Poly[(2,2'-bipyridine)tetrakis(imidazolato)diiron(II)]: structural and spin-state phase transitions and low-temperature magnetic ordering in a unique 2-dimensional material

Poly[(2,2'-bipyridine)tetrakis(imidazolato)diiron(II)] was synthesized by the reaction of ferrocene with imidazole in an excess of 2,2'- bipyridine in a Carius tube at 130 degrees C. Dc magnetic susceptibility studies at an applied field of 1000 G reveal that on cooling from room temperature, the material undergoes two structural phase transitions designated alpha --> beta and beta --> gamma at 151 and 133 K, respectively. On warming, the gamma --> beta and beta --> alpha transitions are observed at 137 and 151 K, respectively; a 4 K thermal hysteresis clearly detectable in the lower temperature beta <--> gamma transition. These structural phase transitions have also been studied by detailed, variable-temperature, ac susceptibility and M?ssbauer spectroscopy techniques. Single-crystal X-ray diffraction studies done at 294, 143, and 113 K reveal 2, 12, and 6 unique iron centers in the alpha-, beta-, and gamma-forms, respectively. All three forms have the same basic structure involving 2D extended double layer sheets (bilayers) of alternating tetrahedral and octahedral irons singly bridged by imidazolate ligands, with the octahedral centers additionally coordinated by 2,2'-bipyridine ligands that occupy positions between the sheets. Magnetic susceptibility and bond length data reveal that in the gamma-phase one in three six-coordinate sites corresponds to spin singlet ground-state iron(II); i.e., the overall alpha --> gamma process involves a spin-crossover transition such that at least 1/6 of the iron sites in the gamma-phase correspond to S = 0. This is supported by the low-temperature M?ssbauer spectra of the gamma-phase, which reveal the simultaneous presence of both S = 2 and S = 0 iron(II) centers. The compound magnetically orders, with a resultant small remnant magnetization, at low temperatures. The magnetic phase transition, studied by a combination of dc and ac susceptibility and M?ssbauer techniques, occurs at T(c) approximately 6.5 K.     

The valence problem of Pd4Br4Te3

Pd(4)Br(4)Te(3) was prepared from Pd, Te, and PdBr(2) at 700 K. Its structure was determined by single-crystal X-ray diffraction to be triclinic, P$\bar 1$, Pearson symbol aP22; a=842.5(2), b=845.0(3), c=864.8(3) pm; alpha=82.55(3), beta=73.36(2), gamma=88.80(2) degrees ; Z=2. The Br and Te atoms are arranged according to the motif of cubic closest-packed spheres in which every 15th position is vacant; the Pd atoms occupy 8/15 of the octahedral voids. The symmetry relations with the packing of spheres are derived. Prominent structural units are hollow cuboctahedral [(PdBrTe)(6)] units, the Pd atoms are positioned near the centers of the square faces of the Br(6)Te(6) cuboctahedra; the cuboctahedra and double-octahedral Pd(2)Br(4)Te(6) units are connected to strands by sharing triangular Te(3) faces. The strands are condensed by common Br atoms into layered assemblies. Conspicuously close Te--Te contacts in the Te(3) triangles indicate attractive Te--Te interactions. The valence puzzle is resolved by the formula Pd(+II)(4)Br(-I)(4)Te(-4/3)(3). Positive Te--Te Mulliken orbital populations and the Pd--K, Br--K, and Te--L(III) XANES spectra of Pd(4)Br(4)Te(3) referenced to the spectra of PdBr(2), K(2)PdBr(6), PdTe, and PdTe(2) are in accord with attractive Te--Te interactions. The measured semiconducting and diamagnetic properties are compatible with the derived picture of chemical bonding in Pd(4)Br(4)Te(3).     

Boron-induced hydrogen localization in the novel metal hydride LaNi(3)BH(x) (x = 2.5-3.0)

