A binuclear CuII/CaII thiocyanate complex with a Schiff base ligand derived from o-vanillin and ammonia

A new title heterometallic Cu/Ca complex, [Cu(C8H8NO2)2Ca(NCS)2(H2O)], with a Schiff base ligand derived from o-vanillin and ammonia has been synthesized by a facile one-pot reaction under ambient conditions. In the crystal, O—H⋯S hydrogen bonds between the coordinating water molecules and thiocyanate groups form a supramolecular chain with a zigzag-shaped calcium skeleton.

The new heterometallic complex, aqua-1O-bis( 2 -2-iminomethyl-6-methoxyphenolato-1 2 O 1 ,O 6 :2 2 O 1 ,N)bis(thiocyanato-1N)calcium(II)copper(II),  NO 2 ) 2 (NCS) 2 (H 2 O)], has been synthesized using a one-pot reaction of copper powder, calcium oxide, o-vanillin and ammonium thiocyanate in methanol under ambient conditions. The Schiff base ligand (C 8 H 9 NO 2 ) is generated in situ from the condensation of o-vanillin and ammonia, which is released from the initial NH 4 SCN. The title compound consists of a discrete binuclear molecule with a {Cu(-O) 2 Ca} core, in which the CuÁ Á ÁCa distance is 3.4275 (6) Å . The coordination geometries of the four-coordinate copper atom in the [CuN 2 O 2 ] chromophore and the seven-coordinate calcium atom in the [CaO 5 N 2 ] chromophore can be described as distorted square planar and pentagonal bipyramidal, respectively. In the crystal, O-HÁ Á ÁS hydrogen bonds between the coordinating water molecules and thiocyanate groups form a supramolecular chain with a zigzag-shaped calcium skeleton.

Chemical context
The coordination chemistry of s-block elements is a fairly new and rapidly growing area of research (Fromm, 2008). Among the many systems studied, special attention is paid to heterometallic Cu/Ca complexes because of their structural diversity, relatively low toxicity, useful properties such as catalytic (Saha et al., 2016;Liu et al., 2017;Mon et al., 2016), magnetic (Sanchis et al., 1992;Zhang et al., 2013), luminescent (Zou & Gao, 2016), sorption (Grancha et al., 2017) and bioactivity (Mon et al., 2018;Grancha et al., 2016), and therefore high potential for applications. In the course of our systematic work on the development of the 'direct synthesis' (DS) approach, we have been successful in preparing different homo-and heterometallic complexes with transition metals (Kokozay et al., 2018). Herein we report the synthesis and crystal structure of the title compound.

Structural commentary
The main structural unit is the heterometallic molecular complex formed by divalent copper and calcium ions with two deprotonated Schiff base ligands (L À = C 8 H 8 NO 2 À ), two thiocyanate ions and one water molecule (Fig. 1). The metal atoms are joined through two -O bridges from the phenolato-groups of the organic ligands, giving a binuclear {Cu(-O) 2 Ca} core with a CuÁ Á ÁCa distance of 3.4275 (6) Å and Cu-O-Ca angles of 106.15 (8) and 106.64 (8) . The copper atom is four-coordinated by two imino N and two phenoxo O atoms from the Schiff base ligands. The coordination geometry of the CuN 2 O 2 chromophore is slightly distorted square planar; the Cu-O and Cu-N bond lengths vary in the range of 1.918 (2)-1.937 (2) Å and the corresponding cis/trans bond angles deviate from ideal symmetry by less than 8 with 4 = 0.112 (Yang et al., 2007). The copper atom is displaced from the N 2 O 2 plane by ca 0.01 Å . All of the O atoms of the {Cu(L) 2 } moiety chelate the calcium atom in a tetradentate manner and the coordination sphere of the Ca center is further completed by two SCN groups and one water molecule giving a coordination number of seven. The CaO 5 N 2 chromophore can be described as having a distorted pentagonalbipyramidal geometry with the oxygen atoms in the equatorial plane and the nitrogen atoms in the axial positions (Fig. 2). The calcium atom is located on the least-squares plane through the five equatorial O atoms, the sum of all O-Ca-O cis angles being 361 . The longest Ca-O bond distances [2.511 (2) and 2.521 (2) Å ] are observed for the coordinating methoxy groups and the shortest ones [2.339 (2)-2.356 (2) Å ] for the phenoxido groups and the water molecule. The values are in accordance with those found in related binuclear Cu/Ca complexes (Mondal et al., 2011;Constable et al., 2010;Chandrasekhar et al., 2012) (Chandrasekhar et al., 2012). The N,O,O,O 0 -tetradentate coordination mode, or [2.11 2 1] in the Harris notation (Coxall et al., 2000), of the HL ligand has been observed previously in [Ni(L) 2 Na(ClO 4 )(H 2 O)] (Costes et al., 1994). The bond-valence-sum (BVS) analysis applied to the corresponding bond lengths leads to the +2 oxidation state for both metals: 2.07 (Cu) and 2.11 (Ca) (Brown & Altermatt, 1985;Chen & Adams, 2017).

