Cyanide Complexes Based on {Mo6i8}4+ and {W6I8}

molecules Article 4+ 4+ Cyanide Complexes Based on {Mo6I8} and {W6I8} Cluster Cores Aleksei S. Pronin 1 , Spartak S. Yarovoy 1, Yakov M. Gayfulin 1,* , Aleksey A. Ryadun 1, Konstantin A. Brylev 1, Denis G. Samsonenko 1 , Ilia V. Eltsov 2 and Yuri V. Mironov 1,* 1 Nikolaev Institute of Inorganic Chemistry SB RAS, 3, Acad. Lavrentiev ave., 630090 Novosibirsk, Russia; [email protected] (A.S.P.); [email protected] (S.S.Y.); [email protected] (A.A.R.); [email protected] (K.A.B.); [email protected] (D.G.S.) 2 Department of Natural Sciences, Novosibirsk State University, 2, Pirogova str., 630090 Novosibirsk, Russia; [email protected] * Correspondence: [email protected] (Y.M.G.); [email protected] (Y.V.M.) Academic Editor: Constantina Papatriantafyllopoulou Received: 10 November 2020; Accepted: 4 December 2020; Published: 8 December 2020 2– Abstract: Compounds based on new cyanide cluster anions [{Mo6I8}(CN)6] , 2– 2 trans-[{Mo6I8}(CN)4(MeO)2] and trans-[{W6I8}(CN)2(MeO)4] − 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 4+ 4+ complexes based on {Mo6I8} and {W6I8} 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. Keywords: cluster compounds; molybdenum; tungsten; cyanide ligand; crystal structure; luminescence; hydrolytic stability 1. Introduction 4+ Chemistry and applications of compounds based on octahedral cluster cores {M6X8} (M = Mo or W; X = Cl, Br or I) have been studied for several decades due to a number of interesting physicochemical properties, including bright luminescence in the red and near infrared regions [1–7] and reversible redox transformations [8,9]. High luminescence quantum yields and lifetimes in combination with tunability of chemical properties by changing ligand environments made it possible to create a number of promising luminescent and photocatalytic materials based on these clusters [10–15]. The phosphorescence of clusters is also associated with the efficient singlet oxygen 4+ production [16,17]. Therefore, compounds and materials based on {M6X8} clusters can potentially be applied in photodynamic therapy [18–22], antibacterial and antiviral applications [23–25]. Besides this, luminescence of clusters was successfully used in bioimaging [18,26,27]. 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 [28–30]. The presence of ambidentate apical cyanide ligands makes it possible to coordinate transition and post-transition metal cations and to obtain Molecules 2020, 25, 5796; doi:10.3390/molecules25245796 www.mdpi.com/journal/molecules Molecules 2020, 25, 5796 2 of 17 crystalline coordination polymers by self-assembly. Using {M6X8}-type cyanoclusters is interesting for obtaining new luminescent coordination polymers. For now, the structure of only one cyanide cluster 4+ 2 based on an {Mo6X8} core has been described in the literature, namely [{Mo6Br8}(CN)6] − [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 4+ 4+ 2 2 on {Mo6I8} and {W6I8} cores, namely [{Mo6I8}(CN)6] −, trans-[{Mo6I8}(CN)4(MeO)2] − and 2 trans-[{W6I8}(CN)2(MeO)4] −. Their crystal structures and spectroscopic properties in the solid state and solutions were investigated together with calculated geometry and electronic structures. To obtain new compounds, two different ways of synthesis were tested, namely, substitution of terminal iodide ligands in solvothermal conditions and mechanochemical depolymerization of Mo6I12. 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. 