Reaction of a P/Al-based frustrated lewis pair with ammonia, borane, and amine-boranes: Adduct formation and catalytic dehydrogenation

Article abstract of DOI:10.1002/anie.201208746

Open wide! The geminal P/Al-based frustrated Lewis pair (Mes ₂P)(tBu₂Al)Ci￡C(H)Ph forms stable Lewis adducts with BH₃ and NH₃. This compound facilitates the dehydrocoupling of the ammonia-borane adduct by unusual N-H bond activation and elimination of dihydrogen at the boron center, and it is a very active main-group-based FLP catalyst for the dehydrogenation of amine-borane H ₃B×NMe₂H. Mes=mesityl. Copyright

Full text of DOI:10.1002/anie.201208746

Angewandte
Chemie
DOI: 10.1002/anie.201208746
Frustrated Lewis Pairs
Reaction of a P/Al-Based Frustrated Lewis Pair with Ammonia,
Borane, and Amine–Boranes: Adduct Formation and Catalytic
Dehydrogenation**
Christian Appelt, J. Chris Slootweg,* Koop Lammertsma, and Werner Uhl*
The discovery of single-bond activation at transition-metal
centers has led to a plethora of catalytic transformations,
which highlights the power of organometallic chemistry.
Recently, main-group systems,[1] such as B/P-based frustrated
Lewis pairs (FLPs)[2] and singlet carbenes,[3] possessing a lone
pair of electrons and a vacant orbital were also shown to be
able to split chemical bonds. Yet, despite this major advance-
ment in main-group chemistry, the transfer of an activated
fragment to a substrate remains challenging. The difficulty
lies in regenerating the active species, which is a necessity for
metal-catalyzed dehydrogenation.[13] Remarkably, just two
stoichiometric main-group-based FLP strategies have been
reported to date.[14,15] As FLP 1 is unreactive towards
dihydrogen,[11] we speculated that it might facilitate the
À
dehydrogenation of amine–boranes bearing hydridic B H
[16,17]
À
and protic N H bonds.
Treatment of 1 with a 1.0m solution of BH3·THF (1 equiv)
in toluene at room temperature afforded, after work-up and
crystallization, BH3 adduct 2B (80% yield, d(31P) = 3.0,
d(11B{1H}) = À24.5 ppm; Scheme 1). The borane moiety of
2B displays dynamic behavior, as illustrated by the broad BH3
resonance in the 1H NMR spectrum at ambient temperature.
At 330 K, the signal becomes a doublet (d(1H{11B}) =
2.26 ppm, 2J(H,P) = 7.2 Hz), indicating a direct interaction
between the phosphorus and boron centers. Interestingly, the
molecular structure of 2B, obtained by a single-crystal X-ray
structure determination (Figure 1, left),[18] displays a BH3
catalytic turnover. So far, FLP catalysis is only known with
[4]
covalent H H and Si H[5,6] bonds and sodium hydride.[7] We
À
À
were keen to extend this series.[8]
[9]
[10]
À
À
Because the heterolytic B H and N H bond activa-
tion of boranes and amines by FLPs has received little
attention, we investigated the formation of the activated
species 2,3A versus the adducts 2,3B (Scheme 1). For this, we
reacted the readily accessible geminal phosphorus/aluminum-
based FLP 1[11] with BH3·THF, ammonia (NH3), and amine–
boranes H3B·NR2H[12] (R = H and Me). The potential of the
B/N adduct as hydrogen carrier was revealed in the transition-
Figure 1. Molecular structures of 2B and 3B. Ellipsoids are set at 30%
probability; hydrogen atoms, except at B1 (2B) and N1 (3B), are
omitted for clarity. Selected average bond lengths [pm] for 2B: P1–C1
180.7(2), C1–Al1 204.3(2), Al1–H11 192(2), H11–B1 124(2), B1–P1
196.7(2), B1–H12 110(2). 3B: P1–C1 182.21(12), C1–Al1 203.68(13),
Al1–N1 201.75(13).
