Reducing CO₂ to Methanol Using Frustrated Lewis Pairs: On the Mechanism of Phosphine–Borane-Mediated Hydroboration of CO₂

The full mechanism of the hydroboration of CO₂ by the highly active ambiphilic organocatalyst 1-Bcat-2-PPh₂–C₆H₄ (Bcat = catecholboryl) was determined using computational and experimental methods. The intramolecular Lewis pair was shown to be involved in every step of the stepwise reduction. In contrast to traditional frustrated Lewis pair systems, the lack of steric hindrance around the Lewis basic fragment allows activation of the reducing agent while moderate Lewis acidity/basicity at the active centers promotes catalysis by releasing the reduction products. Simultaneous activation of both the reducing agent and carbon dioxide is the key to efficient catalysis in every reduction step

A Highly Active Phosphine–Borane Organocatalyst for the Reduction of CO₂ to Methanol Using Hydroboranes

In this work, we report that organocatalyst 1-Bcat-2-PPh₂–C₆H₄ ((1); cat = catechol) acts as an ambiphilic metal-free system for the reduction of carbon dioxide in presence of hydroboranes (HBR₂ = HBcat (catecholborane), HBpin (pinacolborane), 9-BBN (9-borabicyclo[3.3.1]­nonane), BH₃·SMe₂ and BH₃·THF) to generate CH₃OBR₂ or (CH₃OBO)₃, products that can be readily hydrolyzed to methanol. The yields can be as high as 99% with exclusive formation of CH₃OBR₂ or (CH₃OBO)₃ with TON (turnover numbers) and TOF (turnover frequencies) reaching >2950 and 853 h^–1, respectively. Furthermore, the catalyst exhibits “living” behavior: once the first loading is consumed, it resumes its activity on adding another loading of reagents

Synthesis and Reactivity of Novel Mesityl Boratabenzene Ligands and Their Coordination to Transition Metals

The synthesis and characterization of novel bulky mesitylboratabenzene ligands have been achieved. The isolated boracyclohexadienes <b>6a</b>,<b>b</b> were found to be extremely stable and could be selectively desilylated by hydrolysis. These ligands have been successfully coordinated to Fe­(II) and Cr­(II). The selective desilylation of the boratabenzene ring was also observed in the iron complexes, thus furnishing three ironboratabenzene sandwich complexes without a SiMe₃ group (<b>11</b>) and with one (<b>10</b>) and two (<b>12</b>) SiMe₃ groups. Interestingly, species <b>11</b> represents the first structurally characterized bis­(boratabenzene) metallic species not exhibiting the expected trans geometry

A Tris(triphenylphosphine)aluminum Ambiphilic Precatalyst for the Reduction of Carbon Dioxide with Catecholborane

The ambiphilic species Al­(C₆H₄(<i>o</i>-PPh₂))₃ (<b>2</b>) was synthesized and fully characterized, notably using X-ray diffraction. Species <b>2</b> exhibits pseudo-bipyramidal-trigonal geometry caused by the two Al–P interactions. <b>2</b> reacts with CO₂ to generate a CO₂ adduct commonly observed in the activation of CO₂ using frustrated Lewis pairs (FLPs). This ambiphilic species serves as a precatalyst for the reduction of CO₂ in the presence of catecholborane (HBcat) to generate CH₃OBcat, which can be readily hydrolyzed in methanol. The reaction mixture confirms that, in the presence of HBcat, <b>2</b> generates the known CO₂ reduction catalyst 1-Bcat-2-PPh₂-C₆H₄ (<b>1</b>) and intractable catecholate aluminum species. It was, however, possible to isolate a single crystal of Al­(κ²<i>O</i>,<i>O</i>-(MeO)₂Bcat)₃ (<b>5</b>) supporting this hypothesis. Also, a borane-protected analogue of <b>2</b>, Al­(C₆H₄(<i>o</i>-PPh₂·BH₃))₃ (<b>4</b>), does not react with catecholborane, suggesting the influence of the pendant phosphines in the transformation of <b>2</b> into <b>1</b>

Insights into the Formation of Borabenzene Adducts via Ligand Exchange Reactions and TMSCl Elimination from Boracyclohexadiene Precursors

The bonding properties of borabenzene with various neutral Lewis bases have been investigated. 1-Chloro-4-isopropyl-2-(trimethylsilyl)-2,4-boracyclohexadiene reacts with a number of Lewis basesnotably pyridine, PMe₃, PCy₃, and PPh₃to afford 4-isopropylborabenzene–base adducts. These adducts can undergo ligand exchange to afford new borabenzene complexes. The scope and mechanism of the reaction, as well as the steric and electronic properties of different adducts, were studied experimentally and computationally

