Search tips
Search criteria

Results 1-25 (40)

Clipboard (0)

Select a Filter Below

Year of Publication
Document Types
1.  Crystal structure of (4R,5S)-4-methyl-3-methyl­sulfinyl-5-phenyl-1,3-oxazolidin-2-one 
The absolute structure of the chiral asymmetric indole precursor title compound, C11H13NO3S, was confirmed by refinement of the Flack and Hooft parameters and is that expected based on the starting materials for the synthesis. The phenyl group subtends a dihedral angle of 56.40 (5)° with the mean plane of the oxazolidinone ring, which adopts an envelope conformation, with the C atom bearing the methyl group as the flap. In the crystal, no significant directional inter­actions beyond van der Waals contacts are observed.
PMCID: PMC4257383  PMID: 25553031
crystal structure; oxazolidinone; asymmetric indole
2.  Discovery and Biological Characterization of the Auromomycin Chromophore as an Inhibitor of Biofilm Formation in Vibrio cholerae 
Bacterial biofilms pose a significant challenge in clinical environments due to their inherent lack of susceptibility to antibiotic treatment. It is widely recognized that most pathogenic bacterial strains in the clinical setting persist in the biofilm state, and are the root cause of many recrudescent infections. Discovery and development of compounds capable of either inhibiting biofilm formation or initiating biofilm dispersal may provide new therapeutic avenues for reducing the number of hospital acquired, biofilm-mediated infections. We now report the application of our recently reported image-based, high-throughput screen to the discovery of microbially-derived natural products with biofilm inhibitory activity against Vibrio cholerae. Examination of a prefractionated library of microbially-derived marine natural products has lead to the identification of a new biofilm inhibitor that is structurally unrelated to previously reported inhibitors and is one of the most potent inhibitors reported to date against V. cholerae. Combination of this compound with sub-MIC concentrations of a number of clinically relevant antibiotics was shown to improve the biofilm inhibitory efficacy of this new compound compared to monotherapy treatments, and provides evidence for the potential therapeutic benefit of biofilm inhibitors in treating persistent biofilm-mediated infections.
PMCID: PMC3947468  PMID: 24106077
biofilm inhibitor; drug discovery; Vibrio cholerae. natural products; image-based screening
3.  Correlated Ligand Dynamics in Oxyiron Picket Fence Porphyrins: Structural and Mössbauer Investigations 
Journal of the American Chemical Society  2013;135(41):10.1021/ja408431z.
Disorder in the position of the dioxygen ligand is a well-known problem in dioxygen complexes and in particular in those of picket fence porphyrin species. The dynamics of Fe–O2 rotation and tert-butyl motion in three different picket fence porphyrin derivatives has been studied by a combination of multi-temperature X-ray structural studies and Mössbauer spectroscopy. The structural studies show that the motions of the dioxygen ligand also require motions of the protecting pickets of the ligand binding pocket. The two motions appear to be correlated and the temperature-dependent change in the O2 occupancies can not be governed by a simple Boltzmann distribution. The three [Fe(TpivPP)(RIm)(O2)] derivatives studied have RIm = 1-methyl-, 1-ethyl-, or 2-methylimidazole. In all three species there is a preferred orientation of the Fe–O2 moiety with respect to the trans imidazole ligand and the population of this orientation increases with decreasing temperature. In the 1-MeIm and 1-EtIm species, the Fe–O2 unit is approximately perpendicular to the imidazole plane whereas in the 2-MeHIm species the Fe–O2 unit is approximately parallel. This reflects the low energy required for rotation of the Fe–O2 unit and the small energy differences in populating the possible pocket quadrants. All dioxygen complexes have a crystallographically required twofold axis of symmetry that limits the accuracy of the determined Fe–O2 geometry. However, the 80 K structure of the 2-MeHIm derivative allowed for the resolution of the two bonded oxygen atom positions and provided the best geometric description for the Fe–O2 unit. The values determined are: Fe–O = 1.811(5) Å, Fe–O–O = 118.2(9)°, O–O = 1.281(12) Å, and an off-axis tilt of 6.2°. The demonstration of the off-axis tilt is a first. We present detailed temperature-dependent simulations of the Mössbauer spectra that model the changing value of the quadrupole splitting and line widths. Residuals to fits are poorer at higher temperature. We believe that this is consistent with the idea that population of the two conformers are related to the concomitant motions of both Fe–O2 rotations and motions of the protecting tert-butyl pickets.
