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

Exploring Ligand Effects on Structure, Bonding, and Photolytic Hydride Transfer of Cationic Gold(I) Bridging Hydride Complexes of Molybdocene and Tungstenocene

A diverse family of heterobimetallic bridging hydride adducts of the type [LAu(μ-H)2MCp2][X] (L = 1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene, IPr; 1,3-bis(1-adamantyl)imidazole-2-ylidene, IAd; 1,3-bis(2,6-di-iso-propylphenyl)-5,5-dimethyl-4,6-diketopyrimidinyl-2-ylidene, DippDAC; triphenylphosphine, PPh3; 2-di-tert-butylphosphino-2',4',6'-triisopropylbiphenyl, tBuXPhos; X = SbF6-, BF4- or TfO-) was synthesized by reacting group VI metallocene dihydrides Cp2MH2 (Cp = cyclopentadienyl anion; M = Mo, W) with cationic gold(I) complexes [LAu(NCMe)][X]. Trimetallic [L'Au2(μ-H)2WCp2][X]2 and tetrametallic [L'Au2{(μ-H)2WCp2}2] [X]2 complexes (L' = rac-2,2'-bis(diphenylphosphino)-1,1'-binaphthalene or bis(diphenylphosphinomethane)) were obtained by reacting digold [L'{Au(NCMe)}2][X]2 with Cp2WH2 in a 1:1 and a 1:2 stoichiometry. Accessing such a broad structural diversity allowed us to pinpoint roles played by the ancillary ligands and group VI metals on the bonding properties of this family of bridging hydrides. In particular, a clear effect of the ligand on the interaction energy and electronic structure was observed, with important implications on photolytic reactivity. UV or visible light irradiation, indeed, leads to the selective cleavage of the heterobimetallic Au(μ-H)2M arrangement and formation of molecular gold hydrides. The photolysis was found to be chromoselective (wavelength-dependent), which can be ascribed to different charge redistributions upon excitation to the first (Kasha's reactivity) and higher (anti-Kasha's reactivity) excited states.

(P^N^C) Ligands to Stabilize Gold(III): A Straightforward Access to Hydroxo, Formate, and Hydride Complexes

A novel class of (P^N^C) pincer ligands capable of stabilizing elusive gold(III) species is reported here. Straightforward access to (P^N^C)gold(III) hydroxo, formate, and hydride complexes has been streamlined by first incorporating a cycloauration step devoid of toxic metals or harsh conditions. The resulting gold complexes exhibit remarkable stability in solution as well as in the solid state under ambient conditions, which enabled their characterization by X-ray diffraction analyses. Interestingly, the influence of the ligand allowed the preparation of gold(III)-hydrides using mild hydride donors such as H-Bpin, which contrasts with sensitive super hydrides or strong acids and cryogenic conditions employed in previous protocols. A detailed bonding characterization of these species is complemented by reactivity studies.

Ligand-Enabled Oxidative Fluorination of Gold(I) and Light-Induced Aryl-F Coupling at Gold(III)

MeDalphos Au(I) complexes featuring aryl, alkynyl, and alkyl groups readily react with electrophilic fluorinating reagents such as N-fluorobenzenesulfonimide and Selectfluor. The ensuing [(MeDalphos)Au(R)F]+ complexes have been isolated and characterized by multinuclear NMR spectroscopy as well as X-ray diffraction. They adopt a square-planar contra-thermodynamic structure, with F trans to N. DFT/IBO calculations show that the N lone pair of MeDalphos assists and directs the transfer of F+ to gold. The [(MeDalphos)Au(Ar)F]+ (Ar = Mes, 2,6-F2Ph) complexes smoothly engage in C-C cross-coupling with PhCCSiMe3 and Me3SiCN, providing direct evidence for the oxidative fluorination/transmetalation/reductive elimination sequence proposed for F+-promoted gold-catalyzed transformations. Moreover, direct reductive elimination to forge a C-F bond at Au(III) was explored and substantiated. Thermal means proved unsuccessful, leading mostly to decomposition, but irradiation with UV-visible light enabled efficient promotion of aryl-F coupling (up to 90% yield). The light-induced reductive elimination proceeds under mild conditions; it works even with the electron-deprived 2,6-difluorophenyl group, and it is not limited to the contra-thermodynamic form of the aryl Au(III) fluoride complexes.

