CO2 reduction via aluminum complexes of ammonia boranes

Article abstract of DOI:10.1039/c3dt00098b

Reactions of amine-boranes NH₃BH₃, Me ₂NHBH₃, or Me₃NBH₃ with AlX ₃ (X = Cl, Br, I, C₆F₅) have been examined. The species AlBr₃·H₃BNMe

Full text of DOI:10.1039/c3dt00098b

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CO2 reduction via aluminum complexes of ammonia
boranes†
Cite this: DOI: 10.1039/c3dt00098b
Gabriel Ménard and Douglas W. Stephan*
Reactions of amine-boranes NH3BH3, Me2NHBH3, or Me3NBH3 with AlX3 (X = Cl, Br, I, C6F5) have been
examined. The species AlBr3·H3BNMe3 2, Al(C6F5)3·H3BNMe3 3, Al(C6F5)3·H3BNHMe2
4
and
Al(C6F5)3·H3BNH3 5 have been prepared and isolated. The analogous reaction of B(C6F5)3 and H3BNMe3
results in C6F5-transfer and the formation of (C6F5)BH2·NMe3 6. While the adduct 6 was unreactive to
CO2, species 3 reacts with CO2 to give the formate linked Al(C6F5)3(HCO2)H2BNMe3 8. The species R3PC-
(OAl(C6F5)3)2 (R = o-tol (1’-C6F5), R = Mes (1-C6F5)) were prepared, and 1’-C6F5 was shown to react with
amine-boranes to effect the reduction of this bound-CO2 to formate and methoxide-derivatives, proceed-
ing through intermediates including 8 and [(Me3NBH2)2(μ-H)][(HCO2)(Al(C6F5)3)2] 9. The salt [tBu3PH]-
[(HCO2)(Al(C6F5)3)2] 10 was prepared independently.
Received 11th January 2013,
Accepted 12th February 2013
DOI: 10.1039/c3dt00098b
www.rsc.org/dalton
employing FLPs affording reduction to formate, MeOH, CO,
Introduction
and CH4 depending on the reductant used.24–29 In an earlier
Carbon dioxide is ubiquitous in our environment and main- report, we described the formation of Al-based FLP complexes
tained in gentle balance through the combined actions of of CO2 of the form Mes3PC(OAlX3)2 (1-X; X = Cl, Br, I) and their
photosynthesis, animal respiration, decomposing organic subsequent reaction with ammonia borane (AB = NH3BH3) to
material, etc. as part of the carbon cycle. However, since the effect, upon quenching, the stoichiometric reduction to MeOH
industrial revolution, this balance has been altered with (eqn (1)).24
the rapid increase in CO2 emission prompting global climate
Herein, we examine the interaction of Al-based Lewis acids
change.1,2 While mitigation of emission through reduced con- with several amine-boranes and explore the reactivity of these
sumption,3 the use of renewables4 and a shift away from species with CO2. This reactivity is also shown to have impli-
C-based fuels5–7 likely offer the best long-term remedies, strat- cations for the pathway for reduction of the CO2 in 1-X.
egies to carbon-neutral fuels, such as Olah’s “Methanol
Economy”, are being explored.8,9 Essential to such efforts are
fundamental understanding of CO2 activation and reduction
chemistry.
Transition metal and main-group based reductions of CO2
ð1Þ
have recently been explored. A variety of metals, such as Zr,10
Nb,11 Re,12 Fe,13 Ru,14 Ir,15 Ni,16,17 and Cu,18,19 in conjunction
with H2 or an external reductant, have been used to reduce
CO2 to CO, formic acid, MeOH, and even methane. Non-metal
mediated routes are also beginning to emerge. Use of such
reagents to capture CO2 has been demonstrated for carbenes,
frustrated Lewis pairs (FLPs)20 and most recently a seemingly
Results and discussion
Reactions of amine-boranes with Lewis acids
benign dithiocarbamate.21 Furthermore while catalytic
reduction of CO2 has been achieved employing carbenes,22,23
Reactions of amine-boranes AB, Me2NHBH3
Me3NBH3
Me3AB) with AlX3 (X = Cl, Br, I, C6F5) have been
(
Me2AB), or
stoichiometric reductions of CO2 have been demonstrated
(
examined. In the case of AlBr3, an equimolar amount of AlBr3
was combined with the soluble Me3AB in a J-Young tube in
C6D5Br. An 27Al NMR spectrum showed a single broad peak
centered at 79 ppm and the 11B and 1H spectra showed only
slightly shifted and broadened (for BH3) signals in comparison
Department of Chemistry, University of Toronto, 80 St. George St., Toronto, Ontario
M5S 3H6. E-mail: dstephan@chem.utoronto.ca
†Electronic supplementary information (ESI) available: Synthetic and spectro-
scopic data are deposited. CCDC 915110–915116. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c3dt00098b
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Fig. 1 POV-Ray depictions of (a) 2; (b) 3; and (c) 5. C: black, N: blue, B: yellow-green, Al: teal, Br: scarlet, F: pink, H: white. Methyl H-atoms omitted for clarity.
to an authentic sample of Me3AB. The synthesis of this new
species 2 was scaled up and 2 was isolated in 70% yield.
