Homogeneous and Heterogeneous Frustrated Lewis Pairs for the Activation and Transformation of CO2

Due to the serious ecological problems caused by the high CO2 content in the atmosphere, reducing atmospheric CO2 has attracted widespread attention from academia and governments. Among the many ways to mitigate CO2 concentration, the capture and comprehensive utilization of CO2 through chemical methods have obvious advantages, whose key is to develop suitable adsorbents and catalysts. Frustrated Lewis pairs (FLPs) are known to bind CO2 through the interaction between unquenched Lewis acid sites/Lewis base sites with the O/C of CO2, simultaneously achieving CO2 capture and activation, which render FLP better potential for CO2 utilization. However, how to construct efficient FLP targeted for CO2 utilization and the mechanism of CO2 activation have not been systematically reported. This review firstly provides a comprehensive summary of the recent advances in the field of CO2 capture, activation, and transformation with the help of FLP, including the construction of homogeneous and heterogeneous FLPs, their interaction with CO2, reaction activity, and mechanism study. We also illustrated the challenges and opportunities faced in this field to shed light on the prospective research.

Three-Component Reaction to 1,4,2-Diazaborole-Type Heteroarene Systems

The borane FmesBH₂ reacts in a three-component reaction with an isonitrile and a small series of organonitriles to give rare examples of the class of dihydro-1,4,2-diazaborole derivatives. In a related way, annulated BN-indolizine derivatives became conveniently available, as were dihydro-1,4,2-oxaza- or thiazaborole derivatives. The nucleophilic framework of a dihydro-1,4,2-diazaborole example allowed for an uncatalyzed acylation reaction. It also served as a 1,3-dipolar reagent and underwent a [3+2] cycloaddition/[4+2] cycloreversion sequence when treated with methyl propiolate to give the respective pyrrole product. The [3+2] cycloaddition product of a dihydro-1,4,2-diazaborole derivative with N-phenylmaleimide was isolated and its heterobicyclo[2.2.1]heptane derived structure characterized by X-ray diffraction.

The Bis(η6 -benzene)lithium Cation: A Fundamental Main-Group Organometallic Species

The synthesis and characterization of the bis(η⁶ -benzene)lithium cation, the benzene metallocene of the lightest metal, is reported. The boron compound FmesBCl₂ [Fmes: 2,4,6-tris(trifluoromethyl)phenyl] reacted with three molar equivalents of the lithio-acetylene reagent Li-C≡C-Fmxyl [Fmxyl: 3,5-bis(trifluoromethyl)phenyl]. Subsequent crystallization from benzene gave the [bis(η⁶ -benzene)Li]⁺ cation with the [{FmesB(-C≡C-Fmxyl)₃ }₂ Li]^- anion. This parent [(arene)₂ Li]⁺ cation shows an eclipsed arrangement of the pair of benzene ligands at the central lithium cation with uniform carbon-lithium bond lengths. The corresponding [(η⁶ -toluene)₂ Li]⁺ and [(η⁶ -durene)₂ Li]⁺ containing salts were similarly prepared. The bis(arene)lithium cations were characterized by X-ray diffraction, by solid-state ⁷ Li MAS NMR spectroscopy and their bonding features were analyzed by DFT calculations.

Colorimetric Metal-Free Detection of Carbon Monoxide: Reversible CO Uptake by a BNB Frustrated Lewis Pair

We report two BNB‐type frustrated Lewis pairs which feature an acceptor‐donor‐acceptor functionalized cavity, and which differ in the nature of the B‐bound fluoroaryl group (C₆F₅ vs. C₆H₃(CF₃)₂‐3,5, Ar^f). These receptor systems are capable of capturing gaseous CO, and in the case of the ‐BAr^f₂ system this can be shown to occur in reversible fashion at/above room temperature. For both systems, the binding event is accompanied by migration of one of the aryl substituents to the electrophilic carbon of the CO guest. Experiments utilizing an additional equivalent of P^tBu₃ allow the initially formed (non‐migrated) CO adduct to be identified and trapped (via demethylation), while also establishing the reversibility of the B‐to‐C migration process. When partnered with the slightly less Lewis acidic ‐BAr^f₂ substituent, this reversibility allows for release of the captured carbon monoxide in the temperature range 40–70 °C, and the possibility for CO sensing, making use of the associated colourless to orange/red colour change.

Alkyne 1,1-Hydroboration to a Reactive Frustrated P/B-H Lewis Pair

The Mes₂ P-C≡C-SiMe₃ alkyne reacts with the borane H₂ B-Fmes by means of a rare 1,1-hydroboration reaction to give an unsaturated C₂ -bridged frustrated P/B-H Lewis pair. Most of its reactions are determined by the presence of the B-H functionality at the FLP function and the activated connecting carbon-carbon double bond. It reduces carbon monoxide to the formyl stage. With nitriles it reacts in an extraordinary way: it undergoes a reaction sequence that eventually results in the formation of a P-substituted dihydro-1,2-azaborole derivative. Several similar examples were found. In one case a P-ylide was isolated that was related to an intermediate of the reaction sequence. It subsequently opened in an alternative way to give an alkenyl borane product.

Frustrated Lewis Pair Chelation as a Vehicle for Low-Temperature Semiconductor Element and Polymer Deposition

The stabilization of silicon(II) and germanium(II) dihydrides by an intramolecular Frustrated Lewis Pair (FLP) ligand, PB, ⁱ Pr₂ P(C₆ H₄ )BCy₂ (Cy=cyclohexyl) is reported. The resulting hydride complexes [PB{SiH₂ }] and [PB{GeH₂ }] are indefinitely stable at room temperature, yet can deposit films of silicon and germanium, respectively, upon mild thermolysis in solution. Hallmarks of this work include: 1) the ability to recycle the FLP phosphine-borane ligand (PB) after element deposition, and 2) the single-source precursor [PB{SiH₂ }] deposits Si films at a record low temperature from solution (110 °C). The dialkylsilicon(II) adduct [PB{SiMe₂ }] was also prepared, and shown to release poly(dimethylsilane) [SiMe₂ ]_n upon heating. Overall, this study introduces a "closed loop" deposition strategy for semiconductors that steers materials science away from the use of harsh reagents or high temperatures.

Electronically Driven Regioselective Iridium-Catalyzed C-H Borylation of Donor-π-Acceptor Chromophores Containing Triarylboron Acceptors

We observed a surprisingly high electronically driven regioselectivity for the iridium-catalyzed C-H borylation of donor-π-acceptor (D-π-A) systems with diphenylamino (1) or carbazolyl (2) moieties as the donor, bis(2,6-bis(trifluoromethyl)phenyl)boryl (B(F Xyl)2 ) as the acceptor, and 1,4-phenylene as the π-bridge. Under our conditions, borylation was observed only at the sterically least encumbered para-positions of the acceptor group. As boronate esters are versatile building blocks for organic synthesis (C-C coupling, functional group transformations) the C-H borylation represents a simple potential method for post-functionalization by which electronic or other properties of D-π-A systems can be fine-tuned for specific applications. The photophysical and electrochemical properties of the borylated (1-(Bpin)2 ) and unborylated (1) diphenylamino-substituted D-π-A systems were investigated. Interestingly, the borylated derivative exhibits coordination of THF to the boronate ester moieties, influencing the photophysical properties and exemplifying the non-innocence of boronate esters.

Using the Secondary PH/BH Functional Groups of an Active Geminal Frustrated Lewis Pair for Carbon Monoxide Reduction and Reactions with Nitriles and Isonitriles

Reaction of the secondary alkynyl(Mes*)PH phosphane 2 with (Fmes)BH₂ ⋅SMe₂ gives the geminal PH/BH frustrated Lewis pair (FLP) 3. The PH and the BH functions are jointly used in the reduction of carbon monoxide under mild reaction conditions to give the [P]-CH₂ -O-[B] product. A subsequent cycloaddition sequence results in the liberation of formaldehyde. The FLP 3 reacts with benzonitrile to give a P-benzamidine, and it couples two isonitriles at the FLP framework.

Rapid and Scalable Access to Sequence-Controlled DHDM Multiblock Copolymers by FLP Polymerization

An immortal N-(diphenylphosphanyl)-1,3-diisopropyl-4,5-dimethyl-1,3-dihydro-2H-imidazol-2-imine/diisobutyl (2,6-di-tert-butyl-4-methylphenoxy) aluminum (P(NIⁱ Pr)Ph₂ /(BHT)Alⁱ Bu₂ )-based frustrated Lewis pair (FLP) polymerization strategy is presented for rapid and scalable synthesis of the sequence-controlled multiblock copolymers at room temperature. Without addition of extra initiator or catalyst and complex synthetic procedure, this method enabled a tripentacontablock copolymer (n=53, k=4, dp_n =50) to be achieved with the highest reported block number (n=53) and molecular weight (M_n =310 kg mol^-1 ) within 30 min. More importantly, this FLP polymerization strategy provided access to the multiblock copolymers with tailored properties by precisely adjusting the monomer sequence and block numbers.

Multi-Component Synthesis of Rare 1,3-Dihydro-1,3-azaborinine Derivatives: Application of a Bora-Nazarov Type Reaction

The twofold hydroboration products of (Fmes)BH₂ ⋅SMe₂ with a series of alkynes (2-butyne, arylethynes) react with two molar equiv of 2,6-dimethylphenyl isocyanide (CN-Xyl) at 80 °C to give rare examples of 1,3-azaborinine derivatives. A mechanistic study revealed a reaction course involving insertion of one isonitrile followed by a bora-Nazarov type ring-closure reaction and subsequent isonitrile insertion to give the respective 1,3-dihydro-1,3-azaborinines 5.