PMCID: PMC3827975  PMID: 24025123
4.  Probing Heme Vibrational Anisotropy: An Imidazole Orientation Effect? 
Inorganic chemistry  2013;52(19):10.1021/ic401644g.
The complete iron vibrational spectrum of the five-coordinate high-spin complex [Fe(OEP)(2-MeHIm)], where OEP = octaethylporphyrinato and 2-MeHIm = 2-methylimidazole, has been obtained by oriented single-crystal nuclear resonance vibrational spectroscopy (NRVS) data. Measurements have been made in three orthogonal directions, which provides quantitative information for all iron motion. These experimental data, buttressed by DFT calculations, have been used to define the effects of the axial ligand orientation. Although the axial imidazole removes the degeneracy in the in-plane vibrations, the imidazole orientation does not appear to control the direction of the in-plane iron motion. This is in contrast to the effect of the imidazolate ligand, as defined by DFT calculations, which does have substantial effects on the direction of the in-plane iron motion. The axial NO ligand has been found to have the strongest orientational effect (Angew. Chem., Int. Ed., 2010, 49, 4400). Thus the strength of the directional properties are in the order NO > imidazolate > imidazole, consistent with the varying strength of the Fe–ligand bond.
PMCID: PMC3830931  PMID: 24020589
5.  Crystal structure of catena-poly[[potassium-tri-μ-di­methyl­acetamide-κ6 O:O] iodide] 
A one-dimensional chain formed by potassium bridged by μ2-O-di­methyl­acetamide is reported. This represents a small class of di­methyl­acetamide complexes with oxygen bridging two metal centers.
The structure of catena-poly[[potassium-tri-μ-di­methyl­acetamide-κ6 O:O] iodide], {[K(C4H9NO)3]I}n, at 120 K has trigonal (P-3) symmetry. The structure adopts a linear chain motif parallel to the crystallographic c axis. Two crystallographically independent K+ cations are present in the asymmetric unit located on threefold rotoinversion axes at [0, 0, 0] and [0, 0, 1/2] and are bridged by the O atoms of the acetamide moiety. This is an example of a rare μ2-bridging mode for di­methyl­acetamide O atoms. The iodide counter-ion resides on a threefold rotation axis in the channel formed by the [K(C4H9NO)]+ chains.
PMCID: PMC4257214  PMID: 25484650
crystal structure; one-dimensional coordination polymer; symmetry; di­methyl­acetamide; potassium salt
6.  Fluorine NMR Reporter for Phosphate Anions 
A fluorine-labelled zinc(II)-dipicolylamine coordination complex reports the presence of phosphate anions in aqueous solution, especially pyrophosphate and ADP, and is used to monitor the enzymatic hydrolysis of ATP.
PMCID: PMC3957275  PMID: 23619399
7.  Thiolates Chemically Induce Redox Activation of BTZ043 and Related Potent Nitro Aromatic Anti-Tuberculosis Agents 
The development of multidrug resistant (MDR) and extensively drug resistant (XDR) forms of tuberculosis (TB) has stimulated research efforts globally to expand the new drug pipeline. Nitro aromatic compounds, including 1, 3-Benzothiazin-4-ones (BTZs) and related agents, are a promising new class for the treatment of TB. Research has shown that the nitroso intermediates of BTZs that are generated in vivo cause suicide inhibition of decaprenylphosphoryl-β-D-ribose 2′ oxidase (DprE1), which is responsible for cell wall arabinogalactan biosynthesis. We have designed and synthesized novel anti-TB agents inspired from BTZs and other nitroaromatic compounds. Computational studies indicated that the unsubstituted aromatic carbons of BTZ043 and related nitroaromatic compounds are the most electron deficient and might be prone to nucleophilic attack. Our chemical studies on BTZ043 and the additional nitro aromatic compounds synthesized by us and the others confirmed the postulated reactivity. The results indicate that nucleophiles such as thiolates, cyanide and hydride induce non-enzymatic reduction of the nitro groups present in these compounds to the corresponding nitroso intermediates by addition at the unsubstituted electron deficient aromatic carbon present in these compounds. Furthermore we demonstrate here that these compounds are good candidates for the classical von Richter reaction. These chemical studies offer an alternate hypotheses for the mechanism of action of nitro aromatic anti-TB agents in that the cysteine thiol(ate) or a hydride source at the active site of DprE1 may trigger the reduction of the nitro groups in a manner similar to the von Richter reaction to the nitroso intermediates, to initiate the inhibition of DprE1.
