A Highly Asymmetric Gold(III) η3 -Allyl Complex.

A highly asymmetric AuIII η3 -allyl complex has been generated by treating Au(η1 -allyl)Br(tpy) (tpy=2-(p-tolyl)pyridine) with AgNTf2 . The resulting η3 -allyl complex has been characterized by NMR spectroscopy and X-ray crystallography. DFT calculations and variable temperature 1 H NMR suggest that the allyl ligand is highly fluxional.

Synthesis of a (N,C,C) Au(iii) pincer complex via Csp3-H bond activation: increasing catalyst robustness by rational catalyst design.

A (N,CAr,CAlk) Au(iii) pincer complex has been synthesized from Au(OAc)3 (OAc = OCOCH3) and 2-(3,5-di-tert-butylphenyl)pyridine (L1) involving a Csp3-H bond activation by electrophilic substitution. In agreement with DFT calculations, the resulting complex significantly improves the performance of Au(tpy)(OAcF)2 (tpy = 2-(p-tolyl)pyridine, OAcF = OCOCF3) in the catalytic trifluoroacetylation of acetylene.

Small-molecule activation at Au(iii): metallacycle construction from ethylene, water, and acetonitrile.

Incorporation of the simple, readily available, building blocks ethylene, water and acetonitrile into Au(tpy)(OCOCF3)2 (tpy = 2-(p-tolyl)pyridine) in a one-step reaction leads to high yields of a new 6-membered ring gold(iii) metallacycle complex. The metallacycle has been characterized spectroscopically and crystallographically, and the mechanism of its formation has been investigated with the aid of DFT calculations.

New catalysts for carboxylation of propylene glycol to propylene carbonate via high-throughput screening.

High throughput methodologies screened 81 different metal salts and metal salt combinations as catalysts for the carboxylation of propylene glycol to propylene carbonate, as compared to a 5 mol% Zn(OAc)2/p-chlorobenzene sulfonic acid benchmark catalyst. The reactions were run with added acetonitrile (MeCN) as a chemical water trap. Two new catalysts were thereby discovered, zinc trifluoromethanesulfonate (Zn(OTf)2) and zinc p-toluenesulfonate. The optimal reaction parameters for the former catalyst were screened. Zn(OTf)2 gave an overall propylene carbonate yield of greater than 50% in 24 h, twice as large as the previous best literature yield with MeCN as a water trap, with 69% selectivity and 75% conversion of propylene glycol at 145 °C and 50 bar CO2 pressure.

Destabilization effect of transition metal fluorides on sodium borohydride.

The effect of transition metal fluorides on the decomposition of NaBH4 has been investigated for NaBH4 ball milled with TiF3, MnF3 or FeF3. The compounds were examined by thermal programmed desorption with residual gas analysis, thermo gravimetric analysis and volumetric measurements using a Sieverts-type apparatus. The phase formation process during thermal decomposition was studied by in situ synchrotron radiation powder X-ray diffraction on the as-milled powders. NaBF4 was among the products in all mechano-chemical reactions. (11)B-NMR spectra analysis gave NaBF4 : NaBH4 ratios of 1 : 150 for Na-Ti, 1 : 40 for Na-Mn, and 1 : 10 for Na-Fe. Pure NaBH4 possessed a hydrogen release onset temperature of 430 °C. The hydrogen release in the NaBH4-MnF3 system began as low as 130 °C. FeF3 decreased the onset temperature to 161 °C and TiF3 to 200 °C. TiF3 reacted completely with NaBH4 below 320 °C. All the examined systems have negligible emissions of diborane species. H-sorption studies performed at selected temperatures above 300 °C exhibited relatively fast desorption kinetics. Partial hydrogen re-absorption was observed for the Na-Mn and Na-Fe samples.

A gold exchange: a mechanistic study of a reversible, formal ethylene insertion into a gold(III)-oxygen bond.

The Au(III) complex Au(OAc(F))2(tpy) (1, OAc(F) = OCOCF3; tpy = 2-p-tolylpyridine) undergoes reversible dissociation of the OAc(F) ligand trans to C, as seen by (19)F NMR. In dichloromethane or trifluoroacetic acid (TFA), the reaction between 1 and ethylene produces Au(OAc(F))(CH2CH2OAc(F))(tpy) (2). The reaction is a formal insertion of the olefin into the Au-O bond trans to N. In TFA this reaction occurs in less than 5 min at ambient temperature, while 1 day is required in dichloromethane. In trifluoroethanol (TFE), Au(OAc(F))(CH2CH2OCH2CF3)(tpy) (3) is formed as the major product. Both 2 and 3 have been characterized by X-ray crystallography. In TFA/TFE mixtures, 2 and 3 are in equilibrium with a slight thermodynamic preference for 2 over 3. Exposure of 2 to ethylene-d4 in TFA caused exchange of ethylene-d4 for ethylene at room temperature. The reaction of 1 with cis-1,2-dideuterioethylene furnished Au(OAc(F))(threo-CHDCHDOAc(F))(tpy), consistent with an overall anti addition to ethylene. DFT(PBE0-D3) calculations indicate that the first step of the formal insertion is an associative substitution of the OAc(F) trans to N by ethylene. Addition of free (-)OAc(F) to coordinated ethylene furnishes 2. While substitution of OAc(F) by ethylene trans to C has a lower barrier, the kinetic and thermodynamic preference of 2 over the isomer with CH2CH2OAc(F) trans to C accounts for the selective formation of 2. The DFT calculations suggest that the higher reaction rates observed in TFA and TFE compared with CH2Cl2 arise from stabilization of the (-)OAc(F) anion lost during the first reaction step.

