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;D ipp = 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+ )a nd methylated [(2-Im Dipp )PMe(4-Im Dipp )] 2+ (12 2+ )d ications, respectively.E PR/UV/Vis-NIR spectroelectrochemical investigation of the low-coordinated P I cation 10 + predicted astable and "bottleable" P-centered radical dication [(2-Im Dipp )P(4-Im Dipp )] 2+ C (13 2+ C). The reaction of 10 + with the nitrosyl salt NO[OTf] yields the persistent phosphanyl radical dication 13 2+ C as triflate salt in crystalline form. Quantum chemical investigation revealed an exceptional high spin density at the Patom.Among several bonding motifs possible for charged and uncharged P 1 -centered radicals,p hosphanyl radicals are particularly intriguing. [1] Such radicals feature at wo-coordinate Patom in the + II oxidation state in combination with either C-(I) or N-based (II)-or ac ombination of both (III)-substituents L C or L N (Scheme 1). Especially lowcoordinate P-radical species require stabilization by either bulky substituents (kinetic stabilization), and/or spin delocalization (thermodynamic stabilization). [2] However, only af ew examples are known that are stable enough to be isolated in the solid state.Representative examples of isolated and structurally characterized neutral (1C-5C), [3][4][5][6] anionic (6 À C), [7] and cationic (7 + C, 8 + C) [8,9] derivatives are depicted in Scheme 1. Thestability of the radical cations is also explained by the positive charge leading to electrostatic repulsive effects.S urprisingly,a nd to the best of our knowledge,n o example of ap hosphanyl radical dication has been reported so far,although with the isolation of radical cations of type 7 + C and 8 + C those species should be feasible.T hus,adicationic radical species might be accessible by af ormal exchange of the L N substituent in cations of type 7 + C or 8 + C by an imidazoliumyl substituent. Recently,w er eported on the high-yielding synthesis of the cationic phosphanide 10 + , bearing two lone pairs of electrons at the Patom, which we obtained from the reduction of dication 9 2+ (Scheme 2). [10] Cation 10 + is supposed to react with electrophiles such as trifluoromethanesulfonic acid (HOTf) or methyl trifluoromethylsulfonate (MeOTf) to give the respective protonated (11 2+ )a nd methylated dications (12 2+ )a st riflate salts, respectively (Scheme 2). Theu nique combination of the Cbonded N-heterocyclic substituents (L C )o nt he two-coordinate phosphorus atom in cation 10 + makes it an excellent substrate for the synthesis of the hitherto unknown phosphanyl radical dication 13 2+ C (Scheme 2).Reacting 10 + with HOTf or MeOTf in CH 2 Cl 2 gives the expected dications 11 2+ (88 %) and 12 2+ (83 %) as triflate salts in very good yield (Scheme 2). [11] Both compounds are obtained as ...

NHCs in Main Group Chemistry

“…865 The 31 P NMR spectrum of 994 displayed a downfield signal at −66.1 ppm relative to P-imidazolium-2-yl adducts Cation 994 preserves the reactivity characteristic for neutral and cationic NHC−phosphinidene complexes, forming the corresponding mono-and bimetallic complexes with coinage metals, 865 and reacting with electrophilic reagents (TfOH, MeOTf) to form the corresponding dicationic bis-(imidazoliumyl)phosphines. 866 Isolation of the first carbene-stabilized P 3 triphosphaallyl cation 996 and P 2 neutral adduct 997 upon the NHC-induced fragmentation of the P 4 -derived cage [RP 5 Cl] + 995 has also been accomplished by the group of Weigand (Scheme 215). 867 The obtained P 3 cation 996 possesses structural and electronic features similar to those of the discussed 950[Cl], reported later by Gruẗzmacher et al (cf., Scheme 202).…”

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

“…Shortly after we prepared cation 4,W eigand and co-workers reported on the synthesis and redox reactivity of as imilarp hosphamethine cyanine dye, G + + . [18] For G + + ,W eigand,d emonstrated by cyclic voltammetry,t hat the cation could undergo ar eversible 1-electron oxidation (E 1/2 = 0.22 Vi nT HF vs. Fc/Fc + )b ut irreversible reduction (E p = À3.06 V). [18] Cation G + + ,w as then chemically oxidized using nitrosonium to give the phosphorus(II) radicald ication G 2 + + C (Scheme2)w hich was fully characterizeds pectroscopicallya nd by single-crystal XRD, whereas chemical reduction was not attempted.…”

“…[18] For G + + ,W eigand,d emonstrated by cyclic voltammetry,t hat the cation could undergo ar eversible 1-electron oxidation (E 1/2 = 0.22 Vi nT HF vs. Fc/Fc + )b ut irreversible reduction (E p = À3.06 V). [18] Cation G + + ,w as then chemically oxidized using nitrosonium to give the phosphorus(II) radicald ication G 2 + + C (Scheme2)w hich was fully characterizeds pectroscopicallya nd by single-crystal XRD, whereas chemical reduction was not attempted. [18] Given the similarity to G + + ,w en ext focusedo ur efforts on investigating the redox chemistry of 4.…”

“…[18] Cation G + + ,w as then chemically oxidized using nitrosonium to give the phosphorus(II) radicald ication G 2 + + C (Scheme2)w hich was fully characterizeds pectroscopicallya nd by single-crystal XRD, whereas chemical reduction was not attempted. [18] Given the similarity to G + + ,w en ext focusedo ur efforts on investigating the redox chemistry of 4.…”

Enhanced Photoinduced Electron Transfer and Stability of Diradicals in Neutral Extended Pyridine N-Oxides

“…26 However, upon irradiation in the air the BPDO sample produced unknown byproducts, as shown in the 1 H NMR spectra (Figure S4). Common methods to stabilize organic radicals generally include using weak coordination anions, 27 extending molecular conjugation, 28,29 or introducing bulky substituents. 30 In order to improve the stability of the BPDO radical, we took the second method, that is, linking a central πconjugated moiety into BPDO to extend molecular conjugation.…”