PMCID: PMC3677520  PMID: 23402278
8.  Effect of the Ruffled Porphyrin Ring on Electronic Structures: Structure and Characterization of [Fe(TalkylP)(OClO3)] and [Fe(TPrP)(THF)2]ClO4 (alkyl = Ethyl, Et and n-Propyl, Pr) 
We report the synthesis of Fe(TalkylP)(OClO3)] (alkyl = ethyl and propyl) and [Fe(TPrP)(THF)2]ClO4, which are characterized by UV-vis, EPR, X-ray crystallography, and solid-state magnetic susceptibilities. The macrocycles of all three complexes are ruffled, all of the structural features for [Fe(TEtP(OClO3)] and [Fe(TPrP)(OClO3)] are characteristic of the nearly pure S = 3/2 state, while the structural parameters for [Fe(TPrP)(THF)2]ClO4 feature a pure intermediate-spin (S = 3/2) state, which are all consistent with EPR and magnetic data. It is clear from these studies that the ruffled conformation plays a significant role in affecting the extent of S = 3/2 character.
PMCID: PMC3633537  PMID: 23626455
9.  Creation of an HDAC-Based Yeast Screening Method for Evaluation of Marine-Derived Actinomycetes: Discovery of Streptosetin A⊥ 
Journal of natural products  2012;75(12):2193-2199.
A histone deacetylase (HDAC)-based yeast assay employing a URA3 reporter gene was applied as a primary screen to evaluate a marine-derived actinomycete extract library and identify human class III HDAC (SIRT) inhibitors. Based on the bioassay-guided purification, a new compound designated as streptosetin A (1) was obtained from one of the active strains identified through the yeast assay. The gross structure of the new compound was elucidated from the 1D and 2D NMR data. The absolute stereostructure of 1 was determined based on X-ray crystal structure analysis and simulation of ECD spectra using time-dependent density functional theory (TD-DFT) calculations. This compound showed weak inhibitory activity against yeast Sir2p, and human SIRT1 and SIRT2.
PMCID: PMC3532527  PMID: 23167691
10.  Iron(III)-Templated Macrolactonization of Trihydroxamate Siderophores 
Organic letters  2012;14(17):4390-4393.
A method was developed to synthesize macrocyclic trihydroxamate siderophores using optimized Yamaguchi macrolactonization conditions. The natural ability of siderophores to bind iron(III) was exploited to template the reactions and allowed for rapid reaction rates, high product conversions, and the formation of large macrolactone rings up to 35 atoms. An X-ray structure of a 33-membered macrolactone siderophore-Fe(III) complex is presented.
PMCID: PMC3436969  PMID: 22906163
11.  The Aignopsanes, a New Class of Sesquiterpenes from Selected Chemotypes of the Sponge Cacospongia mycofijiensis 
Organic letters  2009;11(9):1975-1978.
A survey of individual specimens of northern Papua New Guinea derived Cacospongia mycofijiensis has yielded novel sesquiterpenes, aignopsanoic acid A (1), methyl aignopsanoate A (2), and isoaignopsanoic acid A (3). The structures and absolute configurations of 1–3 were established using NMR data, X-ray crystallography results, and an analysis of CD properties. Two of these metabolites, 1 and 2, were moderately active against Trypanosoma brucei, the parasite responsible for sleeping sickness.
