Photoinduced and Thermal Single‐Electron Transfer to Generate Radicals from Frustrated Lewis Pairs

Abstract

Archetypal phosphine/borane frustrated Lewis pairs (FLPs) are famed for their ability to activate small molecules. The mechanism is generally believed to involve two‐electron processes. However, the detection of radical intermediates indicates that single‐electron transfer (SET) generating frustrated radical pairs could also play an important role. These highly reactive radical species typically have significantly higher energy than the FLP, which prompted this investigation into their formation. Herein, we provide evidence that the classical phosphine/borane combinations PMes 3 /B(C 6 F 5 ) 3 and P t Bu 3 /B(C 6 F 5 ) 3 both form an electron donor–acceptor (charge‐transfer) complex that undergoes visible‐light‐induced SET to form the corresponding highly reactive radical‐ion pairs. Subsequently, we show that by tuning the properties of the Lewis acid/base pair, the energy required for SET can be reduced to become thermally accessible.

Long-Lived Charge-Transfer State from B–N Frustrated Lewis Pairs Enchained in Supramolecular Copolymers

“…The formation of FLPs between a π-conjugated donor (in this case, N) and acceptor (in this case, B) could result in charge-transfer (CT) processes in the ground and/or excited states. 17 , 40 , 41 Supramolecular copolymerizations were thus performed by mixing poly( S -B1) with an equimolar amount of either poly( S -N1) or poly( S -N2) , giving poly[( S -B1)- co -( S -N1)] and poly[( S- B1)- co -( S -N2)] , respectively. The mixed solution, at a final monomer concentration of 30 μM (i.e., 15 μM/monomer) was heated to 100 °C to reach the molecularly dissolved state and slowly cooled (0.3 °C min –1 ) to 20 °C to perform the polymerization under thermodynamic control.…”

“… 45 Further evidence that confirms the formation of B–N FLP is the presence of a weak CT absorption band (observed as a shoulder at λ ∼ 500 nm) in concentrated solutions of poly[( S -B1)- co -( S -N1)] in decalin ( Figure S8 ), in line with previously reported B–N FLP couples. 41 , 46 …”

Single‐Electron Transfer in Frustrated Lewis Pair Chemistry

“…[13] These data show that the reaction is not light dependent and therefore the formation of the radical ion pair does not significantly influence the reaction kinetics.This finding suggests that the photo-stationary concentration of the radical ion pair [PMes 3 C + ,B(C 6 F 5 ) 3 C À ]istoo low and/or its lifetime is too short to significantly influence the reaction rate. Indeed, this charge-separated state lies much higher in energy (54.4 kcal mol À1 )than the neutral donor-acceptor pair [PMes 3 , B(C 6 F 5 ) 3 ]a nd undergoes rapid back-electron transfer (lifetime = 237 ps) as determined by transient absorption spectroscopy to regenerate the FLP, [10] thus preventing build-up of ac oncentration of radicals large enough to influence the reaction kinetics.This leads to the conclusion that the splitting of dihydrogen by PMes 3 and B(C 6 F 5 ) 3 proceeds via at woelectron, heterolytic pathway,e ven when the reaction is performed in ambient light. [2e-g] Next, we probed the reaction between PMes 3 /B(C 6 F 5 ) 3 and Ph 3 SnH (2 equiv) to analyse whether light affects the formation of phosphonium borate [Mes 3 PH][HB(C 6 F 5 ) 3 ]and Ph 3 SnÀSnPh 3 .W efound that the reaction proceeds rapidly in both darkness and ambient light and, in both cases,w ithin minutes full conversion to [Mes 3 PH][HB(C 6 F 5 ) 3 ]and Ph 3 SnÀ SnPh 3 was observed by multi-nuclear NMR spectroscopy Figure S10), supporting the observations by Stephan et al [2c] We noted, however, that when more Ph 3 SnH (up to 2.5 equiv) was used, the reaction continued and after 20 hours [tBu 3 PH][HB(C 6 F 5 ) 3 ]( d 31 P = 58.1, 1 J P-H = 410 Hz;S upporting Information, Figure S12) as well as Ph 3 Sn-SnPh 3 (Scheme 3; Supporting Information, Figure S14) was observed.…”

Single‐Electron Transfer in Frustrated Lewis Pair Chemistry

“…Recently,wedemonstrated the generality of SET for FLP type donor-acceptor systems [8,9] and showed that for common P/B FLPs (PMes 3 /B(C 6 F 5 ) 3 and PtBu 3 /B(C 6 F 5 ) 3 )a nd analogous N/B systems visible light is required to induce SET to generate the corresponding transient radical ion pairs (Scheme 1e). [10] This light dependence provides an excellent probe for determining whether an FLP reaction proceeds via ar adical mechanism or via concerted, polar pathways,a s carrying out the reaction in the absence of light precludes the formation of the radical ion pair. Thework presented herein focuses on applying this notion to investigate the reaction of archetypal FLPs with the substrates H 2 ,Ph 3 SnH and TCQ.In addition, we analyzed the nature of the initial single-electron transfer event that is responsible for the radical chemistry observed by Melen et al and Ooi et al Fora ll cases,w ed etermine whether the boron Lewis acid is directly involved in SET,o rp lays af acilitating role by enhancing the oxidizing power of the substrate.…”

“…[12] We previously showed that this combination of donor (PMes 3 )a nd acceptor (B(C 6 F 5 ) 3 )f orms av iolet charge-transfer complex in solution from which the corresponding radical ion pair [PMes 3 C + ,B(C 6 F 5 ) 3 C À ]isgenerated by irradiating this electron donor-acceptor (EDA) complex with visible light (534 nm). [10] Thus,ifformation of this radical ion pair is asignificantly contributing factor in hydrogen splitting, the reaction should exhibit ac hange in reaction rate depending on the absence or presence of light. Comparison of reaction samples kept in the dark or irradiated (534 nm, 2.2 W LEDs;S cheme 2) whilst stirring for 2.5 hours showed nearidentical conversions to the phosphonium borate [Mes 3 PH]-[HB(C 6 F 5 ) 3 ], and again after 4hours,a sd etermined by 31 P-NMR spectroscopy (Supporting Information, Figures S1, S2).…”