Direct functionalization of white phosphorus with anionic dicarbenes and mesoionic carbenes: facile access to 1,2,3-triphosphol-2-ides

A series of unique C2P3-ring compounds [(ADCAr)P3] (4) are readily accessible in an almost quantitative yield by the direct functionalization of white phosphorus (P4) with appropriate anionic dicarbenes [Li(ADCAr)].


Introduction
The direct conversion of white phosphorus (P 4 ) into useful organophosphorus compounds (OPCs) is of signicant interest because this excludes the involvement of corrosive Cl 2 gas that is required to convert P 4 into PCl 3 , a common starting material for OPCs, and thus minimizes the waste and energy consumption. 1 The activation and subsequent functionalization of P 4 has therefore become a topical objective. 2 Both transition metal 3 as well as main-group element 4 compounds have been shown to activate or functionalize P 4 . 5 In particular, compounds featuring a lowvalent main-group element have made signicant advances over the past years. 6 Among nonmetals, the use of singlet carbenes 7 has given new impetus to the eld of P 4 activation as it leads to the direct C-P bond formation (Fig. 1). 8 Several stable carbenes (L1-L7) undergo reactions with P 4 and the fate of P 4 fragmentation to give P n (n ¼ 1, 2, 4, 8 or 12) containing products II-IX depends on the relative s-donor/p-acceptor (ambiphilic) property as well as the steric demand of carbenes. 7 Weakly p-accepting NHCs such as IPr (IPr ¼ C{(DippN)CH} 2 ) do not react with P 4 , however, related derivatives containing the [P 2 ] or [P 3 À ] moiety are accessible by alternative methods. 9 Sterically demanding 1,3bis(tBu)imidazol-2-ylidene (IBu t ) activates P 4 in combination with B(C 6 F 5 ) 3 to give X. 6h This frustrated Lewis pair (FLP) type reactivity 10 led to the transformation of the classical NHC (IBu t ) into the mesoionic carbene (iMIC) L8 based on an 1,3-imidazole framework. iMICs are very potent s-donor ligands with almost negligible p-acceptor properties. 11 Nonetheless, no reaction of an iMIC alone with P 4 has been described so far. This is most likely due to their limited synthetic accessibility. 11a We recently reported 12 C5-protonated iMICs Ar (XI) as well as C4/C5-ditopic anionic dicarbenes [Li(ADC Ar )] XII (Fig. 1) by the deprotonation of C2arylated 1,3-imidazolium salts. 13 The dicarbenes XII feature two adjacent C4/C5-nucleophilic sites, and thus are well endowed to affect unique dual P 4 functionalization. 5i,14 Herein, we showcase the direct functionalization of P 4 via unprecedented [3 + 1] fragmentation with [Li(ADC Ar ] and iMICs Ar to give the 1,2,3-triphosphol-2-ide derivatives [(ADC Ar )P 3 ] (ADC Ar ¼ ArC {NDipp)C} 2 ; Dipp ¼ 2,6-iPr 2 C 6 H 3 ; Ar ¼ C 6 H 5 4a, 3-MeC 6 H 4 4b, 4-MeC 6 H 4 4c, and 4-Me 2 NC 6 H 4 4d) (Scheme 1).

