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Boron as a Bridging Ligand.

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Communications
Low-Valent Compounds
Boron as a Bridging Ligand**
Holger Braunschweig,* Krzysztof Radacki,
David Scheschkewitz, and George R. Whittell
Dedicated to Professor Peter Paetzold
on the occasion of his 70th birthday
Borylenes (BR) are highly reactive species that have only
been observed at low temperatures[1] or their presence
inferred from the products of trapping reactions.[2] The
formation of an example stabilized within the coordination
sphere of a transition-metal-ligand fragment thus constituted
a major advance.[3] Both bridging and terminal borylene
coordination modes have now been realized, affording a
comparison with common organometallic ligands.[4] Despite
these advances, however, the range of different borylenes
stabilized by terminal coordination has remained somewhat
narrow. The presence of bulky, p-donating substituents (h5C5Me5,[5] N(SiMe3)2 ,[6] and 2,4,6-Me3C6H2[7]) would appear
almost a prerequisite for stabilization, with the absence of pdonating substituents resulting in the lower thermal stability
of [(OC)5Cr{BSi(SiMe3)3}] (1).[8] A number of theoretical
studies have ventured to quantify the effects of different
substituents on the nature of the transition-metal?boron
bond.[9] Experimental testing of these predictions, however,
requires a wider range of examples. There would appear no
reason as to why the main-group substituent at boron could
not be replaced by another transition-metal-ligand fragment,
as long as the steric and electronic criteria for stabilization are
fulfilled. Compounds containing the heavier Group 13 elements, gallium[10] and thallium,[11] coordinated solely to two
transition-metal-ligand fragments have been reported, these,
however, partially owe their stability to the inert-pair
effect.[12] The synthesis of a boron homologue should hence
pose a much more challenging problem. Such a compound
represents a fundamentally new type of terminal borylene
complex, but also a complex of interstitial boron[13] containing
classical (that is, electron-precise) metal?boron bonds. We
report herein the syntheses, spectroscopic, and structural
characterization of the first compounds to contain a boron
center classically bonded solely to transition metals.
The reaction of dihaloboranes with dianionic transitionmetal carbonylates has proven the most general synthetic
route to terminal borylene complexes.[4b?d] Thus, the recent
realization of the dichloroboryl complex, [(h5-C5H5)Fe(CO)2BCl2] (2),[14] afforded a starting material which could
[*] Prof. Dr. H. Braunschweig, Dr. K. Radacki, Dr. D. Scheschkewitz,
Dr. G. R. Whittell
Institut fr Anorganische Chemie
Bayerische Julius-Maximilians-Universitt Wrzburg
Am Hubland, 97074 Wrzburg (Germany)
Fax: (+ 49) 931-888-4623
E-mail: h.braunschweig@mail.uni-wuerzburg.de
[**] This work was supported by the DFG and EPSRC.
1658
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
potentially be utilized for preparation of a compound
containing a m2-boron bridge.
The reaction of 2 with Na2[Cr(CO)5] in toluene yielded a
single new compound as shown by 11B{1H} NMR spectroscopy. The frequency of the signal (d = 164 ppm), however,
was shifted significantly upfield with respect to that predicted
for the desired product [{(h5-C5H5)Fe(CO)2}(m2-B){Cr(CO)5}], by computational methods (d = 219.7 ppm).[15]
Although we could not isolate this compound, the formulation Na[{(h5-C5H5)Fe(CO)2}(m2-BCl){Cr(CO)5}] is consistent
with the 11B{1H} NMR data.[16] Furthermore, a similar species,
K[{(h5-C5Me5)Fe(CO)2}(m2-GaCl){Fe(CO)4}], was reportedly
formed from reaction of [(h5-C5Me5)Fe(CO)2GaCl2] with
K2[Fe(CO)4].[10a] Changing the reaction solvent from toluene
to THF cleanly afforded a different product, with an 11B{1H}
NMR signal (d = 199.6 ppm) at a frequency much closer to
that predicted by theory. This compound, however, proved
too thermally labile to be isolated.
We and others[17] have observed that the thermal stability
of the half-sandwich iron-boryl complexes, [(h5-C5R5)Fe(CO)2BX2], increases markedly on formal replacement of
the cyclopentadienyl ligand (R = H) for pentamethylcyclopentadienyl (R = Me). Thus, when the reaction was repeated
with [(h5-C5Me5)Fe(CO)2BCl2],[18] the desired compound,
[{(h5-C5Me5)Fe(CO)2}(m2-B){Cr(CO)5}] (3) was obtained in
35 % yield (Scheme 1).
Scheme 1. Syntheses of compounds 3 and 4.
To demonstrate generality, the analogous reaction with
Na2[Fe(CO)4] was attempted and this also yielded a m2-boron
complex, namely [{(h5-C5Me5)Fe(CO)2}(m2-B){Fe(CO)4}] (4),
in comparable yield (36 %). Both compounds were extremely
oxygen and moisture sensitive in the solid state and solution.
However, under a dry argon atmosphere they showed no signs
of decomposition in either THF or toluene solution after 24 h
at room temperature.
The most striking spectroscopic feature of compounds 3
and 4 is the highly deshielded 11B{1H} NMR resonance signals
(d = 204.6 (3) and 190.9 ppm (4)). Compound 1 has a signal at
a similar frequency (d = 204.3 ppm)[8] and the downfield shift
relative to the aminoborylene, [{(OC)5Cr}BN(SiMe3)2] (5)
(d = 92 ppm),[6a] is attributed to the absence of a p-donating
substituent. It is noteworthy, however, that this region of the
spectrum is normally only the domain of interstitial boron
(d = 172?226 ppm).[19] Generally, all other spectroscopic data
for 3 and 4 are as to be expected for the ancillary ligands. The
only exception is that the 13C{1H} NMR signal for the carbonyl
ligand trans to boron is not observed. A similar situation,
however, was reported for 1 and attributed to the high trans
influence of the borylene ligand.[8]
DOI: 10.1002/anie.200463026
Angew. Chem. Int. Ed. 2005, 44, 1658 ?1660
Angewandte
Chemie
Scheme 2. Alternative bonding representations for compounds 3
(M = Cr, n = 5) and 4 (M = Fe, n = 4), see text for details.
fragment is bound to the {Fe(CO)4} unit through the apical
site and thus exhibits a similar gross structure to [(h5C5Me5)B{Fe(CO)4}].[5] However, in 4 the Fe1B bond is
significantly shorter (1.863(2) ) than in [(h5-C5Me5)B{Fe(CO)4}] (2.010(3) ) which contains only an Fe B donor?
acceptor bond. Thus, the bond lengths imply at least a degree
of p bonding in the BFe(CO)4 interaction of 4. These
arguments suggest that the bonding in both 3 and 4 is best
described by a model in which both metal?boron bonds
contain a p component (Scheme 2 b). A comparable partial pbond representation has been proposed for the m2-Ga
complexes on the basis of a similar structural analysis.[10]
In conclusion, the reactions of [(h5-C5Me5)Fe(CO)2BCl2]
with either Na2[Cr(CO)5] or Na2[Fe(CO)4] affords compounds that contain a substituent-free boron center which
bridges the transition metal?ligand fragments.
!
The molecular structures of both compounds 3 and 4 were
determined by single-crystal X-ray diffraction (Figure 1).[20]
The structure of 3 contains half a molecule per asymmetric
unit with a crystallographic mirror plane bisecting the Cr, B,
Fe atoms, and the pentamethylcyclopentadienyl ring. The
structure of 4 has one molecule per asymmetric unit. In both
structures the molecules are well separated and the boron
center forms no close intramolecular contacts. The structures
are characterized by the presence of a linear metal?boron?
