Synthesis and structure of a carbene-stabilized boraallene coordinated to rhodium

Article abstract of DOI:10.1021/ic4005505

Reaction of the (B,C-η²)-1-aza-2-borabutatriene rhodium complex 1 with 1,3-dimethylimidazol-2-ylidene) (IMe, 2) afforded the N-heterocyclic carbene-stabilized (C,C-η²)-1-boraallene rhodium complex 3, which has been characterized in solution and by X-ray crystallography. Density functional theory calculations were carried out to elucidate the observed base-induced B-C to C-C coordination mode shift, which suggested that the latter is 25 kJ/mol lower in energy.

Full text of DOI:10.1021/ic4005505

Communication
pubs.acs.org/IC
Synthesis and Structure of a Carbene-Stabilized Boraallene
Coordinated to Rhodium
Holger Braunschweig,* Qing Ye, and Krzysztof Radacki
Institut fur Anorganische Chemie, Julius-Maximilians-Universitat, Am Hubland, 97074 Wurzburg, Germany
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* Supporting Information
Scheme 1. Thermally Induced Coordination Mode Change
from B−C to C−C
ABSTRACT: Reaction of the (B,C-η2)-1-aza-2-borabuta-
triene rhodium complex 1 with 1,3-dimethylimidazol-2-
ylidene) (IMe, 2) afforded the N-heterocyclic carbene-
stabilized (C,C-η2)-1-boraallene rhodium complex 3,
which has been characterized in solution and by X-ray
crystallography. Density functional theory calculations
were carried out to elucidate the observed base-induced
B−C to C−C coordination mode shift, which suggested
that the latter is 25 kJ/mol lower in energy.
oracumulenes have attracted interest not only because of
Btheir highly unsaturated structure but also because of their
potential application in the synthesis of boron-based com-
pounds, which have in recent years turned out to be of great
importance in materials science.1 However, this class of
compounds is strongly limited to a few examples of amino-
(methylene)boranes that adopt an allene-like structure with a
linear CBN skeleton.2 Here, the highly reactive BC
double bond is electronically (π-electron donation from the
amino group) and kinetically stabilized by the bulky substituents.
Moreover, 1-boraallenes R2CCBR′ were only mentioned as
transient intermediates at −120 °C.3 In 2011, the group of
Aldridge published the first iminoborylene complexes, [LxFe
BNCR2], that feature the cumulenylidene ligands, prepared
by halide abstraction from the corresponding neutral iron boryl
complexes.4a
When the NHC 1,3-dimethylimidazol-2-ylidene (IMe, 2) was
added to an equimolar amount of [CpRh(PCy3){(η2-B,C)-
(SiMe3)2NBCCH2)}] (1) in toluene at room temper-
ature (Scheme 2), the color of the reaction mixture turned from
Scheme 2. Synthesis of the Carbene-Stabilized Boraallene
Rhodium Complex 3
More recently, the first [3]boracumulene H2CCB
N(SiMe3)2 was prepared by coupling of the isoelectronic
vinylidene :CCH2 and aminoborylene :BN(SiMe3)2 moieties
in the coordination sphere of rhodium.4b This highly unsaturated
molecule is effectively stabilized by coordination of the central
B−C bond to the transition metal, which is thermodynamically
favored over C−C coordination according to density functional
theory (DFT) calculations. However, a B−C to C−C
coordination mode shift was observed under thermally forcing
conditions, but the released BC double bond underwent
instantly an intramolecular C−H activation, leading to borylation
of the PCy3 ligand (Scheme 1).
Because recent studies proved N-heterocyclic carbenes
(NHCs) and related carbenes to be versatile stabilizing ligands
for low-valent boron species such as free borylenes,5 diborenes,6
and diborynes,6c we were prompted to synthesize a NHC-
stabilized neutral boraallene on the basis of our previous work.
Herein we describe the synthesis of a carbene-stabilized 1-
boraallene coordinated to a rhodium fragment.
light yellow to deep red immediately. The formation of a new
boron- and phosphorus-containing species was indicated by the
presence of a new resonance at δB 18 in the 11B NMR spectrum
and a new signal at δP 55.1 (1JRh−P = 208.7 Hz) in the 31P NMR
spectrum. The remarkable high-field shift of the 11B resonance by
ca. 50 ppm in comparison to the starting material is most likely
due to an increased coordination number of the boron center,
which strongly suggests the formation of a carbene−boron
adduct. After workup, single crystals were obtained upon storage
of a saturated toluene/hexane solution at −30 °C for 2 weeks.
The product 3 crystallizes in the triclinic space group P1, and the
