Me6TREN-Supported Alkali Metal Hydridotriphenylborates [(L)M][HBPh3] (M = Li, Na, K): Synthesis, Structure, and Reactivity

Article abstract of DOI:10.1021/acs.organomet.7b00308

Triphenylborane (BPh₃) abstracted the β-SiH in [(L)M{N(SiHMe₂)₂}] (L = Me₆TREN; M = Li, Na, K) in THF to give the hydridotriphenylborates [(L)M][HBPh₃]. Reactions in benzene favored silazide instead o

Full text of DOI:10.1021/acs.organomet.7b00308

Article
pubs.acs.org/Organometallics
Me6TREN-Supported Alkali Metal Hydridotriphenylborates
[(L)M][HBPh3] (M = Li, Na, K): Synthesis, Structure, and Reactivity
Hassan Osseili, Debabrata Mukherjee, Klaus Beckerle, Thomas P. Spaniol, and Jun Okuda*
Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, Aachen 52074, Germany
S
* Supporting Information
ABSTRACT: Triphenylborane (BPh3) abstracted the β-SiH
in [(L)M{N(SiHMe2)2}] (L = Me6TREN; M = Li, Na, K) in
THF to give the hydridotriphenylborates [(L)M][HBPh3].
Reactions in benzene favored silazide instead of hydride
abstraction to give [(L)M][Ph3B−N(SiHMe2)2]. The hydri-
dotriphenylborates [(L)M][HBPh3] catalyzed the chemo-
selective hydroboration of benzophenone by pinacolborane
(HBpin), with the lithium derivative being the most active.
The solution structure of [(L)Li][HBPh3] was qualitatively investigated in the context of its superior catalytic activity. Fluxional
coordination of L in [(L)Li(THF)]+ in tandem with a THF solvent molecule was revealed by NMR spectroscopy. para-
Substituents in [HB(C6H4-p-X)3]− influenced the rate which is apparently determined by the addition of the B−H function to
the isolable alkoxy borate intermediate [(L)Li][Ph2CHOBPh3].
Scheme 1. Solvent-Dependent β-SiH vs N(SiHMe2)2
Abstraction from [(L)M{N(SiHMe2)2}] (M = Li, Na, K) by
BPh3
INTRODUCTION
■
Light alkali metal hydridoborates [M(HBR3)] with variable
substituents R (M = Li, Na, K; R = hydride, alkyl, aryl, amido,
alkoxy) are versatile reagents for the stoichiometric reduction of
polar functionalities in organic synthesis.1 [(L)M][HBPh3] was
recently found to catalyze the hydroboration of carbonyls and
CO2 chemoselectively, with the lithium derivative exhibiting
exceptionally high activity.2 Tripodal tris{2-(dimethylamino)-
ethyl}amine (Me6TREN = L) is an effective chelating ligand
that stabilizes reactive organoalkali metal compounds in
monomeric form.3 The alkali metal hydridotriphenylborates
[(L)M][HBPh3] [M = Li (1), Na (2), K (3)] were synthesized
by BPh3-induced β-SiH abstraction from [(L)M{N-
(SiHMe2)2}] in THF.2 We report here on the details of
synthesis, solution structure of the lithium catalyst, along with
some details of the hydroboration catalysis.
RESULTS AND DISCUSSION
■
Reactions of [(L)M{N(SiHMe2)2}] with BPh3. The
tetramethyldisilazide adducts [(L)M{N(SiHMe2)2}], readily
available by mixing L and [M{N(SiHMe2)2}]4 in hydrocarbon
or ethereal solvents, acted as suitable precursors for [(L)M]-
[HBPh3] by reacting with BPh3 under β-hydride abstraction in
THF (Scheme 1). This synthetic route is clean and high
yielding (86−97%). The byproduct cyclodisilazane
(Me2HSiN−SiMe2)2 can be removed by washing with n-
pentane, whereas the products can be recrystallized in pure
form from THF at −35 °C. Hydridotriphenylborates 1−3 are
colorless crystals and highly soluble in THF but insoluble in
aliphatic and aromatic hydrocarbons. They were fully
characterized by spectroscopic and analytical techniques
including X-ray crystallography for 1 and 3.2
Changing the solvent from THF to benzene altered the
reaction pathway. BPh3 abstracted the silazide group N-
(SiHMe2)2 instead of the β-hydride from [(L)M{N-
(SiHMe2)2}] (Scheme 1). The resulting tetramethyldisilazido-
borates [(L)M][Ph3B−N(SiHMe2)2] [M = Li (4), Na (5), K
(6)] are also colorless crystals and show solubility similar to
that of 1−3. Whereas the potassium compound was
quantitatively converted, the lithium and sodium homologues
were contaminated with their [HBPh3]− derivatives by 25 and
1
12%, respectively. Complexes 4−6 were characterized by H,
13C{1H}, and 11B NMR spectroscopy. The 11B NMR spectra in
Received: April 21, 2017