PMCID: PMC3762577  PMID: 19385671
12.  Structural Insights into Ligand Dynamics: Correlated Oxygen and Picket Motion in Oxycobalt Picket Fence Porphyrins 
Journal of the American Chemical Society  2012;134(25):10595-10606.
Two different oxygen-ligated cobalt porphyrins have been synthesized and the solid-state structures have been determined at several temperatures. The solid-state structures provide insight into the dynamics of Co–O2 rotation and correlation with protecting group disorder. [Co(TpivPP)(1-EtIm)(O2)] (TpivPP = picket fence porphyrin) is prepared by oxygenation of [Co(TpivPP)(1-EtIm)2] in benzene solution. The structure at room temperature has the oxygen ligand within the ligand binding pocket and disordered over four sites and the trans imidazole is disordered over two sites. The structure at 100 K, after the crystal has been carefully annealed to yield a reversible phase change, is almost completely ordered. The phase change is reversed upon warming the crystal to 200 K, whereupon the oxygen ligand is again disordered but with quite unequal populations. Further warming to 300 K leads to greater disorder of the oxygen ligands with nearly equal O2 occupancies at all four positions. The disorder of the t-butyl groups of the protecting pickets is correlated with rotation of the O2 around the Co–O(O2) bond. [Co(TpivPP)(2-MeHIm)(O2)] is synthesized by a solid-state oxygenation reaction from the five-coordinate precursor [Co(TpivPP)(2-MeHIm)]. Exposure to 1 atm of O2 leads to incomplete oxygenation, however, exposure at 5 atm yields complete oxygenation. Complete oxygenation leads to picket disorder whereas partial (40%) oxygenation does not. Crystallinity is retained on complete degassing of oxygen in the solid, and complete ordering of the pickets is restored. The results should provide basic information needed to better model M–O2 dynamics in protein environments.
PMCID: PMC3384769  PMID: 22642824
13.  5-Benz­yloxy-3-methyl-1-tosyl-1H-indole 
The title compound, C23H21NO3S, represents one of the few examples of a 5-substituted indole with a toluene­sulfonyl group bonded to the N atom. The benzyl group adopts a synclinal geometry with respect to the indole ring [dihedral angle = 59.95 (4)°], while the tolyl ring is oriented close to perpendicular to the indole ring, making a dihedral angle of 81.85 (3)°. The indole N atom exhibits a slight pyramidalization.
PMCID: PMC3685113  PMID: 23795132
14.  Inhibitors for Bacterial Cell-Wall Recycling 
ACS Medicinal Chemistry Letters  2012;3(3):238-242.
Gram-negative bacteria have evolved an elaborate process for the recycling of their cell wall, which is initiated in the periplasmic space by the action of lytic transglycosylases. The product of this reaction, β-d-N-acetylglucosamine-(1→4)-1,6-anhydro-β-d-N-acetylmuramyl-l-Ala-γ-d-Glu-meso-DAP-d-Ala-d-Ala (compound 1), is internalized to begin the recycling events within the cytoplasm. The first step in the cytoplasmic recycling is catalyzed by the NagZ glycosylase, which cleaves in a hydrolytic reaction the N-acetylglucosamine glycosidic bond of metabolite 1. The reactions catalyzed by both the lytic glycosylases and NagZ are believed to involve oxocarbenium transition species. We describe herein the synthesis and evaluation of four iminosaccharides as possible mimetics of the oxocarbenium species, and we disclose one as a potent (compound 3, Ki = 300 ± 15 nM) competitive inhibitor of NagZ.
PMCID: PMC3404464  PMID: 22844551
Iminosaccharide; NagZ; MltB; peptidoglycan; β-lactam
15.  Inhibitors for Bacterial Cell-Wall Recycling 
ACS medicinal chemistry letters  2012;3(3):238-242.
