Oxychloridoselenites( IV ) with cubane-derived anions and stepwise chlorine-to-oxygen exchange †

The novel oxychloridoselenites( IV ) [BMIm][Se 3 Cl 13 ] ( 1 ), [BMIm][Se 4 Cl 15 O] ( 2 ), [BMIm] 2 [Se 4 Cl 14 O 2 ] ( 3 ), [BMPyr] 2 [Se 4 Cl 14 O 2 ] ( 4 ), [BMPyr] 2 [Se 6 Cl 18 O 4 ] ( 5 ), [BMIm] 2 [SeCl 4 O] ( 6 ), [BMPyr] 2 [Se 2 Cl 6 O 2 ] ( 7 ), and [BMPyr] 2 [Se 6 Cl 14 O 6 ] ( 8 ) are prepared by ionic-liquid-based synthesis. Accordingly, SeCl 4 , SeO 2 ( 1 – 6 ), and/ or SeOCl 2 ( 7 , 8 ) as the starting materials are reacted in [BMIm]Cl or [BMPyr]Cl as ionic liquid (BMIm: 1-butyl-3-methylimidazolium, BMPyr: 1-butyl-1-methylpyrrolidinium; partially with AlCl 3 in addition). Generally, the composition and structure of title compounds can be derived from the tetrameric, hetero-cubane-type (SeCl 4 ) 4 as the initial building unit. Thus, chlorine is successively exchanged by oxygen from 1 to 8 . Moreover, the four edge-sharing (SeCl 6 ) octahedra in (SeCl 4 ) 4 are increasingly dismantled, ending with a [SeCl 4 O] 2 − anion as a single pseudo-octahedron in 6 . Based on the weakly coordinating ionic liquid, it is possible to selectively obtain the di ﬀ erent species via synthesis near room temperature (20 – 80 °C). The oxychloridoselenite anions [Se 4 Cl 15 O] − , [Se 4 Cl 14 O 2 ] 2 − , [Se 6 Cl 18 O 4 ] 2 − , and [Se 6 Cl 14 O 6 ] 2 − are obtained for the ﬁ rst time. The title compounds are characterized by X-ray structure analysis based on single crystals and powders as well as by infrared spectroscopy and thermal analysis.


Introduction
Low-temperature syntheses near room temperature (≤100 °C) often have the limitation of the solvent influencing or even dominating the chemistry and composition of the reaction products. 1On the one hand, the dissolution of the polar starting materials is usually driven by the formation of coordination complexes of cations and solvent molecules (e.g., H 2 O, THF, ethylene diamine). 1,2As a result, solvent molecules often remain coordinated in the product.On the other hand, solvents can initiate redox reactions or acid-base reactions with the starting materials, limiting the electrochemical window or pH range in which a reaction can be performed (e.g., H 2 O, lq-NH 3 , lq-SO 2 ). 1,2Non-coordinating and chemically inert solvents (e.g., heptane, toluene), however, often do not lead to any reaction at all due to the insolubility of polar starting materials and products.In this regard, ionic liquids have turned out to be a most effective alternative. 3They offer high chemical and thermal stability in combination with good solubility and weakly coordinating properties. 4ue to the good solubility of many compounds at moderate temperatures (≤100 °C) and due to their inertness, ionic liquids also offer the option for fine-tuning reactions and obtaining metastable products with comparable composition and structure as well as only slightly different stability.Moreover, a series of compounds with unusual binding situations and/or spectacular building units was described, for instance, including cluster compounds, polyhalides, or new element modifications. 5However, systematic studies on small changes in synthesis parameters and their influence on the obtained reaction product are limited to date.Examples comprise the formation of polybromides, 6 low-valent halides in the system Te-Bi/Al-Cl, 7 the connectivity of (ZnBr 4 ) tetrahedra in bromido zincates, 8 the structure of hexanuclear niobium clusters, 9 or the composition of chalcogenidometallates. 10 These reports mainly focus on halide compounds in ionic liquids, whereas studies on oxides are rare.This finding can be related to the higher lattice energy and the lower solubility of oxides in ionic liquids.Several studies, however, have already shown options to dissolve oxides in ionic liquids. 11ere, we address the reaction of SeCl 4 , SeO 2 and/or SeOCl 2 in [BMIm]Cl or [BMPyr]Cl as the ionic liquid (BMIm: 1-butyl-3methylimidazolium; BMPyr: 1-butyl-1-methylpyrrolidinium; partially with AlCl 3 ).The reactions were performed near room temperature (+20 to +80 °C) and resulted in the novel (oxy) chloridoselenites(IV) [BMIm][Se 3 Cl 13   8).The composition and structure of the title compounds can be derived from the tetrameric cubane-type (SeCl 4 ) 4 12 by stepwise chlorine-to-oxygen exchange.

