Crystal structure of bis{μ2-2,2′-[(4,10-dimethyl-1,4,7,10-tetraazacyclododecane-1,7-diyl)bis(methylene)]bis(4-oxo-4H-pyran-3-olato)}dicobaltcalcium bis(perchlorate) 1.36-hydrate

The title compound is a new heterotrinuclear CoII–CaII–CoII dimer of L1. L1 undergoes a cobalt-driven preorganization, leading to the formation of an electron-rich area able to host a hard metal ion such as CaII. In the dimer, two neutral [Co(H–2 L1)] moieties, held together by the CaII ion, are rotated by 90°. The trinuclear complexes form layers perpendicular to the c axis; the perchlorate anions are located between the layers and interact with the complexes, as well as the lattice water molecules.

On the other hand, hard metal ions also find applications in the biological field. Both rare earth and alkaline earth metal ions are used in the biomedical field, in bioassays and bioimaging applications (Xiao et ISSN 2056-9890 2012; Price et al., 2012). Furthermore, hard metal ions are quite difficult to bind in water because they need a high coordination number without usually showing specific coordination requirements, issues that could be overcome using preorganized receptors bearing oxygenated donor sites. It follows that systems able to bind hard metal ions, both in aqueous solution and in the solid state, are very attractive. Indeed, they have found applications in fields ranging from new materials to medicinal chemistry (Blindauer et al., 2017;Esteves et al., 2016;Lomidze et al., 2016;Yang et al., 2014;Price et al., 2012;Pasatoiu et al., 2011;Pasatoiu et al., 2010;Aime et al., 2006;Bernot et al., 2006;Gatteschi et al., 2006;Malandrino & Fragalà , 2006;Terai et al., 2006).
Ligand L1 {4,10-bis[(3-hydroxy-4-pyron-2-yl)methyl]-1,7dimethyl-1,4,7,10-tetraazacyclododecane} is a Maltol-based macrocycle (Amatori et al., 2012) capable of forming a mononuclear Co II species where both side-arms are forced by the transition metal ion to move and locate on the same part with respect to the macrocyclic plane . Such a cobalt-driven preorganization allows the formation of an electron-rich area formed by the four converging oxygen atoms of the two maltolate functions of L1, suitable to host hard metal ions such as Ln III (Ln = Gd, Eu; Benelli et al., 2013;Rossi et al., 2017), Na I  and Ba II . The resulting heteropolynuclear systems differ in the number of the complexes involved in the coordination, depending on the nature of the hard cation. Indeed, the coordination of the hard ion leads to Co II -Ln III -Co II heterotrinuclear dimers, a Na I -Co II heterodinuclear monomer and a Ba II -Co II heterodinuclear metal coordination polymer.
Herein we present a Co II -Ca II -Co II heterotrinuclear dimer of L1 and a brief comparison with the previous L1-containing structures, highlighting the high versatility of this ligand.

