Overcrowded aminophospanitrenes: a case study

1 Introduction

Phosphinonitrenes, described alternatively as λ⁵-phosphonitriles belong to the family of nucleophilic nitrenes [1], having – together with the aminonitrenes - singlet ground state [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. In the aminonitrene series for example, the bulky N-(2,2,5,5-tetramethylpyrrolidyl)- and N-(2,2,6,6-tetramethylpiperidy1)-substituted derivatives (1 and 2; Scheme 1) were kinetically persistent at –78°C in solution, however, they decomposed rapidly at 0°C [13], [14], [15], [16], [17].

Scheme 1:

Aminonitrenes persistent at −78°C in solution.

In the phosphinonitrene series further stabilization is exerted by substituting phosphorus with imino or amino groups [18]. Using two extremely strong electron donor and bulky imidazolidin-2-iminato substituents on phosphorus, Bertrand and coworkers succeeded in the synthesis [19] and a detailed reactivity study [20] of a persistent phosphinonitrene 3 (Scheme 2).

Scheme 2:

The stable phosphinonitrene isolated by Bertrand et al. [19].

The chemistry of the phosphinonitrene (ⁱPr₂N)₂PN (4, Scheme 3) where the amino moieties are substituted with the sterically demanding di-iso-propylamino groups was also explored [1], [21], [22], [23], [24]. 4 can be formed by photolytical dinitrogen elimination from the corresponding diamino-(azido)phosphine as a primary reaction product. This nitrene, however, subsequently dimerizes to form cyclodiphosphazene 5 (Scheme 3) in a [2+2] cycloaddition reaction [21], [22], [23].

Scheme 3:

Bis-iso-propylamino-phosphinonitrene and its dimer.

The Lewis basicity of the transient species 4, together with the similarly crowded bis(dicyclohexy1amino)phosphanylnitrene was proven by complex formation [25]. Since dimerization is apparently hindered by sterically demanding groups, we considered the (even more bulky than di-iso-propylamino) dimethylpiperidino (DMP) and trimethylpiperidino (TMP) substituents on phosphorus, in an attempt to obtain an observable phosphinonitrene.

Our preliminary quantum chemical calculations using the energy of the isodesmic reaction (1) [26] provided evidence

for a beneficial electronic stabilizing effect of the bulky amino groups in addition to the expected steric effects. The reaction energies obtained at the B3LYP/6-31+G* level were 80, 88 and 96 kcal mol⁻¹ for R=ⁱPr, DMP (2,6-dimethylpiperidino) and TMP (2,2,6,6-tetramethylpiperidino), respectively, showing that the bulky amino groups exert not only steric protection by kinetic hindrance, but also additional electronic stabilization for the nitrenes. This effect is presumably attributable to the planarization of phosphorus, resulting in an increase of the π-donor properties [27], [28].

2 Results and discussion

First we have synthesized the precursor amino-substituted azidophosphines of type 8 [R=2,6-dimethyl-piperidyl (DMP) (a), 2,2′,6,6′-tetramethylpiperidyl (TMP) (b)]. The synthesis of 8a, b was achieved in two steps (Scheme 4). Starting from dichlorophosphine 6 [29] the diazidophosphine 7 is formed as an isolable key intermediate which can be further substituted by replacement of one of the azido groups by DMP or TMP to form 8a, b [30].

Scheme 4:

Synthesis of the precursor azidophosphines 8a, b.

Investigations concerning the thermal stability of 8a, b have shown that their decomposition proceeds at moderate temperatures (at 50–60°C) via loss of dinitrogen. In the case of 8a the stable cyclodiphosphazene 10 (Scheme 5) was generated (apparently via 9a, similarly to the case of 4 [21], [22], [23]). Crystals of 10 were isolated and characterized by X-ray crystallography. The structural parameters are similar to those published for the ⁱPr analogue.

Scheme 5:

Thermolysis reaction of azidophosphines 8a, b.

Fig. 1:

Reduced representation of the molecular structure of 11 in the crystal (without hydrogen atoms). Selected bond lengths (pm) and angles (deg): P(1)–N(1) 1.726(2), P(1)–N(2) 1.734(2), P(1)–N(3) 1.738(2), N(3)–P(2) 1.543(2), P(2)–N(5) 1.523(2), P(2)–N(4) 1.665(2), N(5)–P(3) 1.647(2), P(3)–N(7) 1.640(2), P(3)–N(7)1A 1.656(2), P(3)–N(6) 1.690(2); N(1)–P(1)–N(2) 107.82(11), N(1)–P(1)–N(3) 103.16(11), N(2)–P(1)–N(3) 105.27(11), P(2)–N(3)–P(1) 129.38(14), N(5)–P(2)–N(3) 126.87(12), N(5)–P(2)–N(4) 119.09(12), N(3)–P(2)–N(4) 114.01(12).