Ionic-liquid-based synthesis
The title compounds [BMPyr]Cl as the ionic liquid (Fig. 1).The most important parameters to obtain one or other products comprise the temperature (+20 to +80 °C) and the ratio of the starting materials (Table 1).Here, it needs to be noted that the starting materials, except for SeOCl 2 (T melt : 8.5 °C) but including [BMIm]Cl (T melt : 70 °C) and [BMPyr]Cl (T melt : 65 °C), are solid at room temperature.They only become liquid upon mixing or with moderate heating (Fig. 1).Subsequent to the reaction, the liquid phase usually shows an intense orange to red colour due to slight amounts of elemental selenium.After the removal of the ionic liquid, the title compounds were obtained as colourless, moisture-sensitive crystals.
The formation of the title compounds 1-8 can be ascribed to Lewis acid-base reactions.To this concern, the amphoteric features of chalcogen(IV) halides such as SeCl 4 are wellknown. 13Thus, SeCl 4 can serve as a Lewis base in the presence of strong Lewis acids, which we also used in previously studied reactions of SeCl 4 with GaCl 3 , for instance, resulting in [SeCl 3 ] + [GaCl 4 ] − with non-linear optic effects. 14On the other hand, SeCl 4 can react as Lewis acid if a suitable Lewis base is present.Such behaviour is observed here with the stepwise chlorine-to-oxygen exchange, starting with the tetrameric cubane-like structure of (SeCl 4 ) 4 . 12In these reactions, the ionic liquid supports the dissolution of SeO 2 and serves as a weakly coordinating solvent. 3,4Moreover, the presence of the voluminous cations of the ionic liquid promotes the formation of large oxychloridoselenite anions.
Whereas the compounds 1-6 were prepared by reaction of SeCl 4 and SeO 2 in [BMIm]Cl or [BMPyr]Cl (with the addition of AlCl 3 for 2 and 6) as the ionic liquid, SeOCl 2 was used as additional starting material to obtain compounds 7 and 8 (Table 1).Since the title compounds have comparable composition, structure, and stability, small variations of temperature, stoichiometry, or Lewis acidity are the key to obtaining the one-or-other compound.Here, it needs to be noticed that the SeCl 4 : SeO 2 ratio was varied between 5 : 1 and 1 : 3 with steps of ±1.At high amounts of SeCl 4 or SeO 2 , however, only the respective starting material was observed to re-crystallize.All title compounds are obtained as colourless crystals (Fig. 1), which are highly sensitive to moisture.Thus, strict handling, including synthesis, storage and characterization, with inert conditions was necessary.All title compounds were obtained as phase-pure colourless crystals with a yield of 10-70%.The respective yield relates to the amount of larger crystals and mainly depends on the solubility of the respective starting materials and products in the ionic liquid.Composition, structure, and purity were confirmed by X-ray powder diffraction (XRD) (ESI: Fig. S1 †) as well as by Fourier-transform infrared (FT-IR) spectroscopy and thermogravimetry (TG).

