The chemistry of cationic polyphosphorus cages – syntheses , structure and reactivity

The aim of this review is to provide a comprehensive view of the chemistry of cationic polyphosphorus cages. The synthetic protocols established for their preparation, which are all based on the functionalization of P4, and their intriguing follow-up chemistry are highlighted. In addition, this review intends to foster the interest of the inorganic, organic, catalytic and material oriented chemical communities in the versatile field of polyphosphorus cage compounds. In the long term, this is envisioned to contribute to the development of new synthetic procedures for the functionalization of P4 and its transformation into (organo-)phosphorus compounds and materials of added value.


Introduction
Discovering novel pathways for the activation and transformation of white phosphorus (P 4 ) is important for the ongoing search for new, systematic entries to polyphosphorus and organo-phosphorus compounds.Especially in the realm of polyphosphorus cations methods for the preparation of species featuring a high P to substituent ratio are rare.In contrast, a systematic access to highly substituted cations R n P m (n 4 m) is achieved with synthetic protocols mainly based on the utilization of neutral catena or cyclic polyphosphanes R n P m . 1 Protocols for phosphorus-rich cations R n P m (n o m) often involve P 1 -precursors and are based on the reduction of either P-Cl 2 or P-H bonds. 3Multiple P-P bonds are formed in these reactions giving access to elaborate P-P bonded frameworks.However, in most cases the reaction outcome is unpredictable which hampers the targeted preparation of polyphosphorus cations.Thus, a synthetic approach that takes advantage of the tetrahedral P 4 framework should allow for a targeted and systematic assembly of phosphorus-rich cations R n P m (n o m).Additionally, the application of P 4 in such conversions is of high interest, since it constitutes an important raw material in industrial chemistry and is produced on a megaton-scale nowadays. 4The desire to develop synthetic protocols for the more sustainable production of P-containing bulk chemicals has sparked significant academic and industrial research efforts within the last decades.Progress in the areas of transition metal 5 and main group 6 mediated P 4 activation has been reviewed several times.However, no account was given so far on the importance of P 4 as a starting material for the preparation of polyphosphorus cations.
The aim of this review is to provide a comprehensive view of the chemistry of cationic polyphosphorus cages.
The synthetic protocols established for their preparation, which are all based on the functionalization of P 4 , and their intriguing follow-up chemistry are highlighted.In addition, this review intends to foster the interest of the inorganic, organic, catalytic and material oriented chemical communities in the versatile field of polyphosphorus cage compounds.In the long term, this is envisioned to contribute to the development of new synthetic procedures for the functionalization of P 4 and its transformation into (organo-)phosphorus compounds and materials of added value.
In the following, black dots denote P atoms in order to provide easily comprehensible drawings of complex polyphosphorus frameworks for the reader.These frameworks may give rise to complicated, sometimes higher order, spin systems in their 31 P NMR spectra.Their designation is derived by assigning letters in alphabetical order starting with the resonance at the highest field.The spin systems were considered to be higher order and consecutive letters are assigned if Dd(P i P ii )/ n J(P i P ii ) o 10.For Dd(P i P ii )/ n J(P i P ii ) 4 10, the spin system is considered to be pseudo first order and the assigned letters are separated.However, if a group of similar compounds is discussed, only one spin system is mentioned for the sake of clarity.All cationic polyphosphorus cages presented here are obtained by functionalization of P 4 .Mostly, phosphenium ions or cationic phosphorus species which formally serve as a phosphenium ion source are used for this functionalization.It is of high importance for the reader to be aware of the general reactivity pattern of P 4 and the general characteristics of phosphenium ions.Thus, a brief insight into both fields is given in the first two sections.

P 4 activation pathways
In order to gain an in depth understanding of the reactions of P 4 and main group element compounds, it is crucial to understand the properties of the P 4 tetrahedron.The bonding in P 4 is almost ''cluster-like'', strongly delocalized and mostly effected through 3p atomic orbitals.Interestingly, P 4 shows spherical aromaticity and is virtually unstrained despite acute bond angles of 601. 7Generalized reactions of P 4 with nucleophiles (Nu À ), electrophiles (El + ) and ambiphiles (Ab) are shown in Fig. 1.Radical reactions involving P 4 are excluded.A nucleophile (Nu À ) interacts with the LUMO of P 4 (À1.8 eV), 7 which leads to the rupture of a P-P bond giving butterfly-type bicyclo[1.1.0]tetraphosphaneA (Fig. 1I).The reactions of P 4 with nucleophiles were intensely investigated using an array of organo-alkali and organo-alkali earth reagents. 6However, in many cases the formation of a derivative of A only constitutes the first step of a reaction sequence which ultimately leads to the degradation of P 4 to P 1 -compounds. 6Only a few reactions involving a selective cleavage of only a single bond in the P 4 tetrahedron are reported.
One is the reaction of Mes*Li (Mes* = 2,4,6-tri-tert-butylphenyl) with one equivalent of P 4 yielding a tetraphosphanide intermediate of type A. Subsequent reaction with Mes*Br yields the butterfly-type species 1 (Fig. 2). 8Further degradation of 1 is prevented by the sterically demanding Mes*-groups.Nucleophiles based on silicon, main group 5 or main group 6 elements were also employed. 6An electrophile may attack at a nonbonding orbital of lone pair character (HOMO À 6, À7.5 eV) 7 which results in the formation of compounds of type B (Fig. 1II a).Alternatively, an electrophile may attack at a bonding orbital at one of the edges of the tetrahedron (HOMO, À6.7 eV; Fig. 1II b).However, this mode of attack is commonly less productive for main group element centered electrophiles and is not depicted.In total, only very few reactions with electrophiles were reported due to the low nucleophilicity of P 4 . 9One example constitutes the reaction of P 4 with two equivalents of the sterically encumbered Lewis acid Ga(t-Bu) 3 .This yields compound 2; however, mechanistic details regarding its formation were not reported (Fig. 2). 10 The utilization of ambiphilic main group element compounds (Ab) for the activation of P 4 represents a rather new synthetic approach.Reactions of P 4 with ambiphiles can be divided into two categories assuming an asynchronous process with two consecutive steps.The first category comprises reactions of P 4 with predominantly nucleophilic ambiphiles.Similar to the reactions of P 4 and nucleophiles, intermediate A 0 is obtained in the first step of the reaction.Subsequently, A 0 rearranges to cyclo-triphosphirene derivative C (Fig. 1III).The rearrangement is attributed to the propensity of Ab to accept electron density from the adjacent P atom which formally leads to the formation of an Ab-P double bond.Carbenes are ambiphiles with a predominantly nucleophilic character. 11Two types of carbenes, i.e.N-heterocyclic carbenes (NHC) and cyclic or acyclic alkyl amino carbenes (cAAC or aAAC), were investigated in reactions with P 4 by the research group of Bertrand. 12e formation of an intermediate of type A 0 was confirmed by DFT calculations 12 and of type C by trapping experiments with 2,3-dimethylbutadiene yielding [2+4] cyclo-addition product 3 (Fig. 2, e.g.L 1 = cAAC).
The second category comprises reactions of P 4 with predominantly electrophilic ambiphiles.By analogy with the reactions involving electrophiles, the first step of the reaction is an electrophilic attack of Ab yielding an intermediate B 0 (Fig. 1).Subsequently, B 0 rearranges to a bicyclo[1.1.0]tetraphosphaneD featuring a bridging Ab moiety (IV).This reaction sequence equals the formal insertion of the ambiphile in one of the P-P bonds of the P 4 tetrahedron.P 4 functionalization involving a predominantly electrophilic ambiphile is an experimentally more widespread approach.Monovalent group 13 element compounds with the oxidation state +I are a class of substances that are widely used in such transformations. 13The first type of such a structural motif was achieved by Roesky and coworkers by reacting P 4 with two equivalents of Al(I) compound AlL 2 (L 2 = CH{(CMe)(2,6-i-Pr 2 C 6 H 3 N)} 2 ). 13The formal insertion of AlL 2 into one P-P bond of P 4 is assumed to give an intermediate of type B 0 in the first step.However, the insertion of a second equivalent of AlL 2 into the opposing P-P bond of the P 4 tetrahedron occurs rapidly yielding the two-fold insertion product 4 (Fig. 2).In addition, P 4 activation by predominantly electrophilic silylenes, 14 disilenes, 15 phosphasilenes, 16 and a bis(stannylene) 17 was reported.Reactions of P 4 with phosphenium cations (R 2 P + ) are also classified as P 4 functionalization with predominantly electrophilic ambiphiles.They will be thoroughly discussed within this review from an experimental as well as a mechanistic point of view.

