Palladium-Catalyzed C−H Bond Arylation of Cyclometalated Difluorinated 2-Arylisoquinolinyl Iridium(III) Complexes

The utility of C−H bond functionalization of metalated ligands for the elaboration of aryl-functionalized difluorinated-1-arylisoquinolinyl Ir(III) complexes has been explored. Bis[(3,5-difluorophenyl)isoquinolinyl](2,2,6,6-tetramethyl-3,5-heptanedionato) iridium(III) undergoes Pd-catalyzed C−H bond arylation with aryl bromides. The reaction regioselectively occurred at the C−H bond flanked by the two fluorine atoms of the difluoroaryl unit, and on both cyclometalated ligands. This post-functionalization gives a straightforward access to modified complexes in only one manipulation and allows to introduce thermally sensitive functional groups, such as trifluoromethyl, nitrile, benzoyl, or ester. The X-ray crystallography, photophysical, and electrochemical properties of the diarylated complexes were investigated. Whatever the nature of the incorporated substituted aryl groups is, all obtained complexes emit red phosphorescence (622–632 nm) with similar lifetimes (1.9–2 μs).     

Regioselective B—H/C—H Bond Activation at Azo-Substituted Carboranes Induced by Half-Sandwich Iridium(III)Complex

We report herein a practical approach for regio-selective B—H/C—H bond activation at azo-substituted carboranes. The reaction proceeded through Iridium(III)-induced selective B—H/C—H bond activations based on an azo directing group. Through this strategy, a series of mononuclear, trinuclear and tetranuclear cyclometalated iridium complexes containing Cp*Ir—B or Cp*Ir—C bonds were successfully isolated in a high yield. In this work, efficient routes are developed through one-pot reactions to prepare polynuclear organometallic complexes.     

Rhodium Complexes in P−H Bond Activation Reactions

The feasibility of oxidative addition of the P−H bond of PHPh₂ to a series of rhodium complexes to give mononuclear hydrido-phosphanido complexes has been analyzed. Three main scenarios have been found depending on the nature of the L ligand added to [Rh(Tp)(C₂H₄)(PHPh₂)] (Tp= hydridotris(pyrazolyl)borate): i) clean and quantitative reactions to terminal hydrido-phosphanido complexes [RhTp(H)(PPh₂)(L)] (L=PMe₃, PMe₂Ph and PHPh₂), ii) equilibria between Rh^I and Rh^III species: [RhTp(H)(PPh₂)(L)]⇄[RhTp(PHPh₂)(L)] (L=PMePh₂, PPh₃) and iii) a simple ethylene replacement to give the rhodium(I) complexes [Rh(κ²-Tp)(L)(PHPh₂)] (L=NHCs-type ligands). The position of the P−H oxidative addition–reductive elimination equilibrium is mainly determined by sterics influencing the entropy contribution of the reaction. When ethylene was used as a ligand, the unique rhodaphosphacyclobutane complex [Rh(Tp)(η¹-Et)(κ^C,P-CH₂CH₂PPh₂)] was obtained. DFT calculations revealed that the reaction proceeds through the rate limiting oxidative addition of the P−H bond, followed by a low-barrier sequence of reaction steps involving ethylene insertion into the Rh−H and Rh−P bonds. In addition, oxidative addition of the P−H bond in OPHPh₂ to [Rh(Tp)(C₂H₄)(PHPh₂)] gave the related hydride complex [RhTp(H)(PHPh₂)(POPh₂)], but ethyl complexes resulted from hydride insertion into the Rh−ethylene bond in the reaction with [Rh(Tp)(C₂H₄)₂].     

Pd-Catalyzed Directed Thiocyanation Reaction by C−H Bond Activation

The Pd-catalyzed directed thiocyanation reaction of arenes and heteroarenes by C−H bond activation was achieved. In the presence of an electrophilic SCN source, this original methodology offered an efficient tool to access a panel of functionalized thiocyanated compounds (21 examples, up to 78 % yield). Post-functionalization reactions further demonstrated the synthetic utility of the approach by converting the SCN-containing molecules into value-added scaffolds.     

