Aryldiazonium Salts as Nitrogen‐Based Lewis Acids: Facile Synthesis of Tuneable Azophosphonium Salts

Abstract

Inspired by the commercially available azoimidazolium dyes (e.g., Basic Red 51) that can be obtained from aryldiazonium salts and N-heterocyclic carbenes, we developed the synthesis of a unique set of arylazophosphonium salts. A range of colours were obtained by applying readily tuneable phosphine donor ligands and para-substituted aryldiazonium salts as nitrogen-based Lewis acids. With cyclic voltammetry, a general procedure was designed to establish whether the reaction between a Lewis acid and a Lewis base occurs by single-electron transfer or electron-pair transfer.

Facile Synthesis of Tuneable Azophosphonium Salts

“…To explore the substituent effect on the 31 P NMR chemical shift as well as the colour of this novel class of azo dyes, we expanded the scope of the para substituted arylazophosphonium salts [( p ‐R‐C 6 H 4 )N 2 (P t Bu 3 )][BF 4 ] 1 (Scheme ) . Reaction of 16 p ‐substituted benzenediazonium salts with tri‐ tert ‐butylphosphine (1.1 equiv) in acetonitrile afforded many shades of intensely coloured phosphine–diazonium adducts [( p ‐R‐C 6 H 4 )N 2 (P t Bu 3 )][BF 4 ] 1a – p , ranging from purple, to pink, red, and red/brown (Figure ), which were isolated in moderate to good yields (55–97 %, Scheme , Table ).…”

“…For the π (HOMO‐1) to π* (LUMO) excitation overall a slight decrease is observed ( 1a , n: –12.8, π*: –4.9 eV; 1p , n: –10.3, π*: –3.8 eV), yet the π→π* excitation gap differs for all cases [from 1c : 8.0 (biggest) to 1p : 6.5 (smallest) eV], still, here too overall a decrease is observed from 1a to 1p (–7.9 to –6.5 eV, respectively). The energies obtained from the calculations are not accurate enough to correlate these to UV/Vis absorption bands, for which time‐dependent DFT calculations are required (see SI) …”

Frustrated Radical Pairs: Insights from EPR Spectroscopy

“…Cp 2 Fe 0/+ in THF), [10] and the oxidation potential of tBu 3 P(0.90 Vvs. Cp 2 Fe 0/+ in MeCN) [11] (Table 2), the concentration of the radical ion Lewis pair is expected to Progress in frustrated Lewis pair (FLP) chemistry has revealed the importance of the main group elements in catalysis,o pening new avenues in synthetic chemistry.Recently,new reactivities of frustrated Lewis pairs have been uncovered that disclose that certain combinations of Lewis acids and bases undergo single-electron transfer (SET) processes.H ere an electron can be transferred from the Lewis basic donor to aL ewis acidic acceptor to generate areactive frustrated radical pair (FRP). This minireview aims to showcase the recent advancements in this emerging field covering the synthesis and reactivities of frustrated radical pairs,with extensive highlights of the results from Electron Paramagnetic Resonance (EPR) spectroscopyto explain the nature and stability of the different radical species observed.…”

“…Indeed, while the TEMPOC radical is inert toward dihydrogen, the TEMPO/B(C 6 F 5 ) 3 FLP system was found to be active towards dehydrogenation of 1,4-cyclohexadiene (Path A) as well as dihydrogen activation under mild reaction conditions (Path B). [34] High reactivities of FLPs towards NO were demonstrated to account for the formation of reactive aminoxyl radicals.On the other hand, Slootweg and co-workers [11] explored the formation of the NOC radical using the nitrosonium salt [NO][BF 4 ]and tBu 3 P. Single-electron transfer between tBu 3 P and the nitrosonium salt [NO][BF 4 ]inacetonitrile generated [HPtBu 3 ][BF 4 ]asthe major product (Scheme 16).…”

Aryldiazonium Salts as Nitrogen‐Based Lewis Acids: Facile Synthesis of Tuneable Azophosphonium Salts

“…Gratifyingly,this cationic Lewis adduct is stable towards air,m oisture,a nd even an aqueous 2 m HCl solution (only the tetrafluoroborate anion hydrolysed over time). [21] Themolecular structure of 2a (Scheme 1), [22,23] determined by X-ray crystal-structure analysis of suitable crystals of its tetraphenylborate analogue (obtained after anion exchange with NaBPh 4 in DCM), [10] displays an almost planar (P1-N1-N2-C1 173.4(7)8 8) trans arylazophosphonium moiety with adisordered azo group.The CÀNand NÀNbond lengths (1.437(7) and 1.245(6) ,r espectively) are comparable to those of the related arylazoimidazolium borates [ArN 2 (IMes)][BPh 4 ]( Ar = Mes, o/p-ClC 6 H 4 )r eported by Severin and co-workers (C-N:1 .411(2)/1.395(4)/1.455 (13); N-N:1 .265(2)/1.266(9)/1.24(2) ,r espectively), [11e] illustrating that in these cationic azo dyes,phosphines behave similar to carbenes.…”