Tuning the electronic structure of diboradiferrocenes

Article abstract of DOI:10.1039/b718789k

A series of diboradiferrocenes with different aryl substituents was prepared through reaction of B,B-dichlorodiboradiferrocene 2 with arylcopper and Grignard reagents. The mesityl and pentafluorophenyl derivatives, 3-Mes and 3-Pf, were fully characterized by multinuclear NMR, MALDI-TOF mass spectrometry and X-ray crystallography, and their electronic structure was examined by UV-visible spectroscopy and cyclic voltammetry. A comparison of the data for 3-Mes and 3-Pf with those of the parent compound 3-Ph revealed the importance of electronic and steric effects of the substituents on the electronic structure of the compounds and ultimately the degree of electronic interaction between the two ferrocene moieties. An unusually large redox splitting of ΔE = 703 mV was determined from the cyclic voltammogram of 3-Pf.

Full text of DOI:10.1039/b718789k

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www.rsc.org/dalton | Dalton Transactions
Tuning the electronic structure of diboradiferrocenes†‡
Krishnan Venkatasubbaiah, Thilagar Pakkirisamy, Roger A. Lalancette and Frieder Ja¨kle*
Received 5th December 2007, Accepted 14th January 2008
First published as an Advance Article on the web 10th July 2008
DOI: 10.1039/b718789k
A series of diboradiferrocenes with different aryl substituents was prepared through reaction of
B,B-dichlorodiboradiferrocene 2 with arylcopper and Grignard reagents. The mesityl and
pentafluorophenyl derivatives, 3–Mes and 3–Pf, were fully characterized by multinuclear NMR,
MALDI-TOF mass spectrometry and X-ray crystallography, and their electronic structure was
examined by UV-visible spectroscopy and cyclic voltammetry. A comparison of the data for 3–Mes and
3–Pf with those of the parent compound 3–Ph revealed the importance of electronic and steric effects
of the substituents on the electronic structure of the compounds and ultimately the degree of electronic
interaction between the two ferrocene moieties. An unusually large redox splitting of DE = 703 mV was
determined from the cyclic voltammogram of 3–Pf.
membered ring system.9–11 However, instead of ortho-phenylene
Introduction
groups, two redox-active ortho-ferrocenylene moieties are fused
to this central ring system. The preparation of 3–Ph involves
treatment of 1,1ꢀ-bis(trimethylstannyl)ferrocene with PhBCl2. A
rearranged 1-stannyl-2-borylferrocene12,13 is initally formed, which
in the presence of Al as a catalyst slowly reacts further to give the
final product. The essentially air-stable bifunctional Lewis acid
3–Ph displays interesting redox behavior as a result of effective
electronic communication between the two iron centers via the tri-
coordinate boron bridge.9,11 Moreover, oxidation of the ferrocene
moieties leads to enhanced Lewis acidity of the organoborane
sites.10 Given these highly unusual properties of 3–Ph we set out to
develop an alternative, more general methodology for the synthesis
of diboradiferrocenes. We report here a novel modular synthetic
approach, in which we take advantage of the facile reaction of
the new halogen-substituted diboradiferrocene fc2B2Cl2 (2) for
the preparation of a family of diboradiferrocenes with tunable
properties.
The incorporation of boron into conjugated cyclic p-systems
continues to be an attractive research objective, both from a
synthetic perspective and due to the unusual optical and electronic
properties that are commonly encountered.1 The possibility of
enhancing the Lewis acidity of organoboranes and thereby the ef-
ficiency in catalysis and chemosensor applications is an intriguing
aspect that has attracted much interest. The working hypothesis
is that, for example, in borole or diborabenzene derivatives
participation of the empty pp orbital on boron in what would
become an antiaromatic 4n p-electron system is energetically
unfavorable; the ensuing diminished pp-p overlap in turn leads to
enhanced Lewis acidity of the boron centers.2–4 Among the most
easily accessible and thus most extensively studied examples are
fused boracycles such as borafluorene (A)3,5 and diboraanthracene
(B)2,4,6 derivatives. While these and related species have in the past
been used primarily as building blocks for the generation of (multi-
decker) sandwich complexes,7 much recent attention has focused
on the superior performance of fluorinated derivatives as highly
potent activators in Ziegler–Natta type olefin polymerization
processes.3,4,8
Results and discussion
Initial studies on the reactivity of boron halides toward 1,2-
bis(trimethylstannyl)ferrocene14 revealed large quantities of prod-
ucts, in which the boryl groups are attached at the 1,1ꢀ-positions
of ferrocene based on 1H NMR screening. As noted above,
rearrangements of this type allowed us to prepare 1-stannyl-
2-borylferrocenes from 1,1ꢀ-bis(trimethylstannyl)-ferrocene with
exceptionally high selectivity.12 However, such a reactivity pat-
tern is undesirable in the current case since it prevents
clean conversion of 1,2-bis(trimethylstannyl)-ferrocene to the
desired 1,2-disubstituted products. To circumvent these issues
we decided to investigate as an alternative precursor the 1,2-
bis(chloromercurio)ferrocene derivative (1), which is accessible
with perfect retention of the 1,2-substitution pattern by reaction
of 1,2-bis(trimethylstannyl)ferrocene with HgCl2.14 Compound 1
was treated with BCl3 in 1,2-dichloroethane and the mixture kept
We have recently reported a new type of boracycle, the dib-
oradiferrocene 3–Ph in which, similar as in diboraanthracenes,
two tri-coordinate boryl groups are incorporated into a six-
Department of Chemistry, Rutgers University-Newark, 73 Warren Street,
Newark, NJ, 07102, USA. E-mail: fjaekle@rutgers.edu; Fax: +1 973 353
1264; Tel: +1 973 353 5064
† Based on the presentation given at Dalton Discussion No. 11, 23–25 June
2008, University of California, Berkeley, USA.
