Preparation and structural characterization of transition metal complexes featuring the ferrocenyl(bromo)boryl ligand

Article abstract of DOI:10.1021/om049395b

Transition metal complexes that feature the ferrocenyl(bromo)boryl ligand, -B(Fc)Br, were prepared utilizing salt-elimination and B-Br bond oxidative addition strategies. The stabilizing interaction that causes bending of the boron atom toward the ferrocenyl iron center in the FcBBr₂ precursor (1) weakens upon coordination of boron to the [(η⁵-C ₅R₅)-(OC)₂Fe] and trans-[(Cy₃P) ₂PtBr] fragments. The efficient π-back-bonding capability of the latter is reflected in the absence of any Fe-B interaction in crystalline trans-[(Cy₃P)₂Pt-(Br){B(Fc)Br}] (7), the first platinum haloboryl complex featuring a non-heteroatom-stabilized boryl ligand to be fully characterized and structurally authenticated. In the half-sandwich complexes [(η⁵-C₅R₅)(OC)₂Fe{B(Fc)Br}] (R = H, 2; R = Me, 3) different relative orientations of the [(η⁵- C₅R₅)(OC)₂Fe] and -B(Fc)Br groups are accompanied by different degrees of bending within the boryl ligand. The sensitivity of this parameter to the electronic environment at boron offers an indirect qualitative estimation of the π-bonding component of the TM-B bond.

Full text of DOI:10.1021/om049395b

Organometallics 2004, 23, 5545-5549
5545
Preparation and Structural Characterization of
Transition Metal Complexes Featuring the
Ferrocenyl(bromo)boryl Ligand†
Holger Braunschweig,* Krzysztof Radacki, Daniela Rais, and Fabian Seeler
Institut fu¨r Anorganische Chemie, Universita¨t Wu¨rzburg, Am Hubland,
D-97074, Wu¨rzburg, Germany
Received August 4, 2004
Transition metal complexes that feature the ferrocenyl(bromo)boryl ligand, -B(Fc)Br, were
prepared utilizing salt-elimination and B-Br bond oxidative addition strategies. The
stabilizing interaction that causes bending of the boron atom toward the ferrocenyl iron
center in the FcBBr2 precursor (1) weakens upon coordination of boron to the [(η5-C5R5)-
(OC)2Fe] and trans-[(Cy3P)2PtBr] fragments. The efficient π-back-bonding capability of the
latter is reflected in the absence of any Fe-B interaction in crystalline trans-[(Cy3P)2Pt-
(Br){B(Fc)Br}] (7), the first platinum haloboryl complex featuring a non-heteroatom-stabilized
boryl ligand to be fully characterized and structurally authenticated. In the half-sandwich
complexes [(η5-C5R5)(OC)2Fe{B(Fc)Br}] (R ) H, 2; R ) Me, 3) different relative orientations
of the [(η5-C5R5)(OC)2Fe] and -B(Fc)Br groups are accompanied by different degrees of
bending within the boryl ligand. The sensitivity of this parameter to the electronic
environment at boron offers an indirect qualitative estimation of the π-bonding component
of the TM-B bond.
Introduction
On account of the electronic stabilization offered by
π-donating groups to the Lewis acidic boron center, it
is not surprising that the majority of transition metal
boryl complexes reported to date feature oxy- or ami-
noboryl ligands.1a Alkyl or arylboryl compounds, includ-
ing asymmetric aryl(halo)boryl species,1c still remain
relatively rare. During the course of our studies on the
synthesis and reactivity of haloboryl compounds,3 we
turned our attention to complexes featuring the ferro-
cenyl(bromo)boryl ligand -B(Fc)Br. In light of pre-
vious reports on the electronic stabilization provided by
the iron center to the Lewis acidic boron atom in
dibromoboryl-,4b 1,1′-bis(dibromoboryl)-,4d and 1,1′,3,3′-
tetrakis(dibromoboryl)ferrocene,4e we wondered to
what extent this interaction would be affected by any
[TM]fB(Fc)Br π-back-donation and, in particular,
whether the ferrocenyl(bromo)boryl group could serve
as a probe for the presence of any significant transition
metal-boron π-bonding. Herein we report the synthesis
and structural characterization of iron compounds
[(η5-C5H5)(OC)2Fe{B(Fc)Br}] (2) and [(η5-C5Me5)(OC)2-
Fe{B(Fc)Br}] (3), the synthesis of the ruthenium species
[(η5-C5H4Me)(OC)2Ru{B(Fc)Br}] (4), and the full char-
acterization of trans-[(Cy3P)2Pt(Br){B(Fc)Br}] (7), the
first structurally authenticated platinum haloboryl
complex that features a non-heteroatom-stabilized boryl
ligand.