Insertion of Molecular Oxygen into a Gold(III)-Hydride Bond

The use of molecular oxygen as an oxidant in chemical synthesis has significant environmental and economic benefits, and it is widely used as such in large-scale industrial processes. However, its adoption in highly selective homogeneous catalytic transformations, particularly to produce oxygenated organics, has been hindered by our limited understanding of the mechanisms by which O2 reacts with transition metals. Of particular relevance are the mechanisms of the reactions of oxygen with late transition metal hydrides as these metal centers are better poised to release oxygenated products. Homogeneous catalysis with gold complexes has markedly increased, and herein we report the synthesis and full characterization of a rare AuIII-H, supported by a diphosphine pincer ligand (tBuPCP = 2,6-bis(di-tert-butylphosphinomethyl)benzene). [(tBuPCP)AuIII-H]+ was found to cleanly react with molecular oxygen to yield a stable AuIII-OOH complex that was also fully characterized. Extensive kinetic studies on the reaction via variable temperature NMR spectroscopy have been completed, and the results are consistent with an autoaccelerating radical chain mechanism. The observed kinetic behavior exhibits similarities to that of previously reported PdII-H and PtIV-H reactions with O2 but is not fully consistent with any known O2 insertion mechanism. As such, this study contributes to the nascent fundamental understanding of the mechanisms of aerobic oxidation of late metal hydrides.

Manipulating hydrogen and coordination bond chemistry for reversible zinc metal anodes

Aqueous zinc ion hybrid capacitors (ZHCs) are promising as electrochemical energy storage devices due to their safety and cost-effectiveness. However, the practical application of aqueous ZHCs is impeded by zinc dendrite growth and side reactions induced by H2O during long-term cycling. Herein, an organic small molecule, dimethyl sulfoxide (DMSO), is elaborately introduced into 2 M ZnSO4 electrolyte to simultaneously overcome these challenges. As convincingly evidenced by experimental and theoretical results, the DMSO reconstructs the Zn[(H2O)6]2+ structure and original hydrogen bond networks at the molecular level. By forming coordination bonds with Zn2+ and hydrogen bonds with H2O due to the stronger electron donating ability of oxygen in molecule, DMSO establishes a Zn2+ solvation shell structure that inhibits H2O decomposition and dendrite growth. As a proof of concept, the implementation of this hybrid electrolyte in a Zn||Cu asymmetrical cell results in a high Coulombic efficiency (CE) of over 99.8% for 568 cycles at a current density of 2 mA cm-2. Furthermore, the full cells using this hybrid electrolyte coupled with activated carbon (AC) cathode can operate for over 30,000 cycles. These results suggest that reconstructing the solvation structure and hydrogen bond networks guide the design of electrolytes for the development of high-performance aqueous ZHCs.

Unlocking the Chain-Walking Process in Gold Catalysis

The successful realization of gold-catalyzed chain-walking reactions, facilitated by ligand-enabled Au(I)/Au(III) redox catalysis, has been reported for the first time. This breakthrough has led to the development of gold-catalyzed annulation reaction of alkenes with iodoarenes by leveraging the interplay of chain-walking and π-activation reactivity mode. The reaction mechanism has been elucidated through comprehensive experimental and computational studies.

Reactivity of [AuF3 (SIMes)]: Pathway to Unprecedented Structural Motifs

We report on a comprehensive reactivity study starting from [AuF3 (SIMes)] to synthesize different motifs of monomeric gold(III) fluorides. A plethora of different ligands has been introduced in a mono-substitution yielding trans-[AuF2 X(SIMes)] including alkynido, cyanido, azido, and a set of perfluoroalkoxido complexes. The latter were better accomplished via use of perfluorinated carbonyl-bearing molecules, which is unprecedented in gold chemistry. In case of the cyanide and azide, triple substitution gave rise to the corresponding [AuX3 (SIMes)] complexes. Comparison of the chemical shift of the carbene carbon atom in the 13 C{1 H} NMR spectrum, the calculated SIMes affinity and the Au-C bond length in the solid state with related literature-known complexes yields a classification of trans-influences for a variety of ligands attached to the gold center. Therein, the mixed fluorido perfluoroalkoxido complexes have a similar SIMes affinity to AuF3 with a very low Gibbs energy of formation when using the perfluoro carbonyl route.

Luminescent Au(III)-M(I) (M = Cu, Ag) Aggregates Based on Dicyclometalated Bis(alkynyl) Gold Anions†

The syntheses and structures of a series of complexes based on the C∧C-chelated Au(III) unit (C∧C = 4,4'-bis(t-butyl) 2,2'-biphenyl-1,1'-diyl) are reported, namely, [{(C∧C)Au(C≡CtBu)2}2M2], (C∧C)Au(C≡CR)(C≡NXyl), and [{(C∧C)Au(C≡CR)2}{M(C≡NXyl)}] (M = Ag, Cu; R = tBu, C6H4tBu-4, C6H4OMe-4; Xyl = 3,5-Me2C6H3). The X-ray structures reveal a broad range of dispositions determined by the different coordination modes of Ag(I) or Cu(I). The complexes are bright photoemitters in the solid state and in poly(methyl methacrylate) (PMMA) films. The photoluminescence is dominated by 3IL(C∧C) transitions, with indirect effects from the rest of the molecules, as supported by theoretical calculations. This work opens up the possibility of accessing Au(III) carbon-rich anions to construct photoluminescent aggregates.

