Formation of an imidazoliumyl-substituted [(L_C)₄P₄]⁴⁺ tetracation and transition metal mediated fragmentation and insertion reaction (L_C = NHC)

Abstract

Tetracationic cyclo-tetraphosphane [(L_C)₄P₄]⁴⁺ as triflate salt (3[OTf]₄) (L_C = 4,5-dimethyl-1,3-diisopropyl-imidazol-2-yl) is obtained in high yield from the reduction of [L_CPCl₂]⁺ (4[OTf]) with 1,4-bis(trimethylsilyl)-1,4-dihydropyrazine (6) and represents the first salt of the cationic cyclo-phosphane series with the general formula [L_nP_n]ⁿ⁺. Theoretical calculations reveal the electrophilic nature of the P atoms within the P₄-ring due to the influence of the imidazoliumyl-substituents. Further reduction of 3[OTf]₄ with 6 affords the unexpected formation of the notricyclane P₇-type cation [(L_C)₃P₇]³⁺ (9[OTf]₃). Selective transition metal mediated [2 + 2]-fragmentation of 3⁴⁺ is achieved when 3[OTf]₄ is reacted with Fe₂(CO)₉, Pd(PPh₃)₄ and Pt(PPh₃)₄ leading to the formation of the dicationic diphosphene complexes [(η²-L_CP=PL_C)Fe(CO)₄]²⁺ (12[OTf]₂) and [(η²-L_CP=PL_C)M(PPh₃)₂]²⁺ (13[OTf]₂ for M = Pd; 14[OTf]₂ for M = Pt). In contrast, the reaction of 3[OTf]₄ with an excess of AuCl(tht) gives rise to the formation of the five-membered ring complex [((L_C)₄P₄)AuCl₂]³⁺ (15[OTf]₃), where the Au(I) atom reductively inserts into a P–P bond of 3⁴⁺.

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Introduction

The fact that phosphorus tends to assemble into homo-nuclear cyclic and cage-like structural motifs is not only illustrated by the variety of phosphorus allotropes,¹ but also by the multitude of known polyphosphanes.² Particularly, neutral monocyclic polyphosphanes with the general formula P_nR_n (R = alkyl, aryl, n = 3–5) are already known for decades and their chemistry is well established.^3–5 There are several synthetic protocols for this group of compounds, of which the reduction of a dichlorophosphane RPCl₂ (R = alkyl, aryl) with various reducing agents represents the most common approach.⁶ The steric demand of the substituents at the P atom determines the favored ring-size.^6,7 Thus, the reduction of CyPCl₂ with Mg yields the thermodynamically favored cyclo-Cy₄P₄,⁸ whereas the reduction of PhPCl₂ gives mainly cyclo-Ph₅P₅.⁹ A mixture of compounds with several ring sizes is often observed in the initial reaction, however, a scrambling to the thermodynamically favored ring-size typically occurs over time.^6–9 When the steric demand of the substituent is too large, the formation of cyclo-phosphanes does not proceed, as illustrated by the reduction of Mes*PCl₂ (Mes* = 2,4,6-tri(tert-butyl)phenyl) to diphosphene Mes*–P=P–Mes*.¹⁰ Generally, mixed-substituted or cationic cyclo-phosphanes are very scarce and the only examples of dicationic cyclo-phosphanes of type [R₂(L_C)₂P₄]²⁺ (2a,b²⁺) (L_C = 1,3-bis(2,6-diisopropylphenyl)-imidazol-2-yl, 1a: R = Ph, 1b: R = NMe₂; Scheme 1) were recently introduced by Grützmacher and co-workers as a result of a GaCl₃-induced dimerization reaction of 1a,b.¹¹ Mechanistically, it has been shown that cyclo-phosphanes 2a,b²⁺ are formed via a [2 + 2]-cycloaddition of the generated diphosphene intermediates. We targeted the replacement of all substituents R in cyclo-P_nR_n by an imidazoliumyl-substituent L_C, resulting in the new group of cationic cyclo-phosphanes of type [(L_C)_nP_n]ⁿ⁺ (Scheme 1). These cationic substituents L_C are frequently employed in phosphorus chemistry as they are known to have a significant influence on the reactivity of the directly bonded P atom.^12–15 In this contribution we present the formation of a tetracationic cyclo-tetraphosphane [(L_C)₄P₄]⁴⁺ as triflate salt (3[OTf]₄) via the reduction of [L_CPCl₂]⁺ (4[OTf])¹² and elucidate the mechanism for its formation. Furthermore, the reductive build-up reaction to a larger cationic phosphorus framework and selective transition metal mediated [2 + 2]-fragmentation and ring opening reactions were investigated.

Scheme 1 Cationic cyclo-phosphanes of type [R₂(L_C)₂P₄]²⁺ (2a,b²⁺) and [(L_C)₄P₄]⁴⁺ (3⁴⁺) and subsequent transition metal mediated fragmentation and insertion reactions.

Results and discussion

We first reduced compound 4[OTf] with Mg in THF and observed the formation of a yellow-colored reaction mixture (Scheme 2). After removing MgCl₂ by filtration, the ³¹P NMR spectrum of the obtained clear solution reveals a singlet resonance at δ(P) = −24.0 ppm. This resonance can be attributed to the chloride bridged P₂-compound 5[OTf] which was confirmed by X-ray analysis (see Fig. S2†).¹⁶ As compound 5[OTf] is similar to our previous findings, in which we employed a sterically more demanding imidazoliumyl-substituent (L_C = 1,3-bis(2,6-diisopropylphenyl)-imidazol-2-yl), we abstain from a detailed discussion here.^17,18 It is noteworthy to mention, however, that reduction of 4[OTf] with Na gives the same result. The reduction proceeds completely different when 4[OTf] is reacted with 1,4-bis(trimethylsilyl)-1,4-dihydropyrazine (6) in fluorobenzene as solvent. Within 12 h the formation of a colorless precipitate is observed which after filtration and removal of all volatiles in vacuo gives the analytically pure triflate salt of the tetracationic cyclo-tetraphosphane 3⁴⁺ in very good yield (86%, Scheme 2). Although compound 3[OTf]₄ is well soluble in CH₃CN it slightly decomposes in solution in the course of several days. The ³¹P NMR spectrum of 3[OTf]₄ dissolved in CD₃CN, shows a singlet resonance at δ(P) = −55.7 ppm, which is comparable to ^tBu₄P₄ (δ(P) = −58.1 ppm).¹⁹ Single crystals suitable for X-ray analysis are obtained by slow diffusion of Et₂O into a saturated CH₃CN solution of 3[OTf]₄ at −35 °C. Fig. 1 shows the molecular structure of tetracation 3⁴⁺ and reveals the typical butterfly-shaped P₄ motif with an averaged P–P–P angle of 81.59°. All imidazoliumyl-substituents at the P₄ ring system are arranged in equatorial and all-trans position. The P–P bond lengths in 3⁴⁺ (av. 2.235 Å) are in the range for P–P single bonds (2.22 Å)²⁰ and the P–C bond lengths (av. 1.822 Å) are similar to comparable imidazoliumyl-substituted phosphorus compounds.¹⁵ In order to elucidate whether the formation of tetracation 3⁴⁺ proceeds through a diphosphene intermediate, we monitored the aforementioned reaction by means of ³¹P NMR spectroscopy (see Fig. S3†). Notably, Tamm and co-workers recently reported on the isolation of a diphosphene derivative obtained by the reduction of related dichlorophosphane with compound 6.²¹ The ³¹P NMR spectrum of the reaction mixture shows only one major prominent intermediate with a singlet resonance at δ(P) = −29.1 ppm, which is in the range for diphosphanes.²² We assign this intermediate to dication 7²⁺ which is similar to findings of Goicoechea and co-workers who isolated a related dibromo-diphosphane compound featuring imidazoliumyl-substituents.¹⁸ To further proof our assumption we calculated the chemical shift of intermediate 7²⁺ and found a theoretical ³¹P NMR shift of δ(P) = −33.4 ppm being in a reasonable range compared to the experimentally found shift.^16,23 We confirmed the existence of dication 7²⁺ by the addition of a chloride source, namely nBu₄N[Cl] and observed the formation of chloride-bridged cation 5⁺ from 7²⁺.¹⁶

Scheme 2 Reduction of 4[OTf] with Mg (left) and 6 (right) yields either 5[OTf] or 3[OTf]₄, respectively. Proposed reaction pathway with intermediate 7²⁺ to 3[OTf]₄ (grey); (i): +ex. Mg, 12 h, −2MgCl₂, 99% (determined by ³¹P NMR spectroscopy); (ii): +6, C₆H₅F, 12 h, −pyrazine, −2Me₃SiCl, 86%.

Fig. 1 Molecular structures of tetracation 3⁴⁺ in 3[OTf]₄·2CH₃CN (left) and trication 8³⁺ in 8[OTf]₃ (right; hydrogen atoms, solvate molecules and anions are omitted for clarity and thermal ellipsoids are displayed at 50% probability); selected bond lengths (Å) and angles (°) for 3⁴⁺: P1–P2 2.2382(8), P2–P3 2.2356(8), P3–P4 2.2400(8), P4–P1 2.2298(8), P1–C1 1.822(2), P2–C12 1.824(2), P3–C23 1.823(2), P4–C34 1.822(2); P4–P1–P2 81.69(3), P1–P2–P3 81.54(3), P2–P3–P4 81.53(3), P3–P4–P1 81.62(3); 8³⁺: P1–P2 2.2459(6), P2–P3 2.2293(6), P1–P3 2.2139(6), P1–C1 1.8429(17), P2–C12 1.8373(17), P3–C23 1.8303(17); P1–P2–P3 59.300(18); P2–P3–P1 60.72(2), P3–P1–P2 59.975(18).

