Controlled scrambling reactions to polyphosphanes <i>via</i> bond metathesis reactions
Triphosphanes R 0 2 PP(R)PR 0 2 (9a,c: R ¼ Py; 9b R ¼ BTz), 1,3-diphenyl-2-pyridyl-triphospholane 9d and pentaphospholanes (RP) 5 (13: R ¼ Py; 18: R ¼ BTz) are obtained in high yield of up to 98% from the reaction of dipyrazolylphosphanes RPpyr 2 (5: R ¼ Py; 6: R ¼ BTz; pyr ¼ 1,3-dimethylpyrazolyl) and the respective secondary phosphane (R 0 2 PH, R 0 ¼ Cy (9a,b), t Bu (9c); PhPH(CH 2 ) 2 PHPh (9d)). The formation of derivatives 9a-d proceeds via a condensation reaction while the formation of 13 and 18 can only be explained by a selective scrambling reaction. We realized that the reaction outcome is strongly solvent dependent as outlined by the controlled scrambling reaction pathway towards pentaphospholane 13. In our further investigations to apply these compounds as ligands we first confined ourselves to the coordination chemistry of triphosphane 9a with respect to coinage metal salts and discussed the observation of different synand anti-isomeric metal complexes based on NMR and X-ray analyses as well as quantum chemical calculations. Methylation reactions of 9a with MeOTf yield triphosphan-1-ium Cy 2 MePP(Py)PCy 2 + (10 + ) and triphosphane-1,3-diium Cy 2 MePP(Py)PMeCy 2 2+ (11 2+ ) cations as triflate salts. Salt 11[OTf] 2 reacts with pentaphospholane 13 in an unprecedented chain growth reaction to give the tetraphosphane-1,4-diium triflate salt Cy 2 MePP(Py)P(Py)PMeCy 2 2+ (19[OTf] 2 ) via a P-P/P-P bond metathesis reaction. The latter salt is unstable in solution and rearranges via a rare [1,2]-migration of the Cy 2 MeP-group followed by the elimination of the triphosph-2-en-1-ium cation [Cy 2 MePPPMeCy 2 ] + (20 + ) to yield a novel 1,4,2-diazaphospholium salt (21[OTf]). Scheme 1 Application of tripyrazolylphosphane 1 in P-P bond formation reactions; (i) 2 Cy 2 PH, -2 pyrH, CH 3 CN; (ii) 3 Cy 2 PH, -3 pyrH, and CH 3 CN; (iii) 5 PhPH(CH 2 ) 2 PHPh, 4 Ppyr 3 , -PhP(pyr)(CH 2 ) 2 -P(pyr)Ph, -10 pyrH, CH 3 CN.
Introduction<!>Results and discussion<!>Conclusions<!>Conflicts of interest
<p>A wide range of polyphosphorus compounds, comprising diverse bonding motifs, have been synthesised in the last few decades. 1,2 Many of the observed compounds are oen found to be labile in solution and their rearrangement or decomposition reactions are discussed to proceed via intermolecular scrambling reactions. 3 Even though there are early reports on controlling such scrambling reactions, 4 the selective and high yielding synthesis of certain polyphosphanes is still difficult, hampering their subsequent chemistry. Besides classical approaches for the generation of P-P bonds such as salt metathesis reactions, 2a,b dehalosilylation reactions 2c or condensation reactions, 2d our group focusses on the development of methodologies using tripyrazolylphosphane 1 or derivatives thereof (vide infra) as a P 1 -building unit for P-P bond forming reactions via condensation or P-N/P-P bond metathesis reactions (Scheme 1). 5,6 In this regard, the reaction of 1 with Cy 2 PH to give triphosphane 2 or iso-tetraphosphane 3 depending on the applied stoichiometry has been reported. 5 The reaction of 1 with 1,2-bis(phenylphosphanyl)ethane yields hexaphosphane 4 in a sequence of condensation and P-N/P-P bond metathesis reactions, showing an example of a controlled scrambling reaction. 