Syntheses and characteristics of phosphenium ions
The term phosphenium ion describes a cation featuring a di-coordinated, positively polarized P atom. 18Phosphenium ions reveal a lone pair of electrons and a formally vacant p-type orbital, and thus, they constitute carbene analogues. 11he stability of phosphenium ions strongly depends on their substituents.While aryl-or alkylphosphenium ions R 2 P + (7 + , Fig. 3) are strongly electrophilic and generally elusive, a large series of phosphenium ions bearing amino-substituents (R 2 N) 2 P + (R = alkyl, aryl) are known. 18Three methods for their preparation are mainly reported throughout the literature.Halide abstraction from the corresponding halo-phosphane precursor is the most commonly used synthetic protocol. 18Further methods  constitute the protolysis of P-N single bonds by Brønsted acids and the coordination of strong Lewis acids to P-N double bonds. 18he increased stability of phosphenium ions (R 2 N) 2 P + (5 + , Fig. 3) stems from a lowered electrophilicity due to donation of p-electron density from the lone pair of electrons at the nitrogen atoms to the vacant p-type orbital at the P atom. 19hosphenium ions of type 6 + featuring one amino-substituent are borderline cases between both of the aforementioned types and are only scarcely investigated.Only a few fully characterized derivatives are reported to date bearing either (pseudo-) halogens 20 or sterically demanding aryl-moieties 21 as the second substituent R 0 on phosphorus (Fig. 3).
A phosphenium ion bearing only alkyl-or aryl-substituents has not been isolated to date. 18The reaction of phosphanes bearing organo-and chloro-groups R n PCl (3Àn) (n = 1, 2) and a halide abstracting agent (e.g.Me 3 SiOTf, GaCl 3 , or AlCl 3 ) in the appropriate stoichiometry usually results in the formation of phosphanylphosphonium ions. 1 This is best exemplified by the reaction of Ph 2 PCl and GaCl 3 in a 2 : 1 stoichiometry which yields 8[GaCl 4 ] (Scheme 1). 22wo mechanisms for the formation of 8 + are conceivable.Firstly, Ph 2 PCl reacts with GaCl 3 as a halide abstracting agent giving a transient Ph 2 P + -phosphenium ion.This reacts with the second equivalent of Ph 2 PCl yielding 8 + .The second and in the author's opinion more likely mechanism proceeds via the zwitterionic intermediate 9 which features a Ph 2 PCl molecule donating electron density from its lone pair of electrons to the lobes of the antibonding s*(P-Cl) orbital of a second molecule of Ph 2 PCl.Subsequently, chloride abstraction by GaCl 3 yields 8 + without an intermediary formation of a free Ph 2 P + -phosphenium ion.The phosphoniumyl-moiety in 8 + is easily substituted when 8 + is reacted with phosphanes of higher basicity than the leaving group. 1 This is illustrated by the reaction of 8 + with Ph 3 P yielding 10 + and Ph 2 PCl (Scheme 2, left). 23Other Lewis bases are also suitable as nucleophiles.This is illustrated by the reaction of 8 + with 1,3-di-iso-propyl-4,5dimethylimidazol-2-ylidene (L 3 ) which yields the imidazoliumylsubstituted phosphane 11 + . 23tailed investigations of mixtures of phosphanyl-phosphonium ion 12 + and Ph 3 P revealed second-order kinetics for the exchange process of Ph 3 P consistent with a S N 2-type pathway (Scheme 3). 24his was further supported by quantum chemical calculations which suggested the phosphoranide-type transition state 13 + for the substitution process. 24In contrast, the phosphanylphosphonium ion 14 + , which is formed via the reaction of phosphenium ion 15 + and PMe 3 , was reported to favour a dissociative S N 1-type reaction pathway in substitution reactions (Scheme 4). 25 For phosphanylphosphonium ions such as those described above the term ''ligand stabilized phosphenium ions'' is frequently used in the literature while the described substitution reactions are also called ''ligand exchange'' reactions. 1Independent of any such controversy, however, these distinct points of view are based on the labile P-P bond observed in phosphanylphosphonium ions.This allows for the transfer of R 2 P + -moieties (formally phosphenium ions) between distinct Lewis bases (e.g.phosphanes, carbenes or P 4 ).Thus, for reasons of simplification, phosphanylphosphonium ions will be regarded as ''sources of phosphenium ions'' 1 throughout this review.
Phosphanylphosphonium ions were frequently used as phosphenium ion sources.The reaction of a mixture of Me 2 PCl and Me 3 SiOTf with diphosphane (Ph 2 P) 2 gave diphosphanylphosphonium ion 16 + as a triflate salt (Fig. 4). 26Species 16 + is formally derived from the insertion of a Me 2 P + -phosphenium ion into the P-P bond of the diphosphane (Ph 2 P) 2 .Mixtures of Ph 2 PCl and Me 3 SiOTf with the cyclo-phosphanes (PhP) 4 or (PhP) 5 give in both cases the cyclo-tetraphosphanylphosphonium ion 17 + .
A ring expansion is observed in the reaction with (PhP) 4 whereas a 5-membered ring is retained in the reaction involving (PhP) 5 via an unknown redistribution process. 26Both reactions proceed via the formal transfer of a Ph 2 P + -phosphenium ion from the intermediary formed phosphanylphosphonium ion 8 + .In both cases 17 + is exclusively formed which demonstrates the thermodynamic preference of the five-membered ring over the sixmembered alternative.The highly reactive, cyclic six-membered dication 18 2+ is only obtained by employing a melt approach. 27olvent-free mixtures of Ph 2 PCl and GaCl 3 provide room temperature molten media.These melts represent a powerful source of phosphenium ions Ph 2 P + . 28

