Isomeric distribution and catalyzed isomerization of cobalt(III) complexes with pentadentate macrocyclic ligands. Importance of hydrogen bonding

We have investigated the isomeric distribution and rearrangement of complexes of the type [CoXLn](2+,3+) (where X = Cl-, OH-, H2O, and L-n represents a pentadentate 13-, 14-, and 15-membered tetraaza or diaza-dithia (N-4 or N2S2) macrocycle bearing a pendant primary amine). The preparative procedures for chloro complexes produced almost exclusively kinetically preferred cis isomers (where the pendant primary amine is cis to the chloro ligand) that can be separated by careful cation-exchange chromatography. For L-13 and L-14 the so-called cis-V isomer is isolated as the kinetic product, and for L-15 the cis-VI form (an N-based diastereomer) is the preferred, while for the L-14(S) complex both cis-V and trans-I forms are obtained. All these complexes rearrange to form stable trans isomers in which the pendent primary amine is trans to the monodentate aqua or hydroxo ligand, depending on pH and the workup procedure. In total 11 different complexes have been studied. From these, two different trans isomers of [CoCIL14S](2+) have been characterized crystallographically for the first time in addition to a new structure of cis-V-[CoCIL14S](2+); all were isolated as their chloride perchlorate salts. Two additional isomers have been identified and characterized by NMR as reaction intermediates. The remaining seven forms correspond to the complexes already known, produced in preparative procedures. The kinetic, thermal, and baric activation parameters for all the isomerization reactions have been determined and involve large activation enthalpies and positive volumes of activation. Activation entropies indicate a very important degree of hydrogen bonding in the reactivity of the complexes, confirmed by density functional theory studies on the stability of the different isomeric forms. The isomerization processes are not simple and even some unstable intermediates have been detected and characterized as part of the above-mentioned 11 forms of the complexes. A common reaction mechanism for the isomerization reactions has been proposed for all the complexes derived from the observed kinetic and solution behavior.

Polyamines from polyols - Pathways to azamacrocycles with hydroxymethyl pendent groups

Several pathways to macromonocylic polyamine ligands with pendent hydroxymethyl substituents have been explored. The new ligands have all been characterised by single-crystal, X-ray structure determinations on their complexes with Co(III) (one case) and Cu(II). As in some related systems, four-membered ring species, here oxetanes rather than azetidines, appear to be involved as reaction intermediates and can be quite readily isolated, providing reactants of potential for the construction of even more complicated multidentate ligands. (C) 2005 Elsevier Ltd. All rights reserved.

Synthesis and characterization of Pd(II) and Pt(II) complexes with triazolopyrimidine derivatives: The crystal structure of [Pd2L2Cl4] where L=5,7-dimethyl-1,2,4-triazolo[1,5-a]pyrimidine

The Pd(II) and Pt(II) complexes with triazolopyrimidine C-nucleosides L-1 (5,7-dimethyl-3-(2',3',5'-tri-O-benzoyl-beta-D-ribofuranosyl-s-triazolo)[4,3-a]pyrimidine), L-2 (5,7-dimethyl-3-beta-D-ribofuranosyl-s-triazolo [4,3-a]pyrimidine) and L-3 (5,7-dimethyl[1,5-a]-s-triazolopyrimidine), [Pd(en)(L-1)](NO3)(2), (Pd(bpy)(L-1)](NO3)(2), cis-Pd(L-3)(2)Cl-2, [Pd-2(L-3)(2)Cl-4]center dot H2O, cis-Pd(L-2)(2)Cl-2 and [Pt-3(L-1)(2)Cl-6] were synthesized and characterized by elemental analysis and NMR spectroscopy. The structure of the [Pd-2(L-3)(2)Cl-4]center dot H2O complex was established by Xray crystallography. The two L-3 ligands are found in a head to tail orientation, with a (PdPd)-Pd-... distance of 3.1254(17) angstrom.L-1 coordinates to Pd(II) through N8 and N1 forming polymeric structures. L-2 coordinates to Pd(II) through N8 in acidic solutions (0.1 M HCl) forming complexes of cis-geometry. The Pd(II) coordination to L-2 does not affect the sugar conformation probably due to the high stability of the C-C glycoside bond. (c) 2006 Elsevier B.V. All rights reserved.