6 In our effort to further control these metathesis reactions and driven by our general interest in azole-and azine-substituted phosphanes, we explored the synthesis of phosphanes with a pyridyl-or benzothiazolyl-substituent bound to phosphorus via the carbon atom. This limits the number of P-N bound pyrazolyl-substituents and therefore the possible reaction pathways by P-N/P-P bond metathesis. For a better overview and in thought of a systematic approach we organised our general types of phosphanes with respect to their number of P-N bound substituents as type I to type IV (Scheme 2). In type I phosphanes all three substituents are bound via P-N bonds whereas a type II phosphane is dened by two P-N bonds and one P-C bond. This order is maintained in type III phosphanes with one P-N and two P-C bonds and is nalised in type IV phosphanes featuring only P-C bonds. Importantly, all substituents at the phosphorus atom have an additional sp 2 hybridised donor atom such as nitrogen, being essential as a further reaction or coordination site and for the possible formation of resonance stabilised reaction intermediates. In this contribution, we report the synthesis of the hitherto unknown type II phosphanes carrying either a pyridyl-or benzothiazolyl-substituent and their use for the generation of triphosphanes, a triphospholane and pentaphospholanes and selected one triphosphane to explore its coordination chemistry with respect to coinage metal salts.</p><!><p>The targeted type II phosphanes are synthesised by the reaction of the dichlorophosphanes RPCl 2 (R ¼ Py (5); R ¼ BTz (6)) 7,8 with two equivalents of 3,5-dimethyl-1-(trimethylsilyl)pyrazole (Scheme 3). 9 The released chloro(trimethyl)silane can be removed under reduced pressure aer 16 hours and no further purication is necessary. Both phosphanes 7 and 8 are conveniently prepared on a multi gram scale in excellent yields (97% (7); 96% (8)). The 31  Subsequent washing with CH 3 CN and drying in vacuo give 9ad as colourless solids in good to excellent yield (96% (9a); 93% (9b); 77% (9c); 69% (9d)). The 31 P NMR spectra of all compounds show AX 2 spin systems which are characteristic of triphosphanes and 1,2,3-triphospholanes (Fig. 2). The NMR spectroscopic parameters are listed in Table 1. Single crystals for X-ray analysis are obtained by slow vapour diffusion of npentane into saturated CH 2 Cl 2 solutions of 9a-d. The molecular structures of 9a-d are depicted in Fig. 3. Their structural parameters compare very well with those of similar triphosphanes 12 and are thus not discussed in detail. Only the bond angle of the P-P-P moiety in compound 9c is approximately 10 wider which can be explained by the sterically demanding tertbutyl substituents. In recent years numerous examples of the coordination chemistry of anionic 13 and zwitterionic 14 oligophosphorus compounds towards transition metals have been Scheme 2 Classification of N-heterocyclic substituted phosphanes into type I to type IV phosphanes (n is the number of P-N bonds) with an emphasis on the type II phosphanes presented in this contribution.  reported. Also, few examples of neutral triphosphanes coordinated to transition metals are known, however, the triphosphane moiety was assembled while coordinated to at least one metal centre. 15 Starting from stable triphosphanes only two complexes coordinating to metal carbonyls have been structurally characterised (Scheme 5, top), 5,16 while another example is described in solution. 17 In complex I, the triphosphane ligand chelates the W(CO) 4 moiety while in complex II the coordination of the Fe(CO) 4 moiety proceeds only via the free electron pair of one of the dicylcohexylphosphanyl groups. Featuring an additional nitrogen based donor site we envisioned the formation of multi-dentate coinage metal complexes using triphosphane 9a, exemplarily. This yielded the rst triphosphane coordination complexes of Cu(I), Ag(I) and Au(I) (Scheme 5). Reacting 9a with an equimolar amount of Ag[OTf] in uorobenzene at room temperature leads to the formation of a colourless precipitate which is isolated by ltration to yield the dinuclear complex [(9a*Ag       2+ and subsequent association of the ligand on the opposite side (Fig. 6). This interconversion is the highest in energy for [(9a*Cu) 2 ] 2+ (14.7 kcal mol À1 ), followed by [(9a*Ag) 2 ] 2+ (13.2 kcal mol À1 ) and nally [(9a*Au) 2 ] 2+ (9.6 kcal mol À1 ). 