Revisiting the Preparation and Catalytic Performance of a Phosphine-Modified Co(II) Hydroformylation Precatalyst.

In light of recent conflicting reports regarding the hydroformylation catalytic activity derived from cationic Co(II) precatalysts of the form [Co(acac)(bis(phosphine))]BF4, the synthetic procedures and characterization of [Co(acac)(dppBz)]BF4, 1, are evaluated. Leveraging calibrated ESI-TOF MS methodologies, substantial quantities of Co(acac)2(dppBz), 2, were observed within samples of 1. The source of the impurity, 2, is determined to derive from incomplete protonolysis of the Co(acac)2 precursor and ligand scrambling occurring during the synthesis of 1. Revised synthetic procedures using lower temperature conditions and longer reaction times afford analytically pure samples of 1 based on ESI-TOF MS and NMR spectroscopic analysis. Complex 1 is demonstrated to act as a hydroformylation precatalyst for the conversion of 1-hexene to 1-heptanal under relatively mild conditions at 51.7 bar and 140 °C. The presence of impurity 2 is shown to dramatically decrease the catalytic performance derived from 1.

Direct photocatalytic C-H functionalization mediated by a molybdenum dioxo complex.

Direct photocatalytic C-H activation mediated by MoO2Cl2(bpy-tBu), a unique photoactive metal OXO, is presented. The limiting step, reoxidation to the Mo dioxo, is evaluated and proposed to occur via a key Cl- loss event. Photocatalyst degradation occurs upon substitution of bpy-tBu with H2O generated during catalysis.

Catalyst self-assembly accelerates bimetallic light-driven electrocatalytic H2 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 (H2) evolution via a bimolecular mechanism, providing an opportunity to understand the factors that promote bimetallic H-H coupling. Covalently tethered diiridium catalysts evolve H2 from neutral water faster than monometallic catalysts, even at lower overpotential. The unexpected origin of this improvement is non-covalent supramolecular self-assembly into nanoscale aggregates that efficiently harvest light and form H-H bonds. Monometallic catalysts containing long-chain alkane substituents leverage the self-assembly to evolve H2 from neutral water at low overpotential and with rates close to the expected maximum for this light-driven water splitting reaction. Design parameters for holding multiple catalytic sites in close proximity and tuning catalyst microenvironments emerge from this work.

The Role of Anionic Ligands on Photoreactivity in Mo(VI) Dioxo Complexes of the Form MoO2 X2 (NN).

The photoreactivity of d0 metal dioxo complexes in activating C-H bonds has been recently studied.[1-3] We have previously reported that MoO2 Cl2 (bpy-t Bu) is an effective platform for light initiated C-H activation with unique product selectivity for the overall functionalization.[1] Herein we expand on these studies and report the synthesis and photoreactivity of several new Mo(VI) dioxo complexes with the general formula MoO2 (X)2 (NN); where X=F- , Cl- , Br- , CH3 - , PhO- , t BuO- and NN=2,2'-bipyridine (bpy) or 4,4'-tert-butyl-2,2'bipyridine (bpy-t Bu). Among these compounds, MoO2 Cl2 (bpy-t Bu) and MoO2 Br2 (bpy-t Bu) are able to participate in bimolecular photoreactivity with several substrates containing C-H bonds of various types such as allyls, benzyls, aldehydes (RCHO) and alkanes. MoO2 (CH3 )2 bpy and MoO2 (PhO)2 bpy do not participate in bimolecular photoreactions and instead they undergo photodecompositions. Computational studies indicate that the nature of the HOMO and LUMO is critical in supporting photoreactivity, with access to an LMCT (bpy→Mo) being necessary for tractable hydrocarbon functionalization.

Light-Initiated C-H Activation via Net Hydrogen Atom Transfer to a Molybdenum(VI) Dioxo.