Au(III) Cyclometallated Compounds with 2-Arylpyridines and Their Derivatives or Analogues: 34 Years (1989–2022) of NMR and Single Crystal X-ray Studies

A review paper on Au(III) cyclometallated compounds with 2-arylpyridines (2-phenylpyridine, 2-benzylpyridine, 2-benzoylpyridine, 2-phenoxypyridine, 2-phenylsulfanylpyridine, 2-anilinopyridine, 2-(naphth-2-yl)pyridine, 2-(9,9-dialkylfluoren-2-yl)pyridines, 2-(dibenzofuran-4-yl)pyridine, and their derivatives) and their analogues (2-arylquinolines, 1- and 3-arylisoquinolines, 7,8-benzoquinoline), with 113 references. A total of 554 species, containing κ2-N(1),C(6′)*-Au(III), or analogous moiety (i.e., chelated by nitrogen of the pyridine-like ring and the deprotonated ortho- carbon of the phenyl-like ring) and, thus, possessing a character intermediate between metal complexes and organometallics, studied in the years 1989–2022 by NMR spectroscopy and/or single crystal X-ray diffraction (207 X-ray structures), are described. The compounds for which biological or catalytic activity and the luminescence properties were studied are also quoted.

Tweaking the bridge in metallocene Zr(IV)/W(IV) bimetallic hydrides

Zirconocene cations react with Cp₂WH₂ affording the bimetallic [Cp₂Zr(μ-H)(μ-η¹:η⁵-C₅H₄)WHCp]⁺ bridging hydride 1 (Cp = cyclopentadienyl anion, C₅H₅^-) via σ-bond metathesis. Complex 1 features an atypical out of plane Zr(μ-H)W moiety, where no intermetallic interaction is involved, and a fluxional core. Coordination geometry and bond distances of the bridging hydride interaction can be modulated upon reaction with Lewis bases and unsaturated substrates. PMe₃, P(p-tol)₃, 3,5-dimethylpyridine and THF bind to 1 and shift the hydride bridge on the coordination plane of Zr. Insertion of olefins and alkynes into the Zr-C bond of 1 leads instead to alkyl and vinyl species where the Zr and W coordination planes are perpendicular to each other. Such alterations of the Zr(μ-H)W arrangement are reflected in the average ¹H NMR chemical shift values of the hydride, which correlate linearly with computed Zr-H distances. Reactivity experiments with H₂ showed that the bridging hydride interaction prevents bimetallic cooperativity and that σ-bond metathesis between Zr-C and H-H bonds is the preferred pathway for all the investigated complexes.

Computational Prediction of Tc-99 NMR Chemical Shifts in Technetium Complexes with Radiopharmaceutical Applications

The Tc-99m nucleus is the most used nuclide in radiopharmaceuticals designed for imaging diagnosis. The metal can exist in nine distinct oxidation states and forms distinct coordination complexes with a variety of chelating agents and geometries. These complexes are usually characterized through Tc-99 NMR that is very sensitive to the Tc coordination sphere. Therefore, predicting Tc-99 NMR might be useful to assist experimentalists in structural characterization. In the present study, we propose three computational protocols for predicting Tc-99 NMR chemical shifts based on density functional theory calculations using relativistic and nonrelativistic Hamiltonians: the relativistic Model 1, the nonrelativistic Model 2, and the empirical nonrelativistic Model 3. In Models 2 and 3, the NMR-DKH basis set was used for all atoms, including the Tc, for which it was developed here. All models were applied for a set of 41 Tc-complexes with metal oxidation states 0, I, and V, for which the Tc-99 chemical shift was available experimentally. The mean absolute deviation and the mean relative deviation were 67 ppm and 4.8% (Model 1), 92 ppm and 6.2% (Model 2), and 65 ppm and 4.9% (Model 3), respectively. Last, the effect of the explicit solvent was evaluated for the [TcO₂(en)₂]⁺─Tc(V) complex. The calculated results for the Tc-99 NMR chemical shift at SO-ZORA-SSB-D/TZ2P-ZORA/COSMO//TPSS/def2-SVP/IEF-PCM(UFF) show that the inclusion of 14 water molecules (first solvation shell) together with the implicit solvation model leads to an absolute deviation of only 7 ppm (0.3%) from the experimental value, indicating that the solvent effects play a key role in predicting Tc-99 NMR.

