Synthesis and Characterization of Digerma-closo-dodecaborate: A Higher Homologue of Icosahedral ortho-Carborane
✍ Scribed by Claudia Nickl; Dominik Joosten; Klaus Eichele; Cäcilia Maichle-Mössmer; Karl W. Törnroos; Lars Wesemann
- Publisher
- John Wiley and Sons
- Year
- 2009
- Tongue
- English
- Weight
- 351 KB
- Volume
- 48
- Category
- Article
- ISSN
- 0044-8249
No coin nor oath required. For personal study only.
✦ Synopsis
Dedicated to Professor Ekkehard Lindner on the occasion of his 75th birthday When published in 1990, 1,2-dimethyl-1,2-disila-closo-dodecaborane [1] was the first higher homologue of the famous icosahedral ortho-carbaborane in terms of Group 14 diheteroboranes and borates. Ortho-carbaborane had been synthesized 30 years earlier in 1963 from decaborane and acetylene. [2] Because the triple-bond compounds of silicon, germanium, and tin are not available for chemical synthesis, simple adaptation of this method was not possible. [3] The silicon vertices were incorporated into the decaborane skeleton in one step using bis(dimethylamino)methylsilane, (Me 2 N) 2 SiHMe. In 2006, the dianionic distanna-closododecaborate was prepared in a two-step synthesis. [4] In a simple one-pot procedure with decaborane, tin(II) chloride, proton sponge, and triethylamine, the dimeric closo-cluster 2,2'-bis(1,2-distanna-closo-dodecaborate) was isolated as an intermediate. By varying the addition of the reaction partners, the ions [7-Cl-7-SnB 10 H 12 ] À and [7,7'-(SnB 10 H 12 ) 2 ] 2À could be isolated as well. Cleavage of the intercluster Sn À Sn bond by K[HBEt 3 ] in THF gave the desired dianionic compound 1,2distanna-closo-dodecaborate as its potassium salt. After cation exchange with any common ammonium countercation, the cluster was reprecipitated in aqueous solution. We now introduce the missing germanium analogue with an unprecedented product formation that depends on the kind of base used in the first step.
By applying the same synthetic route as used in the synthesis of the 1,2-distanna-closo-dodecaborate, the corresponding dimeric closo-compound of germanium could not be isolated quantitatively. A product mixture was obtained instead. By layering an acetone solution with hexane, crystals of 7,7'-bis(7-germa-nido-undecaborate) (1) were obtained in a yield smaller than 5 % (Figure 1). [5] Moreover, the ions [B 10 H 13 ] À , [7-Cl-7-GeB 10 H 12 ] À (the iodogermaborate is already known from a simple salt elimination reaction of deprotonated decaborane and GeI 2 in THF [8] ), and the expected product [(Ge 2 B 10 H 10 ) 2 ] 2À could be identified by 11 B{ 1 H} NMR spectroscopy. Dropwise addition of triethylamine to a mixture of germanium(II) bromide and decaborane in THF at room temperature gave 2 within three hours. After removal of [Et 3 NH]Br by filtration, the THF solution of the cluster could be used without further workup. The change of base from proton sponge to triethylamine in THF led to the unprecedented formation of the desired dimeric compound [(Ge 2 B 10 H 10 ) 2 ] 2À in good yields (Scheme 1).
This reaction and the formation of the nido-product proceed with electron transfer. Germanium withdraws electrons to form the GeÀGe bonds, and in theory elemental bromine is formed, because no further boron-containing species are observed. The same problem could not be solved in the case of the tin analogue.