Towards homoleptic naphthalenemetalates of the later transition metals: isolation and characterization of naphthalenecobaltates(1-).
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Cobalt↗
Biomedical subjects
Victor G Young
Publications and source records attributed to Victor G Young.
Mononitrosyl iron complexes supported by sterically hindered carboxylate ligands.
Complexes that feature a single NO bound to Fe, as postulated in various carboxylate-rich metalloproteins, were prepared by mixing Fe(II) salts, NO, and the sterically encumbered 2,6-dimesitylbenzoate (Mes2ArCO2-). Among the compounds isolated are the potentially useful heterobimetallic synthon Tl(mu-Mes2ArCO2)3Fe(NO) and a novel cubane Fe4(Mes2ArCO2)4(NO)4(mu3-OH)4 that forms in the presence of added H2O and features syn-[FeNO]2 units.
Benzoates↗
Alkyl vs. alkoxy chains on ionic liquid cations.
The crystal structures and thermal behavior of the 1-(2-methoxyethyl)-2,3-dimethylimidazolium chloride and hexa-fluorophosphate salts are compared with the analogous 1-butyl-2,3-dimethylimidazolium salts to examine the influence of the ether oxygen on salt thermal properties for a typical constituent cation used in the preparation of ionic liquids.
Journal Article↗
Heterobimetallic activation of dioxygen: characterization and reactivity of novel Cu(I)-Ge(II) complexes.
Reaction of the known germylene Ge[N(SiMe3)2]2 and a new heterocyclic variant Ge[(NMes)2(CH)2] with [L(Me2)Cu]2 (L(Me2) = the beta-diketiminate derived from 2-(2,6-dimethylphenyl)amino-4-(2,6-dimethylphenyl)imino-2-pentene) yielded novel Cu(I)-Ge(II) complexes L(Me2)Cu-Ge[(NMes)2(CH)2] (1a) and L(Me2)Cu-Ge[N(SiMe3)2]2 (1b), which were characterized by spectroscopy and X-ray crystallography. The lability of the Cu(I)-Ge(II) bond in 1a and b was probed by studies of their reactivity with benzil, PPh3, and a N-heterocyclic carbene (NHC). Notably, both complexes are cleaved rapidly by PPh3 and the NHC to yield stable Cu(I) adducts (characterized by X-ray diffraction) and the free germylene. In addition, the complexes are highly reactive with O2 and exhibit chemistry which depends on the bound germylene. Thus, oxygenation of 1a results in scission and formation of thermally unstable L(Me2)CuO2, which subsequently decays to [(L(Me2)Cu)2(mu-O)2], while 1b yields L(Me2)Cu(mu-O)2Ge[N(SiMe3)2]2, a novel heterobimetallic intermediate having a [Cu(III)(mu-O)2Ge(IV)]3+ core. The isolation of the latter species by direct oxygenation of a Cu(I)-Ge(II) precursor represents a new route to heterobimetallic oxidants comprising copper.
Copper↗
A sensitive probe for the detection of Zn(II) by time-resolved fluorescence.
A new, highly sensitive fluorescent sensor for Zn(II) ion (a tris(2-pyridylmethyl)amine derivative) shows very strong binding and Zn(II) concentration-dependent biexponential time-resolved fluorescence (TRF) decay profiles that can be used for ratiometric estimates of Zn(II) concentrations. The ligand-metal complexes were characterized in solution by spectroscopic techniques and in the solid state by X-ray crystallography. The TRF studies revealed that the sensor aggregates in the absence of Zn(II) in a ligand concentration-dependent manner, a complication that is discerned by TRF but not by steady-state fluorescence ratiometric sensing techniques. It is shown that the same TRF methods are highly useful for monitoring Zn(II) concentrations in A549 epithelial lung cells in vitro and that the results were consistent with those in solution.
Cations, Divalent↗
Effects of thioether substituents on the O2 reactivity of beta-diketiminate-Cu(I) complexes: probing the role of the methionine ligand in copper monooxygenases.
