Synthesis and crystal structure of [2,7,12-trimethyl-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),13,15-triene-κ4 N]copper(II) bis(perchlorate)

A copper(II) complex of a pyridine-containing macrocycle (PyMAC) reveals a six-coordinated octahedral CuII complex with a tetradentate aminopyridine macrocycle ligand surrounding the metal centre in a square-planar geometry. Two weakly bound perchlorate counter-ions occupy the axial sites above and below the macrocyclic plane.

Various metal ions have been incorporated into PyMAC ligands, and the resulting complexes often showed interesting catalytic properties. For example, Ni II -PyMAC complexes have been found to exhibit peroxidase-like activity, with NiLCOOH ( Fig. 1) being most active (Organo et al., 2009). Fe II -LMe ( Fig. 1) was also found to have catalytic use in epoxidation reactions of cyclooctene with hydrogen peroxide (Ye et al., 2012). A similar Cu II -PyMAC complex but without methyl groups at the macrocyclic ring was reported by Fernandes et al. (2007) to scavenge superoxide.
Pyridine-containing metallomacrocycles have also found utility beyond synthetic chemistry. For example, Cu-macrocyclic complexes have become increasingly important in radiopharmaceutical applications as contrast agents in positron emission tomographic (PET) imaging (Boros et al., 2014).
While there are known Cu-pyridine macrocycles, only a few have been characterized structurally (Caira et al., 1975;Lindoy et al., 2001;Herrera et al., 2003;Autzen et al., 2003). Here, we report the synthesis and crystal structure of a Cu II -PyMAC perchlorate compound.

Structural commentary
The title compound has the Cu II atom in a distorted octahedral coordination, with the tetradentate aminopyridine macrocyclic ligand surrounding the metal atom in a squareplanar geometry (Fig. 2). Two perchlorate counter-ions occupy the axial sites perpendicular to the macrocyclic plane. The macrocyclic ligand incorporates a 2,6-substituted pyridine unit that is connected on both sides to an aliphatic chain of 11 atoms, including two secondary amines and a tertiary amine bearing a methyl group. When coordinated to the Cu II atom, the macrocycle exhibits approximate molecular mirror symmetry with respect to the plane that bisects the pyridine and tertiary amine nitrogen atoms, and is perpendicular to the macrocyclic plane. The Cu-N distances between Cu II and secondary amine nitrogen atoms [2.0417 (14) and 2.0445 (15) Å ] are similar to each other; the distance between Cu II and the tertiary amine N atom [2.0108 (13) Å ] is slightly shorter. In contrast, the Cu-N py bond length [1.9316 (13) Å ] is much shorter than the Cu-N amine bonds. Both perchlorate anions are only weakly bound, with Cu-O6 and Cu-O3 distances of 2.6478 (13) and 2.4736 (13) Å , respectively.
An intramolecular contact (N4-H4Á Á ÁO5) occurs between a perchlorate O atom and the tertiary amine NH group. The NÁ Á ÁO distance [3.423 (2) Å ] is longer than the sum of van der Waals radii of the two atoms (2.94 Å ), suggesting this is a weaker interaction comparing to normal hydrogen-bonding interactions.

Supramolecular features
In the crystal of the complex (see Fig. 3), several N-HÁ Á ÁO and C py -HÁ Á ÁO hydrogen bonds have longer DÁ Á ÁA distances than the van der Waals radii of the corresponding pairs of atoms (3.25 Å for CÁ Á ÁO). The resulting geometry is a chain along [010]. Numerical details are given in Table 1.

Synthesis and crystallization
The procedure for the synthesis of the title compound was adapted from Karn & Busch (1966) with subsequent reduction using NaBH 4 . 10 mmol of 2,6-diacetylpyridine were dissolved in 160 ml of absolute ethanol, and the resulting solution was mixed with 10 mmol of Cu(ClO 4 ) 2 Á6H 2 O in 240 ml of water. The reaction mixture was heated to 338 K and 10 mmol of N,N-bis(3-aminopropyl)methylamine were added. Subsequently, glacial acetic acid was added to the mixture until the pH was about 4. The mixture was heated to reflux of the solvent for 12 h; a color change from blue to dark blue occurred during that period. After reflux, the mixture was cooled to room temperature and 40 mmol of NaBH 4 were added. The mixture was left to stir for 12 h for complete reduction. Perchloric acid was added until the remaining NaBH 4 was consumed.
The deep-blue solution was concentrated to about a tenth of its original volume by rotary evaporation. The solution was then cooled slowly to room temperature and refrigerated. Dark-purple needle-like crystals formed upon cooling. The crystals were filtered, washed with absolute ethanol and diethyl ether, and allowed to dry. Light-purple crystals were recrystallized from hot water. Single crystals were obtained by dissolving the compound in acetonitrile followed by slow ether diffusion.