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

Abstract

A series of unique C₂P₃-ring compounds [(ADC^Ar)P₃] (ADC^Ar = ArC{(DippN)C}₂; Dipp = 2,6-iPr₂C₆H₃; Ar = Ph 4a, 3-MeC₆H₄4b, 4-MeC₆H₄4c, and 4-Me₂NC₆H₄4d) are readily accessible in an almost quantitative yield by the direct functionalization of white phosphorus (P₄) with appropriate anionic dicarbenes [Li(ADC^Ar)]. The formation of 1,2,3-triphosphol-2-ides (4a–4d) suggests unprecedented [3 + 1] fragmentation of P₄ into P₃⁺ and P⁻. The P₃⁺ cation is trapped by the (ADC^Ar)⁻ to give 4, while the putative P⁻ anion reacts with additional P₄ to yield the Li₃P₇ species, a useful reagent in the synthesis of organophosphorus compounds. Remarkably, the P₄ fragmentation is also viable with the related mesoionic carbenes (iMICs^Ar) (iMIC^Ar = ArC{(DippN)₂CCH}, i stands for imidazole-based) giving rise to 4. DFT calculations reveal that both the C₃N₂ and C₂P₃-rings of 4 are 6π-electron aromatic systems. The natural bonding orbital (NBO) analyses indicate that compounds 4 are mesoionic species featuring a negatively polarized C₂P₃-ring. The HOMO−3 of 4 is mainly the lone-pair at the central phosphorus atom that undergoes σ-bond formation with a variety of metal-electrophiles to yield complexes [{(ADC^Ar)P₃}M(CO)_n] (M = Fe, n = 4, Ar = Ph 5a or 4-Me-C₆H₄5b; M = Mo, n = 5, Ar = Ph 6; M = W, n = 5, Ar = 4-Me₂NC₆H₄7).

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Introduction

The direct conversion of white phosphorus (P₄) into useful organophosphorus compounds (OPCs) is of significant interest because this excludes the involvement of corrosive Cl₂ gas that is required to convert P₄ into PCl₃, a common starting material for OPCs, and thus minimizes the waste and energy consumption.¹ The activation and subsequent functionalization of P₄ has therefore become a topical objective.² Both transition metal³ as well as main-group element⁴ compounds have been shown to activate or functionalize P₄.⁵ In particular, compounds featuring a low-valent main-group element have made significant advances over the past years.⁶

Among nonmetals, the use of singlet carbenes⁷ has given new impetus to the field of P₄ activation as it leads to the direct C–P bond formation (Fig. 1).⁸ Several stable carbenes (L1–L7) undergo reactions with P₄ and the fate of P₄ fragmentation to give P_n (n = 1, 2, 4, 8 or 12) containing products II–IX depends on the relative σ-donor/π-acceptor (ambiphilic) property as well as the steric demand of carbenes.⁷ Weakly π-accepting NHCs such as IPr (IPr = C{(DippN)CH}₂) do not react with P₄, however, related derivatives containing the [P₂] or [P₃⁻] moiety are accessible by alternative methods.⁹ Sterically demanding 1,3-bis(tBu)imidazol-2-ylidene (IBu^t) activates P₄ in combination with B(C₆F₅)₃ to give X.^6h This frustrated Lewis pair (FLP) type reactivity¹⁰ led to the transformation of the classical NHC (IBu^t) into the mesoionic carbene (iMIC) L8 based on an 1,3-imidazole framework.

Fig. 1 Singlet carbene-mediated P₄ activation and fragmentation to II–X and a plausible intermediate I. Mesoionic carbenes (iMICs^Ar, XI) and anionic dicarbenes (XII) ([Li⁺] = solvated lithium ion) investigated in the current study.

iMICs are very potent σ-donor ligands with almost negligible π-acceptor properties.¹¹ Nonetheless, no reaction of an iMIC alone with P₄ has been described so far. This is most likely due to their limited synthetic accessibility.^11a We recently reported¹² C5-protonated iMICs^Ar (XI) as well as C4/C5-ditopic anionic dicarbenes [Li(ADC^Ar)] XII (Fig. 1) by the deprotonation of C2-arylated 1,3-imidazolium salts.¹³ The dicarbenes XII feature two adjacent C4/C5-nucleophilic sites, and thus are well endowed to affect unique dual P₄ functionalization.^5i,14 Herein, we showcase the direct functionalization of P₄via unprecedented [3 + 1] fragmentation with [Li(ADC^Ar] and iMICs^Ar to give the 1,2,3-triphosphol-2-ide derivatives [(ADC^Ar)P₃] (ADC^Ar = ArC{NDipp)C}₂; Dipp = 2,6-iPr₂C₆H₃; Ar = C₆H₅4a, 3-MeC₆H₄4b, 4-MeC₆H₄4c, and 4-Me₂NC₆H₄4d) (Scheme 1).

