Dioxygen Reactivity of Biomimetic Iron-Catecholate and Iron-o-Aminophenolate Complexes of a Tris(2-pyridylthio)methanido Ligand: Aromatic CC Bond Cleavage of Catecholate versus o-Iminobenzosemiquinonate Radical Formation

An iron(III)-catecholate complex [L(1) Fe(III) (DBC)] (2) and an iron(II)-o-aminophenolate complex [L(1) Fe(II) (HAP)] (3; where L(1) =tris(2-pyridylthio)methanido anion, DBC=dianionic 3,5-di-tert-butylcatecholate, and HAP=monoanionic 4,6-di-tert-butyl-2-aminophenolate) have been synthesised from an iron(II)-acetonitrile complex [L(1) Fe(II) (CH(3) CN)(2) ](ClO(4) ) (1). Complex 2 reacts with dioxygen to oxidatively cleave the aromatic C?C bond of DBC giving rise to selective extradiol cleavage products. Controlled chemical or electrochemical oxidation of 2, on the other hand, forms an iron(III)-semiquinone radical complex [L(1) Fe(III) (SQ)](PF(6) ) (2(ox) -PF(6) ; SQ=3,5-di-tert-butylsemiquinonate). The iron(II)-o-aminophenolate complex (3) reacts with dioxygen to afford an iron(III)-o-iminosemiquinonato radical complex [L(1) Fe(III) (ISQ)](ClO(4) ) (3(ox) -ClO(4) ; ISQ=4,6-di-tert-butyl-o-iminobenzosemiquinonato radical) via an iron(III)-o-amidophenolate intermediate species. Structural characterisations of 1, 2, 2(ox) and 3(ox) reveal the presence of a strong iron?carbon bonding interaction in all the complexes. The bond parameters of 2(ox) and 3(ox) clearly establish the radical nature of catecholate- and o-aminophenolate-derived ligand, respectively. The effect of iron?carbon bonding interaction on the dioxygen reactivity of biomimetic iron-catecholate and iron-o-aminophenolate complexes is discussed.     

Stereoselective synthesis of the monomeric unit of actin binding macrolide rhizopodin

An efficient, scalable, and stereocontrolled synthesis of the entire carbon framework of an actin binding dimeric macrolide rhizopodin has been accomplished in its protected form. The key features of our synthesis include a titanium catalyzed anti acetal aldol reaction, a substrate controlled diastereoslelective prenyl stannylation, a Mukaiyama aldol reaction, an indium mediated diastereoselective propargylation, and an advanced stage Stille coupling reaction.     

Dioxygen Reactivity of Biomimetic Iron–Catecholate and Iron–o‐Aminophenolate Complexes of a Tris(2‐pyridylthio)methanido Ligand: Aromatic C?C Bond Cleavage of Catecholate versus o‐Iminobenzosemiquinonate Radical Formation

An iron(III)–catecholate complex [L¹Fe^III(DBC)] ( 2 ) and an iron(II)–o‐aminophenolate complex [L¹Fe^II(HAP)] ( 3 ; where L¹=tris(2‐pyridylthio)methanido anion, DBC=dianionic 3,5‐di‐tert‐butylcatecholate, and HAP=monoanionic 4,6‐di‐tert‐butyl‐2‐aminophenolate) have been synthesised from an iron(II)–acetonitrile complex [L¹Fe^II(CH₃CN)₂](ClO₄) ( 1 ). Complex 2 reacts with dioxygen to oxidatively cleave the aromatic C? C bond of DBC giving rise to selective extradiol cleavage products. Controlled chemical or electrochemical oxidation of 2 , on the other hand, forms an iron(III)–semiquinone radical complex [L¹Fe^III(SQ)](PF₆) ( 2^ox‐PF₆ ; SQ=3,5‐di‐tert‐butylsemiquinonate). The iron(II)–o‐aminophenolate complex ( 3 ) reacts with dioxygen to afford an iron(III)–o‐iminosemiquinonato radical complex [L¹Fe^III(ISQ)](ClO₄) ( 3^ox‐ClO₄ ; ISQ=4,6‐di‐tert‐butyl‐o‐iminobenzosemiquinonato radical) via an iron(III)–o‐amidophenolate intermediate species. Structural characterisations of 1 , 2 , 2^ox and 3^ox reveal the presence of a strong iron? carbon bonding interaction in all the complexes. The bond parameters of 2^ox and 3^ox clearly establish the radical nature of catecholate‐ and o‐aminophenolate‐derived ligand, respectively. The effect of iron? carbon bonding interaction on the dioxygen reactivity of biomimetic iron–catecholate and iron–o‐aminophenolate complexes is discussed.     

A new colorimetric chemodosimeter for mercury ion via specific thioacetal deprotection in aqueous solution and living cells

A sensitive and selective new chemodosimeteric chemosensor (DIBT) for mercury ions was successfully devised and characterized. Upon addition of Hg²⁺ to DIBT, a red-edge absorption band at 972 nm was observed. An important feature for the new chemodosimeter is its high selectivity toward mercury ions over the other competitive species due to Hg²⁺-triggered specific C–S cleavage, making the ‘naked-eye’ detection of mercury ions possible in aqueous solution and living cells such as Candida albicans.     