Cationic homoleptic polyphosphorus cages
For decades the investigation of homoleptic polyphosphorus cations was limited to mass spectroscopy 29 and quantum chemical calculation 30 in the gas phase.Homoleptic P n + cations are paramagnetic if the number of P atoms n is even.In the case of an odd number of P atoms the respective cation is diamagnetic.In general, the paramagnetic series of polyphosphorus cations is less stable.In the odd-membered series, the smaller P n + cations 19 + (n = 5) and 20 + (n = 7) may be described as electron-deficient Wade clusters whereas larger P n + -cages (n Z 9) feature electron-precise Zintl-type structures.According to Wade's rules, a square pyramidal structure is anticipated for cation 19 + (Fig. 5, nido-cluster).Such a structure was confirmed as the most stable isomer by means of quantum chemical calculations.30a The structural motif of the second most stable isomer 19 0+ (34.7 kcal mol À1 higher in energy) does not follow Wade's rules and shows a di-coordinated P atom.The most stable isomer of P 7 + -cage 20 + is a tricapped trigonal prism that is missing two of the capping vertices (arachno-cluster).
A second isomer, which is only slightly higher in energy (20 0+ , 2.0 kcal mol À1 ), shows the P 5 -cage motif of 19 0+ and a three-membered P ring which are both fused by a bridging phosphonium moiety.The P 9 + -cage 21 + , which is composed of two P 4 -moieties fused by a phosphonium moiety, is one of the most stable homoleptic polyphosphorus cations according to quantum chemical calculations (Fig. 5).30b Krossing and co-workers were the first to report evidence for the existence of homoleptic polyphosphorus cations in the condensed phase. 31The attempted oxidation of P 4 with I 2 or Br 2 in the presence of Ag(CH 2 Cl 2 )[A] (A = Al(OC(CF 3 ) 3 ) 4 ) was suggested to proceed via the intermediary formation of P 5 + -cage cation 19 + (Scheme 5). 32However, cation 19 + is highly reactive and reacts with the solvent to give phosphonium ion 22 + as one of the main products.Cation 22 + forms via elimination of P 4 and two-fold insertion into C-Cl bonds of CDCl 3 molecules which was used as solvent.In the case of I 2 as oxidant, P 4 reacts partially to give PI 3 which was suggested to react with intermediate 19 + to give P 4 and the bis(phosphanyl)-substituted phosphonium ion 23 + .Experimental evidence confirming the presence of 19 + in the reaction mixtures was not obtained; however, the suggested reaction pathways are in accordance with quantum chemical calculations. 32The nitrosonium salt [NO][A] (A = Al(OC(CF 3 ) 3 ) 4 ) was also investigated as a possible one electron oxidant.However, the reaction of P 4 with [NO][A] yields P 4 NO + -cage compound 24[A] via insertion of the nitrosonium cation into a P-P bond (Scheme 6). 33lthough X-ray structure determination of compound 24[A] was not successful, the molecular structure is confirmed by spectroscopic data and computational investigations.The theoretical  investigations suggested a two-step mechanism indicating the HOMO of P 4 and a p*-type LUMO at NO + as the interacting frontier orbitals (II b, Fig. 1). 33Similar results were obtained utilizing the carborate salt NO[HCB 11 Cl 11 ]. 34The reaction of P 4 NO + -cage compound 24[A] with additional 1.5 equivalents of P 4 was reported to yield P 9 + -cage compound 21[A] which is the first isolated salt of a homoleptic phosphorus cation (Scheme 6). 35he reaction proceeds very likely via extrusion of 1/n (PNO) n and intermediary formation of a P 3 + -species.The P 9 + cation 21 + is obtained upon reaction of the latter with 1.5 equivalents of P 4 via an unknown reaction mechanism.The 31 P NMR spectrum of cation 21 + shows a characteristic A 2 A 0 2 BC 2 C 0 2 spin system which confirms the D 2d symmetric Zintl-type structure.Despite the electron precise Lewis formula of eight neutral, threecoordinated and one cationic, four-coordinated P atom the charge is almost evenly distributed over all nine atoms according to quantum chemical calculations. 35

Cationic polyphosphorus cages featuring halogen-substituents
The oxidation of Ag(I) complex 25[A] (A = Al(OC(CF 3 ) 3 ) 4 ) featuring two intact P 4 ligands with elemental iodine at low temperatures gives rise to interesting binary PI cations.The P 5 I 2 + -cage 26a + was observed in the reaction mixture at À78 1C together with PI 3 and P 4 (Scheme 7). 36However, on raising the temperature above À40 1C, decomposition of 26a + was observed, leading to the formation of P 3 I 6 + (23 + ) and unidentified by-products.
A proposed reaction mechanism indicates the partial oxidation of the P 4 ligands in 25 + by I 2 to give PI 3 . 36The latter reacts with Ag[A] (A = Al(OC(CF 3 ) 3 ) 4 ) via halide abstraction to give AgI and formally the phosphenium ion PI 2 + .This highly reactive, predominantly electrophilic ambiphile reacts with white phosphorus via insertion in one of the P-P bonds of the P 4 tetrahedron yielding the P 5 I 2 + -cage 26a + .Likewise, according to the observations described in Section 3, a mechanism involving the formation of phosphanylphosphonium ion P 2 I 5 + can also be considered.Here, P 2 I 5 + is assumed to transfer a PI 2 + phosphenium ion to P 4 and, thus, serves as a phosphenium ion source.Upon warming the reaction mixture, the excess of PI 3 reacts with P 4 to yield diphosphane P 2 I 4 in a conproportionation reaction.The diphosphane reacts with 26a + via transfer of the phosphenium ion PI 2 + .This gives P 4 and the P 3 I 6 + cation 23 + which is formed upon insertion of the PI 2 + ion into the P-P bond of P 2 I 4 .On the basis of these observations, a synthetic protocol for the targeted preparation of P 5 X 2 + -cages was developed (Scheme 7). 36,37Thus, white phosphorus reacts with PX 3 (X = I, Br) in the presence of Ag(CH 2 Cl 2 )[A] as a halide abstracting agent and salts of cage cations 26a + and 26b + can be isolated in good yield.However, utilizing PCl 3 , the formation of the respective cation 26c + was observed only in trace amounts since it readily decomposes in the reaction mixture. 38The molecular structure of 26b + is shown in Scheme 7. The structural motif of the P 5 -core of the P 5 X 2 + -cage was unprecedented and was not previously observed as part of the many known polyphosphides and organo-polyphosphanes.

