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(+) subset 1 and La(3+) subset 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 A) distances were observed in the complex Tl(+) subset 1. The C-F bond lengths in the complexes, Tl(+) subset 1 and La(3+) subset 2, are elongated compared to those of the metal-free compounds. Interestingly, no solvent molecules including water molecules were coordinated to La(3+) in the La(3+) subset 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 (-)Delta H 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 (1)H, (13)C, and (19)F 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(+) subset 1, 2 (J(F-Tl) = 2914 Hz for Tl(+) subset 1 and 4558 Hz for Tl(+) subset 2), and these are clear and definitive evidence of the interaction.

Fluorocarbon adsorption in hierarchical porous frameworks

“…The calculated bond length and angle between C-F R12 Cr 3 þ MIL-101 was 2.27 Å and 110°with a slight elongation of C-F R12 bond length (1.39 Å) caused by the weak attractive forces. These C-F M þ interactions are consistent with previous crystallographic and NMR experiments on metal complexes of fluorine-containing cage compounds and ARTICLE suggest short C À F M þ distances and elongation of C À F bonds in the crystal structure 31 .…”

The fluorine-NHC gauche effect: a structural and computational study

“…[27] In the absence of crystallographic data, it was envisaged that determination of the 1 J CF coupling constants might give insights as to the presence or absence of remote metal-fluorine interactions, since the coupling constant would likely decrease upon coordination with the metal centre. [28,29] However, by comparing the coupling constants of the NHC precursor salt 15 (J = 185.0 Hz) with those of the Rh complex 29 (J = 185.0 Hz) and the Ir complex 30 (J = 186.4 Hz), no significant deviation was observed.…”

Intramolecular Metal–Fluorocarbon Coordination, CF Bond Activation and Lanthanoid–Fluoride Clusters with Tethered Polyfluorophenylamide Ligands

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] These range from agostic interactions (ca. 3.0 ) to quite strong bonds (ca.…”

“…+ and fluorobenzene or 1,2-difluorobenzene have been characterised as BPh 4 À salts, [14] whilst the encapsulation of La 3 + in a cage with polyfluoroarene, N-A C H T U N G T R E N N U N G (amine) and OA C H T U N G T R E N N U N G (ether) donor atoms has closer interactions with F than O or N. [16] (There is also related structural chemistry involving fluoroalkyl groups. [17,18] ) Such complexes are poised for CÀF activation, since the very strong CÀF bonds [19,20] are weakened by coordination.…”