Facile Synthesis of an Octagermacubane with Two Three‐Coordinate Germanium(0) Atoms and its Unique Radical Anion

Abstract

Thermolysis of a 1 : 1 mixture of tris(di‐tert‐butylmethylsilyl)germane 9 and bis(di‐tert‐butylmethylsilyl)germane 17 at 100 °C produces unexpectedly octagermacubane 18, having two 3‐coordinate Ge0 atoms (40 % yield). 18 was characterized by X‐ray crystallography and it is a singlet biradical (according to DFT quantum mechanical calculations and the absence of an EPR signal). Reactions of 18 with CH2Cl2 and H2O yield the novel dichloro‐octagermacubane 24 and hydroxy‐octagermacubane 25, respectively. Reduction of 18 with tBuMe2SiNa in THF produces an isolable octagermacubane radical anion 26‐Na. Based on X‐ray crystallography, EPR spectroscopy and DFT quantum mechanical calculations, 26‐Na is classified as a Ge‐centered radical anion.

Bonding Analysis of the Ge‐Ge Bonds in the Octagermacubane Ge8(Sit‐butyl2methyl)6

“…The calculated bond lengths and angles in the two electronic states are not very different, but the triplet state is 15.2 kcal (18.2 kcal/mol with inclusion of dispersion interaction) higher in energy than the singlet state. This is in agreement with the experimental findings [12] and with the previously reported calculated values of other octagermacubanes Ge 8 R 6 . [13a] Geometry optimization of 1 with an open-shell singlet state yielded essentially the same structure with negligible energy difference < 0.1 kcal/mol as for the closed-shell singlet form shown in Figure 1.…”

“…The additional electron occupies a bonding orbital between the threecoordinated Ge atoms. [12] The shape of the HOMO of 1 lends some credibility to the Lewis structure suggested by the NBO analysis, which gives lone-pair orbitals at the Ge(0) atoms. However, the HOMO assigns one electron each at the lobes of Ge4 and Ge4' whereas the NBO calculations indicates an occupation number of 1.7 e. The form of the HOMO also suggests that there is a direct coupling between the formally isolated "left-over" single electrons at the Ge(0) atoms.…”

“…The form of the LUMO of 1 showing a bonding character Ge4‐Ge4’ agrees with the shape of the SOMO of the radical anion 1 − which could also be isolated as a sodium salt by Apeloig, Bravo‐Zhivotovskii and coworkers. The additional electron occupies a bonding orbital between the three‐coordinated Ge atoms [12] …”

“…Very recently, Apeloig, Bravo‐Zhivotovskii and coworkers reported about a facile synthesis of an octagermacubane where six four‐coordinated germanium atoms carry bulky silyl groups whereas there are two three‐coordinate Ge(0) atoms at opposing edges of the octahedron [12] . Octagermacubanes Ge 8 R 6 where R is an amido or aryl ligand have previously been synthesized and structurally characterized earlier by Schnepf et al [13] .…”

“…[10,11] Very recently, Apeloig, Bravo-Zhivotovskii and coworkers reported about a facile synthesis of an octagermacubane where six four-coordinated germanium atoms carry bulky silyl groups whereas there are two three-coordinate Ge(0) atoms at opposing edges of the octahedron. [12] Octagermacubanes Ge 8 R 6 where R is an amido or aryl ligand have previously been synthesized and structurally characterized earlier by Schnepf et al [13] The carbon parent system C 8 H 6 was obtained as an electronic singlet species in a low-temperature argon matrix by Michl and co-workers. [14] The novel octagermacubane Ge 8 R 6 (R = Sit-butyl 2 methyl) (1) bears formally a single electron on the Ge(0) atoms, which are coupled with each other, making the compound a singlet biradical.…”