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

We present the comparative preparation of [Co-7(mu(3)-OH)(6)(L)(6)](ClO4)(2)center dot 12H(2)O (1), [Co-7(mu(3)-CH3O)(6)(L)(6)](ClO4)(2) (2), and [Co7( mu(3)-N-3)(6)(L)(6)](ClO4)(2) (3), where HL is 2-methoxy-6-[(methylimino)methyl]phenol, using traditional (e.g., 120 degrees C, 120 h, 23% yield for 1) and microwave-assisted (e.g., 120 degrees C, 10 min, 46% yield for 1) solvothermal synthesis. The structures contain Co-7 brucite disk [Co-7(mu(3)-X)(6)(mu(2)-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 N-3(-). 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 mu(3)-CH3O- and N-3(-) are replaced partially with mu(3)-CH3O-, indicating that mu(3)-CH3O- has a greater affinity than mu(3)-CH3O or N-3(-) for Coll, and the parental core of [Co-7(OH)(6)(L)(6))(2)(+) is the most stable of the three compounds in solution. The presence of edge-sharing octahedra through mu(3)-O or mu(3)-N provides ferromagnetic coupling between nearest neighbors in all cases. Interestingly, for mu(3)-N, it appears to be stronger than mu(3)-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.