Formation of the spirocyclic, Si-centered cage cations [ClP(NSiMe₃)₂Si(NSiMe₃)₂P₅]⁺ and [P₅(NSiMe₃)₂Si(NSiMe₃)₂P₅]²⁺

Abstract

On account of our interest in P₄ activation by phosphenium ion insertion into P–P bonds we have developed synthetic routes to bicyclic N–P–Si-heterocycle 7 and probed its reactivity towards GaCl₃ and P₄. A GaCl₃-induced rearrangement of 7 leads to the in situ formation of spirocyclic, Si-centered phosphenium ions. Their insertion into P–P bonds of one or two P₄ tetrahedra yields polyphosphorus cages [ClP(NSiMe₃)₂Si(NSiMe₃)₂P₅]⁺ (19⁺) and [P₅(NSiMe₃)₂Si(NSiMe₃)₂P₅]²⁺ (13²⁺).

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Introduction

White phosphorus (P₄) is an archetypal building block for the syntheses of polyphosphorus cations featuring a high P to substituent ratio.¹ In contrast to highly substituted cations R_nP_m (n > m), which are obtained via synthetic methods based on catena or cyclic polyphosphanes,² the targeted preparation of P-rich cations R_nP_m (n < m) is achieved by taking advantage of the tetrahedral P₄ framework.^1,3 In a seminal paper, Krossing et al. reported that dicoordinated phosphenium ions, like other predominantly electrophilic ambiphiles,⁴ are able to insert into a P–P bond of the P₄ tetrahedron.⁵ This was exploited for the preparation of a series of P₅X₂⁺-cages (X = Cl, Br, I).^5,6 We expanded this methodology and prepared a series of symmetrically and unsymmetrically-substituted R₂P₅⁺- and RP₅Cl⁺-cations (R = aryl, alkyl, R₂N).⁷ The additional stability added by organo-substituents allowed for the stepwise insertion of up to three [Ph₂P]⁺ phosphenium ions into P–P bonds of one P₄ molecule yielding mono- to tri-cationic [Ph₂P₅]⁺-, [Ph₄P₆]²⁺- and [Ph₆P₇]³⁺-cages.⁸ A complementary study exploited 1,3-dichloro-cyclo-1,3-diphosphadiazane ClP(NDipp)₂PCl (Dipp = 2,6-diisopropylphenyl), as a twofold phosphenium ion source for the stepwise activation of two P₄ tetrahedra which yielded the mono- and dicationic species [ClP(NDipp)₂P₅]⁺ (1⁺) and [P₅(NDipp)₂P₅]²⁺ (2²⁺, Fig. 1).⁹ The related P₅⁺-species 3[GaCl₄] was obtained by the reaction of P₄ with the four-membered heterocycle Cl₂Si(NSiMe₃)₂PCl and GaCl₃ while the isolobale Al-species 4 is the result of the reaction of phosphenium zwitterion Cl₂Al(NSiMe₃)₂P with P₄.¹⁰

Fig. 1 P₅⁺-cages 1⁺, 2²⁺, 3⁺ and 4 featuring a fused four-membered heterocycle.

Only recently, cationic polyphosphorus cages have emerged as valuable synthetic building blocks for three purposes. First, they can be selectively fragmented in reactions with carbenes which results in the formation of P_n-species (e.g. a P₅⁺-cage cation yields a P₂- and cationic P₃⁺-species).¹¹ Second, they can be oxidized with selenium or sulphur which allows for the targeted preparation of cationic phosphorus–chalcogen cages.^7c Third, they can be used for the controlled release of P₄ due to the reversibility of the phosphenium ion insertion.^10b

With the intention to further expand the range of methods for the in situ generation of phosphenium ions for P₄ activation, we investigated the reactivity of bicyclic phosphorus–nitrogen–silicon heterocycle 7 with GaCl₃ and P₄. A GaCl₃-induced rearrangement reaction was observed which formally gives access to the spirocyclic, Si-centered compound ClP(NSiMe₃)₂Si(NSiMe₃)₂PCl. Chloride abstraction by GaCl₃ and insertion of the respective phosphenium ion in P–P bonds of one or two P₄ molecules yields the Si-centered polyphosphorus cages [ClP(NSiMe₃)₂Si(NSiMe₃)₂P₅]⁺ (19⁺) and [P₅(NSiMe₃)₂Si (NSiMe₃)₂P₅]²⁺ (13²⁺).

Results and discussion

Synthetic routes towards and characterization of 7

As part of our ongoing interest in four-membered phosphorus–nitrogen-element heterocycles as precursors for phosphenium ions¹² that can insert into a P–P bond of P₄ ^10a we revisited the synthesis of heterocycle 6. This compound is synthesized by the reaction of iminophosphane 5 with SiCl₄ according to a procedure reported by Niecke and co-workers (Scheme 1).¹³ Compound 6 was obtained from the reaction mixture by distillation (40 °C, 8 × 10⁻² mbar) and isolated in 41% yield in accordance with the literature report. Surprisingly, however, a second fraction was obtained at higher temperatures (105 °C, 2 × 10⁻³ mbar) and identified as bicyclic compound 7 (18% yield).¹⁴ The ³¹P{¹H} NMR spectrum of 7 dissolved in C₆D₆ shows a singlet resonance at δ(P) = 211.8 ppm which is in the typical chemical shift region of diphosphadiazanes featuring amino-groups on P.¹⁵

Scheme 1 Preparation of compounds 6 and 7; (a) +SiCl₄, −Me₃SiCl, neat, 80 °C, 3 d.

