Supramolecular cis-“Bis(Chelation)” of [M(CN)6]3− (M = CrIII, FeIII, CoIII) by Phloroglucinol (H3PG)

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.


Introduction
Hexacyanidometallates of d block metal ions belong to the most versatile and common building blocks in the construction of functional molecular materials within the frame of various synthetic strategies. Based on their individual properties resulting from the valence electronic structure (magnetic, optical, redox reactivity, communicative molecular orbital system), polynuclear complexes are considered in the construction of switchable materials such as molecular magnets, nanomagnets, and photomagnets or solar energy converting units. These features are very often combined and enhanced with porosity, conductivity, or luminescence under the common flag of the Prussian Blue Analogues (PBAs) family and the related species of various dimensionality [1][2][3].

Structural Studies
Compounds 1-3 crystallized in the monoclinic system, space group C2/c (Table S1). The uniformity of the powder samples and the identity of the crystals examined with SC XRD were confirmed by PXRD ( Figure S1, Supplementary Materials  ]. Importantly, the original double synthon is formed owing to structural and electronic matching between the cis-dicyanido fragment of the complex and resorcinol-like [48] (1,3-dihydroxobenzene) fragment of H 3 PG, as a vivid example of the realization of the molecular tectonics concept [12]. The interatomic distances and the related angles within both synthons are collected in Table 1. The observed N···O and N···H separations, together with the related almost linear N···H-O angles, allow us to classify them as moderate-to-strong hydrogen-bonding interactions [49] (see also calculated interaction energies presented in the Section 2.4 below). Among the N···O and N···H distances, the shortest ones are noted in the single {[M(CN) 6 ] 3− ;(HPGH 2 )} synthon along with the whole 1-3 series, whereas those in the {[M(CN) 6 ] 3− ;(H 2 PGH)} synthon are slightly longer and notably diversified, which is most probably due to steric effects that might accompany the formation of such a complex motif. The double synthons are almost planar, with a ca . 10 Figure 1c is identical to the distribution of ligands in canonical cis stereoisomers of bis(chelated) six-coordinate d metal ion complexes [ML 2 A 2 ] (L = bidentate chelating ligands, A = monodentate ligands), and thus we suggest to describe such aggregation as the non-covalent cis-bischelation. Moreover, the formation of the double synthons leads to the notable deformation of the [M(CN) 6 ] 3− complex from octahedron following the spatial demands imposed by the distribution of the -OH groups in H 3 PG. The C2-M1-C3 and N2···M1···N3 angles in all compounds are close to 85 • and 82 • , respectively, notably smaller compared to other close-to-right angles; this feature resembles the standard biting angles of ca. 72-75 • (Table S3)  The molecular environment of the tectons engaged in the hydrogen-bond networks is completed by the oligomeric arrays of PPh 4 + cations accompanied by some MeCN molecules; these components form multiple weak C-H···A interactions (A = O atoms and ring system of H 3 PG, N atoms of [M(CN) 6 ] 3− ) in the regions not involved in the typical hydrogen bonds ( Figure S3). The {PPh 4 + } ∞ 3D subnetwork itself provides substantial structural stabilization through so-called multiple phenyl embrace (MPE) motifs, here realized mainly by the sextuple phenyl embrace (SPE) or offset sextuple phenyl embrace (OSPE) and other hybrid patterns, with the shortest P···P distances of 6.2, 6.6, and 7.3 Å in all structures [51,52] ( Figure S4).

Hirshfeld Analysis
The square-grid-like anionic subnetwork is possible due to two synthons based on moderateto-strong hydrogen bonds. These features can be observed on the Hirshfeld surfaces [60][61][62] generated for the [Fe(CN) 6 ] 3− anion and H 3 PG molecule (Figure 2a-d). They emerge as red spots marking distances shorter than a sum of van der Waals radii and also as spikes on corresponding fingerprints. This analysis also shows that in the case of the [Fe(CN) 6 ] 3− unit, N···H interactions prevail (74.4%) ( Figure S6). For H 3 PG, it is only 14.7% of created contacts, but they are the shortest ones and correspond to hydrogen bonds, whereas O···H distances are more numerous (23.1%) but much longer, and the tiny spike at (1.4, 1.1) corresponds to two C-H···O hydrogen bonds ( Figure S7). Both synthons can be clearly visible in Figure S8.  6 ] 3− anion and H 3 PG involved in π-π interactions between strongly inclined rings.

