Tuning Lewis acidity using the reactivity of “frustrated Lewis pairs”: facile formation of phosphine-boranes and cationic phosphonium-boranes

Abstract

The concept of "frustrated Lewis pairs" involves donor and acceptor sites in which steric congestion precludes Lewis acid-base adduct formation. In the case of sterically demanding phosphines and boranes, this lack of self-quenching prompts nucleophilic attack at a carbon para to B followed by fluoride transfer affording zwitterionic phosphonium borates [R(3)P(C(6)F(4))BF(C(6)F(5))(2)] and [R(2)PH(C(6)F(4))BF(C(6)F(5))(2)]. These can be easily transformed into the cationic phosphonium-boranes [R(3)P(C(6)F(4))B(C(6)F(5))(2)](+) and [R(2)PH(C(6)F(4))B(C(6)F(5))(2)](+) or into the neutral phosphino-boranes R(2)P(C(6)F(4))B(C(6)F(5))(2). This new reactivity provides a modular route to a family of boranes in which the steric features about the Lewis acidic center remains constant and yet the variation in substitution provides a facile avenue for the tuning of the Lewis acidity. Employing the Gutmann-Beckett and Childs methods for determining Lewis acid strength, it is demonstrated that the cationic boranes are much more Lewis acidic than B(C(6)F(5))(3), while the acidity of the phosphine-boranes is diminished.

Frustrated Lewis Pairs: Metal‐free Hydrogen Activation and More

“…[103] More recently, this reactivity has been shown to be general and the para-substitution was demonstrated to occur for classical phosphine adducts of B(C 6 F 5 ) 3 under warming, yielding the air-and moisture-stable zwitterions 142, 9 a, 143 (Scheme 55). [104] Analogous species are obtained with smaller phosphines after the combined toluene solutions of the reagents are heated under reflux. [105] …”

Frustrierte Lewis‐Paare: metallfreie Wasserstoffaktivierung und mehr

“…9 a und 143 erhalten (Schema 55). [104] Analoge Verbindungen werden auch von weniger sperrigen Phosphinen aus den Reagentien in Toluol bei Siedetemperatur erhalten. [105] …”

Turning Frustration into Bond Activation: A Theoretical Mechanistic Study on Heterolytic Hydrogen Splitting by Frustrated Lewis Pairs

“…[4] The interaction between H 2 and P(tBu) 3 is also found to be repulsive for the chemically relevant d(P-H) range. These results firmly suggest that other reaction channels should be considered to provide a rationale for the reactivity of the P(tBu) 3 /B(C 6 F 5 ) 3 pair with hydrogen. For further discussion we note that both end-on (C 6 F 5 ) 3 B···H 2 and (tBu) 3 P···H 2 interactions induce appreciable polarization of the H 2 molecule already at relatively large X-H distances with an opposite charge separation pattern in these two cases (see Figure 1).…”