Lewis acidity enhancement of organoboranes via oxidation of appended ferrocene moieties

Article abstract of DOI:10.1039/b701807j

The Lewis acidity of boron in diboradiferrocene 1 is strongly enhanced through oxidation of the iron atoms as evident from examination of X-ray structural parameters of the mixed-valent cation 1⁺PF₆ and further confirmed from the str

Full text of DOI:10.1039/b701807j

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Lewis acidity enhancement of organoboranes via oxidation of appended
ferrocene moieties{
Krishnan Venkatasubbaiah,a Israel Nowik,b Rolfe H. Herberb and Frieder Ja¨kle*a
Received (in Berkeley, CA, USA) 8th February 2007, Accepted 10th April 2007
First published as an Advance Article on the web 1st May 2007
DOI: 10.1039/b701807j
provided evidence for two reversible oxidations that were
attributed to changes in the oxidation states of the two iron
centers from Fe(II) to Fe(III). We have also demonstrated from
X-ray crystallography that the mixed-valent species 1+I5, which is
obtained upon treatment of 1 with excess iodine, shows a valence-
detrapped structure in the solid state.11 Mo¨ssbauer studies further
suggest that 1+I5 is delocalized on the Mo¨ssbauer timescale (y107–
109 s21) in the solid state.12 However, the low solubility of 1+I5 in
non-coordinating solvents and the facile loss of I2 prevented
detailed studies on the solution properties of 1+I5.
The Lewis acidity of boron in diboradiferrocene 1 is strongly
enhanced through oxidation of the iron atoms as evident from
examination of X-ray structural parameters of the mixed-valent
cation 1+PF6 and further confirmed from the strong complexa-
tion of MeCN to the dication in 22+(I3)2.
Highly Lewis acidic organoboranes play key roles as sensor
materials for nucleophiles, catalysts in organic and organometallic
synthesis,andasactivatorsinpolymerizationreactions.1,2 Oneofthe
challenges has been to increase the Lewis acidity of the boranes, and
thustofurtherimprovetheirperformance.Inpursuitofthisgoal,the
most widely applied method is the modification of aliphatic or
aromatic side groups with electron-withdrawing fluorine substitu-
ents.2,3 For instance, the Lewis acid strength of tris(pentafluoro-
phenyl)borane, B(C6F5)3, has been shown to be in the same range as
that of BF3, and thus much higher than that of B(C6H5)3.2,3 In
addition, various examples of bidentate4 and polyfunctional5
organoboranes have been reported, and the incorporation of boron
into strained cyclics6 or anti-aromatic ring-systems7 has also been
demonstrated, in some cases, to increase the binding strength. In a
different approach, Gabba¨ı and Chiu,8 and Kawashima et al.9
demonstrated recently the strong binding affinity of boranes in the
presence of covalently linked ammonium or phosphonium ions. We
describe here an alternative method for the Lewis acidity enhance-
ment of triarylboranes that takes advantage of the reversible
oxidation of directly appended metallocene moieties.10
To gain further insight into the properties of the mixed-valent
cation 1+, we performed the oxidation of 1 with AgPF6 in CH2Cl2
and isolated the PF6 salt as a brown solid in 70% yield (Scheme 1).{
A broad signal was observedin the 11BNMR spectrum at 49.7 ppm,
which is slightly upfield shifted compared to the neutral species
(d 57.7), an effect that is likely a result of the paramagnetic shift
exerted by the oxidized ferrocene moiety. Only one set of 1H NMR
signals was observed for 1+PF6 in CDCl3, with three averaged peaks
at d 21.1 (free-Cp), 19.4 (Cp-4) and 15.2 (Cp-3,5) that correspond to
the ferrocene moieties (ferrocene: d4.0; ferrocenium: d31) and three
signals at d 3.78, 3.25, 2.92 for the phenyl groups, consistent with
fast electron transfer on the NMR time scale in solution. Moreover,
the near-IR spectrum of 1+PF6 in CH2Cl2 shows a broad
intervalence charge transfer (IVCT) band at 1543 nm, indicative
of rapid charge transfer between two localized states and thus
consistent with Robin and Day Class II behavior.13,14
An X-ray structure analysis was performed on single crystals of
1+PF6 obtained from a CHCl3–hexanes mixture, and the
molecular structure of one of two independent molecules of
1+PF6 is shown in comparison to that of 1+I5 in Fig. 1.§ The
structural features of the two molecules of 1+PF6 were found to be
similar, but surprisingly different from those of 1+I5. Most notably,
We have recently discovered the formation of the diboradifer-
rocene 1 (Scheme 1) through an unusual rearrangement reaction
from 1,19-distannylferrocene.11 Cyclic voltammetry measurements
for each molecule one ferrocene moiety shows a longer Cpcent
–
Cpcent distance (1+PF6-1: 3.411(1) s; 1+PF6-2: 3.396(1) s), which is
Scheme 1 Synthesis of complexes 1+X. (i) Excess I2 or 1 equiv. AgPF6 in
CH2Cl2.
