Syntheses and crystal structures of three novel oxalate coordination compounds: Rb2Co(C2O4)2·4H2O, Rb2CoCl2(C2O4) and K2Li2Cu(C2O4)3·2H2O

Rb2Co(C2O4)2·4H2O consists of isolated [Co(C2O4)2·2H2O] octahedra which are connected only by hydrogen bonding of the water molecules. Rb2CoCl2(C2O4) consists of chains of Co2+ cations connected via oxalate bridging ligands. In K2Li2Cu(C2O4)3·2H2O, the metal cations are interconnected by oxalate ligands forming a novel three-periodic network.


Chemical context
Oxalate-based transition-metal complexes have long attracted interest because of their promising magnetic and electrochemical properties. Their magnetic properties are in part due to the oxalato ligand, which is known to facilitate magnetic exchange between transition-metal cations, and the compounds are known to exhibit both ferro-and antiferromagnetic interactions (Miller & Drillon, 2002;Baran, 2014). In addition to their magnetic properties, there have also been numerous studies concerning their electrochemical properties, which have shown promising results (Pramanik et al., 2022;Cai et al., 2020;Yao et al., 2019). Part of the appeal of oxalate-based coordination compounds is due to their high degree of structural diversity, as a result of the oxalate ligand, which can adopt 17 different coordination modes and act as a mono-, bi-, tri-or tetradentate ligand (Rao et al., 2004). This has led to a vast compositional area, which is yet to be fully explored. In this context, the crystal structures of three new oxalate-based coordination compounds are reported and discussed herein.

Structural commentary
Rb 2 Co(C 2 O 4 ) 2 Á4H 2 O (I) consists of isolated [Co(C 2 O 4 ) 2 -(H 2 O) 2 ] octahedra. The Co 2+ cation lies on the 2c Wyckoff position with a site symmetry of 1, leading to a trans disposition of the bidentate oxalato and aqua ligands (Fig. 1). The average Co-O bond length was determined as 2.080 Å , with a calculated bond-valence sum of 2.10 valence units. The Rb + cation has a coordination number of 11, defined by oxalate O atoms and water molecules. While the water molecule involving O1 coordinates to both Rb + and Co 2+ , the second water molecule involving O2 solely bonds to the alkali metal cation. The [Co(C 2 O 4 ) 2 (H 2 O) 2 ] octahedra are interlinked by hydrogen bonding of both types of water molecules, as shown in Fig. 2. The mutually trans coordinating water molecules (H3, O1, H4) form hydrogen bonds with the oxalate ligands of the neighbouring [Co(C 2 O 4 ) 2 (H 2 O) 2 ] octahedra, whilst the second type of water molecule (H1, O2, H2) forms hydrogen bonds (in part bifurcated) with the oxalate ligands of two separate [Co(C 2 O 4 ) 2 (H 2 O) 2 ] octahedra. Numerical data for the hydrogen-bonding interactions are given in Table 1.

Figure 4
The crystal structure of Rb 2 CoCl 2 (C 2 O 4 ) (II) in a view approximately along the a axis.
(4i, mm2), Co 2+ (2d, mmm), Cl À (4j, mm2), O (8n,. .m) and C (4h, m2m). The presence of the oxalate-bridged Co 2+ chain could allow for magnetic exchange (García-Couceiro et al., 2004), hence the magnetic properties of the compound should also be investigated in the future. The Cu 2+ and Li + binding environments of K 2 Li 2 Cu-(C 2 O 4 ) 3 Á2H 2 O (III) are shown in Fig. 5. The d 9 Cu 2+ cations display classic Jahn-Teller distortion with elongation of the axial Cu-O bonds. The equatorial Cu-O bond lengths are 1.938 (3) (O2) and 1.942 (3) (O1) Å whilst the axial bonds are significantly longer at 2.473 (4) Å (O6). The Cu 2+ ion lies on a special position with Wyckoff position and site symmetry of 6b and 3, respectively. The Cu 2+ coordination environment consists of four oxalate ligands, two of which act as bidentate bridging ligands and two of which are axially oriented and bind to four metal cations with a tricoordinate oxygen atom. The Li + cation is tetrahedrally coordinated by three oxalate molecules, one of which is bidentate whilst the other two are monodentate. The Cu 2+ and Li + -centred polyhedra are interconnected into a tri-periodic network, as shown in Fig. 6. The coordination environment of the K + cation lies within this network and consists of eight oxygen atoms from the oxalate ligands and two water molecules. These water molecules exhibit disorder of the O7 atom, which is split into two positions. The interatomic distances between the water molecules is $3.7 Å , which is too far apart to facilitate hydrogen bonding.

Figure 6
The crystal structure of K 2 Li 2 Cu(C 2 O 4 ) 3 Á2H 2 O (III) viewed along the a axis.

Synthesis and crystallization
The samples were synthesized via hydrothermal syntheses in the temperature range 433-463 K over four days, from commercially available starting reagents. Compounds (I) and (II) were synthesized as by-products from the reaction of rubidium carbonate, sodium carbonate, cobalt chloride hexahydrate and oxalic acid dihydrate in molar ratios of 2:2:1:1.5 and 1:1.5:1:1.5 at 433 and 463 K, respectively. Compound (III) was synthesized by the reaction of potassium carbonate, lithium carbonate, copper chloride dihydrate and oxalic acid dihydrate (1:3:1:3) at 463 K. Single crystals were isolated from a mixture of products for further analysis. The resulting crystals were filtered and dried overnight at 323 K prior to analysis by X-ray diffraction.

Refinement
Crystal data and refinement details of the three compounds are summarized in Table 2. The H atoms in (I) and (III) were allowed to refine freely. The disordered oxygen atom in compound III (O7) was split over two positions with their occupancies fixed at 0.5 while their atomic coordinates and U ij s were refined independently.  For all structures, data collection: CrystalClear (Rigaku, 2015); cell refinement: CrystalClear (Rigaku, 2015); data reduction: CrystalClear (Rigaku, 2015); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL (Sheldrick, 2015b); molecular graphics: ORTEP for Windows (Farrugia, 2012); software used to prepare material for publication: WinGX (Farrugia, 2012).

catena-Poly[dirubidium [[dichloridocobalt(II)]-µ-oxalato]] (II)
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 ) where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 1.03 e Å −3 Δρ min = −1.01 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.