Cationic polyphosphorus cages featuring alkyl-and aryl-groups
A versatile approach to cationic polyphosphorus cages featuring alkyl-and aryl-groups represents the utilization of dichlorophosphanes RPCl 2 (R = alkyl, aryl) instead of PX 3 (X = I, Br, Cl). 39ixtures of dichlorophosphanes RPCl 2 and a strong Lewis acid (GaCl 3 , AlCl 3 ) as a halide abstracting reagent can be utilized as the source for the phosphenium ion RPCl + .In the presence of P 4 , insertion into one of the P-P bonds takes place, giving access to a series of RP 5 Cl + -cages featuring distinct substituents R. 39 Mixtures of dichlorophosphanes and AlCl 3 were previously utilized for the in situ formation of phosphenium ion salts [RPCl][AlCl 4 ] and subsequent syntheses of various phosphorus heterocycles. 40However, neither free phosphenium ions nor respective phosphenium ion sources could be verified.In some cases, the formation of Lewis acid-base complexes of the type mRPCl 2 ÁnAlCl 3 (n = 1, 2; m = 1, 2) was suggested. 41Detailed investigations of mixtures of mono-and dichlorophosphanes in the presence of Lewis acids revealed the formation of chlorophosphanylchlorophosphonium ions of type 27 + (Fig. 6). 42In most cases, characteristic 1 J(PP) coupling constants were observed by 31 P NMR spectroscopy at ambient temperature.However, the spectra of mixtures of dichlorophosphanes and Lewis acids in CH 2 Cl 2 were less informative and showed in most cases only broad resonances. 42A systematic study based on Raman and 31 P NMR spectroscopy of mixtures of RPCl 2 and GaCl 3 in fluorobenzene applying varying stoichiometries gave important insight into these reactions. 39Depending on the ratio of the reactants and the substituent R in RPCl 2 , mixtures of the structurally distinct species 28 + , 29 + and 30 were formed (Fig. 6).
The classical Lewis acid-base adducts of type 30 are only formed in reaction mixtures involving dichlorophosphanes RPCl 2 featuring alkyl-substituents R. The formation of nonclassical adducts of type 30 0 is not observed and is unlikely according to quantum chemical calculations. 39This is further supported by the isolation and structural characterization of 30a (R = t-Bu), which was proven to form a classical Lewis acidbase adduct.An increasing amount of phosphanylphosphonium ions of type 28 + is formed with decreasing steric demand of the substituent R (t-Bu 4 Cy 4 i-Pr).The formation of cations of type 29 + is observed when the basicity and the steric requirements of the dichlorophosphanes are further reduced (R = Et, Me, Ph).Such cations are the result of adduct formation between GaCl 3 and the phosphane moiety of phosphanylphosphonium ions of type 28 + .Most mixtures show dynamic exchange indicating a possible interconversion of species 28 + , 29 + and 30. 39The exchange rates of these processes strongly depends on the concentration of GaCl 3 .In the reaction mixtures equilibrium dissociation of the GaCl 4 À anion to free GaCl 3 and Cl À occurs.
The dynamic exchange is linked to these chloride anions which nucleophilically attack phosphanylphosphonium species yielding the phosphane starting materials in a back reaction.By using an excess of GaCl 3 the GaCl 4 À forms higher gallates (Ga 2 Cl 7 À or Ga 3 Cl 10 À ) and the concentration of free chloride anions is reduced. 43uantum chemical calculations were carried out to determine which of the observed species serves as the phosphenium ion source in a reaction with P 4 .According to these results, 44 the formation of RP 5 Cl + -cages via a free phosphenium ion RPCl + can be excluded.Attempts to calculate a feasible reaction mechanism from adducts 30 or 30 0 as sources of phosphenium ions were not successful.Thus, the reaction of P 4 with methylsubstituted phosphanylphosphonium derivative 28e + was investigated (Fig. 7).A single step insertion of the phosphenium moiety into a P-P bond of the P 4 tetrahedron is viable and the calculated energy profile of the reaction path is denoted in black.In addition, a two-step reaction pathway is feasible as well (energy profile is shown in red).
The two step reaction pathway proceeds via butterfly-type compound 31 as an intermediate (bottom, Fig. 7).The single step transfer of the phosphenium moiety in 28e[GaCl 4 ] and insertion thereof into a P-P bond of P 4 shows an energy barrier of 27.4 kcal mol À1 (TS1) and is energetically viable.In the light of recent mechanistic studies on the reaction of isoelectronic silylenes with P 4 , 7a this is best understood as a combined electrophilic and nucleophilic attack of the phosphenium moiety.On the one hand the P-P bond of P 4 (HOMO) nucleophilically attacks the p-type orbital of the phosphenium moiety.On the other hand the lone pair of electrons of the phosphenium moiety donates electron density to the LUMO of the P 4 tetrahedron which corresponds to p-orbitals situated perpendicular to the P 4 lone pairs. 7It was found that a lower barrier reaction pathway is possible if 28e + does not act as a nucleophile.Instead, a chloro-substituent of the GaCl All compounds are obtained in almost quantitative yield and high purity.In contrast to the halogen-substituted species 26a-c[A], they are stable in the solid state or when dissolved in non-coordinating solvents at ambient temperature. 36,37The cations 32a-h + show characteristic 31 P NMR spectra.Iterative line shape analysis of the observed spin systems gave chemical shifts and coupling constants in accordance with C S symmetric RP 5 Cl +cages with four chemically non-equivalent phosphorus nuclei.All cages possess a mirror plane which includes the tetracoordinated P atom and both P atoms opposing the former.Due to the reduced symmetry compared to the C 2V -symmetric P 5 X 2 + cages 26a-c + an ABM 2 X spin system is observed for 32a-d + and an ABMX 2 spin system for 32e-h + .Due to the similar geometry of the P 5 + -core in all cations, the respective 1 J(PP) and 2 J(PP) coupling constants deviate only marginally.However, the chemical shifts are strongly dependent on the substituent R attached to the RP 5 Cl +cage (Fig. 8).The P A and P B atoms exhibit characteristic low field resonances at approximately À275 ppm.The assignment of the A and B part to the respective P nuclei is based on the observed coupling pattern.First, the non-symmetrically substituted P 5 + -cage is divided by a plane spanned by the tetra-coordinated and both adjacent P atoms into a H Cl -and H R -hemisphere (Fig. 9).
