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 (μ 3 -OH) 6 (L) 6 ](ClO 4 ) 2 ·12H 2 O ( 1 ), [Co 7 (μ 3 -CH 3 O) 6 (L) 6 ](ClO 4 ) 2 ( 2 ), and [Co 7 (μ 3 -N 3 ) 6 (L) 6 ](ClO 4 ) 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 Co 7 brucite disk [Co 7 (μ 3 -X) 6 (μ 2 -O) 6 ] 2+, where the ligands are arranged in two open hemispheres and the flat inner surface is functionalized when X is OH −, CH 3 O −, 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 μ 3 -CH 3 O − and N 3 − are replaced partially with μ 3 -OH −, indicating that μ 3 -OH − has a greater affinity than μ 3 -CH 3 O − or N 3 − for Co II, 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 μ 3 -O or μ 3 -N provides ferromagnetic coupling between nearest neighbors in all cases. Interestingly, for μ 3 -N, it appears to be stronger than μ 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 .