aRutgers University, 73 Warren Street, Newark, NJ 07102, USA.
E-mail: fjaekle@rutgers.edu; Fax: +1 973 353 5064; Tel: +1 973 353 1264
bCondensed Matter Physics Group, Racah Institute of Physics, The
Hebrew University, 91904 Jerusalem, Israel.
E-mail: herber@vms.huji.ac.il; Fax: +972 2 6586347;
Tel: +972 2 6584244
{ Electronic supplementary information (ESI) available: Experimental
methods, comparison of X-ray data, and ORTEP plots for all X-ray
structures. See DOI: 10.1039/b701807j
Fig. 1 Comparison of the structures of 1+I5-2 and 1+PF6-2. Only one of
two independent molecules in the unit cell is shown, and the counterions,
solvent molecules, and hydrogen atoms are omitted for clarity.
2154 | Chem. Commun., 2007, 2154–2156
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indicative of iron in the +3 oxidation state, and one that is shorter
(1+PF6-1: 3.302(1) s; 1+PF6-2: 3.306(1) s) and thus consistent
with Fe in the +2 oxidation state.15 These observations are in stark
contrast to the structural data of the two independent molecules of
1+I5, where the Cpcent–Cpcent distances for the two molecules of
1+I5 were found to be 3.355(1) and 3.357(2) s, respectively,
intermediate between those of 1 (3.308(1) s) and typical oxidized
ferrocenes (ca. 3.4 s). Thus, while 1+I5 is valence-detrapped
according to the X-ray analysis, the positive charge in 1+PF6
clearly is localized on one ferrocene moiety. The dissimilar
behavior with different counterions can be traced back to
differences in the symmetry, where both molecules of 1+I5 lie
about a crystallographic inversion center, which in turn requires
the ferrocene moieties to be identical. In contrast, the molecules
1+PF6-1 and 1+PF6-2 show no inversion symmetry and the
distances to the anions, though long in all cases, are different for
the two halves of the molecules. Similar anion and symmetry
effects have been reported for a number of mixed-valent species
derived from biferrocenes.16
Scheme 2 Disproportionation of 1+ in acetonitrile.
The latter was filtered off and identified from 1H NMR data as the
neutral, non-coordinated species 1. From the supernatant the
MeCN complex 22+(I3)2 was isolated by recrystallization from
MeCN as a dark green crystalline solid in 88% yield (Scheme 2).19
It is noteworthy that 22+ can be obtained as the sole product upon
addition of excess iodine or another oxidant.
