Three's a charm: The title compound (see picture, right, WO6 purple polyhedra, Mn orange and brown, Si green, O red) contains an embedded mixed‐valence {Mn5O6} cubane core, which is structurally similar to the active site in photosystem II. Solid‐, solution‐, and gas‐phase studies indicate the presence of three lacunary Keggin fragments, thereby giving insight into the complex solution chemistry of plenary POM fragments.
Over the last few years, great interest has emerged in the synthesis and magnetothermal studies of polymetallic molecular clusters based on paramagnetic ions, often referred to as molecular nanomagnets, in view of their potential application as lowtemperature magnetic refrigerants. [1,2] What makes them promising is that their cryogenic magnetocaloric effect (MCE) can be considerably larger than that of any other magnetic refrigerant, e.g. lanthanide alloys and magnetic nanoparticles. [3] The MCE is the change of magnetic entropy (∆S m ) and related adiabatic temperature (∆T ad ) following the change of applied magnetic field and it can be exploited for cooling applications via a field removal process called adiabatic demagnetization. Although the MCE is intrinsic to any magnetic material, in only a few cases are the changes sufficiently large to make them suitable for applications. The ideal molecular refrigerant comprises the following key characteristics: [1] (i) a large spin ground state S, since the magnetic entropy amounts to Rln(2S+1); (ii) a negligible magnetic anisotropy, which permits easy polarization of the net molecular spins in magnetic fields of weak or moderate strength; (iii) the presence of low-lying excited spin states, which enhances the field dependence of the MCE due to the increased number of populated spin states; (iv) dominant ferromagnetic exchange, [3(c)] favouring a large S and hence a large field dependence of the MCE; (v) a relatively low molecular mass (or a large metal:ligand mass ratio) since the non-magnetic ligands contribute passively to the MCE. Although this last point is crucial for obtaining an enhanced effect, it has been mostly ignored to date. Molecular cluster compounds tend to have a very low magnetic density because of the large complex structural frameworks required to encase the multi-metallic core.In this communication we propose a drastically different approach by focusing on the simple and well-known ferromagnetic molecular dimer gadolinium acetate tetrahydrate, [4] [{Gd(OAc) 3 (H 2 O) 2 } 2 ]•4H 2 O (1). The structure of 1 is depicted in Figure 1 and comprises a dimer of Gd 3+ ions bridged through two of the six carboxylate groups which bond in a η 2 :η 1 :µ 2 -fashion. The remaining acetates are chelating with the nine-coordinate [capped square anti-prismatic] geometry of the metal centres being completed by the presence of two terminally bound H 2 O molecules. These partake in intra-molecular H-bonding to the neighbouring chelating acetate ligands, and are responsible for both the direct inter-molecular H-bonds in the a-b plane and the inter-plane Hbonds mediated by the lattice H 2 O molecules ( Fig. S1 and Table S1). Figure 1. The molecular structure of 1. Code: Gd = black, O = dark grey, C = light grey, H = small bullets. H-atoms of the methyl groups are omitted for clarity. Intramolecular hydrogen bonds are depicted as thin lines.Our theoretical and experimental investigations (see Supporting Information for details) of the magnetothermal properties of ...
The reaction of pyridine-2,6-dimethanol (H 2 pdm) with [Mn 3 O(O 2 CMe) 6 (py) 3 ][ClO 4 ] gives the 2Mn II , 2Mn III title compound 1, which has an S = 8 ground state and displays strong out-of-phase signals in ac susceptibility studies that establish 1 as a new class of single-molecule magnet.
Polyoxometalates (POMs), anionic oxide clusters of the early transition metals, [1] represent a vast class of inorganic materials with a virtually unmatched range of properties applicable to biology, [2] magnetism, [3] materials science, [4] or catalysis. [5] This unique span of properties qualifies POMbased materials as prime candidates for the designed construction of electronically interesting materials. Polyoxometalates possess enormous diversity in both size and structure [1b, 6] and thereby provide access to a huge library of readily available and controllable fragments, that is, secondary building units (SBUs) [7] that can be interconnected by electrophiles.The development of novel magnetic polyoxometalates [8] targets either the magnetic functionalization of the metal oxide fragment itself, which is mostly relevant for polyoxovanadates such as {V 15 As 6 }, [9] the interlinking of POM building blocks, as seen for {Mo 72 Fe 30 }, [10] [PMo 12 O 40 -(VO) 2 ] 5À , [11] or the use of lacunary POM fragments as multidentate ligands binding to polynuclear paramagnetic coordination clusters (e.g., {W 18 Cu 6 } [12] and {W 48 Cu 20 } [13] ). In particular, we reasoned that targeting the assembly of a mixed-valence manganese-based cluster [14][15] within a polyoxometalate ligand cage could offer many fantastic new possibilities for design and manipulation. For example, the POM "ligands" could be useful to "dilute" single-molecule magnets (SMMs) to remove unwanted dipolar interactions and also because of the intrinsic redox activity of the POM "ligands" that could allow additional routes to control magnetic-exchange pathways or introduce other functionality for device applications. [11] In addition, the POM shells are themselves surface compatible as well as being excellent ligands and SBUs that will allow a very high degree of reliable design and assembly that is not possible to achieve in SMMs based on first-row transition metals alone.One of the major limitations in the development of SMMs is that the underlying design strategies lie within the boundaries set by the serendipitous self-assembly of metal ions with bridging ligands of different connectivites and the controlled assembly of rigid building blocks typified by metallocyanide (Prussian blue-type) chemistry. [16] Within this scheme there have also been attempts to influence the primary SMM parameters (spin ground state and molecular anisotropy) deliberately through targeted structural and chemical modification. [17] However, despite the comparably precise structural control on the molecular level that characterizes POM chemistry, no single-molecule magnet has yet been derived from a polyoxometalate, as evidenced by hysteresis in magnetization versus field studies. Although several POM-based systems with high spin ground states or significant magnetic anisotropy are known, [18] and hybrid [SMM] n+ [POM] nÀ salts have been isolated, [19] a bona fide polyoxometalate-based SMM has not yet been reported. Herein we report the first two examples of Mn II/III ...
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