Crystal structure of hexakis(urea-κO)chromium(III) dichromate bromide monohydrate from synchrotron X-ray data

The title compound, [Cr(urea)6](Cr2O7)Br·H2O, is isotypic with the corresponding chloride salt and consists of discrete [Cr(urea)6]3+ cations, staggered Cr2O7 4− and Br− anions and water solvent molecules that are held together by an intricate three-dimensional network of hydrogen-bonding interactions.

The title bromide salt, [Cr{CO(NH 2 ) 2 } 6 ](Cr 2 O 7 )BrÁH 2 O, is isotypic to the corresponding chloride salt. Within the complex cation, the Cr III atom is coordinated by six O atoms of six urea ligands, displaying a slightly distorted octahedral coordination environment. The Cr-O bond lengths involving the urea ligands are in the range 1.9534 (13)-1.9776 (12) Å . The Cr 2 O 7 2À anion has a nearly staggered conformation, with a bridging angle of 130.26 (10) . The individual components are arranged in rows extending parallel to [100]. The Br À anion links the complex cation, as well as the solvent water molecule, through N-HÁ Á ÁBr and O-HÁ Á ÁBr hydrogen-bonding interactions. The supramolecular architecture also includes N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen bonding between urea N-H and water O-H donor groups and the O atoms of the Cr 2 O 7 2À anion as acceptor atoms, leading to a three-dimensional network structure.

Chemical context
Counter-ionic species in coordination compounds play important roles in chemistry, pharmacy, molecular assembly, biology and catalysis, as well as contributing significantly to environmental pollution; however, their binding characteristics have not received much recognition (Martínez-Má ñ ez & Sancenó n, 2003;Fabbrizzi & Poggi, 2013). The study of the anion or cation effect in octahedral metal complexes may be expected to yield a great variety of new structures and properties of both chemical and biological significance. Chromium is usually found in trivalent and hexavalent oxidation states in soil, ground water and seawater (Cespon-Romero et al., 1996). Cr III is an essential element in mammals for maintaining efficient glucose, lipid and protein metabolism. On the other hand, Cr VI is toxic and recognized as a carcinogen to humans and wildlife. The dichromate ion is environmentally important due to its high toxicity (Yusof & Malek, 2009) and its use in many industrial processes (Goyal et al., 2003). Recently, the ionic reactions between hexaureachromium(III) and inorganic oxoanions (such as Cr 2 O 7 2À or CrO 4 2À ) in aqueous solution have been investigated. It was found that [Cr(urea) 6 ] 3+ is suitable to target these oxoanions (Bala et al., 2013). Previously, the crystal structure of [Cr(urea) 6 ](Cr 2 O 7 )ClÁH 2 O has been reported (Bondar et al., 1984). This complex crystallizes in the monoclinic space group P2/n with four formula units in a cell of dimensions a = 13.782 (2), b = 10.393 (1), c = 17.794 (3) Å and = 94. 86 (2) . Within our broader study of Cr III complexes as industrial materials (Choi & Lee, 2009;Choi & Moon, 2014;Moon & Choi, 2015), we report herein the preparation and crystal structure of [Cr(urea) 6 ](Cr 2 O 7 )-BrÁH 2 O, (I). ISSN 2056-9890

Structural commentary
In order to check if compound (I) is isotypic to [Cr(urea) 6 ](Cr 2 O 7 )ClÁH 2 O investigated previously (Bondar et al., 1984), a single-crystal X-ray structure determination was performed on the basis of synchrotron data. Compound (I) consists of the isolated complex cation [Cr(urea) 6 ] 3+ , together with Cr 2 O 7 2À and Br À counter-ions and a solvent water molecule. Comparison of the space-group type, metrics and the arrangement of the molecular components reveal (I) to be isotypic to the corresponding chloride salt. An ellipsoid plot of the molecular components of compound (I) is depicted in Fig. 1.
It is of interest to compare the conformation of Cr 2 O 7 2À with that found in other ionic crystals. In the structure of compound (I) it is in a nearly staggered conformation, whereas in K 2 Cr 2 O 7 , the tetrahedral CrO 4 groups are in an almost eclipsed conformation (Brandon & Brown, 1968  The molecular structures of the components in compound (I), showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level.

Supramolecular features
The individual molecular or ionic components of (I) are arranged in rows extending parallel to [100]. The packing in the crystal structure of (I) involves not only hydrogen bonds of the type N-HÁ Á ÁO between urea amino donor groups and the O acceptor atoms of carbonyl groups, the water molecule, or the Cr 2 O 7 2À anion, but also N-HÁ Á ÁBr hydrogen bonding between the urea amino groups and the Br À anion (Table 1). O-HÁ Á ÁBr interactions involving the water molecule are also observed. All these interactions are responsible for the formation of an intricate three-dimensional hydrogen-bonded network in (I) (Fig. 2).

Synthesis and crystallization
All chemicals were reagent-grade materials and used without further purification. Chromium(III) tribromide hexahydrate was obtained from Aldrich Chemical Co. and used as supplied.