An Undecanuclear Ferrimagnetic Cu₉Dy₂ Single Molecule Magnet Achieved through Ligand Fine-Tuning

Abstract

ABSTRACT:We describe the concept of increasing the nuclearity of a previously reported high-spin Cu5Gd2 core using a "fine tuning" ligand approach. Thus two Cu9Ln2 coordination clusters, with Ln = Dy (1), Gd(2) were synthesized with the Gd compound having a ground spin state of 17/2 and the Dy analogue showing SMM behavior in zero field.In the burgeoning area of 3d-4f coordination clusters (CCs) for those showing Single Molecule Magnet (SMM) behavior it is becoming increasingly clear that the correct blend of 3d and 4f ions in conjunction with the resulting topology of the clusters are important factors governing this behavior. 1 For 3d/4f SMMs the magnetic behavior can show two slow relaxation regimes corresponding to the single 4f ion and to the exchange coupled 3d/4f processes as evidenced within the frequency window of routine ac susceptibility studies where, in contrast to 4f single ion systems, 2,3 these can obey an Arrhenius law with two linear dependences corresponding to the two relaxation pathways, as was shown for a Co II 2Dy III 2 system. 4 Generally such systems can be treated within the coupled 3d/4f regime as giant spin systems. As such they show relatively low effective energy barriers to spin inversion (Ueff) when compared with the very high barriers often predicted for single 4f ion systems. 5 However, this is compensated by a suppression of quantum tunneling effects, which are particularly complicated and prevalent for 4f single ions. Thus, for high spin 3d/4f systems it can be possible to observe and analyze the relaxation data within the usual frequency window of ac susceptibility measurements in zero applied dc field for both the single ion and exchange coupled cases. 4 CCs combining highly anisotropic 4f ions such as Dy III with the quantum spin Cu II d 9 ion 6 have received less attention than other combinations, but amongst the relatively few examples of such SMMs, 7 the even rarer cases with high nuclearities above 10 [8][9][10][11][12]7 have the advantage that the molecules possessing high spins and are well-separated within the crystal structure. Although these systems can show slow relaxation of the magnetization, as evidenced by frequency-dependent ac-susceptibilities, the maxima of the characteristic out-of-phase signals are often shifted beyond the available frequency window. 9,10 Only rarely the SMM behaviour can be proved solely by ac measurments. 7 We show here how a ligand engineering approach applied to a system with a favorable core spin-topology leads to Cu II /Dy III SMM. We took our previously reported Cu5Gd2 CC, for which we were unable to isolate the Dy III analogue, as a starting point. This has a high S = 17/2 ground spin state and is synthesized using the Schiffbase ligand H4L1 (Figure 3), 13 based on o-vanillin and tris(hydroxymethyl)aminoenthane (tris). Given that one or even two hydroxyl groups of the tripodal alcohol unit may not be involved in coordination, but form inter-and intramolecular hydrogen-bonds to solvent molecules, neighbouring clusters or internal O...

Stepwise Investigation of the Influences of Steric Groups versus Counterions To Target Cu/Dy Complexes

“…6H2O and the related Schiff base ligands with R = ethyl (L3), methyl (L2) or hydrogen (L1), see Scheme 2 and Table 1. Using the previously reported synthetic method for complex 1, 29 we observed that the combinations containing bromide ions lead to three completely different structures with the three ligands (1 to 3). On the other hand, we can see a clear trend when nitrate counter ions are available.…”

“…In this case, it seems that the differences in steric influence of the different ligands has no impact on the structure and a butterfly Cu2Dy2 compound (4 to 6) is always isolated. Complex 1, a Cu9Dy2 compound, was previously published, 29 but was the origin of this endeavor, and therefore will be mentioned in some parts of the following analysis. Scheme 2.…”

“…Complex 1, [Cu9Dy2(3-OH)4(3-Br)2(L3)2(HL3)4(Br)2 (NO3)2(MeOH)4] will not be described here, but is mentioned for completeness of the analysis. For the structural description see Powell et al 29 Under similar synthetic conditions as used to obtain complex 1, but replacing the ethyl group on the ligand (H3L3) with a methyl group (H3L2) (see Scheme 2), we were able to crystallize a pentanuclear compound. Complex 2, [Cu3Dy2(3-OH)(L2)(HL2)3(H2L2)(MeOH)2]Br .…”

“…27 We previously showed that by replacing the alcohol group of the "tris" ligand, where tris = tris-(hydroxy-methyl)aminoethane, with an ethyl group, leading to the ligand H3L3, 2-ethyl-2-((2-hydroxy-3-methoxybenzyl-idene) amino)propane-1,3-diol and see Scheme 1, it is possible to adapt the synthesis of Cu5Ln2 compound 28 to produce a related, but expanded, Cu9Ln2 compound. 29 Here we describe our effort to test the boundaries of this system by "finetuning" the ligand to test its limits. Thus, we explored the further effects of changing the ethyl group to a methyl group in addition to completely exterminating alkyl group steric influence.…”

Zero Field SMM Behavior and Magnetic Refrigeration in Rare Heterometallic Double Stranded Helicates of Cu2Ln2 (Ln = Dy, Tb, Gd)

“…While in one side, lanthanide ions (especially Dy III and Tb III ) possess Ising type anisotropy and large magnetic moment,, in the other side, 3d metal ions have the ability to participate in strong magnetic exchange interactions. Hence, in these complexes d‐f magnetic exchange can be utilized to increase the ground state spin multiplicity, thereby improving the SMM behavior . Moreover, it would be interesting to impart helical nature into these complexes for a variety of following reasons.…”