pubs.acs.org/IC Published on Web 04/29/2010 r 2010 American Chemical Society 5190 Inorg. Chem. 2010, 49, 5190–5201 DOI: 10.1021/ic100315u Thermodynamics of Core Metal Replacement and Self-Assembly of Ca 2þ 15-Metallacrown-5 † Matteo Tegoni,* ,‡ Michele Furlotti, ‡ Manuel Tropiano, ‡, ) Choong Sun Lim, § and Vincent L. Pecoraro* ,§ ‡ Department of General and Inorganic, Analytical and Physical Chemistry, University of Parma, Parma, 43100, Italy, and § Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109-1055. ) Present address: Chemistry Research Laboratory, University of Oxford, 12 Mansfield Road, Oxford OX1 3TA, United Kingdom Received February 15, 2010 The equilibria for core Ca 2þ replacement by Ln 3þ in copper(II) 15-MC-5 complexes have been investigated using a series of visible spectrophotometric titrations of calcium(II) metallacrowns ({Ca II [15-MC Cu II (N)(L) -5]} 2þ ) with Ln 3þ ions (H 2 L = pheha, (S)-R-phenylalaninehydroxamic acid, or trpha, (S)-R-tryptophanhydroxamic acid). These studies allowed the determination of the equilibrium constants for the reaction {Ca II [15-MC Cu II (N)(L) -5]} 2þ þ Ln 3þ f {Ln III [15- MC Cu II (N)(L) -5]} 3þ þCa 2þ in methanol/water 9:1 (Ln 3þ = La 3þ , Gd 3þ , Dy 3þ , Er 3þ ) or 99:1 (Ln 3þ = La 3þ , Nd 3þ , Gd 3þ , Dy 3þ , Er 3þ , Yb 3þ ), respectively. The log K for these reactions decreases with increasing atomic number of the lanthanide(III), ranging from 6.1 to 3.91 in methanol/water 9:1. The same behavior is observed in methanol/water 99:1, although the constants are uniformly lower (log K = 4.09-2.52). A significant thermodynamic selectivity was observed for the later lanthanides (Gd 3þ -Yb 3þ ) while a smaller selectivity is present throughout the beginning of the series (La 3þ -Gd 3þ ). This observation has been interpreted on the basis of the size correspondence between the metal ions and the metallacrown cavity. The overall stability of the {Ca II [15-MC Cu II (N)(L) -5]} 2þ in methanol/water 9:1 has been determined by pH-spectrophotometric titrations with HCl. The resulting log K values are 63.46(12) and 65.05(13) for pheha and trpha, respectively (Ca 2þ þ 5Cu 2þ þ 5HL - = {Ca II [15-MC Cu II (N)(L) -5]} 2þ þ5H þ ). The stability of both the La 3þ and Ca 2þ 15-metallacrown-5 complexes in the presence of high Na þ concentrations has also been demonstrated by spectophotometric studies. Based upon these observations, the preference of the 15-MC-5 for Ca 2þ complexation compared to crown ethers has been quantitatively demonstrated for the first time. Introduction The application of the lanthanides for a variety of com- mercial applications ranging from hard magnets to lumines- cent sensors has dramatically increased over the past 20 years. One issue influencing the cost of such systems is the difficulty of separating the different lanthanides because of their extremely similar chemistry. Thus, it has been desirable to develop new chelation regiments that could lead to discrimi- nation and, ultimately, facile separation of these ions. Metallamacrocycles represent alternative potential plat- forms for achieving lanthanide ion discrimination. Since the isolation of the first planar 15-metallacrown-5 in 1996, 1,2 there has been a growing interest in the capability of these compounds to encapsulate, with various degrees of selecti- vity, appropriate core metal ions. Metallacrowns (MC, Scheme 1), which are self-assembled metallamacrocycles and the inorganic analogues of crown ethers, encapsulate in their cavity mono-, di-, and trivalent metal ions as a function of the metal coordination and cavity geometries. 3 Investigations on copper(II) metallacrowns (i.e., MCs with Cu 2þ as the ring metal) showed that Cu 2þ , which may adopt distorted octahedral, square pyramidal, or square planar coordination geometries, can be encapsulated in the cavity of {Cu II [12-MC Cu II (N)(L) -4]} 2þ (Scheme 1). 4-8 On the † This paper is dedicated to Professor Francesco Dallavalle in recognition of his many contributions to the field of solution thermodynamics. *To whom correspondence should be addressed. E-mail: matteo. tegoni@unipr.it. (M.T.), vlpec@umich.edu (V.L.P.). Phone: þ39 0521 905427 (M.T.), þ1 734 7631519 (V.L.P.). Fax: þ39 0521 905557 (M.T.), þ1 734 9367628 (V.L.P.). (1) Stemmler, A. J.; Kampf, J. W.; Pecoraro, V. L. Angew. Chem., Int. Ed. Engl. 1996, 35, 2841–2843. (2) Stemmler, A. J.; Barwinski, A.; Baldwin, M. J.; Young, V.; Pecoraro, V. L. J. Am. Chem. Soc. 1996, 118, 11962–11963. (3) Mezei, G.; Zaleski, C. M.; Pecoraro, V. L. Chem. Rev. 2007, 107, 4933– 5003. (4) Gibney, B. R.; Kessissoglou, D. P.; Kampf, J. W.; Pecoraro, V. L. Inorg. Chem. 1994, 33, 4840–4849. (5) Bodwin, J. J.; Cutland, A. D.; Malkani, R. G.; Pecoraro, V. L. Coord. Chem. Rev. 2001, 216-217, 489–512. (6) Bodwin, J. J.; Pecoraro, V. L. Inorg. Chem. 2000, 39, 3434–3435. (7) Pecoraro, V. L.; Stemmler, A. J.; Gibney, B. R.; Bodwin, J. J.; Wang, H.; Kampf, J. W.; Barwinski, A. Prog. Inorg. Chem. 1997, 45, 83–177. (8) Seda, S. H.; Janczak, J.; Lisowski, J. Inorg. Chim. Acta 2006, 359, 1055–1063.