Metal Moiety Research Articles

Chemie metallacyclischer alkenylcarbenkomplexe II. Einfluß des zentralmetalls auf struktur, reaktivität und elektronentransfereigenschaften

The novel metallacyclic alkenylcobaltcarbene complex 2a is readily available by Meerwein alkylation of the underlying cobaltalactone 4a. Although having very similar structures (X-ray analyses) and electron transfer behaviour (cyclic voltammetry), 2a and its iron analogue 1a show significantly different reactivity and regioselectivity towards various nucleophiles. SINDO1, a semiempirical calculation method provides a suitable explanation for this distinct influence of the central metal moiety.

Di- and tri-metallic complexes containing two bridging cyclodiphosphazane ligands: the crystal structure of [MO2(CO)8{µ-cis-[PhNP(OC6H4Me-p)]2}2

The reactions of the mononuclear cyclodiphosphazane complexes, cis-[Mo(CO)4{cis-[PhNP(OR)]2}2] with [Mo(CO)4(nbd)](nbd = norbornadiene), [Mo(CO)4(NHC5H10)2] or [MCl2(cod)](cod = cycloocta-1,5-diene) afforded the homobimetallic complexes, [Mo2(CO)8{µ-cis-[PhNP(OR)]2}2](R = C6H4Me-p5 or CH2CF36) or the heterobimetallic complexes, [Mo(CO)4{µ-cis-[PhNP(OR)]2}2MCl2](R = C6H4Me-p; M = Pd 7 or Pt 8). In all the above complexes, the two metal moieties are bridged by two cyclodiphosphazane ligands. The reactions of the mononuclear complexes, cis-[M(CO)4(A){cis-[PhNP(OC6H4Me-p)]2}] with [M′Cl2(cod)] afforded the trinuclear complexes, cis-[M′Cl2[M(CO)4(A){cis-[PhNP(OC6H4Me-p)]2}]2](M′= Pd, M = Mo, A = P(OMe)310; M′= Pt, M = Mo, A = P(OMe)311; M′= Pd, M = W, A = NHC5H1012; M′= Pt, M = W, A = NHC5H1013). The structure of the complex 5 has been determined by single-crystal X-ray crystallography.

Substituent dependency in pentaamminemetal(II) complexes (M = Ru, Os) of styrenes probed by using carbon-13 nuclear magnetic resonance spectroscopic method and voltammetrical analysis

The 13C NMR spectra of [M(NH3)5(styrene)]2+ (M = Ru, Os) demonstrate the substituent dependency of the interaction between the vinyl group and the metal moiety which suggests the modes of bonding. This phenomenon is also observed from the voltammetric analytical results which confirm the substituent dependency of the oxidation potential.

Langmuir‐blodgett films of stilbazole complexes of iridium(I) and rhodium(I)

AbstractComplexes of 4‐alkoxystilbazoles with iridium and rhodium form stable Langmuir layers at the air‐water interface even when the alkoxy chain is relatively short (C5–C12). The surface pressure‐area isotherms indicate that condensed molecular monolayers are obtained. The area per molecule of each compound in its monolayer form is typically 0.60 nm2, which agrees well with the cross‐sectional area of the [Ir(CO)2CI] or [Rh(CO)2CI] head group predicated using molecular models. This suggests that the molecules are oriented with the metal moiety close to the water surface and their alkoxystilbazole ‘rod’ protruding from the plane of the water surface. Such floating monolayers have been transferred on to solid substrates such as glass, aluminium (AI2O3/AI/Glass) and silicon (SiO2/Si) at relatively high speed (10 mm min −1) to form Y‐type LB assemblies. The UV–Visible absorption properties of these materials in solution and LB film form have been studied. LB films of these complexes yield bathochromically shifted spectra relative to the LB film spectrum of the uncomplexed stilbazole. Additionally, these spectra are often broader and hypsochromically shifted relative to their corresponding solution spectra as a result of the close molecular packing within the LB film and the associated dipole–dipole interactions.The electrically polar nature of the molecules described in this paper suggest that they may be suitable candidates for new pyroelectric materials. Thus the pyroelectric coefficient (the rate of change of electric polarisation with respect to temperature) has been measured for a polar multilayer LB film containing an iridium complex. A pyroelectric coefficient of 3.5 μCm−2K−1 (at 30 °C) has been measured, which is one of the highest reported valued for an LB film. Additionally, a low dielectric loss of around 0.01 has been found over the frequency range 50 Hz–1 kHz, indicating that such LB films may be usfeul materials for pyroelectric sensors.

Theoretical interpretation of the reactivity of highly dispersed rhodium catalysts toward carbon monoxide adsorption

The interaction of CO with precrystalline Rh[sub 4] and Rh[sub 13] structures is analyzed by means of intermediate neglect of differential overlap (INDO) calculations. The strength of the interaction, as well as the relative contribution of the [sigma] and [pi] bonds to the stabilization of CO on the different sites, is studied as a function of the electronic characteristics of the cluster surface. Larger binding energies (BE) are found on the electron-deficient structures and are associated with larger [sigma] interactions. This brings about a greater charge transfer to the metal moiety and a subsequent geometric distortion due to the weakness of the Rh-Rh bonds in comparison to the Rh-C bonds that are formed. The larger activation energy for diffusion between the sites defined on the smaller structures which has been experimentally derived from [sup 13]C NMR determinations is associated with the large BE for adsorption on electron-deficient sites. These sites are present in higher concentration in the smaller structures. 54 refs., 8 figs., 6 tabs.

Theoretical study of the formation of oxide-supported metal particles: strength of the chemical glue as represented by transition metal ions at the metal-oxide interface

The first step of the growth of a metal particle supported on an oxide has been theoretically simulated using the extended Huckel molecular orbital (EHMO) approach. The nucleation site at the oxide surface is modeled by a M(OH)x complex (x=3 and 5) and its interaction with either an isolated metal atom or a dimer theoretically calculated. The latter can approach the nucleation site with its internuclear axis either perpendicular or parallel to the surface. The stabilization energy, E s , expressed as the energy difference between the nucleation site and the metal (mono- or dimer) moieties at infinite separation and at bonding distance (2.25 A), has been plotted as a function of the original number of electrons at the nucleation site

Organoiron-templated stereocontrolled alkylation of enolates: functionalization of cycloheptadienones to give useful synthetic building blocks

Conversion of η 4 -cycloheptatriene-Fe(CO) 2 (OPh) 3 to ketocycloheptadiene-Fe(CO) 2 P(OPh) 3 complex 7 was accomplished by hydroboration followed by Swern oxidation. Methylation and hydroxylation of the enolate from 7 proceeds with complete stereoselectivity, anti to the metal moiety, and introduction of two methyl or hydroxyl groups at the α and α' positions was accomplished in high overall yield. Reduction of the ketone group on these complexes occurs with high stereoselectivity and is controlled by the boat conformation adopted by these complexes

Organische Synthesen mit Übergangsmetall–Komplexen, 61[1] Pyridine und Pyridinium–Salze aus (β‐Aminovinyl)carben–Komplexen (MCr, W) und Alkinen über 4(1H)‐Pyridinyliden–Komplexe

Organic Syntheses via Transition Metal Complexes, 61[1]. – Pyridines and Pyridinium Salts from (β‐Aminovinyl)carbene Complexes (MCr, W) and Alkynes via 4(1H)‐Pyridinylidene ComplexesPyridines and pyridinium salts were obtained in good yields by the condensation of (β‐aminovinyl)carbene complexes with alkynes. The reaction of [(Z)‐β‐(monoalkylamino)vinyl]carbene complexes [ 4‐(monoalkylamino)‐1–metalla–1,3–dienes)] LnMC(OEt)CHCRNHR1 (Z)‐6a–f (R, R1Ph, Me, iPr, nBu, CH2Ph, CH2OMe) with alkynes R2CCH 7a–c (R2  nBu, Ph, CH2OMe) involves the highly regioselective formation of 4(1H)‐pyridinylidene complexes . Protonation of 8 with HBF4 leads to pyridinium salts 13 by the regiospecific substitution of the metal moiety LnM by a proton. The reaction may conveniently be performed in a one–pot procedure. {[(Z)‐β‐(H2‐Amino)vinyl]carbene}chromium complexes, e.g. 14, react with alkynes to give pyridines, e.g. 15. The thermal decomposition of 6 yields enol ethers 11. A regiospecific formation of (E)‐11 is observed in the thermolysis of 6 in the presence of pyridine.

The first examples of 2,2′:6′,2″-terpyridine as a fluxional bidentate ligand

2,2′:6′,2″-Terpyridine (terpy) forms octahedral complexes fac-[ReBr(CO)3(terpy)], cis-[W(CO)4(terpy)] and fac-[PtClMe3(terpy)] in which the ligand oscillates between equivalent bidentate forms by a mechanism involving a ‘tick-tock’ twist of the metal moiety through an angle equal to the N–M–N angle of the octahedral centre and involving a seven-coordinate metal intermediate.

Kinetic studies of the cleavage of M +[M′M″(CO) 9L] − (M +  Na +, PPN +; M′  Cr, W; M″  Mn, Re; L  CO, PR 3)

Heterobimetallic complexes have a donor-acceptor metal-metal bond in which two electrons from the electron rich metal moiety are donated to the other electron-deficient metal moiety. The heterobimetallic species of interest, M +[M′M″(CO) 9L] − (M + = Na +; M′  Cr, W; M″  Mn, Re; L  CO, PR 3) undergoes ligand substitution or metal-metal bond cleavage upon reaction with PR 3 (R  C 6H 5, C 2H 5) or CO. Unprecedented reactions by the counter ion were observed. Factors affecting these dimeric disruptions will be discussed on the basis of the kinetic studies.

EPR studies of the paramagnetic complexes formed in the photochemical reactions of Mo(CO) 6 with o-semiquinones

The photochemical reactions of Mo(CO) 6 with 9,10-phenanthrene and 1,2-acenaphthene semiquinone radicals (PSQ, ASQ) were investigated by EPR. The photoirradiation produces the paramagnetic complexes in which the semiquinones coordinate to Mo(CO) 4 in the chelate form. Most unpaired electron densities populate the quinone ligands, but the spin densities delocalize onto the metal moiety more so in the PSQ complex than in the ASQ complex. This difference in the spin distribution with ligands is interpreted in terms of the MO theory. To confirm the reaction scheme proposed for the above reactions, the reactions between the semiquinone radicals and Mo(CO) 5(s) (s = solvent molecule), which were prepared by photoirradiation to Mo(CO) 6 in tetrahydrofuran or in 1,2-dimethoxyethane, were also examined.

F-Element/crown ether complexes. 20. Synthesis and structure of [Y(NO3)3(OH2)3]�1.5(15-crown-5)�Me2O

The title complex was prepared by the addition of 15-crown-5, dissolved in acetone, to a solution of Y(NO3)3·nH2O in the same solvent. The crystal structure of this complex, determined by single-crystal X-ray diffraction using counter methods, was carried out on a crystal grown by slow evaporation of the reaction mixture at room temperature. [Y(NO3)3(OH2)3]·1.5(15-crown-5) Me2CO crystallizes in the acentric space group, P21, with unit cell dataa=15.900(5),b=16.530(6),c=11.821(5) A,β=92.12(3)°,Dcalc=1.53 g cm−3 forZ=4. Each unit cell consists of two dimers of the formula unit. One crown molecule bridges two [Y(NO3)3(OH2)3] units via hydrogen bonding with one water molecule of each. The two remaining water molecules per metal moiety form three hydrogen bonds to another 15-crown-5 molecule and one to an acetone molecule. The average bonding parameters in the nine-coordinate metal complexes are Y-O(NO3)=2.43(4) A and Y-O(OH2)=2.34(5) A.

Reactivity of Pnicogen Chalcogenides with Metal-Ligand Fragments

Abstract The pnicogen chalcogenides form a large class of molecular and polymeric compounds; the molecules containing pnicogen and chalcogen atoms at the vertices of polyhedral structures exhibit several coordination sites and reactive edges and faces. The pnicogen chalcogenides interact in various ways with metal moieties: depending on the electronic properties of the metal fragments employed, compounds containing the intact cage molecules or fragments originating from their cleavage have been obtained. Different degrees of demolition of the cages have been achieved by tuning the interactions between the cages and the metal fragments through appropriate selection of the reacting species.

Complexes bi- et polymétalliques à base de cobalt dérivés du titanocène et du vanadocène

The bimetallic complexes Cp 2Ti[Co(CO) 4] ( 3) and Cp 2V[Co(CO) 4] ( 5) are obtained from the reaction of Cp 2MCl (M = Ti, V) with [Co(CO) 4] − or from the reaction of Cp 2V with Co 2(CO) 8. The IR data of 3 are indicative of a bridging carbonyl ligand between the two metallic moieties whereas a stretching frequency in the IR spectrum of 5 suggests a direct vanadiumcobalt bond. A fast evolution of these complexes is observed leading to the oxymethylidynetricobalt nonacarbonyl derivatives Cp 2M[PCCo 3(CO) 9]. With substituted cyclopentadienyl complexes, the bimetallic complexes are not isolated and only cobalt cluster derivatives are obtained.

ChemInform Abstract: Carbon‐Carbon Bond Forming Reactions of Organotransition Metal Enolate Complexes

Metal enolates play an important role in stereoselective organic synthesis. Their chemistry is affected profoundly by the metal counterion associated with the enolate fragment. In order to expand the potential of replacing main group with transition metal moieties in such species, methods have been developed for the synthesis of a number of stable, characterizable late transition metal ql-(C)-enolate complexes having the general structure LM-CH2COR (M = Mo, W, Re). The chemistry of these materials (e.g., functional transformations of the organic carbonyl group, transfer of the enolate moietry to organic substrates such as aldehydes and alkynes) has been investigated. The scope and mechanisms of the enolate reactions will be discussed in detail.

1-Alkoxy-1-siloxycyclopropanes as homoenolate nucleophiles of esters

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Carbon-Carbon bond forming reactions of organotransition metal enolate complexes

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ChemInform Abstract: Fragmentation of Realgar, As4S4, Controlled by Transition Metal‐Ligand Systems.

AbstractThe As4S4 molecule yields different fragments in the presence of various (triphos)metal moieties depending on the nature of the metal atom and its oxidation state in the parent compound.

Ultrastructural cytochemical analysis of intranuclear arsenic inclusions

To establish the chemical composition of the arsenic inclusion, freshly isolated preparations of inclusions and epon-embedded thin sections of inclusions were subjected to ultrastructural cytochemical analysis. Intranuclear inclusions are composed of amorphous, arsenic-containing subunits aligned linearly to form a coiled complex. Lipase, ribonuclease, deoxyribonuclease, trypsin, pepsin, protease, amylase, or ethylenediaminetetraacetic acid (EDTA) was used to digest or chelate these inclusions. Following enzymatic digestion or chelation, the electron opacity of inclusions was compared with that of control sections exposed for equal times to equivalent solutions lacking the enzymes. Exposure to amylase caused a consistent reduction in the electron opacity of thin sections of inclusions and almost complete digestion of the freshly isolated preparations of inclusions. This was indicative of the presence of a carbohydrate moiety within arsenic inclusions. Incubation of inclusions with EDTA resulted in solubilization of freshly isolated and thin-sectioned embedded material. These data indicated that the intranuclear arsenic inclusion is composed of both metallic and carbohydrate moieties, confirming earlier studies which identified arsenic within inclusions using instrumental neutron activation analysis and X-ray microprobe analysis.

Porphyrin Iron(III) Mixed Function Oxidases: An Evolutionary Endpoint for Transition Metal(III) Reactions with Oxygen Donors

Peroxidases, catalases, and cytochrome P-450 enzymes have in common iron(III) protoporphyrin-IX as a cofactor. The reactions catalyzed by these enzymes can be, for the most part, duplicated by use of transition metal(III) porphyrins. Porphyrins serve admirably well as conjugated and rigidly planar ligands that prevent other than transformations of the ligated metal moiety at axial positions. The adjacent and distal axial positions serve to separate the enzyme-bound (distal) ligand from the reactive face (adjacent) of the iron(III). These features are not required, however, to mimic the chemical conversions that are catalyzed by peroxidases, catalases, and cytochrome P-450 enzymes. Indeed, other simple ligands may be used with a number of transition metals (cf. Samsel et al. 1985). Porphyrins were selected as components of living organisms at an early time, as attested to by the observation that petroleum is a rich source of porphyrins.