Isocyanide Ligation Enables Electrochemical Ammonia Formation in a Synthetic Cycle for N Fixation.

Transition-metal-mediated splitting of N to form metal nitride complexes could constitute a key step in electrocatalytic nitrogen fixation, if these nitrides can be electrochemically reduced to ammonia under mild conditions. The envisioned nitrogen fixation cycle involves several steps: N binding to form a dinuclear end-on bridging complex with appropriate electronic structure to cleave the N bridge followed by proton/electron transfer to release ammonia and bind another molecule of N. The nitride reduction and N splitting steps in this cycle have differing electronic demands that a catalyst must satisfy.

Room-Temperature Formate Ester Transfer Hydrogenation Enables an Electrochemical/Thermal Organometallic Cascade for Methanol Synthesis from CO.

The reduction of CO to synthetic fuels is a valuable strategy for energy storage. However, the formation of energy-dense liquid fuels such as methanol remains rare, particularly under low-temperature and low-pressure conditions that can be coupled to renewable electricity sources via electrochemistry. Here, a multicatalyst system pairing an electrocatalyst with a thermal organometallic catalyst is introduced, which enables the reduction of CO to methanol at ambient temperature and pressure.

Kinetic Isotope Effects and the Mechanism of CO Insertion into the Metal-Hydride Bond of -(bpy)Re(CO)H.

The 1,2-insertion reaction of CO into metal-hydride bonds of d-octahedral complexes to give κ--metal-formate products is the key step in various CO reduction schemes and as a result has attracted extensive mechanistic investigations. For many octahedral catalysts, CO insertion follows an associative mechanism in which CO interacts directly with the coordinated hydride ligand instead of the more classical dissociative mechanism that opens an empty coordination site to bind the substrate to the metal prior to a hydride migration step. To better understand the associative mechanism, we conducted a systematic quantum chemical investigation on the reaction between CO and -(bpy)Re(CO)H (-Re-H; bpy = 2,2'-bipyridine) starting with the gas phase and then moving to THF and other solvents with increased dielectric constants.

Regulating Access to Active Sites via Hydrogen Bonding and Cation-Dipole Interactions: A Dual Cofactor Approach to Switchable Catalysis.

Hydrogen bonding networks are ubiquitous in biological systems and play a key role in controlling the conformational dynamics and allosteric interactions of enzymes. Yet in small organometallic catalysts, hydrogen bonding rarely controls ligand binding to the metal center. In this work, a hydrogen bonding network within a well-defined organometallic catalyst works in concert with cation-dipole interactions to gate substrate access to the active site.

Catalyst self-assembly accelerates bimetallic light-driven electrocatalytic H evolution in water.

Hydrogen evolution is an important fuel-generating reaction that has been subject to mechanistic debate about the roles of monometallic and bimetallic pathways. The molecular iridium catalysts in this study undergo photoelectrochemical dihydrogen (H) evolution via a bimolecular mechanism, providing an opportunity to understand the factors that promote bimetallic H-H coupling. Covalently tethered diiridium catalysts evolve H from neutral water faster than monometallic catalysts, even at lower overpotential.

Nickel-catalyzed ester carbonylation promoted by imidazole-derived carbenes and salts.

Millions of tons of acetyl derivatives such as acetic acid and acetic anhydride are produced each year. These building blocks of chemical industry are elaborated into esters, amides, and eventually polymer materials, pharmaceuticals, and other consumer products. Most acetyls are produced industrially using homogeneous precious metal catalysts, principally rhodium and iridium complexes.

Synthesis and bonding analysis of pentagonal bipyramidal rhenium carboxamide oxo complexes.

Seven-coordinate rhenium oxo complexes supported by a tetradentate bipyridine carboxamide/carboxamidate ligand are reported. The neutral dicarboxamide Hbpy-da ligand initially coordinates in an L (ONNO) fashion to an octahedral rhenium oxo precursor, yielding a seven-coordinate rhenium oxo complex. Subsequent deprotonation generates a new oxo complex featuring the dianionic (LX) carboxamidate (NNNN) form of the ligand.

Catalytic reduction of dinitrogen to ammonia using molybdenum porphyrin complexes.

Porphyrin complexes are well-known in O and CO reduction, but their application to N reduction is less developed. Here, we show that oxo and nitrido complexes of molybdenum supported by tetramesitylporphyrin (TMP) are effective precatalysts for catalytic N reduction to ammonia, verified by N labeling studies and other control experiments. Spectroscopic and electrochemical studies illuminate some relevant thermodynamic parameters, including the N-H bond dissociation free energy of (TMP)MoNH (43 ± 2 kcal mol).

Tunable and Switchable Catalysis Enabled by Cation-Controlled Gating with Crown Ether Ligands.

ConspectusCatalysis has become an essential tool in science and technology, impacting the discovery of pharmaceuticals, the manufacture of commodity chemicals and plastics, the production of fuels, and much more. In most cases, a particular catalyst is optimized to mediate a particular reaction, continually producing a desired product at a given rate. There is enormous opportunity in developing catalysts that are dynamic, capable of responding to a change in the environment to alter structure and function.