Scheme 1. Reaction of FLP 1 with BH3 and NH3.
À
À
[*] Dr. C. Appelt, Prof. Dr. W. Uhl
fragment (B1 P1 196.7(2) pm) with an elongated B H bond
that participates in a B-H-Al three-center two-electron
Institut fꢀr Anorganische und Analytische Chemie
der Westfꢁlischen Wilhelms-Universitꢁt Mꢀnster
Corrensstrasse 30, 48149 Mꢀnster (Germany)
E-mail: uhlw@uni-muenster.de
bond[19] (B1 H11 124(2), Al1 H11 192(2) pm). This inter-
action and the concomitantly increased coordination number
at aluminum are evident from a) the typical lengthening of
À
À
Dr. J. C. Slootweg, Prof. Dr. K. Lammertsma
Department of Chemistry and Pharmaceutical Sciences
VU University Amsterdam
De Boelelaan 1083, 1081 HV Amsterdam (The Netherlands)
E-mail: j.c.slootweg@vu.nl
À
the Al C bonds from 199.1 in 1 to 202.4 pm in 2B (202.6 pm in
the NH3 adduct 3B, see below); and b) the larger elongation
of the aluminum atom from the plane of its bonded carbon
atoms than in FLP 1 (28.2 pm versus 12.5 pm; 35.0 pm in 3B),
causing the apex of the pyramid to point towards the bridging
hydrogen atom. In contrast, the coordinatively unsaturated
[**] This work was supported by the Deutsche Forschungsgemeinschaft.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201208746.
À
aluminum atom of 1 has only a weak interaction with a C H
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bond of its vinylic phenyl group. The relatively short B···Al
distance of 253.0(2) pm of 2B is in the upper range of related
compounds containing Al-H-B bridges.[19] To provide insight
into this mode of action, we resorted to M06-2X/6-31G(d)
calculations.[20] In accord with the molecular structure, adduct
2B (DG = À16.0 kcalmolÀ1) features a BH···Al hydrogen
bond and does not exist without this additional interaction.
Furthermore, 2A is not a stable minimum on the potential
energy surface, indicating that FLP 1 is not prone to undergo
À
B H bond splitting. The rotational barrier in solution ([D14]n-
Scheme 2. Dehydrocoupling of amine–boranes H3B·NH3 and
H3B·NMe2H with FLP 1.
hexane) seems to be low, and we did not observe a splitting of
1
11
À
the B H resonance in the H{ B} NMR spectrum even at
À908C.
Next, NH3 was bubbled through a solution of 1 in toluene
at room temperature for 15 seconds to afford, after crystal-
lization, the adduct 3B (70% yield; d(31P) = À6.6 ppm;
Scheme 1). A crystal structure determination established
unequivocally the formation of a very rare ammonia–organo-
À
aluminum adduct (Al1 N1 201.75(13) pm; Figure 1,
right).[18,21] Lewis adduct 3B features no NH···P interactions
À
(also computationally) and is favored over N H bond
activation generating 3A (DG = 19.9 kcalmolÀ1).[20] To
increase the potential for NH3 splitting, the Lewis basicity
of the donor site of FLP 1 can easily be enhanced,[10,11] which
is currently under investigation.
Figure 2. Molecular structures of 4 and 5. Ellipsoids are set at 30%
probability; hydrogen atoms, except at B1 and N1, and n-hexane (5)
solvent molecules are omitted for clarity. Selected average bond
lengths [pm] for 4: P1–C1 181.17(11), C1–Al1 204.74(12), Al1–N1
197.95(11), N1–B1 157.01(17), B1–P1 199.84(14). 5: P1–C1
182.51(11), C1–Al1 208.23(11), Al1–N1 202.16(11), N1–C81
149.06(16), N1-C-82 148.85(17), N1–B1 158.83(17), B1–P1 198.81(14).