Scheme 1 Synthesis of 1,2,3-triphosphol-2-ide derivatives 4a–4d by the direct fragmentation of white phosphorus with [Li(ADC^Ar)] (2a–2d). Reaction of iMICs^Ar3a and 3c with P₄ to form 4a and 4c.

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 P₄ 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 indefinitely stable (as solids as well as in solutions) under an inert gas atmosphere. The formation of 4a–4d indicates formal [3 + 1] fragmentation of P₄ into P₃⁺ and P⁻. The cationic P₃⁺ species is captured by the ADCs to give 4a–4d, whereas the P⁻ nucleophile reacts with additional P₄ to eventually form the phosphide (P₇)³⁻ anion, a very common species in metal mediated fragmentation of P₄.¹⁵ Indeed, Li₃P₇ can be isolated as a red-brown solid,^15,16 which was confirmed by its reaction with (IPr)HCl to give (IPr)PH, reported previously using Na₃P₇.¹⁷

Interestingly, treatment of iMICs^Ar3a and 3c with P₄ also afforded, albeit in a lower yield, the corresponding products 4a and 4c, respectively. ¹H 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^Ar3a and 3b to 1,3-imidazolium salts 1a and 1c. Pure 4a and 4c can be extracted from the mixture using toluene.

The ¹H NMR spectra of 4a–4d are very symmetric and show two doublets and one septet for the isopropyl groups along with the signals due to the aryl protons. The ¹³C{¹H} NMR resonances for 4a–4d are fully consistent with their ¹H NMR spectra. The ¹³C{¹H} NMR spectrum of 4a–4d each exhibits a doublet at 167 ppm (J_P–C ≈ 84 Hz) for the backbone carbon atoms due to coupling with the ³¹P nucleus. The ³¹P{¹H} NMR spectrum of 4a–4d each shows a doublet at ∼73 ppm and a triplet at 325 ± 6 ppm in 2 : 1 ratio (J_P–P ≈ 500 Hz), indicating the presence of an AB₂ type system with unsaturated P–P bonds.¹⁸

Solid-state molecular structures¹⁹ of 4a (Fig. 2), 4b (Fig. S47†), and 4c (Fig. S48†) reveal the presence of a C₂P₃-ring that is coplanar with the imidazole C₃N₂-ring plane. The metrical parameters of 4a–4c are comparable (Table S1†) and hence, for brevity, only 4a is discussed here. The P1–P2 bond length of 4a (2.103(1) Å) is intermediate of the sum of covalent radii for P=P double (2.04 Å) and P–P single (2.22 Å) bond lengths,²⁰ indicating a partial π-bond character. Similarly, the C1–P1 (1.757(3) Å) bond length of 4a is shorter compared to a classical C–P single bond length (1.85 Å)¹⁵ but compares well with C=P bond lengths (ca. 1.75 Å) of inversely polarized phosphaalkenes.¹⁷ The C1–C1′ (1.395(5) Å) and C2–N1 (1.404(3) Å) bond lengths of 4a are elongated in comparison with those of 1a (1.350(2) and 1.344(2) Å, respectively).¹³ The C1–C1′, C1/C2–P1 and P1–P2 bond lengths of 4a–4c are comparable with the corresponding bond lengths of triphospholide anions [P₃C₂R₂]⁻ (R = H, C–P 1.726(2) and 1.781(3), and P–P 2.081(1) and 2.094(1) Å; R = Ph, C–P 1.760(2) and 1.762(2), and P–P 2.091(2), 2.098(2) Å).²¹ Thus, 4a–4d may be considered as the neutral analogues of the triphospholide anions.

Fig. 2 Solid-state molecular structure of 4a. Hydrogen atoms are omitted for clarity. Symmetry code: 1 − X, +Y, 3/2 − Z. Selected experimental and calculated [M06-2X/def2SVP] bond lengths (Å) and angles (°): C1–C1′ 1.395(5) [1.402], N1–C1 1.404(3) [1.399], P1–C1 1.757(3) [1.764], P1–P2 2.103(1) [2.112], C1–P1–P2 94.9(1) [94.8], and P1–P2–P1′ 104.0(1) [104.1].

