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Phosphasilene

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Phosphasilene

Phosphasilenes are a class of compounds with silicon-phosphorus double bonds.[1] Since the electronegativity of phosphorus (2.1) is higher than that of silicon (1.9), the "Si=P" moiety of phosphasilene is polarized.[2][3] The degree of polarization can be tuned by altering the coordination numbers of the Si and P centers, or modifying the electronic properties of the substituents.[2] As a deviation from the "double bond rule", the phosphasilene Si=P double bond is highly reactive, yet with the choice of proper substituents, it can be stabilized via donor-acceptor interaction or by steric congestion.[3]

History

Structure of the first phosphasilene prepared by Bickelhaupt et al.[4]

The landmark discovery of the first phosphasilene by NMR spectroscopy was made in 1984 by Bickelhaupt et al.[4] The first phosphasilene came with bulky aryl substituents at the phosphorus and silicon atoms.[4] Almost a decade after this spectroscopic observation, the first structural characterization of phosphasilene was achieved in 1993 by Niecke et al.[5]

Synthesis

β-Elimination

The synthetic pathway towards the first metastable arylphosphasilenes developed by Bickelhaupt et al.[2][4][6][7]

An important synthetic pathway towards phosphasilene is the 1,2-elimination reactions of silylphosphane derivatives. The first metastable arylphosphasilenes were accessed by Bickelhaupt et al. via the deprotonation of in situ formed (chlorosilyl)phosphanes ArP(H)–(Cl)SiAr2 using organolithium bases.[2][4][6][7] As shown in the scheme on the left, it is also possible to use pre-formed lithium phosphanides Ar'P(H)Li as both a phosphorus source and a base. However, the latter synthetic pathway involves formation of primary phosphines ArPH2, which can be difficult of be separated from phosphasilenes.[2] Despite such a drawback, this strategy has been successfully applied by Niecke et al. to obtain a series of phospha-2-silaallyl anions, which serve as precursors for 2-phosphanylphosphasilenes.[2][8]

Applying an analogous strategy, Driess and coworkers developed an effective approach for synthesizing P-silyl phosphasilenes via the thermal elimination of LiF from corresponding lithium (flouorosilyl)phosphanides.

Synthesis of Phosphasilenes with 4-coordinate Silicon

Phosphasilenes with 4-coordinate silicon, which can also be viewed as silylene-stabilized phosphinidene, can be synthesized based on the reactivities of stable silyene complexes.[2]

Structure and Bonding

The parent phosphasilene H2Si=PH

The unstable parent phosphasilene H2Si=PH has been generated in the gas phase by the reacting atomic silicon with phosphine PH3, and identified via matrix isolation spectroscopy methods.[9] DFT calculations suggests that in the ground state, H2Si=PH exists in a singlet spin state, with Cs-symmetric planar geometry.[9] At the B3LYP/6–311+G** level of theory, the Si=P bond length and the Si-P-H bond angle are calculated to be 2.084 Å and 90.7o.[9] The Si=P bond dissociation energy is 75.0 kcal mol-1 at the B97-D/6-31G(d) level of theory; while the π-bond energy, Dπ(Si=P) is 36.6/35.9 kcal mol-1.[10] The frontier orbitals of the parent phosphasilene consists of the π bonding and π anti-bonding orbitals: π(Si=P) and π*(Si=P) correspond to the HOMO and LUMO, respectively. HOMO-1 was calculated to be the lone pair on phosphorus n(P).[11]

"Half"-Parent Phosphasilene R2Si=PH

Driess and coworkers prepared thermally robust "half"-parent phosphasilene R2Si=PH (R2Si = (tBu3Si)(iPr3C6H2)Si), which is the first example of phosphasilene with a terminal PH group.[12] This species was obtained as a mixture of E/Z isomers, thus its 31P NMR spectrum featured two doublets with 29Si satellites (δ=123.1, 1J (P, H)=123 Hz, 1J (P, Si)=157 Hz and δ=134.2 ppm, 1J (P, H)=131 Hz, 1J (P, Si)=130 Hz).[12] These 1J (P, H) coupling constants are much smaller compared to those of secondary phosphanes (R2PH) and phosphaalkenes with a PH group, which indicates that the phosphasilene phosphorus atom possesses more 3p character.[12] X-ray crystallography of this "half"-parent phosphasilene species shows that the silicon atom occupies a trigonal-planar coordination environment.[12] The Si=P bond distance was reported to be 2.094(5) Å, which is about 7% shorter than a typical silicon-phosphorus single bond, but only slightly longer than that of P-silyl-substituted phosphasilenes,[12] which suggests that the potential of Si=P bond on a potential energy surface is relatively shallow.[12][13]

