Anion–π recognition between [M(CN)6]3− complexes and HAT(CN)6: structural matching and electronic charge density modification

Abstract

Hexacyanidometalates (M = Fe^III, Co^III) and multisite anion receptor HAT(CN)₆ (1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile) recognize each other in acetonitrile solution and self-assemble into the novel molecular networks (PPh₄)₃[M(CN)₆][HAT(CN)₆] (M = Fe, 1; Co, 2) and (AsPh₄)₃[M(CN)₆][HAT(CN)₆]·2MeCN·H₂O (M = Fe, 3; Co, 4). 1–4 contain the stacked columns {[M(CN)₆]³⁻;[HAT(CN)₆]}_∞ separated by the organic cations. All of the M–CN vectors point collectively towards the centroids of pyrazine rings on neighboring HAT(CN)₆ molecules, with N_cyanide⋯centroid_pyrazine 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)₆. 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)₆]³⁻ to the HAT(CN)₆ orbital systems and further, toward the peripheral –CN groups of HAT(CN)₆. Solid-state DFT calculations determined the total interaction energy of HAT(CN)₆ to be ca. −125 kcal mol⁻¹, which gives ca. −15 kcal mol⁻¹ per one CN⁻⋯HAT(CN)₆ 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)₆]³⁻;[HAT(CN)₆]} chromophore, with the formation constant K_add = (5.8 ± 6) × 10² dm³ mol⁻¹ and the molar absorption coefficient ε_add = 180 ± 9 cm⁻¹ dm³ mol⁻¹ at 600 nm, as estimated from concentration-dependent studies.