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The thermochemistry of cation–macrocycle interactions in nickel pincer complexes bearing a hemilabile aza-15-crown-5 or aza-18-crown-6 macrocycle is investigated and applied to cation-controlled reversible ligand binding. Cation–crown interactions were examined in a noncoordinating, low polarity solvent (dichloromethane) and a coordinating, polar solvent (acetonitrile). Structural studies provide solid-state information on cation–crown interactions, whereas binding affinity studies in solution provide quantitative thermodynamic information. The different hemilabile ligand coordination modes have vastly different cation binding affinities, with the tridentate pincer coordination mode binding cations more than 100 000 times more strongly than the tetradentate coordination mode with a crown ether oxygen donating to nickel. Dichloromethane enforces strong cation–crown interactions without disrupting the hemilabile ether ligand, whereas acetonitrile disrupts hemilability by displacing the ethers from the nicke...

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.

Rapid, selective, and highly controllable iridium-catalyzed allylbenzene isomerization is described, enabled by tunable hemilability based on alkali metal cation binding with a macrocyclic "pincer-crown ether" ligand. An inactive chloride-ligated complex can be activated by halide abstraction with sodium salts, with the resulting catalyst [κ5 -(15c5 NCOPiPr )Ir(H)]+ exhibiting modest activity. Addition of Li+ provides a further boost in activity, with up to 1000-fold rate enhancement. Ethers and chloride salts dampen or turn off reactivity, leading to three distinct catalyst states with activity spanning several orders of magnitude. Mechanistic studies suggest that the large rate enhancement and high degree of tunability stem from control over substrate binding.

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 [(H-PNP)RuN]+ (H-PNP = HN(CH2CH2PtBu2)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.

Hydricity, or hydride donating ability, is a thermodynamic value that helps define the reactivity of transition metal hydrides. To avoid some of the challenges of experimental hydricity measurements in water, a computational method for the determination of aqueous hydricity values has been developed. With a thermochemical cycle involving deprotonation of the metal hydride (pKa), 2e– oxidation of the metal (E°), and 2e– reduction of the proton, hydricity values are provided along with other valuable thermodynamic information. The impact of empirical corrections (for example, calibrating reduction potentials with 2e– organic versus 1e– inorganic potentials) was assessed in the calculation of the reduction potentials, acidities, and hydricities of a series of iridium hydride complexes. Calculated hydricities are consistent with electronic trends and agree well with experimental values.

A comprehensive mechanistic study of electrocatalytic CO

Transition metal complexes that incorporate crown ethers into the supporting ligands have emerged as a powerful class of catalysts capable of cation-tunable reactivity. Cations held in the secondary coordination sphere of a transition metal catalyst can pre-organize or activate substrates, induce local electric fields, adjust structural conformations, or even modify bonding in the primary coordination sphere of the transition metal. This Feature Article begins with a non-comprehensive review of the structural motifs and catalytic applications of crown ether-containing transition metal catalysts, then proceeds to detail the development of catalysts based on “pincer-crown ether” ligands that bridge the primary and secondary coordination spheres.

The hydricity of the heterobimetallic iridium/ruthenium catalyst [Cp*Ir(H)(μ-bpm)Ru(bpy)2]3+ (1, where Cp* = η5-pentamethylcyclopentadienyl, bpm = 2,2′-bipyrimidine, and bpy = 2,2′-bipyridine) has been determined in both acetonitrile (63.1 kcal mol–1) and water (29.7 kcal mol–1). Hydride 1 features a large increase in the hydride donor ability when the solvent is changed from acetonitrile to water. The acidity of 1, in contrast, is essentially solvent-independent because 1 remains strongly acidic in both solvents. On the basis of an X-ray crystallographic study, spectroscopic analysis, and time-dependent density functional theory calculations, the disparate reactivity trends are ascribed to substantial delocalization of the electron density onto both the bpm and bpy ligands in the conjugate base of 1, [Cp*Ir(μ-bpm)Ru(bpy)2]2+ (3). The H2 evolution tendencies of 1 are considered in the context of thermodynamic parameters.