The crystal structure and hydrogenation properties of the intermetallic boride LaNi(3)B were investigated. The hydrogen-free compound has a novel structure with orthorhombic symmetry, space group Imma, a = 4.9698(8) A, b = 7.1337(8) A, c = 8.3001(9) A, and V = 294.26(7) A(3). Thermal gravimetrical analysis reveals a hydride phase that forms near ambient conditions within the compositional range LaNi(3)BH(2.5)(-)(3.0). Single-crystal X-ray diffraction on both the alloy and the hydride, using the same crystal, shows an expansion in the a-c plane (by up to approximately 8%) and a contraction along b (by approximately 3%), while the symmetry changes from Imma to Bmmb (Cmcm) and the unit cell doubles along a and b. The cell parameters for the composition of LaNi(3)BD(2.73(4)) are a = 10.7709(7) A, b = 16.0852(10) A, c = 7.6365(5) A, V = 1323.03(15) A(3), and space group Cmcm. Four nearly fully occupied interstitial hydrogen sites were located by neutron powder diffraction on deuterides and found to have tetrahedral, La(2)Ni(2) (D1,D2), trigonal-prismatic, La(3)Ni(3) (D3), and trigonal-bipyramidal, La(2)Ni(3) (D4), metal environments. The structure can also be described in terms of alternating quasi two-dimensional [NiD](-) slabs (Ni-D = 1.62-1.97 A) and La-B sheets for which bond-valence sums suggest the limiting formula La(3+)B(0)[Ni(3)D(3)](3)(-). The La-B planes do not accommodate deuterium; the B-D and D-D interactions appear to be repulsive. The shortest B-D and D-D contacts are 2.52(2) and 2.33(2) A, respectively.     

17O NMR in room temperature phase of La2Mo2O9 fast oxide ionic conductor

A room temperature (17)O NMR study of La(2)Mo(2)O(9), a fast oxide ionic conductor exhibiting a phase transition at 580 degrees C between a low-temperature alpha-phase and a high-temperature beta-phase, is presented. Four partly overlapping quasi-continuous distributions of oxygen sites are evidenced from 1D magic angle spinning (MAS) and 2D triple quantum MAS NMR experiments. They can be correlated with the three oxygen sites O1, O2 and O3 of the high-temperature crystal structure. The low-temperature phase is characterized by two distributed sites of type O1, which proves that the symmetry is lower than in the cubic high-temperature phase. Two-dimensional experiments show that there is no dynamic exchange process, on the NMR time-scale, between the different oxygen sites at room temperature, which agrees well with conductivity results.     

Vapor Pressure and Solid Phases of Methanol below Its Triple Point Temperature

We present an experimental work devoted to study of the thermodynamical properties of solid methanol. We combine Fourier transform infrared spectroscopy (FTIR) and mass spectrometry (MS) to measure, for the first time, the vapor pressure of various methanol solid phases and determine their Clausius-Clapeyron equations. We perform our experiments between T = 130 K and the triple point temperature T(t) = 175.61 K. When methanol is condensed from its vapor below T(t), we observe three different solid phases depending on temperature. A condensation at T = 130 K forms a metastable phase with an enthalpy of sublimation deltaH(metastable-vapor) = 42.9 +/- 0.5 kJ.mol(-1). Upon heating, this phase transforms itself at T approximately 145 K to the alpha-phase that has an enthalpy of sublimation deltaH(alpha-vapor) = 46.9 +/- 0.2 kJ.mol(-1). Cooling the alpha-phase does not lead back to the metastable phase, whereas heating this alpha-phase leads to the beta-phase occurrence at T(alpha-beta) = 157.36 K. This latter one is stable until T(t) and has an enthalpy of sublimation deltaH(beta-vapor) = 44.2 +/- 0.5 kJ.mol(-1).     

Tetrahedral D atom coordination of nickel and evidence for anti-isostructural phase transition in orthorhombic Ce2Ni7D approximately 4

Hydrogenation of hexagonal Ce2Ni7 was investigated by synchrotron X-ray and neutron powder diffraction. In contrast to the recently investigated lanthanum analogue, which remains hexagonal (La2Ni7D6.5: space group P63/mmc), the cerium compound becomes orthorhombic (Ce2Ni7D approximately 4: space group Pmcn). As in the structurally related CeNi3D2.8, deuterium occupies CeNi2 slabs only, while the bulk of the CeNi5 slabs remains empty. A significant amount of deuterium is bonded in tetrahedral NiD4 units similar to those in nickel-based complex metal hydrides. These findings provide further evidence for directional bonding effects in hydrides that are traditionally considered as "interstitial". Ce2Ni7D approximately 4 displays various orthorhombic lattice distortions, delta = (b/ radical3 - a)/a. Hydrogen pressures of approximately 30 bar stabilize a phase having a negative distortion (delta < 0). Upon a decrease in the pressure, this phase transforms via a two-phase region into another phase having a positive distortion (delta > 0). Both phases are nearly isostructural and have the same space group symmetry and nearly the same composition. This situation is typical for a so-called anti-isostructural phase transition in which delta is considered to be an order parameter. Neither magnetic nor structural transitions have been detected down to 1.5 K.     

