The preparation and structure of Ge₃F₈ – a new mixed-valence fluoride of germanium, a convenient source of GeF₂

Abstract

The new binary mixed-valence fluoride of germanium, Ge₃F₈, has been obtained by heating GeF₄ with powdered Ge in an autoclave (390 K/4 bar/48 h). The structure contains pyramidal Ge^IIF₃ and octahedral Ge^IVF₆ units, linked by fluoride bridges. The new compound is the missing member of the series (GeF₂)_n·GeF₄ (n = 2, 4, or 6). Sublimation of (GeF₂)_n·GeF₄in vacuo provides a convenient source of GeF₂ in ca. 30% overall yield.

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Although germanium is technologically very important both as the element and in oxide or chalcogenide compounds, with key applications in electronics, ceramics and optics,¹ its chemistry was neglected for many years compared with those of silicon and tin. It is now a very active area of main group chemistry and, in addition to the extensive chemistry of Ge(IV),² recent work has identified a large and complex coordination chemistry of Ge(II);^2,3 the latter contrasting with the limited coordination chemistry of Si(II).² In Group 14 as well as the common tetrahalides MX₄ (M = Si, Ge, Sn; X = F–I),⁴ there are dihalides MX₂ (M = Ge, Sn), and the subhalides, GeBr and SnBr.⁵ Of these, the chemistry of GeF₂ has been very little explored since it is not readily available commercially and its preparation by repeatedly passing GeF₄ over heated germanium, is both inconvenient and time consuming,⁶ while the alternative method, involving the reaction of Ge with anhydrous HF in an autoclave, is hazardous. Both routes also require special equipment.⁷ A number of intermediate halides have also been identified.² The latter are of two types; the most common are those with element–element (E–E) bonds, including Si₂F₆, Si₂Cl₆, Si₃Cl₈, Si₆Cl₁₄, Ge₂Cl₆ and Ge₅Cl₁₂,⁸ with structures analogous to the corresponding alkanes. Much rarer, and limited to Ge and Sn, are a second group of mixed-valence materials, including Sn₃F₈, Ge₅F₁₂, and Ge₇F₁₆, which are without direct E–E bonds, but are fluoride-bridged and contain distinct environments attributable to M^II and M^IV centres.^9–11

We are currently developing new routes for electrodeposition of p-block materials from non-aqueous media, using reagents including halometallate anions as the p-block element source,¹² and have recently reported the electrochemistry of [GeX₃]⁻ (X = Cl, Br or I) and [GeCl₆]²⁻ in CH₂Cl₂ solution.¹³ During the course of this work we have extended our studies to the fluoride systems. We report here the preparation and characterisation of a new binary, mixed-valence fluoride of germanium and its use to provide a convenient route to GeF₂.

Depending upon the experimental conditions, repeatedly passing GeF₄ at low pressure over heated germanium yields either GeF₂,⁶ or mixed valence Ge^II–Ge^IV fluorides.^10,11,14 Two of the latter identified by single crystal X-ray diffraction (XRD) studies are Ge₅F₁₂‡ and Ge₇F₁₆,^10,11 which are members of the series (GeF₂)_n·GeF₄.¹⁴ These flow reactions are inconvenient and low yielding, hence we have investigated the reduction of GeF₄ with Ge powder in an autoclave under modest pressure (390 K/4 bar/48 h, see ESI†). Initial attempts at temperatures <370 K resulted in little reaction, but on increasing the temperature to 390 K/48 h, much of the GeF₄ was consumed (as indicated by the drop in pressure), and upon opening the autoclave in a glove-box, a mass of white microcrystalline material was found on the cooler lid. The crystals are extremely moisture sensitive, converting into a pool of liquid immediately on exposure to air. Single crystal X-ray diffraction data were collected from one of the small crystals and the structure solution identified this product as Ge₃F₈, the missing third member of the series (GeF₂)_n·GeF₄, with n = 2. Unit cell measurements on several other crystals confirmed these as the same compound. Powder X-ray diffraction (PXRD) data were also collected on the bulk material and that showed smaller amounts of Ge₅F₁₂ and Ge₇F₁₆, as well as traces of GeF₂ were also present. The simulated and experimental powder XRD data from this mixture are shown in the ESI.†

Sublimation of the mixture (390 K/0.5 mm) gave ∼30% yield of GeF₂ (based on elemental Ge used in the first step), which was identified by PXRD (see ESI†). Some involatile orange material (cf.ref. 6) was also formed.

Germanium difluoride has a polymeric chain structure based upon trigonal pyramidal GeF₃ units (Ge–F = 1.79(2), 1.91(2), 2.09(2) Å), with a distant fourth fluoride at 2.57(2) Å that cross-links the chains.¹⁵ The new preparation is a convenient way to obtain GeF₂ in useful quantity for further studies of its coordination and organometallic chemistry.