Results and discussion
Treatment of [Li(ADC Ar )] (2a-2d), 12 which are readily accessible by the double deprotonation of C2-arylated 1,3-imidazolium salts 1a-1d with n-BuLi, with P 4 at room temperature afforded the 1,2,3-triphosphol-2-ides 4a-4d as crystalline solids in almost quantitative yields (Scheme 1). Compounds 4a-4d are indenitely stable (as solids as well as in solutions) under an inert gas atmosphere. The formation of 4a-4d indicates formal [3 + 1] fragmentation of P 4 into P 3 + and P À . The cationic P 3 + species is captured by the ADCs to give 4a-4d, whereas the P À nucleophile reacts with additional P 4 to eventually form the phosphide (P 7 ) 3À anion, a very common species in metal mediated fragmentation of P 4 . 15 Indeed, Li 3 P 7 can be isolated as a red-brown solid, 15,16 which was conrmed by its reaction with (IPr)HCl to give (IPr)PH, reported previously using Na 3 P 7 . 17 Interestingly, treatment of iMICs Ar 3a and 3c with P 4 also afforded, albeit in a lower yield, the corresponding products 4a and 4c, respectively. 1 H NMR analyses of the crude reaction product indicate the presence of a 1 : 1 mixture of 4a : 1a and 4c : 1c, suggesting the reprotonation of iMICs Ar 3a and 3b to 1,3imidazolium salts 1a and 1c. Pure 4a and 4c can be extracted from the mixture using toluene.
The anisotropy of current-induced density (AICD) has been used to study the aromatic behavior of several molecules. 23 The AICD plots of 4a (Fig. 3) and 4b-4d (Fig. S62 †) clearly show signicant delocalization of the p-electrons of both the C 3 N 2 and the C 2 P 3 heterocycles, forming one coherent p-system.
The HOMO of compounds 4a ( Fig. 4) and 4b-4c ( Fig. S58-S60 †) corresponds to the p-orbitals of the C-P bonds with a small contribution from the lone pairs at the nitrogen atoms. The HOMOÀ1 corresponds mainly to the p-orbitals of the P 3 and the C 2 moieties of the C 2 P 3 -ring. Like in alkali metal 1,2,3triphospholides, 21b the analyses of frontier molecular orbitals, HOMO and HOMOÀ1 in particular, of 4a-4d reveal the mixing of phosphorus orbitals with lone-pair character amongst the pmanifold frontier orbitals. The HOMOÀ3 and HOMOÀ2 are the lone pairs on the central and neighbouring P atoms, respectively. The LUMO of 4a-4d corresponds to the p* orbital of the aryl group on the C3 carbon atom along with a p-orbital at the central phosphorus atom. The LUMO+2 corresponds mainly to the p*-orbitals of the C 2 P 3 unit.
The intriguing electronic structures of 4 prompted us to investigate their ligand properties as they may function as neutral two electron s-donors (via phosphorus atoms) and/or 6p-electron h 5 -donors (C 2 P 3 -ring) like triphospholide 21 and cyclopentadienyl anions. Treatment of 4a, 4b, and 4c with Fe 2 (CO) 9 or M(CO) 5 (THF) (M ¼ Mo or W) led to the formation of related complexes 5a, 5b, 6, and 7 (Scheme 3). In all complexes, the central phosphorus atom functions as a two-electron sdonor ligand to bind to the M(CO) n moiety. This is consistent with the NBO analysis, which suggests higher charge accumulation at the central phosphorus atom with respect to that of the   AICD plot (based on M06-2X/def2-TZVPP//def2-SVP calculations) of the C 3 N 2 P 3 core of compound 4a. The isovalue was arbitrarily chosen to be 0.03, the magnetic field is orthogonal to the C 2 P 3 -plane and points towards the viewer, and thus clockwise ring currents represent aromatic systems, whereas counter-clockwise ring currents are indicative of antiaromatic systems. AICD plots of the complete molecules 4a-4d are given in the ESI. † neighbouring phosphorus atoms. The 31 P{ 1 H} NMR spectrum of 5a, 5b, 6, and 7 each exhibits one doublet (5a: 145; 5b: 145; 6: 160; 7: 157 ppm) and one triplet (5a: 316; 5b: 315; 6: 299; 7: 250 ppm), which have been upeld shied with respect to that of 4a (173, 332 ppm), 4b (173, 331 ppm), and 4d (173, 319 ppm). In the 31 P{ 1 H} NMR spectrum of 7, the triplet at 250 ppm is accompanied by the 183 W satellites (J P-W ¼ 202 Hz). The iron atom in 5a (Fig. 5) and 5b (Fig. S49 †) each features a trigonal-bipyramidal geometry. Three equatorial positions are occupied by CO ligands, whereas one CO and one 4a or 4b are present at the axial positions. The P-Fe bond length of 5a (2.240(1)Å) compares well with that of triphosphaindanederived P 3 Fe 3 iron-carbonyl clusters (av. 2.244Å). 24 Interestingly, the metrical parameters of the C 3 N 2 -and C 2 P 3 -rings of 5a and 5b are very similar to those of the precursors 4a and 4b, respectively. This indicates that the aromatic p-systems remain virtually intact upon complexation of 4a and 4b with the Fe(CO) 4 fragment. As expected, the molecular structures of 6 ( Fig. S50 †) and 7 (Fig. S51 †) feature six-fold coordinated Mo and W atoms, respectively.
DFT calculations suggest that the HOMO of 5a (Fig. 6) is mainly located at the iron atom and has some contribution from the p-orbitals of the C-C and one P-P bond. The LUMO is comparable to that of 4a but is lower in energy by À0.26 eV, indicating metal-to-ligand p-back bonding. The aromaticity of the C 2 P 3 moiety in 5a remains almost unchanged as indicated by NICS(0) ¼ À9.95, NICS(1) ¼ À9.58, and NICS(2) ¼ À5.12 values. The aromaticity of 5a is also corroborated by the AICD plot (Fig. S62 †).