Mechanistically, we propose that 2 equivalents of 4⁺ react with one equivalent of 6 to give intermediate 7²⁺ which subsequently reacts with another equivalent of 6 ultimately leading to tetracation 3⁴⁺. The first reduction step might also lead to the formation of an [L_CP]⁺-synthon which is supported by the formation of [(L_C)₃P₃]⁺ (8³⁺) in this reaction (Scheme 3). A few crystals of 8[OTf]₃ could be isolated by overlaying the filtrate of the aforementioned reaction with n-hexane at −35 °C. The molecular structure is depicted in Fig. 1. The bonding parameters of trication 8³⁺ are well in the expected range. Accordingly, one imidazoliumyl-substituent at the planar P₃ ring is trans-arranged to the other two imidazoliumyl-substituents which results in the characteristic pattern in the ³¹P NMR spectrum.^3,6 The ³¹P NMR spectrum of 8³⁺ shows the AB₂ spin system (δ(P_A) = −142.5 ppm, δ(P_B) = −130.8 ppm; ¹J(PP) = −162 Hz) in the typical region for cyclo-triphosphanes.¹⁹ To our surprise, time dependent ³¹P NMR spectra reveal the rearrangement of 8³⁺ into cyclo-tetraphosphane 3⁴⁺ in a CD₃CN solution in the course of 24 h (Fig. 2), suggesting that 8³⁺ is the kinetic product of this reaction, while 3⁴⁺ is thermodynamically favorable. Although such ring expansions are known for neutral cyclo-phosphanes, it seems, due to electrostatic repulsive effects, thermodynamically contradicting that a trication (8³⁺) transforms into the corresponding tetracation (3⁴⁺),²⁴ rather than its fragmentation into charge separated species.^24,25 We calculated the HOMO and the LUMO and optimized the structure by DFT methods using the PBE0/def2-TZVP level of theory (Fig. 3).¹⁶ The HOMO of cation 3⁴⁺ is basically located at the imidazoliumyl-substituents, whereas the LUMO is mostly located at the P₄ ring, suggesting an electrophilic nature of the P atoms within the P₄ ring. The small degree of delocalization of the LUMO onto the imidazoliumyl-substituents also contributes to the stability of the tetracation 3⁴⁺. We further reduced cyclo-tetraphosphane 3[OTf]₄ again using the metal free reducing agent 6 (Scheme 4). Upon reaction of 3[OTf]₄ with 6 in CH₃CN, the reaction mixture turned from colorless to orange. The ³¹P NMR spectrum of this mixture showed after 8 hours a broad singlet resonance at δ(P) = −124.6 ppm and a dynamic AA′A′′BXX′X′′ spin system with resonances at δ(P_A) = −178.5 ppm, δ(P_B) = −164.9 ppm and δ(P_X) = 13.2 ppm (see Fig. S5†).¹⁶ The singlet resonance is assigned to cation 10⁺ which was first reported by Macdonald and co-workers²⁶ whereas the spin system of higher order is attributed to the P₇ trication [(L_C)₃P₇]³⁺ (9³⁺). Imidazolium cation 11⁺ has been identified by its characteristic chemical shift in the ²⁹Si NMR spectrum.¹²9[OTf]₃ can be isolated by washing the obtained crude oil with THF and CH₂Cl₂. After removal of all volatiles in vacuo, salt 9[OTf]₃ is obtained as colorless solid in an overall yield of 61%. Fig. 3 shows the ³¹P{H} NMR spectrum of 9³⁺ at 260 K which is iteratively fitted to an AA′A′′BXX′X′′ spin system and indicates the formation of a C₃-symmetric stereoisomer (Fig. 4).²⁷ The A part at δ(P_A) = −178.5 ppm is assigned to the P atoms of the basal P₃-ring, the B part at δ(P_B) = −164.9 ppm to the apical P atom and the X part at δ(P_X) = 13.2 ppm to the bridging P atoms carrying the imidazoliumyl-substituents. Details on coupling constants are included in the ESI in Table S1.† Single crystals suitable for X-ray analysis are obtained by slow diffusion of CH₂Cl₂ into a saturated CH₃CN solution of 9[OTf]₃ at −35 °C. The molecular structure confirms the C₃-symmetric, heptaphosphanortricyclane cage with three imidazoliumyl-substituents at the bridging phosphorus atoms P2, P3 and P4 (Fig. 4). The P–C bond lengths (av. 1.834 Å) are comparable to those of tetracation 3⁴⁺ (av. 1.822 Å) and other imidazoliumyl-substituted phosphorus derivatives.¹⁵ The bonding parameters within the P₇-cage are comparable to those of our previously reported triarsonium-substituted P₇ cage compound,^27ai.e. the P–P bond lengths of the basal P atoms and the bridging P atoms (P2–P5, P3–P6 and P4–P7: av. 2.2238 Å) are slightly longer compared to those in the basal P₃-ring (av. 2.2166 Å) and to those between the apical and the bridging P atoms (P1–P2, P1–P3 and P1–P4: av. 2.1882 Å).

Scheme 3 Formation of 3[OTf]₄ and 8[OTf]₃ from the proposed [L_C–P]⁺-synthon by reduction of 4[OTf]. Compound 8[OTf]₃ readily rearranges in a CH₃CN solution at room temperature within 24 h to 3[OTf]₄.

Fig. 2 Time dependent ³¹P NMR spectra of 8³⁺, revealing a slow conversion of triphosphirane 8³⁺ into cyclo-tetraphosphane 3⁴⁺ over the course of 24 h (300 K, CD₃CN). The resonances can be assigned as follows: δ(P) = −55.7 ppm to 3⁴⁺; and AB₂ spin system δ(P_A) = −142.5 ppm, δ(P_B) = −130.8 ppm to 8³⁺.

Fig. 3 HOMO (a) and LUMO (b) from the DFT optimized structure of 3⁴⁺ using the PBE0/def2-TZVP functional.

Scheme 4 Reaction of 3[OTf]₄ with 6 to 9[OTf]₃, 10[OTf] and 11[OTf]; (i) +2 6, CH₃CN, 8 h, −2 pyrazine, –Me₃SiOTf, 61%.

Fig. 4 ³¹P{H} NMR spectrum of 9³⁺ (CD₃CN, 260 K). Insets show the extension of the experimental (upwards) and the iteratively fitted (downwards) AA′A′′BXX′X′′ spin system (left); molecular structure of trication 9³⁺ in 9[OTf]₃·0.8CH₃CN·1.7CH₂Cl₂ (right); hydrogen atoms, solvate molecules and anions are omitted for clarity and thermal ellipsoids are displayed at 50% probability; selected bond lengths (Å) and angles (°) for: P1–P2 2.1810(6), P1–P3 2.1816(6), P1–P4 2.2022(6), P2–P5 2.2189(7), P3–P6 2.2282(7), P4–P7 2.2243(7), P5–P6 2.2250(7), P5–P7 2.2021(7), P6–P7 2.2227(7), P2–C1 1.8339(19), P3–C12 1.8291(19), P4–C23 1.8418(19).

