Tetraberyllium-oxo-hexabenzoate and -hexa(mesitylcarboxylate) were prepared from benzoic / mesitylcarboxylic acid and freshly precipitated beryllium hydroxide in a tetrahydrofuran / water two-phase system. The crystal structure of the benzoate was deter mined from Be4O(OCOPh)6(C6H6)3 , and that of the mesitylcarboxylate from the phase Be4O(OCOMes)6(CCl4). The two clusters have the μ4-oxo cage structure also detected for the acetate and nitrate complexes, and for the related hexacarbonato hexaanion. The high symmetry of the cluster allows the observation of 9Be and l7O signals in the solution NMR spectra.
The terminal atoms of the rigid carboxylate groups may be expected to lie at the vertices of large octahedra enclosing the cluster molecules. The axes of the six aryl groups would thus coincide with the axes of the cartesian coordinates with the μ4 oxygen atom at the origin. This disposition suggests a connectivity suitable for cubic lattices if p-difunctional arene-dicarb-oxylate anions are employed instead of aryl-monocarboxylate anions. However, the new results show that significant deviations of the OBe2O2C-Ar six-membered rings from planarity cause major deviations from octahedral symmetry and therefore no connectivity in cubic symmetry is to be expected.
Fragmentation of the Be4O(OCOR)6 clusters upon ionization in a mass spectrometer leads predominantly to cations [Be3O(OCOR)3]+ which can be assigned a planar, highly symmetrical tricyclic core structure of D3h symmetry. Quantum chemical calculations confirm dis crete energy minima for these cations and show that their stability is largely due to the favourable arrangement of alternating charges of neighbouring core atoms. The high polar ity of Be-O bonds rules out any significant contributions from aromaticity. The reactions of Be(OH)2 with 4 -HS-C6H4 -COOH and HS(CH2)2COOH give the corresponding cluster com pounds Be4O(OCORSH)6, [R = C6H4 , (CH2)2] with six terminal mercapto functions.
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