The H Cl -hemisphere contains the chloro-substituent and the H R -hemisphere the alkyl-or aryl-substituent.Within the series of cations 32a-h + the P atom located in the H Cl -hemisphere shows values of 1 J(PP) and 2 J(PP) coupling constants which are reminiscent of those of P 5 X 2 + -cages 26a-c + . 36,37Accordingly, the P atom located in the H R -hemisphere reveals one-and  two-bond P-P coupling constants similar to the values observed for the respective R 2 P 5 + -cages.In addition, the former group of P atoms experiences the spatial proximity of the chloro-group, and, therefore, shows similar chemical shifts (marked in green, Fig. 8).In contrast, the P atoms in the H R -hemisphere show resonances in a much larger chemical shift range.This is attributed to the distinct electronic parameters of the substituents.They affect the chemical shifts of the P atoms most likely through ''cross-ring through space'' interactions of the lone pairs on P atoms and the respective group R. 45 For RP 5 Cl +cages featuring alkyl-substituents R (32a-e + ) the resonances of the P atoms adjacent to the phosphonium moiety (marked in blue, Fig. 8) are shifted stepwise to lower field with a decreasing positive inductive effect of the substituent (from 44 ppm (32a + ) to 81 ppm (32e + )).This is in agreement with the increased shielding of a P atom caused by additional alkyl-moieties in the g-position relative to the P nuclei.This trend was previously termed g-effect. 46In contrast, the chemical shifts of tetracoordinated P atoms (marked in red, Fig. 8) exhibit an almost inverse trend (from 99 ppm (32a + ) to 69 ppm (32e + )).This highfield shift correlates with an increasing number of hydrogen atoms at the a-carbon atom of the substituent.This constitutes a characteristic feature of phosphonium moieties and was termed a-effect. 47Overall, these influences are reflected in a change of the spin system between 32e-h + featuring aryl-and methyl-substituents (ABMX 2 spin system) and those bearing alkyl-substituents 32a-d + (ABM 2 X spin system).
Employing dichlorophosphanes R 2 NPCl 2 (R = Cy, i-Pr) in combination with GaCl 3 in reactions with P 4 gave distinct results.In mixtures of R 2 NPCl 2 (R = Cy, i-Pr) and GaCl 3 the corresponding phosphenium ions 33a,b + are the only observable species.20a Indicative of their formation is a resonance in the 31 P NMR which is shifted to remarkable low field. 18 It is interesting to note that a reaction between the two-fold amino-substituted phosphenium ion [(i-Pr 2 N) 2 P] + (35 + ) and P 4 was not observed.20a This is attributed to a significantly lowered electrophilicity of 35 + compared to 33a,b + . 19Also, diaminophosphenium ions of type 35 + reveal frontier orbitals comparable to those of allyl-anions 48 with the HOMO mainly located at the N atoms, and, thus, are not ambiphilic at the P moiety.R 2 P 5 [GaCl 4 ] cage compounds 36[GaCl 4 ] featuring two alkylor aryl-substituents R are obtained in high yield via the reaction of chlorophosphanes R 2 PCl, GaCl 3 and P 4 (Scheme 10). 49he Lewis acid-base adduct 37 and phosphanylphosphonium ion 38 + are commonly formed in mixtures of chlorophosphanes and GaCl 3 in various stoichiometries.22b Cations of type 38 + serve as phosphenium ion sources in the presence of P 4 allowing for the formation of R 2 P 5 + -cage cations 36 + .Most likely, this proceeds in analogy to quantum chemical calculations on the mechanism of the formation of MeP 5 Cl + cage 32e + . 39In contrast to dichlorophosphanes, however, the reaction conditions for the formation of R 2 P 5 + -cages 36a-h + depend strongly on the substituent R. In the case of chlorophosphanes featuring aryl Fig. 9 Definition of the H Cl -and H R -hemisphere in cations 32a-h + .The tetra-coordinated and the adjacent P atoms span a plane.This necessitates a more Lewis acidic environment for the transfer of a phosphenium ion featuring alkyl-groups, which is realized in a solvent free medium.
The molecular structures of all compounds of the series 36a-h[GaCl 4 ] were determined by single crystal X-ray structure determination.This allowed for an in-depth evaluation of the influence of substituents of distinct steric demand on the structural parameters of the P 5 -cage in the solid state (Fig. 10).
The phosphonium P atoms of cations of type 36 + show a distorted tetrahedral environment.If the steric demand of the substituent R is increased a stepwise increase in the corresponding C-P-C angle is observed from the sterically very bulky substituted Dipp 2 P 5 + (36h + ) to the methyl-substituted derivative 36e + .This is accompanied by a decreasing P-P-P angle at the phosphonium moiety and stepwise increase in P-P bond lengths involving the phosphonium P atom.As a consequence, the tetraphosphabicyclo[1.1.0]butanemoieties display a more pronounced folding (distance between both P atoms adjacent to the phosphonium P atom decreases) and the P 5 + -cages are stretched (distance between the bridgehead P-P bond and the phosphonium P atom increases).
The 31 P NMR spectra of cage cations 36 + show A 2 M 2 X or A 2 MX 2 spin systems in accordance with their C 2V symmetry and are comparable to those observed for the P 5 X 2 + cages 26a-c + (Fig. 11).The observation of two different spin systems for R 2 P 5 + -cages of type 36 + may be explained in terms of different steric and electronic influences of the alkyl-or aryl-substituent R. In the series of alkyl-substituted R 2 P 5 + -cages (36a + to 36d + ) the resonances of the phosphonium P atoms are shifted to higher field and the resonances of the adjacent P atoms are shifted to lower field.This can be explained in terms of a combination of a-effect and g-effect (vide infra). 46,47The resonances of the tetra-coordinated P atoms in aryl-substituted cations 36e-h + are shifted to higher field compared to those of the corresponding P atoms in cages 36a-d + .This is due to a positive mesomeric effect, namely the donation of p-electron density from the aryl substituents to the lobes of the antibonding s*(P-P) orbitals at the phosphonium moiety.47a Some main group centered, predominantly electrophilic ambiphiles react with P 4 via multiple insertions into P-P bonds of the P 4 tetrahedron.This is exemplified by SiP 4 -cage compound 40,  which is obtained by the reaction of P 4 with zwitterionic silylene 39.This compound reacts with a second equivalent of 39 to give the Si 2 P 4 -cage compound 41 (Scheme 11). 14The second insertion takes place at a P-P bond opposing the initially inserted main group element.The related product 4 was obtained by the reaction of P 4 with a low valent Al(I) species (Fig. 2). 13istinct results were obtained in the investigation of multiple insertions of phosphenium ions into P-P bonds of P 4 .In this context, solvent-free mixtures of P 4 , Ph 2 PCl and GaCl 3 in various stoichiometries and at different temperatures were investigated.A 1 : 1 : 1 mixture yields quantitatively the Ph 2 P 5 +cage compound 36f[GaCl 4 ] after 45 min at 60 1C (Scheme 12). 51he Ph 4 P 6

