Structure of tetrakis(μ-decanoato-κ2 O:O′)bis[(4-methylpyridine-κN)copper(II)], a dimeric copper(II) complex

The 4-methylpyridine (4-Mepy) based dimeric copper(II) carboxylate complex [Cu2(C10H19O2)4(C6H7N)2] or [Cu2(μ-O2CC9H19)4(4-Mepy)2] crystallizes with triclinic (P ) symmetry. The two CuII ions exhibit a distorted square-pyramidal environment and are connected into a centrosymmetric paddle-wheel dinuclear cluster [Cu⋯Cu = 2.6472 (8) Å] via four bridging carboxylate ligands arranged in the syn–syn coordination mode. The apical positions around the paddle-wheel copper centers are occupied by the N atoms of the 4-methylpyridine ligands. Parts of the decanoate chains are disordered with occupancy ratios of 0.817 (9):0.183 (9) and 0.65 (5):0.35 (5).


Chemical context
Research on metal carboxylates has gained importance in view of their use in the formation of open and porous frameworks and also because of their biological activities and antibacterial properties (Smithenry et al., 2003;Lah et al., 2001). As the number of carboxylate groups increases, so does the complexity of the coordination behaviour. Carboxylate anions are versatile ligands capable of existing as counteranions or as ligands coordinating to the metal ions in different modes (Deacon & Philips, 1980;Tao et al., 2000;Smithenry et al., 2003). Copper complexes containing aliphatic/aromatic carboxylic acid anions as ligands with the general formula [Cu 2 (O 2 CR) 4 ] have been known to adopt a paddle-wheel structure where four bidentate carboxylato ligands bridge the Cu II centres (Baruah et al., 2015;Serrano & Sierra, 2000). Complexes having R = a long-chain alkyl group can make the resultant dimeric carboxylates more soluble in organic solvents and hence can be more effective as catalysts in certain reactions (Baruah et al., 2015). These carboxylates can be prepared either by reaction of basic copper(II) carbonate/ acetate with the corresponding carboxylic acid or by reaction of a copper(II) salt with the sodium salt of the corresponding carboxylic acid (Hamza & Kickelbick, 2009;Moncol et al., 2010;Das & Barman, 2001). Each Cu II centre has four oxygen atoms forming the basal plane, while the axial position is either occupied by a solvent molecule or by a monodentate nitrogen base ligand or sometimes by an oxygen atom of another dimeric unit resulting in an oligomeric chain (Agterberg et al., 1998;Wein et al., 2009). A few members of the family of dicopper(II) tetracarboxylates of the type [Cu 2 (-O 2 CR) 4 L 2 ] have been demonstrated to be active homogeneous catalysts in the oxidation of various alcohols. A dinuclear complex, [Cu 2 (-O 2 CC 5 H 11 ) 4 (C 6 N 2 H 4 ) 2 ] (Baruah et ISSN 2056-9890 al., 2015, was reported as having two crystallographically independent Cu II atoms in a distorted square-pyramidal environment.

Structural commentary
The title compound [Cu 2 (-O 2 CC 9 H 19 ) 4 (4-Mepy) 2 ] crystallizes in the triclinic system, space group P1. The complex has a centrosymmetric structure and consists of a copper(II) dimer having a paddle-wheel structure. The asymmetric unit comprises a Cu II ion coordinated by the N atom of 4methylpyridine and by two deprotonated O-monodentate decanoate ligands. The two Cu II ions are bridged by four carboxylate ligands in the syn-syn coordination mode, resulting in a distorted square-pyramidal environment with the four O atoms forming the square basal plane and the two pyridyl-N atoms of the 4-Mepy ligands occupying the apical positions. The molecular structure of the complex is shown in Fig. 1 (-O 2 CCMe 3 ) 4 (NC 5 H 3 (2-NH 2 )(6-CH 3 )) 2 ] (Fomina et al., 2010) and [Cu 2 (-O 2 CC 6 H 5 ) 4 (py) 2 ] (Iqbal et al., 2014). The CuÁ Á ÁCu distance in the title complex was found to be slightly longer than in the copper(II) carboxylate complex [Cu 2 (-O 2 CC 5 H 11 ) 4 (4-NCpy) 2 ] [2.6055 (9) Å ; Baruah et al., 2015) and in [Cu 2 (-O 2 CC 9 H 19 ) 4 (NC 5 H 4 CO 2 C 12 H 25 ) 2 ] [2.615 (1) Å ; Rusjan et al., 2000). The Cu--N bond in the title complex is slightly shorter than those reported by Rusjan et al. (2000) and Petric et al. (1993). The difference between the CuÁ Á ÁCu and Cu-N distances and those for related complexes is probably due to the difference in the basicity of the pyridinic group in the apical position of the core. The hydrogen atoms at positions 2 and 6 of the aromatic ring establish intramolecular C-HÁ Á ÁO interactions with the closely placed carboxylate oxygen atoms (Table 1).
In the title complex, the two oppositely placed decanoate alkyl chains adopt a fully elongated zigzag conformation, whereas the other pair is distorted, aligning parallel to the first one after a gauche conformation at the C18-C19 bond (Rusjan et al., 2000). This arrangement probably occurs to facilitate efficient packing. The terminal ends of both pairs of alkyl chains are disordered and were modelled as described in the Refinement section.

