117,354 research outputs found
Experimental insights into the structure and reactivity of frustrated Lewis pairs
No full textFrustrated Lewis pairs (FLPs), namely the combination of a Lewis acid (LA) and a Lewis base (LB) that cannot fulfil their natural propensity to form stable Lewis adducts, have been recently exploited to activate small molecules, such as H2 and CO2, and have drawn much interest, due to their applicability in metal‐free catalytic reactions. This review focuses mainly on the experimental studies of both the structure and reactivity of FLPs providing a starting body of evidence to elucidate the mechanism of their action. Important insights coming from NMR spectroscopy, X‐ray diffraction and other experimental techniques are highlighted and discussed. In particular, evidence about the tendency of LA and LB to interact in the absence of a substrate are commented on for both inter‐ and intramolecular FLPs, underlining the role played by the structure of LA and LB in determining the level of interaction. Next, the effect of the structure of FLPs on the kinetics and thermodynamics of substrate activation is illustrated, with a particular focus on catalytic hydrogenation reactions
Recent Advances in Gold(III) Chemistry: Structure, Bonding, Reactivity, and Role in Homogeneous Catalysis
No full textOver the past decade the organometallic chemistry of gold(III) has seen remarkable advances. This includes the synthesis of the first examples of several compound classes that have long been hypothesized as being part of catalytic cycles, such as gold(III) alkene, alkyne, CO and hydride complexes, and important catalysis-relevant reaction steps have at last been demonstrated for gold, like migratory insertion and β-H elimination reactions. Also, reaction pathways that were already known, for example the generation of gold(III) intermediates by oxidative addition and their reductive elimination, are much better understood. A deeper understanding of fundamental organometallic reactivity of gold(III) has revealed unexpected mechanistic avenues, which can open when the barriers for reactions that for other metals would be regarded as "standard" are too high. This review summarizes and evaluates these developments, together with applications of gold(III) in synthesis and catalysis, with emphasis on the mechanistic insight gained in these investigations
Assessing the orbital contribution in the “spodium bond” by natural orbital for chemical valence–charge displacement analysis
No full textThe term "spodium bond"(SpB) has been recently proposed to describe the noncoordinative interaction that can be established between a polarized group 12 metal and a mild Lewis base (LB). Most of the systems showing short metal-donor distances compatible with SpB are characterized by the coexistence of multiple weak interactions, including hydrogen and halogen bonding, making the assessment of real importance of SpB difficult. Here, we show that the relative importance of each contribution can be probed by dissecting the orbital component of the interaction through the extended transition state-natural orbital for chemical valence-charge displacement analysis (ETS-NOCV-CD). The latter gives useful information about relative energies and electrons involved, for model systems ([(thiourea)2MX2]···LB; M = Zn, Cd, and Hg; X = Cl and I; and LB = CH2S, CH2O, CH3CN, and CO) and a variety of structures extracted from experimentally characterized adducts, allowing us to demonstrate the lack of a direct correlation between a favorable metal-base distance and the presence of an orbital contribution for the SpB
Thermally stable gold(III) alkene and alkyne complexes: Synthesis, structures, and assessment of the trans‐influence on gold‐ligand bond enthalpies
No full textThe reaction of [C^C)Au(OEt2)2]+ with 1,5‐cyclooctadiene or norbornadiene affords the corresponding olefin complexes [(C^C)Au(COD)]SbF6 and [(C^C)Au(NBD)]SbF6, which are thermally stable in solution and the solid state (C^C = 4,4′‐di‐t‐butylbiphenyl‐2,2′‐diyl). The crystal structures of these complexes have been determined. By contrast, dienones such as dibenzylideneacetone are O‐ rather than C=C‐bonded. The reactions of (C^C)Au(OAcF)(L) (L = PMe3 or CNxyl) with B(C6F5)3 in the presence of bis(1‐adamantyl)acetylene give the mixed‐ligand alkyne complexes [(C^C)Au(AdC≡CAd)(L)]+, the first complexes of their type in gold chemistry. In the presence of an excess of acetylene these compounds are thermally stable in solution and as solids. The bonding of n‐ and π‐donor ligands to Au(III) fragments and the effect of the trans influence exerted by N‐ and C‐donors was explored with the aid of DFT calculations. Results show that the Au‐L bond enthalpies trans to anionic C are 35 ‐ 60% of the enthalpies trans to N, with strong π‐acceptors being particularly affected. In comparison with [Me2Au]+, the [(C^C)Au]+ fragment is more polar and in bond enthalpy terms resembles Me2Pt
Carbon-sulfur bond formation by reductive elimination of gold(iii) thiolates
