Crowded organometallic compounds of the alkali metals with diphenylphosphino substituents in the organic group

Article abstract of DOI:10.1039/b002488k

The lithium compound [LiC(SiMe₂CH₂PPh₂)₃] 1, obtained by metaliation of the precursor HC(SiMe₂CH₂PPh₂)₃ I with LiMe, crystallised from benzene as a solvate 1-1.5C

Full text of DOI:10.1039/b002488k

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Crowded organometallic compounds of the alkali metals with
diphenylphosphino substituents in the organic group
Anthony G. Avent, Dominique Bonafoux, Colin Eaborn,* Michael S. Hill, Peter B. Hitchcock
and J. David Smith*
School of Chemistry, Physics and Environmental Science, University of Sussex, Brighton,
UK BN1 9QJ. E-mail: C.Eaborn@sussex.ac.uk; J.D.Smith@sussex.ac.uk
Received 29th March 2000, Accepted 9th May 2000
Published on the Web 8th June 2000
The lithium compound [LiC(SiMe2CH2PPh2)3] 1, obtained by metallation of the precursor HC(SiMe2CH2PPh2)3 I
with LiMe, crystallised from benzene as a solvate 1ؒ1.5C6H6 that both in solution and in the solid state has an
unusual tricyclic structure, with lithium bound to the carbanionic centre and the three phosphorus atoms. Lithiation
of the related precursor HC(SiMe3)2(SiMe2CH2PPh2) II under similar conditions gave the dilithium compound
[Li(thf)C(SiMe3)2{SiMe2CH[Li(thf)2]PPh2}] 2, which has a fluxional structure in solution. Metallation of the
precursor HC(SiMe3)2(SiMe2PPh2) III gave the compounds MC(SiMe3)2(SiMe2PPh2) (M = Li 3 or Na 4). Compound
3 is fluxional in benzene with interchange of methyl groups between SiMe2 and SiMe3 fragments. Compound 4, in
contrast, has a non-fluxional molecular structure. In the solid the molecules of 4 form chains in which the sodium is
bound intramolecularly to the carbanionic centre and to phenyl and intermolecularly to phenyl and phosphorus.
Attempts to make the potassium analogue of 3 or 4 led to cleavage of the P–Si bond and formation of KPPh2. This
has a complicated polymeric crystal structure in which molecules are linked by potassium–phenyl interactions.
We have described in previous papers an extensive series of
organometallic compounds containing bulky ligands of the
general type C(SiMe3)n(SiMe2X)3 Ϫ n in which X is a donor group
capable of co-ordinating intra- or inter-molecularly to the
metal.1 The emphasis so far has been on ligands C(SiMe3)2-
(SiMe2X) and C(SiMe2X)3 in which X = OMe,2,3 SMe4 or
NMe2 5,6 but we have also recently made some compounds con-
taining the ligands C(SiMe3)(SiMe2X)2.7 Intramolecular M–X
bonding is found in the highly unusual Grignard reagent
(Me2NMe2Si)3CMgI,6 which appears to have no Mg–C bond,
and in the cage compounds [LiC(SiMe2OMe)2(SiMe2X)]2
(X = Me7 or OMe3) and intermolecular M–X bonding is found
in the compounds MC(SiMe2NMe2)3 (M = Li6 or K7).
This paper describes the extension of the range of ligands to
some analogous phosphorus-donor species, and the formation
of the organometallic compounds [LiC(SiMe2CH2PPh2)3] 1,
Fig. 1 Molecular structure of [LiC(SiMe2CH2PPh2)3] 1.
[Li(thf)C(SiMe3)2{SiMe2CH[Li(thf)2]PPh2}] 2, LiC(SiMe3)2-
(SiMe2PPh2) 3 and NaC(SiMe3)2(SiMe2PPh2) 4. We had previ-
here. Attempts to make the potassium analogue of 4 led to
ously made the tetramethylethane-1,2-diamine (tmen) complex
formation of KPPh2 6, which has a complex structure with
[Li(tmen)2][C(SiMe2PPh2)3] 5, and found it to consist of separ-
an array of potassium–phosphorus and potassium–phenyl
ated [Li(tmen)2]ϩ cations and planar carbanions [C(SiMe2-
interactions.
PPh2)3]Ϫ.8a In contrast to LiC(SiMe2NMe2)3, the phosphorus
compound does not show any interaction between the donor
atom and the metal, reflecting the weaker co-ordination of the
soft P towards the hard Li. Owing to the ease of cleavage of the
Si–P bonds, compound 5 proved not to be useful in syntheses of
derivatives of other metals so we decided to make the ligand
precursor HC(SiMe2CH2PPh2)3 I in which a CH2 group has
been placed between the P and Si atoms. We now describe the
structures of the product 1, obtained by lithiation of I, and that
of the related compound 2, obtained by lithiation of HC-
(SiMe3)2(SiMe2CH2PPh2) II. We were unable to obtain crystals
suitable for an X-ray study of the lithium derivative 3, made
by lithiation of compound HC(SiMe3)2(SiMe2PPh2) III, but we
were able to determine the structure of the sodium derivative 4.