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Received: March 4, 2013
Published: April 30, 2013
© 2013 American Chemical Society
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Inorganic Chemistry
Communication
results of X-ray diffraction analysis partially confirm our
speculation (Figure 1). That is, the carbene−boron bond
Figure 2. Results of DFT calculations on complexes BC and CC. Bond
lengths are shown in blue (Å), Wiberg bond indices in black, and natural
charges in red.
reveals a highly symmetrical η2-coordination of the B−C unit,
as indicated by almost identical Rh−B [2.027(8) Å] and Rh−C
[2.056(7) Å] bond lengths.4
Figure 1. Molecular structure of 3. Hydrogen atoms and ellipsoids of
ligands have been omitted for clarity. Ellipsoids drawn at the 50%
probability level. Selected bond lengths [Å] and angles [deg]: Rh−C2
2.126(2), Rh−C1 2.120(2), C1−C2 1.397(3), C1−B 1.438(3), B−C3
1.585(3), B−N1 1.533(3); C2−C1−B 146.0(2), Rh−C1−C2 71.0 (1),
Rh−C1−B 135.1(2).
In this contribution, we have reported the synthesis and full
characterization of a NHC-stabilized 1-boraallene coordinated to
rhodium by the reaction of (B,C-η2)-1-aza-2-borabutatriene
rhodium complex 1 with 2. The addition of the carbene to the
boron center induces a shift of the coordinated allene from BC
(BC) to a typical η2-CC (CC) mode. Interestingly, the released
BC double bond is in this case effectively stabilized by the
NHC and shows no subsequent intramolecular CH activation, as
observed in previous studies. DFT calculations on model species
CC and BC suggest that C−C coordination is thermodynami-
cally favored over B−C coordination by 25 kJ/mol.
formation is accompanied by a B−C to C−C coordination
1
mode change. Interestingly, while the H NMR signals of the
nitrogen-bound trimethylsilyl groups (δH 0.12) and of the
nitrogen-bound methyl groups (δH 3.63) appear as broad singlets
1
at ambient temperature, the H NMR spectrum shows clearly
two signals for N(SiMe3)2 (δH 0.38 and 0.00) and for NMe (δH
3.98 and 3.02) at −50 °C. This finding implies a certain degree of
rotational barrier about the B−N1 and B−C3 bonds. Never-
theless, the significantly elongated B−N1 bond of 1.533(3) Å
indicates the absence of substantial B−N π interaction. The B−
C3 bond length of 1.585(3) Å is comparable with that of NHC-
coordinated borabenzenes [1.596(2) Å] or 9-boraanthracenes
[1.607(4) Å].7 Hence, it can be anticipated that the rotational
barrier around the B−N1 and B−C3 bonds is due to steric
congestion between the bulky trimethylsilyl groups and the N-
bound methyl groups of the carbene unit. The B−C1 bond
length of 1.438(3) Å is comparable with those of noncoordinated
amino(methylene)boranes [e.g., 1.424(3) Å],2c thus indicating
the presence of a BC double bond. In contrast to the
previously reported thermally induced B−C to C−C coordina-
tion mode shift, the released BC double now displays no
reactivity toward the C−H bond of the coligand PCy3, thus
confirming the stabilizing effect of the carbene.
To gain further information about the observed B−C to C−C
coordination mode shift, DFT calculations on the model species
CC and BC were performed at the B3LYP level of theory (Figure
2). The calculated geometry of the model species CC, which is
derived from the title compound 3 by formal exchange of the
PCy3 ligand by a PMe3 group, is in excellent agreement with the
results of the X-ray structure analysis. Not surprisingly, the
calculations predict that the B−C coordination mode is 25 kJ/
mol higher in energy, thus suggesting that the model species CC
represents the thermodynamically favored product. Further-
more, the central B−C bond in the hypothetical isomer BC
coordinates in a highly distorted η2−π fashion, as indicated by the
calculated geometry. That is, the Rh−B bond (2.381 Å) is
significantly longer than the endocyclic Rh−C bond (2.053 Å).
In contrast, the crystal structure of the aforementioned (carbene-
free) (B,C-η2)-1-aza-2-borabutatriene rhodium complex 1
ASSOCIATED CONTENT
* Supporting Information
Experimental section, X-ray crystallographic details including a
CIF file, and computational details. This material is available free
of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: h.braunschweig@uni-wuerzburg.de.
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Financial support by the European Research Council (Advanced
Investigator Grant to H.B.) is gratefully acknowledged.
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