Activation of Carbon Dioxide by Silyl Triflate‐Based Frustrated Lewis Pairs

Silyl triflates of the form R_4?nSi(OTf)_n (n=1, 2; OTf=OSO₃CF₃) are shown to activate carbon dioxide when paired with bulky alkyl‐substituted Group 15 bases. Combinations of silyl triflates and 2,2,6,6‐tetramethylpiperidine react with CO₂ to afford silyl carbamates via a frustrated Lewis pair‐type mechanism. With trialkylphosphines, the silyl triflates R₃Si(OTf) reversibly bind CO₂ affording [R′₃P(CO₂)SiR₃][OTf] whereas when Ph₂Si(OTf)₂ is used one or two molecules of CO₂ can be sequestered. The latter bis‐CO₂ product is favoured at low temperatures and by excess phosphine.     

Stimuli‐Responsive Frustrated Lewis‐Pair‐Type Reactivity of a Tungsten Iminoazaphosphiridine Complex

Reactions of 3‐imino‐azaphosphiridine complexes 1 a,b with carbodiimides 2 a,b , isocyanates 3 a,b , and carbon dioxide are described. Whereas exchange of the carbodiimide unit occurs in the first case, an overall ring expansion takes place with phenyl isocyanate ( 3 a ) and carbon dioxide to yield complexes 4 and 5 bearing novel 1,3,5‐oxazaphospholane ligands; the isopropyl derivative 3 b did not react under these conditions. DFT calculations provide insight into the pathway of the reaction with carbon dioxide with model complex 1 c , revealing effects of initial non‐covalent interactions with the substrate onto the ring bonding, thus triggering an initially masked frustrated Lewis‐pair‐type behavior.     

Carbene‐9‐BBN Ring Expansions as a Route to Intramolecular Frustrated Lewis Pairs for CO2 Reduction

Reactions of phosphine‐derived carbenes with 9‐borabicyclo[3.3.1]nonane (9‐BBN) result in ring‐expansion reactions to generate novel intramolecular frustrated Lewis pairs (FLPs). These FLPs effect the catalytic reduction of CO₂ in the presence of boranes to give BOB and methoxy‐borate species.     

CO2 and formate complexes of phosphine/borane frustrated Lewis pairs

The reaction of a solution of B(C6F4H)3 and either iPr3P or tBu3P with CO2 afforded the species R3P(CO2)B(C6F4H)3 (R=iPr (1), tBu (2)). In a similar fashion the boranes, RB(C6F5)2 (R=hexyl, cyclohexyl (Cy), norbornyl), ClB(C6F5)2, or PhB(C6F5)2 were combined with tBu3P and CO2 to give the species tBu3P(CO2)BR(C6F5)2 (R=hexyl (3), Cy (4), norbornyl (5), Cl (6), Ph (7)). Similarly, the compounds [tBu3PH][RBH(C6F5)2] (R= hexyl (8), Cy (9), norbornyl (10)) were prepared by reaction of the precursor frustrated Lewis pair (FLP) with H2. Subsequent reactions of 9 and 10 with CO2 afforded the species [((C6F5)2BR)2(μ-HCO2)][tBu3PH] (R= Cy (11), norbornyl (12)). In related chemistry, combinations of the boranes RBG(C6F5)2 (R=hexyl, Cy, norbornyl) with tBu3P treated with an equivalent of formic acid gave [(C6F5)2BR(HCO2)][tBu3PH] (R=hexyl (13), Cy (14), norbornyl (15)). Subsequent addition of an additional equivalent of borane provides a second synthetic route to 11 and 12. Crystallographic studies of compounds 2-6 and 8-14 are reported and discussed. Further understanding of the FLP complexation and activation of CO2 is provided by computational studies.     

A Neutral Silicon/Phosphorus Frustrated Lewis Pair

Frustrated Lewis pairs (FLPs) have a great potential for activation of small molecules. Most known FLP systems are based on boron or aluminum atoms as acid functions, few on zinc, and only two on boron‐isoelectronic silicenium cation systems. The first FLP system based on a neutral silane, (C₂F₅)₃SiCH₂P(tBu)₂ ( 1 ), was prepared from (C₂F₅)₃SiCl with C₂F₅ groups of very high electronegativity and LiCH₂P(tBu)₂. 1 is capable of cleaving hydrogen, and adds CO₂ and SO₂. Hydrogen splitting was confirmed by H/D scrambling reactions. The structures of 1 , its CO₂ and SO₂ adducts, and a decomposition product with CO₂ were elucidated by X‐ray diffraction.     

Transition‐Metal‐Free Catalytic Reduction of Carbon Dioxide

Metal‐free systems, including frustrated Lewis pairs (FLPs) have been shown to bind CO₂. By reducing the Lewis acidity and basicity of the ambiphilic system, it is possible to generate active catalysts for the deoxygenative hydroboration of carbon dioxide to methanol derivatives with conversion rates comparable to those of transition‐metal‐based catalysts.