Synthesis of substituted (N,C) and (N,C,C) Au(III) complexes: the influence of sterics and electronics on cyclometalation reactions

Cyclometalated Au(III) complexes are of interest due to their catalytic, medicinal, and photophysical properties. Herein, we describe the synthesis of derivatives of the type (N,C)Au(OAc^F)₂ (OAc^F = trifluoroacetate) and (N,C,C)AuOAc^F by a cyclometalation route, where (N,C) and (N,C,C) are chelating 2-arylpyridine ligands. The scope of the synthesis is explored by substituting the 2-arylpyridine core with electron donor or acceptor substituents at one or both rings. Notably, a variety of functionalized Au(III) complexes can be obtained in one step from the corresponding ligand and Au(OAc)₃, eliminating the need for organomercury intermediates, which is commonly reported for similar syntheses. The influence of substituents in the ligand backbone on the resulting complexes was assessed using DFT calculations, ¹⁵N NMR spectroscopy and single-crystal X-ray diffraction analysis. A correlation between the electronic properties of the (N,C) ligands and their ability to undergo cyclometalation was found from experimental studies combined with natural charge analysis, suggesting the cyclometalation at Au(III) to take place via an electrophilic aromatic substitution-type mechanism. The formation of Au(III) pincer complexes from tridentate (N,C,C) ligands was investigated by synthesis and DFT calculations, in order to assess the feasibility of C(sp³)-H bond activation as a synthetic pathway to (N,C,C) cyclometalated Au(III) complexes. It was found that C(sp³)-H bond activation is feasible for ligands containing different alkyl groups (isopropyl and ethyl), although the C-H activation is less energetically favored compared to a ligand containing tert-butyl groups.

Chelation-assisted C-C bond activation of biphenylene by gold(i) halides

A chelation-assisted oxidative addition of gold(i) into the C-C bond of biphenylene is reported here. The presence of a coordinating group (pyridine, phosphine) in the biphenylene unit enabled the use of readily available gold(i) halide precursors providing a new, straightforward entry towards cyclometalated (N^C^C)- and (P^C)-gold(iii) complexes. Our study, combining spectroscopic and crystallographic data with DFT calculations, showcases the importance of neighboring, weakly coordinating groups towards the successful activation of strained C-C bonds by gold.

Main Avenues in Gold Coordination Chemistry

In this contribution, we provide an overview of the main avenues that have emerged in gold coordination chemistry during the last years. The unique properties of gold have motivated research in gold chemistry, and especially regarding the properties and applications of gold compounds in catalysis, medicine, and materials chemistry. The advances in the synthesis and knowledge of gold coordination compounds have been possible with the design of novel ligands becoming relevant motifs that have allowed the preparation of elusive complexes in this area of research. Strong donor ligands with easily modulable electronic and steric properties, such as stable singlet carbenes or cyclometalated ligands, have been decisive in the stabilization of gold(0) species, gold fluoride complexes, gold hydrides, unprecedented π complexes, or cluster derivatives. These new ligands have been important not only from the fundamental structure and bonding studies but also for the synthesis of sophisticated catalysts to improve activity and selectivity of organic transformations. Moreover, they have enabled the facile oxidative addition from gold(I) to gold(III) and the design of a plethora of complexes with specific properties.

Non-palindromic (C^C^D) gold(III) pincer complexes are not accessible by intramolecular oxidative addition of biphenylenes - an experimental and quantum chemical study

We herein report on the synthesis of biphenylenes substituted with a pyridine (N), a phosphine (P) and a carbene (C') donor as well as a carbene donor with additional pyridine in the lateral position. We describe the synthesis and structures of derived gold(i) complexes, which we tried to use for the synthesis of non-palindromic [(C^C^D)AuIII] pincer complexes by means of an intramolecular oxidative addition of the strained biphenylene ring. However, the anticipated formation of gold(iii) complexes failed due to kinetic and thermodynamic reasons, which we extensively investigated by quantum chemical calculations. Furthermore, we shed light on the oxidative addition of biphenylene to two different gold(i) systems reported in the literature. Our comprehensive quantum-chemical analysis is complemented by NMR experiments.