23 Further reactivity studies on triphosphane 9a revealed that this compound readily rearranges in solution by means of a P-P/P-P bond metathesis reaction. Aer 24 h the CH 2 Cl 2 solution of 9a shows an additional singlet resonance at d(P) ¼ 21.0 ppm which we assign to tetracyclohexyldiphosphane (12) 24 and an AA 0 BB 0 C spin system at d(P) ¼ 14.0-25.2 ppm being characteristic of pentaphospholanes of type (RP) 5 . 24,25 As this scrambling reaction is in a state of equilibrium the isolation of 13 is hampered. However, the reaction of pure 13 with Cy 4 P 2 yielding 9a illustrates that pentaphospholane 13 might be used as a PyP-synthon as it inserts into the P-P bond of Cy 4 P 2 . The selective synthesis of pentaphospholane 13 is achieved by the equimolar reaction of 7 with Cy 2 PH in Et 2 O as a non-polar solvent (Scheme 6). The formed precipitate is ltered and   dried in vacuo giving 13 in 98% yield as a colourless powder which shows the AA 0 BB 0 C spin system in the 31 P NMR spectrum (Fig. 7). Dicyclohexyl(3,5-dimethylpyrazolyl)phosphane ( 14) is identied as a side product in the supernatant solution by its singlet resonance in the 31 P NMR spectrum (d(P) ¼ 58.4 ppm) indicating a P-N/P-P bond metathesis reaction. 6 To further understand the formation of 13 we studied the reaction in CD 2 Cl 2 by means of 31 P NMR spectroscopy (Fig. 8). Aer 3.5 h compound 7 is still present in the solution while Cy 2 PH is fully consumed by the condensation reaction with 7 to form triphosphane 9a (vide supra) and diphosphane 15 which is identied by an AX spin system (d(P A ) ¼ 10.2 ppm and d(P X ) ¼ 33.4 ppm; 1 J(P A P X ) ¼ À281 Hz). The AMX spin system at d(P A ) ¼ À35.6 ppm, d(P M ) ¼ À7.2 ppm and d(P) X ¼ 29.9 ppm is assigned to triphosphane 16 which is formed via a P-N/P-P bond metathesis reaction of two equivalents of 15 and via a P-N/P-P bond metathesis reaction of 7 and 9a (Scheme 7). In both cases phosphane 14 is liberated. We propose a further chain growth in which triphosphane 16 reacts with 15 with the release of phosphane 14 in a P-N/P-P oligomerisation reaction which ultimately yields pentaphospholane 13. We furthermore propose the formation of tetraphosphetane 17 (d(P) ¼ À49.4 ppm) which can be formed via the reaction of two equivalents 16 with concomitant release of 14 (Scheme 7). Similar to the known ring expansion reactions of certain cyclophosphanes, 26 17 might react to form pentaphospholane 13, being the thermodynamically favoured product. In analogy to 13 we are able to isolate the corresponding benzothiazolyl substituted pentaphospholane 18. 27 Crystals suitable for X-ray analysis are obtained for both compounds by the slow diffusion of npentane into saturated CH 2 Cl 2 solutions at À30 C (Fig. 9). Both compounds reveal the typical envelope conformation of the P 5 -ring featuring the substituents in the all-trans position. The P-P bond lengths observed for both pentaphospholanes 13 (av. P-P 2.218 Å) and 18 (av. P-P 2.246 Å) are in a range known for pentaphospholanes such as (PhP) 5 (av. 2.217 Å); also the P-P-P angles are in good accordance with the reported data for (PhP) 5 . 28 We subsequently envisioned the use of 13 as a PyPsynthon for insertion reactions into P-P bonds which was already indicated by the aforementioned scrambling reaction between 9a, Cy 4 P 2 and 13 (see Scheme 6).