To gain further insight into the electronic structures of 4a–4d, we performed DFT calculations at the M06-2X/def2-TZVPP//M06-2X/def2-SVP level of theory. The computed NPA charges (Table S7†) at the P2 (−0.10e) and the C1/C2 (−0.24e) atoms are negative, whereas both the P1 atoms bear a positive charge (0.12e) (Scheme 2). The Wiberg Bond Indices (WBIs) of 1.40 (P–P), 1.18 (C–P), and 1.31 (C–C) indicate a partial double bond character. The WBI for the C1–C2 bond of 4a (1.31) is significantly smaller compared to that of the imidazolium salt 1a (WBI = 1.64). The WBIs for the C3–N1/2 bonds in 1 (1.28) and 4 (1.26) are, however, almost equal. Thus, compounds 4 may be described as mesoionic species with 6π-electron C₂P₃ and C₃N₂ aromatic systems (Scheme 2c). The nitrogen atoms contribute 4π-electrons to the C₃N₃-ring, whereas the P₃ unit shares 4π-electrons with the C₂P₃-ring. The 2π-electrons of the C1=C2 bond are pooled by both the ring systems. Indeed, calculated nucleus-independent chemical shift (NICS)²² values for 4a–4d (Table 1) suggest the aromaticity of the C₃N₂- and C₂P₃-rings. For comparison, we also calculated the NICS values for C₆H₆ and cyclobutadiene (CBD) molecules.

Scheme 2 (a) Calculated Wiberg Bond Indices (WBIs) and (b) NPA atomic charges of 1,2,3-triphosphol-1,2-ides 4. (c) Schematic representation of 4 with atom numberings.

Table 1 Calculated NICS values for the C₃N₂/C₂P₃ units of 4a–4d at the M06-2X/def2TZVPP//M06-2X/def2SVP level of theory

C3N2/C2P3	4a	4b	4c	4d	5a	C6H6/CBD^a
a CBD (cyclobutadiene).
NICS(0)	−7.08/−10.19	−7.29/−10.31	−7.29/−10.31	−6.77/−10.37	−7.57/−9.95	−7.53/33.21
NICS(1)	−5.94/−10.18	−6.11/−10.28	−6.11/−10.23	−5.64/−10.21	−6.29/−9.58	−10.19/21.09
NICS(2)	−2.43/−5.51	−2.53/−5.21	−2.53/−5.53	−2.36/−5.52	−2.52/−5.12	−5.22/4.98

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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 significant delocalization of the π-electrons of both the C₃N₂ and the C₂P₃ heterocycles, forming one coherent π-system.

Fig. 3 AICD plot (based on M06-2X/def2-TZVPP//def2-SVP calculations) of the C₃N₂P₃ core of compound 4a. The isovalue was arbitrarily chosen to be 0.03, the magnetic field is orthogonal to the C₂P₃-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.†

The HOMO of compounds 4a (Fig. 4) and 4b–4c (Fig. S58–S60†) corresponds to the π-orbitals of the C–P bonds with a small contribution from the lone pairs at the nitrogen atoms. The HOMO−1 corresponds mainly to the π-orbitals of the P₃ and the C₂ moieties of the C₂P₃-ring. Like in alkali metal 1,2,3-triphospholides,^21b the analyses of frontier molecular orbitals, HOMO and HOMO−1 in particular, of 4a–4d reveal the mixing of phosphorus orbitals with lone-pair character amongst the π-manifold frontier orbitals. The HOMO−3 and HOMO−2 are the lone pairs on the central and neighbouring P atoms, respectively. The LUMO of 4a–4d corresponds to the π* 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 π*-orbitals of the C₂P₃ unit.

Fig. 4 Selected MOs of 4a calculated at the M06-2X/def2-TZVPP//def2-SVP level of theory with an isovalue of 0.04. Hydrogen atoms were omitted for clarity.

The intriguing electronic structures of 4 prompted us to investigate their ligand properties as they may function as neutral two electron σ-donors (via phosphorus atoms) and/or 6π-electron η⁵-donors (C₂P₃-ring) like triphospholide²¹ and cyclopentadienyl anions. Treatment of 4a, 4b, and 4c with Fe₂(CO)₉ or M(CO)₅(THF) (M = 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 σ-donor 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 neighbouring phosphorus atoms. The ³¹P{¹H} 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 upfield shifted with respect to that of 4a (173, 332 ppm), 4b (173, 331 ppm), and 4d (173, 319 ppm). In the ³¹P{¹H} NMR spectrum of 7, the triplet at 250 ppm is accompanied by the ¹⁸³W satellites (J_P–W = 202 Hz).

Scheme 3 Synthesis of complexes [{(ADC^Ar)P₃}M(CO)_n] 5a, 5b, 6, and 7.

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 triphosphaindane-derived P₃Fe₃ iron-carbonyl clusters (av. 2.244 Å).²⁴ Interestingly, the metrical parameters of the C₃N₂- and C₂P₃-rings of 5a and 5b are very similar to those of the precursors 4a and 4b, respectively. This indicates that the aromatic π-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.

Fig. 5 Solid-state molecular structure of 5a. Hydrogen atoms and one solvent toluene molecule are omitted for clarity. Selected bond lengths (Å) and angles (°): C2–C3 1.394(2), C2–N1 1.399(1), C3–N2 1.403(1), C2–P1 1.756(1), C3–P3 1.764(1), P1–P2 2.081(1), P2–P3 2.089(1), P2–Fe1 2.240(1), Fe1–C34 1.791(1), Fe1–C35 1.797(2), Fe1–C36 1.810(1), Fe1–C37 1.783(1), P1–P2–P3 109.1(2), and P2–Fe1–C37 178.5(1).

