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.

We present an advanced material exhibiting a spin crossover (SCO) effect generated by the combined application of two external stimuli, temperature and chemical treatment related to the crystallization solvent exchange. The reported material is based on a bimetallic {[FeII(4-Brphpy)4]3[ReV(CN)8]2}⋅7MeOH [1, 4-Brphpy = 4-(4-bromophenyl)pyridine] coordination network built of cyanido-bridged layers and interstitial methanol molecules. A weakly bonded solvent can be exchanged into the water, which results in a hydrated phase, {[FeII(4-Brphpy)4]3[ReV(CN)8]2}⋅5H2O (1-hyd). The difference in solvent content between 1 and 1-hyd is sufficient to induce a remarkable change in spin transition properties. Despite the {N6} coordination environment around Fe(II), 1 reveals a stable high-spin state in the whole 2–300 K temperature range, which was assigned to the stiffening of the framework due to a series of non-covalent interactions involving solvent and 4-Brphpy ligands. Upon the exchange of solvent to water, the framework of 1-hyd becomes more flexible producing a thermally induced SCO effect, which occurs in two distinguishable steps in the broad 70–250 K range. The 1 to 1-hyd transformation can be reversed by immersion in the original solvent, thus, the chemical ON–OFF switching of a thermal SCO effect was achieved. This work shows that solvent exchange processes within bimetallic layered FeII–[ReV(CN)8]3– networks give an efficient route for reversible chemical modulation of thermally induced SCO properties.

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