As the DFT calculations suggest rather electrophilic P atoms in 3⁴⁺, we investigated its reactivity towards a series of low valent transition metal compounds in order to evaluate the ligand properties of 3⁴⁺. 3[OTf]₄ readily reacts with two equivalents of Fe₂(CO)₉ in a 1 : 1 mixture of THF/CH₃CN under the immediate formation of a brown-colored reaction mixture. The addition of Et₂O gives a yellow-colored precipitate in a 98% yield which was identified as complex 12[OTf]₂.¹⁶ In this complex, a Fe(CO)₄ moiety is η²-coordinated by the disphosphene ligand [L_CP=PL_C]²⁺, which is formed via the metal induced [2 + 2]-fragmentation (Scheme 5). Single crystals suitable for X-ray analysis are obtained by slow diffusion of n-hexane into a saturated CH₂Cl₂ solution of 12[OTf]₂. The complex reveals a trigonal core including the P atoms of the [L_CP=PL_C]²⁺ ligand and the Fe atom of the Fe(CO)₄ moiety. The imidazoliumyl-substituents of the [L_CP=PL_C]²⁺ ligand are arranged in an E-configuration (C1–P1–P2–C12 149.205(1)°) and the P1–Fe–P2 angle is relatively acute with a value of 54.490(19)°. The shortest distance between the diphosphene unit and the Fe atom is 2.105 Å as a result of the rather long Fe–P contacts (Fe–P1 2.3774(6) Å and Fe–P2 2.3590(6) Å). In contrast, typically Fe–P distances in η¹-coordination complexes are usually in the region of 2.2 Å.²⁸ The P–P bond length with a value of 2.1684(7) Å is significantly longer compared to those in a typical diphosphene such as Mes*–P=P–Mes*: (2.034(2) Å),¹⁰ but slightly shorter compared to a typical P–P single bond (compare 3⁴⁺: av. 2.235 Å). This also supports the η²-coordination of the P₂ ligand. The IR stretching frequencies of the CO ligands are found at 2048 cm⁻¹, 2074 cm⁻¹ and 2116 cm⁻¹ and are significantly shifted to higher wave numbers compared to related reported phosphorus Fe(CO)₄ complexes (e.g. [(L₂–P–L₂)Fe(CO)₄][BPh₄]: 2029 cm⁻¹, 1947 cm⁻¹, 1907 cm⁻¹).²⁹ According to Tolman analysis,³⁰ this suggests that the [L_CP=PL_C]²⁺ ligand is a very strong π-acceptor which even surpasses PF₃ ([(PF₃)Fe(CO)₄]: 2101 cm⁻¹, 2022 cm⁻¹, 2018 cm⁻¹, 1999 cm⁻¹)³¹ due to the electron withdrawing effect of the imidazoliumyl-substituents.^16,32 We therefore computed the π-accepting properties of the [L_CP=PL_C]²⁺ ligand which is related to the LUMO energy and shape of the ligand (see Fig. S12†).¹⁶ Our calculations show that the LUMO is the π*(P=P) bond which is perfectly pre-organized to interact with the d-orbitals of the Fe atom. Furthermore, the calculations indicated that, due to the dicationic charge, the LUMO is very low in energy (−9.313 eV; compare [L_C–P=P–Dipp]⁺: −5.900 eV;^16,33 L_C = 4,5-dichloro-1,3-bis(Dipp)-imidazol-2-yl; Dipp = 2,6-diisopropylphenyl; see Fig. S12†) which is beneficial for π-bonding with the corresponding transition metal. In addition, the second order perturbation analysis of 12⁺ reveals a strong back-bonding from the d-orbitals of the Fe atom into the σ*(P–C)-orbital (E⁽²⁾ = 35.77 kcal mol⁻¹) indicating the stability of this complex.¹⁶ Similar findings have been recently reported by Macdonald and co-workers for a related diphosphene nickel complex featuring two phosphoniumyl-substituents.³⁴

Scheme 5 Transition metal induced [2 + 2]-fragmentation of 3[OTf]₄ to give diphosphene complexes 12[OTf]₂, 13[OTf]₂ and 14[OTf]₂; (i) +2 Fe₂(CO)₉, CH₃CN/THF, 12 h, −2 Fe(CO)₅, 98%; (ii) +2 M(PPh₃)₄ (M = Pd, Pt), CH₃CN/THF, 12 h, −4PPh₃, 93% for 13[OTf]₂ and 88% for 14[OTf]₂.

