An NHC–Silyl–NHC Pincer Ligand for the Oxidative Addition of C−H,N−H,and O−H Bonds to Cobalt(I) Complexes

N‐Heterocyclic carbene based pincer ligands bearing a central silyl donor, [CSiC]⁻, have been envisioned as a class of strongly σ‐donating ligands that can be used for synthesizing electron‐rich transition‐metal complexes for the activation of inert bonds. However, this type of pincer ligand and complexes thereof have remained elusive owing to their challenging synthesis. We herein describe the first synthesis of a CSiC pincer ligand scaffold through the coupling of a silyl–NHC chelate with a benzyl–NHC chelate induced by one‐electron oxidation in the coordination sphere of a cobalt complex. The monoanionic CSiC ligand stabilizes the Co^I dinitrogen complex [(CSiC)Co(N₂)] with an unusual coordination geometry and enables the challenging oxidative addition of E−H bonds (E=C, N, O) to Co^I to form Co^III complexes. The structure and reactivity of the cobalt(I) complex are ascribed to the unique electronic properties of the CSiC pincer ligand, which provides a strong trans effect and pronounced σ‐donation.     

Insights into the Halogen Oxidative Addition Reaction to Dinuclear Gold(I) Di(NHC) Complexes

Gold(I) dicarbene complexes [Au₂(MeIm‐Y‐ImMe)₂](PF₆)₂ (Y=CH₂ ( 1 ), (CH₂)₂ ( 2 ), (CH₂)₄ ( 4 ), MeIm=1‐methylimidazol‐2‐ylidene) react with iodine to give the mixed‐valence complex [Au(MeIm‐CH₂‐ImMe)₂AuI₂](PF₆)₂ ( 1 a^I ) and the gold(III) complexes [Au₂I₄(MeIm‐Y‐ImMe)₂](PF₆)₂ ( 2 c^I and 4 c^I ). Reaction of complexes 1 and 2 with an excess of ICl allows the isolation of the tetrachloro gold(III) complexes [Au₂Cl₄(MeIm‐CH₂‐ImMe)₂](PF₆)₂ ( 1 c^Cl ) and [Au₂Cl₄(MeIm‐(CH₂)₂‐ImMe)₂](Cl)₂ ( 2 c^Cl‐Cl ) (as main product); remarkably in the case of complex 2 , the X‐ray molecular structure of the crystals also shows the presence of I‐Au‐Cl mixed‐sphere coordination. The same type of coordination has been observed in the main product of the reaction of complexes 3 or 4 with ICl. The study of the reactivity towards the oxidative addition of halogens to a large series of dinuclear bis(dicarbene) gold(I) complexes has been extended and reviewed. The complexes react with Cl₂, Br₂ and I₂ to give the successive formation of the mixed‐valence gold(I)/gold(III) n a^X and gold(III) n c^X (excluding compound 1 c^I ) complexes. However, complex 3 affords with Cl₂ and Br₂ the gold(II) complex 3 b^X [Au₂X₂(MeIm‐(CH₂)₃‐ImMe)₂](PF₆)₂ (X=Cl, Br), which is the predominant species over compound 3 c^X even in the presence of free halogen. The observed different relative stabilities of the oxidised complexes of compounds 1 and 3 have also been confirmed by DFT calculations.     

A Cobalt(I) Pincer Complex with an η2‐Caryl−H Agostic Bond: Facile C−H Bond Cleavage through Deprotonation,Radical Abstraction,and Oxidative Addition