Experimental and theoretical studies on gold(III) carbonyl complexes: reductive C,H- and C,C bond formation

The reactivity of cationic (C^C)gold(iii) carbonyl complexes was investigated. While the in situ-formed IPrAu(bph)CO+ complex (bph = biphenyl-2,2'-diyl) does not undergo a migratory insertion of CO into the neighboring gold-carbon bond, nucleophiles can attack the coordinated CO moiety intermolecularly. Water as a nucleophile initiates a CO2 extrusion combined with a reductive C,H bond formation. The rapid formation of a gold(i) species from an intermediary gold(iii) carbonyl has not been observed before and shows a significant difference in reactivity between (C^C) and (C^N^C)gold(iii) carbonyls. The latter have been reported to form stable gold(iii) hydrides via the WGS reaction. In the case of methanol acting as a nucleophile attacking the gold(iii) carbonyl, no extrusion of CO2 is observed. Instead an intermediary gold(iii) carboxyl complex forms an aryl carboxylate via reductive C-C bond elimination. Experimental and theoretical studies on the mechanism explain the observed selectivities and give new insights into the reactivity of elusive gold(iii) carbonyls.

Selective cytotoxicity of cyclometalated gold(III) complexes on Caco-2 cells is mediated by G2/M cell cycle arrest

New cyclometalated gold(III) complexes with a general structure [Au(C^N)(SR)2] or [Au(C^N)Cl(SR)], where C^N is a biphenyl ligand such as 2-(p-tolyl)pyridinate (tpy), 2-phenylpyridinate (ppy) and 2-benzylpyridinate (bzp) (SR = Spym, S(Me)2pym, 2-thiouracil (2-TU) and thiourea), and also with ethynyl moieties of the type [Au(C^N)(C≡C-Ar)2] (Ar = p-toluene and 2-pyridine) have been synthesized. All of them have been characterized, including X-ray studies of complex [Au(bzp)Cl(Spym)], and these studies have permitted to elucidate that leaving chloride ligand is trans located to CAr atom. After the full characterization, physicochemical properties were measured by evaluating drug-like water solubility and cell permeability (partition coefficient). All these experiments pointed that our complexes present adequate properties to be used as anticancer drugs. Although not all the complexes showed antiproliferative effects on Caco-2 cells, those that did were more cytotoxic than cisplatin; and complex [Au(tpy)Cl(2-TU)] is even more active than auranofin. In addition to this effectiveness, no evidence of cytotoxic effects was observed on considered normal cells (with the exception of [Au(bzp)Cl(2-TU)]. Further action mechanisms studies were performed using these selective complexes, showing cell cycle arrest on the G2/M phase, a proapoptotic behaviour and also the modification of some genes involved in tumorigenesis. Thus, as a result of this investigation, we present a new family of 17 cyclometalated complexes, 6 of them being selective and possible candidates to be used against colon cancer.

Ti Group Metallocene-Catalyzed Synthesis of 1-Hexene Dimers and Tetramers

1-Hexene transformations in the catalytic systems L₂MCl₂–XAlBuⁱ₂ (L = Cp, M = Ti, Zr, Hf; L = Ind, rac-H₄C₂[THInd]₂, M = Zr; X = H, Bu ⁱ) and [Cp₂ZrH₂]₂-ClAlR₂ activated by MMAO-12, B(C₆F₅)₃, or (Ph₃C)[B(C₆F₅)₄] in chlorinated solvents (CH₂Cl₂, CHCl₃, o-Cl₂C₆H₄, ClCH₂CH₂Cl) were studied. The systems [Cp₂ZrH₂]₂-MMAO-12, [Cp₂ZrH₂]₂-ClAlBuⁱ₂-MMAO-12, or Cp₂ZrCl₂-HAlBuⁱ₂-MMAO-12 (B(C₆F₅)₃) in CH₂Cl₂ showed the highest activity and selectivity towards the formation of vinylidene head-to-tail alkene dimers. The use of chloroform as a solvent provides further in situ dimer dimerization to give a tetramer yield of up to 89%. A study of the reaction of [Cp₂ZrH₂]₂ or Cp₂ZrCl₂ with organoaluminum compounds and MMAO-12 by NMR spectroscopy confirmed the formation of Zr,Zr-hydride clusters as key intermediates of the alkene dimerization. The probable structure of the Zr,Zr-hydride clusters and ways of their generation in the catalytic systems were analyzed using a quantum chemical approach (DFT).

