Studies of an enneanuclear manganese single-molecule magnet

The reaction of [Mn(3)O(O(2)CMe)(6)(py)(3)] with the tripodal ligand H(3)thme (1,1,1-tris(hydroxymethyl)ethane) affords the enneanuclear complex [Mn(9)O(7)(O(2)CCH(3))(11)(thme)(py)(3)(H(2)O)(2)] 1.1MeCN.1Et(2)O. The metallic skeleton of complex 1 comprises a series of 10 edge-sharing triangles that describes part of an idealized icosahedron. Variable temperature direct current (dc) magnetic susceptibility data collected in the 1.8-300 K temperature range and in fields up to 5.5 T were fitted to give a spin ground state of S = (17)/(2) with an axial zero-field splitting parameter D = -0.29 cm(-)(1). Ac susceptibility studies indicate frequency-dependent out-of-phase signals below 4 K and an effective barrier for the relaxation of the magnetization of U(eff) = 27 K. Magnetic measurements of single crystals of 1 at low temperature show time- and temperature-dependent hysteresis loops which contain steps at regular intervals of field. Inelastic neutron scattering (INS) studies on complex 1 confirm the S = (17)/(2) ground state and analysis of the INS transitions within the zero-field split ground state leads to determination of the axial anisotropy, D = -0.249 cm(-)(1), and the crystal field parameter, B(4)(0) = 7(4) x 10(-)(6) cm(-)(1). Frequency domain magnetic resonance spectroscopy (FDMRS) determined the same parameters as D = -0.247 cm(-)(1) and B(4)(0) = 4.6 x 10(-)(6) cm(-)(1). DFT calculations are fully consistent with the experimental findings of two Mn(II) and four Mn(III) ions "spin up" and three Mn(IV) ions "spin down" resulting in the S = (17)/(2) spin ground state of the molecule, with D = -0.23 cm(-)(1) and U = 26.2 K.     

Single-molecule magnets: a large Mn30 molecular nanomagnet exhibiting quantum tunneling of magnetization

The largest single-molecule magnet (SMM) to date has been prepared and studied. Recrystallization of known [Mn(12)O(12)(O(2)CCH(2)Bu(t))(16)(H(2)O)(4)] (1; 8Mn(III), 4Mn(IV)) from CH(2)Cl(2)/MeNO(2) causes its conversion to [Mn(30)O(24)(OH)(8)(O(2)CCH(2)Bu(t))(32)(H(2)O)(2)(MeNO(2))(4)] (2; 3Mn(II), 26Mn(III), Mn(IV)). The structure of 2 consists of a central, near-linear [Mn(4)O(6)] backbone, to either side of which are attached two [Mn(13)O(9)(OH)(4)] units. Peripheral ligation around the resulting [Mn(30)O(24)(OH)(8)] core is by 32 Bu(t)CH(2)CO(2)(-), 2 H(2)O, and 4 MeNO(2) groups. The molecule has crystallographically imposed C(2) symmetry. Variable-temperature and -field magnetization (M) data were collected in the 1.8-4.0 K and 0.1-0.4 T ranges and fit by matrix diagonalization assuming only the ground state is occupied at these temperatures. The fit parameters were S = 5, D = -0.51 cm(-1) = -0.73 K, and g = 2.00, where D is the axial zero-field splitting parameter. AC susceptibility measurements in the 1.8-7.0 K range in a zero DC field and a 3.5 G AC field oscillating at frequencies in the 50-997 Hz range revealed a frequency-dependent out-of-phase (chi(M)') signal below 3 K, indicating 2 to be a single-molecule magnet (SMM), the largest yet obtained. Magnetization versus DC field sweeps show hysteresis loops but no clear steps characteristic of quantum tunneling of magnetization (QTM). However, magnetization decay data below 1 K were collected and used to construct an Arrhenius plot that revealed temperature-independent relaxation below 0.3 K. The fit of the thermally activated region above approximately 0.5 K gave U(eff)/k = 15 K, where U(eff) is the effective relaxation barrier. Resonant QTM was confirmed from the appearance of a "quantum hole" when the recent quantum hole digging method was employed. The combined results demonstrate that SMMs can be prepared that are significantly larger than any known to date and that this new, large Mn(30) complex still demonstrates quantum behavior.     

Single-molecule magnets: structural characterization, magnetic properties, and (19)F NMR spectroscopy of a Mn(12) family spanning three oxidation levels

