Research Article: Hexa­aqua­nickel(II) bis­[tri­aqua-μ3-oxalato-di-μ-oxalato-bariumchromate(III)] tetra­hydrate

Date Published: August 01, 2020

Publisher: International Union of Crystallography

Author(s): Yves Alain Mbiangué, Manelsa Lande Ndinga, Jean Pierre Nduga, Emmanuel Wenger, Claude Lecomte.


The structure of the title compound is made up of corrugated anionic layers of formula [BaCr(C2O4)3(H2O)3]nn– that leave voids accommodating the charge-compensating cations, [Ni(H2O)6]2+ (point group symmetry ), as well as the water mol­ecules of crystallization.

Partial Text

Over the past three decades, tris­(oxalato)metalate(III) complex anions, [M(C2O4)3]3–, have been extensively used for the design of many compounds with fascinating physical properties (Zhong et al., 1990 ▸; Bénard et al., 2001 ▸; Coronado et al., 2008 ▸; Pardo et al., 2011 ▸; Martin et al., 2017 ▸; Tsobnang et al., 2019 ▸; Ōkawa et al., 2020 ▸). One of the main reasons for that is the ability of these anions to act like ligands towards a variety of metallic cations and to build a diversity of extended structures in which neighboring metallic ions are linked through bridging oxalate ligands. From the synthetic point of view, the tris­(oxalato)chromate(III) anion, [Cr(C2O4)3]3– or [Cr(ox)3]3–, is most attractive because of its stability and inertness toward ligand substitution. As a source of this anion, the polymeric complex salt {Ba6(H2O)17[Cr(C2O4)3]4}·7H2O (Bélombé et al., 2003 ▸) offers the possibility of easily replacing, in the reaction medium and under daylight, the Ba2+ ions by other cations, provided the latter are brought into that medium as their sulfates. Since Ba2+ has a flexible coordination sphere with coordination numbers ranging from three to twelve (Hancock et al., 2004 ▸), this inspired us to start a research program aimed at exploring the various structures that might arise from different combinations of [Cr(ox)3]3–, Ba2+ and other cations, and possibly studying the physical properties of the corresponding compounds. From an aqueous suspension of {Ba6(H2O)17[Cr(C2O4)3]4}·7H2O, a partial replacement of Ba2+ by Ni2+ led to [Ni(H2O)6][BaCr(C2O4)3(H2O)3]2·4H2O (I), the structure of which is described herein.

The asymmetric unit of (I) is depicted in Fig. 1 ▸. It contains one half of an [Ni(H2O)6]2+ cation situated on an inversion center, one [BaCr(C2O4)3(H2O)3]− anion and two water mol­ecules of crystallization, one of which being equally disordered over two positions (O20A and O20B). The Ba2+ ion is linked to ten O atoms from three water mol­ecules and four oxalate ligands (three chelating, one monodentately binding), with Ba—O bond lengths in the range 2.784 (3)–2.933 (3) Å (Table 1 ▸). These values are typical for ten-coordinate barium complexes with oxalate and water ligands (Alabada et al., 2015 ▸). One of the oxalate ligands (bearing O18) bridges three cations (two Ba and one Cr) while the two others are bis-chelating (one Ba and one Cr). In the crystal, neighboring [Cr(C2O4)3]3– units are linked through barium ions into a ladder-like chain running parallel to [010] (Fig. 2 ▸). Adjacent ladders are then connected, through Ba—O18 coordination bonds, into a corrugated layer extending parallel to (101) (Fig. 3 ▸). The packing of the layers delineates voids that accommodate the cationic complex, [Ni(H2O)6]2+, as well as the water mol­ecules of crystallization (Fig. 4 ▸).

In the crystal, extensive O—H⋯O hydrogen-bonding inter­actions of medium-to-weak strength are observed (Table 2 ▸), with all the water mol­ecules acting as hydrogen-bond donors. The water mol­ecules of crystallization also act as hydrogen-bond acceptors, as well as all of the oxalate O atoms except O12, O14 and O18. Two barium-coordinating water mol­ecules (O1 and O3) behave as hydrogen-bond donors toward both components of the disordered lattice water mol­ecule (O20A and O20B) via three-center bonds, O1—H1B⋯(O20A,O20B) and O3—H3B⋯(O20A,O20B). The cationic complex, [Ni(H2O)6]2+, functions as a hydrogen-bond donor group towards one barium-coordinating water mol­ecule (O3), one water mol­ecule of crystallization (O19) and four oxalate O atoms, viz. O9vi, O13vi, O11iv and O17iv [symmetry codes refer to Table 2 ▸]. Together, these inter­actions lead to a three-dimensional supra­molecular network structure.

A search of the Cambridge Structural Database (CSD version 5.41, May 2020; Groom et al., 2016 ▸) for [M(C2O4)3]n− complexes with each oxalate ligand bis-chelating M and another metal M′ gave 316 hits. Of these hits, 86 contain M = Cr and only one, the parent complex of (I), contains M = Cr and M′ = Ba.

The parent complex of (I), {Ba6(H2O)17[Cr(C2O4)3]4}·7H2O, was prepared as previously described (Bélombé et al., 2003 ▸). The title compound was synthesized as follows: NiSO4·6H2O (0.21 g, 0.8 mmol) was dissolved in water (20 ml) and the resulting green solution added dropwise, under stirring and at 313 K, to a violet suspension of {Ba6(H2O)17[Cr(C2O4)3]4}·7H2O (0.50 g, 0.2 mmol) in water (25 ml). After one h, the colorless precipitate of BaSO4 was filtered off, and the filtrate was left to evaporate at room temperature. Two days later, crystals suitable for X-ray analysis were harvested.

Crystal data, data collection and structure refinement details are summarized in Table 3 ▸. All hydrogen atoms were located in difference-Fourier maps and refined with O—H and H⋯H distance restraints of 0.88 (1) and 1.37 (2) Å, respectively, and with Uiso(H) = 1.5Ueq(O). One lattice water mol­ecule was refined as being disordered over two positions (O20A and O20B), with the occupancy ratio refined to 0.51 (5):0.49 (5). The distances Ba1—H3A and Ba1—H3B were restrained to be equal using a SADI instruction.