Coordination of MeCN to 22+(I3)2 in solution is evident from
the 11B NMR signal at d 20.2 (1+PF6: d 49.7), in a region typical
of tetracoordinate boron species. Moreover, oxidation of both
ferrocenes is reflected in a downfield shift of the Cp resonances to d
32.5 (1+PF6: d 21.1). The crystal structure of 22+(I3)2 shows the
cation lying about an inversion center (Fig. 2).§ The Cpcent–Cpcent
distances of 3.414(2) s are consistent with Fe(III) centers and, as
expected for a tetracoordinate boron complex, the Cp//C4B2 angles
of 2.4u are small. Most intriguingly, the relatively short B–N
distance of 1.594(6) s indicates strong binding of MeCN in the
The difference in oxidation state of the two iron centers has
major consequences on the molecular conformation of 1+PF6. It is
most instructive to compare the tilting between the substituted Cp
rings and the central C4B2 ring that forms the double-bridge
between the two ferrocene moieties. In highly Lewis acidic neutral
ferrocenylboranes tilting of the borane group toward Fe is
commonly observed and has been determined by Wagner and
Holthausen et al. to be due to a delocalized interaction through the
Cp ring, rather than due to direct orbital overlap between the
electron-rich Fe atom and the electron-deficient boron atom.17 For
neutral 1, the Cp//C4B2 interplanar angle was determined to be
15.9u and for the two independent valence delocalized molecules of
1+I5 similar values of 13.8u and 16.5u were determined; due to the
inversion center, the angle is identical for both ferrocenes in each
molecule. In contrast, the tilt angle for the ferrocenyl group in
1+PF6 differs from that of the ferricenyl moiety. For the oxidized
ferricenyl group a relatively small angle was found (1+PF6-1:
8.7(2)u; 1+PF6-2: 12.5(2)u), whereas the angle for the neutral
ferrocenyl group (1+PF6-1: 22.6(2)u; 1+PF6-2: 19.0(2)u) is even
larger than that in 1. As expected based on the bending angles for
the boryl groups, the distances from boron to Fe(II) (2.869 to
2.885 s) are considerably shorter than those to the Fe(III) centers
(3.100 to 3.173 s) in 1+PF6 and also shorter than those in 1
(2.957 s). We attribute these effects to (i) strongly diminished
interaction of Fe(III) of the ferricenyl moiety with the boron
centers and (ii) enhanced interaction of Fe(II) of the ferrocenyl
group with the boron centers. The latter is highly intriguing in that
it provides strong evidence that the boron centers in 1+PF6 become
more electron-deficient due to the presence of an oxidized ferricenyl
moiety. These results are in excellent agreement with theoretical
studies by Wagner and Holthausen et al. on the structural and
electronic effects of oxidation of FcBH2, which they predicted to
lead to a decrease in the dip angle for the boryl group from 25.1 to
5.0u and a reduction of electron density not only at Fe but also B.17
To further investigate the apparent Lewis acidity enhancement
of boron in the oxidized diboradiferrocene, we studied the
complexation with nucleophiles. The neutral molecule 1 coordi-
nates only weakly to pyridine and does not bind to MeCN.18
However, when we treated 1+I5 with excess MeCN, the solution
color turned from brown to green and a red precipitate formed.{
.
dication 22+ Similar B–N distances of 1.610(3) s20 and
1.616(3) s21 were reported by two different groups for the
complex of MeCN with the fluorinated Lewis acid B(C6F5)3. The
C–N bond length of 1.122(7) s for the bound MeCN in 22+(I3)2 is
slightly shortened relative to that of a cocrystallized unbound
molecule of MeCN with 1.163(10) s (literature value for free
MeCN: 1.141(2) s21) and in a similar range as that of
(C6F5)3B?MeCN (1.124(3) s).21 While the relatively large standard
deviations for the structure of 22+(I3)2 do not permit more detailed
discussions, the IR data further support strong binding of MeCN.
The C–N stretch at 2345 cm21 in the IR spectrum of 22+(I3)2 is
Fig. 2 ORTEP plot of 22+(I3)2. The I32 counterions and MeCN solvent
molecules are omitted for clarity. The asterisks indicate atoms in
equivalent positions (1 2 x, 1 2 y, 2 2 z). Selected interatomic distances
(s) and angles (u) for 22+(I3)2: B1–C6 1.607(8), B1–C10* 1.626(8), B1–N1
1.594(6), B1–C11 1.617(7), C17–N1 1.122(7), C6–B1–N1 109.1(4), N1–B1–
C10* 105.4(4), N1–B1–C11 106.7(4), C6–B1–C10* 110.7(4), C6–B1–C11
112.3(4), C10*–B1–C11 112.3(4), Cpcent–C6–B1 175.6(1), Cpcent–C10–B1*
177.6(1), Cpcent–Cpcent 3.414(2), Cp//Cp 5.8, Cp//C4B2 2.4.