The molecular structure of 7 is C_2V-symmetric and features an distorted planar four-membered [NP]₂-ring (largest deviation from the plane 0.195 Å, Fig. 2). All four N atoms exhibit a trigonal planar arrangement (angular sums range from 358.8(3)° to 360.0(3)°) whereas the P atoms are involved in pyramidal bonding environments (angular sums P1: 286(3)° and P2: 286.2(2)°). The P1–N1 and P2–N2 bonds are almost orthogonal to the [NP]₂-plane (N1–P1⋯P2: 95.84(4)°, N2–P2⋯P1: 95.84(4)°). The C_2V-symmetric arrangement of the N₂SiCl₂-moiety in 7 is rare¹⁶ and contrasts other known diamido-cyclodiphosphazane compounds compounds coordinating main group fragments.¹⁵ Compounds of type (PNR)₂NR₂EX_n typically exhibit C_S-symmetric, seco-heterocube type structures in which the main group element fragment (e.g. EX_n = Mg, BPh, AlCl, GaCl, Ge, Sn, SnCl₂, SiCl₂, AsCl, SbCl, BiCl, R = t-Bu, Ph) is coordinated by three N atoms and occupies the edge of a distorted cube.¹⁵

Fig. 2 Molecular structure of 7 (hydrogen atoms are omitted for clarity and thermal ellipsoids are displayed at 50% probability); selected bond lengths [Å] and angles [°]: N1–P1 1.734(1), N3–P1 1.721(1), N4–P1 1.719(4), N2–P2 1.738(1), N3–P2 1.724(1), N4–P2 1.720(1), P1⋯P2 2.5235(6), N1–Si1 1.708(1), N1–Si2 1.773(1), N2–Si1 1.710(1), N2–Si3 1.774(1), N3–Si4 1.728(1), N4–Si5 1.740(1), Si1–Cl2 2.0554(6), Si2–Cl1 2.5235(6); P1–N4–P2 94.39(6), P1–N3–P2 94.19(7), N3–P1–N4 84.06(7), N3–P2–N4 83.96(7), N1–P1–N3 100.48(6), N1–P1–N4 102.10(6), N2–P2–N3 100.29(7), N2–P2–N4 101.91(6), N1–P1⋯P2 95.84(4), N2–P2⋯P1 95.58(4), Si1–N1–P1 117.14(7), Si1–N2–P2 117.18(8), N1–Si1–N2 114.19(6), Cl1–Si1–Cl2 104.35(2).

The molecular arrangement observed for 7 in the solid state also persists in solution. The observation of two sets of resonances for its chemically inequivalent Me₃Si-groups in the ¹H NMR spectrum (δ(H) = 0.21 and 0.40 ppm) confirm the C_2V-symmetry. The ¹³C{¹H} NMR spectrum shows a triplet resonance which is assigned to the carbon atoms of the Me₃Si-groups attached to the [PN]₂-ring (δ(C) = −0.1 ppm, ³J(CP) = 3.6 Hz). A pseudo-triplet is observed for the Me₃Si-groups adjacent to the N₂SiCl₂-moiety (δ(C) = 2.3 ppm, |³J(CP) + ⁵J(CP)| = 5.0 Hz) and is a result of an AA′X₃X′₃ spin system (A = ³¹P, X = ¹³C) with a comparatively large ²J(P_AP_A′) coupling constant.¹⁷ The same arguments account for the pseudo-triplet resonance in the ²⁹Si{¹H} NMR spectrum of 7 (δ(Si) = 7.7 ppm) which is assigned to the Me₃Si-groups adjacent to the N₂SiCl₂-moiety (|²J(SiP) + ⁴J(SiP)| = 12.5 Hz). The triplet resonance corresponding to the Me₃Si-groups attached to the [PN]₂-ring (δ(Si) = 1.8 ppm, ³J(SiP) = 11.7 Hz) and the resonance of the SiCl₂ moiety (δ(Si) = −47.4 ppm) are observed in the expected regions.¹⁸

Due to the low isolated yield of 7, it was of interest to develop an alternative synthetic approach. Compound 8 is the head to tail dimer of iminophosphane 5 and was obtained according to a literature known procedure.¹⁷ The solvent free reaction of 8 with an excess of SiCl₄ gave selectively and quantitatively compound 9via Me₃SiCl elimination (Scheme 2).¹⁹ Compound 9 is the trans-conformer of an unsymmetrically-substituted diphosphadiazane derivative and, thus, shows an AX spin system in its ³¹P{¹H} NMR spectrum (δ(P_A) = 219.5 ppm, δ(P_X) = 232.3 ppm, ²J(P_AP_X) = 12.0 Hz). The ¹H NMR spectrum of 9 shows singlet resonances for the SiMe₃-groups on the [NP]₂-ring and for both pointing towards the plane of the four-membered ring. The SiMe₃ group pointing away from the ring, however, shows a relatively large ⁴J(HP) coupling constant (δ(H) = 0.27 ppm, ⁴J(P_AH) = 3.7 Hz). For structurally related compounds, this was rationalized by the close proximity of the CH₃-groups to the lone pair of electrons on the adjacent P atom.²⁰ Due to the same reason, two relatively large ⁴J(PH) coupling constants (3.4 and 3.7 Hz) were observed for the isomer of 9 in which the SiCl₃-group points towards the face of the four-membered ring (9′).¹⁹ The chloro-substituted Si atom in 9 appears as doublet resonance in the typical range in the ²⁹Si NMR spectrum (δ(Si) = −27.3 ppm)²¹ with a remarkably large coupling constant of ²J(SiP_X) = 26.0 Hz. A coupling constant of similar magnitude is observed for the Me₃Si-group pointing away from the four-membered ring (δ(Si) = 7.8 ppm, ²J(SiP_A) = 32.4 Hz), whereas those pointing to the faces of the [NP]₂-ring exhibit rather small coupling constants (δ(Si) = 4.9 and 0.2 ppm; ²J(SiP) = 4.2 and 3.0 Hz). Similarly the ¹³C{¹H} NMR spectrum reveals rather small coupling constants for the latter Me₃Si-groups (³J(CP) = 8.0 and 10.5 Hz) and a large coupling constant for the Me₃Si-group pointing away from the [NP]₂-ring plane (³J(CP_A) = 19.8 Hz).

