Solid-State Structure and Calculated Electronic Structure, Formation Energy, Chemical Bonding, and Optical Properties of Zn₄O(FMA)₃ and Its Heavier Congener Cd₄O(FMA)₃

The equilibrium solid-state structure of the experimentally synthesized but incompletely characterized Zn₄O­(FMA)₃ is revised with the help of density functional theory computational methods. The electronic structure, formation energy, chemical bonding, and optical properties of Zn₄O­(FMA)₃ and its heavier congener Cd₄O­(FMA)₃ have been systematically investigated. The calculated bulk moduli for Zn₄O­(FMA)₃ and Cd₄O­(FMA)₃ are similarly small (and slightly smaller than the previously reported values for MOF-5), indicative of relatively soft materials. Their estimated band-gap values are ca. 3.2 eV (somewhat lower than that of MOF-5, 3.4–3.5 eV), indicating semiconducting character. The optical properties including dielectric function ε­(ω), refractive index <i>n</i>(ω), absorption coefficient α­(ω), optical conductivity σ­(ω), reflectivity <i>R</i>(ω), and electron energy-loss spectrum <i>L</i>(ω) of M₄O­(FMA)₃ (M = Zn, Cd) were systematically studied. Analysis of chemical bonding reveals that the M–O bonds are largely ionic, with an increase in ionicity from Zn to Cd. The total energy calculations establish that compounds M₄O­(FMA)₃ have large negative formation energies, ca. −80 and −70 kJ·mol^–1 for Zn and Cd, respectively. Whereas Zn₄O­(FMA)₃ has already been synthesized, the results suggest that the heavier congener Cd₄O­(FMA)₃ might be experimentally accessible

On the Mechanism of Cyclopropanation Reactions Catalyzed by a Rhodium(I) Catalyst Bearing a Chelating Imine-Functionalized NHC Ligand: A Computational Study

A computational study at the DFT level using BP86 and dispersion-corrected D-BP86 methods has been performed on the mechanism of a highly <i>cis</i>-diastereoselective cyclopropanation reaction between ethyl diazoacetate and styrene, catalyzed by a Rh­(I) complex bearing a chelating imine-functionalized NHC ligand. The key steps in the mechanism have been elucidated. The favored mechanistic pathway was found to be a stepwise mechanism involving the formation of a Rh metallacyclobutane intermediate. The results from the theoretical study indicate that the diastereoselectivity is determined in the step where styrene coordinates to the Rh­(I) carbenoid and that the high <i>cis</i>-diastereoselectivity can be attributed to an unfavorable steric interaction between styrene and the substituents on the <i>N</i>-aryl ring in the ligand system, which disfavors the formation of the <i>trans</i> cyclopropanation product

Versatile Methods for Preparation of New Cyclometalated Gold(III) Complexes

Versatile methods for the high-yield syntheses of new cyclometalated gold­(III) complexes are described. Mono- or dialkylated or arylated gold­(III) complexes are selectively obtained from reaction of Au­(OCOCF₃)₂(tpy) (tpy = 2-(<i>p</i>-tolyl)­pyridine) with either RMgX or RLi, respectively. Specifically, AuMe₂(tpy) and AuPh₂(tpy) were prepared with the respective lithium reagents, and AuBrMe­(tpy), AuBrEt­(tpy), and AuBrPh­(tpy) were prepared with Grignard reagents. The molecular structures of compounds Au­(OCOCF₃)₂(tpy) and AuMe₂(tpy) were determined by single crystal X-ray diffraction

Versatile Methods for Preparation of New Cyclometalated Gold(III) Complexes

Versatile methods for the high-yield syntheses of new cyclometalated gold­(III) complexes are described. Mono- or dialkylated or arylated gold­(III) complexes are selectively obtained from reaction of Au­(OCOCF₃)₂(tpy) (tpy = 2-(<i>p</i>-tolyl)­pyridine) with either RMgX or RLi, respectively. Specifically, AuMe₂(tpy) and AuPh₂(tpy) were prepared with the respective lithium reagents, and AuBrMe­(tpy), AuBrEt­(tpy), and AuBrPh­(tpy) were prepared with Grignard reagents. The molecular structures of compounds Au­(OCOCF₃)₂(tpy) and AuMe₂(tpy) were determined by single crystal X-ray diffraction

Versatile Methods for Preparation of New Cyclometalated Gold(III) Complexes

Versatile methods for the high-yield syntheses of new cyclometalated gold­(III) complexes are described. Mono- or dialkylated or arylated gold­(III) complexes are selectively obtained from reaction of Au­(OCOCF₃)₂(tpy) (tpy = 2-(<i>p</i>-tolyl)­pyridine) with either RMgX or RLi, respectively. Specifically, AuMe₂(tpy) and AuPh₂(tpy) were prepared with the respective lithium reagents, and AuBrMe­(tpy), AuBrEt­(tpy), and AuBrPh­(tpy) were prepared with Grignard reagents. The molecular structures of compounds Au­(OCOCF₃)₂(tpy) and AuMe₂(tpy) were determined by single crystal X-ray diffraction