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only slightly lower than those of (C6F5)3B?MeCN (2367 cm21 21
)
100, 1391–1434; (c) S. Yamaguchi, S. Akiyama and K. Tamao,
J. Organomet. Chem., 2002, 652, 3–9; (d) F. Ja¨kle, Boron:
Organoboranes, in Encyclopedia of Inorganic Chemistry, ed. R. B.
King, Wiley, Chichester, 2nd edn, 2005.
2 W. E. Piers, Adv. Organomet. Chem., 2005, 52, 1–76.
3 G. Erker, Dalton Trans., 2005, 1883–1890.
and Br3B?MeCN (2362 cm21).22 Importantly, the band is
considerably shifted relative to the cocrystallized free MeCN
(2246 cm21). A new band at 727 cm21 is also observed, which
from comparison to that of Br3B?MeCN (715 cm21 22
)
is
4 For example: (a) W. E. Piers, G. J. Irvine and V. C. Williams, Eur. J.
Inorg. Chem., 2000, 2131–2142; (b) P. A. Chase, L. D. Henderson,
W. E. Piers, M. Parvez, W. Clegg and M. R. J. Elsegood,
Organometallics, 2006, 25, 349–357; (c) M. Melaimi and F. P. Gabba¨ı,
J. Am. Chem. Soc., 2005, 127, 9680–9681; (d) R. Boshra,
A. Sundararaman, L. N. Zakharov, C. D. Incarvito, A. L. Rheingold
and F. Ja¨kle, Chem.–Eur. J., 2005, 11, 2810–2824.
5 (a) R. Roesler, B. J. N. Har and W. E. Piers, Organometallics, 2002, 21,
4300–4302; (b) Y. Qin, G. Cheng, A. Sundararaman and F. Ja¨kle, J. Am.
Chem. Soc., 2002, 124, 12672–12673; (c) M. Schlo¨gl, S. Riethmueller,
C. Troll, M. Mo¨ller and B. Rieger, Macromolecules, 2004, 37, 4004–4007.
6 (a) M. Yasuda, S. Yoshioka, S. Yamasaki, T. Somyo, K. Chiba and
A. Baba, Org. Lett., 2006, 8, 761–764; (b) T. K. Wood, W. E. Piers, B.
A. Keay and M. Parvez, Org. Lett., 2006, 8, 2875–2878.
7 (a) J. J. Eisch and B. W. Kotowicz, Eur. J. Inorg. Chem., 1998, 761–769;
(b) P. A. Chase, W. E. Piers and B. O. Patrick, J. Am. Chem. Soc., 2000,
122, 12911–12912; (c) M. V. Metz, D. J. Schwartz, C. L. Stern, P. N.
Nickias and T. J. Marks, Angew. Chem., Int. Ed., 2000, 39, 1312–1316.
8 C.-W. Chiu and F. P. Gabba¨ı, J. Am. Chem. Soc., 2006, 128,
14248–14249.
tentatively assigned to the B–N stretch.
In conclusion, oxidation of one of the iron centers in dibora-
diferrocene (1) leads to a mixed-valent species that, depending on
the counterion, is valence delocalized or trapped in the solid state.
The structural features of valence trapped 1+PF6 provide direct
evidence that a ferricenyl group acts as a strongly electron-
withdrawing substituent on boron. The enhanced Lewis acidity is
further reflected in the facile binding of MeCN, with formation of
the dioxidized species 22+, which shows spectroscopic and crystal-
lographic features that are consistent with strong Lewis acid–base
interactions. Further studies on the use of oxidized diboradiferro-
cenes as anion sensors and Lewis acid catalysts are in progress.
Acknowledgement is made to the donors of the Petroleum
Research Fund, administered by the American Chemical Society,
for support of this research. FJ thanks the Alfred P. Sloan
Foundation for a research fellowship and the National Science
Foundation for a CAREER award. Funding for an X-ray
diffractometer was provided by NSF (CRIF 0443538). We are
grateful to Prof. Mendelsohn, Dr Carol Flach and Guojin Zhang
for acquisition of IR data of compound 22+(I3)2 and to Prof.
Lalancette and A. Doshi for helpful discussions.
