Single-ion magnet behaviour in homoleptic Co(II) complexes bearing 2-iminopyrrolyl ligands

In this report we present the structural and magnetic characterization of four distorted tetrahedral homoleptic Co(II) complexes bearing two 2-formiminopyrrolyl N,N'-chelating ligands, [Co{kappa N-2,N'-NC4H3-2-C(H)-N(2,6-Pr-i(2)-C6H3)}(2)] (1), [Co{kappa N-2,N'-5-(C6H5)-NC4H2-2-C(H)-N(2,6-Pr-i(2)-C6H3)}(2)] (2), [Co{kappa N-2,N'-5-(2,6-Me-2-C6H3)-NC4H2-2-C(H)= N(2,6-Pr-i(2)-C6H3)}(2)] (3) and [Co{kappa N-2,N'-5-(1-Ad)-NC4H2-2-C(H)= N(1-Ad)}(2)] (Ad = adamantyl) (4), which display Single-Ion Magnet (SIM) behaviour. Static (dc) magnetic susceptibility measurements and high-field EPR spectroscopy showed a large and negative magnetic anisotropy with values of D = -69, -53, -48 and -52 cm(-1) for complexes 1-4, respectively. These values are interpreted and reproduced by means of theoretical calculations (ab initio CASSCF/QD-NEVPT2 methods) where it was shown that the most important source of axial anisotropy stems from the first e -> t(2) electronic transition, in line with other tetrahedrally coordinated Co(II) complexes. Calculations on model systems show that the most favorable magnetostructural modification corresponds to a tetrahedral geometry with a strong distortion towards a trigonal based pyramid. Frequency-dependent (ac) magnetic susceptibility measurements show that the 5-substituted pyrrolyl ring derivatives 2-4 display slow relaxation of the magnetization at zero external magnetic field, whereas the 5-unsubstituted-2-iminopyrrolyl complex 1 requires the presence of a static magnetic field to exhibit this property. By applying a static magnetic field, the quantum tunnelling of magnetization (QTM) process is suppressed revealing large energy barriers (U-eff) for all the complexes studied, exhibiting values of 138, 106, 96 and 104 cm(-1) for 1-4, respectively. These values are higher than the majority of tetracoordinated Co(II)-based SIMs reported in the literature. Despite large values of axial zero-field splitting, as determined by theory, the experimental energy barriers are considerably lower than expected for a pure Orbach process, indicating that other relaxation mechanisms are dominant in the range of temperatures studied.