A [Mn32] double-decker wheel.

Polynuclear clusters of paramagnetic 3d metal ions continue to attract significant interest because of their intriguing geometrical characteristics (large size, high symmetry, aesthetically pleasing shapes and architectures) and fascinating physical properties. Such complexes often combine large and sometimes abnormally large spin ground states with easyaxis-type magnetic anisotropy, resulting in a significant barrier to magnetization relaxation. Thus, at sufficiently low temperatures they function as single-domain magnetic particles displaying magnetization hysteresis and quantum tunneling of the magnetization (QTM). Such singlemolecule magnets (SMMs) represent a molecular approach to nanoscale magnetic materials with potential applications in information storage and molecular spintronics. SMMs with nuclearities up to 84 and structural topologies as diverse as (amongst others) dimers, triangles, cubanes, tetrahedra, icosahedra, and wheels are now known. The latter of these have always attracted intense interest, partly because of their inherent structural beauty, but also because they represent model systems for the study of one-dimensional magnetism, spin frustration, and quantum effects and are promising candidates for use as qubits in quantum computation. Homoand heterometallic molecular wheels are commonly encountered in 3d cluster chemistry, and in general they fall into two distinct structural types. By far the most common are single-stranded wheels, which simply describe linked monometallic units. Multiple-stranded wheels are less common and encompass either 1) wheels built from repeating metal clusters, or 2) multiple-layer wheels, that is, complexes that consist of two or more linked parallel wheels. Those of structural type (2) are extremely rare, being restricted to [V12], [11] [Mn10], [12] [Mn16], [13] and [Cu12] [14] double-deckers, none of which display SMM behavior. Herein we report a beautiful new mixed-valent [Mn32] double-decker wheel, which is the highest-nuclearity cluster possessing this topology, and show that it has a large spin ground state and displays SMM behavior with the largest effective barrier to magnetization relaxation (Ueff 44.5 K) for any molecular wheel. The reaction of MnBr2·4H2O, NaO2CMe, 2-phenyl-1,2propanediol (Ph-pdH2), and 2-hydroxyacetophenone oxime (Me-saoH2) in a 1:1:1:1 molar ratio in MeCN leads to the isolation of [Mn32(m4-O)8(m3-OH)6(Me-sao)14(O2CMe)18Br8(H2O)10](OH)2 (1) in 30% yield after approximately 1 week. The diol does not appear in the final product, but its presence in the reaction mixture is essential, since reactions in its absence lead to the formation of a known [Mn7] cluster. [16] The structure of the cation of 1 reveals it to be a centrosymmetric, mixed-valent [Mn18Mn III 14] doubledecker wheel (Figure 1a), consisting of two linked, parallel [Mn7Mn III 7] crown-shaped wheels (Figure 1b,c) that house a [Mn4] rectangle in their inner cavity. Each [Mn14] unit consists of seven Mn and seven Mn ions arranged alternately in a single-stranded wheel (Figure 1c). All of the Mn ions are six-coordinate, with the Mn ions displaying the expected Jahn–Teller elongations, all of which are (approximately) perpendicular to the planes of the [Mn14] wheels. The four Mn ions in the inner [Mn4] rectangle are five-coordinate. The Mn ions of each [Mn14] crown are held together through a combination of m2-Ophenol atoms from seven fully deprotonatedMe-sao ligands, three m3-OH and four m4-O 2 ions, and seven h:h:m3-acetates. The two [Mn14] units are linked together to form the [Mn28] double-decker wheel through the oximic m2-O atoms of all fourteen Me-sao 2