Showing papers by "Madeleine Helliwell published in 2005"

Trivalent lanthanide lacunary phosphomolybdate complexes : a structural and spectroscopic study across the series [Ln(PMo11O39)2]11-

Abstract: We report the syntheses and crystal structures of (NH4)11[LnIII(PMo11O39)2]·xH2O (where Ln = every trivalent lanthanide cation except promethium) in which two lacunary [PMo11O39]7− anions sandwich an 8-coordinate LnIII cation to yield the complex anion, [LnIII(PMo11O39)2]11−. The 14 salts crystallise in two different space groups, C2/c or P, but the LnIII containing anions are isostructural across the whole series, a very rare example of such a complete study. Solid state and solution 31P NMR, Raman and IR spectroscopies have been used to prove the stability of [Ln(PMo11O39)2]11− in aqueous solution. As expected, the LnIII cation contracts across the series and the Ln–O bond distances decrease uniformly. Interestingly, the splitting in the νP–O mode within the [PMo11O39]7− unit increases uniformly across the series, which we attribute to the stronger interaction with the smaller, higher charge density LnIII cation as the series is traversed. For the 31P NMR measurements a direct comparison of Lanthanide Induced (paramagnetic) Shift could be made with the analogous [P(W11O39)2]11− complexes.

Anomalous scattering in structural chemistry and biology

Abstract: The uses of X-ray anomalous scattering in crystal structure analysis have undergone a major expansion due to the refinement and ease of availability of the necessary X-ray instrumentation and methods. The structural chemistry and biology fields span a similar suite of technical needs but with widely differing molecular systems. The innate synergies between the two research fields brought two of the authors (JRH and VK) together at an Erice Summer School on Synchrotron Radiation in Crystallography in 1985 and took them into a collaboration spanning already 20 years. The authors’ wide perspectives are therefore, if not unique, perhaps rather rare. Thus the breadth of coverage of this review is unusual. However, there are two excellent books on anomalous scattering and its uses that have been published covering the periods up to 1975 and 1994 [S. Ramaseshan, S.C. Abrahams (Eds). Anomalous Scattering, Munksgaard, Copenhagen (1975); G. Materlik, C.J. Sparks, K. Fischer (Eds). Resonant Anomalous X-ray Scatterin...

The synthesis, structural, and spectroscopic characterization of uranium(IV) perrhenate complexes.

Abstract: We report the synthesis, structural, and spectroscopic characterization of a series of uranium(IV)-perrhenato complexes. Three isostructural complexes with general formula [U(ReO4)4(L)4] (where L = tri-n-butylphosphine oxide/TBPO (2), triethyl phosphate/TEP (3), or tri-iso-butyl phosphate/TiBP (4)), have been synthesized, both through the photoreduction of ethanolic {UO2}2+ solutions and also via a novel U(IV) starting material, U(ReO4)4.5H2O (1). Compound 1 has also been used in the preparation of [U(ReO4)4(TPPO)3(CH3CN)].2CH3CN (5) and [U(ReO4)(DPPMO2)3(OH)][ReO4]2.2CH3CN (6), where TPPO represents triphenylphosphine oxide and DPPMO2 represents bis(diphenylphosphino)methane dioxide. All six complexes have been spectroscopically characterized using NMR, UV-vis-NIR, and IR techniques, with 2, 3, 5, and 6 also fully structurally characterized. The U atoms in compounds 2-6 all exhibit eight-coordinate geometry with up to four perrhenate groups in addition to three (DPPMO2 and TPPO) or four (TEP, TiBP, TBPO) coordinated organic ligands. In the case of compounds 5 and 6, the coordination of eight ligands to the U(IV) center is completed by the binding of a solvent molecule (CH3CN) and OH-, respectively. Solid-state physical analysis (elemental and thermogravimetric) and infrared spectroscopy are in agreement with the structural studies. The crystallographic data suggest that the strength of the U(IV)-O-donor ligand bonds decreases across the series R3PO > [ReO4]- > (RO)3PO. Solution-state IR and 31P NMR spectroscopy appear to be in agreement with these solid-state results.

Preference for nitrogen versus oxygen donor coordination in uranyl- and neptunyl(VI) complexes.

