Synthesis and EPR/UV/Vis‐NIR Spectroelectrochemical Investigation of a Persistent Phosphanyl Radical Dication

Abstract

The reaction of the bis(imidazoliumyl)-substituted P(I) cation [(2-Im(Dipp) )P(4-Im(Dipp) )](+) (10(+) ) (2-Im=imidazolium-2-yl; 4-Im=imidazolium-4-yl; Dipp=2,6-di-isopropylphenyl) with trifluoromethanesulfonic acid (HOTf) or methyl trifluoromethylsulfonate (MeOTf) yields the corresponding protonated [(2-Im(Dipp) )PH(4-Im(Dipp) )](2+) (11(2+) ) and methylated [(2-Im(Dipp) )PMe(4-Im(Dipp) )](2+) (12(2+) ) dications, respectively. EPR/UV/Vis-NIR spectroelectrochemical investigation of the low-coordinated P(I) cation 10(+) predicted a stable and "bottleable" P-centered radical dication [(2-Im(Dipp) )P(4-Im(Dipp) )](2+.) (13(2+.) ). The reaction of 10(+) with the nitrosyl salt NO[OTf] yields the persistent phosphanyl radical dication 13(2+.) as triflate salt in crystalline form. Quantum chemical investigation revealed an exceptional high spin density at the P atom.

Synthesis, Spectroscopic Characterization, and Redox Reactivity of a Cyclic (Alkyl) Amino Carbene‐Derived Arsamethine Cyanine Dye

“…Similar to known phosphamethine cyanine dyes, cation 4 is a bright‐red solid that exhibited an intense absorbance in the visible region of the electromagnetic spectrum ( λ max =511 nm, CH 2 Cl 2 , 4.27×10 −5 m , ϵ =2.31×10 4 m cm −1 , Figure S9 in the Supporting Information). Shortly after we prepared cation 4 , Weigand and co‐workers reported on the synthesis and redox reactivity of a similar phosphamethine cyanine dye, G + . For G + , Weigand, demonstrated by cyclic voltammetry, that the cation could undergo a reversible 1‐electron oxidation ( E 1/2 =0.22 V in THF vs. Fc/Fc + ) but irreversible reduction ( E p =−3.06 V) .…”

“…Shortly after we prepared cation 4 , Weigand and co‐workers reported on the synthesis and redox reactivity of a similar phosphamethine cyanine dye, G + . For G + , Weigand, demonstrated by cyclic voltammetry, that the cation could undergo a reversible 1‐electron oxidation ( E 1/2 =0.22 V in THF vs. Fc/Fc + ) but irreversible reduction ( E p =−3.06 V) . Cation G + , was then chemically oxidized using nitrosonium to give the phosphorus(II) radical dication G 2+.…”

Recent Advances in Imidazoliumyl‐Substituted Phosphorus Compounds

“…The redox behavior of cation 125 + was monitored by EPR‐UV/Vis/NIR spectroscopy, which revealed the presence of a persistent radical dication 127 2+ . originating from electrochemical oxidation of 125 + . Thus, the reaction of 125 [OTf] with nitrosyl triflate at −78 °C affords the remarkable and stable radical dication 127 2+ .…”

“…The room‐temperature EPR spectrum of 127 . [OTf] 2 in 1,2‐difluorobenzene shows a doublet resonance ( g= 2.006) associated to a large hyperfine coupling constant of a ( 31 P)=86 G. Interestingly, no hyperfine coupling to the 14 N atoms is observed in the EPR spectrum, which indicates that the singly occupied molecular orbital (SOMO) is barely delocalized over the π‐conjugated substituents but primarily located at the P atom …”

“…can partially be justified by thermodynamic stabilization. More relevant is the kinetic stabilization, including sterically encumbered substituents and electrostatic repulsive effects as a result of the +2 positive charge …”