Crystal structure of the BaII-based CoII-containing one-dimensional coordination polymer poly[[aqua{μ4-2,2′-[(4,10-dimethyl-1,4,7,10-tetraazacyclododecane-1,7-diyl)bis(methylidene)]bis(4-oxo-4H-pyran-3-olato)}perchloratocobaltbarium] perchlorate]

A barium-μ2-oxygen motif develops along the a axis, connecting symmetry-related dinuclear BaII–CoII cationic fragments in a wave-like chain, forming a one-dimensional metal coordination polymer. Non-coordinating ClO4 − anions are located in the space between the chains in this first example of a macrocyclic ligand forming a BaII-based one-dimensional coordination polymer, containing CoII ions surrounded by a N4O2 donor set.

The title compound, {[Ba{Co(H -2 L1)}(ClO 4 )(H 2 O)]ClO 4 } n , L1 = 4,10-bis[(3hydroxy-4-pyron-2-yl)methyl]-1,7-dimethyl-1,4,7,10-tetraazacyclododecane, is a one-dimensional coordination polymer. The asymmetric unit consists of a {Ba[Co(H -2 L1)](ClO 4 )(H 2 O)} + cationic fragment and a non-coordinating ClO 4 À anion. In the neutral [Co(H -2 L1)] moiety, the cobalt ion is hexacoordinated in a trigonal-prismatic fashion by the surrounding N 4 O 2 donor set. The Ba 2+ ion is nine-coordinated and exhibits a distorted [BaO 9 ] monocapped square-antiprismatic geometry, the six oxygen atoms coming from three distinct [Co(H -2 L1)] moieties, while the remaining three vertices are occupied by the oxygen atoms of a bidentate perchlorate anion and a water molecule. A barium-2 -oxygen motif develops along the a axis, connecting symmetry-related dinuclear Ba II -Co II cationic fragments in a wave-like chain, forming a onedimensional metal coordination polymer. Non-coordinating ClO 4 À anions are located in the space between the chains. Weak C-HÁ Á ÁO hydrogen bonds involving both coordinating and non-coordinating perchlorate anions build the whole crystal architecture. To our knowledge, this is the first example of a macrocyclic ligand forming a Ba II -based one-dimensional coordination polymer, containing Co II ions surrounded by a N 4 O 2 donor set.

Chemical context
Metal coordination polymers (CPs) have witnessed continuous growth, owing to their fascinating structural diversity in terms of architecture and topology and also their numerous potential applications, such as gas storage (Banerjee et al., 2016;Fracaroli et al., 2014;Sumida et al., 2012;Suh et al., 2012), chemical sensing (Campbell et al., 2015;Hu et al., 2014;Wang et al., 2013;Kreno et al., 2012), catalysis (Chughtai et al., 2015;Mo et al., 2014;Yoon et al., 2012;Liu, Xuan et al., 2010) and so forth. Recently, the interest in alkaline-earth metal ion-based CPs has been growing due to their unusual advantages such as low toxicity, wide distribution and low cost, which are of benefit for applications in the field of materials science (Raja et al., 2014;Foo et al., 2012Foo et al., , 2013Xiao et al., 2012).
Besides, the ability of a system to bind alkaline-earth metal ions in aqueous solution is highly desirable and can be achieved thanks to the presence of oxygenated ligands and the preorganization of the receptor, which satisfies the need for a high coordination number without specific coordination requirements.
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) and is able to form discrete heteropolynuclear complexes. It has already proved to able form a Co II species  that is able to bind hard metal ions such as Ln III (Ln = Gd, Eu) and Na(I). In the case of Ln III ions, heterotrinuclear Co II -Ln III -Co II systems form, where the Co II cation preorganizes the system and two Co II species are involved in the coordination of one Ln III ion (Benelli et al., 2013;Rossi et al., 2017). In the case of the alkaline ion, a heterodinuclear complex forms, involving only one Co II species .
Herein we present a Ba II -Co II heterodinuclear metal coordination compound of L1, where a one-dimensional wave-like infinite array of barium ions bridges the [Co(H -2 L1)] moieties through a barium-2 -oxygen motif. This is the first time that L1 has proven able to form a coordination polymer and, to our knowledge, this is the first example of a macrocyclic ligand forming a Ba II -based 1D-CP containing Co II ions surrounded by an N 4 O 2 donor set.
In the neutral [Co(H -2 L1)] moiety, the Co 2+ ion is hexacoordinated and exhibits a distorted trigonal-prismatic geometry (Muetterties & Guggenberger, 1974), where the cobalt ion is surrounded by four nitrogen atoms of the macrocyclic base and two deprotonated hydroxyl oxygen atoms provided by both maltolate rings of the ligand. In the distorted trigonal prism, the O1,N2,N3/O4,N1,N4 atoms define the two triangular faces, which are parallel within 12.51 (11) (Fig. 2). The cobalt ion is displaced 1.0971 (5) Å above the mean plane described by the four nitrogen atoms of the tetraazamacrocycle [maximum deviation of 0.068 (4) Å for N3] and falls, together with the Co-N(CH 3 ) and Co-O bond distances (  Benelli et al., 2013;Borgogelli et al., 2013;Rossi et al., 2017]. 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 the N2Á Á ÁN4 distance being longer than the N1Á Á ÁN3 one by 0.26 Å (Table 1), as found only in 36% of cases. This is probably due to the fact that the Maltol units linked to atoms N2 and N4 are involved in chelate sixmembered rings, which stiffen the system and force those nitrogen atoms to move farther apart.
The two maltolate rings are almost orthogonal to each other (dihedral angle between ring mean planes about 71 ); both rings form similar angles (about 55 ) with the mean plane N1,N2,N3,N4. The dimensions of the binding area defined by the four oxygen donor atoms of the ligand, as roughly estimated by the distances separating the opposite O1Á Á ÁO5 and O2Á Á ÁO4 atoms, are quite similar (about 4.5 Å ).
The Ba 2+ and Co 2+ cations are located 3.9799 (7) Å apart from each other, the line connecting them being normal to the mean plane described by the four nitrogen atoms of the macrocycle [angle value: 87.59 (7) ; Fig. 1]. As for the bridged Co-O-Ba moiety (Fig. 3, right), while the Ba-O and Co-O bond distances and the BaÁ Á ÁCo distance are in agreement with those found in the CSD, the corresponding Ba-O-Co angles (Table 1) are outside the observed range (89.5-111.4 ).

