Recoordination of a metal ion in the cavity of a crown compound: a theoretical study 2.* Effect of the metal ion— solvent interaction on the conformations of calcium complexes of arylazacrown ethers

Abstract

The effect of the local interaction of a metal ion with the solvent on the conformations of calcium complexes of arylazacrown ethers and an azacrown containing dye was studied using the density functional method with the PBE and B3LYP functionals. The structures were studied and the interaction energies were determined for the calcium complexes with n = 1-12 water or acetonitrile molecules. It was found that the inner coordination sphere of the free Ca 2+ cation contains six H 2 O or seven MeCN molecules. The cation-acetonitrile interaction energy is higher than the cation-water interaction energy up to the moment the second solvation shell of the cation is almost complete (n = 11). The inner coordination sphere of Ca 2+ in the macrocycle cavity contains at most three water molecules, while the fourth one is displaced to the second coordination sphere. Taking into account the local interaction with the solvent (H 2 O or MeCN), the conformers of the calcium complexes of arylazacrown ethers and the azacrown containing dye were studied. It was shown that the presence of two to four water molecules in the coordination sphere of the cation reduces the relative energies of the conform ers with broken metal-nitrogen bond, thus favoring ground state metal recoordination.Key words: arylazacrown ethers, azacrown containing dyes, complexes with Ca 2+ , photo induced recoordination, solvation, ab initio quantum chemical calculations, density functional theory.UV absorption spectra of the alkaline earth metal com plexes of azacrown containing dyes 1-5 recorded at a large excess of the cation ensuring the absence of the free dye in solution exhibit a long wavelength shoulder, which can not be attributed to the residual absorption of the free ligand. It was shown 4,5 that, in the excited state of the azacrown containing dyes 1a,b, the cation undergoes a photoinduced recoordination in the macrocycle cavity (Scheme 1). It was supposed that this recoordination could also occur in the ground state of the complex.The fact that two types of cation coordination sepa rated by an energy barrier can exist in the macrocycle cavity is nontrivial by itself. Our theoretical density functional study of the model N arylaza 15 crown 5 (ArN15C5) compounds and the real dye 1a (see Ref. 1) has demonstrated that the recoordination can occur only due to the transitions between the axial (Ax) and equato rial (Eq) forms of the arylazacrown ether complex. In the axial type conformers, the aromatic ring axis passing through the crown ether nitrogen N cr and the opposite * For Part 1, see Ref. 1. ** Dedicated to Academician A.L. Buchachenko on the occa sion of his 70th birthday. Scheme 11a: X = S, R = Et, R´ = H, n = 1; 1b: X = CMe 2 , R = Me, R´ = 4,6 Me 2 , n = 2

Probing Ternary Complex Equilibria of Crown Ether Ligands by Time-Resolved Fluorescence Spectroscopy

“…As the stability of crown ether complexes is significantly reduced in protic compared to nonprotic solvents, all complexation studies were conducted in acetonitrile. We selected Ca­(II) as the metal cation to probe ternary complex formation, as it has been frequently used in complexation studies of N -phenylaza-15-crown-5 derivatives. ,− Ca­(II) was supplied as CaClO 4 ·4H 2 O, yielding four water molecules per Ca­(II) center, a number that satisfies the stoichiometry of ternary complexes according to experimental and computational studies. − The pertinent photophysical properties of the pyrazoline probes 1 and 2 are compiled in Table . Due to the low quantum yield of the free probes, the emission maxima were determined in the presence of a large excess of Ca­(ClO 4 ) 2 ·4H 2 O.…”

“…We suspected that such effects are not confined to Cu­(I)–thiazacrown systems but might extend to other ligands and metal ions as well. In fact, Freidzon et al had previously conducted computational studies of ternary complex formation in the Ca­(II)- N -phenylaza-15-crown-5 system in an effort to explain a long wavelength shoulder observed in the absorption spectrum of Ca­(II) responsive styryl dyes containing this ligand motif. − Furthermore, X-ray structural analysis of the Ca­(II)- N -phenylaza-15-crown-5 complex revealed the presence of three water molecules in the first coordination sphere . As illustrated with Scheme A, ternary complex formation with solvent molecules may increase the steric strain to push the equilibrium toward coordination species with weakened Ca–N interaction, thus increasing the electron donating ability of the aniline moiety.…”

Monolayer detection of ion binding at a crown ether-functionalised supramolecular surface via an integrated optical Bragg grating

“…It is known that upon binding of a cation in a crown ether that there is a signicant change in the solvation of the crown ether motif. 30 In addition, it is known that a cation within a crown ether's cavity can still be solvated with solvent molecules together with an associated solvated counter anion. 31 It is proposed that these associated solvent molecules and counter anion increase the steric bulk of the crown ether-cation complexes relative to the uncomplexed crown ether and thus decrease the density of the monolayer; the presence of the lower refractive index solvent molecules in the free volume lowers the effective index of the crown ether monolayer.…”