What Controls the Sign and Magnitude of Magnetic Anisotropy in Tetrahedral Cobalt(II) Single-Ion Magnets?

Abstract

A family of mononuclear tetrahedral cobalt(II) thiourea complexes, [Co(L)](NO) (1) and [Co(L)](ClO) where x = 2 (2), 3 (3), 4 (4) (where L = thiourea, L = 1,3-dibutylthiourea, L = 1,3-phenylethylthiourea, and L = 1,1,3,3-tetramethylthiourea), has been synthesized using a rationally designed synthetic approach, with the aim of stabilizing an Ising-type magnetic anisotropy (-D). On the basis of direct-current, alternating-current, and hysteresis magnetic measurements and theoretical calculations, we have identified the factors that govern the sign and magnitude of D and ultimately the ability to design a single-ion magnet for a tetrahedral cobalt(II) ion. To better understand the magnetization relaxation dynamics, particularly for complexes 1 and 2, dilution experiments were performed using their diamagnetic analogues, which are characterized by single-crystal X-ray diffraction with the general molecular formulas of [Zn(L)](NO) (5) and [Zn(L)](ClO) (6). Interestingly, intermolecular interactions are shown to play a role in quenching the quantum tunneling of magnetization in zero field, as evidenced in the hysteresis loop of 1. Complex 2 exhibits the largest U value of 62 cm and reveals open hysteresis loops below 4 K. Furthermore, the influence of the hyperfine interaction on the magnetization relaxation dynamics is witnessed in the hysteresis loops, allowing us to determine the electron/nuclear spin S(Co) = //I(Co) = / hyperfine coupling constant of 550 MHz, a method ideally suited to determine the hyperfine coupling constant of highly anisotropic metal ions stabilized with large D value, which are otherwise hard to determine by conventional methods such as electron paramagnetic resonance.

Magnetic Anisotropy in CoIIX4 (X=O, S, Se) Single‐Ion Magnets: Role of Structural Distortions versus Heavy Atom Effect

“…Most robust complexes based on Co II belong to four coordinate tetrahedral geometries where various means are proposing to fine tune both the sign and magnitude of the zero‐field splitting parameter. While {CoS 4 } remains one of the thoroughly studied systems and exhibit U eff values as large as 62 cm −1 , how the magnitude and the sign of D values vary if mixed donor ligands are employed has not been explored in detail.…”

“…Ab initio calculations are proven to a powerful tool in this area wherein methodology based on multi‐reference CASSCF/NEVPT2 methods have been utilized to compute zero‐field splitting parameters of various transition metal SIMs successfully . Particularly in tetrahedral Co II complexes, this methodology has been used to even predict suitable target molecules possessing very large negative D values . Here in this manuscript, we attempt to undertake a theoretical study based on ab initio CASSCF methods to understand and predict the anisotropy in twelve Co II complexes containing [CoX 4 ] (X=O, S and Se) cores which are reported in the literature.…”

A Pseudo‐Octahedral Cobalt(II) Complex with Bispyrazolylpyridine Ligands Acting as a Zero‐Field Single‐Molecule Magnet with Easy Axis Anisotropy

“…[6][7][8] Complexes that contain only one metal centerm ay represent the smallestc hemically tuneableS MMs for spin-based devices, [9] and significant progress has been made in this regard by using lanthanides [3,10] and, more recently,3 dm etal ions. However,f ew examples of zero-field SMMs based on mononuclear complexes of iron(I) [12] iron(III), [13] and cobalt(II) have been reported, [14][15][16][17][18][19][20][21][22][23][24][25] which all feature half-integer spin states (S = 3/2 or 5/2). However,f ew examples of zero-field SMMs based on mononuclear complexes of iron(I) [12] iron(III), [13] and cobalt(II) have been reported, [14][15][16][17][18][19][20][21][22][23][24][25] which all feature half-integer spin states (S = 3/2 or 5/2).…”

Drastische Verlangsamung der magnetischen Relaxation durch starke Austauschkopplungen in einem luftstabilen, radikalverbrückten Cobalt(II)‐Einzelmolekülmagneten

“…[9,10,[20][21][22][23][24][25][26][27][28] Die Hysteresekurve weist eine markante Taillierung auf und die Koerzitivfeldstärke ist relativ klein, was auf signifikantes Quantentunneln hinweist. Diese Messungen ergaben eine ausgeprägte Hysterese mit einer Breite von einigen Te sla bei Te mperaturen bis 15 K, was ungewçhnlich fürC obalt-Einzelmolekülmagnete ist.…”

“…Diese Messungen ergaben eine ausgeprägte Hysterese mit einer Breite von einigen Te sla bei Te mperaturen bis 15 K, was ungewçhnlich fürC obalt-Einzelmolekülmagnete ist. [27] Mangan(III) weist interessanterweise ähnliche Werte fürd ie Hyperfeinkopplung auf, [30] aber die auf Mangan(III) basierenden Einzelmolekülmagneten neigen nicht zu einer durch Hyperfeinkopplung verursachten Relaxation. Aufgrund der ungeraden Anzahl an ungepaarten Elektronen ist Komplex 2 ein Kramers-System, was bedeutet, dass Tu nneln nicht durch rhombische Nullfeldaufspaltung verursacht wird.…”