Halogen Photoreductive Elimination from Gold(III) Centers

Redox-Transmetalation Reactions: Easy Access to Homo- and Heterodimetallic d8,d10 Complexes

The ability of the imine PyCH═N-CH2Py (Py = 2-pyridyl, bpi) to behave as a heteroditopic ligand, which is suitable for creating two separate compartments to host metals in different oxidation states, has been developed by studying the reactions of the mixed-valence complexes [(cod)M-Ι(μ-bpi)MΙ(cod)] (M = Rh, Ir) with [M'(Cl)2(PPh3)2] (M' = Pd, Ni). The results depend on the molar ratio of the reagents used (1:1 or 1:2) to give the heterometallic complexes {d10-M',d8-M}-[(PPh3)(Cl)M'0(μ-bpi)MΙ(cod)] (Pd,Rh, 4; Pd,Ir, 5; Ni,Rh, 8; Ni,Ir, 9) and the two-electron mixed-valent compounds [(PPh3)(Cl)M'0(μ-bpi)M'ΙΙ(Cl)] (M' = Ni, 10; Pd, 11), respectively. A redox process occurs in the replacement of the low-valent [(cod)M-I] fragment, whereas the exchange of the [(cod)MI] fragment is redox-neutral. The metal with a d8 configuration in the products exhibits a square-planar geometry coordinated to two (Rh/Ir) or three (Ni/Pd) nitrogen atoms of the bridging bpi ligand. Conversely, the metal with a d10 configuration adopts trigonal-planar geometries, π-bonded to the imine C═N bond. The isolated complexes 4/5 and 10/11, along with the hypothetical heterometallic Pd,Ni compound (12), were studied by DFT methods. Additionally, the T-shaped moiety 'M'ΙΙ(PPh3)(Cl)(η1-CH-N(bpi))', stabilized by a secondary γ-agostic interaction, and the 'M'II(Cl)(κ3N-bpi)' fragment was found to be accessible redoxomers of complexes 10 and 11 by DFT calculations.

Dinuclear Gold-Catalyzed para-Selective C-H Arylation of Undirected Arenes by Noncovalent Interactions

The regioselectivity of C-H functionalization is commonly achieved by directing groups, electronic factors, or steric hindrance, which facilitate the identification of reaction sites. However, such strategies are less effective for reactants such as simple monofluoroarenes due to their relatively low reactivity and the modest steric demands of the fluorine atom. Herein, we present an undirected gold-catalyzed para-C-H arylation of a wide array of monofluoroarenes using air-stable aryl silanes and germanes at room temperature. A high para-regioselectivity (up to 98 : 2) can be realized by utilizing a dinuclear dppm(AuOTs)2 (dppm=bis(diphenylphosphino)methane) as the catalyst and hexafluorobenzene as the solvent. This provides a general and practical protocol for the concise construction of structurally diverse para-arylated monofluoroarenes through C-H activation manner. It features excellent functional group tolerance and a broad substrate scope (>80 examples). Besides, this strategy is also robust for other simple monosubstituted arenes and heteroarenes. Our mechanistic studies and theoretical calculations suggest that para-C-H selectivity arises from highly electrophilic and structurally flexible dinuclear Ar-Au(III)-Au(I) species, coupled with noncovalent interaction induced by hexafluorobenzene.

Trinuclear and Cyclometallated Organometallic Dinuclear Pt-Pyrazolato Complexes: A Combined Experimental and Theoretical Study

Two differently substituted pyrazole ligands have been investigated with regard to the topology of their Pt complexes: upon deprotonation, two mononuclear 1:2 PtII-pyrazole complexes—one of the sterically unhindered 4-Me-pzH and one of the bulky 3,5-tBu-pzH (pzH = pyrazole)—yield the corresponding 1:2 PtII-pyrazolato species; the former a triangular, trinuclear metallacycle (1), and the latter a dinuclear, half-lantern species (2) formed via the unprecedented cyclometallation of a butyl group. Stoichiometric oxidation of the colorless PtII2 complex produces the deep-blue, metal–metal bonded PtIII2 analog (3) with a rarely encountered unsymmetrical coordination across the Pt-Pt bond. All three complexes have been characterized by single crystal X-ray structure determination, 1H-NMR, IR, and UV-vis-NIR spectroscopic methods. The XPS spectra of the PtII2 and PtIII2 species are also reported. Density functional theory calculations were carried out to investigate the electronic structure, spectroscopic properties, and chemical bonding of the new complexes. The calculated natural population analysis charges and Wiberg bonding indices indicate a weak σ-interaction in the case of 2 and a formal Pt-Pt single bond in 3.

