FLP-Mediated Activations and Reductions of CO2 and CO

Coordination of Al(C6F5)3 vs. B(C6F5)3 on group 6 end-on dinitrogen complexes: chemical and structural divergences

The coordination of the Lewis superacid tris(pentafluorophenyl)alane (AlCF) to phosphine-supported, group 6 bis(dinitrogen) complexes [ML2(N2)2] is explored, with M = Cr, Mo or W and L = dppe (1,2-bis(diphenylphosphino)ethane), depe (1,2-bis(diethylphosphino)ethane), dmpe (1,2-bis(dimethylphosphino)ethane) or 2 × PMe2Ph. Akin to tris(pentafluorophenyl)borane (BCF), AlCF can form 1 : 1 adducts by coordination to one distal nitrogen of general formula trans-[ML2(N2){(μ-η1:η1-N2)Al(C6F5)3}]. The boron and aluminium adducts are structurally similar, showing a comparable level of N2 push-pull activation. A notable exception is a bent (BCF adducts) vs. linear (AlCF adducts) M-N-N-LA motif (LA = Lewis acid), explained computationally as the result of steric repulsion. A striking difference arose when the formation of two-fold adducts was conducted. While in the case of BCF the 2 : 1 Lewis pairs could be observed in equilibrium with the 1 : 1 adduct and free borane but resisted isolation, AlCF forms robust 2 : 1 adducts trans-[ML2{(μ-η1:η1-N2)Al(C6F5)3}2] that isomerise into a more stable cis configuration. These compounds could be isolated and structurally characterized, and represent the first examples of trinuclear heterometallic complexes formed by Lewis acid-base interaction exhibiting p and d elements. Calculations also demonstrate that from the bare complex to the two-fold aluminium adduct, substantial decrease of the HOMO-LUMO gap is observed, and, unlike the trans adducts (1 : 1 and 1 : 2) for which the HOMO was computed to be a pure d orbital, the one of the cis-trinuclear compounds mixes a d orbital with a π* one of each N2 ligands. This may translate into a more favourable electrophilic attack on the N2 ligands instead of the metal centre, while a stabilized N2-centered LUMO should ease electron transfer, suggesting Lewis acids could be co-activators for electro-catalysed N2 reduction. Experimental UV-vis spectra for the tungsten family of compounds were compared with TD-DFT calculations (CAM-B3LYP/def2-TZVP), allowing to assign the low extinction bands found in the visible spectrum to unusual low-lying MLCT involving N2-centered orbitals. As significant red-shifts are observed upon LA coordination, this could have important implications for the development of visible light-driven nitrogen fixation.

Understanding the coupling of non-metallic heteroatoms to CO2 from a Conceptual DFT perspective

CONTEXT

A Conceptual DFT (CDFT) study has been carry out to analyse the coupling reactions of the simplest amine (CH3NH2), alcohol (CH3OH), and thiol (CH3SH) compounds with CO2 to form the corresponding adducts CH3NHCO2H, CH3OCO2H, and CH3SCO2H. The reaction mechanism takes place in a single step comprising two chemical events: nucleophilic attack of the non-metallic heteroatoms to CO2 followed by hydrogen atom transfer (HAT). According to our calculations, the participation of an additional nucleophilic molecule as HAT assistant entails important decreases in activation electronic energies. In such cases, the formation of a six-membered ring in the transition state (TS) reduces the angular stress with respect to the non-assisted paths, characterised by four-membered ring TSs. Through the analysis of the energy and reaction force profiles along the intrinsic reaction coordinate (IRC), the ratio of structural reorganisation and electronic rearrangement for both activation and relaxation energies has been computed. In addition, the analysis of the electronic chemical potential and reaction electronic flux profiles confirms that the highest electronic activity as well as their changes take place in the TS region. Finally, the distortion/interaction model using an energy decomposition scheme based on the electron density along the reaction coordinate has been carried out and the relative energy gradient (REG) method has been applied to identify the most important components associated to the barriers.

