Reversible expansion and contraction of a 1,2-diborylated ferrocene dimer promoted by redox chemistry and nucleophile binding

Article abstract of DOI:10.1002/anie.200502148

(Chemical Equation Presented) Two Bs in a pod: The first 1,2-diborylated ferrocene (see structure) was obtained upon reaction of PhBCl₂ with 1,1′-bis(trimethylstannyl)ferrocene. Conformational changes in this highly rigid species are triggered either through redox chemistry or the binding of nucleophiles, to render the complex an interesting candidate for the design of new types of molecular machines.

Full text of DOI:10.1002/anie.200502148

Angewandte
Chemie
DOI: 10.1002/anie.200502148
Communications
The doubly boron-bridged diferrocene [CpFe{C5H3(BPh)2C5H3}FeCp]
represents the first example of a 1,2-diborylated ferrocene. Controlled
conformational changes in this rigid redox-active bifunctional Lewis
acid species can be triggered either through redox chemistry or the
binding of nucleophiles. For more details, see the Communication by F.
Jäkle and co-workers on the following pages.
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Chemie
Ferrocenes
Lewis acidic boron and tin groups adjacent to one another are
readily available through a rearrangement reaction from 1,1’-
bis(trimethylstannyl)ferrocene (1) and boron halides.[9] For
instance, the 1-stannyl-2-borylferrocene 2 is formed as the
DOI: 10.1002/anie.200502148
Reversible Expansion and Contraction of a
1,2-Diborylated Ferrocene Dimer Promoted by
Redox Chemistry and Nucleophile Binding**
major product upon reaction of 1 with an excess ofPhBCl
2
(Scheme 1). However, examples ofefrrocenes that contain
metals or semi-metals in neighboring positions at the same
cyclopentadienyl (Cp) ring are generally still rare.[10] Among
diborylated ferrocenes, for example, only 1,1’-diborylated
species have been isolated and 1,3-diborylated isomers have
been confirmed as minor byproducts by NMR spectrosco-
py.[11] Herein, we report the synthesis and properties ofthe
first example of a 1,2-diborylated bisferrocene, in which two
CpFe fragments are bridged by the novel dibora-s-indacene-
diyl ligand B.
Krishnan Venkatasubbaiah, Lev N. Zakharov,
W. Scott Kassel, Arnold L. Rheingold, and
Frieder Jäkle*
Diboraanthracenes A have been extensively used as precur-
sors to various di- and multimetallic (and multidecker)
When the reaction mixture of 1 and PhBCl2 was kept for
an extended period oftime in an aluminum-capped vial, dark
red crystals ofthe novel 1,2-diborylated efrrocene dimer
3
formed reproducibly in approximately 36% yield. The
presence of aluminum was found to be critical, as confirmed
by the lack offormation ofproduct in the absence ofa source
ofAl. While addition ofAl powder to the reaction mixture
accelerated the reaction, the selectivity for 3 was diminished.
The 1H NMR spectrum of 3 showed a doublet (d = 4.89 ppm,
4H) and a triplet (d = 5.05 ppm, 2H) with a coupling constant
sandwich complexes,[1–3] and more recently a perfluorinated
derivative (X = H, R = C6F5) was employed as an activator in
[4,5]
3
the polymerization ofoleifns.
Boracycles in general also
of J = 2.5 Hz, which is characteristic ofa 1,2-disubstituted
serve as fluorescent materials, as probes for
anions, and as building blocks for the
[6]
assembly ofnew materials.
Finally, the
“doping” ofboron into larger organic
p systems such as fullerenes and conju-
gated organic polymers continues to attract
much interest owing to potentially inter-
esting electronic effects.[7,8]
We previously showed that heteronu-
clear bidendate Lewis acids comprised of
Scheme 1. Formation of diboracycle 3.
ferrocene.[10] A broad signal in the 11B{1H} NMR spectrum at
d = 57.7 ppm is indicative ofthe formation ofa triarylborane.
