Tetravalent metal complexation by Keggin and lacunary phosphomolybdate anions

We report the synthesis, spectroscopic and structural characterization, and computational analysis of a series of phosphomolybdate complexes with tetravalent metal cations. The reaction between Ce (IV) and Th (IV) with phosphomolybdate at the optimum pH for the stabilization of the lacunary heteropolyoxometalate anion, [PMo 11O 39] (7-), results in the formation of compounds containing the anions [Ce(PMo 11O 39) 2] (10-) and [Th(PMo 11O 39) 2] (10-), respectively. Single crystal X-ray diffraction analysis was performed on salts of both species, Cs 10[Ce(PMo 11O 39) 2].20H 2O and (NH 4) 10[Th(PMo 11O 39) 2].22H 2O. In both anionic complexes the f-block metal cation is coordinated to the four unsaturated terminal lacunary site oxygens of each [PMo 11O 39] (7-) anion, yielding 8 coordinate sandwich complexes, analogous to previously prepared related complexes. Spectroscopic characterization points to the stability of these complexes in solution over a reasonably wide pH range. Density functional analysis suggests that the Ce-O bond strength in [Ce(PMo 11O 39) 2] (10-) is greater than the Th-O bond strength in [Th(PMo 11O 39) 2] (10-), with the dominant bonding interaction being ionic in both cases. In contrast, under similar reaction conditions, the dominant solid state Zr (IV) and Hf (IV) complexes formed contain the anions [Zr(PMo 12O 40)(PMo 11O 39)] (6-) and [Hf(PMo 12O 40)(PMo 11O 39)] (6-), respectively. In these complexes the central Group 4 d-block metal cations are coordinated to the four unsaturated terminal lacunary site oxygens of the [PMo 11O 39] (7-) ligand and to four bridging oxygens of a plenary Keggin anion, [PMo 12O 40] (3-). In addition, (NH 4) 5{Hf[PMo 12O 40][(NH 4)PMo 11O 39]}.23.5H 2O can be crystallized as a minor product. The structure of the anion, {Hf[PMo 12O 40][(NH 4)PMo 11O 39]} (5-), reveals coordination of the central Hf (IV) cation via four bridging oxygens on both the coordinated [PMo 11O 39] (7-) and [PMo 12O 40] (3-) anions. Unusually, the highly charged lacunary site remains uncoordinated to the Hf metal center but instead interacts with an ammonium cation. (31)P NMR indicates that complexation of the Keggin anion, [PMo 12O 40] (3-), to Hf (IV) and Zr (IV) will stabilize the Keggin anion to a much higher pH than usually observed.     

Group 3 and lanthanide boryl compounds: syntheses, structures, and bonding analyses of Sc-B, Y-B, and Lu-B σ-coordinated NHC analogues

Reaction of [Ln(CH(2)SiMe(3))(2)(THF)(n)][BPh(4)] (Ln = Sc, Y, Lu ; n = 3, 4) with Li{B(NArCH)(2)}(THF)(2) (Ar = 2,6-C(6)H(3)(i)Pr(2)) formed the first group 3 and lanthanide boryl compounds, Sc{B(NArCH)(2)}(CH(2)SiMe(3))(2)(THF) and Ln{B(NArCH)(2)}(CH(2)SiMe(3))(2)(THF)(2) (Ln = Y, Lu), which contain two-center, two-electron Ln-B σ bonds. All of these systems were crystallographically characterized. Density functional theory analysis of the Ln-B bonding found it to be predominantly ionic, with covalent character in the σ-bonding Ln-B HOMO.     

Heterobimetallic complexes containing Ca-Fe or Yb-Fe bonds: synthesis and molecular and electronic structures of [M{CpFe(CO)2}2(THF)3]2 (M = Ca or Yb)

Reaction of calcium or ytterbium amalgam with [CpFe(CO)(2)](2) (Fp(2)) gave the isostructural heavy alkaline earth or divalent rare earth compounds [MFp(2)(THF)(3)](2) (M = Ca or Yb) containing two direct Ca-Fe (3.0185(6) ?) or Yb-Fe (2.9892(4) ?) bonds. Density functional theory supports experiment in finding shorter Yb-Fe than Ca-Fe distances, and Ziegler-Rauk, molecular orbital, and atoms-in-molecules analyses find the M-Fe bonding to be predominantly electrostatic in nature. The Yb-Fe interaction energy and bond critical point electron density are slightly larger than for Ca-Fe, in agreement with the shorter M-Fe bond in the former. The corresponding reaction for magnesium gave MgFp(2)(THF)(4) with two O-bound Fp moieties and no Mg-Fe bond.     

Flow Potentiometric Injection Analysis of Uric Acid Using Lipid Stabilized Films with Incorporated Uricase on ZnO Nanowires

A novel potentiometric uric acid biosensor was fabricated by immobilization of uricase into stabilized lipid films using zinc oxide (ZnO) nanowires as measuring electrode. Uricase was incorporated into the lipid film prior polymerization on the surface of well aligned ZnO nanowires resulting in a sensitive, selective, stable and reproducible uric acid biosensor. The potentiometric response was twice as large from previously reported values due to the presence of a cationic lipid in the lipid film. The sensor response had no interferences by normal concentrations of ascorbic acid, glucose, urea, proteins and lipids.     

