Synthesis of 1-aza-2-borabutatriene rhodium complexes by thermal borylene transfer from [(OC)5Mo=BN(SiMe3)2]

Article abstract of DOI:10.1002/anie.201104026

Borylene on me: An unprecedented borylene transfer to metal-carbon double bonds afforded the title compounds. Preliminary studies revealed a thermally induced change of coordination mode from B-C to C-C and subsequent highly stereoselective C-H activation by the B=C bond. [(η⁵-C ₅H₅)Rh(PCy₃){(B,C-η²)-(SiMe ₃)₂N=B=C=CH₂)}] and its rearrangement product were both characterized by X-ray crystallography (see picture).

Full text of DOI:10.1002/anie.201104026

Communications
DOI: 10.1002/anie.201104026
Boracumulene Complexes
Synthesis of 1-Aza-2-borabutatriene Rhodium Complexes by Thermal
=
Borylene Transfer from [(OC)5Mo BN(SiMe3)2]**
Holger Braunschweig,* Qing Ye, Alexander Damme, Thomas Kupfer, Krzysztof Radacki, and
Justin Wolf
Since the first reports of allenes more than a century ago[1]
compounds with cumulated double bonds have attracted
interest owing to their highly unsaturated structure.
[3]-Cumulene, or butatriene (I), consists of three cumulated
carbon–carbon double bonds that can potentially ligate to a
synthesize 1-boraallene (VI) by the reaction of an alkynyl-
fluoroborane with tert-butyllithium at À1208C afforded the
corresponding boraallene only as an intermediate, which
=
promptly underwent a hydroalkylation reaction at the B C
bond.[8]
transition-metal center. However, they coordinate mostly in
B-amino-1-boraallenes (II) are isoelectronic to butatriene
(I) and might represent boron-containing [3]-cumulene
systems when the boron–nitrogen p interaction is taken into
account. Currently, very little is known about this class of
compounds, and no successful synthetic approach has been
reported. Ab initio calculations on the parent compound II
and its constitutional isomer aminoborirene III have been
carried out[9] suggesting that II is 12.9 kcalmolÀ1 higher in
energy than III, which is surprising given the borireneꢀs 2p-
electron aromatic stabilization.[10] Ever since a facile synthetic
route to Group 6 terminal borylene complexes was devel-
oped,[11] they have proven excellent sources for the borylene
fragment.[12] This borylene transfer method has developed
quickly in the past decade and has been applied to the
synthesis of borirenes,[13] new transition-metal borylene com-
2
an h -p fashion through the central C C bond.[2] As reported
=
by the groups of Hughes and Lentz, the highly reactive
molecule tetrafluorobutatriene (which decomposes slowly
even at À808C) can be trapped and stabilized by transition-
metal coordination (VII).[3] In 2002, Suzuki et al. reported the
coordination of a butatriene to low-valent zirconocene in a
novel k2-s,s bonding mode, the first example of a five-
membered metallacycloalkyne.[4]
plexes,[12,14] metathesis reactions,[15] and the insertion of the
[16]
À
À
DB R fragment into olefinic C H bonds.
One particularly intriguing and unexplored area of
transition-metal borylene chemistry is their reactivity with
(and perhaps transfer to) metal–carbon multiple bonds. Given
this paucity and our motivation towards the synthesis of
B-amino-1-boraallenes (II), we decided to investigate the
behavior of terminal Group 6 metal borylenes with vinyl-
idenerhodium complexes. Herein, we report our preliminary
results of the borylene transfer to vinylidenerhodium com-
plexes which leads to 1-aza-2-borabutatriene rhodium com-
plexes in good yields.
As boron-based p systems have attracted much attention
owing to their interesting photophysical properties,[5] we
turned our attention to boron-containing cumulene systems.
Amino(methylene)boranes (V), isoelectronic to allenes (IV),
=
can be isolated when the kinetically unstable B C bond is
sterically protected by bulky substituents. Structural charac-
terization revealed that such amino(methylene)boranes
When the vinylidenerhodium complex 2 was added to an
= =
= =
adopt an allene-like structure with a linear N B C skele-
equimolar amount of [(OC)5Mo B N(SiMe3)2] (1) in ben-
ton.[6] Furthermore, there are a few examples of boron-
containing ionic allene analogues such as the 1,3-borataallene
dianion and the 2-borataallene anion.[7] The attempt to
zene and slightly warmed to 408C, NMR spectroscopy
revealed a gradual consumption of the starting materials
and the formation of a mixture of the title compound 6 and
the rhodium monocarbonyl complex [(h5-C5H5)Rh(CO)-
(PiPr3)] (4)[17] in a ratio of approximately 2:1 (1H NMR
spectroscopy). The 11B{1H} and 31P{1H} NMR spectra of 6
[*] Prof. Dr. H. Braunschweig, Q. Ye, A. Damme, Dr. T. Kupfer,
Dr. K. Radacki, Dr. J. Wolf
feature signals at dB = 67.9 ppm and at dP = 66.4 ppm (1JRh-P
=
Institut fꢀr Anorganische Chemie
Julius-Maximilians-Universitꢁt Wꢀrzburg
Am Hubland, 97074 Wꢀrzburg (Germany)
E-mail: h.braunschweig@mail.uni-wuerzburg.de
Homepage: http://www-anorganik.chemie.uni-wuerzburg.de/
braunschweig/index.html
202.5 Hz), respectively, which are both shifted upfield relative
to the signals for the starting materials 1 (dB = 89.1 ppm)[18]
and 2 (dP = 73.5 ppm, 1JRh-P = 209.0 Hz). In the 1H NMR
spectrum of 6, a new set of signals is present in the expected
ratio for one C5H5 ligand and two Me3Si groups, thus
confirming its constitution in solution. Most notably, the
observation of two broad signals for trimethylsilyl groups at
d = 0.30 and 0.62 ppm in a 1:1 ratio and two signals for olefinic
[**] Financial support by the European Research Council (Advanced
Investigator Grant to H.B.) is gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201104026.
9462
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 9462 –9466

