Heterometallic Fe₂^II–U^V and Ni₂^II–U^V Exchange‐Coupled Single‐Molecule Magnets: Effect of the 3 d Ion on the Magnetic Properties

Abstract

Uranium-based compounds have been put forward as ideal candidates for the design of single-molecule magnets (SMMs) with improved properties, but to date, only two examples of exchange-coupled 3d-5f SMM containing uranium have been reported and both are based on the Mn(II) ion. Here we have synthesized the first examples of exchange-coupled uranium SMMs based on Fe(II) and Ni(II) . The SMM behavior of these complexes containing a quasi linear {M-O=U=O-M} core arises from intramolecular Fe-U and Ni-U exchange interactions combined with the high Ising anisotropy of the uranyl(V) moiety. The measured values of the relaxation barrier (53.9±0.9 K in the UFe2 complex and of 27.4±0.5 K in the UNi2 complex) show clearly the dependency on the spin value of the transition metal, providing important new information for the future design of improved uranium-based SMMs.

DFT Investigations of the Magnetic Properties of Actinide Complexes

“…One of the first magnetic systems which was theoretically investigated by DFT/BS computations, is the bis(imido) pentavalent diuranium(V) [U(N t Bu) 2 (I)( t Bu 2 bpy)] 2 complex reported by Kiplinger's group in 2009 [106]. This system exhibits significant AF coupling between the two metallic 5f 1 /5f 1 spin centers, as shown by the magnetic molar χ versus T, through CCI between [U(NR) 2 ] + moieties similar to that observed in poly-uranyl [UO 2 ] + systems [23][24][25][26][27][28][29][30][31][32][46][47][48][49][50][51]59,61,69,[114][115][116]. The authors carried out B3LYP computations, employing the Stuttgart RSC 1997 ECP basis set for uranium.…”

“…As reviewed recently [71,72], a successful route to super-exchange effects in actinide-containing multinuclear species is through cation-cation interactions (CCI) [23][24][25][26][27][28][29][30][31][32][33][46][47][48][49][50][51]59,61,69,[114][115][116]123], mostly between uranyl(V) UO2 + of actinyl units. This linkage forms an oxo-bridge between metal centers affording a great number of oxo-bridged systems exhibiting significant coupling between U V centers [27,49], mixed U V /transition metal [28][29][30][31][32]111] or lanthanide centers [47,112,123], with the largest actinide-based multinuclear complex affording the unique structure of a wheel-shaped cluster {[UO2(salen)]2Mn(Py)3}6 (Py = pyridine) which is assembled through UO2 + and Mn II interactions [29]. Additionally, CCIs were observed between Np IV /Np V ions in neptunyl complexes [75].…”

Differential uranyl(v) oxo-group bonding between the uranium and metal cations from groups 1, 2, 4, and 12; a high energy resolution X-ray absorption, computational, and synthetic study

“…This feature can result in uranium-metal communication, for example in a trinuclear [Co-O]U]O-Co] complex that exhibits a magnetic exchange and a slow relaxation at a reversal barrier temperature of 30.5 AE 0.9 K, resulting in 5f-3d single molecule magnetism. [12][13][14][15][16] We have also reported the exclusively group(I) mono-oxo metalated uranyl(V) complexes [(py) 3 17 We have shown that uranyl(VI) to uranyl(V) reduction is possible by both homolytic bond cleavage 18 and sterically induced reduction, 19 as well as via a direct reductive metalation. 20,21 Importantly, uranyl(VI) complexes in which one oxo group is coordinated by a Lewis acid group, such as a borane or metal complex, are activated towards singleelectron reduction by as much as 0.6 V. 9,10,[22][23][24][25] This has been used to direct the functionalisation of the oxo group by d-and pblock reagents.…”

Theoretical Investigation of the Electronic Structure and Magnetic Properties of Oxo-Bridged Uranyl(V) Dinuclear and Trinuclear Complexes

“…[23][24][25][26][27][28] These studies include mono [29][30][31][32][33][34][35][36][37][38] and polynuclear systems [39][40][41] and heteropolymetallic uranium-3d metal complexes. [42][43][44][45][46][47][48] Notably, uranium not only shows large spin-orbit coupling but in addition the more diffuse nature of 5f orbitals compared to 4f orbitals, 36,40 is expected to allow higher p-f orbital overlap and hence stronger magnetic couplings. 23,25 Therefore, efforts in this area have been directed to synthesize and characterize polynuclear uranium systems, where a successful strategy in promoting electronic interactions between 5f spin centers was to use covalently linked bridging ligands.…”