Showing papers in "Polyhedron in 1988"

Polymerization of olefins with homogeneous zirconocene/alumoxane catalysts

Abstract: The homogeneous Ziegler-Natta catalyst, consisting of zirconocene compounds and methylalumoxane, leads to extremely high polymerization activities. With bis(cyclopentadienyl) zirconium dichloride, polyethylene with activities up to 40 x 106 g PE/g Zr·h is formed. The higher the degree of oligomerization of the alumoxane the more active are the catalysts. The density of polyethylene (LLDPE) can be lowered by using 1-butene or 1- hexene as comonomers. Incorporation of dienes gives EPDM-elastomers. Highly isotactic polypropylene can be prepared by using chiral rac-ethylene bis(tetrahydroindenyl) zirconium dichloride together with methylalumoxane as cocatalyst. The toluene soluble part here consists of less than 0.2 weight per cent. Cyclopentene gives a highly isotactic and insoluble polymer. With the R or S enantiomers of the catalyst it is also possible to produce optically active oligomers. The side reactions and the mechanism are discussed.

Metal clusters and cluster metals

Abstract: The desirable aim to create uniform naked metal clusters still fails. On the other hand, there exist many ligand stabilized clusters. If there was any possibility of eliminating the ligand sphere, a wide field of study, naked clusters of the same size, could be opened. Naked clusters with a magic number of metal atoms (full shell clusters) should have a special stability because of their complete outer geometry. Two-shell clusters of the type M55L12Clx (M = Au, Rh, L = PPh3, x = 6; M Rh, Ru, L = P(t-BU)3, x = 20, M = Pt, L = As(t-Bu)3, x = 20; M = Co, L = PMe3, x = 20) seem to be suited for studying those problems. As can be seen from HRTEM images and from other investigations, they have a cubic close packed structure (ccp) with the outer geometry of a cuboctahedron. The 12 vertices are coordinated by the 12 ligands L, and the chlorine atoms partially fill the gaps between the phosphine or arsine molecules. The nature of the figands makes the cluster molecules soluble in organic solvents. The transformation into water soluble compounds succeeds if PPh3 is exchanged by the hydrophilic derivative Ph2PC6H4SO3Na·2H2O. So, the cluster Au55(Ph2PC6H4SO3Na·2H2O)12Cl6 can be obtained as a very stable (in contrast to Au55(PPh3)12Cl16) water soluble compound which is completely dissociated in aqueous solution. Using a 100 kV electron beam, small crystalline areas of probably intact cluster molecules can be imaged by TEM. The giant cluster Pd56l(Phen)360190–200 (phen = phenanthroline) is obtained by reduction of palladium acetate with H2 in the presence of appropriate amounts of phen with subsequent oxidation by oxygen. The generation of naked clusters indeed succeeds if the M55L12Clx species are degraded in solution by 20 V d.c., using Pt electrodes. The ligand sphere together with the outer shell of metal atoms is eliminated, generating the inner naked M13 nuclei. These full-shell particles are metastable and build up novel structures. The structural principle can be described as an aggregation of 13 M13 particles, combined in a kind of close packed arrangement. The (M13)13 superclusters again act as building blocks and form particles [(M13)13]n. This construction principle is realized for all metals in this investigation, i.e. Au, Rh, Ru, Pt and Co.

Large transition metal clusters—VI. Ligand exchange reactions on Au55(PPh3)12Cl6—the formation of a water soluble Au55 cluster

Abstract: The cluster compound Au55(PPh3)12Cl6 is soluble in organic solvents like pyridine or dichloromethane but decomposes rapidly thus precluding crystal growth and other investigations. Exchange of PPh3 in Au55(PPh3)12Cl6 by Ph2PC6H4SO3Na takes place quantitatively and yields the stable water soluble cluster Au55(Ph2PC6H4SO3Na·2H2O)12Cl6. Molecular weight determinations and conductivity measurements in H2O show that the cluster is completely dissociated into 12Na+ and [Au55(Ph2PC6H4SO3)12]12-. From such aqueous solutions very small, probably crystalline particles are obtained which can, in the dried state, be observed in the transmission electron microscope using a 100 kV electron beam. Here we present images that show columns or layers of cluster molecules with a distance of 2.1± 0.1 nm. The diameters of a cluster molecule including the ligand shell and of the naked cluster are calculated as 2.2±0.1 and 1.3–1.4 nm, respectively. It is concluded that the cluster molecules, forming the layered structures, are intact. This is the first time that M55 clusters could be imaged with an intact ligand shell by means of TEM. Earlier microscopic investigations with a 400 kV beam gave high resolution images of the cluster nuclei only.

