Di-μ-acetato-bis[(acetato-κ2 O,O′)bis(isonicotinamide-κN)copper(II)]

The title centrosymmetric bimetallic complex, [Cu2(C2H3O2)4(C6H6N2O)4], is composed of two copper(II) cations, four acetate anions and four isonicotinamide (INA) ligands. The asymmetric unit contains one copper cation to which two acetate units bind asymmetrically; one of the Cu—O distances is rather long [2.740 (2) Å], almost at the limit of coordination. These Cu—O bonds define an equatorial plane to which the Cu—N bonds to the INA ligands are almost perpendicular, the Cu—N vectors subtending angles of 2.4 (1) and 2.3 (1)° to the normal to the plane. The metal coordination geometry can be described as a slightly distorted trigonal bipyramid if the extremely weak Cu—O bond is disregarded, or as a highly distorted square bipyramid if it is not. The double acetate bridge between the copper ions is not coplanar with the CuO4 equatorial planes, the dihedral angle between the (O—C—O)2 and O—Cu—O groups being 34.3 (1)°, resulting in a sofa-like conformation for the 8-member bridging loop. In the crystal, N—H⋯O hydrogen bonds occur, some of which generate a head-to tail-linkage between INA units, giving raise to chains along [101]; the remaining ones make inter-chain contacts, defining a three-dimensional network. There are in addition a number of C—H⋯O bonds involving aromatic H atoms. Probably due to steric hindrance, the aromatic rings are not involved in significant π⋯π interactions.


Related literature
For the importance of Cu(II) carboxylate complexes in biology, see: Lippard & Berg (1994). For coordination properties of anionic carboxylates, see: Deacon & Phillips (1980). For related compounds obtained from the same (or similar) reaction, see: Aakerö y et al. (2003). For a chloroacetate analogue of the title compound, see: Moncol et al. (2007).
Data collection: MSC/AFC Diffractometer Control Software (Molecular Structure Corporation, 1988); cell refinement: MSC/AFC Diffractometer Control Software; data reduction: MSC/AFC Diffractometer Control Software; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL-NT (Sheldrick, 2008); software used to prepare material for publication: SHELXTL-NT and PLATON (Spek, 2009). We acknowledge the Spanish Research Council (CSIC) for providing us with a free-of-charge licence to the CSD system (Allen, 2002) as well as the donation of a Rigaku AFC6S fourcircle diffractometer by Professor Judith Howard. MP is a member of CONICET.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: HB5326).

