X-ray Magnetic Circular Dichroism (XMCD) Study of a Methoxide-Bridged Dy^III–Cr^III Cluster Obtained by Fluoride Abstraction from cis-[Cr^IIIF₂(phen)₂]⁺

Abstract

An isostructural series of dinuclear chromium-(III)−lanthanide(III) clusters is formed by fluoride abstraction of cis-[CrF 2 (phen) 2 ] + by Ln 3+ resulting in LnF 3 and methoxide-bridged Cr−Ln clusters (Ln = Nd (1), Tb (2), Dy (3 )) of formula [Cr I I I (phen) 2 ( μ -MeO) 2 Ln-(NO 3 ) 4 ]·xMeOH (x = 2−2.73). In contrast to fluoride, methoxide bridges in a nonlinear fashion, which facilitates chelation. For 3, X-ray magnetic circular dichroism (XMCD) provides element-specific magnetization curves that are compared to cluster magnetization and susceptibility data acquired by SQUID magnetometry. The combination of XMCD and SQUID is able to resolve very small magnetic coupling values and reveals a weak Cr III −Dy III coupling of j = −0.04(3) cm −1 . The Dy III ion has a ground-state Kramers doublet of m J = ±13/2, and the first excited doublet is found to be m J = ±11/2 at an energy of δ = 57(21) cm −1 . The Cr III ion exhibits a uniaxial anisotropy of D Cr = −1.7(1.0) cm −1 . Further, we observe that a weak anisotropic coupling of dipolar origin is sufficient to model the data, suggesting that methoxide bridges do not play a significant role in the magnetic coupling for the present systems. ■ INTRODUCTIONMolecular nanomagnets 1 have attracted a lot of interest because of their fascinating properties and their prospects in future applications in quantum information processing and molecular spintronics. 2 In particular, there is increasing activity regarding 3d−4f single-molecule magnets, and numerous species have been reported so far. 3,4 The complete understanding of their magnetic behavior is, however, lagging behind, and for instance, the key to the necessary ingredients that mediate properties such as slow relaxation of magnetization in 3d−4f clusters is still missing. 5 Difficulties in finding this key are aggravated by the complexity of the magnetism of 4f ions with orbital contribution to their ground state. 6 In particular, it is difficult to obtain precise values of the magnetic exchange coupling between 3d and 4f ions: In 3d clusters, this information can be readily extracted from dc magnetic susceptibility measurements; however, the large anisotropy splittings resulting from the 4f ligand-field states can strongly influence the temperature dependence of the magnetic susceptibility. Hence, exchange coupling deduced solely from susceptibility data may be subject to large uncertainties. The problem can be circumvented by using additional methods such as diamagnetic substitution, 7,8 electron paramagnetic resonance, 8 or inelastic neutron scattering studies. 9 X-ray magnetic circular dichroism 10 (XMCD) is able to obtain absolute values of element-specific magnetization, which can in turn be exploited to obtain a sensitive measurement of very small magnetic coupling values, and thus it is among the techniques of choice to quantify such coupling in 3d−4f clusters. 11 Precise knowledge about the magnetic exchange coupling is the prerequisite for obtaining its dependence along the homologous series ...

Field-induced internal Fe and Ln spin reorientation in butterfly{Fe3LnO2}(LnBadía-Romano1, Bartolomé2, Bartolomé3 et al. 2013Phys. Rev. B222140View full textAdd to dashboardCite

“…The combination of these techniques has revealed a weak antiferromagnetic (AF) Dy(III)-Cr(III) coupling. 34 Moreover, in a trimer Dy-Cr-Dy compound the transition from an antiparallel to a parallel orientation of the Cr moment with respect to that of Dy for an increasing field could be shown to occur by the same technique. 35 In the case of molecules adsorbed on metallic surfaces the XMCD technique is extremely useful since, besides being element selective, it has submonolayer sensitivity.…”

“…We denote below the J Ln-M interaction in terms of the real Ln spin, J Dy = 15/2 and S Gd = 7/2, respectively. Recently, in the dimer [Cr(phen) 2 (μ-MeO) 2 Dy-(NO 3 ) 4 ] · xMeOH (x = 2 − 2.73), 34 by means of XMCD measurements a very weak Dy-Cr interaction was found, J Dy-Cr /k B = −0.06(2) K. Moreover, in [Dy(hfac) 3 (H 2 O)-CrF 2 (py) 4 -Dy(hfac) 3 (NO 3 )] the transition from an antiparallel to a parallel orientation of the Cr moment with respect to the Dy one was also observed with the help of XMCD and SQUID techniques. In that case, in spite of the sample's being powdered, the negative contribution to the magnetization due to the Cr is directly observed for an applied field lower than 2 T. The Dy-Cr interaction in terms of real Dy spin (J Dy = 15/2) was found to be J Dy-Cr /k B = −0.25 K. Taking into account that the Cr(III) spin is S Cr = 3/2, a lower value with respect to the {Fe 3 DyO 2 } case J eff (J Dy = 15/2)/k B = −0.4 K, where the Fe 3 (III) spin S Fe 3 = 5/2 is quite reasonable.…”

Fluoride‐Bridged {GdIII3MIII2} (M=Cr, Fe, Ga) Molecular Magnetic Refrigerants

“…[1c] For an exchange-coupled polynuclear complex, if only the ground-spin manifold S T is considered, the magnetic entropy is given by S m = R ln(2S T +1). [5] Unfortunately, this fact imposes strong limitations on the chemistry that can be exploited, and we herein demonstrate that fluoride-bridged 3d-4f polynuclear complexes can indeed also be prepared from labile transition metal fluoride complexes. [1a, 2] Ultimately, on increasing the field change DB, the change of entropy ÀDS m reaches its maximum value, corresponding to the full entropy content of the system, namely, the entropy sum over all single-ion spins S i , that is, R AE i ln(2S i +1).…”

Fluoride‐Bridged {GdIII3MIII2} (M=Cr, Fe, Ga) Molecular Magnetic Refrigerants

“…Recently, Winpenny and co‐workers,3 and we,4 reported the first examples of polynuclear complexes incorporating fluoride‐bridged {Cr III –F–Ln III } units. These systems owe their existence to the robust character of Cr III (d 3 ) complexes by avoiding precipitation of insoluble LnF 3 5. Unfortunately, this fact imposes strong limitations on the chemistry that can be exploited, and we herein demonstrate that fluoride‐bridged 3d–4f polynuclear complexes can indeed also be prepared from labile transition metal fluoride complexes.…”