Microwave and traditional solvothermal syntheses, crystal structures, mass spectrometry and magnetic properties of CoII4O4 cubes

Abstract

We report a study of the comparative solvothermal syntheses, using traditional and microwave ovens, of two complexes [Co₄(μ₃-OMe)₄(L)₄(MeOH)₄], where (HL = Hhmb, 2-hydroxy-3-methoxy-benzaldehyde (1) or Hheb = 2-hydroxy-3-ethoxy-benzaldehyde (2)), their crystal structures and magnetic properties in the solid state and the ESI-MS of the solutions. The microwave heating is more efficient in time and quality of crystals than the traditional one where what is achieved per minute by the former takes an hour by the latter. The structures consist of stacks of high-symmetry [Co₄(μ₃-OMe)₄] cubanes in tetragonal space groups. The inner cores of the clusters have almost the same geometry but the distances between them are longer for the ethoxy than the methoxy derivative. The magnetic interaction within the cluster is ferromagnetic in both cases but spin–orbit competes with it. The different isothermal magnetizations at low temperatures may be due to different intercluster coupling. Electrospray ionization mass spectrometry (ESI-MS) from methanol solution reveals an exchange of methoxide for hydroxide and in ethanol of methoxide for ethoxide at different proportions suggesting a probable “step by step” substitution. Interestingly, besides the [Co₄] cubane a number of higher nuclearity species [Co₅] and [Co₇] were also observed but are not expected under electrospray conditions. This can easily happen by the re-assembly of the smaller fragments produced by ionization process. This unique feature opens up new possibilities for the systematic screening of reactions in search for new cluster architectures, which may then be realised in the solid state.