Traditional and Microwave-Assisted Solvothermal Synthesis and Surface Modification of Co₇ Brucite Disk Clusters and Their Magnetic Properties

Abstract

We present the comparative preparation of [Co7(μ3-OH)6(L)6](ClO4)2·12H2O (1), [Co7(μ3-CH3O)6(L)6](ClO4)2 (2), and [Co7(μ3-N3)6(L)6](ClO4)2 (3), where HL is 2-methoxy-6-[(methylimino)methyl]phenol, using traditional (e.g., 120 °C, 120 h, 23% yield for 1) and microwave-assisted (e.g., 120 °C, 10 min, 46% yield for 1) solvothermal synthesis. The structures contain Co7 brucite disk [Co7(μ3-X)6(μ2-O)6]2+, where the ligands are arranged in two open hemispheres and the flat inner surface is functionalized when X is OH−, CH3O−, and N3−. The symmetry of the core decreases in the order 1 > 2 > 3, according to the shape, size, and rigidity of the inner bridges (X). These units are stacked into a chain, and for 1, the water molecules provide a hydrogen-bonded network through the hydroxyl groups. Interestingly, electrospray ionization mass spectrometry (ESI-MS) indicates that the heptacobalt(II) clusters of 1−3 exist in solution and the their compositions in solution are similar to those in the solid. However, the inner ligands μ3-CH3O− and N3− are replaced partially with μ3-OH−, indicating that μ3-OH− has a greater affinity than μ3-CH3O− or N3− for CoII, and the parental core of [Co7(OH)6(L)6]2+ is the most stable of the three compounds in solution. The presence of edge-sharing octahedra through μ3-O or μ3-N provides ferromagnetic coupling between nearest neighbors in all cases. Interestingly, for μ3-N, it appears to be stronger than μ3-O which resulted in single-molecule magnet (SMM) behavior at a higher temperature of 3 K, while they are below the limit of the SQUID magnetometers (2 K) in the case of 1 and 2.