On the alpha --> beta transition of carbon-coated highly oriented PVDF ultrathin film induced by melt recrystallization

Poly(vinylidene fluoride) (PVDF) is one of the polymers which exhibit pronounced polymorphic crystalline forms, depending on crystallization conditions. Four different crystalline modifications, i.e., alpha, beta, gamma, and delta, have been reported so far. Among them, even though the alpha-form is the most common one, the beta-phase is the one that has attracted the widest interest due to its extensive piezo- and pyroelectric applications. During the past few decades, a substantial amount of work has been done in attempts to characterize these crystal modifications and transformations among them. It was well documented that the alpha-form PVDF can be easily obtained through melt crystallization of the PVDF at atmospheric pressure. Its beta-counterpart can, however, only be directly obtained by growth from solution, molecular epitaxy on the surface of potassium bromide, melt crystallization at high pressures, or by applying with a strong electric field. Transformation from its alpha-phase to beta-phase has been achieved by mechanical deformation, while the retransformation could be conducted through melt recrystallization at atmospheric pressure. In the present work, the recrystallization behavior of carbon-coated melt-drawn oriented PVDF ultrathin films at atmospheric pressure was studied by means of transmission electron microscopy and electron diffraction. The results indicate that through vacuum evaporating a thin carbon layer on the surface of highly oriented alpha-PVDF ultrathin film, not only has its high orientation been preserved after a complete melting and recrystallization process, but an alpha --> beta transition of PVDF has also been achieved through melt recrystallization at atmospheric pressure. This technique can be successfully used for preparing highly oriented beta-PVDF ultrathin films, especially patterned microstructures of PVDF with its highly oriented polar beta-phase and nonoriented nonpolar alpha-phase.     

On the high-temperature phase transition of Gd5Si2Ge2

The first-order monoclinic-to-orthorhombic (beta-->gamma) phase transition of the giant magnetocaloric material Gd(5)Si(2)Ge(2) was studied using in situ high-temperature single-crystal X-ray diffraction. A special crystal mounting procedure was developed to avoid crystal contamination by oxygen or nitrogen at high temperatures. The elastic beta-->gamma transformation occurs at 300-320 degrees C during heating, and it is reversible during fast and slow heating and slow cooling but irreversible during rapid cooling. Contrary to theoretical predictions, the macroscopic distribution of the Si and Ge atoms remains the same in both the orthorhombic gamma-polymorph and the monoclinic beta-phase. It appears that interstitial impurities may affect stability of both the monoclinic and orthorhombic phases. In the presence of small amounts of air, the beta-->gamma transformation is complete only at 600 degrees C. The interslab voids, which can accommodate impurity atoms, have been located in the structure, and an effect of partially filling these voids with oxygen or nitrogen atoms on the beta-gamma transition is discussed.     

Characterization of Pd-Cu alloy nanoparticles on gamma-Al2O3-supported catalysts

Cu-Pd/Al2O3 bimetallic catalysts have been characterized by XRD, TEM, and EDX techniques. The surface structure has been investigated by FT-IR spectroscopy of low-temperature adsorbed CO in the reduced and in the oxidized state. Evidence has been provided of the formation of Cu-Pd alloy nanoparticles, both of the alpha-phase (disordered fcc) and of the beta-phase (ordered CsCl-type). IR spectra suggest that Cu likely decorates the edges while Pd mostly stays at the main faces. Part of copper disperses as Cu+ on the support even after reduction. The presence of copper seems to modify strongly the sate of oxidized Pd centers in oxidized high-Pd content materials. The redox chemistry of the system, where Pd is reduced more easily than Cu, appears to be very complex.     

Deuterium ordering in Laves-phase deuteride YFe2D4.2

The structure of Laves-phase deuteride YFe₂D_4.2 has been investigated by synchrotron and neutron (ToF) powder diffraction experiments between 60 and 370 K. Below 323 K, YFe₂D_4.2 crystallizes in a fully ordered, monoclinic structure (s.g. Pc, Z=8, a=5.50663(4), b=11.4823(1), c=9.42919(6) Å, β=122.3314(5)°, V=503.765(3) Å³ at 290 K) containing 4 yttrium, 8 iron and 18 deuterium atoms. Most D-D distances are, within the precision of the diffraction experiment, longer than 2.1 Å; the shortest ones are of 1.96 Å. Seven of eight iron atoms are coordinated by deuterium in a trigonal bipyramid, similar to that in TiFeD_1.95−2. The eighth iron atom is coordinated by deuterium in a tetrahedral configuration. The coordination of iron by deuterium, and the iron-deuterium distances point to the importance of the directional bonding between iron and deuterium atoms. The lowering of crystal symmetry due to deuterium ordering occurs at much higher temperature than the magnetic ordering, and is therefore one of the parameters that are at the origin of the magnetic transition at lower temperatures.     

