A fragile zwitterionic phosphasilene as a transfer agent of the elusive parent phosphinidene (:PH)

Article abstract of DOI:10.1021/ja4072699

The simplest parent phosphinidene,:PH (1), has been observed only in the gas phase or low temperature matrices and has escaped rigorous characterization because of its high reactivity. Its liberation and transfer to an unsaturated organic molecule in solu

Full text of DOI:10.1021/ja4072699

Communication
pubs.acs.org/JACS
A Fragile Zwitterionic Phosphasilene as a Transfer Agent of the
Elusive Parent Phosphinidene (:PH)
Kerstin Hansen,† Tibor Szilvasi,‡ Burgert Blom,† Shigeyoshi Inoue,† Jan Epping,† and Matthias Driess*,†
́
†Metalorganics and Inorganic Materials, Department of Chemistry, Technische Universitat Berlin, Straße des 17. Juni 135, Sekr. C2,
̈
10623 Berlin, Germany
‡Department of Inorganic and Analytical Chemistry, Budapest University of Technology and Economics, Szent Geller
́ ́
t ter 4, 1111
Budapest, Hungary
S
* Supporting Information
diaminocarbene−parent phosphinidene adduct, and the first
lithiated N-heterocyclic carbene (NHC)−parent phosphini-
dene adduct 2 was recently reported by Robinson and co-
workers.15 To date, only one phosphasilene with a PH group
has been reported, namely, the “half-parent” phosphasilene 3,
which was recently synthesized in our group16 (Scheme 1).
ABSTRACT: The simplest parent phosphinidene, :PH
(1), has been observed only in the gas phase or low
temperature matrices and has escaped rigorous character-
ization because of its high reactivity. Its liberation and
transfer to an unsaturated organic molecule in solution has
now been accomplished by taking advantage of the facile
homolytic bond cleavage of the fragile SiP bond of the
first zwitterionic phosphasilene LSi=PH (8) (L = CH-
[(CCH2)CMe(NAr)2]; Ar = 2,6-iPr2C6H3). The latter
bears two highly localized lone pairs on the phosphorus
atom due to the LSiPH ↔ LSi+−PH− resonance
structures. Strikingly, the dissociation of 8 in hydrocarbon
solutions occurs even at room temperature, affording the
N-heterocyclic silylene LSi: (9) and 1, which leads to
oligomeric [PH]n clusters in the absence of a trapping
agent. However, in the presence of an N-heterocyclic
carbene as an unsaturated organic substrate, the fragile
phosphasilene 8 acts as a :PH transfer reagent, resulting in
the formation of silylene 9 and phosphaalkene 11 bearing a
terminal PH moiety.
Scheme 1. “Free” Parent Phosphinidene 1, Its Lithiated
NHC Adduct 2, and “Half-Parent” Phosphasilene 3
However, the few examples of PH-containing compounds
(Scheme 1) are rather stable and unsuitable for liberation of the
reactive :PH species at ambient temperature in solution.
Inspired by the facile homolytic SiSi bond cleavage reactions
of some disilenes, R1R2SiSiR1R2, to afford the corresponding
“free” silylenes R1R2Si: (R1 = 2,4,6-tris[bis(trimethylsilyl)-
methyl]phenyl; R2 = 2,4,6-trimethylphenyl),17−20 which feature
a large singlet−triplet energy gap, we reasoned that the very
stable zwitterionic silylene LSi: (L = CH[(CCH2)CMe-
(NAr)2]; Ar = 2,6-iPr2C6H3)21 could be a particularly suitable
leaving group to liberate 1 through homolytic dissociation of
the corresponding phosphasilene LSiPH (8) in solution.
Herein we report the facile synthesis and isolation of 8, the first
fragile “half-parent” phosphasilene, which contains a three-
coordinate silicon atom and features the most shielded PH
moiety reported to date as a result of its strikingly zwitterionic
character. In fact, its ability to facilely transfer :PH toward an
NHC has been shown.
he elusive parent phosphinidene :PH (1) has escaped
Tisolation and has been observed only transiently in the gas
phase or in low temperature matrices.1 Only scant examples of
isolated compounds featuring the PH group exist.2 In analogy
to the parent carbene (:CH2), 1 has a triplet electronic ground
state with a singlet−triplet energy gap of −32.9 kcal·mol−1 (vs a
singlet−triplet energy gap of only −9.05 kcal·mol−1 for :CH2),3
which highlights its immense reactivity. Derivatives of 1 of the
type PR, where R is a bulky substituent that affords kinetic
stabilization, have emerged as a crucial class of compounds in
contemporary chemistry, as exemplified by the seminal work of
Lammertsma and Lappert1,4−6 and numerous further exam-
ples.7,8 However, because of the short lifetime and lability of
“free” 1,1 the ability of an isolable molecule bearing a PH
moiety to act as a phosphinidene transfer reagent has never
been demonstrated. The synthetic potential of such a reagent
would enable new vistas in the synthesis of organophosphorus
compounds, such as phospholes9−11 or other compounds
featuring a PH functionality that represent versatile ligands in
coordination chemistry.12,13
The desired phosphasilene 8 was synthesized in a
straightforward manner by a sequence of substitution and
In 1983, Issleib et al.14 reported the phosphaalkene
[(CH3)2N]2CPH, which could be described as an acyclic
Received: July 16, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja4072699 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
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
