Mg-porphyrin complex in photosynthesis and ATP-hydrolase. Calcium ion is more labile resembling sodium and potassium ions and plays a key role in dynamic processes such as nevrotransmission and muscle contraction [5]. The comparison of structural parameters of Ca2+-proteins and Ca2+-small molecules interactions shows many similarities [5,6,7]. The study of complexation equilibria of Ca2+ with small molecules could therefore improve our understanding of dynamic Ca2+-proteins processes. Calcium ion in [{(C5Me4Et)TiF3}4CaF2] 1 (Scheme 1) is coordinated by two tetradentate fluorine-donating [(C5Me4Et)2Ti2F7]- ligands and is capable of extension of coordination sphere by binding an additional ligand [8,9]. We isolated and determined the structure of 1HMPA (HMPA = hexamethylphosphoramide) with a nine-coordinated calcium ion. The variable temperature 19F NMR spectroscopy was used to study the solvatation of 1 [9]. In this paper, we present the study of equilibria of 1 with small ligands tetrahydrofurane, dioxane and hexamethylphosphoramide by use of 19F and 1H variable temperature spectroscopy (Scheme 1). Cp* Fc- Fa Ti Fb.. Fa- Cp* Fc C .-''-Fb—'Ti" ^Ti / Fa"FbTi Fc Fc iFa Cp* Cp* + L Cp* Fc- Fa Fd Fc" Ti / ^Fb-. Ti Fb Cp* Fa :-Car-: Fa-'--Fb- Cp* >Ti-Fc /Fd ¦Fb^Ti Fa" -Fc Cp* Cp* = C5Me4Et L = THF, diox, HMPA Scheme 1. 1-L Experimental The compound [{(C5Me4Et)TiF3}4CaF2] 1 was prepared as previously reported [8]. Deuterated toluene (Aldrich) was dried with potassium and distilled under reduced pressure. The solid samples of 1 were weighed in a dry-box into NMR tubes and solvent and ligand were added in the counter-flow of dry argon. The following amounts of 187 substances were used in the preparation of NMR-samples: a) 12 mg of 1, 1.20 mL of [2H8]toluene, 26 mg of THF; b) 9 mg of 1, 0.82 mL of [2H8]toluene, 16 mg of diox; c) 11 mg of 1, 0.72 mL of [2H8]toluene, 5 mg of HMPA. The 1H and 19F NMR spectra were recorded on a Bruker DPX 300 spectrometer and the respective nuclei chemical shifts referenced to external samples of SiMe4 (1H) and CFCl3 (19F). Variable temperature spectra were recorded using the variable temparature controller of the spectrometer. The sample was allowed to equilibrate for at least 10 minutes before beginning the spectral acquisition. Results and Discussion The 19F NMR spectrum of [2H8]toluene solution of 1 exhibits four resonances assigned to the following fluorine atoms (in the order of increasing shielding, see also Scheme 1): terminal (Fc), doubly bridging (Fa), doubly bridging (Fd) and triply bridging (Fb) [9]. A new set of fluorine resonances appears along the resonances of 1, when THF, diox or HMPA were added (Figures 1-3). The new resonances are ascribed to the complex IL (L = THF, diox, HMPA). The ratio of intensities of 1 and l'L is temperature dependent, suggesting the equilibria shown in Scheme 1. The resonances of Fc and Fa fluorines were used in order to calculate the equilibrium constants: K = C1L/(d -CL) The resonances of fluorine atoms Fb and Fd (not shown in Figures 1-3) of 1 and l'L were not used in equilibrium constant calculations due to overlapping. The plot of ln K versus 1/T (Figure 4) gave the enthalpy and enthropy changes for the complexation of 1. The following )H and )S values for complexation reactions (Scheme 1) were found: -22.8(5) kJ mol–1 and -70.4(3) J mol–1 K–1 (L = THF), -22.7(8) kJ mol–1 and -67.9(5) J mol–1 K–1 (L = diox), -25(1) kJ mol–1 and -56.3(6) J mol–1 K–1 (L = HMPA); respectively. The negative values of )H and )S support the proposed reaction of binding of an additional ligand. The values of thermodynamic parameters for 188 272 K 262 K 252 K «Ml ** 312 K 302 K 292 K 282 K Fc 1THF 1THF Fa 1 200 190 ppm 40 30 20 10 ppm 19 Figure 1. The variable temperature 19F NMR spectra of 1 with added THF. See Scheme 1 for labels. 