© XXXX American Chemical Society
A
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THF-d8 show a singlet at δ −2.7 ppm, which is attributed to the
boron atom in the [Ph3B−N(SiHMe2)2]− anion.5
Complexes 5 and 6 were also characterized by X-ray
crystallography. 5 has an ionic structure with a free [Ph3B−
N(SiHMe2)2]− anion, whereas 6 is zwitterionic (Figure 1). In 6,
alkyls [M{C(SiHMe2)3}2(L′)] (M = Ca, Yb; L′ = 2THF,
TMEDA) in benzene, where the SiH is activated through metal
interactions.7 Hydride abstraction from [Ru(PMe3)4(SiMe3)-
H]8 and the methylene group abstraction from
Cp′2CeCH2OMe (Cp′ = 1,2,4-(Me3C)3C5H2)9 are other
examples. It also interacts reversibly with the Ce−H in
Cp′2CeH9 and reacts with tBuLi to give [Li(HBPh3)] that is
contaminated with the BPh3−• radical anion.1c B-Methyl-9-BBN
reacts with tBuLi in hydrocarbon solvents to a mixture of “ate”
complexes including the hydridoborate.1c Reactions of [Li{N-
(SiHMe2)2}] with group 14 electrophiles such as Me3SiCl are
also solvent-dependent, but in this case, cyclodisilazane is the
major product in hexane, whereas THF favors substitution
rather than elimination.10 A molecular zinc chloride [(ToM)-
ZnCl] (ToM = tris(4,4-dimethyl-2-oxazolinyl)phenylborate)
reacts with [Li{N(SiHMe2)2}] by substitution in benzene but
hydride abstraction/elimination in THF.11
The tetramethyldisilazidoborates 4−6 were compared with
the hydridoborates 1−3. Heating 4−6 in THF-d8 at 60 °C
converted them into 1−3 within a variable amount of time
(Scheme 2). Conversion for the potassium was exceptionally
Scheme 2. Thermolysis of 4−6 to Give 1−3
sluggish. The cyclodisilazane [(Me2HSiN−SiMe2)2] byproduct
was identified by 1H NMR spectroscopy. These reactions were
accelerated by a further equivalent of BPh3. Thus, both
abstractions are independent processes depending on the
solvent, and 4−6 are not the intermediates during the synthesis
of 1−3 in THF. The coordination of THF possibly prevents
BPh3 from accessing the interior M−N bond.
Figure 1. Molecular structures of 5 (top) and 6 (bottom).
Displacement parameters are drawn at 50% probability. Hydrogen
atoms except for the Si-H are omitted for clarity.
Solution Structure of [(L)Li][HBPh3] (1). The lithium
catalyst (1, TOFPh CO = 66.6 × 103 h−1) was >300 times more
2
active than sodium and potassium (2 and 3, TOFPh CO ≥ 0.2 ×
2
103 h−1) in the hydroboration of benzophenone by HBpin (eq
1).2 Moreover, coordination of L appeared to be critical for
one of the three phenyl rings of the BPh3 moiety is η1-bonded
to the potassium through the ipso-carbon [K···Cipso
=
3.2798(14) Å]. Moreover, the orientation of L in 6 is slightly
bent to accommodate an additional three-centered two-electron
K···H−Si bonding interaction (K···H = 2.70 Å) from one of the
two SiH that leads to a six-membered metallacycle. This is also
corroborated by its solid-state IR (KBr) spectrum that exhibits
two distinct νSiH absorptions at 2117 and 2072 cm−1 (see
Supporting Information). [K{N(SiHMe2)2}] in the solid state
has a one-dimensional polymeric structure with each potassium
being involved in double β-agostic K···H−Si bonding
interactions (K···H = 2.67 Å) from a single N(SiHMe2)2
group.6 In comparison, the [HBPh3]− in 3 was η3-bonded to
the potassium through the ipso- and two ortho-carbons of one
phenyl ring, but no K···H−B interaction was found.2 Similar
K+···Cπ noncovalent interactions were reported for [(L)-
KCH2Ph],3b [(L)KCH2(3,5-Me2-C6H3)],3c and [(L)KSiPh3]3e
complexes.
faster catalysis when compared to the ligand-free [Li(HBPh3)].2
That prompted us to analyze the solution structure of 1 in
more details.
1 crystallized from THF as a solvent-separated ion pair with
free [HBPh3]− anion (Figure 2).2 The lithium center adopts a
distorted trigonal bipyramidal geometry with a κ4-coordination
from L, where the apical nitrogen atom (Napical) and a THF
molecule occupy the axial positions. The THF binding is
apparently weak and occurs during the crystallization, whereas
the NMR spectrum and elemental analysis of a rigorously dried
sample correspond to a THF-free complex.
