CoCH3(PMe3)4 is known for a long time for its potential to activate less reactive C–H Bonds under extremely mild conditions by methane elimination. A spontaneous second C–H bond activation at the ...same center of cobalt complex is not expected. In contrast to previous investigations with certain aryl‐iminophosphane chelating ligands, in this work two consecutive metalation steps at the same complex center lead to a rare example of a cobalt metallabicycle. In the first step a C–H activation results in a five‐membered chelate ring, whereas the second ring is formed by an incomplete C–H activation which stops at a stable, rarely observed δ‐C–H agostic interaction. The results are supported by NMR spectroscopy, X ray structure analysis and density functional theory (DFT) calculations. Knowledge of such agostic interactions are essential in order to understand catalytic reaction paths and studies of such molecular structures of their ring sizes, bite angles and electronic properties have shown that these ligands allow sensitive control within the metal coordination sphere.
“Snapshot”: A double metalation through the coordination of an ortho‐iminophosphane anchoring group can initiate a new bicyclometalation reaction affording the mononuclear cobalt complex (16 valence electrons) (1, 2). The first ring closing step involves a C–H activation, while the second one stops at the level of a rarely observed δ‐C–H agostic interaction supported by quantum chemical calculations.
The reactivity of 2‐alkenylpyridine derivatives with trimethylphosphane‐supported iron– and cobalt–methyl adducts were investigated and provided a series of C,N‐cyclometalated complexes through ...smooth vinyl C(sp2)–H activation. The reactions of Co(CH3)(PMe3)4 with 2‐vinylpyridine, 2‐(1‐phenylvinyl)pyridine, and 2‐vinylpyrazine provided dark green crystals of the five‐membered metallacycles (κ2‐C,N‐R1R2C=CH)Co(PMe3)3 (1: R1 = C5H4N, R2 = H; 2: R1 = C5H4N, R2 = Ph; 3: R1 = C4H3N2,R2 = H). The oxidative addition of 1–3 with iodomethane afforded the mer‐trans trivalent cobalt complexes (κ2‐C,N‐R1R2C=CH)Co(CH3)I(PMe3)2 (4: R1 = C5H4N, R2 = H; 5: R1 = C5H4N, R2 = Ph; 6: R1 = C4H3N2, R2 = H) in modest yields. The reaction of 2 with an additional equivalent of 2‐(1‐phenylvinyl)pyridine incorporated a second C,N metallacycle with an η2‐bonded alkenyl moiety to provide (κ2‐C,N‐C5H4N–CH=CH)(κ3,η2‐C,C,N‐C5H4N–CH=CH2)Co(PMe3) (7). No cyclometalation was observed with 8‐vinylquinoline and Co(CH3)(PMe3)4, which afforded (κ3,η2‐C,C,N‐C9H6N–CH=CH2)Co(CH3)(PMe3)2 (8) with retention of the Co–CH3 group; therefore, a suitable bite angle is required for C–H activation. No N coordination was observed with Fe(CH3)2(PMe3)4 and 8‐vinylquinoline (reductive elimination of C2H6), which afforded a low‐valent Fe(PMe3)3 moiety bound in an η4‐fashion with the exocyclic vinyl group and the ortho‐carbon atoms to give (κ4,η4‐C,C,C,C‐C9H6N–CH=CH2)Fe(PMe3)3 (9). In polar solvents, an equilibrium of cyclometalated 7,8‐benzoquinoline exists between the mer‐trans (90 %) and mer‐cis (10 %) configurations of (κ2‐C,N‐C13H6N)Fe(CH3)(PMe3)3 (10). At variance with the previously reported bicyclometalation of 2‐(2‐naphthalene‐1‐ylvinyl)pyridine with iron adducts, similar reactions with methylcobalt species provided the monocyclometalated η2,σ1‐bound coordination motif (κ3,η2‐C,C,N‐C5H4N–CH=CH–C10H6)Co(PMe3)2 (11). The reaction of 11 with carbon monoxide provided the monocarbonyl complex (κ3,η2‐C,C,C‐C5H4N–CH=CH–C10H6)Co(CO)(PMe3)2 (12), accompanied by the release of the N‐coordination site. The carbonylation of (κ3‐C,C,N‐C5H4N–CH=C–C10H6)Fe(PMe3)3 resulted in the exchange of one PMe3 ligand with the retention of the coordination sites to generate (κ3‐C,C,N‐C5H4N–CH=C–C10H6)Fe(CO)(PMe3)2 (13). All new compounds were identified by NMR spectroscopy and structurally characterized by single‐crystal X‐ray crystallography (with the exception of 10 and 13).
The regioselective cyclometalation reactions of 2‐alkenylpyridine/‐pyrazine derivatives with Co(CH3)(PMe3)4 provide cobalt complexes by vinylic C(sp2)–H activation. These cobalt complexes smoothly add iodomethane but readily demetalate with carbon monoxide. Conversely, reductive elimination through the release of ethane is the dominant reaction with the same ligands and Fe(CH3)2(PMe3)4.
Steric control dictates the cyclometalation of aromatic substrates containing imine anchoring groups upon reaction with basic iron or cobalt complexes. As structural isomers, the azadiene systems 1 ...and 2 undergo regiospecific activation of the (vinylic or aromatic) CH bond of the CH group that has switched places with the N atom.
