Recent advances in gold(III) chemistry: Structure, bonding, reactivity and role in homogeneous catalysis

Over 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.

A PGSE NMR approach to the characterization of single and multi-site halogen-bonded adducts in solution

We demonstrate here that the Pulsed field Gradient Spin Echo (PGSE) NMR diffusion technique can be effectively used as a complementary tool for the characterization of mono- and multi-site intermolecular halogen bonding (XB) in solution. The main advantage of this technique is that it provides the possibility of unambiguously determining the stoichiometry of the supramolecular adduct, information that is particularly important when multi-site molecular systems are studied. As an example, PGSE NMR measurements in chloroform indicate that hexamethylenetetramine (HMTA), a potentially four-site XB acceptor, actually exploits only two sites for the interaction with the XB donor N-bromosuccinimide (NBS), leaving the other two nitrogen sites unoccupied. Charge displacement calculations suggest that this is due also to the anti-cooperativity of the XB interaction between HMTA and NBS

Comparative NMR study on the reactions of Hf(IV) organometallic complexes with Al/Zn alkyls

NMR spectroscopy has been exploited to investigate the reactions of Hf(IV) organometallic complexes with trialkylaluminum and dialkylzinc, with the aim of obtaining insights into the elementary steps of coordinative chain transfer polymerization (CCTP). Bis(cyclopentadienyl)hafnium dimethyl (Cp2HfMe2, 1Me2) and [N-(2,6-diisopropylphenyl)-α-(2-isopropylphenyl)-6-(1-naphthalenyl)-2-pyridinemethanaminato]hafnium dimethyl (2Me2) complexes have been chosen as case studies for understanding the differences between poorly performing and highly active CCTP catalysts, in an attempt to assess the effect of the ancillary ligand on the transalkylation rate. 2Me2 was found to react much more quickly with both AlEt3 and ZnEt2 in comparison to 1Me2, mainly due to a remarkably lower activation enthalpy. In addition, while the ethylation rate was found to depend on the nature of the alkylating agent for 1Me2, it does not for 2Me2. This difference in reactivity was observed also in the case of the ion pairs obtained by reacting 1Me2 and 2Me2 with [CPh3][B(C6F5)4]. For the latter species, NMR indicated that two main deactivation pathways, namely anion decomposition and σ-bond methatesis of Hf–alkyl groups, occur

Assessing the Role of Counterion in Gold-Catalyzed Dearomatization of Indoles with Allenamides by NMR Studies

The counterion effect in the gold(I)-catalyzed dearomative condensation of indoles with allenamides is unveiled by means of 1D- and 2D-NMR investigation. The different coordination ability and hydrogen bonding tendency of TFA– and OTf– led to specific interactions with the reaction partners dictating the regiodivergent outcome

Disclosing the multi-faceted world of weakly interacting inorganic systems by means of NMR spectroscopy

The potential of NMR spectroscopy to investigate inorganic systems assembled by, or whose reactivity is affected by, non-covalent interactions is described. Subjects that have received particular attention in recent years (halogen bonding and Frustrated Lewis Pairs) and more classical subjects that remain under-explored (self-aggregation of ion pairs in low polar solvents, behavior of MAO containing metallocenium ion pairs, and hydrogen bonding/ion pairing effects in Au(I) catalysis) are considered, using an innovative approach, always focusing on the crucial information that can be provided by NMR

Carbon-sulfur bond formation by reductive elimination of gold(III) thiolates

Whereas 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 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] and requires at least two molar equivalents of RSH per Au. 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

Unlocking Structural Diversity in Gold(III) Hydrides: Unexpected Interplay of cis/trans-Influence on Stability, Insertion Chemistry, and NMR Chemical Shifts

The synthesis of new families of stable or at least spectroscopically observable gold(III) hydride complexes is reported, including anionic cis-hydrido chloride, hydrido aryl and cis-dihydride complexes. Reactions between (C^C)AuCl(PR3) and LiHBEt3 afford the first examples of gold(III) phosphino hydrides (C^C)AuH(PR3) (R = Me, Ph, p-tolyl; C^C = 4,4′-di-tert-butylbiphenyl-2,2′-diyl). The X-ray structure of (C^C)AuH(PMe3) was determined. LiHBEt3 reacts with (C^C)AuCl(py) to give [(C^C)Au(H)Cl]–, whereas (C^C)AuH(PR3) undergoes phosphine displacement, generating the dihydride [(C^C)AuH2]-. Monohydrido complexes hydroaurate dimethylacetylene dicarboxylate to give Z-vinyls. (C^N^C)Au pincer complexes give the first examples of gold(III) bridging hydrides. Stability, reactivity and bonding characteristics of Au(III)-H complexes crucially depend on the interplay between cis and trans-influence. Remarkably, these new gold(III) hydrides extend the range of observed NMR hydride shifts from δ 8.5 to +7 ppm. Relativistic DFT calculations show that the origin of this wide chemical shift variability as a function of the ligands depends on the different ordering and energy gap between “shielding” Au(dπ)-based orbitals and “deshielding” σ(Au-H)-type MOs, which are mixed to some extent upon inclusion of spin-orbit (SO) coupling. The resulting 1H hydride shifts correlate linearly with the DFT optimized Au-H distances and Au-H bond covalency. The effect of cis ligands follows a nearly inverse ordering to that of trans ligands. This study appears to be the first systematic delineation of cis ligand influence on M-H NMR shifts and provides the experimental evidence for the dramatic change of the 1H hydride shifts, including the sign change, upon mutual cis and trans ligand alternation

Substantial improvement of pyridine-carbene iridium water oxidation catalysts by a simple methyl-to-octyl substitution

The substitution of a methyl to an octyl group in the ancillary triazolylidene ligand—an apparently simple variation—induces a more than 10-fold increase of activity of the corresponding iridium complex in water oxidation catalysis when using cerium(IV) as sacrificial oxidant. Detailed NMR studies suggest that various different molecular species form, all bearing the intact triazolylidene ligand. The octyl substituent is essential for inducing the association of the iridium species, thus generating extraordinarily active multimetallic catalytic sites. Their accessibility and steady-state concentration is critically dependent on the type of sacrificial oxidant and specifically on the cerium ammonium nitrate versus catayst ratio

Gold(III) alkyne complexes: Bonding and reaction pathways

The 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 Photophysical Properties of Au(III)-Ag(I) Aggregates

Cyclometallated gold (III) complexes of the type (C^N^C)AuX [HC^N^CH = 2,6-bis(4-ButC6H4)pyrazine; 2,6-bis(4-ButC6H4)pyridine, or 2,6-bis(4-ButC6H4)4-Butpyridine; X = CN, CH(COMe)2 or CH(CN)2] have been used as building blocks for the construction of the first family of AuIII/AgI aggregates. The crystal structures of these aggregates reveal the formation of complex architectures in which the Ag+ cations are stabilized by the basic centers present on each of the Au precursors. The photophysical properties of these aggregates are reported. Compared to mononuclear pincer complexes, a general red-shift and an increase in the emission intensity are observed. In agreement with DFT calculations the lowest energy absorption and the emission are assigned to 1IL(C^N^C) and 3IL(C^N^C) transitions dominated by the HOMO and the LUMO orbitals