Synthesis and crystal structure of catena-poly[[tetra-μ-acetato-copper(II)]-μ-6-ethoxy-N 2,N 4-bis[2-(pyridin-2-yl)ethyl]-1,3,5-triazine-2,4-diamine]

Green-colored crystals of the title compound [Cu2(C19H23N7O)2(C2H3O2)4]n crystallized in the monoclinic P21/c space group. The dinuclear Cu center is coordinated by both acetate groups and 6-ethoxy-N 2,N 4-bis[2-(pyridin-2-yl)ethyl]-1,3,5-triazine-2,4-diamine ligands.


Chemical context
Dinuclear Cu II groups are recognized for their crucial role as active sites in metalloenzymes and are present in many biological systems (Festa & Thiele, 2011;Solomon et al., 2014). They often constitute the catalytically active sites involved in the stepwise oxidative conversions of many small molecules (Pham & Waite, 2014;Chakraborty et al., 2014). A well-known series of metalloenyzmes containing dinuclear copper active sites is that of the polyphenol oxidases (e.g. catechol oxidase) where the catechol is easily oxidized to quinone products (Ravikiran & Mahalakshmi, 2014). In recent years, there has been an increased effort to carry over this efficient and selective oxidation into biomimetic models of metalloenzymes (Mahadevan et al., 2000;Panda et al., 2011;Marion et al., 2012).
As part of this quest, significant efforts have been made to identify and better understand the specific structural patterns found at these copper-containing active sites. These patterns have often been found to convey functionalities that define a particular enzyme. This has led to a focus on the basic elements of coordination between the ligands and the metal centers. For example, when designing mimics of catechol oxidase, many model catalysts include the same basic structural elements (Koval et al., 2006). These models often contain multidentate ligands with at least five coordinating heteroatoms branched off a central ring, all coordinating to the copper centers. This coordination motif and its orientation often ISSN 2056-9890 provide a unique accessibility for substrate approach, similar to that found in a type-3 active site (Koval et al., 2006).
In this paper, we report the crystal structure of a biomimetic complex (I) of catechol oxidase synthesized from a multidentate ligand that is coordinated to the copper centers in an unexpected fashion. The complex possesses two nitrogen coordinating heteroatoms from triazine ligands, which coordinate to the copper centers of the paddle-wheel unit at the axial positions. Additional coordination by the terminal pendant pyridine group on the ligand to another copper paddle-wheel unit creates a continuous coordinated chain linkage.

Structural commentary
The title compound (I) crystallizes in the space group P2 1 /c. The molecular structure of (I) (Fig. 1) (3) Å ] are a consequence of hydrogen-bonding interactions involving the O5 and O9 oxygen atoms (see text below for further details). Two triazine ligands coordinate to the copper-acetate paddle-wheel unit in an asymmetric manner, with one Cu atom coordinated to the triazyl nitrogen, N1, of the central ring on one ligand (green nitrogen in Scheme 1), and the other Cu coordinated to the terminal pyridyl nitrogen, N6, of a second ligand (pink nitrogen in Scheme 1). The two ligands adopt an almost orthogonal orientation to each other. Each of the ligands has their linking alkyl chain adopting a gauche geometry, making the two terminal pyridine rings twist away from the central triazine ring.

Supramolecular features
The copper centers and ligands are linked into a coordination polymer as a consequence of the presence of the c-glide in the P2 1 /c space group. Intramolecular hydrogen-bonding interactions (Table 2) are observed for only one of the two triazine ligands coordinating to the paddle wheel, as shown in Fig. 2. These occur between the N4-H1Á Á ÁO5 and N5-H2Á Á ÁO9 atoms at (HÁ Á ÁA) distances of 1.89 and 1.99 Å , respectively, with the hydrogens on the nitrogen atoms of the ortho branches off the triazine ring pointing towards the oxygen atoms of two of the acetate groups of the paddle wheel. Closely packed arrays of one-dimensional chains, hypothesized to be held together by dispersion forces, form an extended two-dimensional network in the bc plane (Fig. 3) Figure 1 Molecular structure of (I) drawn with 50% probability displacement ellipsoids. symmetry code (i) x, Ày + 1 2 , z À 1 2 ; (ii) x, Ày + 1 2 , z + 1 2 . Key: carbon, gray; nitrogen, blue; copper, light green; oxygen, red. Table 2 Hydrogen-bond geometry (Å , ).