◦
at 110 C for 12 h (Scheme 1). A dark red powder was isolated
upon removal of the solvents. Recrystallization from hot hexanes
‡ CCDC reference numbers 669587–669589. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b718789k
This journal is
The Royal Society of Chemistry 2008
Dalton Trans., 2008, 4507–4513 | 4507
©

Compound 2 serves as a versatile precursor to other dib-
oradiferrocene derivatives through nucleophilic replacement of
the chlorine substituents as shown in Scheme 1. Reaction with
MesMgBr (Mes = 2,4,6-trimethylphenyl) at 110 ◦C in toluene
gave 3–Mes as a dark red air-stable solid, which was purified by
column chromatography on silica gel. Similarly, highly selective
replacement of the chlorine substituents with C6F5 groups was
accomplished by treatment of 2 with pentafluorophenyl copper in
toluene at 50 ◦C. Recrystallization from toluene/hexanes mixtures
gave pure 3–Mes and 3–Pf as dark red crystalline solids in yields
of ca. 56 and 68%, respectively. The new diboradiferrocenes
were fully characterized by multinuclear NMR, MALDI-TOF
mass spectrometry and X-ray crystallography, and their electronic
structure was examined by UV-visible spectroscopy and cyclic
voltammetry.
The 11B NMR spectra of 3–Mes and 3–Pf display broad
signals at d 62.2 and 52.7, respectively, in the typical region of
Scheme 1 Synthesis of diboradiferrocene derivatives 3.
1
afforded the spectroscopically pure dichlorodiboradiferrocene (2)
as a dark red crystalline solid in a yield of ca. 38%.
triarylboranes (cf. 3–Ph d 57.7). The ferrocene region of the H
NMR spectra is indicative of the ortho-substitution pattern as
evident from the presence of a doublet and a triplet in a 2 : 1
intensity ratio. In CDCl3 as the solvent, the triplet is slightly
downfield (3–Mes d 4.98, 3–Pf d 5.12; cf. 3–Ph d 5.05) from the
doublet (3–Mes d 4.71, 3–Pf d 4.77; cf. 3–Ph d 4.89); all protons and
carbon atoms for the substituted Cp rings are strongly deshielded
relative to ferrocene as expected due to the p-acceptor effect of the
tricoordinate boron. The MALDI-TOF spectra of 3–Mes and
3–Pf show clear evidence of the molecular ion peaks, thereby
confirming the dimeric structures.
The availability of a series of diboradiferrocenes that feature
aryl substituents of different steric and electronic nature provides
a unique opportunity to examine substituent effects on the
geometric parameters by single crystal X-ray diffraction. Plots
of the X-ray crystal structures of 3–Mes and 3–Pf are shown
in Fig. 2 and selected geometric parameters are compared in
Table 1 with those of the parent phenyl-substituted molecule
3–Ph. Compound 3–Mes shows two independent molecules in
the unit cell, both of which sit on crystallographic inversion
centers as does the single unique molecule of 3–Pf. While the
bond lengths and angles are generally similar for 3–Mes and 3–Pf
Single crystals of 2 were obtained from a solution in hexanes
at room temperature. Two different molecules of 2 were found
in the unit cell. One of them sits on an inversion center (2–
A, not shown), but the two halves of the other molecule are
not symmetry related (2–B, Fig. 1). The most striking feature
is the apparent puckering of the bridging dibora-s-indacene
ligand, which was also observed for the phenyl derivative, 3–Ph.9
This unusual geometric feature is not found for the respective
diboraanthracene derivatives, but is a result of the bending of
the individual electron-deficient boryl groups toward iron. The
latter effect has been attributed by Wagner and Holthausen to
a delocalized through-space interaction that involves the iron,
boron, and the Cp rings based on DFT calculations on the mono-
borylated species FcBH2.15 The tilting of the dibora bridge with
respect to the substituted Cp rings is considerably smaller for 2
(2–A, 9.2◦; 2–B, 13.2/13.7◦) than for 3–Ph (15.9◦).
