298 research outputs found
Radical-initiated alkene hydroauration as a route to gold(III) alkyls: an experimental and computational study
No full textThe hydroauration of functionalised 1-alkenes by the gold(III) hydride (C^NOMe^C)AuH is initiated by organic radicals and proceeds via (C^N^C)Au(II) radical intermediates following a bimolecular outer-sphere mechanism. The outcome of these reactions is determined by the stability of the gold-substituted radicals. The reaction is sensitive to steric as well as electronic factors; disubstituted alkenes and alkenes that form unstable radicals give product mixtures or are unreactive. As DFT calculations show, the reactions agree well with the calculated reaction enthalpies and the standard free energy change for the reaction of the gold(II) radical with the respective alkene
Gold(III) alkyne complexes: Bonding and reaction pathways
No full textThe synthesis and characterization of hitherto hypothetical AuIII π-alkyne complexes is reported. Bonding and stability depend strongly on the trans effect and steric factors. Bonding characteristics shed light on the reasons for the very different stabilities between the classical alkyne complexes of PtII and their drastically more reactive AuIII congeners. Lack of back-bonding facilitates alkyne slippage, which is energetically less costly for gold than for platinum and explains the propensity of gold to facilitate C−C bond formation. Cycloaddition followed by aryl migration and reductive deprotonation is presented as a new reaction sequence in gold chemistry
Synthesis and catalytic activity of dinuclear imido titanium complexes: the molecular structure of [Ti(NPh)Cl( mu-Cl)(THF)(2)](2)
No full textReaction of TiCl4 with the reagents RN(SiMe3)(2) in dichloromethane precipitates red to black powders (R = 1-adamantyl, phenyl. pentafluorophenyl, 3,5-bis(trifluoromethyl)phenyl), which on treatment with tetrahydrofuran gives the new THF adducts [Ti(NR)Cl(mu-Cl)(THF)(2)](2). The complexes are dimeric in the solid state with bridging chloride and terminal imido ligands. Activation with MAO (methylaluminoxane) gives ethene polymerisation catalysts with productivities of up to 31 kg PE [(mol complex) h bar](-1)
Stereo- and regioselective alkyne hydrometallation with gold(III) hydrides
No full textThe hydroauration of internal and terminal alkynes by gold(III) hydride complexes [(C^N^C)AuH] was found to be mediated by radicals and proceeds by an unexpected binuclear outer-sphere mechanism to cleanly form trans-insertion products. Radical precursors such as azobisisobutyronitrile lead to a drastic rate enhancement. DFT calculations support the proposed radical mechanism, with very low activation barriers, and rule out mononuclear mechanistic alternatives. These alkyne hydroaurations are highly regio- and stereospecific for the formation of Z-vinyl isomers, with Z/E ratios of >99:1 in most cases
Heterolytic bond activation at gold: Evidence for gold(III) H-B, H-Si complexes, H-H and H-C cleavage
No full textThe coordinatively unsaturated gold(III) chelate complex [(C^N-CH)Au(C6F5)]+ (1+) reacts with main group hydrides H-BPin and H-SiEt3 in dichloromethane solution at 70 °C to form the corresponding σ-complexes, which were spectroscopically characterized (C^N-CH = 2-(C6H3But)-6-(C6H4But)-pyridine anion; Pin = OCMe2CMe2O). In the presence of an external base such as diethyl ether, heterolytic cleavage of the silane H-Si bond leads to the gold hydrides [{(C^N-CH)AuC6F5}2(μ-H)]+ (2+) and (C^N-CH)AuH(C6F5) (5), together with spectroscopically detected [Et3Si-OEt2]+. The activation of dihydrogen also involves heterolytic H-H bond cleavage but requires a higher temperature ( 20 °C). H2 activation proceeds in two mechanistically distinct steps: the first leading to 2 plus [H(OEt2)2]+, the second to protonation of one of the C^N pyridine ligands and reductive elimination of C6F5H. By comparison, formation of gold hydrides by cleavage of suitably activated C-H bonds is very much more facile; e.g. the reaction of 1·OEt2 with Hantzsch ester is essentially instantaneous and quantitative at 30 °C. This is the first experimental observation of species involved in the initial steps of gold catalyzed hydroboration, hydrosilylation and hydrogenation and the first demonstration of the ability of organic C-H bonds to act as hydride donors towards gold
Hydride transfer to gold: Yes or no? Exploring the unexpected versatility of Au···H-M bonding in heterobimetallic dihydrides