Experimental
All syntheses and manipulations were carried out under an inert gas atmosphere (Ar or N 2 ) using standard Schlenk techniques or a glove box (MBraun LABMasterPro). Solvents were dried over appropriate drying agents, distilled, and stored over a 3Å molecular sieve prior to use. Deuterated solvents were dried over appropriate drying agents, distilled, and stored inside a glove box. NMR spectra were recorded on a Bruker Avance III 500 or a Bruker Avance III 500 HD spectrometer. Chemical shis (in d, ppm) are referenced to the solvent residual signals of CD 2 Cl 2 : 1 H 5.32; 13 C 53.84 and C 6 D 6 : 1 H 7.16; 13 C 128.62 ppm. ESI mass spectra were recorded using an Esquire 3000 ion trap mass spectrometer (Bruker Daltonik GmbH, Bremen, Germany) equipped with a nano-ESI source. Samples were dissolved in CH 2 Cl 2 and introduced by static nano-ESI using in-house pulled glass emitters. Nitrogen served as a nebulizer gas as well as a dry gas and was generated by a Bruker nitrogen generator NGM 11. Helium served as a cooling gas for the ion trap. The mass axis was externally calibrated with ESI-L Tuning Mix (Agilent Technologies, Santa Clara, CA, USA) as the calibration standard. UV/ vis spectra were recorded on a ThermoFisher Evolution 300 spectrophotometer. Infrared spectra were recorded using a Bruker Alpha-T FTIR spectrometer equipped with a Bruker Platinum diamond ATR unit. Melting points were measured  with a Büchi B-545 melting point apparatus. (IPr Ar )Br salts 1a-1d (Ar ¼ Ph, 3-MeC 6 H 4 , 4-MeC 6 H 4 or 4-Me 2 NC 6 H 4 ) were synthesized following the reported method. 13a n-BuLi (2.5 M solution in hexanes, Sigma-Aldrich) was used as received. White phosphorus was sublimed and stored inside a glovebox. Commercially available Fe 2 (CO) 9 (Sigma-Aldrich), Mo(CO) 6 (Fluorochem), and W(CO) 6 (Sigma-Aldrich) were used as supplied.

Alternative synthesis of 4a and 4c from iMICs Ar 2a and 2c
To a 15 mL THF suspension of 1a (0.98 g, 1.8 mmol), n-BuLi (2.5 M, 0.8 mL, 2.0 mmol) was added at À40 C. The resulting brown solution was stirred at À20 C for 45 min and then for 15 min at rt. Subsequently, P 4 (0.3 g, 2.4 mmol) was added in one portion and the resulting reaction mixture was stirred overnight at rt. The volatiles were removed under vacuum to obtain a dark residue, which was extracted with toluene (3 Â 10 mL). The ltrate was dried in a vacuum to obtain 4a. Yield: 41% (0.4 g).

Conclusions
In conclusion, the direct functionalization of white phosphorus (P 4 ) with anionic dicarbenes (ADCs) (2a-2d) as well as with mesoionic carbenes (iMICs Ar ) (3a and 3c) that leads to the formation of unique 1,2,3-triphosphol-2-ide derivatives 4a-4d as crystalline solids up to 98% yield has been reported. The isolation of C 2 P 3 -heterocycles 4a-4d is unprecedented in the P 4 activation by singlet carbenes and main-group compounds. The formation of 4a-4d suggests unique [3 + 1] fragmentation of P 4 into P 3 + and P À . The former species combines with an ADC to give 4a-4d, whereas the latter reacts with additional P 4 to form (P 7 ) 3À that can be isolated as Li 3 P 7 . Electronic structures of 4a-4d have been analyzed by computational studies, which, along with the crystallographic data, show that both C 3 N 2 -and C 2 P 3rings of 4a-4d are 6p-electron aromatic systems. Thus, 4a-4d can be considered as neutral analogues of cyclopentadienyl anions. The C 2 P 3 -ring of 4a-4d is negatively polarized towards the central phosphorus atom, and hence 4a-4d may also function as potent two-electron s-donor ligands. This feature has been demonstrated with the isolation of transition metal complexes 5a, 5b, 6, and 7. Consequently, 4a-4d have interesting perspectives as ligands in main-group element as well as transition-metal chemistry and catalysis. Further investigations in this direction are currently underway in this laboratory.

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