Abstract: The first actinyl phosphinimine complexes have been synthesized and, in the case of uranium, exhibit strong U−N interactions. Competition reactions clearly demonstrate a surprising preference for R3PNH ligands over R3PO in the system [AnO2Cl2(R3PX)2] (An = UVI, NpVI; R = Ph, Cy; X = O, NH). Spectroscopic evidence for N-donor coordination to [NpO2]2+ in solution indicates chemical similarities to the [UO2]2+ moiety.

Kinetic and thermodynamic stereocontrol in the atroposelective formation of sulfoxides by oxidation of 2-sulfanyl-1-naphthamides

Abstract: Atropisomeric aromatic amides bearing 2-sulfanyl groups are oxidised by m-CPBA to the corresponding sulfoxides apparently with very high diastereoselectivity. NMR studies and oxidations of chiral benzamides however indicate that the kinetic selectivity of the oxidation is in fact relatively poor, and that the final diastereoisomeric ratio (typically >99∶1) is under thermodynamic control, with relatively unhindered Ar–CO rotation readily converting the less stable to the more stable product diastereoisomer. Molecular mechanics indicates that the thermodynamic diastereoselectivity results principally from electrostatic repulsion between the CO and S–O dipoles.

A non-planar, hexadeca-substituted, metal-free phthalocyanine

Abstract: The readily prepared 2,3,9,10,16,17,23,24-octamethyl-1,4,8,11,15,18,22,25-octaoctyloxyphthalocyanine (Pc 1) adopts a non-planar conformation in the crystalline state as indicated by a dihedral angle of 27.2° formed between isoindole subunits on opposite sides of the core. The ring distortion can be attributed to a combination of steric congestion of the side-chains and the stabilization of the saddle-shaped conformation via pairing of molecules to give the so-called 'overlap effect'. The ring distortion is compared to that found in other metal-free phthalocyanines. Pc 1 adsorbs strongly in the near-IR region, is highly soluble and gives robust spin-coated films of excellent optical clarity.

Ab initio structure determination using dispersive differences from multiple-wavelength synchrotron-radiation powder diffraction data

Abstract: The purpose of this paper and a test case study is to assess a method of ab initio structure solution from powder diffraction data using f′ difference techniques. A theoretical foundation for the approach used is first provided. Then, with a test case (nickel sulfate hexahydrate), it is shown that both the position of the anomalous scatterer (Ni) can be determined and the structure can be developed in full. Specifically, synchrotron-radiation data were collected at two wavelengths close to the K edge for Ni and three wavelengths remote from the Ni absorption edge, at 1.3, 1.8 and 2.16 A. These five wavelengths then allowed various combinations to be tried to establish which wavelength pairs gave the optimum signal in the Patterson maps using dispersive amplitude differences. The initial phases derived from the metal-atom position then allowed the structure to be fully developed by difference Fourier cycling. The relevance of these developments to structure-solution possibilities for proteins via powder dispersive difference data is then outlined.

Exploratory studies to investigate a linked prebiotic origin of RNA and coded peptides. 4th Communication. Further observations concerning pyrimidine nucleoside synthesis by stepwise nucleobase assembly.

Abstract: As part of an ongoing investigation into a possible linked origin of RNA and coded peptides, we have (further) investigated the potentially prebiotic synthesis of pyrimidine ribonucleosides (ribonucleotides), by stepwise nucleobase assembly on sugar (sugar phosphate) scaffolds. In this paper, we complete our description of this assembly process on aldopentose scaffolds. Treatment of D-ribose (14) and D-xylose (20) with cyanamide smoothly produces the furanosylamino-oxazolines 15 and 21, respectively (Schemes 1 and 3). Similar treatment of D-lyxose (26) also produces the furanosylamino-oxazoline 29, but as a minor component of an equilibrating mixture in which the pyranosylamino-oxazoline 28 predominates (Scheme 4). Treatment of the various amino-oxazolines with cyanoacetylene gives furanosylcytidines 16, 25, and 31, and the lyxopyranosylcytidine 30 (Schemes 1 - 4). Minor by-products of the cyanoacetylene reaction include imino lactone 17, acetals 19, 22, and 24, and cyanovinyl adducts 22 and 23 resulting from initial addition of nucleoside OH groups to the terminal acetylenic C-atom of cyanoacetylene (Schemes 2-4).