Ir(IV) Sulfoxide-Pincer Complexes by Three-Electron Oxidative Additions of Br2 and I2. Unprecedented Trap-Free Reductive Elimination of I2 from a formal d5 Metal

Oxidative addition of 1.5 equiv of bromine or iodine to a Ir(I) sulfoxide pincer complex affords the corresponding Ir(IV) tris-bromido or tris-iodido complexes, respectively. The unprecedented trap-free reductive elimination of iodine from the Ir(IV)-iodido complex is induced by coordination of ligands or donor solvents. In the case of added I^-, the isostructural tris-iodo Ir(III)-ate complex is quickly generated, which then can be readily reoxidized to the Ir(IV)-iodido complex with FcPF₆ or electrochemically. DFT calculations indicate an "inverted ligand field" in the Ir(IV) complexes and favor dinuclear pathways for the reductive elimination of iodine from the formal d⁵ metal center.

Ligand-enabled oxidation of gold(i) complexes with o-quinones

Chelating P^P and hemilabile P^N ligands were found to trigger the oxidation of Au(i) complexes by o-benzoquinones. The ensuing Au(iii) catecholate complexes have been characterized by NMR spectroscopy, single crystal X-ray diffraction and X-ray absorption spectroscopy. They adopt tetracoordinate square-planar structures. Reactivity studies substantiate the reversibility of the transformation. In particular, the addition of competing ligands such as chloride and alkenes gives back Au(i) complexes with concomitant release of the o-quinone. DFT calculations provide insight about the structure and relative stability of the Au(i) o-quinone and Au(iii) catecholate forms, and shed light on the 2-electron transfer from gold to the o-quinone.

Chelating P^P and hemilabile P^N ligands were found to trigger the oxidation of Au(i) complexes by o-benzoquinones.

Photodriven Elimination of Chlorine From Germanium and Platinum in a Dinuclear PtII →GeIV Complex

Searching for a connection between the two-electron redox behavior of Group-14 elements and their possible use as platforms for the photoreductive elimination of chlorine, we have studied the photochemistry of [(o-(Ph₂ P)C₆ H₄ )₂ Ge^IV Cl₂ ]Pt^II Cl₂ and [(o-(Ph₂ P)C₆ H₄ )₂ ClGe^III ]Pt^III Cl₃ , two newly isolated isomeric complexes. These studies show that, in the presence of a chlorine trap, both isomers convert cleanly into the platinum germyl complex [(o-(Ph₂ P)C₆ H₄ )₂ ClGe^III ]Pt^I Cl with quantum yields of 1.7 % and 3.2 % for the Ge^IV -Pt^II and Ge^III -Pt^III isomers, respectively. Conversion of the Ge^IV -Pt^II isomer into the platinum germyl complex is a rare example of a light-induced transition-metal/main-group-element bond-forming process. Finally, transient-absorption-spectroscopy studies carried out on the Ge^III -Pt^III isomer point to a ligand arene-Cl^. charge-transfer complex as an intermediate.

Photoreductive chlorine elimination from a Ni(iii)Cl2 complex supported by a tetradentate pyridinophane ligand

Herein we report the isolation, characterization, and photoreactivity of a stable Ni^III dichloride complex supported by a tetradentate pyridinophane N-donor ligand. Upon irradiation, this complex undergoes an efficient photoreductive chlorine elimination reaction, both in solution and the solid-state. Subsequently, the Ni^IIICl₂ species can be regenerated via a reaction with PhICl₂.

Halide Photoredox Chemistry

Halide photoredox chemistry is of both practical and fundamental interest. Practical applications have largely focused on solar energy conversion with hydrogen gas, through HX splitting, and electrical power generation, in regenerative photoelectrochemical and photovoltaic cells. On a more fundamental level, halide photoredox chemistry provides a unique means to generate and characterize one electron transfer chemistry that is intimately coupled with X-X bond-breaking and -forming reactivity. This review aims to deliver a background on the solution chemistry of I, Br, and Cl that enables readers to understand and utilize the most recent advances in halide photoredox chemistry research. These include reactions initiated through outer-sphere, halide-to-metal, and metal-to-ligand charge-transfer excited states. Kosower's salt, 1-methylpyridinium iodide, provides an early outer-sphere charge-transfer excited state that reports on solvent polarity. A plethora of new inner-sphere complexes based on transition and main group metal halide complexes that show promise for HX splitting are described. Long-lived charge-transfer excited states that undergo redox reactions with one or more halogen species are detailed. The review concludes with some key goals for future research that promise to direct the field of halide photoredox chemistry to even greater heights.

