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.

From phosphanylphosphaalkenes to coordination copper and silver polymers containing P-P bonds

This study was focused on the activation of the CP bond via reactions of Ph2CP-PtBu2 (1) with 1,6-hexanediol and selected dithiols (1,4-butanedithiol, 1,4-benzenedithiol and 1,4-benzenedimethanethiol). These reactions proceed according to a 1,2-addition mechanism, providing new compounds with formulas {(Ph)2(H)C-P-PtBu2}{μ2-(O-(CH2)6-O)}{tBu2P-P-C(H)(Ph)2} (2), {(Ph)2(H)C-P-PtBu2}{μ2-(S-(CH2)4-S)}{tBu2P-P-C(H)(Ph)2} (3a), {(Ph)2(H)C-P-PtBu2}{μ2-(S-C6H4-S)}{tBu2P-P-C(H)(Ph)2} (3b), and {(Ph)2(H)C-P-PtBu2}{μ2-(S-CH2-C6H4-CH2-S)}{tBu2P-P-C(H)(Ph)2} (3c). Next, the reactions of 3a and 3c with metal chlorides led to the growth of desired coordination polymers of copper(I) and silver(I). All the obtained compounds remained stable under atmospheric conditions.

Molybdenum-Mediated Insertion of Ketones into the P-P bond of cyclo-(P5 Ph5 ) and Formation of Trinuclear Molybdenum Complexes

The reaction of cyclopentaphosphine cyclo-(P5 Ph5 ) (1) with ketones (acetone and cyclooctanone) in the presence of [Mo(CO)4 (cod)] (cod=cycloocta-1,5-diene) led to air-stable trinuclear complexes in which the bis-phosphanido ligands (PPh-PPh-PPh-PPh-CMe2 O-PPh)2- (complex 2) and (PPh-PPh-PPh-PPh-C(CH2 )7 O-PPh)2- (complex 3) bridge a Mo(CO)3 -Mo(CO)3 unit. This extends the reaction of 1 with transition metal carbonyl complexes to further substrates and represents the first examples of insertion of carbonyl compounds into the P-P bond of cyclic oligophosphorus compounds. Complexes 2 and 3 have been characterized by 31 P NMR spectroscopy and single crystal X-ray diffraction. Furthermore, the thermal properties of the obtained complexes have been studied by differential scanning calorimetry (DSC) and fast scanning calorimetry (FSC).

Reactivity of tetrel functionalized heptapnictogen clusters towards heteroallenes

Despite being known for decades, the solution-state molecular chemistry of heptapnictogen ([Pn₇]^3-; Pn = P, As) clusters is not well established. Here we study heavy element derivatives of tetrel functionalized heptapnictogen clusters towards heteroallene capture, specifically isocyanates, an isothiocyanate and CO₂ are probed. Clusters (Me₃Ge)₃P₇ (1), (Et₃Ge)₃P₇ (2), (ⁿBu₃Sn)₃P₇ (3), and (Me₃Si)₃As₇ (4) were all found to capture isocyanates between all three of their tetrel-pnictogen bonds. In the case of phenyl isocyanate insertion, tetrel coordination at the isocyanate nitrogen atoms is preferred, while in the case of p-toluenesulfonyl isocyanate insertion, tetrel coordination at oxygen is preferred. Furthermore, the reaction of (Me₃Si)₃P₇ with CO₂ gave NMR spectra consistent with the capture of the greenhouse gas. Heteroallene insertion at these clusters was also studied using density functional theory.

Triphenylborane in Metal-Free Catalysis

The development and application of new organoboron reagents as Lewis acids in synthesis and metal-free catalysis have dramatically expanded over the past 20 years. In this context, we will show the recent uses of the simple and relatively weak Lewis acid BPh₃-discovered 100 years ago-as a metal-free catalyst for various organic transformations. The first part will highlight catalytic applications in polymer synthesis such as the copolymerization of epoxides with CO₂, isocyanate, and organic anhydrides to various polycarbonate copolymers and controlled diblock copolymers as well as alternating polyurethanes. This is followed by a discussion of BPh₃ as a Lewis acid component in the frustrated Lewis pair (FLP) mediated cleavage of hydrogen and hydrogenation catalysis. In addition, BPh₃-catalyzed reductive N-methylations and C-methylations with CO₂ and silane to value-added organic products will be covered as well along with BPh₃-catalyzed cycloadditions and insertion reactions. Collectively, this mini-review showcases the underexplored potential of commercially available BPh₃ in metal-free catalysis.