This, together with the structure of its precursor III, is reported
Results and discussion
The lithium compound 1
Compound I was metallated by use of MeLi in tetrahydrofuran
(thf) at room temperature, but the reaction was only 70% com-
plete and a pure bulk sample of the lithium derivative 1 was not
obtained. We have not so far attempted to optimise the reaction
conditions so it is possible that the yield could be improved by
further work. The product was recrystallised from benzene and
a crystal of 1, taken from the mixture, was shown to have a
tricyclic structure in which donor solvent is excluded from
the lithium co-ordination sphere (Fig. 1). A small amount of
benzene (1.5 mol per mol of 1) was occluded in the lattice. We
DOI: 10.1039/b002488k
J. Chem. Soc., Dalton Trans., 2000, 2183–2190
This journal is © The Royal Society of Chemistry 2000
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Table 1 Bond lengths (Å) and angles (Њ) in LiC(SiMe2CH2PPh2)3 1, [Li(thf)C(SiMe3)2{SiMe2CH[Li(thf)2]PPh2}] 2, HC(SiMe3)2(SiMe2PPh2) III and
NaC(SiMe3)2(SiMe2PPh2) 4
2b
IIIc
4d
a
M = Li or Na
1ؒ1.5C6H6
M–C
M–P
2.222(13)
2.652(11)
1.833(7)
1.813(6)
1.897(6)
1.827(6)
1.882(7)
2.148(8)e
2.517(8)
1.854(5)f
1.744(4)
1.849(4)
1.830(4)f
1.886(5)f
2.558(4)
3.371(2)
1.834(4)f
P–C(Ph)
P–CH
Si–CH
C1–Si
Si–Me
P–Si
1.831(3)f
—
—
1.887(3)f
1.866(3)f
2.295(1)
1.819(4)f
1.875(5)f
2.346(2)
C–M–P
95.4(4)
114.4(3)
110.5(3)
115.8(3)
103.1(3)
113.5(3)
103.9(4)
101.4(3)
Si–C1–Si
C–Si–CH
C–Si–Me
Me–Si–Me
P–CH–Si
M–C1–Si
115.4(2)f
113.5(2)
114.4(2)f
116.6(2)f
114.4(2)f
99.8(2)–106.0(2)
114.3(2)f
116.4(3)g
111.7(2)f
104.0(2)–110.1(2)
114.6 [111.9(2)–116.6(2)]
104.3 [101.2(3)–106.0(3)]
91.7(2), 98.9(2), 111.0(2)
a All values except Li–C are averages. E.s.d.s of individual measurements are given in parentheses; none differs significantly from the mean. Mean
P–Li–P 119.1(4)Њ. b Li–O1 1.910(8), Li–O2 1.959(8) and Li2–O 1.915(8) Å, O1–Li–O2 107.1(4), O–Li–C10 118.2(4), O2–Li–C10 132.1(4), Li–P–C10
92.0(2), P–C10–Si3 114.3(2), Li–C1–Si3 116.4(3), O3–Li–P 116.7(4), O3–Li–C1 141.6(4), Li–C10–Si 116.49(3) and C11–P–17 96.36(19)Њ. c C1–Si–P
112.7(1), C2–Si–P 103.4(1), C3–Si–P 108.9(1), C–P–C 103.3(1), Si–P–C4 104.4(1) and Si–P–C10 98.7(1)Њ. d C1–Si–P 111.8(2), C8–Si–P 103.4(2),
C9–Si–P 104.3(2), C–P–C 102.9(2), Si–P–C16 106.8(2) and Si–P–C10 99.2(2)Њ. e Value for Li–C1, Li–C10 2.151(9) Å. f Mean values, e.s.d.s for
individual measurements in parentheses. g Value for Li–C10–Si, Li–C1–Si1 102.0(3), Li–C1–Si2 106.4(3) and Li–C1–Si3 99.2(3)Њ.
had feared that metallation of I might occur at a CH2 centre as
a consequence of stabilisation of the resulting carbanion by the
joint action of the attached P and Si atoms [cf. the ready metal-
lation of a range of phosphinomethanides,9 and the com-
pounds 2-Ph2PC6H4CH2SiMe3 and {Ph (Me SiN᎐)P} CH 10].
᎐
2
3
2
2
The observed preferential metallation of I at the central quat-
ernary carbon may be due not to a greater acidity of the H at
that centre but to the very effective internal solvation of the Li
atom there. A solution of 1 in thf-d8 showed a 1:1:1:1 quartet
(1JLiP = 44 Hz) in the 31P NMR spectrum and a 1:3:3:1 quartet
7
in the Li spectrum, indicating that the co-ordination of the
three P atoms to Li persists even in the oxygen-donor solvent
and pointing to the remarkable stability of the tricyclic species.
Moreover, compound 1 is obtained exclusively even when the
Fig.
2
Molecular structure of [Li(thf)C(SiMe3)2{SiMe2CH[Li-
(thf)2]PPh2}] 2.
ligation of the PPh2 groups of the ligand, which completes the
lithium co-ordination sphere. The P–CH2 bonds are shorter
than the P–Ph bonds as expected,9,10a but, in contrast to data
for lithium phophinomethanides, the Si–CH2 [1.897(6) Å] is
similar to the Si–Me bond length [1.882(7) Å]. The Si–C(1)
bonds [1.827(6) Å] are shorter, suggesting that, as in similar
species,11 the carbanionic charge is delocalised mainly within
lithiation of HC(SiMe2CH2PPh2)3 is carried out in benzene
containing a tenfold excess of tmen. In contrast, the lithium
phosphinomethanides are obtained as solvent-free complexes
only when they are prepared in hydrocarbon media.9
The anionic ligand in compound 1 shows a close skeletal
resemblance to the trianionic tripodal ligands in metallatranes
the CSi3
core. Although this is not planar as in 5, the
mean Si–C–Si angle (114.4Њ) is significantly greater than the
tetrahedral value.
such as the silatranes RSi(OCH2CH2)3N 7, but the co-
ordination to the metal centre is provided by the lone pair of
electrons of the carbanion rather than by that of the nitrogen
atom. The propeller-shaped lithium derivative has no crystallo-
graphic symmetry but there is an approximate C3 axis along
the Li–C bond and the Si3 and P3 planes are parallel. None
of the chemically equivalent bond lengths or angles differs
significantly from the mean values given in Table 1. The
Li–C [2.222(13) Å] and mean Li–P bond lengths are in
the normal range for phosphinomethanides9 suggesting that
the structure of 1 is remarkably strain-free. The mean C–Li–P
angle (95.4Њ) is considerably less than the tetrahedral angle but
ligation of lithium by solvent is prevented by the preferential
The dilithium compound 2
In order to examine the effect of co-ordination of lithium
by only one phosphorus centre we made the precursor
HC(SiMe3)2(SiMe2CH2PPh2) II. When this was treated with
methyllithium in 1:1 molar proportion unchanged II was
recovered together with the dilithium species 2, which could be
made quantitatively by treating II with two equivalents of
LiMe. The structure of 2 is shown in Fig. 2; the lithium atom
Li2 is bound to the Si3-stabilised carbanionic centre C1 and the
atom Li1 to the P,Si-stabilised centre C10. In the light of the
isolation of 1, in which there is clean lithiation at only one of
the potential sites, the formation of the dilithium species 2 is
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J. Chem. Soc., Dalton Trans., 2000, 2183–2190

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surprising. The absence of any detectable monolithium species
indicates that lithiation at the CH centre facilitates attack at the
CH2 group, which is probably activated by the co-ordination of
the adjacent phosphorus to lithium. It is possible that the effect
in the monolithium derivative of II is greater than that in 1
because the single phosphorus can co-ordinate to Li more
strongly than an individual phosphorus atom in 1, which has to
compete with two others. The strong co-ordination persists in
the dilithium compound 2 as shown by the Li–P bond length
[2.517(8) Å cf. 2.652(11) Å in 1]. We attempted to generate the
monolithiated species by treatment of 2 with a stoichiometric
quantity of [NHMe3]Cl or phenol but were not able to isolate a
clean product.
Bond lengths and angles for compound 2 are given in Table 1.
The Li–C bond lengths are at the short end of the usual range,12
but similar to those in Li(pmdta)CH(SiMe3)2 [2.13(5) Å],13
[LiC(SiMe3)2PMe2]2 [2.172(4) Å],9 or [Li(tmen)CH2PR2]2
[2.141(6)–2.174(6) Å] [R2 = Me2, Ph2 or PhMe, pmdta =
Me2N(CH2)2NMe(CH2)2NMe2].10a,14a The C1–Si [mean 1.830(4)
Å] and C10–Si bonds [1.849(4) Å] are short and the Si–C1–Si
angles [114.3(2)–116.9(2)Њ] are all much wider than the tetra-
hedral value, as in 1 and the related compounds LiC(SiMe3)2-
Fig. 3 Molecular structure of HC(SiMe3)2(SiMe2PPh2) III.
Li between the two carbanionic sites in the [C(SiMe3)2{Si-
Me2CHPPh2}] fragment.
Compounds containing the ligand C(SiMe3)2(SiMe2PPh2)
5
(SiMe2X) (X = OMe2 or NMe2 ) where there is delocalisation
of charge from carbon to silicon. The Li–P bond length is simi-
lar to those in phosphanides that contain three-co-ordinate Li,
e.g. LiPPh2ؒOEt2 [2.483–2.496(10) Å],14b [LiC(SiMe3)2PMe2]2
[2.519(4) Å],9b and a fluorophosphanide [2.517(6) Å].14c The
configuration at Li2 is planar and that at Li1 nearly so (sum of
angles 357.4Њ). In the 5-membered chelate ring each of the
atoms P, Li2, C1 and Si3 is displaced slightly and the atom C10
considerably from the mean plane. As in 1 the P–CH2 are much
shorter than the P–Ph bonds.
The structure of HC(SiMe3)2(SiMe2PPh2) III (Fig. 3) requires
little comment. There is considerable variation in chemically
equivalent bond lengths and angles but, for the most part, the
mean values are similar to those in the related compound
HC(SiMe2PPh2)3 IV.8 The P–Si bond length [2.295(1) Å] is
longer than that in IV [2.275(1) Å]. The Me2–Si–Me3 angle
[110.1(2)Њ] is also remarkably wide (cf. the mean for the other
Me–Si–Me angles in III 106.8 and IV 106.0Њ).
Compound III reacts with methyllithium to give the organo-
lithium compound 3. This may be isolated from toluene–hexane
as large yellow crystals suggesting that the species in the solid
state is well defined, but, in spite of numerous attempts, we have
been unable to obtain a sample suitable for an X-ray study.
Owing to the extreme air- and moisture-sensitivity of the com-
pound we have not been able to obtain a satisfactory C and H
The 1H, 7Li, 13C, 29Si and 31P NMR spectra of compound 2 in
toluene-d8 at 182 K suggest that at this temperature the struc-
ture found in the crystal is present also in solution. The reson-
ances in the 13C spectrum attributed to the carbon atoms C1
and C10 were located by INEPT (insensitive nuclei enhanced by
polarisation transfer) and DEPT90 pulse sequences. The reson-
ance ascribed to the proton attached to C10 was identified by
nuclear Overhauser effect measurements. Irradiation of the
protons attached to ortho-hydrogen atoms of the phenyl groups
(C12, C16, C18 and C22) and those attached to the α-hydrogen
atoms of the thf (C23, C26, C27, C30, C31 and C34) signifi-
cantly enhanced the signal from the C10 proton, whereas
irradiation at other positions gave only weak (C8 and C9) or no
enhancement.
1
analysis but the H NMR spectrum at room temperature is
consistent with the composition [Li(thf)nC(SiMe3)2(SiMe2-
PPh2)] (n = 2 or 3). The 1H, 7Li and 29Si NMR spectra of
samples in the temperature range 248–343 K indicate that the
species in solution undergo a series of reversible concentra-
tion-dependent changes reminiscent of those observed for
Li(thf)2C(SiMe3)3 or other organolithium compounds (e.g.
LiPh) which have thoroughly been investigated.18,19 At 338 K
1
As the sample was allowed to warm to 193 K the resonances
ascribed to the protons attached to the carbon atoms C12 and
C16 merged with those ascribed to the protons attached to
C18 and C22, indicating that the two phenyl groups were
exchanging positions rapidly on the NMR timescale with
∆G‡ = 39 kJ molϪ1. At this temperature the resonance of the
atom Li2 in the 7Li spectrum was a doublet (1JLiP = 44 Hz, cf. 45
Hz for 1 and 48.5 Hz for LiN(SiMe2CH2PPri2)215), showing that
the Li2–P bond was intact. At higher temperatures the doublet
ascribed to Li2 became a singlet and at 271 K the signals from
the two lithium atoms merged. A value of ∆G‡ = 50 kJ molϪ1
was calculated for the exchange process. These results can
be rationalised if there is a significant electrostatic (i.e. non-
localised) component in the bonding between the lithium
centres in 2 and the bulky carbanionic [C(SiMe3)2{SiMe2-
CHPPh2}] group (cf. crystals of the alkali metal derivatives
of aromatic hydrocarbons16a,b). If the structure of 2, as deter-
mined by the X-ray study, is preserved in solution, within a
solvent cage, it is possible to envisage that, as the temperature is
raised, the lithium–carbanion bonding weakens in two stages.