π-conjugated phosphasilenes

Tamao et al. reported a series of π-conjugated phosphasilenes stabilized by Eind groups.[14] These systems feature Si=P units that are highly coplanar with the aromatic ring, allowing strong π → π* absorptions. The coplanarity is made possible by the rigidity of the two Eind groups that are oriented trans and perpendicular with respect to the Si=P bond.[14] The Si=P bond length observed by X-ray crystallography are ca. 2.09-2.10 Å, which are typical for phosphasilenes.[14]

Frontier orbitals of π-conjugated phosphasilene reported by Tamao et al.[14]

The bonding of π-conjugated phosphasilenes has been probed by DFT calculations at the B3LYP/6-31G** level. The HOMO was calculated to represent mostly the 3pπ(Si–P), while the LUMO featured significant contribution from the 3pπ*(Si–P)–2pπ*(phenyl) conjugation. The HOMO-1 orbital involves the 3n–2pπ conjugation, which originate from the presence of lone pair on the phosphorus atom and the π-orbital on the Eind benzene ring.[14]

"Push-pull" Phosphasilene

By installing electron-donating substituents on silicon and electron-withdrawing substituents on phosphorus, the Si=P bond polarization can be decreased and even reversed through the "push-pull" interaction of the substituents with opposing electronic effects.[15] Applying this design strategy, Escudié et al. prepared stable "push-pull" phosphasilene (tBu2MeSi)2Si=PMes* (Mes* = 2,4,6-tri-tertbutylphenyl) with electron-donating silyl groups on Si and an electron-withdrawing aryl group on P.[15] Computations on the model compound (Me3Si)2Si=PMes (Mes = 2,4,6-trimethylphenyl) demonstrate that n(P) and π*(Si=P) correspond to the HOMO and LUMO, respectively.[15] The relatively small energy gap between the interacting occupied (n(P)) and vacant (π*(Si=P)) molecular orbitals gives rise to a large paramagnetic contribution,[16] which explains the extreme deshielding of the doubly bonded Si and P atoms, as well as the red shift in the UV spectrum that are observed in (tBu2MeSi)2Si=PMes*.[15]

Metallophosphasilenes R2Si=PM

Driess et al. demonstrated that stable metallophosphasilenes of the type R2Si=PM can be prepared from metalation reaction of "half"-parent phosphasilenes R2Si=PH (R2Si = (tBu3Si)(iPr3C6H2)Si).[12]

The proposed n(P) → σ*(Si–Si) negative hyperconjugation in P-zinciophosphasilene by Driess et al. (left) and analogous n(Si–M) → σ*(Si–Si) hyperconjugation in isoelectronic silyl-substituted alkali-metal disilenylides.[12][17][18]

Reactivity

Reaction with Lewis bases

Reaction with Lewis acids

Metalation

Metalation of phosphasilenes gives rise to either complexes featuring the coordination of the phosphorus lone pair to a metal center [19][3][20] or P-metalated phosphasilenes.[21][12][22] In the former case, the binding of the phosphasilenes to transition metals via the phosphorus lone pair reduces the double-bond character of the Si–P bond.[2] Some examples of this type of phosphasilene transition metal complexes are shown below.

