[(η2-(Si/Ge)4)Zn(η2-(Si/Ge)4)]6– – mixed silicon and germanium tetrahedra bridged by a zinc atom

Thomas Fässler and colleagues at Technische Universität München have recently reported the synthesis and structural characterisation of a novel Zn-bridged dimer of tetrahedral Zintl clusters.

 
The reaction of the ternary Zintl phase K₁₂Si₁₂Ge₅ with Zn(C₆H₅)₂ in liquid ammonia and in the presence of the sequestering agent 2,2,2-crypt yielded crystals of composition K₆ZnSi_4.7Ge_3.3(NH₃)₁₁, which were found to contain [(η²-E₄)Zn(η²-E₄)]^6– ions (E = Si/Ge). These cluster anions consist of two [E₄]^4– tetrahedra bridged by a Zn²⁺ ion.

[(η²-E₄)Zn(η²-E₄)]^6–.

The two Zn-coordinated edges are twisted by approximately 90 ° relative to each other. The Si atoms were found to preferentially occupy the positions coordinated to Zn; the Si occupancies of these positions were found to vary between 68 and 82%, while those of the non-coordinated positions were in the range 36-53%.

Zn-bridged dimers of tetrahedral Zintl clusters have been previously observed in Cs₆Ge₈Zn, which was synthesised in the solid state by Slavi Sevov and co-workers, however [(η²-E₄)Zn(η²-E₄)]^6– is the first metal-bridged dimer to be obtained from solution. Fässler and colleagues hope that this research could potentially open up variable routes to mixed semiconducting nanomaterials.

Synthesis and reactivity of A14ZnGe16 (A = K, Rb)

Thomas Fässler's group have recently published a paper in JACS, in which they describe the synthesis of the A₁₄ZnGe₁₆ (A = K, Rb) intermetallic phases. These compounds were synthesised in the solid state by heating stoichiometric mixtures of the elements for several days. Both phases were found to contain fourteen A⁺ cations, one [(Ge₄)Zn(Ge₄)]^6– cluster anion, and two tetrahedral Ge₄^4– ions. 

The [(Ge₄)Zn(Ge₄)]^6– species consists of two tetrahedral Ge₄^4– clusters bridged by Zn²⁺. One Ge₄ cluster is bonded to Zn via a triangular face (η³-coordination), while the other is bonded via an edge (η²-coordination). Sevov has previously synthesised a Cs₆ZnGe₈ phase, which also features a [(Ge₄)Zn(Ge₄)]^6– cluster anion, however in this case both Ge₄ clusters are bonded to Zn in an η³-fashion.

[(Ge₄)Zn(Ge₄)]^6–.

Next, Fässler tried reacting the K₁₄ZnGe₁₆ phase with Cu(mes) (mes = 2,4,6-trimethylphenyl) in liquid ammonia and in the presence of 18-crown-6. This resulted in the formation of [((mes)Cu)₂Ge₄]^4–, which consists of a Ge₄^4– cluster coordinated to two Cu(mes) fragments via two of its triangular phases. The analogous [((mes)Cu)₂Si₄]^4– species was also synthesised by Fässler, by reacting K₆Rb₆Si₁₇ with Cu(mes).

[((mes)Cu)₂Ge₄]^4–.

These results are impressive because, although the tetrahedral E₄^4– (E = Si, Ge, Sn, Pb) cluster anions have been known about for some time, their solution chemistry has been very little studied. This is possibly because there are very few readily available, soluble precursors. The synthesis of a Ge₄^4–-containing phase that is soluble in liquid ammonia could well result in the formation of many more novel cluster anions.