DFT Investigation of Suzuki–Miyaura Reactions with Aryl Sulfamates Using a Dialkylbiarylphosphine-Ligated Palladium Catalyst

Aryl sulfamates are valuable electrophiles for cross-coupling reactions because they can easily be synthesized from phenols and can act as directing groups for C–H bond functionalization prior to cross-coupling. Recently, it was demonstrated that (1-^tBu-Indenyl)­Pd­(XPhos)­Cl (XPhos = 2-dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl) is a highly active precatalyst for room-temperature Suzuki–Miyaura couplings of a variety of aryl sulfamates. Herein, we report an in-depth computational investigation into the mechanism of Suzuki–Miyaura reactions with aryl sulfamates using an XPhos-ligated palladium catalyst. Particular emphasis is placed on the turnover-limiting oxidative addition of the aryl sulfamate C–O bond, which has not been studied in detail previously. We show that bidentate coordination of the XPhos ligand via an additional interaction between the biaryl ring and palladium plays a key role in lowering the barrier to oxidative addition. This result is supported by NBO and NCI-Plot analysis on the transition states for oxidative addition. After oxidative addition, the catalytic cycle is completed by transmetalation and reductive elimination, which are both calculated to be facile processes. Our computational findings explain a number of experimental results, including why elevated temperatures are required for the coupling of phenyl sulfamates without electron-withdrawing groups and why aryl carbamate electrophiles are not reactive with this catalyst

Insight into the Efficiency of Cinnamyl-Supported Precatalysts for the Suzuki–Miyaura Reaction: Observation of Pd(I) Dimers with Bridging Allyl Ligands During Catalysis

Despite widespread use of complexes of the type Pd­(L)­(η³-allyl)Cl as precatalysts for cross-coupling, the chemistry of related Pd^I dimers of the form (μ-allyl)­(μ-Cl)­Pd₂(L)₂ has been underexplored. Here, the relationship between the monomeric and the dimeric compounds is investigated using both experiment and theory. We report an efficient synthesis of the Pd^I dimers (μ-allyl)­(μ-Cl)­Pd₂(IPr)₂ (allyl = allyl, crotyl, cinnamyl; IPr = 1,3-bis­(2,6-diisopropylphenyl)­imidazol-2-ylidene) through activation of Pd­(IPr)­(η³-allyl)Cl type monomers under mildly basic reaction conditions. The catalytic performance of the Pd^II monomers and their Pd^I μ-allyl dimer congeners for the Suzuki–Miyaura reaction is compared. We propose that the (μ-allyl)­(μ-Cl)­Pd₂(IPr)₂-type dimers are activated for catalysis through disproportionation to Pd­(IPr)­(η³-allyl)Cl and monoligated IPr–Pd⁰. The microscopic reverse comproportionation reaction of monomers of the type Pd­(IPr)­(η³-allyl)Cl with IPr–Pd⁰ to form Pd^I dimers is also studied. It is demonstrated that this is a facile process, and Pd^I dimers are directly observed during catalysis in reactions using Pd^II precatalysts. In these catalytic reactions, Pd^I μ-allyl dimer formation is a deleterious process which removes the IPr–Pd⁰ active species from the reaction mixture. However, increased sterics at the 1-position of the allyl ligand in the Pd­(IPr)­(η³-crotyl)Cl and Pd­(IPr)­(η³-cinnamyl)Cl precatalysts results in a larger kinetic barrier to comproportionation, which allows more of the active IPr–Pd⁰ catalyst to enter the catalytic cycle when these substituted precatalysts are used. Furthermore, we have developed reaction conditions for the Suzuki-Miyaura reaction using Pd­(IPr)­(η³-cinnamyl)Cl which are compatible with mild bases

Insight into the Efficiency of Cinnamyl-Supported Precatalysts for the Suzuki–Miyaura Reaction: Observation of Pd(I) Dimers with Bridging Allyl Ligands During Catalysis

Despite widespread use of complexes of the type Pd­(L)­(η³-allyl)Cl as precatalysts for cross-coupling, the chemistry of related Pd^I dimers of the form (μ-allyl)­(μ-Cl)­Pd₂(L)₂ has been underexplored. Here, the relationship between the monomeric and the dimeric compounds is investigated using both experiment and theory. We report an efficient synthesis of the Pd^I dimers (μ-allyl)­(μ-Cl)­Pd₂(IPr)₂ (allyl = allyl, crotyl, cinnamyl; IPr = 1,3-bis­(2,6-diisopropylphenyl)­imidazol-2-ylidene) through activation of Pd­(IPr)­(η³-allyl)Cl type monomers under mildly basic reaction conditions. The catalytic performance of the Pd^II monomers and their Pd^I μ-allyl dimer congeners for the Suzuki–Miyaura reaction is compared. We propose that the (μ-allyl)­(μ-Cl)­Pd₂(IPr)₂-type dimers are activated for catalysis through disproportionation to Pd­(IPr)­(η³-allyl)Cl and monoligated IPr–Pd⁰. The microscopic reverse comproportionation reaction of monomers of the type Pd­(IPr)­(η³-allyl)Cl with IPr–Pd⁰ to form Pd^I dimers is also studied. It is demonstrated that this is a facile process, and Pd^I dimers are directly observed during catalysis in reactions using Pd^II precatalysts. In these catalytic reactions, Pd^I μ-allyl dimer formation is a deleterious process which removes the IPr–Pd⁰ active species from the reaction mixture. However, increased sterics at the 1-position of the allyl ligand in the Pd­(IPr)­(η³-crotyl)Cl and Pd­(IPr)­(η³-cinnamyl)Cl precatalysts results in a larger kinetic barrier to comproportionation, which allows more of the active IPr–Pd⁰ catalyst to enter the catalytic cycle when these substituted precatalysts are used. Furthermore, we have developed reaction conditions for the Suzuki-Miyaura reaction using Pd­(IPr)­(η³-cinnamyl)Cl which are compatible with mild bases

