Cyanide Complexes Based on {Mo6I8}4+ and {W6I8}4+ Cluster Cores

Compounds based on new cyanide cluster anions [{Mo6I8}(CN)6]2–, trans-[{Mo6I8}(CN)4(MeO)2]2– and trans-[{W6I8}(CN)2(MeO)4]2− were synthesized using mechanochemical or solvothermal synthesis. The crystal and electronic structures as well as spectroscopic properties of the anions were investigated. It was found that the new compounds exhibit red luminescence upon excitation by UV light in the solid state and solutions, as other cluster complexes based on {Mo6I8}4+ and {W6I8}4+ cores do. The compounds can be recrystallized from aqueous methanol solutions; besides this, it was shown using NMR and UV-Vis spectroscopy that anions did not undergo hydrolysis in the solutions for a long time. These facts indicate that hydrolytic stabilization of {Mo6I8} and {W6I8} cluster cores can be achieved by coordination of cyanide ligands.

An interesting and less studied area is the use of luminescent octahedral cluster complexes of molybdenum and tungsten as building blocks for the synthesis of functional coordination polymers. High symmetry and large volume of the cluster complexes make them convenient for the design of crystalline coordination polymers and metal-organic frameworks, while the spectroscopic and redox features of the clusters can be used in order to impart the desired functionality to the solid material. The majority of coordination polymers based on octahedral metal cluster complexes known to date have been obtained based on the cyanide-coordinated clusters [28][29][30]. The presence of ambidentate apical cyanide ligands makes it possible to coordinate transition and post-transition metal cations and to obtain crystalline coordination polymers by self-assembly. Using {M 6 X 8 }-type cyanoclusters is interesting for obtaining new luminescent coordination polymers. For now, the structure of only one cyanide cluster based on an {Mo 6 X 8 } 4+ core has been described in the literature, namely [{Mo 6 Br 8 }(CN) 6 ] 2− [31,32], while the tungsten cyanide clusters of that type are unknown. Therefore, further progress in this field requires the development of methods for the synthesis of cyanide molybdenum and tungsten halide clusters.
In this work, we report on the synthesis of the first cyanide cluster anions based on {Mo 6  To obtain new compounds, two different ways of synthesis were tested, namely, substitution of terminal iodide ligands in solvothermal conditions and mechanochemical depolymerization of Mo 6 I 12 . Each way was found to be usable for obtaining target compounds in preparative amounts. The new anions have shown an outstanding stability in aqueous solutions, demonstrating the stabilizing influence of terminal cyanides.