The solvent-controlled abstraction reactions here are
remarkable, as BPh3 is only moderately Lewis acidic and the
[(L)M{N(SiHMe2)2}] are classical tetramethyldisilazides.
Group abstractions by BPh3 generally are rare. Few examples
include the β-SiH abstraction from the calcium and ytterbium
B
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Supporting Information) provided the activation parameters as
ΔH⧧ = 13.3 0.9 kcal mol−1, ΔS⧧ = 5.2 0.3 cal mol−1 K−1,
and ΔG⧧
= 11.8
0.9 kcal mol−1. The low activation
298 K
barrier underlines the facile exchange process. A positive ΔS⧧ is
consistent with a dissociative mechanism.
Coordination of L at Li is often labile in solution, and both
κ43b−d and κ33a,f modes exist in the solid state. With larger-sized
sodium and potassium, κ4 is the only mode identified so
far.3b,c,e,g Solid [(L)Li{N(SiMe3)2}] shows κ3-bonding from L
with a dangling CH2CH2NMe2 arm, whereas the NMR
spectroscopic data in toluene-d8 over a wide range of
temperatures suggest κ4 in solution by exhibiting one set of
resonances for the three arms.3a [(L)Li(2-tBu-C5H5N)] also
behaves in a similar way.3f Among other metals, [(L)MgMe2],13
[(L)Pd(NCS)][NCS],14 [(L)Ru(Tp)][BPh4] (Tp = trispyr-
azolylborate),15 and [(L)Cu(PPh3)][BPh4]16 all exhibit κ3-
bonding from L in solid state but κ4 in solution. Variable-
Figure 2. [(L)Li(THF)][HBPh3] as a solvent-separated ion pair.
The sodium (2) and potassium (3) derivatives also exhibit
κ4-bonding of L in the solid state.2 NMR spectroscopic data in
THF-d8 support a κ4-mode in solution as well for all three
metals with a pseudo-C3v-symmetric ligand environment.2 A
closer look at the 13C{1H} NMR spectra revealed an interesting
feature. For 1, one of the methylene (CH2) resonances at δ
53.0 ppm is abnormally broad at room temperature (Figure 3),
1
temperature H NMR analysis of [(L)Cu(PPh3)][BPh4] in
acetone-d6 showed resonance broadening upon cooling.16 L is
bonded to zinc in a κ2-fashion in [(L)Zn(tBu)2] and bridges
three gallium centers in [(L){Ga(tBu)3}3] with each metal
bound to one arm.17 Without more evidence for 1, we presently
refrain from specifying the dynamic coordination pattern of L
and the THF solvent molecule at the lithium cation. Notably,
unlike 1, [(L)Li(HBEt3)] and [(L)Li(BH4)] have five-
coordinate lithium centers with bridging hydrides, and both
are soluble in aromatic hydrocarbons.3g
Catalytic Hydroboration. As mentioned earlier, the
lithium catalyst 1 was >300 times more active than sodium 2
and potassium 3 in the hydroboration of benzophenone by
HBpin (eq 1).2 We have attempted to rationalize this
superiority of 1 over 2 and 3, based on the solution behavior
discussed above. Several control experiments were carried out
since fast catalysis prevented rigorous kinetic measurements.
1H and 11B NMR spectra in THF-d8 showed no reaction
between 1 and HBpin or BPh3 and HBpin. However, 1 reacted
rapidly with Ph2CO by insertion to give [(L)Li]-
[Ph2CHOBPh3] (7) (Scheme 3) that is fully characterized
Figure 3. Variable-temperature 13C{1H} NMR spectra of 1 in THF-d8.
Only the L region is shown for clarity.
whereas 2 and 3 show no such peculiarity. It is also apparently
absent in other [(L)Li] complexes known in the literatur-
e.3a−d,g,12 Two-dimensional 1H−13C HSQC and 1H−15N
HMBC spectra of 1 in THF-d8 at room temperature attributed
this broad resonance to the N(CH2CH2NMe2)3 carbons (Cb)
(see Supporting Information). A variable-temperature 13C{1H}
NMR spectroscopic analysis of 1 in THF-d8 was subsequently
performed between 213 and 333 K (Figure 3). The broad
resonance for Cb at δ 53.0 ppm (298 K) gradually became
sharper at both temperature limits, shifting upfield to δ 50.6
ppm (213 K) upon cooling and slightly downfield to δ 53.1
ppm (333 K) upon heating. This gives an overall Δδ(Cb) = 2.5.
In comparison, Δδ values for the other set of methylene
carbons [Δδ(Cc) = 1.0] and the N-methyl carbons [Δδ(Ca) =
0.1] are marginal. The NMe2 (Ca) signal become broader in the
Scheme 3. Isolation of 7 from 1 and Ph2CO
including a crystal structure. Like 1, complex 7 also has a
separate ion pair structure in the solid state with a cation
[(L)Li(THF)]+ with THF bonded and a free anion
[Ph2CHOBPh3]− (Figure 4). The fluxional coordination
pattern described for 1 is maintained in 7. The latter reacted
further with HBpin in THF-d8 to give Ph2CHOBpin under
immediate regeneration of 1. These two stoichiometric
reactions support the two-step mechanism proposed earlier
(Figure 5).