Structural commentary
The title compound is a trinuclear complex cation of formula {Ca[Co(H -2 L1)] 2 }Á2ClO 4 Á1.36H 2 O and crystallizes in the tetragonal system in space group I4. In the {Ca[Co(H -2 L1)] 2 } 2+ trinuclear complex (Fig. 1), two neutral [Co(H -2 L1)] moieties are held together by the Ca 2+ cation, which is coordinated by oxygen atoms provided by the maltolate groups of the two complexes. The asymmetric unit comprises a quarter of the {Ca[Co(H -2 L1)] 2 } 2+ trinuclear complex, half of a perchlorate ion and 0.34 water molecules. The two halves of each cobalt complex are related by a twofold rotation axis, the cobalt ion lying on the symmetry element. The two cobalt complexes are then related by a fourfold rotoinversion axis, the calcium ion lying on the symmetry element. The disordered perchlorate ion and the water molecule lie on a twofold axis, with the chlorine atom (for ClO 4 À ) and the oxygen atom (for H 2 O) lying on the symmetry element.
The conformation of the [12]aneN 4 macrocycle is the usual [3333]C-corners one (Meurant, 1987) with the trans nitrogen distances in agreement with those reported in the CSD for this conformation type, but with the N2Á Á ÁN2 i distance being longer than N1Á Á ÁN1 i by 0.32 Å [ Table 1, symmetry code: (i) Àx, Ày, z], as found only in 12% of cases (88%: Á < 0.32 Å ; 12%: Á > 0.32 Å ). This is probably due to the fact that the Maltol units linked to the nitrogen atoms are involved in chelate six-membered rings, which stiffen the system and force those nitrogen atoms to move farther apart.
The mean planes of the two maltolate rings of the neutral [Co(H -2 L1)] moiety form a dihedral angle of about 55 , while the dihedral angle between the N1,N2,N1 i ,N2 i [symmetry code: (i) Àx, Ày, z] and maltolate ring mean planes is about 63 . The distance between the maltolate ring centroids is 7.8463 (3) Å . The dimension of the binding area defined by the four oxygen donor atoms of the ligand is roughly estimated by the distance separating the opposite O1Á Á ÁO2 i [symmetry code: (i) Àx, Ày, z] atoms (and the other symmetry-related oxygen atoms), which is 4.315 (6) Å . Notably, such a distance is longer than those retrieved for analogous trinuclear complexes (opposite OÁ Á ÁO distances range: 3.98-4.22 Å ; Benelli et al., 2013;Rossi et al., 2017), while it is shorter than those retrieved for the one-dimensional coordination polymer of L1 (opposite OÁ Á ÁO distances: 4.5 Å ; Paoli et al., 2017) and the mononuclear complex of L1 (opposite OÁ Á ÁO distances: 4.49 Å ; Borgogelli et al., 2013). As for the dinuclear complex of L1 , the opposite OÁ Á ÁO distances of the binding area are quite different from each other (4.12 and 4.42 Å ), and are, respectively, shorter and longer than the corresponding distance in the title compound.
The coordination polyhedron around the Ca 2+ ion can be described as a distorted trigonal dodecahedron (Muetterties & Guggenberger, 1974), with all eight deprotonated hydroxyl and carbonyl oxygen atoms of the two [Co(H -2 L1)] moieties of the trinuclear complex situated at the corners of the polyhedron (Fig. 2, right). The maltolate unit acts as a bidentate ligand through the hydroxyl oxygen atom, which bridges the Ca II and Co II cations. All the Ca-O distances are in agreement with data found in the CSD.
The Co 2+ and Ca 2+ cations are located 3.727 (1) Å apart from each other and, because of the symmetry of the system, the line connecting the three cations (Co II -Ca II -Co II ) is normal to the mean plane described by the four nitrogen atoms of the macrocycle (Fig. 1). The values for the CoÁ Á ÁCa distance and the Co-O1-Ca angle are in agreement with data ranges found in the CSD, even if they fall in non-populated regions (only ten hits -corresponding to twenty distances or angle values -are retrieved when the Co-O-Ca fragment is searched). The CoÁ Á ÁCo ii distance and the Co-Ca-Co ii angle value [symmetry code: (ii) y, Àx, Àz] can only be compared with the single hit containing a cobalt-2 -oxygen- Table 1 Selected bond lengths and angles (Å , ).

Figure 2
Coordination polyhedra around the cobalt (left) and calcium (right) ions.
In the present structure and in all the Co-containing structures of L1 published up to now, the cobalt complexes are well superimposable with each other, but for that belonging to the Na I -Co II heterodinuclear complex (r.m.s. deviation values of 0.788 Å and within 0.301 Å for the superimposition of the title compound with the Na I -Co II complex and with all other structures, respectively), where the two maltolate rings show a different arrangement, both rings being tilted toward the same direction (instead of opposite directions) with respect to the cobalt-2 -oxygen-hard metal mean plane (M = Na I , Ca II , Ba II , Gd III , Eu III ; in the case of the mononuclear Co II species, with respect to the cobalt-2 -oxygen mean plane; Fig. 4). Moreover, when considering the heterotrinuclear complexes only, the superimposition of the Co II -Ca II -Co II dimer with the whole structures of the Co II -Ln III -Co II dimers (Ln III = Gd III , Eu III ) shows high r.m.s. deviation values (1.7 Å ), in agreement with a different mutual disposition of the two subunits in the dimers.  Comparison between the overall shapes of the present structure and the other Co-containing structures of L1. Top line, from left to right: Co II -Ca II -Co II , Co II -Eu III -Co II (Rossi et al., 2017), Co II -Gd III -Co II (refcode: FEZBUJ) complexes; bottom line, from left to right: Co II species (refcode: WELGEB), Ba II -Co II coordination polymer (refcode: ZELBAW), Na I -Co II complex (refcode: WELGOL).
The electron-rich area, which forms following the cobaltdriven preorganization of L1, is able to host hard metal ions with different dimensions and coordination requirements, leading to complexes having different stoichiometry (monoand dinuclear monomers and trinuclear dimers) or even a polymeric structure (Fig. 4). In the case of the Na I -Co II structure, a monomer forms, probably because of the lower ionic charge and coordination number (CN) of the Na I cation (CN: 5, Na + ionic radius: 1.00 Å ; Shannon, 1976) with respect to the other cations. Indeed, the low ionic charge and coordination number allow the stabilization of the ion with only one [Co(H -2 L1)] moiety. In the case of the Ba II -Co II structure, the Ba II cation shows the highest coordination number (CN: 9, Ba 2+ ionic radius: 1.47 Å ; Shannon, 1976)