The novel tetranuclear mixed valent iron(II)/iron(0) complex (1) was prepared by ortho-metalation of benzophenone with zerovalent Fe(PMe3)4 (d8) followed by a subsequent insertion of an additional ...equivalent of benzophenone under ring expansion (seven-membered metallacycle). Dimerization and spontaneous dihydrogen loss, constitutes a four membered Fe2O2 core structure in which both incorporated chelating phenylrings are additionally attached by low-valent Fe(PMe3)3-fragments in η4-mode. In a single electron transfer reaction, by C–C-coupling of two molecules benzophenone with zerovalent Co(PMe3)4 (d9), cyclometalated mononuclear five-membered cobalt(II) alcoholate complex (2) is obtained. The crystal structures of 1 and 2 were determined and supported by gas phase DFT calculations and the formation mechanisms of both complexes are discussed.
Reaction cascade: Ortho-metalation of benzophenone with zerovalent iron and cobalt is observed followed by a subsequent addition/insertion of an additional equivalent benzophenone. Display omitted
•Reaction cascade of benzophenone includes C–H activation and C–C coupling steps.•Subsequent insertion/addition sequence with low valent Fe(PMe3)4 and Co(PMe3)4.•Dimerization constitutes a four membered Fe2O2 core structure.•Single electron transfer reaction afforded cyclometalated mononuclear cobalt complex.•Formation mechanisms of both complexes explain the key steps of the proposed sequence.
In this work, magnesium-based composites were obtained by shock-wave compaction of a powder mixture of Mg-5 wt.% AlN at a shock-wave pressure of 2 GPa. Their microstructure was investigated and the ...phase composition was determined, from which it follows that the nanoparticles retain their phase composition and are uniformly distributed in the magnesium matrix. The materials obtained by shock-wave compaction were used as master alloys for the production of magnesium alloys by die casting. The amount of aluminum nitride nanoparticles in the AZ91 magnesium alloy was 0.5 wt.%. Studies of the microstructure of the magnesium alloys showed a decrease in the average grain size of the magnesium matrix from 610 to 420 μm. Studies of mechanical properties have shown that the introduction of aluminum nitride nanoparticles increases the yield strength from 55 to 119 MPa, the tensile strength from 122 to 171 MPa and the plasticity from 4 to 6.5%, respectively. The effect of nanoparticles on the fracture behavior of the magnesium alloy under tension was determined.
Photoproduction of Mesons Schmieden, Hartmut; Klein, Friedrich
EPJ Web of Conferences,
01/2017, Letnik:
134
Journal Article, Conference Proceeding
Recenzirano
Odprti dostop
B.1 is one of the experimental projects within the CRC16. It aims at the systematic investigation of the photoproduction of mesons off nucleons in order to understand reaction mechanisms and the ...relevant degrees of freedom in resonance formation. Of particular interest is the photoproduction of mesons heavier than the pion and resonances involving hidden or open strangeness. Essential hardware contributions have been made to the experimental programme of the CRC16 through tagging systems, and photon-beam polarisation and polarimetry. A new experiment has been set up within the framework of the BGO-OD collaboration. This combines a forward magnetic spectrometer with a central BGO calorimeter with charged particle recognition and identification. The BGO-OD experiment enables reconstruction of complex final states composed of both charged and neutral particles, complementary to the existing CBELSA/TAPS calorimeter which is optimised for multi-photon final states. Selected results of the 12-year CRC period are presented from both experiments.
Even though many different designs for currently available, fluorescence-based fiber optic sensors for measuring oxygen concentration (O2) are well known (and indeed some are commercially available), ...they often are limited by their response time and long-term stability. This will cause problems in the important industrial applications of fiber optic sensors of this type that are developing, with limitations that are evident, for example, in physiology and other fields where rapid sensor responses are required. Research by a number of groups has discussed various new designs of fiber optical sensors, which have been developed in recent years where the key features of such probes to achieve the performance required are, for example, optimization of design features such as tip shape and coating layer thickness. The research reported in this paper represents an evaluation of such key factors to allow the design of better fiber optic-based sensors for oxygen measurement, where the optimized performance of a new, specially tapered tip O2 sensor designed has been compared with the output of conventional and commercially available probe designs. The performance of a group of such sensors has been analyzed and cross-compared, examining the key features of such a probe including sensor accuracy, response time and overall long-term stability, as well as cross-sensitivity to any temperature changes which may occur in ‘real’ measurement situations.
Display omitted
•New fiber-optic probe design with a very fast response time to O2 changes (Δt90max ≈ 0.3 s)•Sensor exhibits exceptional long-term stability (low relative drift of about 0.04%/day)•Cross-comparison of sensor performance with conventional and commercially available probe designs and systems•Cross-Comparison of key characteristics such as sensor response time, long-term stability, and temperature dependency
Four thought: The first nickel(IV) complex that contains only alkyl ligands (see picture) was serendipitously obtained in a reaction between a nickel(0) species and a strained diolefin. The twisted ...conformation of the ligand provides the necessary steric shielding to stabilize the complex. DFT calculations show this conformation to be the ground state and NMR spectra indicate the diamagnetic nature and high oxidation state of the nickel center.
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