Elemental analysis was consistent with the formulation of 2 as
AlBr3·H3BNMe3 and an X-ray diffraction study confirmed the
association of the B of Me3AB to the Al of AlBr3 via two hydride
bridges (Fig. 1a). Efforts to prepare an analogous adduct using
Fig. 2 POV-Ray depiction of 6. C: black, N: blue, B: yellow-green, F: pink,
AB and AlBr3 gave a highly insoluble product, the nature of
H: white. Methyl H’s omitted for clarity. B–N: 1.628(2) Å.
which could not be unambiguously determined.
The analogous synthesis employing Al(C6F5)3 and Me3AB,
Compounds 2–5 are stable in solution for at least a day,
however all compounds readily decompose when heated to
70 °C for several hours. The stability of these compounds
stands in marked contrast to the analogous situation for
B(C6F5)3 in which the Lewis acid has been reported to catalyze
the dehydrogenation of AB.45 However, in the case of Me3AB
and B(C6F5)3, NMR data suggest an alternative reaction
pathway. While a 1 : 1 reaction results in broad signals in the
gave the related species 3 after 15 min in 77% yield. While an
1
27Al NMR signal was not observed, the 11B and H signals for
the Me3AB fragment were again slightly shifted and broadened.
Furthermore, the 19F signals for Al(C6F5)3 were also shifted.
An X-ray structure confirmed 3 as Al(C6F5)3·H3BNMe3 (Fig. 1b).
Similarly, the Me2AB and AB adducts of Al(C6F5)3, 4 and 5 were
prepared in 87% and 55% yield, respectively.
In the case of 5 the structure was again confirmed crystallo-
graphically (Fig. 1c).
1
11B, 19F, and H NMR spectra, a 1 : 2 ratio of B(C6F5)3 to Me3AB
proceeds more cleanly. When monitored by NMR spectro-
scopy, the clean formation of [(µ2-H)(H2BNMe3)2][HB(C6F5)3]
was observed after 20 min. Spectral data were consistent with
the presence of each of these ions.46,47 Attempts to isolate this
salt were unsuccessful as this species reacts further to give two
new compounds 6 and 7. Compound 6 was isolated by crystal-
lization of a toluene–hexanes solution at −38 °C. The 11B NMR
The structural data for 2, 3, and 5, reveal N–B distances of
1.577(5) Å, 1.589(2) Å, and 1.582(4) Å respectively. These are
similar to the values reported for Me3AB (1.617(4) Å) and AB
(1.58(2) Å) in the solid state.30,31 This is attributable to polariz-
ation at B resulting from coordination of the B–H bonds to Al.
The bridging hydrides give rise to differences in the B–Al dis-
tances in 2 and 3 being 2.312(5) Å and 2.498(3) Å, respectively.
This presumably reflects the increased steric congestion
between the C6F5 and Me groups of 3 in comparison to lesser
interaction of the Br and Me substituents of 2. Consistent with
this notion, the B–Al distance in 5 is much shorter than in 3
and similar to 2 at 2.357(3) Å.
Interestingly, while Al has been used in the dehydrogena-
tion chemistry of amine-boranes,32,33 the present compounds
represent rare examples of intermolecular B–H–Al bonds. This
B–H–M bonding motif is reminiscent of several known
early,34,35 late,36–41 and s-block main group42–44 AB complexes.
However, AB complexes of d0 elements typically contain intra-
molecular B–H–M (M = Mg, Ca, Sc)34,42–44 bonds derived from
chelating anions {H3BNMe2BH2Me2N}− or {H3BR2N−} with
both terminal B–H and amide (R2N−) groups bound to the
same metal centre. In contrast, compounds 2–5 represent the
first intermolecular group 13-AB adducts known, where no
chelate is present.
spectrum of the product contained a broadened triplet (JB–H
=
95 Hz) at −10 ppm with corresponding peaks in the 19F spec-
trum at −129, −157, and −164 ppm, indicative of a four-coordi-
1
nate boron centre. The H spectrum contained 2 peaks which
integrated in a 9 : 2 ratio for the Me : B–H peaks. Furthermore,
the B–H peaks formed a triplet (JH–F = 7 Hz) in the 1H{11B}
spectrum indicative of H–F coupling to the ortho-fluorines of
the C6F5 ring. Single crystal X-ray crystallography confirmed
the structure of 6 as (C6F5)BH2·NMe3 (Fig. 2). The second
product (7) is ascribed to the adduct-free (C6F5)BH2 based on
NMR spectroscopy; however, attempts to isolate this species
have thus far failed.