Copper(II) α-hydroxycarboxylate complexes of bis(2-pyridylcarbonyl)amine ligand: From mononuclear complex to one-dimensional coordination polymer

The coordination geometry and supramolecular structures of three copper(II) complexes of two α-hydroxycarboxylates and one α-methoxycarboxylate with nitrogen donor co-ligands are discussed. The complexes have been characterized by elemental analysis, ESI-MS, IR and electronic spectroscopy, thermogravimetric analysis and magnetic measurements. The X-ray structure analysis of all the complexes, namely [(BPCA)Cu^II(MA)] (1), [(BPCA)Cu^II(MPA)(H₂O)] (2) and [(BPCA)Cu^II(BA)]_n (3), where BPCA = bis(2-pyridylcarbonyl)amidate, MA = racemic mandelate, MPA = racemic α-methoxy phenylacetate and BA = benzilate anion, shows the copper(II) ion in a distorted square-pyramidal geometry. In 1 the mandelate anion is coordinated to the copper(II) center in a bidentate fashion while in 2 the α-methoxycarboxylate is monodentate. In both cases a one-dimensional supramolecular array is formed through hydrogen bonds: the mononuclear units are directly connected in 1 by the MA hydroxyl group, whereas in 2 is the coordinated water that operates as H donor towards the MPA carboxylate group and the BPCA carbonyl oxygens of nearby complexes. In 3 the benzilate anion, acting as bridging ligand between copper ions, gives rise to a one-dimensional coordination polymer. In the latter, intra- and inter-chain π?π stacking interactions between pyridines and one phenyl ring of benzilate anions are observed in the packing.     

Zero-field spin splitting in a two-dimensional electron gas with the spin-orbit interaction revisited

We consider a two-dimensional electron gas (2DEG) with the Rashba spin-orbit interaction (SOI) in the presence of a perpendicular magnetic field. We derive analytical expressions of the density of states (DOS) of a 2DEG with the Rashba SOI in the presence of a magnetic field by using the Green's function technique. The DOS allows us to obtain the analytical expressions of the magnetoconductivities for spin-up and spin-down electrons. The conductivities for spin-up and spin-down electrons oscillate with different frequencies and give rise to the beating patterns in the amplitude of the Shubnikov-de Haas (SdH) oscillations. We find a simple equation which determines the zero-field spin splitting energy if the magnetic field corresponding to any beat node is known from the experiment. Our analytical results reproduce well the experimentally observed non-periodic beating patterns, number of oscillations between two successive nodes and the measured zero-field spin splitting energy.     

Zitterbewegung of electrons in quantum wells and dots in the presence of an in-plane magnetic field

We study the effect of an in-plane magnetic field on the zitterbewegung (ZB) of electrons in a semiconductor quantum well (QW) and in a quantum dot (QD) with the Rashba and Dresselhaus spin-orbit interactions (SOIs). We obtain a general expression of the time-evolution of the position vector and current of the electron in a semiconductor QW. The amplitude of the oscillatory motion is directly related to the Berry connection in momentum space. We find that in presence of the magnetic field the ZB in a QW does not vanish when the strengths of the Rashba and Dresselhaus SOIs are equal. The in-plane magnetic field helps to sustain the ZB in QWs even at a low value of k(0)d (where d is the width of the Gaussian wavepacket and k(0) is the initial wavevector). The trembling motion of an electron in a semiconductor QW with high Landé g-factor (e.g. InSb) is sustained over a long time, even at a low value of k(0)d. Further, we study the ZB of an electron in QDs within the two sub-band model numerically. The trembling motion persists in time even when the magnetic field is absent as well as when the strengths of the SOI are equal. The ZB in QDs is due to the superposition of oscillatory motions corresponding to all possible differences of the energy eigenvalues of the system. This is an another example of multi-frequency ZB phenomenon.     

Efficient organotin catalysts for urethanes: kinetic and mechanistic investigations

Dibenzyltin bis(2‐ethylhexanoate) 1 (4‐Y C₆H₄CH₂)₂Sn(OC(O)R¹)₂ [Y = H, 1a; MeO, 1b; Cl, 1c; Me, 1d; and R¹ = MeCH₂CH₂CH₂CH(Et) ] were synthesized either from the reaction of corresponding dibenzyltin dichlorides with silver 2‐ethylhexanoate or from the reaction of dibenzyltin oxides with 2‐ethylhexanoic acid. Compound 1a was further utilized as a catalyst for the reaction of mono‐ and di‐isocyanates [PhNCO, CH₃C₆H₃‐2,4‐(NCO)₂ and OCN(CH₂)₆NCO] with alcohols (primary, secondary, tertiary, cyclohexcyl, alkyl, allyl, benzyl and aryl) leading to the formation of the corresponding urethanes. The catalytic efficiencies of 1 vis‐à‐vis industrially known organotin catalysts have been determined through kinetic studies for the reaction of PhNCO and n‐BuOH at various temperatures. Compounds 1a, 1c and 1d show higher efficiency than dibutyltin bis(2‐ethylhexanoate). FTIR studies further provide mechanistic insights into the catalytic cycle, which comprises pre‐coordination of isocyanate to tin(IV), formation of stannyl carbamate and generation of dibenzyl(alkoxy)carboxylate as the active catalyst. Copyright © 2000 John Wiley & Sons, Ltd.