A Robust Pentacoordinated Iron(II) Proton Reduction Catalyst Stabilized by a Tripodal Phosphine

A pentacoordinated triphosphine benzenedithiolatoiron­(II) complex containing a vacant site for binding has been prepared and characterized. The complex is found to be a robust proton reduction catalyst with an overpotential of 0.56 V and a turnover frequency of 2900 s^–1 with respect to 0.28 M acetic acid as the proton source. A mechanism describing the electroproton reduction process has been proposed

Intramolecular C–C Bond Coupling of Nitriles to a Diimine Ligand in Group 7 Metal Tricarbonyl Complexes

Dissolution of M­(CO)₃(Br)­(L^Ar) [L^Ar = (2,6-Cl₂-C₆H₃-NCMe)₂CH₂] in either acetonitrile [M = Mn, Re] or benzonitrile (M = Re) results in C–C coupling of the nitrile to the diimine ligand. When reacted with acetonitrile, the intermediate adduct [M­(CO)₃­(NCCH₃)­(L^Ar)]Br forms and undergoes an intramolecular C–C coupling reaction between the nitrile carbon and the methylene carbon of the β-diimine ligand

A Nickel-Based, Tandem Catalytic Approach to Isoindolinones from Imines, Aryl Iodides, and CO

We describe herein a modular nickel-catalyzed synthesis of isoindolinones from imines, aryl iodides, and CO. This reaction is catalyzed by Ni­(1,5-cyclooctadiene)₂ in concert with chloride salts and postulated to proceed via a tandem nickel-catalyzed carbonylation to form <i>N</i>-acyl iminium chloride salts, followed by a spontaneous nickel-catalyzed cyclization. A range of aryl iodides and imines have been found to be viable substrates in this reaction, providing a modular route to generate substituted isoindolinones with high atom economy

Solubilizing Metal–Organic Frameworks for an <i>In Situ</i> IR-SEC Study of a CO₂ Reduction Catalyst

Metal–organic frameworks (MOFs) are typically assembled by bridging metal centers with organic linkers for various applications, including providing robust support for heterogeneous catalysts for CO2 reduction. In this study, we have demonstrated the solubilization of a MOF tethered to a CO2-reducing electrocatalyst and studied its fundamental electrochemistry in THF solvent using infrared spectroelectrochemistry (IR-SEC). The fundamental electrochemical properties of this immobilized catalyst were compared to that of its homogeneous counterpart. This approach provides a foundation for future experimental studies to bridge the gap between homogeneous and heterogeneous electrocatalysis

Calculation of Ionization Energy, Electron Affinity, and Hydride Affinity Trends in Pincer-Ligated d⁸‑Ir(^tBu4PXCXP) Complexes: Implications for the Thermodynamics of Oxidative H₂ Addition

DFT methods are used to calculate the ionization energy (IE) and electron affinity (EA) trends in a series of pincer ligated d⁸-Ir­(^tBu4PXCXP) complexes (<b>1</b>-X), where C is a 2,6-disubstituted phenyl ring with X = O, NH, CH₂, BH, S, PH, SiH₂, and GeH₂. Both <i>C</i>_2<i>v</i> and <i>C</i>₂ geometries are considered. Two distinct σ-type (²A₁ or ²A) and π-type (²B₁ or ²B) electronic states are calculated for each of the free radical cation and anion. The results exhibit complex trends, but can be satisfactorily accounted for by invoking a combination of electronegativity and specific π-orbital effects. The calculations are also used to study the effects of varying X on the thermodynamics of oxidative H₂ addition to <b>1</b>-X. Two closed shell singlet states differentiated in the <i>C</i>₂ point group by the d⁶-electon configuration are investigated for the five-coordinate Ir­(III) dihydride product. One electronic state has a d⁶-(a)²(b)²(b)² configuration and a square pyramidal geometry, the other a d⁶-(a)²(b)²(a)² configuration with a distorted-Y trigonal bipyramidal geometry. No simple correlations are found between the computed reaction energies of H₂ addition and either the IEs or EAs. To better understand the origin of the computed trends, the thermodynamics of H₂ addition are analyzed using a cycle of hydride and proton addition steps. The analysis highlights the importance of the electron and hydride affinities, which are not commonly used in rationalizing trends of oxidative addition reactions. Thus, different complexes such as <b>1</b>-O and <b>1</b>-CH₂ can have very similar reaction energies for H₂ addition arising from opposing hydride and proton affinity effects. Additional calculations on methane C–H bond addition to <b>1</b>-X afford reaction and activation energy trends that correlate with the reaction energies of H₂ addition leading to the Y-product

Acrylic Acid Derivatives of Group 8 Metal Carbonyls: A Structural and Kinetic Study

