A Fully Phosphane‐Substituted Disilene

Abstract There is growing interest in compounds containing functionalized E=E multiple bonds (E=Si, Ge, Sn, Pb) because of their potential to exhibit novel physical and chemical properties. However, compounds containing multiple functionalizations are rare, with scarcity increasing with increasing degree of substitution. The first ditetrelene R2E=ER2 in which the E=E bond is substituted by four heteroatoms (other than Si) is described. The tetraphosphadisilene {(Mes)2P}2Si=Si{P(Mes)2}2 (7) is readily isolated from the reaction between SiBr4 and [(Mes)2P]Li, the latter of which acts as a sacrificial reducing agent. The structure of 7 is presented, while the bonding in, and stability of 7 were probed using DFT calculations.

Over the last few years several examples of disilenes and digermenes substituted by either one or two group 15-17 atoms have been reported. Fore xample,S cheschkewitz and co-workers demonstrated that treatment of (Trip) 2 Si = Si-(Trip)(Li)with R 2 PCl or iodine gives (Trip) 2 Si = Si(Trip)(PR 2 ) (A,F igure 1) or (Trip) 2 Si = Si(Trip)(I) (B), respectively (R = Ph, iPr, Cy, tBu, NR' 2 (R' = Me,E t, iPr);T rip = 2,4,6-iPr 3 C 6 H 2 ), [4] while Sekiguchi and co-workers showed that disilynes undergo 1,2-addition with amines to give amino-substituted disilenes [{(Me 3 Si) 2 CH} 2 iPrSi](H)Si= Si(NR 2 )[SiiPr{CH(SiMe 3 ) 2 } 2 ]( C;N R 2 =NEt 2 ,N (CH 2 CH 2 ) 2 , NPh 2 ,a nd NHtBu). [5] In an alternative approach, Jutzi and co-workers . [6] More recently,Roesky and co-workers independently showed that this compound is accessible from the reaction of Cp*SiCl 2 Ha nd K[N(SiMe 3 ) 2 ]. [7] Perhaps the most intriguing observation is that the cyclic diaminosilylene (CH 2 NtBu) 2 SiD aggregates to an unusual amino-substituted disilene E on standing at room temperature,rather than the corresponding tetraaminodisilene; [8] in solution ad ynamic equilibrium between the silylene and E is observed (closely related digermenes have been isolated by Weidenbruch and coworkers). [9] Adynamic equilibrium was also proposed for the highly sterically hindered dibromodisilene F and its bromosilylene analogue. [10] Similarly,Müller, Kira, Apeloig, and coworkers have proposed ad ynamic equilibrium between the diaminosilylene (iPr 2 N) 2 SiD and its disilene dimer, based on variable-temperature UV/Vis spectroscopy;a lthough this latter disilene has not been isolated and these studies suggest that the disilene itself is am inor component over the temperature range measured. [11] Very recently Bacereido,K ato,a nd co-workers reported the dimerization of heteroleptic, intramolecularly base-stabilized amino-chlorosilylenes and amino-hydrosilylenes by [*] Dr.K . Izod insertion of one silylene fragment into the SiÀXb ond of another to give compounds G. [13] To date,there have been no reports of disilenes substituted by three or more group 15-17 atoms,which are isolable in the solid state. Since the tendency for heteroatom-substituted tetrylenes to dimerize to the corresponding ditetrelenes is afunction of the electronegativity and p-donor capacity of the heteroatom, diaminotetrylenes (R 2 N) 2 E( E = Si, Ge,S n, Pb) typically show limited tendencyt owards dimerization. Phosphorus is significantly less electronegative than nitrogen and, while calculations suggest that Pa nd Nh ave similar inherent p-donor capabilities, [14] the large barrier to inversion at phosphorus prevents routine adoption of the planar geometry necessary for efficient pp-pp interactions.T hus,d iphosphatetrylenes (R 2 P) 2 Er epresent an interesting class of compounds in which the tetrel center may not benefit from significant stabilization from the s-withdrawing/p-donating effects of the phosphorus atoms.I ti sp erhaps unsurprising then that few diphosphatetrylenes have been reported and that, until recently,n one of these compounds exhibited P-E pp-pp interactions. [15] Recently,w er eported the first examples of diphosphagermylenes and -stannylenes (H)inwhich phosphorus adopts ap lanar geometry,r esulting in efficient P-E pp-pp interactions ( Figure 2). [16] We now report our attempts to extend this method to the synthesis of ad iphosphasilylene and the consequent isolation of au nique disilene substituted by four phosphorus atoms.