Effect of 2‑Substituents on Allyl-Supported Precatalysts for the Suzuki–Miyaura Reaction: Relating Catalytic Efficiency to the Stability of Palladium(I) Bridging Allyl Dimers

One of the most commonly used classes of precatalysts for cross-coupling are Pd­(II) complexes of the type (η³-allyl)­Pd­(L)­Cl. Here, we report the first full investigation of how the steric and electronic properties of the 2-substituent affect the catalytic properties of precatalysts of the type (η³-allyl)­Pd­(L)­Cl. Specifically, we have prepared and studied a series of well-defined 2-substituted precatalysts of the type (η³-2-R-allyl)­Pd­(IPr)Cl (R = H, Ph, Me, ^tBu, OMe, CN), as well as their related Pd­(I) (μ-2-R-allyl)­(μ-Cl)­Pd₂(IPr)₂ dimers. The catalytic performance of the Pd­(II) monomers and their Pd­(I) μ-allyl dimer congeners is compared for the Suzuki–Miyaura reaction. When Pd­(II) monomers are used as precatalysts, we observe the formation of the Pd­(I) μ-allyl dimers during catalysis. In fact, we find that the catalytic efficiency of (η³-2-R-allyl)­Pd­(IPr)Cl precatalysts correlates inversely with the thermodynamic stability of the related Pd­(I) μ-allyl dimers. Therefore, we have examined the structural and electronic properties of the Pd­(I) μ-allyl dimers in detail and probed the mechanism of the (μ-2-R-allyl)­(μ-Cl)­Pd₂(IPr)₂ dimer/(η³-2-R-allyl)­Pd­(IPr)­Cl monomer interconversion both experimentally and computationally. Overall, this study shows that the formation of Pd­(I) μ-allyl dimers can play a crucial role in determining the catalytic efficiency of precatalysts of the type (η³-allyl)­Pd­(IPr)­Cl

A Gold Exchange: A Mechanistic Study of a Reversible, Formal Ethylene Insertion into a Gold(III)–Oxygen Bond

The Au­(III) complex Au­(OAc^F)₂(tpy) (<b>1</b>, OAc^F = OCOCF₃; tpy = 2-<i>p</i>-tolylpyridine) undergoes reversible dissociation of the OAc^F ligand <i>trans</i> to C, as seen by ¹⁹F NMR. In dichloromethane or trifluoroacetic acid (TFA), the reaction between <b>1</b> and ethylene produces Au­(OAc^F)­(CH₂CH₂OAc^F)­(tpy) (<b>2</b>). The reaction is a formal insertion of the olefin into the Au–O bond <i>trans</i> to N. In TFA this reaction occurs in less than 5 min at ambient temperature, while 1 day is required in dichloromethane. In trifluoroethanol (TFE), Au­(OAc^F)­(CH₂CH₂OCH₂CF₃)­(tpy) (<b>3</b>) is formed as the major product. Both <b>2</b> and <b>3</b> have been characterized by X-ray crystallography. In TFA/TFE mixtures, <b>2</b> and <b>3</b> are in equilibrium with a slight thermodynamic preference for <b>2</b> over <b>3</b>. Exposure of <b>2</b> to ethylene-<i>d</i>₄ in TFA caused exchange of ethylene-<i>d</i>₄ for ethylene at room temperature. The reaction of <b>1</b> with <i>cis</i>-1,2-dideuterioethylene furnished Au­(OAc^F)­(<i>threo</i>-CHDCHDOAc^F)­(tpy), consistent with an overall <i>anti</i> addition to ethylene. DFT­(PBE0-D3) calculations indicate that the first step of the formal insertion is an associative substitution of the OAc^F <i>trans</i> to N by ethylene. Addition of free ^–OAc^F to coordinated ethylene furnishes <b>2</b>. While substitution of OAc^F by ethylene <i>trans</i> to C has a lower barrier, the kinetic and thermodynamic preference of <b>2</b> over the isomer with CH₂CH₂OAc^F <i>trans</i> to C accounts for the selective formation of <b>2</b>. The DFT calculations suggest that the higher reaction rates observed in TFA and TFE compared with CH₂Cl₂ arise from stabilization of the ^–OAc^F anion lost during the first reaction step