◦
˚
Table 1 Comparison of selected interatomic distances (A) and angles ( )
for 3–Mes and 3–Pf with those of 3–Ph
3–Mes
Compound
Molecule A
Molecule B
3–Pf
3–Pha
B–CCp
B–CCp
B–CPh
1.541(7)
1.547(7)
1.571(7)
114.9(4)
3.124
3.127
3.126
5.413
4.1
3.317
172.6
172.2
7.9
1.554(7)
1.531(7)
1.579(7)
115.4(4)
3.156
3.112
3.110
5.442
5.5
3.310
172.6
172.1
7.8
1.537(3)
1.540(3)
1.585(3)
116.9(2)
3.030
3.053
3.072
5.251
3.4
3.311
169.1
167.9
12.6
1.546(2)
1.546(2)
1.564(2)
115.8(1)
3.032
2.957
3.103
5.123
1.2
3.308
167.9
164.4
15.9
CCp–B–CCp
Fe · · · B
Fig. 1 Molecular structure of 2–B; only one of two independent
Fe · · · B*
B · · · B*
molecules in the unit cell is shown and hydrogen atoms are omit-
◦
˚
ted for clarity. Selected interatomic distances (A) and angles ( ) :
Fe · · · Fe*
Cp//Cp tilt
Cpcent · · · Cpcent
Cpcent–Ci–B
Cpcent–Ci–B*
Cp//C4B2
Ph//C4B2
B(1)–C(1) 1.526(5), B(1)–C(11) 1.543(5), B(2)–C(2) 1.533(5), B(2)–C(12)
1.528(5), B(1)–Cl(1) 1.783(3), B(2)–Cl(2) 1.784(3), C(1)–B(1)–C(11)
119.0(3), C(2)–B(2)–C(12) 119.4(3), Fe(1) · · · B(1) 2.984, Fe(1) · · · B(2)
3.021, Fe(2) · · · B(1) 3.025, Fe(2) · · · B(2) 2.994, B(1) · · · B(2) 3.015,
Fe(1) · · · Fe(2) 5.201, Cp//Cp 2.9 and 1.0, Cpcenter · · · Cpcenter 3.300 and
3.312, Cpcent–C(1)–B(1) 167.0, Cpcent–C(2)–B(2) 167.6, Cpcent–C(11)–B(1)
167.7, Cpcent–C(12)–B(2) 166.7, (C1–C5)//C(1)C(2)B(1)B(2)C(11)C(12)
13.7, (C11–C15)//C(1)C(2)B(1)B(2)C(11)C(12) 13.2.
58.3
60.7
47.9
53.4
a Data from ref. 9.
4508 | Dalton Trans., 2008, 4507–4513
This journal is
The Royal Society of Chemistry 2008
©

Fig. 2 ORTEP plots of 3–Mes (left) and 3–Pf (right) with thermal ellipsoids at the 50% probability level. Only one of two independent molecules in
the unit cell is shown for 3–Mes, and a disordered CDCl3 solvent molecule is omitted. Hydrogen atoms are omitted for clarity.
and comparable to those of 3–Ph, there are clear differences in
the interplanar angles between the central diborabenzene and the
Cp rings. The tilt angle is considerably smaller for 3–Mes with
7.8 and 7.9◦ for the two independent molecules in comparison to
the highly tilted parent compound 3–Ph (15.9◦), while the angle
found for 3–Pf lies in-between at 12.6◦. Similarly, the individual
boryl groups are more strongly bent out of the Cp plane for
3–Ph (12.1 and 15.6◦) than for 3–Mes (7.9 to 7.4◦) and 3–Pf
(10.9, 12.1◦). Based only on electronic considerations a different
order would have been expected. Given that the C6F5 moieties
are the least and the mesityl groups the most strongly electron
donating substituents, the most electron deficient boron centers in
Fig. 3 Overlay of the cyclic voltammograms of 3–Ph (black), 3–Mes
3–Pf should interact most strongly with Fe, and hence the largest
(green), and 3–Pf (red). The data are reported vs. FcH/FcH+. Conditions:
angles should be observed for 3–Pf, followed by 3–Ph and 3–Mes.
ca. 1 × 10−3 M solutions in 0.05M [Bu4N][B(C6F5)4]/CH2Cl2 as the
However, experimentally the largest bending is observed for 3–Ph
and this apparent discrepancy indicates that steric factors also
supporting electrolyte, 100 mV s−1. * denotes a trace of ferrocene.
play an important role and ultimately prevent more pronounced
Fe · · · B interactions in 3–Pf and possibly also in 3–Mes, for which
the small angles can be rationalized on both steric and electronic
grounds. Steric effects are also evident from a comparison of the
Cp//Cp tilt angles of the individual ferrocene units. Due to the
steric bulk of the mesityl groups large Cp//Cp tilt angles are
found for 3–Mes with 4.1 and 5.5◦. Importantly, even for 3–Pf
the Cp//Cp tilting of 3.4◦ is significantly more pronounced than
for 3–Ph (1.2◦).
One of the most interesting aspects of compound 3–Ph is the
strong electronic communication between the ferrocene moieties
that is evident from two separate redox events relating to oxidation
of the first and second ferrocene moiety with an unusually large
redox splitting of 510 mV.9,11 The cyclic voltammograms of 3–Mes
and 3–Pf are displayed in Fig. 3 together with that of 3–Ph and
the data are summarized in Table 2. For both 3–Mes and 3–Pf two
redox waves are observed similar to those of 3–Ph. The second
oxidation wave shows far better electrochemical reversibility for
3–Mes than for 3–Pf and 3–Ph. This effect is attributed to the
comparatively higher solubility of 3–Mes, assuming a similar trend
for the mono- and dicationic species in the electrolyte solution
as for the neutral species. While the first redox wave for 3–Mes
(E1/2 = 73 mV vs. Fc/Fc+) is found at a potential similar to that of
3–Ph and hence at only a slightly higher potential than ferrocene
itself, for 3–Pf this first oxidation occurs at a considerably higher
potential of E1/2 = 244 mV. The latter is consistent with attachment
of the electron withdrawing C6F5 moieties and in reasonably
good agreement with data reported by Piers et al. for FcB(C6F5)2
(450 mV in trifluorotoluene/Bu4N[B(C6F5)4]).16
Remarkably, the second oxidation event occurs for both 3–Mes
and 3–Pf at higher potential than for 3–Ph. Consequently, the
redox splitting for 3–Mes (DE = 579 mV) and especially that of
3–Pf (DE = 703 mV) is larger than that determined for 3–Ph. A
similarly large redox coupling has recently been reported for the
ferrocenylborane polymer {1,1ꢀ-fc-B(Mes)-}n with DE = 705 mV
in Bu4N[B(C6F5)4]/CH2Cl2 as the electrolyte.17 In the case of this
polymer, however, a very high second oxidation potential is to be
expected : after oxidation of alternating ferrocene moieties along
the polymer chain during the first oxidation wave, the remaining
ferrocene groups face two (rather than one) neighboring oxidized
ferricenyl moieties, both of which are expected to act as electron
withdrawing groups that in turn enhance the electron deficient
nature of the boron centers.18 In agreement with this analysis is that
a more moderate redox coupling of DE = 422 mV was found for the
respective diferrocenylborane Fc2BMes. The large redox coupling
Table 2 Comparison of CV data for 3–Mes and 3–Pf with 3–Pha
E1/2 (1)
60
73
DEp1
79
90
E1/2 (2)
570
652
DEp2
120
102
137
DE (2–1)
510
579
3–Phb
3–Mes
3–Pf
244
90
947
703
a Data are reported in mV relative to the ferrocene/ferrocenium couple.
b Data from ref. 9.
This journal is
The Royal Society of Chemistry 2008
Dalton Trans., 2008, 4507–4513 | 4509
©

for 3–Pf is thus indicative of generation of highly electron-deficient
boron centers due to the presence of both electron withdrawing
C6F5 and ferricenyl moieties after mono-oxidation.
Conclusions
We have developed a new synthetic approach to diboradiferrocene
species starting from the chloro-substituted boracycle 2, which
serves as a versatile precursor to other derivatives through
nucleophilic substitution reactions. As shown here, the chlorine
substituents can be replaced with different aryl groups to tune
the Lewis acidity of the bridging boryl groups and consequently
influence the degree of electronic interaction between the two
ferrocene moieties as evidenced by an enhanced redox coupling
and red-shifted absorption maximum for 3–Pf relative to 3–
Ph.22 Importantly, we expect that the binding behavior towards
anions and the ability to act as Lewis acid catalysts for organic
transformations can be addressed in a similar manner; these are
new opportunities that we are currently further pursuing.
The analysis given above does not explain the enhanced redox
splitting for 3–Mes relative to 3–Ph. Our interpretation of this
seeming inconsistency takes into consideration the steric crowding
in 3–Mes as evident from the X-ray crystal structure analysis.
A comparison of the interplanar angles between the C4B2 ring
and the (substituted) phenyl rings shows that the largest angles
are found for 3–Mes (58.3◦ and 60.7◦), followed by 3–Ph (53.4◦)
and 3–Pf (47.9◦), suggesting that p-overlap between the boron
p orbital and the exocyclic substituents is least favorable for
3–Mes.19 Similar observations have been made for Fc2BMes,
for which the mesityl group makes an angle of 61.2◦ with the
trigonal boron moiety.17 Importantly, upon mono-oxidation of 3–
Ph to [3–Ph]+ this interplanar angle is considerably lowered (e.g.
for [3–Ph]+PF6 angles of 36.3/45.8◦ and 36.2/45.5◦ are found
−
Experimental
for two independent unsymmetric molecules; for [3–Ph]+SbF6
:
−
38.8/44.6◦).10,11 This change in geometry presumably is a result of
enhanced p-bonding between boron and the phenyl groups that
compensates for the decreased p-interaction with the oxidized
ferrocene moieties. However, such a planarization ought to be
unfavorable in the case of the mesityl-substituted derivative, which
may explain why the second oxidation of 3–Mes occurs at higher
potential than that of the sterically less congested derivative 3–Ph.
Finally, we examined the UV-visible spectra of compounds
3–Mes and 3–Pf, and a comparison with the parent molecule
3–Ph is shown in Fig. 4. The longest wavelength absorption,
which is attributed to a dd transition of the ferrocene moieties
with significant charge transfer character,12 is most bathochromic
1,2-Bis(chloromercury)ferrocene14
and
pentafluorophenyl
copper23 were prepared according to literature procedures, and
mesitylmagnesium bromide (0.5 M in THF) was synthesized
from bromomesitylene and Mg in THF. BCl3 (1M in hexanes)
was purchased from Acros. All reactions and manipulations
involving reactive organoboron species were carried out under
an atmosphere of prepurified nitrogen using either Schlenk
techniques or an inert-atmosphere glovebox (Innovative Techno-
logies). Hydrocarbon and chlorinated solvents were purified using
a solvent purification system (Innovative Technologies), and
the chlorinated solvents were subsequently degassed via several
freeze–pump–thaw cycles.
1
for the pentafluorophenyl-substituted compound 3–Pf (kmax
=
499.9 MHz H NMR, 125.7 MHz 13C NMR, 470.4 MHz 19F
527 nm). However, surprisingly, the absorption maximum of the
mesityl derivative 3–Mes (kmax = 519 nm) is also considerably red-
shifted relative to that of 3–Ph (kmax = 498 nm) and approaches that
of 3–Pf.20 This order reflects that of the CV data, and thus further
suggests that electronic interaction through the diborabenzene
linker is promoted by either electron withdrawing substituents
such as C6F5 groups or by bulky aryl groups that adopt a
conformation orthogonal to the diborabenzene and hence do not
participate significantly in p-bonding with the empty p-orbitals
on boron. The increased Cp//Cp ring tilting for 3–Mes may also
impact the absorption properties, and a correlation of the tilt
angle in strained [1]ferrocenophanes with kmax has been suggested
by Manners et al.21
NMR and 160.4 MHz 11B NMR spectra were recorded on a Varian
INOVA NMR spectrometer (Varian Inc., Palo Alto, CA) equipped
with a 5 mm dual broadband gradient probe (Nalorac, Varian Inc.,
Martinez, CA). Solution 1H and 13C NMR spectra were referenced
internally to the solvent signals. 19F NMR spectra were referenced
ꢀ
ꢀꢀ
externally to a,a ,a -trifluorotoluene (0.05% in C6D6; d = −63.73)
and 11B NMR spectra to BF3·OEt2 (d = 0) in C6D6.
Cyclic voltammetry measurements were carried out with a
BAS CV-50W analyzer. The three-electrode system consisted
of a Au disk as working electrode, a Pt wire as secondary
electrode, and a Ag wire as the pseudo-reference electrode. The
voltammograms were recorded in dichloromethane containing
0.05 M [Bu4N][B(C6F5)4] as the supporting electrolyte. Data were
acquired with decamethylferrocene as an internal reference and are
reported relative to the ferrocene/ferrocenium couple (+610 mV
with 0.1 M [Bu4N][B(C6F5)4] in CH2Cl2 vs. the decamethylfer-
rocene/decamethylferrocenium couple).
UV-visible absorption data were acquired on a Varian Cary 500
UV-vis/NIR spectrophotometer. Solutions were prepared using
a microbalance ( 0.1 mg) and volumetric glassware and then
chargedintoquartz cuvettes withsealingscrew caps (Starna)inside
the glovebox.
GC-MS spectra were acquired on a Hewlett Packard HP 6890
Series GC system equipped with a series 5973 mass selective
detector and a series◦ 7683 injector. A temperature profile with
a heating rate of 20 C min−1 from 50 ◦C to 300 ◦C was used.
Mass spectral data in FAB positive ion mode with NBA (4-
nitrobenzylalcohol) as matrix were obtained at the Michigan State
Fig. 4 Overlay of the UV-visible spectra of 3–Mes, 3–Pf, and 3–Ph.