No full textWhereas the reaction of the gold(iii) pincer complex (C^N^C)AuCl with 1-adamantyl thiol (AdSH) in the presence of base affords (C^N^C)AuSAd, the same reaction in the absence of base leads to formation of aryl thioethers as the products of reductive elimination of the Au-C and Au-S ligands (C^N^C = dianion of 2-6-diphenylpyridine or 2-6-diphenylpyrazine). Although high chemical stability is usually taken as a characteristic of pincer complexes, results show that thiols are capable of cleaving one of the pincer Au-C bonds. This reaction is not simply a function of S-H acidity, since no cleavage takes place with other more acidic X-H compounds, such as carbazole, amides, phenols and malonates. The reductive C-S elimination follows a second-order rate law, -d[1a]/dt = k[1a][AdSH]. Reductive elimination is enabled by displacement of the N-donor by thiol; this provides the conformational flexibility necessary for C-S bond formation to occur. Alternatively, reductive C-S bond formation can be induced by reaction of pre-formed thiolates (C^N^C)AuSR with a strong Brønsted acid, followed by addition of SMe2 as base. On the other hand, treatment of (C^N^C)AuR (R = Me, aryl, alkynyl) with thiols under similar conditions leads to selective C-C rather than C-S bond formation. The reaction of (C^N^C)AuSAd with H+ in the absence of a donor ligand affords the thiolato-bridged complex [{(C^N-CH)Au(μ-SAd)}2]2+ which was crystallographically characterised
Probing the interactions between all components of the catalytic pool for homogeneous olefin polymerisation by diffusion NMR spectroscopy
No full textDiffusion NMR spectroscopy was applied to investigate all individual components and combinations thereof for the Cp2ZrMe2/MAO (DMAO)/TBP (MAO = methylaluminoxane, DMAO = AlMe3 depleted MAO, TBP = 2,6-di-tert-butylphenol) ternary system, selected as a prototypical catalytic pool for homogeneous olefin polymerisation. Both MAO and DMAO were found to self-aggregate in C6D6 with the latter having a higher propensity. TBP reacts with DMAO affording MeAl(2,6-di-tert-butylphenoxide)(2) and causing a structural modification of DMAO, whose aggregates become much larger. The actual dimensions and self-aggregation tendency of (D) MAO, which depend on Al concentration and the possible presence of TBP, turned out to carry over to [Cp2Zr(mu-Me)(2)AlMe2]MeMAO (1) OSIP (outer sphere ion pair) and [Cp2Zr+Me center dot center dot center dot MeMAO(-)] (2) ISIP (inner sphere ion pair) that form upon activation of Cp2ZrMe2. Once the intrinsic self-aggregation tendency of MAO has been subtracted, OSIP 1 and ISIP 2 behave exactly as analogous ion pairs with borate ions: ISIP 2 does not self-aggregate, whereas OSIP 1 exhibits the same self-aggregation trends of zirconocene OSIPs with borate counterions
Self-aggregation tendency of zirconocenium ion pairs which model polymer-chain-carrying species in aromatic and aliphatic solvents with low polarity
No full textFrom pairs to double pairs: Zirconocene ion pairs bearing an aliphatic chain of variable length were synthesized and investigated by means of NOE and diffusion NMR spectroscopy experiments. The presence of long aliphatic chains allowed an unprecedented investigation of their self‐aggregation tendency in cyclohexane (see figure), which has a dielectric constant similar to that of isoparaffins used in industrial plants
Thermally stable gold(III) alkene and alkyne complexes: Synthesis, structures, and assessment of the trans-influence on Gold–Ligand bond enthalpies
No full textThe reaction of [C^C)Au(OEt2)2]+ with 1,5-cyclooc-tadiene or norbornadiene affords the corresponding olefin complexes [(C^C)Au(COD)]SbF6 and [(C^C)Au(NBD)]SbF6, which are thermally stable in solution and the solid state (C^C = 4,4’-di-tert-butylbiphenyl-2,2’-diyl). The crystal structures of these complexes have been determined. By contrast, dienones such as dibenzylideneacetone are O-rather than C=C-bonded. The reactions of (C^C)Au(OAcF)(L) (L = PMe3 or CNxyl) with B(C6F5)3 in the presence of bis(1-ada-mantyl)acetylene give the mixed-ligand alkyne complexes [(C^C)Au(AdC≡CAd)(L)]+, the first complexes of their type in gold chemistry. In the presence of an excess of acetylene these compounds are thermally stable in solution and as solids. The bonding of n-and π-donor ligands to AuIII fragments and the effect of the trans influence exerted by Nand C-donors was explored with the aid of DFT calculations. Results show that the Au L bond enthalpies trans to anionic C are 35–60 % of the enthalpies trans to N, with strong π-ac-ceptors being particularly affected. In comparison with [Me2 Au]+, the [(C^C)Au]+ fragment is more polar and in bond enthalpy terms resembles Me2Pt
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
A Phosphine Gold(I) pi-Alkyne Complex: Tuning the Metal-Alkyne Bond Character and Counterion Position by the Choice of the Ancillary Ligand
No full textThe intra- and interionic structures of a mononuclear phosphine gold(I) alkyne complex [(PAr(3)(F))Au(2-hexyne)]BF(4) [1BF4; Ar(F) = 3,5-bis(trifluoromethyl)phenyl] and its analogous complex [(NHC)Au- (2-hexyne)]BF(4) [2BF(4); NHC = 1,3-bis(dilsopropylphenyl)imidazol-2-ylidene] have been investigated by combining 1D and 20 multinuclear NMR spectroscopy and density functional theory calculations. It has been found that alkyne in 1BF(4) is depleted of its electron density to a greater extent than that in 2BF(4). This correlates with the Delta delta((13)C) NMR of the carbon carbon triple bond. Instead, 2BF(4) is much more "kinetically stable" than 1BF(4). (19)F-(1)H HOESY NMR experiments indicate that the counterion locates close to the gold atom in 1BF(4) (differently from that previously observed in the few other gold(I) ion pairs studied), exactly where the computed Coulomb potential indicates that partial positive charge accumulates
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