Synthesis,Characterization, and Reactivity of Cationic Gold Diarylallenylidene Complexes

Methoxide abstraction from gold acetylide complexes of the form (L)Au[η¹‐C≡CC(OMe)ArAr′] (L=IPr, P(^tBu)₂(ortho‐biphenyl); Ar/Ar′=C₆H₄X where X=H, Cl, Me, OMe) with trimethylsilyl trifluoromethanesulfonate (TMSOTf) at ?78 °C resulted in the formation of the corresponding cationic gold diarylallenylidene complexes [(L)Au=C=C=CArAr′]⁺ OTf^? in ≥85±5 % yield according to ¹H NMR analysis. ¹³C NMR and IR spectroscopic analysis of these complexes established the arene‐dependent delocalization of positive charge on both the C1 and C3 allenylidene carbon atoms. The diphenylallenylidene complex [(IPr)Au=C=C=CPh₂]⁺ OTf^? reacted with heteroatom nucleophiles at the allenylidene C1 and/or C3 carbon atom.     

Gold(I) Vinylidene Complexes as Reactive Intermediates and Their Tendency to π‐Backbond

We herein report a computational study of the bonding in gold(I) vinylidene complexes and compare them to their carbene and CO analogues. The relevance of these intermediates is analysed for the intramolecular cyclisation leading to vinyl sulfonates.     

Dual Gold Catalysis: σ,π‐Propyne Acetylide and Hydroxyl‐Bridged Digold Complexes as Easy‐To‐Prepare and Easy‐To‐Handle Precatalysts

A series of dinuclear gold σ,π‐propyne acetylide complexes were prepared and tested for their catalytic ability in dual gold catalysis that was based on the reaction of an electrophilic π‐complex of gold with a gold acetylide. The air‐stable and storable catalysts can be isolated as silver‐free catalysts in their activated form. These dual catalysts allow a fast initiation phase for the dual catalytic cycles without the need for additional additives for acetylide formation. Because propyne serves as a throw‐away ligand, no traces of the precatalyst are generated. Based on the fast initiation process, side products are minimized and reaction rates are higher for these catalysts. A series of test reactions were used to demonstrate the general applicability of these catalysts. Lower catalyst loadings, faster reaction rates, and better selectivity, combined with the practicability of these catalysts, make them ideal catalysts for dual gold catalysis.     

Enhanced π‐Backdonation from Gold(I): Isolation of Original Carbonyl and Carbene Complexes

The specific electronic properties of bent o‐carborane diphosphine gold(I) fragments were exploited to obtain the first classical carbonyl complex of gold [(DPCb)AuCO]⁺ (ν(CO)=2143 cm^?1) and the diphenylcarbene complex [(DPCb)Au(CPh₂)]⁺, which is stabilized by the gold fragment rather than the carbene substituents. These two complexes were characterized by spectroscopic and crystallographic means. The [(DPCb)Au]⁺ fragment plays a major role in their stability, as substantiated by DFT calculations. The bending induced by the diphosphine ligand substantially enhances π‐backdonation and thereby allows the isolation of carbonyl and carbene complexes featuring significant π‐bond character.     

1,2,4‐Triazol‐3‐ylidenes with an N‐2,4‐Dinitrophenyl Substituent as Strongly π‐Accepting N‐Heterocyclic Carbenes

The synthesis and characterisation of a series of new Rh and Au complexes bearing 1,2,4‐triazol‐3‐ylidenes with a N‐2,4‐dinitrophenyl (N‐DNP) substituent are described. IR, NMR, single‐crystal X‐ray diffraction and computational analyses of the Rh complexes revealed that the N‐heterocyclic carbenes (NHCs) behaved as strong π acceptors and weak σ donors. In particular, a natural bond orbital (NBO) analysis revealed that the contributions of the Rh→C_carbene π backbonding interaction energies (ΔE_bb) to the bond dissociation energies (BDE) of the Rh? C_carbene bond for [RhCl(NHC)(cod)] (cod=1,5‐cyclooctadiene) reached up to 63 %. The Au complex exhibited superior catalytic activity in the intermolecular hydroalkoxylation of cyclohexene with 2‐methoxyethanol. The NBO analysis suggested that the high catalytic activity of the Au^I complex resulted from the enhanced π acidity of the Au atom.     

Synthesis,Structure, and Reactivity of a Gold Carbenoid Complex That Lacks Heteroatom Stabilization

Hydride abstraction from the neutral gold cycloheptatrienyl complex [( P )Au(η¹‐C₇H₇)] ( P =P(tBu)₂(o‐biphenyl)) with triphenylcarbenium tetrafluoroborate at ?80 °C led to the isolation of the cationic gold cycloheptatrienylidene complex [( P )Au(η¹‐C₇H₆)]⁺ BF₄^? in 52 % yield, which was characterized in solution and by single‐crystal X‐ray diffraction. This cycloheptatrienylidene complex represents the first example of a gold carbenoid complex that lacks conjugated heteroatom stabilization of the electron‐deficient C1 carbon atom. The cycloheptatrienylidene ligand of this complex is reactive; it can be reduced by mild hydride donors, and converted to tropone in the presence of pyridine N‐oxide.     