The nature of the metal-boron bond in transition
metal boryl compounds has been the subject of intense
structural and theoretical investigation.1 Particular
emphasis has been placed on attempts to quantify the
relative contributions provided to the metal-boron
linkage by σ- and π-bonding components.2 The majority
of the studies attribute strong σ-donor abilities to the
boryl group. However, the π-acidity of the -BR2 ligands
appears to be highly dependent on the electronic prop-
erties of the transition metal complex fragment and of
the substituents at boron. The effect of π-accepting
ancillary ligands coordinated to the transition metal
center and of π-donating groups linked to the boron
atom generally predominates over any [TM]fBR2 π-back-
donation. Consequently, such interaction is thought to
provide a minor contribution to the overall stability of
the metal-boron bond.2
† Dedicated to Prof. Dr. Johann Weis on the occasion of his 60th
birthday.
* To whom correspondence should be addressed. E-mail:
h.braunschweig@mail.uni-wuerzburg.de. Fax: +49 931 888 5260.
Tel: +49 931 888 4623.
(1) (a) Irvine, G. J.; Lesley, M. G. J.; Marder, T. B.; Norman, N. C.;
Rice, C. R.; Robins, E. G.; Roper, W. R.; Whittell, G. R.; Wright, L. J.
Chem. Rev. 1998, 98, 2685. (b) Braunschweig, H.; Colling, M. Coord.
Chem. Rev. 2001, 223, 1. (c) Aldridge, S. Coombs, D. L. Coord. Chem.
Rev. 2004, 248, 535. (d) Braunschweig, H. Angew. Chem., Int. Ed. 1998,
37, 1786. (e) Braunschweig, H.; Kollann, C.; Englert, U. Eur. J. Inor.
Chem. 1998, 465. (f) Braunschweig, H.; Kollann, C.; Klinkhammer,
K. W. Eur. J. Inor. Chem. 1999, 1523. (g) Braunschweig, H.; Colling,
M.; Kollann, C.; Englert, U. Dalton Trans. 2002, 2289.
(3) Braunschweig, H.; Radacki, K.; Seeler, F.; Whittell, G. R.
Organometallics 2004, 23, 4178.
(2) (a) Frenking, G.; Fro¨hlich, N. Chem. Rev. 2000, 100, 717. (b) Giju,
K. T.; Bickelhaupt, F. M.; Frenking, G. Inorg. Chem. 2000, 39, 4776.
(c) Sivignon, G.; Fleurat-Lassard, P.; Onno, J. M.; Volatron, F. Inorg.
Chem. 2002, 41, 6656. (d) Cundari, T. R.; Zhao, Y. Inorg. Chim. Acta
2003, 345, 70. (e) Dickinson, A. A.; Willock, D. J.; Calder, R. J.;
Aldridge, S. Organometallics 2002, 21, 1146.
(4) (a) Renk, W.; Ruf, W.; Siebert, W. J. Organomet. Chem. 1976,
120, 1. (b) Appel, F.; Ja¨kle, F.; Priermeier, T.; Schmid, R.; Wagner, M.
Organometallics 1996, 15, 1188. (c) Ruf, W.; Renk, T.; Siebert, W. Z.
Naturforsch. 1976, 31B, 1028. (d) Wrackmeyer, B.; Do¨rfler, U.; Milius,
W.; Herberhold, M. Polyhedron 1995, 14, 1425. (e) Appel, A.; No¨th,
H.; Schmidt, M. Chem. Ber. 1995, 128, 621.
10.1021/om049395b CCC: $27.50 © 2004 American Chemical Society
Publication on Web 10/12/2004

5546 Organometallics, Vol. 23, No. 23, 2004
Braunschweig et al.
Scheme 1
Results and Discussion
Iron and Ruthenium Ferrocenyl(bromo)boryl
Compounds. Iron and ruthenium boryl complexes 2-4
were synthesized in moderate to good yields (29-77%)
via reaction of dibromoborylferrocene FcBBr2 (1)4a with
1 equiv of the appropriate organometallic anion in
toluene at room temperature (Scheme 1).
Figure 1. Molecular structure of [(η5-C5H5)(OC)2-
Fe{B(Fc)Br}] (2). Relevant bond lengths (Å): Fe(1)-B(1)
)
1.997(2), Br(1)-B(1)
) 1.9892(22), C(21)-B(1) )
1.5461(31).
Complexes 2-4 are highly oxygen- and moisture-
sensitive red solids but are stable at room temperature
for several days under an inert atmosphere. They are
soluble in toluene but considerably less soluble in
aliphatic hydrocarbons, such as hexane or heptane. The
spectroscopic features of the new compounds are con-
sistent with the presence of a ferrocenyl (bromo)boryl
group directly linked to the transition metal center. In
particular, the 11B{1H} NMR spectra of 2-4 display
characteristic low-field resonances at δ 99.1, 103.0, and
90.1, respectively. Such values are similar to those of
previously reported aryl(halo)boryl complexes, namely,
[(η5-C5R4R′)(CO)2Fe{B(2,4,6-Me3C6H2)Br}] [R ) R′ ) H,
5a, δ 111.4; R ) H, R′ ) Me, 5b, δ 111.3; R ) R′ ) Me,
5c, δ 113.2]5 and [(η5-C5Me5)(CO)2Fe{B(Ph)Cl}] (6) [δ
111.0].6 Compounds 2 and 4 exhibit carbonyl stretching
bands at relatively high wavenumbers, 2015, 1955 cm-1
and 2021, 1958 cm-1, respectively, which resemble those
reported for 5a-c. The analogous bands for compound
3 (1995, 1934 cm-1) are moderately red-shifted and are
very similar to those of 6 (1995, 1929 cm-1).
Single crystals suitable for X-ray structural determi-
nation were obtained by layering heptane on a saturated
solution of 2 in toluene at room temperature and from
a toluene solution of 3 upon cooling to -30 °C. The
molecular structures of 2 and 3 with relevant bond
lengths are shown in Figures 1 and 2, respectively.
The asymmetric unit of 2 contains one unique mol-
ecule, while in that of 3 two independent molecules, 3a
and 3b, are observed. The iron-boron bond distance in
2 [Fe(1)-B(1) ) 1.997(2) Å] is slightly longer than that
in 3 [Fe(12)-B(1) ) 1.972(3) Å, 3a; Fe(22)-B(2) )
1.985(3) Å, 3b] but still in the range of those observed
in mesityl(bromo)boryls 5a-c [1.964(5) Å, 5a; 1.962(4)
Å, 5b; 1.972(2) Å, 5c]5 and in phenyl(chloro)boryl
complex 6 [2.005(10) Å].6 Interestingly, significant dif-
ferences between the two structures arise when the
relative orientations of the [(η5-C5R5)(OC)2Fe] (R ) H,
2; R ) Me, 3) fragments with respect to the boryl ligand
are inspected. In 2, the angle of intersection (θ ) 15.2°)
between the planes defined by the C5H5-ring centroid-
Fe(1)-B(1) and Br(1)-B(1)-C(21) indicates a nearly
Figure 2. Molecular structure of [(η5-C5Me5)(OC)2-
Fe{B(Fc)Br}] (3a). Relevant bond lengths (Å): Fe(12)-B(1)
) 1.972(3), Br(1)-B(1) ) 2.0158(27), C(122)-B(1) )
1.5384(36).
coplanar arrangement of the two fragments. The cor-
responding angles for 3a and 3b, 88.8° and 89.0°,
respectively, determine a virtually orthogonal disposi-
tion of the two groups. It has been previously argued
that the relative orientation of the [(η5-C5R5)(OC)2Fe]
and BX2 units can serve as a probe of π-type interac-
tions.2e These occur preferentially via overlap of the
vacant boryl-based π-orbital with the HOMO of the
transition metal fragment (θ ) 0°) or, less favorably,
with the lower lying perpendicular HOMO-2 (θ ) 90°).
On these grounds, the structural differences between 2
and 3 suggest the possibility for more significant
[(η5-C5R5)(OC)2Fe]fB(Fc)Br π-back-donation in 2. This
hypothesis is corroborated by the analysis of relevant
structural features within the boryl ligand.
The geometry and electronic properties of borylfer-
rocenes have been the subject of structural and theo-
retical investigations.4b A common feature of the
molecular structure of FcBBr2 (1),4b 1,1′-bis(dibromo-
boryl)ferrocene,4d and 1,1′,3,3′-tetrakis(dibromoboryl)-
ferrocene4e is the bending of the -BBr2 substituent(s)
toward the ferrocenyl iron center. The degree of sub-
stituent bending was estimated analyzing the values of
the dip angle R*, which is defined as the angle between
the centroid of the C5H5 ring, the ipso-carbon, and the
boron atom. Such values decrease with increasing
number of boryl substituents on the ferrocenyl moiety,
(5) Coombs, D. L.; Aldridge, S.; Jones, C. Dalton Trans. 2002, 3851.
(6) Coombs, D. L.; Aldridge, S.; Coles, S. J.; Hursthouse, M. B.
Organometallics 2003, 22, 4213.

Ferrocenyl(bromo)boryl Ligand
Organometallics, Vol. 23, No. 23, 2004 5547
going from 17.7° and 18.9° for 1 (two crystallographi-
cally independent molecules were present in the asym-
metric unit of 1)4b to 9.1° for 1,1′-bis(dibromoboryl)-
ferrocene4d and 6.8° (0.1°) for 1,1′,3,3′-tetrakis(dibro-
moboryl)ferrocene.4e This evidence was interpreted in
terms of an interaction between filled d-type orbitals
at iron and the empty p-orbital on boron, capable of
relieving, at least partially, the electron deficiency of
the latter. Such reasoning found further support in the
molecular structure of FcB(NiPr2)2, where electronic
stabilization of the boron center is achieved by N-B
π-bonding and no bending of the boryl substituent
toward the iron atom is observed.4b
Upon coordination of the -B(Fc)Br unit to a transition
metal complex fragment, the electronic deficiency of the
boron center could be expected to be partially relieved
by any π-component of the overall TM-B bond. Inspec-
tion of the molecular structures of 2 and 3 reveals
smaller values of the dip angle R* than in 1, namely,
4.2° for 2 and 8.0° and 7.6° for 3a,b, respectively. When
considered in conjunction with the values of θ discussed
above for π-back-donation arguments, the R* values sug-
gest, consistently, that a more significant [(η5-C5R5)-
(OC)2Fe]fB(Fc)Br π-interaction occurs in 2 than in 3.
This conclusion may seem counterintuitive at first, on
account of the stronger σ-donating properties of the
(η5-C5Me5) ligand with respect to those of (η5-C5H5).
However, the attainment of a coplanar arrangement of
the [(η5-C5R5)(OC)2Fe] and BX2 units, which would allow
optimal π-orbital overlap, might be disfavored in 3 on
steric grounds, due to the increased bulk of the pen-
tamethyl(cyclopentadienyl) ligand. The diminished FefB
π-back-donation would then be reflected in a higher
degree of bending within the ferrocenylboryl substituent
in 3.
Figure 3. Molecular structure of trans-[(Cy3P)2Pt(Br)-
{B(Fc)Br}] (7). Cyclohexyl groups have been omitted for
clarity. Relevant bond lengths (Å): Pt(1)-B(1) ) 1.9963-
(34), Br(2)-B(1) ) 2.0040(35), C(41)-B(1) ) 1.5487(49).
Scheme 2
ucts. The 11B{1H} NMR spectrum of 7 is characterized
by an extremely broad resonance (ω1/2 ) ∼1980 Hz)
centered at δ 82, while the 31P{1H} NMR spectrum
exhibits a sharp singlet at δ 21.51 (1JP-Pt ) 2892 Hz),
significantly upfield shifted when compared to that of
[Pt(PCy3)2] [δ 62.28,1JP-Pt ) 4161 Hz].8b The presence
of the ferrocenyl(bromo)boryl ligand is confirmed in the
1H NMR spectrum by two multiplets at 4.94 and 4.32
ppm, each integrating 2H, that are assigned to the
protons of the boron-substituted cyclopentadienyl ring.
The protons of the unsubstituted cyclopentadienyl ring
resonate as a singlet at 4.22 ppm.
Single crystals of 7 suitable for X-ray structural
determination were obtained by slow evaporation of a
1:1 mixture of benzene and hexane at room tempera-
ture. The molecular structure of 7 with relevant bond
lengths is displayed in Figure 3. It reveals a square
planar platinum center (maximum deviation from the
PtP2BBr plane ) 0.1274 Å), with the ferrocenyl(bromo)-
boryl and the bromide ligands adopting a mutually trans
arrangement. Such a configuration is consistent with
the high trans influence of the boryl ligand and is com-
mon to the other structurally authentic monoboryl plati-
num complexes, namely, trans-[(Ph3P)2Pt(Cl){B(cat)}]
(8)7b and trans-[(Ph3P)2Pt(Cl){BCl(NMe2)}] (9).7a Ac-
cordingly, the Pt(1)-Br(1) bond distance [2.6183(8) Å]
in 7 is considerably longer than, for instance, the ana-
logous distance in trans-[(Cy3P)2PtBr2] [2.435(1) Å].9
The ferrocenyl(bromo)boryl group is oriented in a
nearly orthogonal position (85.0°) with respect to the
mean square plane containing the platinum center. It
has been previously noted that such an orientation
allows for maximum overlap between the filled platinum
Platinum Ferrocenyl(bromo)boryl Compound
trans-[(Cy3P)2Pt(Br){B(Fc)Br}]. With the results
obtained on the synthesis of compounds 2-4 in hand,
we turned our attention to the possibility of introducing
the ferrocenyl(bromo)boryl group into a more electron-
rich system. Preparation of a platinum(II) compound
seemed to be a potentially rewarding target, particularly
given the paucity of structurally characterized monobo-
ryl complexes of platinum7 and the absence of any such
compound featuring a non-heteroatom-stabilized (halo)-
boryl ligand.
Reaction of FcBBr2 (1) with the Pt0 precursor
[Pt(PCy3)2]8a in benzene at room temperature led to
oxidative addition of one B-Br bond of 1 and subse-
quent isomerization to the bright orange PtII ferrocenyl-
(bromo)boryl complex trans-[(Cy3P)2Pt(Br){B(Fc)Br}] (7)
(Scheme 2).
Complex 7 is soluble in benzene, methylene chloride,
and, to a lesser extent, hexane. It is only moderately
oxygen-sensitive but extremely moisture-sensitive, be-
ing rapidly decomposed by traces of water to trans-
1
[(Cy3P)2Pt(H)(Br)] (as judged by H and 31P{1H} NMR
spectroscopy) and unidentified boron-containing prod-
(7) (a) Curtis, D.; Lesley, M. G. J.; Norman, N. C.; Orpen, A. G.;
Starbuck, J. Dalton Trans. 1999, 1687. (b) Clegg, W.; Lawlor, F. J.;
Lesley, G.; Marder, T. B.; Norman, N. C.; Orpen, A. G.; Quayle, M. J.;
Rice, C. R.; Scott, A. J.; Souza, F. E. S. J. Organomet. Chem. 1998,
550, 183.
(8) (a) Otsuka, S.; Yoshida, T.; Matsumoto, M.; Nakatsu, K. J. Am.
Chem. Soc. 1977, 98, 5850. (b) Mann, B. E.; Musco, A. Dalton Trans.
1980, 776.
(9) Cameron, T. S.; Clark, H. C.; Linden, A.; Nicholas, A. M.
Polyhedron 1990, 9, 1683.

5548 Organometallics, Vol. 23, No. 23, 2004
Braunschweig et al.
dxy-orbital and the empty p-orbital on boron, accounting
for any platinum to boron π-back-donation.1a Although
steric arguments are likely to be of some importance in
determining the relative ligand orientations, the plati-
num-boron bond distance Pt(1)-B(1) [1.9963(34) Å] in
7, being shorter than that found in 8 [2.008(8) Å] and 9
[2.075(10) Å], corroborates the presence of a π-contribu-
tion to the bonding between the electron-rich [(Cy3P)2-
PtBr] fragment and the Lewis acidic boron center.
Further qualitative evidence is provided by a compari-
son of the estimated and experimentally determined
Pt-B bond distance in 7. The absence of any structur-
ally authentic alkyl derivative containing the trans-
[(Cy3P)2PtBr] fragment unfortunately prevents a more
accurate comparison. However, given the poor π-acidic
properties of the bromide ligand, an estimated value for
the covalent radius of the trans-[(Cy3P)2PtBr] fragment
(1.295 Å) can be derived by subtraction of the covalent
radius for bromine (1.14 Å) from the Pt-Br bond
distance in trans-[(Cy3P)2PtBr2].9 A similar procedure,
applied to FcBBr2 (1),10 yields a value of 0.792 Å for the
covalent radius of the -B(Fc)Br group. The sum of the
covalent radii for 7 then amounts to 2.09 Å, which is
considerably larger than the experimental value of
1.9963(34) Å, consistent with the presence of a π-com-
ponent to the platinum-boron bond.
This hypothesis is confirmed by analysis of the
structural parameters within the ferrocenylboryl group.
The value of the R* angle in 7 (-6.5°) indicates the
absence of any interaction between the boron and the
iron atoms, in line with sufficient electronic stabilization
of the boron center provided by the trans-[(Cy3P)2PtBr]
fragment. However, even if any direct steric interaction
between the bulky tricyclohexylphosphine and the boryl
ligand is not immediately apparent upon inspection of
the crystal structure, the possibility of a more subtle
influence of the phosphine over the intrinsic orientation
of the ferrocenylboryl group must be borne in mind.
no bending of the boron atom toward the iron center
within the boryl group, in line with the presence of a
significant π-component to the overall Pt-B bond.
Experimental Section
1. General Considerations. All manipulations were con-
ducted either under an atmosphere of dry argon or in vacuo
using standard Schlenk line or glovebox techniques. Solvents
(toluene, benzene, and hexane) were purified by distillation
from appropriate drying agents (sodium and sodium wire)
under dry argon, immediately prior to use. Deuterated solvents
(C6D6 and CD2Cl2) were degassed by three freeze-pump-thaw
cycles and stored over molecular sieves in the glovebox. IR
spectra for compounds 2-4 were recorded as toluene solutions
between KBr plates on a Bruker Vector 22 FT-IR-spectrom-
eter. 1H and 13C{1H} NMR spectra were acquired on a Bruker
AMX 400 NMR spectrometer at 400.14 and 100.63 MHz,
respectively, and referenced to external TMS via the residual
protio solvent (1H) or the solvent itself (13C). 11B{1H} and
31P{1H} NMR spectra were recorded on a Bruker Avance 200
NMR spectrometer at 64.22 and 81.02 MHz, respectively, and
referenced to external BF3‚OEt2 and 85% H3PO4. Microanaly-
ses for C, H, and N were performed by Mr. C. P. Kneis
(University of Wuerzburg) on a Leco CHNS-932 instrument.
2. Synthetic Procedures. [(η5-C5H5)(OC)2Fe{B(Br)Fc}]
(2). K[(η5-C5H5)Fe(CO)2] (0.43 g, 1.99 mmol) was suspended
in toluene (5 mL), and a solution of dibromoborylferrocene (1)
(0.70 g, 1.97 mmol) in toluene (10 mL) was added. After
stirring for 30 min, all volatiles were removed in vacuo and
the residue was treated with hexane (20 mL). The remaining
solid was removed by centrifugation. The red supernatant
solution was concentrated in vacuo to 10 mL and stored at
1
-30 °C for 2 days, yielding 2 as a red solid (0.69 g, 77%). H
NMR (C6D6): δ 4.74 (m, 2H, C5H4B), 4.46 (m, 2H, C5H4B), 4.17
(s, 5H, C5H5), 4.13 (s, 5H, C5H5). 13C{1H} NMR (C6D6): δ 215.3
(CO), 85.1 (C5H5), 77.3 (C5H5FeC5H4B), 75.2 (C5H5FeC5H4B),
69.8 (C5H5FeC5H4B). 11B{1H} NMR (C6D6): δ 99.1 (br, ω1/2
)
∼340 Hz, s). IR ν(CdO) 2015 (vs), 1955 (vs) cm-1. Anal. Calcd
for C17H14BBrFe2O2: C, 45.10; H, 3.12. Found: C, 45.17; H,
3.33.
[(η5-C5Me5)(OC)2Fe{B(Br)Fc}] (3). Na[(η5-C5Me5)Fe(CO)2]
(0.48 g, 1.78 mmol) was suspended in toluene (5 mL), and a
solution of dibromoborylferrocene (1) (0.63 g, 1.77 mmol) in
toluene (10 mL) was added. After stirring for 30 min, all
volatiles were removed in vacuo and the residue was treated
with hexane (20 mL). The remaining solid was removed by
centrifugation. The red supernatant solution was concentrated
in vacuo to 10 mL and stored at -30 °C for 2 days, yielding 3
Conclusions
Coordination of the ferrocenyl(bromo)boryl ligand
-B(Fc)Br to different transition metal complex frag-
ments provides some insight into their π-back-bonding
abilities, thanks to the intrinsic orientational depen-
dence of the ferrocenylboryl ligand on the electronic
situation at boron. The crystal structures of iron com-
pounds 2 and 3 suggest that differences in the steric
bulk of the (η5-C5R5) ligand are likely to affect the
relative orientations of the transition metal and boryl
units, limiting the degree of π-bonding in the (η5-C5Me5)
derivative 3. This is accompanied by a more pronounced
bending of the boron atom toward the iron of the
ferrocenyl moiety in 3, a feature that has been previ-
ously interpreted in terms of a stabilizing interaction
between filled d-type orbitals at iron and the electron-
deficient boron center.
1
as a red solid (0.27 g, 29%). H NMR (C6D6): δ 4.80 (m, 2H,
C5H4B), 4.46 (m, 2H, C5H4B), 4.22 (s, 5H, C5H5), 1.51 (s,
15H, C5Me5). 13C{1H} NMR (C6D6): δ 217.3 (CO), 95.7 (C5Me5),
77.5 (C5H5FeC5H4B), 75.1 (C5H5FeC5H4B), 69.8 (C5H5FeC5H4B),
9.5 (C5Me5). 11B{1H} NMR (C6D6): δ 103.0 (br, ω1/2 ) ∼500 Hz,
s). IR ν(CdO) 1995 (vs), 1934 (vs) cm-1. Anal. Calcd for
C22H24BBrFe2O2: C, 50.54; H, 4.63. Found: C, 50.64; H, 4.72.
[(η5-C5H4Me)(OC)2Ru{B(Br)Fc}] (4). Na[(η5-C5H4Me)Ru-
(CO)2] (0.26 g, 1.00 mmol) were suspended in toluene (5 mL),
and a solution of dibromoborylferrocene (1) (0.36 g, 1.01 mmol)
in toluene (10 mL) was added. After stirring for 30 min, all
volatiles were removed in vacuo and the residue was treated
with hexane (10 mL). The remaining solid was removed by
centrifugation. The orange supernatant solution was concen-
trated in vacuo to 10 mL and stored at -30 °C for 2 days,
yielding 4 as a bright red solid (0.19 g, 37%). 1H NMR (C6D6):
δ 4.73 (m, 2H, C5H4B), 4.62 (m, 4H, C5H4CH3), 4.38 (br s,
2H, C5H4B), 4.20 (s, 5H, C5H5), 1.55 (s, 3H, C5H4CH3).
13C{1H} NMR (C6D6): δ 202.5 (CO), 108.0 (Ci, C5H4CH3),
89.1 (C5H4CH3), 88.2 (C5H4CH3), 77.7 (C5H5FeC5H4B), 75.2
(C5H5FeC5H4B), 69.8 (C5H5FeC5H4B), 13.1 (C5H4CH3). 11B{1H}
NMR (C6D6): δ 90.1 (br, ω1/2 ) ∼410 Hz, s). IR ν(CdO) 2021
Upon oxidative addition of a B-Br bond of 1 to
[Pt(PCy3)2], complex 7 is synthesized, the first haloboryl
platinum compound containing a non-heteroatom-
stabilized boryl ligand to be fully characterized and
structurally authenticated. Its crystal structure reveals
(10) Br-B π-bonding is known to be very weak: Muetterties, E. L.
The Chemistry of Boron and Its Compounds; Wiley: New York, 1967.

Ferrocenyl(bromo)boryl Ligand
Organometallics, Vol. 23, No. 23, 2004 5549
(vs), 1958 (vs) cm-1. Anal. Calcd for C18H16BBrFeO2Ru: C,
42.22; H, 3.15. Found: C, 42.13; H, 3.35.
The data were corrected for Lorentz and polarization effects,
and an empirical absorption correction based on comparison
of redundant and equivalent reflections was applied with
SADABS.14 The structures were solved via direct methods and
refined with the SHELX software package and expanded using
Fourier techniques.15 All non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were assigned idealized posi-
tions and were included in structure factor calculations.
Crystallographic data for the structural analyses have been
deposited with the Cambridge Crystallographic Data Centre,
CCDC, with codes XXXXX. Copies of this information may be
obtained free of charge from The Director, CCDC, 12 Union
Road, Cambridge, CB2 1EZ [fax +44-1223-336033 or e-mail
deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk].
trans-[(Cy3P)2Pt(Br){B(Br)Fc}] (7). A colorless solution
of [Pt(PCy3)2] (0.055 g, 0.073 mmol) in benzene (0.5 mL) was
added to an orange solution of dibromoborylferrocene (1) (0.026
g, 0.073 mmol) in benzene (0.5 mL). The bright orange solution
was stirred for 1 h at room temperature, after which time it
was layered with hexane (1 mL). Slow evaporation of the
resulting solution afforded 7 as bright orange crystals (0.053
g, 66%). 1H NMR (CD2Cl2): δ 4.69 (m, 2H, C5H4B), 4.46 (m,
2H, C5H4B), 4.16 (s, 5H, C5H5), 2.62 (m, 6H, Cy), 2.13-1.22
(m, 60H, Cy). 13C{1H} NMR (CD2Cl2): δ 77.9 (C5H5FeC5H4B),
72.1 (C5H5FeC5H4B), 69.4 (C5H5FeC5H4B), 36.0 (t, 1JC-P ) 27
2
Hz, Ci, Cy), 31.0 (s, Cy), 30.6 (s, Cy), 28.0 (t, JC-P ) 11 Hz,
2
Cy), 27.9 (t, JC-P ) 11 Hz, Cy), 26.8 (s, Cy). 11B{1H} NMR
(CD2Cl2): δ 82 (ω1/2 ) ∼1980 Hz). 31P{1H} NMR (CD2Cl2): δ
21.5 (s, 1JP-Pt ) 2892 Hz). Anal. Calcd for C46H75BBr2FeP2Pt:
C, 49.77; H, 6.67. Found: C, 50.26; H, 6.92.
Acknowledgment. This work was supported by the
DFG and the EPSRC. D.R. thanks the Alexander von
Humboldt Foundation for a postdoctoral fellowship.
General Procedure for X-ray Crystallography. A crys-
tal of appropriate size was mounted on a glass fiber with
silicone grease. The crystal was transferred to a Bruker
SMART APEX diffractometer with CCD area detector, cen-
tered in the beam, and cooled by a nitrogen flow low-
temperature apparatus to -80 °C. Preliminary orientation
matrix and cell constants were determined by collection of 100
frames, followed by spot integration and least-squares refine-
ment. A hemisphere of data was collected.11 The raw data were
integrated with SAINT.12 Cell dimensions were calculated from
the all reflections. Data analysis was performed with XPREP.13
Supporting Information Available: Text detailing the
structural determinations of 2, 3, and 7. This material is
available free of charge via the Internet at http://pubs.acs.org.
OM049395B
(12) Saint+ NT ver. 6.45, Area-Detector Integration Program;
Bruker Advanced X-ray Solutions, Inc.: Madison, WI, 1997-2003.
(13) XPREP ver. 6.10, Part of the SHELXTL Crystal Structure
Determination Package; Bruker Advanced X-ray Solutions, Inc.: Madi-
son, WI, 1997-2001.
(14) Sheldrick, G. SADABS ver. 2.10, Area Detector Absorption
Correction Program; 2002.
(15) Sheldrick, G. SHELXS-97: structure solution and SHELXL-
97: structure refinement programs; 1997.
(11) SMART NT ver.5.63, Area-Detector Software Package; Bruker
Advanced X-ray Solutions, Inc.: Madison, WI, 1997-2001.