Isolation of singlet carbene derived 2-phospha-1,3-butadienes and their sequential one-electron oxidation to radical cations and dications

A synthetic strategy for the 2-phospha-1,3-butadiene derivatives [{(IPr)C(Ph)}P(cAACMe)] (3a) and [{(IPr)C(Ph)}P(cAACCy)] (3b) (IPr = C{(NDipp)CH}2, Dipp = 2,6-iPr2C6H3; cAACMe = C{(NDipp)CMe2CH2CMe2}; cAACCy = C{(NDipp)CMe2CH2C(Cy)}, Cy = cyclohexyl) containing a C 
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Created by potrace 1.16, written by Peter Selinger 2001-2019
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 C–PC framework has been established. Compounds 3a and 3b have a remarkably small HOMO–LUMO energy gap (3a: 5.09; 3b: 5.05 eV) with a very high-lying HOMO (−4.95 eV for each). Consequently, 3a and 3b readily undergo one-electron oxidation with the mild oxidizing agent GaCl3 to afford radical cations [{(IPr)C(Ph)}P(cAACR)]GaCl4 (R = Me 4a, Cy 4b) as crystalline solids. The main UV-vis absorption band for 4a and 4b is red-shifted with respect to that of 3a and 3b, which is associated with the SOMO related transitions. The EPR spectra of compounds 4a and 4b each exhibit a doublet due to coupling of the unpaired electron with the 31P nucleus. Further one-electron removal from the radical cations 4a and 4b is also feasible with GaCl3, affording the dications [{(IPr)C(Ph)}P(cAACR)](GaCl4)2 (R = Me 5a, Cy 5b) as yellow crystals. The molecular structures of compounds 3–5 have been determined by X-ray diffraction and analyzed by DFT calculations.


Introduction
Organic p-conjugated molecules are currently of great academic and signicant technological interest due to their intriguing optoelectronic properties. 1 In this context, p-conjugated systems featuring heavier main-group elements 2 and systems exhibiting a considerable open-shell (radical-type) character 3 are particularly attractive as they display promising optical, electronic, and magnetic properties. Among heavier main-group elements, the choice to incorporate phosphorus into p-conjugated systems has been primarily driven by its semblance to the isoelectronic "CR" (R ¼ H, alkyl or aryl group) unit, which is known as a diagonal relationship. 4 Moreover, while the calculated P]C p-bond strength (43 kcal mol À1 ) is lower than the C]C p-bond of ethene (65 kcal mol À1 ) 5 the conjugative properties of both P]C and C]C bonds are comparable. 6 Among stable main-group radicals, 7 various neutral, 8 cationic, 9 as well as anionic 10 phosphorus radicals have been also isolated and structurally characterized, however, phosphorus radicals based on a p-conjugated framework remain scarce. 1,3-Butadiene I is the simplest molecule with conjugated p-bonds ( Fig. 1) that has also been an important structural motif in phosphorus chemistry. 11 Indeed, unsubstituted as well as alkyl substituted phosphorus containing 1,3-butadiene derivatives were already reported by Appel,11b Regitz, 11f and Denis, 11g however, these compounds are unlikely to afford stable radical compounds on oxidation or reduction. In 2008, Robinson et al. reported a diphosphorus compound II containing a weak p-acceptor N-heterocyclic carbene (NHC). 12 Structural and theoretical data suggest that II should be better described as a base-stabilized diphosphinidene with C (NHC) -P and P-P single bonds. Compound III, reported by Bertrand's group in 2010, features a strong p-acceptor cyclic alkyl amino carbene (cAAC R ) and exhibits short C-P bond lengths, thus it may be regarded as a genuine 2,3-diphospha-1,3-butadiene. 13 The same group also reported the 2-phospha-3-azabutadiene IV by an elegant choice of imine and cAAC precursors. 9c Remarkably, these electron-rich species readily undergo one-electron oxidation to afford the corresponding radical cations (II)c + , (III)c + , and (IV)c + . 9b, 9c We recently reported NHC-derived divinyldiphosphenes V 14 and isolated the corresponding radicals cations (V)c + by one-electron oxidation of V. 15 These and other early results 16 prompted us to reason that stable 2-phospha-1,3-butadienes VI as well as the corresponding radical cations (VI)c + and dications (VI) ++ should be synthetically accessible by a rational choice of substrates and reaction conditions. Herein, we report the synthesis of 2-phospha-1,3-butadienes [{(IPr)C(Ph)}P(cAAC Me )] (3a) and [{(IPr)C(Ph)}P(cAAC Cy )] (3b) based on singlet carbene frameworks (IPr ¼ C{(NDipp)CH} 2 , Dipp ¼ 2,6-iPr 2 C 6 H 3 ; cAAC Me ¼ C{(NDipp)CMe 2 CH 2 CMe 2 }; cAAC Cy ¼ C{(NDipp)CMe 2 CH 2 C(Cy)}, Cy ¼ cyclohexyl) as crystalline solids. Sequential one-electron oxidation of 3a and

