Fischer dinuclear and mononuclear bis-carbene complexes of thiophene and thiophene derivatives

The reaction of dilithiated thiophene and thiophene derivatives with group 6 transition metal carbonyl precursors and subsequent alkylation afforded linearly arranged Fischer 2,5-bis-carbene and the rare unsymmetrical 2,3-bis-carbene chelated complexes. The latter requires a second lithiation to occur at an adjacent, less reactive site on the thiophene ring. The control of reactivity sites was investigated and achieved by either blocking more reactive positions with substituents or activating less reactive positions by lithium−halogen exchange reactions. A series of Fischer bis-carbene chelates were synthesized by manipulating the above variables. Structural features of Fischer mono-carbene, mononuclear bis-carbene, and bis-carbene chelated complexes were studied by IR, NMR, and single-crystal X-ray diffractionThe National Research Foundation of South Africa under Grant No. 73679 (S.L.)http://pubs.acs.org/journal/orgnd7hb2017ChemistryChurch History and Church PolicyCivil EngineeringClinical Epidemiolog

Cobalt silicide formation on a Si(1 0 0) substrate in the presence of an interfacial (Fe90Zr10) interlayer

The reaction between a thin film (126 nm) of Co and Si has been studied at 450 C for 24 h under high vacuum conditions, in the presence of a FeZr barrier layer. Without a diffusion barrier layer between Co and Si, Co2Si forms at 350 C as the initial phase while CoSi2 forms at 550 C. The FeZr barrier layer changed the flux of atoms arriving at the reaction interface. Co reacted with the Si from the substrate and formed a mixed layer of CoSi and CoSi2 in the interlayer region. The use of the FeZr diffusion barrier has been demonstrated to lower the temperature formation of CoSi2 to 450 C. The reactions were characterised by Rutherford backscattering spectrometry, Auger electron spectroscopy depth profiling, X-ray diffraction using CoKa radiation and scanning electron microscopy.http://www.elsevier.com/locate/nimb2016-09-30hb201

Thiophene bimetallic carbene complexes

The syntheses of σ,σ-bimetallic biscarbene- and a, σ π -bimetallic monocarbene complexes, as well as the reactivity of the former, were investigated. The metal carbonyls Cr(C0)6, W(C0)6, Mn ( n5-C5H4Me)(C0)3 and Mn(n5-C5H5)(C0)3 were reacted with dilithiated thiophene to synthesize the novel σ,σ -bimetallic biscarbene complexes: (C0)2L3M{C(OEt)C4H2SC(OEt)}MLlC0)2, where M = Cr, W, Mn; L =CO; = n5-C5H4Me, n5-C5H5. With dilithiated 2,2'-methylene dithiophene; (C0)2L3M{C(OEt)C4H2SCH2C4H2SC( OEt)} ML3 CO )2, were obtained. These complexes were fully characterized and formulations were confirmed by ctystal structure determinations for the former. Spectroscopic data indicate the thiophene rings to act as sources of electron density which stabilize the electrophilic carbene carbons. Treatment ofmonolithiated (n5-thiophene)Cr(C0)3 with hexacarbonyls afforded in addition to the expected a,1t-bimetallic monocarbene complexes, (n1:n5-C4H3SC(OEt)M(C0)5)Cr(C0)3, the complexes (n5 : n5-C4H3SC( O(CH2}40Et)M(C0)5)Cr(C0)3 and M { C(O(CH2)40Et)C4H3S }(C0)5, where M = Cr, W. The formation of the latter two types is most unexpected and indicates that -rt-coordination of Cr(C0)3 to thiophene plays an important electronic and steric role which leads to an unique pathway in which a THF-ring is opened and incorporated into the carbene functionality. The a,1t-bimetallic monocarbene complex of (n6-benzo[b]thiophene) Cr(C0)3 was synthesized to enable a comparison with the former. Monolithiation of (n6-benzo[b]thiophene)Cr(C0)3 occured in the 2-position of the thiophene ring and (n1 : n6 C8H5SC(OEt)Cr(C0)5)Cr( C0)3 was afforded in a quantitative yield, thereby proving that -rt-coordination to the benzene ring is stronger and more distant from the a-bonded metal fragment. Related THF-incorporated complexes were not formed in this reaction. The stability of the thienylene biscarbene complexes at various temperatures, in different solvents and under two different inert atmospheres, led to the characterization of the following classes of complexes: I: (C0)2L3M{C(OEt)C4H2SC(O)OEt}; II: W{C(OEt)C4H2SC(OEt)C4H2SC(OEt)C4H2SC(O)OEt}(C0)5; III: (C0)2L3M{C(OEt)C4H2SC(S)OEt}; IV: M{C(OEt)C4H2S-3-[2,5-{C(S)OEt}2C4HS}(C0)2; V: Cr{C( OEt)C4H2SC(O)C(O)C4H2SC(O)OEt}(C0)5 . I was obtained from the decomposition in acetone under nitrogen and was presumably formed in the reaction between the carbene moiety and oxygen which had diffused into the system, as no proof of an oxygen transfer from acetone could be found. II was isolated from the analogous decomposition under argon, thus indicating that the complete elimination of oxygen from a better equiped system would lead to the formation of new and different types of oligomeric products. III was afforded by the reaction in CS2, with the exception of IV, which was yielded, for the chromium biscarbene in CS2, as well as, hexane. V was isolated from the decomposition in hexane. The result of these studies indicates that as opposed to monocarbene complexes, biscarbene complexes displayed enhanced reactivity and the reaction conditions control conversion. The reaction of the thienylene chromium and tungsten biscarbenes with hex-3-yne yielded analogous monocarbene products, M{C(OEt)CCHCC(OH)C(Et)C(Et)C(OEt)CS}(C0)5, lacking a TI-coordinated Cr(CO)3. This is a surprising result for tungsten as the expected product should be the cyclopentathienyl complex. The result suggests an alternative mechanism for the Dotz reaction and a possible reaction route is proposed. The reaction of biphenyl acetylene, however, afforded the expected cyclopentathienyl complex, M{C(OEt)CCHCC(Ph)C(Ph)CH(OEt)CS}(CO)5, in two isomeric forms. The analogous chromium reaction gave the related cyclopentathienyl product, as well as, the expected benzothienyl complex, as two isomers, with Cr(CO)3 TI-coordinated to either the benzene- or thiophene ring , and the end product lacking the Cr(CO)3-fragment. The isolation of the alkyne inserted intermediate, (CO)5Cr{C(OEt)C4H2SC(OEt)C(Ph)C(Ph) }Cr(CO)5, emphasises that the stabilization of intermediates by coordination to more than one metal fragment, highlights the value of bimetallic systems in studying reaction mechanisms.Thesis (DPhil)--University of Pretoria, 1996.ChemistryDPhilUnrestricte

Modellering en eksperimentele ondersoek van Sb-oppervlak segregasie in Cu-enkelkristalle

In this study, the segregation of Sb to a Cu(111) and a Cu(110) surface was studied by (i) modelling the segregation process theoretically and (ii) measuring it experimentally. The aim of this study was to determine the influence of surface orientation on the segregation kinetics. Theoretically, the segregation of Sb from the bulk to the surface was modelled by an improved Darken model, since Darken's current segregation model does not include vacancy diffusion. Darken's model was improved by adding vacancy diffusion and rewriting the activation energy (E) in terms of the energy of migration (Em) and the vacancy formation energy (Ev). From the literature, it is clear that the energy (ES) of a surface depends on the surface orientation and in this study, the surface energy was linked to the vacancy formation energy and an improved Darken model was developed. This improved Darken model was used and the segregation kinetics to different surface orientations were modelled. The results of these calculations showed that the rate of segregation to the (110) surface is higher than segregation to the (111)-surface and that the activation energy for diffusion in the bulk just under the (111)-surface is approximately 15% higher than under the (110)-surface. Experimentally, the segregation of Sb from the bulk to the surface of two Cu single crystals were measured. The surface orientations of the two Cu single crystals were (110) and (111) and the bulk concentrations of Sb were ¼ 0.09 at%. The Sb surface concentration was monitored with Auger Electron Spectroscopy (AES) while the temperature of the crystal was increased linearly. Both the AES data and the temperature of the crystal were controlled and recorded by a computer. The conclusion is that the bulk diffusion coefficient in a crystal is position dependent and it is determined by the surface orientation of the crystal

Addressing the challenges of standalone multi-core simulations in molecular dynamics

AbstractComputational modelling in material science involves mathematical abstractions of force fields between particles with the aim to postulate, develop and understand materials by simulation. The aggregated pairwise interactions of the material’s particles lead to a deduction of its macroscopic behaviours. For practically meaningful macroscopic scales, a large amount of data are generated, leading to vast execution times. Simulation times of hours, days or weeks for moderately sized problems are not uncommon. The reduction of simulation times, improved result accuracy and the associated software and hardware engineering challenges are the main motivations for many of the ongoing researches in the computational sciences. This contribution is concerned mainly with simulations that can be done on a “standalone” computer based on Message Passing Interfaces (MPI), parallel code running on hardware platforms with wide specifications, such as single/multi- processor, multi-core machines with minimal reconfiguration for upward scaling of computational power. The widely available, documented and standardized MPI library provides this functionality through the MPI_Comm_size (), MPI_Comm_rank () and MPI_Reduce () functions. A survey of the literature shows that relatively little is written with respect to the efficient extraction of the inherent computational power in a cluster. In this work, we discuss the main avenues available to tap into this extra power without compromising computational accuracy. We also present methods to overcome the high inertia encountered in single-node-based computational molecular dynamics. We begin by surveying the current state of the art and discuss what it takes to achieve parallelism, efficiency and enhanced computational accuracy through program threads and message passing interfaces. Several code illustrations are given. The pros and cons of writing raw code as opposed to using heuristic, third-party code are also discussed. The growing trend towards graphical processor units and virtual computing clouds for high-performance computing is also discussed. Finally, we present the comparative results of vacancy formation energy calculations using our own parallelized standalone code called Verlet–Stormer velocity (VSV) operating on 30,000 copper atoms. The code is based on the Sutton–Chen implementation of the Finnis–Sinclair pairwise embedded atom potential. A link to the code is also given.</jats:p