Scheme 2 Synthesis of 9via reaction of 8 and SiCl₄; (a) +SiCl₄, −Me₃SiCl, rt, 12 h, 96%; and subsequent transformation to 7; (b) neat, 185 °C, 10 min, 25%.

It is assumed, that 7 is obtained from 9via an intermediate of type 10 which forms upon rotation²² of the P–N bond involving the SiCl₃-substituted N atom and inversion²³ of one of the P centers in 9 (Scheme 2). The arrangement of the SiCl₃-group and one of the SiMe₃-groups in intermediate 10 allows for Me₃SiCl elimination yielding 7. Indeed, heating 9 for ten minutes to 185 °C results in conversion to 7 in 25% yield.²⁴

Reactivity of 7 towards GaCl₃ and P₄

Diphosphadiazanes and related compounds bearing chloro- and Me₃Si-substituents are known to undergo a variety of distinct reactions with Lewis acids. Next to halide abstraction,¹⁰ also elimination of Me₃SiCl,²⁵ migration of Me₃Si-,²⁶ chloro- or methyl-substituents²⁷ and P–N bond cleavage reactions are reported.²⁸ Especially the latter reaction is of interest, since it is assumed to proceed via phosphenium ion intermediates. Thus, bicyclic compound 7 is a promising substrate for the generation of phosphenium ions and, subsequently, insertion of the latter into P–P bonds of P₄.

Thus, the reaction of 7 with the Lewis acid GaCl₃ was probed (Scheme 3). The addition of one equivalent of GaCl₃ to a solution of 7 in C₆H₅F gave a reddish colored reaction mixture which was investigated by means of ³¹P{¹H} NMR spectroscopy. The formation of the Lewis-acid/base adduct 11 is proposed on the basis of the observation of two broad resonances in an approximate ratio of 1 : 1 (δ(P_A) = 115 ppm, Δν_1/2 = 1200 Hz; δ(P_X) = 179.6 ppm, Δν_1/2 = 600 Hz). The coordination of GaCl₃ to a P atom is very likely, since they have been identified as the most basic sites in related compounds.²⁹ The resonance at low field is tentatively assigned to the tri-coordinated P atom. Accordingly, the resonance at high field corresponds to the tetra-coordinated P atom and exhibits a stronger line broadening due to the coordination of GaCl₃.

Scheme 3 Formation of Lewis-acid/base adduct 11via the reaction of 7 and GaCl₃; (a) +GaCl₃, C₆H₅F, r.t., 1 h.

Interestingly, P₄ does not react with 11 which indicates that the latter is not a suitable phosphenium ion source. Adding two equivalents of GaCl₃ to a solution of 7 in C₆H₅F, however, results in the rapid consumption of in situ generated 11. This reaction yields a complex mixture of not identified products and bodes well for the generation of reactive intermediates that might be able to activate P₄. Indeed, mixtures of 7, P₄ and GaCl₃ in 1 : 1 : 2 and 1 : 2 : 4 stoichiometries were bright red and the consumption of P₄ accompanied by a color change to brown was observed. Subsequent investigation by means of ³¹P{¹H} NMR spectroscopy revealed the formation of P₅⁺ cage cation 12⁺ and bridged bis(P₅⁺)-cage dication 13²⁺ by their characteristic A₂MOX₂ and A₂MX₂ spin systems (Scheme 4).

Scheme 4 Reaction of 7 with P₄ and GaCl₃ in various stoichiometries; (a) +P₄, +2, GaCl₃, C₆H₅F, r.t., 24 h; (b) +2, P₄, +4, GaCl₃, r.t., C₆H₅F, 24 h; the equation depicts the major products and is not balanced; anions of the products are not depicted.

In addition, both spectra reveal significant amounts of P₄ but neither remaining 7 nor the corresponding adduct 11 were observed which is attributed to unidentified side reactions. The products 12⁺ and 13²⁺ are formally obtained by the insertion of phosphenium ions based on ClP(NSiMe₃)₂Si(NSiMe₃)₂PCl which features a Si-centered spiro[3.3]heptane-motif, into P–P bonds of P₄. Interestingly, both reactions yield 12⁺ and 13²⁺ in comparable ratios (1 : 1.8 for (a) and 1 : 1.5 for (b), Scheme 4). This contrasts the anticipated increase of the amount of dication 13²⁺ in the presence of excess P₄ and GaCl₃. In addition, the formation of large quantities of 13²⁺ in the reaction of 1 : 1 : 2 stoichiometry indicates that the reaction rate of a GaCl₃-induced rearrangement of 7 yielding the spiro-motif is slow compared to that of the subsequent phosphenium ion insertion.

A tentative mechanism for the formation of such a Si-centered spiro[3.3]heptane-type species from 7 is illustrated in Scheme 5. While the reaction of 7 with one equivalent GaCl₃ gives the Lewis-acid/base adduct 11 (vide supra), an excess of GaCl₃ is assumed to initiate a chloride abstraction from the SiCl₂-moiety. This is facilitated by the tendency of bicyclic compounds of type 7 to form a seco-heterocube-type structure.¹⁵ Thus, compound 7 might be in equilibrium with the related derivative 7^I. Compound 7^I features a hypervalent penta-coordinated Si-moiety which might favor the sequestering of a chloride anion. Thus, it is assumed that the reaction with GaCl₃ proceeds via chloride abstraction and formation of the cationic seco-heterocube 14⁺ featuring a tetra-coordinated N atom. The next two steps constitute P–N bond cleavage and formation reactions yielding intermediates 15⁺ and 16⁺via formal retention of the seco-heterocube-type structure. Similar P–N bond ruptures were reported as decomposition pathways of diamino-cyclo-diphosphadiazanes.²⁶ Intermediate 16⁺ features a diphosphadiazane [NP]₂-ring which is assumed to react via a cyclo-reversion reaction to intermediate 17⁺. Cation 17⁺ features an aminoiminophosphane moiety similar to 5 which is tethered to a four-membered SiN₂P-ring. Intramolecular nucleophilic attack of the imino-N atom on the chloro-substituted Si atom initiates a GaCl₃-mediated transfer of a chloride anion from the Si atom to a di-coordinated P atom to give formally the phosphenium ion 18⁺. Cation 18⁺ features the Si-centered spiro[3.3]heptane-motif and, thus, is assumed to be accountable for the formation of cages cations 12⁺ and 13²⁺via insertion into a P–P bond of P₄, chloride abstraction by GaCl₃ and subsequent insertion into a P–P bond of a second P₄ tetrahedron.