Is gluconate a good model for isosaccharinate in uranyl(VI) chemistry? A DFT study

The geometries, relative energies and spectroscopic properties of a range of α-isosaccharinate complexes of uranyl(VI) are studied computationally using ground state and time-dependent density functional theory. The effect of pH is accommodated by varying the number of water and hydroxide ligands accompanying isosaccharinate in the equatorial plane of the uranyl unit. For 1 : 1 complexes, the calculated uranyl ν(asym) stretching frequency decreases as pH increases, in agreement with previous experimental data. Three different isosaccharinate chelating modes are studied. Their relative energies are found to be pH dependent, although the energetic differences between them are not sufficient to exclude the possibility of multiple speciation. At higher pH, the uranyl-ligand interactions are dominated more by the equatorial OH(-) than by the organic ligands. Calculated electronic excitation energies support experiment in finding the lowest energy transitions to be ligand → metal charge transfer. (13)C NMR chemical shifts are calculated for the coordinated isosaccharinate in the high pH mimics, and show good agreement with experimental data, supporting the experimental conclusion that the five-membered chelate ring is favoured at high pH. The effect of increasing the isosaccharinate concentration is modelled by calculating 1 : 2 and 1 : 3 uranyl : α-isosaccharinate complexes. Comparison of the results of the present study with those from our closely related investigation of uranyl(VI)-D-gluconate complexes (Dalton Transactions 40 (2011) 11248) reveals strong similarities in structure, bonding, coordination geometry and electronic excitations, but also differences in ΔG for key ligand replacement reactions, suggesting that caution should be exercised when using gluconate as a thermodynamic model for isosaccharinate in uranyl(vi) chemistry.     

Multiple positive solutions for nonlinear eigenvalue problems with the p-Laplacian

In this paper we consider a nonlinear eigenvalue problem driven by the p$p$-Laplacian differential operator and with a nonsmooth potential. Using degree theoretic arguments based on the degree map for certain operators of monotone type, we show that the problem has at least two nontrivial positive solutions as the parameter λ>0$\lambda >0$ varies in a half-line.     

Molecular structures of two metal tetrakis(tetrahydroborates), Zr(BH4)(4) and U(BH4)(4): equilibrium conformations and barriers to internal rotation of the triply bridging BH4 groups

The molecular structures of Zr[(mu-H)(3)BH](4) and U[(mu-H)(3)BH](4) have been investigated by density functional theory (DFT) calculations and gas electron diffraction (GED). The triply bridged bonding mode of the tetrahydroborate groups in the former is confirmed, but both DFT calculations and GED structure refinements indicate that the BH(4) groups are rotated some 12 degrees away from the orientation in which the three bridging B-H bonds are staggered with respect to the opposing ZrB(3) fragment. As a result the symmetry of the equilibrium conformation is reduced from T(d) to T. Bond distances and valence angles are as follows (DFT/GED): Zr-B = 232.2/232.4(5) pm; Zr-H(b) = 214.8/214.4(6) pm; B-H(b) = 125.3/127.8(8) pm; B-H(t) = 119.4/118.8(17) pm; angle ZrBH(b) = 66.2/65.6(3) degrees; the smallest dihedral angle of type tau(BZrBH(b)) = 48/45(2) degrees. DFT calculations on Hf(BH(4))(4) indicate that the structure of this molecule is very similar to that of the Zr analogue. Matrix-isolation IR spectroscopy and DFT calculations on U(BH(4))(4) show that while the polymeric solid-state structure is characterized by terminal triply bridging and metal-metal bridging bidentate BH(4) groups, all BH(4) groups are triply bridging in the gaseous monomer. Calculations with one of the two nonbonding 5f electrons on U occupying an a(1) and the other distributed equally among the three t(2) orbitals indicate that the equilibrium conformation has T(d) symmetry, i.e. that the three B-H(b) bonds of each tetrahydroborate group are exactly staggered with respect to the opposing UB(3) fragment with tau(BUBH(b)) = 60 degrees. Calculations including spin-orbit interactions indicate that Jahn-Teller distortions from T(d) symmetry are either absent or very small. The best agreement between observed and calculated GED intensity data was obtained for a model of T(d) symmetry, but models of T symmetry with dihedral angles tau(BUBH(b)) > 42 degrees cannot be ruled out. Bond distances and valence angles are as follows (DFT/GED): U-B = 248.8/251.2(4) pm; U-H(b) = 227.7/231.5(6) pm; B-H(b) = 126.0/131.6(5) pm, B-H(t) = 119.5/117.8(11) pm; angle UBH(b) = 65.6/63.1(3) degrees. It is suggested that the different equilibrium conformations of the three molecules are determined primarily by repulsion between bridging H atoms in different tetrahydroborate groups.