METHODS

The theoretical calculation were performed with Gaussian-16 scientific program. The B3LYP-D3(BJ)/aug-cc-pVDZ level was used for optimization of the minima and TSs. IRC calculations has also been carried out connecting the TS with the associated minima. Conceptual-DFT (CDFT) calculations have been carried out with the Eyringpy program and in-house code. The distortion/interaction model along the reaction coordinate have used the decomposition scheme of Mandado et al. and the analysis of the importance of each components have been done with the relative energy gradient (REG) method.

Carbon dioxide capture and functionalization by bis(N-heterocyclic carbene)-borylene complexes

Derivatives of free monocoordinated borylenes have attracted considerable interest due to their ability to exhibit transition-metal-like reactivity, in particular small molecules capture. However, such complexes are rare as the formation is either endergonic, or the resulting adduct is a transient intermediate that is prone to reaction. Here, we present the synthesis of two bis(N-heterocyclic carbene)-borylene complexes capable of capturing and functionalizing carbon dioxide. The capture and subsequent functionalization of CO2 by the bis(NHC)-disilylamidoborylene 1 is demonstrated by the formation of the bis(NHC)-isocyanatoborylene-carbon dioxide complex 3. Reversible capture of CO2 is observed using the bis(NHC)-mesitylborylene 2, and the persistent bis(NHC)-mesitylborylene-carbon dioxide adduct 4 can be stabilized by hydrogen bonding with boric acid. The reactions of 4 with ammonia-borane and aniline demonstrate that the captured CO2 can be further functionalized.

Main group elements in electrochemical hydrogen evolution and carbon dioxide reduction

Main-group elements are renowned for their versatile reactivities in organometallic chemistry, including CO2 insertion and H2 activation. However, electrocatalysts comprising a main-group element active site have not yet been widely developed for activating CO2 or producing H2. Recently, research has focused on main-group element-based electrocatalysts that are active in redox systems related to fuel-forming reactions. These studies have determined that the catalytic performances of heavier main-group element-based electrocatalysts are often similar to those of transition-metal-based electrocatalysts. Our group has recently reported the scope of including the main-group elements in the design of molecular catalysts and explored their applications in redox catalysis, such as the generation of H2 upon coupling of two protons (H+) and two electrons (e-). This feature article summarizes our research efforts in developing molecular electrocatalysts comprising main-group elements at their active sites. Furthermore, we highlight their influence on the rate-determining step, thereby enhancing the reaction rate and product selectivity for multi-H+/multi-e- transfer catalysis. Particularly, we focus on the performance of our recently reported molecular Sn- or Sb-centered macrocycles for electrocatalytic H2 evolution reaction (HER) and on how their mechanisms resemble those of transition-metal-based electrocatalysts. Moreover, we discuss the CO2 reduction reaction (CO2RR), another promising fuel-forming reaction, and emphasize the recent progress in including the main-group elements in the CO2RR. Although the main-group elements are found at the active sites of the molecular catalysts and are embedded in the electrode materials for studying the HER, molecular catalysts bearing main-group elements are not commonly used for CO2RR. However, the main-group elements assist the CO2RR by acting as co-catalysts. For example, alkali and alkaline earth metal ions (e.g., Li+, Na+, K+, Rb+, Cs+, Mg2+, Ca2+, and Ba2+) are known for their Lewis acidities, which influence the thermodynamic landscape of the CO2RR and product selectivity. In contrast, the elements in groups 13, 14, and 15 are primarily used as dopants in the preparation of catalytic materials. Overall, this article identifies main-group element-based molecular electrocatalysts and materials for HER and CO2RR.

A theoretical study of the reaction of borata derivatives of benzene, anthracene and pentacene with CO2

A theoretical study of the reaction between several borataacenes (1-methylboratabenzene, 9-methyl-9-borataanthracene and cis and trans diboratapentacene) and CO2 has been carried out at the M06-2X computational level. The influence of a counterion (potassium cation), the cation complexation by 18-crown-6-ether and solvent effects have been explored. The computational results predict anti/syn selectivity as found experimentally in the cis- and trans-diboratapentacene reaction with CO2 (Baker et al., J. Am. Chem. Soc., 2023, 145, 2028).

Tin-catalyzed reductive coupling of amines with CO2 and H2

Tin-based FLPs catalyze reductive coupling reactions of amines with CO2 and H2. Water produced by the reaction is well tolerated and TONs up to 300 can be achieved.