Comment
Lewis based coordinated Cu(II) carboxylate complexes are an important class of coordination compounds due to their relevance as structural and functional models for biologically important metalloenzymes (Lippard & Berg, 1994). Anionic carboxylates are highly flexible and versatile O-donor ligands since a range of substituents may be introduced on the alkyl chain to modulate its reactivity and coordination propensity and result in a variety of coordination modes such as monodentate, bidentate bridging, chelating, monoatomic bridging and chelating bridging (Deacon & Phillips, 1980). The Lewis base isonicotinamide acts as an effective tool for assembling coordination building units of Cu(II) into infinite 1-D chains.
It has been reported that the reaction of Cu(II) acetate with isonicotinamide in acetonitrile (molar ratio 1:10) and drops of glacial acetic acid afforded the tetrakis (µ-acetato-O,O')-bis(isonicotinamide-N) dicopper (II) acetonitrile, whereas the same rection in methanol affords bis{bis(µ 2 -acetato-O)-aceticacid-O-bis(isonicotinamide-N)copper}bis(methanol) (Aakeröy et al., 2003). The crystal structure of the former contains the classical "paddle-wheel" core and peripheral isonicotinamide ligands with the amides oriented linearly and pointing in opposite directions. In the latter, two monodentate acetates and two isonicotinamides are in a plane in trans-geometry with a third acetate completing a square-pyramidal arrangement, two acetates coordinate to neighbouring coppers in a µ 2 coordination, creating the dinuclear species. We now report a third structure obtained from the reaction of isonicotinamide and Cu(II) acetate (1:1) in methanol, (C 16 H 18 CuN 4 O 6 ) 2 (I). The structure here consists of a dinuclear unit with two bridging µ 2 acetate ligands, two peripheral acetate ligands, and four axial isonicotinamide ligands. The common feature in the three structures with different Cu(II) coordination geometries is the role played by the isonicotinamide units as rigid structures to guide the direction of propagation of the hydrogen-bonded links in the 1-D constructions.
The dimeric title compound (I) ( Fig. 1) is built up around a center of symmetry; the independent unit is composed of one cation to which two acetate units bind, both of them in rather asymmetric way: the one with trailing number 3, bridging both copper cations in a double bridge (Cu1-O13: 1.952 (2) Å; Cu1-O23 i : 2.271 (2) Å, (i): 1 -x,1 -y,1 -z); the remaining one (trailing number 4) binding each cation in a chelating manner, with a Cu-O bond in the normal range (Cu1-O14: 2.020 (2) Å) and a second, extremely long contact almost in the limit of coordination (Cu1-O24: 2.740 (2) Å). These bonds define an equatorial plane to which the Cu-N bonds provided by the INA groups (Cu1-N11: 2.047 (2); Cu1-N12: 2.027 (2) Å) are almost perpendicular, the preceeding Cu-N vectors subtending angles of 2.4 (1) and 2.3 (1)° to the plane normal. The coordination geometry thus described could be defined as as lightly distorted trigonal bipyramid, if the weak Cu1-O24 bond is disregarded, or as a highly distorted square bipyramid, if not. The double acetate bridge is non-coplanar to the cation equatorial planes, the corresponding O-C-O and O-Cu-O planes forming a dihedral angle of 34.3 (1)° and resulting in a sofa-like conformation for the 8-member bridging loop.
The packing organization is governed by N-H···O interactions (  (Fig. 2). This particular disposition leaves H21b and H22b pointing outwards the chains, in a favourable disposition to make interchain contacts to define a strong three-dimensional network. There are in addition a number of C-H···O bonds involving aromatic H atoms.
supplementary materials sup-2 One of them (fifth entry in Table 1), the only one involving the bridging acetate, is rather strong for a non conventional H-bond and provides to interchain cohesion, while the remaining four, involving the chelating acetate O atoms, are intradimeric. Probably due to steric hindrance, the aromatic rings are not involved in significant π···πinteractions.
A chloroacetate isolog of the title compound has been recently described in the literature (Moncol et al., 2007), and in spite of presenting an anisotropic cell expansion/contraction as compared to (I) (Unit cell differences: -1% in a, +5% in b, +1% in c) the general trend both in the dimer metrics as well as in packing interactions is extremely similar.

Experimental
To a solution of Cu(CH 3 CO 2 ) 2 .H 2 O (0.20 g, 0.01 mol) in methanol (40 cm 3 ) at troom temperature was added solid isonicotinamide (INA) (0.14 g, 0.01 mol) in small portions under constant stirring. It was then filtered and the solution allowed to stand for two days, after which small blue blocks of (I) were filtered and dried under vacuum. Yield: 0.28 g (80%

Refinement
All H atoms were located at idealized positions (C-H: 0.93 Å, N-H: 0.85 Å) after being confirmed by inspection in a difference map. They were allowed to ride, with U iso (H) = 1.2U eq (host) Figures   Fig. 1. Molecular view of a dimer, with displacement ellipsoids at a 40% level. Atoms in the asymmetric unit drawn in full ellipsoids and full bonds; symmetry related ones (through the i: 1 -x, 1 -y, 1 -z operation), in empty ellipsoids and simple bonds. In double broken lines, the extremely weak Cu-O interaction. H atoms bound to carbon not shown, for clarity.

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.