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.

Spin crossover (SCO) materials, revealing the externally tunable transition between two different spin states, arouse great scientific interest due to their perspective application in information storage, display devices and sensing. Of special importance are the molecular systems offering the possibility of multimodal switching within many spin centers. This is achievable in polynuclear clusters consisting of several SCO-active complexes, however, such molecules are very rare. Herein, we report a unique pair of nanometric pentadecanuclear {Fe[M(CN)](Metacn)}·14MeOH (Metacn = 1,4,7-trimethyl-1,4,7-triazacyclononane, M = Re, 1; M = W, 2) clusters exhibiting a thermally induced spin crossover effect on Fe(ii) complexes, that is on both central and external Fe sites embedded in the cyanido-bridged cluster core. The spin transition occurs gradually in the 120-300 K range, and it is not fully completed even at room temperature. We show that facial coordination of an N,N,N-tridentate Metacn ligand dramatically modifies the character of the spin transition phenomenon when confronted with the previously reported {Fe[M(CN)](MeOH)}·nMeOH (M = Re, W) clusters by (i) engaging, for the first time, not only central but also external Fe intracluster units in the SCO effect, (ii) cancelling the Fe-W charge transfer pathway, and (iii) decreasing the cooperativity within the supramolecular network.

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Studies on molecular co-crystal type materials are important in the design and preparation of easy-to-absorb drugs, non-centrosymmetric, and chiral crystals for optical performance, liquid crystals, or plastic phases. From a fundamental point of view, such studies also provide useful information on various supramolecular synthons and molecular ordering, including metric parameters, molecular matching, energetical hierarchy, and combinatorial potential, appealing to the rational design of functional materials through structure–properties–application schemes. Co-crystal salts involving anionic d-metallate coordination complexes are moderately explored (compared to the generality of co-crystals), and in this context, we present a new series of isomorphous co-crystalline salts (PPh4)3[M(CN)6](H3PG)2·2MeCN (M = Cr, 1; Fe, 2; Co 3; H3PG = phloroglucinol, 1,3,5-trihydroxobenzene). In this study, 1–3 were characterized experimentally using SC XRD, Hirshfeld analysis, ESI-MS spectrometry, vibrational IR and Raman, 57Fe Mössbauer, electronic absorption UV-Vis-NIR, and photoluminescence spectroscopies, and theoretically with density functional theory calculations. The two-dimensional square grid-like hydrogen-bond {[M(CN)6]3–;(H3PG)2}¥ network features original {[M(CN)6]3–;(H3PG)4} supramolecular cis-bis(chelate) motifs involving: (i) two double cyclic hydrogen bond synthons M(-CN×××HO-)2Ar, {[M(CN)6]3–;H2PGH}, between cis-oriented cyanido ligands of [M(CN)6]3– and resorcinol-like face of H3PG, and (ii) two single hydrogen bonds M-CN×××HO-Ar, {[M(CN)6]3–;HPGH2}, involving the remaining two cyanide ligands. The occurrence of the above tectonic motif is discussed with regard to the relevant data existing in the CCDC database, including the multisite H-bond binding of [M(CN)6]3– by organic species, mononuclear coordination complexes, and polynuclear complexes. The physicochemical and computational characterization discloses notable spectral modifications under the regime of an extended hydrogen bond network.