DFT calculations suggest that the HOMO of 5a (Fig. 6) is mainly located at the iron atom and has some contribution from the π-orbitals of the C–C and one P–P bond. The LUMO is comparable to that of 4a but is lower in energy by −0.26 eV, indicating metal-to-ligand π-back bonding. The aromaticity of the C₂P₃ moiety in 5a remains almost unchanged as indicated by NICS(0) = −9.95, NICS(1) = −9.58, and NICS(2) = −5.12 values. The aromaticity of 5a is also corroborated by the AICD plot (Fig. S62†).

Fig. 6 Frontier molecular orbitals of 5a calculated at the M06-2X/def2-TZVPP//def2-SVP level of theory. The isovalue was arbitrarily chosen to be 0.04. Hydrogen atoms were omitted for clarity.

Experimental

All syntheses and manipulations were carried out under an inert gas atmosphere (Ar or N₂) using standard Schlenk techniques or a glove box (MBraun LABMasterPro). Solvents were dried over appropriate drying agents, distilled, and stored over a 3 Å 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 shifts (in δ, ppm) are referenced to the solvent residual signals of CD₂Cl₂: ¹H 5.32; ¹³C 53.84 and C₆D₆: ¹H 7.16; ¹³C 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 CH₂Cl₂ 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 = Ph, 3-MeC₆H₄, 4-MeC₆H₄ or 4-Me₂NC₆H₄) were synthesized following the reported method.^13an-BuLi (2.5 M solution in hexanes, Sigma-Aldrich) was used as received. White phosphorus was sublimed and stored inside a glovebox. Commercially available Fe₂(CO)₉ (Sigma-Aldrich), Mo(CO)₆ (Fluorochem), and W(CO)₆ (Sigma-Aldrich) were used as supplied.

Synthesis of compound (ADC^Ph)P₃ (4a)

To a 15 mL THF suspension of 1a (0.88 g, 1.6 mmol), n-BuLi (2.5 M, 1.4 mL, 3.5 mmol) was added at −40 °C. The resulting reaction mixture was stirred at −20 °C for 1 h and then at room temperature (25 °C) for 15 minutes to obtain a clear light brown solution of 2a.¹² To this solution, solid P₄ (0.4 g, 3.2 mmol) was added in one portion and then stirred overnight at rt. The resulting dark suspension was refluxed for 2 h and the red insoluble material (probably a mixture of Li₃P₇ and other polyphosphides) was removed by filtration. The volatiles from the filtrate were removed under vacuum to give a brown residue, which was extracted with dichloromethane, dried under vacuum, washed with toluene (2 × 10 mL), and re-dried to obtain compound 4a as a yellow solid. Yield: 96% (0.86 g). Single crystals suitable for X-ray diffraction analysis were grown by storing a saturated toluene solution of 4a at −24 °C for three days. Mp: 343 °C. Elem. anal. (%), calcd for C₃₃H₃₉N₂P₃ (556.6): C, 71.21; H, 7.06; N, 5.03; found: C, 71.02; H, 6.84; N, 4.87. ¹H NMR (500 MHz, CD₂Cl₂, 298 K): δ = 7.59 (t, J = 7.7 Hz, 2H, p-C₆H₃), 7.38 (d, J = 7.8 Hz, 4H, m-C₆H₃), 7.34 (t, J = 6.6 Hz, 1H, p-C₆H₅), 7.23–7.18 (m, 4H, o-, m-C₆H₅), 2.62 (sept, J = 6.6 Hz, 4H, CH(CH₃)₂), 1.26 (d, J = 6.6 Hz, 12H, CH(CH₃)₂), and 1.03 (d, J = 6.7 Hz, 12H, CH(CH₃)₂) ppm. ¹³C{¹H} NMR (126 MHz, CD₂Cl₂, 298 K): δ = 167.6 (d, J_P–C = 84.4 Hz, CP); 149.2 (NCN); 146.2, 133.5, 132.0, 131.8, 129.8, 129.1, 125.8, and 123.8 (C₆H₃, C₆H₅); 29.7 (CH(CH₃)₂); 26.1 and 23.5 (CH(CH₃)₂) ppm. ³¹P{¹H} NMR (202 MHz, CD₂Cl₂, 298 K): δ = 332.3 (t, J_P–P = 506 Hz) and 173.7 (d, J_P–P = 506 Hz) ppm. MS (ESI, positive mode): m/z = 557.3 [4a + H]⁺. UV-vis (λ/nm ε (M⁻¹ cm⁻¹)): 282 (22336), 346 (31017), and 361 (31397).