No full textThe potential for coordination and H-transfer from Cp2MH2 (M = Zr, W) to gold(I) and gold(III) complexes was explored in a combined experimental and computational study. [(L)Au]+ cations react with Cp2WH2 giving [(L)Au(κ2-H2WCp2)]+ (L = IPr ( 1 ), cyclic (alkyl)(amino)carbene ( 2) , or phosphines PPh3 (3) and Dalphos-Me (4) [IPr = 1,3-bis(diisopropylphenyl)imidazolylidene; Dalphos-Me = di(1-adamantyl)-2-(dimethylamino)phenyl-phosphine], while [Au(DMAP)2]+ (DMAP = p-dimethylaminopyridine) affords the C2-symmetric [Au(κ-H2WCp2)2]+ (5) . The Dalphos complex 4 can be protonated to give the bicationic adduct 4H , showing Au(I)…H+-N hydrogen bonding. The gold(III) Lewis acid [(C^N-CH)Au(C6F5)(OEt2)]+ binds Cp2WH2 to give an Au-H-W σ-complex. By contrast, the pincer species [(C^N^C)Au]+ adds Cp2WH2 by a purely dative W→Au bond, without Au···H interaction. The biphenyl-based chelate [(C^C)Au]+ forms [(C^C)Au(µH)2WCp2]+, with two 2-electron-3-centre W-H…Au interactions and practically no Au-W donor acceptor contribution. In all these complexes strong but polarised W-H bonds are maintained, without H-transfer to gold. On the other hand, the reactions of Cp2ZrH2 with gold complexes led in all cases to rapid H-transfer and formation of gold hydrides. Relativistic DFT calculations were used to rationalize the striking reactivity and bonding differences in these heterobimetallic hydride complexes along with an analysis of their characteristic NMR parameters and UV-Vis absorption properties
Hybrid catalysts: the synthesis, structure and ethene polymerisation activity of (salicylaldiminato)(pyrrolylaldiminato) titanium complexes
No full textThe mono(salicylaldiminato) complexes Ti{3-Bu-t-2-(O)C6H3CH=N(R)}Cl-3(THF) (where R = C6H5, C6F5) react with the metallated pyrrolylaldiminato ligand, K[2-(C6H5NCH)C4H3N], to afford the first examples of hybrid salicylaldiminato-ligated octahedral titanium complexes; the pre-catalysts give from very high to extremely high ethene polymerisation productivities when activated with MAO
Group 4 salicyloxazolines are potent polymerization catalysts
No full textOctahedral titanium and zirconium complexes based on salicyloxazoline ligands with sterically demanding ortho-substituents provide a new family of extremely active ethene polymerization catalysts [up to 10(8) g PE (mol bar h)–1] which are in some cases "single site"
The synthesis of new weakly coordinating diborate anions: anion stability as a function of linker structure and steric bulk
No full textThe successive addition of KCN and Ph3CCl to B(C6F4-C6F5-2)(3) (PBB) affords triphenylmethyl salts of the [NC-PBB](-) anion. By contrast, the analogous reaction with sodium dicyanamide followed by treatment with Ph3CCl leads to the zwitterionic aminoborane H2NB(C12F9)(2)C12F8, via nucleophilic attack on an o-F atom, together with CPh3[F-PBB]. Whereas treatment of [NC-PBB](-) with either PBB or B(C6F5)(3) fails to give isolable cyano-bridged diborates, the reaction of Me3SiNC-B(C6F5)(3) with PBB in the presence of Ph3CCl affords [Ph3C][PBB-NC-B(C6F5)(3)]. Due to steric hindrance this anion is prone to borane dissociation. The longer linking group N(CN)(2)(-) gives the very voluminous anions [N{CNB(C6F5)(3)}(2)](-) and [N(CN-PBB)(2)](-). A comparison of propylene polymerisations with rac-Me2Si(Ind)(2)ZrMe2 activated with the various boranes or trityl borates gives an anion-dependent activity sequence, in the order [NC-PBB](-) < [MeB(C6F5)(3)](-) < [MePBB](-) approximate to [PBB-NCB(C6F5)(3)](-) approximate to [N{CNB(C6F5)(3)}(2)](-) < [F-PBB](-) << [B(C6F5)(4)](-) < [N(CN-PBB)(2)](-). The anion [N(CN-PBB)(2)](-) gives a catalyst productivity about 2500 times higher than that of [NC-PBB](-) and exceeds that of [B(C6F5)(4)](-) based catalysts. The van der Waals volumes and surface areas of the anions have been calculated and provide a rationale for the observed reactivity trends in polymerisation reactions
Highly electrophilic main group compounds: Ether and arene thallium and zinc complexes
No full textThis review describes aspects of the chemistry of highly electrophilic compounds of thallium and zinc. Extremely weakly coordinating anions based on perfluorophenyl borates allow the synthesis and structural characterization of complexes in which cation–anion interactions are minimized. The metals are therefore able to coordinate weakly donating ligands without the structural distortions and limitations in coordination geometry that are always encountered when coordinating anions are present. This has led to the isolation of thallium mono-, bis- and tris-arene complexes, thallium–ferrocene multidecker structures and the determination of their bond energies. New types of ether and crown ether adducts, as well as the first examples of cationic alkylzinc derivatives have become similarly accessible. These ionic Zn compounds very effectively polymerize cyclic esters and epoxides. Arene coordination is also a feature of strongly Lewis acidic bis(perfluoroaryl)zinc compounds, which when combined with activated alkyl chlorides provide an excellent new type of initiator for the homo and copolymerization of isoalkenes. These systems have given rise to a new family of elastomer materials. Keywords: Thallium; Zinc; Arene complex; Ether complex; Weakly coordinating anion; Crystal structure; DFT calculation
- …