ESI-MS
The ESI-MS spectrograms in the negative ionization mode measured for MeOH solutions

DFT Calculations
To shed some light on the strength and nature of the interaction between the [M(CN) 6 ] 3− anion and H 3 PG molecule(s) in 1-3, dispersion-corrected density functional theory (DFT + D4) [63] 6 ] 3− ;HPGH 2 } motifs) and between 4,4 bpy or dpe and H 3 PG in {4,4 bpy;HPGH 2 } and {dpe;HPGH 2 } used as a reference. As can be seen, ∆E int computed for a given motif does not show a strong dependence on the basis set nor the density functional employed in the calculations, although the double-hybrid functionals (expected to give the most accurate results [65]) systematically indicate somewhat stronger interactions between hydrogen-bond acceptor and hydrogenbond donor molecules in the examined clusters compared to standard gradient and global hybrid functionals. Note also that tremendously decreased magnitude interaction energies were obtained for the motifs extracted from 1-3 when the acetonitrile continuum solvent model was employed in the calculations (see Table S4), in line with additional energetic stabilization of the charged [M(CN) 6 ] 3− fragment in such electrostatic medium; as in the crystal structures, the charge of the hexacyanidometallate anion is also screened to some extent by the surrounding moieties (cations in particular), and we expect the [M(CN) 6 ] 3− /H 3 PG interaction energies to be smaller in magnitude than those determined by the gas-phase calculations although definitely not so diminished as indicated by solvation ones. Nevertheless, all the methods uniformly demonstrate: (i) a slight increase in the magnitude of interaction energies between [M(CN) 6 6 ] 3− ;HPGH 2 } is determined by both electrostatic ∆E elstat and orbital-interaction ∆E orb components [67], with the latter being represented not only by the σ-CT hydrogen-bonding channel but primarily, as indicated by the analysis of other energetically relevant NOCV contributions, by the polarization (intra-CT) of the π-electron system within H 3 PG, enhanced likely due to ion-dipole interaction imposed by the negative charge of the [Co(CN) 6 ] 3− . Such electron-transfer channels in {4,4 bpy;HPGH 2 } and {dpe;HPGH 2 } are visibly diminished, which seems to be directly responsible for a pronounced decrease in the magnitude of the orbital-interaction contribution (even for {dpe;HPGH 2 }, for which a shorter hydrogen-bond distance significantly strengthens its corresponding orbital component, see Figure S17). This decrease in ∆E orb along with less stabilizing ∆E elstat provides less counterbalance for the repulsive Pauli interaction and, accordingly, a significant drop in the absolute values of ∆E int for {4,4 bpy;HPGH 2 } and {dpe;HPGH 2 } was observed. For comparison, the corresponding interaction energies between 4,4 -bipyridyl (4,4 bpy) and H 3 PG in molecular cluster {4,4 bpy;HPGH 2 } extracted from the crystal structure reported in Ref. [64] are listed.   6 ] 3− ;HPGH 2 } molecular clusters extracted from the crystal structure of 3 and between 4,4 -bipyridyl (4,4 bpy) and H 3 PG in {4,4 bpy;HPGH 2 } molecular cluster extracted from the crystal structure reported in Ref. [64] used here as a reference. Isosurfaces (±0.0005 au) of dominant NOCV contributions to the differential electron density ∆ρ describing hydrogen bonding along with their charge (q in e) and orbital energy (∆E in kcal mol −1 ) assessment. 6 ] 3− ;H 2 PGH} motif correspond to the total assessment for both hydrogen bonds and to the shortest one only (given in parentheses). Numbers listed close to the O-H···N contacts are the hydrogen-bond distances, in Å. In the table: The corresponding interaction energy components (in kcal mol −1 ) as obtained using the ETS energy decomposition scheme are presented. Based on BLYP + D4//TZP calculations.

