Single crystals of [Cr(H
2O)
6]
2[B
12H
12]
3 · 15H
2O and [In(H
2O)
6]
2[B
12H
12]
3 · 15H
2O were obtained by reactions of aqueous solutions of the acid (H
3O)
2[B
12H
12] with chromium(III) hydroxide and indium metal shot, respectively. The title compounds crystallize isotypically in the trigonal system with space group
R$\bar{3}$
c (
a = 1157.62(3),
c = 6730.48(9) pm for the chromium,
a = 1171.71(3),
c = 6740.04(9) pm for the indium compound,
Z = 6). The arrangement of the
quasi‐icosahedral [B
12H
12]
2– dianions can be considered as stacking of two times nine layers with the sequence …
ABCCABBCA… and the metal trications arrange in a cubic closest packed …
abc… stacking sequence. The metal trications are octahedrally coordinated by six water molecules of hydration, while another fifteen H
2O molecules fill up the structures as zeolitic crystal water or second‐sphere hydrating species. Between these free and the metal‐bonded water molecules, bridging hydrogen bonds are found. Furthermore, there is also evidence of hydrogen bonding between the anionic [B
12H
12]
2– clusters and the free zeolitic water molecules according to B–H
δ– ···
δ+H–O interactions. Vibrational spectroscopy studies prove the presence of these hydrogen bonds and also show slight distortions of the dodecahydro‐
closo‐dodecaborate anions from their ideal icosahedral symmetry (
Ih). Thermal decomposition studies for the example of [Cr(H
2O)
6]
2[B
12H
12]
3 · 15H
2O gave no hints for just a simple multi‐stepwise dehydration process.
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