MoO2Cl2(bpy-tBu) (1) is shown to be a potent one-electron oxidant upon irradiation with 365 nm light in various solvents, while being a weak two-electron oxidant in the dark. Complex 1 is characterized to activate various types of C-H bonds photochemically, including allylic and benzylic positions as well as alkanes and aldehydes. In all of these oxidations, 1 ultimately forms a bimetallic Mo(V)/Mo(V) species with a µ-oxo ligand (2). Depending on the substrate, the major organic product is identified as either an oxygenated or a C-C coupled (homodimerized) compound along with a minor chlorinated species. The product selectivity is proposed to be dependent upon the relative values between the bond dissociation enthalpy (BDE) of a potentially new C-OH bond within the product versus the BDE of a Mo-OH motif within a Mo(V)O(OH) intermediate. Based on this, we can estimate the BDE for Mo-OH to be 83-93 kcal/mol. Mechanistic studies suggest that the C-H activation occurs via a net hydrogen atom transfer (HAT) from 1* occurring either asynchronously or via a stepwise electron-proton transfer (ET-PT) process. Complex 2 is further demonstrated to reform dioxo 1 in the presence of chemical oxidants.

"May the Force Be with You!" Force-Volume Mapping with Atomic Force Microscopy.

Information of the chemical, mechanical, and electrical properties of materials can be obtained using force volume mapping (FVM), a measurement mode of scanning probe microscopy (SPM). Protocols have been developed with FVM for a broad range of materials, including polymers, organic films, inorganic materials, and biological samples. Multiple force measurements are acquired with the FVM mode within a defined 3D volume of the sample to map interactions (i.e., chemical, electrical, or physical) between the probe and the sample. Forces of adhesion, elasticity, stiffness, deformation, chemical binding interactions, viscoelasticity, and electrical properties have all been mapped at the nanoscale with FVM. Subsequently, force maps can be correlated with features of topographic images for identifying certain chemical groups presented at a sample interface. The SPM tip can be coated to investigate-specific reactions; for example, biological interactions can be probed when the tip is coated with biomolecules such as for recognition of ligand-receptor pairs or antigen-antibody interactions. This review highlights the versatility and diverse measurement protocols that have emerged for studies applying FVM for the analysis of material properties at the nanoscale.

Mechanistic basis for tuning iridium hydride photochemistry from H2 evolution to hydride transfer hydrodechlorination.

The photochemistry of metal hydride complexes is dominated by H2 evolution, limiting access to reductive transformations based on photochemical hydride transfer. In this article, the innate H2 evolution photochemistry of the iridium hydride complexes [Cp*Ir(bpy-OMe)H]+ (1, bpy-OMe = 4,4'-dimethoxy-2,2'-bipyridine) and [Cp*Ir(bpy)H]+ (2, bpy = 2,2'-bipyridine) is diverted towards photochemical hydrodechlorination. Net hydride transfer from 1 and 2 to dichloromethane produces chloromethane with high selectivity and exceptional photochemical quantum yield (Φ ≤ 1.3). Thermodynamic and kinetic mechanistic studies are consistent with a non-radical-chain reaction sequence initiated by "self-quenching" electron transfer between excited state and ground state hydride complexes, followed by proton-coupled electron transfer (PCET) hydrodechlorination that outcompetes H-H coupling. This unique photochemical mechanism provides a new hope for the development of light-driven hydride transfer reactions.

Maximizing the Photocatalytic Activity of Metal-Organic Frameworks with Aminated-Functionalized Linkers: Substoichiometric Effects in MIL-125-NH2.

Despite the promise of utilizing metal-organic frameworks (MOFs) as highly tunable photocatalytic materials, systematic studies that interrogate the relationship between their catalytic performances and the amount of functionalized linkers are lacking. Aminated linkers are known to enhance the absorption of light and afford photocatalysis with MOFs under visible-light irradiation. However, the manner in which the photocatalytic performances are impacted by the amount of such linkers is poorly understood. Here, we assess the photocatalytic activity of MIL-125, a TiO2/1,4-benzenedicarboxylate (bdc) MOF for the oxidation of benzyl alcohol to benzaldehyde when increasing amounts of bdc-NH2 linkers (0%, 20%, 46%, 70%, and 100%) are incorporated in the framework. Analytical TEM allowed assessing the homogeneous localization of bdc-NH2 in these mixed-linker MOFs. Steady state reaction rates reveal two regimes of catalytic performances: a first linear regime up to ∼50% bdc-NH2 into the hybrid framework whereby increased amounts of bdc-NH2 yielded increased photocatalytic rates, followed by a plateau up to 100% bdc-NH2. This unexpected "saturation" of the catalytic activity above ∼50% bdc-NH2 content in the framework whatever the wavelength filters used demonstrates that amination of all linkers of the MOF is not required to obtain the maximum photocatalytic activity. This is rationalized on the basis of mixed-valence Ti3+/Ti4+ intermediate catalytic centers revealed by electron spin resonance (ESR) measurements and recent knowledge of lifetime excited states in MIL-125-type of solids.

Molecular polypyridine-based metal complexes as catalysts for the reduction of CO2.

Polypyridyl transition metal complexes represent one of the more thoroughly studied classes of molecular catalysts towards CO2 reduction to date. Initial reports in the 1980s began with an emphasis on 2nd and 3rd row late transition metals, but more recently the focus has shifted towards earlier metals and base metals. Polypyridyl platforms have proven quite versatile and amenable to studying various parameters that govern product distribution for CO2 reduction. However, open questions remain regarding the key mechanistic steps that govern product selectivity and efficiency. Polypyridyl complexes have also been immobilized through a variety of methods to afford active catalytic materials for CO2 reductions. While still an emerging field, materials incorporating molecular catalysts represent a promising strategy for electrochemical and photoelectrochemical devices capable of CO2 reduction. In general, this class of compounds remains the most promising for the continued development of molecular systems for CO2 reduction and an inspiration for the design of related non-polypyridyl catalysts.

Efficient Photochemical Dihydrogen Generation Initiated by a Bimetallic Self-Quenching Mechanism.

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.

Thermodynamic Hydricity of Transition Metal Hydrides.

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.

A Simple and Non-Destructive Method for Assessing the Incorporation of Bipyridine Dicarboxylates as Linkers within Metal-Organic Frameworks.

As a novel avenue for applications, metal-organic frameworks (MOFs) are increasingly used for heterogenizing catalytic molecular species as linkers into their crystalline framework. These multifunctional compounds can be accessed with mixed linkers synthesis or postsynthetic-exchange strategies. Major limitations still reside in their challenging characterization; in particular, to provide evidence of the genuine incorporation of the functionalized linkers into the framework and their quantification. Herein, we demonstrate that a combination of computational chemistry, spectroscopy and X-ray diffraction allows access to a non-destructive analysis of mixed-linker UiO-67-type materials featuring biphenyl- and bipyridine-dicarboxylates. Our UV/Vis-based methodology has been further applied to characterize a series of Rh-functionalized UiO-67-type catalysts. The proposed approach allows a recurrent key issue in the characterization of similar supported organometallic systems to be solved.

Versatile functionalization of carbon electrodes with a polypyridine ligand: metallation and electrocatalytic H(+) and CO2 reduction.

A strategy is proposed for immobilization of homogeneous catalysts whereby a glassy carbon electrode is functionalized by electro-grafting of a ligand, terpyridine. The modified electrode can easily be metallated with cobalt and shows activity towards catalytic proton and CO2 reduction. The metal can be removed and the electrode re-metallated at will.

Photocatalytic carbon dioxide reduction with rhodium-based catalysts in solution and heterogenized within metal-organic frameworks.

The first photosensitization of a rhodium-based catalytic system for CO2 reduction is reported, with formate as the sole carbon-containing product. Formate has wide industrial applications and is seen as valuable within fuel cell technologies as well as an interesting H2 -storage compound. Heterogenization of molecular rhodium catalysts is accomplished via the synthesis, post-synthetic linker exchange, and characterization of a new metal-organic framework (MOF) Cp*Rh@UiO-67. While the catalytic activities of the homogeneous and heterogeneous systems are found to be comparable, the MOF-based system is more stable and selective. Furthermore it can be recycled without loss of activity. For formate production, an optimal catalyst loading of ∼10 % molar Rh incorporation is determined. Increased incorporation of rhodium catalyst favors thermal decomposition of formate into H2 . There is no precedent for a MOF catalyzing the latter reaction so far.

Turning it off! Disfavouring hydrogen evolution to enhance selectivity for CO production during homogeneous CO2 reduction by cobalt-terpyridine complexes.

Understanding the activity and selectivity of molecular catalysts for CO2 reduction to fuels is an important scientific endeavour in addressing the growing global energy demand. Cobalt-terpyridine compounds have been shown to be catalysts for CO2 reduction to CO while simultaneously producing H2 from the requisite proton source. To investigate the parameters governing the competition for H+ reduction versus CO2 reduction, the cobalt bisterpyridine class of compounds is first evaluated as H+ reduction catalysts. We report that electronic tuning of the ancillary ligand sphere can result in a wide range of second-order rate constants for H+ reduction. When this class of compounds is next submitted to CO2 reduction conditions, a trend is found in which the less active catalysts for H+ reduction are the more selective towards CO2 reduction to CO. This represents the first report of the selectivity of a molecular system for CO2 reduction being controlled through turning off one of the competing reactions. The activities of the series of catalysts are evaluated through foot-of-the-wave analysis and a catalytic Tafel plot is provided.

