Is the phosphaethynolate anion, (OCP)-, an ambident nucleophile? A spectroscopic and computational study

Article abstract of DOI:10.1039/c3dt53569j

The reactivity of Na(OCP) was investigated towards triorganyl compounds of the heavier group 14 elements (R₃EX R = Ph or ⁱPr; E = Si, Ge, Sn, Pb; X = Cl, OTf). In the case of E = Si two constitutional isomers were formed and characterised in situ: R₃Si-O-CP is the kinetic and R₃Si-PCO is the thermodynamic product, representing experimental evidence of the ambident character of the (OCP)^- anion. Applying theoretical calculations and spectroscopic methods, the compound previously reported as ⁱPr₃Si-O-CP can now unambiguously be identified as ⁱPr₃Si-PCO. The heavier analogues form exclusively the phosphaketene isomer R₃E-PCO (E = Ge, Sn, Pb). DFT calculations were performed to gain deeper insight into the bonding and thermodynamic stability of these compounds.

Full text of DOI:10.1039/c3dt53569j

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Is the phosphaethynolate anion, (OCP)−, an
ambident nucleophile? A spectroscopic and
computational study†
Cite this: Dalton Trans., 2014, 43,
5920
Dominikus Heift,a Zoltán Benkő*a and Hansjörg Grützmacher*a,b
The reactivity of Na(OCP) was investigated towards triorganyl compounds of the heavier group 14
i
elements (R3EX R = Ph or Pr; E = Si, Ge, Sn, Pb; X = Cl, OTf). In the case of E = Si two constitutional
isomers were formed and characterised in situ: R3Si–O–CuP is the kinetic and R3Si–PvCvO is the
thermodynamic product, representing experimental evidence of the ambident character of the (OCP)−
anion. Applying theoretical calculations and spectroscopic methods, the compound previously reported
Received 19th December 2013,
Accepted 30th January 2014
i
i
as Pr3Si–O–CuP can now unambiguously be identified as Pr3Si–PvCvO. The heavier analogues form
exclusively the phosphaketene isomer R3E–PvCvO (E = Ge, Sn, Pb). DFT calculations were performed
to gain deeper insight into the bonding and thermodynamic stability of these compounds.
DOI: 10.1039/c3dt53569j
www.rsc.org/dalton
assumed in the reaction of (triisopropylsilyl)bis(trimethylsilyl)-
phosphane [(iPr3Si)(Me3Si)2P] with phosgene; however, no
Introduction
Ambident nucleophiles have not only attracted scientific atten- details have been reported.8 The authors identified the product
1
tion but also play an important role in various synthetic pro- as iPr3Si–O–CuP by extensive 31P-, 13C-, 17O-, 29Si- and H-NMR
cedures.1 For example, enolate anions are remarkably useful spectroscopic investigations and – among others – mentioned a
3
synthons in a plethora of organic transformations. According surprisingly large JPSi coupling constant (51.3 Hz). Further-
to the HSAB concept, soft electrophiles (e.g. alkyl halides) are more, it was reported that iPr3Si–O–CuP is not accessible
attacked by the carbon centre of the enolate anion, which is directly from iPr3SiCl and the lithium salt of the (OCP)−
widely utilised to construct a carbon–carbon bond.2 In con- anion.6,8 In contrast, the nitrogen analogue of the latter,
i
trast, silyl halides as hard electrophiles react with the oxygen (iPrNCP)−, reacts with R3SiCl (R = Ph, Me) to form Pr(R3Si)N–
site, which enables the application of silyl enol ethers as pro- CuP.9,10 The formation of the Si–N bond was explained by the
tecting groups for carbonyl compounds.3 The driving force is stronger affinity of silicon for nitrogen than for phosphorus
the high oxophilicity of silicon, manifested in the exceptionally (σ-bond energies: Si–N: 92.0 vs. Si–P: 62.7 kcal mol−1).4
large bond energy of the Si–O σ-bond (111.2 kcal mol−1),
Recently, the reactivity of the (OCP)− anion11–14 was investi-
which exceeds the Si–C σ-bond energy (82.6 kcal mol−1).4 In gated experimentally towards different electrophiles,15,16
accordance with the diagonal relationship between carbon ammonium and imidazolium salts17 as well as in cycloaddi-
and phosphorus, the ambident character of phosphaenolate tions.14,15,18 Different theoretical studies have already pro-
anions has been reported in the literature. In silylation reac- posed the ambident character of the (OCP)− anion;16,19
tions, the oxygen centre is the preferred site of reactivity: silyl however, there has been no solid experimental evidence to
substituted acylphosphanes, (Me3Si)2P–CO–R, undergo easily a date to verify this. Alkylation of this anion at the phosphorus
1,3 silyl shift whereby Me3Si–PvCR–O–SiMe3 is obtained.5,6 atom has been assumed in the formation of transient phos-
This observation was explained by the weakness of the Si–P phaketenes.15 We recently reported a (OCP)− rhenium complex
σ-bond (bond energy: 62.7 kcal mol−1 4
compared to the where the phosphorus atom binds to the transition metal
)
strength of the Si–O σ-bond.7 Similarly, silyl migration was centre and which is best described as a metallaphosphaketene
[Re–PvCvO(triphos)(CO)2].16 Herein, we report a systematic
study on the reactivity of the sodium salt of the (OCP)− anion
aDepartment of Chemistry and Applied Biosciences, ETH Zurich, CH-8093 Zurich,
[Na(OCP)] towards various triorganyl tetrel compounds R3EX
i
Switzerland. E-mail: hgruetzmacher@ethz.ch, benkoe@inorg.chem.ethz.ch
bLehn Institute of Functional Materials (LIFM), Sun Yat-Sen University,
where E = Si, Ge, Sn, Pb; R = Ph or Pr; X = Cl or OTf, trifluoro-
methanesulfonate. The results from these experiments will
510275 Guangzhou, China
†Electronic supplementary information (ESI) available. See DOI:
help clarify to what extent the (OCP)− anion can be regarded
as an ambident nucleophile (Scheme 1).
10.1039/c3dt53569j
5920 | Dalton Trans., 2014, 43, 5920–5928
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i
Scheme 2 Reaction of Pr3SiOTf with NaOCP (*: combined amount of
3 and 3’).
spectroscopic data and the expected oxophilicity of silicon, the
Scheme 1 Possible reactivity of Na(OCP) towards triorganyl tetrel
compounds.
i
authors assumed that Pr3Si–O–CuP had formed rather than
i
the Pr3Si–PvCvO isomer. In the light of our calculations,
however, there is no thermodynamic driving force for the for-
mation of this less stable oxyphosphaalkyne isomer. A pre-
vious study found the reaction between Li(OCP) and iPr3SiCl to
be unsuccessful.8 We reacted the more stable sodium salt Na-
(OCP) with the more reactive silyl trifluoromethanesulfonates
R3SiOTf (R = Ph, iPr) in order to elucidate this contradiction.
Results and discussion
We have performed density functional calculations to compute
the relative energies of the phosphaketenes (R3E–PvCvO, R =
H, Ph) versus the corresponding heterosubstituted oxyphospha-
alkynes (R3E–O–CuP). At the B3LYP/cc-pVDZ(-PP) level of
theory the phosphorus bonded analogues (phosphaketenes)
are always more stable than the oxygen bonded isomers for
both R = H and Ph (Fig. 1), and similar tendencies have also
been found at other levels of theory (included in the ESI†).
In the case of carbon (E = C), the energy difference between
the two isomers is remarkably large (e.g. 18.5 kcal mol−1 for
R = Ph). This agrees with our previous experimental obser-
vations: the reaction of Na(OCP) with 4,4′,4″-trimethoxytri-
phenylmethyl chloride was assumed to initially deliver the
phosphaketene, R3C–PvCvO, which was not detected but
dimerises to yield a stable four-membered (R3C–P)2(CvO)2
heterocycle.16
Among the group 14 analogues the smallest relative energy
difference between the two isomers is found in the case of the
silicon compounds (E = Si). This energy difference (7.9 kcal
mol−1 for R = Ph) is outside the range of the computational
error and the silyl phosphaketene is predicted to be more
stable than the silyl oxyphosphaalkyne. This result is inconsist-
ent with a previous experimental study.8 Based on NMR
i
The reaction of Na(OCP) (1) with Pr3SiOTf at room temp-
erature in ethereal solvent leads to three products revealed by
three singlets in the 31P-NMR spectrum (Scheme 2): The reso-
nance at −307.2 ppm has no Si satellites, while the ones at
−371.3 and −368.4 ppm show Si satellites with coupling con-
stants of 48.3 and 46.3 Hz, respectively. As will be discussed
below, the most downfield signal can be assigned unambigu-
i
ously to Pr3Si–O–CuP (2), while the two close upfield peaks
represent two isomers of iPr3Si–PvCvO (3 and the metastable
3′). Although the NMR measurements did not allow us to
identify the exact structures of these two isomers, their very
similar NMR characteristics suggest that 3 and 3′ are two rota-
mers of the same compound. The relative integrals of the peak
areas revealed that at room temperature 2 is formed in 95%
while 3 is present in small amounts (5%). Most remarkably,
when the reaction was performed at 45 °C, the isomer 3 was
formed exclusively.
The concomitant formation of the two constitutional
isomers 2 and 3 provides the first experimental evidence for
the ambident character of the (OCP)− anion. Isomer 2 is not
stable at room temperature; it rearranges slowly to 3. This iso-
merisation was monitored by 31P-NMR spectroscopy at room
temperature (Fig. 2) and the kinetic behaviour was studied
Fig. 1 Relative energy [ΔErel = E(R3E–O–CuP) – E(R3E–PvCvO)] of
the oxyphosphaalkynes (R3E–O–CuP) compared to that of the corres-
ponding phosphaketenes (R3E–PvCvO) at the B3LYP/cc-pVDZ(-PP)
level [E = C, Si, Ge, Sn, Pb; R = H (dotted line), Ph (solid line)].
Fig. 2 Details of the 162 MHz 31P-NMR spectra for the isomerisation
reaction. The time difference between the spectra is 50 min.
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charged controlled reaction product, because the oxygen atom
of the (OCP)− anion is slightly more negatively charged (NPA
charges: O −0.65e, P −0.44e).16
2 and 3 were characterised by NMR- and IR-spectroscopy
and the analytical data were compared to the data of related
compounds reported in the literature. Additionally, theoretical
calculations were performed to bolster the assignments (see
Table 1).20
Scheme 3 Rearrangement of 2 to 3 and 3’.
Based on the similar charge distribution and HOMO orbital
coefficients,9 (iPrNCP)− is very similar to the (OCP)− anion
(Fig. 3) and therefore was included in our computational
studies in order to broaden the data set for our analyses.
using non-linear regression to the data obtained by
integration.
According to the fitted model (for details see ESI†) the
1
Since the 31P-NMR chemical shifts and JPC coupling con-
1
decay of 2 follows first order kinetics with a half-life of t ≈
2
stants in phosphaalkynes (R–CuP) vary within a broad range,
their values are not very informative.8,22 Therefore, we
compare the changes in the chemical shifts and coupling con-
stants, which accompany the formation of iPr3Si–O–CuP from
1.5 h. Both phosphaketene rotamers 3′ and 3 form simul-
taneously from 2 with rate constants of k1 = 2.5 × 10−3 min−1
and k2 = 4.8 × 10−3 min−1, respectively (Scheme 3). Further-
more, the less stable rotamer 3′ slowly converts in a first order
process to 3 (k3 = 1.3 × 10−3 min−1). Thus 3 can either form
directly from the oxyphosphaalkyne or via the intermediate 3′.
The oxyphosphaalkyne 2 is the kinetic and the silyl phos-
phaketene 3 the thermodynamic product of the nucleophilic
substitution reaction (see Scheme 2), which is in agreement
with the lower energy of the latter (vide supra). A simple expla-
nation for the parallel formation of the two constitutional
isomers can be given as follows: Since the largest contribution
in the HOMO of the (OCP)− anion is on the phosphorus atom
(Fig. 3), the orbital controlled product is 3. In contrast, 2 is the
i
(OCP)− and Pr(Me3Si)N–CuP from (iPrNCP)−: Δδ = δ(silylated
compound) – δ(anion). According to the 31P-NMR spectra, sily-
lation of (OCP)− to form 2 is accompanied by a chemical shift
change of Δδ = 84.8 ppm to higher frequency.8 A similar silyla-
tion-shift of Δδ = 88.5 ppm is observed for the (iPrNCP)− ana-
logue. The 13C-NMR spectra show similar shift changes, but in
this case to lower frequency: Δδ = −11.8 ppm for the (OCP)−
and Δδ = −24.9 ppm for the (iPrNCP)− analogue. Furthermore,
1
the JPC coupling constants are also comparable for 2 (9.8 Hz)
i
and Pr(Me3Si)N–CuP (18.2 Hz). In the IR-spectrum of 2 the
frequency of ν = 1656 cm−1 can be assigned to the asymmetric
stretching vibration of the O–CuP unit.23
These experimental findings contradict the previously
i
reported data for Pr3Si–O–CuP.8 Indeed they do fit its consti-
tutional isomer iPr3Si–PvCvO based on the following
arguments:
(A) The P–Si coupling constant of 48.3 Hz is in the range of
typical 1JPSi values (15.5–73.0 Hz).7,24
1
(B) The 13C-NMR chemical shift of 182.5 ppm and the JPC
value of 90.8 Hz correspond to reported data of phospha-
1
1
ketenes (δ = 175.4 ppm, JPC = 91 Hz;16 δ = 206.9 ppm, JPC
=
Fig. 3 HOMO of (OCP)− (left) and (iPrNCP)− (right) anions at the B3LYP/
107.7 Hz25) with the JPC coupling constants much larger than
typical 1JPC couplings in phosphaalkynes.8
1
aug-cc-pVDZ level. Partial NPA charges: (OCP)− anion:
O −0.67,
C +0.12, P −0.45; (iPrNCP)− anion: N −0.69, C −0.13, P −0.28e.
Table 1 Experimental and calculated (B3LYP/IGLOIII//B3LYP/aug-cc-pVDZ level, in parentheses) spectroscopic data of 2, 3 and related compounds
known from the literature. Chemical shifts (δ) are given in ppm and the absolute values of the coupling constants (J) are given in Hz. IR-vibrational
frequencies [νasym(PCO)] are given in cm−1
1/3
δ
31P
δ
13C
1JPC
δ
29Si
J
Psi
δ
17Ο
νasym(PCO)
iPr3SiOCP (2)a
iPr3SiPCO (3)d
iPr3SiOCPLit.d,e
Me3SiOCP
−307.2 (−296.2)
−370.1 (−353.7)
−369.9
145.5 (157.9)
183.4 (199.1)
183.2
9.8 (17.8)
91.8 (150.9)
91.9
30.2 (37.2)
33.1 (43.2)
33.1
−(0.5)b
48.3 (87.8)
48.3
−(128.6)c
288.9 (345.3)
4.9
1656 (1648)
1947 (1963)
—
(−330)f
Na(OCP)g
−392.0 (−428.1)
−137.6 (−114.5)
−226.1 (−224.9)
166.3 (181.1)
153.9 (167.0)
178.8 (189.9)
46.5 (101.9)
18.2 (1.0)
45.7 (60.8)
—
—
3.5 (1.9)
—
155.3 (210.4)
—
—
1755 (1817)
1588 (1606)
—
iPr(Me3Si)NCPd,h
K(iPrNCP)h,i
13.2 (11.2)
—
a In THF-d8–Et2O mixture. b No J coupling detectable due to the large line width (40 Hz). c No signal detectable due to instability of the
compound. d In C6D6. e Reported values for a compound, which was incorrectly described as iPr3SiOCP; for more information see text.
f Calculated at the HF/MP2/aug-cc-pVTZ level.8,19 g Measured in THF-d8,13 IR-spectrum measured with the ATR unit.13,16 h NMR data,21 IR data.10
i In THF-d8.
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(C) The measured 31P-NMR chemical shift (−370.1 ppm) PvCvO (7), the tristannylphosphane (Ph3Sn)3P was identified
fits better the calculated value for iPr3Si–PvCvO as one of the decomposition products. (Ph3Sn)3P was also
(−353.7 ppm) than that for iPr3Si–O–CuP (−296.2 ppm).26
obtained when 7 was heated under vacuum. All phosphake-
(D) Our experimentally obtained 17O-NMR chemical shift of tenes Ph3E–PvCvO [E = Si (5), Ge (6), Sn (7), Pb (8)] were thus
3 (δ = 288.9 ppm) is consistent with the computed value (δ = characterised by NMR- and IR-spectroscopy, and selected
345.3 ppm) and lies in the typical range of ketenes (e.g. δ = analytical data are collected in Table 2.
340 ppm for Ph2CvCvO27 and δ = 255.0 ppm for Me3Si–
CHvCvO28).29
In Table 3 selected computed structural data are listed for
the Ph3E–PvCvO and Ph3E–O–CuP (E = C–Pb) molecules.
(E) The IR-spectrum shows the characteristic stretching The CP bond distances of the phosphaketenes show the
vibration of a phosphaketene at ν = 1947 cm−1 (cf. Mes*– typical values for CvP double bonds, and only minor elonga-
PvCvO: ν = 1970 cm−1: Mes* = 2,4,6-tri-tert-butylphenyl)),25 tion compared to H2CvPH (1.679 Å, B3LYP/cc-pVDZ) can be
which is in good agreement with the calculated value (1963 cm−1
.
observed. The slight shortening of the CO bonds in the phos-
i
Analogous to the reaction of Na(OCP) with Pr3SiOTf, we phaketenes compared to H2CvO (1.204 Å) is due to the de-
have also reacted Na(OCP) with Ph3SiOTf at room temperature localisation of the p-type oxygen lone pair into the π*(CP)
in ethereal solvent. Based on the 31P-NMR spectrum both orbital (vide infra). The bonding parameters do not vary signifi-
Ph3Si–O–CuP (4, δ 31P = −318.3 ppm) and Ph3Si–PvCvO (5, cantly with the variation of the heteroatom E = Si–Pb. The
1
δ
31P = −340.5 ppm, JPSi = 42.7 Hz) isomers were formed in a E–P–C angle is around 90° as was observed in the experi-
1 : 9 ratio. Even when the reaction was performed at low temp- mentally determined structure of the rhenium complex [Re–
erature it was not possible to obtain larger amounts of 4. So PvCvO-(triphos)(CO)2].16 The delocalisation within the OCP
far, due to their high sensitivity, the compounds 4 and 5 have moiety can also be observed in the case of the oxyphosphaalk-
not been isolated and have only been characterised by NMR ynes: the CO bonds are shorter than an average CO single
spectroscopy.
bond (1.416 Å for H3C–OH, B3LYP/cc-pVDZ), while the CP
In contrast to the silyl compounds, the heavier triorganyl bonds are somewhat longer than in H–CuP (1.550 Å, B3LYP/
tetrel chlorides (R3ECl, E = Ge, Sn, Pb) form exclusively the cc-pVDZ).
phosphaketene isomers as predicted by the computations
(Fig. 1). When the reaction mixtures are rapidly worked up, the
germyl Ph3Ge–PvCvO (6) and stannyl Ph3Sn–PvCvO (7)
phosphaketenes can be isolated in good yields as yellow
powders. The lead analogue Ph3Pb–PvCvO (8), even though
it forms cleanly, decomposes when isolation is attempted.
Remarkably, no dimerisation of any of these hetero phospha-
ketenes was observed while all known organyl phosphaketenes
do form dimers.15,25,30–32
All the attempts to grow single crystals of the phospha-
ketenes remained unsuccessful because the compounds
decompose slowly in solution. In experiments with Ph3Sn–
Table 2 NMR- and IR-spectroscopic data for Ph3E–PvCvO (E = Si,
Ge, Sn, Pb). Chemical shifts (δ) are given in ppm and the absolute values
of the coupling constants (J) are given in Hz. IR-vibrational frequencies
[νasym(PCO)] are given in cm−1. Calculated values are shown in paren-
theses [B3LYP/cc-pVDZ(-PP) level]
E
δ
31P
δ
13C
1JPC
νasym (PCO)
Si (5)
Ge (6)
Sn (7)
Pb (8)
−340.5
−344.0
−378.0
−364.0
184.0
182.5
180.6
181.7
99.8
1962 (1977)
1954 (1971)
1946 (1969)
1923 (1950)
105.0
108.0
110.0
Table 3 Selected geometrical parameters for Ph3E–PvCvO (left) and Ph3E–O–CuP (right) (E = C, Si, Ge, Sn, Pb) calculated at the B3LYP/
cc-pVDZ(-PP) level. Bond lengths (d) are given in Å and angles (α) are given in °
E
d(EP)
d(PC)
d(CO)
α(EPC)
α(PCO)
d(EO)
d(OC)
d(CP)
α(EOC)
α(OCP)
C
Si
Ge
Sn
Pb
1.996
2.327
2.398
2.592
2.678
1.698
1.685
1.684
1.683
1.681
1.163
1.164
1.165
1.167
1.169
98.8
92.6
91.5
89.5
88.9
173.7
176.3
176.4
176.3
176.4
1.535
1.770
1.909
2.118
2.262
1.285
1.287
1.279
1.272
1.262
1.571
1.572
1.577
1.582
1.588
122.0
126.8
123.4
123.4
121.4
175.1
177.8
178.0
178.5
179.0
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Fig. 6 Homolytic bond dissociation energies (D0) of the E–O and E–P
σ-bonds (E = C, Si, Ge, Sn, Pb), calculated at the B3LYP/cc-pVDZ(-PP)
level for the model compounds H3E–OH and H3E–PH2, respectively.
Fig. 4 Correlation of the 31P-NMR chemical shifts of the phospha-
ketenes (solid line) with the Allen electronegativity34 of the heteroatom
E (dotted line). The chemical shift for E = C is for Mes*–PvCvO.33
significantly stronger than the corresponding E–P bonds
(based on the homolytic bond dissociation energies, D0, calcu-
lated for the model compounds H3E–PH2 and H3E–OH).
However, even in the case of silyl substituents, R3Si, which are
especially oxophilic, the strength of the Si–O bond and for-
mation of the CuP triple bond in R3Si–O–CuP does not com-
pensate for the loss of the CvO double and (weak) Si–P bond
in R3Si–PvCvO.
For a simple, approximate energy analysis based on bond
increments, the Lewis structures of the two model isomers
H3E–PvCvO and H3E–O–CuP were inspected (see Table 4).
The “O–CvP” moiety with one C–O σ-bond, one C–P σ-bond,
and one CP π-bond is the common motif in both isomers, and
for our analysis we assume that the bonding energies within
this increment remain the same. The differences rely in an
E–P σ- and a CO π-bond present in R3E–PvCvO, while an
E–O σ- and a “second” CP π2-bond can be found in R3E–O–
CuP. The E–P and E–O σ-bond energies are estimated with the
The 31P-NMR chemical shifts lie within a relatively narrow
range (from −340.5 ppm to −378.0 ppm), which is shifted to
lower frequencies compared to Mes*–PvCvO (−207.4 ppm).33
As Fig. 4 shows, the chemical shifts correlate remarkably well
with the electronegativity values as defined by Allen.34
In contrast, the 13C-NMR chemical shifts are almost inde-
pendent of the nature of the heteroatom
E =
(δ 13C
180.6–184.0 ppm) and slightly shifted to lower frequencies
when compared to Mes*–PvCvO (206.9 ppm).33
The 1JPC coupling constants increase slightly in the order Si
< Ge < Sn < Pb, which indicates an increasing s-orbital charac-
ter and moderate strengthening of the PvC bonds. The
decreasing IR frequencies show a concomitant slight weaken-
ing of the CO bonds in the same order 5, 6, 7, 8. These ten-
dencies are also somewhat reflected in the calculated bond
lengths even though the changes are very minor (less than
0.01 Å, see Table 3). A possible explanation for these trends is
the concept of neutral hyperconjugation,28 which is the reason
for the enhanced stability of silyl ketenes.28,35 Fig. 5 illustrates
two possible mesomeric structures (A and B) for compounds
5–8. Resonance structure B shows the hyperconjugative elec-
tron donation from the E–P bond to the PvCvO moiety.
The contribution of the zwitterionic resonance structure B
to the electronic ground state increases with increasing elec-
tron donation of the EPh3 group, which rises from Si to Pb.
Note, however, that the weight of the phosphaketene reso-
nance structure (A) is by far dominant in all compounds.
Why are the phosphaketene isomers more stable than the
oxyphosphaalkynes? As Fig. 6 shows, the E–O bonds are
Table 4 Stabilisation energy increments (kcal mol−1) for H3E–PvCvO
and H3E–O–CuP at the B3LYP/cc-pVDZ(-PP) level. D0: bond dis-
sociation energy of E–P and E–O bonds (for details see text and Fig. 6),
Eπ: CO π-bond energy for H3E–PvCvO and the CP π2-bond energy for
H3E–O–CuP, respectively (for details see the Computational details
section). Edeloc: isodesmic reaction energies based on the reactions
shown in Scheme 4. The total stabilisation energy is calculated from
these three increments as Etotal = D0 + Eπ + Edeloc. The difference
between the total stabilisation energies is calculated as ΔEtotal
=
Etotal(H3E–PvCvO) − Etotal(H3E–O–CuP). The relative energy of the
two isomers (H3E–O–CuP compared to H3E–PvCvO) is ΔErel
E(R3E–O–CuP) – E(R3E–PvCvO), see also Fig. 1
=
E
Compound
D0
Eπ
Edeloc Etotal
ΔEtotal ΔErel
C
Si
H3C–PvCvO
H3C–O–CuP
H3Si–PvCvO
H3Si–O–CuP
70.6 94.6 26.4
95.2 55.3 13.2
66.8 94.6 37.1
114.8 55.3 14.7
62.4 94.6 36.3
97.4 55.3 18.8
55.9 94.6 38.5
85.8 55.3 23.6
48.0 94.6 37.5
70.0 55.3 27.7
191.7 28.1
163.6
198.5 13.7
184.8
193.3 21.7
171.6
189.0 24.4
164.6
27.6
14.1
22.7
25.3
28.2
Ge H3Ge–PvCvO
H3Ge–O–CuP
Sn H3Sn–PvCvO
H3Sn–O–CuP
Pb H3Pb–PvCvO
H3Pb–O–CuP
180.1 27.1
153.0
Fig. 5 Two possible mesomeric structures of phosphaketenes.
5924 | Dalton Trans., 2014, 43, 5920–5928
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PvCvO isomers. The former is the kinetic product, while the
latter is the thermodynamically favoured isomer. This is in
agreement with the DFT calculations, which also show that the
phosphaketenes are lower in energy than the corresponding
oxyphosphaalkynes, and previous reports were corrected in
view of these results. In reactions with alkyl halides, R–X, and
the triorganyl chlorides of the heavier group 14 elements
Ph3ECl (E = Ge, Sn, Pb) exclusively the phosphaketene isomer
(R–PvCvO or Ph3E–PvCvO) was formed, which is – accord-
ing to the calculated relative energies – significantly more
stable than the oxyphosphaalkyne isomer. With a simple
energy decomposition scheme, the relative stability of the two
isomers can be explained. First, the CO π-bond in the phos-
phaketenes, R–PvCvO, is much stronger than the “second”
additional CP π2-bond of the CuP triple bond in the corres-
ponding phosphaalkyne, R–O–CuP. Secondly, the electron
delocalisation within the PvCvO unit of the phosphaketene
delivers approximately twice as much stabilisation energy than
the interaction between the CuP triple bond and the lone
pairs at the oxygen atom in the oxyphosphaalkyne.
