Influence of the Ligand Field on Slow Magnetization Relaxation versus Spin Crossover in Mononuclear Cobalt Complexes

Abstract

The electronic and magnetic properties of the complexes [Co(terpy)Cl2 ] (1), [Co(terpy)(NCS)2 ] (2), and [Co(terpy)2 ](NCS)2 (3) were investigated. The coordination environment around Co(II) in 1 and 2 leads to a high-spin complex at low temperature and single-molecule magnet properties with multiple relaxation pathways. Changing the ligand field and geometry with an additional terpy ligand leads to spin-crossover behavior in 3 with a gradual transition from high spin to low spin.

Magnetic anisotropy in two- to eight-coordinated transition–metal complexes: Recent developments in molecular magnetism

“…The d orbital energies of these complexes were computed using DFT, which showed that no partially filled degenerate or almost degenerate orbitals were present. Detailed ab initio calculations for 70 indicated significantly high anisotropy, which was supported by HF-EPR studies performed using a powder sample, where D = 100 cm -1 , g x = 1.35, g y = 1.93, and g z = 7.75 [85]. Both complexes exhibited modest field-induced SMM-behavior (Table 5).…”

“…This suggests that other relaxation mechanisms due to environmental degrees of freedom might operate, such as Raman, hyperfine, and dipolar coupling. The detailed structural and magnetic features of related terpyridine-chelated Co II complexes have also been reported [85]. [Co II (terpy)Cl 2 ] (70) has a distorted square-based TP geometry with C s symmetry while [Co II (terpy)(NCS) 2 ] (71) possesses a distorted TBP geometry with C 2v symmetry (Fig.…”

“…Penta-coordinated complexes usually possess trigonal bipyramidal (ideal and distorted cases) [52,[82][83][84][85], tetragonal pyramidal [86,87], and distorted square-based tetragonal pyramidal [84][85][86]88] geometries (Scheme 5). The magnetic anisotropy of a metal ion was expected in this environment and it was demonstrated as early as in 1975 [89][90][91].…”

“…The crystal field d orbital splitting patterns for distorted TBP or TP geometries depend mainly on the degree of distortion as well as the ligands involved. Nevertheless, the qualitative splitting patterns for transition metal ions with distorted TBP geometry and C 2v symmetry [85,94] as well as with distorted square-based TP geometry and C 4v symmetry [86] are shown in Scheme 5 to provide an overview of how the relative positions of the d orbitals change after the modification of the coordination geometry. Scheme 5.…”

Effect of Low Spin Excited States for Magnetic Anisotropy of Transition Metal Mononuclear Single Molecule Magnets

“…Concretely, complete active space self-consistent field (CASSCF) calculations permitted the direct calculation of magnetic anisotropy, which is the physical phenomenon responsible for the development of an energy barrier blocking magnetic relaxation. Nowadays, CASSCF calculations are widely applied for the rationalization of magnetic properties of mononuclear single molecule magnets based on transition metal ions and lanthanides [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. Besides energies and wave functions, ab initio calculations can be analyzed in terms of Spin-Hamiltonian and ligand field parameters [19][20][21].…”