Periodic trends in gas phase MH and MC bond energies

Abstract: Thermochemistry of simple transition metal species as determined by guided ion beam mass spectrometry is reviewed. Compounds which are discussed include metal hydrides (both ionic and neutral), metal methyls (ionic and neutral), ionic metal methylidenes and metal methylidynes, and bis-methyl metal ions. The periodic trends in this thermochemistry are discussed with an eye on developing ways of applying the data to condensed phase species. The measurement of gas phase BDEs of transition metal species is just emerging from its infancy. The technological and theoretical tools necessary to obtain both accurate and precise values using beam techniques have only recently been developed, as have competing methods (such as photodisso ciation 28 and proton affinity measurements 43,44 ). In combination with values from more established techniques (such as high temperature mass spectrometry 45 and spectroscopy), this gas phase thermochemistry provides a wealth of data which can be analysed for its periodic trends. As outlined above, these analyses can provide intrinsic bond energies for ionic and neutral metal hydrides (57–60 kcal mol −1 ), ionic and neutral metal methyls [2–8 kcal mol −1 higher than D 0 (MH + ) for cationic species, and ∼ 2 kcal mol −1 less than D 0 (MH) for neutral species], ionic metal methylidenes ( ∼ 94 kcal mol −1 ) and ionic metal methylidynes (∼ 130 kcal mol −1 ). Further, the periodic trends can help to identify influential factors in determining the strength or weakness of a bond. These include electronic (essentially hybridization) and polarization effects. Continued studies on even more complex transition metal species should enable even closer relationships between gas and condensed phase thermochemistry to be forged.

The influence of copper on the transformation of ferrihydrite (5Fe2O3 · 9H2O) into crystalline products in alkaline media

Abstract: The presence of Cu retards the transformation of ferrihydrite (5Fe 2 O 3 · 9H 2 O) into crystalline products Low levels of Cu (9 mol%) suppress formation of goethite [(α-FeO(OH)] and lead to a product consisting entirely of haematite (α-Fe 2 O 3 ) Cu prevents formation of goethite by hindering dissolution of ferrihydrite rather than by interfering with nucleation and growth of goethite in solution Chemical analysis showed that as much as 9 mol% of Cu replaces Fe in the haematite structure The a o value of the unit cell of haematite increases from 05040 mn for unsubstituted haematite to 05048 nm for haematite with 9 mol% Cu incorporation Transmission electron microscopy showed that Cu-haematite grows as rhombohedral crystals instead of the hexagonal plates typically obtained from ferrihydrite As the level of Cu in the system increases above 9 mol%, Cu-magnetite (Fe 3 O 4 ) forms together with Cu-haematite At 33 mol% Cu, tenorite (CuO) as well as Cu-magnetite is obtained

Thermolysis of the CoC bond in adenosylcobalamin (coenzyme B12)—IV. Products, kinetics and CoC bond dissociation energy studies in ethylene glycol