The single crystals of the mixed-valence Ge₃F₈ are isomorphous with Sn₃F₈,⁹ adopting the monoclinic space group P2₁/n. The structure is composed (Fig. 1) of slightly distorted GeF₆ octahedra with four terminal Ge–F bonds (1.767(1), 1.782(1) Å), and two slightly longer Ge–F bonds (1.855(1) Å) that are involved in bridging to the Ge^II units. The germanium(II) core environment is trigonal pyramidal, composed of one terminal (Ge–F = 1.938(1) Å) and two bridging (Ge–F = 1.980(1), 2.010(1) Å) fluorides, one linked to Ge^IV and one to a second Ge^II centre. There are also longer Ge^II⋯F contacts (2.56 Å), and if these are included, the germanium(II) geometry is a distorted saw-horse shape, reminiscent of GeF₂. Overall, the packing is best considered as sheets in the (101) planes (Fig. 2a), with each sheet being made up of puckered chains of GeF₃ units along [010] connected together by the GeF₆ octahedra (Fig. 2b).

Fig. 1 The Ge^II and Ge^IV units in Ge₃F₈ with ellipsoids drawn at the 50% level. Selected bond lengths (Å) and angles (°): Ge2–F1 = 1.855(1), Ge2–F2 = 1.767(1), Ge2–F3 = 1.782(1), Ge1–F1 = 2.010(1), Ge1–F4ⁱⁱ = 1.938(1), Ge1–F4 = 1.980(1), F1–Ge1–F4 = 83.28(5), F4–Ge–F4ⁱⁱ = 86.35(3), F4ⁱⁱ–Ge1–F1 = 85.42(6). Symmetry codes: (i) 1 − x, 1 − y, 1 − z; (ii) 0.5 − x, −0.5 + y, 1.5 − z; (iii) x, −1 + y, z.

Fig. 2 The Ge₃F₈ structure viewed along: (a) the b axis to observe the sheets, and (b) the a axis, showing the connectivity within the sheets.

Considering the structures of Ge₅F₁₂ ¹⁰ and Ge₇F₁₆,¹¹ the same basic building blocks are present (trigonal pyramidal GeF₃ and octahedral GeF₆), but as the F/Ge ratio declines, the structures become more distorted to maintain the germanium coordination numbers. In Ge₅F₁₂, if we ignore the distant fourth fluoride at 2.44 Å, the GeF₃ trigonal pyramids (Ge–F = 1.80(2), 1.99(2), 2.20(2) Å) form corrugated sheets in (001), based on GeF₆ octahedra linked to dimers of two corner-linked GeF₃ pyramids. As a consequence of the 4 : 1 Ge^II : Ge^IV constitution, the GeF₆ units are linked to four dimers (rather than two as in Ge₃F₈) (Fig. 3a). The structure of Ge₇F₁₆ is complicated in that there are seven distinct germanium sites,¹¹ but again, the building blocks are trigonal pyramidal GeF₃ and octahedral GeF₆ units. The structure is best described as chains of GeF₃ pyramids along [001] with side chains of four GeF₃ units terminated by a GeF₆ octahedron attached to every second GeF₃ of the main chain (Fig. 3b).

Fig. 3 (a) View of the structure of Ge₅F₁₂.¹⁰ (b) View of the structure of Ge₇F₁₆.¹¹

In conclusion, the missing member of the unique series of mixed-valence germanium fluorides (GeF₂)_n·GeF₄ (n = 2, 4, or 6) has been obtained by reaction of GeF₄ and Ge powder under modest pressure and temperature, its structure determined and the structural relationships within the series established. Sublimation of the (GeF₂)_n·GeF₄in vacuo provides a convenient route to the previously rather inaccessible GeF₂. Further work to explore the chemistry of GeF₂ formed by this route is underway and will be reported in due course.

Acknowledgements

We thank EPSRC for support (EP/1033394/1 and EP/1010890/1). The SCFED Project (http://www.scfed.net) is a multidisciplinary collaboration of British universities investigating the fundamental and applied aspects of supercritical fluids.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Experimental details for the syntheses of Ge₃F₈ and GeF₂, and the PXRD data for all the products. Further details of the crystal structure investigation may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: (+49)7247-808-666; e-mail: crysdata@fiz-karlsruhe.de, http://www.fiz-karlsruhe.de/request_for_deposited_data.html) on quoting CSD number 427896. See DOI: 10.1039/c4dt02265c

‡ Ge₅F₁₂ was originally formulated as Ge₂F₅,¹⁴ the correct formula subsequently being established from the crystal structure determination.¹⁰

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