Homolytic C−H Bond Activation by Phosphine−Quinone-Based Radical Ion Pairs

Herein, we present the formation of transient radical ion pairs (RIPs) by single-electron transfer (SET) in phosphine−quinone systems and explore their potential for the activation of C−H bonds. PMes₃ (Mes=2,4,6-Me₃C₆H₂) reacts with DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone) with formation of the P−O bonded zwitterionic adduct Mes₃P−DDQ ( 1 ), while the reaction with the sterically more crowded PTip₃ (Tip=2,4,6-iPr₃C₆H₂) afforded C−H bond activation product Tip₂P(H)(2-[CMe₂(DDQ)]-4,6-iPr₂-C₆H₂) ( 2 ). UV/Vis and EPR spectroscopic studies showed that the latter reaction proceeds via initial SET, forming RIP [PTip₃]⋅⁺[DDQ]⋅⁻, and subsequent homolytic C−H bond activation, which was supported by DFT calculations. The isolation of analogous products, Tip₂P(H)(2-[CMe₂{TCQ−B(C₆F₅)₃}]-4,6-iPr₂-C₆H₂) ( 4 , TCQ=tetrachloro-1,4-benzoquinone) and Tip₂P(H)(2-[CMe₂{oQ^tBu−B(C₆F₅)₃}]-4,6-iPr₂-C₆H₂) ( 8 , oQ^tBu=3,5-di-tert-butyl-1,2-benzoquinone), from reactions of PTip₃ with Lewis-acid activated quinones, TCQ−B(C₆F₅)₃ and oQ^tBu−B(C₆F₅)₃, respectively, further supports the proposed radical mechanism. As such, this study presents key mechanistic insights into the homolytic C−H bond activation by the synergistic action of radical ion pairs.     

Intermolecular C?H Bond Activation by a Cationic Iridium(III) Dichloride Phenanthroline Complex

It is demonstrated that a cationic iridium(III) dichloride phenanthroline complex is capable of C H activation and H/D exchange. It can cleave benzylic and unactivated secondary C H bonds, but exhibits unique selectivity when compared to similar systems that have been studied in the condensed phase. Gas‐phase rate constants and kinetic isotope effects are reported for a variety of substrates and the analysis is supported by DFT calculations at the M06/QZVP level.     

Oxidative Coordination versus C3−C(O)Me Bond Cleavage in Acetylacetonate Iridium Complexes

Iridabicycles [Ir{κ³-N,C,O-(pyC(H)=C(C(O)Me)₂}(Cl)(L−L)](L−L=cod (cod=1,5-cyclooctadiene), 1 a ; bipy (bipy=2,2’-bipyridine), 1 b ) have been obtained by oxidative coordination of 3-(pyridine-2-yl-methylene)pentane-2,4-dione L1 , to the complexes [{Ir(μ-Cl)(cod)}₂] and [{Ir(μ-Cl)(coe)₂}₂] (coe=cis-cyclooctene), the latter in the presence of bipy. Remarkably, cleavage of the C³−C(O)Me bond of L1 has instead been achieved in the reaction with [Ir(Cl)(dmb)₂] (dmb=2,3-dimethylbutadiene), yielding a compound formulated as [Ir{κ²-N,C-(pyC(H)C(C(O)Me))}(CO)(μ-Cl)(Me)]₂, 2 . Treatment of dimer 2 with DMSO or PMe₃ produced the complexes[Ir{κ²-N,C-(pyC(H)C(C(O)Me)}(CO)Cl(Me)L] (L=DMSO, 3 a ; PMe₃, 3 b ). Plausible mechanisms for the reactions leading to complexes 1 and 2 are proposed by means of DFT calculations.     

C−F Bond Activation Mediated by Phosphorus Compounds

The activation and functionalization of C−F bonds has garnered significant attention in the scientific community as a strategy to mitigate toxicity and environmental concerns, as well as provide new pathways to agro- and pharmaceutical chemicals and materials. Although several transition-metal-based systems have been developed for this transformation, the use of main-group compounds remains less explored. In recent years, several strategies for C−F bond activation have focused on the use of phosphorus-based reagents. In this Minireview, an overview of strategies is provided that exploits P^V and P^III-based Lewis acids as well as P^III Lewis bases in frustrated Lewis pair (FLP) protocols for hydrodefluorination, C−C couplings and C−F derivatizations.     

