Cation-Modulated Reactivity of Iridium Hydride Pincer-Crown Ether Complexes

Complexes of a new multidentate ligand combining a rigid, strongly donating pincer scaffold with a flexible, weakly donating aza-crown ether moiety are reported. The pincer-crown ether ligand exhibits tridentate, tetradentate, and pentadentate coordination modes. The coordination mode can be changed by Lewis base displacement of the chelating ethers, with binding equilibria dramatically altered through lithium and sodium cation–macrocycle interactions. Cation-promoted hydrogen activation was accomplished by an iridium monohydride cation ligated in a pentadentate fashion by the pincer-crown ether ligand. The rate can be controlled on the basis of the choice of cation (with lithium-containing reactions proceeding about 10 times faster than sodium-containing reactions) or on the basis of the concentration of the cation. Up to 250-fold rate enhancements in H/D exchange rates are observed when catalytic amounts of Li⁺ are added

Connecting Neutral and Cationic Pathways in Nickel-Catalyzed Insertion of Benzaldehyde into a C–H Bond of Acetonitrile

Nickel catalysts supported by diethylamine- or aza-crown ether-containing aminophosphinite (NCOP) pincer ligands catalyze the insertion of benzaldehyde into a C–H bond of acetonitrile. The catalytic activity of neutral (NCOP)­Ni­(O^<i>t</i>Bu) and cationic [(NCOP)­Ni­(NCCH₃)]⁺ are starkly different. The neutral <i>tert</i>-butoxide precatalysts are active without any added base and give good yields of product after 24 h, while the cationic precatalysts require a base cocatalyst and still operate much more slowly (120 h in typical runs). A series of in situ spectroscopic studies identified several intermediates, including a nickel cyanoalkoxide complex that was observed in all of the reactions regardless of the choice of precatalyst. Reaction monitoring also revealed that the neutral <i>tert</i>-butoxide precatalysts decompose to form the cationic acetonitrile complex during catalysis; this deactivation involves alkoxide abstraction and can be hastened by the addition of lithium salts. While the deactivated cationic species is inactive under standard base-free conditions, catalysis can be reinitiated by the addition of catalytic amounts of base

Photochemical Production of Ethane from an Iridium Methyl Complex

An iridium methyl complex, [Cp*Ir­(bpy)­(CH₃)]⁺, was prepared by electrophilic methylation of Cp*Ir­(bpy) with CH₃I and characterized electrochemically, photophysically, crystallographically, and computationally. Irradiation of the MLCT transition of [Cp*Ir­(bpy)­(CH₃)]⁺ in the presence of CH₃I in acetonitrile produces ethane, methane, propionitrile, and succinonitrile. A series of mechanistic studies indicates that C–C bond formation is mediated by free methyl radicals produced through monometallic photochemical homolysis of the Ir–CH₃ bond

Molecular Photoelectrocatalysts for Visible Light-Driven Hydrogen Evolution from Neutral Water

A light-activated hydrogen evolution electrocatalyst is reported. Hydrogen evolves near the thermodynamic potential when aqueous solutions of the iridium chloride complex [Cp*Ir­(bpy)­(Cl)]­[Cl] (<b>1</b>, bpy = 2,2′-bipyridine) are illuminated by visible light. In the dark, no electrocatalytic activity is observed. This unique hydrogen evolution mechanism is made possible because a single transition metal complex is the active light absorber and active electrocatalyst. Optimization by tuning the electronic structure of the catalyst and varying reaction conditions resulted in H₂ evolution with faster rates, even at milder applied potentials (<i>k</i>_obs ∼ 0.1 s^–1 at 100 mV electrochemical overpotential)

Arene Activation at Iridium Facilitates C–O Bond Cleavage of Aryl Ethers

An arene activation strategy for the selective disassembly of aryl ethers is reported. A variety of aryl ethers readily bind an electrophilic pentamethylcyclopentadienyl iridium center by η⁶-arene coordination, generating complexes that are activated toward hydrolysis and cleavage of the Ar–OR bond (R = Me, Et, Ph). Hydrolysis occurs rapidly at room temperature in aqueous pH 7 phosphate buffer (or upon modest heating under acidic conditions), releasing alcohol while forming cyclohexadienyl-one products. Under strongly acidic conditions, protonation of the dienyl-one followed by substitution with starting aryl ether completes a hydrolysis cycle. Mechanistic studies suggest that the key hydrolysis step proceeds via nucleophilic attack at the ipso position of the arene (S_NAr mechanism). The observed mechanism is considered in the context of lignocellulosic biomass conversion