Use of 5,10-Disubstituted Dibenzoazaborines and Dibenzophosphaborines as Cyclic Supports of Frustrated Lewis Pairs for the Capture of CO2

The reactivity of 5,10-disubstituted dibenzoazaborines and dibenzophosphaborines towards carbon dioxide was studied at the DFT, M06-2X/def2-TZVP, computational level. The profile of this reaction comprises of three stationary points: the pre-reactive complex and adduct minima and the transition state(TS) linking both minima. Initial results show that dibenzoazaborines derivatives are less suitable to form adducts with CO2 than dibenzophosphaborine systems. The influence of the basicity on the P atom and the acidity on the B center of the dibenzophosphaborine in the reaction with CO2 was also explored. Thus, an equation was developed relating the properties (acidity, basicity and boron hybridization) of the isolated dibenzophosphaborine derivatives with the adduct energy. We found that modulation of the boron acidity allows to obtain more stable adducts than the pre-reactive complexes and isolated monomers.

Evidence for Borylene Carbonyl (LHB═C═O) and Base-Stabilized (LHB═O) and Base-Free Oxoborane (RB≡O) Intermediates in the Reactions of Diborenes with CO2

Doubly N-heterocyclic-carbene-stabilized diborenes undergo facile reactions with CO₂, initially providing dibora-β-lactones. These lactones convert over time to their 2,4-diboraoxetan-3-one isomers through a presumed dissociative pathway and hypovalent boron species borylene carbonyls (LHB═C═O) and base-stabilized oxoboranes (LHB═O). Repeating these reactions with doubly cyclic(alkyl)(amino)carbene-stabilized diborenes allowed the isolation of a borylene carbonyl intermediate, whereas a base-stabilized oxoborane could be inferred by the isolation of a boroxine from the reaction mixture. These results, supported by calculations, confirm the presumed mechanism of the diboralactone-to-diboraoxetanone isomerization while also establishing a surprising level of stability for three unknown or very rare hypovalent boron species: base-stabilized derivatives of the parent borylene carbonyl (LHB═C═O) and parent oxoborane (LHB═O) as well as base-free oxoboranes (RB≡O).

Exploring the Reactivity of a Frustrated Sn/P Lewis Pair: The Highly Selective Complexation of the cis-Azobenzene Photoisomer

The reactivity of the geminal frustrated Lewis pair (FLP) (F5 C2 )3 SnCH2 P(tBu)2 (1) was explored by reacting it with a variety of small molecules (PhOCN, PhNCS, PhCCH, tBuCCH, H3 CC(O)CH=CH2 , Ph[C(O)]2 Ph, PhN=NPh and Me3 SiCHN2 ), featuring polar or non-polar multiple bonds and/or represent α,β-unsaturated systems. While most adducts are formed readily, the binding of azobenzene requires UV-induced photoisomerization, which results in the highly selective complexation of cis-azobenzene. In the case of benzil, the reaction does not lead to the expected 1,2- or 1,4-addition products, but to the non-stereoselective (tBu)2 PCH2 -transfer to a prochiral keto function of benzil. All adducts of 1 were characterised by means of multinuclear NMR spectroscopy, elemental analyses and X-ray diffraction experiments.

Enantioselective Synthesis of N-Alkylamines through β-Amino C-H Functionalization Promoted by Cooperative Actions of B(C6F5)3 and a Chiral Lewis Acid Co-Catalyst

We disclose a catalytic method for β-C(sp³)-H functionalization of N-alkylamines for the synthesis of enantiomerically enriched β-substituted amines, entities prevalent in pharmaceutical compounds and used to generate different families of chiral catalysts. We demonstrate that a catalyst system comprising of seemingly competitive Lewis acids, B(C₆F₅)₃, and a chiral Mg- or Sc-based complex, promotes the highly enantioselective union of N-alkylamines and α,β-unsaturated compounds. An array of δ-amino carbonyl compounds was synthesized under redox-neutral conditions by enantioselective reaction of a N-alkylamine-derived enamine and an electrophile activated by the chiral Lewis acid co-catalyst. The utility of the approach is highlighted by late-stage β-C-H functionalization of bioactive amines. Investigations in regard to the mechanistic nuances of the catalytic processes are described.