The activation of dioxygen by dopamine beta-monooxygenase (DbetaM) and peptidylglycine alpha-hydroxylating monooxygenase (PHM) is postulated to occur at a copper site ligated by two histidine imidazoles and a methionine thioether, which is unusual because such thioether ligation is not present in other O2-activating copper proteins. To assess the possible role of the thioether ligand in O2 activation by DbetaM and PHM, two new ligands comprising beta-diketiminates with thioether substituents were synthesized and Cu(I) and Cu(II) complexes were isolated. The Cu(II) compounds are monomeric and exhibit intramolecular thioether coordination. While the Cu(I) complexes exhibit a multinuclear topology in the solid state, variable-temperature 1H NMR studies implicate equilibria in solution, possibly including monomers with intramolecular thioether coordination that are structurally defined by DFT calculations. Low-temperature oxygenation of solutions of the Cu(I) complexes generates stable 1:1 Cu/O2 adducts, which on the basis of combined experimental and theoretical studies adopt side-on "eta(2)" structures with negligible Cu-thioether bonding and significant peroxo-Cu(III) character. In contrast to previously reported findings with related ligands lacking the thioether group, however (cf., Aboelella; et al. J. Am. Chem. Soc. 2004, 126, 16896), purging the solutions of the thioether-containing adducts with argon results in conversion to bis(mu-oxo)dicopper(III) species. A role for the thioether in promoting loss of O2 from the 1:1 Cu/O2 adduct and facilitating trapping of the resulting Cu(I) complex to yield the bis(mu-oxo) species is proposed, and the possible relevance of this role to that of the methionine in the active sites of DbetaM and PHM is discussed.
Copper↗
New disordering mode for TFSI- anions: the nonequilibrium, plastic crystalline structure of Et4NTFSI.
A new TFSI- anion disordering mode has been discovered in a supercooled plastic crystalline phase of Et4NTFSI, which may, in part, account for the low melting points of TFSI- salts with organic cations, thereby forming ionic liquids, and the intriguing properties of LiTFSI for lithium battery applications.
Journal Article↗
Comparison of structurally analogous Zn2, Co2, and Mg2 catalysts for the polymerization of cyclic esters.
Three dimetallic monoethoxide complexes supported by a binucleating phenoxide ligand, LM2Cl2OEt (M = Zn, Co, or Mg), were prepared and shown by X-ray crystallography to be structurally analogous. Comparative studies of their cyclic ester polymerization reactivity revealed different trends for reactions with epsilon-caprolactone and lactide, however, implicating complicated effects of metal ion variation in these polymerizations.
Journal Article↗
Syntheses of T(N) building blocks Nalpha-(9-fluorenylmethoxycarbonyl)-O-(3,4,6-tri-O-acetyl-2-azido-2-deoxy-alpha-D-galactopyranosyl)-L-serine/L-threonine pentafluorophenyl esters: comparison of protocols and elucidation of side reactions.
T(N) antigen building blocks Nalpha-(9-fluorenylmethoxycarbonyl)-O-(3,4,6-tri-O-acetyl-2-azido-2-deoxy-alpha-D-galactopyranosyl)-L-serine/L-threonine pentafluorophenyl ester [Fmoc-L-Ser/L-Thr(Ac3-alpha-D-GalN3)-OPfp, 13/14] have been synthesized by two different routes, which have been compared. Overall isolated yields [three or four chemical steps, and minimal intermediary purification steps] of enantiopure 13 and 14 were 5-18% and 6-10%, respectively, based on 3,4,6-tri-O-acetyl-D-galactal (1). A byproduct of the initial azidonitration reaction of the synthetic sequence, that is, N-acetyl-3,4,6-tri-O-acetyl-2-azido-2-deoxy-alpha-D-galactopyranosylamine (5), has been characterized by X-ray crystallography, and shown by 1H NMR spectroscopy to form complexes with lithium bromide, lithium iodide, or sodium iodide in acetonitrile-d3. Intermediates 3,4,6-tri-O-acetyl-2-azido-2-deoxy-alpha-D-galactopyranosyl bromide (6) and 3,4,6-tri-O-acetyl-2-azido-2-deoxy-beta-D-galactopyranosyl chloride (7) were used to glycosylate Nalpha-(9-fluorenylmethoxycarbonyl)-L-serine/L-threonine pentafluorophenyl esters [Fmoc-L-Ser/L-Thr-OPfp, 11/12]. Previously undescribed low-level dehydration side reactions were observed at this stage; the unwanted byproducts were easily removed by column chromatography.
Antigens, Tumor-Associated, Carbohydrate↗
Structural variability and dynamics in carboxylato- and carbamatomagnesium bromides. Relationship to the carboxylate shift.