Unexpectedly, during the vacum “thermolysis” (at 60°C) of 8b a volatile compound was detected and condensed as a yellow solid in a cooled (−78°C) receiving flask. Alcoholysis of this yellow residue yielded the hydroxylamino derivative of 2,2′,6,6′-tetramethylpiperidine. These findings verify the elimination of the highly reactive aminyl radical, TMP˙ [31]. From the remaining polymeric residue, we extracted a yellow crystalline compound 11, which is stable under an atmosphere of argon, but is extremely sensitive to air (Scheme 5). The mass spectrum of 11 shows the radical cations [(TMP–PN)_n]⁺˙ (n=1,2,3,4) as well as the cation (TMP)₂P⁺ as fragments of high intensity in the higher mass range (m/z>300). The ³¹P NMR spectrum proved the presence of three chemically different phosphorus atoms [δ=121.3 (br); 51.3 (d); 41.4 (d); ²J(PP)=52.3 Hz]. Single crystals of 11 were obtained at −30°C by crystallization from n-hexane. Figure 1 shows the crystal structure of 11:

Fig. 2:

Substituent dependent reactivity of phosphinonitrenes stabilized by bulky amino groups.

Compound 11 crystallizes as a centrosymmetric molecule with a planar central P₂N₂ ring. The tetracoordinated phosphorus atoms exhibit an exocyclic TMP substituent as well as a bis(imino)phosphorane ligand. One of the imino nitrogen atoms of the bis(imino)phosphorane is additionally connected to a (TMP)₂P substituent. The bond distances in the P₂N₂ four membered rings (10, 11: 164–168 pm) are significantly longer than the PN distances in the well-known, comparably substituted cyclotriphosphazenes (e.g. 157–158 pm in P₃N₃(NMe₂)₆) [32], [33]. This could be due to the electrostatic transannular repulsion [6] between the oppositely spaced ionic charges of the ylidic cyclodiphosphazene system [24].

Theoretical calculations were carried out to understand the differences in reactivity for 8a and 8b via cycloaddition (Scheme 5): 9a→10 or radical elimination of TMP  (Scheme 5): 9b→11.

Calculation of the B3LYP/6-31+G* Gibbs free energies (at 298 K) explains the stability of the phosphino nitrenes 4a, b and shows that [2+2] cycloaddition is preferred in the case of R=DMP (4a: −22.4 vs. 4b: 14.2 kcal mol⁻¹) whereas a TMP elimination is energetically favored for 4b (4b: −4.8 vs. 4a: +10.7 kcal mol⁻¹). The energetics of these two reaction pathways is shown in Fig. 2.

Thus, in accordance with our preliminary calculations, the nitrene (TMP)₂PN (9b) becomes indeed stabilized, as it is shown by the endoergic dimerization energy. However, the steric strain – which by flattening the phosphorus pyramid increases the π-donor stabilizing effect on the nitrene – also weakens the PN bond, resulting in bond scission and the formation of radical 12.

12, which is apparently an unstable species, is likely to dimerize. One possible product is 13 (Scheme 6), which is formed by a [2+2] cycloaddition.

Scheme 6:

Formation of 1,3-diaza-2,4-diphosphetane-diyl 13, and its isomer, phosphinonitrene 14. The ΔG value for the TS^# is given relative to 13.

Indeed, the B3LYP/6-31+G* Gibbs free energy of this reaction is –35.8 kcal mol⁻¹. This compound belongs to biradicaloids (I- Scheme 7) [34], and is isolobal with 1,3-diphosphetane-2,4-diyls [35], [36] (II). Compound II and their anionic derivatives (1,3-diphosphetane-2,4-diyle-2-ides [37], [38]) have received much attention in the literature. These compounds have unusual electronic structure [39], [40] and reactivity [38], [41], [42], [43], [44]. An example of the N-substituted isomeric ring system III has already been reported by Schultz [45], [46] (Scheme 7), together with the arsenic analogue [47], and also four-membered phosphorus containing biradicaloids with boron [48], [49] and aluminum [50] are known in the literature. Also the parent ring system (P₂N₂H₂) was investigated computationally [51].

Scheme 7:

Four-membered ring biradicaloid molecules containing phosphorus and C or N (I–III).

The possible structures of 13 were explored computationally, and the results are shown in Fig. 3. One of the structures (13′) is analogous to those of 1,3-diphosphetane-2,4-diyls [37], [38], [39], [40], [41], [42] bearing equal (and bulky) substituents at the phosphorus atom. The structure has nearly C_i symmetry [52]. The PN bond lengths in the ring are between 168.7 and 168.9 pm. The sum of the bond angles about the two phosphorus atoms is 334.5°, showing the flattening effect of the bulky TMP substituent. Surprisingly, the endocyclic NPN bond angle (94.9°) is larger than the PNP angle (85.1°). The configuration of one of the phosphorus atoms of the other conformer 13″ is nearly planar (bond angle sum: 352.2°) and the corresponding PN distances are short (161.4 pm). The second phosphorus atom of 13″, however, is strongly pyramidalized (sum of the bond angles: 314.6 pm) with rather long PN single bonds (178.7 pm), showing a similar structure to those observed in 1,3-diphosphadiyls with different substituents at the phosphorus atoms (e.g. 1-methyl-3-tert-butyl-2,4-bis[2,4,6-tri-tert-butylphenyl]-1,3-diphosphetane-2,4-diyl) reported by Yoshifuji and coworkers [43], [44]. Compound 13′ is more stable by 0.3 kcal mol⁻¹ at B3LYP/6-31+G* than 13″. At B3LYP/6-311+G**//B3LYP/6-31+G* the energy ordering is reversed, the preference of 13″ being 0.5 kcal mol⁻¹.