Optimization of Photocatalyst Excited- and Ground-State Reduction Potentials for Dye-Sensitized HBr Splitting

Dye-sensitized bromide oxidation was investigated using a series of four ruthenium polypyridyl photocatalysts anchored to SnO₂/TiO₂ core/shell mesoporous thin films through 2,2'-bipyridine-4,4'-diphosphonic acid anchoring groups. The ground- and excited-state reduction potentials were tuned over 500 mV by the introduction of electron withdrawing groups in the 4 and 4' positions of the ancillary bipyridine ligands. Upon light excitation of the surface-bound photocatalysts, excited-state electron injection yielded an oxidized photocatalyst that was regenerated through bromide oxidation. High injection quantum yields (Φ_inj) and regeneration quantum yields (Φ_reg) were essential to obtain efficient bromide oxidation yet required a photocatalyst that is both a potent photoreductant and a strong oxidant after excited-state injection. The four photocatalysts utilized in this manuscript ranged from unity Φ_inj (1.0) and minimal Φ_reg (0.037) to minimal Φ_inj (0.09) and unity Φ_reg (1.0). The photocatalyst that displayed the highest overall dye-sensitized photoelectrosynthesis cell performances exhibited near unity Φ_reg (0.99), while a significant Φ_inj was still preserved (0.59). Thus, these results highlighted the delicate interplay between the ground- and excited-state reduction potentials of photocatalysts for dye-sensitized hydrobromic acid splitting.

Synthesis and characterization of a gold(i) bis(triazolylidene) complex featuring a large [(TpMe2)2K] anion

We report the synthesis, charaterization and catalytic performance of a unique bis(triazolylidene) gold(i) complex featuring a large [(Tp^Me2)₂K] anion.

A renaissance of ligand-to-metal charge transfer by cerium photocatalysis

Diverse alcohols, serving as substrates or co-catalysts, and Ce(iv) achieve photoinduced LMCT to generate highly active alkoxy radicals, which opens a new avenue for many difficult HAT reactions.

Photochemical Water-Splitting with Organomanganese Complexes

Certain organometallic chromophores with water-derived ligands, such as the known [Mn(CO)₃(μ₃-OH)]₄ (1) tetramer, drew our attention as possible platforms to study water-splitting reactions. Herein, we investigate the UV irradiation of various tricarbonyl organomanganese complexes, including 1, and demonstrate that dihydrogen, CO, and hydrogen peroxide form as products in a photochemical water-splitting decomposition reaction. The organic and manganese-containing side products are also characterized. Labeling studies with ¹⁸O-1 suggest that the source of oxygen atoms in H₂O₂ originates from free water that interacts with 1 after photochemical dissociation of CO (1-CO) constituting the oxidative half-reaction of water splitting mediated by 1. Hydrogen production from 1 is the result of several different processes, one of which involves the protons derived from the hydroxido ligands in 1 constituting the reductive half-reaction of water splitting mediated by 1. Other processes that generate H₂ are also operative and are described. Collectively the results from the photochemical decomposition of 1 provide an opportunity to propose a mechanism, and it is discussed within the context of developing new strategies for water-splitting reactions with organomanganese complexes.

Stereoelectronic Effects in Cl2 Elimination from Binuclear Pt(III) Complexes

Halogen photoelimination is the critical energy-storing step of metal-catalyzed HX-splitting photocycles. Homo- and heterobimetallic Pt(III) complexes display among the highest quantum efficiencies for halogen elimination reactions. Herein, we examine in detail the mechanism and energetics of halogen elimination from a family of binuclear Pt(III) complexes featuring meridionally coordinated Pt(III) trichlorides. Transient absorption spectroscopy, steady-state photocrystallography, and far-infrared vibrational spectroscopy suggest a halogen elimination mechanism that proceeds via two sequential halogen-atom-extrusion steps. Solution-phase calorimetry experiments of the meridional complexes have defined the thermodynamics of halogen elimination, which show a decrease in the photoelimination quantum efficiency with an increase in the thermochemically defined Pt-X bond strength. Conversely, when compared to an isomeric facial Pt(III) trichloride, a much more efficient photoelimination is observed for the fac isomer than would be predicted based on thermochemistry. This difference in the fac vs mer isomer photochemistry highlights the importance of stereochemistry on halogen elimination efficiency and points to a mechanism-based strategy for achieving halogen elimination reactions that are both efficient and energy storing.

Marriage of heavy main group elements with π-conjugated materials for optoelectronic applications

This review article summarizes recent progress in the synthesis and optoelectronic properties of conjugated materials containing heavy main group elements from Group 13-16 as integral components. As will be discussed, the introduction of these elements can promote novel phosphorescent behavior and support desirable molecular and polymeric properties such as low optical band gaps and high charge mobilities for photovoltaic and thin film transistor applications.