Interestingly, addition of BH3·THF (1 equiv) to NH3
adduct 3B at room temperature resulted in the elimination
of ammonia and the formation of BH3 adduct 2B (65% yield),
together with small amounts of amine–borane H3B·NH3. The
inverse reaction of BH3 adduct 2B with ammonia yielded
a complex mixture of which only ammonia adduct 3B and
H3B·NH3 could be identified as minor components. Treat-
ment of FLP 1 with H3B·NH3 (1 equiv) in toluene at room
temperature did not afford the stable double P/B, Al/N
adduct (DG = À14.5 kcalmolÀ1).[20] Instead, this reaction
resulted in evolution of dihydrogen to afford, after work-up
and crystallization, adduct 4 (80% yield,
uct (DG° = 20.6; DG = À7.7 kcalmolÀ1; Figure 3);[23] com-
pound 6 bears short, intramolecular P Hd+···dÀH B contacts
À
À
[24]
À
(for example, H1 H2 197.5 pm) and is much more stable
À
than the corresponding B H activated species (DDG =
16.2 kcalmolÀ1). Next, protonation (by H1) of the hydridic
[25]
yields the loosely bound h2-H2 borane
À
H2 B1 bond
d31P = 15.2,
d(11B{1H}) = À12.0 ppm;
Scheme 2), which shows in the 1H NMR
spectrum both the BH and NH protons
(d(1H) = 3.17 and 1.31 ppm, respectively).
Single-crystal X-ray diffraction analysis
revealed that 4 is the P/Al adduct of the
=
dehydrocoupling
product
H2B NH2
unique
(Figure 2, left),[18,22] showing
a
five-membered heterocycle with a slightly
distorted envelope conformation and a syn-
À
clinal arrangement of the B/N H bonds
(HBNH torsion angles of 31 and 458).
To gain insight into the underlying
mechanism of the FLP-mediated dehydro-
genation of H3B·NH3, we conducted M06-
2X/6-31G(d) calculations[20] on the full
system. In contrast to the transition-
Figure 3. Relative M06-2X/6-31G(d) Gibbs free energies (in kcalmolÀ1) for the dehydrogen-
ation of H3B·NH3 by FLP 1 and formation of aminoborane adduct 4. Hydrogen atoms, except
at B1 and N1, are omitted for clarity. Selected average bond lengths [pm] for TS1–6: P1–H1
167.3, H1–N1 133.7, B1–N1 170.0, N1–Al1 233.3. 6: P1–H1 139.2, H2–B1 123.0, H3–B1
122.1, H1–H2 197.5, H1–H3 216.2, B1–N1 163.3, N1–Al1 197.9. TS6–7: P1–H1 218.7, H1–B1
157.6, H2–B1 143.4, H1–H2 82.4, B1–N1 154.9. 7: H1–B1 280.2, H2–B1 296.4, H1–H2 73.9,
metal-catalyzed
dehydrogenation
of
À
amine–boranes that proceed by B H
bond activation,[13] FLP 1 promotes the
À
heterolytic N H bond splitting to afford
phosphonium aluminate 6 as initial prod- B1–N1 144.6. 8: B1–N1 144.4.
2
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Chemie
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readily undergoes elimination of dihydrogen to afford amino–
borane adduct
8
(DDG° = 0.2; DDG = À7.0 kcalmolÀ1).
À
Finally, ring closure by P B bond formation generates the
experimentally ascertained heterocycle
4
(DDG° = 4.6;
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DDG = À22.1 kcalmolÀ1).