Two new carbamatomagnesium bromide complexes 1 (=[Mg(O2CN(Me)Ph)(THF)2Br]2) and 2 (=[Mg3(O2CNPh2)4(THF)5Br][(THF)MgBr3]) were prepared and crystallographically characterized. Complex 1 consists of a dinuclear core with two syn-syn-mu(1,3) bridging carbamato ligands. In solution, the 1H and 13C NMR spectra revealed a two-site fluxional behavior for the carbamato ligands between 0 degrees C and 25 degrees C that is consistent with a bridge-mode isomerization relating the syn-syn-mu(1,3) form and an alternative syn-anti-mu(1,1) form. At temperatures below 0 degrees C, a more complex NMR signal is observed that is ascribed to further resolution of C-N rotational isomers, which has the effect of increasing the number of molecular isomers that can be resolved on the NMR time scale. New temperature-dependent NMR spectra of the previously known diethylcarbamato- and benzoato-bridged complexes [Mg(O2CNEt2)(THF)2Br]2, 3, and [Mg(O2CPh)(THF)2Br]2, 5, revealed fluxional behavior that was also interpreted in terms of bridge-mode isomerization. Kinetic activation parameters for 1, 3, and 5 are consistent with an intramolecular motion relating the syn-syn-mu(1,3) and syn-anti-mu(1,1) isomers. These results provide new insight into bridge-mode isomerism that is closely related to the carboxylate shift concept. Complex 2 is compositionally related to 1 but exists as a salt consisting of a trinuclear cationic unit [Mg3(O2CNPh2)4(THF)5Br]+ and a solvated tribromomagnesiate anion [(THF)MgBr3]-. This shows that changing the hydrocarbon substituents on the carbamato ligand changes the structure and dynamics of multinuclear carbamatomagnesium complexes in ways that limit our ability to predict structural topologies in this system.
Journal Article↗
Oligomeric rods of alkyl- and hydridogallium imides.
Reaction of [RGa(NMe(2))(2)](2), where R = Me, Et, Bu, and Hx, with ammonia at 150 degrees C in an autoclave produced insoluble white powders formulated as oligomers of [RGaNH](n). The analogous reaction between NH(3) and MeGa[N(SiMe(3))(2)](2) at low temperature (<25 degrees C) formed an isolable intermediate, [MeGa(mu-NH(2))N(SiMe(3))(2)](2), that was characterized using single-crystal X-ray diffraction. Infrared spectroscopy and X-ray diffraction of the oligomers were consistent with a rodlike structure comprised of six-membered, [RGaNH](3) rings stacked perpendicular to the long axis of the rod. The method of synthesis, formula, and diffraction results suggested a structural similarity between the alkyl, [RGaNH](n)(), and the previously reported hydride, [HGaNH](n). The structural and electronic properties of rods having the general formula H(3)[(HXYH)(3)](n)H(3) (XY = GaN, GeC; n = 1-9) were investigated using density functional theory. Atomic electronegativity differences between the group 13/15 and 14/14 systems were found to play important roles in the geometrical structures of the two rods and also caused significant differences in the electronic structures. Energetically, it was found to be increasingly favorable to add additional cyclotrigallazane rings to the GaN rods, while for the GeC rods, there was a roughly constant energy cost associated with each additional ring. The electric dipole moments of the GaN rods increased substantially with length; in the GeC rods, charge separation occurred to a much smaller extent and had a polarization opposite to that found in GaN. In addition, increased dipole moments correlated with smaller electronic excitation energies, as predicted by time-dependent density functional theory. All of the powders exhibited luminescence in the visible spectrum at room temperature. Structure observed in the photoluminescence spectra of [HGaNH](n) and [MeGaNH](n) was interpreted as arising from rods of different length.
Journal Article↗
Stereoselective synthesis of alpha,alpha'-biprolines.
A means to induce dehydrodimerization of Seebach's oxazolidinone (5), the stereochemical outcome of which is entirely temperature dependent, is described. The resultant dimers 3 and 4 are precursors to (R,R)-alpha,alpha'-biproline (1) and meso-alpha,alpha'-biproline (2), respectively. An organohypobromite and an iminium halide are proposed to serve as electrophiles in the reaction with the enolate of 5 to give 3 and 4, respectively.
Crystallography, X-Ray↗
Zirconium sandwich complexes with eta(9) indenyl ligands: well-defined precursors for zirconocene-mediated coupling reactions.
