Crystal structure of trans-(1,8-dibutyl-1,3,6,8,10,13-hexaazacyclotetradecane-κ4 N 3,N 6,N 10,N 13)bis(isonicotinato-κO)copper(II) from synchrotron data

The CuII ion in the title compound shows a distorted octahedral coordination environment defined by four N atoms of the azamacrocylic ligand in the equatorial plane and two O atoms of the isonicotinate ions in the axial sites. In the crystal, the molecules are connected by hydrogen bonds and π–π interactions, forming rods parallel to [001].

The title compound, [Cu(C 6 H 4 NO 2 ) 2 (C 16 H 38 N 6 )] has been synthesized and characterized by structure analysis based on synchrotron data and by FT-IR spectroscopy. The asymmetric unit consists of half of the Cu II complex, the other half being completed by inversion symmetry. The Cu II ion has a tetragonally distorted octahedral coordination sphere with four secondary N atoms of the azamacrocyclic ligand in the equatorial plane [Cu-N eq = 2.018 (12) Å ] and two O atoms of the isonicotinate anions at the axial positions [Cu-O ax = 2.4100 (11) Å ]. Intramolecular N-HÁ Á ÁO hydrogen bonds between one of the secondary amine N-H groups of the azamacrocyclic ligand and the noncoordinating O atom of the isonicotinate ions stabilize the molecular structure. Intermolecular N-HÁ Á ÁN hydrogen bonds between the other macrocyclic N-H group and the pyridine N atom of an adjacent isonicotinate anion as well asinteractions [centroid-to-centroid distance 3.711 (2) Å ] lead to the formation of rods parallel to [001].

Chemical context
The coordination chemistry of macrocyclic ligands has attracted extensive interest due to their potential applications in material science, chemistry and metalloenzymes (Lehn, 1995;Carnes et al., 2014). In particular, Cu II macrocylic complexes involving vacant sites in an axial position are feasible candidates for assembling supramolecular materials, with potential applications as gas-storage materials (Suh et al., 2012) as well as catalysts for co-polymerization of carbon dioxide and cyclohexene oxide (Tsai et al., 2014). Moreover, Cu II complexes with tetraazamacrocyclic ligands involving alkyl moieties have been investigated as magnetic materials with various auxiliary ligands such as metal cyanide, azide, and dicyanamide (Bi et al., 2012).
Isonicotinic acid is a versatile anion which can easily bind to transition metals via the carboxyl group or the pyridine N atom, thus allowing the assembly of multidimensionally structured compounds or heterometallic complexes (Liu et al., 2006).

Structural commentary
In compound (I), the Cu II ion lies on an inversion center and is coordinated by the four secondary amine N atoms of the azamacrocyclic ligand in the equatorial plane and by two O atoms from the isonicotinate anions at the axial positions, resulting in a tetragonally distorted octahedral geometry, as shown in Fig. 1. The average Cu-N eq bond length is 2.018 (12) and the Cu-O ax bond length is 2.4100 (11) Å . This difference can be attributed either to a large Jahn-Teller distortion effect of the Cu II ion and/or to a ring contraction of the azamacrocyclic ligand (Halcrow, 2013). The six-membered chelate ring (Cu1-N1-C2-N3-C3-N2) adopts a chair confor-mation and the five-membered chelate ring (Cu1-N1-C1-C4-N2) a gauche conformation (Min & Suh, 2001). The two C-O bond lengths of the carboxylate group are 1.255 (2) and 1.258 (2) Å , indicating that this group is fully delocalized with a bond angle (O1-C9-O2) of 126.8 (1) . Intramolecular N1-H1Á Á ÁO2 hydrogen bonds between one of the secondary amine groups of the azamacrocyclic ligand and the O atoms of a coordinating isonicotinate anion stabilize the molecular structure ( Fig. 1 and Table 1).

Figure 1
The molecular structure of (I), showing the atom-labelling scheme, with displacement ellipsoids drawn at the 50% probability level. Intramolecular N-HÁ Á ÁO hydrogen bonds are shown as red dashed lines.

Figure 2
View of the crystal packing of (I), with N-HÁ Á ÁO hydrogen bonds and interactions shown as dashed lines (red: intramolecular hydrogen bonds, green: intermolecular hydrogen bonds, cyan:interactions).

Synthesis and crystallization
Compound ( Safety note: Although we have experienced no problem with the compounds involved in this study, perchlorate salts of metal complexes are often explosive and should be handled with great caution.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms, with C-H distances of 0.95 (ring H atoms) or 0.98-0.99 Å (open-chain H atoms) and an N-H distance of 1.0 Å with U iso (H) values of 1.2 or 1.5U eq of the parent atoms.  -(1,8-dibutyl-1,3,6,8,10,13-hexaazacyclotetradecaneκ 4 N 3 ,N 6 ,N 10 ,N 13 )

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.