Anion-π recognition between [M(CN)] complexes and HAT(CN): structural matching and electronic charge density modification.

Hexacyanidometalates (M = Fe, Co) and multisite anion receptor HAT(CN) (1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile) recognize each other in acetonitrile solution and self-assemble into the novel molecular networks (PPh)[M(CN)][HAT(CN)] (M = Fe, 1; Co, 2) and (AsPh)[M(CN)][HAT(CN)]·2MeCN·HO (M = Fe, 3; Co, 4). 1-4 contain the stacked columns {[M(CN)];[HAT(CN)]} separated by the organic cations. All of the M-C[triple bond, length as m-dash]N vectors point collectively towards the centroids of pyrazine rings on neighboring HAT(CN) molecules, with Ncentroid distances that are under 3 Å. The directional character and structural parameters of the new supramolecular synthons correspond to collective triple anion-π interactions between the CN ligands of the metal complexes and the π-deficient areas of HAT(CN). Physicochemical characterisation (IR spectroscopy, UV-Vis spectroscopy, cyclic voltammetry) and dispersion-corrected DFT studies reveal the dominating charge-transfer (CT) and polarisation characters of the interactions. The electronic density flow occurs from the CN ligands of [M(CN)] to the HAT(CN) orbital systems and further, toward the peripheral -CN groups of HAT(CN). Solid-state DFT calculations determined the total interaction energy of HAT(CN) to be ca. -125 kcal mol, which gives ca. -15 kcal mol per one CNHAT(CN) contact after subtraction of the interaction with organic cations. The UV-Vis electronic absorption measurements prove that the intermolecular interactions persist in solution and suggest a 1 : 1 composition of the anion-π {[M(CN)];[HAT(CN)]} chromophore, with the formation constant K = (5.8 ± 6) × 10 dm mol and the molar absorption coefficient ε = 180 ± 9 cm dm mol at 600 nm, as estimated from concentration-dependent studies.