Compounds 4b–4d were prepared by employing a similar protocol to that described for 4a using the appropriate precursor 1b, 1c or 1d, n-BuLi, and P₄.

(ADC^3-Tol)P₃ (4b)

Yield: 98% (0.90 g). Mp: 338–341 °C. Elem. anal. (%), calcd for 4b, C₃₄H₄₁N₂P₃, (570.6): C, 71.56; H, 7.24; N 4.91; found C, 70.64; H, 7.33; N 4.68. ¹H NMR (500 MHz, CD₂Cl₂, 298 K): δ = 7.57 (t, J = 7.8 Hz, 2H, p-C₆H₃), 7.35 (d, J = 7.8 Hz, 4H, m-C₆H₃), 7.14 (d, J = 7.6 Hz, 1H, o-C₆H₄), 7.08 (t, J = 7.8 Hz, 1H, m-C₆H₄), 7.02 (s, 1H, o-C₆H₄), 6.96 (d, J = 7.8 Hz, 1H, p-C₆H₄), 2.60 (sept, J = 6.7 Hz, 4H, CH(CH₃)₂), 2.10 (s, 3H, CH₃), 1.24 (d, J = 6.7 Hz, 12H, CH(CH₃)₂), and 1.03 (d, J = 6.8 Hz, 12H, CH(CH₃)₂) ppm. ¹³C{¹H} NMR (126 MHz, CD₂Cl₂, 298 K): δ = 167.5 (d, J_P–C = 84.5 Hz, CP); 149.6 (NCN); 146.3, 139.3, and 133.6 (i-C₆H₄ and i-, m-C₆H₃); 132.7 (o-C₆H₄); 131.7 (p-C₆H₃); 130.5 (o-C₆H₄); 129.5 and 128.9 (m-C₆H₄); 126.9, 125.8, and 123.71 (m-C₆H₃); 29.7 (C(CH₃)₂); 26.2 and 23.5 (C(CH₃)₂); 21.2 (CH₃) ppm. ³¹P{¹H} NMR (202 MHz, CD₂Cl₂, 298 K): δ = 331.1 (t, J_P–P = 506 Hz) and 173.6 (d, J_P–P = 506 Hz) ppm. MS (ESI, positive mode): m/z = 571.3 [4b + H]⁺. UV-vis (λ/nm ε (M⁻¹ cm⁻¹)): 280 (25637), 345 (36323), and 361 (36539).

(ADC^4-Tol)P₃ (4c)

Yield: 93% (0.85 g). Single crystals suitable for X-ray diffraction were obtained by storing a saturated toluene solution of 4c for three days at −24 °C. Mp: 339–343 °C. Elem. anal. (%), calcd for C₃₄H₄₁N₂P₃ (570.6): C, 71.56; H, 7.24; N, 4.91; found C, 71.11; H, 7.06; N, 4.65. ¹H NMR (500 MHz, CD₂Cl₂, 298 K): δ = 7.59 (t, J = 7.8 Hz, 2H, p-C₆H₃), 7.38 (d, J = 7.8 Hz, 4H, m-C₆H₃), 7.05 (d, J = 8.4 Hz, 2H, C₆H₄), 7.02 (d, J = 8.3 Hz, 2H, C₆H₄), 2.61 (sept, J = 6.8 Hz, 4H, CH(CH₃)₂), 2.23 (s, 3H, CH₃), 1.26 (d, J = 6.7 Hz, 12H, CH(CH₃)₂), and 1.03 (d, J = 6.9 Hz, 12H, CH(CH₃)₂) ppm. ¹³C{¹H} NMR (126 MHz, CD₂Cl₂, 298 K): δ = 167.5 (d, J_P–C = 84.1 Hz, CP); 146.2 (NCN); 143.1, 133.7, 131.7, 129.8, 129.7, 125.8, and 120.9 (C₆H₃ and C₆H₄); 29.7 (CH(CH₃)₂); 26.1 and 23.5 (CH(CH₃)₂); 21.7 (CH₃) ppm. ³¹P{¹H} NMR (202 MHz, CD₂Cl₂, 298 K): δ = 329.9 (t, J_P–P = 506 Hz) and 173.6 (d, J_P–P = 506 Hz) ppm. MS (ESI, positive mode): m/z = 571.3 [4c + H]⁺. UV-vis (λ/nm ε (M⁻¹ cm⁻¹)): 283 (23295), 336 (28771), 346 (29238), and 362 (29676).