The syntheses, crystal structures, and magnetic properties of [Mn(12)O(12)(O(2)CC(6)F(5))(16)(H(2)O)(4)] (2), (NMe(4))[Mn(12)O(12)(O(2)CC(6)F(5))(16)(H(2)O)(4)] (3), and (NMe(4))(2)[Mn(12)O(12)(O(2)CC(6)F(5))(16)(H(2)O)(4)] (4) are reported. Complex 2 displays quasi-reversible redox couples when examined by cyclic voltammetry in CH(2)Cl(2): one-electron reductions are observed at 0.64 and 0.30 V vs ferrocene. The reaction of complex 2 with 1 and 2 equiv of NMe(4)I yields the one- and two-electron reduced analogues, 3 and 4, respectively. Complexes 2.3CH(2)Cl(2), 3.4.5CH(2)Cl(2).(1)/(2)H(2)O, and 4.6C(7)H(8) crystallize in the triclinic P, monoclinic P2/c, and monoclinic C2/c space groups, respectively. The molecular structures are all very similar, consisting of a central [Mn(IV)O(4)] cubane surrounded by a nonplanar alternating ring of eight Mn and eight mu(3)-O(2)(-) ions. Peripheral ligation is provided by 16 bridging C(6)F(5)CO(2)(-) and 4 H(2)O ligands. Bond valence sum calculations establish that the added electrons in 3 and 4 are localized on former Mn(III) ions giving trapped-valence Mn(IV)(4)Mn(III)(7)Mn(II) and Mn(IV)(4)Mn(III)(6)Mn(II)(2) anions, respectively. (19)F NMR spectroscopy in CD(2)Cl(2) shows retention of the solid-state structure upon dissolution and detrapping of the added electrons in 3 and 4 among the outer ring of Mn ions on the (19)F NMR time scale. DC studies on dried microcrystalline samples of 2, 3, and 4.2.5C(7)H(8) restrained in eicosane in the 1.80-10.0 K and 1-70 kG ranges were fit to give S = 10, D = -0.40 cm(-)(1), g = 1.87, D/g = 0.21 cm(-)(1) for 2, S = 19/2, D = -0.34 cm(-)(1), g = 2.04, D/g = 0.17 cm(-)(1) for 3, and S = 10, D = -0.29 cm(-)(1), g = 2.05, D/g = 0.14 cm(-)(1) for 4, where D is the axial zero-field splitting parameter. The clusters exhibit out-of-phase AC susceptibility signals (chi(M)' ') indicative of slow magnetization relaxation in the 6-8 K range for 2, 4-6 K range for 3, and 2-4 K range for 4; the shift to lower temperatures reflects the decreasing magnetic anisotropy upon successive reduction and, hence, the decreasing energy barrier to magnetization relaxation. Relaxation rate vs T data obtained from chi(M)' ' vs AC oscillation frequency studies down to 1.8 K were combined with rate vs T data from DC magnetization decay vs time measurements at lower temperatures to generate an Arrhenius plot from which the effective barrier (U(eff)) to magnetization reversal was obtained; the U(eff) values are 59 K for 2, 49 and 21 K for the slower- and faster-relaxing species of 3, respectively, and 25 K for 4. Hysteresis loops obtained from single-crystal magnetization vs DC field scans are typical of single-molecule magnets with the coercivities increasing with decreasing T and increasing field sweep rate and containing steps caused by the quantum tunneling of magnetization (QTM). The step separations gave D/g values of 0.22 cm(-)(1) for 2, 0.15 and 0.042 cm(-)(1) for the slower- and faster-relaxing species of 3, and 0.15 cm(-)(1) for 4.     

Synthesis and structure of tetranuclear orthometallated Pd(II) complexes derived from bis-iminophosphoranes

The reaction of Pd(OAc)2 with bis-iminophosphoranes Ph3P=NCH2CH2CH2N=PPh3 (1a), [C6H4(C(O)N=PPh3)2-1,3] (1b) and [C6H4(C(O)N=PPh3)2-1,2] (1c), gives the orthopalladated tetranuclear complexes [{Pd(mu-Cl){C6H4(PPh2=NCH2-kappa-C,N)-2}}2CH2]2 (2a) [{Pd(mu-OAc){C6H4(PPh2=NC(O)-kappa-C,N)-2}}2C6H4-1',3']2 (2b) and [{Pd(mu-OAc){C6H4(PPh2=NC(O)-kappa-C,N)-2}}2C6H4-1',2']2 (2c). The reaction takes place in CH2Cl2 for 1a, but must be performed in glacial acetic acid for 1b and 1c. The process implies in all cases the activation of a C-H bond on a Ph ring of the phosphonium group, with concomitant formation of endo complexes. This is the expected behaviour for 1a, but for 1b and 1c reverses the exo orientation observed in other ketostabilized iminophosphoranes. The influence of the solvent in the orientation of the reaction is discussed. The dinuclear acetylacetonate complexes [{Pd(acac-O,O'){C6H4(PPh2=NCH2-kappa-C,N)-2}}2CH2] (3a), [{Pd(acac-O,O'){C6H4(PPh2=NC(O)-kappa-C,N)-2}}2C6H4-1',3'] (3b) and [{Pd(acac-O,O'){C6H4(PPh2=NC(O)-kappa-C,N)-2}}2C6H4-1',2'] (3c) have been obtained from the halide-bridging tetranuclear derivatives. The X-ray crystal structure of [3c.4CHCl3] is also reported.     

Tip and surface determination from experiments and simulations of scanning tunneling microscopy and spectroscopy

We present a very efficient and accurate method to simulate scanning tunneling microscopy images and spectra from first-principles density functional calculations. The wave functions of the tip and sample are calculated separately on the same footing and propagated far from the surface using the vacuum Green function. This allows us to express the Bardeen matrix elements in terms of convolutions and to obtain the tunneling current at all tip positions and bias voltages in a single calculation. The efficiency of the method opens the door to real time determination of both tip and surface composition and structure, by comparing experiments to simulated images for a variety of precomputed tips. Comparison with the experimental topography and spectra of the Si111-(7 x 7) surface shows a much better agreement with Si than with W tips, implying that the metallic tip is terminated by silicon.     

Outstanding Methane Oxidation Performance of Palladium‐Embedded Ceria Catalysts Prepared by a One‐Step Dry Ball‐Milling Method

By carefully mixing Pd metal nanoparticles with CeO₂ polycrystalline powder under dry conditions, an unpredicted arrangement of the Pd‐O‐Ce interface is obtained in which an amorphous shell containing palladium species dissolved in ceria is covering a core of CeO₂ particles. The robust contact that is generated at the nanoscale, along with mechanical forces generated during mixing, promotes the redox exchange between Pd and CeO₂ and creates highly reactive and stable sites constituted by PdO_x embedded into CeO₂ surface layers. This specific arrangement outperforms conventional Pd/CeO₂ reference catalysts in methane oxidation by lowering light‐off temperature by more than 50°C and boosting the reaction rate. The origin of the outstanding activity is traced to the structural properties of the interface, modified at the nanoscale by mechanochemical interaction.