The abundance of short proteins in the mammalian proteome

Short proteins play key roles in cell signalling and other processes, but their abundance in the mammalian proteome is unknown. Current catalogues of mammalian proteins exhibit an artefactual discontinuity at a length of 100 aa, so that protein abundance peaks just above this length and falls off sharply below it. To clarify the abundance of short proteins, we identify proteins in the FANTOM collection of mouse cDNAs by analysing synonymous and nonsynonymous substitutions with the computer program CRITICA. This analysis confirms that there is no real discontinuity at length 100. Roughly 10% of mouse proteins are shorter than 100 aa, although the majority of these are variants of proteins longer than 100 aa. We identify many novel short proteins, including a dark matter'' subset containing ones that lack detectable homology to other known proteins. Translation assays confirm that some of these novel proteins can be translated and localised to the secretory pathway.

Cobalt complexes of tripodal hexadentate ligands: Electrochemically driven rearrangements

The Co-III complexes of the hexadentate tripodal ligands HOsen (3-(2'-aminoethylamino)-2,2-bis((2 ''-aminoethylamino) methyl) propan-1-ol) and HOten (3-(2'-aminoethylthia)-2,2-bis((2 ''-aminoethylthia) methyl) propan-1-ol) have been synthesized and fully characterized. The crystal structures of [Co(HOsen)]Cl-3 center dot H2O and [Co(HOten)](ClO4)Cl-2 are reported and in both cases the ligands coordinate as tripodal hexadentate N-6 and N3S3 donors, respectively. Cyclic voltammetry of the N3S3 coordinated complex [Co(HOten)](3+) is complicated and electrode dependent. On a Pt working electrode an irreversible Co-III/II couple ( formal potential - 157 mV versus Ag-AgCl) is seen, which is indicative of dissociation of the divalent complex formed at the electrode. The free HOten released by the dissociation of [Co(HOten)](2+) can be recaptured by Hg as shown by cyclic voltammetry experiments on a static Hg drop electrode ( or in the presence of Hg2+ ions), which leads to the formation of an electroactive Hg-II complex of the N3S3 ligand (formal potential + 60 mV versus Ag-AgCl). This behaviour is in contrast to the facile and totally reversible voltammetry of the hexaamine complex [Co(HOsen)](3+) ( formal potential (Co-III/II) - 519 mV versus Ag-AgCl), which is uncomplicated by any coupled chemical reactions. Akinetic and thermodynamic analysis of the [Co(HOten)](2+)/[Hg(HOten)](2+) system is presented on the basis of digital simulation of the experimental voltammetric data.

Environmental degradation of polyethylene-based plastics

The criteria involved in the degradation of polyethylene-based degradable polymer samples have been investigated, with a view to obtaining a clearer mechanism of photo-biodegradation. The compatibility of degradable polymer samples during materials recycling was also studied. Commercial and laboratory prepared degradable polymer samples were oxidised in different environments and the oxidation products formed were studied using various analytical chromatographic and spectroscopic techniques such as HPLC, FT-IR and NMR. It was found that commercial degradable polymer samples which are based on the ECO systems, degrade predominantly via the Norrish II process, whereas the other degradable systems studied (starch-filled polyethylene systems, transition metal systems, including metal carboxylate based polyethylene systems and the photoantioxidant-activator systems) photodegrade essentially via the Norrish I process. In all cases, the major photoxidation products extracted from the degradable polymer samples were found to be carboxylic acids, although, in the polymer itself a mixture of carbonyl containing products such as esters, lactones, ketones and aldehydes was observed. The study also found that the formation of these hydrophilic carbonyl products causes surface swelling of the polymer, thus making bioerosion possible. It was thus concluded that environmental degradation of LDPE is a two step process, the initiation stage being oxidation of the polymer which gives rise to bioassimilable products, which are consequently bioeroded in the second stage, (the biodegradation step). Recycling of the degradable polymer samples as 10% homogeneous and heterogeneous blends was carried out using a single screw extruder (180°C and 210°C) and an internal mixer (190°C). The study showed that commercial degradable polymer samples may be recycled with a minimal loss in their properties.