Following on from our earlier report, [16] we initially attempted the synthesis of the crowded precursor {(Dipp) 2 P} 2 SiCl 2 (1), aiming to reduce this to the corresponding diphosphasilylene.However,while we were able to isolate afew crystals of 1 (Supporting Information), the difficulty of isolating this compound cleanly in acceptable yield prevented further exploitation. Consequently,wesought aless sterically demanding substituent at phosphorus and so synthesized the dichlorosilane {(Mes) 2 P} 2 SiCl 2 (2), which was isolated in good yield and purity from the reaction between SiCl 4 and 2equivalents of [(Mes) 2 P]Li (Mes = 2,4,6-Me 3 C 6 H 2 ), as a colorless solid.
However,treatment of 2 with 2equivalents of KC 8 did not lead to the corresponding diphosphasilylene {(Mes) 2 P} 2 SiD (3), but instead gave am ixture of the potassium phosphanide [(Mes) 2 P]K, along with as mall amount of the diphosphane (Mes) 2 PÀP(Mes) 2 and av ery small number of red crystals, which were shown by X-ray crystallography to be the triphosphasilanate complex {(Mes) 2 P} 3 SiK(THF) 3 (4)( Supporting Information). Theformation of 4 during this reaction clearly indicates that the Si IV center is reduced to Si II in this process.H owever,g iven that 4 is effectively an adduct between 3 and [(Mes) 2 P]K, its formation suggests in situ reduction of 3 (or the corresponding disilene,see proceeding text) by the KC 8 to give [(Mes) 2 P]K and an unidentified silicon-containing species (Scheme 1). Attempts to circumvent this over-reduction by using lithium as amilder reducing agent gave the triphosphasilanate complex {(Mes) 2 P} 3 SiLi-(THF) (5.THF;i dentified by multielement NMR spectroscopy), whereas magnesium was found not to react with 2.We have not been able to synthesize 4 by amore systematic route, but 5 may be isolated in good yield and purity (see proceeding text).
It has been determined that the dehydrochlorination of chlorosilanes by strong bases is an effective route to Si II species. [7,17] Therefore,asanalternative strategy we prepared the chlorosilane {(Mes) 2 P} 2 SiClH (6). However,r eactions between 6 and av ariety of strong non-nucleophilic bases (such as LiN(SiMe 3 ) 2 or LiN(CMe 2 CH 2 ) 2 CH 2 )g ave complex mixtures of products,asjudged by 31 P{ 1 H} NMR spectroscopy, from which we were not able to isolate any silicon-containing species.
Since the over-reduction of 2 by alkali metals is in direct competition with the formation of the diphosphasilylene 3,we sought to prepare the dibromosilane {(Mes) 2 P} 2 SiBr 2 in the expectation that reduction of this species to adiphosphasilylene would be more competitive with the over-reduction (PÀSi cleavage) process.T oo ur surprise,w ef ound that the reaction between SiBr 4 and 2equiv of [(Mes) 2 P]Li in diethyl ether gave adark blue solution, which slowly decolorized and deposited asmall amount of dark-purple crystals;these were shown by X-ray crystallography to be the tetraphosphadisilene {(Mes) 2 P} 2 Si = Si{P(Mes) 2 } 2 (7)( see proceeding text).
This clearly indicated that the lithium phosphanide was itself acting as ar educing agent and so we adjusted the stoichiometry accordingly. [18] Thus,t he reaction between SiBr 4 and four equivalents of [(Mes) 2 P]Li in diethyl ether yields as imilar dark-blue solution. Removal of the solvent and extraction of the residue into light petroleum gave ab rown solution after removal of the LiBr side-product (Scheme 2). A 31 P{ 1 H} NMR spectrum of this crude solution exhibits as inglet at À31.4 ppm that is due to (Mes) 2 PÀ P(Mes) 2 and ab road singlet at À41.3 ppm, which we

Angewandte Chemie
Communications 5594 www.angewandte.org tentatively assign to the silylene 3 (although we have not yet been able to isolate this species), along with a1:1:1:1 quartet at À62.5 ppm corresponding to 5,and asinglet at À90.8 ppm that is due to an unknown species (approximate ratio of peaks 1.0:2.2:1.3:1.0). We were unable to locate as ignal corresponding to 3 in the 29 Si{ 1 H} NMR spectrum of this crude solution;t his may be due to extensive signal broadening for this species,a sn oted for the analogous diphosphastannylene G for which no 119 Sn NMR signal could be found at room temperature either in the solid state or solution. [16] On standing at room temperature for several days,t his brown solution deposits dark-purple crystals of 7 in reasonable yield; heating the solution under reflux accelerates deposition such that it is complete within 4hours.
