Chemical Detection of Hydrogen Fluoride by the Phosphorus Congener of Cyclobutane-1,3-diyl

Abstract

Heteroaryl-substituted air-tolerant 2,4-bis(2,4,6-tri-t-butylphenyl)-1,3-diphosphacyclobutane-2,4-diyls in the open-shell singlet state were synthesized by a sterically promoted regioselective S(N)Ar process. Here we demonstrate that these diyls are effective for capturing hydrogen fluoride (HF) generated by intermediary base-coordinated HF and amine-stabilized HF reagents. The hydrofluorination reaction predominantly occurred on the λ(3)σ(3)-phosphorus atoms to afford the energetically disfavored 1λ(5),3λ(5)-diphosphete. The positively charged t-butyl-substituted phosphorus atom trapped the fluoride anion, and the subsequent protonation was controlled by the steric effect. X-ray crystallographic analysis and an Atoms in Molecule study of the air-stable 1λ(5),3λ(5)-diphosphete bearing P-H and P-F bonds revealed that the delocalized ylidic linkages in the four-membered ring were almost identical, in contrast to the nonsymmetrically substituted 2,4-bis(2,4,6-tri-t-butylphenyl)-1,3-diphosphacyclobutane-2,4-diyl. Hydrofluorination efficiently induced a remarkable exchange of visible photoabsorption. The charge-transfer-type transition from highest occupied molecular orbital to lowest unoccupied molecular orbital was highly tuned, which is advantageous for the facile identification of HF. In contrast to hitherto known trapping reagents for HF based on cleavage of the H-F bond, several hydrofluorinated P-heterocycles were reconverted into the 1,3-diphosphacyclobutane-2,4-diyl by treatment with sodium hydride. However, in the hydrofluorination of the benzoyl-substituted 1,3-diphosphacyclobutane-2,4-diyl, fluorination and protonation occurred at the t-butyl-substituted phosphorus atom and the skeletal carbon atom, respectively, and the energetically preferable 1λ(5),3λ(3)-dihydrodiphosphete was isolated as a purple-blue crystalline compound. These findings are promising not only for the practical detection of HF but also for the development of fluorine technology based on the chemistry of phosphorus heterocycles.

Access to Air‐Stable 1,3‐Diphosphacyclobutane‐2,4‐diyls by an Arylation Reaction with Arynes

“…[7] In our study of HF trapping by the open-shell P-heterocyclic system, the electron-rich 1,3-diphosphacyclobutane-2,4-diyl system showed ar emarkable blueshift of photoabsorption assigned as the transition from the HOMO to the LUMO, which enabled the identification of the presence of HF by visual observation. Compound 2d showed aw eak visible absorption (595 nm) due to this HOMO-LUMO transition, whereas 7 (Scheme 4), with ah igher-lying LUMO as compared to that of 2d,s howed remarkably different photoabsorption characteristics (Figure 2).…”

“…Nevertheless,t he aryl substituent on the phosphorus atom affects the physicochemical properties. [6,7] Thee ffect of the aromatic substituent on the 4-membered molecular system was almost identical for all compounds 2.Onthe other hand, 6 showed ar emarkable blueshift of photoabsorption as compared to 2a,w hereas the oxidation potential of 6 was almost identical to that of 2a.The HOMO-LUMO energetic difference (4.91 eV) of 6,optimized at the M06-2X/6-31G(d)/ SDD level, was larger than that of 2a (4.68 eV; M06-2X/6-31G(d)).…”

“…[7] However,the S N Ar process was only successful when electron-deficient N-heterocyclic aryl halides were employed as the electrophiles,b ecause the sp 2 phosphorus atom of 1 is positive in nature and scarcely contributes to the HOMO of 1. [7] Thed evelopment of novel methods for the direct arylation of 1,3-diphosphacyclobuten-4-yl anions with relatively electron rich aromatic substituents is quite attractive,asthe experimentally characterized strong electron-donating nature of the resulting air-stable 1,3-diphosphacyclobutane-2,4-diyls makes these compounds suitable as p-type semiconductors and for the practical sensing of HF.F urthermore,i nstallation of the electron-rich aromatic substituent on the phosphorus atom induces high stability owing to the enlarged HOMO-LUMO gap.…”

“…5 -diphosphetes, [7] were observed in the reaction mixture (Table 1, entry 3). On the other hand, the equivalent reaction at À20 8 8Ca fforded 3a together with the hydrofluorinated products,t hus indicating that the subsequent hydrofluorination was minimized at lower temperature (Table 1, entry 4).…”

Exploration of 1-Arylmethyl-1,3-diphosphacyclobutane-2,4-diyls as Hole Transfer Materials

“…Compounds 6–8 were obtained by using arylmethyl bromide, whereas 9 and 10 were synthesized by the corresponding mesylate owing to the instability of the bromides. Table S1 summarizes the visible photoabsorption wavelengths of the transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) ( ϵ =700–1300) and the oxidation potentials . The visible photoabsorption and electron‐donating voltage were essentially unaffected by the aryl group.…”