57 Fe Mössbauer Spectra
The solid-state 57 Fe Mössbauer spectra for 2 and the (PPh 4 ) 3 [Fe(CN) 6 ]·7H 2 O reference are presented in Figure 6b. Both spectra were reproduced using single doublets assignable to the low-spin (LS) Fe III state expected for the LS [Fe(CN) 6 ] 3− anion. Co-crystal salt 2 reveals the isomeric shift δ 2 = -0.11 mm s −1 , and quadrupole splitting QS 2 = 0.55 mm s −1 . δ 2 is smaller than δ ref = -0.09 mm s −1 for the reference salt, which is in line with more significant electron density removal from the 3 d metal valence orbitals expected for more extended and stronger hydrogen bonds CN N···H-O H3PG in 2 compared to those present in the reference salt. QS 2 is considerably larger than QS ref = 0.33 mm s −1 for the reference, which might be related to a specific distribution of the cyanido ligands of the local C 2 symmetry and slightly decreased degeneracy of the t 2g orbital set due to the bis(chelate)like arrangement of four H 3 PG hydrogen bond donors. The observed negative correlation between δ and QS change is in line with the tendency found for a set of hydrated and dehydrated PBAs [1,73].

UV-Vis Electronic Absorption Spectra
The colors of the starting materials are: white for H 3 6 ]·6H 2 O, respectively. For 1, the energies of the 4 A 2g → 4 T 2g ( 4 F) and 4 A 2g → 4 T 1g ( 4 F) transitions [9,69,[74][75][76][77] were increased from 388 to 380 nm (∆E = 550 cm −1 ) and from 315 to ca. 290 nm (∆E of at least of 2700 cm −1 ; this estimation lacks exactness due to the bands' overlap). For 3, the energy of the 1 A 1g → 1 T 1g transition [70,[76][77][78][79] was shifted from 324 to ca. 297 nm (∆ of at least 2800 cm −1 ). For the Fe(CN) 6 ] 3− analogue, the lowest-energy range was dominated by ligand-to-metal charge-transfer (LMCT) σ(CN -) → π(2t 2g ) transitions with a possible admixture of one of the ligand-field (LF) spin-forbidden transitions [71,76,77,80]. While the whole band was also shifted to a higher energy from 433 nm for the reference towards 422 nm for 2 (∆E = 600 cm −1 ), another feature centered at ca. 490 nm appeared for 2, covering the range up to 750 nm. All observed changes should be interpreted in terms of the stabilization or destabilization of the relevant metal and cyano-ligand orbitals involved in the transitions due to the relocation of electronic density along the σand π-channels under the impact of the electrostatic field provided by the species surrounding cyanido-ligands. For 1 and 3, the relative increase of ∆ O splitting might be directly inferred due to the stronger electrostatic field imposed by six CN − ···H-O H3PG hydrogen bonds, compared to the CN − ···H-O water hydrogen bonds observed in the crystal structures of (PPh 4 ) 3 [M(CN) 6 ]·nH 2 O salts [58,59]. The range of the ∆ O change is comparable to those reported for [Cr(CN) 6 ] 3− and [Co(CN) 6 ] 3− anions in the solid matrices of alkali metal halides; however, a sign of this change might depend on the distance and geometry of CN···cation motif [69,70,75]. The theoretical consideration of the energy levels for the mono-ionized [Co(CN) 6 ] 4anion suggests that our observations for 1 and 3 might be due to the relative stabilization of 2t 2g orbitals, from where the electrons are excited in the LF states [79]. In the case of 2, the interpretation is not so straightforward as the 2t 2g orbitals (the incomplete configuration 2t 2g 5 ) involved in the LMCT transitions are the electron recipient levels. Thus, in this case, one should also consider the relocation of electronic density on the relevant lower energy orbitals (of 3t 1u and/or 2a 1g type) or some splitting of the involved orbitally degenerated states under the electrostatic field of hydrogen bonds [77]. The low energy spin forbidden bands of [Cr(CN) 6 ] 3− and [Co(CN) 6 ] 3− were scarcely detectable in our setup and were not considered in the analysis. A more precise description of the electronic structure of our co-crystal salts might be obtained with the application of more advanced experimental methods based on X-ray absorption and emission, or ultrafast photoelectron spectroscopy coupled with transient infrared studies combined with modern computational methods [71,78,[80][81][82][83].