Structural and spectroscopic characterization of potassium fluoroborohydrides.

Mechanochemical reactions between KBH4 and KBF4 result in the formation of potassium fluoroborohydrides K(BH(x)F(4-x)) (x = 0-4), as determined by (11)B and (19)F solid state NMR. The materials maintain the cubic KBH4 structure. Thermogravimetric (TG) data for a ball-milled sample with KBH4 : KBF4 = 3 : 1 are consistent with only desorption of hydrogen.

Influence of nanoconfinement on morphology and dehydrogenation of the Li11BD4-Mg(11BD4)2 system.

The decomposition of a nanoconfined mixture of lithium-magnesium borohydride, Li(11)BD(4)-Mg((11)BD(4))(2), has been investigated and compared to the corresponding mixture in the bulk form. The systems were investigated by thermal analysis, small-angle neutron scattering, (11)B nuclear magnetic resonance and transmission electron microscopy. The dehydrogenation temperatures decreased by up to 60 °C in the nanoconfined system, with gas evolution following different steps, compared to the behaviour of the bulk material under the same conditions. Most importantly, desorption from the nanoconfined hydride proceeds without formation of diborane, B(2)D(6), which evolves from the bulk mixture. From small-angle neutron scattering, differences in morphology between the bulk and the nanoconfined systems are also demonstrated. Evidence of a complete decomposition has been found in the nanoconfined system, after heating up to 460 °C. Furthermore, (11)B NMR data show that nanoconfinement inhibits the formation of dodecaborane, [B(12)D(12)](2-), during decomposition, a result which is important for practical applications of borohydrides.

Ping-pong at gold: proton jump between coordinated phenyl and eta1-benzene ligands, a computational study.

A DFT computational investigation predicts that the Au(III) complex (bpy)Au(C(6)H(5))(2+) reacts with benzene to furnish square planar (bpy)Au(C(6)H(5))(eta(1)-C(6)H(6))(2+). Intramolecular processes that occur within this species have been located, and the energetics of all processes have been quantified. The dynamic processes that have been identified are (1) benzene ring rotation with respect to Au, (2) direct hydrogen transfer from the benzene to the phenyl ligand, (3) hydrogen transfer from the ipso to the ortho positions in the coordinated benzene ligand, and (4) hydrogen transfer from the benzenium ligand formed by the ipso/ortho isomerization to the phenyl ligand. Similarities and differences are seen between the behavior of (bpy)Au(C(6)H(5))(eta(1)-C(6)H(6))(2+) and previously reported isoelectronic Pt(II) complexes. Preliminary experimental results related to this chemistry are reported, and possible consequences for C-H bond activation mediated by gold are discussed.

A general one-pot synthesis for elusive 2-substituted indenes: does bis[2-(tert-butyl)indenyl]zirconium(IV) dichloride/MAO polymerise ethene?

Among the 2-substituted indenes, 2-trifluoroindene and 2-tert-butylindene are poorly or incompletely described in the open literature. We herein describe an efficient one-pot synthesis of these compounds as a variation of the Perkin reaction which allows us to refute an earlier claim that bis(2-tert-butylindenyl)zirconium(IV) dichloride (2a) will not polymerise ethene. In fact, 2a/MAO polymerises ethene to extremely high molecular weights. Extensive DFT calculations on the polymerisation mechanism revealed an unprecedented suppression of the otherwise predominant chain termination by beta-hydride transfer due to steric congestion of the active site.

Synthesis, structure, and ethene polymerisation catalysis of 1- or 2-silyl substituted bis[indenyl]zirconium(IV) dichlorides.

The systematic syntheses of 1- and 2-substituted silylindenes, with a wide variety of substitution patterns on the silyl moiety, and their corresponding zirconocene dichlorides are presented. The rac- and meso-diastereomers of the 1-substituted zirconocene dichlorides can in most cases be separated. Instable zirconocenes were observed for certain substitution patterns. Two of the obtained zirconocene dichlorides, bis[2-(dimethylsilyl)indenyl]zirconium dichloride (4a) and bis[2-(trimethylsilyl)indenyl]zirconium dichloride (4b), were characterised by single crystal X-ray diffraction. On the basis of DFT results, the two compounds are geometrically similar, i.e. the additional methyl group on the silyl moiety only affects the conformational energy profile. Differences in their catalyst performance in the homopolymerisation studies with ethane are thus attributed to conformational control. For the remaining complexes, sterically less demanding silyl groups seem to be favoured with respect to the catalyst performance. All the 2-isomers have lower polymerisation activities than the unsubstituted bis[indenyl]zirconium dichloride/MAO system. Curiously, the rac-bis[1-(dimethylphenylsilyl)indenyl]zirconium dichloride/MAO system is found to be the most active catalyst in ethene homopolymerisations.