Lewis Structures and the Bonding Classification of End-on Bridging Dinitrogen Transition Metal Complexes.

The activation of dinitrogen by coordination to transition metal ions is a widely used and promising approach to the utilization of Earth's most abundant nitrogen source for chemical synthesis. End-on bridging N complexes (μ-η:η-N) are key species in nitrogen fixation chemistry, but a lack of consensus on the seemingly simple task of assigning a Lewis structure for such complexes has prevented application of valence electron counting and other tools for understanding and predicting reactivity trends. The Lewis structures of bridging N complexes have traditionally been determined by comparing the experimentally observed NN distance to the bond lengths of free N, diazene, and hydrazine.

Oxidative Addition of a Phosphinite P-O Bond at Nickel.

Oxidative addition is an essential elementary reaction in organometallic chemistry and catalysis. While a diverse array of oxidative addition reactions has been reported to date, examples of P-O bond activation are surprisingly rare. Herein, we report the ligand-templated oxidative addition of a phosphinite P-O bond in the diphosphinito aniline compound HN(2-OPPr-3,5-Bu-CH) [H(PONO)] at Ni to form (PONO)Ni(HPPr) after proton rearrangement.

Mechanisms of Electrochemical N Splitting by a Molybdenum Pincer Complex.

Molybdenum complexes supported by tridentate pincer ligands are exceptional catalysts for dinitrogen fixation using chemical reductants, but little is known about their prospects for electrochemical reduction of dinitrogen. The viability of electrochemical N binding and splitting by a molybdenum(III) pincer complex, (PNP)MoBr (PNP = 2,6-bis(BuPCH)-CHN)), is established in this work, providing a foundation for a detailed mechanistic study of electrode-driven formation of the nitride complex (PNP)Mo(N)Br. Electrochemical kinetic analysis, optical and vibrational spectroelectrochemical monitoring, and computational studies point to two concurrent reaction pathways: In the reaction-diffusion layer near the electrode surface, the molybdenum(III) precursor is reduced by 2e and generates a bimetallic molybdenum(I) Mo(μ-N) species capable of N-N bond scission; and in the bulk solution away from the electrode surface, over-reduced molybdenum(0) species undergo chemical redox reactions via comproportionation to generate the same bimetallic molybdenum(I) species capable of N cleavage.

Photochemical H Evolution from Bis(diphosphine)nickel Hydrides Enables Low-Overpotential Electrocatalysis.

Molecules capable of both harvesting light and forming new chemical bonds hold promise for applications in the generation of solar fuels, but such first-row transition metal photoelectrocatalysts are lacking. Here we report nickel photoelectrocatalysts for H evolution, leveraging visible-light-driven photochemical H evolution from bis(diphosphine)nickel hydride complexes. A suite of experimental and theoretical analyses, including time-resolved spectroscopy and continuous irradiation quantum yield measurements, led to a proposed mechanism of H evolution involving a short-lived singlet excited state that undergoes homolysis of the Ni-H bond.

Stepwise Iodide-Free Methanol Carbonylation via Methyl Acetate Activation by Pincer Iridium Complexes.

Iodide is an essential promoter in the industrial production of acetic acid via methanol carbonylation, but it also contributes to reactor corrosion and catalyst deactivation. Here we report that iridium pincer complexes mediate the individual steps of methanol carbonylation to methyl acetate in the absence of methyl iodide or iodide salts. Iodide-free methylation is achieved under mild conditions by an aminophenylphosphinite pincer iridium(I) dinitrogen complex through net C-O oxidative addition of methyl acetate to produce an isolable methyliridium(III) acetate complex.

Understanding Terminal versus Bridging End-on N Coordination in Transition Metal Complexes.

Terminal and bridging end-on coordination of N to transition metal complexes offer possibilities for distinct pathways in ammonia synthesis and N functionalization. Here we elucidate the fundamental factors controlling the two binding modes and determining which is favored for a given metal-ligand system, using both quantitative density functional theory (DFT) and qualitative molecular orbital (MO) analyses. The Gibbs free energy for converting two terminal MN complexes into a bridging MNNM complex and a free N molecule (2Δ) is examined through systematic variations of the metal and ligands; values of Δ range between +9.

Decarbonylative ether dissection by iridium pincer complexes.

A unique chain-rupturing transformation that converts an ether functionality into two hydrocarbyl units and carbon monoxide is reported, mediated by iridium(i) complexes supported by aminophenylphosphinite (NCOP) pincer ligands. The decarbonylation, which involves the cleavage of one C-C bond, one C-O bond, and two C-H bonds, along with formation of two new C-H bonds, was serendipitously discovered upon dehydrochlorination of an iridium(iii) complex containing an aza-18-crown-6 ether macrocycle. Intramolecular cleavage of macrocyclic and acyclic ethers was also found in analogous complexes featuring aza-15-crown-5 ether or bis(2-methoxyethyl)amino groups.

