Crystal structure of catena-poly[diammonium [di-μ-oxalato-cuprate(II)]]

The polymeric title compound was formed unexpectedly from the reaction of bis(diisopropylphosphanyl)amine with copper oxalate hemihydrate. It shows extensive N—H⋯O hydrogen bonding, as well as long Cu⋯O interactions.


Chemical context
Metal oxalate salts are ubiquitous in nature (Baran, 2014) and are also of great interest to synthetic chemists and materials scientists because they often display unusual magnetic and conductive properties (Nenwa et al., 2015;Robinson et al., 2015;Zhang et al., 2012;Clemente-Leó n et al., 2011;Gruselle et al., 2006). Other areas of study for metal oxalates include, but are not limited to, metallogels (Feldner et al., 2016), coordination polymers and networks (Guo et al., 2016;Mizzi & LaDuca, 2016;Yeşilel et al., 2010), and precursors for nanomaterials and metallic inks (Yadav et al., 2013;Cheng et al., 2016). The properties of metal oxalates are often tuned by using a combination of different cations. These may be simply metal cations, but often they are more complex, such as quaternary nitrogen cations. Surprisingly, the structure of the simplest of the (NR 4 ) 2 [Cu(C 2 O 4 ) 2 ] family, (NH 4 ) 2 [Cu(C 2 O 4 ) 2 ], has not previously been reported. ISSN 2056-9890

Structural commentary
The title compound crystallizes in the monoclinic space group P2 1 /c with the copper atom on an inversion center (Fig. 1). As is true for all but one copper oxalate complex (Gu & Xue, 2007) found in the CSD (Version 5.37, May 2016 update; Groom et al., 2016), the copper atom is chelated by oxygen atoms from the adjacent carbon atoms to form a five membered ring, rather than oxygen atoms from the same carbon. The coordination environment of copper is nearly perfectly square planar, with the O1-Cu1-O2 bond angle measuring 85.44 (3) within the asymmetric unit, and 94.56 (3) across the inversion center. Within the plane of the oxalate ligand, O1 and O2 form bonds to Cu1 measuring 1.9326 (7) and 1.9301 (7) Å , respectively. O3 interacts weakly, at a distance of 2.7057 (8) Å , with the symmetry-related Cu atoms above and below the ligand plane, giving an elongated octahedron. O4 has no bonding interactions with Cu, but does engage in hydrogen bonding with the ammonium cation (see below). The different ways that the oxygen atoms do or do not interact with copper is reflected in the C-O bonds. The two oxygen atoms that are strongly bound to copper, O1 and O2, have slightly longer bonds to carbon of 1.2798 (11) and 1.2895 (12) Å for C1-O1 and C2-O2, respectively. The weakly interacting O3 and non-bonded O4 have shorter C-O bonds of 1.2355 (12) and 1.2249 (12) Å for C1-O3 and C2-O4, respectively.

Supramolecular features
As noted above, the coordination sphere of the copper atoms is completed by a long interaction of 2.7057 (8) Å between O3 and Cu1 in the planes above and below the ligand, giving rise to polymeric copper oxalate chains along the a axis (Fig. 2). These chains do not interact directly with one another. Instead, they are linked into a three-dimensional network by partly bifurcated N-HÁ Á ÁO hydrogen bonds (Fig. 3) between all four protons of the ammonium cation and the oxalate oxygen atoms indicated by the symmetry operations in Table 1.

Database survey
There are three published reports of hydrated ammonium copper oxalate but to the best of our knowledge, the anhydrous title compound has not been reported previously. The earliest report, for (NH 4 ) 4n [Cu 2 (C 2 O 4 ) 4 (H 2 O) 2 ] n Á2nH 2 O (Viswamitra, 1962) was reinterpreted (Novosad et al., 2000) as a polymeric complex with the repeat unit consisting of two [Cu(C 2 O 4 ) 2 ] moieties. One copper atom forms long Cu-O bonds to the next unit, similar to the way in which the title compound forms its chains, while the other is capped by two water molecules. A different hydrate, (NH 4 has also been reported (Kadir et al., 2006), but it does not feature chains of polymeric copper oxalate. Instead, it exists as a discrete water-capped tetramer. In each case, these hydrates also display hydrogen bonding between the oxalate, ammonium and water molecules. A single chain of the copper oxalate complex with the ammonium cation omitted.

Figure 1
The molecular structure of the title compound, with non-H atoms shown as displacement ellipsoids at the 50% probability level.

Synthesis and crystallization
A solution of bis(diisopropylphosphanyl)amine (0.25 g, 1.0 mmol) in 1 mL MeOH was added to a slurry of copper(II) oxalate hemihydrate (0.15 g, 1.0 mmol) in 1 mL MeOH. The mixture was heated to reflux for 5 min and then allowed to cool to room temperature. After three days, the blue supernatant solution was decanted from an insoluble powder and cooled to 248 K. Block-like blue crystals of the title compound were isolated after six weeks. The mechanism by which bis-(diisopropylphosphanyl)amine decomposes into ammonium is under investigation.

catena-Poly[diammonium [di-µ-oxalato-cuprate(II)]]
Crystal data 2(H 4 N + )·C 4 CuO 8  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.