9 T. Agou, J. Kobayashi and T. Kawashima, Inorg. Chem., 2006, 45,
9137–9144.
10 The reversible oxidation of ferrocenylboronates has been exploited for
the sensing of fluoride. See, for example: (a) C. Dusemund, K. R. A. S.
Sandanayake and S. Shinkai, J. Chem. Soc., Chem. Commun., 1995,
333–334; (b) S. Aldridge, C. Bresner, I. A. Fallis, S. J. Coles and
M. B. Hursthouse, Chem. Commun., 2002, 740–741; (c) C. Bresner,
S. Aldridge, I. A. Fallis, C. Jones and L.-L. Ooi, Angew. Chem., Int. Ed.,
2005, 44, 3606–3609. Notably, in the case of other acyclic ferrocene-
based triarylboranes, oxidation of ferrocene was reported to be an
irreversible process (see citations in ref. 18).
Notes and references
{ Oxidation of 1 with AgPF6: to a solution of 1 (100 mg, 0.184 mmol) in
CH2Cl2 (6 mL) was added AgPF6 (46.4 mg, 0.184 mmol) in CH2Cl2 (3 mL)
at 235 uC. The mixture was stirred at RT for 10 min, filtered, and the
solvent removed under vacuum to give a crystalline brown solid. Yield:
111 mg (70%). X-Ray quality crystals were grown from a mixture of CHCl3
11 K. Venkatasubbaiah, L. N. Zakharov, W. S. Kassel, A. L. Rheingold
and F. Ja¨kle, Angew. Chem., Int. Ed., 2005, 44, 5428–5433.
12 Details of the Mo¨ssbauer studies will be reported elsewhere.
13 M.B.RobinandP.Day,Adv.Inorg.Chem.Radiochem.,1967,10,247–422.
14 The corresponding mixed-valent silicon-bridged diferrocene
[Cp2Fe2(m-C5H3SiMe2)2]+ does not show any detectable IVCT bands
in the NIR region. See: (a) H. Atzkern, J. Hiermeier, F. H. Ko¨hler and
A. Steck, J. Organomet. Chem., 1991, 408, 281–296; (b) U. Siemeling,
P. Jutzi, E. Bill and A. X. Trautwein, J. Organomet. Chem., 1993, 463,
151–154; (c) J. Kreisz, R. U. Kirss and W. M. Reiff, Inorg. Chem., 1994,
33, 1562–1565; (d) F. H. Ko¨hler, A. Schell and B. Weber, J. Organomet.
Chem., 1999, 575, 33–38.
15 The zwitterion ferricenyl(III)tris(ferrocenyl(II))borate has been reported
to be valence trapped and shows similar structural features:
D. O. Cowan, P. Shu, F. L. Hedberg, M. Rossi and T. J.
Kistenmacher, J. Am. Chem. Soc., 1979, 101, 1304–1306.
16 For example: (a)R.J.Webb,S.J.Geib,D.L.Staley,A.L.Rheingoldand
D. N. Hendrickson, J. Am. Chem. Soc., 1990, 112, 5031–5042; (b) T. Oda,
S. Nakashima and T. Okuda, Inorg. Chem., 2003, 42, 5376–5383.
17 M. Scheibitz, M. Bolte, J. W. Bats, H.-W. Lerner, I. Nowik, R. H.
Herber, A. Krapp, M. Lein, M. C. Holthausen and M. Wagner, Chem.–
Eur. J., 2005, 11, 584–603 and references therein.
1
and hexanes at 235 uC. H NMR (500 MHz, CDCl3, 25 uC) d 21.1 (br,
10H, free-Cp), 19.4 (br, 2H, Cp-4), 15.2 (br, 4H, Cp-3,5), 5.31 (CH2Cl2),
3.78, 3.25, 2.92 (br, 10H, Ph). 11B NMR (160 MHz, CD2Cl2, 25 uC) d 49.7
(w1/2 = 1600 Hz). UV-Vis (CH2Cl2, 2.60 6 1023 M): lmax = 1543 nm (e =
330 M21 cm21). Calcd for C32H26B2Fe2PF6?2CH2Cl2: C 47.56, H 3.52;
found C 48.20, H 3.74%.
