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Borylene Transfer under Thermal Conditions Synthesis and Structure of a Tetrarhodium Bisborylene Complex.

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Communications
Coordination Chemistry
DOI: 10.1002/anie.200504093
Borylene Transfer under Thermal Conditions:
Synthesis and Structure of a Tetrarhodium
Bisborylene Complex**
Holger Braunschweig,* Melanie Forster, and
Krzysztof Radacki
Transition-metal borylene complexes have received considerable attention, owing to their close relationship to pivotal
organometallic compounds, such as carbonyl and carbene
complexes,[1–4] as well as their potential as sources of
borylenes (that is, BR) for organic reactions.[5] Despite
recent successes in the realization of novel coordination
modes in metalloborylene complexes[6, 7] and heterodinuclear
borylene complexes,[8, 9] the scope of coordination modes
encountered in borylene complexes is still very limited, as
only combinations between a single borylene ligand and one
or two metal centers are known. Given the parallels in the
structural chemistry of borylene and carbonyl complexes, this
fact is surprising, especially when the wealth of polynuclear
carbonyl complexes is considered.[10] The main reason for this
difference seems to be the ready availablity of the metastable
CO, which reacts in various stoichiometries with metal
precursors to yield polynuclear carbonyl compounds with
complex structures. Similarly, the heavier congeners of
borylenes, in particular Cp*Ga and Cp*In (Cp* = C5Me5),
are also available for the synthesis of mono- or polynuclear
complexes.[11] As borylenes are extremely reactive species,
they cannot be handled under standard conditions,[3] but must
be generated within the coordination sphere of a transition
metal. Salt-elimination reactions between anionic metal
carbonylates and haloboranes have been used for the
purpose, and the first bridged and terminal borylene complexes were thus obtained.[12, 13] However, this method may
have reached its limits, because it is restricted to a few specific
combinations of transition metals and boranes.[14–16]
More recently, we were able to show that photochemically
induced borylene transfer is a valuable method for the
synthesis of borylene complexes that cannot be obtained by
salt elimination.[17, 18] Furthermore, this approach allows the
transfer of the borylene unit to organic substrates.[5] The
reactivity of borylene complexes toward transition-metal
species under nonphotolytic (that is, thermal) conditions
was previously restricted to the addition of the {Pd(PCy3)}
fragment (Cy = cyclohexyl) to [(OC)5M=BN(SiMe3)2] (M =
Cr, W), which resulted in the clean formation of the first
[*] Prof. H. Braunschweig, M. Forster, Dr. K. Radacki
Institut f)r Anorganische Chemie
Bayerische Julius-Maximilians-Universit4t W)rzburg
Am Hubland, 97074 W)rzburg (Germany)
Fax: (+ 49) 931-888-4623
E-mail: h.braunschweig@mail.uni-wuerzburg.de
[**] This work was supported by the Deutsche Forschungsgemeinschaft.
2132
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 2132 –2134
Angewandte
Chemie
complexes with semibridging borylene ligands.[9] Herein, we
report the selective synthesis of a novel tetranuclear rhodium
complex, which is the first example of a bisborylene complex.
This compound is formed in high yield by an unprecedented
metal–metal borylene transfer at ambient temperature.
The reaction of the terminal borylene complexes
[(OC)5M=BN(SiMe3)2] (M = Cr, 1; M = W, 2) with
[{RhCl(CO)2}2] (3) in C6D6 yields, within minutes, the
tetranuclear bisborylene complex [Rh4{m-BN(SiMe3)2}2(mCl)4(m-CO)(CO)4] (4; Scheme 1). After recrystallization in
hexane, 4 was isolated as air- and moisture-sensitive black
crystals in 43 % (M = Cr, 1) or 41 % (M = W, 2) yield.
dine).[20] For both this compound and 4, the presence of a
RhRh bond may be assumed, as the formal electron count of
each rhodium center is odd. The distances between the central
and terminal rhodium atoms, Rh1Rh3 (3.0415(4) B) and
Rh2Rh4 (3.0472(4) B), are significantly longer than the
Rh1Rh2 distance. Both distances are, however, significantly
shorter than the RhRh bond in the starting compound 3
(3.31 B).[21]
The length of the B1N1 and B2N2 bonds (1.369(4) B)
is slightly shorter than that observed in the borylene-bridged
manganese
complex
[(m-BNMe2){(h5-C5H5)Mn(CO)2}2]
[12]
(1.390(1) B), indicating significant BN double-bond character. The presence of the bulky SiMe3 groups requires that
the Si1-B1-Si2 and Si3-B2-Si4 planes twist by 58.6(14)8 or
42.3(5)8 out of the Rh1-B1-Rh2 and Rh1-B2-Rh2 planes,
respectively, as previously described for similar structures.[22]
Interestingly, in the solid state, the individual tetramer
units aggregate into neutral linear chains (Figure 2), featuring
Scheme 1. Synthesis of the doubly borylene-bridged complex 4.
The structure of 4 was unequivocally determined by
single-crystal X-ray diffraction (Figure 1).[19] The core of the
molecule is composed of a chain of four rhodium atoms, the
internal metal centers are bridged by two borylene moieties
and a CO ligand. The remarkably short Rh1Rh2 distance
between the two central rhodium atoms (2.5786(3) B) is
comparable to RhRh distances of dinuclear complexes in
which the rhodium atoms are bridged by three CO ligands, for
example, 2.566(5) B in [Rh2(m-CO)3Cl2(py)4] (py = pyri-
Figure 1. Molecular structure of 4 in the solid state. Bond lengths [C]
and angles [8]: Rh1Rh2 2.5786(3), Rh1Rh3 3.0415(4), Rh2Rh4
3.0472(4), Rh3Rh4 3.1833(4), B1Rh1 2.004(3), B1Rh2 2.051(3),
B2Rh1 2.076(3), B2Rh2 2.003(3), B1N1 and B2N2 1.369(4);
Si2-Si1-N1-B1 58.62(14), Si3-Si4-N2-B2 43.30(5). Thermal ellipsoids
are set at 50 % probability. Hydrogen atoms are omitted for clarity.
Dashed lines indicate intermolecular RhRh contacts.
Angew. Chem. Int. Ed. 2006, 45, 2132 –2134
Figure 2. Linear chains of the tetranuclear complex 4 in the solid state.
Thermal ellipsoids are set at 50 % probability. Hydrogen atoms are
omitted for clarity.
a relatively short intermolecular Rh3Rh4 distance
(3.1833(4) B). This distance is longer than the RhRh
distances within the tetramers (2.5786(3)–3.0472(4) B) and
is approximately 0.3 B longer than those in the mixed-valent
polymer
[Rh(3,6-dbdiox-4,5-Cl2)(CO)2]1
(2.8629(2)–
2.8984(3) B; 3,6-dbdiox-4,5-Cl2 = 3,6-di-tert-butyl-4,5-dichlorocatecholate),[23] but is similar to the RhRh distances in the
rhodium(i) complex [Rh(3,6-dbsq)(CO)2]1 (3.252(4) and
3.304(5) B; 3,6-dbsq = 3,6-di-tert-butyl-1,2-benzosemiquinonate).[24]
Clearly, 4 is a product of a complicated reaction, in which
[M(CO)6] (M = Cr, W) is also probably formed as a byproduct. However, the 11B{1H} and 1H NMR spectra of the
reaction solutions reveal that they are surprisingly free from
boron- or hydrogen-containing side products, with the
exception of one equivalent of Cl2BN(SiMe3)2 (d(11B) =
36 ppm; d(1H) = 0.21 ppm).[25] Since no loss of chloride
ligands from the starting material 3 is required for the
formation of 4, it is likely that the side product Cl2BN(SiMe3)2
is produced in an independent process. The 11B{1H} NMR
spectrum of the new compound 4 features a broad singlet at
d = 74 ppm (w1/2 = 1071 Hz), which is shifted upfield with
respect to those of the starting materials (d = 92 ppm, 1; d =
86 ppm, 2).[16] Such an upfield shift is unusual, as borylenebridged complexes usually exhibit downfield-shifted 11B
resonances between d = 98 and 120 ppm.[2] The CO ligand
remains in a bridging position in solution, as demonstrated by
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
2133
Communications
the presence of a band at ñ = 1856 cm1 in the IR spectrum of
4 in toluene.
Herein, we have described the first transition-metal
borylene complex featuring two bridging borylene ligands.
The compound was obtained by an unprecedented thermal
borylene-transfer reaction, and comprises a linear chain of
four rhodium atoms, a relatively rare structural motif.
Experimental Section
All manipulations were conducted under an atmosphere of dry argon
using either standard Schlenk techniques or a glove box.
4: Solid 3 (0.054 g, 0.138 mmol) was added to an orange solution
of 1 (0.075 g, 0.207 mmol) in C6D6 (0.6 mL) at ambient temperature.
The color of the solution changed immediately to dark brown. After
30 min, the solvent was removed in vacuo. The black solid obtained
was dissolved in hexane (2 mL). After filtration, cooling the solution
to 35 8C yielded black crystals of 4 (0.061 g, 43 %, based on
rhodium). 1H NMR (500 MHz, C6D6, 25 8C, TMS): d = 0.45 ppm (s,
36 H, SiMe3); 13C{1H} NMR (126 MHz, C6D6, 25 8C): d = 178.3 (d,
1
J(C,Rh) = 76 Hz, CO), 3.1 ppm (s, SiMe3), a signal for the bridging
CO was not observed; 11B{1H} NMR (64 MHz, C6D6, 25 8C): d =
74 ppm (s, w1/2 = 1071 Hz). IR (toluene): ñ = 2089, 2032, 1944,
1856 cm1
(C=O).
C,H,N
analysis
(%)
calcd
for
C17H36N2B2Cl4O5Si4Rh4 : C 19.71, H 3.50, N 2.70; found: C 20.07, H
3.18, N 2.30.
In an analogous manner, 4 was also obtained through the reaction
of 2 (0.075 g, 0.152 mmol) and 3 (0.039 g, 0.101 mmol) in 41 % yield
(based on rhodium, 0.040 g).
Received: November 17, 2005
Published online: February 24, 2006
.
Keywords: boron · borylene complexes · bridging ligands ·
rhodium
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Crystal data for 4: C17H36B2Cl4N2O5Rh4Si4, Mr = 1035.90, translucent black blocks, 0.24 N 0.15 N 0.08 mm3, orthorhombic, Pbca,
a = 14.5097(12),
b = 21.4616(18),
c = 23.971(2) B,
V=
7464.7(11) B3, Z = 8, 1calcd = 1.844 g cm3, m = 2.180 cm2, F(000) = 4048, T = 153(2) K, R1 = 0.0335, wR2 = 0.0767, 7597
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2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 2132 –2134
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