Expansion of the Family of Molecular Nanoparticles of Cerium Dioxide and Their Catalytic Scavenging of Hydroxyl Radicals.

The syntheses, crystal structures, and catalytic radical scavenging activity are reported for four new molecular clusters that have resulted from a bottom-up molecular approach to nanoscale CeO2. They are [Ce6O4(OH)4(dmb)12(H2O)4] (dmb- = 2,6-dimethoxybenzoate), [Ce16O17(OH)6(O2CPh)24(HO2CPh)4], [Ce19O18(OH)9(O2CPh)27(H2O)(py)3], and [Ce24O27(OH)9(O2CPh)30(py)4]. They represent a major expansion of our family of so-called "molecular nanoparticles" of this metal oxide to seven members, and their crystal structures confirm that their cores all possess the fluorite structure of bulk CeO2. In addition, they have allowed the identification of surface features such as the close location of multiple Ce3+ ions and organic ligand binding modes not seen previously. The ability of all seven members to catalytically scavenge reactive oxygen species has been investigated using HO• radicals, an important test reaction in the ceria nanoparticle biomedical literature, and most have been found to exhibit excellent antioxidant activities compared to traditional ceria nanoparticles, with their activity correlating inversely with their surface Ce3+ content.

Atomically-precise colloidal nanoparticles of cerium dioxide.

Synthesis of truly monodisperse nanoparticles and their structural characterization to atomic precision are important challenges in nanoscience. Success has recently been achieved for metal nanoparticles, particularly Au, with diameters up to 3 nm, the size regime referred to as nanoclusters. In contrast, families of atomically precise metal oxide nanoparticles are currently lacking, but would have a major impact since metal oxides are of widespread importance for their magnetic, catalytic and other properties. One such material is colloidal CeO2 (ceria), whose applications include catalysis, new energy technologies, photochemistry, and medicine, among others. Here we report a family of atomically precise ceria nanoclusters with ultra-small dimensions up to ~1.6 nm (~100 core atoms). X-ray crystallography confirms they have the fluorite structure of bulk CeO2, and identifies surface features, H+ binding sites, Ce3+ locations, and O vacancies on (100) facets. Monodisperse ceria nanoclusters now permit investigation of their properties as a function of exact size, surface morphology, and Ce3+:Ce4+ composition.

Three-Dimensional (3-D) Ferromagnetic Network of Mn12 Single-Molecule Magnets: Subtle Environmental Effects and Switching to Antiferromagnetic.

A new member of the Mn12 family of single-molecule magnets (SMMs) has been prepared and found to be the first of this family to give a 3-D ferromagnetic network. [Mn12O12(O2CC6H4-p-F)16(H2O)4] (2) was prepared by carboxylate substitution on the acetate derivative with p-F-benzoic acid and crystallizes as 2·8MeCN in space group I4̅2m with extensive formation of intermolecular C-H···F hydrogen-bonding. The latter leads to a combination of ferromagnetic (F) and antiferromagnetic (AF) interactions and an overall F network that gives a χMT value at low T that is abnormally high for an S = 10 ground state. 2·8MeCN undergoes solvent loss under vacuum to 2, with a decrease in unit-cell volume of 17%, primarily due to a 13% decrease in the c-axis. The χMT vs T plot for 2 indicates a switch to a net AF network. Exposure to air causes hydration to 2·3H2O, a concomitant increase in unit cell volume, and a switch back to a F network. The same conversion of 2·8MeCN to 2·3H2O can also be accomplished in one step rather than two steps, by leaving crystals of the former exposed to air at ambient temperature and pressure for 10 days, giving the same magnetic plots. Interestingly, the desolvation/solvation processes cause Jahn-Teller isomerism to occur, but the ratio of the faster-relaxing isomer to the normal slowly relaxing one does not change monotonically. Single-crystal micro-SQUID studies on 2·8MeCN show the expected magnetization hysteresis loops for a SMM and a small exchange-bias from the intermolecular interactions that is unexpectedly AF. Since the micro-SQUID study only identifies interactions along the easy-axis (z-axis) of the crystal, this is readily rationalized as due to the Jz components of the intermolecular interactions in 2·8MeCN being net AF. The combined results offer useful insights into the degree of sensitivity of the magnetic properties to small environmental perturbations.

Magnetostructural Correlation for High-Nuclearity Iron(III)/Oxo Complexes and Application to Fe5, Fe6, and Fe8 Clusters.

The synthesis and characterization are reported of two new polynuclear Fe(III) complexes containing the anion of 8-hydroxyquinoline (hqnH), an N,O-chelating ligand. The complexes are [Fe8O4(O2CPh)10(hqn)4(OMe)2] (1) and [Fe6O2(OH)2(O2CPh)10(hqn)2] (2) and were obtained from reactions in MeOH (1) or H2O (2) using either low-nuclearity preformed clusters or simple metal salts as starting materials. Variable-temperature, solid-state dc and ac magnetic susceptibility studies were carried out and indicate S = 0 and S = 5 ground states for 1 and 2, respectively. In order to rationalize the ground states of these and other higher-nuclearity Fe(III)/O clusters, a magnetostructural correlation (MSC) has been developed specifically for polynuclear Fe(III)/O systems that predicts the exchange interaction constant (Jij) between two Fe(III) atoms based on the Fe-O distances and Fe-O-Fe angles at monoatomically bridging ligands. This correlation was refined using selected tri- and tetranuclear complexes in the literature for which both crystal structures and reliable experimentally determined Jij values were available. The predictive capability of the MSC was evaluated by rationalizing the ground-state spins of 1, 2, and other Fe5-Fe8 clusters, simulating the dc magnetic susceptibility data of polynuclear Fe(III) complexes, and fitting experimental dc magnetic susceptibility vs T data. The latter fits were evaluated to identify and eliminate systematic errors, and this allowed a protocol to be developed for application of this MSC to other polynuclear Fe(III)/oxo clusters.

A new "offset" analogue of the classical oxime-bridged [Mn(III)6] single-molecule magnets.

A new "offset" analogue of the classical [Mn6O2]-core oxime-bridged single-molecule magnets is introduced with a modified stacking arrangement of the [Mn3O] units. Studies of the magnetic properties reveal antiferromagnetic exchange interactions, a spin S = 4 ground state and population of low-lying excited states. Slow relaxation of the magnetization can be detected, with a corresponding energy barrier of 35.8 K. Interpretation of these features is supported with high-frequency EPR studies, quantifying the easy-axis type magnetic anisotropy, leading to a biaxial system. Redox properties investigated by cyclic and differential pulse voltammetry reveal multiple irreversible redox processes.

Manganese/cerium clusters spanning a range of oxidation levels and CeMn(8), Ce(2)Mn(4), and Ce(6)Mn(4) nuclearities: structural, magnetic, and EPR properties.

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 = -2JSi·Sj 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.