The mass spectrum of 3 clearly shows a peak for the molecular
ion at m/z = 544, which is consistent with the proposed
dimeric structure.
[*] Dr. K. Venkatasubbaiah, Prof. F. Jäkle
Department of Chemistry
Rutgers University Newark
73 Warren Street, Newark, NJ 07102 (USA)
Fax: (+1)973-353-1264
E-mail: fjaekle@rutgers.edu
Single-crystal X-ray diffraction analysis of 3 confirmed
that two ferrocene units are doubly bridged with two
tricoordinate boron atoms (Figure 1).[12] The CpFe moieties
adopt a trans configuration relative to the central B2C4 ring.[13]
A pronounced tilting ofthe boron atoms towards the iron
center (Cpcent-C6-B1 = 167.98; Cpcent-C10-B1* = 164.48) is
evident, as is also observed in other ferrocenyl boranes;
recent studies based on a combination ofX-ray crystallog-
raphy, cyclic voltammetry, Mössbauer spectroscopy, and
theoretical calculations show that this feature results from a
delocalized through-space interaction between iron and
boron.[14] This interaction leads to a strong deviation from
planarity for the diboraindacene ligand in 3 with an inter-
planar angle between the substituted Cp rings and the central
B2C4 cycle ofaround 15.9 8 (see Figure 3a). In comparison, the
related diboraanthracenes, A, are planar in the absence of
Dr. L. N. Zakharov, Prof. A. L. Rheingold
Department of Chemistry and Biochemistry
University of California, San Diego
9500 Gilman Drive, La Jolla, CA 92093-0358 (USA)
Prof. W. S. Kassel
Department of Chemistry
Villanova University
Villanova, PA 19085 (USA)
[**] Acknowledgement is made to the National Science Foundation
(CAREER award CHE-0346828 to F.J.), to the donors of the
Petroleum Research Fund, administered by the American Chemical
Society, and to the Rutgers University Research Council for support
of this research. We are grateful to Prof. Nicholas J. Turro and
Alberto Moscatelli for recording of the EPR spectrum and to Dr.
Kakalis for acquisition of 2D NMR spectra. We would also like to
thank Prof. Jack Norton for insightful discussions.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
steric strain.[2,4,15] The B C bond lengths in 3 are similar to
À
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Communications
Figure 1. Molecular structure of 3 with thermal ellipsoids at the 50%
probability level; hydrogen atoms are omitted for clarity. Selected
À
À
interatomic distances [Š] and angles [8]: B1 C6 1.546(2), B1 C10*
À
1.546(2), B1 C11 1.564(2), B1···B1* 3.103, Fe1···B1 3.032, Fe1···B1*
2.957, Fe1···Fe1* 5.123, C6-B1-C10* 115.8(1), C6-B1-C11 123.7(1),
C10*-B1-C11 120.4(1), C10-C6-B1 121.6(1), C6-C10-B1* 122.5(1).
Figure 2. Cyclic voltammogram of 3 (vs. FcH/FcH+). a) Oxidation
those in 9,10-diboraanthracenes A with organic substituents
waves of 3 (10À3 m) at 100 mVsÀ1 with Bu4N+B(C6F5)4 (0.1m) in
À
at the boron center (1.554–1.580 Š).[2,4,15] However, the B C
À
CH2Cl2 as the supporting electrolyte; b) reduction waves of 3 (10À3 m)
at 100 mVsÀ1 with Bu4N+PF6 (0.1m) in DME as the supporting elec-
À
bonds to the Cp rings (1.546(2) Š) are significantly shorter
À
than the exocyclic B C bonds to the phenyl groups
(1.564(2) Š), which suggests a significant contribution of the
trolyte.