Compound 7 has limited solubility in common organic solvents,p reventing characterization by solution-state NMR spectroscopy.H owever,t he solid-state cross-polarization magic angle spinning (CP-MAS) 31 P{ 1 H} NMR spectrum of 7 consists of ap air of singlets at À55.9 and À77.9 ppm, consistent with the two distinct phosphorus environments observed by X-ray crystallography (see proceeding text), while the solid-state CP-MAS 29 Si{ 1 H} NMR spectrum of 7 consists of ab road singlet at 111.7 ppm; 31 P-29 Si coupling is not resolved. The 29 Si chemical shift of 7 is in the typical range for disilenes; [2,3] theo bserved 31 P{ 1 H} and 29 Si{ 1 H} chemical shifts correlate reasonably well with those obtained from DFT calculations (Supporting Information).
Single crystals suitable for X-ray crystallography were grown from n-hexane solutions of 3 that were left to stand at room temperature for several days.T he structure of 7 is shown in Figure 3, along with selected bond lengths and angles.C ompound 7 crystallizes as ad iscrete molecular species in which the silicon atoms are disordered over two positions with 92:8 occupancy, with ac enter of inversion midway along each of the two SiÀSi vectors.T he major disorder component has astrongly trans-bent geometry (40.68 8 deviation of the SiP 2 mean plane from the SiÀSi vector). This contrasts with the near-planar geometries adopted by most silicon-substituted disilenes, [2,3] although af ew carbon-substituted disilenes do exhibit large trans-bending angles. [19] The trans-bending angle in the major disorder component of 7 is similar to those observed in af ew heteroatom-substituted systems;f or example in (Trip) 2 Si=Si(Trip){P(NiPr 2 ) 2 }t he trans-bending angle at the phosphorus-substituted silicon center is 30. 88 8, [4] while the trans-bending angles in (Bbt)BrSi = SiBr(Bbt) are 32.4 and 39.88 8 (Bbt = 2,6-{(Me 3 Si) 2 CH} 2 -4-{(Me 3 Si) 3 C}C 6 H 2 ). [10] Thep hosphorus atoms adopt ap yramidal configuration (sum of angles at P(1) 325.628 8,P (2) 337.948 8), and the SiÀP distances are 2.2666(8) and 2.2392 (8) ,w hich are compara-ble to Si À Pd istances reported for the few other compounds with adirect bond between Pand Si II ;for example,the Si À P distances in (Trip) 2 Si = Si(Trip)(PCy 2 )a nd {PhC(NtBu) 2 }Si-(PiPr 2 )a re 2.2367(12) and 2.307(8) ,r espectively. [4,20] The relatively short SiÀPd istances in 7 may suggest ad egree of conjugation between the phosphorus lone pairs and the Si=Si bond. Thep lane of one aromatic ring on each phosphorus center lies parallel to the corresponding ring in the opposite Si(PR 2 ) 2 moiety with an interplane separation of 3.6 , consistent with an offset p-p interaction. This interaction may contribute to the overall stability of the disilene.
As ac onsequence of the restraints used in solving the crystal structure,a ny discussion of the minor disorder  component of 7 must necessarily be more circumspect; however this disorder component appears to have a transbending angle of 23.88 8,w hile the Si(1B) À Si(1C) distance of 2.109(11) appears identical to that in the major disorder component. Theminor disorder component was not observed in the solid-state 29 Si{ 1 H} NMR spectrum of 7,b ut as mall peak is present at À67.4 ppm in the corresponding CP-MAS 31 P{ 1 H} NMR spectrum, which we tentatively ascribe to this component.
Tr eatment of 7 with either lithium or KC 8 yields the phosphanides [(Mes) 2 P]M (M = Li, K) as the sole identifiable phosphorus-containing products.W ealso find that treatment of as lurry of 7 in THF with two equivalents of [(Mes) 2 P]Li-(THF) cleanly yields the triphosphasilanate complex 5.THF. Thef oregoing is consistent with our premise that the triphosphasilanate anions result from cleavage of aP À Si bond in 3 (or its dimer 7), to generate [(Mes) 2 P]Li (or [(Mes) 2 P]K), followed by adduct formation with another molecule of 3 (or 7)togive 4 or 5.
To better understand the bonding in and stability of 7 we have undertaken ad ensity functional theory (DFT) study.
