Phosphido-borane and phosphido-bis(borane) complexes of the alkali metals, a comparative study

Article abstract of DOI:10.1021/ic302205b

Treatment of the secondary phosphine {(Me₃Si) ₂CH}(Ph)PH with BH₃·SMe₂ yields the phosphine-borane {(Me₃Si)₂CH}(Ph)PH(BH₃) (12) as colorless crystals. The reactions between 12 and n-BuLi, PhCH₂Na or PhCH₂K yield the corresponding phosphido-borane complexes [[{(Me₃Si)₂CH}(Ph)P(BH₃)]Li(THF) ₂]_∞ (13), [{(Me₃Si)₂CH}(Ph) P(BH₃)][Na(12-crown-4)₂] (14), or [[{(Me ₃Si)₂CH}(Ph)P(BH₃)]K(pmdeta)]₂ (15), respectively, after crystallization in the presence of the appropriate co-ligand. While 13 crystallizes as a chain polymer, 14 crystallizes as a separated ion pair and 15 as a dimer. In both 13 and 15 the phosphido-borane ligands bind the alkali metal cations via both P-M and B-H·M contacts. Unexpectedly, NMR spectroscopy suggests that the separated ion pair 14 undergoes rapid inversion at phosphorus, while 13 and 15 do not. The phosphido- bis(borane) complexes [{(Me₃Si)₂CH}(Ph)P(BH ₃)₂]Li(12-crown-4) (16b), [[{(Me₃Si) ₂CH}(Ph)P(BH₃)₂]Na(THF)₂] ₂ (17), and [[{(Me₃Si)₂CH}(Ph)P(BH ₃)₂]K(THF)_0.5]_∞ (18a) were prepared by treatment of the corresponding in situ-generated phosphido-borane complexes with BH₃·SMe₂ and crystallization in the presence of the appropriate co-ligand. Compound 16b crystallizes as a monomer, while 17 crystallizes as a dimer and 18a crystallizes as a ribbon polymer. These crystallographic studies reveal entirely new binding modes for both phosphido-borane and phosphido-bis(borane) ligands and allow a direct comparison between these two ligand types.

Full text of DOI:10.1021/ic302205b

Article
pubs.acs.org/IC
Phosphido-Borane and Phosphido-Bis(Borane) Complexes of the
Alkali Metals, a Comparative Study
Keith Izod,* James M. Watson, William Clegg, and Ross W. Harrington
Main Group Chemistry Laboratories, School of Chemistry, Bedson Building, Newcastle University, Newcastle upon Tyne, NE1 7RU,
U.K.
S
* Supporting Information
ABSTRACT: Treatment of the secondary phosphine
{(Me3Si)2CH}(Ph)PH with BH3·SMe2 yields the phosphine-
borane {(Me3Si)2CH}(Ph)PH(BH3) (12) as colorless crystals.
The reactions between 12 and n-BuLi, PhCH2Na or PhCH2K
yield the corresponding phosphido-borane complexes
[[{(Me3Si)2CH}(Ph)P(BH3)]Li(THF)2]∞ (13), [{(Me3Si)2CH}(Ph)P(BH3)][Na(12-crown-4)2] (14), or [[{(Me3Si)2CH}-
(Ph)P(BH3)]K(pmdeta)]2 (15), respectively, after crystallization in the presence of the appropriate co-ligand. While 13
crystallizes as a chain polymer, 14 crystallizes as a separated ion pair and 15 as a dimer. In both 13 and 15 the phosphido-borane
ligands bind the alkali metal cations via both P−M and B−H···M contacts. Unexpectedly, NMR spectroscopy suggests that the
separated ion pair 14 undergoes rapid inversion at phosphorus, while 13 and 15 do not. The phosphido-bis(borane) complexes
[{(Me3Si)2CH}(Ph)P(BH3)2]Li(12-crown-4) (16b), [[{(Me3Si)2CH}(Ph)P(BH3)2]Na(THF)2]2 (17), and [[{(Me3Si)2CH}-
(Ph)P(BH3)2]K(THF)0.5]∞ (18a) were prepared by treatment of the corresponding in situ-generated phosphido-borane
complexes with BH3·SMe2 and crystallization in the presence of the appropriate co-ligand. Compound 16b crystallizes as a
monomer, while 17 crystallizes as a dimer and 18a crystallizes as a ribbon polymer. These crystallographic studies reveal entirely
new binding modes for both phosphido-borane and phosphido-bis(borane) ligands and allow a direct comparison between these
two ligand types.
atures the same reaction yields the phosphine-free secondary
alcohols (i.e., the phosphido-borane anion acts as a hydride
transfer reagent).
INTRODUCTION
■
In spite of the isoelectronic relationship between silyl ligands,
R3Si−, and phosphido-borane anions, R2P(BH3)−, the latter
compounds (also known as mono(borane)phosphides or
phosphanylborohydrides) have received relatively little atten-
tion. This is particularly surprising given the relatively
straightforward synthesis of these compounds and their
potential utility: phosphido-boranes are key intermediates in
the synthesis of novel (chiral) phosphines and in the catalytic
dehydrocoupling of phosphine-boranes to give polymeric
materials.1,2 Very recently, Gaumont and co-workers have
shown that copper(I) phosphido-borane complexes are pre-
catalysts for the formation of P−C(sp) bonds.3 Frequently, in
these applications, the phosphido-borane intermediates are
generated and used in situ; few such compounds have been
isolated in the solid state. Those phosphido-borane complexes
which have been isolated and structurally characterized may be
divided into two classes: alkali metal complexes in which the
hard metal center is coordinated principally by the borane
hydrogen atoms of the ligand,4 and transition metal complexes
in which the phosphido-borane ligand binds the softer metal
center via its phosphorus donor atom.2,3,5 In this regard, it is
notable that a recent report indicates that the lithium
phosphido-borane complex [Ph2P(BH3)]Li exhibits ditopic
character.6 This compound reacts with aldehydes at low
temperature to give the corresponding phosphine-borane-
substituted alcohols (i.e., the phosphido-borane undergoes
1,2-addition across the CO bond), while at higher temper-
Even less numerous are complexes of phosphido-bis(borane)
−
anions, R2P(BH3)2 , which are typically prepared by the
addition of BH3 to an alkali metal phosphido-borane complex,
and which are the isoelectronic, negatively charged analogues of
silanes. To date, only three such compounds have been
structurally characterized, namely, {R2P(BH3)2}K(18-crown-6)
[R = Ph (1), t-Bu (2)]4b,7 and {(2,4,6-t-Bu3C6H2)PH(BH3)2}-
Li(THF)3 (3);8 all three compounds crystallize as contact ion
pairs in which the alkali metal cations are coordinated by the
ether or tetrahydrofuran (THF) co-ligands and by only one of
the BH3 groups of the phosphido-bis(borane) ligand. In
addition, Lancaster and co-workers have reported the crystal
structure of the mixed phosphido-bis(borane) complex [Ph2P-
{B(C6F5)3}(BH3)]Li(Et2O)3 (4),9 while Bertrand and co-
workers have reported the crystal structure of the unusual
hydride-bridged complex [{(2,4,6-t-Bu3C6H2)P(BH3){(μ-
BH2)2H}}Li(THF)2]2.8
We recently reported the synthesis and crystal structures of a
series of alkali metal phosphido-borane complexes possessing
peripheral donor functionalization through the incorporation of
a benzyl ether or thioether group in the ligand (5−10, Chart
1).10,11 This donor functionalization leads to additional M−O
Received: October 10, 2012
Published: January 15, 2013
© 2013 American Chemical Society
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Chart 1
or M−S interactions and the consequent formation of chelate
rings in their complexes. For the thioether-functionalized
ligands we noted a borane-redistribution reaction between the
lithium phosphido-borane complex (8) and free phosphine-
borane, which yields a phosphido-bis(borane) complex (11)
and free secondary phosphine (Scheme 1);10 however,
(Ph)PH(BH3) (12), after removal of volatiles, as a colorless,
crystalline solid in good yield. Treatment of a solution of 12 in
THF with 1 equiv of n-BuLi leads to rapid deprotonation and
the formation of the lithium complex [[{(Me3Si)2CH}(Ph)P-
(BH3)]Li(THF)2]∞ (13), which crystallizes from methylcyclo-
hexane/THF as colorless blocks (Scheme 2). Similarly, the
reaction between 12 and 1 equiv of either PhCH2Na or
PhCH2K in THF gives the corresponding heavier alkali metal
phosphido-borane complexes, which are isolated as the
separated ion pair [{(Me3Si)2CH}(Ph)P(BH3)][Na(12-
crown-4)2] (14) and the dimer [[{(Me3Si)2CH}(Ph)P(BH3)]-
K(pmdeta)]2 (15), respectively, in good yields, after crystal-
lization in the presence of the appropriate co-ligand [pmdeta =
N,N,N′,N″,N″-pentamethyldiethylenetriamine].
