Low-temperature measurement of the unexpected hydrate Cs2[B12H12]·2H2O

The closo-dodecaborate anion [B12H12] 2 -is one of the most stable structures among all hydroboron clusters and well known since the 1960's.The crystal structure of the cesium salt has been determined in 2000 be isotypic with K2[B12H12] [1] adopting the cubic space group Fm3 ̅ (a = 1128.12(7) pm, Z = 4) [2].The single crystal was obtained by fast crystallization of a supersaturated aqueous solution of Cs2[B12H12].The collected crystal from diluted solution was not stable in air at room temperature, so due to that the single crystal X-ray diffraction of what turned out to be Cs2[B12H12] • 2 H2O was performed at 100 K.This unexpected hydrate crystallizes in the orthorhombic space group Pbcm with a = 956.31(6)pm, b = 1365.72(8)pm and c = 1027.68(6)pm for Z = 4.The unit-cell volume of Cs2[B12H12] • 2 H2O is almost 7 % smaller than the one of Cs2[B12H12] (1342 Å 3 versus 1436 Å 3 ) despite containing more particles.In contrast to the anhydrate Cs2[B12H12], there are two crystallographically different Cs + cations present.(Cs1) + carries three and (Cs2) + one water molecule at distances of 318 pm and 333 pm, respectively, while ten or twelve hydrogen atoms 306 -335 pm apart complete both coordination numbers to C.N. = 13.This diverse behavior of cesium and water leads to an endless chain along [001] (see Figure 1).Cs2[B12H12] has only one crystallographically unique Cs + cation with twelve coordinating hydrogen atoms at a distance of 313.4 pm (12⨯).The same special crystallographic behavior was observed in a similar network of ammonia and cesium reported in the low-temperature measurement of Cs2[B12H12] • 6 NH3 (Pbca, a = 775.69(8)pm, b = 1408.65(14)pm, c = 1807.48(17)pm, Z = 4) [3] with Cs•••N distances of 313, 326 and 378 pm.There is also a chain formation observed, although only two thirds of the ammonia molecules are involved.With V = 1975 Å 3 the unit-cell is larger than in the dihydrate case, since one third of the NH3 particles represent interstitials.Reason for the contraction of Cs2[B12H12] • 2 H2O are not only the extra water ligands at the Cs + cations but also rather strong classical hydrogen bonds (O δ--H δ+ •••O δ-) among water molecules and non-classical dihydrogen bonds (B δ+ -H δ-•••H δ+ -O δ-) between the negatively polarized hydrogen atoms of the [B12H12] 2-clusters and positively polarized hydrogen atoms of the H2O molecules.