Synthesis, structure, and solution behavior of a phosphine-borane- stabilized 1,3-dicarbanion

Article abstract of DOI:10.1021/om100454t

The reaction between the phosphine-borane {(Me₃Si)CH ₂}₂P(BH₃)Ph (2) and 2 equiv of n-BuLi in THF yields the 1,3-dicarbanion complex (THF)₂Li{(Me₃SiCH) ₂P(BH₃)Ph}Li(THF)₃ (3), which crystallizes as an unusual contact ion pair. In solution 3 is subject to dynamic exchange which interconverts the two lithium environments.

Full text of DOI:10.1021/om100454t

4774 Organometallics 2010, 29, 4774–4777
DOI: 10.1021/om100454t
Synthesis, Structure, and Solution Behavior of a
Phosphine-Borane-Stabilized 1,3-Dicarbanion†
Keith Izod,* Corinne Wills, William Clegg, and Ross W. Harrington
Main Group Chemistry Laboratories, School of Chemistry, Bedson Building, Newcastle University,
Newcastle upon Tyne NE1 7RU, U.K.
Received May 11, 2010
Summary: The reaction between the phosphine-borane
{(Me3Si)CH2}2P(BH3)Ph (2) and 2 equiv of n-BuLi in THF
yields the 1,3-dicarbanion complex (THF)2Li{(Me3SiCH)2-
P(BH3)Ph}Li(THF)3 (3), which crystallizes as an unusual con-
tact ion pair. In solution 3 is subject to dynamic exchange which
interconverts the two lithium environments.
the exact nature of the binding mode depends on the sub-
stituents at the P and C centers, the metal ion, and the pre-
sence of coligands.
Very recently the groups of Westerhausen and Harder
have independently demonstrated that CH2{PPh2(BH3)}2
undergoes deprotonation at the central carbon atom to give
novel alkali-metal and alkaline-earth-metal derivatives.3,4 In
the latter case Harder and co-workers have shown that the
reaction between CH2{PPh2(BH3)}2 and (4-t-BuC6H4CH2)2-
Ca(THF)4 yields the methandiide complex [[C{PPh2(BH3)}2]-
Ca]2 (1),4 in which two adjacent phosphine-borane groups
support a formal 1,1-dicarbanion; this prompted us to inves-
tigate whether a single phosphine-borane group would sup-
port two adjacent carbanion centers to give a 1,3-dicarbanion.
We describe herein the results of our initial investigations in
this area and report the unusual structure and solution beha-
vior of a novel lithium complex of a 1,3-dicarbanion supported
by both phosphine-borane and silyl groups.
Phosphine-borane-stabilized carbanions are key inter-
mediates in the synthesis of a range of important phosphorus
compounds, including chiral mono- and polyphosphines
with applications as ligands in catalytically active transi-
tion-metal complexes.1 However, until recently phosphine-
borane-stabilized carbanions were almost exclusively pre-
pared and used in situ and the structures of, and bonding in,
these species had not been investigated. Indeed, prior to our
and other groups’ recent efforts,2-5 the coordination chem-
istry of these potentially ambidentate ligands was confined to
a single example, [Ph2P(BH3)CHPPh2(BH3)][Li(tmeda)2],6
which crystallizes as a separated ion pair complex (tmeda =
N,N,N0,N0-tetramethylethylenediamine).
Over the last 3 years, as part of an ongoing project
investigating the coordination chemistry of phosphine-
borane-stabilized carbanions, we have shown that these
ligands adopt a wide variety of coordination modes in their
complexes with the alkali and alkaline-earth metals.2 Ob-
served binding modes include terminal BH3- and C-donor
coordination, chelating modes, and various bridging modes;
Treatment of PhPCl2 with 2 equiv of Me3SiCH2MgCl in
diethyl ether, followed by 1 equiv of BH3 SMe2, yields the
3
air-stable phosphine-borane {(Me3Si)CH2}2P(BH3)Ph (2)
as a colorless, crystalline solid (Scheme 1);7 single crystals of
2 were obtained from cold methylcyclohexane.8 The struc-
ture of 2 (Figure 1) is unexceptional; the P-C(7) and P-C-
˚
(11) distances are 1.8110(17) and 1.8130(17) A, respectively,
˚
whereas the P-B distance is 1.924(2) A.