Similar [2 + 2]-fragmentation reactions are observed when 3[OTf]₄ is reacted with 2 equivalents of M(PPh₃)₄ (M = Pd, Pt) in a 1 : 1 mixture of THF/CH₃CN which leads to the formation of diphosphene complexes 13[OTf]₂ (Pd(PPh₃)₄) and 14[OTf]₂ (Pt(PPh₃)₄), respectively, under concomitant release of 4 equivalents PPh₃ (Scheme 5). After workup, complexes 13[OTf]₂ and 14[OTf]₂ are obtained as analytically pure red-colored (13[OTf]₂, 93%) or yellow-colored (14[OTf]₂, 88%) crystalline powders. The ³¹P NMR spectrum of 13[OTf]₂ at 300 K shows a broad singlet resonance at δ(P) = 10.2 ppm for the phosphorus atoms of the diphosphene moiety which upon cooling to 270 K appears as pseudo triplet with a ²J(PP) coupling constant of 31 Hz. The two PPh₃ ligands give rise to a pseudo-triplet resonance at δ(P) = 23.4 ppm (²J(PP) = 31 Hz). The chemical shift is comparable to the related diphosphene complex [(η²-F₃C–P=P–CF₃)Pd(PPh₃)₂].³⁵ In contrast to the trans-²J(PP) coupling, the magnitude of the cis²J(PP) coupling is not resolved, resulting in the observed pseudo-triplet splitting of this resonance. The ³¹P NMR spectrum of 14[OTf]₂ shows dynamic behavior at 300 K which upon cooling resolves to an AA′BB′X spin system. The high field shifted A part at δ(P_A) = – 42.9 ppm is assigned to the diphosphene moiety and shows a ²J(PP) coupling constant of 29 Hz similar to that of 13[OTf]₂. The ¹J(PPt) coupling with a value of 235 Hz is significantly smaller compared to typical ¹J(PPt) coupling constants,³⁵ indicating the η²-coordination of the disphosphene ligand and the Pt atom.^35,36 The B part at δ(P_B) = 25.1 ppm (²J(PP) = 29 Hz) is assigned to the PPh₃ ligands and reveals a typical ¹J(PPt) coupling constant of 3279 Hz. The ¹⁹⁵Pt NMR spectrum shows a triplet of triplet resonance at δ(Pt) = −5015.7 ppm (¹J(PtP) = 3279 Hz; ¹J(PtP) = 235 Hz; see Fig. S8†). For both complexes a set of resonances with low intensities indicates the presence of rotational isomers in the NMR spectra which, however, only appears at temperatures below 270 K (see Fig. S6 and S7†). Single crystals suitable for X-ray analysis are obtained for both complexes by slow diffusion of Et₂O into a saturated CH₃CN solutions (Fig. 5). Similar to complex 12[OTf]₂, the η²-coordination of the corresponding metal by the diphosphene ligand is observed with the metal atoms being in a distorted square planar bonding environment. The averaged distance between the diphosphene unit and metals are in the same range (13²⁺: Pd⋯P–P 2.124 Å, Pd–P1 2.3543(6) Å, Pd–P2 2.4003(8) Å; 14²⁺: Pt⋯P–P 2.114 Å, Pt–P1 2.3530(7) Å, Pt–P2 2.3930(6) Å). The P–P bond lengths are only marginally shorter (13²⁺ 2.1340(12) Å and 14²⁺ 2.1562(9) Å) compared to that of complex 12²⁺ (2.1684(7) Å). Similar to the Fe complex the [L_CP=PL_C]²⁺ ligand is in both cases also in the E-configuration (13²⁺: C1–P1–P2–C12 148.393°; 14²⁺: C1–P1–P2–C12 147.088°).

Fig. 5 Molecular structures of diphosphene complexes 12²⁺ in 12[OTf]₂, 13²⁺ in 13[OTf]₂·3C₆H₅F and 14²⁺ in 14[OTf]₂·3C₆H₅F; hydrogen atoms, solvate molecules and anions are omitted for clarity and thermal ellipsoids are displayed at 50% probability; selected bond lengths (Å) and angles (°) for 12²⁺(Fe): P1–P2 2.1684(7), Fe⋯P–P 2.105, Fe–P1 2.3774(6), Fe–P2 2.3590(6); C1–P1–P2–C12 149.205(1), P1–Fe–P2 54.490(19); 13²⁺(Pd): P1–P2 2.1340(12), Pd⋯P–P 2.124, Pd–P1 2.3543(6), Pd–P2 2.4003(8); C1–P1–P2–C12 148.393; 14²⁺ (Pt): 2.1562(9), Pt⋯P–P 2.114, Pt–P1 2.3530(7), Pt–P2 2.3930(6); C1–P1–P2–C12 147.088.

Notably, the known diphosphene complexes [(η²-F₃C–P=P–CF₃)M(PPh₃)₂] (M = Pd, Pt)^35,37 are also synthesized from the reaction of the tetraphosphetane (CF₃)₄P₄ and the corresponding transition metal precursor M(PPh₃)₄ (M = Pd, Pt) suggesting the necessity of electron withdrawing substituents at the P atoms for the observed [2 + 2]-fragmentation. However, the formation of the dicationic diphosphene complexes (12²⁺, 13²⁺ and 14²⁺), featuring imidazoliumyl-substituents at the P atom, illustrate the potential use of cyclo-tetraphosphane 3⁴⁺ to synthesize a variety of novel cationic transition metal complexes. When 3[OTf]₄ is reacted with one equivalent AuCl(tht) in THF/CH₃CN (1 : 1) the formation of a small set of two coupled broad resonances at δ(P) = −49.9 ppm and δ(P) = −24.7 ppm along with a singlet resonance for 1[OTf] (δ(P) = 107.8 ppm) can be observed in the ³¹P NMR spectrum of the reaction mixture (see Fig. S9†). A complete consumption of 3[OTf]₄ is achieved with 4 equivalents of AuCl(tht) (see Fig. S10†). Filtration of the reaction mixture after 12 h and recrystallization from THF, complex 15[OTf]₃ can be isolated as a yellow powder. The yield can be increased by further recrystallization steps up to 57%. The ³¹P NMR spectrum of 15[OTf]₃ dissolved in CD₂Cl₂ shows at 300 K two broad resonances which resolve to an AA′BB′ spin system at 260 K (Fig. 6). The A part at δ(P) = −49.9 ppm can be assigned to the backbone P atoms and the B part at δ(P) = −24.7 ppm to the Au-coordinating P atoms. Details on coupling constants are included in the ESI in Table S2.† Single crystals suitable for X-ray analysis are obtained by overlaying a saturated CH₂Cl₂ solution of 15[OTf]₃ with pentane at −35 °C (Fig. 7). The Au atom is coordinated by two P atoms resulting in a five-membered P₄Au-core structure which reveals an envelope conformation. Hey-Hawkins and co-workers reported on a similar structural motif where a catena-tetraphophosphane-1,4-diide coordinates Ni(0) and Pd(0), respectively.³⁸ The gold atom in 15³⁺ reveals a square-planar geometry where the P₄-chain chelates the gold atom via P1 and P4. The phosphorus gold bond lengths (P1–Au 2.3053(11) Å and P4–Au 2.3103(10) Å) are well in the range for related P–Au^III bonds (av. 2.314).³⁹ The four P atoms exhibit a pyramidal bonding environment with P–P bond lengths (av. 2.237 Å) being in the range of typical P–P single bonds (2.22 Å).²⁰ As expected, the bond angles around the phosphorus atoms P1 and P4 (Au–P1–P2 107.79(5) ° and P3–P4–Au 107.90(5) Å) are wider compared to the other angles within the P₄Au-core structure (P1–Au–P4 92.63(4) Å, P1–P2–P3 93.22(5) Å, P2–P3–P4 90.66(5) Å). Different from the above mentioned reactions, the reaction of 3[OTf]₄ with an excess of AuCl leads to a reductive insertion of the gold atom into the P₄-ring of 3[OTf]₄ leading to the formation of a five-membered P₄Au-core structure (Scheme 6).