Gram-negative bacteria have evolved an elaborate process for the recycling of their cell wall, which is initiated in the periplasmic space by the action of lytic transglycosylases. The product of this reaction, β-D-N-acetylglucosamine-(1→4)-1,6-anhydro-β-D-N-acetylmuramyl-L-Ala-γ-D-Glu-meso-DAP-D-Ala-D-Ala (compound 1), is internalized to begin the recycling events within the cytoplasm. The first step in the cytoplasmic recycling is catalyzed by the NagZ glycosylase, which cleaves in a hydrolytic reaction the N-acetylglucosamine glycosidic bond of metabolite 1. The reactions catalyzed by both the lytic glycosylases and NagZ are believed to involve oxocarbenium transition species. We describe herein the synthesis and evaluation of four iminosaccharides as possible mimetics of the oxocarbenium species, and disclose one as a potent (compound 3, Ki = 300 ± 15 nM) competitive inhibitor of NagZ.
PMCID: PMC3404464  PMID: 22844551
Iminosaccharide; NagZ; MltB; Peptidoglycan; β-Lactam
16.  Synthetic Peptidoglycan Motifs for Germination of Bacterial Spores 
PMCID: PMC3519099  PMID: 21117117
bioorganic chemistry; biological activity; bacteria; spores; carbohydrates
17.  N-O Chemistry for Antibiotics: Discovery of N-Alkyl-N-(pyridin-2-yl)hydroxylamine Scaffolds as Selective Antibacterial Agents Using Nitroso Diels-Alder and Ene Chemistry 
Journal of medicinal chemistry  2011;54(19):6843-6858.
The discovery, syntheses, and structure-activity relationships (SAR) of a new family of heterocyclic antibacterial compounds based on N-alkyl-N-(pyridin-2-yl)hydroxylamine scaffolds are described. A structurally diverse library of ~100 heterocyclic molecules generated from Lewis acid-mediated nucleophilic ring opening reactions with nitroso Diels-Alder cycloadducts and nitroso ene reactions with substituted alkenes was evaluated in whole cell antibacterial assays. Compounds containing the N-alkyl-N-(pyridin-2-yl)hydroxylamine structure demonstrated selective and potent antibacterial activity against the Gram-positive bacterium Micrococcus luteus ATCC 10240 (MIC90 = 2.0 μM or 0.41 μg/mL) and moderate activity against other Gram-positive strains including antibiotic resistant strains of Staphylococcus aureus (MRSA) and Enterococcus faecalis (VRE). A new synthetic route to the active core was developed using palladium-catalyzed Buchwald-Hartwig amination reactions of N-alkyl-O-(4-methoxybenzyl)hydroxylamines with 2-halo-pyridines that facilitated SAR studies and revealed the simplest active structural fragment. This work shows the value of using a combination of diversity-oriented synthesis (DOS) and parallel synthesis for identifying new antibacterial scaffolds.
PMCID: PMC3188665  PMID: 21859126
18.  Metalloporphyrin Mixed-Valence π-Cation Radicals: [Fe(oxoOEC./2)(Cl)]2SbCl6, Structure, Magnetic Properties, and Near-IR Spectra 
Inorganic chemistry  2011;50(18):9114-9121.
The preparation and characterization of a mixed-valence π-cation radical derivative of an iron(III) oxochlorinato complex is reported. The new complex has been synthesized by the one-electron oxidation of a pair of [Fe(oxoOEC)(Cl)] molecules to form the dimeric cation [Fe(oxoOEC)(Cl)]+2. The cation has been characterized by an X-ray analysis, Mössbauer spectroscopy, UV-vis and near-IR spectroscopy, and magnetic susceptibility measurements from 6–300 K. The crystal structure shows that the two rings have a smaller overlap area that those of the formally related nickel and copper octaethylporphyrinateethylporphinate derivatives, reflecting the larger steric congestion at the periphery in part of the oxochlorin rings. The Mössbauer data is consistent with two equivalent iron(III) centers. The unpaired electron is delocalized over the two oxochlorin rings and mediates a strong antiferromagnetic interaction between the high-spin iron(III) centers.
PMCID: PMC3171574  PMID: 21809820
19.  (2E)-3-(4-Methyl­phen­yl)-1-(pyridin-3-yl)prop-2-en-1-one 
The title compound, C15H13NO, has two crystallographically independent mol­ecules in the asymmetric unit which differ principally in the periplanar angle formed by the benzene and pyridine rings [41.41 (3) and 17.92 (5)°]. The mol­ecules exhibit an E conformation between the keto group with respect to the olefin double bond.