Synthesis of Carboxylate Cp*Zr(IV) Species: Toward the Formation of Novel Metallocavitands

With the intent of generating metallocavitands isostructural to species [(CpZr)₃(μ³-O)­(μ²-OH)₃­(κ_O,O,μ²-O₂C­(R))₃]⁺, the reaction of Cp*₂ZrCl₂ and Cp*ZrCl₃ with phenylcarboxylic acids was carried out. Depending on the reaction conditions, five new complexes were obtained, which consisted of Cp*₂ZrCl­(κ²-OOCPh) (<b>1</b>), (Cp*ZrCl­(κ²-OOCPh))₂­(μ-κ²-OOCPh)₂ (<b>2</b>), [(Cp*Zr­(κ²-OOCPh))₂­(μ-κ²-OOCPh)₂­(μ²-OH)₂]·Et₂O (<b>3</b>·<b>Et</b>_<b>2</b><b>O</b>), [[Cp*ZrCl₂]­(μ-Cl)­(μ-OH)­(μ-O₂CC₆H₅)­[Cp*Zr]]₂­(μ-O₂CC₆H₅)₂ (<b>4</b>), and [Cp*ZrCl₄]­[(Cp*Zr)₃­(κ₂-OOC­(C₆H₄Br)₃­(μ₃-O)­(μ₂-Cl)₂­(μ₂-O<i>H</i>)] [<b>5</b>]⁺[<b>Cp*ZrCl</b>_<b>4</b>]⁻. The structural characterization of the five complexes was carried out. Species <b>3</b>·<b>Et</b>_<b>2</b><b>O</b> exhibits host–guest properties where the diethyl ether molecule is included in a cavity formed by two carboxylate moieties. The secondary interactions between the cavity and the diethyl ether molecule affect the structural parameters of the complex, as demonstrated be the comparison of the density functional theory models for <b>3</b> and <b>3</b>·<b>Et</b>_<b>2</b><b>O</b>. Species <b>5</b> was shown to be isostructural to the [(CpZr)₃­(μ³-O)­(μ²-OH)₃­(κ_O,O,μ²-O₂C­(R))₃]⁺ metallocavitands

Main-Group Metallomimetics: Transition Metal-like Photolytic CO Substitution at Boron

The carbon monoxide adduct of an unhindered and highly reactive CAAC-bound arylborylene, [(CAAC)­B­(CO)­Ar] (CAAC = cyclic (alkyl) (amino)­carbene), has been prepared using a transfer reaction from the linear iron borylene complex [(PMe₃) (CO)₃Fe=BAr]. [(CAAC)­B­(CO)­Ar] is a source of the dicoordinate [(CAAC)­ArB:] borylene that can be liberated by selective photolytic CO extrusion and that, although highly reactive, is sufficiently long-lived to react intermolecularly. Through trapping of the borylene generated in this manner, we present, among others, the first metal-free borylene­(I) species containing a nitrogen-based donor, as well as a new boron-containing radical

Hydroboration of Carbon Dioxide Using Ambiphilic Phosphine–Borane Catalysts: On the Role of the Formaldehyde Adduct

Ambiphilic phosphine–borane derivatives 1-B­(OR)₂-2-PR′₂–C₆H₄ (R′ = Ph (<b>1</b>), <i>i</i>Pr (<b>2</b>); (OR)₂ = (OMe)₂ (<b>1a, 2a</b>); catechol (<b>1b, 2b</b>) pinacol (<b>1c, 2c</b>), −OCH₂C­(CH₃)₂CH₂O– (<b>1d</b>)) were tested as catalysts for the hydroboration of CO₂ using HBcat or BH₃·SMe₂ to generate methoxyboranes. It was shown that the most active species were the catechol derivatives <b>1b</b> and <b>2b</b>. In the presence of HBcat, without CO₂, ambiphilic species <b>1a</b>, <b>1c</b>, and <b>1d</b> were shown to transform to <b>1b</b>, whereas <b>2a</b> and <b>2c</b> were shown to transform to <b>2b</b>. The formaldehyde adducts <b>1b·CH</b>_<b>2</b><b>O</b> and <b>2b·CH</b>_<b>2</b><b>O</b> are postulated to be the active catalysts in the reduction of CO₂ rather than being simple resting states. Isotope labeling experiments and density functional theory (DFT) studies show that once the formaldehyde adduct is generated, the CH₂O moiety remains on the ambiphilic system through catalysis. Species <b>2b·CH</b>_<b>2</b><b>O</b> was shown to exhibit turnover frequencies for the CO₂ reduction using BH₃·SMe₂ up to 228 h^–1 at ambient temperature and up to 873 h^–1 at 70 °C, mirroring the catalytic activity of <b>1b</b>