Selecting Double Bond Positions with a Single Cation-Responsive Iridium Olefin Isomerization Catalyst.

The catalytic transposition of double bonds holds promise as an ideal route to alkenes of value as fragrances, commodity chemicals, and pharmaceuticals; yet, selective access to specific isomers is a challenge, normally requiring independent development of different catalysts for different products. In this work, a single cation-responsive iridium catalyst selectively produces either of two different internal alkene isomers. In the absence of salts, a single positional isomerization of 1-butene derivatives furnishes 2-alkenes with exceptional regioselectivity and stereoselectivity.

Temperature and Solvent Effects on H Splitting and Hydricity: Ramifications on CO Hydrogenation by a Rhenium Pincer Catalyst.

The catalytic hydrogenation of carbon dioxide holds immense promise for applications in sustainable fuel synthesis and hydrogen storage. Mechanistic studies that connect thermodynamic parameters with the kinetics of catalysis can provide new understanding and guide predictive design of improved catalysts. Reported here are thermochemical and kinetic analyses of a new pincer-ligated rhenium complex (POCOP)Re(CO) (POCOP = 2,6-bis(di--butylphosphinito)phenyl) that catalyzes CO hydrogenation to formate with faster rates at lower temperatures.

Thermodynamic and kinetic hydricity of transition metal hydrides.

The prevalence of transition metal-mediated hydride transfer reactions in chemical synthesis, catalysis, and biology has inspired the development of methods for characterizing the reactivity of transition metal hydride complexes. Thermodynamic hydricity represents the free energy required for heterolytic cleavage of the metal-hydride bond to release a free hydride ion, H, as determined through equilibrium measurements and thermochemical cycles. Kinetic hydricity represents the rate of hydride transfer from one species to another, as measured through kinetic analysis.

(Electro-)chemical Splitting of Dinitrogen with a Rhenium Pincer Complex.

The splitting of N into well-defined terminal nitride complexes is a key reaction for nitrogen fixation at ambient conditions. In continuation of our previous work on rhenium pincer mediated N splitting, nitrogen activation and cleavage upon (electro)chemical reduction of [ReCl(2)] {2 = N(CHCHPBu) } is reported. The electrochemical characterization of [ReCl(2)] and comparison with our previously reported platform [ReCl(1)] {1 = N(CHCHPBu) } provides mechanistic insight to rationalize the dependence of nitride yield on the reductant.

Kinetics of the Effect in Ruthenium Complexes Provide Insight into the Factors That Control Activity and Stability in CO Electroreduction.

Comparative kinetic studies of a series of new ruthenium complexes provide a platform for understanding how strong effect ligands and redox-active ligands work together to enable rapid electrochemical CO reduction at moderate overpotential. After synthesizing isomeric pairs of ruthenium complexes featuring 2'-picolinyl-methyl-benzimidazol-2-ylidene (Mebim-pic) as a strong effect ligand and 2,2':6',2″-terpyridine (tpy) as a redox-active ligand, chemical and electrochemical kinetic studies examined how complex geometry and charge affect the individual steps and overall catalysis of CO reduction. The relative effect of picoline vs the N-heterocyclic carbene (NHC) was quantified through a kinetic analysis of reductively triggered chloride dissociation, revealing that chloride loss is 1000 times faster in the isomer with the NHC to chloride.

Mechanistic basis for tuning iridium hydride photochemistry from H evolution to hydride transfer hydrodechlorination.

The photochemistry of metal hydride complexes is dominated by H evolution, limiting access to reductive transformations based on photochemical hydride transfer. In this article, the innate H evolution photochemistry of the iridium hydride complexes [Cp*Ir(bpy-OMe)H] (, bpy-OMe = 4,4'-dimethoxy-2,2'-bipyridine) and [Cp*Ir(bpy)H] (, bpy = 2,2'-bipyridine) is diverted towards photochemical hydrodechlorination. Net hydride transfer from and to dichloromethane produces chloromethane with high selectivity and exceptional photochemical quantum yield ( ≤ 1.

Electrochemical C-H bond activation cationic iridium hydride pincer complexes.

A C-H bond activation strategy based on electrochemical activation of a metal hydride is introduced. Electrochemical oxidation of ( PCP)IrH ( PCP is [1,3-( BuPCH)-CH]) in the presence of pyridine derivatives generates cationic Ir hydride complexes of the type [( PCP)IrH(L)] (where L = pyridine, 2,6-lutidine, or 2-phenylpyridine). Facile deprotonation of [( PCP)IrH(2,6-lutidine)] with the phosphazene base -butylimino-tris(pyrrolidino)phosphorane, BuP(pyrr), results in selective C-H activation of 1,2-difluorobenzene (1,2-DFB) solvent to generate ( PCP)Ir(H)(2,3-CFH).