Silica Nanopowder Supported Frustrated Lewis Pairs for CO2 Capture and Conversion to Formic Acid

Treatment of hydroxylated silica nanopowders S1 and allyl-functionalized silica nanopowders S2 with 3-(diphenylborano)- or 3-bis(pentafluorophenylborano)propyltrimethoxysilane or 2-(diphenylphosphino)- or 2-(dicyclohexylphosphino)ethyltriethoxysilane generates silica nanopowder supported Lewis acids S3 and silica nanopowder supported Lewis bases S4. These surfaces were characterized by ¹³C, ¹¹B, and ³¹P cross-polarization magic angle spinning nuclear magnetic resonance (CP MAS NMR), X-ray photoelectron spectroscopy (XPS), and attenuated total reflection Fourier transform infrared (ATR FTIR). When S3 is combined with solution-phase Lewis bases PR₃ (R = C₆F₅, C₆H₅, mesityl), six associated silica nanopowder supported frustrated Lewis pairs (FLPs) are formed. In another set of six reactions, the interactions between the supported Lewis bases S4 and solution-phase Lewis acids BR₃ with R = C₆F₅, C₆H₅, mesityl produced six more associated supported FLPs. The capture of CO₂ by these FLPs producing FLP-CO₂ Lewis pair adducts S5 and S6 were highlighted by ATR FTIR, and it was found that FLP S5e with R = C₆H₅ on both the supported Lewis acid and solution-phase Lewis base trapped the largest quantities of CO₂ on the silica nanopowder supports. Conversion of CO₂ to HCOOH was achieved by first activating H₂ to generate activated FLP-H₂ surfaces S7 and S9. Addition of CO₂ then generated HCOOH via the silica nanopowder supported FLP-HCOOH adducts S8 and S10. Qualitative identification of HCOOH generation was achieved by ATR FTIR measurements, and surface 10b with R = C₆H₅ proved to be the most successful silica nanopowder surface bound FLP in HCOOH generation. In some cases, diborano formates (-BO(CH)OB-) S11 and S12 were also identified as side products during HCOOH formation. Spectroscopic characterization of purposefully synthesized S11 and S12 included ¹¹B and ³¹P CP MAS NMR.

Sequential Conia-ene-type cyclization and Negishi coupling by cooperative functions of B(C6F5)3, ZnI2, Pd(PPh3)4 and an amine

We disclose a method for sequential Conia-ene-type cyclization/Negishi coupling for the union of alkynyl ketones and aryl iodides. This process is promoted through cooperative actions of Lewis acidic B(C6F5)3, ZnI2, Pd-based complex, and a Brønsted basic amine. The three Lewis acid catalysts with potential overlapping functions play their independent roles as activators of carbonyl group, alkyne moiety, and alkenyl zinc intermediate, respectively. A variety of 1,2,3-substituted cyclopentenes can be synthesized with high efficiency.

Direct Conversion of N-Alkylamines to N-Propargylamines through C-H Activation Promoted by Lewis Acid/Organocopper Catalysis: Application to Late-Stage Functionalization of Bioactive Molecules

An efficient catalytic method to convert an α-C-H bond of N-alkylamines into an α-C-alkynyl bond was developed. In the past, such transformations were carried out under oxidative conditions, and the enantioselective variants were confined to tetrahydroisoquinoline derivatives. Here, we disclose a method for the union of N-alkylamines and trimethylsilyl alkynes, without the presence of an external oxidant and promoted through cooperative actions of two Lewis acids, B(C6F5)3 and a Cu-based complex. A variety of propargylamines can be synthesized in high diastereo- and enantioselectivity. The utility of the approach is demonstrated by the late-stage site-selective modification of bioactive amines. Kinetic investigations that shed light on various mechanistic nuances of the catalytic process are presented.