Role of pendant proton relays and proton-coupled electron transfer on the hydrogen evolution reaction by nickel hangman porphyrins.

The hangman motif provides mechanistic insights into the role of pendant proton relays in governing proton-coupled electron transfer (PCET) involved in the hydrogen evolution reaction (HER). We now show improved HER activity of Ni compared with Co hangman porphyrins. Cyclic voltammogram data and simulations, together with computational studies using density functional theory, implicate a shift in electrokinetic zone between Co and Ni hangman porphyrins due to a change in the PCET mechanism. Unlike the Co hangman porphyrin, the Ni hangman porphyrin does not require reduction to the formally metal(0) species before protonation by weak acids in acetonitrile. We conclude that protonation likely occurs at the Ni(I) state followed by reduction, in a stepwise proton transfer-electron transfer pathway. Spectroelectrochemical and computational studies reveal that upon reduction of the Ni(II) compound, the first electron is transferred to a metal-based orbital, whereas the second electron is transferred to a molecular orbital on the porphyrin ring.

Terpyridine complexes of first row transition metals and electrochemical reduction of CO2 to CO.

Homoleptic terpyridine complexes of first row transition metals are evaluated as catalysts for the electrocatalytic reduction of CO2. Ni and Co-based catalytic systems are shown to reduce CO2 to CO under the conditions tested. The Ni complex was found to exhibit selectivity for CO2 over proton reduction while the Co-based system generates mixtures of CO and H2 with CO : H2 ratios being tuneable through variation of the applied potential.

Halogen Photoelimination from Dirhodium Phosphazane Complexes via Chloride-Bridged Intermediates.

Halogen photoelimination is a critical step in HX-splitting photocatalysis. Herein, we report the photoreduction of a pair of valence-isomeric dirhodium phosphazane complexes, and suggest that a common intermediate is accessed in the photochemistry of both mixed-valent and valence-symmetric complexes. The results of these investigations suggest that halogen photoelimination proceeds by two sequential photochemical reactions: ligand dissociation followed by subsequent halogen elimination.

Iron in a trigonal tris(alkoxide) ligand environment.

Mononuclear Fe(II) and Fe(III) complexes residing in a trigonal tris(ditox) (ditox = (t)Bu2(Me)CO(-)) ligand environment have been synthesized and characterized. The Fe(III) ditox complex does not react with oxidants such as PhIO, whereas NMe3O substitutes a coordinated tetrahydrofuran (THF) in the apical position without undergoing oxo transfer. In contrast, the Fe(II) ditox complex reacts rapidly with PhIO or Me3NO in THF or cyclohexadiene to furnish a highly reactive intermediate, which cleaves C-H bonds to afford the Fe(III)-hydroxide complex. When generated in 1,2-difluorobenze, this intermediate can be intercepted to oxidize phosphines to phosphine oxide. The fast rates at which these reactions occur is attributed to a particularly weak ligand field imparted by the tris(alkoxide) ancillary ligand environment.

Cobalt in a bis-ß-diketiminate environment.

The reaction of Co(2)(mesityl)(4) with acetonitrile leads to the formation of a planar, low spin, bis-ß-diketiminate cobalt(II) complex, (1-mesitylbutane-1,3-diimine)(2)Co (1). EPR spectroscopy, magnetic studies, and DFT calculations reveal the Co(II) ion to reside in a tetragonal ligand field with a (2)B(2)(d(yz))(1) ground state electronic configuration. Oxidation of 1 with ferrocenium hexafluorophosphate furnishes (1-mesitylbutane-1,3-diimine)(2)Co(THF)(2)PF(6) (2). The absence of significant changes in the metal-ligand bond metrics of the X-ray crystal structures of 1 and 2 supports ligand participation in the oxidation event. Moreover, no significant changes in C-C or C-N bond lengths are observed by X-ray crystallography upon oxidation of a ß-diketiminate ligand, in contrast to typical redox noninnocent ligand platforms.