Reactivity of (ferrocenylmethyl)phosphine toward palladium and platinum chlorides. X-ray structure of [Pd(PH2CH2Fc)Cl(mu-PHCH2Fc)]4 (Fc = ferrocenyl), a unique complex containing a Pd4P4 cycle

The reaction of 2 equiv of the air-stable primary phosphine (ferrocenylmethyl)phosphine (PH2CH2Fc, 1) with [Pd(cod)Cl2] (Fc = ferrocenyl; cod = 1,5-cyclooctadiene) at 298 K gave the phosphanido-bridged Pd(II) tetramer [Pd(PH2CH2Fc)Cl(mu-PHCH2Fc)]4 (2), which shows an unprecedented arrangement of four Pd atoms embedded in an eight-membered Pd4P4 ring. An X-ray diffraction study showed that 2 crystallizes in the triclinic space group P with a = 17.607(7) A, b = 17.944(7) A, c = 18.792(7) A, alpha = 107.120(12) degrees, beta = 96.344(13) degrees, gamma = 117.087(15) degrees . Each molecule contains four palladium atoms in a distorted square-planar coordination formed by one chlorine and three phosphorus atoms. Two of the latter belong to bridging primary phosphanides and the remaining one is contributed by a terminal PH2CH2Fc ligand. The coordination environments of neighboring metal centers adopt an almost perpendicular mutual orientation. The reaction of 2 equiv of 1 with [Pt(cod)Cl2] at 323 K yielded the analogous Pt(II) tetramer of formula [Pt(PH2CH2Fc)Cl(mu-PHCH2Fc)]4 (3), which was fully characterized by multinuclear and dynamic NMR, IR, and elemental analyses. Single-crystal X-ray diffraction on 3 confirmed the tetranuclear arrangement in the solid state, but orientational disorder of the molecule precludes a more detailed discussion of the structure. Low-temperature NMR experiments in CD2Cl2 showed the presence of two slowly interconnecting conformers. Reaction of 1 and [M(cod)Cl2] (M = Pd or Pt) at lower temperatures (273 K for Pd, 295 K for Pt) in dichloromethane allowed the detection in solution of the mononuclear species cis-[M(PH2CH2Fc)2Cl2] (M = Pd, 4; M = Pt, 5) which, upon heating, transformed into the tetramers 2 and 3, respectively. Solid samples of 4 and 5 could be isolated after workup at low temperature and were characterized by conventional spectroscopic methods.     

Direct UHV-TEM observation of palladium clusters on a silicon surface.

About 1 monolayer of palladium was deposited onto a silicon (111) 7 x 7 surface at a temperature of about 550 K inside an ultrahigh vacuum transmission electron microscope, resulting in formation of Pd2Si nanoislands and a 1 x 1 surface layer. Pd clusters created from an excess of Pd atoms on the 1 x 1 surface layer were directly observed by in situ plan view high-resolution transmission electron microscopy. When an objective aperture was introduced so that electron diffractions less than 0.20 nm were filtered out, the lattice structure of the 1 x 1 surface with 0.33 nm spacing and the Pd clusters with a trimer shape were visualized. It was found that image contrast of the 1 x 1 lattice on the specific height terraces disappeared, and thereby an atomic structure of the Pd clusters was clearly observed. The appearance and disappearance of the 1 x 1 lattice was explained by the effect of the kinematical diffraction. It was identified that a Pd cluster was composed of three Pd atoms without a centered Si atom, which is consistent with the model proposed previously. The feature of the Pd clusters stuck at the surface step was also described.     

Matrix reactivity of Al and Ga atoms (M) in the presence of silane: generation and characterization of the eta2-coordinated complex M.SiH4, the insertion product HMSiH3, and the MI species MSiH3 in a solid argon matrix

Matrix isolation experiments give evidence for the formation of the loosely bonded metal-silane complex M.SiH(4) by the spontaneous reaction of Al or Ga atoms (M) with silane in a solid Ar matrix at 12 K; however, Ga(2) appears to insert spontaneously into an Si--H bond to form HGaGaSiH(3), probably with the structure HGa(micro-SiH(3))Ga. In M.SiH(4) the metal atom is eta(2)-coordinated by the silane, resulting in a species with C(2v) symmetry. The complex has a distinctive photochemistry: it can be converted on photolysis at lambda approximately 410 or approximately 254 nm to its tautomer, HMSiH(3), which also has a doublet ground electronic state and from which it can be regenerated with lambda approximately 580 nm radiation. Broadband UV-visible photolysis (lambda=200-800 nm) results in decomposition of HMSiH(3), the univalent species MSiH(3) being the only detectable product. The experimental data collected for several silane isotopomers (SiH(4), SiD(4), and SiD(3)H) and different reagent concentrations, together with the results of sophisticated quantum chemical calculations, are used to explore in detail the properties of the detected species and the reaction pathways compassing their formation.     

Equilibrium isotope effects in the preparation and isothermal decomposition of ternary hydrides Pd(HxD1-x)y (0 < x < 1 and y > 0.6)

A Sieverts' apparatus coupled with a residual gas analysis is used to measure the concentration variations of hydrogen isotopes in the gas and solid phases during exchange and isothermal decomposition of mixed hydrides. beta-phase palladium hydrides with known ratios of H:D, Pd(H x D 1- x ) y (0 < x < 1, y > 0.6), are prepared by H 2 with PdD y or D 2 with PdH y exchange, and their desorption isotherms are reported here at 323 K. A higher equilibrium pressure in isothermal desorption of mixed hydrides is associated with a higher ratio of D/H in the initial mixed hydrides in beta-phase. The composition of the gas desorbed from a mixed hydride varies; i.e., the ratio of D/H in gas decreases with the sum of (H + D) in Pd. The values of the separation factor alpha during desorption at 323 K and during H-D exchange at 248 K are discussed and compared with those in the literature. Desorption isotherms of mixed isotope hydrides are between those of the single isotope hydrides of H-Pd and D-Pd, however, plateaus slope more than those of pure isotope hydrides. The origin of the plateau sloping in the mixed hydrides can be attributed to the compositional variations during desorption, i.e., the equilibrium pressure is greater when D/H ratio in solid is greater. A simple model is proposed in this study that agrees well with experimental results.     

Neutron powder diffraction with (nat)Sm: crystal structures and magnetism of a binary samarium deuteride and a ternary samarium magnesium deuteride

Binary SmH(3) (trigonal, a=656.7(3), c=680.1(3) pm, P$\bar 3$c1, Z=6), ternary SmMg2H7 (tetragonal, a=626.47(6), c=937.2(2) pm, P4(1)2(1)2, Z=4) and the corresponding deuterides SmD3 (a=653.9(1)m, c=676.7(2) pm) and SmMg2D7 (a=624.10(1), c=934.81(2) pm) have been prepared by hydrogenation (deuteration) of elemental samarium and the Laves phase SmMg2, respectively, and investigated by X-ray and neutron powder diffraction and SQUID and vibration magnetometry. The problem of the enormous neutron absorption of the natural isotopic mixture (natSm) is circumvented by carefully choosing the neutron wavelength (approximately 50 pm) and the use of double-walled cylindrical sample holders and a high-intensity neutron diffractometer (D4c at ILL). SmD3 crystallises with a tysonite-type structure and has three independently ordered deuterium atom sites with trigonal-planar, trigonal-pyramidal and tetrahedral metal environments and Sm--D bond lengths in the range 220(1)-258(1) pm (average: 235 pm). SmMg2D7 is a new deuteride that crystallises with an LaMg2D7-type structure. It displays four fully occupied deuterium sites having triangular and tetrahedral metal environments and Sm--D bond lengths in the range 227.6(5)-246.8(8) pm (average: 239 pm). These are the first samarium-deuterium bond lengths to be reported. Both deuterides are paramagnetic down to 2 K (SmD3: mueff=0.63(1) muB, thetap approximately -4 K; SmMg2D7: mueff=0.57(2) muB, thetap approximately -4 K). Their crystal structures and chemical and physical properties suggest mainly ionic bonding according to the limiting ionic formulae Sm3+(H-)3 and Sm3+(Mg2+)2(H-)7.     