</p><p>As similar scrambling reactions might be expected for triphosphanium and triphosphanediium salts, we methylated triphosphane 9a with different amounts of MeOTf. The 31 P Scheme 6 Equilibrium reaction of 9a with 13 and 12 and selective synthesis of 13 by the reaction of 7 with Cy 2 PH; (i) Et 2 O, -pyrH. Fig. 7 31 P NMR spectrum of pentaphospholane 13 (CD 3 CN, 300 K, the inset shows the AA 0 BB 0 C spin system of the experimental (upwards) and fitted spectra (downwards)). NMR spectrum of the 1 : 1 reaction of 9a with MeOTf shows an AMX spin system (d(P A ) ¼ À46.2 ppm, d(P M ) ¼ À12.9 ppm and d(P X ) ¼ 34.2 ppm ( 1 J(P A P M ) ¼ À290 Hz, 1 J(P A P X ) ¼ À281 Hz, and 2 J(P M P X ) ¼ 58 Hz)), suggesting the methylation of the phosphorus atom rather than the nitrogen atom of the pyridylsubstituent. Aer the removal of all volatiles in vacuo triphosphan-1-ium triate salt 10[OTf] can be isolated in quantitative yield as a colourless powder. A second methylation is achieved when 9a is reacted with an excess of MeOTf under solvent free conditions (Scheme 8). Similar to the aforementioned reaction, only the phosphorus atoms are methylated giving triphosphane-1,3-diiumtriate salt 11[OTf] 2 . As both dicyclohexylphosphanyl moieties are methylated, the 31 P NMR spectrum of 10[OTf] 2 shows an AX 2 spin system with resonances at d(P A ) ¼ À67.9 ppm and d(P X ) ¼ 44.0 ppm ( 1 J(P A P X ) ¼ À315 Hz). The molecular structures of cation 10 + and dication 11 2+ are depicted in Fig. 10. Note that a small number of triphosphanediium salts are reported in the literature and compound 10[OTf] extends the library of cationic, catenated phosphorus compounds as a structurally characterised triphosphan-1-ium salt. [29][30][31] The P1-P2 bond length in cation 10 + (P1-P2 2.1956(5) Å) is shortened whereas the P2-P3 bond (P2-P3 2.2362(5) Å) is comparable to those in 9a (P1-P2 2.2301(3) Å, P1-P3 2.2371(3) Å) or similar diphosphanium cations (compare [Ph 2 P-PPh 3 ] + : 2.230(1) Å). 29b The P-P-P bond angle in 10 + (97.89(2) ) is similar to that in compound 9a (97.16( 6) ). The P-P bond lengths in 11 2+ (P1-P2 2.2152(4) Å and P2-P3 2.2182(4) Å) are marginally shorter compared to those in 9a but in accordance with bond lengths reported for similar triphosphane-1,3-diium cations (compare [Me 3 P-P(Cy)-PMe 3 ] 2+ : 2.1979(5) Å and 2.1976(6) Å). 29b The P-P-P bond angle in 11 2+ (107.43(1) ) is signicantly wider compared to those in 9a and [Me 3 P-P(Cy)-PMe 3 ] 2+ (103.11(2) ). In order to investigate the envisioned PyP-transfer into a P-P bond, we reacted 13 with 5 equivalents of 11[OTf] 2 in CH 3 CN (Scheme 9). Upon addition of 13 to a colourless CH 3 CN solution of 11[OTf] 2 the reaction mixture turns from deep red to pale yellow. The 31 P NMR spectrum of the reaction mixture aer 24 h shows two AA 0 XX 0 spin systems which are attributed to two isomers of tetraphosphane-1,4-diium cations 19 2+ (Fig. 11), indicating the successful PyP-transfer via a P-P/P-P bond metathesis reaction. The A part of the prominent AA 0 XX 0 spin system at d(P A ) ¼ À55.1 ppm is assigned to the inner pyridylsubstituted P nuclei and the X part at d(P X ) ¼ 40.7 ppm to the tetra-coordinate phosphorus atoms which is similar to known tetraphosphane-1,4-diium salts. 32 The minor spin system resonates at d(P A ) ¼ À40.1 ppm and d(P X ) ¼ 39.8 ppm, respectively. Aer work-up compound 19[OTf] 2 can be isolated analytically pure in 51% yield. X-ray analysis revealed that the centrosymmetric meso-isomer 19 2+ crystallized showing a central torsion angle of 180 (Fig. 11). The P-P bond lengths in 19 2+ (P1-P2 2.234(1) Å and P2-P2 0 2.232(2) Å) are in good agreement with the reported data for comparable tetraphosphane-1,4-diium cations (compare [Ph 3 P-(PPh) 2 -PPh 3 ] 2+ : P1-P2 2.258(1) Å, P2-P2 0 2.221(1) Å). 