Synthesis, characterization, and X-ray crystal structure of the octabromodiosmate(III) salt [(C5Me4H)2OsBr]2[Os2Br8] and its conversion to [(C5Me4H)2OsBr][Os2HBr4(cod)2]

Treatment of hexabromoosmic acid, H2OsBr6, with 4 equiv. of tetramethylcyclopentadiene (C5Me4H2) in tert -butanol at reflux for 8 h affords the unusual salt [(C5Me4H)2OsBr]2[Os2Br8], 1, which is the bis(tetramethylcyclopentadienyl)bromoosmocinium(IV) salt of the octabromodiosmate(III) dianion. The brown color of the salt suggests that the anion adopts an eclipsed conformation (D 4h symmetry) and this conclusion has been confirmed by a single-crystal X-ray diffraction experiment. The X-ray crystal structure indicates that the osmium atoms in the View the MathML source[Os2Br82-] anion are disordered over two sites. The bromine atoms show no evidence of disorder and are disposed in a quasi-cubic arrangement; the two Os–Os vectors are almost exactly orthogonal to each other and each vector points toward a different pair of opposite square faces of the Br8 cube. The Os–Os bond distances are 2.219(5) and 2.229(1) Å; the average Os–Br distance in the anion is 2.417(2) Å. Treatment of [(C5Me4H)2OsBr]2[Os2Br8] with excess 1,5-cyclooctadiene in ethanol at gentle reflux for 3 h affords [(C5Me4H)2OsBr][Os2HBr4(cod)2], 2. An X-ray crystallographic study was carried out on a sample in which the cation was a mixture of [(C5Me4H)2OsBr+] and [(C5Me4H)2OsH+]. The results demonstrate that the anion adopts a confacial bioctahedral structure in which the hydride ligand and two bromides bridge between the two osmium centers. The Cdouble bond; length as m-dashC bonds of the cod ligands are trans to the bridging bromide groups. The Os–Os bond distance in the anion is 2.874(1) Å. The average Os–Br distance is 2.596(2) Å for the bridging bromides and 2.565(2) Å for the terminal bromides. Compound 2 is the first example of an anionic diosmium complex containing a bridging hydride. The reaction of 1 with cod also results in the formation of bis(tetramethylcyclopentadienyl)osmocene, (C5Me4H)2Os, 3, which has been isolated and characterized. Treatment of (C5Me4H)2Os with 1.0 equiv. of HBF4 · Et2O affords the osmocinium salt [(C5Me4H)2OsH][BF4].