The synthesis and reactivity of a Co^I pincer complex [Co(?³P,CH,P‐P(CH)P^NMe‐iPr)(CO)₂]⁺ featuring an η²‐ C_aryl?H agostic bond is described. This complex was obtained by protonation of the Co^I complex [Co(PCP^NMe‐iPr)(CO)₂]. The Co^III hydride complex [Co(PCP^NMe‐iPr)(CNtBu)₂(H)]⁺ was obtained upon protonation of [Co(PCP^NMe‐iPr)(CNtBu)₂]. Three ways to cleave the agostic C?H bond are presented. First, owing to the acidity of the agostic proton, treatment with pyridine results in facile deprotonation (C?H bond cleavage) and reformation of [Co(PCP^NMe‐iPr)(CO)₂]. Second, C?H bond cleavage is achieved upon exposure of [Co(?³P,CH,P‐P(CH)P^NMe‐iPr)(CO)₂]⁺ to oxygen or TEMPO to yield the paramagnetic Co^II PCP complex [Co(PCP^NMe‐iPr)(CO)₂]⁺. Finally, replacement of one CO ligand in [Co(?³P,CH,P‐P(CH)P^NMe‐iPr)(CO)₂]⁺ by CNtBu promotes the rapid oxidative addition of the agostic η²‐C_aryl?H bond to give two isomeric hydride complexes of the type [Co(PCP^NMe‐iPr)(CNtBu)(CO)(H)]⁺.     

Experimental and Theoretical Study of NiII- and PdII-Promoted Double Geminal C(sp3)−H Bond Activation Providing Facile Access to NHC Pincer Complexes: Isolated Intermediates and Mechanism

We report the first examples of metal-promoted double geminal activation of C(sp³)−H bonds of the N−CH₂−N moiety in an imidazole-type heterocycle, leading to nickel and palladium N-heterocyclic carbene complexes under mild conditions. Reaction of the new electron-rich diphosphine 1,3-bis((di-tert-butylphosphaneyl)methyl)-2,3-dihydro-1H-benzo[d]imidazole ( 1 ) with [PdCl₂(cod)] occurred in a stepwise fashion, first by single C−H bond activation yielding the alkyl pincer complex [PdCl(PC ^HP)] ( 3 ) with two trans phosphane donors and a covalent Pd−C bond. Activation of the C−H bond of the resulting α-methine C H−M group occurred subsequently when 3 was treated with HCl to yield the NHC pincer complex [PdCl(PC^NHCP)]Cl ( 2 ). Treatment of 1 with [NiBr₂(dme)] also afforded a NHC pincer complex, [NiBr(PC^NHCP)]Br ( 6 ), but the reactions leading to the double geminal C−H bond activation of the N−CH₂−N group were too fast to allow identification or isolation of an intermediate analogous to 3 . The determination of six crystal structures, the isolation of reaction intermediates and DFT calculations provided the basis for suggesting the mechanism of the stepwise transformation of a N−CH₂−N moiety in the N−C^NHC−N unit of NHC pincer complexes and explain the key differences observed between the Pd and Ni chemistries.     

Facile Oxidative Addition of O−H Bonds of Methanol and Water to IrI Complexes Having Peraryldiphosphane Ligands

A new piece in the puzzle. Hydrido(alkoxo) complexes of late transition metals have often been postulated as intermediates or transition states in the catalytic hydrogenation of carbonyl compounds and catalytic hydrogen transfer reactions. Here the facile oxidative addition of the O−H bonds of alcohol and water to iridium(I ) complexes of peraryldiphosphanes [Eq. (a)] is reported and the resulting complexes are described. R=H, Me.     

Oxidative P−P Bond Addition to Cobalt(−I): Formation of a Low‐Spin Cobalt(III) Phosphanido Complex

The first homoleptic cobalt phosphanido complex [K(thf)₄][Co{1,2‐(Pt Bu₂)₂C₂B₁₀H₁₂}₂] ( 1 ) was prepared by an unprecedented oxidative P−P bond addition of an ortho ‐carborane‐substituted 1,2‐diphosphetane to cobalt(−I) in [K(thf)_0.2][Co(η⁴‐cod)₂)] (cod=1,5‐cycloctadiene). Compound 1 is a rare distorted tetrahedral 3d⁶ complex with a low‐spin ground state configuration. Magnetic measurements revealed that the complex is diamagnetic between 2 to 270 K in the solid state and at 298 K in [D₈]THF solution. Based on DFT calculations, the unusual singlet ground state is caused by the strong σ‐donor and moderate π‐donor properties of the bis(phosphanido) ligand.     