B(C6F5)3-Catalyzed α-Deuteration of Bioactive Carbonyl Compounds with D2O

An efficient deuteration process of α-C-H bonds in various carbonyl-based pharmaceutical compounds has been developed. Catalytic reactions are initiated by the action of Lewis acidic B(C₆F₅)₃ and D₂O, converting a drug molecule into the corresponding boron-enolate. Ensuing deuteration of the enolate by in situ-generated D₂O⁺-H then results in the formation of α-deuterated bioactive carbonyl compounds with up to >98% deuterium incorporation.

Metal-Organic Frameworks as Platform for Lewis-Acid-Catalyzed Organic Transformations

Metal-organic frameworks (MOFs) are highly promising Lewis acid catalysts; they either inherently possess Lewis acid sites (LASs) on it or the LASs can be generated through various post-synthetic methods, the later can be performed in MOFs in a trivial fashion. MOFs are suitable platform for catalysis because of its highly crystalline and porous nature. Moreover, with recent advancements, thermal and chemical stability is not a problem with many MOFs. In this Minireview, an enormous versatility of MOFs, in terms of their microporosity/mesoporosity, size/shape selectivity, chirality, pore size, etc., has been highlighted. These are advantageous for designing and performing various targeted organic transformations. Although, many organic transformations catalyzed by MOFs with LASs have been reported in the recent past. In this Minireview, we have restricted ourselves to four important organic reactions: (i) cyanosilylation, (ii) Diels-Alder reaction, (iii) C-H activation, and (iv) CO₂ -addition. The discussion focuses mostly on the recent reports (42 examples).

Catalytic Deuterium Incorporation within Metabolically Stable β-Amino C-H Bonds of Drug Molecules

An efficient deuteration process of β-amino C-H bonds in various N-alkylamine-based pharmaceutical compounds has been developed. Catalytic reactions begin with the action of Lewis acidic B(C₆F₅)₃ and Brønsted basic N-alkylamine, converting a drug molecule into the corresponding enamine. The acid/base catalysts also promote the dedeuteration of acetone-d₆ to afford a deuterated ammonium ion. Ensuing deuteration of the enamine then leads to the formation of β-deuterated bioactive amines with up to 99% deuterium incorporation.

Preparation of the Borane (Fmes)BH2 and its Utilization in the FLP Reduction of Carbon Monoxide and Carbon Dioxide

Treatment of 1,3,5-tris(trifluoromethyl)benzene with n-BuLi, followed by H₃ B⋅SMe₂ and subsequent hydride removal gave the (Fmes)BH₂ reagent, which was isolated as a SMe₂ stabilized monomer or a ligand free (μ-H)₂ -bridged dimer. Reaction with Mes₂ P(vinyl) gave the respective ethylene-bridged P/B(Fmes)H FLP. It reduced carbon monoxide to the formyl stage and carbon dioxide to the formaldehyde oxidation state. Most new compounds were characterized by X-ray diffraction.

Investigation of main group promoted carbon dioxide reduction

The reduction of carbon dioxide (CO₂) is of interest to the chemical industry, as many synthetic materials can be derived from CO₂. To help determine the reagents needed for the functionalization of carbon dioxide this experimental and computational study describes the reduction of CO₂ to formate and CO with hydride, electron, and proton sources in the presence of sterically bulky Lewis acids and bases. The insertion of carbon dioxide into a main group hydride, generating a main group formate, was computed to be more thermodynamically favorable for more hydridic (reducing) main group hydrides. A ten kcal/mol increase in hydricity (more reducing) of a main group hydride resulted in a 35% increase in the main group hydride's ability to insert CO₂ into the main group hydride bond. The resulting main group formate exhibited a hydricity (reducing ability) about 10% less than the respective main group hydride prior to CO₂ insertion. Coordination of a second identical Lewis acid to a main group formate complex further reduced the hydricity by about another 20%. The addition of electrons to the CO adduct of ^t Bu₃P and B(C₆F₅)₃ resulted in converting the sequestered CO₂ molecule to CO. Reduction of the CO₂ adduct of ^t Bu₃P and B(C₆F₅)₃ with both electrons and protons resulted in only proton reduction.