2+
-cage cation 42 2+ is observed in a mixture of 1 : 8 : 5 stoichiometry (P 4 : Ph 2 PCl : GaCl 3 ) as the main product after a reaction time of seven hours at 1C.The 31 P NMR spectrum of 42 2+ shows a characteristic ABMM 0 XX 0 spin system which is in accordance with the insertion of a second Ph 2 P +phosphenium ion into a P-P bond adjacent to the phosphonium moiety in 36f + .Two second-order resonances corresponding to an AA 0 XX 0 X 00 X 0 0 0 spin system are expected for the isomer of 42 2+ formed via formal insertion into two opposing P-P bonds of P 4 .Such a species is not formed in the melt reaction.The formation of dication 42 2+ can only be observed if the ratio of Ph 2 PCl and GaCl 3 is higher than 0.5.In these mixtures, the dominant gallium species is GaCl 4 À ; hence, the melt can be considered as basic medium.In a more Lewis acidic melt, composed of P 4 , Ph 2 PCl and GaCl 3 in a 1 : 3 : 6 stoichiometry, the tricationic Ph 6 P 7

3+
-cage 43 3+ is formed exclusively.Large single crystals of 43 3+ as a heptachlorodigallate salt are formed in the respective melt after 12 h at 100 1C..6 ]heptane) framework.It is composed of a basal ring of three-coordinated P atoms, three tetra-coordinated P atoms at the bridging positions and a three-coordinated P atom at the apex of the cage.This skeleton is reminiscent of the trianionic phosphide P 7 3À , 52 several polyphosphanes R 3 P 7 53 and many polyphosphorus-chalcogenides like e.g.P 4 S 3 . 54The 31 P NMR spectrum of 43 3+ shows an AA 0 A 00 BXX 0 X 00 spin system resulting from the C 3 symmetry of the cage.A 2 J or 3 J P-P bond coupling to the apex of the cage is not observed which might be a result of the adjacent phosphonium P atoms.This leads to a first-order quartet resonance for the apical P atom.The highly electrophilic cation 43 3+ is stable only in the presence of excess GaCl 3 .This prevents the detrimental presence of chloride anions which decompose 43 3+ by nucleophilic attack and subsequent degradation via 42 2+ to 36f + .This illustrates that the consecutive insertion of up to three Ph 2 P + -moieties into P-P bonds of P 4 is directed by the Lewis acidity of the reaction mixture.

Cationic polyphosphorus cages featuring four-membered heterocycles
Cyclic diaminohalophosphanes are important precursors for the preparation of cyclic phosphenium ions via halide abstraction. 55Within this class of compounds, phosphazanes, like the diphosphadiazane 44, are of particular interest (Scheme 13).These compounds feature two chloro-substituted P moieties and, thus, offer a versatile reactivity. 56The diphosphadiazenium ion 45 + is generated from 44 upon chloride abstraction with GaCl . 57The molecular structure of cation 46 + shows a planar four-membered (NP) 2 ring and an almost orthogonal oriented P-Cl bond (Scheme 13).This arrangement is also reflected by the A 2 MVXZ spin system observed in the 31 P NMR spectrum of C S -symmetric cation 46 + .Interestingly, the P 5 + -cage does not couple with the chloro-substituted P atom resulting in the observation of a singlet resonance for the latter.This P-Cl functionality was used for the in situ generation of a phosphenium ion upon addition of three equivalents of GaCl 3 to the reaction mixture.The resulting dicationic intermediate was not detected.However, upon addition of P 4 , the formation of the corresponding insertion product 47 2+ is observed.The 31 P NMR spectrum of 47 2+ shows an A 2 MX 2 spin system which is consistent with two C 2V -symmtric P 5 + -cages bridged by two imido-groups.The dication can be isolated as heptachlorodigallate salt 47[Ga 2 Cl 7 ] 2 and the molecular structure of the N 2 P 10 -cage was confirmed by single crystal structure determination (Scheme 13).This illustrates that the stepwise insertion of the disguised bifunctional Lewis acid [DippNP] 2 2+ into P-P bonds of two P 4 tetrahedra can be mediated by the Lewis acidity of the reaction mixture.The utilization of an excess of GaCl 3 allows for the preparation of the more electrophilic, higher charged species 47 2+ , similar to the reaction sequence yielding 43 3+ (Scheme 12).It is interesting to note that related NHC analogues, fivemembered 1,3,2-diazaphospholenium ions, do not react with P 4 under various reaction conditions 58 similar to acyclic, diaminophosphenium ion (i-Pr 2 N) 2 P + (vide infra).This indicates that the strained four-membered ring geometry present in diphosphadiazenium ions is crucial for its reactivity towards P 4 .
Other cyclic, four-membered phosphorus containing heterocycles can be employed in reactions with P 4 as well. 59The cyclic chlorophosphane 48, featuring a SiCl 2 -backbone, 60 reacts with GaCl 3 to give the corresponding Lewis acid-base adduct 49 (Scheme 14).The formation of related phosphenium ion 50 + is observed only upon addition of a second equivalent of GaCl 3 .This can be explained by the suppression of detrimental concentrations of nucleophilic chloride anions through the formation of Ga 2 Cl 7 À .Cation 50 + is not stable in solution and decomposes via Lewis acid mediated Me 3 SiCl elimination.However, the insertion reaction with P 4 requires only the use of one equivalent of GaCl 3 .In 1 : 1 : 1 mixtures of 48, GaCl 3 and P 4 the corresponding P 5 + -cage compound 51[GaCl 4 ] is formed slowly within four days presumably due to the presence of small amounts of 50 + formed from 49 in a series of equilibrium reactions. 59The related zwitterionic phosphenium ion 52 features a formally anionic AlCl 2 -backbone. 60It reacts with P 4 in toluene giving the formally neutral P 5 -cage compound 53.A conversion of only 30% to the respective product is observed in the reaction mixture, presumably due to the low electrophilicity of 52.However, the developed synthetic protocol includes removal of unreacted starting materials 52 and P 4 by sublimation which can be used in additional synthetic cycles increasing the overall isolated yield.