Examples of complexes with phosphasilene coordinated to transition metals via the phosphorus lone pair.[19][3][20]

Driess and coworkers first observed the formation of P-metalated phosphasilenes of the latter case: P-ferrio-substituted phosphasilene R2Si=P[Fe(CO)2(η5-C5H5)] (R = 2,4,6-iPr3C6H2).[21] They further demonstrated that P-metalated phosphasilenes R2Si=PM can be obtained by metalating "half"-parent phosphasilenes, which substitutes the R2Si=PH hydrogen atom with transition metal-containing fragments.[3][12]

Phosphinidene transfer

Phosphasilenes with significant zwitterionic characters undergoes facile hemolytic cleavage of the fragile Si=P bond. This can be utilized for the liberation and transfer of phosphinidene (:PH) to unsaturated organic molecules.[23] Driess et al. demonstrated that a fragile "half-parent" phosphasilene LSi=PH (L = CH[(C=CH2)CMe(NAr)2]; Ar = 2,6-iPr2C6H3) with highly shielded PH moiety is capable of transferring :PH to NHC.[23]

Phosphasilene as a phosphinidene (:PH) transfer agent.[23]
The polarized Si=P π orbital (HOMO-1) of phosphasilene LSi=PH (L = CH[(C=CH2)CMe(NAr)2]; Ar = 2,6-iPr2C6H3) prepared by Driess et al.[23]

Theoretical investigation by DFT (B3LYP/6-31G(d) level) revealed that this phosphasilene bears two highly localized lone pairs on the phosphorus atom due to the LSi=PH ↔ LSi–P+H- resonance. Based on natural bond orbital (NBO) analysis, the σ bond of Si=P involves even contributions from Si and P, while the π bond (HOMO-1) is strongly polarized to the phosphorus atom. This indicates that the π bond between silicon and phosphorus is not predominant, supporting the significance of the zwitterionic resonance structure in the description of Si–P bonding.[23]

