Base-Promoted, Remote C-H Activation at a Cationic (η5-C5Me5)Ir(III) Center Involving Reversible C-C Bond Formation of Bound C5Me5

Broad-Scope Amination of Aryl Sulfamates Catalyzed by a Palladium Phosphine Complex

Among phenol-derived electrophiles, aryl sulfamates are attractive substrates since they can be employed as directing groups for C-H functionalization prior to catalysis. However, their use in C-N coupling is limited only to Ni catalysis. Here, we describe a Pd-based catalyst with a broad scope for the amination of aryl sulfamates. We show that the N-methyl-2-aminobiphenyl palladacycle supported by the PCyp2ArXyl2 ligand (Cyp = cyclopentyl; ArXyl2 = 2,6-bis(2,6-dimethylphenyl)phenyl) efficiently catalyzes the C-N coupling of aryl sulfamates with a variety of nitrogen nucleophiles, including anilines, primary and secondary alkyl amines, heteroaryl amines, N-heterocycles, and primary amides. DFT calculations support that the oxidative addition of the aryl sulfamate is the rate-determining step. The C-N coupling takes place through a cationic pathway in the polar protic medium.

Polarized Au(I)/Rh(I) bimetallic pairs cooperatively trigger ligand non-innocence and bond activation

The combination of molecular metallic fragments of contrasting Lewis character offers many possibilities for cooperative bond activation and for the disclosure of unusual reactivity. Here we provide a systematic investigation on the partnership of Lewis basic Rh(I) compounds of type [(η5-L)Rh(PR3)2] (η5-L = (C5Me5)- or (C9H7)-) with highly congested Lewis acidic Au(I) species. For the cyclopentadienyl Rh(I) compounds, we demonstrate the non-innocent role of the typically robust (C5Me5)- ligand through migration of a hydride to the Rh site and provide evidence for the direct implication of the gold fragment in this unusual bimetallic ligand activation event. This process competes with the formation of dinuclear Lewis adducts defined by a dative Rh → Au bond, with selectivity being under kinetic control and tunable by modifying the stereoelectronic and chelating properties of the phosphine ligands bound to the two metals. We provide a thorough computational study on the unusual Cp* non-innocent behavior and the divergent bimetallic pathways observed. The cooperative FLP-type reactivity of all bimetallic pairs has been investigated and computationally examined for the case of N-H bond activation in ammonia.

Divergent Regioselective Csp2-H Difluoromethylation of Aromatic Amines Enabled by Nickel Catalysis

Herein, the first catalytic protocol for nickel-catalyzed ortho or para position difluoromethylation of various aromatic amines has been developed with the assistance of a bidentate phosphine ligand, offering an invaluable synthesis means to construct extensive p-difluoromethylated products and difluorooxindole derivatives with significant functional fragments. Furthermore, the gram-scale reaction, broad substrate scope, excellent functional-group compatibility, late-stage difluoromethylation of pesticides, and even formal synthesis of HDAC6 inhibitors further demonstrate the usefulness of this method.

Dicoordinate Au(I)-Ethylene Complexes as Hydroamination Catalysts

A series of gold(I)-ethylene π-complexes containing a family of bulky phosphine ligands has been prepared. The use of these sterically congested ligands is crucial to stabilize the gold(I)-ethylene bond and prevent decomposition, boosting up their catalytic performance in the highly underexplored hydroamination of ethylene. The precatalysts bearing the most sterically demanding phosphines showed the best results reaching full conversion to the hydroaminated products under notably mild conditions (1 bar of ethylene pressure at 60 °C). Kinetic analysis together with density functional theory calculations revealed that the assistance of a second molecule of the nucleophile as a proton shuttle is preferred even when using an extremely congested cavity-shaped Au(I) complex. In addition, we have measured a strong primary kinetic isotopic effect that is consistent with the involvement of X-H bond-breaking events in the protodeauration turnover-limiting step.

Ligand Protonation at Carbon, not Nitrogen, during H2 Production with Amine-Rich Iron Electrocatalysts

We present monometallic H₂ production electrocatalysts containing electron-rich triamine-cyclopentadienyl (Cp) ligands coordinated to iron. After selective CO extrusion from the iron tricarbonyl precursors, electrocatalysis is observed via cyclic voltammetry in the presence of an exogenous acid. Contrary to the fact that amines in the secondary coordination sphere are often protonated during electrocatalysis, comprehensive quantum-chemical calculations indicate that the amines likely do not function as proton relays; instead, endo-Cp ring protonation is most favorable after 1e^- reduction. This unusual mechanistic pathway emphasizes the need to consider a broad domain of H⁺/e^- addition products by synergistically combining experimental and theoretical resources.