(ADC^4-DMP)P₃ (4d)

Yield: 94% (0.93 g). Mp: 270–273 °C (decomp.). Elem. anal. (%), calcd for C₃₅H₄₄N₃P₃ (599.3): C, 70.10; H, 7.40; N, 7.01; found: C, 69.66; H, 7.18; N 6.59. ¹H NMR (500 MHz, CD₂Cl₂, 298 K): δ = 7.56 (t, J = 7.8 Hz, 2H, p-C₆H₃), 7.35 (d, J = 7.8 Hz, 4H, m-C₆H₃), 6.90 (d, J = 9.0 Hz, 2H, C₆H₄), 6.28 (d, J = 9.0 Hz, 2H, C₆H₄), 2.88 (s, 6H, N(CH₃)₂), 2.67 (sept, J = 6.8 Hz, 4H, CH(CH₃)₂), 1.24 (d, J = 6.7 Hz, 12H, CH(CH₃)₂), and 0.97 (d, J = 6.8 Hz, 12H, CH(CH₃)₂) ppm. ¹³C{¹H} NMR (126 MHz, CD₂Cl₂, 298 K): δ = 167.0 (d, J_P–C = 82.8 Hz, CP); 151.9 (NCN); 146.2, 138.4, 134.6, 131.4, 130.8, 126.3, 125.8, and 111.1 (C₆H₃ and C₆H₄); 40.0 (N(CH₃)₂); 29.6 (CH(CH₃)₂); 25.7 and 23.5 (CH(CH₃)₂) ppm. ³¹P{¹H} NMR (202 MHz, CD₂Cl₂, 298 K): δ = 319.5 (t, J_P–P = 504 Hz) and 173.3 (d, J_P–P = 504 Hz) ppm. MS (ESI, positive mode): m/z = 600.3 [4d + H]⁺. UV-vis (λ/nm (ε M⁻¹ cm⁻¹)): 285 (37768), 322 (46655), 343 (47543), 366 (48310), and 398 (45288).

Experimental identification of the insoluble material. A mixture of the insoluble material (20 mg, 80 μmol, calcd for Li₃P₇) and IPrHCl (80 mg, 188 μmol) was stirred in 3 mL of THF for three days at rt, resulting in a dark red suspension. A black solid was removed by filtration and the filtrate was dried in a vacuum, affording a dark red solid which was identified as IPr=PH¹⁷ by NMR spectroscopy. ¹H NMR (500 MHz, C₆D₆, 298 K): δ = 7.23 (t, J = 7.7 Hz, 2H, p-C₆H₃), 7.14 (d, J = 7.6 Hz, 4H, m-C₆H₃), 6.18 (s, 2H, NCH), 3.06 (sept, J = 6.7 Hz, 4H, CH(CH₃)₂), 1.92 (d, J_PH = 165.2 Hz, 1H, PH), 1.47 (d, J = 6.8 Hz, 12H, CH(CH₃)₂), and 1.15 (d, J = 6.9 Hz, 12H, CH(CH₃)₂) ppm. ³¹P NMR (C₆D₆, 298 K, 500 MHz): δ = −134.4 (d, J_P–H = 165.2 Hz) ppm. ³¹P{¹H} NMR (C₆D₆, 298 K, 500 MHz): δ = −134.4 ppm.

Alternative synthesis of 4a and 4c from iMICs^Ar2a 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 −40 °C. The resulting brown solution was stirred at −20 °C for 45 min and then for 15 min at rt. Subsequently, P₄ (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 × 10 mL). The filtrate was dried in a vacuum to obtain 4a. Yield: 41% (0.4 g).

(ADC^4-Tol)P₃ (4c). Similarly, treatment of 3c with P₄ gave 4c. Yield: 36% (0.4 g).

Syntheses of complexes 5a, 5b, 6, and 7

[(ADC^Ph)P₃]Fe(CO)₄ (5a). To a mixture of 4a (651 mg, 1.2 mmol) and Fe₂(CO)₉ (510 mg, 1.4 mmol), 30 mL THF was added at rt. The brown colored solution changed to a dark red colored solution after 15 min, which was further stirred overnight. The volatiles were removed in a vacuum to afford a red solid, which was extracted with 30 mL toluene. The volume of the filtrate was reduced to 10 mL and stored at −30° for one week to obtained orange needles of 5a (696 mg, 80%), which were also suitable for X-ray diffraction. Mp: 167–172 °C (decomp.). Elem. anal. (%), calcd for C₃₇H₃₉FeN₂O₄P₃ (724.5): C, 61.34; H, 5.43; N, 3.87; found: C, 59.66; H, 5.24; N, 3.71. ¹H NMR (500 MHz, CD₂Cl₂, 298 K): δ = 7.60 (t, J = 7.8 Hz, 2H, p-C₆H₃), 7.38 (d, J = 7.8 Hz, 4H, m-C₆H₃), 7.34 (t, J = 7.5 Hz, 1H, p-C₆H₅), 7.22–7.15 (m, 4H, o-, m-C₆H₅), 2.57 (sept, J = 6.7 Hz, 4H, CH(CH₃)₂), 1.27 (d, J = 6.7 Hz, 12H, CH(CH₃)₂), and 1.01 (d, J = 6.8 Hz, 12H, CH(CH₃)₂) ppm. ¹³C{¹H} NMR (126 MHz, CD₂Cl₂, 298 K): δ = 215.1 (CO); 161.7 (d, J_P–C = 70.3 Hz, CP); 146.0 (NCN), 133.1, 132.2, 129.7, 129.5, 129.3, 128.7, 126.1, and 123.1 (C₆H₃ and C₆H₅); 29.8 (CH(CH₃)₂); 26.0 and 23.5 (CH(CH₃)₂) ppm. ³¹P{¹H} NMR (202 MHz, CD₂Cl₂, 298 K): δ = 316.8 (t, J_P–P = 531 Hz) and 145.4 (d, J_P–P = 531 Hz) ppm. MS (ESI, positive mode): m/z = 725.1 [5a + H]⁺. UV-vis (λ/nm ε (M⁻¹ cm⁻¹)): 285 (33061), 328 (31345), and 428 (37184). IR (ATR, diamond): [small nu, Greek, tilde]/cm⁻¹ = 2041, 1966, 1937, and 1919.