Phosphinophosphoranes: Mixed-Valent Phosphorus Compounds with Ambiphilic Properties

Herein, we present a simple synthesis of mixed-valent phosphinophosphoranes bearing three- and five-coordinate phosphorus centers. Compounds with phosphorus-phosphorus bonds were synthesized via a reaction of lithium phosphides RR'PLi with cat₂PCl (cat = catecholate), whereas derivatives with methylene-linked phosphorus centers were obtained via a reaction of phosphanylmethanides RR'CH₂Li with cat₂PCl. The presence of accessible lone-pair electrons on the P-phosphanyl atom of phosphinophosphoranes during the reaction of the title compounds with H₃B·SMe₂, where phosphinophosphorane-borane adducts were formed quantitatively, was confirmed. Furthermore, the Lewis basic and Lewis acidic properties of the phosphinophosphoranes in reactions with phenyl isothiocyanate were tested. Depending on the structure of the starting phosphinophosphorane, phosphinophosphorylation of PhNCS or formation of a five-membered zwitterionic adduct was observed. The structures of the isolated compounds were unambiguously determined by heteronuclear nuclear magnetic resonance spectroscopy and single-crystal X-ray diffraction. Moreover, by applying density functional theory calculations, we compared the Lewis basicity and nucleophilicity of diversified trivalent P-centers.

Exploring the Reactivity of Unsymmetrical Diphosphanes toward Heterocumulenes: Access to Phosphanyl and Phosphoryl Derivatives of Amides, Imines, and Iminoamides

We present a comprehensive study on the diphosphanation of iso(thio)cyanates by unsymmetrical diphosphanes. The reactions involving unsymmetrical diphosphanes and phenyl isocyanate or phenyl thioisocyanate gave rise to phosphanyl, phosphoryl, and thiophosphoryl derivatives of amides, imines, and iminoamides. The structures of the diphosphanation products were confirmed through NMR spectroscopy, IR spectroscopy, and single-crystal X-ray diffraction. We showed that unsymmetrical diphosphanes could be used as building blocks to synthesize phosphorus analogues of important classes of organic molecules. The described transformations provided a new methodology for the synthesis of organophosphorus compounds bearing phosphanyl, phosphoryl, or thiophosphoryl functional groups. Moreover, theoretical studies on diphosphanation reactions explained the influence of the steric and electronic properties of the parent diphosphanes on the structures of the diphosphanation products.

We provided synthetic access to phosphanyl, phosphoryl, or thiophosphoryl derivatives of amides, imines, and iminoamides starting from simple building blocks such as unsymmetrical diphosphanes and heterocumulenes.

Aromaticity-promoted CS2 activation by heterocycle-bridged P/N-FLPs: a comparative DFT study with CO2 capture

Carbon dioxide (CO₂) capture has attracted considerable attention from both experimental and theoretical chemists. In comparison, carbon disulfide (CS₂) activation is less developed. Here, we carry out a thorough comparative density functional theory study to examine the reaction mechanisms of CS₂ activation by five-membered heterocycle-bridged P/N frustrated Lewis pairs (FLPs). Calculations suggest that despite a weaker carbon-sulfur bond, all the CS₂ activations have higher reaction barriers than the CO₂ capture, which could be attributed to electrostatic repulsion between FLPs and CS₂ caused by the reversed polarity of CS in CS₂ rather than the electrostatic attraction in CO₂ capture. In addition, aromaticity is found to play an important role in CS₂ capture as it stabilizes both the transition states and products in heterocycle-bridged FLPs. All these findings could be useful for experimentalists to realize small molecule activations by FLPs.

Heterometathesis of diphosphanes (R2P–PR2) with dichalcogenides (R'E–ER', E = O, S, Se, Te)

The reactions of R2P–PR2 with R'E–ER', (where E = Se, S, O, Te) to give R2P–ER' have been explored experimentally and computationally. The reaction of Ph2P–PPh2 with PhSe–SePh gives Ph2P–SePh...

Insertion of CS2 into a Phosphorus-Arsenic Single Bond and Investigations on Phosphane Arsanyldithiocarboxylates

The synthesis and reactivity of sterically demanding phosphaarsanes TerR¹P-AsR² (3) is described. These species were selectively synthesized via metathesis reactions of Ter-stabilized [Ter = 2,6-bis(2,4,6-trimethylphenyl)phenyl] potassium phosphides TerR¹PK (1) with the N-heterocyclic chloroarsane ClAs{N(tBu)CH₂}₂ (2). Conversion of the n-butyl-substituted phosphaarsane 3c with the reactive heterocumulene CS₂ leads to an insertion into the P-As bond, yielding the phosphane arsanyldithiocarboxylate TerR¹P-C(S)S-AsR² (4c) as a new structural motif. Because full conversion of 3c with CS₂ requires long reaction times, an alternative synthetic route is reported herein, involving the conversion of 2 with phosphane dithiocarboxylates TerR¹-C(S)SK (5), enabling synthetic access to a wider range of phosphane arsanyldithiocarboxylates. These interpnictogen dithiocarboxylates show an interesting bonding situation with a significantly elongated As-S bond due to negative hyperconjugation within the molecule. All products along the reaction path were fully characterized.