Whereas 4 is stable in solution and does not undergo
further loss of dihydrogen in the presence of excess 1, the
reaction of FLP 1 with H3B·NMe2H (1 equiv) affords the
31
=
thermally unstable H2B NMe2 adduct 5 (59% yield, d( P) =
12.5, d(11B{1H}) = À5.4 ppm; Scheme 2), which only could be
obtained when the reaction mixture was kept below À308C.[27]
Single-crystal X-ray analysis of crystals grown at À458C
revealed that the molecular structure of 5 is similar to that of 4
À
except for the elongated Al1 N1 bond (5: 202.16(11), 4:
197.95(11) pm; Figure 2).[18] This elongated bond facilitates
exergonic fragmentation at room temperature into dimeric
[28]
À
=
[(H2B NMe2)2] (via transient H2B NMe2)
and 1 (DG =
À20.3 kcalmolÀ1).[20] Note that in accord with these findings,
À
the formation of cyclodiborazane [(H2B NH2)2] from 4 is
endergonic (DG = 15.0 kcalmolÀ1).[20,29] Interestingly, the
complete regeneration of FLP 1 from 5 suggests the feasibility
of a catalytic dehydrocoupling. Indeed, heating neat dime-
thylamine–borane with 9.3 mol% of 1 in the melt at 458C
resulted in vigorous gas evolution and the immediate
À
sublimation of [(H2B NMe2)2] onto
a
cold finger
(Scheme 2). Careful heating to 908C completes the reaction,
À
affording cyclodiborazane [(H2B NMe2)2] in 71% yield of
isolated product after just 45 min (TON = 7.6, TOF = 10.2 hÀ1
based on isolated material). Lower catalyst loadings are also
feasible. For example, treatment of H3B·NMe2H with only
0.4 mol% of 1 gave the four-membered cyclodiborazane in
77% yield of isolated product after 44 h (TON = 198.3,
TOF = 4.5 hÀ1).[30] Heating H3B·NMe2H without FLP catalyst
1 at 908C (24 h) gave only traces (5%) of [(H2B-NMe2)2],
À
À
À
together with the linear dimer Me2NH BH2 NMe2 BH3
(10%)[28] and unreacted amine–borane (85%), which under-
scores the efficiency of FLP 1 in this catalytic dehydrogen-
ation reaction.
In summary, FLP 1 readily forms adducts with ammonia
À
and the borane BH3, but reacts via N H bond activation with
amine–boranes. We demonstrated a new application of main-
group-based FLPs, namely the catalytic dehydrocoupling of
H3B·NMe2H, which proceeds by elimination of dihydrogen at
the boron center and subsequent dimerization of the inter-
=
mediate aminoborane H2B NMe2.
Received: October 31, 2012
Revised: February 7, 2013
Published online: && &&, &&&&
Keywords: amine–boranes · dehydrogenation ·
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Robertson, H. Helten, R. J. Cox, G. C. Lloyd-Jones, I. Manners,
J. Am. Chem. Soc. 2012, 134, 16805 – 16816.
À
[29] The formation of the experimentally observed trimer [(H2B
NH2)3], see Ref. [28], is also endergonic (DG = 11.8 kcalmolÀ1).
[30] After analysis of the residual material that did not sublime, the
total degree of dehydrogenation was estimated at 98%
(9.3 mol% of 1) and 89% (0.4 mol%), respectively.
4
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ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
These are not the final page numbers!

Angewandte
Chemie
Communications
Frustrated Lewis Pairs
Open wide! The geminal P/Al-based
=
frustrated Lewis pair (Mes2P)(tBu2Al)C
C. Appelt, J. C. Slootweg,*
K. Lammertsma, W. Uhl*
C(H)Ph forms stable Lewis adducts with
BH3 and NH3. This compound facilitates
the dehydrocoupling of the ammonia–
&&&& — &&&&
À
Reaction of a P/Al-Based Frustrated Lewis
Pair with Ammonia, Borane, and Amine–
Boranes: Adduct Formation and Catalytic
Dehydrogenation
borane adduct by unusual N H bond
activation and elimination of dihydrogen
at the boron center, and it is a very active
main-group-based FLP catalyst for the
dehydrogenation of amine–borane
H3B·NMe2H.
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!