Structural, magnetic and spectral properties of tetrahedral cobalt(ii) silanethiolates: a variety of structures and manifestation of field-induced slow magnetic relaxation

Abstract

Blue crystals of five heteroleptic cobalt(II) silanethiolates 1–5 have been obtained by the reaction of [Co{SSi(tBuO)₃}₂(NH₃)]₂ with aminopyridines and aminomethylpyridines at an appropriate molar ratio and their structural, spectral, thermal and magnetic properties have been established and described. All complexes 1–5 contain Co(II) ions in a tetrahedral CoN₂S₂ environment formed by (tBuO)₃SiS⁻ residues and pyridines and present variable structures. Complexes 1–3 are mononuclear [Co{SSi(tBuO)₃}₂(L1)₂] (L1 = 2-aminopyridine 2AP, 3-aminopyridine 3AP, and 4-aminopyridine 4AP). The application of 3AMP and 4AMP (3-aminomethylpyridine and 4-aminomethylpyridine) allows either dinuclear complex 4 [Co{SSi(tBuO)₃}₂(μ-3AMP)]₂ or 1D coordination polymer 5 with the formula of [Co{SSi(tBuO)₃}₂(μ-4AMP)]_n to be obtained. The molecular structures of 1–5 were determined by single-crystal X-ray and powder diffraction, UV-vis and FTIR spectrocopy for solid samples and their thermal properties were characterized by TG-DSC and TG-FTIR methods. The dc and ac magnetic and EPR studies of polycrystalline samples have been performed. For all complexes, the obtained data show a behavior typical of paramagnetic high–spin Co(II) ions in a tetrahedral geometry, with a considerable contribution of the ZFS effect in a low temperature range. All complexes were also probed for SIM behavior. The modeling of the magnetic and EPR data was done for samples 1, 3, 4 and 5 to estimate ZFS parameters. The obtained results imply a negative value of the axial parameter D in complex 4 and positive D values for the rest of the compounds. A comparative magneto-structural analysis of complexes 4 and 5 points to the high sensitivity of the single-ion magnetic anisotropy of tetrahedral Co(II) complexes to subtle changes in the first and second coordination spheres of Co(II) ions.