Spectroscopic and Magnetic Studies of Co(II) Scorpionate Complexes: Is There a Halide Effect on Magnetic Anisotropy?

Abstract

The observation of single-molecule magnetism in transition-metal complexes relies on the phenomenon of zero-field splitting (ZFS), which arises from the interplay of spin−orbit coupling (SOC) with ligand-field-induced symmetry lowering. Previous studies have demonstrated that the magnitude of ZFS in complexes with 3d metal ions is sometimes enhanced through coordination with heavy halide ligands (Br and I) that possess large free-atom SOC constants. In this study, we systematically probe this "heavy-atom effect" in high-spin cobalt(II)−halide complexes supported by substituted hydrotris(pyrazol-1-yl)borate ligands (Tp tBu,Me and Tp Ph,Me ). Two series of complexes were prepared: [Co II X(Tp tBu,Me )] (1-X; X = F, Cl, Br, and I) and [Co II X(Tp Ph,Me )(Hpz Ph,Me )] (2-X; X = Cl, Br, and I), where Hpz Ph,Me is a monodentate pyrazole ligand. Examination with dc magnetometry, high-frequency and -field electron paramagnetic resonance, and far-infrared magnetic spectroscopy yielded axial (D) and rhombic (E) ZFS parameters for each complex. With the exception of 1-F, complexes in the four-coordinate 1-X series exhibit positive D-values between 10 and 13 cm −1 , with no dependence on halide size. The five-coordinate 2-X series exhibit large and negative D-values between −60 and −90 cm −1 . Interpretation of the magnetic parameters with the aid of ligand-field theory and ab initio calculations elucidated the roles of molecular geometry, ligand-field effects, and metal−ligand covalency in controlling the magnitude of ZFS in cobalt−halide complexes.

Tuning Magnetic Anisotropy in Co(II) Tetrahedral Carbazole-Modified Phosphine Oxide Single-Ion Magnets: Importance of Structural Distortion versus Heavy-Ion Effect

“…The orbitals d xy , d yz , d xz are essentially nondegenerate for all three complexes, but the gap dramatically widens to ∼2100 cm –1 as we go from Cl/Br to I due to the higher degree of structural distortion in the 3 as can be seen from the XRD data. This further contributes to the observation of the geometry of the complex being the key factor in deciding the sign of D. Previous studies have also found that the impact of halide substitution on ZFS is dependent on multiple factors and is not the only factor that determines the trend in the D value of complexes. ,− …”

“…Thus, the effects of the heavy donor atom, halide ions, − and a secondary coordination sphere have been investigated to glean information for the rational design of newer SIMs. Fiedler and co-workers recently studied the “heavy-atom effect” in high-spin cobalt­(II)–halide complexes [Co II X­(Tp tBu,Me )] (Tp tBu,Me is sterically hindered tris­(pyrazolyl)­borato ligand), which have provided experimental basis to the fact that the choice of halide ions can modulate ZFS owing to the ligand field effects.…”

Synthesis, structure, and characterisation of a ferromagnetically coupled dinuclear complex containing Co(ii) ions in a high spin configuration and thiodiacetate and phenanthroline as ligands and of a series of isomorphous heterodinuclear complexes containing different Co : Zn ratios

“…The g-matrix was assumed to be isotropic as usual; interdimeric isotropic exchange interactions were neglected in our analysis (see Co-Co interactions section). Since the magnetic susceptibility data analysis with eqn (3) could be subjected to over-parametrisation, we conducted least squares fitting of eqn (3) to the data constraining the E/Dvalue obtained by EPR (see above) and leaving unconstrained g-, J-, and D-parameters. Best fitting parameters obtained were g = 2.595 (7), D = 61(1) cm −1 , and J = 1.39(4) cm −1 , which indicated an intradimer ferromagnetic coupling.…”

“…The physicochemical characterisation of high-spin (S = 3/2) Co (II) ions in coordination compounds is of continuing interest because of their relevance in solid-state physical chemistry. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] The high-spin Co(II) ion in these compounds presents magnetic and spectroscopic properties sensitive to the coordination environment, giving rise to countless applications. For instance, the large magnetic anisotropy of high-spin Co(II) complexes is essential for their use as molecular magnets since these compounds can form fundamental blocks with special and very exciting magnetic properties.…”

Spectroscopic techniques to probe magnetic anisotropy and spin–phonon coupling in metal complexes

“…However, it is an indirect method using best-fit analysis which may give unreliable results including the sign of D , unless multi-field and magnetization data are included. 37 The magnitude of the rhombic parameter E is even more difficult to predict without in-depth single-crystal analysis. The fits of DC susceptibility data also often require the inclusion of additional terms such as intermolecular interaction ( zJ ) and temperature independent paramagnetism (TIP) which may complicate the determination of ZFS parameters.…”