Since the X-ray crystal structure of 7 contains two distinct disorder components,w hich differ chiefly in the degree of trans-bending,w eh ave modeled both of these molecules using the crystallographic coordinates as as tarting point for the optimization;t hese are referred to hereafter as 7 maj and 7 min for the minimum energy geometries corresponding to the major and minor disorder components,respectively (Table 1). Additionally,w eh ave located am inimum energy geometry for the alternative phosphanide-bridged dimer {(Mes) 2 P}Si{m-P(Mes) 2 } 2 Si{P(Mes) 2 }(7 alt ).
Them inimum energy geometry of 7 maj is similar to the crystallographically determined structure,but exhibits atwist between the two SiP 2 units (14.758 8 dihedral angle between the two normals of the SiP 2 planes). Thecalculated trans-bending angles of 38.42 and 40.048 8 are close to that determined crystallographically,but the calculated SiÀSi bond distance of 2.2386 is somewhat longer than that observed in the solid state;t his is likely ac onsequence of the twisted geometry of 7 maj ,which would reduce the SiÀSi p-overlap.T he calculated trans-bending angle of 7 min (6.228 8)d iffers significantly from that in the solid-state structure (23.88 8), although the calculated Si À Si distance (2.182 )i sc lose to that determined crystallographically (2.190(11) ). TheD FT calculations reveal that 7 min and 7 maj are almost isoenergetic,w ith the former just 9.0 kJ mol À1 less stable than the latter.I n comparison, the alternative dimeric form 7 alt ,c ontaining aP 2 Si 2 core,lies 82.7 kJ mol À1 higher in free energy than 7 maj .
Inspection of the molecular orbitals of both 7 maj and 7 min reveals that the HOMO and LUMO are essentially the Si=Si p and p*orbitals,although in both cases there is asignificant component of these orbitals on the phosphorus atoms (Supporting Information). Natural Bond Orbital (NBO) analysis yields Wiberg Bond Indices (WBIs) for the Si = Si bonds in 7 maj and 7 min of 1.411 and 1.551, respectively, consistent with substantial double bond character. TheWBIs for the SiÀPbonds in 7 maj and 7 min range from 0.934 to 1.016, significantly greater than we calculate for as traightforward P À Si II s-bond (see proceeding text), again suggesting some interaction between the phosphorus lone pairs and the Si = Si bond.
To explore the dimerization energy of the putative diphosphasilylene 3 to the tetraphosphadisilene 7 we have calculated the minimum energy geometries of the two extreme forms of the silylene.T hese are 3 plan ,i nw hich the two phosphorus centers approach planarity,and 3 pyr ,inwhich both phosphorus centers adopt ap yramidal configuration (Table 1);all attempts to obtain aminimum energy geometry for am olecule possessing one planar and one pyramidal phosphorus center,a so bserved in the diphosphagermylenes and -stannylenes H,c onverged to 3 plan .F or 3 plan both phosphorus centers are close to planar (sum of angles at P = 352.71 and 352.768 8)a nd the Si À Pd istances (2.208 and 2.207 )a re shorter than is typical for aS i ÀPs ingle bond; however, the SiÀPd istances are longer than previously reported Si=Pb onds in phosphasilenes such as (tBu)-(Trip)Si = P-Si(iPr) 3 (Si = P2 .062(1), Si À P2 .255(1) ), [21] although we note that the latter involves Si IV rather than Si II and fully sp 2 -hybridized and hence smaller phosphorus and silicon centers.The foregoing,along with SiÀPWBIs of 1.221 and 1.222, suggest as ignificant SiÀP p-interaction in 3 plan , despite the slight variation from ap lanar geometry of the phosphorus centers.Incontrast, the SiÀPdistances in 3 pyr are both 2.337 and the WBIs for these bonds are both 0.804, consistent with Si À Psingle bonds.
Our calculations reveal that 3 plan is more stable than 3 pyr by 16.4 kJ mol À1 .These calculations also show dimerization to the disilene 7 maj is strongly favored;t he difference in Gibbs free energy between the disilene 7 maj and two equivalents of 3 plan is + 71.0 kJ mol À1 .T his is in marked contrast to recent calculations on the putative tetraamino-substituted ditetrelenes {(Me 3 Si) 2 N} 2 E = E{N(SiMe 3 ) 2 } 2 ,w hich suggests that dissociation to the corresponding tetrylene monomers {(Me 3 Si) 2 N} 2 E: is strongly favored (E = Ge, DG = À69 kJ mol À1 ;E = Sn, DG = À75 kJ mol À1 ;E = Pb, DG = À45 kJ mol À1 ). [22] In summary,w eh ave shown that au nique tetraphosphadisilene is accessible by reduction of as imple Si IV starting material, using sacrificial lithium phosphanide as the reducing agent. This is the first example of astructurally characterized ditetrelene substituted by more than two heteroatoms from groups 15-17.