The adsorption of phenolic and organotin compounds by clays and cation exchanged clays

Quaternary ammonium exchanged laponites (Quat-laponites) show selectivity in the adsorption of phenols and chlorinated phenols. Strong adsorbate-adsorbent interactions are indicated by adsorption isotherms. Adsorption of phenols and chlorinated phenols by Quat-smectites is greater than that by the Bi Quat-Smectites prepared in this study. It is thought that the quaternary ammonium exchanged smectite components of the Bi Quat-smectites interact with each other (adsorbent-adsorbent interactions) reducing the number of sites available for adsorbate-adsorbent interactions. Solidification/stabilisation studies of 2-chlorophenol show that a blend of ground granulated blast furnace slag and ordinary Portland cement attenuates 2-chlorophenol more effectively than ordinary Portland cement alone. Tetramethyl ammonium- (TMA-) and tetramethyl phosphonium- (TMP-) montmorillonites were exposed to solutions of phenol or chlorinated phenols. TMP- montmorillonite was the better adsorbent and preferentially adsorbed 4-chlorophenol over phenol. Hydration of the interlayer cations occurs to a greater extent in the TMA-montmorillonite than the TMP-montmorillonite restricting interlayer adsorption. Contrary to that observed for phenols and chlorinated phenols, the Quat-smectites were ineffective as adsorbents for triphenyltin hydroxide and bis(tributyltin) oxide at room temperature. Under microwave conditions, only bis(tributyltin) oxide was adsorbed by the quaternary ammonium exchanged smectites. Bis(tributyltin) oxide was adsorbed from ethanol on the surface of the smectite clays at room temperature and under microwave conditions. The adsorbate-adsorbent interactions were weak. Adsorption is accompanied by conversion of bis(tributyltin) oxide to a different tin(IV) species and the release of sodium cations from the montmorillonite interlayer region. Attempts to introduce conditions suitable for charge transfer interactions between synthesised quaternary ammonium compounds and 2,4,6-trichlorophenol are documented. Transition metal complex exchanged clays adsorb 2,4,6-trichlorophenol and phenol. Strong adsorbate-adsorbent interactions (Type I isotherms) occur when the adsorbate is 2,4,6-trichlorophenol and when the adsorbent is [Fe(bipy)3]2+ exchanged montmorillonite or [Co(bipy)3]3+ exchanged montmorillonite. The 2,2'-bipyridyl ligands of the adsorbents are electron rich and the 2,4,6-trichlorophenol is electron deficient. This may have enhanced adsorbate-adsorbent interactions.

The geochemistry of radioactive waste disposal

The present study attempted to identify the significant parameters which affect radionuclide migration from a low level radioactive waste disposal site located in a clay deposit. From initial sorption studies on smectite minerals, increased Kd with decreasing initial cation concentration was observed, and three sorption mechanisms were identified. The observation of anion dependent sorption was related to the existence of a mechanism in which an anion-cation pair are bound to the clay surface through the anion. The influence of competing cations, typical of inorganic groundwater constituents, depended on: (1) Ni/Co:Mn+(Mn+ = competing cation) ratio, (2) nature of M^n+, (3) total solution ionic strength. The presence of organic material in groundwater is well documented, but its effect on cation sorption has not been established. An initial qualitative investigation involving addition of simple organic ligands to Ni(Co)-hectorite samples demonstrated the formation of metal complexes in the clay interlayers, although some modified behaviour was observed. Further quantitative examination involving likely groundwater organic constituents and more comprehensive physical investigation confirmed this behaviour and enabled separation of the organic compounds used into two classes, according to their effect on cation sorption; (i) acids, (ii) amine compounds. X-ray photoelectron spectroscopy, scanning electron microscopy and Mossbauer spectroscopy were used to investigate the nature of transition metal ions sorbed onto montmorillonite and hectorite. Evidence strongly favoured the sorption of the hexaaquo cation, although a series of sorption sites of slightly different chemical characteristics were responsible for broadened peak widths observed in XPS and Mossbauer investigations. The surface sensitivity of XPS enabled recognition of the two surface sorption sites proposed in earlier sorption studies. Although thermal treatment of Fe^3+/Fe^2+-hectorite samples left iron atoms bonded to the silicate sheet structure, Mossbauer evidence indicated the presence of both ferric and ferrous iron in all samples.