Cationic polyphosphorus-chalcogen cages
A multitude of phosphorus-chalcogenides have been characterized to date and many of their structural motifs are displayed even in undergraduate textbooks. 61However, until recently, only very few examples of polyphosphorus-chalcogen cations were known which was due to the lack of established synthetic routes for their preparation.To the best of our knowledge only three distinct protocols have been reported so far.The first is based on the reaction of P 4 S 3 with in situ generated phosphenium ion PI 2 + (Scheme 15).44b The phosphenium ion formally inserts into a P-P bond of the basal P 3 -ring accompanied by migration of one of the iodosubstituents to an adjacent P atom giving cation 54 + .However, 54 + is not stable and subsequently disproportionates via an unknown reaction pathway to form 55 + and 56 + .This process involves the extrusion of a very reactive iodo-phosphinidene [PI] and redistribution of the sulfur atoms.
The second protocol is based on halide abstraction from a-P 4 S 3 I 2 with Ag(CH 2 Cl 2 )[Al(OC(CF 3 ) 3 ) 4 ] and yields the spirocyclic cage cation 58 + (Scheme 16). 62The initial step involves the formation of 57 + via iodide abstraction from a-P 4 S 3 I 2 .Cation 57 + subsequently reacts with a second equivalent of a-P 4 S 3 I 2 and this in association with the formal extrusion of phosphinidene [PI] gives rise to spiro-cyclic cage 58 + .However, detailed information on the mechanism of the formation of 58 + was not gained.The structural motif of this cation is unprecedented and contains the first tetra-coordinated P atom exclusively bonded to P and S atoms. 62ecently, a third approach garnered interest which is based on using cationic polyphosphorus cages as starting materials for the preparation of cationic polyphosphorus-chalcogen cages.They constitute potentially versatile reagents due to the multitude of distinctly substituted derivatives which are all conveniently obtained in one step procedures from white phosphorus. 49halcogenation reactions of R This journal is © The Royal Society of Chemistry 2014 the addition of one equivalent of GaCl 3 is beneficial since it lowers the melting point of the respective melt.Both cations are formed upon insertion of two selenium atoms into two P-P bonds adjacent to the phosphonium moieties in 36a,f + .Their structural motif resembles that of nortricyclane, with a basal P 3 -ring, the tetra-coordinated P atom and the selenium atoms occupying the bridging positions, and one P atom defining the apex of the cage.This class of compounds feature interesting 31 P and 77 Se NMR characteristics.Cages 59a,f + reveal an AM 2 OX spin system for the C S -symmetric isotopomer without a 77 Se nucleus.These resonances are superimposed by the C 1 -symmetric isotopomer featuring one 77 Se atom in one of the bridging positions.This isotopomer gives rise to an AMNOXZ spin system which is highly influenced by higher order effects.However, in the case of 59a + , the spin systems of both isotopomers were successfully simulated allowing for the exact determination of chemical shifts and coupling constants.A series of experiments employing varying temperatures, reaction times and stoichiometries gave meaningful insights into the mechanism of the chalcogenation.These experiments indicate that the insertion of Se atoms into P-P bonds of 36a,f + proceeds in a stepwise manner via the intermediates 60a,f + .In the case of alkyl-substituted cage 36a + the insertion of a second equivalent grey selenium is fast, yielding the respective product 59a + quantitatively.If the aryl-substituted starting material 36f + is employed, the intermediate formation of dication 61 2+ is observed.This species forms via the transfer of a [Ph 2 P] + moiety from a second equivalent of 36f + to the reactive intermediate 60f + .Due to the higher stability of the corresponding phosphenium ion Ph 2 P + , 19,50 this transfer is faster than the insertion of the second selenium atom.Subsequently, one of the [Ph 2 P] + -moieties of 61 2+ is substituted by a selenium atom giving rise to 59f + .The formally liberated Ph 2 P + -phosphenium ion is not stable and reacts with a GaCl 4 À anion to give the Lewis acid-base adduct 37 (Ph 2 PCl-GaCl 3 ).This is in accordance with the observation of only 50% conversion and the quantitative formation of P 4 and 37 or the respective oxidation product Ph 2 P(Se)Cl-GaCl 3 in the case of reactions involving 36f + as a starting material.The targeted preparation of 61 2+ as GaCl 4 À salt is achieved by utilizing a 2 : 1 stoichiometry of 36f + and grey selenium.Another synthetic approach for the preparation of 61 2+ is the targeted substitution of one [Ph 2 P] + -moiety in the tricationic cage 43 3+ .This is achieved by reacting 43 3+ with grey selenium under solvent-free conditions (Scheme 17). 49ication 61 2+ was comprehensively characterized by X-ray crystallography (Fig. 12) as well as 31 P and 77 Se NMR spectroscopy.The 31 P NMR spectrum reveals a characteristic AA 0 MOXX 0spin system for the isotopomer without a 77 Se nucleus which is superimposed by the respective AA 0 MOXX 0 Z-spin system of the 77 Se containing species.A similar reactivity was observed for reactions of the P 5 + -cage 36a + or the P 7

3+
-cage 43 3+ with elemental a-S 8 . 49The polyphosphorus cation 43 3+ and cationic polyphosphorus-chalcogen cages 61 2+ and 59f + are formally derived from the stepwise isolobal exchange of [Se] atoms by [Ph 2 P] + units in the bridging positions of the nortricyclane-type structure of P 4 Se 3 .This allows for an in-depth study of the 31 P NMR characteristics of the whole series of compounds and a correlation with the observed structural features in the solid state.Fig. 13 shows the dependence of the chemical shifts of 43 3+ , 61 2+ and 59f + , the related sulfur-containing cages 62a + and 63 2+ , and P 4 Ch 3 (Ch = Se, S) 63 on the number of chalcogen atoms in the corresponding molecules.The stepwise exchange of tetracoordinated P atoms in 43 3+ by Se or S atoms is accompanied by a high-field shift of the resonances of the P atoms of the basal three-membered ring.The chemical shifts of basal P atoms in nortricyclane-type cages are influenced by the exocyclic angles of the P 3 -ring. 63The observed high-field shift correlates well with decreasing exocyclic angles observed in the solid state structures of the respective compounds.The resonances of apical P atoms exhibit the widest range of chemical shifts and reveal a stepwise down-field shift upon the substitution of tetra-coordinated P atoms by chalcogen atoms.This is consistent with different electronegativities of directly bonded phosphorus or chalcogen atoms.Moreover, apical P atoms show a high dependency of their chemical shift on elongation or compression of the nortricyclane framework. 64longation is accompanied by a decrease in the P-P-P angles involving the apical P atom.This increases the s-orbital contribution to the lone pair of electrons and leads to an upfield shift of the corresponding resonance in the 31 P NMR spectrum. 65n this basis, the observed downfield shift indicates a stepwise elongation of the cages which is observed in the respective molecular structures in the solid state.