Photoluminescence Studies
The photoluminescence spectra of 1 and the (PPh 4 ) 3 [Cr(CN) 6 ]·2H 2 O reference at 77 K are presented in Figure 8, both revealing three distinguishable bands of the location and maximum lines specified in Table 3. The photoluminescence pattern observed in the range 750-875 nm is assigned to the 2 E g → 4 A 2g phosphorescence characteristic of various inorganic solids and hybrid molecular solids involving [Cr(CN) 6 ] 3− moiety [9,69,75,84,85] and may be interpreted in terms of its vibronic properties. The positions of these bands are usually indicated in respect to the R 1 (0 -0) emission line (here not measured, however, expected to be located at ca. 795-800 nm) and might be attributed to the specific fundamental modes involving some of the skeletal δ(C-Cr-C), δ(Cr-C-N) and δ(Cr-C) vibrations (below 450-500 cm −1 ; ν 9 , ν 13 , ν 7 , ν 8 , and ν 12 in the increasing energy order) and combination modes (above 500 cm −1 ) [9,69,75]. The systematic bathochromic shift of ca. 5 nm was observed, going from (PPh 4 ) 3 [Cr(CN) 6 ]·2H 2 O to 1, which might be interpreted in terms of the modification of molecular surroundings described above, and correlates with the hypsochromic shift of the 4 A 2g → 4 T 2g ( 4 F) and 4 A 2g → 4 T 1g ( 4 F) transitions in the UV-Vis spectra. This nicely corresponds with the systematic increase of the relevant absorption energy and decrease of the 2 E g → 4 A 2g phosphorescence energy observed for the solid phases of (PPh 4 ) 3 [Cr(CN) 6 ]·2H 2 O, 1, and K 3 [Cr(CN) 6 ], coming from the first one to the last one [69,75,84]. The emission lifetimes τ 1 determined using the equation corresponding to the single decay process are (all in ms): 6.8 (77 K) and 5.5 (298 K) for (PPh 4 ) 3 [Cr(CN) 6 ]·2H 2 O, and 5.1 (77 K) and 4.5 (298 K) for 1 (λ exc = 395 nm followed at the various accessible emission lines) (Figures S23 and S24). The observed slight decrease of τ 1 for 1 might be tentatively attributed to the specific character of hydrogen bond architecture in 1 described above. The shortening of lifetimes coincides with the decrease of average luminescence quantum yields measured at room temperature, from 10.9(2)% for (PPh 4 ) 3 [Cr(CN) 6 ]·2H 2 O to 8.84(4)% for 1. More detailed information could be inferred from LHe measurements, supported by computational methods, which are beyond the scope of this study.

Conclusions and Perspectives
The PPh 4 + cation-assisted supramolecular self-assembly between the [M(CN) 6  The above findings might be important from the standpoint of the design of modular multisite anion receptors dedicated to binding of d-metallates and the development of alternative pathways towards the controlled synthesis of new multicomponent (coordinationbased, hybrid organic-inorganic, etc.) architectures and materials of functional features. In particular, the targeted CCDC search indicates the existence of several very interesting complex molecular motifs exhibiting multiple resorcinol groups [86][87][88][89][90][91][92][93]; they may be definitely considered as potential platforms for multisite anions receptors exploiting the synthons described in this manuscript. Advanced research in this direction is underway in our group. 2 mmol in 15 mL of CH 3 CN) were mixed to obtain a colorless solution immediately, and the mixture was tightly closed in the vessel. After one day, colorless crystals of 3 appeared. The crystals were filtered and washed with cold acetonitrile (10 mL, 2 • C) and dried in air. The composition of (PPh 4 ) 3 [Co(CN) 6 ](H 3 PG) 2 ·2MeCN was defined by a single-crystal X-ray diffraction analysis. Phase purity was proved by XRD data. Yield: 0.126 mg, 44 Figure S25).

