Linkage Isomerization Reactions of M(CO)₂L Complexes (M = (η⁵-C₅H₅)Mn, (η⁵-C₅H₅)Re, or (η⁶-C₆H₆)Cr; L = 2,3-Dihydrofuran): A Step-Scan FTIR and DFT Study

The linkage isomers, M(CO)2-(η1-(O)-2,3 DHF) and M(CO)2-(η2-(C,C)-2,3 DHF) [M = (η5-C5H5)Mn, (η5-C5H5)Re, (η6-C6H6)Cr; DHF = dihydrofuran] are formed upon photolysis of the parent M(CO)3 complexes in the presence of 2,3-DHF. The rearrangement of the oxygen bound to the thermodynamically favored π bound complex is followed on the millisecond to microsecond time scale using step-scan FTIR. The rate of the isomerization reaction increases in the order Re < Mn < Cr primarily due to a decrease in the activation enthalpy. The experimental data along with theoretical calculations suggest that the rearrangement proceeds intramolecularly in which the metal migrates from one functional group to another

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

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

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

Photochemically Generated Transients from κ²- and κ³-Triphos Derivatives of Group 6 Metal Carbonyls and Their Reactivity with Olefins

The synthesis and characterization of (κ2-Triphos)­M­(CO)4 derivatives, where M = Mo, W and Triphos = MeC­(CH2PPh2)3, are reported. Photolyses of these metal carbonyls in dichloromethane or CO2-saturated dichloromethane readily afford the (κ3-Triphos)­M­(CO)3 complexes with no evidence of significant solvent or carbon dioxide interactions with the site vacated by CO. However, in the presence of 1-hexene a transient (κ2-Triphos)­M­(CO)3(1-hexene) adduct was observed, which subsequently releases the olefin with formation of the stable κ3-tricarbonyl species. In the case of M = W the kinetic parameters for this process were assessed, with the rate of olefin replacement being inversely proportional to [1-hexene]. A dissociative rate constant of 25.6 ± 1.1 s–1 at 298 K was determined for olefin loss, with the selectivity for 1-hexene vs free phosphine arm addition to the unsaturated intermediate being somewhat surprisingly large at 22. The activation parameters measured were ΔH⧧ = 26.1 ± 0.4 kcal/mol and ΔS⧧ = 36 ± 3 eu, which are consistent with a dissociative substitution reaction. The kinetic parameters for this transformation were unaffected in the presence of excess quantities of CO2. Although no interaction of CO2 with the transient species resulting from CO loss in the κ2 complex was noted on the time scale of 50 ms, an intermediate described as an η2-HSiEt3 complex was observed upon addition of triethylsilane. This latter transient species underwent dissociation with κ3-complex formation about 15 times as fast as its 1-hexene analogue. X-ray structures of the κ2 complexes of Mo and W where the dangling phosphine arm has undergone oxidation are also reported

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

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

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

Manganese Tricarbonyl Diimine Bromide Complexes as Electrocatalysts for Proton Reduction

Manganese tricarbonyl diimine complexes bearing pyridine and imidazole ligands have been prepared as electrocatalysts for proton reduction using acetic acid as the proton source. The electron-donor ability of the diimine ligand is found to play an important role in determining the efficiency of the electrocatalysts with [MnBr­(pybz)­(CO)3] (pybz = 2-(2-pyridyl)­benzimidazole) exhibiting the lowest overpotential (0.28 V) toward proton reduction. The [Mn­(pybz)­(CO)3(MeCN)]+ cationic complex prepared via debromination of [MnBr­(pybz)­(CO)3] by a silver salt has also been shown to catalyze proton reduction upon its electrochemical reduction. A neutral complex [Mn­(pyridine-benzimidazolate)­(CO)3(MeCN)], which can be synthesized by reacting [MnBr­(pybz)­(CO)3] with a strong base, has been detected using IR-SEC (infrared spectroelectrochemistry) as an intermediate species in the catalytic process. Using [MnBr­(pybz)­(CO)3] as the model electrocatalyst, we have carried out density functional calculations to propose a proton reduction mechanism consistent with our experimental observations

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