The development of a sustainable ammonia synthesis by proton-coupled electroreduction of dinitrogen (N2) requires knowledge of the thermodynamics described by standard reduction potentials. The first collection of N2 reduction standard potentials in an organic solvent are reported here. The potentials for reduction of N2 to ammonia (NH3), hydrazine (N2H4), and diazene (N2H2) in acetonitrile (MeCN) solution are derived using thermochemical cycles. Ammonia is thermodynamically favored, with a 0.43 V difference between NH3 and N2H4 and a 1.26 V difference between NH3 and N2H2. The thermodynamics for reduction of N2 to the protonated products ammonium (NH4+) and hydrazinium (N2H5+) under acidic conditions are also presented. Comparison with the H+/H2 potential in MeCN reveals a 63 mV thermodynamic preference for N2 reduction to NH3 over H2 production. Combined with knowledge of the kinetics of electrode-catalyzed H2 evolution, a wide working region is identified to guide future electrocatalytic studies.

The rate of catalytic methanol carbonylation to acetic acid is typically limited by either the oxidative addition of methyl iodide or the subsequent C–C bond-forming migratory insertion step. These elementary steps have been studied independently in acetonitrile solution for iridium aminophenylphosphinite (NCOP) complexes. The modular synthesis of NCOP ligands containing a macrocyclic aza-crown ether arm enables a direct comparison of two complementary catalyst optimization strategies: synthetic modification of the phenyl backbone and noncovalent modification through cation–crown interactions with Lewis acids in the surrounding environment. The oxidative addition of methyl iodide to iridium(I) carbonyl complexes proceeds readily at room temperature to form iridium(III) methylcarbonyliodide complexes. The methyl complexes undergo migratory insertion under 1 atm CO at 70 °C to produce iridium(III) acetyl species. Synthetic tuning, by incorporation of a methoxy group into the ligand backbone, had little infl...

Transition metal hydrides play a critical role in stoichiometric and catalytic transformations. Knowledge of free energies for cleaving metal hydride bonds enables the prediction of chemical reactivity, such as for the bond-forming and bond-breaking events that occur in a catalytic reaction. Thermodynamic hydricity is the free energy required to cleave an M-H bond to generate a hydride ion (H(-)). Three primary methods have been developed for hydricity determination: the hydride transfer method establishes hydride transfer equilibrium with a hydride donor/acceptor pair of known hydricity, the H2 heterolysis method involves measuring the equilibrium of heterolytic cleavage of H2 in the presence of a base, and the potential-pKa method considers stepwise transfer of a proton and two electrons to give a net hydride transfer. Using these methods, over 100 thermodynamic hydricity values for transition metal hydrides have been determined in acetonitrile or water. In acetonitrile, the hydricity of metal hydrides spans a range of more than 50 kcal/mol. Methods for using hydricity values to predict chemical reactivity are also discussed, including organic transformations, the reduction of CO2, and the production and oxidation of hydrogen.

An introductory guide to deducing the mechanism of chemical reactions is presented. Following a typical workflow for probing reaction mechanism, the guide introduces a wide range of kinetic and mechanistic tools. In addition to serving as a broad introduction to mechanistic analysis for students and researchers, the guide has also been used by instructors to provide the organizational structure for an upper-level course on organic and inorganic reaction mechanism. After providing students with the tools of mechanistic study, student-led discussions of case studies and an independent proposal project provide preparation for understanding the mechanism of new reactions encountered in independent research.

Attempts to generate a proposed rhodium hydride catalytic intermediate instead resulted in isolation of (Cp*H)Rh(bpy)Cl (1), a pentamethylcyclopentadiene complex, formed by C-H bond-forming reductive elimination from the fleeting rhodium hydride. The hydride transfer ability of diene 1 was explored through thermochemistry and hydride transfer reactions, including the reduction of NAD(+).

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 continuum in water: the free energy of hydride transfer changes with pH, buffer composition, and salts present in solution.

With a view towards replacing sacrificial hydrogen acceptors in alkane dehydrogenation catalysis, electrochemical methods for oxidative activation of a pincer-ligated iridium hydride intermediate were explored. A 1H(+)/2e(-) oxidation process was observed in THF solvent, with net hydride loss leading to a reactive cationic intermediate that can be trapped by chloride. Analogous reactivity was observed with the concerted hydride transfer reagent Ph3C(+), connecting chemical and electrochemical hydride loss pathways.

The synthesis of a bis(arylphosphinito)amide pincer ligand P2ONO− designed to form two six-membered rings upon metallation is reported. Phosphination of a known bis(phenolato)amide ONO3− scaffold led to isolation of two isomers: the intended H(P2ONO) preligand with an amine moiety is formed as a kinetic product, which isomerizes via net oxidative addition of the amine N−H to phosphorous to yield a benzoxazaphosphole-containing thermodynamic product. Both isomers undergo productive metallation with NiBr2(dimethoxyethane) to produce the same complex, (P2ONO)NiBr. Treatment with triethylborohydride furnished the terminal hydride complex (P2ONO)NiH. The bromide and hydride complexes enabled comparisons of the steric and electronic properties of the P2ONO− ligand with other amide-based pincers.