A family of isolable, well-defined bis-indenyl zirconium sandwich complexes, (eta(5)-C(9)H(5)-1,3-R(2))(eta(9)-C(9)H(5)-1,3-R(2))Zr (R = silyl, alkyl), have been prepared by either alkane reductive elimination or alkali metal reduction of a suitable zirconium(IV) dihalide precursor. Crystallographic characterization of two of these derivatives, R = SiMe(2)CMe(3) and CHMe(2), reveals unprecedented eta(9) coordination of one of the indenyl ligands. Variable-temperature and EXSY NMR studies establish that the eta(5) and eta(9) rings are rapidly interconverting in solution. The sandwich complexes serve as effective sources of low-valent zirconium reacting rapidly with both olefins and alkynes at ambient temperature. In contrast to bis-cyclopentadienyl chemistry, the olefin adducts of the bis-indenyl zirconium sandwiches undergo preferential C-H activation to yield the corresponding allyl hydride compounds, although reaction with excess olefin proceeds through the eta(2)-olefin adduct, forming the corresponding zirconacyclopentane.
Journal Article↗
Dioxygen activation at a single copper site: structure, bonding, and mechanism of formation of 1:1 Cu-O2 adducts.
To evaluate the fundamental process of O(2) activation at a single copper site that occurs in biological and catalytic systems, a detailed study of O(2) binding to Cu(I) complexes of beta-diketiminate ligands L (L(1) = backbone Me; L(2) = backbone tBu) by X-ray crystallography, X-ray absorption spectroscopy (XAS), cryogenic stopped-flow kinetics, and theoretical calculations was performed. Using synchrotron radiation, an X-ray diffraction data set for L(2)CuO(2) was acquired, which led to structural parameters in close agreement to theoretical predictions. Significant Cu(III)-peroxo character for the complex was corroborated by XAS. On the basis of stopped-flow kinetics data and theoretical calculations for the oxygenation of L(1)Cu(RCN) (R = alkyl, aryl) in THF and THF/RCN mixtures between 193 and 233 K, a dual pathway mechanism is proposed involving (a) rate-determining solvolysis of RCN by THF followed by rapid oxygenation of L(1)Cu(THF) and (b) direct, bimolecular oxygenation of L(1)Cu(RCN) via an associative process.
Binding Sites↗
Preparation and characterization of pi-stacking quinodimethane oligothiophenes. Predicting semiconductor behavior and bandwidths from crystal structures and molecular orbital calculations.
A series of new quinodimethane-substituted terthiophene and quaterthiophene oligomers has been investigated for comparison with a previously studied quinoid oligothiophene that has demonstrated high mobilities and ambipolar transport behavior in thin-film transistor devices. Each new quinoidal thiophene derivative shows a reversible one-electron oxidation between 0.85 and 1.32 V, a quasi-reversible one-electron second oxidation between 1.37 and 1.96 V, and a reversible two-electron reduction between -0.05 and -0.23 V. The solution UV-vis-NIR spectrum of each compound is dominated by an intense (epsilon congruent with 100 000 M(-1) cm(-1)) low energy pi-pi transition that has a lambda(max) ranging between 648 and 790 nm. All X-ray crystal structures exhibit very planar quinoidal backbones and short intermolecular pi-stacking distances (3.335-3.492 A). Structures exhibit a single pi-stacking distance with parallel cofacial stacking (sulfur atoms of equivalent rings pointed in the same direction) or with alternating distances and antiparallel cofacial stacking (sulfur atoms of equivalent rings pointed in the opposite direction). Examples of the layered and herringbone-packing motifs are observed for both the parallel and the antiparallel cofacial stacking. Analysis of the X-ray structures and molecular orbital calculations indicates that all of these compounds have one-dimensional electronic band structures as a result of the pi-stacking. For structures with a unique pi-stacking distance, a simple geometric overlap parameter calculated from the shape of the molecule and the slip from perfect registry in the pi-stack correlates well with the transfer integrals (t) calculated using molecular orbital theory. The calculated valence (633 meV) and conduction (834 meV) bandwidths for a quinoid quaterthiophene structure are similar to those calculated for the benchmark pentacene and indicate that both hole and electron mobilities could be significant.
Journal Article↗
Experimental and theoretical studies of fused-ring persistent [1,2,4]thiadiazinyl radicals.
Five persistent radicals 1a-1e were generated by the oxidation of 4H-[1,2,4]thiadiazines 2a-2e and studied with ESR and UV-vis spectroscopy. Three of the radicals, 1a, 1d, and 1e, were generated in high yields (>90%) using either SO(2)Cl(2)/amine in toluene or AgO/K(2)CO(3) in a toluene/MeCN mixture. Halogenated radicals 1d and 1e were sufficiently stable for chromatographic isolation and vacuum sublimation. The solution stability of the fluorinated 1d was measured at t(1/2) approximately 4 months in the absence of oxygen, and 1e at t(1/2) approximately 40 min in the presence of air. The crystal and molecular structures of 1d were determined by X-ray crystallography showing columns of parallel almost evenly spaced planar heterocycles connected by infinite ...N...S... chains. Cyclic voltammetry of 1d and 1e shows reversible reduction waves at about 0 V and irreversible oxidations at about 1.2 V. Spectral and electrochemical properties of 1 were well reproduced by DFT methods.