Oxidative Addition of 2‐Halogenoazoles—Direct Synthesis of Palladium(II) Complexes Bearing Protic NH,NH‐Functionalized NHC Ligands

The chemistry of N‐heterocyclic carbenes (NHCs) is dominated by N,N′‐dialkylated or ‐diarylated derivatives. Such NHC ligands are normally obtained by C2‐deprotonation of azolium cations or by reductive elimination from azol‐2‐thiones. A simple one‐step procedure is described that leads to complexes with NH,NH‐functionalized NHC ligands by the oxidative addition of 2‐halogenoazoles to complexes of zero‐valent transition metals.     

Synthesis,Characterization, and Reactivity of Cobalt(III)–Oxygen Complexes Bearing a Macrocyclic N‐Tetramethylated Cyclam Ligand

Mononuclear metal–dioxygen species are key intermediates that are frequently observed in the catalytic cycles of dioxygen activation by metalloenzymes and their biomimetic compounds. In this work, a side‐on cobalt(III)–peroxo complex bearing a macrocyclic N‐tetramethylated cyclam (TMC) ligand, [Co^III(15‐TMC)(O₂)]⁺, was synthesized and characterized with various spectroscopic methods. Upon protonation, this cobalt(III)–peroxo complex was cleanly converted into an end‐on cobalt(III)–hydroperoxo complex, [Co^III(15‐TMC)(OOH)]²⁺. The cobalt(III)–hydroperoxo complex was further converted to [Co^III(15‐TMC‐CH₂‐O)]²⁺ by hydroxylation of a methyl group of the 15‐TMC ligand. Kinetic studies and ¹⁸O‐labeling experiments proposed that the aliphatic hydroxylation occurred via a Co^IV–oxo (or Co^III–oxyl) species, which was formed by O? O bond homolysis of the cobalt(III)–hydroperoxo complex. In conclusion, we have shown the synthesis, structural and spectroscopic characterization, and reactivities of mononuclear cobalt complexes with peroxo, hydroperoxo, and oxo ligands.     

Oxidative Addition Reactions of Element–Hydrogen Bonds with Different Polarities to a Gallium(I) Compound

The gallium(I) derivative [Ga({N(dipp)CMe}₂CH)] ( 1 ; dipp=2,6‐diisopropylphenyl) undergoes facile oxidative addition reactions with various element–hydrogen bonds including N? H, P? H, O? H, Sn? H, and H? H bonds. This was demonstrated by its reaction with triphenyltin hydride, ethanol, water, diethylamine, diphenylphosphane, and dihydrogen. All products were characterized by means of single‐crystal X‐ray structure determination, NMR spectroscopy, IR spectroscopy, and mass spectrometry.     

Oxidative Coupling of Anionic Abnormal N‐Heterocyclic Carbenes: Efficient Access to Janus‐Type 4,4′‐Bis(2H‐imidazol‐2‐ylidene)s

The oxidative coupling of anionic imidazol‐4‐ylidenes protected at the C2 position with [MnCp(CO)₂] or BH₃ led to the corresponding 4,4′‐bis(2H‐imidazol‐2‐ylidene) complexes or adducts, in which the two carbene moieties are connected through a single C?C bond. Subsequent acidic treatment of the later species led to the corresponding 4,4′‐bis(imidazolium) salts in good yields. The overall procedure offers practical access to a novel class of Janus‐type bis(NHC)s. Strikingly, the coplanarity of the two NHC rings within the mesityl derivative 4,4′‐bis(IMes), favored by steric hindrance along with stabilizing intramolecular C?H???π aryl interactions, allows the alignment of the π‐systems and, as a direct consequence, significant electron communication through the bis(carbene) scaffold.     

N‐Heterocyclic Carbene–Gold(I) Complexes Conjugated to a Leukemia‐Specific DNA Aptamer for Targeted Drug Delivery

This report describes the synthesis and characterization of novel N‐heterocyclic carbene (NHC)–gold(I) complexes and their bioconjugation to the CCRF‐CEM‐leukemia‐specific aptamer sgc8c. Successful bioconjugation was confirmed by the use of fluorescent tags on both the NHC–Au^I complex and the aptamer. Cell‐viability assays indicated that the NHC–Au^I–aptamer conjugate was more cytotoxic than the NHC–gold complex alone. A combination of flow cytometry, confocal microscopy, and cell‐viability assays provided clear evidence that the NHC–Au^I–aptamer conjugate was selective for targeted CCRF‐CEM leukemia cells.     