Supramolecular features
The title compound forms wave-like chains with a repeating unit comprising a dinuclear Ba II -Co II cationic fragment with associated coordinating water molecules and perchlorate ions (Fig. 4). Non-coordinating ClO 4 À anions are located in the space between the chains.

Figure 3
Fragments searched in the CSD.  (Desiraju & Steiner, 1999) involving both coordinating and non-coordinating perchlorate anions build the whole crystal architecture (Table 2). Distinct 1D-CPs are held together by weak C-HÁ Á ÁO interactions between the coordinating perchlorate anions belonging to a CP and methylene hydrogen atoms belonging to the adjacent CPs (Fig. 5).

Database survey
Five structures containing L1 were found in a search of the CSD (Version 5.38, May 2017; Groom et al., 2016), three of them containing Co II : a hetero-trinuclear Gd III -Co II -Gd III dimer, a hetero-dinuclear Na I -Co II complex and a Co II complex (Benelli et al., 2013;Amatori et al., 2012;. In addition, our group recently published the    Wave-like one-dimensional Ba II -based coordination polymer that develops along the a axis. The oxygen and barium atoms belonging to the barium-2oxygen motif are depicted in ball and stick mode. Only one component of the disordered perchlorate anion and water molecule is shown. H atoms and the non-coordinating ClO 4 À anions have been omitted for clarity. [Symmetry codes: (i) Àx + 1, Ày, Àz; (ii) Àx + 2, Ày, Àz.] corresponding hetero-trinuclear Eu-Co-Eu dimer (Rossi et al., 2017). A general search for structures containing both Co II and Ba II ions revealed 61 hits, 20 of which are polymeric structures formed by organic ligands containing both oxygen and nitrogen donor atoms and only two being 1D-CPs. It is noteworthy that none of the 20 structures contains either macrocyclic ligands or an N 4 O 2 donor set around the Co II ion. In eight out of those 20 polymeric structures, the Ba II and Co II ions are bridged by oxygen atoms and ten out of 20 show oxygenbridged Ba II ions (only eight forming an infinite chain). Finally, only six out of the 20 polymeric structures contain both oxygen-bridged Ba II ions and oxygen-bridged Ba II and Co II ions.
All these data suggest that structures containing both oxygen-bridged Ba II ions and oxygen-bridged Ba II and Co II ions are not common and that no Ba II -based 1D-CPs formed by macrocyclic ligands and containing Co II ions surrounded by an N 4 O 2 donor set are present in the CSD.

Synthesis and crystallization
Compound L1 was obtained following the synthetic procedure previously reported (Amatori et al., 2012).
To obtain the Ba II -based Co II -containing 1D-CP of L1, {{Ba[Co(H -2 L1)](ClO 4 )(H 2 O)}ÁClO 4 } n , 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 BaCl 2 Á 2H 2 O were added. The solution was saturated with NaClO 4 . The Ba II -Co II 1D-CP of L1 quickly precipitated as a microcrystalline pink solid. Crystals suitable for X-ray analysis were instead 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 (Cmethyl) and = 1. , the hydrogen atoms were not found in the Fourier-difference map and they were not introduced in the refinement. All non-hydrogen atoms were anisotropically refined: as for the disordered perchlorate anions, the SIMU instruction was used to restrain the anisotropic displacement parameters of the disordered atoms, while the ISOR instruction was used to model the disordered water oxygen atoms.