Scheme 5 Suggested GaCl₃-induced rearrangement mechanism of 7 to phosphenium ion 18⁺ featuring a Si-centered spiro[3.3]heptane-type motif; R denotes Me₃Si-substituents; for reasons of simplification the reactions of intermediates or products possessing a di-coordinated P atom with P₄ as well as retransfer of a chloride ion to a di-coordinated P atom are not considered.

Attempts to isolate a gallate salt of 12⁺ from both reaction mixtures were unsuccessful, possibly due to the fluxional coordination behavior of the GaCl₃-molecule to the PCl-function. However, slow diffusion of n-hexane into the reaction mixture of 1 : 1 : 2 stoichiometry yielded compound 19[GaCl₄] in low yields (10%, Scheme 6). The ³¹P{¹H} NMR spectrum of 19[GaCl₄] dissolved in CD₂Cl₂ shows an ABMX₂Y spin system which is in accordance with the C_s symmetry of the molecule (Fig. 3). The mirror plane is defined by the tetra-coordinated P atom and the two adjacent P atoms. The P–Cl unit is included in the plane and exhibits a spatial proximity to one of the two bridge-head P atoms. The chemical shifts and coupling constants involving the P₅⁺-cage motif are similar to those observed for the cage cation 3⁺.^10a The chloro-substituted P atom in 19⁺ exhibits a singlet resonance (δ(P_Y) = 166.8 ppm) in the typical range of silyl-substituted diamino-chlorophosphanes.^13,30 A ⁴J(PP) coupling between this P atom and the tetra-coordinated P atom of the P₅⁺-cage is not resolved. The ¹H NMR spectrum of 19⁺ reveals three singlet resonances assigned to the chemically different Me₃Si-groups which integrate in a 2 : 1 : 1 ratio. The high-field resonance (δ(H) = 0.31 ppm) exhibits the highest intensity and is assigned to the two Me₃Si-groups bonded to the four-membered ring which incorporates the P–Cl moiety. The chemical shift is comparable to related four-membered ring compounds (6: δ(H) = 0.18 ppm,¹³7: δ(H) = 0.21 ppm). Both resonances assigned to the Me₃Si-groups bonded to the N atoms adjacent to the P₅⁺-moiety exhibit a low field shift (δ(H) = 0.67 ppm and 0.76 ppm) and are comparable to the corresponding resonance of 3⁺ (δ(H) = 0.68 ppm).^10a The ²⁹Si{¹H} NMR spectrum exhibits a resonance at δ(Si) = −58.0 ppm which is assigned to the Si spiro-atom. This resonance reveals a doublet of doublet splitting caused by ²J(SiP)-couplings to the tetra-coordinated P atom (²J(SiP_M) = 8.5 Hz) and the chloro-substituted P atom (²J(SiP_Y) = 18.5 Hz).

Scheme 6 Reaction of 19[GaCl₄] with GaCl₃; (a) +GaCl₃, CD₂Cl₂, r.t., 30 min.

Fig. 3 ³¹P{¹H} NMR spectrum of 19[GaCl₄] (CD₂Cl₂, r.t.); insets show experimental (upwards) and fitted spectra (downwards); ABMX₂Y spin system of 19⁺: δ(P_A) = −322.9 ppm, δ(P_B) = −320.2 ppm, δ(P_M) = 30.4 ppm, δ(P_X) = 165.5 ppm, δ(P_Y) = 166.8 ppm, ¹J(P_AP_B) = −189.8 Hz, ¹J(P_AP_X) = −143.4 Hz, ¹J(P_BP_X) = −147.7 Hz, ¹J(P_MP_X) = −245.1 Hz, ²J(P_AP_M) = 18.8 Hz, ²J(P_BP_M) = 18.0 Hz.

The molecular structure of 19⁺ is depicted in Fig. 4 and the P–P bond lengths and angles in the P₅⁺-moiety are comparable to those of 3⁺.^10a Both four-membered rings are almost planar (largest deviation from the planes N1: 0.026 Å and N3: 0.022 Å) and exhibit a perpendicular arrangement (angle between both planes: 89.79(9)°). Due to the steric limitations of the four-membered heterocycles the spiro-Si atom exhibits a distorted tetrahedral arrangement with two rather small (N1–Si3–N2: 85.4(1)°, N3–Si3–N4: 87.7(1)°) and two widened (N1–Si3–N3: 122.9(1)°, N2–Si3–N4: 118.8(1)°) N–Si–N angles. Alternating bond lengths are observed within the two four-membered rings. The P–N bonds involving the tetra-coordinated P atom are shorter (N2–P1: 1.656(3) Å, N1–P1: 1.661(3) Å) than those involving the tri-coordinated P atom (N3–P6: 1.716(3) Å, N4–P6: 1.711(3) Å) and both magnitudes of bond lengths are also observed in the related cages 3⁺ and 2⁺.^9,10a The Si–N bonds in the [SiN₂P]-ring fused to the P₅⁺-cage (N1–Si3: 1.755(3) Å, N2–Si3: 1.751(3) Å) are of similar lengths as observed for 3⁺.^10a In contrast, the Si–N bonds in the second [SiN2P]-ring are shorter (N3–Si3: 1.711(3) Å, N4–Si3: 1.704(3) Å).

Fig. 4 Molecular structure of 19⁺ in compound 19[GaCl₄]·C₆H₅F (hydrogen atoms are omitted for clarity and thermal ellipsoids are displayed at 50% probability); selected bond lengths [Å] and angles [°]: N2–P1 1.656(3), N1–P1 1.661(3), N1–Si3 1.755(3), N2–Si3 1.751(3), N3–Si3 1.711(3), N4–Si3 1.704(3), N3–P6 1.716(3), N4–P6 1.711(3), Cl1–P6 2.238(2), P1⋯Si3 2.445(1), P6⋯Si3 2.472(1), P1–P2 2.167(1), P1–P3 2.165(1), P2–P4 2.249(2), P2–P5 2.238(2), P3–P4 2.251(2), P3–P5 2.229(2), P4–P5 2.169(2); N1–P1–N2 91.5(1), P1–N1–Si3 91.4(1), P1–N2–Si3 91.7(1), N1–Si3–N2 85.4(1), N3–Si3–N4 87.7(1), N1–Si3–N3 122.9(1), N2–Si3–N4 118.8(1), N3–P6–N4 87.2(1), N3–P6–Cl1 102.2(1), N4–P6–Cl1 101.7(1), P3–P1–P2 91.15(5), P1–P2–P5 83.95(5), P1–P2–P4 82.53(5), P5–P2–P4 57.82(5), P5–P4–P2 60.83(5), P2–P5–P3 87.67(5).

The addition of GaCl₃ to a solution of 19[GaCl₄] in CD₂Cl₂ yields the previously mentioned Lewis-acid base adduct 12⁺ (Scheme 6). The A₂MOX₂ spin system observed in the ³¹P{¹H} NMR spectrum of 12⁺ suggests a time averaged C_2v-symmetry of the molecule in solution which can be explained by a fluxional behavior of the coordinated GaCl₃ molecule (Fig. 5). The A₂MX₂ part of the A₂MOX₂ spin system corresponds to the P₅⁺-cage moiety and reveals comparable chemical shifts and coupling constants as observed for 3⁺.^10a The resonance corresponding to the O part of the spin system is very broad (δ(P_O) = 144 ppm, Δν_1/2 = ∼1200 Hz) which is caused by the fluxional behavior of the coordinated GaCl₃ molecule and its quadrupole moment.

Fig. 5 ³¹P{¹H} NMR spectrum of 12[GaCl₄] (CD₂Cl₂, r.t.); A₂MOX₂ spin system of 12⁺: δ(P_A) = −318.3 ppm, δ(P_M) = 33.4 ppm, δ(P_O) = 144 ppm (Δν_1/2 = ∼1200 Hz), δ(P_X) = 170.8 ppm, ¹J(P_AP_X) = −144.8 Hz, ¹J(P_MP_X) = −248.6 Hz, ²J(P_AP_M) = 19.0 Hz.

Dication 13²⁺ was isolated as a [Ga₂Cl₇]⁻ salt from the reaction of 7, P₄ and GaCl₃ in a 1 : 2 : 4 stoichiometry (Scheme 4) by the addition of n-hexane. This gave a brown oil which was isolated by decanting the supernatant and upon addition of small amounts of 1,2-C₆H₄F₂ yielded a suspension containing a yellow microcrystalline material of 13[Ga₂Cl₇]₂. This compound was isolated by filtration in low yields (20%) in an approximate purity of 75%. Further purification by recrystallization from CH₂Cl₂/n-hexane leads to significantly decreased yields (7% yield in >90% purity). The ³¹P{¹H} NMR spectrum of 13[Ga₂Cl₇]₂ dissolved in CD₂Cl₂ shows an A₂MX₂-spin system in accordance with the C_2v-symmetry of the molecule (Fig. 6). The C₂-axis includes both tetra-coordinated P atoms and the spiro-Si atom and the mirror planes are defined by the four-membered [SiN₂P]-rings. The resonances and coupling constants of 13²⁺ are similar to those observed for the related compounds 19⁺ and 3⁺.^10a

Fig. 6 ³¹P{¹H} NMR spectrum of 13[Ga₂Cl₇]₂ (CD₂Cl₂, r.t.); unidentified side products are marked with asterisks; A₂MX₂ spin system of 13²⁺: δ(P_A) = −320.4 ppm, δ(P_M) = 32.2 ppm, δ(P_X) = 158.8 ppm, ¹J(P_AP_X) = −142.7 Hz, ¹J(P_MP_X) = −259.1 Hz, ²J(P_AP_M) = 20.0 Hz.

Single crystals of compound 13[Ga₂Cl₇]₂ were obtained by diffusion of n-hexane in a CH₂Cl₂ solution of 13[Ga₂Cl₇]₂ at −35 °C. The compound crystallizes with two independent formula units in the asymmetric unit. Two of the four Ga₂Cl₇⁻ anions are highly disordered exhibiting unusually high thermal displacement parameters (see ESI† for details). Single crystals of 13[Ga₂Cl₇][GaCl₄] were obtained by layering the supernatant solution of the reaction mixture of the synthesis of 13[Ga₂Cl₇]₂ with n-hexane at −35 °C. The data obtained by X-ray single crystal structure determination was of higher quality and, thus, the molecular structure of 13[Ga₂Cl₇][GaCl₄] is discussed (Fig. 7). The P–P bond lengths and angles in the P₅⁺-moieties are comparable to those of related P₅⁺-cage compounds.^9,10a The P–N bonds in 13²⁺ are rather short (av. P–N: 1.666(8) Å) which is a typical feature of P–N bonds involving phosphonium moieties. The Si–N bond lengths in the four-membered rings are nearly identical (av. Si–N: 1.732(8) Å) and are between the two types of bond lengths observed for 19⁺ (av. Si–N: 1.753(6) Å and Si–N: 1.7007(6) Å).