Cyclopentadienyl Complexes of IrIII for Attempted C−D Bond Activation

The complexes Cp(MeIm)IrI₂ and Cp^Me4(MeIm)IrCl₂ have been prepared and subsequently methylated to form Cp(MeIm)IrMe₂ and Cp^Me4(MeIm)IrMe₂ (Cp=η⁵-C₅H₅, Cp^Me4=η⁵-C₅HMe₄, MeIm=1,3-dimethylimidazol-2-ylidene). We attempted unsuccessfully to use the dimethyl complexes to study C−D bond activation via methyl-group abstraction. Protonation with one equivalent of a weak acid, such as 2,6-dimethylpyridinium chloride, affords methane and Ir^III methyl chloride complexes. ¹H-NMR experiments show addition of pyridinium [BArF₂₀]⁻ (BArF₂₀=[B(C₆F₅)₄]⁻) to the dimethyl species forms [Cp(MeIm)IrMe(py)]⁺[BArF₂₀]⁻ (py=pyridine) or [Cp^Me4(MeIm)IrMe(py)]⁺[BArF₂₀]⁻ respectively, alongside methane, while use of the [BArF₂₀]⁻ salts of more bulky 2,6-dimethylpyridinium and 2,6-di-tert-butylpyridinium gave an intractable mixture. Likewise, the generation of 16 e⁻ species [Cp^Me4(MeIm)IrMe]⁺[BArF₂₀]⁻ or [Cp(MeIm)IrMe]⁺[BarF₂₀]⁻ at low temperature using 2,6-dimethylpyridinium or 2,6-di-tert-butylpyridinium in thawing C₆D₆ or toluene-d₈ formed an intractable mixture and did not lead to C−D bond activation. X-ray structures of several Ir^III complexes show similar sterics as that found for the previously reported Cp* analogue.     

Facile and Reversible Formation of Iron(III)–Oxo–Cerium(IV) Adducts from Nonheme Oxoiron(IV) Complexes and Cerium(III)

Ceric ammonium nitrate (CAN) or Ce^IV(NH₄)₂(NO₃)₆ is often used in artificial water oxidation and generally considered to be an outer-sphere oxidant. Herein we report the spectroscopic and crystallographic characterization of [(N4Py)Fe^III-O-Ce^IV(OH₂)(NO₃)₄]⁺ ( 3 ), a complex obtained from the reaction of [(N4Py)Fe^II(NCMe)]²⁺ with 2 equiv CAN or [(N4Py)Fe^IV=O]²⁺ ( 2 ) with Ce^III(NO₃)₃ in MeCN. Surprisingly, the formation of 3 is reversible, the position of the equilibrium being dependent on the MeCN/water ratio of the solvent. These results suggest that the Fe^IV and Ce^IV centers have comparable reduction potentials. Moreover, the equilibrium entails a change in iron spin state, from S=1 Fe^IV in 2 to S=5/2 in 3 , which is found to be facile despite the formal spin-forbidden nature of this process. This observation suggests that Fe^IV=O complexes may avail of reaction pathways involving multiple spin states having little or no barrier.     

Carbon Monoxide Activation by a Molecular Aluminium Imide: C−O Bond Cleavage and C−C Bond Formation

Anionic molecular imide complexes of aluminium are accessible via a rational synthetic approach involving the reactions of organo azides with a potassium aluminyl reagent. In the case of K₂[( NON )Al(NDipp)]₂ ( NON =4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethyl-xanthene; Dipp=2,6-diisopropylphenyl) structural characterization by X-ray crystallography reveals a short Al−N distance, which is thought primarily to be due to the low coordinate nature of the nitrogen centre. The Al−N unit is highly polar, and capable of the activation of relatively inert chemical bonds, such as those found in dihydrogen and carbon monoxide. In the case of CO, uptake of two molecules of the substrate leads to C−C coupling and C≡O bond cleavage. Thermodynamically, this is driven, at least in part, by Al−O bond formation. Mechanistically, a combination of quantum chemical and experimental observations suggests that the reaction proceeds via exchange of the NR and O substituents through intermediates featuring an aluminium-bound isocyanate fragment.     

Iridium(III) Catalysts with an Amide-Pendant Cyclopentadienyl Ligand: Double Aromatic Homologation Reactions of Benzamides by Fourfold C−H Activation

The synthesis, characterization, and catalytic performance of iridium(III) catalysts that bear an amide-pendant cyclopentadienyl ligand ([Cp^AIrI₂]₂) is reported. These [Cp^AIrI₂]₂ catalysts were obtained from the complexation of a Cp^A ligand precursor with [Ir(cod)OAc]₂ followed by oxidation. Double aromatic homologation reactions of benzamides with alkynes by fourfold C−H activation proceeded in good to high yield with these [Cp^AIrI₂]₂ catalysts, demonstrating their superior catalytic performance in this challenging transformation.     

Methyleneimidazoline Complexes of Iridium,Rhodium, and Palladium from Selective C(sp3)?H Bond Activation

Phosphine‐assisted C? H activation of the methyl group of 2‐methylimidazolium compounds led to a series of iridium, rhodium, and palladium complexes of 2‐methyleneimidazolines. Experimental results confirmed that the product of methyl C? H activation is the kinetic product, whereas the aryl C? H activation product is the thermodynamic product (see scheme).

     

Multicomponent Aromatic and Benzylic Mannich Reactions through C−H Bond Activation

Multicomponent Mannich reactions through C−H bond activation are described. These transformations allowed for the straightforward generation of densely substituted benzylic and homo-benzylic amines in good yields. The reaction involves a reaction between two transient species: an organometallic species, generated by transition-metal-catalyzed sp² or sp³ C−H bond activation and an in situ generated imine. The use of an acetal as an aldehyde surrogate was found essential for the reaction to proceed. The process could be successfully applied to Rh^III-catalyzed sp² C−H bond functionalization and extended to Cu^II-catalyzed sp³ C−H bond functionalization.     

Rh(III)-Catalyzed Oxidative Annulation of 2-Arylquinoxalines with Cyclic 2-Diazo-1,3-diketones by C−H Bond Activation

Rh(III)-catalyzed C−H bond annulation of 2-arylquinoxalines with cyclic 2-diazo-1,3-diketones has been accomplished for the first time to synthesize a novel series of 2,3-dihydrodibenzo[a,c]phenazin-4(1H)-one frameworks by means of carbene insertion followed by condensation. The reaction proceeds through the C−H bond activation and functionalization of 2-arylquinoxalines using Rh(III)/AgSbF₆ complex to produce highly substituted 2,3-dihydrodibenzo[a,c]phenazin-4(1H)-one and benzo[5,6][1,2,4]thiadiazino[2,3-f]phenanthridin-5(6H)-one-10,10-dioxide derivatives in good to excellent yields.     

Photoredox-Catalyzed Isomerization of Highly Substituted Allylic Alcohols by C−H Bond Activation

Photoredox-catalyzed isomerization of γ-carbonyl-substituted allylic alcohols to their corresponding carbonyl compounds was achieved for the first time by C−H bond activation. This catalytic redox-neutral process resulted in the synthesis of 1,4-dicarbonyl compounds. Notably, allylic alcohols bearing tetrasubstituted olefins can also be transformed into their corresponding carbonyl compounds. Density functional theory calculations show that the carbonyl group at the γ-position of allylic alcohols are beneficial to the formation of their corresponding allylic alcohol radicals with high vertical electron affinity, which contributes to the completion of the photoredox catalytic cycle.     

Iridium(I)-Catalyzed C−H Borylation in Air by Using Mechanochemistry

Mechanochemistry has been applied for the first time to an iridium(I)-catalyzed C−H borylation reaction. By using either none or just a catalytic amount of a liquid, the mechanochemical C−H borylation of a series of heteroaromatic compounds proceeded in air to afford the corresponding arylboronates in good-to-excellent yields. A one-pot mechanochemical C−H borylation/Suzuki–Miyaura cross-coupling sequence for the direct synthesis of 2-aryl indole derivatives is also described. The present study constitutes an important milestone towards the development of industrially attractive solvent-free C−H bond functionalization processes in air.     

