The crystal structures of the ligand N-(quinolin-8-yl)pyrazine-2-carboxamide and of a tetranuclear copper(II) complex

The title tridentate ligand (HL1), crystallizes with three independent molecules in the asymmetric unit. Its reaction with Cu(Ac)2 produced a tetranuclear complex with a central tetrakis(μ-acetato)dicopper paddle-wheel moiety linked on either side via bridging acetate anions to a mononuclear copper(II)–(L1) complex.


Chemical context
The crystal structures of a number of hetero bimetallic ironmanganese cyano complexes of the ligand HL1 have been synthesized in order to explore their super-exchange magnetic properties (Kim et al., 2007;Zhou et al., 2014). To the best of our knowledge (Cambridge Structural Database; Groom et al., 2016), the crystal structure of the ligand itself has never been described, although the structure of the pyridine analogue, N-(8-quinolyl)pyridine-2-carboxamide, has been reported (Zhang et al., 2001). There is only one previous report of a copper(II) complex of ligand HL1,viz. (acetato)[N-(quinolin-8-yl)pyrazine-2-carboxamidato]copper(II) monohydrate, a mononuclear complex with the ligand coordinating in a tridentate fashion (Meghdadi et al., 2013). It has been shown previously that pyrazine carboxamide ligands are useful for the synthesis of transition-metal complexes that exhibit magnetic super-exchange and anion encapsulation (Hausmann et al., 2003;Cati et al., 2004;Klingele et al., 2007). During ISSN 2056-9890 further work in this area (Cati, 2002), the title copper(II) complex, I, of ligand HL1 was synthesized, and we report herein on the crystal structures of ligand HL1 and complex I. The various intermolecular interactions in the crystal of HL1 have been studied by Hirshfeld surface analysis.

Structural commentary
The ligand HL1 crystallized with three independent molecules (A, B and C) in the asymmetric unit, and their molecular structures are illustrated in Fig. 1. In each molecule the carboxamide NH H atom forms three-centered (bifurcated) intramolecular N-HÁ Á ÁN hydrogen bonds involving the quinoline and the adjacent pyrazine N atoms ( Fig. 1 and Table 1). This arrangement is similar to that observed in 1,3bis(2-pyridylimino)isoindoline (Schilf, 2004) and its pyrazine analogue, bis(pyridin-2-yl)-6,7-dihydro-pyrrolo[3,4-b]pyrazine-5,7-diimine (Posel & Stoeckli-Evans, 2018). There is also a short C-HÁ Á ÁO contact present in each molecule ( Fig. 1 and Table 1). Hence, the three molecules have similar conformations, with the pyrazine ring being inclined to the quinoline ring by 4.5 (4) in molecule A, 3.1 (4) in B and 4.1 (4) in C. For the three molecules, the r.m.s. deviations for the mean planes of the non-H atoms are 0.068, 0.055 and 0.06 Å , respectively. Inverted molecule A on molecule B has an r.m.s. deviation of 0.054 Å for the 19 non-H atoms, while inverted molecule B on molecule C has an r.m.s. deviation of 0.054 Å , and molecule A and molecule C have an r.m.s. deviation of 0.057 Å .
Reaction of HL1 with Cu(CH 3 CO 2 ) 2 produced a tetranuclear complex, I, with a central tetrakis(-acetato)dicopper paddle-wheel moiety linked on either side via a bridging acetate anion to a mononuclear copper(II)-(L1) complex, illustrated in Fig. 2. Selected geometrical parameters are given in