A comprehensive mechanistic study of N2 activation and splitting into terminal nitride ligands upon reduction of the rhenium dichloride complex [ReCl2(PNP)] is presented (PNP– = N(CH2CH2PtBu2)2–). Low-temperature studies using chemical reductants enabled full characterization of the N2-bridged intermediate [{(PNP)ClRe}2(N2)] and kinetic analysis of the N–N bond scission process. Controlled potential electrolysis at room temperature also resulted in formation of the nitride product [Re(N)Cl(PNP)]. This first example of molecular electrochemical N2 splitting into nitride complexes enabled the use of cyclic voltammetry (CV) methods to establish the mechanism of reductive N2 activation to form the N2-bridged intermediate. CV data was acquired under Ar and N2, and with varying chloride concentration, rhenium concentration, and N2 pressure. A series of kinetic models was vetted against the CV data using digital simulations, leading to the assignment of an ECCEC mechanism (where “E” is an electrochemical step and “C” is a chemical step) for N2 activation that proceeds via initial reduction to ReII, N2 binding, chloride dissociation, and further reduction to ReI before formation of the N2-bridged, dinuclear intermediate by comproportionation with the ReIII precursor. Experimental kinetic data for all individual steps could be obtained. The mechanism is supported by density functional theory computations, which provide further insight into the electronic structure requirements for N2 splitting in the tetragonal frameworks enforced by rigid pincer ligands.

A series of rhenium diimine carbonyl complexes was prepared and characterized in order to examine the influence of axial ligands on electronic structure. Systematic substitution of the axial carbonyl and acetonitrile ligands of [Re(deeb)(CO)3(NCCH3)]+ (deeb = 4,4′-diethylester-2,2′-bipyridine) with trimethylphosphine and chloride, respectively, gives rise to red-shifted absorbance features. These bathochromic shifts result from destabilization of the occupied d-orbitals involved in metal-to-ligand charge-transfer transitions. Time-Dependent Density Functional Theory identified the orbitals involved in each transition and provided support for the changes in orbital energies induced by ligand substitution.

The development of molecular organometallic electrocatalysts for the reduction of carbon dioxide (CO2) is reviewed. Organometallic complexes are a promising subset of the many catalysts capable of electrochemical reduction of carbon dioxide to organic small molecules such as carbon monoxide (CO), formate (HCO2−), and oxalate (C2O42−). The carbon-based supporting ligands of organometallic complexes provide electron-rich metal centers and engender good chemical stability, factors that help place organometallic catalysts among the most impressive molecular systems for CO2 reduction. This chapter provides a comprehensive review of CO2 electroreduction by organometallic complexes, defined as those containing metal–carbon bonds in the supporting ligands.

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-tert-butylphosphinoethyl)amine (H-PNP) with reductant or base generates new five-coordinate cis-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 cis-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 s...

A comprehensive mechanistic study of N2 activation and splitting into terminal nitride ligands upon reduction of the rhenium dichloride complex [ReCl2(PNP)] is presented (PNP– = N(CH2CH2PtBu2)2–). Low-temperature studies using chemical reductants enabled full characterization of the N2-bridged intermediate [{(PNP)ClRe}2(N2)] and kinetic analysis of the N–N bond scission process. Controlled potential electrolysis at room temperature also resulted in formation of the nitride product [Re(N)Cl(PNP)]. This first example of molecular electrochemical N2 splitting into nitride complexes enabled the use of cyclic voltammetry (CV) methods to establish the mechanism of reductive N2 activation to form the N2-bridged intermediate. CV data was acquired under Ar and N2, and with varying chloride concentration, rhenium concentration, and N2 pressure. A series of kinetic models was vetted against the CV data using digital simulations, leading to the assignment of an ECCEC mechanism (where “E” is an electrochemical step an...

Several new iridium(I) and iridium(III) carbonyl complexes supported by aminophosphinite pincer ligands have been prepared and characterized. A surprising diversity of reaction pathways was encountered upon treatment of Ir carbonyl complexes with Li+, Na+, Ca2+, and La3+ salts. Iridium(III) hydridocarbonyl chloride complexes underwent either halide abstraction or halide substitution reactions, whereas iridium(I) carbonyl complexes underwent protonative oxidative addition reactions. When the nitrogen donor of the pincer ligand is an aza-crown ether macrocycle, cation–macrocycle interactions could be supported, leading to divergent reactivity in some cases.

