An oxide-bridged Dy–ReV–Dy single-molecule magnet
Graphical abstract
Use of diamagnetic trans-[ReV(O)2(py)4]+ moiety as a bridging metallo-ligand, provides a magnetically simpler analogue of a fluoride-bridged {Dy–Cr–Dy} complex. Magnetic studies of this compound and a magnetically diluted analogue allow for a direct observation of weak Dy–Dy interactions.
Highlights
► First use of diamagnetic trans-[ReV(O)2(py)4]+ moiety as a bridging metallo-ligand. ► Direct observation of weak Dy–Dy interaction by magnetic dilution experiments. ► Observation of two distinct magnetic relaxation pathways.
Introduction
The discovery that magnetically isolated molecules can exhibit a meta-stable magnetization, yielding so-called single molecule magnets (SMMs) [1], has attracted much interest in the synthesis of polynuclear metal-ion complexes [2]. The origin of SMM behavior in most 3d-based polynuclear systems is the combination of a large spin ground state and an easy axis-type anisotropy, giving rise to an energy barrier for spin-reversal. In lanthanide(III) ions the ligand-field perturbations are much weaker than the inter-electronic repulsion to the extent that the orbital angular momentum in general is left unquenched [3]. This in combination with strong spin–orbit coupling gives rise to significant anisotropy and hence renders lanthanides promising building blocks for molecular magnetic materials. Several mononuclear lanthanide SMMs with very large spin-relaxation barriers have recently been realized [4]. Interestingly, fewer polynuclear 4f complexes have shown high relaxation barriers [5] and for mixed nd–4f complexes, only recently the first example having a barrier exceeding 100 K was reported [6a]. One major drawback resulting from the largely shielded 4f electrons in lanthanides is the presence of only weak exchange interactions in 3d–4f and 4f polynuclear complexes. Despite the small magnitude of these interactions, the magnetic properties can be strongly affected by even very weak interactions. The vast majority of the studied nd–4f complexes encompass alkoxides, phenolates or carboxylates as bridging ligands with only few examples of e.g. oxide or fluoride bridges. The scarcity of oxide-bridged systems is certainly related to the extreme basicity of oxide leading to hydroxide bridges or precipitation of lanthanide hydroxides. However, oxide ligands can effectively be stabilized by high-oxidation state transition metal ions. Recently, Pointillart et al. [7] reported on an oxide-bridged ReV2Dy2 complex exhibiting SMM behavior owing to the strong magnetic anisotropy of the Dy3+ ions.
We recently reported a multi-technique study of a trinuclear, fluoride-bridged DyIII–CrIII–DyIII (DyCrDy) complex [8]. This compound incorporates as bridge between the dysprosium ions, the trans-[CrIIIF2(py)4]+ (py = pyridine) unit of structural similarity to trans-[ReV(O)2(py)4]+, which however, in contrast to the former, is diamagnetic (dxy2). Herein, we show that the reaction of trans-[ReV(O)2(py)4]NO3 with [Dy(hfac)3(H2O)2] affords a trinuclear, oxide-bridged DyIII–ReV–DyIII cluster: [Dy(hfac)3(NO3)(H2O)–Re(O)2(py)4–Dy(hfac)3(H2O)] (DyReDy). Interestingly, DyReDy is isomorphous and isostructural with the trans-difluoridochromium(III) analogue (DyCrDy). This facilitates a magnetic analysis of the constituent dysprosium ions without the complicating effects of exchange interactions and the (small) magnetic anisotropy of Cr(III). Notably, we find that the magnetic properties of DyReDy can be described consistently by the same set of parameters as derived for DyCrDy.
Section snippets
Synthesis
Trans-[Re(O)2(py)4]I was prepared by boiling ReO2I(PPh3)2 [9] in pyridine and [Ln(hfac)3(H2O)2] (Ln = Y, Tb, Dy) was synthesized according to a published method [10]. Chloroform was reagent grade (Lab-Scan, stabilized with EtOH, 1%). All other reagents were purchased from commercial sources and used without further purification. Trans-[Re(O)2(py)4]NO3 was prepared from trans-[Re(O)2(py)4]I by metathesis with Pb(NO3)2 in acetonitrile, filtration, and evaporation of the solvent at room temperature.
Results and discussion
Addition of an excess of [Dy(hfac)3(H2O)2] (hfac− = 1,1,1,5,5,5-hexafluoroacetylacetonate(1–)) in chloroform to a solution of trans-[ReV(O)2(py)4]NO3 in chloroform affords orange needles of DyReDy. No polynuclear ensembles incorporating trans-[ReV(O)2(py)4]+ or derivatives are found in the literature. The coordination of trans-[ReV(O)2(py)4]+ to DyIII may seem surprising taking into account the positive charge and, especially, the ReV that effectively reduces the basicity of the oxido ligand by
Conclusions
In summary, DyReDy is a new member of nd–4f clusters with mono-atomic bridges. Only few polynuclear clusters incorporating 5d and 4f metal ions have been reported [23] despite the promising properties of such materials owing to the diffuse character of 4d and 5d orbitals over 3d [24]. This may overcome the problem of only weak interactions observed in the vast majority of 3d-4f clusters. Despite the fact that rhenium is already present in a high oxidation state of +V, Re(VI) is relatively
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O and ReV–N bonds lengths being ca. 1.77 and ca. 2.14 Å, respectively]. The magnetic properties were investigated from susceptibility measurements performed on a polycrystalline sample of 1. Its magnetic behavior is that of a magnetically diluted Re(IV) complex and is interpreted in terms of magnetically isolated spin quartets with a high value of the zero-field splitting (|2D| is ca. 8.6 cm−1).