Small molecule activation

Physical Properties

Nuclear Magnetic Resonance (NMR) spectroscopy

Ultraviolet-visible (UV-Vis) spectroscopy

References

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  1. ^ Driess, M. (1994). "Some Aspects of the Chemistry of Silylidene-Phosphanes and -Arsanes". Coord. Chem. Rev. 145: 1–25 – via Science Direct.
  2. ^ a b c d e f g h Nesterov, V.; Breit, N.; Inoue, S. (2017). "Advances in Phosphasilene Chemistry". Chem. Eur. J. 23 (50): 12014–12039 – via Wiley Online Library.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  3. ^ a b c d e Hansen, K.; Szilvási, T.; Blom, B.; Driess, M. (2015). "Transition Metal Complexes of a "Half-Parent" Phosphasilene Adduct Representing Silylene→Phosphinidene→Metal Complexes". Organometallics. 34 (24): 5703–5708.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  4. ^ a b c d e Smit, C. N.; Lock, F. M.; Bickelhaupt, F. (1984). "2,4,6-Tri-tert-butylphenylphosphene-Dimesitylsilene; The First Phosphasilaalkene". Tetrahedron Lett. 25 (28): 3011–3014 – via Science Direct.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  5. ^ Bender, H. R. G.; Niecke, E.; Nieger, M. (1993). "The First X-ray Structure of a Phosphasilene: 1,3,4-Triphospha-2-silabutene-(1)". J. Am. Chem. Soc. 115: 3314–3315.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  6. ^ a b Smit, C. N.; Bickelhaupt, F. (1987). "Phosphasilenes: synthesis and spectroscopic characterization". Organometallics. 6 (6): 1156–1163.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  7. ^ a b van den Winkel, Y.; Bastiaans, H. M. M.; Bickelhaupt, F. (1991). "Phosphasilene synthesis and reactivity: an improved route to 1-(2,4,6-tri-tert-butylphenyl)-2-tert-butyl-2-(2,4,6-tri-isopropylphenyl)phosphasilene". J. Organomet. Chem. 405 (2): 183–194 – via Science Direct.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  8. ^ Lange, D.; Klein, E.; Bender, H.; Niecke, E.; Nieger, M. Pietschnig, R. (1998). "1,3-Diphospha-2-silaallylic Lithium Complexes and Anions:  Synthesis, Crystal Structures, Reactivity, and Bonding Properties". Organometallics. 17 (12): 2425–2432.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  9. ^ a b c Glatthaar, J.; Maier, G. (2004). "Reaktion von atomarem Silicium mit Phosphan: eine matrix‐spektroskopische Studie". Angew. Chem. 116 (10): 1314–1317 – via Wiley Online Library.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  10. ^ Avakyan, V. G.; Sidorkin, V. F.; Belogolova, E. F.; Guselnikov, S. L.; Gusel’nikov, L. E. (2006). "AIM and ELF Electronic Structure/G2 and G3 π-Bond Energy Relationship for Doubly Bonded Silicon Species, H2SiX (X = E14H2, E15H, E16)". Organometallics. 25 (26): 6007–6013.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  11. ^ Driess, M.; Janoschek, R. (1994). "A comparison of silylidene-amines, -phosphanes and -arsanes: syntheses and quantum chemical calculations". J. Mol. Struct. 313 (1): 129–139 – via Science Direct.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  12. ^ a b c d e f g h i j Driess, M.; Block, S.; Brym, M.; Gamer, M. T. (2006). "Synthesis of a "half"-parent phosphasilene R2Si=PH and its metalation to the corresponding P zinciophosphasilene [R2Si=PM]". Angew. Chem., Int. Ed. 45 (14): 2293–2296.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  13. ^ Dykema, K. J.; Truong, T. N.; Gordon, M. S. (1985). "Studies of silicon-phosphorus bonding". J. Am. Chem. Soc. 107: 4535–4541.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  14. ^ a b c d e Li, B.; Matsuo, T.; Hashizume, D.; Fueno, H.; Tanaka, K.; Tamao, K. (2009). "π-Conjugated Phosphasilenes Stabilized by Fused-Ring Bulky Groups". J. Am. Chem. Soc. 131 (37): 13222–13223.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  15. ^ a b c d Lee, V. Y.; Kawai, M.; Sekiguchi, A.; Ranaivonjatovo, H.; Escudié, J. (2009). "A "Push-Pull" Phosphasilene and Phosphagermene and Their Anion-Radicals". Organometallics. 28 (15): 4262–4265.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  16. ^ Ramsey, N. F. (1950). "Magnetic Shielding of Nuclei in Molecules". Phys. Rev. 78 (6): 699–703.
  17. ^ Inoue, S.; Ichinohe, M.; Sekiguchi, A. (2005). "Disilenyl Anions Derived from Reduction of Tetrakis(di-tert-butylmethylsilyl)disilene with Metal Naphthalenide through a Disilene Dianion Intermediate: Synthesis and Characterization". Chem. Lett. 34 (11): 1564–1565.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  18. ^ Ichinohe, M.; Sanuki, K.; Inoue, S.; Sekiguchi, A. (2004). "Disilenyllithium from Tetrasila-1,3-butadiene:  A Silicon Analogue of a Vinyllithium". Organometallics. 23 (13): 3088–3090.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  19. ^ a b Li, B.; Matsuo,, T.; Fukunaga, T.; Hashizume, D.; Fueno, H.; Tanaka, K.; Tamao, K. (2011). "Neutral and Cationic Gold(I) Complexes with π-Conjugated Phosphasilene Ligands". Organometallics. 30 (13): 3453–3456.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  20. ^ a b Hansen, K.; Szilvási, T.; Blom, B.; Driess, M. (2015). "A Persistent 1,2‐Dihydrophosphasilene Adduct". Angew. Chem. Int. Ed. 54 (50): 15060–15063.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  21. ^ a b Driess, M.; Pritzkow, H.; Winkler, U. (1997). "Synthesis of (fluorosilyl)phosphido iron and nickel complexes and ferriosilyl-substituted (fluorosilyl)phosphanes and evidence for the formation of a metallo phosphasilene of the type [M]-P=SiR2". J. Organomet. Chem. 529: 313–321 – via Science Direct.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  22. ^ Yao, S.; Block, S.; Brym, M.; Driess, M. (2007). "A new type of heteroleptic complex of divalent lead and synthesis of the P-plumbyleniophosphasilene, R2Si=P–Pb(L): (L = β-diketiminate)". Chem. Commun. 37: 3844–3846.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  23. ^ a b c d e Hansen, K.; Szilvási, T.; Blom, B.; Inoue, S.; Epping, J.; Driess, M. (2013). "A Fragile Zwitterionic Phosphasilene as a Transfer Agent of the Elusive Parent Phosphinidene (:PH)". J. Am. Chem. Soc. 135 (32): 11795–11798.{{cite journal}}: CS1 maint: multiple names: authors list (link)
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