Heptadentate, Octadentate, Or Even Nonadentate? Denticity in the Unexpected Formation of an All-Carbon Donor-Atom Ligand in RhIII(Cp*)(Anthracenyl-NHC) Complexes

Investigations on incorporating an N-flanking anthracenyl moiety to [Rh(Cp*)(NHC)Cl₂] complexes surprisingly led to the formation of an intramolecular C-C bond between the Cp* and anthracenyl moieties, with additional auxiliary interactions between the metal and the anthracenyl ring system. In silico modeling supports a reaction mechanism whereby Rh(η⁴-tetramethylfulvene) intermediates undergo metallocycloaddition and the abstraction of a chlorido ligand, affording unique cationic complexes that feature Rh centers coordinated by a nonadentate ligand with exclusively carbon donor atoms. Some Rh-C interactions were extremely weak but nevertheless exhibited covalent bonding character. These weak Rh-C interactions were readily displaced by stronger electron donors, and the nonadentate ligand reverted to the heptadentate coordination mode observed in the intermediate. As far as we are aware, this study provides the first conclusive evidence of complexes bearing a single nonadentate κ⁹-coordinating ligand that features only carbon donors bound to a metal center.

Reversible Hydride Migration from C5Me5 to RhI Revealed by a Cooperative Bimetallic Approach

The use of cyclopentadienyl ligands in organometallic chemistry and catalysis is ubiquitous, mostly due to their robust spectator role. Nonetheless, increasing examples of non-innocent behaviour are being documented. Here, we provide evidence for reversible intramolecular C-H activation at one methyl terminus of C5Me5 in [(η-C5Me5)Rh(PMe3)2] to form a new Rh-H bond, a process so far restricted to early transition metals. Experimental evidence was acquired from bimetallic rhodium/gold structures in which the gold center binds either to the rhodium atom or to the activated Cp* ring. Reversibility of the C-H activation event regenerates the RhI and AuI monometallic precursors, whose cooperative reactivity towards polar E-H bonds (E=O, N), including the N-H bonds in ammonia, can be understood in terms of bimetallic frustration.

Dialkylterphenyl Phosphine-Based Palladium Precatalysts for Efficient Aryl Amination of N-Nucleophiles

A series of 2-aminobiphenyl palladacycles supported by dialkylterphenyl phosphines, PR₂ Ar' (R=Me, Et, iPr, Cyp (cyclopentyl), Ar'=Ar^Dipp2 , Ar^Xyl2f , Dipp (2,6-C6H3-(2,6-C6H3-(CHMe2)2)2), Xyl=xylyl) have been prepared and structurally characterized. Neutral palladacycles were obtained with less bulky terphenyl phosphines (i.e., Me and Et substituents) whereas the largest phosphines provided cationic palladacycles in which the phosphines adopted a bidentate hemilabile k¹ -P,η¹ -C_arene coordination mode. The influence of the ligand structure on the catalytic performance of these Pd precatalysts was evaluated in aryl amination reactions. Cationic complexes bearing the phosphines PiPr₂ Ar^Xyl2 and PCyp₂ Ar^Xyl2 were the most active of the series. These precatalysts have demonstrated a high versatility and efficiency in the coupling of a variety of nitrogen nucleophiles, including secondary amines, alkyl amines, anilines, and indoles, with electronically deactivated and ortho-substituted aryl chlorides at low catalyst loadings (0.25-0.75 mol % Pd) and without excess ligand.

Electron acceptors promote proton–hydride tautomerism in low valent rhenium β-diketiminates

We report a series of β-diketiminate (BDI) complexes in which tautomeric rhenium(iii) hydride and rhenium(i) protio-BDI species readily interconvert between the solid and solution states.

Bonding rearrangements in organometallic reactions: from orbitals to curly arrows

Reaction mechanisms include a description of both the nuclear and electronic rearrangements along the energetically most favourable pathway. Extracting the nuclear rearrangements from the outcome of quantum chemical calculations is straightforward, while it is more intricate for the electronic rearrangements. This is particularly true when changes in the bonding pattern are of interest, just as in the arrow-pushing formalism used in chemical schemes. Here, we report on our use of a simple and highly visual procedure to recover the bonding rearrangements along a reaction pathway from DFT calculations and to draw curly arrows. We show that the procedure allows us discern among mechanistic proposals in the context of organometallic reactions featuring the forming and breaking of bonds.

Snapshots of a Migrating H-Atom: Characterization of a Reactive Iron(III) Indenide Hydride and its Nearly Isoenergetic Ring-Protonated Iron(I) Isomer

We report the characterization of an S= 1 / 2 iron π-complex, [Fe(η⁶ -IndH)(depe)]⁺ (Ind=Indenide (C₉ H₇ ^- ), depe=1,2-bis(diethylphosphino)ethane), which results via C-H elimination from a transient Fe^III hydride, [Fe(η³ :η² -Ind)(depe)H]⁺ . Owing to weak M-H/C-H bonds, these species appear to undergo proton-coupled electron transfer (PCET) to release H₂ through bimolecular recombination. Mechanistic information, gained from stoichiometric as well as computational studies, reveal the open-shell π-arene complex to have a BDFE_C-H value of ≈50 kcal mol^-1 , roughly equal to the BDFE_Fe-H of its Fe^III -H precursor (ΔG°≈0 between them). Markedly, this reactivity differs from related Fe(η⁵ -Cp/Cp*) compounds, for which terminal Fe^III -H cations are isolable and have been structurally characterized, highlighting the effect of a benzannulated ring (indene). Overall, this study provides a structural, thermochemical, and mechanistic foundation for the characterization of indenide/indene PCET precursors and outlines a valuable approach for the differentiation of a ring- versus a metal-bound H-atom by way of continuous-wave (CW) and pulse EPR (HYSCORE) spectroscopic measurements.