metal unit (Cr-B-Fe 177.75(11)8 and Fe-B-Fe 175.38(12)8),
confirming the bridging mode of the formally sp-hybridized
boron atom. A linear bridging mode is also common to the m2Ga and m2-Tl complexes, [{(h5-C5Me5)Fe(dppe)}(m2-Ga){Fe(CO)4}] {dppe = 1,2-bis(diphenylphospino)ethane},[10a] [{(h5C5Me5)Fe(CO)2}2(m2-Ga)]+ ,[10b] and [{Cr(CO)5}2(m2-Tl)] ,[11b]
that exhibit corresponding angles that are similarly close to
1808 (176.01(4), 178.99(2), and 178.5(2)8, respectively). Only
the m2-thallium complex, [Ir2(m2-Tl)(CO)2Cl2(m-dmpa)2]+
(dmpa = bis(diphenylphosphinomethyl)phenylarsine),[11a] has
a bridging unit which differs significantly from linearity [Ir-TlIr 139.4(1)8]. This distortion, however, probably originates
from the IrиииIr separation being constrained somewhat by the
bridging bidentate ligands.
For a discussion of the bonding in 3 and 4, it is convenient
to start from a formulation that depicts a single bond from
boron to the half-sandwich moiety and a double bond from
boron to the metal carbonyl fragment, thus affording each
metal 18 valence electrons (Scheme 2 a). The FeB bond
length in 3 (1.8617(9) ) is intermediate between those of the
cationic borylene complex, [(h5-C5Me5)Fe(CO)2(BMes)]+
(Mes = 2,4,6-trimethylphenyl),[7] for which an FeB double
bond has been proposed, and 2 (1.942(3) ),[14] which has the
shortest reported FeB(sp2) separation.
DFT calculations[21] and structural studies[14] indicate that
even the FeB bond in 2 contains an appreciable p component, and thus the formal single bond proposed for 3 should
be augmented by at least a modest p interaction. Similarly,
the CrB bond length of 1.975(2) in 3 is intermediate
between those of 5 (1.996(6) )[6b] and 1 (1.878(10) ).[8]
Compound 1 has a relatively strong CrB p interaction
owing to the absence of a competing p-donor group at boron.
The p contribution to the CrB bond in 3 is therefore less
than that required for a formal double bond.
In 4, the Fe2B bond length (1.867(2) ) is comparable to
that in 3, hence implying a similar p component. This
Experimental Section
All manipulations were conducted either under an atmosphere of dry
argon or in vacuo using standard Schlenk line or glove-box techniques.
3: A solution of Na2[Cr(CO)5][22] (2.354 mmol) in THF (25 mL)
was prepared in a Schlenk tube and cooled to about 90 8C. This
solution was then transferred, by cannula, to another Schlenk tube,
similarly chilled and containing solid [(h5-C5Me5)Fe(CO)2BCl2][18]
(0.702 g, 2.140 mmol). The reaction mixture was slowly allowed to
warm to about 30 8C over approximately 1 h, resulting in the
formation of a dark yellow/orange solution. Subsequent removal of
all volatiles in vacuo, at room temperature, afforded a dark orange
solid. Hexane (50 mL) was added and residual solids separated by
centrifugation. The clear, orange supernatant liquors were then
decanted, reduced in volume by about 50 % and cooled to 25 8C.
After about 7 days, the pale orange crystals of pure 3 that had formed
were isolated by removal of the mother liquors and dried in vacuo
(yield 0.58 g, 31 %). IR (toluene): n? = 2055 (s), 2012 (s), 1976 (br s),
1932 (br s) cm1, n(CO). 1H NMR (400 MHz, C6D6, 25 8C): d =
1.40 ppm (s, 15 H, C5Me5); 13C{1H} NMR (101 MHz, C6D6, 25 8C):
d = 9.48 (s, C5Me5), 98.45(s, C5Me5), 210.32 (s, CO), 216.79 ppm (s,
Figure 1. Molecular structures of 3 (left) and 4 (right). Selected bond lengths [] and angles [8]. 3: Cr-B 1.975(2), Fe-B 1.8617(19); Cr-B-Fe
177.75(11). 4: Fe1-B 1.863(2), Fe2-B 1.867(2); Fe1-B-Fe2 175.38(12).