protons at d = 7.31 (dd, 3JRh-H = 4.1 Hz, 4JP-H = 1.1 Hz) and
3
4
6.48 ppm (dd, JRh-H = 2.9 Hz, JP-H = 2.2 Hz) suggests both a
considerable rotational barrier of the nitrogen boron double
bond and the asymmetry of the product, which is in good
accordance with the proposed structure. This coupling pattern
of olefinic protons is confirmed by comparison with a 1H{31P}
NMR spectrum, which features doublet instead of doublet-of-
À
doublet signals and thus geminal H H coupling is excluded.
The assignment of the resonances for endo- and exo-H or
endo- and exo-SiMe3 is based on the NOESY correlation with
protons of the PiPr3 ligand. Unfortunately, as a result of its
oily consistency, single crystals of 6 suitable for X-ray
diffraction could not be obtained.
To probe the versatility of the synthetic method and to
structurally characterize the target compound, we synthesized
the sterically more demanding PCy3-substituted rhodium
Figure 1. Molecular structure of 7. Except for the two olefinic protons,
hydrogen atoms have been omitted for clarity. Ellipsoids set at 50%
probability. Selected bond lengths [ꢂ] and angles [8]: N1–B1 1.400(10),
B1–C1 1.489(12), C1–C2 1.331(10), Rh1–B1 2.027(8), Rh1–C1 2.056(7),
N1–Si1 1.769(6), N1–Si2 1.765(6), Rh1–P1 2.2799(17); N1-B1-C1
142.1(7), B1-C1-C2 152.9(7).
= =
vinylidene 3. The reaction of [(OC)5Mo B N(SiMe3)2] (1)
with 3 was carried out under analogous conditions to those
applied for the synthesis of compound 6 and was monitored
by NMR spectroscopy. The formation of 7 was indicated by a
new resonance signal at d = 68.7 ppm in the 11B{1H} NMR
spectrum and at d = 56.6 ppm (1JRh-P = 201.9 Hz) in the
1
31P{1H} NMR spectrum. Resembling that of 6, the H NMR
bond in comparison to the free tetrafluorobutatriene was
observed for VII.[3] In the case of 7, coordination of the 1-aza-
2-bora-butatriene is accompanied by significant bending. The
N1-B1-C1 angle of 142.1(7)8 is comparable to the values of
corresponding C-C-C angles (137.5–145.58) in VII,[3] while the
B1-C1-C2 angle of 152.9(7)8 is widened by more than 5%
compared to those values, thus suggesting some boron–
carbon double bond character. The B1–N1 (1.400(10) ꢁ) and
C1–C2 (1.331(10) ꢁ) distances are both slightly elongated in
comparison to those of amino(methylene)borane (e.g.
1.363(4) ꢁ)[6b] and free triene (1.3162(3) ꢁ)[3]), which can be
spectrum is characterized by two signals for trimethylsilyl
groups at d = 0.33 and 0.64 ppm and two signals for olefinic
protons at d = 7.37 (d, 3JRh-H = 3.5 Hz) and 6.55 ppm (d,
3JRh-H = 2.9 Hz). Notably, and in contrast to the common
behavior of aminoboranes,[19] the amino group in the title
=
compound shows no rotation about the B N bond up to 808C,
as judged by variable-temperature NMR experiments in
C6D6, thus suggesting a significant double bond character.
Complex 5, analogous to [(h5-C5H5)Rh(CO)(PiPr3)] (4), was
detected as a byproduct in a ratio of 1:2 relative to 7, as
indicated by the resonance signal at dP = 68.7 ppm (1JRh-P
=
explained by a decrease of the s character in the B C s-bond
À
189.6 Hz) and dH = 5.32 ppm (s, C5H5) in the 31P{1H} NMR
and 1H NMR spectra, respectively. Moreover, the reaction is
accompanied by the concomitant formation of [Mo(CO)6] as
indicated by a resonance at d = 201.49 ppm in the 13C NMR
spectrum, which is in accordance with the reported results on
borylene transfer reactions of Group 6 carbonyl species.[14d]
Both of the title compounds (6 and 7) possess considerable
stability. No sign of decomposition in solution at ambient
temperature was observed. The complexes were also stable
towards chromatography at room temperature without sig-
nificant loss of material. Complex 7 was isolated in the form of
pale yellow crystals by crystallization from hexane at À308C.
The result of X-ray diffraction analysis of 7 is displayed in
Figure 1.
orbitals upon bending of the [3]-boracumulene. Comparison
of the structural characteristics of 7 with those of free
amino(methylene)boranes (V), butatrienes (I), and buta-
triene complexes (VII) suggests an overall bonding situation
to which both mesomeric forms A and B contribute
(Scheme 1).
To provide further insight into the electronic structure of
7, DFT calculations were performed. We studied two model
species by geometry optimization at the B3LYP level of
=
theory – one with side-on coordination of the B C bond
derived from the crystal structure of 7 (BC), and a hypo-
=
thetical isomer with side-on coordination of the C C bond
(CC; Figure 2). The calculations indicated a distinct prefer-
=
ence for the observed B C coordination mode, that is, BC is
Complex 7 crystallizes in the monoclinic space group