Fig. 7 Molecular structure of 13²⁺ in compound 13[Ga₂Cl₇][GaCl₄](hydrogen atoms are omitted for clarity and thermal ellipsoids are displayed at 50% probability); selected bond lengths [Å] and angles [°]: N1–P1 1.667(2), N2–P1 1.671(2), N3–P6 1.663(2), N4–P6 1.665(2), Si1–N1 1.731(2), Si1–N2 1.732(2), S1–N3 1.736(2), Si1–N4 1.728(2), P1⋯Si1 2.4452(8), P6⋯Si1 2.4277(8), P1–P2 2.1540(8), P1–P3 2.1499(8), P2–P4 2.245(1), P2–P5 2.245(1), P3–P4 2.246(1), P3–P5 2.2464(9), P4–P5 2.168(1), P6–P7 2.1511(8), P6–P8 2.1572(2), P7–P9 2.240(1), P7–P10 2.252(1), P8–P9 2.2415(9), P8–P10 2.242(1), P9–P10 2.172(1); N1–P1–N2 90.81(9), N3–P6–N4 90.99(9), N1–Si1–N2 86.73(9), N3–Si1–N4 86.49(9), P1–N1–Si1 91.26(9), P1–N2–Si1 91.12(9), P6–N3–Si1 91.14(9), P6–N4–Si1 91.34(9), P3–P1–P2 92.59(3), P1–P2–P5 81.73(3), P1–P2–P4 83.19(3), P5–P2–P4 57.69(3), P5–P4–P2 61.25(3), P2–P5–P3 87.60(3), P7–P6–P8 92.30(3), P6–P7–P10 82.39(3), P6–P7–P9 82.89(3), P10–P7–P9 57.83(3), P10–P9–P7 61.36(3), P7–P10–P8 87.48(3).

Conclusions

The bicyclic P–N–Si heterocycle 7 was targeted as source for the in situ generation of phosphenium cations for P₄ activation. In this context, two distinct synthetic protocols for its preparation were thoroughly investigated and gave insights into the formation of the bicycle. The reaction of 7 with GaCl₃ initially yields adduct 11. This adduct is not stable and subsequently rearranges to give in situ spirocyclic, Si-centered compound ClP(NSiMe₃)₂Si(NSiMe₃)₂PCl. The latter species gives access to polyphosphorus cage cations [ClP(NSiMe₃)₂Si(NSiMe₃)₂P₅]⁺ (19⁺) and [P₅(NSiMe₃)₂Si(NSiMe₃)₂P₅]²⁺ (13²⁺) in the presence of GaCl₃ and P₄. We are continuing to investigate the Lewis-acid mediated generation of phosphenium ions for P₄ activation from related phosphorus–nitrogen-element bicycles. Furthermore, studies directed towards the utilization of 12⁺, 19⁺ and 13²⁺ as synthetic building blocks will be the target of future efforts.

Experimental

General

General information on materials and methods as well as ³¹P{¹H} NMR spectra of reaction mixtures are given in the ESI.†

Synthesis of Cl₂Si(NSiMe₃)₂(PNSiMe₃)₂ (7)

Method A: The literature reported synthesis of 5 was performed on a 20 mmol scale.¹³ Compound 5 was removed by distillation from the reaction mixture (40 °C, 8 × 10⁻² mbar). The remaining colorless, slushy residue was dissolved in C₆H₅F (5 mL) yielding a turbid suspension. The solvent was removed in vacuo yielding a sludgy residue which was redistilled employing a short Vigreux column (5 cm). Compound 7 was obtained as colorless oil (1.789 g, 3.51 mmol, 18%, 105 °C, 2 × 10⁻³ mbar) which solidified shortly after distillation. Method B: 9 (305 mg, 0.50 mmol, 1.0 eq.) was heated to 185 °C for 10 min. In the course of the reaction a colorless liquid is formed accompanied by the condensation of Me₃SiCl on colder parts of the reaction vessel. After cooling to ambient temperature the reaction mixture remains a liquid. Isolation of 7 from this mixture proceeds as described in method A.

m.p.: 55.6–57.8 °C; Raman (300 mW, [cm⁻¹]): ν = 2959 (390), 2899 (100), 1410 (11), 690 (10), 645 (22), 613 (51), 562 (4), 349 (34), 185 (30), 141 (6), 75 (10); IR (ATR, [cm⁻¹]): ν = 2956 (w), 1408 (vw), 1249 (s), 1098 (vw), 973 (vw), 942 (w), 883 (w), 830 (vs), 776 (vw), 754 (vw), 713 (w), 682 (w), 643 (vw), 556 (s), 450 (m); ¹H NMR (C₆D₆, [ppm]): δ = 0.21 (18H, s, H1), 0.39 (18H, s, H2); ¹³C{¹H} NMR (C₆D₆, [ppm]): δ = −0.08 (6C, t, C1, ³J(CP) = 3.6 Hz), 2.7 (6C, pseudo-t, C2, ³J(CP) = 5.0 Hz); ²⁹Si{¹H} NMR (C₆D₆, [ppm]): δ = −47.4 (1Si, s, Si3), 1.8 (2Si, t, Si1, ²J(SiP) = 11.7 Hz), 7.7 (2Si, pseudo-t, Si2, ²J(SiP) = 11.8 Hz); ¹⁵N NMR (C₆D₆, [ppm]): δ = −397 (t, N1, ¹J(NP) = 55 Hz), −374 (d, N2, ¹J(NP) = 75 Hz); ³¹P{¹H} NMR (C₆D₆, [ppm]): δ = 211.8 (s); elemental analysis for C₁₂H₃₆Cl₂N₄P₂Si₅: calcd: C 28.3, H 7.1, N 11.0; found: C 28.5, H 7.3, N 10.6; MS-ESI-EM: 473.0943 [M-Cl⁻], calcd: for C₁₂H₃₆Cl₁N₄P₂Si₅: 473.0945.