The insertion step appeared to be equally fast for all three
metals, at least qualitatively. While HBpin reacted with 7
immediately, the reaction with in situ prepared [(L)Na]-
[Ph2CHOBPh3] required ca. 15 min to complete. Therefore,
the group transfer is probably the rate-limiting step. Orbital
limitations make it unlikely that hydride and alkoxide units are
directly exchanged between the two boron centers by σ-bond
metathesis. We rather propose that the cationic lithium center
1
low-temperature range. In the corresponding H NMR spectra
under variable temperature, the methylene (CH2) resonances,
particularly the CbH2 at δ 2.44 ppm, become broader upon
cooling and shifted upfield (see Supporting Information). In
1
addition, the H NMR spectrum of a recrystallized and mildly
dried sample of 1 at 233 K showed distinct resonances of
protio-THF separated from the residual solvent signal (see
Supporting Information).
This behavior indicates a fluxional coordination from L and
the THF molecule, possibly to maintain a four-coordinate
lithium center in solution at room temperature. At the low-
temperature limit, all donor sites appear to bind to give a five-
coordinate lithium, consistent with the solid state, whereas at
higher temperature, a faster exchange results in a time average
picture. An Eyring plot derived from a line-shape analysis (see
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is similar to that observed for benzophenone hydroboration
catalyzed by 1,3,2-diazaphospholene (KIE = 2.69)18 and a
magnesium-catalyzed dihydroboration of alkyl nitriles (KIE =
2.79).19 An electronic effect on the catalysis rate was also
noticed with two other modified lithium catalysts [(L)Li][HB-
(C6H4-p-Me)3] (8) and [(L)Li][HB(C6H4-p-CF3)3] (9). Both
8 and 9 were synthesized following β-SiH abstraction from the
tetramethyldisilazide precursor [(L)M{N(SiHMe2)2}] by the
respective boranes (Scheme 1). They have solubility and
spectroscopic behavior similar to that of 1. Complex 8 was also
characterized by X-ray crystallography that shows a similar ion
pair structure as 1 (see Supporting Information). Whereas 0.01
mol % of 8 was sufficient to complete the hydroboration of
benzophenone within 10 min, catalyst 9 gave only 35%
conversion after 5 h under identical conditions.
Figure 4. Molecular structure of 7. Displacement parameters are
drawn at 50% probability. Hydrogen atoms are omitted for clarity.
CONCLUSION
■
Moderately Lewis acidic BPh3 mediates β-SiH and N(SiHMe2)2
group abstractions from a series of Me6TREN-coordinated
group 1 tetramethyldisilazides. The two abstraction pathways
are independent and solvent-controlled, with polar THF
favoring the former and nonpolar benzene promoting the
latter. The hydridotriphenylborates 1−3 catalyze the hydro-
boration of carbonyls and CO2; the order of activity being Li ≫
Na ∼ K. Solution behavior of the most active lithium catalyst
(1) was studied in some detail, which divulged a dynamic
coordination from Me6TREN to the cationic lithium center in
tandem with a labile THF molecule. Based on that, the superior
performance of the lithium catalyst was elucidated. The actual
role of Me6TREN is quite intriguing due to its rate-enhancing
ability when 1 is compared to the ligand-free [Li(HBPh3)]
catalyst. In principle, chelation from Me6TREN is expected to
coordinatively saturate the relatively small lithium cation and
decrease its Lewis acidity significantly. Most likely, it renders
lithium mononuclear3g,20 that is otherwise prone to form
aggregates.21 At the same time, lithium probably retains
sufficient Lewis acidity to become a powerful catalyst. Effects
of other chelating ligands in comparison to Me6TREN are
currently under investigation.
Figure 5. Proposed catalytic cycle and group transfer step in the
hydroboration of Ph2CO using 1.
mediates this group exchange by abstracting the hydride from
HBpin (Figure 4). Concurrent alkoxide transfer to “Bpin”
generates the product Ph2CHOBpin along with an incipient
lithium hydride and BPh3. The last two species should
recombine instantly to regenerate the catalyst. BPh3 has a
pronounced hydride affinity as it can easily break down the
ionic lattice of LiH. Likewise, the electrophilic “Bpin” moiety
would prefer to bind to the alkoxy (OCHPh2) group to form
the boronic ester, adding to the driving force of the reaction. A
tighter dative coordination from THF in the case of sodium (2)
or arene coordination from [HBPh3]− in the case of potassium
(3), in addition to a nonlabile κ4-bonding of L, would inhibit a
metal participation. This probably explains why the higher
Lewis acidity and better accessibility of lithium make 1 a far
better catalyst than 2 and 3.
EXPERIMENTAL SECTION
■
All reactions were performed under a dry argon atmosphere using
standard Schlenk techniques or under argon atmosphere in a glovebox.
Prior to use, glassware were dried overnight at 130 °C and solvents
were dried, distilled, and degassed using standard methods. [(L)M-
{N(SiHMe2)2}],2 complexes 1−3,2 B(C6H4-p-Me)3,22 and B(C6H4-p-
22
CF3)3 were prepared following literature procedure. LiD was
A direct hydride transfer from HBpin to Ph2CO induced
by the Lewis acidic metal center is another possibility. We ruled
this out as using [(L)M][BPh4] (M = Li, 0.001 mol %; M = Na,
0.1 mol %) as catalysts were inactive. Initial carbonyl insertion
into the B−H bond is therefore important.
purchased from Sigma-Aldrich. BPh3 (95% pure) was purchased
from abcr and purified by sublimation. 1H, 13C{1H}, 11B, and 29Si{1H}
NMR spectra were recorded on a Bruker Avance-III spectrometer at
ambient temperature unless otherwise mentioned. Chemical shifts (δ
1
ppm) in the H and 13C{1H} NMR spectra were referenced to the
The hydroboration catalysis was further probed with a
deuterium labeling study. A deuterated lithium catalyst
[(L)Li][DBPh3] (1-d) was synthesized by mixing L with
[Li(DBPh3)] in THF. 1-d was found to be equally active as 1 in
the hydroboration of benzophenone. Catalytic hydroboration
of benzophenone by HBpin using 0.001 mol % of 1 was
reported earlier to be accomplished within 1.5 h at room
temperature. The same catalysis with DBpin as the reducing
agent took 4 h to complete under the same condition. Thus,
the kinetic isotope effect (KIE), qualitatively estimated as 2.67,
indicates that the B−H bond breaking is involved in the rate-
determining step (see Supporting Information). This KIE value
residual signals of the deuterated solvents. Abbreviations for NMR
spectra are as follows: s (singlet), d (doublet), t (triplet), q (quartet),
sep (septet), br (broad). IR spectra were recorded on KBr pellets using
an AVATAR 360 FT-IR spectrometer. Abbreviations for IR spectra are
as follows: w (weak), m (medium), s (strong), br (broad). Elemental
analyses were performed on an elementar vario EL machine. X-ray
diffraction data for 5−7 were collected on a Bruker APEX II
diffractometer and reported in a crystallographic information file.
[(L)Li][Ph3B−N(SiHMe2)2] (4). A solution of BPh3 (0.034 g, 0.140
mmol) in 2 mL of benzene was added to a solution of
[(L)Li{N(SiHMe2)2}] (0.051 g, 0.138 mmol) in 3 mL of benzene.
A colorless crystalline solid precipitated immediately upon mixing. The
suspension was stirred for a further 30 min. The precipitate was
D
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isolated and washed with n-pentane (3 × 5 mL). Drying the precipitate
7Li{1H} NMR (156 MHz, THF-d8): δ −0.4. Anal. Calcd for
C43H56BN4OLi: C, 77.93; H, 8.52; N, 8.45. Found: C, 77.62; H,
8.35; N, 8.40.
1
under vacuum afforded 0.068 g of colorless crystals. H NMR of this
solid in THF-d8 indicates a mixture of 4 and 1 in ∼3:1 ratio. Attempts
to isolate pure 4 from this mixture by recrystallization were not
successful, and analytical data were not collected. 4 is characterized
[(L)Li][DBPh3] (1-d). A suspension of LiD (0.026 g, 2.905 mmol)
and BPh3 (0.350 g, 1.445 mmol) in 5 mL of THF was heated to reflux
for 24 h. The suspension was filtered, and the filtrate was evaporated
under reduced pressure to give a pale-yellow solid. The solid was
washed with n-pentane (3 × 5 mL) and dried under vacuum to obtain
[Li(DBPh3)] (0.250 g, 0.996 mmol, 69%) as an off-white powder. L
(0.230 g, 0.998 mmol) and [Li(DBPh3)] (0.200 g, 0.797 mmol) were
mixed in 2 mL of THF to give a clear solution. Slow n-pentane
diffusion into this solution at −35 °C precipitated colorless crystals
within 24 h. The solid was isolated, washed with n-pentane (3 × 5
mL), and dried under vacuum to afford analytically pure [(L)Li]-
1
only by NMR spectroscopy in solution. H NMR (400 MHz, THF-
d8): δ 7.48 (m, 6 H, o-Ph), 6.88 (m, 6 H, m-Ph), 6.73 (m, 3 H, p-Ph),
4.40 (sep, 2 H, 3JHH = 3.2 Hz, SiHMe2), 2.51 (m, 6 H, CH2), 2.36 (m,
3
6 H, CH2) 2.22 (s, 18 H, NMe2), −0.26 (d, JHH = 3.2 Hz, 12 H,
SiHMe2). 13C{1H} NMR (100 MHz,THF-d8): δ 166.4 (ipso-Ph),
137.5 (o-Ph), 125.6 (m-Ph), 122.6 (p-aryl), 58.5 (CH2), 52.6 (CH2),
46.3 (NMe2), 3.4 (SiHMe2). 11B NMR (128 MHz, THF-d8): δ −2.7.