The formation of 6 and 7 is the result of a redistribution
reaction (eqn (2)) and stands in contrast to that seen for the
corresponding reaction with Al(C6F5)3. It should be noted that
a similar combination of B(C6F5)3 and BH3·SMe2 has been
reported to generate (C6F5)BH2·SMe2.48
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ð2Þ
Reactions of Al–AB species with CO2
Scheme 1 Proposed reaction pathways of 3 and 8 (X = C6F5 and R = Me).
While the adduct 6 was unreactive to CO2, species 2–5 react
with CO2. In the case of 2 or 4 exposure to 1 atm of 13CO2
leads to evidence of reduction to several formate and methoxy
species as evidenced by the 13C resonances in the
170–175 ppm and 50–70 ppm ranges, respectively. However no
single species could be isolated from these mixtures. In con-
trast, 3 reacts with 13CO2 to cleanly form a single formate
derivative 8. This species exhibits a resonance at 173 ppm in
the 13C NMR spectrum that correlates to a peak at 8.14 ppm in
the 1H NMR spectrum. While the corresponding 27Al NMR
spectrum again showed no discernible signal, the 11B signal
was shifted to 3.8 ppm. Compound 8 was isolated in 72% yield
and subsequently crystallographically characterized (Fig. 3)
and shown to be Al(C6F5)3(HCO2)H2BNMe3 in which a formate
group links the Al(C6F5)3 and Me3NBH2 fragments. Similar bis-
boron formate derivatives have been reported by Piers28 and by
our group.26 The C–O bond lengths in 8 are equal at 1.252(2) Å
and the Al–O (1.823(1) Å) and B–O (1.536(2) Å) are similar to
those in previous reports.24–26,28 Compound 8 represents the
first example, to our knowledge, of a formate moiety bound
between two different group 13 elements.
Scheme 2 Synthesis of 10.
While 8 is stable in solution, it is noteworthy that the corre-
sponding reactions employing 5 with 13CO2 show evidence of
further reaction. A broad signal in the 13C NMR spectrum at
172 ppm, typical for a formate fragment was observed 15 min
after mixing. In addition, broad signals in the 11B NMR spec-
trum at 34 and −15 ppm were attributed to the dehydroge-
nated species (HNBH)n and (H2NBH2)n,49 respectively.
Quenching this sample with D2O and subsequent 1H NMR
Fig. 4 POV-Ray depiction of the anion of 10. C: black, O: red, Al: teal, F: pink,
H: white.
3.34 ppm and a less intense doublet at 8.43 ppm consistent
with methanol and formic acid, respectively. Attempts to experi-
mentally identify intermediates in the conversion of formate
to methanol were inconclusive.
analysis showed
a
doublet (due to H–13C coupling) at
Subsequently, the reaction of 8 with 1 equivalent of 3 was
monitored over time by NMR spectroscopy. These data suggest
a Lewis acid exchange reaction generates the species [(µ2-H)-
(H2BNMe3)2][HCO2(Al(C6F5)3)2] (9) although this species could
not be isolated (Scheme 1). Nonetheless, the cation of 9 [(µ2-H)-
(H2BNMe3)2]+ is known47 and exhibits the expected reson-
ances. The anion was independently synthesized by reacting
the known salt50 [tBu3PH][(µ2-H)(Al(C6F5)3)2] with CO2 to gen-
1
erate [tBu3PH][(HCO2)(Al(C6F5)3)2] 10 (Scheme 2). The H, 13C
and 19F spectra for the anion of 10 were identical to those seen
for 9. The structure of 10 was unambiguously determined
(Fig. 4), confirming the bridging nature of the formate
fragment in the anion.
Reduction of CO2 in Mes3PC(OAlX3)2
Related to the above reductions of CO2 with AB, we previously
communicated that solutions of 1-Cl or 1-Br treated with
excess (3 eq.) AB and subsequent water quench result in the
Fig. 3 POV-Ray depiction of 8. C: black, N: blue, B: yellow-green, O: red,
Al: teal, F: pink, H: white. Methyl H’s omitted for clarity.
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Scheme 3 Reaction of 1’-C6F5 with Me3AB.
Fig. 5 POV-Ray depiction of 1’-C6F5. C: black, O: red, Al: teal, F: pink, P: orange.