Angew. Chem. Int. Ed. 2005, 44, 1658 ?1660
www.angewandte.org
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1659
Communications
CO); 11B NMR (64 MHz, C6D6, 25 8C): d = 204.6 ppm (w1/2 = 488 Hz).
Elemental analysis (%) calcd for C17H15O7BCrFe: C 45.38; H 3.36;
found: C 45.34, H 3.41.
4: A suspension of Na2[Fe(CO)4][23] (0.329 g, 1.537 mmol) in THF
(20 mL) was prepared in a 100-cm3 round-bottomed flask. This
suspension was cooled to about 100 8C and then transferred, by
cannula, to a similarly chilled Schlenk tube containing solid [(h5C5Me5)Fe(CO)2BCl2][18] (0.480 g, 1.464 mmol). The resulting brown
suspension was slowly allowed to warm to around 15 8C over about
90 min affording a dark red solution. This solution was then allowed
to warm to room temperature and all volatiles were removed
in vacuo. Hexane (30 mL) was subsequently added to the residual
dark red solid affording a cloudy, red solution. The insoluble
components were separated by centrifugation and the supernatant
liquors cooled to 25 8C. After about 2 days, the yellow crystals of
pure 4 that had formed were isolated by removal of the mother
liquors and dried in vacuo (yield 0.223 g, 36 %). IR (toluene): n? =
2048 (s), 2020 (s), 1979 (br s), 1929 (br s) cm1, n(CO). NMR 1H
(400 MHz, C6D6, 25 8C): d = 1.38 ppm (s, 15 H, C5Me5); 13C{1H} NMR
(101 MHz, C6D6, 25 8C): d = 9.29 (s, C5Me5), 98.64 (s, C5Me5), 209.92
(s, CO), 215.36 ppm (s, CO); 11B NMR (64 MHz, C6D6, 25 8C): d =
190.9 ppm (w1/2 = 521 Hz). Elemental analysis (%) calcd for
C16H15O6BFe2 : C 45.13; H 3.55; found: C 45.23, H 3.65.
[11]
[12]
[13]
[14]
[15]
Received: December 22, 2004
.
Keywords: boron и bridging ligands и chromium и iron и
low-valent compounds
[16]
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Crystal structure data for 3: orange blocks from heptane;
C17H15O7BCrFe, orthorhombic, space group Pnma; a =
17.721(6), b = 12.968(4), c = 8.552(3) , a = b = g = 908, V =
1965.3(12) 3 ; Z = 4, 1calcd = 1.521 g cm3 ; crystal dimensions:
0.25 0.25 0.2 mm; diffractometer: Bruker SMART APEX
CCD; MoKa radiation, 173(2) K; 2qmax = 56.568; 19 159 reflections, 2545 unique (Rint = 0.0261), direct methods; absorption
correction SADABS (m = 1.325 mm1); refinement (against F 2o)
with SHELXTL-97, 142 parameters, 5 restraints, R1 = 0.0248 (I >
2s) and wR2 = 0.0701 (all data), Goof = 1.066, max/min residual
electron density: 0.381/0.291 e 3. Crystal structure data for 4:
yellow plates from heptane; C16H15O6BFe2, monoclinic, space
group P21/n; a = 8.058(5), b = 14.638(10), c = 15.664(8) , a =
90, b = 93.87(3), g = 908, V = 1849(2) 3 ; Z = 4, 1calcd =
1.530 g cm3 ; crystal dimensions: 0.05 0.15 0.15 mm; diffractometer: Bruker SMART APEX CCD; MoKa radiation,
173(2) K; 2qmax = 55.788; 18 309 reflections, 4420 unique (Rint =
0.0312), direct methods; absorption correction SADABS (m =
1.595 mm1); refinement (against F 2o) with SHELXTL-97, 226
parameters, 0 restraints, R1 = 0.0329 (I > 2s) and wR2 = 0.0826
(all data), Goof = 1.042, max/min residual electron density:
0.488/0.211 e 3. CCDC-258544 (3) and CCDC-258433 (4)
contain the supplementary crystallographic data for this paper.
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