Journal Article↗
Fused-ring thiadiazines: preparation and crystallographic characterization of 3-phenyl derivative of benzo-, pyridio[2,3-e]-, pyrazino[2,3-e]-, and tetrafluorobenzo-[1,2,4]thiadiazines.
Four bicyclic 4H-[1,2,4]thiadiazines 1a-d were prepared in 74-88% yields in two steps from the corresponding amidines 2. Three of them, 1a, 1b, and 1d, were obtained by thermal elimination of propene from the intermediate S-propylsulfilimines 12. The pyrazino derivative 1c was formed upon thermolysis of sulfoxide 14c obtained from 2c. The E(i) mechanism was investigated using DFT methods. The elimination in the sulfilimine appears to be more favorable by about 2 kcal/mol than in the analogous sulfoxide. Crystal and molecular structures of three out of the four thiadiazines were established by single-crystal X-ray analysis. All thiadiazines were found as the 4H tautomers with the heterocyclic ring puckered along the S(1)...N(4) line. The benzo derivative 1a forms a unidimensional N(4)-H...N(2) chain, the pyrazino derivative 1c forms dimeric pairs with two synergistic hydrogen bonds, and the crystal structure of 1d is characterized by strong C(6)F(4)...C(6)H(5) quadrupolar interactions.
Journal Article↗
Utilization of hydrogen bonds to stabilize M-O(H) units: synthesis and properties of monomeric iron and manganese complexes with terminal oxo and hydroxo ligands.
Non-heme iron and manganese species with terminal oxo ligands are proposed to be key intermediates in a variety of biological and synthetic systems; however, the stabilization of these types of complexes has proven difficult because of the tendency to form oxo-bridged complexes. Described herein are the design, isolation, and properties for a series of mononuclear Fe(III) and Mn(III) complexes with terminal oxo or hydroxo ligands. Isolation of the complexes was facilitated by the tripodal ligand tris[(N'-tert-butylureaylato)-N-ethyl]aminato ([H(3)1](3-)), which creates a protective hydrogen bond cavity around the M(III)-O(H) units (M(III) = Fe and Mn). The M(III)-O(H) complexes are prepared by the activation of dioxygen and deprotonation of water. In addition, the M(III)-O(H) complexes can be synthesized using oxygen atom transfer reagents such as N-oxides and hydroxylamines. The [Fe(III)H(3)1(O)](2-) complex also can be made using sulfoxides. These findings support the proposal of a high valent M(IV)-oxo species as an intermediate during dioxygen cleavage. Isotopic labeling studies show that oxo ligands in the [M(III)H(3)1(O)](2-) complexes come directly from the cleavage of dioxygen: for [Fe(III)H(3)1(O)](2-) the nu(Fe-(16)O) = 671 cm(-1), which shifts 26 cm(-1) in [Fe(III)H(3)1((18)O)](2-) (nu(Fe-(18)O) = 645 cm(-1)); a nu(Mn-(16)O) = 700 cm(-1) was observed for [Mn(III)H(3)1((16)O)](2-), which shifts to 672 cm(-1) in the Mn-(18)O isotopomer. X-ray diffraction studies show that the Fe-O distance is 1.813(3) A in [Fe(III)H(3)1(O)](2-), while a longer bond is found in [Fe(III)H(3)1(OH)](-) (Fe-O at 1.926(2) A); a similar trend was found for the Mn(III)-O(H) complexes, where a Mn-O distance of 1.771(5) A is observed for [Mn(III)H(3)1(O)](2-) and 1.873(2) A for [Mn(III)H(3)1(OH)](-). Strong intramolecular hydrogen bonds between the urea NH groups of [H(3)1](3-) and the oxo and oxygen of the hydroxo ligand are observed in all the complexes. These findings, along with density functional theory calculations, indicate that a single sigma-bond exists between the M(III) centers and the oxo ligands, and additional interactions to the oxo ligands arise from intramolecular H-bonds, which illustrates that noncovalent interactions may replace pi-bonds in stabilizing oxometal complexes.
Ethylamines↗