H atoms omitted for clarity.
of Al acceptors on each oxygen of the formate in 9 prompts
hydride transfer to the central carbon.
Interestingly, treating 1′-C6F5 with the sterically less
crowded amine-boranes Me2AB or AB led to more rapid
reduction to methoxy derivatives under milder conditions.
Space-filling models of 3 and 5 (Fig. S1†) show decreasing
steric congestion around the B–H–Al moiety providing more
accessible hydride centres. This is thought to facilitate further
reduction consistent with the increasing reactivity of 2, 4, and
5 compared to 3 with CO2, as well as observed in our initial
report with 1-X and AB.24
reduction of the CO2 moiety to MeOH in 37–51% yield
(eqn (1)).24 However, the study of the reduction of CO2 in com-
plexes 1-X (X = Cl, Br, I)24 was plagued by the poor solubility of
AB in bromobenzene, the very fast reaction rate, and the
complex mixture of products formed. Nonetheless, treatment
of 1-Cl with increasing amounts of Me2AB or Me3AB to 1 equiv.
results in increasing formation of methoxy species from an
intermediate formate species.
To provide a further spectroscopic handle, the species
Mes3PC(OAl(C6F5)3)2 (1-C6F5) and o-tol3PC(OAl(C6F5)3)2 (1′-C6F5)
were prepared employing an analogous method to the one
used for 1-X.24,25 The latter species was structurally character-
ized (Fig. 5) confirming the coordination of a Al(C6F5)3 frag-
Conclusions
ment coordinated to each of the O-atoms of the CO2 moiety. In this work, we have demonstrated that amine-boranes form
The O–C–O angle in 1′-C6F5 (127.1(3)°) and O–Al bond lengths adducts with Al-based Lewis acids and undergo subsequent
of 1.858(2) Å are similar to those reported for 1-X. This stands reactions with CO2 affording formate-bridged species. In
in contrast to the reported complex51 tBu3P(CO2)Al(C6F5)3 addition, the reduction of the CO2 fragment in 1′-C6F5 is
where only one Al centre is present. Indeed, although a 1 : 1- shown to proceed through the formate species 9, described
CO2 complex could be observed by NMR spectroscopy, reaction above. This demonstrates that, while the species 1-X are
mixtures of o-tol3P and Al(C6F5)3 in the presence of CO2 led derived from FLP capture of CO2,24 these species serve only as
only to the isolation of 1′-C6F5 regardless of the stoichiometry.
stoichiometric sources of CO2 and Lewis acid upon reaction
The reaction of 13C-labelled 1′-C6F5 (13C-1′-C6F5) with with AB. The rapidity described for the reduction of 1-X,24 with
0.33 equiv. of Me3AB was monitored by NMR spectroscopy. The AB is consistent with reduced reactivity observed for the more
initial spectra show broadening of the 31P peak attributable to sterically demanding reagents Me2AB or Me3AB. Finally, Paul
1′-C6F5, as well as some free o-tol3P, inferring an equilibrium et al.52 proposed a complex mechanism involving direct attack
involving dissociation of Al(C6F5)3 from 1′-C6F5. This was con- of AB on the CO2 moiety of 1-X, based on computational
current with the formation of the formate species 8 as evi- studies. The present experimental results stand in contrast
denced by 11B{1H}, 13C{1H}, 19F{1H} and 1H NMR spectroscopy. and support a mechanism resulting from dissociative reactions
The liberated alane also generates 3 in situ which can react of 1-X.
with 8 to form the salt 9 (Scheme 3). After 12 h the reaction
mixture is observed to contain 13C-1′-C6F5, 9, and the cation
[o-tol3P-BH2NMe3]+. The identity of the latter species was con-
Experimental section
General considerations
firmed by the independent reaction of the known salt47 [(µ2-H)-
(H2BNMe3)2][B(C6F5)4] with excess o-tol3P. Altering the stoichio-
metry to a full equivalent of Me3AB resulted in the consump-
All manipulations were performed under an atmosphere of
tion of 1′-C6F5 converting it to 8 and 9. Heating this reaction
mixture to 60 °C for several hours led to further reduction
from formate to methoxy derivatives. Presumably the presence
dry, oxygen-free N2 by means of standard Schlenk or glovebox
techniques (Innovative Technology glovebox equipped with a
−38 °C freezer). Hexanes, pentane, and toluene (Aldrich) were
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dried using an Innovative Technologies solvent system and (s, p-CH3Mes). Anal. Calc. for C64H33Al2F30O2P: C, 51.63; H,
degassed prior to use. Fluorobenzene and bromobenzene (–H5 2.23. Found: C, 51.24; H, 2.56.
and –D5) were purchased from Aldrich and dried over P2O5 for
Synthesis of (o-tol)3PC(OAl(C6F5)3)2 (1′-C6F5). Synthesized in
several days and vacuum distilled onto 4 Å molecular sieves an analogous fashion to 1-C6F5 using 76 mg (0.25 mmol)
prior to use. Dichloromethane-d2 and toluene-d8 were pur- (o-tol)3P and 311 mg (0.50 mmol) Al(C6F5)3·tol. Isolated yield is
chased from Aldrich, dried over CaH2 and vacuum distilled 325 mg (0.22 mmol, 90%). Vapour diffusion of a bromo-
onto 4 Å molecular sieves prior to use. D2O was purchased benzene solution of the compound with pentane yielded
from Cambridge Isotopes. NMR spectra were obtained on a single crystals suitable for X-ray crystallography.
Bruker Avance 400 MHz or a Varian 400 MHz and spectra were
1H NMR (400 MHz, C6D5Br): δ 7.60–6.85 (m, 12H), 1.81 (bs,
referenced to residual solvent of C6D5Br (1H = 7.28 ppm 9H, o-CH3), 1.27–1.13 (m, 4H, 0.5·(CH3(CH2)CH3)), 0.84 (t,
for meta proton; 13C = 122.4 ppm for ipso carbon), C7H8 (1H = 3JH–H = 7 Hz, 3H, 0.5·(CH3(CH2)CH3)). 31P{1H} NMR (161 MHz,
2.08 ppm for methyl; 13C = 20.43 ppm for methyl), D2O (1H = C6D5Br): δ 30.0. 27Al NMR (104 MHz, C6D5Br): blank. 19F{1H}
4.79 ppm for residual HDO peak), and CD2Cl2 (1H = 5.32 ppm; NMR (376 MHz, C6D5Br): δ −121.8 (bd, JF–F = 19 Hz, 12F,
3
13C = 53.84 ppm), or externally (27Al: Al(NO3)3,11B: (Et2O)BF3, o-C6F5), −152.5 (t, JF–F = 20 Hz, 6F, p-C6F5), −161.4 (m, 12F,
3
31P: 85% H3PO4, 19F: CFCl3). Chemical shifts (δ) listed are in m-C6F5). 13C{1H} NMR (100 MHz, C6D5Br): δ 167.3 (d, JC–P
=
1
ppm and absolute values of the coupling constants are in Hz. 128 Hz, CO2), 149.9 (dm, 1JC–F = 234 Hz), 144.2 (d, 2JC–P = 8 Hz,
1
1
NMR assignments are supported by additional 2D experi- C-Me), 141.7 (dm, JC–F = 252 Hz), 136.7 (dm, JC–F = 253 Hz),
ments. Elemental analyses (C, N, H) and X-ray crystallography 136.4 (d, JC–P = 2 Hz), 135.2 (d, JC–P = 12 Hz), 133.7 (d, JC–P
=
1
were performed in house. (o-tol)3P and Mes3P were 11 Hz), 127.7 (d, JC–P = 14 Hz), 112.9 (m, i-C6F5), 112.1 (d, JC–P
purchased from Strem and used without further purification. = 78 Hz, i-C6H4), 31.8 (s, C6H14), 22.9 (s, C6H14), 22.4 (bs,
NH3BH3 was purchased from Aldrich and used without further o-CH3), 14.4 (s, C6H14). Anal. Calc. for C61H28Al2F30O2P
purification. AlBr3, Me2NHBH3, and Me3NBH3 were purchased (4 + 0.5·C6H14): C, 50.61; H, 1.95. Found: C, 50.36; H, 2.06.
from Strem and sublimed prior to use. 13CO2 and Me2Si(H)Cl
Synthesis of Me3NBH3·AlBr3 (2). AlBr3 (300 mg, 1.1 mmol)
were purchased from Aldrich and used without further and Me3NBH3 (82 mg, 1.1 mmol) were combined in bromo-
purification. B(C6F5)3 was purchased from Boulder Scientific, benzene (5 mL) in a screw cap vial. After stirring for 5 min,
sublimed under vacuum, then treated with excess Me2Si(H)Cl hexanes (ca. 10 mL) were added to precipitate a product. The
for 4 h and re-sublimed after removal of volatiles. CO2 product was filtered on a glass frit, washed with pentane and
(grade 4.0) was purchased from Linde and passed through dried (265 mg, 0.78 mmol, 70%). Vapour diffusion of a bromo-
a Drierite column prior to use. Al(C6F5)3·tol,24a [tBu3PH][H(Al- benzene solution of the compound with pentane yielded
(C6F5)3)2]50 and 1-X (X = Cl, Br, I)24,25 were prepared according single crystals suitable for X-ray crystallography.
