Syntheses and structure of bridged haloborylene complexes

Article abstract of DOI:10.1021/om700486d

The homodinuclear, bridged haloborylene transition metal complexes [(μ-BCl){(η⁵-C₅H₅)Fe(CO) ₂}₂] and [(μ-BX){Mn(CO)₅}₂] (X = Cl, Br) were prepared by salt elimination reactions a

Full text of DOI:10.1021/om700486d

4700
Organometallics 2007, 26, 4700-4701
Syntheses and Structure of Bridged Haloborylene Complexes
Philipp Bissinger, Holger Braunschweig,* and Fabian Seeler
Institut f u¨ r Anorganische Chemie, UniVersit a¨ t W u¨ rzburg, Am Hubland, D-97074, W u¨ rzburg, Germany
ReceiVed May 16, 2007
Summary: The homodinuclear, bridged haloborylene transition
5
metal complexes [(µ-BCl){(η -C5H5)Fe(CO)2}2] and [(µ-BX)-
{
Mn(CO)5}2] (X ) Cl, Br) were prepared by salt elimination
reactions and characterized by single-crystal X-ray diffraction.
Since the advent of the first structurally authenticated borylene
complex,1 the chemistry of this class of transition metal
2
complexes of boron has received considerable interest. In
particular, complexes derived from fluoroborylene have been
subject to numerous theoretical studies due to the close
electronic relationship between BF and the ubiquitous CO
3
ligand. These studies revealed BF to possess enhanced σ-donor
Figure 1. Molecular structure of 1. Selected bond lengths [Å] and
angles [deg]: Fe(1)-B ) 2.019(2), Fe(2)-B ) 2.006(2), B-Cl
and π-acceptor properties with respect to CO, thus predicting
an increased thermodynamic stability of BF complexes with
respect to homolytic M-B dissociation. Due to the buildup of
positive charge at the BF ligand, however, borylene complexes
are known to be readily susceptible to nucleophilic attack at
the boron center. Hence, the vast majority of experimentally
realized species of that type have to rely both on electron-
releasing substituents at boron and on sterically demanding
)
1.841(2); Fe(1)-B-Fe(2) ) 131.27(12), Fe(1)-B-Cl )
113.91(12), Fe(2)-B-Cl ) 114.82(12).
represents the only fully characterized haloborylene complex.7
5
Additionally, the heterodinuclear complex [{(η -C5Me5)Fe-
(CO)}(µ-BBr)(µ-CO){Pd(Br)(PCy3)}] was obtained by a salt
2
elimination-oxidative addition sequence and characterized by
groups, thus increasing their kinetic stability. Indeed, corre-
8
multinuclear NMR spectroscopy and elemental analyses. In the
sponding complexes with boron-bound groups lacking π-donor
5
present paper we describe the direct synthesis of [(µ-BCl){(η -
properties are very rare and restricted to only two examples,
4
5
C5H5)Fe(CO)2}2] and of [(µ-BX){Mn(CO)5}2] (X ) Cl, Br),
the latter representing the first example of a structurally
characterized bromoborylene complex.
namely, [(OC)5CrdB-Si(SiMe3)3] and [(µ-BtBu){(η -C5H4R)-
Mn(CO)2}2] (R ) H, Me).1,5 Likewise, chloro- and bromo-
borylene complexes, which can be considered as the closest
relatives of the elusive B-F species, represent peculiar cases,
obviously due to the lack of steric protection provided by the
small halide substituent. While terminal complexes of the type
During the course of our studies about the reactivity of
monoanionic transition metal carbonylates toward trihalo-
boranes, which recently led to a general access to dihaloboryl
9
[
LxMdB-Hal] are generally unknown, the bridged chloro-
complexes, we turned our attention to the preparation of bridged
5
5
borylene species [(µ-BCl){(η -C5H4Me)Mn(CO)2}2], which was
synthesized from its amino-substituted precursor [(µ-BNMe2)-
{
haloborylene complexes. From the reaction of 2 equiv of K[(η -
C5H5)Fe(CO)2] with BCl3, the bridged chloroborylene complex
5
6
5
(η -C5H4Me)Mn(CO)2}2] upon reaction with gaseous HCl,
[(µ-BCl){(η -C5H5)Fe(CO)2}2] (1) is obtained. The synthesis
is accompanied by the formation of an almost equimolar amount
of [(η -C5H5)Fe(CO)2]2, and therefore 1 was only isolated in
*
E-mail: h.braunschweig@mail.uni-wuerzburg.de. Fax: +49 931 888
260. Tel: +49 931 888 4623.
1) (a) Braunschweig, H.; Wagner, T. Angew. Chem. 1995, 107, 904;
5
5
1
0% yield as an analytically pure red, air- and moisture-sensitive
(
11
Angew. Chem., Int. Ed. 1995, 34, 825. (b) Braunschweig, H.; Ganter, B. J.
Organomet. Chem. 1996, 545, 163.
solid. The B NMR resonance (δ ) 146.6) shows the expected
low-field shift in comparison to the dichloroboryl complex [(η -
C5H5)Fe(CO)2(BCl2)] (δ ) 90.0). The constitution of 1 was
confirmed by single-crystal X-ray diffraction. Suitable crystals
5
(2) For reviews see: (a) Braunschweig, H. Angew. Chem. 1998, 110,
9
1
882; Angew. Chem., Int. Ed. 1998, 37, 1786. (b) Braunschweig, H.; Colling,
M. Eur. J. Inorg. Chem. 2003, 3, 393. (c) Braunschweig, H. AdV.
Organomet. Chem. 2004, 51, 163. (d) Aldridge, S.; Coombs, D. L. Coord.
Chem. ReV. 2004, 248, 535. (e) Braunschweig, H.; Rais, D. Heteroat. Chem.
005, 16, 566. (f) Braunschweig, H.; Kollann, C.; Rais, D. Angew. Chem.
006, 118, 5380; Angew. Chem., Int. Ed. 2006, 45, 5254. (g) Braunschweig,
were grown from a solution of 1 in hexane at -35 °C.
2
2
The complex crystallizes in the triclinic space group P 1h , and
the asymmetric unit contains one unique molecule. The Fe-B
bond distances [2.019(2) and 2.006(2) Å] are elongated com-
H.; Colling, J. Organomet. Chem. 2000, 614, 18.
3) (a) Bickelhaupt, F. M.; Radius, U.; Ehlers, A. W.; Hoffmann, R.;
(
5
Baerends, E. J. New J. Chem. 1998, 22, 1. (b) Radius, U.; Bickelhaupt, F.
M.; Ehlers, A. W.; Goldberg, N.; Hoffmann, R. Inorg. Chem. 1998, 37,
080. (c) Ehlers, A. W.; Baerends, E. J.; Bickelhaupt, F. M.; Radius, U.
pared to that of the boryl species [(η -C5H5)Fe(CO)2(BCl2)]
9
[
1.942(3) Å] but are still shorter than those of iron boryl
1
Chem.-Eur. J. 1998, 4, 210. (d) Uddin, J; Boehme, C.; Frenking, G. Coord.
Chem. ReV. 2000, 197, 249. (e) Uddin, J; Boehme, C.; Frenking, G.
Organometallics 2000, 19, 571.
(7) Braunschweig, H.; Colling, M.; Hu, C.; Radacki, K. Angew. Chem.
2002, 114, 1415; Angew. Chem., Int. Ed. 2002, 41, 1359.
(8) Braunschweig, H.; Radacki, K.; Rais, D.; Seeler, F.; Uttinger, K. J.
Am. Chem. Soc. 2005, 127, 1386.
(9) (a) Braunschweig, H.; Radacki, K.; Seeler, F.; Whittell, G. R.
Organometallics 2004, 23, 4178. (b) Braunschweig, H.; Radacki, K.; Seeler,
F.; Whittell, G. R. Organometallics 2006, 25, 4605.
(
4) Braunschweig, H.; Colling, M.; Kollann, C.; Merz, K.; Radacki, K.
Angew. Chem. 2001, 113, 4327; Angew. Chem., Int. Ed. 2001, 40, 4198.
5) Braunschweig, H.; Burschka, C.; Burzler, M.; Metz, S.; Radacki, K.
Angew. Chem. 2006, 118, 4458; Angew. Chem., Int. Ed. 2006, 45, 4352.
6) Braunschweig, H.; M u¨ ller, M. Chem. Ber./Recl. 1997, 130, 1295.
(
(
1
0.1021/om700486d CCC: $37.00 © 2007 American Chemical Society
Publication on Web 08/09/2007

Communications
Organometallics, Vol. 26, No. 19, 2007 4701
Figure 2. Molecular structures of 2 and 3. Selected bond lengths [Å] and angles [deg]: 2: Mn(1)-B ) 2.164(2), Mn(2)-B ) 2.161(2),
B-Cl ) 1.804(2); Mn(1)-B-Mn(2) ) 131.65(10), Mn(1)-B-Cl ) 113.87(11), Mn(2)-B-Cl ) 114.48(11); 3: Mn(1)-B ) 2.149(3),
Mn(2)-B ) 2.163(3), B-Br ) 1.997(3); Mn(1)-B-Mn(2) ) 132.98(15), Mn(1)-B-Br ) 114.20(15), Mn(2)-B-Br ) 112.82(15).
Scheme 1a
of 3, which differ only marginally in the arrangement of the
borylene ligand with respect to the (OC)5Mn fragments. The
difference between the Br-B-Mn-Ccis torsion angles amounts
to approximately 6°, and for simplicity reasons only one of the
molecular structures is discussed below. In both compounds,
the boron centers display distorted trigonal-planar coordination
geometries with Mn-B-Mn angles of 131.65(10)° and
1
2
32.98(15)°. The Mn-B bond distances [2, 2.164(2) and
.161(2) Å; 3, 2.149(3) and 2.163(3) Å] are longer than in the
9
boryl complexes [(OC)5Mn(BCl2)] [2.060(5) Å] and [(OC)5MnB-
1
3
(1,2-O2C6H4)] [2.108(6) Å] and even exceed the Mn-B
5
separation in the half-sandwich complex [(η -C5H4Me)(OC)2-
H)Mn-B(Cl){Si(SiMe3)3}] [2.138(16) Å], which is character-
ized by a hydrogen atom adopting a bridging position between
(
14
the metal and boron centers. The B-Cl bond length [1.804(2)
Å] in 2 is slightly longer than those found in [(OC)5Mn(BCl2)]
a
5
Reaction conditions: (i) 2 equiv of K[(η -C
5 5 2
H )Fe(CO) ], toluene,
-
)
60 °C; (ii) 2 equiv of Na[Mn(CO)
Br), -30 °C.
5
], toluene (X ) Cl) or hexane (X
9
[
1.777(3) Å], while the B-Br bond length [1.997(3) Å] in 3
is almost identical to that found in the bisboryloxide [{(OC)5Mn-
9
complexes with π-stabilizing nitrogen substituents at boron.10
The Fe-Fe distance [3.667 Å] is shorter than in [(µ-BMes)-
(BBr)} O] [1.981(2) Å]. The Mn-Mn distances [3.945 and
2
5
3.954 Å] are significantly longer than in [(µ-BNMe2){(η -C5H5)-
1
5
11
2 2
Mn(CO) } ] [2.03(1) Å], clearly indicating the absence of any
{
(η -C5H5)Fe(CO)2}2] [3.802 Å], but significantly longer than
5
Mn-Mn interactions. The Mn-Ctrans bond distances [2, 1.862(2)
and 1.864(2) Å; 3, 1.862(3) and 1.863(3) Å] are within the range
of the Mn-Ccis bond distances [2, 1.844(2)-1.874(2) Å; 3,
1.848(3)-1.872(3) Å], and thus the trans influence of the
borylene ligand is similar to that of CO.
in [{µ-BN(SiMe3)2}{µ-CO}{(η -C5H5)Fe(CO)}2] [2.548(1) Å],
which in contrast to the aforementioned homodinuclear iron
species is characterized by the presence of only two terminal
but one bridging CO ligand,12 and therefore precludes any Fe-
Fe interaction.
Crystallographic data (excluding structure factors) for the
structures reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary
publication nos. CCDC-646830 (1), CCDC-646829 (2), and
CCDC-622966 (3). Copies of these data can be obtained free
of charge on application to CCDC, 12 Union Road, Cambridge
CB21EZ, UK (fax: (+44)1223-336-033; e-mail: deposit@
ccdc.cam.ac.uk).
The reaction of 2 equiv of Na[Mn(CO)5] with BX3 (X ) Cl,
Br) leads to the formation of the homodinuclear haloborylene
complexes [(µ-BX){Mn(CO)5}2] (2, X ) Cl; 3, X ) Br), which
were isolated as orange, air- and moisture-sensitive solids in
8% and 65% yield, respectively. The 1 B NMR resonances
1
3
(
2, δ ) 160.1; 3, δ ) 163.6) are significantly low-field shifted
with respect to those of the corresponding dihaloboryl species
9
[
(OC)5Mn(BX2)] (X ) Cl, δ ) 94.2; X ) Br, δ ) 92.9).
Recrystallization of 2 from hexane and 3 from toluene/hexane
1:1) afforded yellow crystals suitable for X-ray structure
Acknowledgment. This work was supported by the DFG
and the Fonds der Chemischen Industrie.
(
determination.
Supporting Information Available: Text detailing the synthetic
procedures, spectroscopic data, and structural determinations for
Both compounds crystallize in the monoclinic space group
P21/c. While the asymmetric unit of 2 contains one unique
molecule, two independent molecules are present in the case
1, 2, and 3. This material is available free of charge via the Internet
at http://pubs.acs.org.
(
10) Braunschweig, H.; Kollann, C.; M u¨ ller, M. Eur. J. Inorg. Chem.
OM700486D
1
8
1
998, 291.
(11) Aldridge, S.; Coombs, D. L.; Jones, C. Chem. Commun. 2002, 8,
(13) Waltz, K. M.; He, X.; Muhoro, C.; Hartwig, J. F. J. Am. Chem.
Soc. 1994, 117, 11357.
(14) Braunschweig, H.; Kollann, C.; Colling, M.; Englert, U. J. Chem.
Soc., Dalton Trans. 2002, 2289.
56.
(12) Braunschweig, H.; Kollann, C.; Englert, U. Eur. J. Inorg. Chem.
998, 465.