29Si{1H} NMR (80 MHz, THF-d8): δ −19.1. 7Li{1H} NMR (156
MHz, THF-d8): δ −2.1.
1
[(L)Na][Ph3B−N(SiHMe2)2] (5). A solution of BPh3 (0.051 g, 0.211
mmol) in 2 of mL of benzene was added to a solution of
[(L)Na{N(SiHMe2)2}] (0.080 g, 0.207 mmol) in 3 mL of benzene.
A colorless crystalline solid precipitated immediately upon mixing. The
suspension was stirred for a further 30 min. The precipitate was
isolated and washed with n-pentane (3 × 5 mL). Drying the precipitate
[DBPh3] (0.100 g, 0.208 mmol, 26%) as a white powder. H NMR
(400 MHz, THF-d8): δ 7.25 (m, 6 H, o-Ph), 6.83 (m, 6 H, m-Ph), 6.66
(m, 3 H, p-Ph), 2.47 (m, 6 H, CH2), 2.31 (m, 6 H, CH2) 2.16 (s, 18 H,
NMe2). 13C{1H} NMR (100 MHz,THF-d8): δ 136.7 (o-Ph), 126.1 (m-
Ph), 121.7 (p-Ph), 58.8 (CH2), 52.9 (CH2), 46.4 (NMe2). 11B NMR
7
(128 MHz, THF-d8): δ −10.1 (s). Li{1H} NMR (156 MHz, THF-
1
under vacuum afforded 0.011 g of colorless crystals. H NMR of this
d8): δ −2.3. Anal. Calcd for C30H45DBN4Li: C, 74.84; H, 9.84; N,
11.64. Found: C, 74.40; H, 9.55; N, 11.42.
solid in THF-d8 indicates a mixture of 5 and 2 in ∼88:12 ratio. X-ray-
quality single crystals were obtained from slow n-pentane diffusion
[(L)Li][HB(C6H4-p-Me)3] (8). A solution of B(C6H4-p-Me)3 (0.200
g, 0.704 mmol) in 2 mL of THF was added dropwise to a solution of
[(L)Li{N(SiHMe2)2}] (0.260 g, 0.703 mmol) in 1 mL of THF. The
resulting mixture was stirred for 2 h at room temperature. The solution
was then concentrated under reduced pressure. Addition of n-pentane
(5 mL) precipitated a colorless solid, which was washed with n-
pentane (3 × 5 mL) and dried under vacuum to give analytically pure
[(L)Li][HB(C6H4-p-Me)3] (8, 0.261 g, 0.499 mmol, 71%) as a white
powder. X-ray-quality single crystals were obtained from slow n-
1
into a concentrated THF solution at −35 °C. H NMR (400 MHz,
THF-d8): δ 7.48 (m, 6 H, o-Ph), 6.87 (m, 6 H, m-Ph), 6.73 (m, 3 H, p-
Ph), 4.40 (sep, 2 H, 3JHH = 3.2 Hz, SiHMe2), 2.51 (m, 6 H, CH2), 2.34
3
(m, 6 H, CH2), 2.19 (s, 18H, NMe2), −0.26 (d, JHH = 3.2 Hz, 12 H,
SiHMe2). 13C{1H} NMR (100 MHz, THF-d8): δ 164.5 (ipso-Ph),
137.5 (o-Ph), 125.6 (m-Ph), 122.5 (p-Ph), 58.7 (CH2), 53.1 (CH2),
46.2 (NMe2), 3.4 (SiHMe2). 11B NMR (128 MHz, THF-d8): δ −2.7.
29Si{1H} NMR (79.5 MHz, THF-d8): δ −17.3. Satisfactory CHN
analysis data could not be obtained for 5.
1
pentane diffusion into a concentrated THF solution at −35 °C. H
[(L)K][Ph3B−N(SiHMe2)2] (6). A solution of BPh3 (0.031 g, 0.128
mmol) in 2 mL of benzene was added to a solution of
[(L)K{N(SiHMe2)2}] (0.050 g, 0.124 mmol) in 3 mL of benzene.
A colorless crystalline solid precipitated immediately after mixing. The
suspension was then stirred for a further 30 min. The precipitate was
isolated and washed with n-pentane (3 × 5 mL). Drying the solid
under vacuum afforded analytically pure 6 (0.073 g, 0.113 mmol, 91%)
as a colorless powder. X-ray-quality single crystals were obtained from
slow n-pentane diffusion into a concentrated THF solution at −35 °C.