1
to literature procedure.
1H NMR (400 MHz, C6D5Br): δ 2.62 (bq, JH–B = 89 Hz, 3H,
Synthesis of Mes3PC(OAl(C6F5)3)2 (1-C6F5). A Schlenk flask BH3), 2.07 (s, 9H, 3 × CH3). 11B NMR (128 MHz, C6D5Br):
equipped with a Teflon screw cap was charged with Mes3P δ −10.2 (bq, JB–H = 89 Hz). 27Al NMR (104 MHz, C6D5Br): 78.5
1
(125 mg, 0.32 mmol), Al(C6F5)3·tol (400 mg, 0.64 mmol) (bs). 13C{1H} NMR (100 MHz, C6D5Br): δ 53.5. Anal. Calc. for
and bromobenzene (5 mL). The bomb was transferred C3H12BAlBr3N: C, 10.61; H, 3.56; N, 4.12. Found: C, 10.83;
to the Schlenk line equipped with a CO2 outlet. The bomb H, 3.22; N, 4.11.
was degassed, filled with CO2 (1 atm), and sealed. The
Synthesis of Me3NBH3·Al(C6F5)3 (3). Al(C6F5)3·tol (750 mg,
solution became a mixture and was stirred for 30 min. 1.2 mmol) and Me3NBH3 (88 mg, 1.2 mmol) were combined in
after which time the CO2 atmosphere was removed and toluene (10 mL) in a 50 mL Schlenk flask. After stirring for
hexanes (ca. 5–10 mL) were added dropwise to the stirring 5 min, pentane (ca. 30 mL) was added to precipitate a product.
mixture in the glovebox. The precipitate was filtered on a glass The product was filtered on a glass frit, washed with pentane
frit, washed with hexanes and dried in vacuo (410 mg, and dried (560 mg, 0.93 mmol, 77%). Slow cooling a saturated
0.27 mmol, 85%).
toluene solution to −38 °C yielded single crystals suitable for
4
1H NMR (400 MHz, CD2Cl2): δ 7.21 (d, JH–H = 4 Hz, 3H, X-ray crystallography.
4
m-Mes), 7.05 (d, JH–H = 4 Hz, 3H, m-Mes), 2.38 (s, 9H, CH3),
1H NMR (400 MHz, C6D5Br): δ 2.14 (overlapping s (3 × CH3)
2.28 (s, 9H, CH3), 1.97 (s, 9H, CH3). 31P{1H} NMR (161 MHz, and bs (BH3), 12H). 11B NMR (128 MHz, C6D5Br): δ −9.1 (bs).
CD2Cl2): δ 17.0. 27Al NMR (104 MHz, CD2Cl2): blank. 19F{1H} 27Al NMR (104 MHz, C6D5Br): blank. 19F{1H} NMR (376 MHz,
3
4
3
4
NMR (376 MHz, CD2Cl2): δ −122.2 (dd, JF–F = 26 Hz, JF–F
=
C6D5Br): δ −122.4 (dd, JF–F = 26 Hz, JF–F = 11 Hz, 6F, o-C6F5),
10 Hz, 12F, o-C6F5), −154.7 (bs, 6F, p-C6F5), −163.3 (m, 12F, −152.4 (t, 3JF–F = 19 Hz, 3F, p-C6F5), −161.3 (m, 6F, m-C6F5). 13
C
m-C6F5). 13C{1H} NMR (100 MHz, CD2Cl2): δ 150.2 (dm, JC–F
=
{1H} NMR (100 MHz, C6D5Br): δ 150.0 (dm, JC–F = 235 Hz),
1
1
232 Hz), 146.8 (d, 4JC–P = 3 Hz, p-C6H2), 145.2 (d, 2JC–P = 11 Hz, 141.9 (dm, JC–F = 253 Hz), 136.9 (dm, JC–F = 253 Hz), 113.2
1
1
2
1
o-C6H2), 144.2 (d, JC–P = 10 Hz, o-C6H2), 141.8 (dm, JC–F
250 Hz), 136.9 (dm, JC–F = 252 Hz), 133.9 (d, JC–P = 12 Hz, 41.96; H, 2.01; N, 2.33. Found: C, 41.72; H, 1.99; N, 2.36.
m-C6H2), 133.3 (d, JC–P = 12 Hz, m-C6H2), 131.2 (d, JC–P
=
(m, i-C6F5), 53.3 (s, N(CH3)3). Anal. Calc. for C21H12BAlF15N: C,
1
3
3
1
=
=
Synthesis of Me2NHBH3·Al(C6F5)3 (4). This compound was
synthesized in an analogous fashion to 3 using Al(C6F5)3
2
3
135 Hz, CO2), 117.6 (d, JC–P = 78 Hz, i-C6H2), 24.7 (d, JC–P
3
4 Hz, o-CH3Mes), 24.2 (d, JC–P = 5 Hz, o-CH3Mes), 21.3 (500 mg, 0.8 mmol), Me2NHBH3 (47 mg, 0.8 mmol), and
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toluene (5 mL). Following filtration of the solid, the filtrate was degassed, filled with CO2 (1 atm), and sealed. The solution was
stored at −38 °C to obtain a second crop (410 mg (total), stirred for 30 min after which time the CO2 atmosphere was
0.70 mmol, 87%).