Fig. 6 ³¹P{H} NMR spectrum of 15³⁺ (CD₂Cl₂, 260 K). Insets show the extension of the experimental (upwards) and the iteratively fitted (downwards) AA′XX′ spin system; trace amounts of unidentified side product is marked with an asterisk.

Fig. 7 Molecular structure of gold complex 15³⁺ in 15[OTf]₃·3CH₂Cl₂; hydrogen atoms, solvate molecules and anions are omitted for clarity and thermal ellipsoids are displayed at 50% probability; selected bond lengths (Å) and angles (°): P1–P2 2.2364(14), P2–P3 2.2304(13), P3–P4 2.2467(15), P1–Au 2.3053(11), P4–Au 2.3103(10); P1–Au–P4 92.63(4), Au–P1–P2 107.79(5), P1–P2–P3 93.22(5), P2–P3–P4 90.66(5), P3–P4–Au 107.90(5).

Scheme 6 Formation of the 5-membered P₄Au-complex 15[OTf]₃; (i) +4 AuCl(tht), THF/CH₃CN, 8 h 57%; (ii) +AuCl(tht), +nBu₄N[Cl], THF/CH₃CN, −nBu₄N[OTf], 76% determined by ³¹P NMR spectroscopy.

In this reaction, the Au(I) atom reductively inserts into a P–P bond of 3⁴⁺ and is oxidized to Au(III). The free coordination site at the Au atom is saturated by a further chloride anion, leading to the square planar coordination environment at the Au(III) atom. Thus, it appears that the excess of the AuCl(tht) merely serves as chloride source in this reaction. We found that the reaction can be carried out with one equivalent AuCl(tht) and one equivalent nBu₄N[Cl] as chloride source which leads to the consumption of 3[OTf]₄ up to 90% and to the formation of the P₄Au-complex 15[OTf]₃ in 70% yield as judged by ³¹P NMR spectroscopy (see Fig. S11†).

Conclusions

In summary, we successfully synthesized the first example of a cationic cyclo-phosphane with the general formula [(L_C)_nP_n]ⁿ⁺ (3[OTf]₄) via the reduction of 4[OTf] using 1,4-bis(trimethylsilyl)-1,4-dihydropyrazine (6). Due to the electrophilic nature of the P atoms within the P₄ ring, which was shown by theoretical calculations, we further reduced compound 3[OTf]₄ to give the notricyclane P₇ compound 9[OTf]₃. The reaction of 3[OTf]₄ with low oxidation state transition metal complexes Fe₂(CO₉), Pd(PPh₃)₄ and Pt(PPh₃)₄ leads to the formation of dicationic diphosphene complexes 12[OTf]₂, 13[OTf]₂ and 14[OTf]₂, respectively. The transition metal mediated [2 + 2]-fragmentation reaction of 3⁴⁺ is attributed to the cationic imidazoliumyl-substituents causing the electrophilic nature of the P atoms in 3⁴⁺. Due to the dicationic charge, the P₂ ligand in the aforementioned complexes is a very good π-acceptor with an exceptionally low lying LUMO (π*-P=P bond) rendering this ligand optimal for the complexation of a π-basic metal center. In contrast, the reaction of 3[OTf]₄ with an excess of AuCl(tht) gives rise to the formation of complex 15[OTf]₃ featuring a five-membered P₄Au-core structure. In this reaction, the Au(I) atom reductively inserts into a P–P bond of 3⁴⁺ and is oxidized to Au(III). As the DFT calculations of the optimized structure of 3⁴⁺ reveal rather Lewis acidic P atoms, the reactivity towards nucleophiles promises a variety of novel phosphorus compounds featuring interesting bonding motifs and properties.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the German Science Foundation (DFG Grant number WE 4621/3-1). A. F. and A. B. thank MINECO/AEI of Spain (project CTQ2017-85821-R FEDER funds) for financial support. We also thank Philipp Lange for experimental assistance and EA measurements.