Synthesis
Until now, only one structure of a cyanide cluster based on halide octahedral molybdenum core has been reported, namely [Mo 6 Br 8 (CN) 6 ] 2− [31]. It was shown that this complex formed polymeric compounds with transition metal cations [32]. For octahedral tungsten halides, cyanide compounds have not been described. At the same time, complexes based on {Mo 6 I 8 } 4+ and {W 6 I 8 } 4+ cluster cores are usually very bright luminophores, which makes them interesting as components of supramolecular systems and coordination polymers.
In this work, two approaches were realized for the preparation of cyanide cluster complexes based on {Mo 6 I 8 } 4+ and {W 6 I 8 } 4+ cores (Scheme 1). The first approach is depolymerization of M 6 I 12 compounds by mechanochemical activation. The reaction between polymeric Mo 6 I 12 and NaCN in the ball mill led to the formation of water-soluble products. After filtration of the solution, addition of CsCl and evaporation, an orange microcrystalline powder was isolated, which was identified as Cs 1 . These reactions were carried out under solvothermal conditions in methanol. In this case, orange-red solutions were formed, which, after cooling, were filtered from an excess of NaCN, mixed with an equal volume of H2O and evaporated. Addition of H2O was found to be necessary in order to prevent the formation of oil after concentration of solutions in pure methanol. The obtained red crystalline solids were isolated and investigated. The use of methanol in solvothermal synthesis made it possible to avoid the problem of hydrolysis of clusters at elevated temperatures. However, in this case, the methylate anion competes with cyanide, so that compounds based on apically heteroleptic anions trans-[Mo6I8(CN)4(MeO)2] 2− and trans-[W6I8(CN)2(MeO)4] 2− were obtained in high yields. The reaction products demonstrated phase purity (Figure 1), and the study of their solutions by 13 C-NMR in d6-DMSO did not reveal the presence of geometric isomers or clusters with a different ratio of CN − and MeO − anions ( Figure 2). Note that hexamolybdenum cluster complexes with methylate anions as apical ligands were described earlier [3], while similar W6 clusters are reported for the first time. Taking into account the high yields of reactions (86% and 60% for 4 and 5, respectively), we can declare the successful preparation of heteroleptic cyanide complexes based on {Mo6I8} 4+ and {W6I8} 4+ cluster cores with the defined mutual orientation of CN − and MeO − ligands. This is of great importance not only for the chemistry of coordination polymers based on cluster complexes. Since methylate ligands are easily leaving ligands that are removed in the presence of acidic protons [3,34], synthesis of the new cluster anions opens the way to the design of new heteroleptic luminescent octahedral clusters of molybdenum and tungsten with predetermined geometries and charges through ligand exchange reactions. The second approach is the substitution of the terminal iodide ligands in the discrete cluster anions [{Mo 6 I 8 }I 6 ] 2− and [{W 6 I 8 }I 6 ] 2− . These reactions were carried out under solvothermal conditions in methanol. In this case, orange-red solutions were formed, which, after cooling, were filtered from an excess of NaCN, mixed with an equal volume of H 2 O and evaporated. Addition of H 2 O was found to be necessary in order to prevent the formation of oil after concentration of solutions in pure methanol. The obtained red crystalline solids were isolated and investigated. The use of methanol in solvothermal synthesis made it possible to avoid the problem of hydrolysis of clusters at elevated temperatures. However, in this case, the methylate anion competes with cyanide, so that compounds based on apically heteroleptic anions trans-[Mo 6 I 8 (CN) 4 (MeO) 2 ] 2− and trans-[W 6 I 8 (CN) 2 (MeO) 4 ] 2− were obtained in high yields. The reaction products demonstrated phase purity (Figure 1), and the study of their solutions by 13 C-NMR in d 6 -DMSO did not reveal the presence of geometric isomers or clusters with a different ratio of CN − and MeO − anions ( Figure 2). Note that hexamolybdenum cluster complexes with methylate anions as apical ligands were described earlier [3], while similar W 6 clusters are reported for the first time. Taking into account the high yields of reactions (86% and 60% for 4 and 5, respectively), we can declare the successful preparation of heteroleptic cyanide complexes based on {Mo 6 I 8 } 4+ and {W 6 I 8 } 4+ cluster cores with the defined mutual orientation of CN − and MeO − ligands. This is of great importance not only for the chemistry of coordination polymers based on cluster complexes. Since methylate ligands are easily leaving ligands that are removed in the presence of acidic protons [3,34], synthesis of the new cluster anions opens the way to the design of new heteroleptic luminescent octahedral clusters of molybdenum and tungsten with predetermined geometries and charges through ligand exchange reactions.

Stability in Aqueous Solutions
The compounds based on anion [Mo 6 I 8 (CN) 6 ] 2− belong to a small family of water-soluble molybdenum and tungsten halide clusters, which are the goal of many researches in the field [35,36]. There is also a challenge to prepare hydrolytically stable clusters of molybdenum and tungsten.  4 , which exists in aqueous solution for several days [38]. The latter complex exhibits phototoxicity, which is the reason for the high interest in such compounds.  13 C-NMR spectra for recrystallized samples match those for the freshly prepared compounds. Octahedral halide cluster complexes of molybdenum and tungsten usually have low hydrolytic stability because of replacement of the apical ligands by H 2 O or OH − over time or hydrolysis of the cluster core itself [39]. Thus, the coordination of cyanide ligands significantly increased the hydrolytic stability of the clusters, which may open the way for investigation of their potential biological applications.     Table 1). The average Mo-Mo and Mo-I bond distances (2.680(5) and 2.773(8) Å, respectively) agree well with the corresponding values observed for clusters with {Mo6I8} 4+ core [40][41][42]. Each Mo atom of the cluster core is coordinated by apical CN − ligand. The Mo-C distances display the average length of 2.188(8) Å, which is in the range of values that were reported for hexanuclear cyanide clusters of Mo [32,43,44]. The crystal packing includes the cluster anions forming a column along the a axis and Bu4N + cations, which fill the space between anions forming a cationic sublattice with channel cavities (Figure 4b).