[(ADC^3-Tol)P₃]Fe(CO)₄ (5b). Compound 5b was synthesized following a similar procedure to that described above for 5a using 4b (300 mg, 0.53 mmol) and Fe₂(CO)₉ (191 mg, 0.53 mmol) as an orange crystalline solid. Yield: 84% (333 mg). Crystals suitable for X-ray diffraction were obtained by storing a saturated toluene solution of 5b overnight at rt. Mp: 180–182 °C (decomp.) Elem. anal. (%), calcd for 5b, C₃₈H₄₁FeN₂O₄P₃ (738.5): C, 61.80; H, 7.56; N, 3.79; found C, 62.69; H, 5.95; N, 3.45. ¹H NMR (500 MHz, CD₂Cl₂, 298 K): δ = 7.59 (t, J = 7.8 Hz, 2H, p-C₆H₃), 7.37 (d, J = 7.8 Hz, 4H, m-C₆H₃), 7.15 (d, J = 7.4 Hz, 1H, o-C₆H₄), 7.08 (t, J = 7.7 Hz, 1H, m-C₆H₄), 7.00 (s, 1H, o-C₆H₄), 6.94 (d, J = 7.7 Hz, 1H, p-C₆H₄), 2.56 (sept, J = 6.6 Hz, 4H, CH(CH₃)₂), 2.09 (s, 3H, CH₃), 1.27 (d, J = 6.6 Hz, 12H, CH(CH₃)₂), and 1.04 (d, J = 6.7 Hz, 12H, CH(CH₃)₂) ppm. ¹³C{¹H} NMR (126 MHz, CD₂Cl₂, 298 K): δ = 215.1 and 214.9 (CO); 161.6 (d, J_P–C = 71 Hz, NCP); 147.6 (NCN); 146.1, 139.5, 133.1, 133.0, 132.1, 130.3, 129.9, 129.0, 126.8, 126.0, and 122.9 (C₆H₃ and C₆H₅); 29.8 (CH(CH₃)₂); 26.1 and 23.6 (CH(CH₃)₂); 21.2 (CH₃) ppm. ³¹P{¹H} NMR (202 MHz, CD₂Cl₂, 298 K): δ = 315.5 (t, J_P–P = 531 Hz) and 145.4 (d, J_P–P = 532 Hz) ppm. MS (ESI, positive mode): m/z = 739.1 [5b + H]⁺. UV-vis (λ/nm ε (M⁻¹ cm⁻¹)): 282 (35189), 327 (29758), and 416 (37568). IR (ATR, diamond): [small nu, Greek, tilde]/cm⁻¹ = 2039, 2007, 1962, and 1921.