Notes and references

1. (a) D. E. C. Corbridge, Phosphorus Chemistry, Biochemistry and Technology, Elsevier, Amsterdam, 2000; (b) N. Wiberg, Lehrbuch der Anorganischen Chemie, de Gruyter, Berlin, 102 edn, 2007, pp. 758–775.
2. (a) A. H. Cowley, Chem. Rev., 1965, 65, 617; (b) L. Lamandé, K. Dillon and R. Wolf, Phosphorus, Sulfur Silicon Relat. Elem., 1995, 103, 1; (c) M. Donath, F. Hennersdorf and J. J. Weigand, Chem. Soc. Rev., 2016, 45, 1145.
3. Selected examples for the chemistry of neutral triphosphiranes: (a) M. Bentner and W. Uhl, Dalton Trans., 2000, 3133; (b) C. A. Dyker, N. Burford, G. Menard, M. D. Lumsden and A. Decken, Inorg. Chem., 2007, 46, 4277; (c) M. H. Holtausen, D. Knackstedt, N. Burford and J. J. Weigand, Aust. J. Chem., 2013, 66, 1155; (d) A. P. M. Robertson, C. A. Dyker, P. Gray, B. O. Patrick, A. Decken and N. Burford, J. Am. Chem. Soc., 2014, 136, 14941; (e) S. S. Chitnis, H. A. Sparkes, V. T. Annibale, N. E. Pridmore, E. Natalie, A. M. Oliver and I. Manners, Angew. Chem., Int. Ed., 2017, 56, 9536 ( Angew. Chem. , 2017 , 129 , 9664 ).
4. Selected examples for the chemistry of neutral tetraphosphetanes: (a) S. Parveen, P. Kilian, A. M. Z. Slawin and J. D. Woollins, Dalton Trans., 2006, 2586; (b) C. A. Dyker, S. D. Riegel, N. Burford, M. D. Lumsden and A. Decken, J. Am. Chem. Soc., 2007, 129, 7464; (c) J. J. Weigand, N. Burford, R. J. Davidson, T. S. Cameron and P. Seelheim, J. Am. Chem. Soc., 2009, 131, 17953; (d) E. Conrad, N. Burford, U. Werner-Zwanziger, R. McDonald and M. J. Ferguson, Chem. Commun., 2010, 46, 2465.
5. Selected examples for the chemistry of neutral pentaphospholanes: (a) A. J. Arduengo III, H. V. R. Dias and J. C. Calabrese, Chem. Lett., 1997, 143; (b) I. P. Gray, P. Bhattacharyya, A. M. Z. Slawin and J. D. Woollins, Chem.–Eur. J., 2005, 11, 6221; (c) R. B. Baker, C. Jones, D. P. Mills, D. M. Murphy, E. Hey-Hawkins and R. Wolf, Dalton Trans., 2006, 64; (d) R. Wolf, M. Finger, C. Limburg, C. A. Willis, S. B. Wild and E. Hey-Hawkins, Dalton Trans., 2006, 831; (e) S. Geier and D. W. Stephan, Chem. Commun., 2010, 46, 1026; (f) J. H. Barnard, P. A. Brown, K. L. Shuford and C. D. Martin, Angew. Chem., Int. Ed., 2015, 54, 12083 ( Angew. Chem. , 2015 , 127 , 12251 ).
6. For reviews see: (a) M. Baudler and K. Glinka, Chem. Rev., 1993, 93, 1623; (b) G. He, O. Shynkaruk, M. W. Lui and E. Rivard, Chem. Rev., 2014, 114, 7815.
7. (a) L. Weber, Chem. Rev., 1992, 92, 1839; (b) A.-M. Caminade, J.-P. Majoral and R. Mathieu, Chem. Rev., 1991, 91, 575.
8. L. R. Smith and J. L. Mills, J. Am. Chem. Soc., 1976, 98, 3852.
9. (a) M. Baudler and K. Hammerstroem, Z. Naturforsch., B: J. Chem. Sci., 1965, 20, 810; (b) A. H. Cowley and R. P. Pinnell, Inorg. Chem., 1966, 5, 459.
10. M. Yoshifuji, I. Shima, N. Inamoto, K. Hirotsu and T. Higuchi, J. Am. Chem. Soc., 1981, 103, 4587.
11. A. Beil, R. J. Gilliard Jr and H. Grützmacher, Dalton Trans., 2016, 45, 2044.
12. (a) J. J. Weigand, K.-O. Feldmann and F. D. Henne, J. Am. Chem. Soc., 2010, 132, 16321; (b) F. D. Henne, A. T. Dickschat, F. Hennersdorf, K.-O. Feldmann and J. J. Weigand, Inorg. Chem., 2015, 54, 6849.
13. F. D. Henne, F. A. Watt, K. Schwedtmann, F. Hennersdorf, M. Kokoschka and J. J. Weigand, Chem. Commun., 2016, 52, 2023.
14. K. Schwedtmann, M. H. Holthausen, K.-O. Feldmann and J. J. Weigand, Angew. Chem., Int. Ed., 2013, 52, 14204 ( Angew. Chem. , 2013 , 125 , 14454 ).
15. For reviews see: (a) K. Schwedtmann, G. Zanoni and J. J. Weigand, Chem.–Asian J., 2018, 13, 1388; (b) T. Krachko and C. Slootweg, Eur. J. Inorg. Chem., 2018, 2734.
16. For details see ESI.†.
17. F. D. Henne, E.-M. Schnoeckelborg, K.-O. Feldmann, J. Grunenberg, R. Wolf and J. J. Weigand, Organometallics, 2013, 32, 6674.
18. For the bromide-bridged analogue see: J. B. Waters, T. A. Everitt, W. K. Myers and J. M. Goicoechea, Chem. Sci., 2016, 4, 77.
19. M. Baudler, J. Hahn, H. Dietsch and G. Fürstenberg, Z. Naturforsch., B: J. Chem. Sci., 1976, 31, 1305.
