Synthesis, characterization and reactivity of tetranuclear ruthenium hydrido clusters containing chiral phosphine ligands

The chiral clusters [H(4)Ru(4)(CO)(12-n)(L)(n)] (n = 1, 2; L = NMDPP), 1,1-[H(4)Ru(4)(CO)(10)(L-L)] (L-L = DUPHOS, DIPAMP), 1,2-[H(4)Ru(4)(CO)(10)(DIOP)] and [{H(4)Ru(4)(CO)(10)(DIOP)}(2)] have been synthesized by derivatizing the parent carbonyl cluster [H(4)Ru(4)(CO)(12)] with the appropriate mono- or didentate chiral phosphine ligand. The phosphine-substituted clusters were found to be able to catalyze the (asymmetric) hydrogenation of tiglic acid albeit with relatively low selectivity (enantiomeric excesses varying from 0 to 23%). It was found that the stability of the chiral ruthenium hydride clusters and the product distribution obtained in the catalytic reactions are dependent on the nature of the chiral phosphine. The crystal structures of [H(4)Ru(4)(CO)(12-n)(L)(n)] (n = 1, 2; L = NMDPP), 1,1-[H(4)Ru(4)(CO)(10)(L-L)] (L-L = DUPHOS, O-DUPHOS (partially oxygenated ligand), DIPAMP), 1,2-[H(4)Ru(4)(CO)(10)(DIOP)] and [{H(4)Ru(4)(CO)(10)(DIOP)}(2)] are presented.     

Propagation of a reaction front accompanied by island formation: CO/Au/Ni(1 1 1)

Recent high-pressure scanning tunneling microscopy studies, performed at room temperature, have explicitly demonstrated the specifics of the CO-mediated removal of Ni atoms from the topmost layer of an Au/Ni(1 1 1) surface alloy. After an incubation period, the reaction is found to start at step edges. On each edge, a large fraction of Ni atoms is removed from the terrace in certain areas, whereas other areas are nearly intact after a given time. With increasing time, the former areas begin to overlap and the reaction front becomes somewhat more homogeneous. The Au atoms remaining behind the front form nm-sized islands. Here, we present Monte Carlo simulations reproducing all these observations.     

Synthesis, characterization, and reactivity studies of heterodinuclear complexes modeling active sites in purple acid phospatases

To model the heterodinuclear active sites in plant purple acid phosphatases, a mononuclear synthon, [Fe(III)(H(2)IPCPMP)(Cl(2))][PF(6)] (1), has been generated in situ from the ligand 2-(N-isopropyl-N-((2-pyridyl)methyl)aminomethyl)-6-(N-(carboxylmethyl)-N-((2-pyridyl)methyl)amino methyl)-4-methylphenol (IPCPMP) and used to synthesize heterodinuclear complexes of the formulas [Fe(III)M(II)(IPCPMP)(OAc)(2)(CH(3)OH)][PF(6)] (M = Zn (2), Co (3), Ni (4), Mn (5)), [Fe(III)Zn(II)(IPCPMP)(mpdp)][PF(6)] (6) (mpdp = meta-phenylene-dipropionate), and [Fe(III)Cu(II)(IPCPMP) (OAc)}(2)(μ-O)][PF(6)] (7). Complexes 2-4, 6, and 7 have been crystallographically characterized. The structure of 6 is a solid state coordination polymer with heterodinuclear monomeric units, and 7 is a tetranuclear complex consisting of two heterodinuclear phenolate-bridged Fe(III)Cu(II) units bridged through a μ-oxido group between the two Fe(III) ions. Mo?ssbauer spectra confirm the presence of high spin Fe(III) in an octahedral environment for 1, 3, and 5 while 2 and 4 display relaxation effects. Magnetic susceptibility measurements indicate weak antiferromagnetic coupling for 3, 4, and 5 and confirm the assignment of the metal centers in 2-5 as high spin Fe(III)-M(II) (M = Zn, Co (high spin), Ni (high spin), Mn (high spin)). Complexes 2-5 are intact in acetonitrile solution as indicated by IR spectroscopy (for 2-4) and electrospray ionization mass spectrometry (ESI-MS) but partly dissociate to hydroxide species and a mononuclear complex in water/acetonitrile solutions. UV-vis spectroscopy reveal pH-dependent behavior, and species that form upon increasing the pH have been assigned to μ-hydroxido-bridged Fe(III)M(II) complexes for 2-5 although 2 and 3 is further transformed into what is propsed to be a μ-oxido-bridged tetranuclear complex similar to 7. Complexes 2-5 enhance phosphodiester cleavage of 2-hydroxy-propyl-p-nitrophenyl phosphate (HPNP) and bis(2,4-dinitrophenyl)phosphate (BDNPP), but the reactivities are different for different complexes and generally show strong pH dependence.     