Abstract: Following an introduction outlining the key questions surrounding the adenosylcobalamin (AdoB 12 ) bond homolysis problem, the full details of product, kinetic and CoC5′ bond dissociation energy studies of the thermolysis of AdoB 12 in ethylene glycol are presented. The anaerobic thermolysis of AdoB 12 in the absence of the nitroxide radical trap TEMPO proceeds with four isosbestic points to yield 100±2% Co(II)B 12 and two nucleoside products, 8,5′-anhydroadenosine and 5′-deoxyadenosine. In the presence of >10 −2 M TEMPO, the TEMPO-trapped Ado . product (T-Ado) and Co(II)B 12 account quantitatively for the starting AdoB 12 . From HPLC studies of the concentration of the nucleoside products vs [TEMPO], absolute rate constants at 110°C for Ado . cyclization and its H . abstraction from glycol solvent are obtained for the first time, k c (110°C) ⋍ 5 × 10 5 s −1 and k a (110°C) ⋍ 7 × 10 3 M −1 s −1 . Kinetic studies in the presence of added, authentic Co(II)B 12 show an inverse, linear dependence of 1/ k obs vs [Co(II)B 12 ], thereby providing quantitative evidence for the long sought demonstration of the AdoB 12 ⇌ Ado . +Co(II)B 12 equilibrium outside of the enzyme-cofactor complex. The Co(II)B 12 dependence data also provide the previously unavailable rate constant for recombination of Co(II)B 12 and Ado . in ethylene glycol of k t ⋍ 3 × 10 8 M −1 s −1 . Next, the assumption used previously of slow homolysis by the base-off form, k h ,off ⪡ k h ,on , an assumption necessary to simplify the AdoB 12 thermolysis kinetics, is tested directly by the synthesis and thermolysis of adenosylcobinamide + OH − and found to be valid. The base-on homolysis rate constants ( k h ,on ) were measured from 110 to 80°C. When combined with independently measured temperature-dependence parameters for the off ⇌gon axial base equilibrium, Δ H = −7.6±0.2 kcal mol −1 and Δ S = −20.2±0.7 e.u., values for the base-on homolysis activation parameters of ethylene glycol are obtained, Δ H h ,on ‡ = 34.5±0.8 kcal mol −1 , and Δ S ‡ = 14±1 e.u. Use of the Δ H h ,on ‡ data to provide an estimate of the AdoB 12 CoC5′ BDE in ethylene glycol is discussed, as are a number of additional points and conclusions that result from the present work. Key considerations that led to the nitroxide radical trapping method, and the desirable features of this method, are then discussed, with an eye towards aiding future kinetic studies and BDE determinations of CoR and other MR compounds.

Cage effects in organotransition metal chemistry: their importance in the kinetic estimation of bond dissociation energies in solution

Abstract: Following a brief review of the literature of radical cage effects in solution, the available evidence for the operation of solvent cage effects in organotransition metal chemistry is summarized. Kinetic determination of ML bond dissociation energies (BDEs) in solution are examined within this context, emphasizing a comparison of the current gas phase reaction coordinate model vs a model more appropriate for solution work that includes solvent cage effects. The temperature dependence of the cage effect is presented in its proper mathematical form which shows that solution kinetics cannot be connected to BDEs without knowledge of the cage efficiency factor (Fc), except in special cases. It is pointed out that the observed activation enthalpy in solution, ΔHobs‡(soln), is a composite containing variable amounts of the difference between the activation enthalpy for the cage combination (ΔHc‡) and that for diffusive separation of the cage pair (ΔHd‡), depending on the cage efficiency factor (Fc). No constant correction to ΔHobs‡(soln) can be expected; a better first approximation would be to use the activation enthalpy for viscous flow of the solvent, ΔHη‡, although this includes the implicit assumption that Fc = 1. The text also attempts to analyse critically the available, relevant literature and to note areas requiring further attention. A short section in which the equations are applied to Ni(CO)4 and PhCH(CH3)Co(DMG)2(Base) (and thus NiCO and CoC BDEs, respectively) is included. A brief list of major points is also provided in the Summary.

Cooperative effect between two metal centres in hydroformylation: Routes towards heterobimetallic catalysis

Abstract: The Rh2(μ-SR)2(CO)2L2 complexes are good catalyst precursors for the hydroformylation reaction of alkenes under mild conditions; high conversion rates are obtained, but when L is a phosphite or triphenylphosphine ligand no more than 85% of linear aldehyde is obtained for a full selectivity of the conversion of alkenes into the corresponding aldehydes. Introduction of the water-soluble P(mSO3C6 H4)3 ligand leads to 96% of selectivity in linear aldehyde and solves the problem of catalyst recovery; in this case the CO/H2 or CO/H2O couples can be used as feedstock. Various bisphosphine L2 ligands can be introduced. These include several containing a transition metal centre (Zr, Fe, Ru). The major effect observed is electronic. Extension to heterobimetallic complexes was carried out. Various rhodium-palladium complexes have been synthesized and the first results of hydroformylation show that aldehydes are produced due to the presence of the rhodium centre.

