A mixed-valence [CoII 4CoIII 2] cluster with defect disk-shaped topology

A mixed-valence Co6 cluster was synthesized and characterized. It exhibits a defect disk-shaped architecture. DC magnetic property studies in the 2.0–300 K range revealed antiferromagnetic interactions between the CoII ions.

Several synthetic methodologies towards polynuclear cobalt clusters were established and one of the most efficient approaches involves the employment of hydroxy-containing Schiff base ligands. Schiff base ligands are easy to synthesize and their steric properties can be tuned by varying the size of the amine or carbonyl substituents (Qin et al., 2017Ge et al., 2018;Li et al., 2021). More importantly, the hydroxy moieties of the Schiff base ligands can combine many metal ions with -O bridges, resulting in the formation of large polynuclear clusters.

Synthesis of complex 1 and IR spectral analysis
The reaction of H 2 L and Co(CH 3 COO) 2 Á4H 2 O in MeOH in the presence of NEt 3 under solvothermal conditions led to the isolation of 1 in moderate yield. Co(CH 3 COO) 2 Á4H 2 O is a good starting material because it not only serves as a convenient metal source, but also provides CH 3 COO À bridging ligands. In the solid state, complex 1 is stable in air and its elemental analysis is consistent with the given molecular formula.
The vibrational bands in the IR spectrum agree well with the formulation of complex 1 (see Fig. S1 in the supporting information). The signals of the carboxyl as (CO 2 ) and s (CO 2 ) vibrations were found in the 1637-1419 cm À1 range. The vibrations of the C N bond appear at 1450 cm À1 . Several bands in the 1286-1185 cm À1 range were assigned to the vibrations of the aromatic rings. The sharp signals in the 979-766 cm À1 range were ascribed to the vibrations of C-H bonds.

Structure description of 1
Single crystals of 1 were obtained from MeOH under solvothermal conditions. Complex 1 crystallized in the orthorhombic space group P2 1 2 1 2 1 . The structure is shown in Fig. 1. The structure analysis shows that complex 1 is composed of six cobalt ions, four 2-[(4-chloro-2-oxidobenzylideneamino)methyl]phenolate (L 2À ) ligands, two acetate ligands and four methanol-solvent-derived MeO À ligands. There exists an approximate C 2 symmetry in the molecule. The imine N atom and both phenolate O-atom donors of each L 2À ligand coordinate each cobalt centre. Bond valence calculations (Brese & O'Keeffe, 1991;Brown & Altermatt, 1985)   and Co6 ions are in 2+ oxidation states. The formation of four fused defect cubes confirms the involvement of four methanolsolvent-derived 3 -O À groups, giving the Co 6 O 10 structure. Thus, the molecular structure of 1 displays a defect diskshaped topology [ Fig. 1 The geometries of the five-coordinated Co2 and Co6 atoms were analyzed with the program SHAPE (Version 2.0; Pinsky & Avnir, 1998). The calculated values revealed trigonal bipyramid (D 3h ) geometry for both atoms, with a minimum CShM (contunuous shape measure) value of 1.065 for Co2 and 1.172 for Co6.

Magnetic properties of 1
Magnetic susceptibility data as a function of temperature for complex 1 are shown in Fig. 2. The room temperature M T value is 10.96 cm 3 mol À1 K, which is greater than the value of 7.50 cm 3 mol À1 K for four uncoupled S = 3/2 Co II centres, possibly owing to the orbital contributions of the metal ions (Cao et al., 2013). Upon lowering the temperature, the M T value drops slightly to a minimum value of 3.49 cm 3 mol À1 K at 2 K, which suggests possible antiferromagnetic couplings between the unpaired spins. The data of 1/ M in the temperature range 2-300 K were fitted by the Curie-Weiss Law of 1/ M = (T À )/C. The Curie constant C = 12.09 cm 3 mol À1 K and the Weiss constant = À37.24 K were obtained. The negative value proves the antiferromagnetic interactions.
The magnetic dynamic behaviour of 1 was also explored. The ac magnetic susceptibilities for 1 at 1000 Hz under a zerodc field in the temperature range 2-25 K were shown in Fig. S2 (see supporting information). The 00 susceptibilities at 1000 Hz did not increase upon lowering the temperature and no peaks were determined. These phenomena revealed that complex 1 is not a single-molecule magnet.

Conclusion
A hexanuclear cobalt complex of composition [Co 2 III Co 4 II -(L) 4 (CH 3 COO) 2 (MeO) 4 ] (1), based on the hydroxy-containing Schiff base ligand 2-[(4-chloro-2-hydroxybenzylideneamino)methyl]phenol (H 2 L) was prepared and characterized. Complex 1 exhibits a defect disk-shaped topology. Four cobalt ions are six-coordinated and two cobalt ions are five-coordinated. An investigation of the magnetic properties revealed that there exist antiferromagnetic interactions between the Co II ions.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. SQUEEZE, constraints & restraints, applied to the data, 17 reflections OMITted Data for 1 was collected on a Bruker SMART APEXII diffractometer equipped with a graphite monochromator utilizing Mo Kα radiation (λ = 0.71073). The crystal structure of 1 was solved with the OLEX2 program (Dolomanov et al., 2009) and refined by SHELXL package (Sheldrick, 2015). The crystal structure of 1 contained disordered solvent that could not be satisfactorily refined. The SQUEEZE routine of PLATON was used in the treatment of the crystallographic data. CCDC-1991979 (1) contains the supplementary crystallographic data for this paper. The data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Centre, 12 Union Road, Cambridge CB21EZ, UK; Fax: (+44) 1223-336-033; or deposit@ccdc.cam.ac.uk). The crystallographic data was shown in Table 1. Selected bond lengths and bond angles of 1 are listed in Table 2.