Cationic,Two‐Coordinate Gold π Complexes

Cationic, two‐coordinate gold π complexes that contain a phosphine or N‐heterocyclic supporting ligand have attracted considerable attention recently owing to the potential relevance of these species as intermediates in the gold‐catalyzed functionalization of C? C multiple bonds. Although neutral two‐coordinate gold π complexes have been known for over 40 years, examples of the cationic two‐coordinate gold(I) π complexes germane to catalysis remained undocumented prior to 2006. This situation has changed dramatically in recent years and well‐defined examples of two‐coordinate, cationic gold π complexes containing alkene, alkyne, diene, allene, and enol ether ligands have been documented. This Minireview highlights this recent work with a focus on the structure, bonding, and ligand exchange behavior of these complexes.     

Cyclization of Gold Acetylides: Synthesis of Vinyl Sulfonates via Gold Vinylidene Complexes

Differently substituted terminal alkynes that bear sulfonate leaving groups at an appropriate distance were converted in the presence of a propynyl gold(I) precatalyst. After initial formation of a gold acetylide, a cyclization takes place at the β‐carbon atom of this species. Mechanistic studies support a mechanism that is related to that of dual gold‐catalyzed reactions, but for the new substrates, only one gold atom is needed for substrate activation. After formation of a gold vinylidene complex, which forms a tight contact ion pair with the sulfonate leaving group, recombination of the two parts delivers vinyl sulfonates, which are valuable targets that can serve as precursors for cross‐coupling reactions, for example.     

Boryl‐Functionalized σ‐Alkynyl and Vinylidene Rhodium Complexes: Synthesis and Electronic Properties

The synthesis, reactivity, and properties of boryl‐functionalized σ‐alkynyl and vinylidene rhodium complexes such as trans‐[RhCl(?C?CHBMes₂)(PiPr₃)₂] and trans‐[Rh(C?CBMes₂)(IMe)(PiPr₃)₂] are reported. An equilibrium was found to exist between rhodium vinylidene complexes and the corresponding hydrido σ‐alkynyl complexes in solution. The complex trans‐[Rh(C?CBMes₂)(IMe)(PiPr₃)₂] (IMe=1,3‐dimethylimidazol‐2‐ylidene) was found to exhibit solvatochromism and can be quasireversibly oxidized and reduced electrochemically. Density functional calculations were performed to determine the reaction mechanism and to help rationalize the photophysical properties of trans‐[Rh(C?CBMes₂)(IMe)(PiPr₃)₂].     

Stabilization and Large Nonlinearity of Gold Nanoparticles Functionalized with a π‐Conjugated Polymer

Gold nanoparticles protected by a novel π‐conjugated polymer [poly(p‐phenylene ethynylene) containing pendent disulfide and bipyridine groups] are synthesized and characterized. The polymer can stabilize the gold nanoparticles effectively. The nonlinear optical properties of the gold nanoparticle colloid solutions in toluene are investigated by using the Z‐scan technique at a wavelength of 532 nm and pulse width of 4 ns. The gold‐nanoparticle colloid solutions show an exceptional nonlinear absorption effect, which simultaneously contains the saturated absorption resulting from third‐order nonlinearity and a large reverse‐saturated absorption resulting from fifth‐order nonlinearity. In addition, asymmetric self‐focusing refractive effects are investigated in the colloid solutions.     

Gold(III) π‐Allyl Complexes

Gold(III) π‐complexes have been authenticated recently with alkenes, alkynes, and arenes. The key importance of Pd^II π‐allyl complexes in organometallic chemistry (Tsuji–Trost reaction) prompted us to explore gold(III) π‐allyl complexes, which have remained elusive so far. The (P,C)Au^III(allyl) and (methallyl) complexes 3 and 3′ were readily prepared and isolated as thermally and air‐stable solids. Spectroscopic and crystallographic analyses combined with detailed DFT calculations support tight quasi‐symmetric η³‐coordination of the allyl moiety. The π‐allyl gold(III) complexes are activated towards nucleophilic additions, as substantiated with β‐diketo enolates.     

Synthesis,Characterization, and Reactivity of a Uranium(VI) Carbene Imido Oxo Complex

We report the uranium(VI) carbene imido oxo complex [U(BIPM^TMS)(NMes)(O)(DMAP)₂] ( 5 , BIPM^TMS=C(PPh₂NSiMe₃)₂; Mes=2,4,6‐Me₃C₆H₂; DMAP=4‐(dimethylamino)pyridine) which exhibits the unprecedented arrangement of three formal multiply bonded ligands to one metal center where the coordinated heteroatoms derive from different element groups. This complex was prepared by incorporation of carbene, imido, and then oxo groups at the uranium center by salt elimination, protonolysis, and two‐electron oxidation, respectively. The oxo and imido groups adopt axial positions in a T‐shaped motif with respect to the carbene, which is consistent with an inverse trans‐influence. Complex 5 reacts with tert‐butylisocyanate at the imido rather than carbene group to afford the uranyl(VI) carbene complex [U(BIPM^TMS)(O)₂(DMAP)₂] ( 6 ).     