The mechanism of photochemical formic acid dehydrogenation catalyzed by [Cp*Ir(bpy)(Cl)]+ (1, bpy = 2,2′-bipyridine) and [Cp*Ir(bpy-OMe)(Cl)]+ (1-OMe, 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), ...

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(OtBu) and cationic [(NCOP)Ni(NCCH3)]+ are starkly different. The neutral tert-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 tert-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 inac...

A protocol for identifying ligand binding modes in a series of iridium pincer complexes bearing hemilabile aza-crown ether ligands has been developed using readily accessible NMR methods. The approach was tested on a collection of 13 structurally diverse pincer–crown ether complexes that include several newly characterized species. New synthetic routes enable facile interconversion of coordination modes and supporting ligands. Detailed structural assignments of five complexes reveal that the difference in chemical shift (Δδ) between geminal protons in the crown ether is influenced by diamagnetic anisotropy arising from halides and other ligands in the primary coordination sphere. The average difference in chemical shift between diastereotopic geminal protons in the crown ether macrocycle (Δδavg), as determined through a single 1H–13C HSQC experiment, provides information on the pincer ligand binding mode by establishing whether the macrocycle is in close proximity to the metal center. The Δδavg values for...

Methods for in situ reversible control over ligand binding processes at transition metal complexes can enable advances in switchable and tunable catalysis. After a brief overview of different approaches to controlling ligand binding, this Perspective details the development of “pincer-crown ether” ligands that contain a rigid pincer backbone and a hemilabile aza-crown ether unit that enables cation-modulated hemilability. Applications of pincer-crown ether complexes in small molecule activation and catalysis are discussed, culminating in a set of design principles for ligands capable of cation-modulated ligand binding.

Visible light-triggered hydride transfer from [Cp*Ir(bpy)(H)](+) (1) to organic acids and 1-methylnicotinamide (MNA(+)) is reported (Cp* = pentamethylcyclopentadienyl; bpy = 2,2'-bipyridine). A new thermochemical cycle for determining excited-state hydride donor ability (hydricity) predicted that 1 would be an incredibly potent photohydride in acetonitrile. Phototriggered H2 release was indeed observed from 1 in the presence of various organic acids, providing experimental evidence for an increase in hydricity of at least 18 kcal/mol in the excited state. The rate and product selectivity of hydride transfer to MNA(+) are photoswitchable: 1,4-dihydro-1-methylnicotinamide forms slowly in the dark but rapidly under illumination, and photolysis can also produce doubly reduced 1,4,5,6-tetrahydro-1-methylnicotinamide.

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.

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] (1, 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 (kobs ∼ 0.1 s–1 at 100 mV electrochemical overpotential).

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 (SNAr mechanism). The observed mechanism is considered in the context of lignocellulosic biomass conversion.

The hydricity of the heterobimetallic iridium/ruthenium catalyst [Cp*Ir(H)(μ-bpm)Ru(bpy)

Transition metal hydrides play a critical role in stoichiometric and catalytic transformations. Knowledge of free energies for cleaving metal hydride bonds enables the prediction of chemical reactivity, such as for the bond-forming and bond-breaking events that occur in a catalytic reaction. Thermodynamic hydricity is the free energy required to cleave an M–H bond to generate a hydride ion (H–). Three primary methods have been developed for hydricity determination: the hydride transfer method establishes hydride transfer equilibrium with a hydride donor/acceptor pair of known hydricity, the H2 heterolysis method involves measuring the equilibrium of heterolytic cleavage of H2 in the presence of a base, and the potential–pKa method considers stepwise transfer of a proton and two electrons to give a net hydride transfer. Using these methods, over 100 thermodynamic hydricity values for transition metal hydrides have been determined in acetonitrile or water. In acetonitrile, the hydricity of metal hydrides sp...

Artificial photosynthesis relies on coupling light absorption with chemical fuel generation. A mechanistic study of visible light-driven H2 production from [Cp*Ir(bpy)H]+ (1) has revealed a new, highly efficient pathway for integrating light absorption with bond formation. The net reaction of 1 with a proton source produces H2, but the rate of excited state quenching is surprisingly acid-independent and displays no observable deuterium kinetic isotopic effect. Time-resolved photoluminescence and labeling studies are consistent with diffusion-limited bimetallic self-quenching by electron transfer. Accordingly, the quantum yield of H2 release nearly reaches unity as the concentration of 1 increases. This unique pathway for photochemical H2 generation provides insight into transformations catalyzed by 1.