Synthesis of (SiMe₃)₂N(PNSiMe₃)₂N(SiMe₃)(SiCl₃) (9)

Compound 8 (1.114 g, 2.00 mmol, 1.0 eq.) was suspended in SiCl₄ (6.796 g, 40.0 mmol, 40.0 eq.) and stirred for 12 h at ambient temperature. After removal of all volatiles in vacuo9 was isolated in quantitative yields as colorless solid (1.187 g, 1.92 mmol, 96%).

m.p.: 180.2–182.5 °C; Raman (300 mW, [cm⁻¹]): ν = 2957 (37), 2906 (100), 1410 (15), 686 (13), 651 (4), 641 (64), 586 (21), 488 (4), 436 (38), 351 (21), 206 (37), 107 (27), 78 (18); IR (ATR, [cm⁻¹]): ν = 3139 (vw), 3047 (m), 2958 (vw), 1406 (m), 1251 (s), 1060 (m), 923 (m), 834 (vs), 778 (vw), 755 (w), 712 (w), 677 (w), 572 (vw), 559 (m), 506 (w), 432 (vw); ¹H NMR (C₆D₆, [ppm]): δ = 0.17 (18H, s, H5), 0.27 (9H, d, H3, ⁴J(HP) = 3.7 Hz), 0.52 (9H, s, H4), 0.64 (9H, s, H1); ¹³C{¹H} NMR (C₆D₆, [ppm]): δ = 1.4 (6C, t, C5, ³J(CP) = 2.5 Hz), 4.7 (3C, d, C1, ³J(CP) = 10.5 Hz), 4.8 (3C, d, C3, ³J(CP) = 19.8 Hz), 5.0 (3C, d, C4, ³J(CP) = 8.0 Hz); ²⁹Si{¹H} NMR (C₆D₆, [ppm]): δ = −27.3 (1Si, d, Si2, ²J(SiP) = 26.0 Hz), 0.2 (1Si, d, Si4, ²J(SiP) = 3.0 Hz), 1.5 (2Si, t, Si5, ²J(SiP) = 7.4 Hz), 4.5 (1Si, d, Si2, ²J(SiP) = 4.2 Hz), 7.6 (1Si, d, Si3, ²J(SiP) = 32.4 Hz); ¹⁵N NMR (C₆D₆, [ppm]): δ = 67.1 (t, N2, ¹J(NP) = 50 Hz), 93.8 (d, N1, ¹J(NP_X) = 90 Hz), 113.9 (d, N3, ¹J(NP_A) = 90 Hz); ³¹P{¹H} NMR (C₆D₆, [ppm]): AX spin system: δ(P_A) = 218.6 (d, P1, Δν_1/2 = 42 Hz, ²J(P_AP_X) = 12 Hz), δ(P_X) = 232.2 (d, P2, Δν_1/2 = 37 Hz, ²J(P_AP_X) = 12 Hz); elemental analysis for C₁₅H₄₅Cl₃N₄P₂Si₆: calcd: C 29.1, H 7.3, N 9.1; found: C 28.8, H 7.3, N 8.6.

Reaction of 7, P₄ and GaCl₃ in 1 : 1 : 2 and 1 : 2 : 4 stoichiometries

1 : 1 : 2: Compound 7 (256 mg, 0.50 mmol, 1.0 eq.) was added to a suspension of P₄ (62 mg, 0.50 mmol, 1.0 eq.) in C₆H₅F (5 mL). A solution of GaCl₃ (176 mg, 1.00 mmol, 2.0 eq.) in C₆H₅F (2 mL) was added dropwise to the suspension giving a red colored reaction mixture which was stirred for 12 h at ambient temperature. In the course of the reaction the color of the reaction mixture changed to yellow and the dissolving of P₄ was observed. The reaction mixture was investigated by means of ³¹P{¹H} NMR spectroscopy (see ESI†). 1 : 2 : 4: Compound 7 (256 mg, 0.50 mmol, 1.0 eq.) was added to a suspension of P₄ (124 mg, 1.00 mmol, 2.0 eq.) in C₆H₅F (5 mL). A solution of GaCl₃ (352 mg, 2.00 mmol, 4 eq.) in C₆H₅F (4 mL) was added dropwise to the suspension giving a red colored reaction mixture which was stirred for 12 h at ambient temperature. In the course of the reaction the color of the reaction mixture changed to brown and the dissolving of P₄ was observed. The reaction mixture was investigated by means of ³¹P{¹H} NMR spectroscopy (see ESI†). n-Hexane (2 mL) was added leading to the formation of a brown oil. The supernatant was removed, diluted with C₆H₅F (6 mL) and layered with n-hexane (3 mL) at −35 °C. Small amounts of crystalline material of 19[GaCl₄] (41 mg, 10%), suitable for X-ray single crystal structure determination, were obtained within a few days. The remaining oil was washed with n-hexane (3 × 3 mL) transforming it into a brown sludge. All volatiles were removed in vacuo and the sludge was suspended in 1,2-C₆H₄F₂ (2 mL) leading to the formation of a yellow, microcrystalline solid. The supernatant was removed and the yellow powder was washed with 1,2-C₆H₄F₂/n-hexane (1 : 1 mixture, 2 × 2 mL). The obtained yellow powder consisting of 13[Ga₂Cl₇]₂ in an approximate purity of 75% (determined by ³¹P{¹H} NMR spectroscopy, 20% yield, 145 mg) was isolated by filtration and dried in vacuo. Recrystallization from a CH₂Cl₂ solution by slow diffusion of n-hexane yielded crystalline material of 19[Ga₂Cl₇]₂ (purity > 90%) which was suitable for single crystal structure determination. Isolation was conducted via filtration and removal of all volatiles in vacuo (7% yield, 55 mg). Single crystals of 13[Ga₂Cl₇][GaCl₄] were obtained by layering the diluted supernatant of the reaction mixture with n-hexane at −35 °C.

