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

Abstract

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)N(2,6-ⁱPr₂-C₆H₃)}₂] (1), [Co{κ²N,N′-5-(C₆H₅)-NC₄H₂-2-C(H)N(2,6-ⁱPr₂-C₆H₃)}₂] (2), [Co{κ²N,N′-5-(2,6-Me₂-C₆H₃)-NC₄H₂-2-C(H)N(2,6-ⁱPr₂-C₆H₃)}₂] (3) and [Co{κ²N,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 → 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 (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.