The effective design of an artificial photosynthetic system entails the optimization of several important interactions. Herein we report stopped-flow UV-visible (UV-vis) spectroscopy, X-ray crystallographic, density functional theory (DFT), and electrochemical kinetic studies of the Re(bipy- t Bu)(CO) 3 (L) catalyst for the reduction of CO 2 to CO. A remarkable selectivity for CO 2 over H + was observed by stopped-flow UV-vis spectroscopy of [Re(bipy- t Bu)(CO) 3 ] -1 . The reaction with CO 2 is about 25 times faster than the reaction with water or methanol at the same concentrations. X-ray crystallography and DFT studies of the doubly reduced anionic species suggest that the highest occupied molecular orbital (HOMO) has mixed metal-ligand character rather than being purely doubly occupied , which is believed to determine selectivity by favoring CO 2 ( σ + π ) over H + ( σ only) binding. Electrocatalytic studies performed with the addition of Brönsted acids reveal a primary H/D kinetic isotope effect, indicating that transfer of protons to Re -CO 2 is involved in the rate limiting step. Lastly, the effects of electrode surface modification on interfacial electron transfer between a semiconductor and catalyst were investigated and found to affect the observed current densities for catalysis more than threefold, indicating that the properties of the electrode surface need to be addressed when developing a homogeneous artificial photosynthetic system.

Recent approaches to the homogeneous conversion of synthesis gas to organic chemicals and fuels are reviewed. Progress in this field is placed in the context of important industrially practiced transformations, such as the Fischer–Tropsch process for conversion of synthesis gas to higher alkanes, as well as previous attempts to produce a viable homogeneous alternative. Approaches to homogeneous syngas conversion discussed in some detail include the reduction of transition metal carbonyl complexes by main group hydrides or transition metal hydrides; unusual routes to formyls including radical-based late transition metal chemistry; early metal and lanthanide reductive couplings; cluster chemistry; and Lewis acid-assisted transformations. Our current research combines a number of these concepts in an attempt to convert syngas selectively to multicarbon organic fragments.

A highly emissive bis(phosphine)diarylamido dinuclear copper(I) complex (quantum yield = 57%) was shown to exhibit E-type delayed fluorescence by variable temperature emission spectroscopy and photoluminescence decay measurement of doped vapor-deposited films. The lowest energy singlet and triplet excited states were assigned as charge transfer states on the basis of theoretical calculations and the small observed S(1)-T(1) energy gap. Vapor-deposited OLEDs doped with the complex in the emissive layer gave a maximum external quantum efficiency of 16.1%, demonstrating that triplet excitons can be harvested very efficiently through the delayed fluorescence channel. The function of the emissive dopant in OLEDs was further probed by several physical methods, including electrically detected EPR, cyclic voltammetry, and photoluminescence in the presence of applied current.

Thermodynamic studies of a series of [H(2)Rh(PP)(2)](+) and [HRh(PP)(2)(CH(3)CN)](2+) complexes have been carried out in acetonitrile. Seven different diphosphine (PP) ligands were selected to allow variation of the electronic properties of the ligand substituents, the cone angles, and the natural bite angles (NBAs). Oxidative addition of H(2) to [Rh(PP)(2)](+) complexes is favored by diphosphine ligands with large NBAs, small cone angles, and electron donating substituents, with the NBA being the dominant factor. Large pK(a) values for [HRh(PP)(2)(CH(3)CN)](2+) complexes are favored by small ligand cone angles, small NBAs, and electron donating substituents with the cone angles playing a major role. The hydride donor abilities of [H(2)Rh(PP)(2)](+) complexes increase as the NBAs decrease, the cone angles decrease, and the electron donor abilities of the substituents increase. These results indicate that if solvent coordination is involved in hydride transfer or proton transfer reactions, the observed trends can be understood in terms of a combination of two different steric effects, NBAs and cone angles, and electron-donor effects of the ligand substituents.