Results and discussion
For the synthesis of desired 2-phospha-1,3-butadienes, Nheterocyclic vinyl (NHV)-substituted dichlorophosphine {(IPr) C(Ph)}PCl 2 (1) 14 and strong p-acceptor cAAC R (ref. 17) were chosen as the appropriate precursors (Scheme 1). 18 Treatment of Scheme 1 Synthesis of 2-phospha-1,3-butadiene derivatives 3a and 3b. * cAACs were prepared by the deprotonation of their triflate salts with LDA and the side-product LiOTf was not separated. a colorless THF solution of 1 with one equivalent of cAAC Me or cAAC Cy immediately resulted in the formation of dark blue solutions (Scheme 1). Aer workup, the ionic compounds [{(IPr)C(Ph)}P(Cl)(cAAC R )](OTf) (R ¼ Me 2a, Cy 2b) were isolated as violet crystalline solids. Compounds 2a and 2b are highly air sensitive solids and have been characterized by elemental analysis and NMR spectroscopy. The solid state molecular structure of a typical compound 2a (Fig. S31 †) was determined by X-ray diffraction. Reduction of 2a and 2b with magnesium turnings afforded the target compound 3a and 3b, respectively, as orange solids. Interestingly, 3a and 3b are also accessible in a one-pot reaction of 1 and cAAC R with magnesium. Both 3a and 3b are soluble in common organic solvents (n-hexane, Et 2 O, benzene, toluene, THF) and are stable under an inert gas atmosphere.
The 1 H NMR spectra of compounds 2a and 2b as well as 3a and 3b show expected resonances for the NHV and cAAC R moieties. The 13 C{ 1 H} NMR spectrum of 2a and 2b as well as 3a and 3b each is consistent with the 1 H NMR resonances and exhibits expected doublets for the phosphorus bound carbon atoms (see the ESI †). The 31 P{ 1 H} NMR spectrum of 2a (+100.9 ppm) and 2b (+102.9 ppm) each shows a singlet, which is high-eld shied with respect to that of the 1 (+167 ppm). This is most likely due to the coordination of electron-rich cAAC R to the phosphorus atoms in 2a and 2b. The 31 P{ 1 H} NMR signal for 3a (+102.5 ppm) and 3b (+108.6 ppm), respectively, appears at a higher eld compared to that of IV (+134.0 ppm), 9c which is expected because of the electronegativity difference between carbon and nitrogen.
In the dications 5a and 5b, the HOMO-2 is a p-type orbital with contributions from the aryl groups and C vinyl -P bond (Fig. 10). The LUMO of 5a and 5b is the p-orbital of the C IPr ] C vinyl and P]C cAAC bonds with a small contribution from the nitrogen atoms of pyrrolidine and imidazole rings. Upon removal of one and two electrons from the p-type orbital of 3a and 3b, the HOMO of 3a and 3b (Fig. 3) becomes SOMO in the radical cations 4a and 4b (Fig. 6) and LUMO in the dications 5a and 5b (Fig. 10).  This journal is © The Royal Society of Chemistry 2020 Chem. Sci., 2020Sci., , 11, 1975Sci., -1984Sci., | 1981

Conclusions
In conclusion, we have isolated the crystalline 2-phospha-1,3butadiene derivatives 3a and 3b by a rational choice of combining a weak p-acceptor (IPr) and a strong p-acceptor (cAAC R ) singlet carbene scaffolds. Sequential one-electron oxidation of 3a and 3b affords the radical cations 4a and 4b and the dications 5a and 5b. The isolation of 4a, 4b and 5a, 5b as crystalline solids is consistent with the redox properties of 3a and 3b analyzed by electrochemical studies. Molecular structures of all compounds in the solid-state were established by single crystal X-ray diffraction. Computational and EPR spectroscopic data indicate that the unpaired electron in 4a and 4b is delocalized over the CPCN p-conjugated framework. The study emphasizes the advantage of merging singlet carbenes with dissimilar donor-acceptor properties in accessing stable open-shell p-conjugated systems. As a variety of stable singlet carbenes with adaptable properties are readily accessible, it is very likely that many p-conjugated systems with other heavier main-group elements, which are hitherto believed to be synthetically challenging targets, may be isolated.

Conflicts of interest
There are no conicts to declare.