32 Similar to the reported PhP-transfer from (PhP) 5 to a NHC, 33 13 can be used as a PyP-synthon, thus featuring an  additional reaction or coordination site. When compound 19 [OTf] 2 was kept in a CD 3 CN solution for 14 days, the 31 P NMR spectrum shows three rearrangement products (Fig. 12). Next to an AX 2 spin system (d(P A ) ¼ À235.3 ppm and d(P X ) ¼ 38.8 ppm ( 1 J(P A P X ) ¼ À501 Hz)), a pseudo triplet resonance (d(P) ¼ 153.1 ppm; 1 J(PN) ¼ 50 Hz) and an AMXY spin system (d(P A ) ¼ À208.2 ppm, d(P M ) ¼ À30.0 ppm, d(P X ) ¼ 31.1 ppm, and d(P Y ) ¼ 39.8 ppm; 1 J(P A P y ) ¼ À490 Hz, 1 J(P M P X ) ¼ À342 Hz, 1 J(P A P M ) ¼ À298 Hz, 2 J(P M P Y ) ¼ 91 Hz, 2 J(P A P X ) ¼ 45 Hz, 3 J(P X P Y ) ¼ 34 Hz) are observed. The AX 2 spin system can be attributed to the 3l 5triphosph-2-en-1-ium cation (20 + ), as the spectroscopic parameters are in the range of those observed for [Ph 3 P-P-PPh 3 ] + (d(P A ) ¼ À174 ppm and d(P X ) ¼ 30 ppm; 1 J(P A P X ) ¼ À500 Hz) which was rst reported by Schmidpeter and co-workers. 34 A variety of similar derivatives has been synthesised and reported by the group of Macdonald. 35 Slow vapour diffusion of Et 2 O into the CD 3 CN solution yielded crystals suitable for X-ray analysis conrming the structural connectivity of 20 + (Fig. 13). The structural parameters of 20 + are similar to those observed for the cation [Ph 3 P-P-PPh 3 ] + , i.e. slightly shortened P-P bond lengths (P1-P2 2.134(4), P2-P3 2.1382( 4)) and a P-P-P bond angle of 103.85(2) . 34 By co-crystallisation, a further product was identied as the triate salt of diazaphospholium cation 21 + (Fig. 13). We assign this species to the pseudo triplet resonance observed in the 31 P NMR spectrum. The P-C bond distance of 1.733(1) Å slightly exceeds the upper limit of typical P]C bond lengths (1.61-1.71 Å), 36 while the P-N bond length of P1-N2 1.741(1) Å indicates a P-N single bond (P-N: 1.78 Å). 37 Furthermore, the structure of 21 + reveals an acute angle around the P atom (C1-P1-N2 87.08( 6) ) and a planar arrangement, suggesting a delocalized psystem. We assign the AMXY spin system to asymmetric tetraphosphanediium dication 22 2+ , most likely formed by an intramolecular aromatic substitution reaction of 19 2+ . This can be attributed to be the rst step of the rearrangement reaction of 19 2+ to form 20 + and 21 + . We further studied this rearrangement reaction using the TURBOMOLE 7.0 program at the PB86-D3/ def2-TZVP level of theory (see ESI for details †) and taking into consideration solvent effects by using the Conductor-like Screening Model (COSMO). Our calculations started from 22 + and show that the cleavage of Cy 2 PMe is energetically quite costly. This is in accordance with the 31 P NMR spectrum as Cy 2 PMe is not observed (d(P) ¼ À19.8 ppm). 38 More likely and energetically favoured is a [1,2]-shi of the Cy 2 PMe group to form iso-22 2+ with a reaction barrier of +22.0 kcal mol À1 . Fig. 14 shows the optimised structures for this [1,2]-Cy 2 PMe shi reaction. It is noteworthy that similar [1,2]-and [1,3]-PR 3 shis have been reported. 39 Compared to 22 2+ , iso-22 2+ is almost isoenergetic and    readily reacts in an exergonic step (À31.9 kcal mol À1 ) to form cations 20 + and 21 + with a low barrier of only 1.9 kcal mol À1 (see Scheme 10 and ESI for details †).</p><!><p>In summary we have shown that phosphanes which we denote as type II phosphanes (two P-N bonds and one P-C bond) can be used to build polyphosphorus compounds featuring a further nitrogen donor functionality at the C bound substituent. The reactions of type II phosphanes 7 and 8 with secondary phosphanes of type R 0 2 PH in a 1 : 2 ratio in MeCN give triphosphanes 9a-c and triphospholane 9d via protolysis reactions. With the multi-gram scale synthesis of 9a we were able to conveniently investigate its coordination chemistry towards coinage metal salts. The synthesised coordination complexes are studied by X-ray analysis, multi nuclear NMR spectroscopy and quantum chemical calculations. Further reactivity studies of 9a focused on the reaction with the electrophilic MeOTf. Depending on the amount of MeOTf, the reaction yields triphosphane-1,3-diium triate salt 11 [OTf]</p><!><p>There are no conicts to declare.</p>