Substituted versus Naked Thiourea Ligand Containing Pseudotetrahedral Cobalt(II) Complexes: A Comparative Study on Its Magnetization Relaxation Dynamics Phenomenon

Abstract

A series of mononuclear tetrahedral cobalt(II) complexes with the general molecular formula [Co(L)X] [where L = tetramethylthiourea ([(CH)N]C═S) and X = Cl (1), Br (2), and I (3)] were isolated, and their structures were characterized by single-crystal X-ray diffraction. The experimental direct-current magnetic data are excellently reproduced by fitting both χ T( T) and M( H) simultaneously using the spin Hamiltonian (SH) parameters D = -18.1 cm and g = 2.26, D = -16.4 cm and g = 2.33, and D = -22 cm and g = 2.4 for 1-3, respectively, and the sign of D was unambiguously confirmed from X-band electron paramagnetic resonance measurements. The effective energy barrier extracted for the magnetically diluted complexes 1-3 (10%) is larger than the barrier observed for the pure samples and implies a nonzero contribution of dipolar interaction to the magnetization relaxation dynamics. The SH parameters extracted for the three complexes drastically differ from their respective parent complexes that possess the general molecular formula [Co(L)X] [where L = thiourea [(NH)C═S] and X = Cl (1a), Br (2a), and I (3a)], which is rationalized by detailed ab initio calculations. An exhaustive theoretical study reveals that both the ground and excited states are not pure but rather multideterminental in nature (1-3). Noticeably, the substitution of L by L induces structural distortion in 1-3 on the level of the secondary coordination sphere compared to 1a-3a. This distortion leads to an overall reduction in | E/ D| of 1-3 compared to 1a-3a. This may be one of the reasons for the origin of the slower relaxation times of 1-3 compared to 1a-3a.

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.…”

Mechanistic Investigation of Well-Defined Cobalt Catalyzed Formal E-Selective Hydrophosphination of Alkynes

“…The rationale for EPR silent signature of Co(II) ion in a high-spin state ( S = 3/2) is well established already in the literature. 58,[69][70][71][72][73][74] When an alkyne is added to the in-situ generated species, the EPR signal is re-generated, The spin Hamiltonian parameters extracted for A', reproduce only part of the experimental spectrum, which leave out some of the spectral features in 320-350 mT range (see Figure 3 marked with # symbol). Origin of these EPR signals is due to an unknown radical impurity.…”

Influence of ligand field on magnetic anisotropy in a family of pentacoordinate CoII complexes

“…The best fit is obtained with B = 0.77(15) s -1 K -6. 35 , n = 6.35 (12) and A = 3.41(4)•10 -10 s -1 Oe -4 K -1 and m = 4 (fixed) (Figure 6). At high temperatures, the relaxation is governed by the Raman process and by the direct at low temperatures.…”

“…25,26 Being a Kramers ion, Co II would have a bistable ground electronic state, so, independent of the sign of D, its complexes can show slow relaxation of magnetization for a wide range of coordination numbers ranging from 2 to 8. [27][28][29][30][31][32][33][34][35][36][37][38][39][40][41] However, fine-tuning the coordination number and geometry of Co II complexes to achieve significant magnetic properties is still a work in progress.…”