Gold carbonyl cations and beyond: homoleptic gold(I) complexes with P4 and P4S3 and the half-sandwich cation [Au(C6H6)(CO)]. Chem Commun (Camb) 2024;60:5403-5406. [PMID: 38682872 DOI: 10.1039/d4cc01374c] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Oxidation of Au0 with the synergistic Ag+/0.5 I2 system in the commercial organic solvent 1,2,3,4-tetrafluorobenzene led to the perfluoroalkoxyaluminate salt of the [Au(CO)2]+ cation known from superacid chemistry. This [Au(CO)2]+ salt proved to be an excellent 'naked' Au+-synthon yielding complex salts with [Au(η2-P4)2]+, [Au(η1-P4S3)2]+ and half-sandwich [Au(η2-C6H6)(CO)]+ cation.

Time-resolved control of nanoparticle integration in titanium-organic frameworks for enhanced catalytic performance

Among the multiple applications of metal-organic frameworks (MOFs), their use as a porous platform for the support of metallic nanoparticles stands out for the possibility of integrating a good anchorage, that improves the stability of the catalyst, with the presence of a porous network that allows the diffusion of substrates and products. Here we introduce an alternative way to control the injection of Au nanoparticles at variable stages of nucleation of a titanium(iv) MOF crystal (MUV-10). This allows the analysis of the different modes of nanoparticle integration into the porous matrix as a function of the crystal formation stage and their correlation with the catalytic performance of the resulting composite. Our results reveal a direct effect of the stage at which the Au nanoparticles are integrated into MUV-10 crystals not only on their catalytic activity for the cyclotrimerization of propargyl esters and the hydrochlorination of alkynes, but also on the selectivity and recyclability of the final solid catalyst, which are far superior than those reported for the same reactions with TiO2 supports.

(Hexafluoroacetylacetonato)copper(I)-cycloalkyne complexes as protected cycloalkynes

A protection method for cycloalkynes by the formation of (hexafluoroacetylacetonato)copper(i)-cycloalkyne complexes is disclosed. Various complexes having functional groups were efficiently prepared, which are easily purified by silica-gel column chromatography. Selective click reactions were realized through the complexation of cycloalkynes with copper.

Selective strain-promoted azide-alkyne cycloadditions through transient protection of bicyclo[6.1.0]nonynes with silver or gold

Complexation of bicyclo[6.1.0]nonynes with a cationic silver or gold salt results in protection from a click reaction with azides. The cycloalkyne protection using the silver or gold salt enables selective strain-promoted azide-alkyne cycloadditions of diynes keeping the bicyclo[6.1.0]nonyne moiety unreacted.

Globally stabilized bent carbon-carbon triple bond by hydrogen-free inorganic-metallic scaffolding Al4F6

For over 100 years, known bent C[triple bond, length as m-dash]C compounds have been limited to those with organic (I) and all-carbon (II) scaffoldings. Here, we computationally report a novel type (III) of bent C[triple bond, length as m-dash]C compound, i.e., C2Al4F6-01, which is the energetically global minimum isomer and bears an inorganic-metallic scaffolding and unexpected click reactivity.

Sequential conjugation methods based on triazole formation and related reactions using azides

The recent remarkable progress in azide chemistry has realized sequential conjugation methods with selective 1,2,3-triazole formation. On the basis of the diverse reactivities of azides and azidophiles, including terminal alkynes and cyclooctynes, various selective reactions to furnish triazoles and a wide range of platform molecules, such as diynes, diazides, triynes, and triazides, have been developed so far for bis- and tris(triazole) syntheses. This review highlights recent transformations involving selective triazole formation, allowing the efficient preparation of unsymmetric bis- and tris(triazole)s using diverse platform molecules.

Role of σ,π-Digold(I) Alkyne Complexes in Reactions of Enynes

Gold(I) acetylide and σ,π-digold(I) alkyne complexes derived from one prototypical 1,6-enyne and from 7-ethynyl-1,3,5-cycloheptatriene have been prepared and structurally characterized. Their possible role in gold(I)-catalyzed cycloisomerizations has been studied by experiment and by DFT calculations. Gold(I) acetylides are totally unproductive complexes in the absence of Brønsted acids. Similarly, no cyclizations were observed by heating σ,π-digold(I) alkyne digold(I) at least up to 130 °C. Theoretical studies provide a rationale for the much lower reactivity of digold species in reactions of enynes.

Heterobimetallic Complexes Featuring Fe(CO)5 as a Ligand on Gold

Iron(0) pentacarbonyl complexes of gold(I), [Mes₃ PAu-Fe(CO)₅ ][SbF₆ ] (1) and [(IPr*)Au-Fe(CO)₅ ][SbF₆ ] (2) (Mes=2,4,6-trimethylphenyl; IPr*=1,3-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazol-2-ylidene) have been synthesized using Mes₃ PAuCl and (IPr*)AuCl as the gold(I) precursor, AgSbF₆ halide ion abstractor, and the Lewis base Fe(CO)₅ . The Au-Fe bond lengths of these metal-only Lewis pair complexes are significantly shorter than the sum of the experimentally derived covalent radii of Au and Fe. The v̄(CO) bands of the molecules show a notable blueshift relative to those observed for free Fe(CO)₅ , indicating a substantial reduction in Fe→CO backbonding upon its coordination to gold(I) with either Mes₃ P or IPr* supporting ligands (L). The analysis of the electronic structure with quantum chemical method suggests that the Au-Fe bond consists mainly of [LAu]⁺ ←Fe(CO)₅ σ-donation and weaker [LAu]⁺ →Fe(CO)₅ π-backdonation. The donor strength of Fe(CO)₅ is similar to that of CO.