Scheme 1
Upon metalation, the 31P{1H} NMR signal observed for 12
(−8.6 ppm) moves to significantly higher field [−56.8 (13),
−41.0 (14), −50.6 (15) ppm]. In our previous studies on
phosphido-borane complexes we have found the 31P{1H}
chemical shifts to be essentially independent of the nature of
the alkali metal cation;10,11 however, each of the compounds in
our earlier studies crystallizes as a contact ion pair. We attribute
the somewhat lower-field chemical shift of 14 compared to 13
and 15 to the formation of a separated ion pair in the former
compound. The 31P-11B coupling constants of 12−15 exhibit a
significant decrease with increasing ionic character of the P−H/
M bond [JPB = 52 (12), 44 (13), 26 (14), 20 (15) Hz]. This is
consistent with Bent’s rule:13 increasing ionic character leads to
increasing phosphorus s-character in the P−H/M bond and
thus increasing p-character in the P−B bond; this results in a
decrease in the Fermi contact term and so a decrease in the
31P-11B coupling constant. The rather similar 31P-11B coupling
although we subsequently prepared 11 by a more logical
route (addition of BH3·SMe2 to the phosphido-borane 8), we
were not able to obtain any suitable crystals of this compound
to investigate its structure.
We now report the synthesis and structural characterization
of a new phosphine-borane, free from peripheral donor
functionalization, its lithium, sodium, and potassium derivatives,
and their conversion into the corresponding phosphido-
bis(borane) complexes. This provides the first opportunity to
compare the coordination behavior of phosphido-borane and
phosphido-bis(borane) ligands within a closely related family of
compounds.
RESULTS AND DISCUSSION
■
The reaction between {(Me3Si)2CH}(Ph)PH12 and 1 equiv of
BH3·SMe2 in THF gives the phosphine-borane {(Me3Si)2CH}-
Scheme 2. [R = CH(SiMe3)2]
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constants of the potassium complex 15 and the separated ion
pair 14 suggest that the P−K bonding in the former is
essentially ionic in nature. Coupling between 31P and 7Li is not
observed in 13, indicating rapid exchange on the NMR time-
scale.
state is rather large, and hence chiral tertiary phosphines (and,
by inference, chiral phosphido-borane anions) are largely
configurationally stable,16 it has been reported that the presence
of electropositive substituents acts to greatly reduce this
barrier.16,17 For example, the inversion barriers for iPrPhP-
(EMe3) have been measured as 136.8, 89.5, and 80.8 kJ mol−1
for E = C, Ge, and Sn, respectively.17 With particular relevance
to the current system, we have shown that inversion at
phosphorus in the phosphatetrylenes [{(Me3Si)2CH}P(C6H4-
2-CH2 NMe2 )]ECl and [{(Me3 Si)2 CH}P(C6 H4 -2-
CH2NMe2)]2E (E = Ge, Sn) is rapid on the NMR time-
scale.18 Calculations on the model phosphatetrylenes [MeP-
(C6H4-2-CH2NMe2)]GeX (X = F, Cl, Br, H, Me) indicate that
the barrier to inversion at phosphorus via a planar transition
state ranges from 66.1 to 98.7 kJ mol−1, depending on the
substituent X.19 The Pauling electronegativity of boron (2.04)
is similar to that of germanium (2.01) and so a borane group
should exert an influence over the inversion barrier of
phosphorus similar to that of a germyl group. We note that
the barrier to inversion at phosphorus calculated for [MeP-
(C6H4-2-CH2NMe2)]GeF (66.1 kJ mol−1) lies close to that
observed for 14 and so tentatively favor this mechanism as an
explanation for the high-temperature equivalence of the
diastereotopic SiMe3 groups in the latter compound. The
temperature-invariant nature of the 1H NMR spectra of 13 and
15 indicates that, where the cation is not fully separated from
the anion, inversion at phosphorus is inhibited.
1
The room-temperature H NMR spectra of the contact ion
pairs 13 and 15 in d8-THF are as expected and are consistent
with the above formulations: the diastereotopic SiMe3 groups
give rise to a pair of singlets in each case at 0.29 and 0.39 ppm
1
(13) and 0.28 and 0.42 ppm (15), respectively, and the H
NMR spectra of 13 and 15 do not vary with temperature.
1
Unexpectedly, however, in the room-temperature H NMR
spectrum of 14 in d8-THF the SiMe3 groups appear as two
rather broad singlets. At lower temperatures these signals
sharpen, such that, below −10 °C, two sharp singlets are
observed at −0.09 and 0.15 ppm; at temperatures above
ambient these signals broaden and coalesce until, at 55 °C, a
broad singlet at 0.10 ppm is observed. The remaining signals
are unaffected by temperature.
This behavior may be attributed to a dynamic equilibrium
which interconverts the two diastereotopic SiMe3 groups and
suggests that, at ambient temperature and above, 14 is subject
to rapid inversion at the phosphorus center. Such an inversion
process may reasonably occur via one of two distinct
mechanisms: (i) dissociation of the borane group and
subsequent re-coordination at phosphorus to give the
alternative enantiomeric form, or (ii) inversion at phosphorus
via a planar transition state (Scheme 3). With regard to the first
To provide a comparison with the metalated derivatives we
obtained an X-ray crystal structure of the free phosphine-
borane 12; the molecular structure of 12 is shown in Figure 1,
Scheme 3
mechanism, Imamoto and co-workers have suggested that the
racemization of o-anisyl-substituted tertiary phosphine-boranes
occurs via a dynamic equilibrium in which the anisyl oxygen
atom abstracts the borane group from phosphorus, generating
an intermediate tertiary phosphine, which then racemizes prior
to re-coordination of the borane group.14 In light of this, it is
feasible that the dissociation of BH3 from 14 to form a two-
coordinate (achiral) phosphide anion would be assisted by the
formation of a THF−BH3 adduct, reducing the barrier to
dissociation. However, theoretical calculations yield a bond
dissociation energy of 166.5 kJ mol−1 for the neutral
phosphine-borane adduct Me3P−BH3,15 and it is likely that
the negative charge localized on phosphorus in 14 would
increase the strength of the P−B σ-bond, and so rapid
dissociation of BH3 from 14 appears unlikely. In this regard,
line-shape analysis of the variable-temperature 1H NMR spectra
of 14 gives values of ΔH⧧ = 52.8 5.0 kJ mol−1 and ΔS⧧ =
−28 10 J K−1 mol−1, and consequently ΔG⧧ = 61.1 5.0 kJ
mol−1, for this process at 298 K, substantially less than the likely
P−B bond dissociation energy.