20. (a) M. Nieger, E. Niecke and J. Tirree, Private Communication, 2002, CCD, 178050; (b) L. Heurer, D. Schromburg and R. Schmutzler, Phosphorus, Sulfur Silicon Relat. Elem., 1989, 45, 217.
21. (a) L. P. Ho, A. Nasr, P. G. Jones, A. Altun, F. Neese, G. Bistoni and M. Tamm, Chem.–Eur. J., 2018, 24, 18922; (b) Y.-y. Carpenter, N. Burford, M. D. Lumsden and R. McDonald, Inorg. Chem., 2011, 50, 3342.
22. S. Aime, R. K. Harris, E. M. McVicker and M. Fild, J. Chem. Soc., Dalton Trans., 1976, 2144.
23. (a) V. Robert, S. Petit, S. A. Borshch and B. Bigot, J. Phys. Chem. A, 2000, 104, 4586; (b) S. K. Latypov, F. M. Polyancev, D. G. Yakhvarov and O. G. Sinyashin, Phys. Chem. Chem. Phys., 2015, 17, 6976.
24. (a) D. A. Klumpp, Chem.–Eur. J., 2008, 14, 2004; (b) D. Schröder and H. Schwarz, J. Phys. Chem. A, 1999, 103, 7385.
25. (a) G. A. Olah, G. Rasul, A. K. Yudin, A. Burrichter, G. K. S. Prakash, A. L. Chistyakov, I. V. Stankevich, I. S. Akhrem, N. P. Gambaryan and M. E. Vol'pin, J. Am. Chem. Soc., 1996, 118, 1446; (b) G. K. S. Prakash, T. Mathew, D. Hoole, P. M. Esteves, Q. Wang, G. Rasul and G. A. Olah, J. Am. Chem. Soc., 2004, 126, 15770; (c) R. Weiss and S. Engel, Angew. Chem., Int. Ed., 1992, 31, 216 ( Angew. Chem. , 1992 , 104 , 239 ); (d) F. G. Pühlhofer and R. Weiss, Eur. J. Inorg. Chem., 2004, 1002.
26. B. D. Ellis, C. A. Dyker, A. Decken and C. L. B. Macdonald, Chem. Commun., 2005, 1965.
27. For related P₇ compounds see: (a) M. Donath, M. Bodensteiner and J. J. Weigand, Chem.–Eur. J., 2014, 20, 17306; (b) J. J. Weigand, M. H. Holthausen and R. Fröhlich, Angew. Chem., Int. Ed., 2009, 48, 295 ( Angew. Chem. , 2009 , 121 , 301 ); (c) W. Hönle, H. G. von Schnering, A. Schmidpeter and G. Burget, Angew. Chem., Int. Ed. Engl., 1984, 23, 817 ( Angew. Chem. , 1984 , 96 , 796 ); (d) C. Mujica, D. Weber and H.-G. von Schnering, Z. Naturforsch., B: J. Chem. Sci., 1986, 41b, 991.
28. (a) S. Roy, S. K. S. Mazinani, T. L. Groy, L. Gan, P. Tarakeshwar, V. Mujica and A. K. Jones, Inorg. Chem., 2014, 53, 8919; (b) L. D. Field, R. W. Guest and P. Turner, Inorg. Chem., 2010, 49, 9086; (c) C. Federsel, A. Boddien, R. Jackstell, R. Jennerhahn, P. J. Dyson, R. Scopelliti, G. Laurenczy and M. Beller, Angew. Chem., Int. Ed., 2010, 49, 9777 ( Angew. Chem. , 2010 , 122 , 9971 ).
29. J. F. Binder, S. C. Kosnik and C. L. B. Macdonald, Chem.–Eur. J., 2018, 24, 3556.
30. C. A. Tolman, Chem. Rev., 1977, 77, 313.
31. H. Mahnke, R. J. Clark, R. Rosanske and R. K. Sheline, J. Chem. Phys., 1974, 60, 2997.
32. (a) M. Alcarazo, Chem.–Eur. J., 2014, 20, 7968; (b) M. Alcarazo, Acc. Chem. Res., 2016, 49, 1797.
33. K. Schwedtmann, M. H. Holthausen, C. H. Sala, F. Hennersdorf, R. Fröhlich and J. J. Weigand, Chem. Commun., 2016, 52, 1409.
34. S. C. Kosnik, J. F. Binder, M. C. Nascimento, A. Swidan and C. L. B. Macdonald, Chem.–Eur. J., 2019, 25, 1208.
35. (a) J. Chatt, P. B. Hitchcock, A. Pidcock, C. P. Warrens and K. R. Dixon, J. Chem. Soc., Dalton Trans., 1984, 2237; (b) J. Chatt, P. B. Hitchcock, A. Pidcock, C. P. Warrens and K. R. Dixon, J. Chem. Soc., Dalton Trans., 1982, 932.
36. (a) O. Kühl, Phosphorus-31 NMR Spectroscopy, Springer, Heidelberg, 2008; (b) A. Pidcock, R. E. Richards and L. M. Venanzi, J. Chem. Soc. A, 1966, 1707.
37. I. G. Phillips, R. G. Ball and R. G. Cavell, Inorg. Chem., 1992, 31, 1633.
38. (a) S. Gómez-Ruiz and E. Hey-Hawkins, Dalton Trans., 2007, 5678; (b) S. Gómez-Ruiz and E. Hey-Hawkins, Coord. Chem. Rev., 2011, 255, 1360.
39. (a) H. Kawai, W. J. Wolf, A. G. DiPasquale, M. S. Winston and F. D. Toste, J. Am. Chem. Soc., 2016, 138, 587; (b) O. Schuster and H. Schmidbaur, Organometallics, 2005, 24, 2289; (c) O. Schuster, R.-Y. Liau, A. Schier and H. Schmidbaur, Inorg. Chim. Acta, 2005, 358, 1429.

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 1884156–1884162 and 1904570. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9sc01701a

‡ These authors contributed equally to this work.

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