The 1H, 13C{1H}, 31P{1H}, and 11B{1H} NMR spectra of 2
are as expected; the 31P{1H} spectrum exhibits a broad quar-
tet at 11.7 ppm, while the 11B{1H} spectrum exhibits a broad
doublet at -38.2 ppm (JPB = 58.8 Hz).
† Part of the Dietmar Seyferth Festschrift.
*To whom correspondence should be addressed. E-mail: k.j.izod@
ncl.ac.uk.
(1) For reviews see: (a) Ohf, M.; Holz, J.; Quirmbach, M.; Borner, A.
Synthesis 1998, 1391. (b) Brunel, J. M.; Faure, B.; Maffei, M. Coord. Chem.
Rev. 1998, 178-180, 665. (c) Carboni, B.; Monnier, L. Tetrahedron 1999,
55, 1197. (d) Gaumont, A. C.; Carboni, B. In Science of Synthesis;
Kaufmann, D., Matteson, D. S., Eds.; Thieme: Stuttgart, 2004; Vol. 6, pp
485-512.
The reaction between 2 and 2 equiv of n-BuLi in THF
proceeds cleanly to give the corresponding 1,3-dicarbanion
in good yield as a pale yellow solid, which may be recrys-
tallized from cold THF to give single crystals of the adduct
(2) (a) Izod, K.; McFarlane, W.; Tyson, B. V.; Clegg, W.; Harrington,
R. W. Chem. Commun. 2004, 570. (b) Izod, K.; Wills, C.; Clegg, W.;
Harrington, R. W. Organometallics 2006, 25, 38. (c) Izod, K.; Wills, C.;
Clegg, W.; Harrington, R. W. Organometallics 2006, 25, 5326. (d) Izod, K.;
Wills, C.; Clegg, W.; Harrington, R. W. Inorg. Chem. 2007, 46, 4320.
(e) Izod, K.; Wills, C.; Clegg, W.; Harrington, R. W. Organometallics 2007,
26, 2861. (f) Izod, K.; Wills, C.; Clegg, W.; Harrington, R. W. Dalton Trans.
2007, 3669. (g) Izod, K.; Wills, C.; Clegg, W.; Harrington, R. W.
J. Organomet. Chem. 2007, 692, 5060. (h) Izod, K.; Bowman, L. J.; Wills,
C.; Clegg, W.; Harrington, R. W. Dalton Trans. 2009, 3340.
(7) Synthesis of 2: to a cold (-78 ꢀC) solution of PhPCl2 (1.28 mL, 9.4
mmol) in diethyl ether (30 mL) was added Me3SiCH2MgCl (40 mL of a
0.47 M solution in diethyl ether, 18.8 mmol). The resulting solution was
warmed to room temperature and was stirred overnight. BH3 SMe2
3
(4.7 mL, 9.4 mmol) was added, the solution was stirred for 2 h, and then
water (40 mL) was added. The organic layer was extracted into diethyl
ether (3 ꢀ 30 mL), the combined organic extracts were dried over MgSO4
and filtered, and the solvent was removed in vacuo from the filtrate to
yield 2 as a colorless solid which was purified by crystallization from cold
(-30 ꢀC) methylcyclohexane (20 mL). Isolated yield: 2.24 g, 80%. Anal.
Calcd for C14H30BPSi2: C, 56.74; H, 10.20. Found: C, 56.88; H, 10.15.
1H{11B} NMR (d8-toluene, 25 ꢀC): δ -0.02 (s, 18H, SiMe3), 0.95 (m, 4H,
€
(3) Langer, J.; Wimmer, K.; Gorls, H.; Westerhausen, M. Dalton
Trans. 2009, 2951.
(4) Orzechowski, L.; Jansen, G.; Lutz, M.; Harder, S. Dalton Trans.
2009, 2958.
(5) Sun, X.-M.; Manabe, K.; Lam, W. W.-L.; Shiraishi, N.; Kobayashi,
J.; Shiro, M.; Utsumi, H.; Kobayashi, S. Chem. Eur. J. 2005, 11, 361.
(6) Schmidbaur, H.; Weiss, E.; Zimmer-Gasse, B. Angew. Chem., Int.
Ed. Engl. 1979, 18, 782.
2
CH2), 1.41 (d, JPH = 14.2 Hz, 3H, BH3), 7.09-7.66 (m, 5H, Ph).
13C{1H} NMR (d8-toluene, 25 ꢀC): δ 0.18 (d, JPC = 1.9 Hz, SiMe3),
18.62 (d, JPC = 23.0 Hz, CH2), 128.26 (d, JPC = 9.6 Hz, m-Ph), 130.56
(p-Ph), 131.65 (d, JPC = 8.6 Hz, o-Ph), 134.07 (d, JPC = 50.9 Hz, ipso-
Ph). 11B{1H} NMR (d8-toluene, 25 ꢀC): δ -38.2 (d, JPB = 58.8 Hz).
31P{1H} NMR (d8-toluene, 25 ꢀC): δ 11.7 (q, JPB = 58.8 Hz).
r
pubs.acs.org/Organometallics
Published on Web 07/12/2010
2010 American Chemical Society

Communication
Organometallics, Vol. 29, No. 21, 2010 4775
Figure 2. Molecular structure of 3 with 40% probability ellip-
soids and with C-bound H atoms and THF minor disorder
˚
components omitted for clarity. Selected bond lengths (A) and
Figure 1. Molecular structure of 2 with 40% probability ellip-
˚
angles (deg): Li(1)-C(7) = 2.218(3), Li(1)-C(11) = 2.234(3),
Li(1)-O(1) = 1.975(3), Li(1)-O(2) = 2.014(3), Li(2)-H(0A) =
soids. Selected bond lengths (A) andangles(deg):P-B = 1.924(2),
P-C(1) = 1.8203(18), P-C(7) = 1.8110(17), P-C(11) =
1.8130(17), Si(1)-C(7) = 1.8853(17), Si(2)-C(11) = 1.8852(17);
C(7)-P-C(11) = 106.55(8).
2.01(2), Li(2)-H(0B) = 2.03(2), Li(2) B = 2.384(4), Li(2)-
3 3 3
O(3) = 1.976(3), Li(2)-O(4) = 1.950(3), Li(2)-O(5) = 1.961(3),
P-B = 1.943(2), P-C(1) = 1.8685(16), P-C(7) = 1.7571(17),
P-C(11) = 1.7620(16), Si(1)-C(7) = 1.8179(18), Si(2)-C(11) =
1.8115(16); C(7)-P-C(11) = 109.51(8), C(7)-Li(1)-C(11) =
80.42(11).
a
Scheme 1.
Crude 31P{1H}, 11B{1H}, and 7Li{1H} NMR spectra of this
oil are consistent with the presence of equal amounts of 3 and
unreacted 2, rather than formation of the expected mono-
lithiated species.
a Reagents and conditions: (i) 2 Me3SiCH2MgCl/Et2O; (ii) BH3.
SMe2/Et2O; (iii) 2 n-BuLi/THF.
To our knowledge, with the exception of the dilithium
complex (THF)Li[C(SiMe3)2[SiMe2CH(PPh2){Li(THF)2}]],10
isolated serendipitously from the reaction between HC-
(SiMe3)2(SiMe2CH2PPh2) and MeLi, compound 3 is the first
example of a formal 1,3-dicarbanion with an E-C--
E0-C--E skeleton. The molecular structure of 3 is shown
in Figure 2, along with selected bond lengths and angles.
Compound 3 crystallizes as a contact ion pair ate complex
in which one lithium ion is bound by the two carbanion
centers and by the oxygen atoms of two molecules of THF,
affording a four-coordinate, distorted-tetrahedral geometry
at lithium; the second lithium ion is coordinated by three
molecules of THF and in an η2 manner by the BH3 group of
the dicarbanion, affording a pseudotetrahedral geometry at
this lithium center. Thus, the carbanion ligand acts as a
bidentate C,C0-donor to one lithium ion, generating a dia-
lkyllithate “anion”, while simultaneously bridging via its
borane hydrogen atoms to the lithium “counterion”. The
SiMe3 groups of the ligand are arranged in a cis configura-
tion with respect to the PC2Li ring, positioning them on the
opposite face of the ring to the phenyl group.
(THF)2Li{(Me3SiCH)2P(BH3)Ph}Li(THF)3 (3) (Scheme 1).9
Somewhat surprisingly, 3 appears to be formed irrespective
of the reaction stoichiometry: treatment of 2 with 1 equiv of
n-BuLi in THF, followed by removal of the solvent, yields a
sticky yellow oil, which we were unable to purify further.
(8) Crystallographic data for 2 and 3 are as follows. Data were
˚
collected on a Nonius KappaCCD diffractometer (λ = 0.710 73 A) at
150 K, and structures were solved by direct methods; H atoms of the BH3
groups were refined freely, while others were constrained as riding
atoms. Disorder was satisfactorily resolved for some THF ligands of
3, with the aid of displacement parameter restraints. 2: C14H30BPSi2,
˚
fw = 296.34, monoclinic, space group P21/c, a = 13.792(3) A, b =
3
˚
˚
˚
10.686(3) A, c = 13.1277(15) A, β = 92.447(10)ꢀ, V = 1933.0(7) A ,
Z = 4, μ = 0.252 mm-1, crystal size 0.18 ꢀ 0.10 ꢀ 0.05 mm, Rint = 0.058
(27 151 measured and 4411 unique data), R = 0.039 on F values of 3322
reflections with F2 > 2σ, Rw = 0.095 on all F2 values, goodness of fit
1.062, final difference synthesis within (0.36 e A-3. 3: C34H68BLi2O5P-
˚
˚
Si2, fw = 668.72, triclinic, space group P1, a = 10.193(4) A, b =
˚
˚
11.3220(14) A, c = 19.021(4) A, R = 86.058(13)ꢀ, β = 82.76(2)ꢀ, γ =
75.133(12)ꢀ, V = 2103.2(9) A , Z = 2, μ = 0.156 mm-1, crystal size
3
˚
0.34 ꢀ 0.30 ꢀ 0.30 mm, Rint = 0.036 (32 091 measured and 9453 unique
data), R = 0.045 on F values of 7164 reflections with F2 > 2σ, Rw
=
0.125 on all F2 values, goodness of fit 1.030, final difference synthesis
The Li(1)-C(7) and Li(1)-C(11) distances in 3 (2.218(3)
-3
˚
˚
within (0.80 e A
.
and 2.234(3) A, respectively) compare with Li-C distances
of 2.249(8) and 2.252(8) A in the related ate complex
(9) Synthesis of 3: to a solution of 2 (1.04 g, 3.51 mmol) in THF
(30 mL) was added n-BuLi (2.81 mL, 7.03 mmol), and the resulting
solution was stirred for 1 h. The solution was concentrated to approxi-
mately 5 mL and was cooled to -30 ꢀC for 16 h, yielding pale yellow
blocks of 3 suitable for X-ray crystallography. Isolated yield: 1.41 g,
60%. Anal. Calcd for C34H68BLi2O5PSi2: C, 61.06; H, 10.25. Found: C,
˚
(THF)3Li{(Me3Si)2CPMe2(BH3)}2Li (4),2b which contains
a monodentate, sterically hindered phosphine-borane-sta-
˚
bilized carbanion, and with the Li-C distance of 2.213(5) A
in the anion of [{(Me3Si)3C}2Li][Li(tmeda)2] (tmeda = N,N,
60.88; H, 10.14. 1H{11B} NMR (d8-toluene, 25 ꢀC): δ -0.25 (d, JPH
=
N0,N0-tetramethylethylenediamine);11 the C(7)-Li(1)-C(11)
17.4 Hz, 2H, CH), 0.22 (s, 18H, SiMe3), 1.07 (d, JPH = 12.9 Hz, BH3),
1.49 (m, 20H, THF), 3.61 (m, 20H, THF), 7.14 (m, 1H, p-Ph), 7.27 (m,
2H, m-Ph), 8.10 (m, 2H, o-Ph). 13C{1H} NMR (d8-THF, 25 ꢀC): δ 4.17
(d, JPC = 3.8 Hz, SiMe3), 14.74 (d, JPC = 18.2 Hz, CH), 25.40 (THF),
(10) Avent, A. G.; Bonafoux, D.; Eaborn, C.; Hill, M. S.; Hitchcock,
P. B.; Smith, J. D. Dalton Trans. 2000, 2183.
(11) Avent, A. G.; Eaborn, C.; Hitchcock, P. B.; Lawless, G. A.;
Lickiss, P. D.; Mallien, M.; Smith, J. D.; Webb, A.; Wrackmeyer, B.
Dalton Trans. 1993, 3259.
67.86 (THF), 126.64 (Ar), 127.05 (d, JPC = 7.7 Hz, Ar), 129.89 (d, JPC
=
8.7 Hz, Ar) (ipso carbon not observed). 7Li{1H} NMR (d8-toluene, 25
ꢀC): δ 4.5. 11B{1H} NMR (d8-toluene, 25 ꢀC): δ -34.4 (d, JPB = 107.9
Hz). 31P{1H} NMR (d8-THF, 25 ꢀC): δ 3.4 (q, JPB = 107.9 Hz).

4776 Organometallics, Vol. 29, No. 21, 2010
Izod et al.
bite angle for 3 is 80.42(11)ꢀ. The Li(2) H distances of
3 3 3
˚
2.01(2) and 2.03(2) A are similar to the Li H distances of
3 3 3
˚
1.94(3) and 2.05(3) A in the 1,4-dicarbanion complex
[(Me3Si){n-Pr2P(BH3)}C(CH2)]2Li2(THF)4,2a in which the
Li cations are also bound by the BH3 groups in an η2 manner.
Metalation of 2 leads to a significant shortening of the
P-C(7) and P-C(11) distances from 1.8110(17) and 1.8130(17)
A, respectively, in 2 to 1.7571(17) and 1.7620(16) A in 3;
˚
˚
similarly, the Si(1)-C(7) and Si(2)-C(11) distances decrease
from 1.8853(17) and 1.8852(17) A, respectively, in 2 to
˚
˚
1.8179(18) and 1.8115(16) A in 3. This is consistent with
significant delocalization of the carbanion charges onto the
phosphorus and silicon atoms. In contrast, the P-B distance
˚
˚
increases from 1.924(2) A in 2 to 1.943(2) A in 3.
The 31P{1H} and 11B{1H} NMR spectra of 3 consist of a
broad quartet at 3.4 ppm and a broad doublet at -34.4 ppm,
respectively (JPB = 107.9 Hz). We have previously observed
that R-deprotonation of phosphine-borane adducts results
in a significant increase in the 31P-11B coupling constant in
the corresponding anion;2 for example, the 31P-11B coupling
constant in (Me3Si)2{Me2P(BH3)}CH is 59.4 Hz, whereas
the corresponding coupling constant in 4 is 89.4 Hz. The
magnitude of the increase in JPB upon R-metalation appears
to be a function of the degree of charge delocalization in the
metalated form. In 4 the highly charge-localizing lithium ion
inhibits the delocalization of charge onto phosphorus and
the 31P-11B coupling constant is 89.4 Hz;2b in the heavier
alkali-metal derivatives [[(Me3Si)2{Me2P(BH3)}C]M]n (M =
Na, K, Rb, n = ¥; M = Cs, n = 2), in which charge loca-
lization is less pronounced, the corresponding coupling
constants lie between 95.2 and 101.6 Hz.2c Similarly, in
[(Me3Si)2{Me2P(BH3)}C]2Mg, in which the pyramidal car-
banion centers of the ligands are coordinated to the charge-
localizing Mg2þ ion, the 31P-11B coupling constant (88.9
Hz) is significantly less than in [(Me3Si)2{Me2P(BH3)}C]2-
M(THF)n (M= Ca, n = 4; M = Sr, Ba, n = 5; JPB = 95.6-
105.2 Hz), in which there is no direct contact between the
metals and the essentially planar carbanion centers.2d The
change in the magnitude of the 31P-11B coupling constant
observed on metalation of 2 (49.1 Hz) represents the largest
such increase reported to date and may be attributed to
increased charge delocalization onto phosphorus due to the
stabilization of two adjacent negative charges by the single
phosphorus center in 3. Unfortunately, the 31P-11B cou-
pling constant for 1 was not reported, and so we are unable to
compare the effect of a single phosphine-borane center sup-
porting two adjacent negative charges with the effect of two
phosphine-borane groups supporting an adjacent metha-
nediide center.
Figure 3. Variable-temperature 7Li NMR spectra of 3 in
d8-toluene (referenced to external 1.0 M LiCl in D2O).
sharpening somewhat to a broad doublet centered at -33.4
ppm. The room-temperature 1H NMR spectrum of 3 in d8-
toluene is consistent with the solid-state structure, although a
single pair of signals is observed for the THF protons,
suggesting rapid dynamic exchange of these ligands on the
NMR time scale. In addition to these signals and signals due
to the phenyl ring, the spectrum exhibits a broad doublet
at -0.25 ppm (JPH = 17.4 Hz), due to the methine protons at
the carbanion centers, and a singlet at 0.22 ppm, due to the
trimethylsilyl protons; the BH3 protons give rise to a very
broad signal at 1.07 ppm, which collapses to a doublet (JPH
=
12.9 Hz) on decoupling of the 11B nucleus. As the tempera-
ture is reduced, the signal due to the SiMe3 protons broadens
and shifts to lower field, before sharpening at -90 ꢀC to a
singlet at 0.87 ppm. Concurrently, the signal at 8.10 ppm, due
to the ortho protons of the phenyl ring, broadens and shifts
to lower field, until at -90 ꢀC this signal sharpens to a poorly
resolved multiplet centered at 8.80 ppm. The remaining
signals change little with temperature.
The room-temperature 7Li NMR spectrum of 3 consists of
a singlet at 1.6 ppm (Figure 3), consistent with rapid ex-
change between the two lithium environments on the NMR
time scale. However, as the temperature is reduced this signal
broadens and begins to decoalesce, until at -90 ꢀC the
spectrum consists of two relatively sharp singlets of approxi-
mately equal intensity at -0.6 and 1.9 ppm.
Previous variable-temperature, multielement NMR stud-
ies of the sterically hindered organolithium compounds
[{(Me3Si)3C}2Li][LiLn] (Ln = (THF)4, (tmeda)2) have re-
vealed that these may be subject to dynamic equilibria
between a range of species in solution, including ate com-
plexes and both contact and separated ion pairs.11,12 The
variable-temperature 7Li NMR data for 3 indicate that there
The 31P{1H} and 11B{1H} NMR spectra of 3 do not change
significantly with temperature: the signal at 3.4 ppm in the
31P{1H} spectrum merely broadens slightly as the tempera-
ture is reduced to -60 ꢀC, before sharpening again to a broad
quartet centered at 5.4 ppm (JPB = 108.3 Hz) at -90 ꢀC,
while the signal at -34.4 ppm in the 11B{1H} spectrum also
broadens as the temperature is reduced to -60 ꢀC before

Communication
Organometallics, Vol. 29, No. 21, 2010 4777
Scheme 2
is rapid exchange between the two lithium environments at
1
component and the signal at 1.6 ppm to the formally anionic
dialkyllithate component. For comparison, the solid-state
7Li NMR spectrum of the dialkyllithate complex [{(Me3-
Si)3C}2Li][Li(THF)4] exhibits signals at 1.7 and 2.8 ppm, assig-
ned to the cation and dialkyllithate anion, respectively.11 It is
informative to compare the low-temperature 7Li NMR
spectrum of 3 with that of 4, which adopts a closely related
dialkyllithate contact ion multiple structure in the solid
state.2b Whereas for 4 signals due to both the ate com-
plex (for which 7Li signals are observed at 0.5 and 2.5 ppm
for the cationic and anionic components, respectively) and a
mononuclear (or symmetrical oligonuclear) complex (which
room temperature, while the variable-temperature H, 31P-
{1H}, and 11B{1H} NMR spectra suggest that the species
present at low temperature are different from the species
present at high temperatures, although not dramatically so.
This behavior may be attributed to the operation of a dy-
namic equilibrium between the contact ion pair form obser-
ved in the solid state (3) and either a solvent-separated ion
pair (3a) and/or solvent-separated ion triple form (3b) or an
alternative molecular species in which both lithium ions are
each bound to a carbanion center (3c) (Scheme 2). At higher
temperatures this equilibrium is rapid on the NMR time
scale and so a time-averaged spectrum is observed; as the
temperature is reduced the rate of exchange between these
forms decreases, leading to broadening of some of the NMR
signals, while concurrently the equilibrium increasingly
favors the contact ion pair form 3. Thus, although the rate of
exchange is reduced with decreasing temperature, individual
signals due to 3a, 3b, and/or 3c are not observed, since at low
temperatures the contact ion pair form 3 predominates.
Unfortunately, with the present data, it is not possible to
distinguish unambiguously between these processes; how-
ever, we tentatively favor an equilibrium involving 3c, since
the ionic compounds 3a and 3b are likely to be less favored in
the nondonor solvent toluene used in this study.
7
has a Li signal at 0.9 ppm) may be observed at low tem-
perature, for 3 only the ate complex is observed, reflecting
stability of this species due to the bidentate, dianionic nature
of the ligand.
In summary, we have shown that a single phosphine-borane
center, in conjunction with silyl groups, is able to support
two adjacent carbanion centers. The reported dilithium
derivative crystallizes as a contact ion pair ate complex, which
is subject to dynamic exchange which inter-converts the two
lithium environments.
Acknowledgment. We thank the EPSRC for support.
The 7Li signal for the cationic component of ate complexes
typically lies to higher field than that of the anionic compo-
nent, and so we attribute the signal at -0.6 ppm in the low-
temperature 7Li NMR spectrum of 3 to the formally cationic
Supporting Information Available: CIF files giving details of
the structure determination, atomic coordinates, bond lengths
and angles, and displacement parameters for 2 and 3. This
material is available free of charge via the Internet at http://
pubs.acs.org. Observed and calculated structure factor details
are available from the authors upon request.
(12) Reich, H. J.; Sikorski, W. H.; Sanders, A. W.; Jones, A. C.;
Plessel, K. N. J. Org. Chem. 2009, 74, 719.