PMCID: PMC3415023  PMID: 22905010
20.  catena-Poly[[penta-μ-benzoato-μ-chlorido-dioxanedineodymium(III)] dioxane 2.5-solvate] 
The asymmetric unit of the title compound, [Nd2(C6H5COO)5Cl(C4H8O2)]·2.5C4H8O2, consists of two NdIII ions bridged by one Cl− ion, five benzoate ions and one coordinating 1,4-dioxane mol­ecule. One NdIII ion is nine-coordinate, with a very distorted monocapped square-anti­prismatic geometry. It is coordinated by two chelating carboxyl­ate groups, three monodentate carboxyl­ate groups, one chloride ion and one dioxane mol­ecule. A second independent NdIII ion is eight-coordinated in a distorted square-anti­prismatic geometry by one chelating carboxyl­ate group, five monodentate carboxyl­ate groups and one chloride ion. The chains of the extended structure are parallel to the crystallographic b axis. There is a small amount of void space which is filled with five disordered dioxane solvent mol­ecules per unit cell. The intensity contribution of the disordered solvent molecules was removed by applying the SQUEEZE procedure in PLATON [Spek (2009). Acta Cryst. D65, 148–155].
PMCID: PMC3344409  PMID: 22590171
21.  Structure and compatibility of a magnesium electrolyte with a sulphur cathode 
Nature Communications  2011;2:427-.
Magnesium metal is an ideal rechargeable battery anode material because of its high volumetric energy density, high negative reduction potential and natural abundance. Coupling Mg with high capacity, low-cost cathode materials such as electrophilic sulphur is only possible with a non-nucleophilic electrolyte. Here we show how the crystallization of the electrochemically active species formed from the reaction between hexamethyldisilazide magnesium chloride and aluminum trichloride enables the synthesis of a non-nucleophilic electrolyte. Furthermore, crystallization was essential in the identification of the electroactive species, [Mg2(μ-Cl)3·6THF]+, and vital to improvements in the voltage stability and coulombic efficiency of the electrolyte. X-ray photoelectron spectroscopy analysis of the sulphur electrode confirmed that the electrochemical conversion between sulphur and magnesium sulfide can be successfully performed using this electrolyte.
Magnesium is an ideal rechargeable battery anode material, but coupling it with a low-cost sulphur cathode, requires a non-nucleophilic electrolyte. Kim et al. prepare a non-nucleophilic electrolyte from hexamethyldisilazide magnesium chloride and aluminium trichloride, and show its compatibility with a sulphur cathode.
PMCID: PMC3266610  PMID: 21829189
22.  Co-crystals of 3-de­oxy-3-fluoro-α-d-glucopyran­ose and 3-de­oxy-3-fluoro-β-d-glucopyran­ose 
3-De­oxy-3-fluoro-d-glucopyran­ose crystallizes from acetone to give a unit cell containing two crystallographically independent mol­ecules. One of these mol­ecules (at site A) is structurally homogeneous and corresponds to 3-de­oxy-3-fluoro-β-d-glucopyran­ose, C6H11FO5, (I). The second mol­ecule (at site B) is structurally heterogeneous and corresponds to a mixture of (I) and 3-de­oxy-3-fluoro-α-d-glucopyran­ose, (II); treatment of the diffraction data using partial-occupancy oxygen at the anomeric center gave a high-quality packing model with an occupancy ratio of 0.84:0.16 for (II):(I) at site B. The mixture of α- and β-anomers at site B appears to be accommodated in the lattice because hydrogen-bonding partners are present to hydrogen bond to the anomeric OH group in either an axial or equatorial orientation. Cremer–Pople analysis of (I) and (II) shows the pyranosyl ring of (II) to be slightly more distorted than that of (I) [θ(I) = 3.85 (15)° and θ(II) = 6.35 (16)°], but the general direction of distortion is similar in both structures [ϕ(I) = 67 (2)° (B C1,C4) and ϕ(II) = 26.0 (15)° (C3 TB C1); B = boat conformation and TB = twist-boat conformation]. The exocyclic hy­droxy­methyl (–CH2OH) conformation is gg (gauche–gauche) (H5 anti to O6) in both (I) and (II). Structural comparisons of (I) and (II) to related unsubstituted, de­oxy and fluorine-substituted monosaccharides show that the gluco ring can assume a wide range of distorted chair structures in the crystalline state depending on ring substitution patterns.
