Comment

The title compound, (I), was prepared as part of our ongoing studies of second-sphere hydrogen-bonding interactions in compounds containing cationic metal complexes and organic counter-anions (Sharma, Bala et al., 2006; Sharma, Sharma et al., 2006).

The geometrical parameters for the component species in (I) fall within their expected ranges (Allen et al., 1987). The well known [Cu(C₂N₂H₈)₂(H₂O)₂]²⁺ complex cation in (I) is built up from a central copper(II) ion (site symmetry ) chelated by two ethylenediamine molecules to form an approximate CuN₄ square. The Jahn-Teller distorted copper coordination is completed by two trans water molecules (Table 1). The Cu-N and Cu-O bond lengths in (I) are very similar to the equivalent values observed for the same complex cation in its bis(naphthalene-2-sulfonate) (Sharma et al., 2005) and bis(4-fluorobenzoate) (Liu et al., 2004) salts.

The 4-nitrobenzoate anion in (I) is almost planar, the dihedral angles between the mean plane of the C3-C8 benzene ring and the planes of its attached C9/O2/O3 carboxylate and N3/O4/O5 nitro groups being 2.14 (17) and 1.9 (2)°, respectively. The carboxylate C-O bond lengths are almost equal, suggesting charge delocalization.

As well as electrostatic forces, the component species in (I) interact by way of O-HO and N-HO hydrogen bonds (Table 2). Firstly, adjacent complex cations are linked into chains propagating along [100] by way of translation-related pairs of N1-H1O1ⁱ bonds (see Table 2 for symmetry code). A bridging carboxylate atom O3 also helps to consolidate the chains (Fig. 2). Then, adjacent cations and anions form a distinctive bridged chain propagating along [010] (Fig. 3), where each carboxylate group in the chain accepts no fewer than four hydrogen bonds from its two adjoining cations. Combining these hydrogen-bonding motifs results in (001) sheets of tightly bound cations and anions. It is notable that the nitro O atoms do not serve as acceptors for any of the hydrogen bonds.

Figure 1 View of the molecular structure of (I), showing 50% probability displacement ellipsoids (arbitrary spheres for the H atoms). [Symmetry code: (i) 1 - x, 1 - y, 1 - z.]

Figure 2 Detail of (I), showing part of a [100] chain arising from hydrogen-bonding interactions (dashed lines). C-bound H atoms have been omitted. [Symmetry codes: (i) 1 - x, 1 - y, 1 - z; (ii) 2 - x, 1 - y, 1 - z; (iii) 1 + x, y, z; (iv) 1 + x, 1 + y, z.]

Figure 3 Detail of (I), showing part of a [010] chain arising from hydrogen-bonding interactions (dashed lines). C-bound H atoms have been omitted. [Symmetry codes: (i) 1 - x, 1 - y, 1 - z; (ii) x, 1 + y, z; (iii) 1 - x, 2 - y, 1 - z; (iv) x, 1 + y, z.]

Experimental

Compound (I) was prepared by taking a suspension of [Cu(H₂O)₆](C₇H₄NO₄)₂ [obtained by reacting basic copper(II) carbonate with p-nitrobenzoic acid in water] and adding a methanol solution of ethylenediamine dropwise until a slight excess of a 1:2 Cu-en stoichiometry was achieved, resulting in a deep-blue solution, which was allowed to evaporate at room temperature to obtain purple crystals of (I) after a few days. Crystals were filtered off and dried in air.

Crystal data

[Cu(C2H8N2)2(H2O)2](C7H4NO4)2 Mr = 552.00 Triclinic, a = 6.0019 (5) Å b = 7.1230 (4) Å c = 15.370 (2) Å = 95.48 (2)° = 98.43 (2)° = 114.26 (2)° V = 583.69 (15) Å3 Z = 1 Dx = 1.570 Mg m-3 Mo K radiation = 1.00 mm-1 T = 93 (2) K Cube, purple 0.10 × 0.10 × 0.10 mm

Data collection

Rigaku Mercury CCD diffractometer and scans Absorption correction: multi-scan (ABSCOR; Higashi, 1995) Tmin = 0.856, Tmax = 1.000 (expected range = 0.774-0.905) 3754 measured reflections 2005 independent reflections 1930 reflections with I > 2(I) Rint = 0.015 max = 25.3°

Refinement

Refinement on F2 R[F2 > 2(F2)] = 0.035 wR(F2) = 0.077 S = 1.08 2005 reflections 160 parameters H-atom parameters constrained w = 1/[2(Fo2) + (0.0337P)2 + 0.5044P] where P = (Fo2 + 2Fc2)/3 (/)max = 0.002 max = 0.43 e Å-3 min = -0.44 e Å-3

Table 1 Selected bond lengths (Å) Cu1-N1 2.0146 (18) Cu1-N2 2.0272 (19) Cu1-O1 2.5369 (17) C9-O2 1.256 (3) C9-O3 1.261 (3)

Table 2 Hydrogen-bond geometry (Å, °) D-HA D-H HA DA D-HA N1-H1O1i 0.92 2.11 3.018 (2) 170 N1-H2O3 0.92 2.20 3.081 (2) 161 N2-H3O3ii 0.92 2.24 3.037 (3) 145 N2-H4O2iii 0.92 2.07 2.960 (2) 162 O1-H5O3iv 0.89 1.88 2.754 (2) 166 O1-H6O2 0.86 1.93 2.767 (2) 164 Symmetry codes: (i) x-1, y, z; (ii) x+1, y+1, z; (iii) x, y+1, z; (iv) -x+1, -y, -z+1.

The O-bound H atoms were located in a difference map and refined as riding in their as-found relative positions with U_iso(H) = 1.2U_eq(O). The C- and N-bound H atoms were geometrically placed (C-H = 0.95-0.99 Å, N-H = 0.92 Å) and refined as riding with U_iso(H) = 1.2U_eq(C,N).

Data collection: CrystalClear (Rigaku, 2004); cell refinement: CrystalClear; data reduction: CrystalClear; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP-3 (Farrugia, 1997); software used to prepare material for publication: SHELXL97.