J. Phys. Chem. B 1998, 102, 4871-4874
4871
Molecular Design of Crown Ethers. 16.1 Calorimetric Titration of Complexation of Light Lanthanoid Nitrates with N-Benzylaza-16-crown-5 and 15,15-Dimethyl-16-crown-5 in Acetonitrile: Enhanced Selectivity for Pr3+ Yu Liu,*,† Xiao-Peng Bai,† Yoshihisa Inoue,*,‡ and Mikio Ouchi§ Department of Chemistry, Nankai UniVersity, Tianjin 300071, China, Department of Molecular Chemistry, Faculty of Engineering, Osaka UniVersity, Yamadaoka, Suita 565, Japan, and Department of Applied Chemistry, Himeji Institute of Technology, Shosha, Himeji 671-22, Japan ReceiVed: NoVember 11, 1997; In Final Form: March 30, 1998
Calorimetric titrations have been performed in anhydrous acetonitrile at 25 °C to give the complex stability constants (Ks) and the thermodynamic parameters (∆G°, ∆H°, and ∆S°) for the stoichiometric 1:1 complexation of light lanthanoid(III) nitrates (La-Gd) with N-benzylaza-16-crown-5 (1) and 15,15-dimethyl-16-crown-5 (3). Using the present and previous data, the effects of substitution and ring enlargement of crown-5 derivatives (1-4) upon complexation behavior are discussed comparatively from the thermodynamic point of view. Possessing a less-symmetrical skeleton and the same N-substitution as compared with N-benzylaza-15-crown-5 (2), N-benzylaza-16-crown-5 (1) gave lower complex stabilities by 1-2 orders of magnitude for all light lanthanoids examined, but exhibited significantly altered relative cation selectivity with much enhanced preference up to 8 for Pr3+ over the other light lanthanoids. The lower complex stabilities for 3 than for the parent 16-crown-5 4 are attributed to the reduced enthalpic gains caused by the steric hindrance between the axial methyl group introduced at C-15 and the accommodated cation, but the methyl substitution appears to help lock the ring conformation such that complexation can occur, as indicated by the positive entropic changes for most light lanthanoids. These results indicate that the introduction of a nondonating sidearm to C- and N-pivot 16-crown-5 alters significantly not only the binding constant but also the relative cation selectivity.
Introduction
CHART 1
A wide variety of lariat ethers with donating or nondonating sidearm(s) attached to the parent crown ethers through a C- or N-pivot have been designed and synthesized in order to alter the original cation-binding ability and/or selectivity.2-7 Indeed, lariat ethers with additional binding sites in their sidearms give much enhanced cation-binding ability and selectivity as compared with the parent crown ether.8-13 However, the studies on the complexation thermodynamics with lariat ethers have been concentrated mostly on alkali, alkaline earth, and some heavy metal salts, while little attention has been paid so far to the complexation thermodynamics of trivalent lanthanoids.14 We have recently shown that both C- and N-pivot lariat ethers display distinctly different cation-binding ability and selectivity for the light lanthanoid series.1c,15 In particular, the N-pivot lariat 16-crown-5, which carries a donating sidearm, shows specific cation-binding ability and selectivity for Nd3+ in the light lanthanoids. This indicates that the increased conformational freedom of the ligand is more suitable for the recognition of trivalent lanthanoid ions by lariat ethers. On the other hand, we also revealed that the introduction of nondonating alkyl groups to 16-crown-5 at C-15 alters not only the complexing ability with alkali, alkaline earth, and some heavy metal salts but also the relative cation selectivity significantly.16 Unfortunately, thermodynamic study of less-symmetrical crown ethers with nondonating sidearms upon complexation with trivalent †
Nankai University. Osaka University. § Himeji Institute of Technology. ‡
lanthanoid ions has been not performed, to our best knowledge. Therefore, these results prompted us to investigate the complexation behavior of less-symmetrical C- and N-pivot 16crown-5 bearing a nondonating sidearm with lanthanoid nitrates from a thermodynamic viewpoint. In the present study, we synthesized N- and C-pivot 16crown-5 with nondonating sidearm(s) (1 and 3) (Chart 1) and investigated their complexation thermodynamics with light lanthanoid(III) nitrates (Ln ) La, Ce, Pr, Nd, Sm, Eu, and Gd) in acetonitrile, using titration calorimetry. The thermodynamic quantities obtained, together with those reported for N-benzylaza-15-crown-5 (2)17 and 16-crown-5 (4),15 will provide further understanding of the complexation behavior of N- and C-pivot 16-crown-5 possessing nondonating sidearm(s) with light lanthanoid nitrates. It is another point of interest to examine the influence of the ligand’s cavity size, substitution, and the pivot
S1089-5647(98)00152-7 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/28/1998
4872 J. Phys. Chem. B, Vol. 102, No. 25, 1998 atom upon complexation of less-symmetrical 16-crown-5 with light lanthanoids from the thermondynamic point of view. Experimental Section General Procedure. IR spectra were recorded on a JASCO A-100 spectrometer. Mass spectra were obtained on a Hitachi RMU-6E instrument. 1H NMR spectra were recorded on a JEOL GX-400 spectrometer. Materials. Less-symmetrical 1 was synthesized in 11% (5.3 g) yield by the reaction of N-benzyldiethanolamine (in 200 mL THF) (29.9 g, 0.15 mol) with the ditosylate of 3,7-dioxanonane1,9-diol (in 200 mL THF) (70.0 g, 0.14 mol) for 24 h in the presence of NaH (60% in oil) as a base in tetrahydrofuran. Purification by column chromatography (alumina, 0.5% ethyl acetate in n-hexane) and the subsequent distillation under reduced pressure gave the pure product (1):18 bp 175-181 °C/ 0.5 Torr; MS (70 eV) m/z 323 (M+); NMR (400 MHz; CDCl3) d1.83-1.86 (q, 2H), 2.80-2.83 (t, 4H), 3.59-3.61 (t, 6H), 3.62 (s, 8H), 3.65-3.68 (t, 4H), and 7.22-7.34 (m, 5H); IR/cm-1 2860, 1600, 1460, 1110, and 740. Anal. Calc for C18H29O4N: C, 66.84; H, 9.04; N, 4.33; Found: C, 66.30; H, 9.06; N, 4.46. Compound 3 was synthesized from tetraethylene glycol bis(ptoluenesulfonate) and 2,2-dimethyl-1,3-propandiol according to the method reported previously.16 Analytical-grade acetonitrile was dried over calcium hydride and then distilled fractionally to give the anhydrous solvent (
