Influence of a Counteranion on the Zero-Field Splitting of Tetrahedral Cobalt(II) Thiourea Complexes

Abstract

Four mononuclear cobalt­(II) complexes with pseudo tetrahedral geometry were isolated with different counteranions; their structure solution reveals the molecular formula as [Co­(L1)4]­X2 [where L1 = thiourea (NH2CSNH2) and X = NO3 (1), Br (2), and I (3)] and [Co­(L1)4]­(SiF6) (4). The detailed analysis of direct-current (dc) magnetic data reveals a zero-field splitting (ZFS; D) with m S = ±3/2 as the ground levels (D < 0) for the four complexes. The magnitude of the ZFS parameter is larger, in absolute value, for 1 (D = −61.7 cm–1) than the other three complexes (−5.4, −5.1, and −12.2 cm–1 for 2–4, respectively). The sign of D for 1, 2, and 4 was unambiguously determined by X-band electron paramagnetic resonance (EPR) spectroscopy of the diluted samples (10%) at 5 K. For 3, the sign of D was naturally endorsed from the frequency-dependent out-of-phase signal (χM″) observed in the absence of an external dc magnetic field and confirmed by high-frequency EPR (70–600 GHz) experiments performed on a representative pure polycrystalline 3, which gave a quantitative D value of −5.10(7) cm–1. Further, the drastic changes in the spin Hamiltonian parameters and their related relaxation dynamics phenomena (of 2–4 compared to 1) were rationalized using ab initio complete-active-space self-consistent field/n-electron valence perturbation theory calculations. Calculations disclose that the anion-induced structural distortion observed in 2–4 leads to a nonfavorable overlap between the π orbital of cobalt­(II) and the π* orbital of the sulfur atom that reduces the overall |D| value in these complexes compared to 1. The present study demonstrates that not only the first but also the second coordination sphere significantly influences the magnitude of the ZFS parameters. Particularly, a reduction of D of up to ∼90% occurs (in 2–4 compared to 1) upon a simple variation of the counteranions and offers a viable approach to modulate ZFS in transition-metal-containing single-molecule magnets.

Quantifying magnetic anisotropy using X-ray and neutron diffraction

“…It has been shown on several occasions that four-coordinate Co(II) complexes in distorted tetrahedral geometries exhibit very high negative D-values, and some explanations have been presented by us and others (Zadrozny et al, 2013;Zadrozny & Long, 2011;Tripathi et al, 2019;Vaidya et al, 2018Vaidya et al, , 2016Vaidya et al, , 2014Rechkemmer et al, 2016;Damgaard-Møller et al, 2020c). In the current work, we combine results from neutron and synchrotron diffraction to provide irrefutable experimental evidence for the origin of this behavior.…”

Role of Coordination Number and Geometry in Controlling the Magnetic Anisotropy in FeII, CoII, and NiII Single‐Ion Magnets

“…on this complex predicted a very large negative D value of −70 cm −1 [9a] . Magneto‐structural correlations performed on this type of {CoS 4 } system by us and others further revealed that the magnetic anisotropy of these complexes is controlled by the following factors: i) structural parameters such as bond angle and torsional angle around the first and second coordination sphere atoms, [12c, 39] ii) effect of donor atoms or ligand field strength, [39b, 40] iii) spin‐orbit coupling of the donor atoms [41] and also iv) counterions [39d, 42] . Soft donor groups such as S, Se/Br, I coordination bring down the 4 A 2 → 4 T 2 (ligand field states in T d symmetry) transition energy gap, and thus the D switches to negative from positive.…”

The Quest for Optimal 3 d Orbital Splitting in Tetrahedral Cobalt Single‐Molecule Magnets Featuring Colossal Anisotropy and Hysteresis

“…A nearly degenerate orbital pair was obtained for 2’ , with =20.8 cm −1 and D =−155 cm −1 . In order to further validate our hypothesis on a larger population of compounds, we performed similar calculations for additional 19 [CoE 4 ] structures (with E=O, N, S, Se) available from the CSD database (Sections S4 and S5) [17–19,22,25,31,42] . They hold different ligand types and allow to cover a broader range of E−Co−E angles from 66 to 107°.…”

“…Nevertheless, 3d based systems are still arousing interest because of their highly‐tunable properties, contrary to lanthanide‐containing compounds [11,12] . In the past decade, remarkable 3d mononuclear SMMs were synthesized, [13] including two‐coordinate iron [14] and cobalt [15] systems, as well as highly distorted tetrahedral cobalt complexes [16–22] . Common to all these compounds is that the ligand field distortions recreate the OAM [15] giving a large spin‐orbit coupling (SOC) and therefore large zero‐field splitting (ZFS).…”

“…It does not, however, discuss the possibility for the N−Co−N bite angle to become too small or the existence of an optimal angle. Instead, theoretical [19,24–26] or experimental magneto‐structural investigations focused on other structural variations, [26,27] such as changing the donor strength and the counter ion [19,28] or applying high‐pressure [29,30] . None of these studies considers the possibility of an ideal bite angle [12,17,24–27,29,31] .…”