A Study of C−F···M+ Interaction: Metal Complexes of Fluorine-Containing Cage Compounds

Abstract

The C−F···M⁺ interaction was investigated by observation of the NMR spectroscopic changes and complexation studies between metal cations and the cage compounds 1 and 2 which have fluorobenzene units as donor atoms. As a result, the interaction was detected with all of the metal cations which form complexes with 1 and 2. The stability of the complexes of 1 and 2 was determined by the properties of the metal cations (ionic radii and degree of hydrolysis), not by the hard−soft nature of the cations. Crystallographic analyses of Tl⁺ ⊂ 1 and La³⁺ ⊂ 2 provided structural information (interatomic distances and bond angles), and the bond strengths, C−F···M⁺, O···M⁺, and N···M⁺, were estimated by Brown's equation based on the structural data. Short C−F···Tl⁺ (2.952−3.048 Å) distances were observed in the complex Tl⁺ ⊂ 1. The C−F bond lengths in the complexes, Tl⁺ ⊂ 1 and La³⁺ ⊂ 2, are elongated compared to those of the metal-free compounds. Interestingly, no solvent molecules including water molecules were coordinated to La³⁺ in the La³⁺ ⊂ 2. The stabilization energy of cation−dipole interaction was calculated on the basis of the data from X-ray crystallographic analysis, and it is roughly consistent with the −ΔH values estimated in solution. Thus, the C−F···M⁺ interaction can be expressed by the cation−dipole interaction. This result explains the fact that compound 1 which has fluorine atom as hard donor strongly binds soft metals such as Ag⁺ and Tl⁺. Furthermore, it was concluded that the fluorobenzene unit has a poor electron-donating ability compared to that of ether oxygen or amine nitrogen, and thus the ratio of the coordination bond in C−F···M⁺ is small. The specific and remarkable changes in the ¹H, ¹³C, and ¹⁹F NMR spectra were observed accompanied by the complexation between M⁺ and the hosts 1 and 2. These spectral features are important tools for the investigation of the C−F···M⁺ interaction. Furthermore, F···Tl⁺ spin couplings were observed at room temperature in the Tl⁺ ⊂ 1, 2 (J_F-Tl = 2914 Hz for Tl⁺ ⊂ 1 and 4558 Hz for Tl⁺ ⊂ 2), and these are clear and definitive evidence of the interaction.