Scheme 4 Calculated isodesmic reactions, E = C, Si, Ge, Sn, Pb.
computed homolytic bond dissociation energies (D0) of
H3E–PH2 and H3E–OH (see Table 4).36 The CO π-bond energy in
R3E–PvCvO is approximated by that of formaldehyde (H2CO,
94.6 kcal mol−1 at the B3LYP/cc-pVDZ level). For the estimation
of the “second” CP π2-bond energy in R3E–O–CuP, phos-
phaethyne is used as a model compound (H–CuP, 55.3 kcal
mol−1 at the B3LYP/cc-pVDZ level).36 Both π-bond energies (see
Eπ in Table 4) have been computed based on a previously
reported method4 (for details see the Computational details
section).
Besides the σ- and π-bond energies discussed above, the
delocalisation within the phosphorus–carbon–oxygen unit has
to be taken into account. To gain quantitative information on
the extent of delocalisation, the energies (Edeloc) of the iso-
desmic reactions below (Scheme 4) have been calculated.
A large positive reaction energy indicates significant stabil-
isation due to electron delocalisation in the phosphorus–
carbon–oxygen unit. Note that the NBO analyses show remark-
able delocalisation from the oxygen lone pairs into the π*(CP)
orbitals of the CvP or CuP bond in all compounds. The
calculated isodesmic reaction energies (Edeloc) are also col-
Experimental part
Materials and methods
All reactions were performed using standard Schlenk tech-
niques under a dry argon atmosphere. Sensitive chemicals
were stored and weighed in a glove box under an argon atmos-
phere. All solvents were purified and dried by standard
methods. [Na(OCP)(dioxane)2.5] was prepared following the
procedure published recently.13,15 All other reagents were used
without further purification as received from commercial
suppliers. NMR spectra were recorded on BRUKER 200, 250,
300, 400, 500 or 700 spectrometers. Deuterated solvents were
further dried prior to use. Chemical shifts are reported in ppm
lected in Table 4. The total relative stabilisation energy (Etotal
between both isomers can then be estimated by a comparison
between the sum of the following increments: Etotal = D0 + Eπ +
)
Edeloc
.
According to this simple analysis the reasons for the higher
stability of phosphaketenes compared to the corresponding
phosphaalkynes are: (1) As expected and known from a pre-
vious study the CvO double bond is exceptionally strong,4
and the CO π-bond in R3E–PvCvO is much stronger
(by 39.3 kcal mol−1) than the second CP π2-bond in R3E–O–
CuP. (2) Remarkably, the delocalisation energy (Edeloc) also
strongly favours the phosphaketene isomer and is approxi-
mately twice as high as in the oxyphosphaalkynes. As a conse-
quence of these two effects, all phosphaketenes possess larger
total stabilisation energies compared to the corresponding oxy-
phosphaalkyne isomers and are the thermodynamically more
stable isomers.
1
relative to SiMe4 and 85% H3PO4 for H, 13C and 31P, respecti-
vely. IR spectra were recorded on a Perkin-Elmer-Spectrum
2000 FT-IR spectrometer; spectra in the solid state were col-
lected using an ATR device under an inert atmosphere; spectra
in solution were collected using a liquid IR cell with KBr
windows. The absorption bands are described as follows: very
strong (vs), strong (s), medium (m) and weak (w); br stands for
broad. Elemental analyses were performed at the Microanalysis
Laboratory of the ETH Zurich.
Note that the difference in total stabilisation energies
(ΔEtotal) correlates well with the computed relative energies
[B3LYP/cc-pVDZ(-PP) level] of the two isomers (ΔErel) (see
Table 4) and thereby supports the reliability of the rather crude
energy increment procedure which we applied here.37
Generation of iPr3Si–O–CuP (2). At 25 °C a solution of
[Na(OCP)(dioxane)2.5] (30 mg, 0.1 mmol) in THF (0.3 mL) was
added dropwise to a solution of triisopropylsilyl trifluoro-
methanesulfonate (37 mg, 0.12 mmol) in diethyl ether
(0.3 mL). Besides 2, the solution contained 3 and 3′ (5%). A
17O-NMR spectrum could not be obtained, presumably due to
the low concentration of the sample.
Conclusion
31P{1H}-NMR (81.0 MHz, THF-d8–Et2O, 25 °C): δ (ppm) =
We have provided the first experimental evidence for the ambi- −307.2 (s);
dent character of the (OCP)− anion: The reaction of Na(OCP)
31P{1H}-NMR (81.0 MHz, THF-d8–Et2O, −40 °C): δ (ppm) =
with silyl triflates delivers both the R3Si–O–CuP and R3Si– −308.5 (s);
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13C{1H}-NMR (100.61 MHz, THF-d8–Et2O, −40 °C): δ (ppm) reaction solution was concentrated to 3 mL under reduced
1
= 145.5 (d, JPC = 9.8 Hz, 1C, PCO), 16.7 (s, 6C, CH3), 11.7 (s, pressure to facilitate the precipitation of sodium chloride. The
3C, CH);
solution was passed through a PTFE syringe filter and the
1H-NMR (400.1 MHz, THF-d8–Et2O, −40 °C): δ (ppm) = solvent was removed under reduced pressure to obtain a yellow
1.46–1.32 (m, CH, overlapping), 1.10–1.03 (m, CH3, powder. The product was washed with hexane and dried
overlapping);
in vacuo yielding 274 mg (64%) yellow solid. The compound is
29Si{1H}-NMR (99.4 MHz, THF-d8–Et2O, −50 °C): δ (ppm) = well soluble in THF and slightly soluble in hexane. MP: 84
1
30.2 (br s, ν = 40 Hz);
2 °C.
