Phosphasilene

Phosphasilenes or silylidenephosphanes are a class of compounds with silicon-phosphorus double bonds. Since the electronegativity of phosphorus (2.1) is higher than that of silicon (1.9), the "Si=P" moiety of phosphasilene is polarized. The degree of polarization can be tuned by altering the coordination numbers of the Si and P centers, or by modifying the electronic properties of the substituents. 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. The landmark discovery of the first phosphasilene by NMR spectroscopy was made in 1984 by Bickelhaupt et al. The first phosphasilene came with bulky aryl substituents at the phosphorus and silicon atoms. Almost a decade after this spectroscopic observation, the first structural characterization of phosphasilene was achieved in 1993 by Niecke et al. The successful isolation of phosphasilenes with silicon-phosphorus double bonds represents one of the discoveries that challenged and disproved the "double-bond rule".

Synthesis

Synthesis via β-elimination

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. 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. Despite such a drawback, this strategy has been successfully applied by Niecke et al. to obtain a series of 1,3-diphospha-2-silaallyl anions, which serve as precursors for 2-phosphanylphosphasilenes.

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 based on reactivities of stable silyene

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. For example, Inoue and coworkers demonstrated that benzamidinate-stabilized phosphanylsilylene can give rise to corresponding Si- and P-trimethylsilyl-substituted phosphasilene via thermal rearrangement, while the reaction can also yield 4-disila-1,3-diphosphacyclobuta-diene with the addition of a mild chlorinating agent Ph3PCl2. In these phosphasilenes with four-coordinate silicon, even though formally five bonds are drawn around silicon, the Si–P π bonds are calculated to be strongly polarized towards the P atoms. The contribution from phosphorus and silicon to the π orbitals are calculated to be 87.53% and 12.47%, respectively.

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. Density functional theory (DFT) calculations suggests that in the ground state, H2Si=PH exists in a singlet spin state, with Cs-symmetric planar geometry. 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. 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. 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).

"Half-Parent" Phosphasilene R2Si=PH Driess and coworkers prepared thermally stable "half"-parent phosphasilene R2Si=PH (R2Si = (tBu3Si)(iPr3C6H2)Si), which is the first example of phosphasilene with a terminal PH group. 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). 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. X-ray crystallography of this "half"-parent phosphasilene species shows that the silicon atom occupies a trigonal-planar coordination environment. 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, which suggests that the potential of Si=P bond on a potential energy surface is relatively shallow.

π-conjugated phosphasilenes

Tamao et al. reported a series of π-conjugated phosphasilenes stabilized by Eind groups (Eind=1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl). 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. The Si=P bond length observed by X-ray crystallography are ca. 2.09-2.10 Å, which are typical for phosphasilenes.

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.

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