Self-assembly of D-penicillaminato M6M'8 (M = Ni(II), Pd(II), Pt(II); M' = Cu(I), Ag(I)) clusters and their organization into extended La(III)M6M'8 supramolecular structures

A series of chiral M(6)M'(8) cluster compounds having twelve free carboxylate groups, [M(6)M'(8)(D-pen-N,S)(12)X](5-) (M/M'/X = Pd(II)/Ag(I)/Cl(-) ([1](5-)), Pd(II)/Ag(I)/Br(-) ([2](5-)), Pd(II)/Ag(I)/I(-) ([3](5-)), Ni(II)/Ag(I)/Cl(-) ([4](5-)), Pt(II)/Ag(I)/Cl(-) ([5](5-)), Pd(II)/Cu(I)/Cl(-) ([6](5-)); D-H(2)pen = D-penicillamine), in which six cis-[M(D-pen-N,S)(2)](2-) square-planar units are bound to a [M'(8)X](7+) cubic core through sulfur-bridges, was synthesized by the reactions of cis-[M(D-pen-N,S)(2)](2-) with M' in water in the presence of halide ions. These M(6)M'(8) clusters readily reacted with La(3+) in aqueous buffer to form La(III)(2)M(6)M'(8) heterotrimetallic compounds, La(2)[1](CH(3)COO), La(2)[2](CH(3)COO), La(2)[3](CH(3)COO), La(2)[4](CH(3)COO), La(2)[5](CH(3)COO) and La(2)[6]Cl, in which the M(6)M'(8) cluster units are linked by La(3+) ions through carboxylate groups in a 1?:?2 ratio. While the La(III)(2)M(6)Ag(I)(8) compounds derived from [1](5-), [2](5-), [3](5-), [4](5-) and [5](5-) have a 1D helix supramolecular structure with a right-handedness, the La(III)(2)Pd(II)(6)Cu(I)(8) compound derived from [6](5-) has a 2D sheet-like structure with a triangular grid of the Pd(II)(6)Cu(I)(8) cluster units. When aqueous HCl was added to the reaction solution of [6](5-) and La(3+), another La(III)(2)Pd(II)(6)Cu(I)(8) heterotrimetallic compound, La(2)[6]Cl·HCl, in which the Pd(II)(6)Cu(I)(8) cluster units are linked by La(3+) ions to form a 2D structure with a rectangular grid, was produced. The solid-state structures of these La(III)(2)M(6)M'(8) compounds, determined by single-crystal X-ray crystallography, along with the spectroscopic properties of the M(6)M'(8) cluster compounds in solution, are described.     

Flux synthesis, crystal structures, and solid-state NMR spectroscopy of two indium silicates containing varied In-O coordination geometries

Two novel indium silicates, K5In3Si7O21 (1) and K4In2Si8O21 (2), have been synthesized by a flux-growth method and characterized by single-crystal X-ray diffraction. The structure of 1 consists of siebener single chains of corner-sharing SiO4 tetrahedra running along the b axis linked via corner-sharing by In2O9 face-sharing octahedral dimers and InO5 trigonal bipyramids to form a 3D framework. The structure of 2 consists of a 3D silicate framework containing 6- and 14-ring channels. InO5 square pyramids are located within the 14-ring channels sharing corners with the silicate framework. The solid-state 29Si MAS NMR spectrum of compound 1 was recorded; it shows the influence of the indium atoms in the second coordination sphere of the silicon on the chemical shift. Crystal data: 1, orthorhombic, Pna21 (No. 33), a = 12.4914(3) A, b = 16.8849(3) A, c = 10.2275(2) A, V = 2157.1(1) A3 and Z = 4; 2, monoclinic, P21/n (No. 14), a = 8.4041(3) A, b = 11.4919(4) A, c = 10.4841(3) A, beta = 90.478(2) degrees , V = 1012.5(1) A3 and Z = 2.     

Formation of Metal Clusters or Nitrogen-Bridged Adducts by Reaction of a Bis(amino)stannylene with Halides of Two-Valent Transition Metals

When the cyclic bis(amino)stannylene Me(2)Si(NtBu)(2)Sn is allowed to react with metal halides MX(2) (M = Cr, Fe, Co, Zn; X = Cl, Br [Zn]) adducts of the general formula [Me(2)Si(NtBu)(2)Sn.MX(2)](n) are obtained. The compounds are generally dimeric (n = 2) except the ZnBr(2) adduct, which is monomeric in benzene. The crystal structures of [Me(2)Si(NtBu)(2)Sn.CoCl(2)](2) (triclinic, space group &Pmacr;1; a = 8.620(9) ?, b = 9.160(9) ?, c = 12.280(9) ?, alpha = 101.2(1) degrees, beta = 97.6(1) degrees, gamma = 105.9(1) degrees, Z = 1) and of [Me(2)Si(NtBu)(2)Sn.ZnCl(2)](2) (monoclinic, space group P2(1)/c; a = 8.156(9) ?, b = 16.835(12) ?, c = 13.206(9) ?, beta = 94.27(6) degrees, Z = 2) were determined by X-ray diffraction techniques. The two compounds form similar polycyclic, centrosymmetrical assemblies of metal atoms bridged by chlorine or nitrogen atoms. While in the case of the cobalt compound Co is pentacoordinated by three chlorine and two nitrogen atoms, in the zinc derivative Zn is almost tetrahedrally coordinated by three chlorine atoms and one nitrogen atom. The iron derivative [Me(2)Si(NtBu)(2)Sn.FeCl(2)](2) seems to be isostructural with the cobalt compound as can be deduced from the crystal data (triclinic, a = 8.622(7) ?, b = 9.158(8) ?, c = 12.353(8) ?, alpha = 101.8(1) degrees, beta = 96.9(1) degrees, gamma = 105.9(1) degrees, Z = 1). If NiBr(2), PdCl(2), or PtCl(2) is combined with the stannylene, the reaction product is totally different: 4 equiv of the stannylene are coordinating per metal halide, forming the molecular compound [Me(2)Si(NtBu)(2)Sn](4)MX(2), which crystallizes with half a mole of benzene per molecular formula. The crystal structures of [Me(2)Si(NtBu)(2)Sn](4).NiBr(2).(1)/(2)C(6)H(6) (tetragonal, space group I4(1)/a, a = b = 43.86(4) ?, c = 14.32(2) ?, Z = 16) and [Me(2)Si(NtBu)(2)Sn](4).PdCl(2).(1)/(2)C(6)H(6) (tetragonal, space group I4(1)/a, a = b = 43.99(4) ?, c = 14.318(14) ?, Z = 16) reveal the two compounds to be isostructural. The molecules have an inner Sn(4)M pentametallic core (mean distances: Sn-Ni 2.463 ?, Sn-Pd 2.544 ?) with the transition metal in the center of a slightly distorted square formed by the four tin atoms, the distortion from planarity resulting in a weak paramagnetism of 0.2 &mgr;(B) for the nickel compound. The halogen atoms form bridges between two of the tin atoms and have no bonding interaction with the transition metal. The nickel compound has also been prepared by direct interaction of Br(2) or NR(4)Br(3) with [Me(2)Si(NtBu)(2)Sn](4)Ni as a minor product, the main products being Me(2)Si(NtBu)(2)Sn(NtBu)(2)SiMe(2,) Me(2)Si(NtBu)(2)SnBr(2), NiBr(2) and SnBr(2). Other metal clusters have been obtained by the reaction of Me(2)Si(NtBu)(2)Sn with tetrakis(triphenyphosphine)palladium or by the reaction of Me(2)Si(NtBu)(2)Ge with RhCl(PPh(3))(3). In the first case Ph(3)PPd[Sn(NtBu)(2)SiMe(2)](3)PdPPh(3) (rhombohedral, space group R3c, a = b = 21.397(12) ?, c = 57.01(5) ?, alpha = beta = 90 degrees, gamma = 120 degrees, Z = 12) is formed and is characterized by X-ray techniques to be composed of a central PdSn(3)Pd trigonal bipyramid with the tin atoms occupying the equatorial positions (Pd-Sn = 2.702(5) ?). In the second reaction all the triphenylphosphine ligands are replaced from rhodium and Rh[Ge(NtBu)(2)SiMe(2)](4)Cl is formed (monoclinic, space group P2(1)/n, a = 12.164(2) ?, b = 23.625(5) ?, c = 24.128(5) ?, beta = 102.74(3) degrees, Z = 4). The central core of this molecule is made up of a rhodium atom which is almost square planarly coordinated by the germanium atoms, two of which are bridged by chlorine (mean Ge-Rh = 2.355 ?).     

Growth and sintering of Pd clusters on alpha-Al2O3(0001)

The growth and sintering of Pd nanoparticles on alpha-Al(2)O(3)(0001) have been studied by noncontact atomic force microscopy (NC-AFM), low-energy ion scattering spectroscopy (LEIS), temperature-programmed desorption (TPD) and x-ray photoelectron spectroscopy (XPS). This is the first study of metal nanoparticles on a well-defined oxide surface where both NC-AFM and LEIS are used for characterization. These prove to be a powerful combination in assessing particle dimensions. The clean alumina surface showed atomically flat, 200-700 nm wide terraces. The sharp step edges are straight (within our resolution) for lengths of >300 nm and have heights in multiples of 0.2 nm. The Pd grows initially as two-dimensional (2D) islands at 300 K, with the transition to 3D particle growth at 0.25 ML (ML=monolayers). Upon heating at 1 K/s, the Pd starts to sinter below 400 K, and sinters at a nearly constant rate with increasing temperature, covering approximately 50% less of the alumina surface by approximately 1000 K, with a doubling in particle diameter and an eightfold decrease in particle number density. By approximately 1000 K, the number density was approximately 9 x 10(11)cm(2) for 0.8 ML of Pd, with an average diameter of 5 nm and an average thickness of 1 nm.     

Synthesis and structural characterization of configurational isomers of tungsten-palladium complexes with bridging diphenyl(dithioalkoxycarbonyl)phosphine as a ligand and phosphine transfer from tungsten to palladium

Treatment of the complex [W(CO)5[PPh2(CS2Me)]] (2) with [Pd(PPh3)4] (1) affords binuclear complexes such as anti-[(Ph3P)2Pd[mu-eta 1,eta 2-(CS2Me)PPh2]W(CO)5] (3), syn-[(Ph3P)2Pd[mu-eta 1,eta 2-(CS2Me)PPh2]W(CO)5] (4), and trans-[W(CO)4(PPh3)2] (5). In 3 and 4, respectively, the W and Pd atoms are in anti and syn configurations with respect to the P-CS2 bond of the diphenyl(dithiomethoxycarbonyl)phosphine ligand, PPh2(CS2Me). Complex 3 undergoes extensive rearrangement in CHCl3 at room temperature by transfer of a PPh3 ligand from Pd to W, eliminating [W(CO)5(PPh3)] (7), while the PPh2CS2Me ligand transfers from W to Pd to give [[(Ph3P)Pd[mu-eta 1,eta 2-(CS2Me)PPh2]]2] (6). In complex 6, the [Pd(PPh3)] fragments are held together by two bridging PPh2(CS2Me) ligands. Each PPh2(CS2Me) ligand is pi-bonded to one Pd atom through the C=S linkage and sigma-bonded to the other Pd through the phosphorus atom, resulting in a six-membered ring. Treatment of Pd(PPh3)4 with [W(CO)5[PPh2[CS2(CH2)nCN]]] (n = 1, 8a; n = 2, 8b) in CH2Cl2 affords syn-[(Ph3P)2Pd[mu-eta 1,eta 2-[CS2(CH2)nCN]PPh2]W(CO)5] (n = 1, 9a; n = 2, 9b). Similar configurational products syn-[(Ph3P)2Pd[mu-eta 1,eta 2-(CS2R)PPh2]W(CO)5] (R = C2H5, C3H5, C2H4OH, C3H6CN, 11a-d) are synthesized by the reaction of Pd(PPh3)4 with [W(CO)5[PPh2(CS2R)]] (R = C2H5, C3H5, C2H4OH, C3H6CN, 10a-d). Although complexes 11a-d have the same configuration as 9a,b, the SR group is oriented away from Pd in the former and near Pd in the latter. In these complexes, the diphenyl(dithioalkoxycarbonyl)phosphine ligand is bound to the two metals through the C=S pi-bonding and to phosphorus through the sigma-bonding. All of the complexes are identified by spectroscopic methods, and the structures of complexes 3, 6, 9a, and 11d are determined by single-crystal X-ray diffraction. Complexes 3, 9, and 11d crystallize in the triclinic space group P1 with Z = 2, whereas 6 belongs to the monoclinic space group P2/c with Z = 4. The cell dimensions are as follows: for 3, a = 10.920(3) A, b = 14.707(5) A, c = 16.654(5) A, alpha = 99.98(3) degrees, beta = 93.75(3) degrees, gamma = 99.44(3) degrees; for 6, a = 15.106(3) A, b = 9.848(3) A, c = 20.528(4) A, beta = 104.85(2) degrees; for 9a, a = 11.125(3) A, b = 14.089(4) A, c = 17.947(7) A, alpha = 80.13(3) degrees, beta = 80.39(3) degrees, gamma = 89.76(2) degrees; for 11d, a = 11.692(3) A, b = 13.602(9) A, c = 18.471(10) A, alpha = 81.29(5) degrees, beta = 80.88(3) degrees, gamma = 88.82(1) degrees.