D-HÁ
linked by a pair of C6-H6BÁ Á ÁO2 interactions that form infinite chains along the b-axis direction. The interlinking between them gives rise to the crystal packing in the complex, as shown in Fig. 2. The crystal packing is also supported by C6-H6CÁ Á Á interactions between a pyridine ring-bound methyl group and the pyridine ring (-x, 2 -y, 1 -z) of a neighbouring 4-Mepy unit with an HÁ Á Ácentroid distance of 2.94 Å and C-HÁ Á Ácentroid angle of 134 ( Fig. 3). At the same time, the centroidÁ Á Ácentroid distances of 4.4183 (14) Å and 4.6957 (15) Å with slippage of 2.909 and 2.913 Å , respectively, between neighbouring pyridine rings ( Fig. 3) are too long for meaningfulinteractions (Tsuzuki et al., 2002). More details on the mutual arrangement of the pyridine rings can be found in Table 2.

Database survey
A survey of the Cambridge Structural Database (CSD version 2020.2; Groom et al., 2016) for dimeric copper complexes of alkyl carboxylates revealed that most of the complexes adopt a paddle-wheel structure with a slightly distorted squarepyramidal environment around the Cu II ions. The crystal structure of tetrakis(-heptanoato-O,O 0 )bis(nicotinamide)-)bis(nicotinamide)dicopper(II) (CSD refcodes: CAYHIT and CAYHIT01; Kozlevcar et al., 1999) and tetrakis(- Kozlevcar et al., 2000) were reported as having normal zigzag as well as distorted alkyl chains. Riesco and coworkers reported on the preparation of three polymorphs of Cu II decanoate, which differ in the cell parameters and the packing of chains following crystallization using different solvents (CUDECN01 and CUDECN02; Riesco et al., 2008Riesco et al., , 2015. In the dodecylnicotinate bis-adduct of a centrosymmetric dinuclear copper decanoate (XADREZ; Rusjan et al., 2000) with average Cu-O, Cu-N and CuÁ Á ÁCu distances of 1.960 (6), 2.183 (3) and 2.615 (1) Å , respectively, the alkyl chains in the complex lead to the formation of two different layers along the crystal: one defined by the polar copper carboxylate cores and the second, non-polar one containing the alkyl chains. The Cu II octanoate adduct with pyridine, viz.

Synthesis and crystallization
All reagents were purchased from E. Merck and used as received without further purification. CuSO 4 Á5H 2 O (0.4994 g, 2.0 mmol) and sodium decanoate (0.7708 g, 4.0 mmol) were stirred in 25 mL of methanol. After 30 minutes, 4-methyl pyridine (0.1863 g, 2.0 mmol) was added to the reaction mixture, and stirring was continued for 3 h. The resulting green product was filtered off, washed repeatedly with small volumes of methanol and dried in a vacuum desiccator over fused CaCl 2 (yield 0.8180 g, 82%). The product was dissolved in acetonitrile to give a greenish homogeneous solution, which was allowed to concentrate by evaporation at room temperature. Single crystals suitable for X-ray diffraction analysis were obtained from this solution after one day and were collected by filtration. The compound is insoluble in water but soluble in methanol and acetonitrile. IR spectroscopic data (KBr disc, cm À1 ): asym (COO À ) 1580, sym (COO À ) 1381, stretch (C-H) 2800-2950, stretch (py) 1682, 1489, 1445.  Table 2 Geometry (Å , ) of the stacking of the pyridine rings.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. C-bound hydrogen atoms were placed in idealized positions with C-H = 0.95-0.99 Å , and refined as riding with U iso (H) = 1.2U eq (C) or 1.5U eq (C-methyl). The twofold disordered parts of the decanoate chains (C15-C16, C24-C26 and C15A-C16A, C24A-C26A) have been completed through successive electron density difference-Fourier maps and were refined with a sum of their occupancies restrained to unity using geometry (SAME) and U ij restraints (SIMU and RIGU) implemented in SHELXL. The refinement converged with the relative occupancies of 0.817 (9) and 0.183 (9) for the C15-C16 section and 0.65 (5) and 0.35 (5) for the C24-C26 section.

Tetrakis(µ-decanoato-κ 2 O:O)bis[(4-methylpyridine-κN)copper(II)]
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.24 e Å −3 Δρ min = −0.28 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (