Single-ion magnet behaviour in homoleptic Co(ii) complexes bearing 2-iminopyrrolyl ligands. Issue 17 (17th June 2022)

• Record Type: Journal Article
• Authors: Ferreira, Patrícia S.; Cerdeira, Ana C.; Cruz, Tiago F. C.; Bandeira, Nuno A. G.; Hunger, David; Allgaier, Alexander; van Slageren, Joris; Almeida, Manuel; Pereira, Laura C. J.; Gomes, Pedro T.
• Is Part Of: Inorganic chemistry frontiers. Volume 9:Issue 17(2022)
• Page Start: 4302
• Page End: 4319
• Publication Date: 2022-06-17
• Subjects: Chemistry, Inorganic -- Periodicals; 546.05
• DOI: 10.1039/d2qi00601d
• Languages: English
• ISSNs: 2052-1553

Abstract

Abstract : Four-coordinate distorted tetrahedral bis(2-iminopyrrolyl)cobalt(ii ) complexes behave as Single-Ion Magnets (SIMs) in the absence of an external magnetic field. 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{κ 2 N, N ′-NC4 H3 -2-C(H)N(2, 6- i Pr2 -C6 H3 )}2 ] (1 ), [Co{κ 2 N, N ′-5-(C6 H5 )-NC4 H2 -2-C(H)N(2, 6- i Pr2 -C6 H3 )}2 ] (2 ), [Co{κ 2 N, N ′-5-(2, 6-Me2 -C6 H3 )-NC4 H2 -2-C(H)N(2, 6- i Pr2 -C6 H3 )}2 ] (3 ) and [Co{κ 2 N, N ′-5-(1-Ad)-NC4 H2 -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.