4510 | Dalton Trans., 2008, 4507–4513
This journal is
The Royal Society of Chemistry 2008
©

University Mass Spectrometry Facility, which is supported, in
part, by a grant (DRR-00480) from the Biotechnology Research
Technology Program, National Center for Research Resources,
National Institutes of Health. MALDI-TOF measurements were
performed on an Applied Biosystems 4700 Proteomics Analyzer
in reflectron (+) mode with delayed extraction. Benzo[a]pyrene
was used as the matrix (10 mg mL−1 in toluene). Samples were
prepared in toluene (10 mg mL−1), mixed with the matrix in a
1 : 10 ratio, and then spotted on the wells of a sample plate inside
a glove box. Elemental analyses were obtained from Quantitative
Technologies Inc., Whitehouse, NJ.
Copies of the data can be obtained free of charge on application
to CCDC, 12 Union Road, Cambridge CB21EZ, UK (fax: (+44)
1223-336-033; email: deposit@ccdc.cam.ac.uk).
Synthesis of Fc2B2Cl2 (2)
To a suspension of 1,2-bis(chloromercury)ferrocene (1) (8.00 g,
12.2 mmol) in dichloroethane (180 mL) in a Teflon-stoppered
glass tube was added BCl3 (14 mL, 1M solution in hexanes) inside
a glove box. The tube was closed and immersed into an oil bath
at 110 ◦C for 12 h. After cooling to room temperature the volatile
components were removed under reduced pressure. The product
was extracted with toluene, hexanes were added, and the mixture
was filtered. The solvents were removed under high vacuum to give
a red solid. Recrystallization from hot hexanes gave 2 as a dark
red crystalline solid. Yield: 1.08 g (38%). X-Ray quality crystals
were obtained from a hexanes solution at room temperature. For
Details of X-ray diffraction experiments and crystal structure
refinements for 3–Mes and 3–Pf are provided in Table 3. Data
were collected at 100(2) K on a Bruker SMART APEX CCD
˚
diffractrometer using Cu Ka (1.54178 A) radiation. Numerical
absorption corrections were applied in all cases. The structures
were solved using direct methods, completed by subsequent
difference Fourier syntheses and refined by full matrix least
squares procedures on F2. All non-hydrogen atoms were refined
with anisotropic displacement coefficients. The H atoms were
placed at calculated positions and were refined as riding atoms.
All software and source scattering factors are contained in the
SHELXTL program package.24 There is a disordered solvent
chloroform molecule included in the crystal of 3–Mes. It was split
over two positions and refined anisotropically. The occupancy
factors for the major domain of the CHCl3 site refined to 0.643.
Crystallographic data for the structures of 2, 3–Mes and 3–Pf
have been deposited with the Cambridge Crystallographic Data
Center as supplementary publication no. CCDC 669587–669589.
2: H NMR (499.9 MHz, CDCl3, 25 ◦C) d 5.12 (t, J = 2.5 Hz,
1
2H, Cp–4), 5.05 (d, J = 2.5 Hz, 4H, Cp–3,5), 4.11 (s, 10H, free
◦
1
Cp). H NMR (499.9 MHz, C6D6, 25 C) d 5.04 (d, J = 2.5 Hz,
4H, Cp–3,5), 4.69 (t, J = 2.5 Hz, 2H, Cp–4), 3.88 (s, 10H, free
◦
Cp). 13C NMR (125.7 MHz, C6D6, 25 C) d 80.3 (Cp–3,5), 80.1
(Cp–4), 71.4 (free Cp), n.o. ipso–Cp–B. 11B NMR (160.4 MHz,
C6D6, 25 ◦C) d 49.9 (w1/2 = 600 Hz). UV-Vis (CH2Cl2, 2.5 × 10−4
M) kmax (e) = 492 nm (4540 M−1 cm−1), 405 (3730 M−1 cm−1).
Synthesis of Fc2B2Mes2 (3-Mes)
A solution of mesitylmagnesium bromide (0.8 mL, 0.5 M in THF)
was transferred to a Teflon-stoppered glass tube and the solvent
Table 3 Details of X-ray crystal structure determinations
Compound
2
3–Mes
3–Pf
Formula
M
C20H16B2Cl2Fe2
460.55
Triclinic
C39H39B2Cl3Fe2
747.37
Triclinic
C32H16B2F10Fe2
723.77
Monoclinic
C2/c
23.7946(2)
7.0879(1)
18.0370(2)
90
119.329(2)
90
Crystal system
Space group
P-1
P-1
˚
a/A
8.1595(2)
12.9933(2)
14.0037(2)
112.665(2)
97.722(2)
95.855(2)
1338.24(7)
3
11.2730(2)
11.9306(2)
14.7300(2)
90.4950(10)
103.3200(10)
114.8210(10)
1737.22(5)
2
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
V/A
2652.09(5)
4
1.813
Z
qcalc/g cm−3
1.714
15.776
1.429
9.018
l (Cu Ka)/mm−1
Crystal size/mm
h range/◦
9.645
0.49 × 0.34 × 0.29
3.74 to 68.39
−9 ≤ h ≤ 9
−15 ≤ k ≤ 15
−16 ≤ l ≤ 16
8355
0.23 × 0.21 × 0.06
3.11 to 59.99
−12 ≤ h ≤ 12
−12 ≤ k ≤ 13
−16 ≤ l ≤ 13
10934
0.26 × 0.19 × 0.11
4.26 to 67.89
−28 ≤ h ≤ 28
−8 ≤ k ≤ 8
−21 ≤ l ≤ 21
13982
Limiting indices
Reflns collected
Unique reflns
4409
4720
2400
R(int)
0.0358
4409/0/352
1.051
0.0399
4720/0/450
1.062
0.0431
2400/0/208
1.042
Data/restraints/parameters
GOF on F2
Final R indices
[I >2r(I)]a
R1 = 0.0462
wR2 = 0.1198
R1 = 0.0474
wR2 = 0.1211
1.465/−0.607
R1 = 0.0528
wR2 = 0.1468
R1 = 0.0660
wR2 = 0.1595
0.696/−0.740
R1 = 0.0301
wR2 = 0.0756
R1 = 0.0337
wR2 = 0.0778
0.493/−0.268
R indices (all data)a
−3
˚
Peak/hole/e A
1/2
a R1 = RꢁFo|–|Fcꢁ/R|Fo|; wR2 = {R[w(Fo2–Fc2)2]/R[w(Fo2)2]}
.
This journal is
The Royal Society of Chemistry 2008
Dalton Trans., 2008, 4507–4513 | 4511
©

was removed under high vacuum. The residue was suspended
in toluene (5 mL), and a solution of 2 (40.5 mg, 0.088 mmol)
in toluene was added. The reaction mixture was kept stirring at
110 ◦C for 12 h. After cooling to room temperature the solvent was
removed under high vacuum. The compound was extracted with
a toluene/hexanes mixture and passed through a silica gel column
(hexanes : CH2Cl2 9 : 1 as eluent). The product was recrystallized
from a toluene/hexanes mixture at −35 ◦C. Yield: 31 mg (56%). X-
Ray quality crystals were obtained by slow evaporation of a CDCl3
solution. For 3–Mes: 1H NMR (499.9 MHz, CDCl3, 25 ◦C) d 6.98
(s, 4H, meta-Mes), 4.98 (t, J = 2.5 Hz, 2H, Cp–4), 4.71 (d, J =
2.5 Hz, 4H, Cp–3,5), 4.03 (s, 10H, free Cp), 2.56 (s, 12H, ortho-Me),
2.39 (s, 6H, para-Me). 13C NMR (125.7 MHz, CDCl3, 25 ◦C) d
140.5 (ortho-Mes), 139.4 (br, ipso-B-Mes), 137.3 (para-Mes), 128.0
(meta-Mes), 83.3 (br, ipso-Cp–B), 81.3 (Cp–3,5), 79.2 (Cp–4), 70.4
(free Cp), 24.3 (ortho-Me), 21.4 (para-Me). 11B NMR (160.4 MHz,
CDCl3, 25 ◦C) d 62.2 (w1/2 = 1600 Hz). UV-Vis (CH2Cl2, 2.4 × 10−4
M) kmax (e) = 519 nm (5560 M−1 cm−1), 424 (3190 M−1 cm−1). FAB-
MS (NBA): m/z (%) 628 (100) [M+], 563 (5) [M+–Cp], 509 (7) [M+–
Mes]; small amounts of higher aggregates and their fragments are
0346828). We are grateful to Haiyan Li for acquisition of cyclic
voltammetry data.
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+
found: 1256 (5) [M2 ]. MALDI-TOF MS: m/z 628.1934 (calcd
12
for
C
38
1H3811B256Fe2: 628.1858). Calcd for C38H38B2Fe2·C7H8 (1
equiv toluene by 1H NMR): C 75.05, H 6.44%; found C 74.62, H
6.51%.
