Influence of the Lewis Acidity of Gallium Atoms on the Reactivity of a Frustrated Lewis Pair: Experimental and Theoretical Studies

Abstract

The reactivity of the Ga/P-based frustrated Lewis pair (FLP) Mes 2 P− C[C(H)−Ph]−Ga t Bu 2 (3) is influenced by the relatively weak Lewis acidity of its Ga atom and differs significantly from that of the analogous Al compound 1. The adduct of 3 with CO 2 was only detectable at low temperature by NMR spectroscopy. Benzaldehyde was coordinated only via a Ga−O bond; the P atom was not involved. In contrast, a relatively persistent adduct was formed with soft CS 2 to yield a fivemembered GaCPCS heterocycle. Dehydrocoupling with H 3 B←NHMe 2 afforded the dimeric amidoborane (H 2 B−NMe 2 ) 2 , while an adduct with a GaCPBN heterocycle was isolated with the sterically less shielded ammonia−borane H 3 B←NH 3 . The latter product was unstable in solution and decomposed by H 2 elimination and formation of oligomeric BN compounds. Small quantities of 3 catalyzed hydrogen transfer from H 3 B←NH 3 to an imine. The Lewis acidities of the Al/P-and Ga/P-based FLPs were examined by experiments (Gutmann−Beckett method) and by calculation of the fluoride ion affinity (including the B and In analogues). The Al compound is the strongest Lewis acid; the Ga FLP is significantly weaker but is a stronger F − acceptor in comparison to the unknown analogues of B and In. These results reflect the different reactivities of these FLPs and may help to develop FLPs with finely adjusted properties.

Dihalides of Sterically Constrained Tricyclic Phosphines, Lewis Acidity and Fluoride Affinity, Chloride Abstraction, and a Phosphonium Cation, Dimethylphosphorane

“…Ring strain in compounds 5 may influence the Lewis acidity of the P atoms. Quantum chemical calculations (PBE0 def2‐TZVP+D3BJ) of the fluoride anion affinity (FIA) of the organodifluorophosphorane 5a afforded values of 322.6 (gas phase) and 237.9 kJ/mol in THF, which characterize 5a as a relatively weak Lewis acid comparable to BF 3 (338 kJ/mol; gas phase) or a recently published Ga‐based frustrated Lewis pair (302 kJ/mol) . PF 5 is expectedly a stronger Lewis acid and has a FIA of 384 kJ/mol in the gas phase .…”

Dehydrogenation of Amine–Boranes Using p‐Block Compounds

“…Gallium analogue 69 showed similar reactivity towards Me 2 NH⋅BH 3 , yet in this case no aminoborane adduct intermediate ( 70 ) could be detected during the reaction and solely FLP 69 and cyclic diborazane 68 were observed . Treatment of FLP 69 with the sterically less hindered NH 3 ⋅BH 3 did afford the five‐membered heterocycle 71 , next to the evolution of dihydrogen gas (Scheme ).…”

“…Gallium analogue 69 showed similar reactivity towards Me 2 NH·BH 3 ,y et in this case no aminoborane adduct intermediate (70)c ould be detected during the reactiona nd solely FLP 69 and cyclic diborazane 68 were observed. [71] Treatment of FLP 69 with the sterically less hindered NH 3 ·BH 3 dida fford the five-membered heterocycle 71,n ext to the evolution of dihydrogen gas (Scheme 34). This aminoborane adduct is not stable at elevated temperatures (75 8C) and full recovery of the P/Ga FLP 69 was observed together with the formation of dihydrogen, which promptedt he question of whether 69 could act as ah ydrogent ransfer catalyst.…”

Influence of the Lewis Acid/Base Pairs on the Reactivity of Geminal E‐CH2‐E′ Frustrated Lewis Pairs

“…Indeed, geminal P/Al [3] and P/B [4,5] Lewis pairs have attracted considerable interestq uite recently due to their remarkable activity in the activation of small molecules. In addition, other geminal FLPs based on N/B, [6] N/Al, [7] P/X [8] (X = Group 14 element), and even P/Ga [9] have been prepared in order to tune the Lewis acidity/basicity of the FLP antagonists and consequently,t om odify the activity of the system. Despite that, the influence of the nature of the Lewisp airs on the reactivity of the FLP is so far not fully understood, which is of crucial importance for furtherd evelopment of FLPs.…”