m.p.: 115.0–116.9 °C; Raman (300 mW, [cm⁻¹]): ν = 2961 (37), 2998 (100), 1412 (24), 636 (37), 552 (63), 442 (22), 398 (24), 376 (14), 344 (43), 152 (44), 121 (13); IR (ATR, [cm⁻¹]): ν = 2956 (vw), 2897 (vw), 1411 (vw), 1254 (m), 1123 (vw), 993 (s), 905 (s), 822 (vs), 756 (vw), 724 (s), 690 (w), 639 (w), 548 (vw), 509 (w), 462 (m); ¹H NMR (CD₂Cl₂, [ppm]): δ = 0.31 (18H, s, H1), 0.67 (9H, s, H2), 0.76 (9H, s, H3); ¹³C{¹H} NMR (CD₂Cl₂, [ppm]): δ = 1.4 (6C, d, C1, ³J(CP) = 3.1 Hz), 2.2 (3C, m, C2), 3.1 (3C, m, C3); ²⁹Si{¹H} NMR (CD₂Cl₂, [ppm]): δ = −58.0 (dd, Si4, ²J(SiP_Y) = 18.5 Hz, ²J(SiP_M) = 8.5 Hz), 7.4 (d, Si1, ²J(SiP) = 8.4 Hz), 10.9 (s, Si2), 11.7 (s, Si3); ¹⁵N NMR (CD₂Cl₂, [ppm]): δ = 96 (s, N1), 115 (s, N2), 125 (s(br), N3); ⁷¹Ga{¹H} NMR (CD₂Cl₂, [ppm]): δ = 249.6 (s); ³¹P{¹H} NMR (CD₂Cl₂, [ppm]): ABMX₂Y spin system: δ(P_A) = −322.9, δ(P_B) = −320.2, δ(P_M) = 30.4, δ(P_X) = 165.5, δ(P_Y) = 166.8, ¹J(P_AP_B) = −189.8 Hz, ¹J(P_AP_X) = −143.4 Hz, ¹J(P_BP_X) = −147.7 Hz, ¹J(P_MP_X) = −245.1 Hz, ²J(P_AP_M) = 18.8 Hz, ²J(P_BP_M) = 18.0 Hz; elemental analysis for C₁₂H₃₆GaCl₅P₆N₄Si₅: calcd: C 17.8, H 4.5, N 6.9; found: C 17.1, H 4.4, N 5.5.

¹ H NMR (CD ₂ Cl ₂ , [ppm]): δ = 0.37 (18H, s, H2), 0.75 (18H, s, H1); ³¹P{¹H} NMR (CD₂Cl₂, [ppm]): ABMOX₂ spin system: δ(P_A) = −318.3, δ(P_M) = 33.4, δ(P_O) = 144 (Δν_1/2 = ∼1200 Hz), δ(P_X) = 170.8, ¹J(P_AP_X) = −144.8 Hz, ¹J(P_MP_X) = −248.6 Hz, ²J(P_AP_M) = 19.0 Hz. Compound 12[GaCl₄] was independently synthesized by addition of GaCl₃ (18 mg, 0.10 mmol, 1.0 eq.) to a solution of 19[GaCl₄] (64 mg, 0.10 mmol, 1.0 eq.) in CD₂Cl₂ (1 mL). The obtained colorless solution was stirred for 30 min at ambient temperature and subsequently investigated by ¹H and ³¹P{¹H} NMR spectroscopy.

m.p.: 164.5–167.5 °C (decomposition); Raman (250 mW, [cm⁻¹]): ν = 2962 (10), 2898 (17), 1095 (10), 633 (13), 549 (100), 441 (11), 397 (17), 384 (10), 354 (10), 138 (81) the Raman measurement was hampered by strong fluorescence effects; IR (ATR, [cm⁻¹]): ν = 2956 (vw), 2898 (vw), 1409 (w), 1257 (s), 994 (vs), 899 (m), 814 (vs), 759 (vw), 729 (w), 692 (vw), 637 (w), 544 (w), 490 (w), 441 (vw), 409 (w); ¹H NMR (CD₂Cl₂, [ppm]): δ = 0.72 (36H, s, CH₃); ¹³C{¹H} NMR (CD₂Cl₂, [ppm]): δ = 2.7 (12C, s, CH₃); ²⁹Si{¹H} NMR (CD₂Cl₂, [ppm]): δ = 13.7 (s, Si(CH₃)₃), the Si atom of the SiN₄-moiety was not detected; ¹⁵N NMR (CD₂Cl₂, [ppm]): δ = 115 (s); ³¹P{¹H} NMR (CD₂Cl₂, [ppm]): A₂MX₂ spin system: δ(P_A) = −320.4, δ(P_M) = 32.2, δ(P_X) = 158.8, ¹J(P_AP_X) = −142.7 Hz, ¹J(P_MP_X) = −259.1 Hz, ²J(P_AP_M) = 20.0 Hz; elemental analysis for C₁₂H₃₆Ga₄Cl₁₄P₁₀N₄Si₅: calcd: C 10.8, H 2.5, N 3.8; found: C 10.9, H 2.5, N 4.0.

Acknowledgements

We gratefully acknowledge financial support from the Fonds der Chemischen Industrie (fellowship to M.H.H.) and the DFG (WE4621/2-1).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Crystallographic data, ³¹P NMR spectra of reaction mixtures and general experimental information. CCDC 1060461–1060464. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5dt01512j

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