Single Pyramid Magnets: Dy₅ Pyramids with Slow Magnetic Relaxation to 40 K

Abstract

There has recently been a huge renaissance in the study of the magnetism of 4f-coordination complexes.[1] There have been remarkable results, such as slow relaxation of magnetization in the "single-ion magnets" (Bu 4 N)[Tb(Pc) 2 ] (H 2 Pc = phthalocyanine), for which the thermal energy barrier for relaxation is 330 K.[2] Equally remarkable has been the slow relaxation brought about by the toroidal arrangement of local magnetization vectors in a {Dy 3 } triangle ("spin chirality"). [3] In parallel, studies of polymetallic dysprosium cages have shown slow relaxation in a variety of cages with energy barriers as high as 200 K, [4a] and showing magnetic hysteresis to 8 K.[4b]Much of the fascinating physics of (Bu 4 N)[Tb(Pc) 2 ][2] and other single-ion magnets, such as Na 9 [Er(W 5 O 18 ) 2 ], [5] is associated with their fourfold symmetry. Equally, the toroidal magnetism of the {Dy 3 } cage is associated with the triangular array of 4f-ions.[3] Therefore, we targeted a molecule that had fourfold symmetry and metal triangles. The obvious polyhedron is a square-based pyramid. Oxo-centered {Ln 5 } pyramids of general formula [Ln 5 (m 5 -O)(m 3 -OR) 4 (m 2 -OR) 4 (OR) 5 ] are known for around half of the lanthanoids (R = iPr, Ln = Nd, [6,7] Eu, [8] Gd, [6] Er, [6,9] Yb;[10] R = tBu, Ln = La, [11] Nd [11,12] ) but not with dysprosium. The iso-propoxide-bridged dysprosium square-based pyramid [Dy 5 O(OiPr) 13 ] (1) is made by the reaction of freshly generated KOiPr with DyCl 3 in iPrOH/toluene with a stoichiometric amount of H 2 O (see the Experimental Section). Crystals of 1 form in two different crystal systems: one is isostructural with the previously reported [9] {Er 5 } cage whereas the second has a new unit cell.[13] Both polymorphs have essentially identical magnetic behavior. There is no evidence, either visual or by X-ray diffraction, [14] that samples ever contain a mixture of polymorphs, that is, we have studied pure samples of each. Polymorphs have been previously reported for lanthanide alkoxides. [6,9] In the new structure the square-based pyramid is disordered, with four of the Dy sites common to both disorder forms. The final Dy site is 50:50 disordered over two positions, however it is clear that the square-based pyramid is slightly elongated (Figure 1), with the average distance between the apical dysprosium and the basal dysprosia 3.43 , while the average distance between the adjacent dysprosia within the basal plane is 3.37 in one of the two models and 3.40 in the second. The structure has no crystallographic symmetry.All Dy sites are six-coordinate. The geometry at each site is based on octahedral, but with the Dy shifted towards the terminal alkoxide and away from the central m 5 -oxide-thus each Dy site has local, but non-crystallographic, C 4v symmetry. The distances of the central oxide to the Dy sites fall in the range 2.25-2.60 . The thirteen alkoxides fall into three groups: there is a terminal alkoxide on each metal site; a second group of four alkoxides bridges on each of the four triangul...

An electrostatic model for the determination of magnetic anisotropy in dysprosium complexes

“…The equatorial plane of each ion is formed by four bridging isopropoxide ( i PrO À ) ligands and the axial positions are occupied by the single m 5 -oxide bridge at the centre of the molecule and a terminal isopropoxide ligand 32 . The oxygen atom of the terminal i PrO À ligand is substantially closer to the Dy III ion than all other donor atoms, at 2.04(1) Å compared with 2.35(8) Å, and the central m 5-oxide has a double negative charge.…”

“…Calculating the ab initio properties of the following polymetallic complexes is extremely computationally expensive. Hence, to demonstrate the power of our simple 32,33). The methodology for the calculation of the electrostatic anisotropy axes in polymetallic complexes is identical to that of monometallic complexes and is performed for each Dy III ion independently.…”

Computational Studies on SAMs of {Mn6} SMMs on Au(111): Do Properties Change upon Grafting?

“…The reversal of magnetization in SMMs is hampered by a barrier reversal whose height is given by |D|S 2 for integer spin systems with S being the spin ground state and D a negative axial anisotropy parameter. 3 For {Mn 12 } family of complexes the theoretical barrier height is estimated to be 60 K. Recently many lanthanide complexes reported to have higher barrier height, 4 in particular a {Dy 5 (III)} cluster with a barrier of 540 K. 5 Moreover monomeric Tb(III) complex with phthalocyaninate ligands reported to have a barrier height of about 800 K, 6 the largest known for such class of molecules. However, in most lanthanide based SMMs the quantum tunneling of magnetization is very fast leading the very small coercitivity in the hysteresis loop, a remarkable exception being the lanthanide dimers bridged by the N 3 -radical 7,8 Regarding clusters of transition metal complexes, a breakthrough has been achieved with the discovery of a family of {Mn 6 } complexes [Mn(III) 6 O 2 (sao) 6 (O 2 C-th) 2 L 4-6 ], 9 where HO 2 C-ph=3-phenil carboxylic acid, L = EtOH, H 2 O, possessing a barrier height of ca.…”

Multiple Magnetization Reversal Channels Observed in a 3d-4f Single Molecule Magnet

“…This provided breakthroughs in the understanding of the role of the magnetic anisotropy and the crystal field effects in determining high barriers to the relaxation of magnetization, and provided key inputs for the design and synthesis of compounds showing relatively high magnetization blocking temperatures [12][13][14]. In this framework, it is interesting to note that most of the systems earlier synthesized for the study of their static magnetic properties have not yet been characterized for the dynamics of their magnetization, despite the few exceptions that have provided clear evidence of their potentialities in this regard [15,16].…”