chinoid resonance structure of B.[9,16]
An intriguing feature of 3 is the presence ofboth redox-
active ferrocene and triorganoborane moieties, which may
allow one to electrochemically trigger conformational
changes based on changes in the nature ofthe Fe···B
interaction as suggested by DFT calculations by Wagner
and co-workers[14] Such processes have attracted much
interest recently with regard to the design ofmolecular
machines that can be addressed through external stimuli.[17]
Cyclic voltammograms of 3 were recorded in CH2Cl2/
Bu4N+PF6À as well as CH2Cl2/Bu4N+B(C6F5)4À (Figure 2a). In
both cases two separate one-electron oxidation events were
observed. However, improved chemical reversibility was
The mono-oxidized mixed-valence species (3+)I3·I2 was
prepared in 85% yield upon treatment of 3 with an excess of
I2 in CH2Cl2/toluene and crystallization at À378C. The X-ray
crystal structure shows two independent molecules, (3+)-1 and
(3+)-2, along with I3 counterions and co-crystallized mole-
À
cules ofI .[12,21] There are two independent halfcations on
2
inversion centers, which render the Fe atoms in each ofthe
dimers (3+)-1 and (3+)-2 equivalent. The latter has been
shown to be indicative ofvalence delocalization owing to very
rapid electron transfer.[22] In line with this interpretation, the
Cpcent–Cpcent distances of3.355 Š and 3.357 Š for ( 3+)-1 and
(3+)-2, respectively, are intermediate between those of 3
(3.308 Š) and those oftypical oxidized ferrocenes (ca. 3.4 Š).
In contrast, the mixed-valent zwitterion ferricenyl(iii)-
trisferrocenyl(ii)borate (Fc4B), which contains a coordina-
tively saturated boron bridge, shows distinct ferrocene and
ferrocenium moieties with Cpcent–Cpcent distances of3.291–
3.320 Š for the former and 3.428 Š for the latter.[23] The
geometric parameters for (3+)-1 and (3+)-2 (Figure 3b) are
similar apart from the tilting of the C4B2 ring relative to the
Cp ring.[21] The interplanar angle of16.5 8 for (3+)-1 is larger
than that of13.8 8 for (3+)-2 and even larger than in the free
acid 3 itself. This unexpected feature is likely a result of
packing effects involving the counterions, which form infinite
helical chains {I3À·I2}n through short intermolecular I···I
contacts.[21]
achieved with Bu4N+B(C6F5)4 as reported by Geiger and
À
co-workers for related systems.[18] The first oxidation of 3
occurs at E1/2 = + 60 mV (versus the ferrocene/ferrocenium
(FcH/FcH+) couple; E1/2 = + 88 mV with PF6 ), which is
À
slightly higher than that reported for FcBMe2 (E =+
9 mV).[14] The second oxidation takes place at a much
higher potential of E1/2 = + 570 mV (E1/2 = + 432 mV with
À
PF6 ). The pronounced difference between the results for the
two electrolytes is due to differences in the degree of ion-
pairing interactions.[18] In either case the separation between
the two oxidation waves (DE = 510 mV in CH2Cl2/Bu4N+B-
(C6F5)4À; DE = 344 mV in CH2Cl2/Bu4N+PF6À) is considerably
larger than those for the related doubly silyl-bridged ferro-
cenes [CpFe{Cp(SiR2)2Cp}FeCp] (DE = 195–210 mV for R =
Me, Et, and nBu in propionitrile/Bu4N+PF6 ),[19] which
Hendrickson and co-workers have shown that IR spec-
troscopy may also serve as a convenient tool to evaluate the
valence detrapping in diferrocene-type species. Particularly
À
indicates unusually strong electronic coupling through the
dibora-s-indacene bridge. However, the interaction is smaller
than that for the all-carbon analogue, the s-indacene-bridged
dimer [Cp*Fe{Cp(CH)2Cp}FeCp*] (Cp* = pentamethylcy-
clopentadienyl), which shows a splitting of DE = 820 mV
and forms a fully valence-delocalized cation upon mono-