Figure 1
A view of the molecular structure of the three independent molecules (A, B and C) of ligand HL1, with the atom labelling. Displacement ellipsoids are drawn at the 50% probability level. Intramolecular N-HÁ Á ÁO and C-HÁ Á ÁO contacts (see Table 1) are shown as dashed lines.  (7) Symmetry code: (i) Àx þ 1; Ày; Àz þ 1.
symmetry code: (i) Àx + 1, Ày, Àz + 1)] of the paddle-wheel moiety (Table 2). Both copper atoms are fivefold coordinate; CuN 3 O 2 for Cu1 and CuO 5 for Cu2. Atom Cu1 is ligated in the equatorial plane by the three N atoms of the ligand and an O atom, O3, of the bridging acetate ion, and with a coordinated methanol O atom, O2, in the apical position. It has an irregular coordination sphere with a 5 factor of 0.17 ( 5 = 0 for an ideal square-pyramidal coordination sphere, and = 1 for an ideal trigonal-pyramidal coordination sphere; Addison et al., 1984). Atom Cu2 is ligated by four acetate O atoms (O5, O6, O7 and O8) of the paddle-wheel moiety in the equatorial plane and by atom O4 of the bridging acetate ion in the apical position. It has a perfect square-pyramidal coordination sphere with a 5 factor of 0.01. There are two intramolecular C-HÁ Á ÁO contacts present involving the quinoline unit and oxygen atoms O1 of the carboxymide group and O4 of the bridging acetate ion ( Fig. 2 and Table 3).

Supramolecular features
In the crystal of ligand HL1, and as can be seen from Fig. 1, molecule B is closely related to molecules A and C by noncrystallographic inversion symmetry, while molecules A and C are closely related by non-space group translation. An analysis with PLATON/ADDSYM (Spek, 2009), however, concluded that no obvious extra crystallographic symmetry was present and no change in the space group (Cc) was required. In the crystal, packets of the three molecules stack in the order (ABC), (ABC) etc ( Fig. 3; A blue, B red, C green). They are linked by offsetinteractions, so forming layers lying parallel to the ab plane ( Fig. 4 and Table 4).
In the crystal of I, molecules are linked by pairs of O-HÁ Á ÁO hydrogen bonds involving the coordinated methanol molecule and the carboxamide O atom, O1, forming chains propagating along [011]; see Table 3 and Fig. 5. The chains thus formed enclose R 2 2 (12) ring motifs, as illustrated in Fig. 5. The methanol solvent molecule is linked to the chain via bifurcated O-HÁ Á ÁO/O hydrogen bonds, which enclose an R 2 1 (4) ring motif ( Fig. 5 and A view of the molecular structure of complex I, with the atom labelling. Displacement ellipsoids are drawn at the 50% probability level. The unlabelled atoms are related to labelled atoms by inversion symmetry [symmetry code: (i) Àx + 1, Ày, Àz + 1]. The intramolecular C-H...O contacts are shown as dashed lines (Table 3). For clarity, the methanol solvate molecules have been omitted.

Figure 4
A view normal to plane (110) of the crystal packing of ligand HL1 (colour code: A molecules are blue, B are red and C are green).

Hirshfeld surface analysis of ligand HL1
The Hirshfeld surface analysis (Spackman & Jayatilaka, 2009) and the associated two-dimensional fingerprint plots (McKinnon et al., 2007) were performed with Crystal-Explorer17 (Turner et al., 2017). A recent article by Tiekink and collaborators (Tan et al., 2019) 'outlines the various procedures and what can be learned by using CrystalExplorer'. The Hirshfeld surface of HL1 mapped over d norm is given in Fig. 7a, where short interatomic contacts are indicated by the faint red spots. Thestacking is confirmed by the small blue regions surrounding bright red spots in the various aromatic rings in Fig. 7b, the Hirshfeld surface mapped over the shape-index, and by the flat regions around the aromatic regions in Fig. 7c, the Hirshfeld surface mapped over the curvedness.
The full two-dimensional fingerprint plots for HL1 and for the individual molecules are given in Fig. 8a. The principal intermolecular interactions for HL1 (Fig. 8b), are delineated into HÁ Á ÁH at 43.0%, NÁ Á ÁH/HÁ Á ÁN at 14.5%, followed by CÁ Á ÁH/HÁ Á ÁC interactions at 11.8%. The contributions of the CÁ Á ÁC and CÁ Á ÁN interactions, which are 10.8 and 10.7%, respectively, are superior to the contribution of the OÁ Á ÁH/ HÁ Á ÁO interactions at 8.1%. The relative percentage contributions of close contacts to the Hirshfeld surface for HL1 and for the individual molecules are similar, as indicated in Table 5  A partial view along the a axis of the crystal packing of complex I. The hydrogen bonds (see Table 3) are shown as dashed lines and C-bound H atoms have been omitted.