Reactivity, photochemistry and thermochemistry of simple metal—ligand ions in the gas phase

Abstract: In this review we discuss recent results from our laboratory on the chemistry, photochemistry and thermodynamics of simple metal—ligand ionic species in the gas phase. Using laser desorption coupled to Fourier transform mass spectrometry, we have successfully generated numerous highly unsaturated metal—ligand ions including bare metal carbenes, methyls, hydrides, nitrenes and amides. The combination of photodissociation, ion—molecule reactions and other techniques has made possible the determination of a wide variety of metal—ligand ion bond dissociation energies. Reactivity studies on these species reveal gas phase catalytic cycles including olefin homologation and Fe+ catalysed production of NC bonds from NH3, olefins and N2O. General trends are observed in the reactions of MOH+, MH+, MCH3+, and MNH2+ with alkanes; MO+ and MS+ with alkanes; and MNH+, MCH2+ and MO+ with alkenes.

A theoretical study of metal—ligand bond strengths (ML: L = OH, OCH3, SH, NH2, PH2, CH3, SiH3, CN and H) in the early transition metal systems Cl3ML (M = Ti, Zr and Hf) and late transition metal systems LCo(CO)4

Abstract: The strength of metal—ligand bonds (ML: L = OH, OCH 3 , SH, NH 2 , PH 2 , CN, CH 3 , SiH 3 and H) in the early transition metal systems Cl 3 ML (M = Ti, Zr and Hf) and late transition metal systems LCo(CO) 4 have been calculated by a non-local density functional method. For the early transition metal systems the TiL bond in Cl 3 TiL was found to be quite polar, in particular for OCH 3 , OH and NH 2 . The order of the D (TiL) bond strength was calculated to be OH(453) > OCH 3 (427) > CN(410) > NH 2 (365) > SH(283) > CH 3 (268) > H(251) > SiH 3 (211) > PH 2 (191), where the numbers in parentheses are the bond energies in kJ mol −1 . The corresponding bond energies of the Cl 3 ZrL systems were calculated to be between 25 kJ mol −1 and 50 kJ mol −1 higher. An additional increase in the ML bond energy of 10 kJ mol −1 to 20 kJ mol −1 was calculated in going from M = Zr to M = Hf. The ML bonds in the late transition metal systems LCo(CO) 4 were calculated to be weaker and less polar than the corresponding bonds in Cl 3 TiL. The order for the bond strengths in LCo(CO) 4 , was calculated to be CN(304) > OH(232) > H(230) > SiH 3 (212) > SH(169) > CH 3 (160) > NH 2 (146) > PH 2 (145). The quite different order of stability for the ML bond strength in early and late transition metal systems was analysed in terms of electronic and steric factors.

A new type of pyridine-2-thionato bridge: X-ray crystal structure of the complex [Re2(MepyS)2(CO)6] where MepyS is the 6-methylpyridine-2-thionato ligand

Abstract: [Re2(CO)10] reacts with pyridine-2-thione (pySH) in refluxing xylene to give the dinuclear cluster [Re2(pyS)2(CO)6] which contains three fused four-membered rings. The novel five-electron donating pyS bridges are easily cleaved by ligand addition and in redistribution reactions with [Re2(MepyS)2(CO)6], the 6-methyl-substituted derivative for which the X-ray structure is reported.

Preparation and characterization of ruthenium complexes with the new 4,4′,4″-tri-tert-butylterpyridine ligand and with 4,4′-di-tert-butylbipyridine

Abstract: An efficient procedure for the preparation of 4,4′,4″-tri- tert -butylterpyridine (trpy*) which is formed together with 4,4′-di- tert -butylbipyridine (bipy*) is described, and the preparation of the complexes (trpy*)RuCl 3 , [(trpy*)L 2 RuCl]PF 6 (L 2 = bipy, bipy*), and (bipy*) 2 RuCl 2 and their characterization by cyclic-voltametry, UV—vis and 1 H NMR spectroscopy are reported. It is shown that introduction of tert -butyl substituents increases the solubility of the resultant complexes and enhances the electron donating influence of the trpy ligand.