Oxidative additions of alkynyl/vinyl iodides to gold and gold-catalyzed vinylation reactions triggered by the MeDalphos ligand

The hemilabile Ad2P(o-C6H4)NMe2 ligand promotes fast, quantitative and irreversible oxidative addition of alkynyl and vinyl iodides to gold. The reaction is general. It works with a broad range of substrates of various electronic bias and steric demand, and proceeds with complete retention of stereochemistry from Z and E vinyl iodides. Both alkynyl and vinyl iodides react faster than aryl iodides. The elementary step is amenable to catalysis. Oxidative addition of vinyl iodides to gold and π-activation of alkenols (and N-alkenyl amines) at gold have been combined to achieve hetero-vinylation reactions. A number of functionalized heterocycles, i.e. tetrahydrofuranes, tetrahydropyranes, oxepanes and pyrrolidines were obtained thereby (24 examples, 87% average yield). Taking advantage of the chemoselectivity for vinyl iodides over aryl iodides, sequential transformations involving first a hetero-vinylation step and then a C-N coupling, a C-C coupling or an heteroarylation were achieved from a vinyl/aryl bis-iodide substrate.

Synthesis of New Donor-Substituted Biphenyls: Pre-ligands for Highly Luminescent (C^C^D) Gold(III) Pincer Complexes

We herein report on new synthetic strategies for the preparation of pyridine and imidazole substituted 2,2'-dihalo biphenyls. These structures are pre-ligands suitable for the preparation of respective stannoles. The latter can successfully be transmetalated to K[AuCl4 ] forming non-palindromic [(C^C^D)AuIII ] pincer complexes featuring a lateral pyridine (D=N) or N-heterocyclic carbene (NHC, D=C') donor. The latter is the first report on a pincer complex with two formally anionic sp2 and one carbenic carbon donor. The [(C^C^D)AuIII ] complexes show intense phosphorescence in solution at room temperature. We discuss the developed multistep strategy and touch upon synthetic challenges. The prepared complexes have been fully characterized including X-ray diffraction analysis. The gold(III) complexes' photophysical properties have been investigated by absorption and emission spectroscopy as well as quantum chemical calculations on the quasi-relativistic two-component TD-DFT and GW/Bethe-Salpeter level including spin-orbit coupling. Thus, we shed light on the electronic influence of the non-palindromic pincer ligand and reveal non-radiative relaxation pathways of the different ligands employed.

Trifluoromethylation of [AuF3 (SIMes)]: Preparation and Characterization of [Au(CF3 )x F3-x (SIMes)] (x=1-3) Complexes

Trifluoromethylation of [AuF3 (SIMes)] with the Ruppert-Prakash reagent TMSCF3 in the presence of CsF yields the product series [Au(CF3 )x F3-x (SIMes)] (x=1-3). The degree of trifluoromethylation is solvent dependent and the ratio of the species can be controlled by varying the stoichiometry of the reaction, as evidenced from the 19 F NMR spectra of the corresponding reaction mixtures. The molecular structures in the solid state of trans-[Au(CF3 )F2 (SIMes)] and [Au(CF3 )3 (SIMes)] are presented, together with a selective route for the synthesis of the latter complex. Correlation of the calculated SIMes affinity with the carbene carbon chemical shift in the 13 C NMR spectrum reveals that trans-[Au(CF3 )F2 (SIMes)] and [Au(CF3 )3 (SIMes)] nicely follow the trend in Lewis acidities of related organo gold(III) complexes. Furthermore, a new correlation between the Au-Ccarbene bond length of the molecular structure in the solid state and the chemical shift of the carbene carbon in the 13 C NMR spectrum is presented.

Visible light driven generation and alkyne insertion reactions of stable bis-cyclometalated Pt(iv) hydrides

Hydride complexes resulting from the oxidative addition of C–H bonds are intermediates in hydrocarbon activation and functionalization reactions. The discovery of metal systems that enable their direct formation through photoexcitation with visible light could lead to advantageous synthetic methodologies. In this study, easily accessible dimers [Pt₂(μ-Cl)₂(C^N)₂] (C^N = cyclometalated 2-arylpyridine) are demonstrated as a very convenient source of Pt(C^N) subunits, which promote photooxidative C–H addition reactions with different 2-arylpyridines (N′^C′H) upon irradiation with blue light. The resulting [PtH(Cl)(C^N)(C′^N′)] complexes are the first isolable Pt(iv) hydrides arising from a cyclometalation reaction. A transcyclometalation process involving three photochemical steps is elucidated, which occurs when the C^N ligand is a monocyclometalated 2,6-diarylpyridine, and a detailed analysis of the photoreactivity associated with the Pt(C^N) moiety is provided. Alkyne insertions into the Pt–H bond of a photogenerated Pt(iv) hydride are also reported as a demonstration of the ability of this class of compounds to undergo subsequent organometallic reactions.

The photochemical generation of isolable bis-cyclometalated Pt(iv) hydrides via photooxidative C–H addition reactions is demonstrated from easily accessible Pt(ii) precursors using visible light.

