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

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 GeF3 and octahedral GeF6 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·GeF4 in vacuo provides a convenient source of GeF2 in ca. 30% overall yield.

rarer, and limited to Ge and Sn, are a second group of mixedvalence materials, including Sn 3 F 8 , Ge 5 F 12 , and Ge 7 F 16 , which are without direct E-E bonds, but are fluoride-bridged and contain distinct environments attributable to M II and M IV centres. [9][10][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 [GeX 3 ] − (X = Cl, Br or I) and [GeCl 6 ] 2− in CH 2 Cl 2 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 GeF 2 .
Depending upon the experimental conditions, repeatedly passing GeF 4 at low pressure over heated germanium yields either GeF 2 , 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 Ge 5 F 12 ‡ and Ge 7 F 16 , 10,11 which are members of the series (GeF 2 ) n ·GeF 4 . 14 These flow reactions are inconvenient and low yielding, hence we have investigated the reduction of GeF 4 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 GeF 4 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 Ge 3 F 8 , the missing third member of the series (GeF 2 ) n ·GeF 4 , 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 Ge 5 F 12 and Ge 7 F 16 , as well as traces of GeF 2 were also present. The simulated and experimental powder XRD data from this mixture are shown in the ESI. † † Electronic supplementary information (ESI) available: Experimental details for the syntheses of Ge 3 F 8 and GeF 2 , and the PXRD data for all the products. Sublimation of the mixture (390 K/0.5 mm) gave ∼30% yield of GeF 2 (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 GeF 3 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 GeF 2 in useful quantity for further studies of its coordination and organometallic chemistry.
The single crystals of the mixed-valence Ge 3 F 8 are isomorphous with Sn 3 F 8 , 9 adopting the monoclinic space group P2 1 /n. The structure is composed (Fig. 1) of slightly distorted GeF 6 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 GeF 2 . Overall, the packing is best considered as sheets in the (101) planes (Fig. 2a), with each sheet being made up of puckered chains of GeF 3 units along [010] connected together by the GeF 6 octahedra (Fig. 2b).
Considering the structures of Ge 5 F 12 10 and Ge 7 F 16 , 11 the same basic building blocks are present (trigonal pyramidal GeF 3 and octahedral GeF 6 ), but as the F/Ge ratio declines, the   structures become more distorted to maintain the germanium coordination numbers. In Ge 5 F 12 , if we ignore the distant fourth fluoride at 2.44 Å, the GeF 3 trigonal pyramids (Ge-F = 1.80(2), 1.99(2), 2.20(2) Å) form corrugated sheets in (001), based on GeF 6 octahedra linked to dimers of two corner-linked GeF 3 pyramids. As a consequence of the 4 : 1 Ge II : Ge IV constitution, the GeF 6 units are linked to four dimers (rather than two as in Ge 3 F 8 ) (Fig. 3a). The structure of Ge 7 F 16 is complicated in that there are seven distinct germanium sites, 11 but again, the building blocks are trigonal pyramidal GeF 3 and octahedral GeF 6 units. The structure is best described as chains of GeF 3 pyramids along [001] with side chains of four GeF 3 units terminated by a GeF 6 octahedron attached to every second GeF 3 of the main chain (Fig. 3b).
In conclusion, the missing member of the unique series of mixed-valence germanium fluorides (GeF 2 ) n ·GeF 4 (n = 2, 4, or 6) has been obtained by reaction of GeF 4 and Ge powder under modest pressure and temperature, its structure determined and the structural relationships within the series established. Sublimation of the (GeF 2 ) n ·GeF 4 in vacuo provides a convenient route to the previously rather inaccessible GeF 2 . Further work to explore the chemistry of GeF 2 formed by this route is underway and will be reported in due course.