oxidation.[20]
À
sensitive to oxidation ofFe is the perpendicular C H bending
vibration, which occurs at 815 cmÀ1 in ferrocene and is shifted
to approximately 850 cmÀ1 in ferrocenium salts.[22a,b] A KBr
pellet ofthe mixed-valence species ( 3+)I3·I2 shows a single
broad band at 841 cmÀ1, which is shifted relative to the neutral
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Chemie
Fe···Fe separation from 5.123 to 5.509 Š (Figure 3c). Both
boryl groups are bent towards Fe at a similar angle (170.58 and
171.48) which suggests that the electron is delocalized. A
À
shortening ofthe B C bond lengths within the six-membered
diboracycle, as reported by Siebert and co-workers for the
reduced diboraanthracene [K(18-c-6)(THF)2][ACÀ] in com-
parison to the neutral diboraanthrancene A (R = Me),[2] is not
observed. In this context, note that reduction of1,8-bis(di-
phenylboryl)naphthalene, which shows a B···B separation of
3.002 Š similar to that of 3, has been studied by Gabbaï and
co-workers.[24] Upon chemical or electrochemical reduction, a
monoanionic species was detected, for which a one-electron
s bond between the two boron centers has been proposed on
the basis ofEPR spectroscopy and DFT calculations. The
calculations predict a shortening ofthe B···B distance upon
reduction.[25] Although reduction apparently leads to a
delocalized radical anion in our system, a decrease in the
B···B distance (3.111 Š) in 3CÀ is not observed. The latter is
due to incorporation ofboron into the rigid ring system in 3CÀ
which does not allow for direct overlap ofthe p orbitals ofthe
two boron atoms.
Lewis acid/Lewis base binding offers yet another oppor-
tunity to trigger conformational changes in 3.[14,26] Adducts 5a
and 5b were obtained upon treatment of 3 with two equiva-
Figure 3. Comparison of structures of 3, (3+)-2, 3CÀ, and trans-5b;
hydrogen atoms are omitted for clarity; Cp//C4B2 denotes the interpla-
nar angle between the Cp and C4B2 rings.
molecule 3 (820 cmÀ1) and thus further indicates rapid
electron transfer on the IR timescale (ca. 1012 sÀ1).[21,22a,b]
The electrochemical reduction of 3 was studied by cyclic
voltammetry in 1,2-dimethoxyethane (DME, Figure 2b) and
revealed a quasi-reversible reduction wave at E1/2 = À2.52 V
(vs. FcH/FcH+) followed by an irreversible reduction at Ep2
=
lents ofpyridine (py) and tert-butylpyridine (tBu-py), respec-
tively.[21] Weak binding ofpyridine to 3 is evident from partial
dissociation into the monoadducts 4 and the free acid 3 in
solution. Single crystals of trans-5b were obtained by slow
À3.02 V. These data suggest the boracycle in 3 to be
considerably more electron-rich than the respective dibora-
anthracene (A, R = Me), which was reported to show
reversible reduction waves at À1.41 and À2.07 V in DME
versus the saturated calomel electrode (SCE; or ca. À1.80/
À2.46 V vs. FcH/FcH+).[2] The latter is to be expected if
extensive delocalization between the electron-rich ferrocene
moieties and the boron atoms is present, which is evident
evaporation ofsolvent rfom a 1:2 mixture of
3 and tert-
butylpyridine in chloroform.[12,21] A strong effect of the
binding ofpyridine on the geometric efatures is observed.
In contrast to the free acid 3, the central C4B2 ring ofthe
dibora-s-indacene moiety is slightly bent away from the CpFe
fragments, with interplanar angles between the Cp and C4B2
rings of5.5 8 (Figure 3d). The latter is a result ofelectronic
saturation and steric congestion around boron[26] which
ultimately leads to expansion ofthe molecule and a much
greater Fe···Fe distance of5.940 Š relative to that of5.123 Š
in 3.