Nucleophilic fragmentation of cationic polyphosphorus cages
The activation of white phosphorus with carbenes, which belong to the class of predominantly nucleophilic ambiphiles, displays one of the most diverse fields of P 4 chemistry. 6The cyclo-triphosphirene derivative C constitutes a key intermediate in all transformations, independent of the characteristic of the respective carbene employed (Fig. 1).However, intermediate C is elusive and distinct reaction pathways occur depending on the electronic and steric features of carbene L (Scheme 18).Bertrand and co-workers reacted P 4 with carbenes L 1 and L 3 in a 1 : 2 stoichiometry and obtained E/Z isomers 64a,b via an intermediate of type C. 12 Bicyclic species 65 is the result of a cyclo-addition reaction involving the phosphorus double bond of an intermediate of type C and the alkyl amino carbene L 4 . 66ompound 66 results from a ring-opening reaction of an intermediate C with two equivalents of L 5 . 66This reaction is accompanied by the formation of 67 as a side product.This P -species is formed by the formal [2+2] fragmentation of P 4 by carbene L 5 .A [3+1]-fragmentation of the P 4 tetrahedron was achieved using the sterically less demanding carbene L 6 in a reaction with P 4 in a 3 : 1 stoichiometry. 66The P 1 -fragment was identified as 68 + and isolated as chloride salt.The presence of chloride anions is explained by the decomposition of CHCl 3 solvent molecules.
A compound of unknown constitution featuring a P 3 moiety was indicated in the 31 P NMR spectrum of the respective reaction mixture. 66 combination of phosphenium ion and carbene mediated P 4 activation constitutes a novel, potentially versatile approach for the preparation of cationic polyphosphorus cages.This strategy allows for the preparation of polyphosphorus cations featuring imidazoliumyl-substituents.These substituents are valuable for two purposes.First, they serve well for the stabilization of cations by delocalization of the positive charge. 70econd, they stabilize low-coordinated P moieties by reducing the nucleophilicity of directly bonded P atoms. 71The reaction of P 5 + cage compound 32h[GaCl 4 ] with carbene L 7 in a 1 : 1 stoichiometry yields the bicyclo[1.1.0]tetraphosphane69[GaCl 4 ] (Scheme 19). 67he bicyclic framework is substituted with an imidazoliumylgroup in an exo-position and a phosphanyl-group in an endoposition.This is reminiscent of the intermediate 31 observed in the formation of RP 5 Cl + -cages.Cation 69 + features an ACEMX spin system indicating a non-symmetrical molecular structure due to hindered rotation around the P-P bond involving the Dipp-substituted P atom.The endo,exo-substitution of 69 + causes a short intermolecular distance between the Dipp-and the imidazoliumyl-substituted P atoms in the solid state (see molecular structure in Scheme 19).This spatial proximity is also indicated in solution by an extraordinarily large 3 J(PP) coupling constant of 244.6 Hz in the 31 P NMR spectrum.
The reaction of 32h[GaCl 4 ] with carbene L 7 in a 1 : 3 stoichiometry proceeds via a quantitative [3+2]-fragmentation of the P 5 + -cage (Scheme 20). 67he P 2 fragment was identified as the neutral P 2 species 70 featuring an inversely-polarized 68 phosphaalkene moiety.The di-coordinated P atom bears a phosphanyl-substituent which originates from the tetra-coordinated P atom of starting material 32h + .The P 3 fragment was identified as GaCl 4 À salt of cation 71 + which features a chain of three di-coordinated P atoms terminated by two imidazoliumyl-substituents.This compound is characterized by a deep green colour that results from np* and pp* transitions similar to those observed in diphosphenes. 69uantum chemical calculations elucidated the bonding in 71 + . 67he frontier orbital arrangement of the cation is closely related to the classical p-system of the C 3 -allyl anion.Thus, 71 + features a local triphosphaallylanion moiety substituted with imidazoliumylgroups.The mechanism of the [3+2] fragmentation is explained by the reaction sequence in Scheme 21 on the basis of experimental evidence and quantum chemical calculations. 67The reaction of 32h + with the first equivalent of L 7 yields the experimentally verified species 69 + .The nucleophilic attack of L 7 occurs at a P atom adjacent to the phosphonium moiety in 32h + and initiates a P-P bond cleavage.This reaction step is the reverse of the last step in the formation of RP 5 Cl + -cages (Fig. 7) and is in accordance with the observed reversibility of phosphenium ion insertion into P-P bonds of P 4 (vide infra).The nucleophilic attack of a second carbene L 7 occurs at the endosubstituted P atom of 69 + and initiates a P-P bond cleavage in the respective P 3 -ring.This yields intermediate 72 + according to quantum chemical calculations. 67Subsequently, this intermediate intramolecularly eliminates the P 2 fragment 70.This yields the elusive triphosphirene derivative 73 + which is related to the key intermediate C (Fig. 1) of carbene-induced P 4 activation. 12,66The nucleophilic attack of a third carbene L 7 on the PP double bond of 73 + initiates a ring-opening and yields the second fragment 71 + .The ease of fragmentation (high yields, multi-gram scale) together with the facile accessibility of cationic phosphorus cages from P 4 and the multitude of carbenes available render this approach suitable for the preparation of a plethora of interesting polyphosphorus compounds.

4 À 4 À
anion nucleophilically attacks the P 4 tetrahedron along with the electrophilic attack of the phosphenium moiety of 28e + on P 4 .This leads to the slightly endothermic formation of the intermediate 31 (15.6 kcal mol À1 ) via transition state TS2 (17.7 kcal mol À1 ).Compound 31 reveals a butterfly-type structure featuring a chloro-substituent in an exo-position and a phosphanyl-substituent in an endo-position.Finally, 31 reacts via TS3 (16.3 kcal mol À1 ), which shows only a very low energy barrier.This step proceeds via the intramolecular nucleophilic attack of the phosphanyl-substituent on the chloro-substituted P atom.This eliminates the GaCl anion and leads to the formation of the P 5 + -cage cation 32e + .Despite their different compositions 1 : 1 mixtures of RPCl 2 and a Lewis acid ECl 3 (E = Al, Ga) in fluorobenzene are potent sources of reactive phosphenium ion RPCl + equivalents, which insert formally into P-P bonds of P 4 . 39Dissolution of P 4 in these mixtures yielded white to yellowish precipitates of the corresponding RP 5 Cl + -cage salts for a large range of different alkyl-and aryl-substituents R (32a-h[GaCl 4 ], Scheme 8).

Fig. 7
Fig. 7 Calculated reaction pathways for the reaction of 28e[GaCl 4 ] and P 4 ; calculated differences of the enthalpies at 298.15 K (DH 298 ) are given for the optimized structures of MP2/6-31G(d) and the optimized structures of 28e[GaCl 4 ] + P 4 were defined as 0 kcal mol À1 .
It is highly influenced by the nature of the respective anion (compare 33a[GaCl 4 ]: d = 310 ppm, 33a[Ga 2 Cl 7 ]: d = 350 ppm).The GaCl 4 À salt of 33a + can be isolated and constitutes a rare example of a structurally characterized mono-amino substituted phosphenium ion (Scheme 9).Upon reacting phosphenium ions 33a,b + with P 4 insertion into a P-P bond is observed giving the C S -symmetric RP 5 Cl + -cage cations 34a,b + .However, these cages are in equilibrium with the respective free phosphenium ions and P 4 which hampers the isolation of pure compounds 34a,b[GaCl 4 ].The observation of an equilibrium can be attributed to the relative stability of free 33a,b + .A similar reversibility of the phosphenium ion insertion was observed in the case of RP 5 Cl + compounds.The addition of coordinating solvents like acetonitrile to solutions of 32[ECl 4 ] (E = Ga, Al) decomposes the respective metallate anion via chloride liberation.Nucleophilic attack of free chloride anions on 32 + yields mainly the starting materials P 4 and RPCl 2 (R = alkyl, aryl) in a back reaction.

Scheme 10 Fig. 10
Scheme 10 Preparation of compounds 36a-h[GaCl 4 ] from P 4 , R 2 PCl and GaCl 3 in fluorobenzene (R = aryl) or according to a solvent free approach (R = alkyl) and species 37 and 38 + commonly observed in mixtures of R 2 PCl and GaCl 3 .

3 . 31 P
Solutions of 45 + are characterized by a bright red colour and the NMR spectrum shows a broad resonance at characteristic low field (d = 242.3ppm) indicating the formation of a di-coordinated P moiety.Subsequent addition of P 4 to this solution leads to discolouration and quantitative formation of the P 5 + -cage compound 46[GaCl 4 ]

Scheme 13
Scheme 13 Stepwise synthesis of N 2 P 10 -cage compound 47[Ga 2 Cl 7 ] 2 via insertion of phosphenium ions generated in situ by the reaction of diphosphadiazane 44 with GaCl 3 .

Fig. 13
Fig. 13 Family of cationic polyphosphorus-chalcogen cages formally derived from stepwise isolobal exchange of [Ch] by [R 2 P] + units in P 4 Ch 3 (Ch = Se, S, left), their 31 P NMR shifts versus the number of chalcogen atoms (n, middle) and the assignment of P or Ch atoms to the positions of a nortricyclanetype cage (right).
Scheme 18 Carbene-induced transformation and fragmentation reactions of P 4 .