Crystal Structures
The crystallographically observable lattice H2O molecules are located in proximity with cluster anions forming hydrogen bonds (2.96-3.07 Å) with N atoms of CN − ligands.  Table 1). The average Mo-Mo and Mo-I bond distances (2.680(5) and 2.773(8) Å, respectively) agree well with the corresponding values observed for clusters with {Mo 6 I 8 } 4+ core [40][41][42]. Each Mo atom of the cluster core is coordinated by apical CN − ligand. The Mo-C distances display the average length of 2.188(8) Å, which is in the range of values that were reported for hexanuclear cyanide clusters of Mo [32,43,44]. The crystal packing includes the cluster anions forming a column along the a axis and Bu 4 N + cations, which fill the space between anions forming a cationic sublattice with channel cavities (Figure 4b). The crystallographically observable lattice H 2 O molecules are located in proximity with cluster anions forming hydrogen bonds (2.96-3.07 Å) with N atoms of CN − ligands.
Compound (Bu 4 N) 2 [{Mo 6 I 8 }(CN) 6 ]·2DMSO (3) crystallizes in the triclinic crystal system, P1 space group. Asymmetric unit contains half of the cluster anions (three Mo atoms, four I atoms and atoms of three CN − groups) as well as one Bu 4 N + cation and one DMSO molecule ( Figure S1). All positions are fully occupied. The structure of the cluster anion is similar to the one found in structure 2. The crystal packing ( Figure S2) is formed by the numerous interactions of hydrogen bond acceptors (O atoms of DMSO molecules and N atoms of CN − groups) with -CH 2 -and CH 3 -groups of Bu 4 N + cations and DMSO molecules. The corresponding -CH···O and -CH···N distances are within the range of 3.26-3.60 Å. In addition, weak hydrogen bonds were observed between -CH 3 and -CH 2 -groups of Bu 4 N + cation and µ 3 -I ligands of cluster anions. The C-I distances are 3.88 and 3.77 Å for -CH 3 and -CH 2 -groups, respectively.     (Table 2). Energy level diagrams in the near frontier region (Figure 6) show some differences between electronic structures of heteroligand clusters and [{Mo 6 I 8 }(CN) 6 ] 2− anions. Frontier orbitals of the latter include almost degenerated HOMO and HOMO-1, which are localized primarily on the {Mo 6 I 8 } 4+ core with a small contribution (~3%) of atomic orbitals of C and N atoms. In combination with cluster core-centered LUMO, electronic structure of this anion is typical for face-capped clusters of the [{M 6 X 8 }L 6 ] type [48][49][50][51]

Materials and Methods
Mo 6 I 12 was synthesized by a high-temperature treatment of stoichiometric amounts of Mo and I 2 at 700 • C in a sealed silica tube for 4 days [55] [56,57]. Other reagents and solvents employed were commercially available and used as received without further purification.
Synthesis of compound 1 was carried out using a vibratory ball mill of the following construction: steel balls with a diameter of 3 mm (total mass 200 g) were charged into a cylindrical steel reactor (100 cm 3 volume and 50 mm height) fitted with a flange cover. The frequency of the vertical reciprocating motion of the reactor was 100 Hz, the amplitude was 10 mm. Elemental (CHN) analysis was performed on a Euro EA3000 CHNS-O Analyzer (EuroVector, Pavia, Lombardy, Italy). Energy-dispersive X-ray spectroscopy (EDS) was performed on TM3000 TableTop Scanning Electron Microscope (Hitachi, Ltd., Marunouchi, Chiyoda-ku, Tokyo, Japan) with QUANTAX 70 EDS for SEM equipment (Bruker Corporation, Billerica, MA, USA). Infrared spectra were recorded with a Vertex 80 FT-IR spectrometer (Bruker Corporation, Billerica, MA, USA) in a KBr pellet. UV-Vis spectra were recorded in H 2 O with a Cary 60 spectrophotometer (Agilent Technologies, Inc., Santa Clara, California, USA) at room temperature in the range 200-600 nm. X-ray powder diffraction data were collected on a PW1820/1710 diffractometer (Philips, Amsterdam, Netherlands) (Cu Kα radiation, graphite monochromator, silicon plate used as an external standard). The thermogravimetric properties were studied using a TG 209 F1 Iris Thermo Microbalance (NETZSCH-Gerätebau GmbH, Selb, Germany) in the temperature range of 25-800 • C at a rate of 10 • ·min −1 in a He flow (30 mL·min −1 ). The 13 C-NMR spectra were recorded from a DMSO-d 6 solution at room temperature on Avance III 500 FT-spectrometer (Bruker Corporation, Billerica, MA, USA) with working frequency 125.73 MHz. The 13 C-NMR chemical shifts are reported in ppm of the δ scale and referred to signals of the solvents (39.50 ppm). Excitation and emission photoluminescence spectra were recorded with a spectrofluorometer Fluorolog 3 spectrofluorometer (Horiba, Ltd., Kyoto, Japan) equipped with ozone-free 450 W Xe lamp, cooled R928/1860 photon detector (Hamamatsu Photonics K.K., Hamamatsu City, Shizuoka, Japan) with refrigerated chamber PC177CE-010 (Products for Research, Inc., Danvers, MA, USA) and double grating monochromators. Excitation and emission spectra were corrected for source intensity and detector spectral response by standard correction curves. The same instrument was used for determination emission lifetimes. The relative emission quantum yields (Φ em ) for the deaerated acetonitrile solutions were estimated by using [Ru(bpy) 3 ]Cl 2 as a standard: Φ em = 0.04 in aerated water [58].