2. Results and Discussion 2.1. Synthesis Until now, only one structure of a cyanide cluster based on halide octahedral molybdenum core 2 has been reported, namely [Mo6Br8(CN)6] − [31]. It was shown that this complex formed polymeric compounds with transition metal cations [32]. For octahedral tungsten halides, cyanide compounds 4+ 4+ have not been described. At the same time, complexes based on {Mo6I8} and {W6I8} 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 4+ 4+ based on {Mo6I8} and {W6I8} cores (Scheme1). The first approach is depolymerization of M 6I12 compounds by mechanochemical activation. The reaction between polymeric Mo6I12 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 Na [{Mo I }(CN) ] 2H O(1) based on elemental analysis and EDS. The reaction of an aqueous 1.3 0.7 6 8 6 · 2 solution of this compound with an aqueous solution of Bu4NI led to the precipitation of the compound (Bu N) [{Mo I }(CN) ] 0.7H O(2), which is soluble in polar organic solvents and can be easily 4 2 6 8 6 · 2 recrystallized from CH3CN/H2O or MeOH/H2O mixtures. Recrystallization from dimethyl sulfoxide (DMSO) led to removal of the solvate H2O molecules and precipitation of compound 3. The yield of compound 2 is about 50% with respect to the initially taken Mo6I12, which makes it possible to obtain it in preparative amounts. It was also shown that this compound can be obtained with a very low yield in a solvothermal reaction between the salt (Bu4N)2[{Mo6I8}I6] and NaCN in an aqueous solution. The reaction gave an insoluble brown precipitate and a cloudy yellow solution, from which, after filtration and evaporation, several crystals of compound 2 were isolated. An attempt to obtain 2 a tungsten analog of the [{Mo6I8}(CN)6] − anion by mechanochemical activation of W6I12 in the presence of NaCN was unsuccessful: the resulting brown-black product was unstable to hydrolysis. It is interesting to note that soluble trinuclear clusters of both molybdenum and tungsten, namely (Et4N)2[W3S7Br6], (Et4N)2[W3Se7Br6], and [Mo3Se7(dtc)3]dtc (dtc = diethyldithiocarbamate), were synthesized earlier using mechanochemical activation of polymeric solids W3S7Br4,W3Se7Br4 and Mo3Se7Br4, respectively [33]. Molecules 2020, 25, 5796 3 of 17 Molecules 2020, 25, x 3 of 19 Scheme 1. The synthetic ways of obtaining the compounds 1–5. Scheme 1. The synthetic ways of obtaining the compounds 1–5. The second approach is the substitution of the terminal iodide ligands in the discrete cluster The second approach2 is the substitution2 of the terminal iodide ligands in the discrete cluster anions [{Mo6I8}I6] − and [{W6I8}I6] −. These reactions were carried out under solvothermal conditions anions [{Mo6I8}I6]2− and [{W6I8}I6]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 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 an excess of NaCN, mixed with an equal volume of H2O and evaporated. Addition of H2O was found found to be necessary in order to prevent the formation of oil after concentration of solutions in pure 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 methanol. The obtained red crystalline solids were isolated and investigated. The use of methanol in in solvothermal synthesis made it possible to avoid the problem of hydrolysis of clusters at elevated 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 temperatures. However, in this case, the methylate anion competes2 with cyanide, so that compounds2 based on apically heteroleptic anions trans-[Mo6I8(CN)4(MeO)2] − and trans-[W6I8(CN)2(MeO)4] − based on apically heteroleptic anions trans-[Mo6I8(CN)4(MeO)2]2− and trans-[W6I8(CN)2(MeO)4]2− were were obtained in high yields. The reaction products demonstrated phase purity (Figure1), and the obtained in high yields. The13 reaction products demonstrated phase purity (Figure 1), and the study study of their solutions by C-NMR in d6-DMSO did not reveal the presence of geometric isomers or of their solutions by 13C-NMR in d6-DMSO did not reveal the presence of geometric isomers or clusters with a different ratio of CN− and MeO− anions (Figure2). Note that hexamolybdenum cluster clusters with a different ratio of CN− and MeO− anions (Figure 2).

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