Exploiting Strong {Cr^III–Dy^III} Ferromagnetic Exchange Coupling to Quench Quantum Tunneling of Magnetization in a Novel {Cr^III₂Dy^III₃} Single-Molecule Magnet

Abstract

The {3d–4f} pentanuclear complexes with the formula [CrIII 2LnIII 3(PhCO2)7(OH)6 (iPrO)(NO3)(H2O)3] (iPrO = isopropoxide) (where Ln = Dy (1), Gd (2)) have been synthesized and characterized using magnetic and theoretical studies. The metal core of complexes 1 and 2 has a trigonal bipyramidal arrangement with three LnIII ions in the triangular plane and two CrIII ions occupying the axial positions. These ions are held together by six μ3-OH bridges and seven carboxylate bridges. The dc magnetic susceptibility data reveal ferromagnetic interactions presiding between CrIII and LnIII ions in 1 and 2. The fitting of the susceptibility curve employing the DFT calculated J (vide infra), yield J Gd III –Gd III = +0.008 cm–1, J Cr III –Gd III = +0.27 cm–1, and J Cr III –Cr III = −0.007 cm–1 for 2. The dynamic (ac) magnetic susceptibility studies on 1 indicate slow relaxation of magnetization with a U eff value of 30.9 K (21.4 cm–1) and τ0 = 4.09 × 10–10 s. These extracted parameters are among the highest for any reported {CrIIIDyIII} complexes. DFT and ab initio CASSCF/RASSI–SO/SINGLE_ANISO/POLY_ANISO calculations were carried out to estimate the exchange interactions and their role in quenching the quantum tunneling of magnetization (QTM) behavior. Ab initio calculations on the DyIII ions reveal three asymmetric DyIII centers with the estimated single ion barrier in the range of 78–184 cm–1, however, with a large QTM probability. Despite a triangular {Dy3} motif, the g zz axes do not align in the triangular plane as observed in the {CrIIIDyIII 6} single-molecule toroids (SMT) reported earlier by us. This is essentially due to the presence/absence of the iPrO–/carboxylate group that alters the charge distribution around the DyIII ion and hence the orientation of the corresponding g zz axis. The combination of DFT and ab initio CASSCF calculations yield J Dy III –Dy III = +0.012 cm–1, J Cr III –Dy III = +1.20 cm–1, and J Cr III –Cr III = −0.95 cm–1 for 1. The mechanism of magnetization relaxation developed for the {CrIII 2DyIII 3} cluster reveals that the relatively strong ferromagnetic CrIII–DyIII exchange interaction reduces the ground state QTM significantly yielding a U eff of 38.6 cm–1, which agrees with the experimental value. Thus, our study iterates the importance of CrIII ion to enhance the exchange coupling in the {3d–4f} family of clusters.

Hourglass-Shaped Homo- and Heteronuclear Nonanuclear Lanthanide Clusters: Structures, Magnetism, Photoluminescence, and Theoretical Analysis

“…Earlier magneto-structural correlation predicts that the Gd–Gd exchange is expected to be ferromagnetic only when the Gd–O–Gd angle lies in the range of 105–135° and here all of the angles are found to lie outside this range and, therefore, expected to be antiferromagnetic as computed from DFT calculations. , The earlier observed parabolic behavior suggests that a lower angle would yield strong antiferromagnetic Gd···Gd interactions, which is observed here with J 1 > J 3 ≫ J 2 . This is essentially due to weak ferromagnetic contribution J s that arise due to charge-transfer interaction to the 5d/6s orbital and weak but non-negligible 4f–4f orbital overlap that tends to get larger both in acute and wider angles, as illustrated earlier. , As the simulation of susceptibility for this cluster is not possible using the exact diagonalization method, we have adopted a fragment approach and have considered the {Gd5} fragment interacting with a {Gd4} fragment employing DFT J s . This is found to reproduce the experimental susceptibility, offering confidence in the methodology chosen.…”

“…This is essentially due to weak ferromagnetic contribution J s that arise due to charge-transfer interaction to the 5d/6s orbital and weak but non-negligible 4f−4f orbital overlap that tends to get larger both in acute and wider angles, as illustrated earlier. 72,74 As the simulation of susceptibility for this cluster is not possible using the exact diagonalization method, we have adopted a fragment approach and have considered the {Gd5} fragment interacting with a {Gd4} fragment employing DFT J s . This is found to reproduce the experimental susceptibility, offering confidence in the methodology chosen.…”

Magnetic materials based on heterometallic CrII/III–LnIII complexes

“…20). 161 It has been well demonstrated with the aid of theoretical investigations that the stronger ferromagnetic {Cr III –Dy III } exchange interaction with J Cr III –Dy III = +1.20 cm −1 facilitates reducing the ground state QTM to generate the SMM behaviour in the Dy analogue, with U eff value of 30.9 K (21.4 cm −1 ) and τ 0 = 4.09 × 10 −10 s.…”

Insight into ferromagnetic interactions in CuII–LnIII dimers with a compartmental ligand

“…In several 3d–4f compounds, the exchange interaction between 3d and 4f ions has been found to effectively suppress the fast zero-field Quantum Tunnelling of Magnetization (QTM) of lanthanide ions, which in turn enhances the SMM behaviour. 12–14 In general, the magnetic interaction between transition-metal and lanthanide ions plays a crucial role in defining the magnetic behaviour and applicability of such magnetic materials and hence needs to be evaluated. The pioneering work in the field of 3d–4f systems by Gatteschi et al .…”