Figure 1. Molecular structure of 12 with C-bound H atoms and the
minor disorder component omitted for clarity. Selected bond lengths
(Å) and angles (deg): P−H(1P) 1.35(4), P−B 1.920(7), P−C(1)
1.814(4), P−C(8) 1.805(4), C(1)−Si(1) 1.898(4), C(1)−Si(2)
1.910(4), B−H(1A) 1.17(6), B−H(1B) 1.04(6), B−H(1C) 1.04(5),
B−P−C(1) 117.7(3), B−P−C(8) 113.9(3), C(1)−P−C(8)
108.34(17).
along with selected bond lengths and angles. The P−C(1), P−
C(8), and P−B distances [1.814(4), 1.805(4), and 1.920(7) Å,
respectively] are similar to the corresponding distances in
previously reported phosphine-borane adducts; the sum of
angles in the C2PB core is 339.9°.
The lithium complex 13 crystallizes as infinite chains of
alternating lithium cations and phosphido-borane anions; the
structure of 13 is shown in Figure 2, along with selected bond
lengths and angles. Each lithium cation is coordinated by the
phosphorus atom of one phosphido-borane ligand and, in an
η2-manner, by two borane hydrogen atoms of an adjacent
phosphido-borane ligand in the chain. The coordination of each
lithium ion is completed by two molecules of THF. Thus, each
phosphido-borane anion bridges adjacent lithium cations via its
The alternative mechanism for the inversion of 14 involves a
trigonal planar transition state. Although calculations indicate
that the barrier to inversion at phosphorus via such a transition
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Figure 3. Structure of 14 with C-bound H atoms omitted for clarity.
Selected bond lengths (Å) and angles (deg): Na−O 2.466 (average),
P−B 1.9688(15), P−C(1), 1.8899(13), P−C(8) 1.8400(14), B−P−
C(1) 110.57(6), B−P−C(8) 106.06(7), C(1)−P−C(8) 101.50(6).
Figure 2. Section of the structure of polymeric 13 with C-bound H
atoms omitted for clarity. Selected bond lengths (Å) and angles (deg):
Li−P 2.741(3), Li−H(1A′) 2.086(18), Li−H(1B′) 2.183(19), Li···H-
(1C′) 2.79(2), Li···B′ 2.440(3), Li−O(1) 1.985(3), Li−O(2)
1.949(3), P−B 1.9611(17), P−C(1) 1.8851(14), P−C(8)
1.8395(14), B−P−C(1) 110.55(7), B−P−C(8) 106.09(7), C(1)−
P−C(8) 102.95(6). Primed atoms are related by the symmetry
operator x−1, y, z (translation by one unit cell along the a axis).
The P−B, P−C(1), and P−C(8) distances in 14
[1.9688(15), 1.8899(13), and 1.8400(14) Å, respectively] are
almost identical to the corresponding distances in 13
[1.9611(17), 1.8850(14), and 1.8394(14) Å, respectively],
while the sum of angles in the PBC2 framework in 14 [318.67°]
is also very similar to that for 13 [319.60°]. This clearly implies
that coordination of the lithium cation in 13 has little impact on
the bonding in the phosphido-borane anion itself.
P and BH3 centers. The P−Li distance of 2.741(3) Å is
somewhat longer than the corresponding distances in 5 and 8
[2.611(6) and 2.527(3) Å, respectively],10,11 reflecting the fact
that the P−Li bond is incorporated into a chelate ring in the
latter two compounds. Indeed, the P−Li distance in 13 is
significantly longer than the corresponding distance in
[{Me2P(BH3)}Li(tmeda)]∞ [2.620(4) Å],4a which is, with
the exception of 5 and 8, the only other lithium complex of a
phosphido-borane anion, and lies at the longer end of the range
of typical P−Li distances in lithium phosphides (R2P)Li [tmeda
= N,N,N′,N′-tetramethylethylenediamine].20 The Li−H dis-
tances [2.086(18) and 2.183(19) Å] and the Li···B distance
[2.440(3) Å] are typical of η2-BHn-Li contacts. For example,
the Li−H and Li···B distances in [{Me2P(BH3)}Li(tmeda)]∞
are 2.03(2) and 2.372(4) Å, respectively,4a while the Li−H and
Li···B distances in 5 are 1.99(4) and 2.05(4) Å and 2.453(7) Å,
respectively.10 The next shortest Li···H(B) distance in 13 is
2.79(2) Å, so that an η3 formulation is not appropriate.
Metalation of 12 results in a significant lengthening of the
P−B bond from 1.920(7) Å in 12 to 1.9611(17) Å in 13;
similarly the P−C(1) and P−C(2) distances increase from
1.814(4) and 1.805(4) Å in 12 to 1.8851(14) and 1.8395(14)
Å in 13, respectively. Metalation also leads to increasing
pyramidalization at phosphorus: in 12 the sum of angles within
the C2PB framework is 339.9°, whereas the sum of angles at
phosphorus in 13 is 319.6°.
The polymeric chains of 13 pack such that the phenyl rings
are oriented toward each other; however, there are no
significant π-stacking interactions between the rings, which
are arranged in a substantially offset inter-digitated fashion
along the chains.
In contrast to the polymeric structure of 13, compound 14
crystallizes as a separated ion pair; this permits a direct
comparison of the structure of the isolated phosphido-borane
anion with those of its lithium complex and of the parent
phosphine-borane 12. The structure of 14 is shown in Figure 3,
along with selected bond lengths and angles. The [Na(12-
crown-4)2]+ cation is unexceptional and requires no further
comment.
In contrast to 13 and 14, the potassium salt 15 crystallizes as
a discrete molecular species; the molecular structure of 15 is
shown in Figure 4, along with selected bond lengths and angles.
Figure 4. Structure of 15 with C-bound H atoms omitted for clarity.
Selected bond lengths (Å) and angles (deg): P−B 1.969(3), P−C(1)
1.874(2), P−C(8) 1.831(2), K−P 3.2754(8), K−H(1A) 2.71(2), K−
H(1A′) 2.76(2), K−H(1B′) 2.77(3), K···H(1B) 3.22(3), K···H(1C′)
3.25(3), K···B 3.262(3), K···B′ 3.107(3), K−N(1) 2.830(2), K−N(2)
2.9132(18), K−N(3) 2.801(2), K···C(15) 3.275(4), K···C(22)
3.418(3), P−K···B 35.06(5), B···K···B′ 96.32(7), K···B···K′ 83.69(7),
B−P−C(1) 111.20(11), B−P−C(8) 107.46(12), C(1)−P−C(8)
102.57(10). Primed atoms are related by the inversion operator 1−
x, 1−y, 1−z.
Compound 15 crystallizes as a dimer in which each potassium
ion is bound by the phosphorus atom and one of the borane
hydrogen atoms of a phosphido-borane ligand; this “side-on”
coordination mode closely resembles the P,B binding mode
observed in the related potassium complex [Ph2P(BH3)]K(18-
crown-6).4c The coordination sphere of each potassium ion in
15 is completed by the three nitrogen atoms of a molecule of
pmdeta, an η2-BH3 group from the second phosphido-borane
ligand in the dimer, and two short K···Me contacts to two of
the methyl groups of the tmeda co-ligand. Thus, the borane
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groups act as μ2-η1:η2-bridges between the two potassium ions
in the dimer, generating a planar K2(BH3)2 rhombus-shaped
core. The next shortest K···H(B) distances are 3.22(3) and
3.25(3) Å, almost 0.5 Å longer than the most significant
interactions described here.