Tetrakisdimethylamidozirconium and its dimethylamido lithium adduct: Structures of [zr(nme2)4]2 and zr(nme2)6li2(thf)2

Abstract: The molecular structure of [Zr(NMe2)4]2 has been determined by an x-ray study and shown to involve a central Zr2N8 moiety involving the fusing of two trigonal bipyramidal units along a common axial-equatorial edge. The terminal ZrNMe2 units have trigonal planar coordination about the nitrogen atoms: ZrN = 2.050(5) and 2.104(5) A, and ZrN (bridge) = 2.224(3) and 2.453(4) A for equatorial and axial bonds, respectively. The ZrZr distance is 3.704(1) A as expected for a non-MM bonding bridged compound. In tetrahydrofuran solution, Zr(NMe2)4 and LiNMe2 react irreversibly giving Zr(NMe2)6 Li2(THF)2 which has been isolated and characterized by an X-ray study. The central ZrN6 octahedral moiety is capped on two opposite faces by Li atoms which are also coordinated to an oxygen atom of a THF molecule. Pertinent distances are: ZrN = 2.22(7) (av.), NLi = 2.155(25) (av.) and LiO = 1.915(10) A.

The thermodynamic driving force for CH activation at iridium

Abstract: This paper discusses the relationship between the intermolecular oxidative addition reaction of carbon-hydrogen bonds in organic molecules to transition metal centres, and the dissociation energies of the CH, MH and MR bonds that undergo changes during this process. Earlier studies of transition metal MH and MR bond energies are reviewed, followed by a summary of relative and absolute bond energies measured more recently for the (η 5 -C 5 Me 5 )(PMe 3 )Ir(X)(Y) system. The MH and MR energies are unusually large in this system compared with most others that are presently known; an important exception are those in the thorium series, where intermolecular CH activation is also observed. The IrC and IrH bond energy values are utilized in discussing the propensity of iridium for intermolecular CH insertion, and in predicting thermochemistries for its RH insertion and M(H)(R) reductive elimination reactions. Finally, the physical basis for the strong metal-carbon and -hydrogen bonds in the iridium system is discussed.

Oxovanadium(IV) and amino acids—I. The system L-alanine+VO2+ ; a potentiometric and spectroscopic study

Abstract: The equilibria in aqueous solution in the system L -alanine+VO 2+ have been studied by a combination of pH-potentiometric and spectroscopic methods (EPR, visible absorption and circular dichroism) in the pH range 1.5–13. For pH > 4, high ligand to metal ratios were used. The results of the various methods are made self-consistent, then rationalized assuming an equilibrium model including the species MAH, MA, MAH −2 , MA 2 H 2 , MA 2 H, MA 2 , MA 2 H −1 , M 2 A 2 H −2 , M 2 A 2 H −3 (where HA denotes L -alanine) and several hydrolysis products; their formation constants and individual electronic spectra (isotropic and circular dichroism) are given. The isomerism arising from the combination of the lop-sided oxovanadium ion with the asymmetric carbon ligand is analysed.

The pentacyanonitrosylferrate ion—V. The course of the reactions of nitroprusside with a range of thiols

Abstract: Thiols RSH (RSH = MeSH, cysteine, N-acetylcysteine, 2-methylcysteine, N-acetyl-2-methylcysteine, penicillamine, N-acetylpenicillamine and glutathione) react with nitroprusside ([Fe(CN)5NO]2−, pentacyanonitrosylferrate (2−)) to give, via the intermediates [Fe(CN)5N(O)SR]3− and [Fe(CN)5NO]3−, the pentacoordinate iron(I) complex [Fe(CN)4NO]2−. The fate of this complex depends crucially upon the reaction conditions; in the presence of oxygen, [Fe(CN)4NO]2− can reform [Fe(CN)5NO]2− to give an effective stoichiometry of RS− to [Fe(CN)5NO]2− substantially greater than 1 : 1 ; in the absence of oxygen and with a stoichiometric ratio of 1 : 1, the [Fe(CN)4NO]2− follows a well established pathway to yield [Fe(CN)6]4−, Fe2+ and NO; in the absence of oxygen and with an excess of the RS− ligand, substitution occurs at [Fe(CN)4NO]2− to give firstly [Fe(NO)2(SR)2]− and subsequently [Fe2(SMe)[in2(NO)4] (where R = Me) or [Fe4S3(NO)7]− (when R = H). For every R except R = H, decomposition of the initial adduct [Fe(CN)5N(O)SR]3− yields the disulphide RSSR; when R = H, elemental sulphur is formed.

Polyphosphorus ligands—V. The synthesis, phosphorus-31 NMR spectra and conformations of the polykis(diphenylphosphino) benzens (Ph2P)nC6H6−n (n = 1–4)

Abstract: All isomers of (Ph2P)nC6H6−n (n = 1–4) have been prepared by the reaction between the corresponding isomer of C6H6−nFn and Ph2PNa in liquid ammonia. In many cases their phosphorus-31 NMR spectra are structurally diagnostic, with δ(31P) having values characteristic of the stereochemical environment of the Ph2P unit, and the couplings 3J(PP) being conformationally dependent and lying in the range 35–154 Hz, while 4J(PP) and 5J(PP) are much smaller.

Bimetallic cluster-derived heterogeneous catalysts—heteronuclear two-site activation of CO in syngas conversion to oxygenates

Abstract: Tailored bimetal catalysts have been prepared by using SiO2-supported Rh4Fe2, Rh5Fe,Ir4Fe and Pd6Fe6 carbonyl clusters and NaY (or NaX) zeolite-entrapped RhFe and Rh6-xIrx(x = 0–6) carbonyl clusters as the molecular precursors. They exhibited markedly high activity and selectivities for C1 + C2 alcohols in a CO + H2 reaction and higher alcohols in olefin hydroformylation. The bimetal cluster-derived catalysts are structurally characterized by EXAFS, Mossbauer and FTIR spectroscopies in terms of their cluster frameworks and metal compositions including their oxidation states in catalysis. Iron promotion is proposed to be associated with the heteronuclear activation of CO with the adjacent RhFe3+, PdFe3+ and IrFe3+ located at the metal/oxide interfaces to enhance the migratory insertion of CO into MH and Malkyl, reflected in the marked increase of C1 + C2 alcohol formation from CO and H2.

New, improved syntheses of the group 6 oxyhalides, W(O)Cl4, W(O)2Cl2 and Mo(O)2Cl2

Abstract: Hexamethyldisiloxane is a useful reagent for the convenient, high yield preparation of oxyhalide derivatives of molybdenum and tungsten.

Relative bond dissociation energies for early transition metal alkyl, aryl, alkynyl and hydride compounds. Equilibration of metallated cyclopentadienyl derivatives of peralkylated hafnocene and scandocene with hydrocarbons and dihydrogen

Abstract: Relative bond dissociation energies (BDEs) have been obtained by equilibrating early transition metal alkyls and hydrides with H2 or the CH bonds of hydrocarbons. Thus, in benzene solution Cp2HfH2 (CP = (η5-C5Me5)) equilibrates with Cp2Hf(C6H5)H and dihydrogen. From the enthalpy of the reaction, ΔHO = +6.0(3), the HfH (BDE) is calculated to be 0.8(3) kcal mol−1 stronger than the HfC6H5BDE. Relative ScC6H5 and Scalkyl BDEs have been estimated from the equilibration of the metallated complex Cp(η5,η1-C5Me4CH2CH2CH2)Sc, C6H6 and Cp(η5-C5Me4CH2CH2CH3)ScC6H5, the ScC6H5 BDE being 16.6(3) kcal mol−1 stronger than the ScCH2CH2CH2C5Me4 BDE. From a similar reversible intramolecular metallation of Cp(η5-C5Me4CH2CH2CH3)HfH2 to give Cp(η5,η1-C5Me4CH2CH2CH2)HfH and dihydrogen, the HfH BDE is estimated to be 23.0(3) kcal mol−1 stronger than the HfCH2CH2CH2C5Me4 BDE. The equilibration of Cp(η5-C5Me4CH2C6H5)ScCCCMe3 with metallated scandocene derivative Cp(η5,η1-C5Me4CH2-o-C6H4)Sc and tert-butylacetylene lies very far towards Cp(η5- C5Me4CH2C6H5)ScCCCMe3, so that only a lower limit for the relative Scalkynyl and Scaryl BDEs may be determined: BDE(Scalkynyl)-BDE(Scaryl) >29(5) kcal mol−1. These early transition metal-hydrocarbyl (MR) BDEs correlate with the corresponding HR BDEs (i.e. Malkynyl > Maryl > Malkyl); however, the MR BDEs increase more rapidly with s character for R than do the HR BDEs. The origin of this effect is not known, but it is undoubtedly also responsible for the characteristically high MH BDEs for transition metal hydride compounds. In order to probe the polarity of Scaryl bonds a series of scandocene derivatives capable of reversibly metallating at either of two differently substituted benzyl groups was prepared. The equilibrium constants for these metallated derivatives: (η5,η1-C5Me4CH2-o-C6H3-p-X)(η5-C5Me4CH2C6H4-m- CH3)Sc (η5-C5Me4CH2C6H4-m-X)(η5,η1-C5Me4CH2-o-C6H3-p-CH3)Sc (X = H, CF3, NMe2) were determined. The small dependence of Keq on the nature of X suggests that the Scaryl bond is essentially covalent with only a small ionic contribution.

The reversible adduct formation of group III trialkyls with relatively involatile phosphines; X-ray crystal structures of (Me3M)2·(Ph2PCH2)2 (M = Al, Ga, In) and (Me3Al)3·(Ph2PCH2CH2)2PPh

Abstract: The following adducts of Group III trialkyls with phosphines have been prepared, either by direct reaction in hydrocarbon solution or by displacement of ether from the metal trialkyl etherate: Me3M·PPh3 (M = Ga, In); Me3In·P(2-MeC6H4)3; (R3M)2·(Ph2PCH2)2 (R = Me, M = Al, Ga, In; R = Et, M = Ga, In; R = Bui, M = Al); (Me3M)3·(Ph2PCH2CH2)2PPh (M = Al, Ga, In) and (Me3M)4·(Ph2PCH2CH2PPhCH2)2 (M = Al, Ga, In). The compounds were analysed by 1H and 31P NMR spectra of (Me3M)2·(Ph2PCH2)2 (M = Ga, In) showed little change between 193 K and room temperature. Thermal dissociation of the adducts in vacuo gave the free metal trialkyl with no detectable contamination by the respective phosphine. Crystals of (Me3M)2·(Ph2PCH2)2 (M = Al, Ga, In) are iso-structural and the molecules contain two distorted tetrahedral metals bridged by the (Ph2PCH2)2; the MP distances are 2.544(4), 2.546(4) and 2.755(4) A, respectively. The X-ray crystal structure of (Me3Al)3·(Ph2PCH2CH2)2PPh shows the molecule to contain distorted tetrahedral aluminium atoms bonded to each of the three phosphorus atoms, with AlP distances of 2.536(9) and 2.510(9) A for the terminal and central moieties, respectively; the unit cell contains two such molecules plus one benzene molecule (the crystallizing solvent).

Synthesis and characterization of Fe(III), Co(II), Ni(II), Cu(II) and Zn(II) complexes with N-salicydene-o-hydroxymethyleneaniline

Abstract: The new Fe(III), Co(II), Ni(II), Cu(II) and Zn(II) complexes with tridentate Schiff base, the product of condensation of o-aminobenzyl alcohol with salicylaldehyde have been synthesized and characterized by elemental analysis, IR, electronic, EPR and Mossbauer spectra, thermal analysis, magnetic susceptibility and molecular weight measurements. Dimeric or polymeric structures for the investigated complexes were proposed. The interaction of the cobalt complex with dioxygen is also described.

Nitrido complexes of technetium with tertiary phosphines and arsines

Abstract: Diamagnetic Technetium(V) complexes of general formulae [TcNX 2 (Ph 3 Y) 2 ] and [TcNX 3 (Me 2 PhP) 3 ] (X = Cl, Br; Y = P, As) were prepared starting from TcNCl 4 − and TcNBr 4 − , respectively and were characterized. Ligand exchange rates with diethyldithiocarbamate, et 2 dtc − and N-(N″-morpholinylthiocarbonyl)benzamidinate, morphtcb − , were determined.