Enantioselective Conia-Ene-Type Cyclizations of Alkynyl Ketones through Cooperative Action of B(C6F5)3, N-Alkylamine and a Zn-Based Catalyst

An efficient and highly enantioselective Conia-ene-type process has been developed. Reactions are catalyzed by a combination of B(C₆F₅)₃, an N-alkylamine and a BOX-ZnI₂ complex. Specifically, through cooperative action of B(C₆F₅)₃ and amine, ketones with poorly acidic α-C-H bonds can be converted in situ to the corresponding enolates. Subsequent enantioselective cyclization involving a BOX-ZnI₂-activated alkyne leads to the formation of various cyclopentenes in up to 99% yield and 99:1 er.

B(C6F5)3-Catalyzed C-H Alkylation of N-Alkylamines Using Silicon Enolates without External Oxidant

An efficient method for the coupling of N-alkylamines with silicon enolates to generate β-amino carbonyl compounds is disclosed. These reactions proceed by activation of α-amino C-H bonds by B(C₆F₅)₃, which likely generates a "frustrated" acid/base complex in the presence of large N-alkylamines. The transformation requires no external oxidant and releases hydrosilane as a byproduct. The utility of this method is demonstrated in the late-stage functionalization of bioactive molecules such as citalopram, atomoxetine, and fluoxetine.

C-H Functionalization of Amines via Alkene-Derived Nucleophiles through Cooperative Action of Chiral and Achiral Lewis Acid Catalysts: Applications in Enantioselective Synthesis

Catalytic transformations of α-amino C-H bonds to afford valuable enantiomerically enriched α-substituted amines, entities that are prevalent in pharmaceuticals and bioactive natural products, have been developed. Typically, such processes are carried out under oxidative conditions and require precious metal-based catalysts. Here, we disclose a strategy for an enantioselective union of N-alkylamines and α,β-unsaturated compounds, performed under redox-neutral conditions, and promoted through concerted action of seemingly competitive Lewis acids, B(C₆F₅)₃, and a chiral Mg-PyBOX complex. Thus, a wide variety of β-amino carbonyl compounds may be synthesized, with complete atom economy, through stereoselective reaction of an in situ-generated enantiomerically enriched Mg-enolate and an appropriate electrophile.

Cooperative carbon monoxide to formyl reduction at a trifunctional PBB frustrated Lewis pair

Twofold hydroboration of the bulky Mes*P(vinyl)₂ phosphane with Piers' borane [HB(C₆F₅)₂] followed by C₆F₅/H exchange with 9-BBN generated a reactive P/B/BH FLP structure that cleanly reduced carbon monoxide at the trifunctional frustrated Lewis pair framework to the [B]-formyl stage.

A hydroboration route to geminal P/B frustrated Lewis pairs with a bulky secondary phosphane component and their reaction with carbon dioxide

The secondary aryl-P(H) phosphanyl substituted tert-butylacetylenes 7a,b (aryl: Mes or Mes*) undergo hydroboration with [HB(C₆F₅)₂] to give the geminal vinylidene-bridged P/B Lewis pairs 8a,b. The treatment of 8a,b with benzonitrile, N-sulfinylaniline, and phenyl isothiocyanate, respectively, gives the addition products 12a,b, 13a,b, and 14 with proton transfer from the phosphorus to the more basic nitrogen site. The reaction of the FLPs 8a,b with carbon dioxide yields a doubly boron bonded addition product. The reaction of 8b with a conjugated ynone formally proceeded by trans-1,2-hydrophosphination of the alkyne at the geminal FLP framework to give the seven-membered heterocycle 21.

CO-Reduction Chemistry: Reaction of a CO-Derived Formylhydridoborate with Carbon Monoxide, with Carbon Dioxide, and with Dihydrogen