Plasmon hybridization in nanoshells with a nonconcentric core

We apply the plasmon hybridization method to a nanoshell with a nonconcentric (offset) core and investigate how the energy and excitation cross section of the plasmon modes depend on the offset distance D of the inner core from the nanoshell center. A two-center spherical coordinate system is used for mathematical convenience. It is shown that the presence of an offset core shifts the plasmon energies and makes higher multipolar nanoshell plasmons dipole active and visible in the optical spectrum. The dependence of the plasmon shifts on D is weak for small offsets but strong for large offsets. The polarization dependence of the optical absorption spectra is found to be relatively weak. The electromagnetic field enhancements are shown to be much larger than on a concentric nanoshell. The results agree very well with results from finite difference time domain simulations.     

Abnormal nuclear matter and many-body forces

Qualitative aspects of quantum corrections to the Lee-Wick abnormal nuclear matter are studied in terms of many-body forces in the normal nuclear matter implied by the σ-model Lagrangian field theory. Using a simplified model for the scalar meson self-energy in the nuclear medium and restricting to a set of graphs which in non-relativistic normal nuclear matter reduces to the well-known random phase approximation (RPA), we have found that an abnormal nuclear state can be bound or unbound depending upon whether strongly attractive multi-body forces are present or absent in the normal matter. This is in support of our previous result obtained heuristically from some general considerations of quantum corrections. A strongly bound abnormal matter with an equilibrium density of a few times the normal nuclear matter density ρ₀ can be formed if large attractive manybody forces can be accommodated in the normal nuclear matter. However if one accepts the present status of theories of nuclear matter binding energy in which no attractive many-body forces are called for, then the abnormal state can occur only at large densities (perhaps 8 to 10 times ρ₀) and is expected to be unbound by several hundred MeV per particle.     

Plasmon modes of curvilinear metallic core/shell particles

The plasmon hybridization method is generalized to calculate the plasmon modes and optical properties of solid and dielectric-core/metallic-shell particles of geometrical structures that can be described using separable curvilinear coordinates. The authors present a detailed discussion of the plasmonic properties of hollow metallic nanowires with dielectric cores and core/shell structures of oblate and prolate spheroidal shapes. They show that the plasmon frequencies of these particles can be expressed in a common form and that the plasmon modes of the core/shell structures can be viewed as resulting from the hybridization of the solid particle plasmons associated with the outer surface of the shell and of the cavity plasmons associated with the inner surface.     

Two modes of C-H bond activation of tris(2-thienyl)phosphine in trinuclear osmium carbonyl clusters

