Single-ion magnet behaviour in homoleptic Co() 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{κ 2 N , N ′-NC 4 H 3 -2-C(H)&z.dbd;N(2,6- i Pr 2 -C 6 H 3 )} 2 ] ( 1 ), [Co{κ 2 N , N ′-5-(C 6 H 5 )-NC 4 H 2 -2-C(H)&z.dbd;N(2,6- i Pr 2 -C 6 H 3 )} 2 ] ( 2 ), [Co{κ 2 N , N ′-5-(2,6-Me 2 -C 6 H 3 )-NC 4 H 2 -2-C(H)&z.dbd;N(2,6- i Pr 2 -C 6 H 3 )} 2 ] ( 3 ) and [Co{κ 2 N , N ′-5-(1-Ad)-NC 4 H 2 -2-C(H)&z.dbd;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. Four-coordinate distorted tetrahedral bis(2-iminopyrrolyl)cobalt( ii ) complexes behave as Single-Ion Magnets (SIMs) in the absence of an external magnetic field.