À
from the relatively short endocyclic B C bonds and the
significant tilting of the boryl groups towards the iron atoms
in the X-ray crystal structure of 3.
The diboracycle 3 was chemically reduced by treatment
with potassium in THF in the presence of18-crown-6 (18-c-6).
ESR spectroscopy ofthe solution in THF at À508C showed a
signal, from which the 11B hyperfine coupling could not be
resolved. Dark brown thermally labile single crystals ofthe
monoreduced species [K(18-c-6)(THF)2][3CÀ] were obtained
by crystallization from THFat À358C.[12,21] The bending ofthe
boryl groups in 3CÀ towards Fe and the interplanar angle of
9.28 between the Cp rings and the C4B2 ring are significantly
smaller than those in the parent compound 3 (interplanar
angle: 15.98), which in turn results in partial flattening of the
dibora-s-indacene moiety and a significant increase in the
In conclusion, the first 1,2-diboradiferrocene 3 was readily
prepared from 1,1’-distannylferrocene and shows unusually
strong electronic coupling between the ferrocene groups upon
partial oxidation, which is indicative of fast electron transfer
through the dibora-s-indacene ligand framework. Conforma-
tional changes in this highly rigid diferrocene can be triggered
either through redox chemistry or through the binding of
nucleophiles to render 3 an interesting candidate for the
design ofnew types ofmolecular machines.
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Communications
wR2 (I > 2s(I)) = 0.0777, GOF = 1.484, 163 parameters, final
Experimental Section
À3
difference map within 0.517 and À0.353 eŠ
.
(3+)I3·I2:
Experimental procedure for 3: Neat PhBCl2 (0.62 g, 3.90 mmol) was
C32H26B2Fe2I5, Mr = 1178.35, monoclinic, space group P21/n, a =
18.141(3), b = 11.5344(19), c = 18.752(3) Š, b = 118.474(2)8, V=
3449.1(10) Š3, Z = 4, 1calcd = 2.269 gcmÀ3, l(MoKa) = 0.71073 Š,
T= 213(2) K, crystal dimensions 0.30 ” 0.10 ” 0.10 mm3, m-
added to a solution of 1 (2.00 g, 3.90 mmol) in toluene (15 mL) at
room temperature. The reaction mixture was kept for three months in
an aluminum-capped vial. Large dark red crystals of 3 formed and
were filtered off, washed with hexanes, and dried under high vacuum
(0.38 g, 36%). 1H NMR (500 MHz, CD2Cl2, 258C): d = 7.94, 7.49–7.48
(m, 10H, Ph), 5.05 (t, J = 2.5 Hz, 2H, Cp-4), 4.89 (d, J = 2.5 Hz, 4H,
Cp-3,5), 3.88 ppm (s, 10H, C5H5); 13C NMR (125.69 MHz, CD2Cl2,
258C): d = 144.5 (br, i-Ph), 135.5 (o-Ph), 130.9 (p-Ph), 129.5 (m-Ph),
84.1 (br, Cp-1,2), 83.3 (Cp-3,5), 81.4 (Cp-4), 72.1 ppm (C5H5);
11B NMR (160 MHz, CD2Cl2, 258C) d = 57.7 (w1/2 = 1280 Hz); GC-
MS: m/z: 544 [M+] (100%); UV/Vis (CH2Cl2, 10À4 m): lmax (e) = 498
(6040), 411 (4360), 335 nm (5810 cmÀ1mÀ1); Elemental analysis (%)
calcd for C32H26Fe2B2: C 70.67, H 4.82; found: C 70.72, H 4.78. See
Supporting Information for further experimental details.
(MoKa) = 5.338 mmÀ1
,
q
range from 2.15 to 28.198, 25675
measured reflections, 8083 independent reflections (Rint
=
0.0300), R1 (I > 2s(I)) = 0.0333, wR2 (I > 2s(I)) = 0.0946,
GOF = 1.025, 370 parameters, final difference map within
1.532 and À0.735 eŠÀ3. There are two independent halfcations
on inversion centers and one co-crystallized molecule ofI
.