Isolation of a Reactive Tricoordinate α‐Oxo Gold Carbene Complex

The [(P,P)Au=C(Ph)CO₂Et]⁺ complex 3 [where (P,P) is an o‐carboranyl diphosphine ligand] was prepared by diazo decomposition at ?40 °C. It is the first α‐oxo gold carbene complex to be characterized. Its crystallographic structure was determined and DFT calculations have been performed, unraveling the key influence of the chelating (P,P) ligand. The gold center is tricoordinate and the electrophilicity of the carbene center is decreased. Complex 3 mimics transient α‐oxo gold carbenes in a series of catalytic transformations, and provides support for the critical role of electrophilicity in the chemoselectivity of phenol functionalization (O?H vs. C?H insertion).     

Synthesis of π‐Conjugated Benzocyclotrimers

Polycyclic aromatic hydrocarbons (PAHs), especially three branchphene benzocyclotrimers represent a series of molecules with intriguing physical and chemical properties. Benzocyclotrimers are also important precursors to construct fullerenes and graphenes. In this article, we review the recent progress in the preparation methods of π‐conjugated benzocyclotrimers. In particular, cyclotrimerization reactions to construct varying shaped and edged benzocyclotrimers are illustrated. Various typical characterization methods for these materials, such as variable‐temperature ¹H‐NMR, single crystal X‐ray analysis, density functional theory (DFT) calculations and atomic force microscope (AFM) measurements are included for discussion.     

Organometallic Intramolecular π-Olefin-Metal Coordination Compounds

In this review article complexes of the type X_nM—R are dealt with in which a σ-bonded organic ligand R is additionally linked by a π-bond with the metal M. In other words, the complexes contain metallacycles with three and a half, four and a half, five and a half members, etc. Well-Known five and a half membered ring systems are those with 4-pentenyl, 1,4-pentadienyl, norbornyl norbornenyl, cyclooctenyl, and endo-dicyclopentadienyl ligands. Metallacycles of this size are strain-free and stable. Donor ligands such as amines and phosphanes can displace the coordinated double bond of the π-ligand from the metal. This C?C double bond is lengthened from ca. 133 pm to 136–146 pm by the coordination.     

Controlling Electrical Conductance through a π‐Conjugated Cruciform Molecule by Selective Anchoring to Gold Electrodes

Tuning charge transport at the single‐molecule level plays a crucial role in the construction of molecular electronic devices. Introduced herein is a promising and operationally simple approach to tune two distinct charge‐transport pathways through a cruciform molecule. Upon in situ cleavage of triisopropylsilyl groups, complete conversion from one junction type to another is achieved with a conductance increase by more than one order of magnitude, and it is consistent with predictions from ab initio transport calculations. Although molecules are well known to conduct through different orbitals (either HOMO or LUMO), the present study represents the first experimental realization of switching between HOMO‐ and LUMO‐dominated transport within the same molecule.     

Selectively Measuring π Back‐Donation in Gold(I) Complexes by NMR Spectroscopy

Even though the Dewar–Chatt–Duncanson model has been successfully used by chemists since the 1950s, no experimental methodology is yet known to unambiguously estimate the constituents (donation and back‐donation) of a metal–ligand interaction. It is demonstrated here that one of these components, the metal‐to‐ligand π back‐donation, can be effectively probed by NMR measurements aimed at determining the rotational barrier of a C?N bond (ΔH_r^≠) of a nitrogen acyclic carbene ligand. A large series of gold(I) complexes have been synthesized and analyzed, and it was found that the above experimental observables show an accurate correlation with back‐donation, as defined theoretically by the appropriate charge displacement originated upon bond formation. The proposed method is potentially of wide applicability for analyzing the ligand effect in metal catalysts and guiding their design.     

Synthesis,Characterization and Optical Properties of a Novel Si‐Containing σ‐π‐Conjugated Hyperbranched Polymer

The polymerization of bis(4‐ethynylphenyl)methylsilane catalyzed by RhI(PPh₃)₃ afforded a regio‐ and stereoregular hyperbranched polymer, hb‐poly[(methylsilylene)bis(1,4‐phenylene‐trans‐vinylene)] (poly( 1 )), containing 95% trans‐vinylene moieties. The weight loss of this polymer at 900°C in N₂ was 9%. Poly( 1 ) displayed an absorption due to π‐π* transition around 275 nm as a shoulder and a weak absorption around 330 nm due to π‐to‐σ charge transfer, which was hardly seen in the corresponding linear polymer.