Reduction of Cu(II) in matrix-assisted laser desorption/ionization mass spectrometry

The mechanisms of the reduction of Cu(II) in matrix-assisted laser desorption/ionization mass spectrometry (MALDI) are studied. In MALDI mass spectra, ions cationized by copper mostly contain Cu(I) even if Cu(II) salts are added to the sample. It was found that Cu(II) was reduced to Cu(I) by gas-phase charge exchange with matrix molecules, which is a thermodynamically favorable process. Under some conditions, large amounts of free electrons are present in the plume. Cu(II) can be even more efficiently reduced to Cu(I) by free electron capture in the gas phase. The matrices studied in this work are nicotinic acid, dithranol, and 2,5-dihydroxybenzoic acid.     

Modulation of the pK(a) of metal-bound water via oxidation of thiolato sulfur in model complexes of Co(III) containing nitrile hydratase: insight into possible effect of cysteine oxidation in Co-nitrile hydratase

The Co(III) complexes of N,N'-bis(2-mercaptophenyl)pyridine-2,6-dicarboxamide (PyPSH(4)), a designed pentadentate ligand with built-in carboxamide and thiolate groups, have been synthesized and studied to gain insight into the role of Cys-S oxidation in Co-containing nitrile hydratase (Co-NHase). Reaction of [Co(NH(3))(5)Cl]Cl(2) with PyPS(4)(-) in DMF affords the thiolato-bridged dimeric Co(III) complex (Et(4)N)(2)[Co(2)(PyPS)(2)] (1). Although the bridged structure is quite robust, reaction of (Et(4)N)(CN) with 1 in acetonitrile affords the monomeric species (Et(4)N)(2)[Co(PyPS)(CN)] (2). Oxidation of 2 with H(2)O(2) in acetonitrile gives rise to a mixture which, upon chromatographic purification, yields K(2)[Co(PyPSO(2)(OSO(2))(CN] (3), a species containing asymmetrically oxidized thiolates. The Co(III) metal center in 3 is coordinated to a S-bound sulfinate and an O-bound sulfonate (OSO(2)) group. Upon oxidation with H(2)O(2), 1 affords an asymmetrically oxidized dimer (Et(4)N)(2)[Co(2)(PyPS(SO(2)))(2)] (4) in which only the terminal thiolates are oxidized to form S-bound sulfinate groups while the bridging thiolates remain unchanged. The thiolato-bridge in 4 is also cleaved upon reaction with (Et(4)N)(CN) in acetonitrile, and one obtains (Et(4)N)(2)[Co(PyPS(SO(2)))(CN)] (5), a species that contains both coordinated thiolate and S-bound sulfinate around Co(III). The structures of 1-4 have been determined. The spectroscopic properties and reactivity of all the complexes have been studied to understand the behavior of the Co(III) site in Co-NHase. Unlike typical Co(III) complexes with bound CN(-) ligands, the Co(III) centers in 2 and 5 are labile and rapidly lose CN(-) in aqueous solutions. Since 3 does not show this lability, it appears that at least one thiolato sulfur donor is required in the first coordination sphere for the Co(III) center in such species to exhibit lability. Both 2 and 5 are converted to the aqua complexes [Co(PyPS)(H(2)O)](-) and [Co(PyPS(SO(2))(H(2)O)](-) in aqueous solutions. The pK(a) values of the bound water in these two species, determined by spectrophotometry, are 8.3 +/- 0.03 and 7.2 +/- 0.06, respectively. Oxidation of the thiolato sulfur (to sulfinate) therefore increases the acidity of the bound water. Since 2 and 5 promote hydrolysis of acetonitrile at pH values above their corresponding pK(a) values, it is also evident that a metal-bound hydroxide is a key player in the mechanism of hydrolysis by these model complexes of Co-NHase. The required presence of a Cys-sulfinic residue and one water molecule at the Co(III) site of Co-NHase as well as the optimal pH of the enzyme near 7 suggests that (i) modulation of the pK(a) of the bound water molecule at the active site of the enzyme could be one role of the oxidized Cys-S residue(s) and (ii) a cobalt-bound hydroxide could be responsible for the hydrolysis of nitriles by Co-NHase.     

Microscopic approach to the pseudo-spin-1/2 Hamiltonian for Kramers doublets in exchange coupled Co(II) pairs

The microscopic theory of the magnetically anisotropic effective pseudo-spin-(1)/(2) Hamiltonian for a pair of Co(2+) ions is reported. In the framework of the second-order perturbation approach, the analytical expressions are found for the components of the tensor of the exchange interaction, g-tensor, and the factor in the temperature independent paramagnetic contribution. The parameters of the Hamiltonian are expressed in terms of the basic intra- and intercenter parameters of the pair, namely, the spin-orbit coupling constant, orbital reduction factor, exchange integral, and low symmetry crystal field parameters including axial and rhombic terms.     

A novel organic-inorganic hybrid based on a dinuclear copper complex supported on a keggin polyoxometalate

The autoassembly process of copper-oxalate dimers and Keggin type polyoxometalates leads to the first example of a new family of organic-inorganic hybrids, K(14)[(Cu(2)(bpy)(2)(mu-ox))(SiW(11)O(39)Cu(H(2)O))](2)[SiW(11)O(39)Cu(H(2)O)]. approximately 55H(2)O. This compound crystallizes in the monoclinic space group C2/m, a = 37.932(6) A, b = 21.303(3) A, c = 12.546(2) A, beta = 106.16(1) degrees, Z = 2. The crystal structure reveals the presence of a polymeric hybrid built up of alternating dimer and polyoxometalate entities.     