2
[K(18-c-6)(THF)2][3CÀ]: C52H66B2Fe2KO8, Mr = 991.47, triclinic,
¯
space group P1, a = 10.049(5), b = 10.913(5), c = 12.313(6) Š,
a = 67.864(8)8, b = 79.656(8)8, g = 86.403(8)8, V= 1230.4(10) Š3,
Z = 1, 1calcd = 1.338 gcmÀ3, l(MoKa) = 0.71073 Š, T= 100(2) K,
crystal
dimensions
0.14 ” 0.04 ” 0.04 mm3,
m(MoKa) =
0.727 mmÀ1
, q
range from 2.01 to 24.008, 3900 measured
Received: June 20, 2005
reflections, 3480 independent reflections (Rint = 0.0211), R1 (I >
2s(I)) = 0.0967, wR2 (I > 2s(I)) = 0.2614, GOF = 1.092, 285
parameters, final difference map within 1.371 and
À1.241 eŠÀ3. trans-5b: C50H52B2Fe2N2, Mr = 814.26, monoclinic,
space group C2/c, a = 28.2223(18), b = 8.3188(5), c =
21.8954(14) Š, b = 127.6860(10)8, V= 4068.1(4) Š3, Z = 4,
1calcd = 1.329 gcmÀ3, l(MoKa) = 0.71073 Š, T= 213(2) K, crystal
Keywords: boron · cyclopentadienyl ligands · iron ·
Lewis acids · mixed-valent compounds
.
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dimensions 0.30 ” 0.20 ” 0.15 mm3, m(MoKa) = 0.751 mmÀ1
, q
range from 1.82 to 28.228, 14480 measured reflections, 4785
independent reflections (Rint = 0.0362), R1 (I > 2s(I)) = 0.0527,
wR2 (I > 2s(I)) = 0.1257, GOF = 1.150, 253 parameters, final
difference map within 0.517 and À0.423 eŠÀ3. For all four
structures, SADABS (Sheldrick, G. M. SADABS (2.01), Bruker/
Siemens Area Detector Absorption Correction Program;
Bruker AXS: Madison, WI, 1998) absorption correction was
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275646 (3, (3+)I3·I2, [K(18-c-6)(THF)2][3CÀ], and trans-5b,
respectively) contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from
the Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
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[12] X-ray crystal structure analyses: 3: C32H26B2Fe2, Mr = 543.85,
monoclinic, space group P21/c, a = 9.1414(6), b = 8.7739(6), c =
15.1355(10) Š, b = 101.4410(10)8, V= 1189.83(14) Š3, Z = 2,
1calcd = 1.518 gcmÀ3, l(MoKa) = 0.71073 Š, T= 150(2) K, crystal
dimensions 0.40 ” 0.30 ” 0.20 mm3, m(MoKa) = 1.240 mmÀ1
, q
range from 2.27 to 28.288, 7302 measured reflections, 2789
independent reflections (Rint = 0.0206), R1 (I > 2s(I)) = 0.0313,
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5432
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Angewandte
Chemie
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[21] See Supporting Information for experimental details, ORTEP
plots of( 3+)I3·I2, [K(18-c-6)(THF)2][3CÀ], and trans-5b, a plot of
the extended structure of( 3+)I3·I2, and a comparative plot ofthe
IR spectra of 3 and (3+)I3·I2.
[22] a) J. A. Kramer, D. N. Hendrickson, Inorg. Chem. 1980, 19, 3330;
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Hendrickson, J. Am. Chem. Soc. 1990, 112, 5031.
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[25] The X-ray crystal structure ofthe radical anion (BMes 3)CÀ
(Mes = mesityl) has been reported; see: M. M. Olmstead P. P.
Power, J. Am. Chem. Soc. 1986, 108, 4235.
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Angew. Chem. Int. Ed. 2005, 44, 5428 –5433
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