31P{1H}-NMR (101.26 MHz, THF, 25 °C): δ (ppm) = −344 (s);
13C{1H}-NMR (176.07 MHz, THF, 25 °C): δ (ppm) = 182.5 (d,
2
IR(THF–Et2O): ν = 1656 cm−1 (vs, νasym(PCO)).
i
Generation of Pr3Si–PvCvO (3). A solution of [Na(OCP)-
(dioxane)2.5] (200 mg, 0.66 mmol) in THF (15 mL) was added 1JPC = 105 Hz, PCO), 135–125 (Carom, overlapping);
dropwise very slowly to a solution of triisopropylsilyl trifluoro-
methanesulfonate (255 mg, 0.79 mmol) in diisopropylether (m, Harom);
(2 mL) at 45 °C. To minimise the amount of byproducts, a
diluted solution of Na(OCP) was added to a concentrated solu-
1H-NMR (300.1 MHz, THF-d8, 25 °C): δ (ppm) = 7.90–7.00
IR(hexane): ν = 1954 cm−1 (vs, νasym(PCO));
Elemental analysis (%) calculated for C19H15GeOP: C, 62.88;
tion of the silicon reagent. The reaction solution was stirred H, 4.17; Found: C, 62.61; H, 4.26.
for 15 min at 45 °C, and then the solvent was removed under
Synthesis of Ph3Sn–PvCvO (7). A solution of [Na(OCP)-
reduced pressure. The obtained yellow oily residue was sus- (dioxane)2.5] (2.00 g, 6.6 mmol) in THF (15 mL) was added
pended in 1 mL benzene-d6 and the suspension was filtered slowly, dropwise at room temperature to
a solution of
over celite. The yellow filtrate was investigated by NMR spectro- triphenyltin chloride (2.55 g, 6.6 mmol) in THF (10 mL). Note
scopy. In the 29Si- and 17O-NMR spectra an excess of triisopro- that local high concentrations of Na(OCP) in the reaction solu-
pylsilyl trifluoromethanesulfonate was observed (δ 29Si = 41.6, tion can lead to a consecutive reaction. The yellow reaction
δ
17O = 167.5 ppm, respectively).
solution was concentrated to 5 mL under reduced pressure to
31P{1H}-NMR (101.28 MHz, C6D6, 25 °C): δ (ppm) = −370.1 precipitate the sodium chloride. The solution was filtered and
(s, 1JPSi = 48.3 Hz);
the solvent was removed under reduced pressure to obtain a
13C{1H}-NMR (125.8 MHz, C6D6, 25 °C): δ (ppm) = 183.4 (d, yellow powder. The product was washed with hexane and dried
1JPC = 91.8 Hz, 1C, PCO), 18.8 (d, JPC = 3.5 Hz, 6C, CH3), 14.6 in vacuo. Yield: 2.19 g (78%).
2
(d, 3JPC = 8.7 Hz, 3C, CH);
31P{1H}-NMR (101.22 MHz, THF, 25 °C): δ (ppm) = −378 (s,
1H-NMR (500.3 MHz, C6D6, 25 °C): δ (ppm) = 1.20–1.07 1JSnP = 678 Hz);
(m, CH3, overlapping), 0.99 (m, CH, overlapping);
13C{1H}-NMR (75.47 MHz, THF-d8, 25 °C): δ (ppm) = 180.6
29Si{1H}-NMR (79.49 MHz, C6D6, 25 °C): δ (ppm) = 33.1 (d, JPC = 108 Hz, JSnC = 23.8 Hz, PCO), 140.8–127.2 (Carom
1
2
(d, 1JPSi = 48.3 Hz);
overlapping);
17O-NMR (64.80 MHz, C6D6, 25 °C): δ (ppm) = 289.7 (s, ν1/2
= 60 Hz);
1H-NMR (300.13 MHz, THF-d8, 25°): δ (ppm) = 8.60–6.85
(m, Harom);
IR(THF–Et2O): ν = 1947 cm−1 (vs, νasym(PCO)).
119Sn-NMR (93.28 MHz, THF, 25 °C): δ (ppm) = −50.7 (d,
Reaction of Na(OCP) with Ph3SiOTf. A solution of [Na(OCP)- 1JSnP = 678 Hz);
(dioxane)2.5] (15 mg, 0.05 mmol) in THF-d8 (0.2 mL) was
added dropwise at room temperature to
solution of 1944 cm−1 (vs, νasym(PCO)); IR(solid, ATR): ν = 1930 cm−1 (vs,
triphenylsilyl trifluoromethanesulfonate (25 mg, 0.06 mmol) νasym(PCO));
IR(hexane): ν = 1946 cm−1 (vs, νasym(PCO)); IR(CCl4): ν =
a
in diisopropyl ether (0.3 mL). The reaction mixture contained
Elemental analysis (%) calculated for C19H15SnOP: C, 55.79;
two reaction products according to the 31P-NMR spectrum. All H, 3.70; O, 3.91; Found: C, 54.82; H, 3.86; O, 3.33.
the attempts at isolation failed due to the high sensitivity of
The reaction was repeated following the same synthetic pro-
the products. The 31P{1H}-NMR spectrum (202.5 MHz, THF- cedure with different organotin chlorides. The 31P-NMR chemical
d8–iPr2O, 25 °C) showed two singlet resonances at δ = shifts of these compounds R3Sn–PvCvO are collected below.