Synthesis and Characterization of Iron,Cobalt, and Nickel [PNP] Pincer Amido Complexes by N–H Activation

The N–H bond activation product [PNP]‐Fe^I(PMe₃)₂ ( 2 ) was obtained at room temperature by the reaction of diphosphinito [PNP] pincer ligand ((Ph₂P(C₆H₄))₂NH ( 1 )) with Fe(PMe₃)₄. Treatment of 1 with Co(PMe₃)₄, CoCl(PMe₃)₃ and CoMe(PMe₃)₄ afforded the same N–H bond activation product [PNP]‐Co^I(PMe₃)₂ ( 3 ). In order to have a better understanding of the mechanism of formation of 3 , in situ IR and ¹H NMR spectroscopic investigations were conducted.The reaction of 1 with Ni(PMe₃)₄ afforded the ligand replacement complex 4 while a [PNP]‐Ni^IIMe complex 5 was obtained via deprotonation through the reaction of 1 with NiMe₂(PMe₃)₃. The molecular structures of 2 – 4 were confirmed by X‐ray diffraction analysis.     

[Cu(NHC)]‐Catalyzed C−H Allylation and Alkenylation of both Electron‐Deficient and Electron‐Rich (Hetero)arenes with Allyl Halides

New reactivity of a [Cu(NHC)] (NHC=N‐heterocyclic carbene) catalyst is disclosed for the efficient C?H allylation of polyfluoroarenes using allyl halides in benzene at room temperature. The same catalyst system also promotes an isomerization‐induced alkenylation of initially the generated allyl arenes when the reaction is run in tetrahydrofuran. Significantly, not only electron‐deficient but also electron‐rich (hetero)arenes undergo this double‐bond migration process, thus leading to alkenylated products. The present system features mild reaction conditions, broad scope with respect to the arene substrates and allyl halide reactants, good functional‐group tolerance, and high stereoselectivity.     

[Ru(NHC)(P-P)(CO)HF] (NHC = N-heterocyclic carbene; P-P = xantphos, dppf) complexes: Efforts to prepare new hydrodefluorination catalysts

The ruthenium N-heterocyclic carbene (NHC) hydride fluoride complexes Ru(NHC)(P-P)(CO)HF (NHC = ICy (3), IEt₂Me₂ (5), P-P = xantphos; NHC = ICy (7), P-P = dppf) have been prepared by treatment of the corresponding dihydride complexes [Ru(NHC)(P-P)(CO)H₂] (NHC = ICy (2), IEt₂Me₂ (4) P-P = xantphos; NHC = ICy (6), P-P = dppf) with Et₃N·3HF. In all cases, the hydride fluoride complexes exist in solution as two conformers or isomers. Although 3, 5 and 7 could be converted back to 2, 4 and 6, respectively, by heating with Et₃SiH, efforts to generate a catalytic cycle for the hydrodefluorination of aromatic fluorocarbons by subsequent reaction of Ru(NHC)(P-P)(CO)H₂ with C₆F₆ were prevented by the much more favourable cyclometallation of the carbene ligand.     

Group 13 Ligand Supported Heavy‐Metal Complexes: First Structural Evidence for Gallium–Lead and Gallium–Mercury Bonds