Comparison of metal-hydrogen, -oxygen, -nitrogen and -carbon bond strengths and evaluation of functional group additivity principles for organoruthenium and organoplatinum compounds

Abstract: The equilibria: LnMX + HY ⇁ LnMY + HX (LnM = (DPPE)MePt or Cp(PMe3)2Ru; X, Y = hydride, alkoxide, hydroxide, amide, alkyl, alkynyl, hydrosulphide, cyanide) have been examined. The equilibrium constants allow for the determination of relative MX, MY bond dissociation energies (BDEs) for each series of compounds. A linear correlation of LnMX to HX BDEs is found for the two dissimilar metal centres. Activation barriers for phosphine dissociation from Cp(PMe3)2RuX complexes have been measured and suggest the principle of functional group additivity has limited applicability in organometallic thermochemistry. The generality and predictive value of this correlation and the observations on functional group additivity are discussed.

Solution synthesis of calcium, strontium and barium metallocenes

Abstract: Bis(cyclopentadienyl)metal complexes of calcium, strontium and barium can be prepared in synthetically useful yields (> 65%) from the reaction of lithium or potassium cyclopentadienides and the appropriate metal dihalides in THF. The THF-soluble complexes (C5H5)2Ca(THF)2 and (Me5C5)2M(THF)2 (M = Ca, Sr, Ba) are extracted from the reaction of the appropriate potassium cyclopentadienide with a metal halide; the THFinsoluble (C5H5)2M(THF)x(x ≈ 1 for Sr; x ≈ 0.25 for Ba) remain after the reaction of a lithium cyclopentadienide and the metal iodide.

Syngas and HDS catalysts derived from sulphido bimetallic clusters

Abstract: The clusters, CP2′Mo2Fe2S2(CO)8 (MoFeS) and Cp2′Mo2CO2S3(CO)4 (MoCoS) (Cp′ = η-C5H4Me) have been supported on the refractory oxides, Al2O3, SiO2, TiO2, and MgO, and subjected to temperature programmed decomposition (TPDE) under flowing H2. Typically, CO evolution commences near 100°C, followed by evolution of 1–2 Cp-ligands from 180 to 400°C along with small amounts of CO2, CH4, and H2S or Me2S. The resulting compositions are shown to be active catalysts for CO hydrogenation and hydrodesulphurization (HDS) of thiophene. Methane is the principal hydrocarbon product from CO hydrogenation except for MoFeS/MgO where high selectivity for C2 products was observed. The activity and selectivity of MoCoS/Al2O3 for thiophene HDS closely resembles those of conventionally prepared “cobalt molybdate” catalysts. The cluster derived catalysts have been characterized by Mossbauer and X-ray absorption (XANES) and EXAFS) spectroscopies. It is concluded that the clusters undergo oxidation by the surface upon loss of organic ligands. The results obtained to date show that sulphido bimetallic clusters are excellent precursors for the formation of uniform catalytic surfaces. The uniformity of the surface species facilitates physical characterization of the active site(s). Our results show that the supported clusters are transformed to surface oxo-ensembles which are active for CO hydrogenation and HDS of organic sulphur compounds.

Synthesis, characterization and fungitoxicities of metal chelates of a pentadentate N3S2 ligand

Abstract: The Schiff base obtained by reaction of S-methyldithiocarbazate with 2,6-di-acetylpyridine behaves as a dinegatively charged pentadentate N 3 S 2 chelating agent producing stable crystalline complexes of the general formula, M(SNNNS) (where M = Ni, Cu, Co, Zn and Cd; SNNNS = the dinegative anion of the Schiff base). The complex Fe(SNNNS)C1 was also isolated. Conductivity data and magnetic and spectroscopic evidence support a five-coordinate configuration for the M(SNNNS) complexes and an octahedral configuration for the Fe(SNNNS)Cl complex. Fungitoxicities of the Schiff base and its metal complexes against three plant pathogens viz. Alternaria solani, Curvularia geniculata and Colletotrichum capsici have been studied. The ligand and its complexes display marked antifungal activities against all the test fungi.