Radical-initiated alkene hydroauration as a route to gold(III) alkyls: an experimental and computational study

The 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

Photochemical Disproportionation of an Au(II) Pincer Complex:Synthesis and Structure of an Au(I)4Au(III)4 Macrocycle

The Au(II) C^N^C pincer complex [(C^N^C)Au]2 is stable under thermal conditions but disproportionates on irradiation in solution to give an Au(I)4Au(III)4 mixed-valence aggregate with a 20-membered macrocyclic structure, consisting of four linear Au(I) C-Au-N building blocks, each of which is decorated with a square planar (C^N^C)Au(III) substituent. In the crystal, the rings are stacked to form solvent-filled channels with an internal diameter of 8.3 Å and a cross-channel AuIAuI distance of 7.7 Å

Gold(III) complexes for antitumor applications: An overview

Gold(III) complexes have emerged as a versatile and effective class of metal‐based anticancer agents. The development of various types of ligands capable of stabilizing the AuIII cation and preventing its reduction under physiological conditions, such as chelating nitrogen‐donors, dithiocarbamates and C^N cyclometalled ligands, has opened the way for the exploration of their potential intracellular targets and action mechanisms. At the same time, the bioconjugation of AuIII complexes has emerged as a promising strategy for improving the selectivity of this class of compounds for cancer cells over healthy tissues, and recent developments have shown that combining gold complexes with molecular structures that are specifically recognized by the cell can exploit the cell's own transport mechanisms to improve selective metal uptake

Synthesis of Copper(I) Cyclic (Alkyl)(Amino)Carbene Complexes with Potentially Bidentate N^N, N^S and S^S Ligands for Efficient White Photoluminescence

The reaction of (Me2L)CuCl with either NaS2CX [X = OEt, NEt2 or carbazolate (Cz)] or with 1,3-diarylguanidine, 1,3-diarylformamidine or thioacetaniline in the presence of KOtBu affords the corresponding S- or N-bound copper complexes (Me2L)Cu(S^S) 1–3, (Me2L)Cu(N^N) 4/5 and (Me2L)Cu(N^S) 6 (aryl = 2,6-diisopropylphenyl; Me2L = 2,6-bis(isopropyl)phenyl-3,3,5,5-tetramethyl-2-pyrrolidinylidene). The crystal structure of (Me2L)Cu(S2CCz) (3) confirmed the three-coordinate geometry with S^S chelation and perpendicular orientation of the carbene and S^S ligands. On heating 3 cleanly eliminates CS2 and forms (Me2L)CuCz. The N-bound complexes show strongly distorted T-shaped (4) or undistorted linear (5) geometries. On excitation with UV light the S-bound complexes proved non-emissive, while the guanidinato and formamidinato complexes are strongly phosphorescent, with excited state lifetimes in the range of 11–24 µs in the solid state. The conformationally flexible formamidinato complex 5 shows intense green-white phosphorescence with a solid-state quantum yield of >96%

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.

Synthesis, Structures and Properties of Luminescent (C^N^C)gold(III) Alkyl Complexes: Correlation between Photoemission Energies and C-H Acidity

The reaction of (C^Npz^C)AuCl with C-H acidic compounds H2CR1R2 in the presence of base readily affords the corresponding alkyl complexes (C^Npz^C)AuCHR1R2 [2a, R1 = R2 = CN; 3a, R1 = R2 = C(O)Me; 4, R1 = R2 = CO2Et; 5, R1 = R2 = C(O)Ph; 6a, R1 = H, R2 = C(O)Me]. The analogous pyridine-pincer complexes 2b, 3b and 6b were obtained similarly [C^Npz^C = 2,6-bis(4′-tBuC6H3)2pyrazine dianion; C^Npy^C = 2,6-bis(4′-tBuC6H3)2pyridine dianion]. The reactions of (C^Npz^C)AuOAcF (7a) and (C^Npy^C)AuOAcF (7b) with MeMgI gave the methyl complexes (C^Npz^C)AuMe (8a) and (C^Npy^C)AuMe (8b), respectively. The crystal structures of 2a, 2b, 3a, 6a, and 7a have been determined. The photophysical properties of the new complexes and those of the previously reported gold hydride (C^Npz^C)AuH (AuH) are reported. The lower-energy absorption and the emission maxima follow the energy sequence 2a < 3a < 4 < 5 < 6a < AuH ≈ 8a for the pyrazine series, and 2b < 3b < 6b ≈ 8b for the pyridine series. These values provide a correlation with the pKa values of the corresponding ancillary ligand precursors. In agreement with DFT calculations, the emissions are assigned to 3IL(C^N^C)/3ILCT {(C6H4tBu-4’)→pz/py*} transitions, dominated by the HOMO and the LUMO orbitals. The LUMO is located in the heterocycle (py/pz) in trans position to the ancillary ligand, which makes this orbital more sensitive to the electronic nature of the ancillary ligand than the HOMO. The calculations establish that the charge transfer from the tBuC6H3 ligand fragments to the central heterocycle ring in the dominant transition explains the modulation of the properties with the σ-donor characteristics of the alkyl or hydride ligands

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

Zwitterionic mixed-carbene coinage metal complexes: Synthesis, structures and photophysical studies

A series of three zwitterionic mixed-ligand bis(carbene) complexes of copper (1), silver (2) and gold (3) have been synthesized, based on a combination of ethyl-substituted cyclic (alkyl)(amino)carbene (Et2CAAC) and an anionic methyl-malonate-derived N-heterocyclic carbene (maloNHC). The crystal structures confirm the linear two-coordinate geometry without close intermolecular contacts. The compounds show blue-white phosphorescence consistent with a wide HOMO-LUMO energy gap (2.87–3.07 eV) estimated by cyclic voltammetry. The excited state lifetimes of crystalline powders decrease in the series Cu > Ag > Au, from 400 µs for copper 1 to 50 µs for the gold complex 3. DFT calculations indicate a large change in the transition dipole moment on excitation, of up to 16 D

Ultrafast Structure and Dynamics in the Thermally Activated Delayed Fluorescence of a Carbene-Metal-Amide

Thermally activated delayed fluorescence has enormous potential for the development of efficient light emitting diodes. A recently discovered class of molecules (the carbene – metal – amides, CMAs) are exceptionally promising as they combine the small singlet - triplet energy gap required for thermal activation with a large transition moment for emission. Calculations suggest that excited state structural dynamics modulate the critical coupling between singlet and triplet states, but do not agree on the nature of those dynamics. Here we report ultrafast time resolved transient absorption and Raman studies of CMA photodynamics. The measurements reveal complex structural evolution following intersystem crossing on the tens to hundreds of picoseconds timescale, and a change in the low frequency vibrational spectrum between singlet and triplet states. The latter is assigned to a change in frequency or amplitude associated with a Raman active mode localized on the metal centre

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