Magnetic Anisotropy in “Scorpionate” First‐Row Transition‐Metal Complexes: A Theoretical Investigation

Abstract

In this work we have analyzed in detail the magnetic anisotropy in a series of hydrotris(pyrazolyl)borate (Tp(-)) metal complexes, namely [VTpCl](+), [CrTpCl](+), [MnTpCl](+), [FeTpCl], [CoTpCl], and [NiTpCl], and their substituted methyl and tert-butyl analogues with the goal of observing the effect of the ligand field on the magnetic properties. In the [VTpCl](+), [CrTpCl](+), [CoTpCl], and [NiTpCl] complexes, the magnetic anisotropy arises as a consequence of out-of-state spin-orbit coupling, and covalent changes induced by the substitution of hydrogen atoms on the pyrazolyl rings does not lead to drastic changes in the magnetic anisotropy. On the other hand, much larger magnetic anisotropies were predicted in complexes displaying a degenerate ground state, namely [MnTpCl](+) and [FeTpCl], due to in-state spin-orbit coupling. The anisotropy in these systems was shown to be very sensitive to perturbations, for example, chemical substitution and distortions due to the Jahn-Teller effect. We found that by substituting the hydrogen atoms in [MnTpCl](+) and [FeTpCl] by methyl and tert-butyl groups, certain covalent contributions to the magnetic anisotropy energy (MAE) could be controlled, thereby achieving higher values. Moreover, we showed that the selection of ion has important consequences for the symmetry of the ground spin-orbit term, opening the possibility of achieving zero magnetic tunneling even in non-Kramers ions. We have also shown that substitution may also contribute to a quenching of the Jahn-Teller effect, which could significantly reduce the magnetic anisotropy of the complexes studied.

Combined Experimental and Theoretical Investigation of the Origin of Magnetic Anisotropy in Pentagonal Bipyramidal Isothiocyanato Co(II), Ni(II), and Fe(III) Complexes with Quaternary-Ammonium-Functionalized 2,6-Diacetylpyridine Bisacylhydrazone

“…LFDFT includes both dynamical correlation (through the DFT exchange-correlation energy) and non-dynamical correlation (via LF Configuration Interaction). With LFDFT procedure, all customary molecular properties can be calculated, [88][89][90][91][92][93][94] including ZFS parameters, 19,20,95,96 however, its accuracy decreases with increasing metal-ligand covalency. 97 Comparison between experimentally determined and calculated ZFS parameters in PBPY-7…”

“…Therefore, design, control and fine-tuning of the magnetic anisotropy is a challenging task. [13][14][15][16][17][18][19][20][21][22] It is noteworthy to mention that magnetic anisotropy is very sensitive to the surrounding of a central metal ion and is therefore also employed to explore the oxidation states, spin states and coordination modes of transition metal complexes. 23 Examples of systems with very large magnetic anisotropy are complexes of transition metals in linear geometry.…”

Variable Borohydride Hapticity in Nickel(II) Scorpionate Complexes [(TpR,Me)Ni(ηn‐BH4)]: TpR,Me = hydrotris{3‐R‐5‐methyl‐1‐pyrazolyl}borate; R = Ph, n = 3 vs. R = Me, n = 4

“…The pair of singly‐occupied e orbitals (SOMOs), nominally 3d xz and 3d yz , is destabilized by σ* interaction with the pyrazole nitrogens, as well as π* overlap with B–H bonds. C 3 v ‐symmetric chloride complex analogues adopt a similar electronic conformation, (e) 4 (a 1 ) 2 (e) 2 ,, wherein the chloride lone pairs are essentially isolobal with the B–H bonds, except that the 3d z ² orbital is relatively destabilized by overlap with the axially‐constrained chloride 3p z orbital , . The η 4 ‐borohydride ligand therefore acts as a six‐electron donor and the [(Tp Me,Me )Ni(η 4 ‐BH 4 )] complex has 20 valence electrons …”

Calculation of the Jahn-Teller parameters with DFT

“…9 They are essential in the design of single molecular magnets. [17][18][19] The vibronic coupling is responsible for the observation of symmetry-forbidden transitions in spectroscopy, 20 conical intersections in photochemistry, and is indispensable for understanding the spectroscopy of linear molecules. [21][22][23][24][25][26] The cause of any structural distortion of a polyatomic system is due to the JT, RT, PJT effect, or their combination.…”

“…It has been shown that controlling the geometry of a transition metal complex is the way to chemically control their magnetic properties, particularly that spin-orbit coupling (SOC) could be used to tune the magnetic anisotropy. 18,19 If the SOC is higher, the magnetic anisotropy has a greater value. Knowing the fact that both coordination of the ligands and the partial quenching of the SOC due to JT distortions reduce the magnetic anisotropy from its ideal, the free ion value, the system ([NiCl 3 (Hdabco) 2 ] + , dabco is 1,4-diazabicyclo[2.2.2]-octane), was found in which the bulky dabco axial ligands prevent distortion.…”