272 K 262 K 252 K ¦ ' i ' ' ' ' 200 190 1 F 1.diox MiMI 19 ¦ ' i " ' ppm 312 K 302 K 282 K 1.diox F 1 "i.........i.........i.........i.........i 40 30 20 10 ppm Figure 2. The variable temperature 19F NMR spectra of 1 with added diox. See Scheme 1 for labels. 1 189 312 K 302 K 292 K 282 K 272 K 262 K 1 Fc 1HMPA 1HMPA Fa 1 200 190 ppm 50 40 30 20 ppm 19 Figure 3. The variable temperature 19F NMR spectra of 1 with added HMPA. See Scheme 1 for labels. 5 4 3 2 1 0 0,0032 0,0034 0,0036 1/T 0,0038 0,004 Figure 4. The temperature dependency of equilibrium constants for the complexation of 1 with THF (a), diox (b) and HMPA (c). Figure 5. The 1H NMR spectra (C5Me4Et resonances) of 1 (a), 1 with added THF (b), 1 with added di ox (c), 1 with added HMPA (d). The multiplet is the residual resonance of the solvent. complexation with tetrahydrofurane and dioxane are very similar. Although the complexation constants of 1 with hexamethylphosphoramide are higher, they result in only about ten percent higher enthalpy change. The complexation of calcium ion with aqua ligands has been studied by ab initio methods [1, 10] and the calculation for the reaction: [Ca(H2O)8]2+ + H2O —> [Ca(H2O)9]2+ resulted in enthalpy change of -57 kJ mol–1 [1]. On the other hand there is a lack of experimental results for the complexation of calcium ion with a single ligand. The complexation and decomplexation reactions are not fast on the 19F NMR timescale, since separate resonances were observed for each species. The 1H NMR spectrum of 1 has two resonances of methyl protons of the organic ligand C5Me4Et. The intensity of the resonances decrease after addition of THF, diox or HMPA (Figure 5). A possible explanation is the coalescence due to complexation-decomplexation reaction 191 with the rate comparable to the 1H NMR timescale. However, the line shape analysis that would result in the rate of reactions was not possible due to overlapping of 1, 1L and residual solvent resonances. Acknowledgements The work was supported by the Ministry of Science and Technology, Republic of Slovenia, through grant J1-8914-103. References [1] A. K. Katz, J. P. Glusker, S. A. Beebe, C. W. Bock, J. Am. Chem. Soc 1996, 118, 5752-5763. [2] H. Einspahr, C. E. Bugg, Acta Crystallogr. 1981, B37, 1044-1052. [3] H. Einspahr, C. E. Bugg, Acta Crystallogr. 1980, B36, 264-271. [4] D. T. Richens, The Chemistry of Aqua Ions; Wiley, Chichester, 1997, chapter 2. [5] N. C. J. Strynadka, M. N. G. James, in Encyclopedia of Inorganic Chemistry, ed. R. B. King; Wiley,Chichester, 1994, vol. 1, pp 477-507. [6] C. A. McPhalen, N. C. J. Strynadka, M. N. G. James, Adv. Protein Chem. 1991, 42, 77-144. [7] N. C. J. Strynadka, M. N. G. James, Ann. Rev. Biochem. 1989, 58, 951-998. [8] A. Pevec, A. Demsar, V. Gramlich, S. Petricek, H. W. Roesky, J. Chem. Soc, Dalton Trans. 1997, 2215-2216. [9] A. Demšar, A. Pevec, S. Petriček, L. Golič, A. Pétrie, M. Björgvinsson, H. W. Roesky, J. Chem. Soc, Dalton Trans. 1998, 4043-4047. [10] S. E. Rodriguez-Cruz, R. A. Jockusch, E. R. Williams, J. Am. Chem. Soc. 1999, 121, 1986-1987. Povzetek Kalcijev ion v spojini [{(C5Me4Et)TiF3}4CaF2] 1 lahko veže dodatni ligand in tako poveča koordinacijsko število z osem na devet. S spreminjanjem temperature smo študirali ravnotežja kompleksacije spojine 1 z ligandi tetrahidrofuranom, dioksanom in heksametilfosforamidom. Ravnotežja smo opazovali z 19F NMR spectroskopijo . Določili smo sledeče )H in )S vrednosti za reakcije vezave liganda: -22.8(5) kJ mol–1 in -70.4(3) J mol–1 K–1 (tetrahidrofuran); -22.7(8) kJ mol–1 in -67.9(5) J mol–1 K–1 (dioksan); -25(1) kJ mol–1 in -56.3(6) J mol–1 K–1 (heksametilfosforamid).