1H NMR (400 MHz, THF-d8): δ 7.51 (m, 6 H, o-Ph), 6.91 (m, 6 H,
NMR (400 MHz, THF-d8): δ 6.74 (m, 6 H, o-aryl), 6.60 (m, 6 H, m-
1
aryl), 3.68 (br, q, JBH = 79 Hz, 1 H, HB), 2.52 (m, 6 H, CH2), 2.35
(m, 6 H, CH2) 2.19 (s, 18 H, NMe2), 2.13 (s, 9 H, p-Me). 13C{1H}
NMR (100 MHz,THF-d8): δ 164.2 (ipso-aryl), 136.0 (o-aryl), 127.9
(m-aryl), 123.7 (p-aryl), 58.8 (CH2), 52.9 (CH2), 46.4 (NMe2), 23.9
1
(p-Me). 11B NMR (128 MHz, THF-d8): δ −15.7 (d, JBH = 79 Hz).
7Li{1H} NMR (156 MHz, THF-d8): δ −2.3. Anal. Calcd for
C33H52BN4Li: C, 75.85; H, 10.03; N, 10.72. Found: C, 75.30; H,
9.73; N, 10.66.
[(L)Li][HB(C6H4−p-CF3)3] (9). A solution of B(C6H4-p-CF3)3
(0.223 g, 0.500 mmol) in 2 mL of THF was added dropwise to an
1 mL of THF solution of [(L)Li{N(SiHMe2)2}] (0.185 g, 0.500
mmol). The resulting mixture was stirred for 2 h at room temperature.
The solution was then concentrated under reduced pressure. Addition
of n-pentane (5 mL) precipitated a light yellow solid, which was
washed with n-pentane (3 × 5 mL) and dried under vacuum to give
analytically pure [(L)Li][HB(C6H4-p-CF3)3] (9, 0.216 g, 0.316 mmol,
63%) as a light yellow powder. 1H NMR (400 MHz, THF-d8): δ 7.36
(m, 6 H, o-aryl), 7.18 (m, 6 H, m-aryl), 3.65 (br, q, 1JBH = 79 Hz, 1 H,
HB), 2.53 (m, 6 H, CH2), 2.36 (m, 6 H, CH2), 2.20 (s, 18 H, NMe2).
13C{1H} NMR (100 MHz,THF-d8): δ 170.29 (ipso-aryl), 136.4 (o-
aryl), 135.0 (m-aryl), 124.8 (p-aryl), 123.0 (p-CF3), 58.8 (CH2), 52.9
3
m-Ph), 6.77 (m, 3 H, p-Ph), 4.36 (sep, 2 H, JHH = 3.2 Hz, SiHMe2),
2.55 (m, 6 H, CH2), 2.30 (m, 6 H, CH2), 2.15 (s, 18 H, NMe2), −0.24
3
(d, JHH = 3.2 Hz, 12 H, SiHMe2). 13C{1H} NMR (100 MHz, THF-
d8): δ 163.9 (ipso-Ph), 137.3 (o-Ph), 125.9 (m-Ph), 122.8 (p-Ph), 59.4
(CH2), 54.6 (CH2), 46.4 (NMe2), 3.4 (SiHMe2). 11B NMR (128 MHz,
THF-d8): δ −2.7. 29Si{1H} NMR (79.5 MHz, THF-d8): δ −17.1. IR
(KBr, cm−1): 3058 (m), 3041 (m), 2970 (s), 2862 (s), 2828 (m), 2777
(s), 2117 (s, νSiH), 2072 (s, νSiH), 1578 (w), 1462 (s), 1427 (m), 1357
(m), 1303 (s), 1281 (m), 1237 (s), 1161 (m), 1131 (m), 1115 (s),
1069 (m), 1036 (m), 1019 (m), 987 (s), 922 (s), 885 (s), 833 (s), 789
(m), 764 (s), 744 (m), 732 (m), 706 (s), 612 (s). Anal. Calcd for
C34H59BN5Si2K: C, 63.42; H, 9.24; N, 10.88. Found: C, 63.40; H,
9.18; N, 11.01.
1
(CH2), 46.3 (NMe2). 11B NMR (128 MHz, THF-d8): δ −8.7 (d, JBH
7
= 83 Hz). 19F NMR (377 MHz, THF-d8): δ −61.9. Li{1H} NMR
[(L)Li][Ph3B−OCHPh2] (7). A 2 mL THF solution of 1 (0.100 g,
0.208 mmol) and Ph2CO (0.038 g, 0.208 mmol) was stirred for 10
min at room temperature. Volatiles were removed under reduced
pressure to obtain a white solid. The solid was washed with n-pentane
(3 × 5 mL) and dried under vacuum to give analytically pure 7 (0.131
g, 0.201 mmol, 97%) as a colorless powder. X-ray-quality single
crystals were obtained from slow n-pentane diffusion into a
(156 MHz, THF-d8): δ −0.4. Anal. Calcd for C33H43BF9N4Li: C,
57.91; H, 6.33; N, 8.19. Found: C, 57.50; H, 6.42; N, 8.44.