A Mechanistic Study of Halogen Addition and Photoelimination from π-Conjugated Tellurophenes

The ability to drive reactivity using visible light is of importance for many disciplines of chemistry and has significant implications for sustainable chemistry. Identifying photochemically active compounds and understanding photochemical mechanisms is important for the development of useful materials for synthesis and catalysis. Here we report a series of photoactive diphenyltellurophene compounds bearing electron-withdrawing and electron-donating substituents synthesized by alkyne coupling/ring closing or palladium-catalyzed ipso-arylation chemistry. The redox chemistry of these compounds was studied with respect to oxidative addition and photoelimination of bromine, which is of importance for energy storage reactions involving X2. The oxidative addition reaction mechanism was studied using density functional theory, the results of which support a three-step mechanism involving the formation of an initial η(1) association complex, a monobrominated intermediate, and finally the dibrominated product. All of the tellurophene derivatives undergo photoreduction using 430, 447, or 617 nm light depending on the absorption properties of the compound. Compounds bearing electron-withdrawing substituents have the highest photochemical quantum efficiencies in the presence of an alkene trap, with efficiencies of up to 42.4% for a pentafluorophenyl-functionalized tellurophene. The photoelimination reaction was studied in detail through bromine trapping experiments and laser flash photolysis, and a mechanism is proposed. The photoreaction, which occurs by release of bromine radicals, is competitive with intersystem crossing to the triplet state of the brominated species, as evidenced by the formation of singlet oxygen. These findings should be useful for the design of new photochemically active compounds supported by main-group elements.

Phosphanchalkogenide und ihre Metallkomplexe. IV. Halogenierungsprodukte der Gold(I)halogenidkomplexe einiger Diphosphanmonochalkogenide

Diphosphanegold(I) complexes of the form dppmEAuX [dppm = bis(diphenylphosphano)methane, E = S, Se; X = Br, I], dppeEAuX [dppe = 1,2-bis(diphenylphosphano)ethane; E = O, S; X = Br, I] and dppbzEAuX [dppbz = 1,2-bis(diphenylphosphano)benzene; E = S, Se, X = Br, I] were treated with elemental X2. With dppm, the three products [dppmEAuX2]+X3 – (E = S, X = Br (1), I (2); E = Se, X = I (3) were obtained in quantitative yield. These are gold(III) complexes involving a five-membered ring . With dppe, the only related product was [dppeEAuBr2]+Br3 – (4), in which the central ring is six-membered with two carbon atoms. These dppe systems are very sensitive to oxidation/hydrolysis of the ligand, and several such unintended products were isolated and identified. The reaction of dppbzSAuBr with bromine leads to [dppbzS]2+[AuBr4]–Br– (5), the dication of which is formally 1,1,3,3-tetraphenylbenzo[d]-2-thia-1, 3-diphosphol-1,3-diium and contains a central five-membered ring . The dications are associated with the bromide anions via S…Br contacts of ca. 3.1 Å to form inversion-symmetric S2Br2 rings. The halogenation of the dppbzSe derivatives leads to loss of selenium and formation of dppbzAuBr3 (6), with [4+1] coordination at gold, or the known compound [dppbzAuI2]+I3 – (7). All products 1–6 were subjected to X-ray diffraction analyses, as were four hydrolysis products 4a–d and two further by-products [5(thtBr+)·2Br3 –·3(AuBr4 –)] (1a) and (tht)AuBr3 (1b). Compound 1a displays unusually short Br…Br contacts of 3.2398(8) Å between neighbouring tetrabromidoaurate(III) ions.

Spotlight on the ligand: luminescent cyclometalated Pt(iv) complexes containing a fluorenyl moiety

A family of Pt(iv) complexes with 2-(9,9-dimethylfluoren-2-yl)pyridine is reported, including bis- and tris-cyclometalated derivatives, which exhibit intense ³LC phosphorescent emissions in fluid solution.