Binding of acetonitrile to 1+I5: synthesis of 22+(I3)2: MeCN (5 mL) was
added to 1+I5 (34.3 mg, 29.1 mmol) at RT and the mixture stirred for
30 min. The color of the solution turned green with a red precipitate that
was identified as 1 from 1H NMR data (yield: 2.3 mg, 87%). The solution
was filtered, the volume reduced to 2 mL, and the mixture kept for
crystallization at 235 uC. Yield 22+(I3)2(MeCN)2: 31.4 mg (88%). X-Ray
quality crystals were grown by recrystallization from MeCN at 235 uC. 1H
NMR (500 MHz, CD3CN, 25 uC) d 32.5 (br, Cp), 5.42, 4.71, 23.32 (br,
Ph), 1.94 (MeCN). 11B NMR (160 MHz, CD3CN, 25 uC) d 20.2 (w1/2
=
640 Hz). UV-Vis (MeCN, 3.29 6 1024 M): lmax (nm) = 553 (e =
760 M21 cm21), 644 (e = 920 M21 cm21). IR (KBr): n (cm21) = 2345 (CN,
bound MeCN), 2246 (CN, free MeCN), 727 (B–N). Calcd for
C36H32B2Fe2I6N2?2MeCN: C 32.69, H 2.61, N 3.81; found C 32.46, H
2.27, N 3.62%.
18 FcB(C6F5)2 does not significantly bind MeCN, but 1,19-Fc(B(C6F5)2)2
has been reported to bind MeCN even at ambient temperature: (a)
B. E. Carpenter, W. E. Piers, M. Parvez, G. P. A. Yap and S. J. Rettig,
Can. J. Chem., 2001, 79, 857–867; (b) B. E. Carpenter, W. E. Piers and
R. McDonald, Can. J. Chem., 2001, 79, 291–295.
§ Crystallographic data: 1+PF6?2CHCl3, CCDC 635884:
C34H28B2Cl6F6Fe2P, M = 927.55, T = 100(2) K, monoclinic, P21/c, a =
18.5711(3), b = 25.9704(4), c = 15.3244(3) s, b = 100.0530(10)u, V =
7277.5(2) s3, Z = 8, m = 11.374 mm21, Rint = 0.0493, R1 = 0.0513,
19 Note that the excess iodine in the precursor 1+I5 serves to reoxidize the
neutral complex 1, thus resulting in an overall reaction stoichiometry of 6
1+I5 +10MeCNA522+(I3)2 +1. Removal of solid 1from the equilibrium
promotes disproportionation and nucleophile binding facilitates the
oxidation process as evident from considerably lower oxidation potentials
in the cyclic voltammogram of 1 in coordinating solvents.
20 C. Bergquist, B. M. Bridgewater, C. J. Harlan, J. R. Norton, R. A.
Friesner and G. Parkin, J. Am. Chem. Soc., 2000, 122, 10581–10590.
21 H. Jacobsen, H. Berke, S. Do¨ring, G. Kehr, G. Erker, R. Fro¨hlich and
O. Meyer, Organometallics, 1999, 18, 1724–1735.
wR2
= 0.1220 (I . 2s(I)). 2
2+(I3)2?2MeCN, CCDC 635885:
C40H38B2Fe2I6N4, M = 1469.46, T = 100(2) K, monoclinic, P21/c, a =
11.7183(9), b = 8.9579(7), c = 21.3539(15) s, b = 92.612(4)u, V =
2239.2(3) s3, Z = 2, m = 37.924 mm21, Rint = 0.0353, R1 = 0.0455,
wR2 = 0.1172 (I . 2s(I)). For crystallographic data in CIF or other
electronic format, see DOI: 10.1039/b701807j
1 (a) E. H. Yamamoto, Lewis Acids in Organic Synthesis, Wiley-VCH,
New York, 2000; (b) E. Y.-X. Chen and T. J. Marks, Chem. Rev., 2000,
22 D. F. Shriver and B. Swanson, Inorg. Chem., 1971, 10, 1354–1365.
2156 | Chem. Commun., 2007, 2154–2156
This journal is ß The Royal Society of Chemistry 2007