A new binuclear Cr(III) citrate anion: Synthesis, characterization and crystal structure of [Cr(CO(NH 2 ) 2 ) 6 ] 2 [Cr 2 (CO(NH 2 ) 2 ) 2 (C 6 H 4 O 7 ) 2 ] 3 Ritu Bala a, ⁎, Monika Kashyap a , Amrinder Kaur a , Amalija Golobič b, ⁎⁎ a Department of Chemistry, UGC Centre for Advanced Studies-1, Guru Nanak Dev University, Amritsar-143005, Punjab, India b Faculty of Chemistry and Chemical Technology, University of Ljubljana, Ašerčeva 5, 1000, Ljubljana, Slovenia abstract article info Article history: Received 23 October 2012 Accepted 12 December 2012 Available online 21 December 2012 Keywords: Chromium(III) Coordination chemistry One pot synthesis Spectroscopy X-ray crystallography The new anion [Cr 2 (CO(NH 2 ) 2 ) 2 (C 6 H 4 O 7 ) 2 ] 2− has been isolated in the complex [Cr(CO(NH 2 ) 2 ) 6 ] 2 [Cr 2 (CO(NH 2 ) 2 ) 2 (C 6 H 4 O 7 ) 2 ] 3 . The crystals of this complex were obtained by a one-pot synthetic method using 1:1 molar ratio of hexaureachromium(III) chloride and trisodium citrate in aqueous medium. It was characterized by elemental anal- ysis, IR spectroscopy and single crystal X-ray diffraction analysis. The compound crystallizes in space group R – 3 of the trigonal system with three formula units in a cell of dimensions a =b = 24.7461(4) Å, c = 13.7204(3) Å and γ=120°. The crystal structure comprises the columns consisting of [Cr(CO(NH 2 ) 2 ) 6 ] 3+ cations surrounded by a cylinder of complex anions, [Cr 2 (CO(NH 2 ) 2 ) 2 (C 6 H 4 O 7 ) 2 ] 2− which are packed in a honeycomb-like manner. This supramolecular architecture is formed and stabilized by inter- and intramolecular N\H⋯O hydrogen bonds. © 2012 Elsevier B.V. All rights reserved. Chromium(III) complexes has significant role in catalytical [1], electroplating [2] and photochemical reactions [3]. Biologically, chromium(III) is an essential micronutrient required for proper carbohydrate and lipid metabolism in mammals [4] and protein precipitation processes [5]. It is an active ingredient of the glucose tolerance factor (GTF) and its deficiency leads to moderate diabetes mellitus [6]. Supplementation of a GTF-deficient diet with chromium (in the form of hexaureachromium(III) chloride) results in a significant increase of intravenous glucose tolerance [7]. As hexaureachromium(III) chloride can form number of complexes [8–15] by reacting with various metal salts, we envisaged that octahedral hexaureachromium(III) cation should provide two triangular urea faces with twenty four \NH bonds (from twelve \NH 2 groups) that can act as potential hydrogen bond donors. The biologically important anion, citrate was selected because the oxygen atoms of three carboxylate groups act as hydrogen bond acceptors and hydroxyl groups act as hydrogen bond donor. Including all the probabilities in the solid state, these two ions should form an in- tricate network of hydrogen bonds and electrostatic force of attractions stabilizing the entire lattice. Understanding of such network interactions of judicially chosen cations and anions would be rewarding as it can pro- vide means of constructing intricate and novel molecular entities based on second-sphere coordination. Expecting such network we have reacted hexaureachromium(III) chloride with a trisodium citrate in 1:1 molar ratio in aqueous medium with the expectation of hexaureachromium(III) citrate. The grayish green product was obtained after two days which is insoluble in water, DMSO and any other commonly used organic solvents (like (CH 3 ) 2 CO, EtOH, CHCl 3 etc.). The elemental analysis was not in the agreement with the expected product i.e., [Cr(CO(NH 2 ) 2 ) 6 ](C 6 H 5 O 7 ). Infrared spectrum of the complex was recorded in KBr pellets and it shows typical absorption bands for coordinated urea ligands which are comparable with hexaureachromium(III) chloride [16]. In anion, [Cr 2 (CO(NH 2 ) 2 ) 2 (C 6 H 4 O 7 ) 2 ] 2− the frequencies of the observed carbonyl vibrations were shifted to lower values in comparison to the corre- sponding vibrations in free citrate ion and citric acid, indicating changes in the vibrational status of the citrate ligand upon binding to Cr(III) [17,18]. In the title complex, sharp peaks for asymmetric and symmetric COO − stretch were observed at 1624 and 1419 cm −l respectively. The difference between ν as and ν s i.e., Δν =(ν as (COO − ) −ν s (COO − )) is 205 cm −1 , which is greater than 200 cm −1 indicating that the carboxyl- ate groups of the citrate ligand are either free or coordinated to Cr(III) in a monodentate fashion [17,18]. The observation is consistent with X-ray crystal structure of title complex showing that the carboxylate groups of citrate ion are bonded in monodentate fashion. Single crystal X-ray structure determination of the title com- plex revealed the presence of two symmetry independent com- plex cations, [Cr(CO(NH 2 ) 2 ) 6 ] 3+ and dimeric complex anions, [Cr 2 (CO(NH 2 ) 2 ) 2 (C 6 H 4 O 7 ) 2 ] 2− shown in Fig. 1. Coordination bond lengths are given in caption of Fig. 1. The remaining bond lengths and angles are available in supporting material. The primary coordination sphere of Cr1 and Cr2 atoms within cations is composed of six urea mol- ecules and each urea molecule is coordinated terminally through O atom forming approximately octahedral geometry. Cr1 and Cr2 atoms lie on 3-fold rotoinversion axis. The Cr\O bond distances are in range 1.958 (2) and 1.966 (3) Å for Cr1 and Cr2 respectively, which is in Inorganic Chemistry Communications 29 (2013) 56–59 ⁎ Corresponding author. Tel.: +91 183 2258802 09x3197 (O); fax: +91 183 2258820. ⁎⁎ Corresponding author. Tel.: +386 12419 104. E-mail addresses: rituhatta@yahoo.com (R. Bala), Amalija.Golobic@fkkt.uni-lj.si (A. Golobič). 1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.inoche.2012.12.001 Contents lists available at SciVerse ScienceDirect Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche