Synthesis and electronic structure of a ferroborirene

Article abstract of DOI:10.1002/anie.200700382

(Chemical Equation Presented) The photochemically induced transfer of a ferroborylene yielded the first metal-bound borirene (see scheme and product structure, in which the methyl substituents are omitted). Experimental and theoretical data suggest significant it derealization in the three-membered BC₂ ring.

Full text of DOI:10.1002/anie.200700382

Angewandte
Chemie
DOI: 10.1002/anie.200700382
Boron Ligands
Synthesis and Electronic Structure of a Ferroborirene
Holger Braunschweig,* Israel Fernµndez, Gernot Frenking,* Krzysztof Radacki, and
Fabian Seeler
Dedicated to Professor Walter Siebert on the occasion of his 70th birthday.
Recently, it was demonstrated that borylene complexes,[11]
[12]
= =
Unsaturated boron-containing heterocycles such as boroles
(I)[1] or borepines (II)[2] have been intensively studied, in
particular with regard to the question of how the sp2-
hybridized boron center affects the p-electron delocalization
in these conjugated systems.
such as [(OC)5Cr B N(SiMe3)2], act as a facile source for
elusive borylenes under ambient conditions and effectively
transfer BN(SiMe3)2 not only to different transition metals[13]
but also to alkynes, thus providing a high-yield synthesis for
various B-amino borirenes.[14] To study the influence of the
exocyclic substitutents on the properties of borirenes, it was
necessary to search for borylene sources that provide an
alternative substitution pattern at the boron center. Since
= ꢀ
[(OC)5Cr B Si(SiMe3)3] is known to be thermally rather
labile,[15] which attenuates its potential for borylene transfer
reactions, we turned our attention to the metalloborylene
5
complex [(OC)5Cr B Fe(CO)2(h -C5Me5)] (1).[16] Herein, we
= ꢀ
report the synthesis of a novel B-ferroborirene obtained by
unprecedented metalloborylene transfer and the elucidation
of its electronic structure by DFT methods.
Experimental studies on derivatives of I and II revealed
electronic interaction between the p electrons of the carbon
backbone with the empty pz orbital of the boron atom.[1,3] As
a consequence of extended p conjugation across the boron
center, substituted, electron-rich boroles have already
attracted interest owing to their potentially useful photo-
physical and electrochemical properties.[4]
Although delocalization of p electrons in boroles and
borepines was demonstrated, the degree of aromatic stabili-
zation, or antiaromatic destabilization in the case of of I,
appears to be less pronounced than in the respective carbon
analogues, that is, cyclopentadienyl- and tropylium cations.[5,6]
The chemistry of I and II is well-established, in particular with
respect to transition-metal complexes derived from boroles.[7]
Corresponding borirenes (III) represent boron analogues of
cyclopropenyl cations, the smallest cyclic aromatic system,
and were, on the basis of earlier ab initio studies, predicted to
exhibit p delocalization.[8] Presumably because of problems
associated with their isolation and crystallization, however,
borirenes have been scarcely investigated experimentally.
Moreover, some earlier proposed syntheses were found to be
difficult to reproduce.[9,10]
Photolysis of a mixture of 1 with 1,2-bis(trimethylsilyl)-
ethyne in hexane, THF, or benzene leads, as judged by
11B NMR spectroscopy, to the quantitative formation of the
new boron-containing compound 2 (d = 63.5 ppm) within
0.5 h. In the 1H NMR spectrum, a new set of signals is present
in the expected ratio for one C5Me5 ligand and two Me3Si
groups. After extraction with hexane and removal of
[Cr(CO)6] by crystallization, a red solution was obtained.
Storage at ꢀ308C yielded yellow crystals of 2 suitable for
X-ray structure determination.[17] The molecular structure of
2 is shown in Figure 1, and relevant bond lengths and angles
are given in the caption. Analogous to the aminoborirene
(Me3Si)2NBC2(SiMe3)2 (3), compound 2 is characterized by a
ꢀ
C-B-C three-membered ring, which results from the [B
5
ꢀ
Fe(CO)2(h -C5Me5)] borylene transfer to the C C triple
ꢀ
ꢀ
bond. The B1 C3/C4 (149.0(4) and 149.3(4) pm) and C3
C4 (137.1(3) pm) bond lengths are equivalent within exper-
ꢀ
imental error to those in 3. The Fe1 B1 (197.9(3) pm) bond
length is in the lower range for neutral iron half-sandwich
boryl complexes (194–204 pm),[11b] which indicates a substan-
ꢀ
tial degree of Fe B dp–pp back-bonding. However, the IR
data of 2 (1983, 1927 cmꢀ1) are almost identical to those of the
corresponding iron methyl complex (1988, 1936 cmꢀ1).[18] This
[*] Prof. Dr. H. Braunschweig,Dr. K. Radacki,Dipl.-Chem. F. Seeler
Institut für Anorganische Chemie
ꢀ
similarity suggests a different bonding situation with no Fe B
Julius-Maximilians Universität Würzburg
Am Hubland,97074 Würzburg (Germany)
Fax : (+49)931-888-4623
dp–pp interactions, probably owing to significant p delocali-
zation in the borirene ring. An explanation for these contra-
dictory data may be found in the high strain of the borirene
ring, which is reflected by the C3-B1-C4 (54.75(16)8) and Fe1-
B1-C3/C4 (152.8(2)8 and 152.5(2)8) angles and results in a
E-mail: h.braunschweig@mail.uni-wuerzburg.de
Dr. I. Fernµndez,Prof. Dr. G. Frenking
Fachbereich Chemie
Philipps-Universität Marburg
Hans-Meerwein-Strasse,35043 Marburg (Germany)
Fax : (+49)6421-282-5566
E-mail: frenking@chemie.uni-marburg.de
ꢀ
shorter Fe B bond owing to low steric bulk.
To analyze the bonding situation in 2 in more detail, we
carried out DFT calculations (BP86/TZ2P) of the model
compound [(h5-Cp)(OC)2FeBC2(SiH3)2] (2M; Cp = C5H5).[19]
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Figure 1. Molecular structure of 2,thermal ellipsoids set at 50%,
hydrogen atoms omitted for clarity. Selected bond lengths [pm] and
angles [8]: Fe1-B1 197.9(3),B1-C3 149.0(4),C3-C4 137.1(3); C3-B1-C4
54.75(16),Fe1-B1-C3 152.8(2),Fe1-B1-C4 152.5(2).
Figure 2. a) Calculated structure of 2M and comparison of theoretical
and experimental (in parentheses) bond lengths [pm]. b) Contour-line
2
diagram 5 1(r) for 2M in the BC2 ring plane. Solid lines indicate areas
2
The nature of the bonding interactions of the boron atom was
investigated with the AIM (atoms in molecules)[20] method
and with the EDA (energy decomposition analysis) partition-
ing procedure,[19e,f,21] which we have used previously to
investigate chemical bonds in boron compounds.[22,23]
of charge concentration (5 1(r)<0),while dashed lines show areas of
2
charge depletion (5 1(r)>0). The solid lines connecting atomic nulei
are bond paths. The solid lines separating atomic basins indicate zero-
flux surfaces crossing the molecular planes.
Figure 2 shows the optimized geometry of 2M and the
closed-shell donor–acceptor bond, which was previously
found for transition-metal complexes with Group 13 diyl
ligands ER (E = B–Tl).[24] Compound 2M is clearly charac-
terized by the AIM method as a three-membered cyclic
2
contour line diagrams of the Laplacian distribution 5 1(r) in
the plane of the three-membered ring. The calculated bond
lengths for 2M are in very good agreement with the
experimental values for 2. Note that the conformation of
ꢀ
ꢀ
species possessing two B C and one C C bond path and one
BC2 ring critical point.
ꢀ
2M with regard to rotation about the B Fe bond is different
from the conformation found in the experimental structure of
2. In the latter compound, both CO ligands are on the same
side of the BC2 ring (Figure 1), while in 2M they are on
opposite sides of the ring plane. We calculated 2M using the
conformation of 2 as the starting geometry. The geometry
optimization yielded a shallow minimum, which is 0.9 kcal
molꢀ1 higher in energy than the structure shown in Figure 2.
The calculated bond lengths of the
More detailed information about the bonding situation at
the boron atom in 2M is given by the EDA results, which are
ꢀ
summarized in Table 1. We analyzed the B Fe bond using two
different fragments that are in accord with an electron-
sharing bond and a donor–acceptor bond, respectively. The
interacting fragments for the electron-sharing bond are {(h5-
C5H5)(OC)2Fe} and BC2(SiH3)2 in the electronic doublet
two conformations were nearly
Table 1: Results of the EDA for 2M at BP86/TZ2P. Energy values in kcalmolꢀ1
.
ꢀ
identical, and the C O stretching
frequencies differed by less than
1 cmꢀ1. Therefore, we used the
optimized structure shown in
Figure 2 for the analysis of the DEint
ꢀ75.4
ꢀ225.8
ꢀ229.8
DEPauli
DEelstat
152.0
276.8
445.8
bonding situation.
[a]
ꢀ127.3 (56.0%)
ꢀ100.2 (44.0%)
ꢀ89.4 (89.2%)
ꢀ10.8 (10.8%)
4.6
ꢀ341.4 (67.9%)
ꢀ161.2 (32.1%)
ꢀ148.4 (92.1%)
ꢀ12.8 (7.9%)
9.3
ꢀ252.1 (37.3%)
ꢀ423.4 (62.6%)
ꢀ393.4 (92.9%)
ꢀ30.0 (7.1%)
161.3
The Laplacian distribution of
[a]
DEorb
ꢀ
2M in the B Fe bonding region
[b]
[b]
DEs
DEp
exhibits an area of charge concen-
2
tration at the boron atom (5 1(r) <
DEprep
DE (=ꢀDe)
0, solid lines), which has the shape
of a droplet-like appendix directed
towards the iron atom, while the
ꢀ70.8
ꢀ216.5
ꢀ68.5
fragments[c]
{Fe(CO)2Cp}(d)
BC2Si2H6(d)
[Fe(CO)2Cp]+(s)
[BC2Si2H6]ꢀ(s)
{B Fe(CO)2Cp}(os)
ꢀ
C2Si2H6(os)
iron end carries an area of charge
[a] The percentage values in parentheses give the contribution to the total attractive interactions
DEelstat +DEorb. [b] The percentage values in parentheses give the contribution to the total orbital
interactions DEorb. [c] d=doublet; s=singlet; os=open-shell singlet.
2
depletion
(5 1(r) > 0,
dashed
lines). This situation is typical for a
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Angewandte
Chemie
2: A yellow solution of 1 (0.050 g, 0.11 mmol) and 1,2-bis(trime-
states, while the fragments for the donor–acceptor bond are
[(h5-C5H5)(OC)2Fe]+ and [BC2(SiH3)2]ꢀ in the electronic
singlet states.
thylsilyl)ethyne (0.034 g, 0.20 mmol) in benzene (0.5 mL) was irradi-
ated for 0.5 h at room temperature. All volatiles were removed
in vacuo, and the dark brown residue was extracted with hexane
(0.5 mL). The red solution was filtered and stored overnight at
ꢀ308C. The solution was then decanted from crystallized [Cr(CO)6]
and filtered through silica gel, and all volatiles were removed
in vacuo, yielding 2 as a red solid (0.017 g, 35%). IR (hexane): n˜ =
ꢀ
The calculations suggest that the B Fe bond in 2M is
rather strong. The theoretically predicted bond-dissociation
energy (BDE) is De = 70.8 kcalmolꢀ1. The EDA results
ꢀ
indicate that the attractive B Fe interactions mainly result
1
1983 (s; CO), 1927 (s; CO) cmꢀ1; H NMR (500 MHz, C6D6, 178C,
ꢀ
from electrostatic attraction. The covalent character of the B
TMS): d = 1.63 (s, 15H; C5Me5), 0.42 ppm (s, 18H; SiMe3);
13C{1H} NMR (125.8 MHz, C6D6, 178C, TMS): d = 217.3 (s; CO),
94.95 (s; C5Me5), 10.30 (s; C5Me5), 0.01 ppm (s; SiMe3), BC
resonances were not observed; 11B NMR (64.22 MHz, C6D6, 178C,
Et2O·BF3): d = 63.5 ppm (w1/2 = 170 Hz). Elemental analysis (%)
calcd for C20H33BFeO2Si2: C 56.09, H 7.77; found: C 55.40, H 7.47.
Fe bond in 2M is less pronounced, as shown by the percentage
contribution of the orbital interactions in the electron-sharing
bond (44.0%) and the donor–acceptor bond (32.1%). The
most interesting EDA data come from the breakdown of the
orbital term DEorb into s and p contributions. Table 1 shows
that the strength of DEorb(p) in 2M is only ꢀ10.8 kcalmolꢀ1
when the electron-sharing model is employed. The DEorb(p)
Received: January 29, 2007
Published online: May 30, 2007
ꢀ
value for a B Fe donor–acceptor bond is very similar
(ꢀ12.8 kcalmolꢀ1). The weakness of the Fe!B p bonding in
Keywords: borirenes · boron · boryl complexes ·
borylene complexes · density functional calculations
2M becomes obvious when its DEorb(p) value is compared
.
with the previous EDA results for the borylene complexes
[23]
ꢀ
ꢀ
[(CO)4Fe BR]. The DEorb(p) values for the axial Fe BR
bonds are between ꢀ22.1 kcalmolꢀ1 (R = Cp) and ꢀ52.2 kcal
molꢀ1 (R = phenyl).[22a]
ꢀ
Table 1 also gives the EDA results for the B C2(SiH3)2
bonds in 2M using the appropriate fragments in open-shell
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ꢀ
a similar value as the BDE for the B Fe bond (De =
70.8 kcalmolꢀ1), which could be interpreted as an indication
of a similar bond strength. However, the intrinsic strength of
ꢀ
the B C2(SiH3)2 interactions is much higher. This effect is
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ꢀ
shows a much larger value for the B C2(SiH3)2 bonding
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interactions in 2M (DEint = ꢀ229.8 kcalmolꢀ1) than for the
B Fe bond (DEint = ꢀ75.4 kcalmolꢀ1). The preparation ener-
ꢀ
gies of the fragments {BFe(CO)2Cp} and C2(SiH3)2 are very
large (DEprep = 161.3 kcalmolꢀ1), particularly for the latter
species, because the closed-shell molecule is excited into an
ꢀ
ꢀ
open-shell singlet diradical. Unlike the B Fe bond, the B
C2(SiH3)2 bonds in 2M have a larger covalent character
(62.6%) than electrostatic character (37.3%). Note that the
ꢀ
calculated values for the B C2(SiH3)2 p interactions in the
three-membered ring (ꢀ30.0 kcalmolꢀ1) are clearly stronger
ꢀ
than the B Fe p interactions, which indicates substantial
cyclic delocalization.
In conclusion, 1 can be used as a source for the metal-
loborylene {B Fe(CO)2(h -C5Me5)}. Reaction with 1,2-bis-
(trimethylsilyl)ethyne under photolytic conditions leads to
the formation of the metalloborirene 2. Both spectroscopic
data and theoretical calculations suggest a bonding situation
in 2 with significant p delocalization in the borirene ring and
5
ꢀ
ꢀ
no relevant Fe B dp–pp back-bonding.
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Experimental Section
All manipulations were performed in an atmosphere of dry argon
using standard Schlenk line and glovebox techniques. Photolysis
experiments were performed with quartz NMR tubes using a Hg/Xe
arc lamp (400–550 W) equipped with IR filters as the light source.
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Communications
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[17] The crystal data for 2 were collected on a Bruker x8 apex
diffractometer with
a CCD area detector and multilayer
mirror monochromated MoKa radiation. The structure was
solved using direct methods, refined with the Shelx software
package (G. Sheldrick, Universität Göttingen, 1997), and
expanded using Fourier techniques. All non-hydrogen atoms
were refined anisotropically. Hydrogen atoms were assigned
idealized positions and were included in structure-factor calcu-
lations. Crystal data for 2: C20H33BFeO2Si2, Mr = 428.30, yellow
3
¯
block, 0.18 ” 0.12 ” 0.05 mm , triclinic, space group P1, a =
7.0700(3), b = 9.8671(4), c = 17.4633(7) Š, a = 86.591(2), b =
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=
1.186 gcmꢀ3, m = 0.739 mmꢀ1, F(000) = 456, T= 100(2) K, R1 =
0.0749, wR2 = 0.1678, 11075 independent reflections (2q =
74.348) and 246 parameters. CCDC-633058 (2) contains the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
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[19] a) The geometry of the molecule was first optimized without
symmetry constraints (C1). The optimized structure had nearly
Cs symmetry. Reoptimization of 2M with a Cs-symmetry con-
straint gave a structure which was only slightly (< 0.2 kcalmolꢀ1
)
higher in energy than the C1 energy minimum. The Cs structure
was used for the EDA in order to divide the orbital interactions
into s (a’) and p (a’’) contributions. All calculations were carried
out at the BP86 level: b) A. D. Becke, Phys. Rev. A 1988, 38,
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the basis sets in this work have TZ2P quality using uncontracted
Slater-type orbitals (STOs) as basis functions: d) J. G. Snijders, P.
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