Tris(2-thienyl)phosphine, P(C₄H₃S)₃, reacts with [Os₃(CO)₁₂] at 110 °C to give the phosphine-substituted derivatives [Os₃(CO)₁₁{P(C₄H₃S)₃}] (1), [Os₃(CO)₁₀{P(C₄H₃S)₃}₂] (2) and [Os₃(CO)₉{P(C₄H₃S)₃}₃] (4), as well as the C-H activated product [Os₃(μ-H)(CO)₉{μ-P(C₄H₂S)(C₄H₃S)₂}{P(C₄H₃S)₃}] (3), in which the bridging ligand is equatorially coordinated to two osmium atoms. Thermolysis of 2 in refluxing toluene results in the formation of 3. Compound 1 can also be prepared in high yield from [Os₃(CO)₁₁(NCMe)]. The reaction of [Os₃(μ-H)₂(CO)₁₀] with tris(2-thienyl)phosphine at room temperature afforded [Os₃(μ-H)₂(CO)₉{P(C₄H₃S)₃}] (5) and [Os₃H(μ-H)(CO)₁₀{P(C₄H₃S)₃}] (6), with the ligand coordinated through the phosphorus atom whereas at elevated temperature the cyclometallated compounds [Os₃(μ-H)(CO)₉{μ₃-P(C₄H₂S)(C₄H₃S)₂}] (7) and [Os₃(μ-H)(CO)₈{μ₃-P(C₄H₂S)(C₄H₃S)₂{P(C₄H₃S)₃}] (8) were obtained in addition to 5. Heating 6 in refluxing heptane furnished 5 via loss of one carbonyl ligand. Thermolysis of 1 and 3 in refluxing toluene gives 7 and 8, respectively, in good yields. In 3, the μ-P(C₄H₂S)(C₄H₃S)₂ ligand is coordinated through the phosphorus to one Os atom and through a σ-Os-C bond to the second osmium atom. Compound 7 contains the μ₃-P(C₄H₂S)(C₄H₃S)₂ ligand bound through phosphorus to one Os atom, through a σ-Os-C bond to another and by an η² (π)-interaction to the third osmium atom. Compounds 1, 2 and 4 contain the ligand coordinated exclusively through the phosphorus atom. The crystal and molecular structures of 2, 3, 5, 6 and 7 are reported.     

Efficient Cluster‐Based Catalysts for Asymmetric Hydrogenation of α‐Unsaturated Carboxylic Acids

The new clusters [H₄Ru₄(CO)₁₀(μ‐1,2‐P‐P)], [H₄Ru₄(CO)₁₀(1,1‐P‐P)] and [H₄Ru₄(CO)₁₁(P‐P)] (P‐P=chiral diphosphine of the ferrocene‐based Josiphos or Walphos ligand families) have been synthesised and characterised. The crystal and molecular structures of eleven clusters reveal that the coordination modes of the diphosphine in the [H₄Ru₄(CO)₁₀(μ‐1,2‐P‐P)] clusters are different for the Josiphos and the Walphos ligands. The Josiphos ligands bridge a metal–metal bond of the ruthenium tetrahedron in the “conventional” manner, that is, with both phosphine moieties coordinated in equatorial positions relative to a triangular face of the tetrahedron, whereas the phosphine moieties of the Walphos ligands coordinate in one axial and one equatorial position. The differences in the ligand size and the coordination mode between the two types of ligands appear to be reflected in a relative propensity for isomerisation; in solution, the [H₄Ru₄(CO)₁₀(1,1‐Walphos)] clusters isomerise to the corresponding [H₄Ru₄(CO)₁₀(μ‐1,2‐Walphos)] clusters, whereas the Josiphos‐containing clusters show no tendency to isomerisation in solution. The clusters have been tested as catalysts for asymmetric hydrogenation of four prochiral α‐unsaturated carboxylic acids and the prochiral methyl ester (E)‐methyl 2‐methylbut‐2‐enoate. High conversion rates (>94 %) and selectivities of product formation were observed for almost all catalysts/catalyst precursors. The observed enantioselectivities were low or nonexistent for the Josiphos‐containing clusters and catalyst (cluster) recovery was low, suggesting that cluster fragmentation takes place. On the other hand, excellent conversion rates (99–100 %), product selectivities (99–100 % in most cases) and good enantioselectivities, reaching 90 % enantiomeric excess (ee) in certain cases, were observed for the Walphos‐containing clusters, and the clusters could be recovered in good yield after completed catalysis. Results from high‐pressure NMR and IR studies, catalyst poisoning tests and comparison of catalytic properties of two [H₄Ru₄(CO)₁₀(μ‐1,2‐P‐P)] clusters (P‐P=Walphos ligands) with the analogous mononuclear catalysts [Ru(P‐P)(carboxylato)₂] suggest that these clusters may be the active catalytic species, or direct precursors of an active catalytic cluster species.