The K−P distance of 3.2754(8) Å in 15 is somewhat shorter
than the corresponding distance in [Ph2P(BH3)]K(18-crown-
6) [3.320(2) Å]4c and is substantially shorter than the
corresponding distances in the closely related compounds 7
and 10 [K−P 3.7619(10) and 3.6617(6) Å, respectively].10,11
In contrast, the K···B distance of 3.262(3) Å in 15 is slightly
longer than the corresponding distance in [Ph2P(BH3)]K(18-
crown-6) [3.162 Å], which also exhibits an η2-BH3 binding
mode, but is similar to the corresponding distances in 7 and 10
[K···B 3.322(3) and 3.234(2) Å, respectively].
toluene with 1 equiv of 12-crown-4 gave the adduct
[{(Me3Si)2CH}(Ph)P(BH3)2]Li(12-crown-4) (16b), which
was crystallized from cold toluene as colorless blocks suitable
for X-ray crystallography. The 1H and 13C{1H} NMR spectra of
16b are consistent with the proposed formulation; the 31P{1H}
spectrum consists of a broad, unresolved multiplet at −8.8 ppm
due to coupling to two quadrupolar 11B nuclei, while the
11B{1H} spectrum exhibits a broad doublet (JPB = 46 Hz) at
−32.5 ppm.
The sodium analogue [[{(Me3Si)2CH}(Ph)P(BH3)2]Na-
(THF)2]2 (17) was synthesized by the reaction of 12 with
benzylsodium and subsequent treatment with 1 equiv of
BH3·SMe2; single crystals were obtained from a cold mixture of
methylcyclohexane, toluene, and THF (10:2:5). The corre-
sponding potassium salt [[{(Me3Si)2CH}(Ph)P(BH3)2]K-
(THF)0.5]∞ (18a) was prepared by a similar reaction between
12, benzylpotassium, and BH3·SMe2 in THF and was
crystallized from cold toluene/THF; a second batch of crystals,
obtained in a similar fashion, proved to be of the alternative
solvate [[[{(Me3Si)2CH}(Ph)P(BH3)2]K(THF)2].1/2PhMe]∞
Since phosphido-borane ligands RR′P(BH3)− are isoelec-
tronic with the corresponding methyl-substituted silyl ligands
RR′SiMe−, it is pertinent to compare the structures of their
alkali metal derivatives. While the compounds [{(Me3Si)2CH}-
(Ph)(Me)Si]M [M = Li, Na, K] are unknown and so a direct
comparison with the structures of these species is not possible,
several alkali metal complexes of methyl-substituted silyl ligands
have been crystallographically characterized.21 Typically, in
these compounds the alkali metal is coordinated by the silicon
center of the silyl ligand, along with the donor atoms of any
coligands, and, in some cases, these contacts are supplemented
by weak C−H···M contacts with organic groups at the
periphery of the molecule. For example, in the complex
[(Me3Si)2(Me)Si]K(PhH)2,21a alongside the K−Si contact,
there are short contacts between the potassium ion and two
η6-benzene molecules along with short C−H···K contacts
between the potassium ion and a methyl group on two of the t-
Bu substituents of the silyl ligand; there are no short contacts in
this complex between the potassium ion and the Si−Me group.
To the best of our knowledge, the only crystallographically
characterized Si−Me···M contact, where the silicon center bears
a negative charge, is found in the mixed aggregate [{([t-
BuMe2Si]2Si)Li2}2{(t-BuMe2Si)Li}2,21b in which the Li···C
distance is 2.355 Å [cf. Li···B 2.440(3) in 13]. This contrasts
with the prevalence of P−BH3···M contacts found in
phosphido-borane complexes of the alkali metals, clearly
illustrating the greater hydridic character of P−BH3 compared
to Si−CH3 hydrogen atoms.
1
(18b). The H, 13C{1H}, 11B{1H}, and 31P{1H} NMR spectra
of 17 and 18 are as expected; the 11B{1H} spectra of 17 and 18
consist of broad doublets at −33.8 (JBP = 69 Hz) and −32.9
ppm (JBP = 64 Hz), respectively, while the 31P{1H} NMR
spectra consist of broad, unresolved multiplets at −14.9 and
−14.1 ppm, respectively.
The structure of 16b is shown in Figure 5, along with
selected bond lengths and angles. Compound 16b crystallizes
We noted above the scarcity of well-characterized complexes
of phosphido-bis(borane) anions. This is particularly surprising
given that these anions are direct homologues of the amido-
Figure 5. Structure of 16b with C-bound H atoms and the minor
disorder component omitted for clarity. Selected bond lengths (Å) and
angles (deg): Li−H(1A) 2.01(4), Li−H(1B) 2.28(5), Li−H(1C)
2.27(5), Li···B 2.281(8), Li−O(1) 2.157(8), Li−O(2) 2.072(8), Li−
O(3) 2.090(7), Li−O(4) 2.129(8), P−B(1) 1.929(4), P−B(2)
1.948(5), P−C(1) 1.854(3), P−C(8) 1.832(4), B(1)−P−B(2)
111.0(2), B(1)−P−C(1) 115.39(19), B(1)−P−C(8) 102.48(18),
B(2)−P−C(1) 111.68(18), B(2)−P−C(8) 113.6(2), C(1)−P−C(8)
102.20(16).
bis(borane) ligands developed by Noth and subsequently
̈
employed by Girolami and co-workers for the synthesis of a
range of highly volatile, high coordination number lanthanide
and actinide compounds.22,23 We previously observed that
thermolysis of 7 in the presence of free phosphine-borane leads
to a borane-redistribution reaction, ultimately yielding the
corresponding alkali metal phosphine-bis(borane) (11) and
free secondary phosphine (Scheme 1).10 Although we were
unable to crystallize 11 its constitution was unambiguously
verified by NMR spectroscopy.
as discrete monomers in which the lithium ion is coordinated
by the four oxygen atoms of a molecule of 12-crown-4 and, in
an η3 manner, by one of the two borane groups of the
phosphido-bis(borane) ligand; the second borane group has no
short contacts to lithium. The binding mode of the phosphido-
bis(borane) ligand in 16b is similar to that observed in the
three previously reported alkali metal phosphido-bis(borane)
complexes 1−3.
To gain more insight into the structures and binding modes
of this under-developed ligand class, we sought to prepare the
phosphido-bis(borane) analogues of 13−15. Treatment of a
solution of 13 in THF with 1 equiv of BH3·SMe2 gives the
phosphido-bis(borane) complex [{(Me3Si)2CH}(Ph)P-
(BH3)2]Li(THF)n (16a). Although we were unable to
crystallize this compound, treatment of a solution of 16a in
The Li−H distances of 2.01(4), 2.28(5) and 2.27(5) Å and
the Li···B distance of 2.281(8) Å are consistent with the η3
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binding mode of the borane group in 16b; for example, the Li−
H distances in (4-MeC5H4N)3Li(η3-BH4) are 2.10(2), 2.20(2),
and 2.10(2), while the Li···B distance in this compound is
2.319(5) Å.24 The P−B(1) distance of 1.929(4) Å is slightly
shorter than the “free” P−B(2) distance [1.948(5) Å], although
it is difficult to attribute this to any steric or electronic effects; a
similar difference between the P−B distances of the “free” and
coordinated BH3 groups was observed for 3. However, both P−
B distances in 16b are significantly shorter than the P−B
distances in 3 [1.955(3) and 1.984(3) Å], and in the
phosphido-borane complexes 8 [1.9617(18) Å] and 13
[1.9611(17) Å].
These distances are similar to the corresponding distances in 6
[Na···B 2.987(5) and 2.845(5) Å] and in (py)3Na(η2-BH4)
[Na···B 2.927(3) Å].25 The two P−B distances in 17 are
essentially identical [1.950(3) and 1.958(3) Å] and are similar
to those observed in 16b and 3.
Both 18a and 18b crystallize as ribbon-type polymers; the
structures of 18a and 18b are shown in Figure 7, along with
selected bond lengths and angles. These two compounds differ
in the degree of solvation of the potassium ions: in 18a half of
the potassium ions in the chain are coordinated by a single
molecule of THF each, while in 18b each potassium ion is
coordinated by two molecules of THF; additionally, the crystal
of 18b contains uncoordinated solvent, highly disordered,
which is probably toluene. Nevertheless, the overall structures
of both 18a and 18b are very similar.
In 18a, allowing a generous range for K−H distances, each
potassium ion is coordinated by two η2-BH3 groups from a
chelating phosphido-bis(borane) ligand and two η3-BH3 groups
from two adjacent phosphido-bis(borane) ligands in the chain;
additionally, half of the potassium ions are each coordinated by
one molecule of THF, the packing of the chains providing
insufficient space for THF coordination of all the cations; the
THF molecules are disordered over inversion centers located
between the chains.
In contrast to monomeric 16b, compound 17 crystallizes as
discrete dimers with a crystallographic inversion center mid-
way along the Na···Na vector; the molecular structure of 17 is
shown in Figure 6, along with selected bond lengths and angles.
Compound 18b crystallizes with a very similar structure to
that of 18a, but here each potassium ion is coordinated by two
molecules of THF, in addition to the two η2-BH3 groups from a
chelating phosphido-bis(borane) ligand and two η3-BH3 groups
from adjacent phosphido-bis(borane) ligands in the chain. The
greater mean K−H distance (and larger range of distances) in
18b than in 18a reflects the increased coordination by THF in
the former compound.
The K−H and K···B distances in 18a and 18b [18a: K−H
2.698(18)−3.191(19) Å, K···B 3.039(2)− 3.224(2) Å; 18b: K−
H 2.77(3)−3.40(3), K···B 3.132(4)−3.416(4) Å] are similar to
the corresponding distances in previously reported phosphido-
borane and phosphido-bis(borane) complexes of potassium.
For example, the K−H distances in 7 range from 2.796(18) to
3.06(2) Å, while the K···B distances in this compound are
3.103(3) and 3.322(3) Å.
Comparison of the structures of the contact ion pair lithium
phosphido-borane and phosphido-bis(borane) complexes 13
and 16b reveals that, while there is little difference in the Li−H
distances in these two compounds [Li−H 2.086(18), 2.183(19)
Å (13); 2.01(4), 2.27(5), 2.28(5) Å (16b)], the P−B distance
in 13 [1.9611(17) Å] is somewhat longer than the
corresponding distances in 16b [1.929(4) and 1.948(5) Å].
Furthermore, while the K−H distances in 15 and 18a are also
rather similar, the P−B distance in the potassium phosphido-
borane complex 15 [1.969(3) Å] is significantly longer than the
P−B distances in the potassium phosphido-bis(borane)
complex 18a [1.940(2) and 1.941(2) Å]. This is a reflection
of the increased phosphorus p-character in the P−B bonds in
13 and 15, which arises from the highly ionic nature and
consequent high phosphorus s-character of the P−Li and P−K
bonds in these compounds.
Figure 6. Structure of 17 with C-bound H atoms omitted for clarity.
Selected bond lengths (Å) and angles (deg): Na−O(1) 2.339(2), Na−
O(2) 2.369(2), Na−H(1A) 2.63(2), Na−H(1B) 2.67(3), Na−H(2A)
2.42(2), Na−H(2B) 2.54(3), Na−H(1B′) 2.42(3), Na−H(1C′)
2.59(3), Na···H(1A′) 2.95(3), Na···B(1) 3.011(3), Na···B(2)
2.864(3), Na···B(1′) 2.782(3), P−B(1) 1.950(3), P−B(2) 1.958(3),
P−C(1) 1.862(2), P−C(8) 1.846(2), B(1)···Na···B(2) 65.79(8),
B(1′)···Na···B(2) 144.87(9), B(1)···Na···B(1′) 88.88(9),
Na···B(1)···Na′ 91.12(9). Primed atoms are related by the inversion
symmetry operation 1−x, 1−y, 1−z.
Each sodium ion is coordinated by two molecules of THF,
two η2-BH3 groups from one phosphido-bis(borane) ligand and
an η2-BH3 group from the second phosphido-bis(borane)
ligand in the dimer. Thus, each phosphido-bis(borane) ligand
chelates one sodium ion via two η2-BH3···Na contacts, while
one of the BH3 groups acts as an η2:η2-bridge between the two
sodium ions in the dimer. Neither chelating nor bridging
bonding motifs have been observed previously for phosphido-
bis(borane) ligands, although the related amido-bis(borane)
ligands developed by Noth and co-workers are known to adopt
̈
similar chelating modes.22,23 A slightly longer Na···H distance
(by 0.28 Å, similar to the range of the primary Na−H
distances) means that the links between the two halves of the
dimer through the bridging borane group are actually
intermediate between η2 and η3, as shown in Figure 6.
CONCLUSIONS
■
Phosphido-boranes R2P(BH3)− represent a versatile class of
potentially ambidentate and/or polydentate anionic ligand.
While these ligands may adopt a wide variety of coordination
modes there appears to be a marked preference for the
formation of BH3···M contacts in their complexes with hard
The dimer is constructed around a central Na2(BH3)2
rectangle with primary Na−H contacts ranging from 2.42(2)
to 2.67(3) Å and Na···B distances of 3.011(3) and 2.782(3) Å.
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separated ion pair results in rapid inversion at the phosphorus
center, most likely via a trigonal planar transition state; this
behavior is suppressed in the contact ion pair species 13 and
15.
Addition of a further equivalent of BH3 to the phosphido-
borane salts 13−15 yields the corresponding phosphido-
bis(borane) complexes 16−18; in spite of the formal analogy
−
between phosphido-bis(borane) anions R2P(BH3)2 and
tetraorganosilanes this ligand class remains largely uninvesti-
gated. The present study reveals that phosphido-bis(borane)
anions may adopt a “monodentate” BH3-donor mode or a
variety of chelating/bridging modes, depending on the nature
of the metal center.
The coordination chemistry of phosphido-borane and
phosphido-bis(borane) anions is at a very early stage of
development, but shows excellent promise for the future. These
highly under-utilized ligands, analogues of triorganosilyl anions
and tetraorganosilanes, offer an abundance of coordination
modes and metal−ligand interactions. We are currently
exploring the use of these ligands in main group and lanthanide
chemistry.
EXPERIMENTAL SECTION
■
All manipulations were carried out using standard Schlenk techniques
under an atmosphere of dry nitrogen. THF, light petroleum (bp 40−
60 °C), toluene, and methylcyclohexane were dried prior to use by
distillation under nitrogen from sodium, potassium, or sodium/
potassium alloy as appropriate. THF was stored over activated 4A
molecular sieves; all other solvents were stored over a potassium film.
Deuterated toluene and THF were distilled from potassium, and
CDCl3 was distilled from CaH2 under nitrogen; all NMR solvents
were deoxygenated by three freeze-pump-thaw cycles and were stored
over activated 4A molecular sieves. Benzylsodium,26 benzylpotassi-
um,27 and {(Me3Si)2CH}(Ph)PH12 were prepared by previously
published procedures; n-butyllithium was purchased from Aldrich as a
2.5 M solution in hexanes, and BH3·SMe2 was purchased from Aldrich
as a 2.0 M solution in THF. Pmdeta was distilled from CaH2 under
nitrogen and was stored over activated 4A molecular sieves. All other
compounds were used as supplied by the manufacturer.
Figure 7. (a) Structure of 18a and (b) structure of 18b with C-bound
H atoms omitted for clarity. Selected bond lengths (Å) and angles
(deg) for 18a: K−O 2.762(3), K−H(1A) 2.813(18), K−H(1B)
2.78(2), K−H(2A) 2.822(19), K−H(2B) 2.698(18), K−H(1A′)
2.766(18), K−H(1B′) 3.191(19), K−H(1C′) 2.762(18), K−H(2A″)
2.924(18), K−H(2B″) 3.084(18), K−H(2C″) 2.79(2), K···B(1)
3.224(2), K···B(2) 3.174(2), K···B(1′) 3.039(2), K···B(2′) 3.083(2),
P−B(1) 1.941(2), P−B(2) 1.940(2), P−C(1) 1.8410(16), P−C(8)
1.8326(16), K···B(1)···K′ 93.06(5), K···B(2)···K″ 889.59(5). Primed
and double-primed atoms are related by the inversion symmetry
operators −x, −y, 1−z and 1−x, −y, 1−z, respectively. For 18b: K−
O(1) 2.730(2), K−O(2) 2.735(2), K−H(1A) 3.05(3), K−H(1B)
2.77(3), K−H(2A) 2.89(3), K−H(2B) 3.13(2), K−H(1A′) 2.94(3),
K−H(1B′) 3.23(3), K−H(1C′) 2.79(3), K−H(2A″) 2.86(3), K−
H(2B″) 3.40(3), K−H(2C″) 2.97(3), K···B(1) 3.315(4), K···B(2)
3.416(4), K···B(1′) 3.132(4), K···B(2″) 3.183(4), P−B(1) 1.933(4),
P−B(2) 1.949(4), P−C(1) 1.852(3), P−C(8) 1.839(3), K···B(1)···K″
91.06(9), K···B(2)···K′ 88.37(8). Primed and double-primed atoms
are related by the screw-axis symmetry operators x−1/2, 1−y, 1/2−z
and x+1/2, 1−y, 1/2−z, respectively.
1H and 13C{1H} NMR spectra were recorded on a JEOL ECS500
spectrometer operating at 500.16 and 125.65 MHz, respectively, or a
Bruker Avance300 spectrometer operating at 300.15 and 75.47 MHz,
respectively; chemical shifts are quoted in parts per million (ppm)
relative to tetramethylsilane. 31P{1H}, 11B{1H}, and 7Li{1H} NMR
spectra were recorded on a JEOL ECS500 spectrometer operating at
202.35, 160.16, and 194.38 MHz, respectively; chemical shifts are
quoted in ppm relative to external 85% H3PO4, BF3.Et2O, and 0.1 M
LiCl, respectively. Elemental analyses were obtained by the Elemental
Analysis Service of London Metropolitan University.
{(Me3Si)2CH}(Ph)PH(BH3) (12). To a solution of {(Me3Si)2CH}-
(Ph)PH (9.78 g, 36.43 mmol) in THF (20 mL) was added BH3·SMe2
(18.2 mL of a 2.0 M solution in THF, 36.40 mmol). After 1 h stirring
the solvent was removed in vacuo to yield a white solid, which was
crystallized from cold (−30 °C) THF as colorless blocks. Isolated
1
yield: 8.89 g, 63%. H{11B} NMR (CDCl3, 25 °C) δ 0.29 (s, 9H,
SiMe3), 0.36 (s, 9H, SiMe3), 0.76 (d, JPH = 19.3 Hz, 1H, CHP), 0.94
1
3
1
(dd, JPH = 15.3 Hz, JHH = 6.4 Hz, 3H, BH3), 5.72 (dq, JPH = 368.9
3
Hz, JHH = 6.4 Hz, 1H, PH), 7.46 (m, 3H, ArH), 0.77 (m, 2H, ArH).
13C{1H} NMR (CDCl3, 25 °C) δ 1.26 (d, 3JPC = 2.9 Hz, SiMe3), 2.10
(d, 3JPC = 2.9 Hz, SiMe3), 9.41 (d, 1JPC = 3.0 Hz, CHP), 128.81 (d, J PC
= 10.5 Hz, ArC), 129.21 (d, JPC = 51.8 Hz, ArC), 131.29 (d, JPC = 2.4
Hz, ArC), 133.16 (d, JPC = 9.6 Hz, ArC). 11B{1H} NMR (CDCl3, 25
°C): δ −40.6 (d, br, 1JPB = 52 Hz). 31P{1H} NMR (CDCl3, 25 °C): δ
alkali metal cations; in the present case these contacts result in
oligomerization to afford both dimeric (15) and polymeric (13)
species. The structures of the phosphido-borane anions in these
compounds are remarkably similar to that of the anion in the
separated ion pair complex 14, suggesting that coordination of
an alkali metal cation has little impact on the bonding within
the phosphido-borane anion itself. However, formation of a
1
−8.6 (q, br, JPB = 52 Hz).
[[{(Me3Si)2CH}(Ph)P(BH3)]Li(THF)2]∞ (13). To a solution of 12
(0.41 g, 1.26 mmol) in THF (15 mL) was added nBuLi (0.5 mL of 2.5
M solution in hexane, 1.25 mmol). After 1 h stirring the solvent was
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removed in vacuo to yield an off-white solid. Colorless blocks of 13
suitable for X-ray crystallography were obtained from cold (−30 °C)
methylcyclohexane/THF (approx 10:1). Isolated crystalline yield: 0.34
g, 63%. Anal. Calcd for [[{(Me3Si)2CH}(Ph)P(BH3)]Li(THF)2]∞: C
58.32, H 10.02%. Found C 58.29, H 9.95%. 1H{11B} NMR (d8-
toluene, 25 °C): δ 0.29 (s, 9H, SiMe3), 0.39 (s, 9H, SiMe3), 0.63 (d,
2JPH = 7.8 Hz, 1H, CHP), 1.04 (s, 3H, BH3), 1.40 (m, 8H, THF), 3.54
(m, 8H, THF), 6.99 (m, 1H, ArH), 7.17 (m, 2H, ArH), 7.78 (m, 2H,
7.12 (m, 2H, ArH), 7.79 (m, 1H, ArH). 13C{1H} NMR (d8-THF, 25
°C): δ 3.20 (SiMe3), 11.85 (d, 1JPC = 6.7 Hz, CHP), 67.15 (12-crown-
4), 126.02 (ArC), 126.31 (d, JPC = 7.7 Hz, ArC), 132.32 (d, JPC = 6.7
Hz, ArC), 146.78 (d, JPC = 29.7 Hz, ArC). 11B{1H} NMR (d8-THF, 25
°C): δ −32.5 (d, br, 1JBP = 46 Hz). 31P{1H} NMR (d8-THF, 25 °C): δ
7
−8.8 (br). Li{1H} NMR (d8-THF, 25 °C): δ −0.7.
[[{(Me3Si)2CH}(Ph)P(BH3)2]Na(THF)2]2 (17). To a solution of 12
(0.45 g, 1.59 mmol) in THF (10 mL) was added a solution of
benzylsodium (184 mg, 1.61 mmol). This mixture was stirred for 1 h
and then a solution of BH3·SMe2 (0.8 mL, of a 2.0 M solution in THF,
1.60 mmol) was added. This mixture was stirred for a further 1 h and
then the solvent was removed in vacuo to yield a colorless solid. Single
crystals suitable for X-ray crystallography were obtained from a cold
(−30 °C) mixture of methylcyclohexane (10 mL), toluene (2 mL),
and THF (5 mL). Isolated crystalline yield: 0.46 g, 63%. Anal. Calcd
for [[{(Me3Si)2CH}(Ph)P(BH3)2]Na(THF)2]2: C 54.55, H 10.03%.
Found C 54.25, H 9.65%. 1H{11B} NMR (d8-THF, 25 °C): δ 0.07 (s,
3
ArH). 13C{1H} NMR (d8-toluene, 25 °C): δ 2.29 (d, JPC = 7.7 Hz,
1
SiMe3), 3.32 (SiMe3), 8.86 (d, JPC = 34.6 Hz, CHP), 25.12 (THF),
68.13 (THF), 124.23 (ArC), 127.14 (d, JPC = 4.8 Hz, ArC), 131.84 (d,
JPC = 11.5 Hz, ArC), 149.62 (d, JPC = 16.3 Hz, ArC). 11B{1H} NMR
(d8-toluene, 25 °C): δ −32.7 (d, br, 1JBP = 44 Hz). 31P{1H} NMR (d8-
1
7
toluene, 25 °C): δ −56.8 (q, br, JPB = 44 Hz). Li{1H} NMR (d8-
toluene, 25 °C): δ 3.1.
[{(Me3Si)2CH}(Ph)P(BH3)][Na(12-crown-4)2] (14). To a solution
of 12 (0.37 g, 1.31 mmol) in THF (10 mL) was added, dropwise, a
solution of benzylsodium (0.15 g, 1.31 mmol) in THF (20 mL), and
this mixture was stirred for 1 h. 12-crown-4 (0.4 mL, 2.62 mmol) was
added to the solution, and this mixture was stirred for 10 min. Solvent
was removed in vacuo to give a pale yellow solid, which was
crystallized from cold (−30 °C) toluene/THF (approx 10:1) as
colorless crystals suitable for X-ray crystallography. Isolated crystalline
yield: 0.73 g, 86%. Anal. Calcd for [{(Me3Si)2CH}(Ph)P(BH3)][Na-
(12-crown-4)2]: C 53.48, H 8.96%. Found C 53.04, H 9.06%. 1H{11B}
NMR (d8-THF, 25 °C): δ −0.05 (s, br, 9H, SiMe3), 0.14 (s, br, 9H,
SiMe3), 0.20 (d, 2JPH = 5.5 Hz, 1H, CHP), 0.97 (d, 2JPH = 4.1 Hz, 3H,
BH3), 3.63 (s, 32H, 12-crown-4), 6.70 (m, 1H, ArH), 6.91 (m, 2H,
ArH), 7.55 (m, 2H, ArH). 13C{1H} NMR (d8-THF, 25 °C): δ 1.81 (s,
br, SiMe3), 2.91 (s, br, SiMe3), 10.51 (d, 1JPC = 47.9 Hz, CHP), 66.13
(12-crown-4), 121.11, 125.73 (ArC), 131.84 (d, JPC = 11.5 Hz, ArC),
2
2
18H, SiMe3), 0.63 (d, JPH = 8.7 Hz, 6H, BH3), 0.64 (d, JPH = 15.1
Hz, 1H, CHP), 1.78 (m, 8H, THF), 3.61 (m, 8H, THF), 7.12 (m, 1H,
ArH), 7.17 (m, 2H, ArH), 7.74 (m, 2H, ArH). 13C{1H} NMR (d8-
THF, 25 °C): δ 2.79 (d, 3JPC = 1.9 Hz, SiMe3), 10.55 (d, 1JPC = 2.9 Hz,
CHP), 25.30 (THF), 67.14 (THF), 126.58 (ArC), 126.68 (d, JPC = 7.7
Hz, ArC), 132.03 (d, JPC = 7.7 Hz, ArC), 143.84 (d, JPC = 36.4 Hz,
ArC). 11B{1H} NMR (d8-THF, 25 °C): δ −33.8 (d, br, 1JBP = 69 Hz).
31P{1H} NMR (d8-THF, 25 °C): δ −14.9 (br).
[[{(Me3Si)2CH}(Ph)P(BH3)2]K(THF)0.5]∞ (18a). To a solution of 12
(0.42 g, 1.49 mmol) in THF (10 mL) was added a solution of
benzylpotassium (194 mg, 1.49 mmol) in THF (10 mL). This mixture
was stirred for 1 h and then a solution of BH3·SMe2 (0.8 mL of a 2.0
M solution in THF, 1.60 mmol) was added. The solution was stirred
for a further 1 h and then the solvent was removed in vacuo to yield a
colorless solid. Colorless crystals suitable of 18a for X-ray
crystallography were obtained from cold (5 °C) toluene/THF. A
second batch of crystals, isolated in a similar manner, proved to be of
the alternate solvate [[[{(Me3Si)2CH}(Ph)P(BH3)2]K(THF)2].1/
2PhMe]∞ (18b), which was characterized solely by X-ray crystallog-
raphy. The following data refer to 18a. Isolated crystalline yield: 0.48
g, 88%. Anal. Calcd for [[{(Me3Si)2CH}(Ph)P(BH3)2]K(THF)0.5]∞:
C 48.65, H 9.25%. Found C 48.71, H 9.05%. 1H{11B} NMR (d8-THF,
157.07 (d, JPC = 35.5 Hz, ArC). 11B{1H} NMR (d8-THF, 25 °C): δ
1
−32.4 (s, br). 31P{1H} NMR (d8-THF, 25 °C): δ −41.0 (d, br, JPB
=
26 Hz).
[[{(Me3Si)2CH}(Ph)P(BH3)]K(pmdeta)]2 (15). To a solution of 12
(0.45 g, 1.59 mmol) in THF (15 mL) was added a solution of
benzylpotassium (0.21 g, 1.59 mmol) in THF (10 mL). After 1 h
stirring the solvent was removed in vacuo, yielding an orange powder.
The product was dissolved in toluene (20 mL), and pmdeta (0.3 mL,
1.44 mmol) was added. The solvent volume was then carefully reduced
in vacuo until the product began to precipitate. The turbid solution
was warmed slightly to dissolve the product, and upon cooling to −30
°C colorless crystals of 15 suitable for X-ray crystallography were
deposited. Isolated crystalline yield: 0.46 g, 59%. Anal. Calcd for
[[{(Me3Si)2CH}(Ph)P(BH3)]K(pmdeta)]2: C 53.52, H 10.21, N
8.51%. Found C 53.41, H 10.14, N 8.42%. 1H{11B} NMR (d8-toluene,
25 °C): δ 0.28 (s, 9H, SiMe3), 0.42 (s, 9H, SiMe3), 0.61 (d, 2JPH = 6.9
2
25 °C): δ 0.07 (s, 18H, SiMe3), 0.62 (d, JPH = 14.7 Hz, 1H, CHP),
2
0.70 (d, JPH = 9.6 Hz, 6H, BH3), 1.78 (m, 8H, THF), 3.62 (m, 8H,
THF), 7.10 (m, 1H, ArH), 7.14 (m, 2H, ArH), 7.77 (m, 2H, ArH).
13C{1H} NMR (d8-THF, 25 °C): δ 2.91 (d, JPC = 1.9 Hz, SiMe3),
11.37 (d, JPC = 3.8 Hz, CHP), 25.01 (THF), 66.85 (THF), 126.38,
3
126.46 (ArC), 132.27 (d, JPC = 6.7 Hz, ArC), 144.58 (d, JPC = 34.5
1
Hz, ArC). 11B{1H} NMR (d8-THF, 25 °C): δ −32.9 (d, br, JBP = 64
Hz). 31P{1H} NMR (d8-THF, 25 °C): δ −14.1 (br).
2
Hz, 1H, CHP), 1.08 (d, JPH = 4.1 Hz, 3H, BH3), 1.93 (m, 4H,
Crystal Structure Determinations of 12−18. For 12, 14, 15,
16b, 17, 18a, and 18b measurements were made at 150 K on an
Oxford Diffraction (Agilent Technologies) Gemini A Ultra diffrac-
tometer or a Nonius KappaCCD diffractometer, using MoKα radiation
(λ = 0.71073 Å); for 13 measurements were made at 120 K using
synchrotron radiation (λ = 0.6941 Å). Cell parameters were refined
from the observed positions of all strong reflections. Intensities were
corrected semi-empirically for absorption, based on symmetry-
equivalent and repeated reflections. The structures were solved by
direct methods and refined on F2 values for all unique data. Table 1
gives further details. All non-hydrogen atoms were refined anisotropi-
cally, and C-bound H atoms were constrained with a riding model,
while B-bound H atoms were freely refined; U(H) was set at 1.2 (1.5
for methyl groups) times Ueq for the parent C atom. Disorder in one
SiMe3 group of 12 was successfully modeled with both components
refined freely; the disordered crown ligand of 16b was refined with the
aid of restraints on geometry and displacement parameters, and the
same applies to the disordered THF of 18a. Highly disordered solvent
in 18b could not be modeled with discrete atoms and was treated by
the SQUEEZE procedure of PLATON;28 while the volume of the
disorder region is appropriate for toluene (approximately half a
molecule per potassium ion), the electron density calculated in this
region is anomalously low, suggesting that the solvent is readily lost
NCH2), 1.95 (m, 4H, NCH2), 1.99 (s, 3H, NMe), 2.09 (s, 12H,
NMe2), 6.98 (m, 1H, ArH), 7.20 (m, 2H, ArH), 7.80 (m, 2H, ArH).
3
13C{1H}NMR (d8-toluene, 25 °C): δ 2.25 (d, JPC = 7.7 Hz, SiMe3),
1
3.20, (SiMe3), 9.78 (d, JPC = 43.1 Hz, CHP), 41.00 (NMe), 44.80
(NMe2), 55.60 (NCH2), 56.76 (NCH2), 123.42 (ArC), 127.09 (d, JPC
= 2.9 Hz, ArC), 131.83 (d, JPC = 11.5 Hz, ArC), 153.07 (d, JPC = 25.9
Hz). 11B{1H} NMR (d8-toluene, 25 °C): δ −31.6 (d, br, 1JBP = 20 Hz).
1
31P{1H} NMR (d8-toluene, 25 °C): δ −50.6 (d, br, JPB = 20 Hz).
[[{(Me3Si)2CH}(Ph)P(BH3)2]Li(12-crown-4)] (16b). To a solution
of 12 (0.54 g, 1.91 mmol) in THF (10 mL) was added nBuLi (0.8 mL
of a 2.5 M solution in hexane, 2.0 mmol). This mixture was stirred for
1 h and then a solution of BH3·SMe2 (1.0 mL, of a 2.0 M solution in
THF, 2.0 mmol) was added. This mixture was stirred for a further 1 h
and then the solvent was removed in vacuo to yield a colorless solid.
The product was dissolved in toluene (5 mL), and 12-crown-4 (0.3
mL, 1.85 mmol) was added. Colorless crystals of 16b suitable for X-ray
crystallography were obtained upon cooling this solution to 5 °C for
16 h. Isolated crystalline yield: 0.70 g, 77%. Anal. Calcd for
[[{(Me3Si)2CH}(Ph)P(BH3)2]Li(12-crown-4)]: C 52.73, H 9.69%.
1
Found C 52.67, H 9.81%. H{11B} NMR (d8-THF, 25 °C): δ −0.05
2
2
(s, 18H, SiMe3), 0.57 (d, JPH = 13.8 Hz, 1H, CHP), 0.77 (d, JPH
=
10.1 Hz, 6H, BH3), 3.75 (s, 16H, 12-crown-4), 7.07 (m, 2H, ArH),
1474
dx.doi.org/10.1021/ic302205b | Inorg. Chem. 2013, 52, 1466−1475

Inorganic Chemistry
Article
from the crystals before diffraction data collection. Other programs
were Oxford Diffraction CrysAlisPro, Nonius COLLECT/EvalCCD
and Bruker APEX2 for data collection and processing, and SHELXTL
for structure solution, refinement, and molecular graphics.29
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ASSOCIATED CONTENT
■
Chem. Phys. 1992, 96, 6807. (f) Schwerdtfeger, P.; Hunt, P. Adv. Mol.
Struct. Res. 1999, 5, 223. (g) Schwerdtfeger, P.; Boyd, P. D. W.;
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* Supporting Information
For 12−15, 16b, 17, 18a, and 18b details of structure
determination, atomic coordinates, bond lengths and angles,
and displacement parameters in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
(h) Goller, A.; Clark, T. Chem. Commun. 1997, 1033.
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(17) (a) Beachler, R. D.; Casey, J. P.; Cook, R. J.; Senkler, G. H., Jr.;
Mislow, K. J. Am. Chem. Soc. 1987, 109, 338. (b) Beachler, R. D.;
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626, 2264.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: k.j.izod@ncl.ac.uk.
Notes
(18) (a) Izod, K.; McFarlane, W.; Allen, B.; Clegg, W.; Harrington, R.
W. Organometallics 2005, 24, 2157. (b) Izod, K.; Stewart, J.; Clark, E.
R.; McFarlane, W.; Allen, B.; Clegg, W.; Harrington, R. W.
Organometallics 2009, 28, 3327.
The authors declare no competing financial interest.
(19) Izod, K.; Clark, E. R.; Stewart, J. Inorg. Chem. 2011, 50, 3651.
(20) Izod, K. Adv. Inorg. Chem. 2000, 50, 33.
(21) For examples see: (a) Wiberg, N.; Neidermayer, W.; Noth, H.;
Warchold, M. J. Organomet. Chem. 2001, 628, 46. (b) Bravo-
Zhivotovskii, D.; Molev, G.; Kravchenko, V.; Botoshansky, M.;
Schmidt, A.; Apeloig, Y. Organometallics 2006, 25, 4719. (c) Stroh-
ACKNOWLEDGMENTS
■
̈
The authors are grateful to the EPSRC and Newcastle
University for financial support, EPSRC also for funding the
U.K. National Crystallography Service and STFC for access to
synchrotron facilities at SRS Daresbury Laboratory.
mann, C.; Daschlein, C. Chem. Commun. 2008, 2791. (d) Sekiguchi,
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A.; Nanjo, M.; Kabuto, C.; Sakurai, H. Organometallics 1995, 14, 2630.
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(e) Tecle, B.; Ilsley, W. H.; Oliver, J. P. Organometallics 1982, 1, 875.
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