Database survey
A structure survey was carried out on the Cambridge Structural Database (CSD version 5.41, update of August 2020; Groom et al., 2016). Search results show that although 1,3,5triazine-2,4-diamine-derivative complexes with copper, ruthenium and rhodium have been reported (Singh et al., 2010;Chu et al., 2011;Massoud et al., 2011;Chakraborty et al., 2014), none of these complexes contains a copper(II) acetate [Cu 2 (OAc) 4 L 2 ] paddle wheel, as is found in compound (I). In all the previously reported structures, each ligand is coordinated to the metal using at least four of the nitrogen heteroatoms present. The structure of compound (I) presented here is rather different, as each ligand is coordinated to copper through only one nitrogen heteroatom. In addition, whilst some of the previously reported derivatives contain ortho-branched tertiary amines, compound (I) contains secondary amines.

Synthesis, crystallization and catalytic activity
The triazine ligand (Fig. 5, c) was synthesized by substituting all three chlorines on the cyanuric chloride ring (Fig. 5 The two-dimensional array of the coordinating networks of (I) viewed along the a axis.

Figure 4
Separated planes of the neighboring one-dimensional networks viewed slightly off the c axis. Key: carbon, gray; nitrogen, blue; copper, light green; oxygen, red.

Figure 2
A packing diagram of (I) viewed along the a axis showing the onedimensional network. The N-HÁ Á ÁÁO hydrogen bonds are shown with the dashed light-blue lines. Key: carbon, gray; nitrogen, blue; copper, light green; oxygen, red.
by chilling 40 mL (0.69 mol) of ethanol in an ice bath. Cyanuric chloride (5.00 g, 27 mmol) and sodium bicarbonate (2.35 g, 28 mmol) were added to the chilled ethanol and stirred in an ice bath for 45 minutes. The reaction mixture was then taken out of the ice bath, stirred at room temperature for 3.5 h and then poured over 20 mL of ice. The resulting precipitate was collected by vacuum filtration. The second and third substitutions were completed by taking the product from step 1 (2.30 g, 12 mmol) (Fig. 5, b) and dissolving it in CHCl 3 . The solution was chilled in an ice bath. 2-(2-Aminoethyl)pyridine (3.60 g, 29 mmol) and N,N-diisopropylethylamine (DIPEA) (3.80 g, 29 mmol) were dissolved in CHCl 3 and added dropwise to the chilled solution. The reaction was stirred at room temperature for 1 h and stirred at reflux for 12 h. The final product was purified by removing the solvent and taking up the residue in chilled DMF. The product was collected by vacuum filtration and washed at least three times with 15 mL of chilled DMF. The product was obtained as a white powder [yield: 1.75g, 4.8 mmol (40% yield)] and was characterized using 1 H NMR. The triazine ligand (339.4 mg, 1.0 mmol) was dissolved in chloroform (20 mL) and a stoichiometric amount of copper(II) acetate (367.3 mg, 1.0 mmol) was dissolved in methanol (20 mL). The two solutions were mixed, and the resulting solution was placed in an ether diffusion chamber for at least four days. Green crystals of (I) were filtered off and washed with methanol. The melting point of the crystals was 639-643 K.
Catalytic activity of [Cu 2 (C 19 H 23 N 7 O) 2 (C 2 H 3 O 2 ) 4 ] n (I) The catechol, 1,4-dihydroxybenzene, was used to test the catalytic activity of compound (I). This catechol is cheap and has good solubility in water. 2 mL of 10 À4 M of compound (I) in a chloroform: methanol (1:1) solution was placed in a cuvette and 10 mL of a 1 M solution of the catechol injected. The conversion of the catechol into its quinone derivative (benzoquinone) was monitored by measuring the absorbance at 403 nm over a period of time. Fig. 6 shows a continuous increment in absorption at this wavelength, indicating the formation of the product.

catena-Poly[[tetra-µ-acetato-copper(II)]-µ-6-ethoxy-N 2 ,N 4 -bis[2-(pyridin-2-yl)ethyl]-1,3,5-triazine-2,4-diamine]
Crystal data 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.