more stable than CC by 65.06 kJmolÀ1 (Figure 2).
À
P21/c. The coordinated B C bond (1.489(12) ꢁ) is ca. 6%
The relevant Wiberg bond indices (WBI, Table 1) for BC
=
=
longer than the B C bonds in non-coordinated amino-
and CC are in accordance with a preference for the B C over
(methylene)boranes V (e.g. 1.391(4)[6b] and 1.424(3) ꢁ[6c]
)
the C C coordination mode. Thus, the WBIs in BC for the
=
À
and approximately 3% shorter than the B C single bond
found between two-coordinate boron and four-coordinate
carbon atoms (1.531(11) ꢁ).[20] This finding indicates consid-
erable back-bonding from rhodium to an antibonding
p* orbital of the ligand; a bonding situation, which was also
found for the butatriene complex VII. In particular, a similar
Rh1–B1 and Rh1–C1 linkages are slightly higher than the
corresponding WBIs in CC, that is, Rh1–C1 and Rh1–C2.
The major MO contribution to the unprecedented side-on
coordination of a boron–carbon double bond is represented
by the HOMO+2 in the case of BC, which comprises an
À
interaction of the pz-orbital of the B C bond with the dz2 and
À
increase by around 10% for the coordinated C C double
dx2Ày2 orbitals of the rhodium center (Figure 3).
Angew. Chem. Int. Ed. 2011, 50, 9462 –9466
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Communications
After demonstrating the facile synthetic route to the novel
1-aza-2-bora-butatriene-rhodium complexes, we addressed
the question, whether a ligand-exchange process would occur
in the presence of CO to afford the free monocarbonyl
complexes and the free B-amino 1-boraallene species. Owing
to the poor steric protection, the B-amino 1-boraallene
species is expected to be labile and might undergo secondary
reactions. However, in stark contrast to butatriene rhodium
complexes,[2c,e] 1-aza-2-bora-butatriene complex 7 proved to
be stable in the presence of CO at ambient temperature, even
under photolytic conditions. However, upon warming to
858C, NMR spectroscopy revealed a gradual and almost
quantitative transformation of the starting material, as
indicated by 11B{1H} (dB = 42.7 ppm) and 31P{1H} NMR
1
(dP = 74.3 ppm, JRh-P = 187.8 Hz) spectra. Unexpectedly, nei-
ther 31P{1H} NMR nor 13C{1H} NMR spectra suggested the
formation of the corresponding carbonyl complex. Moreover,
the significant upfield shift of the 11B NMR resonance by
À
À
26 ppm implies a change from B C to C C coordination,
which could obviously take place without CO as well. Thus, an
analogous experiment in the absence of CO was carried out,
which confirmed our assumption.
Scheme 1. Synthesis of 1-aza-2-bora-butatriene rhodium complexes 6
and 7 by borylene transfer to 2 and 3, respectively.
Complete conversion of 7 required two weeks at 858C.
After workup, single crystals were obtained from a hexane
solution at ambient temperature. The product 8 (Scheme 2)
Figure 2. Energy difference between hypothetical isomers BC and CC.
Table 1: Calculated relevant WBIs for complexes BC and CC.
Scheme 2. Quantitative formation of 8 (RSRR/SRSS) by a thermally
À
À
induced change of coordination mode from B C to C C and subse-
À
=
quent C H activation by the B C bond.
À
À
À
À
B1 C1
À
C1 C2
Comp.
Rh1 B1
Rh1 C1
Rh1 C2
BC
CC
0.6024
0.0966
0.5648
0.3977
0.0900
0.5010
1.0982
1.7788
1.8788
1.3135
ꢀ
crystallizes in the triclinic space group P1 with two independ-
ent molecules in the asymmetric unit, both featuring very
similar structural parameters. The results of X-ray diffraction
analysis partially confirm our proposed structure. As shown in
2
=
Figure 4, the C C bond coordinates in a h -fashion to the
rhodium center. Despite the presence of the N(SiMe3)2 group
that provides both steric shielding and p-electron donation,
=
the released B C bond is highly reactive and undergoes an
À
addition reaction with the C4 H bond of one cyclohexyl
group of the PCy3ligand, affording the Rh1-C1-B1-C4-C3-P1
À
six-membered ring. The C1 C2 bond (1.417(6) ꢁ) is elon-
gated by approximately 6% as a result of the side-on
coordination, which is comparable to that observed for the
=
À
À
B C bond in 7. The B1 C1 (1.533(7) ꢁ) and B1 C4
(1.615(8) ꢁ) separations fall within the expected range for
the corresponding single bonds.[20] The elongation of the B1
À
À
C4 in comparison to the B1 C1 bond can be attributed to a
lower amount of s-character in C4. Both boron and nitrogen
atoms adopt a trigonal-planar geometry as indicated by the
sum of angles of 359.1 and 359.9, respectively. The Si2-N1-B1-
Figure 3. Graphical representation of HOMO+2 of BC.
9464 www.angewandte.org
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Angew. Chem. Int. Ed. 2011, 50, 9462 –9466