The process with the lower activation energy leads to inversion
at the carbanionic centre C10 (we have recently described
inversions at similar centres17) and the process with slightly
higher activation energy involves interchange of (thf-solvated)
the H, 13C and 29Si signals assigned to the distinct SiMe2 and
SiMe3 groups give single sharp peaks showing that there is
interchange between methyl and PPh2 groups attached to
silicon. There is no evidence for coupling between the P
and the Si, C, or H nuclei, but both the 31P and 7Li signals are
broad, possibly due to unresolved Li–P coupling as well as
quadrupolar relaxation. The NMR spectra at 338 K can be
interpreted in terms of the reversible formation of a solvent-
caged silethene–lithium diphenylphosphanide donor–acceptor
complex, eqn. (1), similar to that proposed to account for the
(1)
chemistry of the compounds MC(SiMe3)2(SiPh2X) (M = Li or
Na; X = F or Br).20a Scrambling of organic groups between SiR2
and SiR3 fragments could proceed through the silethene 820b
but it is not possible to ascertain whether the methyl transfer is
concerted over the CSi3 system. Alternatively, a methyl group
could migrate from SiMe3 to a positive Si centre (cf. ref. 20c)
Ϫ
formed by transfer of PPh2 to lithium without breaking the
Li–C bond. Further work is required to understand the com-
plex equilibria between species (possibly involving oligomers
J. Chem. Soc., Dalton Trans., 2000, 2183–2190
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Fig. 4 Structure of NaC(SiMe3)2(SiMe2PPh2) 4.
Fig. 5 The asymmetric unit in KPPh2 6.
and ate complexes) evident from the NMR spectra obtained
from 3 at low temperatures.
Although we could not determine the structure of compound
3 we were able to isolate the corresponding sodium derivative 4.
This was readily made in 55% yield by treatment of III with
sodium tert-butoxide and methyllithium in thf, and in higher
yield, but less conveniently, from III and methylsodium. The
crystals obtained from pentane were sufficiently air-sensitive to
preclude satisfactory C and H analysis but were more easily
handled than those of 3. Their identity was confirmed by an
X-ray structural determination which indicated that they were
solvent-free. The structure is shown in Fig. 4 and bond lengths
and angles are given in Table 1. The lattice consists essentially
of monomers in which each sodium is co-ordinated to the cen-
tral carbanion and to a phenyl group, with Na–C distances
similar to those in numerous other organometallic com-
pounds.16a The sodium atoms interact more weakly with phos-
phorus atoms (Na–P 3.371(2) Å, cf. 2.883(8) and 2.936(7) Å in
[Na(pmdeta){PH(C6H11)}]2 21a), and with ipso-carbon atoms of
phenyl groups in neighbouring molecules, to give chains along
the screw axis parallel to b. Table 1 shows that the P–C, C1–Si
and Si–Me bond lengths are similar to those in 1–3, but the
P–Si bond length [2.346(2) Å] is unusually long, perhaps as a
consequence of the delocalisation of carbanionic charge on to
silicon [cf. P–Si 2.275(2) Å in HC(SiMe2PPh2)3 IV, 2.323(3) Å
Fig. 6 The K–P framework in compound 6.
so that, in contrast to the situation for 3, there is no evidence
that the P–Si bonds are broken. It appears that the molecular
units found in the crystal are retained in solution.
Reactions between compound 3 and potassium tert-butoxide
or between III and methylpotassium in thf at room temperature
gave an orange solution. The solvent was removed, the residual
solid washed with hexane then dissolved in hot toluene (ca.
50 ЊC). When this solution was allowed to cool orange crystals
separated and a structure determination showed that they were
of potassium diphenylphosphanide KPPh2, 6. This could be
formed by direct replacement of lithium in the LiPPh2 shown
on the right of eqn. (1) or from a potassium analogue of 4,
which is unstable in solution. We have not isolated the silicon-
containing compound but assume it to be the dimer of 8,
in [Li(tmen)2][C(SiMe2PPh2)3],8 2.266(4)
(SiMe3)4,21b 2.17 in [Li(tmen)2][P(SiH3)2],21c 2.110(8)–
Å in Zr(Cp)2P6-
[(Me3Si)2CSiMe2C(SiMe3)2SiMe2],22 which is
a
common
Å
decomposition product of alkali metal compounds containing
organosilyl-substituted carbanions C(SiMe3)2(SiMe2X) in
which X is a good leaving group.20a The disilacyclobutane
would have been discarded in the hexane washings during
work-up.
2.2146(7) Å in P(SiMe3)2 derivatives21d and 2.24–2.29 Å in
most organosilylphosphines21e]. The Si–C1–Si, Me–Si–Me,
C1–Si–Me and C1–Si–P angles are all in the usual range; aver-
age values are given in Table 1 since none of the individual meas-
urements differs significantly from the mean. The Si3–P–C10
angle [99.2(1)Њ] is narrower than the tetrahedral value presum-
ably because the phenyl group C10–C15 is pulled towards the
sodium. The Na ؒ ؒ ؒ C6 and Na ؒ ؒ ؒ C2 distances are short but
not unusually so;16 the methyl groups show no distortion from
tetrahedral geometry and thus there is no indication of a
bonding interaction with the metal.
The structure of KPPh2
Since the crystal structure of KPPh2 had not apparently been
described we decided to refine our data. The structure is exceed-
ingly complicated with seven molecules in the asymmetric unit
shown in Fig. 5. In order to describe the K–P framework
it is necessary to set arbitrary upper limits for the lengths of
constituent bonds. Interactions with K–P <3.5 Å are shown
in Fig. 6 which indicates that 4-membered K1P7K2P1 and
6-membered K4P6K5P4K7P3 rings are linked by K3 which
bridges P1 and P6 and by K6P2 which link P7 and K4. The
Compound 4 dissolves in both aromatic and aliphatic hydro-
carbons, indicating that the weak intermolecular Na ؒ ؒ ؒ P and
Na ؒ ؒ ؒ Ph interactions are broken upon dissolution. In the
NMR spectra (in C6D6) each of the 1H, 13C and 29Si resonances
of the C(SiMe3)2SiMe2 fragment shows coupling to phosphorus
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Table 2 Bond lengths in KPPh2 6
K1–P7 3.3232(13), K1–P1 3.4142(13)
K1–C
K2–C
η5 3.086(4)–3.334(4); η5 3.138(4)–3.315(4)
η6 3.186(4)–3.278(2); η2 3.251(4), 3.302(3)
K2–P1 3.2585(13), K2–P7 3.3386(13),
K2–P5 3.3459(14)
K3–P6 3.3281(14), K3–P1 3.3350(13)
K3–C
η2 3.199(4), 3.337(4); η4 3.279(5)–3.413(4)
η4 3.284(5)–3.390(5)
K4–P2 3.2062(13), K4–P3 3.2291(14)
K5–P6 3.2615(14), K5–P4 3.4019(15)
K6–P2 3.3205(15), K6–P7Ј 3.4352(14),
K7–P3 3.2430(17), K7–P4Љ 3.2831(15),
K7–P4 3.4949(16)
K4–C
K5–C
K6–C
K7–C
η6 3.086(4)–3.290(4); η2 3.335(3), 3.382(4)
η4 3.132(5)–3.494(4); η1 3.316(4)
η2 3.118(4), 3.189(3); η2 3.163(4), 3.218(4)
η2 3.077(4), 3.385(5); η2 3.168(5), 3.238(7)
η1 3.385(5); η1 3.446(4)
1
1
–
–
Symmetry transformations: Ј 3 Ϫ x, y Ϫ 2, Ϫz ϩ 2; Љ Ϫx, Ϫy, Ϫz.
asymmetric units are linked into helices about the 21 screw
axis of the space group and the helices are linked by inversions
at the centres of the (P4K7)2 rings. The 4-membered rings
linking the asymmetric units are reminiscent of those in the
described here show an interesting and varied range of chem-
istry. The lithium compound is markedly more air-sensitive and
the potassium compound much more thermally unstable than
the sodium analogue, which dissolves in hydrocarbons to give
stable molecular species. That 3 is more air-sensitive than 4
may perhaps be because attack at the sodium centre is slowed
down by the intramolecular co-ordination of phenyl groups.
The fluxional properties of the lithium compound 3 are not
surprising in view of previous work,20 but it is noteworthy that a
similar behaviour is not observed with 4. It is possible that co-
ordination of phenyl may push the phosphorus away from the
sodium to make intramolecular elimination of NaPPh2 more
difficult. In contrast, elimination of KPPh2 from the potassium
analogue of 3 or 4 appears to be extremely facile. The process
takes place in thf so the species formed are probably KPPh2
solvates or oligomers like those found for LiPPh2,14b,30 but it is
not possible to discern whether the K–P bonds are formed
intra- or inter-molecularly. The K ؒ ؒ ؒ P, K ؒ ؒ ؒ Ph and K ؒ ؒ ؒ O
interactions appear to be of similar strength so that the balance
between them is a subtle one. When the thf solution of KPPh2
is concentrated the K ؒ ؒ ؒ P and K ؒ ؒ ؒ Ph interactions are suf-
ficiently strong to exclude thf from the crystalline solid that
separates. This is in contrast to the behaviour of LiPPh2, which
crystallises from Et2O or thf as an ether solvate.14b
ladder structures of e.g. KPHC6H3(C6H2Me3-2,4,6)2-2,6 9 or
21d
K(thf)P(SiMe3)2
or lithium phosphanides such as [Li2(µ3-
But2P)(µ-But2P)(thf)]2, or [LiP(SiMe3)2]6.24 Helical structures
have been found in Li{Me(OCH2CH2)2OMe}PHMe.24c Each
phosphorus interacts strongly with two or three potassium
atoms and the framework is held together by further, slightly
weaker, interactions between phosphorus and potassium
neighbours. The co-ordination sphere round potassium is
completed by potassium–phenyl interactions. Those which give
K–C <3.5 Å are listed in Table 2; the range of hapticities
(η1–2 or η4–6) is noteworthy and reflects the complexity of the
K–P framework. The K–P bond lengths [3.2062(13)–3.4949(16)
Å] are in the range associated with electrostatic interactions25
in a number of other phosphanides, e.g. 3.043(2)–3.438 in 9,
3.497(4) in [Cp2ZrHP(C6H2But3-2,4,6)K(thf)2],25 3.306(2)–
3.669(1) in K3(thf)2(PHC6H2Me3-2,4,6)3,26 3.251(2)–3.658(3) in
[K(pmdta){(ButP)2H}]27 and 3.256(1)–3.264(1) Å in phosphole
derivatives.28 Potassium–phenyl interactions have now been
documented in a wide range of organometallic derivatives,16
including a number of phosphanides.23,25 Individual K–C dis-
tances in 6 differ significantly from each other but it appears
that (a) for η5- and η6-co-ordinated Ph groups the bond dis-
tances are shortest (ca. 3.2 Å), (b) for η1- and η4-co-ordinated
groups the distances are longer (3.5–3.6 Å) and (c) for η2-co-
ordinated groups the distances are very variable. These general-
isations are consistent with data tabulated previously.16a
Experimental
Air and moisture were excluded as far as possible by the use of
Schlenk techniques and Ar as blanket gas. Glassware was
flame-dried under vacuum and solvents were dried and distilled
immediately before use. NMR spectra were recorded at 500.1
(1H), 125.6 (13C), 194.5 (7Li), 99.4 (29Si) and 202.4 (31P) MHz
and chemical shifts are given relative to SiMe4 for H, C and Si,
aqueous LiCl for Li, and H3PO4 for P. Except where indicated
solutions in C6D6 at 298 K were used.
