Enhancing the barrier height for magnetization reversal in 4d/4f Ru2IIILn2III butterfly single molecule magnets (Ln = Gd, Dy) via targeted structural alterations

A series of 4d-4f {(Ru2Dy2III)-Dy-III} and {(Ru2Gd2III)-Gd-III} 'butterfly' (rhombohedral) complexes have been synthesized and characterized and their magnetic properties investigated. Earlier, we have reported the first 4d/4f SMM - [(Ru2Dy2III)-Dy-III(OMe)(2)(O2CPh)(4)(mdea)(2)(NO3)(2)] (1(Dy)) with a U-eff value of 10.7 cm(-1). As the structural distortion around the Dy-III centres and the Ru-III center dot center dot center dot Dy-III exchange interactions are key to enhancing the anisotropy, in this work we have synthesised three more {Ru2Dy2} butterfly complexes where structural alteration around the Dy-III centres and alterations to the bridging groups are performed with an aim to improve the magnetic properties. The new complexes reported here are [Ru2Dy2(OMe)(2)(O2C(4-Me-Ph)(4)(mdea)(2)(MeOH)(4)], 2(Dy), [Ru2Dy2(OMe)(2)(O2C(2-Cl,4,5-F-Ph)(4)(mdea(2)(NO3)(2)], 3(Dy), and an acac derivative (Ru2DY2(OMe)(2)(acac)(4)(NO3)(2)(edea)(2)], 4(Dy), where acac(-) = acetylacetonate, edea(2-) = N-ethyldiethanolamine dianion. Complex to y describes alteration in the Dy-III centers, while complexes 3(Dy) and 4(Dy) are aimed to alter the Ru-III center dot center dot center dot Dy-III exchange pathways. To ascertain the 4d-4f exchange, the Gd-analogues of 1(Dy) and 4(Dy) were synthesised [Ru2Dy2(OMe)(2)(O2CPh)(4)(mdea)(2)(NO3)(2)], 1(Gd), [Ru2Dy2(OMe)(2)(acac)(4)(NO3)(2)(edea)(2)], 4(Gd). Both ac and dc susceptibility studies were performed on all these complexes, and out-of-phase signals were observed for 3(Dy) in zero-field while 2(Dy) and 4(Dy) show out-of-phase signals in the presence of an applied field. Complex 3(D)(y) reveals a barrier height U-eff of 45 K. To understand the difference in the magnetic dynamic behavior compared to our earlier reported {(Ru2Dy2III)-Dy-III} analogue, detailed theoretical calculations based on ab initio CASSCF/RASSI-SO calculations have been performed. Calculations reveal that the J(Ru center dot center dot center dot Dy) value varies from -1.8 cm(-1) (4(Dy)) to -2.4 cm(-1) (3(Dy)). These values are also affirmed by DFT calculations performed on the corresponding Gd analogues. The origin of the largest barrier and observation of slow magnetic relaxation in 3(Dy) is routed back to the stronger single-ion anisotropy and stronger J(Ru center dot center dot center dot Dy) exchange which quenches the QTM effects more efficiently. This study thus paves the way forward to tune local structure around the Ln(III) center and the exchange pathway to enhance the SMM characteristics in other {3d-4f)/{4d-4f) SMMs.

Automated summary

A series of novel {Ru2Dy2} butterfly complexes have been synthesized with alterations to the structure around the Dy-III centers and bridging groups to enhance magnetic properties. Studies show successful tuning of these complexes to improve their single-molecule magnet characteristics.