removed and pentane (ca. 20 mL) was added. After stirring
1H NMR (400 MHz, C6D5Br): δ 3.73 (bs, 1H, NH), 2.15 (over- rapidly for several minutes, a product precipitated and was fil-
lapping d (3JH–H = 5.6 Hz, N(CH3)2) and bs (BH3), 9H total). 11
B
tered, washed with pentane, and dried (120 mg, 0.19 mmol,
NMR (128 MHz, C6D5Br): δ −16.5 (bs). 27Al NMR (104 MHz, 72%). Slow cooling a toluene–pentane solution to −38 °C
C6D5Br): blank. 19F{1H} NMR (376 MHz, C6D5Br): δ −123.2 (dd, yielded single crystals suitable for X-ray crystallography.
4
3
3JF–F = 26 Hz, JF–F = 8 Hz, 6F, o-C6F5), −151.5 (t, JF–F = 21 Hz,
1H NMR (400 MHz, C7D8): δ 7.55 (s, 1H, H-CO2), 2.17 (bs,
3F, p-C6F5), −160.0 (m, 6F, m-C6F5). 13C{1H} NMR (100 MHz, 2H, BH2), 12H), 1.33 (s, 9H, 3 × CH3). 11B NMR (128 MHz,
C6D5Br): δ 149.7 (dm, JC–F = 234 Hz), 141.7 (dm, JC–F = 252 C7D8): δ 3.2 (bs). 27Al NMR (104 MHz, C7D8): 121 (bs, ν1/2 = ca.
1
1
Hz), 136.9 (dm, JC–F = 255 Hz), 113.5 (m, i-C6F5), 43.8 (s, 2000 Hz). 19F{1H} NMR (376 MHz, C7D8): δ −124.1 (dd, JF–F
=
1
3
4
3
N(CH3)2). Anal. Calc. for C20H10BAlF15N: C, 40.92; H, 1.72; 26 Hz, JF–F = 11 Hz, 6F, o-C6F5), −154.0 (t, JF–F = 21 Hz, 3F,
N, 2.39. Found: C, 40.58; H, 1.70; N, 2.39.
p-C6F5), −162.7 (m, 6F, m-C6F5). 13C{1H} NMR (100 MHz,
Synthesis of NH3BH3·Al(C6F5)3 (5). Al(C6F5)3 (300 mg, C7D8): δ 173.3 (s, HCO2), 150.5 (dm, 1JC–F = 235 Hz), 142.0 (dm,
0.48 mmol) and H3NBH3 (15 mg, 0.48 mmol) were combined 1JC–F = 251 Hz), 137.3 (dm, JC–F = 253 Hz), 48.1 (s, 3 × CH3).
1
in fluorobenzene (10 mL) in a vial. After stirring for 20 min, Anal. Calc. for C22H12BAlF15NO2: C, 40.96; H, 1.87; N, 2.16.
the mixture was filtered through Celite. The solvent volume Found: C, 40.37; H, 1.94; N, 2.17.
was reduced to ca. 2–3 mL and pentane (ca. 10 mL) was added
Synthesis of [tBu3PH][HCO2(Al(C6F5)3)2] (10). A Schlenk
to precipitate a product. The product was filtered on a glass flask was charged with [tBu3PH][H(Al(C6F5)3)2] (400 mg,
frit, washed with pentane and dried (150 mg, 0.27 mmol, 0.32 mmol) dissolved in fluorobenzene (ca. 5 mL). The bomb
55%). Slow cooling a fluorobenzene/pentane solution to was transferred to the Schlenk line equipped with a CO2
−38 °C yielded single crystals suitable for X-ray outlet, degassed and filled with CO2 (1 atm). The solution was
crystallography.
stirred for 12 h after which the solvent was removed in vacuo.