[(ADC^Ph)P₃]Mo(CO)₅ (6). To a mixture of 4a (447 mg, 0.8 mmol) and Mo(CO)₆ (212 mg, 0.8 mmol), 20 mL THF was added at rt. The yellow suspension was stirred for three days at 60 °C. Filtration through a plug of Celite afforded an orange solution. The volatiles were removed under vacuum to obtain 6 as a yellow solid (523 mg, 81%). Crystals suitable for X-ray diffraction were obtained by a slow diffusion of n-hexane into a saturated toluene solution of 6. Elem. anal. (%), calcd for 6, C₃₈H₃₉MoN₂O₅P₃ (792.6): C, 57.58; H, 4.96; N, 3.53; found: C, 57.06; H, 4.73; N, 3.25. ¹H NMR (500 MHz, CD₂Cl₂, 298 K): δ = 7.60 (t, J = 7.8 Hz, 2H, p-C₆H₃), 7.38 (d, J = 7.8 Hz, 4H, m-C₆H₃), 7.35 (t, J = 7.6 Hz, 1H, p-C₆H₅), 7.21 (t, J = 7.8 Hz, 2H, m-C₆H₅), 7.17 (d, J = 7.9 Hz, 2H, o-C₆H₅), 2.58 (sept, J = 6.7 Hz, 4H, CH(CH₃)₂), 1.27 (d, J = 6.7 Hz, 12H, CH(CH₃)₂), and 1.02 (d, J = 6.8 Hz, 12H, CH(CH₃)₂). ¹³C{¹H} NMR (126 MHz, CD₂Cl₂, 298 K): δ = 206.0 and 201.8 (CO); 164.4 (d, J_P–C = 73 Hz, NCP); 146.1 (NCN); 133.2, 132.2, 132.1, 129.7, 129.2, and 126.1 (C₆H₃ and C₆H₅); 29.8 (CH(CH₃)₂); 26.0 and 23.6 (CH(CH₃)₂) ppm. ³¹P{¹H} NMR (202 MHz, CD₂Cl₂, 298 K): δ = 299.1 (t, J_P–P = 510 Hz) and 160.2 (d, J_P–P = 511 Hz) ppm. IR (ATR, diamond): [small nu, Greek, tilde]/cm⁻¹ = 2065, 2051, 1945, 1925, and 1911.

[(ADC^4-DMP)P₃]W(CO)₅ (7). A 10 mL THF solution of W(CO)₆ (212 mg, 0.8 mmol) was irradiated under UV light for 3 h and then combined with a 6 mL THF solution of 4d (447 mg, 0.8 mmol). The yellow solution was stirred overnight at rt. The volatiles were removed under vacuum to obtain 7 as a yellow solid (256 mg, 88%). Crystals suitable for X-ray diffraction were obtained by slow evaporation of a saturated toluene solution of 7 at rt. Elem. anal. (%), calcd for 7, C₄₀H₄₄N₃O₅P₃W (923.6): C, 52.02; H, 4.80; N, 4.55; found C, 51.40; H, 4.39; N, 4.10. ¹H NMR (500 MHz, CD₂Cl₂, 298 K): δ = 7.61 (t, J = 7.8 Hz, 2H, p-C₆H₃), 7.41 (d, J = 7.8 Hz, 4H, m-C₆H₃), 6.91 (d, J = 9.2 Hz, 2H, C₆H₄), 6.33 (d, J = 9.2 Hz, 2H, C₆H₄), 2.88 (s, 6H, N(CH₃)₂), 2.60 (sept, J = 6.8 Hz, 4H, CH(CH₃)₂), 1.26 (d, J = 6.9 Hz, 12H, CH(CH₃)₂), and 1.00 (d, J = 6.8 Hz, 12H, CH(CH₃)₂) ppm. ¹³C{¹H} NMR (126 MHz, CD₂Cl₂, 298 K): δ = 197.3 and 192.0 (CO); 152.1 (NCP); 146.1 (NCN); 134.3, 131.8, 130.7, 129.9, 126.1, 111.2, and 108.7 (C₆H₃ and C₆H₅); 40.0 (N(CH₃)₂); 29.7 (CH(CH₃)₂); 25.6 and 23.6 (CH(CH₃)₂) ppm. ³¹P{¹H} NMR (202 MHz, CD₂Cl₂, 298 K): δ = 250.9 (t, J_P–P = 512 Hz, with ¹⁸³W satellites, J_W–P = 202 Hz) and 157.0 (d, J_P–P = 505 Hz) ppm. MS (ESI): m/z = 924.2 [7 + H]⁺. IR (ATR, diamond): [small nu, Greek, tilde]/cm⁻¹ = 2063, 1978, 1925, and 1907.

Conclusions

In conclusion, the direct functionalization of white phosphorus (P₄) 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 C₂P₃-heterocycles 4a–4d is unprecedented in the P₄ activation by singlet carbenes and main-group compounds. The formation of 4a–4d suggests unique [3 + 1] fragmentation of P₄ into P₃⁺ and P⁻. The former species combines with an ADC to give 4a–4d, whereas the latter reacts with additional P₄ to form (P₇)³⁻ that can be isolated as Li₃P₇. Electronic structures of 4a–4d have been analyzed by computational studies, which, along with the crystallographic data, show that both C₃N₂- and C₂P₃-rings of 4a–4d are 6π-electron aromatic systems. Thus, 4a–4d can be considered as neutral analogues of cyclopentadienyl anions. The C₂P₃-ring of 4a–4d is negatively polarized towards the central phosphorus atom, and hence 4a–4d may also function as potent two-electron σ-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 conflicts to declare.

Acknowledgements

We gratefully acknowledge the support from the Deutsche Forschungsgemeinschaft (DFG) and thank Professor Norbert W. Mitzel for his continuous support. The authors gratefully acknowledge the support by computing time provided by the Paderborn Center for Parallel Computing (PC2).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1939608–1939615. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9sc04441h

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