Slow Magnetic Relaxation in Dinuclear CoIIYIII Complexes

Four new dinuclear complexes, Co(μ-L)(μ-CCl3COO)Y(NO3)2·2CHCl3·CH3CN·2H2O (1), Co(μ-L)(μ-CH3COO)Y(NO3)2·CH3CN (2), Co(μ-L)(μ-PhCOO)Y(NO3)2·3CH3CN·2H2O (3), and Co(μ-L)(μ-tBuCOO)Y(NO3)2·CHCl3·2H2O (4), having a CoIIYIII core, have been synthesized by employing a ferrocene based compartmental ligand which was synthesized by the reaction of diacetyl ferrocene with hydrazine hydrate followed by a condensation reaction with o-vanillin. A general synthetic protocol was employed to synthesize complexes 1-4, where the metallic core was kept the same with changing the bridging carboxylate groups. In all the complexes, the main structural motif is kept similar by only slightly varying the substitution on the bridging acetate groups. This variation has resulted in a small but subtle influence on the magnetic relaxation of all these four compounds. Ab initio CASSCF/NEVPT2 calculations were carried out to assess the effect of the different substitutions of the bridging ligands on the magnetic anisotropy parameters and on orbital arrangements. Ab initio calculations yield a very large positive D value, which is consistent with the geometry around the CoII ion and easy plane anisotropy (gxx, gyy > gzz), with the order of the calculated D in the range of 72.4 to 91.7 cm-1 being estimated in this set of complexes. To ascertain the sign of zero-field splitting in these complexes, EPR spectra were recorded, which support the sign of D values estimated from ab initio calculations.