Treatment of the bulky metallocene hydride Cp*₂Zr(H)OMes (Cp* = pentamethylcyclopentadienyl, Mes = mesityl) with Piers' borane [HB(C₆F₅)₂] and carbon monoxide (CO) gave the formylhydridoborate complex [Zr]-O═CH-BH(C₆F₅)₂ ([Zr] = Cp*₂Zr-OMes). From the dynamic NMR behavior, its endergonic equilibration with the [Zr]-O-CH₂-B(C₆F₅)₂ isomer was deduced, which showed typical reactions of an oxygen/boron frustrated Lewis pair. It was trapped with CO to give an O-[Zr] bonded borata-β-lactone. Trapping with carbon dioxide (CO₂) gave the respective O-[Zr] bonded cyclic boratacarbonate product. These reaction pathways were analyzed by density functional theory calculation. The formylhydridoborate complex was further reduced by dihydrogen via two steps; it reacted rapidly with H₂ to give Cp*₂Zr(OH)OMes and H₃C-B(C₆F₅)₂, which then slowly reacted further with H₂ to eventually give [Zr]-O(H)-B(H)(C₆F₅)₂ and methane (CH₄). Most complexes were characterized by X-ray diffraction.

The Reductive Activation of CO2 Across a Ti=Ti Double Bond: Synthetic, Structural, and Mechanistic Studies

The reactivity of the bis(pentalene)dititanium double-sandwich compound Ti2Pn†2 (1) (Pn† = 1,4-{SiiPr3}2C8H4) with CO2 is investigated in detail using spectroscopic, X-ray crystallographic, and computational studies. When the CO2 reaction is performed at -78 °C, the 1:1 adduct 4 is formed, and low-temperature spectroscopic measurements are consistent with a CO2 molecule bound symmetrically to the two Ti centers in a μ:η2,η2 binding mode, a structure also indicated by theory. Upon warming to room temperature the coordinated CO2 is quantitatively reduced over a period of minutes to give the bis(oxo)-bridged dimer 2 and the dicarbonyl complex 3. In situ NMR studies indicated that this decomposition proceeds in a stepwise process via monooxo (5) and monocarbonyl (7) double-sandwich complexes, which have been independently synthesized and structurally characterized. 5 is thermally unstable with respect to a μ-O dimer in which the Ti-Ti bond has been cleaved and one pentalene ligand binds in an η8 fashion to each of the formally TiIII centers. The molecular structure of 7 shows a "side-on" bound carbonyl ligand. Bonding of the double-sandwich species Ti2Pn2 (Pn = C8H6) to other fragments has been investigated by density functional theory calculations and fragment analysis, providing insight into the CO2 reaction pathway consistent with the experimentally observed intermediates. A key step in the proposed mechanism is disproportionation of a mono(oxo) di-TiIII species to yield di-TiII and di-TiIV products. 1 forms a structurally characterized, thermally stable CS2 adduct 8 that shows symmetrical binding to the Ti2 unit and supports the formulation of 4. The reaction of 1 with COS forms a thermally unstable complex 9 that undergoes scission to give mono(μ-S) mono(CO) species 10. Ph3PS is an effective sulfur transfer agent for 1, enabling the synthesis of mono(μ-S) complex 11 with a double-sandwich structure and bis(μ-S) dimer 12 in which the Ti-Ti bond has been cleaved.

Alkaline-Earth-Promoted CO Homologation and Reductive Catalysis

Reaction between a β-diketiminato magnesium hydride and carbon monoxide results in the isolation of a dimeric cis-enediolate species through the reductive coupling of two CO molecules. Under catalytic conditions with PhSiH₃, an observable magnesium formyl species may be intercepted for the mild reductive cleavage of the CO triple bond.

Intramolecular B/N frustrated Lewis pairs and the hydrogenation of carbon dioxide

The FLP species 1-BR2-2-NMe2-C6H4 (R = 2,4,6-Me3C6H2, 2,4,5-Me3C6H2) reacts with H2 in sequential hydrogen activation and protodeborylation reactions to give (1-BH2-2-NMe2-C6H4)2. While reacts with H2/CO2 to give formyl, acetal and methoxy-derivatives, reacts with H2/CO2 to give C6H4(NMe2)(B(2,4,5-Me3C6H2)O)2CH2. The mechanism of CO2 reduction is considered.

Controlling selectivity in the reductive activation of CO2 by mixed sandwich uranium(iii) complexes

The uranium complexes [U(η⁸-C₈H₆(1,4-SiMe₃)₂)(η⁵-Cp^Me4R)] can be tuned to selectively reduce CO₂, giving U(iv) complexes incorporating bridging oxo, carbonate, or oxalate groups.