Structural characterization
From a more general perspective, all title compounds can be derived from the tetrameric, heterocubane-type (SeCl 4 ) 4 as the initial structure. 12Starting with (SeCl 4 ) 4 , on the one hand,

Dalton Transactions Paper
chlorine is exchanged by oxygen with increasing oxygen content from compound 1 to compound 8 (Fig. 2).On the other hand, the four edge-sharing (SeCl 6 ) octahedra in (SeCl 4 ) 4 are more and more dismantled, ending with the [SeCl 4 O] 2− anion as a single pseudo-octahedron in 6.The structure of all compounds was determined based on single-crystal structure analysis (ESI: Table S1 †).Space-group symmetry and structure were further validated by X-ray powder diffraction (XRD) with Rietveld analysis (ESI: Fig. S1 †).
[BMIm][Se 3 Cl 13 ] (1) as the first compound crystallizes in the monoclinic, non-centrosymmetric space group P2 1 (ESI: Table S1, Fig. S2 † 15 As expected, Se-Cl distances with bridging chlorine atoms are longer than for terminal chlorine atoms (Tables 2  and 3).Although present in the synthesis of 1, it must be noted that SeO 2 is not involved in the product formation.

Material properties
In addition to single-crystal structure analysis, the title compounds were examined by X-ray powder diffraction (XRD, ESI: Fig. S1 †), Fourier-transform infrared (FT-IR) spectroscopy (Fig. 6), and thermal analysis (DTA/TG, ESI: Fig. S12 and S13 †).Here, we have focused on the new compounds 2-5 and 8.In the case of compounds 1, 6, and 7, the specific combination of    6).Moreover, the absence of O-H or CvO vibrations shows the absence of typical impurities such as moisture and/or carbonates due to insufficient inert conditions.By comparison with the FT-IR spectra of SeOCl 2 and SeCl 4 , the characteristic ν(Se-O) and δ(Se-O) vibrations at 1000-900 cm −1 and 500-400 cm −1 can be identified for the title compounds. 23Specifically, ν(Se-O) occurs as a sharp, intense absorption with a wavenumber increasing with the number of oxygen atoms (Table 4).Moreover, the intensity of the ν(Se-O) vibration increases with the number of oxygen atoms in the respective oxychloridoselenite anion.In contrast to the ν(Se-O) and δ(Se-O) vibrations, the ν(Se-Cl) vibrations only sum up to a broad, non-specific absorption.
Thermal analysis with thermogravimetry (TG) shows a step decomposition with only slightly different for the compounds 2-5 and 8 (ESI: Fig. S12 †).The first decomposition step at 100-300 °C can be related to the decomposition and sublimation of SeCl 4 and SeOCl 2 (Table 5).The second decomposition step relates to a fragmentation of [BMIm]Cl and [BMPyr]Cl.Since TG was performed in a nitrogen atmosphere, the organic cations cannot be oxidized, so that amorphous carbon remains as a solid residue.Indeed, the crucibles were covered with an amorphous deep black film after TG analysis.Finally, differential thermal analysis (DTA) indicates the melting point of 3, 4, 5 at 70-80 °C, whereas 2 and 8 obviously decompose prior to melting (ESI: Fig. S13 †).Taken together, X-ray diffraction based on single crystals and powders as well as FT-IR and TG confirm the composition and purity of the title compounds.All compounds are of similar stability and prepared in a narrow temperature range.Temperature and ratio of starting materials are the main parameters to realize the one or other title compound.The ionic-liquid-based synthesis offers the option to perform reactions near room temperature in a weakly coordinating solvent so that the reaction and products are not dominated by the coordination and/or redox chemistry of the solvent.Such synthesis conditions, of course, offer the option to realize many additional metastable and new compounds.

Fig. 2
Fig. 2 Chemical and structural relation of the oxychloridoselenites(IV) in the title compounds 1-8 via chlorine-to-oxygen exchange and stepwise dismantling of the tetrameric cubane-type (SeCl 4 ) 4 .

Table 5
Evaluation of the TG data by comparing the experimentally observed weight loss with calculated data