A total synthesis of milbemycin G: Approaches to the C(1)-C(10)-fragment and completion of the synthesis

Abstract: A synthesis of the hydroxybutenolide (−)-6 required for synthesis of α-milbemycins and the completion of a total synthesis of milbemycin G 7 is reported.Following preliminary studies, an optimised synthesis of the hydroxybutenolide (−)-6 from the hydroxyketone 38 was developed which involved the resolution of 38 by separation of the 3-(O-chloroacetyl)-(S)-mandelates 80 and 83. Ester 80, which corresponded to the required enantiomer of the hydroxyketone 38, crystallized from the mixture of the diastereoisomeric esters 80 and 83 giving the (−)-hydroxyketone (−)-38 in an overall yield of 47% (based on racemic 38) after ethanolysis. Hydroxyketone (−)-38 was oxidised to the enolic diketone (−)-39 and phenylselenation and stereoselective reduction gave the trihydroxycyclohexyl selenide (−)-43. The regioselective introduction of the non-conjugated double-bond into the six-membered ring was then achieved by esterification of the 4-hydroxyl group using trichloroacetic acid to give the trichloroacetate (−)-69. Oxidative elimination from the trichloroacetate using tert-butyl hydroperoxide was highly regioselective and gave the endo- and exocyclic alkenes (−)-44 and (−)-46 in a ratio of 95 : 5 after ethanolysis of the trichloroacetates. Selective O-methylation of the 4-hydroxyl group via the cyclic stannylene 55 and protection of the 3-hydroxyl group as its 2-trimethylsilylethoxymethyl (SEM) ether gave the ester (−)-57. Following saponification of the ethyl ester, re-esterification using 2-trimethylsilylethanol and oxidation of the 2-trimethylsilylfuryl fragment using singlet oxygen gave the required hydroxybutenolide (−)-6.The Wittig reaction between the phosphonium salt 2 and the hydroxybutenolide (−)-6 gave a ca. 2 : 1 mixture of the (4Z)- and (4E)-isomers of the ester 84 which on treatment with a catalytic amount of iodine was converted into the (4E)-isomer (4E)-84. Deprotection gave the seco-acid 85 but attempts to macrocyclise this were unsuccessful, the elimination product 86 being the only product isolated. The Wittig product 84 was taken through to the butenolide (2′E)-91 by removal of the SEM group, cyclisation to form the butenolide ring and diene isomerization, but this could not be converted into the corresponding seco-acid 92. However, removal of the SEM group from the seco-acid 85 gave the trihydroxy-acid 93 which was cyclized under modified Yamaguchi conditions to give the macrolide 94 together with a small amount of the macrocyclic butenolide 95. Reduction of this mixture using diisobutylaluminium hydride gave (6R)-6-hydroxymilbemycin E 96 which was converted to milbemycin G 7 by cyclisation of the primary chloride 97. The synthetic milbemycin G 7 was identical to a sample prepared by methylation of a commercial sample of milbemycin D 98, 7-O-methylmilbemycin G 99 being a side-product of this methylation.

The use of softer X-rays in the structure elucidation of microporous materials

Abstract: Microporous materials, such as zeolites and aluminophosphates, have many applications as molecular sieves and shape-selective catalysts. This is due to their three-dimensional frameworks, which contain regular pores and channels, to their high acidity, arising from Bronsted and Lewis acid active sites, and to the incorporation of transition metal atoms into framework sites. This review firstly provides an introduction into the nature and properties of these materials, and their important applications; the difficulties in their full characterization and possible methods of elucidating their structures are then outlined; finally, methods of characterization, utilizing `softer X-rays' are introduced. The first method is the determination of low concentrations of transition metals, incorporated into the frameworks using single crystal anomalous dispersion crystallography; synchrotron radiation is used to tune to the absorption edge of the metal atom in question, in order to change its signal relative to that of the rest of the structure, thereby allowing the pinpointing of its positions and the determination of its concentration at each site in the framework. Secondly, the use of longer wavelengths in powder diffraction studies is described, which, by stretching out the powder pattern, thereby reduces the overlapping of the diffraction peaks, thus allowing the structure to be solved by conventional direct methods. Finally, the use of X-ray absorption spectroscopy to determine the metal incorporation and the nature of coordination at the metal atom sites, in Mn silicalite-1 and FAPO-36, are described.