Fig. 3:

B3LYP/6-31+G* computed structures of 13.

Apparently, the potential surface for 14 is very flat allowing to combine the out of plane movement of the phosphorus substituents with rotation about the PN bond. B3LYP/6-31+G* calculations of the ring opening reaction 13→14 predict the formation of the butadiene analogous compound 14 (Scheme 6) with a 3.7 kcal mol⁻¹ higher energy (B3LYP/6-31+G*) than 13, connected via the transition state 13^# (at +10.7 kcal mol⁻¹ higher energy than 13 – see Scheme 6). The reaction pathway is analogous to the formation of phosphino carbenes from 1,3-diphosphetane-2,4-diyls [37], [53]. Apparently, 14 might also form directly from the dimerization of 12 (Scheme 6), although it is by 3.7 kcal mol⁻¹ more rich in energy than 13. Nevertheless, the energetically favored unhindered [2+2] cycloaddition to cyclodiphosphazene 15 from 14 (Scheme 8) opens this reaction pathway. The easy oxidation of both iminophosphine fragments 15 with azidophosphine 3b stops the reaction with the formation of compound 11. The formation of the observed unidentified polymeric compounds can also easily be explained from 15 as a precursor.

Scheme 8:

Formation of oligomeric phosphazene 11.

3 Experimental, calculations

4 Syntheses

5 X-ray structure determinations

Crystal structure determination of 10: C₃₂H₆₄N₆P₂, colorless crystals, crystal dimensions: 0.20×0.30×0.50 mm³; M=594.83; monoclinic, space group P2₁/n (No. 14), a=9.0824(1), b=9.0695(1), c=19.6207(3) Å, α=90°, β=94.435(1)°, γ=90°, V=1611.37(4) ų, Z=2, μ(MoK_α)=0.167 mm⁻¹, T=123(2) K, F(000)=656.0 e. Of 32256 reflections which were collected on a Nonius Kappa-CCD diffractometer using MoK_α radiation up to 2θ_max=50° 2830 were independent and used in all further calculations. The structure was solved with Direct Methods. The non-H atoms were refined anisotropically against F²; hydrogen atoms were refined using a riding model (programs Shelxs-97 [54], Shelxl-97 [55], [56]). The final wR2(F²) was 0.2036 and the conventional R(F) was 0.0832 for 181 parameters and 0 restraints. Selected bond lengths (pm) and angles (deg) of compound 10: P1–N1: 165.9(4), P1–N1A: 166.2(4), P1–N2: 168.0(4), P1–N3: 167.2(4); N1–P1–N1A: 94.1(2), P1–N1–P1A: 85.9(2); N1–P1–N3: 114.6(2), N1–P1–N2: 115.7(2).

Crystal structure determination of 11: C₇₂H₁₄₄N₁₄P₆, yellow crystals (n-hexane), crystal dimensions: 0.45×0.35×0.20 mm³; M=1564.18; triclinic, space group P1̅ (No. 2), a=11.8836(5), b=12.3639(6), c=17.8457(7) Å, α=100.363(2)°, β=98.858(2)°, γ=113.400(2)°, V=2292.1(2) ų, Z=1, μ(MoKα)=0.166 mm⁻¹, T=123(2) K, F(000)=864.0 e. Of 19799 reflections which were collected on a Nonius Kappa-CCD diffractometer using MoK_α radiation up to 2θ_max=50° 8020 were independent and used in all further calculations. The structure was solved with Direct Methods. The non-H atoms were refined anisotropically against F²; hydrogen atoms were refined using a riding model (programs Shelxs-97 [54], Shelxl-97 [55], [56]). The final wR2(F²) was 0.1718 and the conventional R(F) was 0.0585 for 443 parameters with 29 restraints.

CCDC 249487 (10) and 246713 (11) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Center viawww.ccdc.cam.ac.uk/data_request/cif.

6 Theoretical calculations

Density functional calculations were carried out by the Gaussian 98 program package [57]. The B3LYP functional has been used with the 3-21G(*) and 6-31+G* basis sets. The structures were first optimized with the 3-21G(*) basis, and the stationary points obtained were characterized as minima (NIMAG=0) or first order saddle points (NIMAG=1) by calculation of the second derivatives at the stationary points obtained. The wave functions at the minima turned out to be stable as closed shell for the singlet species (including the nitrenes). The S² value for the doublet species was not larger than 0.77. From the first order saddle points subsequent IRC calculations were carried out to locate the minima which are connected by the saddle point. Energy corrections for Gibbs free energies were obtained by using the B3LYP/3-21G(*) harmonic frequencies at 298 K and atmospheric pressure. Subsequent optimization was carried out at the B3LYP/6-31+G* level without calculating the second derivatives. To visualize the molecular structures obtained, the Molden program has been used [58].