Gold(i)-catalyzed synthesis of 2-substituted indoles from 2-alkynylnitroarenes with diboron as reductant

An efficient method for the synthesis of 2-substituted indoles via a diboron/base promoted tandem reductive cyclization of o-alkynylnitroarene under Au catalysis conditions has been disclosed.

Selective formation of silver(i) bis-phospholane macrocycles and further evidence that gold(i) is smaller than silver(i)

Highly flexible bis-phospholane ligands form 16- to 28-membered dinuclear macrocyclic silver(i) complexes selectively without using high-dilution techniques. Comparison with gold(i) complexes gives further evidence that gold(i) is significantly smaller than silver(i).

An element through the looking glass: exploring the Au-C, Au-H and Au-O energy landscape

Gold, the archetypal "noble metal", used to be considered of little interest in catalysis. It is now clear that this was a misconception, and a multitude of gold-catalysed transformations has been reported. However, one consequence of the long-held view of gold as inert metal is that its organometallic chemistry contains many "unknowns", and catalytic cycles devised to explain gold's reactivity draw largely on analogies with other transition metals. How realistic are such mechanistic assumptions? In the last few years a number of key compound classes have been discovered that can provide some answers. This Perspective attempts to summarise these developments, with particular emphasis on recently discovered gold(iii) complexes with bonds to hydrogen, oxygen, alkenes and CO ligands.

Dispersion, solvent and metal effects in the binding of gold cations to alkynyl ligands: implications for Au(I) catalysis

The coordination modes of the [Au(PPh3)](+) cation to metal alkynyl complexes have been investigated. On addition to ruthenium, a vinylidene complex, [Ru(η(5)-C5H5)(PPh3)2([double bond, length as m-dash]C[double bond, length as m-dash]CPh{AuPPh3})](+), is obtained while addition to a gold(iii) compound gives di- and trinuclear gold complexes depending on the conditions employed. In the trinuclear species, a gold(i) cation is sandwiched between two gold(iii) alkynyl complexes, suggesting that coordination of multiple C-C triple bonds to gold is facile.

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.

NHC-AuCl/selectfluor: a highly efficient catalytic system for carbene-transfer reactions

The combination of NHC-gold complex and Selectfluor has been found to be a highly efficient catalyst system for carbene-transfer reactions, with a turnover number (TON) up to 990000 and a turnover frequency (TOF) up to 82500 h(-1).

Dual gold catalysis

For more than a decade the innovative field of homogeneous catalysis by gold was dominated by the interaction of the substrate molecule with one gold center, in most cases in mononuclear gold complexes. The initial interaction was typically a π-coordination of a carbon-carbon double bond to the gold, an activation of the unsaturated substrate molecule by a π-acidic metal center. Only recently clear evidence for reactions that involve the activation of organic substrates by two gold centers was obtained. In that new class of gold-catalyzed reactions the two gold centers interact with the substrate in a very different way. One gold complex is σ-bonded to a terminal alkynyl group in the substrate, the other one is π-coordinated. Only in a few cases, a combination π-coordination and σ-coordination to the same alkyne, which is the energetically preferred mode of interaction with two gold centers, initiates the reaction. In most of the cases, the reaction proceeds through an intermediate with one alkyne σ-bonded to one gold complex and a different alkyne π-coordinated to the second gold complex. Experimental and computational results for many new reactions provide a clear picture of the overall sequence of elemental steps of these conversions; some of the steps are unprecedented in organometallic catalysis and chemistry. For example, the reaction of diynes can involve gold vinylidene-like species as very reactive substructures formed by a 5-exo-dig cyclization. NBO analysis indicates no gold-carbon double bond character in these "vinylidenes". In other reactions, a 6-endo-dig cyclization is energetically preferred; after that gold aryne complexes are not local minima but transition states of a 1,2-shift of gold. Computational studies showed a good correlation of the cyclization mode with the aromaticity of the intermediate. For both the 5-exo-dig and the 6-endo-dig cyclization modes, the intermediates are able to react even with unactivated alkyl-C,H bonds, in low yields even in intermolecular reactions. The final step of the catalytic cycles is also remarkable, because the protodeauration has to occur with the next alkyne substrate molecule. Only then the next gold acetylide is formed directly and a loss of selectivity can be avoided. A computational study suggests that two gold complexes are on the substrate throughout the catalytic cycle. The most recent results indicate that analogous intermediates can be accessible by the reaction of other electrophiles with gold acetylides. With regard to organic synthesis, the overall catalytic conversions open up a universe of new possibilities. Selective C,H-activations now allow to one use usually innocent alkyl side chains for additional anellation reactions by an sp(3)-C,H activation. The C,H activation can even be combined with halogen transfer reactions, directly providing vinyl iodides as versatile building blocks. Short and efficient routes to different carbo- and heterocycles including benzocyclobutenes, fulvenes, and pentalenes demonstrate the synthetic potential not only for total synthesis but also for material science.

Intriguing mechanistic labyrinths in gold(I) catalysis

Many mechanistically intriguing reactions have been developed in the last decade using gold(I) as catalyst. Here we review the main mechanistic proposals in gold-catalysed activation of alkynes and allenes, in which this metal plays a central role by stabilising a variety of complex cationic intermediates.