Abstract
AbstractWe 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, Arf). These receptor systems are capable of capturing gaseous CO, and in the case of the ‐BArf2 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 PtBu3 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 ‐BArf2 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.
Highlights
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, Arf)
Weak OCÀB s-bonding and lack of p-backbonding in these systems typically leads to low thermodynamic stability: OC·B(C6F5)3, OC·B(CF3)3, and OC·B(C6F5)2H all decompose at room temperature or below, with only the perfluoropentaphenylborole adduct retaining CO at elevated temperatures.[6]
Erker et al have reported single-component borane/ phosphine frustrated Lewis pairs (FLPs) which react in solution to give CO adducts which are stable at low temperatures (e.g. Figure 1 C);[9] CO capture by analogous borane/amine FLPs has not been reported
Summary
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, Arf). We hypothesized that a structural modification to “simple” bifunctional FLPs involving the incorporation of an additional Lewis acid site to bind the Oterminus of the CO molecule might allow for enhanced binding affinities (Figure 2).[14] we report on the synthesis and mode of action of BNB bis-borane FLPs, one of which can reversibly take up CO at room temperature.
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