Abstract
Chiral bipyrimidine-bridged dinuclear LnIII complexes of general formula [(μ-bipym){((+)-tfacam)3Ln}2] and [(μ-bipym){((-)-tfacam)3Ln}2], have been prepared from the assembly of Ln(AcO)3·nH2O (LnIII = Dy, Gd), (+)/(−)-3-(trifluoroacetyl)camphor enantiopure ligands ((+)/(-)-Htfacam) and bipyrimidine (bipym). The structure and chirality of these complexes have been supported by single-crystal X-Ray diffraction and circular dichroism. The study of the magnetic properties of the GdIII complexes revealed a very weak antiferromagnetic interaction between the GdIII ions through the bipyrimidine bridging ligand. Ab initio CASSCF calculations indicated that the ground Kramers doublet (KD) of both DyIII centers is almost purely axial with the anisotropy axis located close to the two tfacam−ligands at opposite sides of each DyIIIatom, which create an axial crystal field. In keeping with this, ac dynamic measurements indicated slow relaxation of the magnetization at zero field with Ueff = 55.1 K, a pre-exponential factor of τo = 2.17·10−6 s and τQTM = 8 μs. When an optimal dc field of 0.1 T is applied, QTM is quenched and Ueff increases to 75.9 K with τo = 6.16 × 10−7 s. The DyN2O8 coordination spheres and SMM properties of [(μ-bipym){((+)-tfacam)3Ln}2] and their achiral [(Dy(β-diketonate)3)2(μ-bpym)]analogous have been compared and a magneto-structural correlation has been established, which has been supported by theoretical calculations. In contrast to the GdIII compounds, the magnetic exchange interaction between the DyIII ions has been calculated to be very weak and, generally, ferromagnetic in nature. Relaxation mechanisms for [(μ-bipym){((+)-tfacam)3Ln}2] and previously reported analogous have been proposed from ab initio calculations. As the magnetic exchange interaction found to be very weak, the observed magnetization blockade in these systems are primarily dictated by the single ion anisotropy of DyIII ions.
Highlights
During the last few decades, molecular materials in which coexist two or more physical properties have attracted much attention mainly due to their promising potential applications (Coronado et al, 2003a,b, 2008; Gómez-Romero and Sánchez, 2004; Fahmi et al, 2009; Rocha et al, 2011; Sanchez et al, 2011; Ouahab, 2012). It is well-known that a molecule without any improper axis of symmetry (Sn) is chiral, that is to say is not superimposable with its mirror image
Single-Molecule Magnet (SMM) are of current interest due to the above indicated outstanding physical properties and due to their envisaged applications in molecular spintronics (Bogani and Wernsdorfer, 2008; Dediu et al, 2009; Mannini et al, 2010; Vincent et al, 2012; Ganzhorn et al, 2013; Jenkins et al, 2013; Prezioso et al, 2013; Thiele et al, 2014; Lumetti et al, 2016; Cornia and Seneor, 2017), ultra-high density magnetic information storage (Rocha et al, 2005; Affronte, 2009), magneto-optics (Sessoli et al, 2015) and as qubits for quantum computing at molecular level (Leuenberger and Loss, 2001; Ardavan et al, 2007; Stamp and Gaita-Ariño, 2009; Martínez-Pérez et al, 2012; Ghirri et al, 2017)
In the absence of quantum tunneling of the magnetization (QTM) and TA-QTM, which shortcut the relaxation barrier to an effective value (Ueff), TB increases with the height of the energy barrier
Summary
During the last few decades, molecular materials in which coexist two or more physical properties (multifunctional molecular materials) have attracted much attention mainly due to their promising potential applications (Coronado et al, 2003a,b, 2008; Gómez-Romero and Sánchez, 2004; Fahmi et al, 2009; Rocha et al, 2011; Sanchez et al, 2011; Ouahab, 2012). It has been shown that the incorporation of a Dy2 unit with an strong Dy-electron coupling inside a fullerene cage gives rise to a blocking temperature of 18 K This fact demonstrate that the magnetic exchange between the DyIII ions and the ligand environment is crucial in dictating both the effective energy barrier and eventually the blocking temperature for this structural motif (Singh et al, 2015; Singh and Rajaraman, 2016; Liu F. et al, 2017)
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