Phosphanchalkogenide und ihre Metallkomplexe. III. Halogenierungsprodukte der Gold(I)komplexe Ph3PEAuX (E = S oder Se; X = Cl, Br oder I)

The complexes Ph3PEAuI (E = S, Se; 1, 2) were obtained from the reaction of Ph3PEAuCl with KI; they are appreciably less stable than their chloro and bromo analogues. The X-ray structures were determined, whereby 1 proved to be contaminated by a small amount of Ph3PS·I2. Oxidation of 1 and 2 with elemental iodine led to the adducts Ph3PEAuI·0.5I2 (3 and 4), but X-ray investigation of a crystal initially assumed to be 3 proved it to be a 1:1 mixture of 3 with the adduct Ph3PS·1.5I2, while in 4 the iodine molecule was severely disordered, which prevented successful refinement of the structure. Decomposition of 4 by loss of gold led to Ph3PSeI2·1.5I2 4a. Complexes Ph3PEAuX (E = S, Se; X = Br, Cl) were oxidized by elemental bromine (X = Br) or PhICl2 (X = Cl) to Ph3PEAuX 3 (5, 6, 9, 10); none of these X-ray structures could be refined satisfactorily because of diffuse scattering phenomena. Further oxidation led to the ionic compounds [Ph3PEX]+ [AuX 4]– (X = Br, E = S, Se: 7, 8; X = Cl, E = S, 11) or [Ph3PSeCl]+ 0.5[Au4Cl10Se2]2– (12), containing the novel groupings P–E–X. X-ray structures confirmed the nature of all four of these compounds, which display longer P–E bonds than the gold(I) starting materials and short X···X and/or E···X contacts between cations and anions.

Gold(I) and Gold(III) Complexes of Cyclic (Alkyl)(amino)carbenes

The chemistry of Au(I) complexes with two types of cyclic (alkyl)(amino)carbene (CAAC) ligands has been explored, using the sterically less demanding dimethyl derivative Me2CAAC and the 2-adamantyl ligand AdCAAC. The conversion of (AdCAAC)AuCl into (AdCAAC)AuOH by treatment with KOH is significantly accelerated by the addition of tBuOH. (AdCAAC)AuOH is a convenient starting material for the high-yield syntheses of (AdCAAC)AuX complexes by acid/base and C-H activation reactions (X = OAryl, CF3CO2, N(Tf)2, C2Ph, C6F5, C6HF4, C6H2F3, CH2C(O)C6H4OMe, CH(Ph)C(O)Ph, CH2SO2Ph), while the cationic complexes [(AdCAAC)AuL]+ (L = CO, CN t Bu) and (AdCAAC)AuCN were obtained by chloride substitution from (AdCAAC)AuCl. The reactivity toward variously substituted fluoroarenes suggests that (AdCAAC)AuOH is able to react with C-H bonds with pKa values lower than about 31.5. This, together with the spectroscopic data, confirm the somewhat stronger electron-donor properties of CAAC ligands in comparison to imidazolylidene-type N-heterocyclic carbenes (NHCs). In spite of this, the oxidation of Me2CAAC and AdCAAC gold compounds is much less facile. Oxidations proceed with C-Au cleavage by halogens unless light is strictly excluded. The oxidation of (AdCAAC)AuCl with PhICl2 in the dark gives near-quantitative yields of (AdCAAC)AuCl3, while [Au(Me2CAAC)2]Cl leads to trans-[AuCl2(Me2CAAC)2]Cl. In contrast to the chemistry of imidazolylidene-type gold NHC complexes, oxidation products containing Au-Br or Au-I bonds could not be obtained; whereas the reaction with CsBr3 cleaves the Au-C bond to give mixtures of [AdCAAC-Br]+[AuBr2]- and [(AdCAAC-Br)]+ [AuBr4]-, the oxidation of (AdCAAC)AuI with I2 leads to the adduct (AdCAAC)AuI·I2. Irrespective of the steric demands of the CAAC ligands, their gold complexes proved more resistant to oxidation and more prone to halogen cleavage of the Au-C bonds than gold(I) complexes of imidazole-based NHC ligands.

Tandem redox mediator/Ni(ii) trihalide complex photocycle for hydrogen evolution from HCl

Photoactivation of M-X bonds is a challenge for photochemical HX splitting, particularly with first-row transition metal complexes because of short intrinsic excited state lifetimes. Herein, we report a tandem H2 photocycle based on combination of a non-basic photoredox phosphine mediator and nickel metal catalyst. Synthetic studies and time-resolved photochemical studies have revealed that phosphines serve as photochemical H-atom donors to Ni(ii) trihalide complexes to deliver a Ni(i) centre. The H2 evolution catalytic cycle is closed by sequential disproportionation of Ni(i) to afford Ni(0) and Ni(ii) and protolytic H2 evolution from the Ni(0) intermediate. The results of these investigations suggest that H2 photogeneration proceeds by two sequential catalytic cycles: a photoredox cycle catalyzed by phosphines and an H2-evolution cycle catalyzed by Ni complexes to circumvent challenges of photochemistry with first-row transition metal complexes.