Manganese/Cerium Clusters Spanning a Range of Oxidation Levels and CeMn₈, Ce₂Mn₄, and Ce₆Mn₄Nuclearities: Structural, Magnetic, and EPR Properties

Abstract

The syntheses, structures, and magnetic properties are reported for three new Ce/Mn clusters with different Ce/Mn ratios: [Ce6Mn4O12(O2CMe)10(NO3)4(py)4] (py = pyridine) (1), [CeMn8O8(O2CCH2(t)Bu)12(DMF)14] (DMF = dimethylformamide) (2), and [Ce2Mn4O2(O2CMe)6(NO3)4(hmp)4] (3; hmp(-) is the anion of 2-(hydroxymethyl)pyridine). 1 and 2 were obtained from the reaction of Ce(IV) with [Mn12O12(O2CMe)16(H2O)4] (Mn(III)8Mn(IV)4) and [Mn8O2(O2CCH2(t)Bu)14((t)BuCH2CO2H)4] (Mn(II)6Mn(III)2), respectively, whereas 3 resulted from the oxidation of Mn(II) acetate with Ce(IV) in the presence of hmpH. Cluster 1 possesses an unusual [Ce6Mn4O12](14+) core topology consisting of a [Ce6O8] face-capped octahedron, which is face-fused at each end to a [Ce(IV)2Mn(III)Mn(IV)O4] cubane. Cluster 2 possesses a nonplanar, saddlelike loop of eight Mn(III) atoms bridged by eight μ3-O(2-) ions to a central Ce(IV) atom. Cluster 3 is similar to 1 in possessing an octahedral core, but this is now a [Ce2Mn4] octahedron consisting of a Ce(III) atom on either side of a Mn4 parallelogram, with the metal atoms bridged by two μ4-O(2-) ions, the alkoxide arms of four hmp(-) groups, and six acetates. Clusters 1, 2, and 3 are thus at the Ce(IV)6Mn(III)2Mn(IV)2, Ce(IV)Mn(III)8, and Ce(III)2Mn(III)4 oxidation levels, respectively. Variable-temperature, solid-state direct current (DC) and alternating current (AC) magnetization studies on 1-3 in the 5.0-300 K range revealed predominantly antiferromagnetic exchange interactions within the complexes. For 1, fitting of the DC data to the theoretical expression for a dinuclear Mn(III)Mn(IV) complex derived using the Van Vleck equation and an isotropic spin Hamiltonian (ℋ = -2JŜi·Ŝj convention) gave a value for the exchange coupling parameter (J) of -60.4(7) cm(-1) and a Landé factor g = 2.00(1), indicating an S = 1/2 ground state. For 2, both DC and AC data indicate an S = 0 ground state, which is unprecedented for a member of the CeMn8 family and now means members of the latter have been made that span the whole range of possible ground states from S = 0 to the maximum S = 16. Cluster 3 possesses an S = 0 ground state for its Mn4 fragment, with the paramagnetism remaining at low temperature coming from the weakly coupled Ce(III) centers. These three species are new additions to the Mn-Ce family of clusters and the broader class of 3d/4f molecular systems.

Reactivity of a Chloride Decorated, Mixed Valent CeIII/IV38–Oxo Cluster

“…These clusters provide a handle to better understand reactivity on the molecular scale and also exhibit interesting properties. For example, cerium nanoclusters have been engineered into metal–organic frameworks, acting as metal nodes, with applications in gas storage, catalytic transformations, and nerve agent detoxification. − Mixed cerium–oxo/transition metal–oxo clusters possess catalytic oxidation properties and unique magnetic behaviors, with some displaying single-molecule magnet properties. − Cerium oxide nanoclusters and nanoparticles are also relevant for the actinide (An) community. Such clusters may lend insight into the formation and chemical behavior of actinide nanoclusters that have historically impeded separations and reprocessing efforts and have been responsible for the migration of radionuclides in environmental systems. − The impact and scope of cerium oxide nanoclusters continue to expand, moving from research and development into industrial and medical applications.…”

Structural Diversities in Heterometallic Mn–Ca Cluster Chemistry from the Use of Salicylhydroxamic Acid: {MnIII4Ca2}, {MnII/III6Ca2}, {MnIII/IV8Ca}, and {MnIII8Ca2} Complexes with Relevance to Both High- and Low-Valent States of the Oxygen-Evolving Complex

“…In addition, work in progress includes the synthesis and systematic investigation of various new hydroxamic acids and oxime-based ligands in Mn–Ca coordination chemistry. Finally, the {Mn 4 Ca 2 } octahedral arrangement of 1 is very similar to the one seen before in Mn–Ce chemistry albeit with different bridging and ancillary ligands . This is important when targeting the deliberate replacement of Ca II atoms by paramagnetic and anisotropic 4f metal ions in an attempt to synthesize molecule-based magnets.…”

“…Finally, the {Mn 4 Ca 2 } octahedral arrangement of 1 is very similar to the one seen before in Mn−Ce chemistry albeit with different bridging and ancillary ligands. 74 This is important when targeting the deliberate replacement of Ca II atoms by paramagnetic and anisotropic 4f metal ions in an attempt to synthesize molecule-based magnets.…”

Synthesis and characterization of mixed-valence manganese fluorophosphate and analogues with clathrate-like structures: MnIII6F12(PO3(OH))8[Na8(Kx(H3O)4−x(H2O)2)MIV(OH)6] (MIV = Mn, Ti, Ge)

“…11 Thus, in an attempt to enrich the structural chemistry of mixed-valence compounds and obtain high valence manganese compounds, we initiated a comprehensive synthesis program to explore manganese phosphates. 12 This mixed-and high-valence manganese phosphate Mn III 6 F 12 (PO 3 (OH)) 8 6 guest clusters. Mixed Mn III and Mn IV organic com-pounds are not uncommon.…”

“…Mixed Mn III and Mn IV organic com-pounds are not uncommon. 12 Of particular interest, the structure of the new mixed-and high-valence manganese phosphate resembles that of the clathrate compounds. Clathrates have been known for over two hundred years and are of growing interests for diverse applications from hydrogen storage 13 to superconductivity 14 and semiconductivity, 15 etc.…”