Supramolecular features
In the crystal, the heterotrinuclear Co II -Ca II -Co II complexes are connected in the three dimensions via weak C-HÁ Á ÁO hydrogen bonds (Desiraju & Steiner, 1999).
The perchlorate anion interacts with five complexes: four out of five (magenta in Figs. 5 and 6) are connected to form a layer perpendicular to the c axis, the fifth complex also belongs to a layer (blue in Figs. 5 and 6) perpendicular to the c axis, adjacent layers being staggered relative to one other (Fig. 6). All interactions are weak C-HÁ Á ÁO-Cl hydrogen bonds (Table 2)  Crystal packing of the title compound viewed along the a axis. Staggered layers of complexes (in magenta and blue) perpendicular to the c axis are present, which are interconnected thanks to hydrogen bonds in the c-axis direction. The perchlorate anions are located between the layers. Interactions with water molecules are also shown. Hydrogen bonds involving ClO 4 À anions are depicted as light-blue dotted lines. Hydrogen bonds involving water molecules are depicted as green (along the a axis) and red (along the b axis) dotted lines. The ClO 4 À anions and water molecules are depicted in ball-and-stick mode.

Figure 6
Crystal packing of the title compound viewed along the c axis. Staggered layers of complexes (in magenta and blue) perpendicular to the c axis are visible. The ClO 4 À anions and water molecules are depicted in ball-andstick mode. Table 2 Hydrogen-bond geometry (Å , ).
Water molecules also interact with the complexes via weak C-HÁ Á ÁO hydrogen bonds (Table 2) along the a and b axes (Fig. 5). These interactions also involve the methylene hydrogen atoms of the macrocycle.

Synthesis and crystallization
Compound L1 was obtained following the previously reported synthetic procedure (Amatori et al., 2012).
To obtain the title compound, {Ca[Co(H -2 L1)] 2 }Á-2ClO 4 Á1.36H 2 O, 0.1 mmol of CoCl 2 Á 6H 2 O in water (10 ml) were added to an aqueous solution (20 ml) containing 0.1 mmol of L1Á3HClO 4 ÁH 2 O. The solution was adjusted to pH 7 with 0.1 M N(CH 3 ) 4 OH and then 0.05 mmol of CaCl 2 were added. The solution was saturated with NaClO 4 . The title compound quickly precipitated as a microcrystalline pink solid. Crystals suitable for X-ray analysis were obtained by slow evaporation of a more diluted aqueous solution.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3.
All hydrogen atoms of the macrocycle were positioned geometrically and refined as riding with C-H = 0.95-0.99 Å with U iso (H) = 1.5U eq (C-methyl) and = 1.2U eq (C) for other H atoms.
The perchlorate anion is disordered about a twofold rotation axis and was refined giving the two positions a fixed occupancy factor of 0.5. The chlorine atom is located on a twofold rotation axis.
The oxygen atom of the water molecule lies on a twofold rotation axis, the refined occupancy factor is 0.34 (2); the hydrogen atoms were not found in the difference-Fourier map and they were not introduced in the refinement.
All non-hydrogen atoms were refined anisotropically: as for the disordered perchlorate anion, the SIMU instruction was used to restrain the anisotropic displacement parameters of the disordered atoms, while the ISOR instruction was used to restrain the anisotropic displacement parameters of the isolated water oxygen atom.

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. Refinement. Refined as a 2-component inversion twin.