A ruthenium complex of the tripodal ligand tris(2-pyridyl)phosphine oxide exhibits rapid water oxidation electrocatalysis over a wide pH range., The tris(2-pyridyl)phosphine oxide (Py3PO) complex [Ru(Py3PO)(bpy)(OH2)]2+ (bpy is 2,2′-bipyridine) is a pH-dependent water oxidation electrocatalyst that accelerates dramatically with increasing pH—up to 780 s–1 at pH 10 (∼1 V overpotential). Despite retaining the pentakis(pyridine) ligand arrangement common to previously reported catalysts, the tripodal Py3PO ligand framework supports much faster electrocatalysis. The early stages of the catalytic cycle are proposed to follow the typical pattern of single-site ruthenium catalysts, with two sequential 1H+/1e– proton-coupled electron transfer (PCET) oxidations, but the pH-dependent onset of catalysis and rapid rates are distinguishing features of the present system.

A series of half-sandwich Ir and Rh compounds are demonstrated to be competent catalysts for the hydrogenation of carboxylic acids under relatively mild conditions. Of the structurally diverse group of catalysts tested for activity, a Cp*Ir complex supported by an electron-releasing 2,2'-bipyridine ligand was the most active. Higher activity was achieved with employment of Bronsted or Lewis acid promoters. Mechanistic studies suggest a possible reaction pathway involving activated carboxylic acid substrates. The hydrogenation reaction was shown to be general to a variety of aliphatic acids.

Trialkylborane additives promote reduction of CO(2) to formate by bis(diphosphine) Ni(II) and Rh(III) hydride complexes. The late transition metal hydrides, which can be formed from dihydrogen, transfer hydride to CO(2) to give a formate-borane adduct. The borane must be of appropriate Lewis acidity: weaker acids do not show significant hydride transfer enhancement, while stronger acids abstract hydride without CO(2) reduction. The mechanism likely involves a pre-equilibrium hydride transfer followed by formation of a stabilizing formate-borane adduct.

A new family of low-coordinate nickel imides supported by 1,2-bis(di-tert-butylphosphino)ethane was synthesized. The oxidation of nickel(II) complexes led to the formation of both aryl- and alkyl-substituted nickel(III) imides, and examples of both types have been isolated and fully characterized. The aryl substituent that proved most useful in stabilizing the Ni(III)-imide moiety was the bulky 2,6-dimesitylphenyl. The two nickel(III)-imide compounds showed different variable-temperature magnetic properties, but analogous EPR spectra at low temperatures. In order to account for this discrepancy, a low-spin/high-spin equilibrium was proposed to take place for the alkyl-substituted imide nickel(III) complex. This proposal was supported by DFT calculations. DFT calculations also indicated that the unpaired electron is mostly localized on the imide nitrogen for the nickel(III) complexes. The results of reactions carried out in the presence of hydrogen donors supported the findings from DFT calculations that the adamantyl substituent was a significantly more reactive hydrogen-atom abstractor. Interestingly, the steric properties of the 2,6-dimesitylphenyl substituent are important not only in protecting the Ni=N core but also in favoring one rotamer of the resulting nickel(III) imide, by locking in the phenyl ring in a perpendicular orientation with respect to the NiPP plane.

An exceptionally low coordinate nickel imido complex, (IPr*)Ni═N(dmp) (2) (dmp = 2,6-dimesitylphenyl), has been prepared by the elimination of N_2 from a bulky aryl azide in its reaction with (IPr*)Ni(η^6-C_7H_8) (1). The solid-state structure of 2 features two-coordinate nickel with a linear C−Ni−N core and a short Ni−N distance, both indicative of multiple-bond character. Computational studies using density functional theory showed a Ni═N bond dominated by Ni(dπ)−N(pπ) interactions, resulting in two nearly degenerate singly occupied molecular orbitals (SOMOs) that are Ni−N π* in character. Reaction of 2 with CO resulted in nitrene-group transfer to form (dmp)NCO and (IPr*)Ni(CO)_3 (3). Net C−H insertion was observed in the reaction of 2 with ethene, forming the vinylamine (dmp)NH(CH═CH_2) (5) via an azanickelacyclobutane intermediate, (IPr*)Ni{N,C:κ^2-N(dmp)CH_2CH_2} (4).

Tables of ^1H and ^(13)C NMR chemical shifts have been compiled for common organic compounds often used as reagents or found as products or contaminants in deuterated organic solvents. Building upon the work of Gottlieb, Kotlyar, and Nudelman in the Journal of Organic Chemistry, signals for common impurities are now reported in additional NMR solvents (tetrahydrofuran-d_8, toluene-d_8, dichloromethane-d_2, chlorobenzene-d_5, and 2,2,2-trifluoroethanol-d_3) which are frequently used in organometallic laboratories. Chemical shifts for other organics which are often used as reagents or internal standards or are found as products in organometallic chemistry are also reported for all the listed solvents.