Conformational preferences driven by the C-methyl substituent in chelated o-diphenylphosphino-alpha-methyl-N,N-dimethylbenzylamine rhodium complexes

The stereochemistry of the chelate rings of a number of rhodium aminophosphine complexes is studied by NMR spectroscopy. The similarity in the variable-temperature behavior for the different compounds is consistent with them having in common highly preferred chelate ring conformations. The six-membered metallacycle of coordinated (R)-PN (PN = o-diphenylphosphino-alpha-methyl-N,N-dimethylbenzylamine) adopts a delta conformation in the solid state. NMR experiments indicate that this conformation is strongly favored in solution as well. The preferred sense of helicity is imposed by the absolute configuration of the stereogenic carbon atom on the ligand, which exerts an important steric control. The complex [Rh(TFB)((C(6)H(4)CHMeNMe(2))(2)P(C(6)H(4)CHMeNHMe(2)))](BF(4))(2).H(2)O.Me(2)CO crystallizes in the monoclinic space group P2(1) with a = 12.0548(11) A, b = 16.139(2) A, c = 12.1804(10) A, beta = 100.742(9) degrees, Z = 4.     

Dioxygen activation by an osmium-dihydride: preparation and characterization of a d4 square-planar complex

Exposure of OsH2Cl2(PiPr3)2 to air or bubbling with pure O2 affords the dioxo-Os(VI) compound OsO2Cl2(PiPr3)2 as result of a direct O=O double bond activation reaction. This Os(VI) species is reduced in the presence of n-BuLi to the novel dioxo-Os(IV) derivative OsO2(PiPr3)2, a rare example of a d4 square-planar compound.     

Measurements of electron-transfer rates of charge-storage molecular monolayers on Si(100). Toward hybrid molecular/semiconductor information storage devices

Redox kinetics were measured for two electroactive molecules attached to Si(100) surfaces, a ferrocene (Fc-BzOH) and a Zn(II) trimesitylporphyrin (Por-BzOH). Each molecule was derivatized with a benzyl alcohol linker for attachment to the Si surface via the formation of a Si-O bond. A complete protocol was developed for the preparation of stable Si(100) surfaces derivatized with the electroactive molecules. The redox-kinetic measurements were performed on the resulting Fc-BzOH and Por-BzOH monolayers to probe (1) the rate of electron transfer (k0) for oxidation in the presence of applied potentials and (2) the rate of charge dissipation after the applied potential is disconnected (in the form of a charge-retention half-life t1/2). The k0 values for the two types of monolayers were found to be similar to one another as were the t1/2 values. Perhaps more importantly, the electron-transfer rates for both the Fc-BzOH and the Por-BzOH monolayers differ from the charge-dissipation rates by approximately 6 orders of magnitude and are strongly dependent on the surface concentration of the electroactive species. For the Por-BzOH monolayers on Si(100), the k0 and t1/2 values and their trends as a function of surface coverage were determined to be similar to those previously measured for the analogous thiol-derivatized molecule assembled on Au(111). In contrast, the Fc-BzOH monolayers on Si(100) were found to exhibit much slower electron-transfer and charge-dissipation rates than those in the corresponding thiol-Au(111) case. Two alternative hypotheses are advanced to explain both the diminution in rates with increased surface coverage and the contrasting behavior with the analogous thiols on Au, one based on space-charge effects at the monolayer-solution interface, and a second relying on changes in distance of the redox centers from the surface as modulated by the orientation of the linking chains. Collectively, the ability to prepare and study stable, electroactive molecular media on Si(100) is likely to be key in the development of hybrid molecular/semiconductor devices.     

Switching of alcohol oxidation mechanism on nickel surfaces by fluorine substitution

A partial switch in mechanism for the partial oxidation of alcohols on nickel surfaces can be induced by substitution of gamma-hydrogens with more electronegative fluorine atoms. While exclusive dehydrogenation to acetone via beta-hydride elimination from 2-propoxide surface species is seen with 2-propanol, some dehydration to 3,3,3-trfluoropropene is observed with 1,1,1-trifluoro-2-propanol. The latter reaction involves a gamma-hydride elimination rate-limiting step.     

Asymmetric intermolecular Pauson-Khand reactions of unstrained olefins: the (o-dimethylamino)phenylsulfinyl group as an efficient chiral auxiliary

The first asymmetric version of intermolecular Pauson-Khand reactions of unstrained alkenes is described. Generally simple acyclic alkenes exhibit low reactivity and regioselectivity in intermolecular Pauson-Khand reactions; however, o-(dimethylamino)phenyl vinyl sulfoxide reacts under very mild conditions with a wide variety of terminal alkynes in a completely regioselective and highly stereoselective manner. The utility of the resulting 5-sulfinyl-2-cyclopentenones in asymmetric synthesis is illustrated by a very short enantioselective synthesis of the antibiotic (-)-pentenomycin I.