1
−318.3 ppm (s) and δ = −340.5 ppm (s, JPSi = 42.7 Hz), which
R = cyclohexyl: 31P{1H}-NMR (101.26 MHz, THF, 25 °C):
we assigned to the compounds Ph3Si–O–CuP (4) and Ph3Si– δ (ppm) = −403.9 (s,1JSnP = 720 Hz);
PvCvO (5), respectively. In the 13C{1H}-NMR spectrum
R = isopropyl: 31P{1H}-NMR (101.26 MHz, THF, 25 °C):
(125.8 MHz, THF-d8–iPr2O, 25 °C) a doublet at δ = 184.0 ppm δ (ppm) = −404.7 (s,1JSnP = 700 Hz);
1
(d, JPC = 99.8 Hz, CuP) was assigned to the phosphaketene
R = benzyl: 31P{1H}-NMR (101.26 MHz, THF, 25 °C):
carbonyl carbon of 5. The IR spectrum (THF–Et2O) of the reac- δ (ppm) = −383.8 (s,1JSnP = 736 Hz);
tion mixture showed two characteristic bands at ν = 1962 cm−1
(vs, νasym(PCO), 5) and ν = 1591 cm−1 (s, νasym(PCO), 4).
Synthesis of Ph3Ge–PvCvO (6). A solution of [Na(OCP)-
R = methyl: 31P{1H}-NMR (202.5 MHz, THF, 25 °C): δ (ppm)
= −378.6 (s).
Reaction of [Na(OCP)(dioxane)2.5] with Ph3PbCl (8). A solu-
(dioxane)2.5] (356 mg, 1.18 mmol) in THF (4 mL) was added tion of [Na(OCP)(dioxane)2.5] (0.663 g, 2.19 mmol) in THF (5 mL)
dropwise at room temperature to a solution of triphenylgerma- was added dropwise at room temperature to a solution of
nium chloride (400 mg, 1.18 mmol) in THF (4 mL). The triphenyllead chloride (1.04 g, 2.19 mmol) in THF (10 mL). The
5926 | Dalton Trans., 2014, 43, 5920–5928
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yellow orange solution was concentrated to 5 mL and was investi-
It has been previously reported that the homolytic dis-
gated by 31P-, 13C-, and 207Pb-NMR spectroscopy. (The highly sen- sociation at a double or triple bond does not give relevant
sitive 8 decomposed partially during the attempts for isolation.)
bond energies;4,46 thus we estimated the π-bond energies with
31P{1H}-NMR (202.46 MHz, THF, 25 °C): δ (ppm) = −364.2 isodesmic reactions.4 The reaction energy (ΔE1) of the follow-
(s, 1JPbP = 1224 Hz);
ing reaction compares the energy of a CvY double bond with
13C{1H}-NMR (100.6 MHz, THF, 25 °C): δ (ppm) = 181.7 (d, that of two C–Y single bonds:
1JPC = 110 Hz, PCO), 159.8–128.1 (Carom, overlapping);
H2CvY þ CH4 þ H2Y ! 2H3C–YH;
1H-NMR (500.23 MHz, THF-d8, 25°): δ (ppm) = 8.1–7.0 (m,
15 H, Harom);
where Y = O or PH. The energy of the double bond (Eσ+π) is
approximated as Eσ+π = 2D0 + ΔE1 and thus the π-contribution
as Eπ = D0 + ΔE1.
207Pb-NMR (104.7 MHz, THF, 25 °C): δ (ppm) = −15.4
(d,1JPbP = 1224 Hz);
IR(THF): ν = 1923 cm−1 (vs, νasym(PCO)).
Similarly, for a CuP triple bond the following isodesmic
reaction can be written:
Computational details
HCP þ 2CH4 þ 2PH3 ! 3H3C–PH2:
The computations were carried out with the Gaussian 09
program package.38 All structures were optimised using the
B3LYP functional. The all valence cc-pVDZ basis set was
applied for H, C, N, O, Si, P and Ge, while for Sn and Pb the
cc-pVDZ-PP basis sets were used, which include pseudopoten-
tials for the simulation of relativistic effects. All the basis sets
with pseudo potentials were obtained from the EMSL Basis Set
Library.39,40 At each of the optimised structures, vibrational
analysis was performed to check that the stationary point
located is a minimum of the potential energy hypersurface. To
investigate the convergence of the basis set on the optimised
geometries, single point calculations have been performed
with the larger aug-cc-pVDZ(-PP) basis set using the B3LYP
functional, and the difference was found to be negligible.
Further calculations have been carried out applying the
ωB97XD and M06-2X functionals and the cc-pVDZ(-PP) basis
set, and the difference between the relative energies obtained
with different functionals proved to be not larger than 4 kcal
mol−1 (see ESI†). For the model compounds H3SiOCP and
H3SiPCO the relative energy of the two isomers was tested
using the CBS-QB3 and CCSD(T)/aug-cc-pVXZ (X = T, Q) levels,
which reproduced well the results obtained at the B3LYP/cc-
pVDZ level. NMR chemical shifts were computed at the B3LYP/
IGLOIII//aug-cc-pVDZ level. The computed 31P chemical shifts
were determined based on a previously reported method,41
and the only difference was that the chemical shift of PH3 in
THF (−239.0 ppm)42 was used as a reference for the conversion
of the absolute isotropic shielding (instead of −266.1 ppm).
For the other elements the computed chemical shifts of
common standards (SiMe4, H2O) were applied as references.
The coupling constants were calculated using the method
reported by Deng et al.43 The harmonic vibrational frequencies
were multiplied by a scaling factor of 0.970. The NBO analysis
was performed with the NBO 5.0 program.44 For the visualisa-
tion of the molecules the Molden program was used.45
The reaction energy of this reaction (ΔE2) accounts for the
energy difference between a CuP triple bond and three C–P
single bonds. The triple bond energy: Eσ+2π = 3D0 + ΔE2. The
“second” π-bond energy was calculated as Eσ+2π − Eσ+π. The
obtained results proved to be similar to previously reported
data (see text).
Acknowledgements
This work was supported by the Swiss National Science Foun-
dation (SNF) and the ETH Zurich. The authors are grateful to
Dr Simone Alidori, Dr Florian Frank Puschmann and Dr René
Verel for their help in some experiments.
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