Radical-initiated P,P-metathesis reactions of diphosphanes: evidence from experimental and computational studies

By combining the diphosphanes Ar2P-PAr2, where Ar = C6H5, 4-C6H4Me, 4-C6H4OMe, 3,5-C6H3(CF3)2, it has been shown that P,P-metathesis generally occurs rapidly under ambient conditions. DFT calculations have shown that the stability of unsymmetrical diphosphanes Z2P-PZ'2 is a function of the difference between the Z and Z' substituents in terms of size and electronegativity. Of the mechanisms that were calculated for the P,P-metathesis, the most likely was considered to be one involving Ar2P˙ radicals. The observations that photolysis increases the rate of the P,P-metatheses and TEMPO inhibits it, are consistent with a radical chain process. The P,P-metathesis reactions that involve (o-Tol)2P-P(o-Tol)2 are anomalously slow and, in the absence of photolysis, were only observed to take place in CHCl3 and CH2Cl2. The role of the chlorinated solvent is ascribed to the formation of Ar2PCl which catalyses the P,P-metathesis. The slow kinetics observed with (o-Tol)2P-P(o-Tol)2 is tentatively attributed to the o-CH3 groups quenching the (o-Tol)2P˙ radicals or inhibiting the metathesis reaction sterically.

Diphosphinoboranes as Intramolecular Frustrated Lewis Pairs: P-B-P Bond Systems for the Activation of Dihydrogen, Carbon Dioxide, and Phenyl Isocyanate

Herein, we present the first example of the activation of small molecules by P-B-P bond systems. The reactivity study involves reactions of two selected diphosphinoboranes, (t-Bu2P)2BPh (1') and (Cy2P)2BNiPr2 (2), that differ in terms of their structural and electronic properties for the activation of dihydrogen, carbon dioxide, and phenyl isocyanate. Diphosphinoborane 1' activates H2 under very mild conditions in the absence of a catalyst with the formation of the dimer (t-Bu2PB(Ph)H)2 and t-Bu2PH. Conversely, diphosphinoborane 2 did not react with H2 under the same conditions. The reaction of 1' with CO2 led to the formation of a compound with an unusual structure, where two phosphinoformate units were coordinated to the PhBOBPh moiety. In addition, 2 reacted with CO2 to insert two CO2 molecules into the P-B bonds of the parent diphosphinoborane. Both diphosphinoboranes activated PhNCO, yielding products resulting from the addition of two and/or three PhNCO molecules and the formation of new P-C, B-O, B-N, and C-N bonds. The products of the activation of small molecules by diphosphinoboranes were characterized with nuclear magnetic resonance (NMR) and infrared (IR) spectroscopy, single-crystal X-ray diffraction, and elemental analysis. Additionally, the reaction mechanisms of the activation of small molecules by diphosphinoboranes were elucidated by theoretical methods.

Synthesis of Sterically Demanding Secondary Phosphides and Diphosphanes and Their Utilization in Small-Molecule Activation

Sterically demanding secondary potassium phosphides (4) were synthesized and investigated. Reaction with halophosphanes (5) yields diphosphanes (6), whereas reaction with CS₂ yields phosphanyl dithioformates (10). These can be further converted to the corresponding phosphanyl esters of dithioformic acid R₂P-C(S)S-PR₂ (8). One of these thioesters (8) was found to undergo a migration reaction, resulting in the formation of a phosphanylthioketone with an additional phosphanylthiolate group (9), which was used as a chiral ligand in gold coordination chemistry. The phosphanyl migration reaction was investigated by spectroscopic and theoretical methods, revealing a first-order reaction via a cyclic transition state. All species mentioned were fully characterized.

Activation of N2O and SO2 by the P-B Bond System. Reversible Binding of SO2 by the P-O-B Geminal Frustrated Lewis Pair

Herein, we present the first transformation of borylphosphine into borylphosphinite using nitrous oxide. Borylphosphine reacts with N2O via insertion of a single oxygen atom into the P-B bond and formation of a P-O-B bond system. Borylphosphine and borylphosphinite capture SO2 and activate it in an irreversible and reversible manner, respectively.

Diaminophosphinoboranes: effective reagents for phosphinoboration of CO2

The monomeric diaminophosphinoboranes readily react with CO₂ under mild conditions to cleanly form products of the general formula in the absence of a catalyst. The isolated products from the CO₂-phosphinoboration were fully characterized by NMR spectroscopy, IR spectroscopy, and X-ray diffraction. The mechanism of CO₂ phosphinoboration with diaminophosphinoboranes was elucidated by DFT calculations.

We present the activation of CO₂ by aminophosphinoboranes along with the mechanism of CO₂ phosphinoboration.