The synthesis, spectroscopic, and X-ray structural studies of acrylic acid complexes of iron and ruthenium tetracarbonyls are reported. In addition, the deprotonated η²-olefin bound acrylic acid derivative of iron as well as its alkylated species were fully characterized by X-ray crystallography. Kinetic data were determined for the replacement of acrylic acid, acrylate, and methylacrylate for the group 8 metal carbonyls by triphenylphosphine. These processes were found to be first-order in the concentration of metal complex with the rates for dissociative loss of the olefinic ligands from ruthenium being much faster than their iron analogues. However, the ruthenium derivatives afforded formation of primarily <i>mono</i>-phosphine metal tetracarbonyls, whereas the iron complexes led largely to <i>trans</i>-<i>di</i>-phosphine tricarbonyls. This difference in behavior was ascribed to a more stable spin crossover species ³Fe­(CO)₄ which undergoes rapid CO loss to afford the <i>bis</i> phosphine derivative. The activation enthalpies for dissociative loss of the deprotonated η²-bound acrylic acid ligand were found to be larger than their corresponding values in the protonated derivatives. For example, for dissociative loss of the protonated and deprotonated acrylic acid derivatives of iron(0) the Δ<i>H</i>^⧧ values determined were 28.0 ± 1.2 and 34.1 ± 1.5 kcal·mol^–1, respectively. Density functional theory (DFT) computations of the bond dissociation energies (BDEs) in these acrylic acids and closely related complexes were in good agreement with enthalpies of activation for these ligand substitution reactions, supportive of a dissociative mechanism for olefin displacement. Processes related to catalytic production of acrylic acid from CO₂ and ethylene are considered

Oxidative Addition of Haloalkanes to Metal Centers: A Mechanistic Investigation

Photolysis of CpRe­(CO)₃ in the presence of dichloromethane results in the initial formation of the CpRe­(CO)₂(ClCH₂Cl) complex followed by insertion of the metal into the C–Cl bond. The activation enthalpy is determined to be 20.4 kcal/mol, and with the assistance of DFT calculations, a radical mechanism is proposed for the oxidative addition reaction. Photolysis of Ni­(CO)₂(PPh₃)₂ with dihalomethanes also results in oxidative addition, but the intermediacy of a halogen-bound adduct has not been established

Oxidative Addition of Haloalkanes to Metal Centers: A Mechanistic Investigation

Photolysis of CpRe­(CO)₃ in the presence of dichloromethane results in the initial formation of the CpRe­(CO)₂(ClCH₂Cl) complex followed by insertion of the metal into the C–Cl bond. The activation enthalpy is determined to be 20.4 kcal/mol, and with the assistance of DFT calculations, a radical mechanism is proposed for the oxidative addition reaction. Photolysis of Ni­(CO)₂(PPh₃)₂ with dihalomethanes also results in oxidative addition, but the intermediacy of a halogen-bound adduct has not been established

Ancillary Ligand Effects upon the Photochemistry of Mn(bpy)(CO)₃X Complexes (X = Br^–, PhCC^–)

The photochemistry of two Mn­(bpy)­(CO)₃X complexes (X = PhCC^–, Br^–) has been studied in the coordinating solvents THF (terahydrofuran) and MeCN (acetonitrile) employing time-resolved infrared spectroscopy. The two complexes are found to exhibit strikingly different photoreactivities and solvent dependencies. In MeCN, photolysis of <b>1</b>-(CO)­(Br) [<b>1</b> = Mn­(bpy)­(CO)₂] affords the ionic complex [<b>1</b>-(MeCN)₂]­Br as a final product. In contrast, photolysis of <b>1</b>-(CO)­(CCPh) in MeCN results in facial to meridional isomerization of the parent complex. When THF is used as solvent, photolysis results in facial to meridional isomerization in both complexes, though the isomerization rate is larger for X = Br^–. Pronounced differences are also observed in the photosubstitution chemistry of the two complexes where both the rate of MeCN exchange from <b>1</b>-(MeCN)­(X) by THFA (tetrahydrofurfurylamine) and the nature of the intermediates generated in the reaction are dependent upon X. DFT calculations are used to support analysis of some of the experiments

Time Resolved Infrared Spectroscopy: Kinetic Studies of Weakly Binding Ligands in an Iron–Iron Hydrogenase Model Compound

Solution photochemistry of (μ-pdt)­[Fe­(CO)₃]₂ (pdt = μ₂-S­(CH₂)₃S), a precursor model of the 2-Fe subsite of the H-cluster of the hydrogenase enzyme, has been studied using time-resolved infrared spectroscopy. Following the loss of CO, solvation of the Fe center by the weakly binding ligands cyclohexene, 3-hexyne, THF, and 2,3-dihydrofuran (DHF) occurred. Subsequent ligand substitution of these weakly bound ligands by pyridine or cyclooctene to afford a more stable complex was found to take place via a dissociative mechanism on a seconds time scale with activation parameters consistent with such a pathway. That is, the Δ<i>S</i>^⧧ values were positive and the Δ<i>H</i>^⧧ parameters closely agreed with bond dissociation enthalpies (BDEs) obtained from DFT calculations. For example, for cyclohexene replacement by pyridine, experimental Δ<i>H</i>^⧧ and Δ<i>S</i>^⧧ values were determined to be 19.7 ± 0.6 kcal/mol (versus a theoretical prediction of 19.8 kcal/mol) and 15 ± 2 eu, respectively. The ambidentate ligand 2,3-DHF was shown to initially bind to the iron center via its oxygen atom followed by an intramolecular rearrangement to the more stable η²-olefin bound species. DFT calculations revealed a transition state structure with the iron atom almost equidistant from the oxygen and one edge of the olefinic bond. The computed Δ<i>H</i>^⧧ of 10.7 kcal/mol for this isomerization process was found to be in excellent agreement with the experimental value of 11.2 ± 0.3 kcal/mol