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dx.doi.org/10.1021/ja4072699 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
associated with the dissociation of 8 into silylene 9 and the
parent phosphinidene 1 is rather high (ΔG = +42.4
kcal·mol−1).22 However, the formation of the [PH]4 oligomer,
for example, is thermodynamically favorable (ΔG = −3.9
kcal·mol−1), suggesting that the formation of larger [PH]n
clusters is likely even more favorable. In light of the
experimentally observed lability of the PH moiety in 8,
supported by the aforementioned calculations, we tested the
propensity of 8 to act as a :PH transfer agent. In this context, 1
molar equiv of NHC 10 was added into an NMR tube with
phosphasilene 8 in C6D6 at room temperature. Immediately,
the :PH species of 8 was transferred to 10 with concomitant
liberation of silylene 9 (Scheme 3), demonstrating facile :PH
transfer for the first time.
Scheme 4. Calculated Gibbs Free Energies for the Reaction
of 8 with 10
Scheme 3. Transfer of :PH from phosphasilene 8 to NHC 10
(Ar = 2,6-diisopropylphenyl)
barrier of the P−H moiety about the C−P and Si−P axes. The
rotation is almost free when the bond character is close to
phosphinidene with a charge-transfer double bond, while for
standard double bonds the rotation barrier is very high
[ΔG(H2CCH2) = 65 kcal·mol−1].26,27 For phosphaalkene
11 and phosphasilene 8, these barriers are ΔG = 14.0 and 11.3
kcal·mol−1, respectively. These low free energy barriers
demonstrate once more that the π bond between the silicon
and phosphorus atoms in 8 and that between the carbon and
the phosphorus atoms in 11 are not predominant, again
highlighting the ylidic nature represented by structure 8A.
In summary, we synthesized the first zwitterionic “half-
parent” phosphasilene 8 featuring a highly polarized Si−P
bond, which is capable of transferring the elusive :PH species to
an unsaturated organic substrate. The behavior of 8 enables
facile access to PH-functionalized organic molecules, which
have been shown to be useful tools for ligand synthesis in
coordination chemistry. Moreover, the strategy to liberate
elusive low-valent species by facile homolytic bond cleavage of
fragile SiX compounds could be an attractive new method,
for example, to generate the parent borylene (:BH)28,29,8 using
LSiBH.
The formation of phosphaalkene 11 was confirmed by high-
resolution electrospray ionization mass spectrometry (HR-ESI-
1
MS) (m/z 421.27606, 1.5 ppm deviation) and H, 31P, and
13C{1H} NMR spectroscopy. The 31P NMR spectrum shows a
1
doublet at −134.3 ppm [1J(P,H) = 165.5 Hz]. The H NMR
spectrum exhibits a doublet for the proton of the PH group at
1.9 ppm [1J(P,H) = 165.5 Hz]. In the 13C{1H} NMR spectrum,
the doublet for the carbon atom bound to the phosphorus atom
is at 180.5 ppm [1J(P,C) = 85.0 Hz]. These analytical data,
particularly that of the 31P NMR spectrum, are quite similar to
ASSOCIATED CONTENT
■
S
* Supporting Information
1
those of compound 2 [δ(31P) = −143.0 ppm, J(P,H) = 171.0
Details of single-crystal XRD analysis; selected NMR and HR-
ESI-MS spectra of 6−11; and Cartesian coordinates, NBO
analysis, and optimized structures and additional MOs of 8, 11,
and TS. This material is available free of charge via the Internet
at http://pubs.acs.org.
Hz].15
This astonishingly facile :PH transfer reaction to an
unsaturated organic substrate (NHC) is certainly facilitated
by the zwitterionic nature of 8. The reaction mechanism,
according to DFT studies at the B97-D/cc-pVTZ-
(PCM=Benzene)//B97-D/6-31G* level,22 is a one-step
reaction. The Gibbs free energy of activation of the transition
state TS (ΔG⧧ = 11.8 kcal·mol−1) is low enough that the
reaction can proceed at room temperature, resulting in
thermodynamically stable products (9 + 11, ΔG = −13.2
kcal·mol−1) (Scheme 4). In the transition state (TS), the silicon
atom is pyramidalized and adopts a silylene character while the
Si−P bond is elongated to 235.4 pm and the NHC 10 attacks
the phosphorus center (C−P bond distance = 225.2 pm). The
carbon atom in compound 11 has a stronger σ-donor capacity
than the silicon atom in phosphasilene 8. Therefore, it can form
a stronger interaction with the PH moiety. The WBI of
phosphaalkene 11 (1.36) indicates that it has a lower double-
bond character than 8 (WBI = 1.68). In addition to the NBO
analysis, these partial Si−P and C−P double-bond characters
are suggested by transition-state calculations on the rotational
AUTHOR INFORMATION
Corresponding Author
matthias.driess@tu-berlin.de
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The paper is dedicated to Professor Helmut Schwarz on the
occasion of his 70th birthday. We are grateful to the Cluster of
Excellence “UniCat” (sponsored by the Deutsche Forschungs-
gemeinschaft and administered by TU Berlin), the Deutsche
Forschungsgemeinschaft (DR 226/17-2), and the Alexander
von Humboldt Foundation (S.I.) for financial support. We
thank Carsten Prasang and Miriam Stoelzel for fruitful
̈
discussions.
C
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Communication
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