ASSOCIATED CONTENT
■
S
* Supporting Information
1
concentrated THF solution at −35 °C. H NMR (400 MHz, THF-
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00308.
d8): δ 7.40 (m, 6 H, o-aryl), 7.29 (m, 4 H, o-aryl), 6.92 (m, 4 H, m-Ph),
6.81 (m, 2 H, p-Ph), 6.78 (m, 6 H, m-Ph), 6.67 (m, 3 H, p-H), 5.53 (s,
1 H, OCHPh2), 2.46 (m, 6 H, CH2), 2.29 (m, 6 H, CH2), 2.15 (s, 18
H, NMe2). 13C{1H} NMR (100 MHz, THF-d8): δ 164.0 (ipso-Ph),
152.1 (ipso-Ph), 135.9 (o-Ph), 128.2 (o-Ph), 127.6 (m-Ph), 126.0 (p-
Ph), 125.1 (m-Ph), 122.9 (p-Ph), 78.6 (OCHPh2), 58.8 (CH2), 53.1
(CH2), 46.3 (NMe2). 11B NMR (128 MHz, THF-d8): δ 2.1 (s).
Spectroscopic characterization of compounds 4−9,
B(C6H4-p-Me)3, and B(C6H4-p-CF3)3, and crystallo-
graphic details for compounds 5−8 (PDF)
E
DOI: 10.1021/acs.organomet.7b00308
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(7) Yan, K.; Schoendorff, G.; Upton, B. M.; Ellern, A.; Windus, T. L.;
Accession Codes
Sadow, A. D. Organometallics 2013, 32, 1300−1316.
CCDC 1498800 and 1545138−1545140 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(8) Dioumaev, V. K.; Plossl, K.; Carroll, P. J.; Berry, D. H.
̈
Organometallics 2000, 19, 3374−3378.
(9) Werkema, E. L.; Andersen, R. A.; Yahia, A.; Maron, L.; Eisenstein,
O. Organometallics 2009, 28, 3173−3185.
(10) Wiseman, G. H.; Wheeler, D. R.; Seyferth, D. Organometallics
1986, 5, 146−152.
(11) Mukherjee, D.; Ellern, A.; Sadow, A. D. J. Am. Chem. Soc. 2010,
132, 7582−7583.
AUTHOR INFORMATION
■
(12) Kennedy, A. R.; Mulvey, R. E.; O’Hara, C. T.; Robertson, G. M.;
Robertson, S. D. Angew. Chem., Int. Ed. 2011, 50, 8375−8378.
(13) Guard, L. M.; Hazari, N. Organometallics 2013, 32, 2787−2794.
(14) Ferguson, G.; Parvez, M. Acta Crystallogr., Sect. B: Struct.
Crystallogr. Cryst. Chem. 1979, 35, 2207−2210.
Corresponding Author
*E-mail: jun.okuda@ac.rwth-aachen.de.
ORCID
Hassan Osseili: 0000-0002-2255-9230
Jun Okuda: 0000-0002-1636-5464
Notes
(15) Slugovc, C.; Gemel, C.; Shen, J.-Y.; Doberer, D.; Schmid, R.;
Kirchner, K.; Mereiter, K. Monatsh. Chem. 1999, 130, 363−375.
(16) Eckenhoff, W. T.; Pintauer, T. Dalton Trans. 2011, 40, 4909−
4917.
The authors declare no competing financial interest.
(17) Cadenbach, T.; Hevia, E.; Kennedy, A. R.; Mulvey, R. E.;
Pickrell, J.-A.; Robertson, S. D. Dalton Trans. 2012, 41, 10141−10144.
(18) Chong, C. C.; Hirao, H.; Kinjo, R. Angew. Chem., Int. Ed. 2015,
54, 190−194.
ACKNOWLEDGMENTS
■
We thank the Deutsche Forschungsgemeinschaft through the
International Research Training Group “Selectivity in Chemo-
and Biocatalysis” for financial support and the Alexander von
Humboldt Foundation for a fellowship to D.M. We thank Khai-
Nghi Truong for collecting crystallographic data, Dr. Gerhard
Fink for NMR spectroscopic, and Wataru Hato and Takafumi
Higuchi for experimental assistance.
(19) Weetman, C.; Anker, M. D.; Arrowsmith, M.; Hill, M. S.;
Kociok-Kohn, G.; Liptrot, D. J.; Mahon, M. F. Chem. Sci. 2016, 7,
628−641.
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(b) Giese, H.-H.; Noth, H.; Schwenk, H.; Thomas, S. Eur. J. Inorg.
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DOI: 10.1021/acs.organomet.7b00308
Organometallics XXXX, XXX, XXX−XXX