Supramolecular features
The coordinating water molecule and thiocyanate ions of each binuclear complex are involved in four O-HÁ Á ÁS hydrogen bonds (Table 1)  Coordination polyhedron of the calcium atom in the title compound.

Figure 1
Molecular structure of the title compound, with the numbering scheme and displacement ellipsoids drawn at the 50% probability level.
A fragment of the crystal structure showing the chain skeleton based on the [R 2 2 (12)] 2 synthon is shown in Fig. 4. It should be noted that the arrangement of calcium atoms within the chain has a zigzag shape with all metal atoms lying in the same plane. The shortest CaÁ Á ÁCa distance is 7.792 (7) Å and the angle formed by the three nearest metal centers is 85.093 (7) . The supramolecular chains run parallel to the b-axis (Fig. 5). Weak N-HÁ Á ÁS hydrogen bonds (Table 1) and astacking interaction between the C1-C6 ring and the adjacent C9-C14(x À 1, y, z) ring [dihedral angle between the rings 4.6 (1) , mean interplanar separation 3.40 Å and plane shift 0.69 (1) Å ] link neighbouring chains, increasing the whole dimensionality of the crystal framework.

Database survey
To date, the crystal structures of 72 complexes containing copper and calcium are known (CSD, version 5.40, last update February 2019; Groom et al., 2016). Most of them possess polymeric or ionic frameworks. Only five examples were found of molecular binuclear Cu/Ca complexes, including two formed by carboxylate ligands (Smith et al., 1985;Breeze & Wang, 1994) and three with symmetric salen-type Schiff base ligands (Constable et al., 2010;Mondal et al., 2011;Chandrasekhar et al., 2012). To the best of our knowledge, [Cu(L) 2 Ca(NCS) 2 (H 2 O)] is the first molecular binuclear Cu/ Ca complex with an asymmetric Schiff base ligand to have been characterized crystallographically.

Synthesis and crystallization
The following system has been investigated: Cu 0 -CaO-o-vanillin-NH 4 SCN-methanol (open air), and the heterometallic complex [Cu(L) 2 Ca(NCS) 2 (H 2 O)] was obtained. Its formation can be described by the following scheme: where the Schiff base HL can be regarded as a product of the condensation of o-vanillin and NH 3 , which is released from NH 4 SCN in the basic environment.
Copper powder (0.06 g, 1 mmol), CaO (0.11 g, 2 mmol), o-vanillin (0.3 g, 2 mmol) and NH 4 SCN (0.15 g, 2 mmol) were added to 30 ml of methanol. The reaction mixture was stirred magnetically at 323-333 K for ca 5 h until the complete dissolution of the copper powder was observed. The solution was filtered and left for 1 d, and then light-orange crystals were formed. Yield: 0.26 g (48.3%, Cu

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  (7)