Cobalt(Ii)-Based Single-Ion Magnets With Distorted Pseudotetrahedral [N2o2] Coordination: Experimental And Theoretical Investigations

The synthesis and magnetic properties of cobalt(II) complexes with sterically demanding Schiff-base ligands are reported. The compounds [Co(L-Br)(2)] (1) and [Co(L-Ph)(2)]center dot CH2Cl2 (2 center dot CH2Cl2) are obtained by the reaction of cobalt(II) acetate with the ligands HLBr and HLPh in a dichloromethane/methanol mixture. 1 and 2 crystallize in the space groups P2(1)2(1)2 and PT, respectively. X-ray diffraction studies revealed mononuclear constitution of both complexes. For 1, relatively short intermolecular Co-Co distances of 569 pm are observed. In compound 2, a hydrogen-bonded dichloromethane molecule is present, leading to a solvent aggregate with remarkable thermal stability for which desolvation is taking place between 150 and 210 degrees C. Magnetic measurements were performed to determine the zero-field-splitting (ZFS) parameter D for both complexes. Frequency-dependent susceptibility measurements revealed Slow magnetic relaxation behavior with spin-reversal barriers of 36 cm(-1) for 1 and 43 cm(-1) for 2 at an applied:external field of 400 Oe. This observation is related to an increasing distortion of the pseudotetrahedral coordination geometry for complex 2. These distortions can be decomposed in two major contributions. One is the elongation effect described by the parameter epsilon(T), which is the ratio of the averaged obtuse and acute bond angles. The other effect is related to a twisting distortion of the chelate coordination planes at the cobalt center. A comparison with literature examples reveals that the elongation effect seems to govern the overall magnetic behavior in pseudotetrahedral complexes with two bidentate chelate ligands. Ab initio calculations for complexes 1 and 2 using the CASPT2 method show strong splitting of the excited T-4(2) term, which explains the observed strong ZFS. Spin orbit calculations with the RASSI-SO method confirm the single-molecule-magnet behavior because only small transversal elements are found for the lowest Kramers doublet for both complexes.