Ferreira PS, Cerdeira AC, Cruz TFC, Bandeira NAG, Hunger D, Allgaier A, van Slageren J, Almeida M, Pereira LCJ, Gomes PT.

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{κ2N,N′-NC4H3-2-C(H)[double bond, length as m-dash]N(2,6-iPr2-C6H3)}2] (1), [Co{κ2N,N′-5-(C6H5)-NC4H2-2-C(H)[double bond, length as m-dash]N(2,6-iPr2-C6H3)}2] (2), [Co{κ2N,N′-5-(2,6-Me2-C6H3)-NC4H2-2-C(H)[double bond, length as m-dash]N(2,6-iPr2-C6H3)}2] (3) and [Co{κ2N,N′-5-(1-Ad)-NC4H2-2-C(H)[double bond, length as m-dash]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 → t2 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 (Ueff) 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.

Doi: 10.1039/D2QI00601D

Ferreira PS, Cerdeira AC, Cruz TFC, Bandeira NAG, Hunger D, Allgaier A, van Slageren J, Almeida M, Pereira LCJ, Gomes PT (2022) Single-ion magnet behaviour in homoleptic Co(ii) complexes bearing 2-iminopyrrolyl ligands. Inorganic Chemistry Frontiers, , . doi: 10.1039/D2QI00601D