Conclusions
We have concentrated in this paper on describing the syntheses
and structures of the alkali metal derivatives derived from
the diphenylphosphino ligand precursors I–III, but it seems
appropriate to consider briefly their potential as ligand transfer
reagents for the syntheses of further organometallic com-
pounds. The ligand C(SiMe2CH2PPh2)3 has the most potential
for the development of further chemistry and we know of no
other strain-free tetradentate ligand containing a hard carb-
anionic centre and three soft phosphorus centres, though
ligands containing one silicon and three phosphorus or
nitrogen centres have been described.29 Compound 1 should be
particularly valuable in making derivatives of later transition
metals but its stability indicates that derivatives of other hard
metals should be accessible also.
The other ligands discussed here have less potential. The pre-
cursor HC(SiMe3)2(SiMe2CH2PPh2) II reacts with LiMe at
both CH and CH2 centres, making subsequent synthetic pro-
cedures complicated, and the ligand C(SiMe3)2(SiMe2PPh2),
like C(SiMe2PPh2)3,8 is easily cleaved at Si–P bonds. Reactions
of compound 3 with a number of metal halides gave complex
mixtures containing a variety of phosphorus compounds simi-
lar to those formed from reactions of 5 with metal halides.8
Nevertheless the lithium, sodium and potassium compounds
Preparations
Tris[(diphenylphosphinomethyl)dimethylsilyl]methane I.
A
31
solution of HC(SiMe2Br)3 (1.75 g, 4.1 mmol) in benzene (40
cm3) was added at room temperature to a suspension of
LiCH2PPh2ؒtmen (5.9 g, 18 mmol)32 in benzene (40 cm3). When
the solution became clear the solvent was pumped off to leave
a white solid, which was crystallised from hexanes to give
HC(SiMe2CH2PPh2)3 I as a white solid (2.22 g, 69%) (Found: C,
66.7; H, 7.0; P, 11.2. C46H55P3Si3 requires C, 70.4; H, 7.1; P,
11.8%). In spite of the low C analysis, no impurities were
detected by NMR spectroscopy: δH (thf-d8) Ϫ0.15 (1 H, s, CH),
0.00 (18 H, s, SiMe2), 1.47 (2 H, s, CH2), 7.23–7.30 and 7.41–7.45
2
3
(30 H, m, Ph). δC 3.2 (d, JCP = 5.4, SiMe2), 4.7 (q, JPC = 43,
1
3
CH), 17.8 (d, JCP = 31.9, CH2), 128.8 (d, JCP = 3, m-C), 128.9
2
1
(s, p-C), 133.0 (d, JPC = 20, o-C) and 142.3 (d, JCP = 18.8 Hz,
ipso-C). δP Ϫ17.4. m/z 784 (20, M), 769 (5, M Ϫ Me), 707 (30,
M Ϫ Ph) and 585 (100%, M Ϫ Ph2PCH2).
Tris[(diphenylphosphinomethyl)dimethylsilyl]methyllithium 1.
A solution of LiMe (9.6 mmol) in Et2O (0.6 cm3) was added to
J. Chem. Soc., Dalton Trans., 2000, 2183–2190
2187

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a solution of compound I (0.61 g, 7.7 mmol) in thf at room
temperature. The mixture was stirred overnight to give a solu-
tion shown by NMR spectroscopy to contain I (34%) and the
lithium compound 1 (66%). δH (thf-d8) 0.28 (18 H, s, SiMe2),
1.61 (6 H, d, 2JPH = 6.6 Hz, CH2), 6.83–7.05 and 7.39–7.44 (m,
Ph). δC (C6D6) 7.1 (s, SiMe2), 30.1 (s, CH2), 128.6 (d, Ph), 133.0
M Ϫ Me), 217 [100, (Me Si) C᎐SiMe ], 201 [25, Me SiC-
᎐
3 2 2 2
(SiMe3)SiMe2], 186 (35, PPh2), 129 (75), 108 (40, PPh) and 73
(75%, SiMe3).
Li(thf)2C(SiMe3)2(SiMe2PPh2) 3. A solution of LiMe (3.28
mmol) in thf (10 cm3) was added to a solution of compound III
(1.23 g, 3.28 mmol) in thf (20 cm3) at room temperature, and
the yellow solution then stirred for 3 h and the solvent removed
under vacuum to leave a dark yellow paste. This was dissolved
in toluene (5 cm3) and the solution carefully layered with hex-
ane, to give large yellow crystals (1.26 g, 75%), mp 88 ЊC (Found:
C, 57.7; H, 9.6. C21H34LiPSi3ؒ2C4H8O requires C, 63.0, H,
9.1%). δH 0.48 (18 H, s, SiMe3), 0.70 (6 H, s, SiMe2), 1.33 (8 H,
m, thf), 3.34 (8 H, m, thf), 6.91–7.15 (6 H, m, Ph) and 7.68–7.72
(4 H, m, o-H). δC 7.0 (s, SiMe3), 8.4 (s, SiMe2), 25.3 (s, thf), 68.4
2
1
(d, JPC = 7.2 Hz, o-C) and 140.0 (ipso-C). δLi 4.2 (q, JLiP = 45
1
Hz). δP –13.0 (q, JLiP = 44.7 Hz). The solvent was pumped off
to leave a white solid mixture of I and 1, which was recrystal-
lised from benzene. A single crystal of 1 was extracted for an
X-ray study.
{(Diphenylphosphinomethyl)dimethylsilyl}bis(trimethylsilyl)-
methane II. A solution of HC(SiMe3)2(SiMe2Br) (3.88 g, 1.30
mmol) in thf (20 cm3) was added at room temperature to a
stirred solution of Li(tmen)CH2PPh2 (4.20 g, 1.30 mmol) in thf
(15 cm3). The yellow solution was stirred for 48 h, and solvent
removed to leave a sticky solid, which was extracted with pent-
ane (2 × 30 cm3). The solvent was removed from the extract
and the residue distilled at 145–148 ЊC at 10Ϫ3 Torr to give com-
pound II as a colourless oil (3.8 g, 70%) (Found: C, 63.3; H, 9.0.
C22H37PSi3 requires C, 63.4; H, 9.0%). δH Ϫ0.59 (1 H, s, CH),
0.14 (18 H, s, SiMe3), 0.16 (6 H, s, SiMe2), 1.44 (2 H, s, CH2),
7.10 (6 H, m, m- and p-H) and 7.51 (4 H, t, o-H). δC 3.0
3
(s, thf), 122.7 (s, p-C), 128.1 (d, JPC = 5.8, m-C), 131.6 (d,
1
2JPC = 15.7) and 149.6 (d, JPC = 31.4 Hz, ipso-C). δLi 0.89.
δSi Ϫ10.2 (SiMe3) and Ϫ10.0 (SiMe2). δP Ϫ38.9. The 1H, 13C and
29Si signals assigned to SiMe2 and SiMe3 groups merge as the
sample is heated from 305 to 338 K.
NaC(SiMe3)2(SiMe2PPh2) 4. A solution of LiMe (4.27
mmol) in thf (10 cm3) was added at room temperature to a
solution of compound III (1.72 g, 4.27 mmol) and NaOBut
(0.41 g, 4.27 mmol) in thf (20 cm3). The cloudy orange mixture
was stirred for 14 h and the solvent then pumped off to give a
sticky residue, which was extracted with pentane (20 cm3). The
extract was filtered to remove a small amount of solid. Some
solvent was removed from the filtrate and slow cooling gave
large pale yellow crystals (1.0 g, 55%), mp 125 ЊC (decomp.)
(Found: C, 57.1; H, 8.1. C21H34NaPSi3 requires C, 59.4; H,
8.1%). No impurities were detected by NMR spectroscopy.
3
(d, JCP = 5.2, SiMe2), 3.3 (s, Si3C), 3.5 (s, SiMe3), 17.3 (d,
3
1JCP = 31.7, CH2), 128.5 (d, JCP = 2.3 Hz, m-C), 128.6 (s, p-C),
133.2 (d, 2JPC = 19.8, o-C) and 142.0 (d, 1JCP = 15.9 Hz, ipso-C).
δP Ϫ16.7. m/z 416 (30, M), 401 (25, M Ϫ Me), 381 (15,
M Ϫ 2Me), 325 (15), 293 (20), 217 (100), 200 (80), 183 (55), 129
(50) and 73 (80%).
Li(thf){C(SiMe3)2[SiMe2CHLi(thf)2PPh2]} 2. A solution of
compound II (0.78 g, 1.88 mmol) in thf (20 cm3) was treated
with LiMe (1.88 mmol) in thf (10 cm3) at Ϫ78 ЊC and the pale
yellow solution allowed to warm to room temperature. The
solvent was removed to leave a sticky glass, from which
unchanged II was removed by washing with cold pentane
(2 × 10 cm3, 0 ЊC). The residual solid was recrystallised from
toluene at Ϫ30 ЊC to give pale yellow blocks of 2, suitable for
an X-ray study (0.29 g. 57% based on LiMe), mp 120–125 ЊC
(softens 80 ЊC). The yield of 2 can be increased to ca. 90%
by using 2LiMe/II stoichiometry (Found: C, 62.8; H, 9.0.
C34H59Li2O3PSi3 requires C, 63.3; H, 9.2%). δH (toluene-d8, 182
3
δH 0.30 (18 H, s, SiMe3), 0.57 (6 H, d, SiMe2, JPH = 3.6 Hz),
6.91–7.08 (6 H, m, m- and p-H) and 7.57 (4 H, m, o-H). δC 2.8
1
(2JPC = 24.1, JSiC = 75.6, CSi3), 6.2 (2JPC = 17.5, SiMe2), 8.4
1
(4JPC = 3.3, JSiC = 56.0, SiMe3), 126.7 (p-C), 128.3 (3JPC = 6.4,
m-C), 134.4 (2JPC = 15.9, o-C) and 141.4 (1JPC = 22.8 Hz, ipso-
C). δSi Ϫ10.6 (3JPSi = 3.7, SiMe3) and Ϫ2.2 (1JPSi = 26.1 Hz,
SiMe2). δP Ϫ38.6. m/z 424 (75, M), 402 [60, RH, R = C(SiMe3)2-
(SiMePPh2)], 386 (70, R Ϫ Me), 217 (R Ϫ PPh2, 95), 201 (100),
185 (60, PPh2), 129 (80), 108 (45) and 73 (90%).
2
K) 0.24 (1 H, d, JPH = 25 Hz, CH), 0.78 (6 H, s, SiMe2), 0.85
Attempted synthesis of KC(SiMe3)2(SiMe2PPh2). A slurry of
KOBut (0.45 g, 4.02 mmol) and compound III (1.63 g, 4.05
mmol) in thf (20 cm3) was treated with a solution of LiMe
(4.02 mmol) in thf (10 cm3). Gas was evolved immediately. The
orange solution was stirred for 14 h, the solvent removed, and
the sticky residue washed with hexane (2 × 20 cm3) then
extracted with hot toluene (30 cm3, 50 ЊC). The extract was
filtered and the filtrate cooled to give orange crystals, which
were identified as KPPh2 6, by an X-ray study.
(18 H, s, SiMe3), 1.04 (12 H, s, thf), 3.15 (12 H, s, thf), 6.94–7.31
(6 H, m, Ph), 7.65 and 7.77 (2 H, s, o-H). δC 3.2 (s, CSi3), 8.1 (s,
3
1
SiMe3), 11.4 (d, JPC = 6.5, SiMe2), 17.3 (d, JPC = 25 Hz, CH),
3
25.3 (s, thf), 68.2 (s, thf), 126.2 (s, p-C), 128.0 (d, JPC = 6.2,
2
1
m-C), 131.7 (d, JPC = 15.8, o-C) and 151.9 (d, JPC = 3.8 Hz,
ipso-C). δLi (toluene-d8, 223 K) 0.47 (s, Li1) and 2.83 (d, 1JLiP 44
2
Hz, Li2). δSi Ϫ11.7 (s, SiMe3) and Ϫ10.2 (d, JPSi = 46.1 Hz).
δP 1.6 (br q, 1JLiP = ca. 48 Hz).
[(Diphenylphosphino)dimethylsilyl]bis(trimethylsilyl)methane
III. A solution of KPPh2 (59.0 mmol) in thf (120 cm3) was
added dropwise at room temperature to a stirred solution of
HC(SiMe3)2(SiMe2Br) (17.6 g, 59.2 mmol) in thf (60 cm3). The
solvent was removed under vacuum to leave a yellow solid
which was extracted with hexane (2 × 100 cm3) and the extract
filtered to remove KBr. The solvent was evaporated from the
filtrate to give a paste, which on standing at room temperature
for one week gave pale yellow crystals of III (22 g, 93%), mp
60–61 ЊC (Found: C, 62.3; H, 8.5. C21H35PSi3 requires C, 62.6;
H, 8.6%). δH Ϫ0.52 (1 H, s, CH), 0.19 (18 H, s, SiMe3), 0.31
Crystallography
For compounds 1, 2, III and 4, data were collected on an Enraf
Nonius CAD4 diffractometer and the structures solved by
direct methods (SHELXS 86) and refined by full matrix least
squares (SHELXL 93).33 Data for structure 6 were collected on
a KappaCCD instrument and refinement was by SHELXL
97.33 All non-hydrogen atoms were anisotropic; hydrogen atom
positions were refined in riding mode for 1, III and Ph and CH2
groups of 2. For 1 one C6H6 solvate molecule was on an inver-
sion centre and the other was at a general position and dis-
ordered; it was modelled as two rigid bodies with isotropic C
atoms in slightly different orientations in the ratio 0.57:0.43.
The Me groups were fixed at idealised geometry but the torsion
angles defining the H atom positions were refined. The H
attached to C(10) in 2 and all those in 4 were freely refined. In 6
a poorly defined molecule of toluene was disordered about an
3
(6 H, d, JPH = 3.3, SiMe2), 7.08 (6 H, m, m-, p-H) and 7.52
2
(4 H, t, J = 9 Hz, o-H). δC 1.1 (d, JCP = 8.9, SiMe2), 2.8 (d,
2
2JCP = 11.4, CSi3), 3.7 (d, JCP = 3.3, SiMe3), 127.8 (p-C), 128.6
(d, 3JCP = 6.4, m-C), 134.3 (d, 2JCP = 17.4, o-C) and 137.1 (ipso-
1
3
C, JCP = 19.1 Hz). δSi Ϫ0.1 (d, JSiP = 4.2, SiMe3) and 2.5 (d,
1JSiP = 27.5 Hz, SiMe2). δP Ϫ46.9. m/z 402 (30, M), 387 (45,
2188
J. Chem. Soc., Dalton Trans., 2000, 2183–2190

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Table 3 Summary of crystallographic data for compounds 1, 2, III, 4 and 6
1ؒ1.5C6H6
2
III
4
6
1
––
Chemical formula
Formula weight
C46H54LiPSi3ؒ1.5C6H6
908.2
C34H59Li2O3PSi3
644.9
C21H35PSi3
402.7
C21H34NaPSi3
424.7
C12H10KPؒ14C7H8
230.85
T/K
Crystal system
Space group
a/Å
b/Å
c/Å
α/Њ
β/Њ
173(2)
173(2)
173(2)
173(2)
173(2)
Triclinic
Triclinic
Monoclinic
P21/c (no. 14)
6.852(2)
19.404(7)
18.012(7)
Orthorhombic
P212121 (no. 19)
10.770(2)
13.683(2)
16.542(4)
Monoclinic
P21/c (no. 14)
15.1313(2)
13.7513(2)
39.8676(6)
¯
¯
P1 (no. 2)
P1 (no. 2)
13.967(3)
13.981(2)
16.707(5)
67.62(2)
65.41(2)
60.03(1)
2507(1)
11.095(6)
11.356(3)
17.505(5)
93.19(2)
91.03(3)
118.29(4)
1937(1)
94.31(2)
94.851(1)
γ/Њ
U/Å3
2388(1)
4
0.27
2438(1)
4
0.28
8266(1)
28
0.55
Z
2
0.23
2
0.19
µ/mmϪ1
R1, wR2 [I > 2σ(I)]
all data
Measured/independent
0.071, 0.116
0.148, 0.145
6125/6125
0.061, 0.130
0.108, 0.152
5383/5383
0.049, 0.098
0.085, 0.115
4555/4190
(0.0399)
2940
0.049, 0.096
0.076, 0.109
4699/4284
(0.0400)
3347
0.055, 0.140
0.094, 0.163
41955/14434
(0.060)
reflections (Rint
)
Reflections with I > 2σ(I)
3424
3570
9528
13 M. F. Lappert, L. M. Engelhardt, C. L. Raston and A. H. White,
J. Chem. Soc., Chem. Commun., 1982, 1323.
14 (a) S. Blaurock, O. Kühl and E. Hey-Hawkins, Organometallics,
1987, 16, 807; (b) R. A. Bartlett, M. M. Olmstead and P. P. Power,
Inorg. Chem., 1986, 25, 1243; (c) M. Driess, S. Rell, H. Pritzkow and
R. Janoschek, Chem. Commun., 1996, 305.
inversion centre; the Me groups were not located. Further
details are given in Table 3.
CCDC reference number 186/1976.
See http://www.rsc.org/suppdata/dt/b0/b002488k/ for crystal-
lographic files in .cif format.
15 M. D. Fryzuk, G. R. Giesbrecht and S. J. Rettig, Organometallics,
1997, 16, 725.
Acknowledgements
We thank the EPSRC for financial support and the EU for a
Marie Curie Fellowship for D. B.
16 (a) J. D. Smith, Adv. Organomet. Chem., 1998, 43, 267; (b) H. Bock,
K. Ruppert, C. Näther, Z. Havlas, H.-F. Herrmann, C. Arad,
I. Göbel, A. John, J. Meuret, S. Nick, A. Rauschenbach, W. Seitz,
T. Vaupel and B. Solouki, Angew. Chem., Int. Ed. Engl., 1992, 31,
550; (c) D. Hoffmann, W. Bauer, F. Hampel, N. J. R. van Eikema
Hommes, P. v. R. Schleyer, P. Otto, U. Pieper, D. Stalke, D. S.
Wright and R. Snaith, J. Am. Chem. Soc., 1994, 116, 528, and
references therein.
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