Journal of the American Chemical Society
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subsequent elimination reactions (Scheme 2). Starting with the
Si(IV) precursor 4,21 the reaction with LiPH2·dme (dme = 1,2-
Scheme 2. Synthesis of Compounds 5−8
Figure 1. ORTEP representation of the molecular structure of 8
(thermal ellipsoids show 50% probability; H atoms have been omitted
for clarity, except those of C1, C3, C5, and P1). Selected bond
distances (pm) and angles (deg) in 8: Si1−P1, 207.12(10); Si1−N1,
169.4(2); Si1−N2, 170.1(2); C1−C2, 140.9(4); C4−C5, 142.8(4);
N1−Si1−N2, 106.50(11); Si1−P1−H1, 86(2).
Si−P single bond in compound 6 and slightly (2.3 pm) shorter
than the Si−P bond length in phosphasilene 3 [209.4(5) pm].
To further elucidate the bonding situation in the highly ylidic
system of 8, density functional theory (DFT) studies at the
B3LYP/6-31G(d) level were performed. Natural bond orbital
(NBO) analysis showed that the Si−P bond consists of a σ
bond and a π bond. The Si and P atoms each contribute 50% to
the σ bond. For this bonding, the orbital of the silicon atom
mainly has strong s character (64.58% s, 35.15% p, 0.27% d),
whereas the orbital of the phosphorus atom exhibits high p
character. In Figure 2, the HOMO−7 illustrates this Si−P σ
dimethoxyethane) afforded lithiated bis(phosphido)silane 5.
The bis(phosphido)silane intermediate 6 competes with
unconverted LiPH2·dme and is deprotonated immediately.
Therefore, 3 equiv of LiPH2·dme and a mild proton source
(NH4Cl) were needed to form 6 as a key starting material
(Scheme 2). The following synthetic step involved the
replacement of one PH2 group by chlorine. Accordingly, the
reaction of 6 with HCl afforded the product LSi(Cl)PH2 (7) by
PH3 elimination. Compounds 6 and 7 were isolated as colorless
crystals and characterized by NMR spectroscopy, electron
impact mass spectrometry (EI-MS), elemental analysis, IR
spectroscopy, and single-crystal X-ray diffraction (XRD).22
Subsequently, treatment with lithium diisopropylamide (LDA)
enabled the base-assisted dehydrochlorination of 7, affording
the desired phosphasilene 8 at low temperature as a colorless
solid in 35% yield (Scheme 2). Notably, the 31P NMR spectrum
of 8 shows a doublet signal at −293.9 ppm with 29Si satellites
1
[1J(P,H) = 143.0 Hz, J(Si,P) = 186.4 Hz], which is by far the
most shielded signal for a PH-containing compound reported
to date and provides strong evidence for its formulation as a
highly ylidic phosphasilene.23 In comparison with the “half-
parent” phosphasilene 3 [δ(31P) = 123.1 pm], the 31P signal of
compound 8 is shifted dramatically by 417 pm toward higher
field, indicative of a more covalent bonding situation in the
former compound. Accordingly, the 1H NMR spectrum exhibits
a high-field doublet at −0.7 ppm with 29Si satellites [1J(P,H) =
143.0 Hz, 2J(Si,H) = 18.3 Hz] for the proton of the PH moiety.
Figure 2. (left) HOMO−1 and (right) HOMO−7 of 8.
bond, with an interaction of the hydrogen atom and the lone
pair of the phosphorus, which has strong s character (69.58% s,
30.38% p, 0.04% d). In contrast to the σ bond, the π bond
(HOMO−1) is strongly polarized to the phosphorus atom
(73.45%), which explains the dramatic high-field shift of the 31
P
NMR resonance and provides further evidence for its
formulation as the highly ylidic phosphasilene 8A (Scheme 2)
with two essentially localized lone pairs residing on the
phosphorus atom (Figure 2). The Wiberg bond index (WBI) of
the Si−P bond of 8 is 1.68. These computational results
demonstrate that the π bond between the silicon and
phosphorus atoms is not predominant and further supports
the importance of the zwitterionic resonance structure 8A for
the description of the bonding situation in 8.
Remarkably, phosphasilene 8 was unstable in solution at
room temperature. After a few hours in C6D6, it dissociated into
silylene 9 [δ(29Si) = 88.5 ppm]21 and an insoluble red-brown
solid. The solid-state 31P NMR analysis of the latter revealed
broad signals between +150 and −300 ppm, potentially
corresponding to polyphosphanes of large [PH]n clusters.25
According to DFT calculations [B97-D/cc-pVTZ-
(PCM=Benzene)//B97-D/6-31G*], the Gibbs free energy
1
The small J(P,H) coupling constant indicates that the P 3s
character of the P−H bond is decreased and therefore that the
phosphorus atom adopts a large 3p character in the P−H bond.
The 29Si{1H} NMR spectrum exhibits a low-field doublet at
101.5 ppm [1J(Si,P) = 186.4 Hz], which is characteristic for
24
1
coordinatively unsaturated λ3-Si atoms. The large J(Si,P)
coupling constant of 186.4 Hz is typical for phosphasilenes with
three-coordinate silicon and results from the increase in the s
character of the Si−P bond. These NMR results, particularly
the chemical shift in the 31P NMR spectrum, reveal that
phosphasilene 8 is highly ylidic and that the Si−P double bond
is strongly polarized to the phosphorus atom (resonance form
8A in Scheme 2).
Single-crystal XRD analysis revealed that the silicon atom
adopts a trigonal-planar coordination geometry (Figure 1). The
Si−P bond distance of 207.12(10) pm is ∼8% shorter than the
B
dx.doi.org/10.1021/ja4072699 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX