Tetraberyllium-eta(4)-oxo-hexa(arylcarboxylates)

Tetraberyllium-oxo-hexabenzoate and -hexa(mesitylearboxylate) were prepared from benzoic/mesitylcarboxylic acid and freshly precipitated beryllium hyd roxide in a tetrahydrofuran/water two-phase system. The crystal structure o f the benzoate was determined from Be4O(OCOPh)(6)(C6H6)(3), and that of the mesitylearboxylate from the phase Be4O(OCOMes)(6)(CCl4). The two clusters have the mu (4)-oxo cage structure also detected for the acetate and nitrat e complexes, and for the related hexacarbonato hexaanion. The high symmetry of the cluster allows the observation of Be-9 and O-17 signals in the solu tion NMR spectra. The terminal atoms of the rigid carboxylate groups may be expected to lie a t 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 oxygen atom at the origin. This disposition suggests a connectivity suitable for cubic lattices if p-difunctional arene-dicarboxy late anions are employed instead of aryl-monocarboxylate anions. However, t he new results show that significant deviations of the OBe2O2C-Ar six-membe red rings from planarity pause major deviations from octahedral symmetry an d therefore no connectivity in cubic symmetry is to be expected. Fragmentation of the Be4O(OCOR)(6) clusters upon ionization in a mass spect rometer leads predominantly to cations [Be3O(OCOR)(3)](+) which can be assi gned a planar, highly symmetrical tricyclic core structure of D-3h symmetry . Quantum chemical calculations confirm discrete energy minima for these ca tions and show that their stability is largely due to the favourable arrang ement of alternating charges of neighbouring core atoms. The high polarity 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)(2)COOH give the cor responding cluster compounds Be4O(OCORSH)(6), [R = C6H4, (CH2)(2)] with six terminal mercapto functions.