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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 (PPh 4 ) 3 [M(CN) 6 ](H 3 PG) 2 ·2MeCN (M = Cr, 1 ; Fe, 2 ; Co 3 ; H 3 PG = 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, 57 Fe 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- ;(H 3 PG) 2 } ∞ network features original {[M(CN) 6 ] 3- ;(H 3 PG) 4 } supramolecular cis -bis(chelate) motifs involving: (i) two double cyclic hydrogen bond synthons M(-CN⋅⋅⋅HO-) 2 Ar, {[M(CN) 6 ] 3- ; H 2 PGH}, between cis -oriented cyanido ligands of [M(CN) 6 ] 3- and resorcinol-like face of H 3 PG, and (ii) two single hydrogen bonds M-CN⋅⋅⋅HO-Ar, {[M(CN) 6 ] 3- ; H PGH 2 }, 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.

Functional molecule-based solids built of metal complexes can reveal a great impact of external stimuli upon their optical, magnetic, electric, and mechanical properties. We report a novel molecular material, {[Eu III (H 2 O) 3 (pyrone) 4 ][Co III (CN) 6 ]}· n H 2 O ( 1 , n = 2; 2 , n = 1), which was obtained by the self-assembly of Eu 3+ and [Co(CN) 6 ] 3- ions in the presence of a small 2-pyrrolidinone (pyrone) ligand in an aqueous medium. The as-synthesized material, 1 , consists of dinuclear cyanido-bridged {EuCo} molecules accompanied by two H-bonded water molecules. By lowering the relative humidity (RH) below 30% at room temperature, 1 undergoes a single-crystal-to-single-crystal transformation related to the partial removal of crystallization water molecules which results in the new crystalline phase, 2 . Both 1 and 2 solvates exhibit pronounced Eu III -centered visible photoluminescence. However, they differ in the energy splitting of the main emission band of a 5 D 0 → 7 F 2 origin, and the emission lifetime, which is longer in the partially dehydrated 2 . As the 1 ↔ 2 structural transformation can be repeatedly reversed by changing the RH value, the reported material shows a room-temperature switching of detailed luminescent features including the ratio between emission components and the emission lifetime values.