A solid-state ¹³³Cs nuclear magnetic resonance and X-ray crystallographic study of cesium complexes with macrocyclic ligands

Abstract

We report solid-state NMR determination of the 133 Cs chemical shift anisotropy (CSA) for a series of cesium complexes with macrocyclic ligands. It was found that the isotropic 133 Cs chemical shifts are related to the number of oxygen atoms to which the Cs + ion is coordinated. The 133 Cs chemical shifts were found to correlate with average Cs-O distances. We also attempt to use the established correlation to deduce Cs + coordination environment for compounds with unknown structures. We also report the X-ray determination of the crystal structure for Cs(DB18C6) 2 SCN•1/2CH 3 OH•1/2H 2 O. The compound crystallizes in monoclinic, a = 14.503(2), b = 15.152(3), c = 39.989(6) Å, β = 90.796(8)°, space group P2 1 /c, Z = 8. There are two independent molecules in the asymmetric unit cell where each of the two Cs + ions is coordinated to two DB18C6 ligand molecules forming a sandwich-type structure.Résumé : On a mesuré l'anisotropie du déplacement chimique du 133 Cs dans les spectres RMN à l'état solide d'une série de complexes du césium avec des ligands macrocycliques. On a trouvé que les déplacements chimiques isotropes du 133 Cs sont reliés au nombre d'atomes d'oxygène formant une coordination avec le Cs + . On a trouvé qu'il existe une corrélation entre les déplacements chimiques du 133 Cs et les distances moyennes Cs-O. On a aussi essayé d'établir une corrélation qui permettrait de déduire l'environnement de coordination du Cs + dans des composés de structures inconnues. On a aussi déterminé la structure du Cs(DB18C6) 2 SCN•1/2CH 3 OH•1/2H 2 O par diffraction des rayons X. Les cristaux sont monocliniques, groupe d'espace P2 1 /c, avec a = 14,503 (2), b = 15,152(3) et c = 39,989(6) D, β = 90,796(8)°et Z = 8. Il y a deux molécules indépendantes dans la maille élémentaire asymétrique dans laquelle chacun des deux ions Cs + est coordiné à deux molécules de ligand DB18C6 formant une structure de type sandwiche.Mots clés : RMN à l'état solide, métal alcalin, déplacement chimique du 133 Cs, ligand macrocyclique, structure cristalline. Wong et al. 985Can. J. Chem. 78: 975-985 (2000) binding environment. We also report the crystal structure for Cs(DB18C6) 2 SCN. Experimental MaterialsCesium thiocyanate, cesium chloride, and macrocyclic ligands were purchased from Aldrich and used without further purification. All complexes were prepared according to the literature methods (15). In general, ligand and cesium salt are mixed in a suitable molar ratio in various solvents (methanol, ethanol, or acetone) and crystallized at either room temperature or 5°C. The melting points of the compounds were consistent with the literature values: mp: 201-202°C for Cs(18C6) SCN,[191][192][193] for Cs(18C6) 2 SCN, 152-153°C for Cs(DB18C6) 2 SCN, 99-100°C for Cs(C222)SCN, and 146-148°C for Cs(C22)SCN. Nonactin (purchased from Sigma) was mixed with CsSCN in a molar ratio of 1:1.5 in methanol. Gramicidin D [Dubos] was purchased from Sigma (80% gramicidin A, 6% gramicidin B, and 14% gramicidin C). Gramicidin A (25 mg/mL) was crystallized at 5°C from...

Solid state nuclear magnetic resonance as a tool to explore solvent-free MALDI samples

“…In particular, the high field shift in 133 Cs NMR spectrum is consistent with a coordination of the Cs ϩ cation by oxygen atoms. Chemical shift measured in Figure 1b suggests approximately eight to ten oxygen atoms around the Cs ϩ cation [31], which is consistent with the irregular coordination sphere of Cs ϩ ion reported for caesium salicylate crystal [32], exclusively consisting of eight oxygen atoms. However, although CsF is a highly hygroscopic salt, observed 133 Cs chemical shifts are neither compatible with surface hydrated species (expected at 74 ppm and 101 ppm) nor with fully hydrated species (usually observed near 45 ppm).…”

“…The main 19 F resonance observed at Ϫ110 ppm ( Figure 1c) actually consists of two overlapping signals, one of which is particularly broad and reflects a large distribution of the chemical shift, suggesting a substantial participation of atmospheric water molecules in the reaction [37]. The 133 Cs MAS NMR spectrum still exhibits a typical signal of Cs ϩ coordinated with multiple oxygen atoms [31], but the chemical shift observed for this Form II is weakly down-fielded compared with Form I (Ϫ5 ppm versus Ϫ11 ppm, respectively). This difference is due to a slightly modified organization and participation of ligands in the caesium coordination sphere, in terms of number and distance.…”

Complexation of the caesium cation by the host p-tert-butylcalix[6]arene hexaacetamide

“…28 Downfield shifts are observed for shorter Cs-O distances. 28 The average of the 15 Cs-O distances in the crystal structure of the 2 : 1 complex (see below) is 3.1 ± 0.2 Å. On the basis of the correlation, 28 this value corresponds to an isotropic 133 Cs chemical shift in the range of 70-90 ppm, in excellent agreement with the experimental value for the 2 : 1 complex.…”

“…28 The average of the 15 Cs-O distances in the crystal structure of the 2 : 1 complex (see below) is 3.1 ± 0.2 Å. On the basis of the correlation, 28 this value corresponds to an isotropic 133 Cs chemical shift in the range of 70-90 ppm, in excellent agreement with the experimental value for the 2 : 1 complex. Since the two coordination sites of the caesium cations in the solid state 2 : 1 complex are not equivalent, the observation of one 133 Cs NMR signal for the two Cs ϩ in solution indicates a rapid intramolecular exchange between the two sites.…”

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“…This trend is consistent with previous observations made in solid‐state 23 Na and 133 Cs NMR studies of similar Na + ‐ and Cs + ‐ionophore compounds. [ 37,38 ]…”