bond. Moreover, the borylene-based functionalization of
vinylidene rhodium complexes allows straightforward access
to novel h2-1-aza-2-borabutatriene rhodium complexes in
satisfactory yields. Preliminary reactivity studies revealed a
À
À
thermally induced B C to C C coordination mode shift with
À
=
subsequent highly stereoselective C H activation by the B C
bond.
Received: June 12, 2011
Published online: August 31, 2011
Keywords: boraallene · boron · borylene complexes · rhodium ·
.
vinylidene complexes
Figure 4. Molecular structure of 8 (RRh,SC1,RC3,RC4). Hydrogen atoms,
the co-crystallized solvent molecules and the second independent
molecule (SRh,RC1,SC3,SC4) in asymmetric unit have been omitted for
clarity. Ellipsoids set at 50% probability. Selected bond lengths [ꢂ] and
angles [8]: C1–C2 1.417(6), C1–B1 1.533(7), B1–C4 1.615(8), C3–C4
1.546(6), P1–Rh1 2.2631(13), B1–N1 1.492(6); C2-C1-B1 131.6(5), C1-
B1-C4 125.1(4), C1-B1-N1 116.6(4), N1-B1-C4 117.4(4), B1-N1-Si1
115.9(3), B1-N1-Si2 123.2(3), Si1-N1-Si2 120.8(2).
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=
a reduced B N
C4 torsion angle of 51.768 suggests
p-contribution, which corresponds to the significant elonga-
À
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7 (1.400(10) ꢁ) and is presumably a result of the pronounced
steric congestion imposed by the bulky N(SiMe3)2, Cp, and Cy
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8 exists as a racemate (as the
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1
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À
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= =
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Angew. Chem. Int. Ed. 2011, 50, 9462 –9466
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www.angewandte.org 9465

Communications
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