Mechanistic Insights into C(sp2 )-C(sp)N Reductive Elimination from Gold(III) Cyanide Complexes

A new family of phosphine-ligated dicyanoarylgold(III) complexes has been prepared and their reactivity towards reductive elimination has been studied in detail. Both, a highly positive entropy of activation and a primary ^12/13 C KIE suggest a late concerted transition state while Hammett analysis and DFT calculations indicate that the process is asynchronous. As a result, a distinct mechanism involving an asynchronous concerted reductive elimination for the overall C(sp² )-C(sp)N bond forming reaction is characterized herein, for the first time, complementing previous studies reported for C(sp³ )-C(sp³ ), C(sp² )-C(sp² ), and C(sp³ )-C(sp² ) bond formation processes taking place on gold(III) species.

Site-Dependent Spin Delocalization and Evidence of Ferrimagnetism in Atomically Precise Au25(SR)180 Clusters as Seen by Solution 13C NMR Spectroscopy

We report a simple but detailed solution ¹³C nuclear magnetic resonance spectroscopic study of atomically precise neutral Au₂₅(SR)₁₈⁰ (SR = alkyl thiolate) clusters. The paramagnetic ¹³C Knight shift of alkyl chain carbons, which is proportional to the local electron spin density, exhibits an electron spin delocalization that exponentially decays along the alkyl chain. The magnitude and decay constant of the observed electron spin delocalization, although largely independent of alkyl chain length, depend on where, that is, "in" versus "out" (vide infra) position, the alkyl chain is bound, in agreement with density functional theory calculations. Notably, the determined position-dependent decay constants, 1.70/Å and 0.41/Å for "in" and "out" ligands, respectively, not only could have important ramifications in molecular spintronics but are also comparable to measured decay constants in molecular electrical conductance of alkyl chains, potentially offering an alternative, simple method for estimating the latter. Moreover, the negative intercept temperatures of linear fits of reciprocal ¹³C (as well its bound ¹H) Knight shift versus temperature strongly suggest the existence of local ferrimagnetism in individual Au₂₅(SR)₁₈⁰ clusters.

Gold Catalyzed Decarboxylative Cross-Coupling of Iodoarenes

This report details a decarboxylative cross-coupling of (hetero)aryl carboxylates with iodoarenes in the presence of a gold catalyst (>25 examples, up to 96% yield). This reaction is site specific, which overcomes prior limitations associated with gold catalyzed oxidative coupling reactions. The reactivity of the (hetero)aryl carboxylate correlates qualitatively to the field effect parameter (F_ortho). Reactions with isolated gold complexes and DFT calculations support a mechanism proceeding through oxidative addition at a gold(I) cation with decarboxylation being viable at either a gold(I) or a silver(I) species.

Oxidative Addition of Alkenyl and Alkynyl Iodides to a AuI Complex

The first isolated examples of intermolecular oxidative addition of alkenyl and alkynyl iodides to Au^I are reported. Using a 5,5'-difluoro-2,2'-bipyridyl ligated complex, oxidative addition of geometrically defined alkenyl iodides occurs readily, reversibly and stereospecifically to give alkenyl-Au^III complexes. Conversely, reversible alkynyl iodide oxidative addition generates bimetallic complexes containing both Au^III and Au^I centers. Stoichiometric studies show that both new initiation modes can form the basis for the development of C-C bond forming cross-couplings.

Synthetic access to a phosphorescent non-palindromic pincer complex of palladium by a double oxidative addition – comproportionation sequence

A highly phosphorescent non-palindromic (C^C^N) palladium complex may be prepared by means of a double oxidative addition – comproportionation sequence, which is a new approach for the synthesis of non-palindromic pincer complexes.

Trans-philicity (trans-influence/trans-effect) ladders for square planar platinum(II) complexes constructed by 35 Cl NMR probe

The unified term of trans-philicity is proposed to cover the trans-effect/trans-influence concepts. NMR trans-philicity ladders are built for a broad series of square planar trans-Pt(NH₃ )₂ (Cl)L and trans-Pt(CO)₂ (Cl)L complexes employing ³⁵ Cl NMR probe and quantified by calculation of NMR trans-philicity indicators. The trans-philicity is linearly correlated with the ligand electronic P_L constant, a measure of the net donor power of the ligand. The nature of cis-ligands does not affect trans-philicity ladders but strongly affects trans-philicity strength. Solvent has significant effect on the σ_calcd ³⁵ Cl shielding constants, with the polar Dimethylformamide (DMF) solvent inducing downfield shifts relative to σ_calcd ³⁵ Cl with nonpolar benzene solvent. Good correlations between σ_calcd ³⁵ Cl shielding constants and the estimated R(Pt-Cl) bond distances demonstrate the relation of trans-philicity with trans-influence and trans-effect phenomena and put the grounds for the establishment of the new concept of trans-philicity in the realm of square planar Pt(II) and other transition metal complexes. © 2019 Wiley Periodicals, Inc.