PMCID: PMC3089378  PMID: 21051824
23.  4-De­oxy-4-fluoro-β-d-gluco­pyranose 
4-De­oxy-4-fluoro-β-d-glucopyran­ose, C6H11FO5, (I), crystallizes from water at room temperature in a slightly distorted 4 C 1 chair con­formation. The observed chair distortion differs from that observed in β-d-glucopyran­ose [Kouwijzer, van Eijck, Kooijman & Kroon (1995 ▶). Acta Cryst. B51, 209–220], (II), with the former skewed toward a B C3,O5 (boat) conformer and the latter toward an O5 TB C2 (twist–boat) conformer, based on Cremer–Pople analysis. The exocyclic hy­droxy­methyl group conformations in (I) and (II) are similar; in both cases, the O—C—C—O torsion angle is ∼−60° (gg con­former). Inter­molecular hydrogen bonding in the crystal structures of (I) and (II) is conserved in that identical patterns of donors and acceptors are observed for the exocyclic substituents and the ring O atom of each monosaccharide. Inspection of the crystal packing structures of (I) and (II) reveals an essentially identical packing configuration.
PMCID: PMC3089016  PMID: 20921614
24.  A cationic rhodium(I) N-heterocyclic carbene complex isolated as an aqua adduct 
The title complex, aqua­[1,3-bis­(2,6-diiso­propyl­phen­yl)imid­az­ol-2-yl­idene](η4-cyclo­octa-1,5-diene)rhodium(I) tetra­fluor­ido­borate, [Rh(C8H12)(C27H36N2)(H2O)]BF4, exihibits a square-planar geometry around the Rh(I) atom, formed by a bidentate cyclo­octa-1,5-diene (cod) ligand, an N-heterocylcic carbene and an aqua ligand. The complex is cationic and a BF4 − anion balances the charge. The structure exists as a hydrogen-bonded dimer in the solid state, formed via inter­actions between the aqua ligand H atoms and the BF4 − F atoms.
PMCID: PMC3200919  PMID: 22058874
25.  The Lanthanide Contraction Revisited 
Journal of the American Chemical Society  2007;129(36):11153-11160.
A complete, isostructural series of complexes with La-Lu (except Pm) with the ligand TREN-1,2-HOIQO has been synthesized and structurally characterized by means of single-crystal X-ray analysis. All complexes are 1D-polymeric species in the solid state, with the lanthanide being in an eight-coordinate, distorted trigonal-dodecahedral environment with a donor set of eight unique oxygen atoms. This series constitutes the first complete set of isostructural complexes from La-Lu (without Pm) with a ligand of denticity greater than two. The geometric arrangement of the chelating moieties slightly deviates across the lanthanide series, as analyzed by a shape parameter metric based on the comparison of the dihedral angles along all edges of the coordination polyhedron. The apparent lanthanide contraction in the individual Ln-O bond lengths deviates considerably from the expected quadratic decrease that was found previously in a number of complexes with ligands of low denticity. The sum of all bond lengths around the trivalent metal cation, however, is more regular, showing an almost ideal quadratic behavior across the entire series. The quadratic nature of the lanthanide contraction is derived theoretically from Slater’s model for the calculation of ionic radii. In addition, the sum of all distances along the edges of the coordination polyhedron show exactly the same quadratic dependence as the Ln-X bond lengths. The universal validity of this coordination sphere contraction, concomitant with the quadratic decrease in Ln-X bond lengths, was confirmed by reexamination of four other, previously published series of lanthanide complexes. Due to the importance of multidentate ligands for the chelation of rare-earth metals, this result provides a significant advance for the prediction and rationalization of the geometric features of the corresponding lanthanide complexes, with great potential impact for all aspects of lanthanide coordination.
PMCID: PMC3159577  PMID: 17705483

Results 1-25 (40)