1H NMR (400 MHz, C6D5Br): δ 2.99 (bs, 3H, NH3), 2.10 (bs, Hexanes (ca. 10 mL) were added to the residue and the precipi-
3H, BH3). 11B NMR (128 MHz, C6D5Br): δ −24.0 (bs). 27Al NMR tate that forms was stirred in hexanes for 30 min before being
(104 MHz, C6D5Br): blank. 19F{1H} NMR (376 MHz, C6D5Br): δ filtered on a glass frit (260 mg, 0.20 mmol, 63%). Vapour
3
4
−123.1 (dd, JF–F = 28 Hz, JF–F = 19 Hz, 6F, o-C6F5), −151.6 (t, diffusion of a bromobenzene solution of the compound with
3JF–F = 19 Hz, 3F, p-C6F5), −159.9 (m, 6F, m-C6F5). 13C{1H} NMR hexanes yielded single crystals suitable for X-ray
1
(100 MHz, C6D5Br): δ 149.7 (dm, JC–F = 235 Hz), 141.7 (dm, crystallography.
1JC–F = 252 Hz), 136.9 (dm, JC–F = 255 Hz), 113.5 (m, i-C6F5).
1H NMR (400 MHz, C6D5Br): δ 8.65 (s, 1H, HCO2–), 4.12 (d,
1
Anal. Calc. for C18H6AlBF15N: C, 38.67; H, 1.08; N, 2.51. 1JH–P = 426 Hz, 1H, P-H), 0.96 (d, JH–P = 16 Hz, 27H, tBu).
3
Found: C, 38.30; H, 0.95; N, 3.26.
31P{1H} NMR (161 MHz, C6D5Br): δ 60.0. 27Al NMR (104 MHz,
Synthesis of (Me3N)·H2B(C6F5) (6). In a vial in the glovebox C6D5Br): blank. 19F{1H} NMR (376 MHz, C6D5Br): δ −122.4 (dd,
were combined B(C6F5)3 (200 mg, 0.39 mmol) and Me3NBH3 3JF–F = 26 Hz, JF–F = 11 Hz, 12F, o-C6F5), −153.6 (t, JF–F
=
4
3
(57 mg, 0.78 mmol) in a 1 : 2 solution of toluene (5 mL) and 21 Hz, 6F, p-C6F5), −161.6 (m, 12F, m-C6F5). 13C{1H} NMR
hexanes (10 mL). Precipitation initially occurs but the mixture (100 MHz, C6D5Br), partial: δ 172.8 (s, HCO2–), 150.1 (dm, 1JC–F
1
1
becomes a solution after stirring overnight. The next morning, = 235 Hz), 141.4 (dm, JC–F = 250 Hz), 136.7 (dm, JC–F
=
1
the solution was put in the −38 °C freezer. The crystals that 252 Hz), 114.5 (m, i-C6F5), 36.9 (d, JC–P = 26.4 Hz, PCMe3),
formed were filtered on a glass frit and washed with minimal 29.3 (s, PCMe3). Anal. Calc. for C49H29Al2F30O2P: C, 45.11; H,
cold hexanes and dried in vacuo (100 mg, 0.42 mmol, 54%). 2.24. Found: C, 44.58; H, 2.40.
Slow cooling a concentrated toluene–hexanes solution of the
compound to −38 °C yielded single crystals suitable for X-ray
crystallography.
Acknowledgements
1H NMR (400 MHz, C6D5Br): δ 2.49 (bq, 2H, (C6F5)BH2),
2.07 (s, 9H, N(CH3)3). 11B{1H} NMR (128 MHz, C6D5Br): −9.9 (s). D.W.S. gratefully acknowledges the financial support of the
19F{1H} NMR (376 MHz, C6D5Br): δ −129.0 (dd, JF–F = 26 Hz, NSERC of Canada and the award of a Canada Research Chair.
3
4JF–F = 11 Hz, 2F, o-C6F5), −157.4 (t, JF–F = 21 Hz, 1F, G.M. is grateful for the support of NSERC and Walter
3
p-C6F5), −163.6 (m, 2F, m-C6F5). 13C{1H} NMR (100 MHz, C. Sumner Fellowships.
1
1
C6D5Br): δ 148.8 (dm, JC–F = 236 Hz), 139.9 (dm, JC–F = 249
Hz), 137.0 (dm, JC–F = 249 Hz), 117.4 (bs, i-C6F5), 51.7 (s,
1
N(CH3)3). Anal. Calc. for C9H11BF5N: C, 45.23; H, 4.64; N, 5.86.
Found: C, 45.27; H, 4.92; N, 5.99.
Notes and references
Synthesis of Al(C6F5)3(HCO2)H2BNMe3 (8). Me3NBH3·Al-
(C6F5)3 (155 mg, 0.26 mmol) was dissolved in toluene (5 mL)
in a 50 mL Schlenk flask. The flask was transferred to the
Schlenk line equipped with a CO2 outlet. The bomb was
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