Figure 6
A view along the a axis of the crystal pack of complex I. The hydrogen bonds (see Table 3) are shown as dashed lines and only the H atoms involved in these intermolecular interactions have been included (the two methanol hydroxyl H atoms are shown as grey balls).

Database survey
A search of the Cambridge Structural Database (Version 5.40, update February 2019; Groom et al., 2016) of ligand HL1 yielded nine hits. The majority of these compounds are hetero bimetallic iron-manganese cyano complexes that exhibit super-exchange magnetic properties [e.g. CSD refcodes JIVGIF and JIVGOL (Kim et al., 2007) and BOLJOD, BOLJUJ and BOLKIY (Zhou et al., 2014)]. Only one hit concerns a copper(II) complex, namely (acetato)(N-(quinolin-8-yl)pyrazine-2-carboxamidato)copper(II) monohydrate, with the ligand coordinating in a tridentate fashion (AYIFOF; Meghdadi et al., 2013). The copper ion is ligated by the three N atoms of the ligand, and the two O atoms of the acetate anion, hence the copper atom is CuN 3 O 2 five-coordinate with an irregular coordination sphere; 5 = 0.17. This value is similar to that for atom Cu1 in the title complex I ( 5 factor of 0.17).
A search for the tetrakis(-acetato)dicopper paddle-wheel moiety gave 356 hits. Limiting the search for a tetrakis(acetato)-dicopper paddle-wheel moiety bridged on either side by an acetato group to a second copper atom gave 15 hits for 14 structures (see supporting information file S1). Eight of these compounds are polymeric structures, for example, the -TUP;Neels et al., 1995]. Only six are tetranuclear compounds similar to compound I; for example, hexakis( 2 -acetato)bis[1-(5-bromosalicylaldimino)-3-(2-methylpiperidino)propane]tetracopper(II) (PIBXOU; Chiari et al., 1993), hexakis- ]. The CuÁ Á ÁCu distance in the paddle-wheel unit varies from ca 2.604 to 2.669 Å ; in I this distance, Cu2Á Á ÁCu2 i , is 2.6201 (6) Å . The CuÁ Á ÁCu distance involving the two copper atoms bridged by a single acetato group varies from ca 3.772 to 5.441 Å . The longer distance is observed when only one O atom bridges the two copper atoms as in compound I, where distance Cu2Á Á ÁCu1 is ca 5.147 Å , close to the distance of ca 5.392 Å observed in UJOWEX. A shorter distance is observed when one O atom bridges the two copper atoms and the second O atom coordinates to the second copper atom, in a ( 2 -acetato-O,O,O 0 ) manner, as in CERTOI/CERTOI10 where this CuÁ Á ÁCu distance is ca 3.772 Å .

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 6. Intensity data for ligand HL1 were measured at 223 K on a four-circle diffractometer assuming a C-centered unit cell and only one equivalent of data were measured; hence R int = 0 and the h,k,l reflections for which h + k = 2n + 1 were not measured. For compound I, data were measured at 173 K on a Stoe IPDS1, a one-circle image-plate diffractometer. For compound I a small cusp of data is missing. This is common with data measured using the IPDS1 for monoclinic and triclinic crystal systems.
For ligand HL1 the NH H atoms could be located in a difference-Fourier map, but during refinement they were included in calculated positions and treated as riding: N-H = 0.87 Å with U iso (H) = 1.2U eq (N). The OH H atom of the coordinated methanol molecule in complex I was located in a difference-Fourier map and freely refined. The OH H atom of the solvent methanol molecule in I was included in a calcu- SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015); molecular graphics: PLATON (Spek, 2009) and Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXL2018/3 (Sheldrick, 2015), PLATON (Spek, 2009) and publCIF (Westrip, 2010).

N-(quinolin-8-yl)pyrazine-2-carboxamide (HL1)
Crystal data   (2) 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 O1 0.7337 (5) (7) 0.056 (7) 0.012 (5)   where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.66 e Å −3 Δρ min = −0.66 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.