Switching nuclearity and Co(II) content through stoichiometry adjustment: CoII6CoIII3 and CoIICo4III mixed valent complexes and a study of their magnetic properties

We are reporting two new mixed valent Co(II)/Co(III) polynuclear complexes, {(Co6Co3III)-Co-II} and {(CoCo4III)-Co-II}, bearing different amount of Co(II) ions in their cores, through the employment of the multidentate triethanolamine (teaH(3)) ligand in different stoichiometric ratios. We present a complete picture of the magnetic behaviour of both complexes through a combined usage of the susceptibility, magnetization and X-band EPR data as well as broken-symmetry DFT calculations. Compound 1 shows an atypical spin-only behaviour, probably due to the presence of four and five coordinated Co(II) sites as well as highly distorted six coordinated Co(II) ions, promoting a high degree of orbital contribution quenching. Through the usage of a simplified exchange coupling scheme and relying on DFT based magneto-structural correlation we have been able to explain the observed diamagnetic ground state. Concerning compound 2, DC magnetic data supported by X-band EPR measurements suggest the existence of anisotropy with a zero-field splitting parameter D, at least in the range of 2-10 cm(-1). In agreement with this description, a slow relaxation of magnetization is observed after applying a small external magnetic field, under AC measurements. Field and temperature dependence of the characteristic relaxation time establishes a thermal barrier for magnetization reversal of about 25 cm(-1), which is in good agreement with the energy splitting of the vertical bar +/- 1/2 > and vertical bar +/- 3/2 > doublets established from static magnetic measurements.