Quantum chemical calculations of 31P NMR chemical shifts of P-donor ligands in platinum(II) complexes

This work aims to find the most suitable method that is practically applicable for the calculation of ³¹P NMR chemical shifts of Pt(II) complexes. The influence of various all-electron and ECP basis sets, DFT functionals, and solvent effects on the optimized geometry was tested. A variety of combinations of DFT functionals BP86, B3LYP, PBE0, TPSSh, CAM-B3LYP, and ωB97XD with all-electron basis sets 6-31G, 6-31G(d), 6-31G(d,p), 6-311G(d,p), and TZVP and ECP basis sets SDD, LanL2DZ, and CEP-31G were used. Chemical shielding constants were then calculated using BP86, PBE0, and B3LYP functionals in combination with the TZ2P basis. The magnitude of spin-orbit interactions was also evaluated.

13C NMR Shifts as an Indicator of U-C Bond Covalency in Uranium(VI) Acetylide Complexes: An Experimental and Computational Study

A series of uranium(VI)-acetylide complexes of the general formula U^VI(O)(C≡C-C₆H₄-R)[N(SiMe₃)₂]₃, with variation of the para substituent (R = NMe₂, OMe, Me, Ph, H, Cl) on the aryl(acetylide) ring, was prepared. These compounds were analyzed by ¹³C NMR spectroscopy, which showed that the acetylide carbon bound to the uranium(VI) center, U- C≡C-Ar, was shifted strongly downfield, with δ(¹³C) values ranging from 392.1 to 409.7 ppm for Cl and NMe₂ substituted complexes, respectively. These extreme high-frequency ¹³C resonances are attributed to large negative paramagnetic (σ^para) and relativistic spin-orbit (σ^SO) shielding contributions, associated with extensive U(5f) and C(2s) orbital contributions to the U-C bonding in title complexes. The trend in the ¹³C chemical shift of the terminal acetylide carbon is opposite that observed in the series of parent (aryl)acetylenes, due to shielding effects of the para substituent. The ¹³C chemical shifts of the acetylide carbon instead correlate with DFT computed U-C bond lengths and corresponding QTAIM delocalization indices or Wiberg bond orders. SQUID magnetic susceptibility measurements were indicative of the Van Vleck temperature independent paramagnetism (TIP) of the uranium(VI) complexes, suggesting a magnetic field-induced mixing of the singlet ground-state (f⁰) of the U(VI) ion with low-lying (thermally inaccessible) paramagnetic excited states (involved also in the perturbation-theoretical treatment of the unusually large paramagnetic and SO contributions to the ¹³C shifts). Thus, together with reported data, we demonstrate that the sensitive ¹³C NMR shifts serve as a direct, simple, and accessible measure of uranium(VI)-carbon bond covalency.

Heterolytic bond activation at gold: evidence for gold(iii) H-B, H-Si complexes, H-H and H-C cleavage

The 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-(C6H3Bu t )-6-(C6H4Bu t )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.

Evidence for genuine hydrogen bonding in gold(I) complexes

The ability of gold to act as proton acceptor and participate in hydrogen bonding remains an open question. Here, we report the synthesis and characterization of cationic gold(I) complexes featuring ditopic phosphine-ammonium (P,NH+) ligands. In addition to the presence of short Au∙∙∙H contacts in the solid state, the presence of Au∙∙∙H-N hydrogen bonds was inferred by NMR and IR spectroscopies. The bonding situation was extensively analyzed computationally. All features were consistent with the presence of three-center four-electron attractive interactions combining electrostatic and orbital components. The role of relativistic effects was examined, and the analysis is extended to other recently described gold(I) complexes.

Isocyanide insertion into Au–H bonds: first gold iminoformyl complexes

The reaction of gold hydrides with isocyanides leads to η¹-iminoformyl complexes, the first example of an isocyanide insertion in gold chemistry. Key intermediates are gold(ii) isocyanide adducts.

Tuning the Lewis acidity of difluorido gold(iii) complexes: the synthesis of [AuClF2(SIMes)] and [AuF2(OTeF5)(SIMes)]

A route to [Au^IIIClF₂(SIMes)] and [Au^IIIF₂(OTeF₅)(SIMes)] including structural characterization to tune the ligand arrangement and Lewis acidity of gold(iii) centres was developed.