Arene Activation at Iridium Facilitates C–O Bond Cleavage of Aryl Ethers

An arene activation strategy for the selective disassembly of aryl ethers is reported. A variety of aryl ethers readily bind an electrophilic pentamethylcyclopentadienyl iridium center by η⁶-arene coordination, generating complexes that are activated toward hydrolysis and cleavage of the Ar–OR bond (R = Me, Et, Ph). Hydrolysis occurs rapidly at room temperature in aqueous pH 7 phosphate buffer (or upon modest heating under acidic conditions), releasing alcohol while forming cyclohexadienyl-one products. Under strongly acidic conditions, protonation of the dienyl-one followed by substitution with starting aryl ether completes a hydrolysis cycle. Mechanistic studies suggest that the key hydrolysis step proceeds via nucleophilic attack at the ipso position of the arene (S_NAr mechanism). The observed mechanism is considered in the context of lignocellulosic biomass conversion

Photochemical Formic Acid Dehydrogenation by Iridium Complexes: Understanding Mechanism and Overcoming Deactivation

The mechanism of photochemical formic acid dehydrogenation catalyzed by [Cp*Ir­(bpy)­(Cl)]⁺ (<b>1</b>, bpy = 2,2′-bipyridine) and [Cp*Ir­(bpy-OMe)­(Cl)]⁺ (<b>1-OMe</b>, bpy-OMe = 4,4′-dimethoxy-2,2′-bipyridine) is examined. The catalysts operate with good turnover frequency (TOF) across an unusually wide pH range. Above pH 7, the evolved gas is >95% pure H₂ (along with traces of CO₂ but no detectable CO). Light-triggered H₂ release from a metal hydride intermediate is found to be the turnover-limiting step, based on the observed first-order dependence on catalyst concentration, saturation behavior in formate concentration, and direct in situ observation of a metal hydride resting state during turnover. Deactivation pathways are identified, including ligand loss and aggregate formation, precipitation of insoluble forms of the catalyst, and deprotonation of the iridium hydride intermediate. Guided by mechanistic insights, improved catalytic activity (initial TOF exceeding 50 h^–1), stability (>500 turnovers at nearly 5 atm), and selectivity (>95% H₂ gas) are achieved

Aqueous Hydricity of Late Metal Catalysts as a Continuum Tuned by Ligands and the Medium

Aqueous hydride transfer is a fundamental step in emerging alternative energy transformations such as H₂ evolution and CO₂ reduction. “Hydricity,” the hydride donor ability of a species, is a key metric for understanding transition metal hydride reactivity, but comprehensive studies of aqueous hydricity are scarce. An extensive and self-consistent aqueous hydricity scale is constructed for a family of Ru and Ir hydrides that are key intermediates in aqueous catalysis. A reference hydricity is determined using redox potentiometry and spectrophotometric titration for a particularly water-soluble species. Then, relative hydricity values for a range of species are measured using hydride transfer equilibria, taking advantage of expedient new synthetic procedures for Ru and Ir hydrides. This large collection of hydricity values provides the most comprehensive picture so far of how ligands impact hydricity in water. Strikingly, we also find that hydricity can be viewed as a <i>continuum</i> in water: the free energy of hydride transfer changes with pH, buffer composition, and salts present in solution

A Ruthenium Hydrido Dinitrogen Core Conserved across Multielectron/Multiproton Changes to the Pincer Ligand Backbone

A series of ruthenium­(II) hydrido dinitrogen complexes supported by pincer ligands in different formal oxidation states have been prepared and characterized. Treating a ruthenium dichloride complex supported by the pincer ligand bis­(di-<i>tert</i>-butylphosphinoethyl)­amine (H-PNP) with reductant or base generates new five-coordinate <i>cis</i>-hydridodinitrogen ruthenium complexes each containing different forms of the pincer ligand. Further ligand transformations provide access to the first isostructural set of complexes featuring all six different forms of the pincer ligand. The conserved <i>cis</i>-hydridodinitrogen structure facilitates characterization of the π-donor, π-acceptor, and/or σ-donor properties of the ligands and assessment of the impact of ligand-centered multielectron/multiproton changes on N₂ activation. Crystallographic studies, infrared spectroscopy, and ¹⁵N NMR spectroscopy indicate that N₂ remains weakly activated in all cases, providing insight into the donor properties of the different pincer ligand states. Ramifications on applications of (pincer)Ru species in catalysis are considered

Ammonia Synthesis from a Pincer Ruthenium Nitride via Metal–Ligand Cooperative Proton-Coupled Electron Transfer

The conversion of metal nitride complexes to ammonia may be essential to dinitrogen fixation. We report a new reduction pathway that utilizes ligating acids and metal–ligand cooperation to effect this conversion without external reductants. Weak acids such as 4-methoxybenzoic acid and 2-pyridone react with nitride complex [(<i>H</i>-PNP)­RuN]⁺ (<i>H-</i>PNP = HN­(CH₂CH₂P^tBu₂)₂) to generate octahedral ammine complexes that are κ²-chelated by the conjugate base. Experimental and computational mechanistic studies reveal the important role of Lewis basic sites proximal to the acidic proton in facilitating protonation of the nitride. The subsequent reduction to ammonia is enabled by intramolecular 2H⁺/2e^– proton-coupled electron transfer from the saturated pincer ligand backbone