Solvothermal Ligand Exchange Reaction
A mixture of (Bu 4 N) 2 [{Mo 6 I 8 }I 6 ] (150 mg, 0.05 mmol) and NaCN (150 mg, 3.06 mmol) were mixed with 3 mL of H 2 O in a glass tube. The tube was sealed, heated to 120 • C, held at this temperature for 24 h and then cooled to room temperature at a natural rate (about 50 • C/h). The reaction led to a dark brown insoluble powder and a pale yellow solution. The solution was filtered, evaporated to a volume of about 1 mL, cooled to the room temperature and kept in air. One day later, a few bright red needle crystals of compound 2 precipitated from the solution.

Preparation of (Bu 4 N) 2 [{Mo 6 I 8 }(CN) 4 (MeO) 2 ] (4)
A mixture of (Bu 4 N) 2 [{Mo 6 I 8 }I 6 ] (150 mg, 0.05 mmol) and NaCN (100 mg, 2.04 mmol) were mixed with 5 mL of MeOH in a glass tube. The tube was sealed, heated to 100 • C, held at this temperature for 12 h and then cooled to room temperature at a natural rate (about 50 • C/h). The bright red solution was filtered from excess of NaCN and 5 mL of H 2 O were added. The solution was evaporated to about 3 mL, cooled to the room temperature and kept in air. Two days later, dark red block crystals of compound 4 were precipitated.

Single Crystal Diffraction Studies
Single crystals of compounds 2-5 were picked up directly from the reaction mixtures. Diffraction data for 2-4 were obtained on an Xcalibur diffractometer (Agilent Technologies, Inc., Santa Clara, California, USA) equipped with an area CCD AtlasS2 detector (Mo Kα, λ = 0.71073 Å, graphite monochromator, ω-scans). Integration, absorption correction, and determination of unit cell parameters were performed using the CrysAlisPro program package [59]. Single crystal XRD data for compound 5 were collected with a D8 Venture diffractometer (Bruker Corporation, Billerica, MA, USA) equipped with an area CMOS PHOTON III detector and IµS 3.0 source (Mo Kα, λ = 0.71073 Å, ϕand ω-scan). Absorption corrections were applied with the use of the SADABS program [60]. All measurements were conducted at 150 K. The structures were solved by a dual space algorithm (SHELXT) [61] and refined by the full-matrix least squares technique (SHELXL) [62] in the anisotropic approximation (except hydrogen atoms). Positions of hydrogen atoms of organic ligands were calculated geometrically and refined in the riding model. Hydrogen atoms of the water molecules were not located. The crystallographic data and details of the structure refinement are summarized in Table 4. Selected bond distances are listed in Table 1. CCDC 2042364-2042367 contain the crystallographic data for 2-5, respectively. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures.  [63,64]. Optimization of geometric parameters for the cluster anions in C 1 symmetry was carried out using VWN + S12g dispersion corrected density functional [65][66][67] and the all-electron TZP basis set [68]. The calculated vibrational spectra contained no imaginary frequencies. Single point calculations of bonding energies and molecular orbitals with geometries from the VWN + S12g/TZP level of theory were carried out with the dispersion-corrected hybrid density functional S12h [67] and the all-electron TZP basis set. The zero-order regular approximation (ZORA) was used in all calculations to take into account the scalar relativistic effects [69]. The calculations were performed using the CH 3 CN environment effects, which were added with a Conductor-like Screening Model (COSMO) [70]. The selected calculated interatomic distances are presented in Table 2.

Conclusions
In summary, the new octahedral anionic complexes based on {Mo 6 I 8 } 4+ and {W 6 I 8 } 4+ cluster cores with apically homoleptic or heteroleptic coordination environment including ambidentate CN − ligands were obtained. The [{Mo 6 I 8 }(CN) 6 ] 2− cluster anion was synthesized by implementation of mechanochemical activation to the polymeric MoI 2 , while two heteroleptic anionic complexes trans-[{Mo 6 I 8 }(CN) 4 (MeO) 2 ] 2− and trans-[{W 6 I 8 }(CN) 2 (MeO) 4 ] 2− were synthesized using solvothermal method. Compounds based on the new anions were isolated as crystalline salts in preparative amounts. It was found that the compounds could be recrystallized from aqueous solutions and H 2 O/MeOH mixtures, showing excellent hydrolytic stability. Investigation of spectroscopic properties of the new compounds revealed the bright red luminescence with typical lifetimes of the order of 100 microseconds. The relative quantum yield for complex [{Mo 6 I 8 }(CN) 4 (MeO) 2 ] 2− reached 0.14 in a deoxygenated acetonitrile solution. Note that this is the first report of the luminescence properties of cyanide octahedral clusters of molybdenum and tungsten. Owing to the combination of including ambidentate inert cyanide and labile methylate ligands, the heteroleptic anions can be used as functional building blocks for coordination polymers, as a basis for the directed synthesis of more complex heteroligand clusters, and as promising objects for phototoxicity studies.