A New Family of Homo‐ and Heterometallic Manganese Complexes at High Oxidation States Derived from the Oxidation of Mn II with Ce IV : Syntheses, Structures, and Magnetic Properties

MnII/III and CeIII/IV Units Supported on an Octahedral Molecular Nanoparticle of CeO2

The preparation of three new heterometallic clusters [Ce₆Mn₁₂O₁₇(O₂CPh)₂₆] (1), [Ce₁₀Mn₁₄O₂₄(O₂CPh)₃₂] (2), and [Ce₂₃Mn₂₀O₄₈(OH)₂(tbb)₄₆(H₂O)₄](NO₃)₂ (3; tbb^- = 4-^tBu-benzoate) is reported. They all possess unprecedented structures with a common feature being the presence of an octahedral Ce^IV-oxo core: a Ce₆ in 1, two edge-fused Ce₆ giving a Ce₁₀ bioctahedron in 2, or a larger Ce₁₉ octahedron in 3. Complex 1 is the first Ce₆ cluster with a central μ₆-O^2-. 2 and the cation of 3 are molecular nanoparticles of CeO₂ (ceria) because they possess the fluorite structure of bulk ceria and are thus ultrasmall ceria nanoparticles in molecular form. The {Ce₁₉O₃₂} octahedral subunit of the cation of 3 had been predicted from density functional theory studies to be one of the stable fragments of the CeO₂ lattice, but has never been previously synthesized in molecular chemistry. Around the Ce/O core of 1-3 is an incomplete monolayer of Mn_n ions disposed as four Mn₃, two Mn₇, and four Mn₅ units, respectively. This represents a clear structural similarity with composite (phase-separated) CeO₂/MnO_x mixtures where at high Ce:Mn ratios the Mn atoms segregate on the surface of CeO₂ phases. Variable-temperature dc and ac magnetic susceptibility studies have revealed S = 2, S = ¹/₂, and S = ³/₂ ground states for 1-3, respectively. Fitting of the 5.0-300 K dc data for 1 to a two-J model for an asymmetrical V-shaped Mn₃ unit with no interaction between the end Mn^III ions gave an excellent fit with the following values: J₁ = 5.2(3) cm^-1, J₂ = -7.4(3) cm^-1, and g = 1.96(2).

Long-Range Ferromagnetic Exchange Interactions Mediated by Mn-CeIV-Mn Superexchange Involving Empty 4f Orbitals

Reactions involving reductive aggregation of MnO₄^- in methanol in the presence of Ce^IV and an excess of carboxylic acid have led to the synthesis of structurally related Ce/Mn clusters, [Ce₃Mn₅O₈(OMe)(O₂CBu^t)₁₃(MeOH)] (1) and [Ce₂Mn₃O₅(O₂CPh)₉(MeOH)₃] (2), containing at least one {Mn₂Ce₂O₄} cubane unit. The cores of both clusters contain Mn_x units separated by three (1) or two (2) Ce^IV ions. Fits of variable-temperature, solid-state dc and ac magnetic susceptibility data reveal dominant ferromagnetic interactions within 1 and 2, resulting in the maximum S = ¹⁷/₂ and S = 5 ground state spins, respectively, and thus suggesting significant ferromagnetic (F) interactions between the Mn_x units that are ≥6 Å apart and separated by four intervening bonds through diamagnetic Ce^IV. Fits of magnetic susceptibility data also revealed unusual long-range F interactions, and this finding was further supported by high-field EPR measurements and simulations. Density functional theory calculations and a Wannier function analysis confirm long-range interactions and indicate a Mn-Ce-Mn superexchange pathway via Mn-d/Ce-f orbital overlap/hybridization.