Easy Access to Versatile Catalytic Systems for C−H Activation and Reductive Amination Based on Tetrahydrofluorenyl Rhodium(III) Complexes

On the basis of the 1,2,3,4-tetrahydrofluorenyl ligand, a simple approach was developed to new effective rhodium catalysts for the construction of C−C and C−N bonds. The halide compounds [(η⁵-tetrahydrofluorenyl)RhX₂]₂ ( 2 a : X=Br; 2 b : X=I) were synthesized by treatment of the bis(ethylene) derivative (η⁵-tetrahydrofluorenyl)Rh(C₂H₄)₂ ( 1 a ) with halogens. An analogous reaction of the cyclooctadiene complex (η⁵-tetrahydrofluorenyl)Rh(cod) ( 1 b ) with I₂ is complicated by the side formation of [(cod)RhI]₂. The reaction of 2 b with 2,2′-bipyridyl leads to cation [(η⁵-tetrahydrofluorenyl)Rh(2,2′-bipyridyl)I]⁺ ( 3 ). The halide abstraction from 2 a , b with thallium or silver salts allowed us to prepare sandwich compounds with incoming cyclopentadienyl, dicarbollide and mesityleneligands [(η⁵-tetrahydrofluorenyl)RhCp]⁺ ( 4 ), (η⁵-tetrahydrofluorenyl)Rh(η-7,8-C₂B₉H₁₁) ( 5 ), and [(η⁵-tetrahydrofluorenyl)Rh(η-mesitylene)]²⁺ ( 6 ). The structures of 1 b , 2 b ⋅ 2I₂, 3 PF₆, 4 TlI₄, 5 , and [(cod)RhI]₂ were determined by X-ray diffraction. Compounds 2 a , b efficiently catalyze the oxidative coupling of benzoic acids with alkynes to selectively give isocoumarins or naphthalenes, depending on the reaction temperature. Moreover, they showed moderate catalytic activity in other annulations of alkynes with aromatic compounds (such as benzamide, acetanilide, etc.) which proceed through CH activation. Compound 2 b also effectively catalyzes the reductive amination of aldehydes and ketones in the presence of carbon monoxide and water via water-gas shift reaction, giving amines in high yields (67–99 %).     

H2, B−H,and Si−H Bond Activation and Facile Protonolysis Driven by Pt-Base Metal Cooperation

We report a series of heterobimetallic Pt/Zn and Pt/Ca complexes to study the effect of proximity of a dicationic base metal on the organometallic Pt species. Varying degrees of Pt⋅⋅⋅Zn and Zn interaction with the bridging Me group are achieved, showcasing snapshots of a hypothetical process of retrotransmetalation from Pt to Zn. In contrast, only weak interactions were observed for Ca with a Pt-bound Me group. Activation of H₂, B−H and Si−H bonds leads to the formation of hydride-bridged Pt−H−Zn complexes, which is not observed in the absence of Zn, pointing out the importance of metal-metal cooperation. Reactivity of PtMe₂/M²⁺ with terminal acetylene, water and methanol is also studied, leading to facile protonation of one of the Me groups at the Pt center only when Zn is present. This study sheds light on various ways in which the presence of a 2+ metal cation significantly affects the reactivity of a common organoplatinum complex.     

C−H and Si−H Activation Reactions at Ru/Ga Complexes: A Combined Experimental and Theoretical Case Study on the Ru−Ga Bond

Treatment of [Ru(COD)(MeAllyl)₂] and [Ru(COD)(COT)] with GaCp* under hydrogenolytic conditions leads to reactive intermediates which activate Si−H or C−H bonds, respectively. The product complexes [Ru(GaCp*)₃(SiEt₃)H₃] ( 1 ) and [Ru(GaCp*)₃(C₇H₇)H₃] ( 2 ) are formed with HSiEt₃ or with toluene as the solvent, respectively. While 1 was isolated and fully characterized by NMR, MS, IR and SC-XRD, 2 was too labile to be isolated and was observed and characterized in situ by using mass spectrometry, including labelling experiments for the unambiguous assignment of the elemental composition. The structural assignment was confirmed by DFT calculations. The relative energies of the four isomers possible upon toluene activation at the ortho-, meta-, para- and CH₃-positions have been determined and point to aromatic C−H activation. The Ru−Ga bond was analyzed by EDA and QTAIM and compared to the Ru−P bond in the analogue phosphine compound. Bonding analyses indicate that the Ru-GaCp* bond is weaker than the Ru-PR₃ bond.