Heavy‐metal complexes of lead and mercury stabilized by Group 13 ligands were derived from the oxidative addition of Ga(ddp) (ddp=HC(CMeNC₆H₃‐2,6‐iPr₂)₂, 2‐diisopropylphenylamino‐4‐diisopropyl phenylimino‐2‐pentene) with corresponding metal precursors. The reaction of Me₃PbCl and Ga(ddp) afforded compound [{(ddp)Ga(Cl)}PbMe₃] ( 1 ) composed of Ga? Pb^IV bonds. In addition, the monomeric plumbylene‐type compound [{(ddp)Ga(OSO₂CF₃)}₂Pb(thf)] ( 2 a ) with an unsupported Ga‐Pb^II‐Ga linkage was obtained by the reaction of [Pb(OSO₂CF₃)₃] with Ga(ddp) (2 equiv). Compound 2 a falls under the rare example of a discrete plumbylene‐type compound supported by a nonclassical ligand. Interesting structural changes were observed when [Pb(OSO₂CF₃)₃] ? 2 H₂O was treated with Ga(ddp) in a 1:2 ratio to yield [{(ddp)Ga(μ‐OSO₂CF₃)}₂(OH₂)Pb] ( 2 b ) at below ?10 °C. Compound 2 b consists of a bent Ga‐Pb‐Ga backbone with a bridging triflate group between the Ga? Pb bond and a weakly interacting water molecule at the gallium center. Similarly, the reaction of mercury thiolate Hg(SC₆F₅) with Ga(ddp) (2 equiv) produced the bimetallic homoleptic compounds anti‐[{(ddp)Ga(SC₆F₅)}₂Hg] ( 3 a ) and gauche‐[{(ddp)Ga(SC₆F₅)}₂Hg] ( 3 b ), respectively, with a linear Ga‐Hg‐Ga linkage. Compounds 1 – 3 were structurally characterized and these are the first examples of compounds comprised of Ga? Pb^II, Ga? Pb^IV, and Ga? Hg bonds.     

Structural Diversity of Copper(I)–N‐Heterocyclic Carbene Complexes; Ligand Tuning Facilitates Isolation of the First Structurally Characterised Copper(I)–NHC Containing a Copper(I)–Alkene Interaction

The preparation of a series of imidazolium salts bearing N‐allyl substituents, and a range of substituents on the second nitrogen atom that have varying electronic and steric properties, is reported. The ligands have been coordinated to a copper(I) centre and the resulting copper(I)–NHC (NHC=N‐heterocyclic carbene) complexes have been thoroughly examined, both in solution and in the solid‐state. The solid‐state structures are highly diverse and exhibit a range of unusual geometries and cuprophilic interactions. The first structurally characterised copper(I)–NHC complex containing a copper(I)–alkene interaction is reported. An N‐pyridyl substituent, which forms a dative bond with the copper(I) centre, stabilises an interaction between the metal centre and the allyl substituent of a neighbouring ligand, to form a 1D coordination polymer. The stabilisation is attributed to the pyridyl substituent increasing the electron density at the copper(I) centre, and thus enhancing the metal(d)‐to‐alkene(π*) back‐bonding. In addition, components other than charge transfer appear to have a role in copper(I)–alkene stabilisation because further increases in the Lewis basicity of the ligand disfavours copper(I)–alkene binding.     

A Cobalt(I) Pincer Complex with an η2‐Caryl−H Agostic Bond: Facile C−H Bond Cleavage through Deprotonation,Radical Abstraction,and Oxidative Addition

The synthesis and reactivity of a Co^I pincer complex [Co(ϰ³P,CH,P‐P(CH)P^NMe‐iPr)(CO)₂]⁺ featuring an η²‐ C_aryl−H agostic bond is described. This complex was obtained by protonation of the Co^I complex [Co(PCP^NMe‐iPr)(CO)₂]. The Co^III hydride complex [Co(PCP^NMe‐iPr)(CNtBu)₂(H)]⁺ was obtained upon protonation of [Co(PCP^NMe‐iPr)(CNtBu)₂]. Three ways to cleave the agostic C−H bond are presented. First, owing to the acidity of the agostic proton, treatment with pyridine results in facile deprotonation (C−H bond cleavage) and reformation of [Co(PCP^NMe‐iPr)(CO)₂]. Second, C−H bond cleavage is achieved upon exposure of [Co(ϰ³P,CH,P‐P(CH)P^NMe‐iPr)(CO)₂]⁺ to oxygen or TEMPO to yield the paramagnetic Co^II PCP complex [Co(PCP^NMe‐iPr)(CO)₂]⁺. Finally, replacement of one CO ligand in [Co(ϰ³P,CH,P‐P(CH)P^NMe‐iPr)(CO)₂]⁺ by CNtBu promotes the rapid oxidative addition of the agostic η²‐C_aryl−H bond to give two isomeric hydride complexes of the type [Co(PCP^NMe‐iPr)(CNtBu)(CO)(H)]⁺.