X-ray Diffraction Analysis
Single crystal X-ray diffraction data for all compounds were collected using a Bruker D8 Quest Eco diffractometer equipped with a Photon II detector and a Mo Kα (λ = 0.71073 Å) radiation source with a graphite monochromator and Oxford Cryostream cooling system. Crystals for measurement were taken from the mother solution and covered by NVH immersion oil. All measurements were performed in 100.0 K. Data reduction and cell parameter refinement were performed using Apex software with included SAINT and SADABS programs. Intensities of reflections for the sample absorption were corrected using the multiscan method. Structures were solved by the intrinsic phasing method and refined anisotropically with weighted full-matrix least-squares on F 2 using SHELXT [94] and SHELXL [95] programs with Olex 2 graphic interface [96].
Hydrogen atoms within structures were placed in idealized positions and refined using a riding coordinate model with isotropic displacement parameter set at 1.2-1.5 times U eq of appropriate carrier atoms. Crystal data and structure refinement parameters are summarized in Table S1. The structural figures in the article were prepared using the latest Mercury software [97]. The crystal structures are deposited in the CCDC database. The deposition numbers are 2,175,600 (1), 2,175,601 (2), and 2,175,602 (3).

Physical Techniques and Calculations
Elemental analyses of CHNS were performed on the air-dried samples using the Elemental Vario Micro Cube CHNS analyzer. Powder X-ray diffraction patterns for 1, 2, and 3 in 0.5 mm glass capillary were collected on a D8 Advance Eco (Bruker) using a Cu−Kα radiation source. The thermogravimetric (TGA) curves for the polycrystalline samples were collected using TG209 F1 Libra thermogravimetric analyzer with aluminum pans as holders. The data were collected in the temperature range of 21-400 • C under a nitrogen atmosphere with a heating rate of 1 • C per minute. Infrared (IR) absorption spectra in the range 4000-675 cm −1 were measured on the selected single-crystals using a Nicolet iN10 MX Fourier transform infrared microscope. Far infrared (FIR) spectra in the range 600-100 cm −1 were measured on the powder samples dispersed Apiezon N grease using an FT-IR Bruker Vertex 70 V spectrometer. Raman spectra in the range 3200-100 cm −1 were recorded on the microcrystalline samples with a Renishaw inVia Raman spectrometer with the excitation line 514.5 nm of Ar laser. The transmission 57 Fe Mössbauer spectra were collected in 1024 channels, with a 10 mCi 57 Co source in an Rh matrix, at room temperature using a Wissel spectrometer. The velocity scale was calibrated using the α-Fe foil standard. The powder sample was directly placed in copper rings and sealed with Kapton foil. The background spectra of the sample holders did not reveal any significant contribution to the main spectra. Mössbauer spectra were fitted with the use of the WinNormos-for-Igor software package, assuming the Lorentzian shape of the resonance lines, i.e., the saturation effects were not included. For each compound, one quadrupole doublet was considered in the fitted model, assigned to the Fe III (LS) electron state. Diffuse reflectance spectra in the UV-Vis-NIR range were performed for the ground powder samples mixed with BaSO 4 (2 mass %) using a Shimadzu UV-3600i Plus spectrophotometer equipped with the 50 mm integrating sphere. The spectra were recalculated according to the Kubelka-Munk equation. Solid-state photoluminescent characterization for all reported compounds was performed using an FS5 spectrofluorometer (Edinburgh Instruments) equipped with a Xe arc lamp (150 W, excitation spectra) or a 365 nm diode flashlight (10 W, emission spectra) serving as excitation sources, and a Hamamatsu photomultiplier of the R928P type as a detector. All lifetime measurements were conducted using the same spectrofluorometer employing a multichannel scaling module with a microsecond Xe flashlamp (5 W), while the collected data curves were fitted using a monoexponential decay function within a Fluoracle software (Edinburgh Instruments). Absolute quantum yields (QYs) were measured by a direct excitation method using the Xe arc lamp, an integrating sphere module (SC-30), and a Teflon pin as a reference, while the related calculations were performed within the implemented software. The Fluoracle program was also employed for the background corrections and a smoothing procedure, while for the data collected using the 365 nm flashlight, the application of a non-linear baseline was determined using the Asymmetric Least-Square Smoothing method (OriginPro 2021b) was found necessary.
Author Contributions: K.J.: investigation-syntheses, measurements, data analysis and visualization of PXRD, TGA, IR spectroscopy, and UV-Vis electronic absorption spectroscopy, writing-original draft fragments preparation; J.K.: investigations-SC XRD structural measurements, crystal structure solution and refinement, data curation; D.G.: investigation-preliminary synthesis, structural data curation and description, participation in photoluminescence measurements, writing-original draft fragments preparation, review; E. K  Data Availability Statement: The insight into detailed data might be obtained after the contact with the corresponding author.