Synthesis of Fc2B2(C6F5)2 (3-Pf)
To a solution of 2 (30 mg, 0.065 mmol) in toluene (2 mL) in a
Teflon-stoppered glass tube was added pentafluorophenyl copper
(30 mg, 0.13 mmol) in toluene (2 mL) inside a glove box. The
reaction mixture was heated to 50 ◦C for 12 h. After cooling
to room temperature the reaction mixture was filtered and the
solvents were removed under reduced pressure. The residue shows
1
about 95% purity by H and 19F NMR and was recrystallized
from toluene/hexanes mixture at −35 ◦C. Yield: 32 mg (68%). X-
Ray quality crystals were obtained from a toluene/hexanes (3 : 1)
mixture at room temperature. For 3–Pf : 1H NMR (499.9 MHz,
CDCl3, 25 ◦C) d 5.12 (t, J = 2.5 Hz, 2H, Cp–4), 4.77 (br, 4H, Cp–
3,5), 4.14 (s, 10H, free Cp). 19F NMR (470.4 MHz, CDCl3, 25 ◦C)
d −129.3 (dd, J = 11, 25 Hz, ortho-F), −153.9 (t, J = 20, para-F),
−162.9 (br m, meta-F). 13C NMR (125.7 MHz, CDCl3, 25 ◦C) d
147.1 (d, J(C,F) = 252 Hz, ortho–C6F5), 141.8 (d, J(C,F) = 250 Hz,
para–C6F5), 137.8 (d, J(C,F) = 247 Hz, meta-C6F5), 113.8 (br, ipso-
C6F5), 82 (very br, ipso-Cp–B), 81.8 (d, J(C,F) = 3.9 Hz, Cp–3,5),
80.9 (Cp–4), 70.7 (free Cp). 11B NMR (160.4 MHz, CDCl3, 25 ◦C)
d 52.7 (w1/2 = 1280 Hz). UV-Vis (CH2Cl2, 2.5 × 10−4 M) kmax (e) =
527 nm (5460 M−1 cm−1), 420 (2650 M−1 cm−1). GC-MS (m/z,
(%)): 724 [M+] (100). MALDI-TOF MS: m/z 723.9857 (calcd
1
19
for 12C32 H1611B2
F
56Fe2: 723.9977). Calcd for C32H16B2F10Fe2: C
10
53.10, H 2.23%; found C 53.01, H 2.08%.
Acknowledgements
16 B. E. Carpenter, W. E. Piers, M. Parvez, G. P. A. Yap and S. J. Rettig,
Can. J. Chem., 2001, 79, 857–867; B. E. Carpenter, W. E. Piers and R.
McDonald, Can. J. Chem., 2001, 79, 291–295.
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Wagner, Dalton Trans., 2005, 159–170; J. B. Heilmann, M. Scheibitz,
Y. Qin, A. Sundararaman, F. Ja¨kle, T. Kretz, M. Bolte, H.-W. Lerner,
M. C. Holthausen and M. Wagner, Angew. Chem., Int. Ed., 2006, 45,
920–925; J. B. Heilmann, Y. Qin, F. Ja¨kle, H. W. Lerner and M. Wagner,
Inorg. Chim. Acta, 2006, 359, 4802–4806.
We are grateful to the donors of the Petroleum Research Fund,
administered by the American Chemical Society for support of
this research and to the National Science Foundation for partial
funding of an X-ray diffractomer (NSF CRIF-0443538). F.J.
thanks the Alfred P. Sloan foundation for a research fellowship
and the National Science foundation for a CAREER award (CHE-
4512 | Dalton Trans., 2008, 4507–4513
This journal is
The Royal Society of Chemistry 2008
©

18 For a discussion of the Lewis acidity enhancement of ferrocenylboranes
upon oxidation of the iron centers see ref. 10.
19 The B–CPh distances of 1.585(3) A for 3-Pf and of 1.571(7)/1.579(6)
5774; D. E. Herbert, U. F. J. Mayer and I. Manners, Angew. Chem., Int.
Ed., 2007, 46, 5060–5081.
22 For a more detailed discussion of the electronic communication via the
diboron bridge in 3-Ph see ref. 11.
23 A. Cairncross, H. Omura and W. A. Sheppard, J. Am. Chem. Soc.,
1971, 93, 248–249; A. Cairncross, W. A. Sheppard, E. Wonchoba,
W. J. Guildford, C. B. House and R. M. Coates, Org. Synth., 1979,
59, 122–131; A. Sundararaman, R. A. Lalancette, L. N. Zakharov,
A. L. Rheingold and F. Ja¨kle, Organometallics, 2003, 22, 3526–3532;
A. Sundararaman and F. Ja¨kle, J. Organomet. Chem., 2003, 681, 134–
142; F. Ja¨kle, Dalton Trans., 2007, 2851–2858.
˚
˚
˚
A for 3-Mes are longer than that of 1.564(2) A for 3-Ph, which
is consistent with a decreased p-overlap of the empty pp-orbital on
boron with the mesityl and C6F5 substituents, respectively; however,
the standard deviations for the B–C distances for 3-Mes prevent any
further conclusions.
20 In comparison, absorption maxima of 490 nm and 505 nm have
been determined for Fc2BMes and the polymer {1,1ꢀ-fc-B(Mes)-}n,
respectively; see ref. 17.
21 A. Berenbaum, H. Braunschweig, R. Dirk, U. Englert, J. C. Green, F.
Ja¨kle, A. J. Lough and I. Manners, J. Am. Chem. Soc., 2000, 122, 5765–
24 G. Sheldrick, SHELXTL, Version 6.14, Bruker AXS Inc., Madison,
WI, 2004.
This journal is
The Royal Society of Chemistry 2008
Dalton Trans., 2008, 4507–4513 | 4513
©