Syntheses and anion binding capabilities of bis(diarylboryl) ferrocenes and related systems

Article abstract of DOI:10.1021/om400152x

Isomeric diborylated ferrocenes featuring 1,1′-, 1,2-, and 1,3-substitution patterns have been targeted via a combination of electrophilic aromatic substitution and directed ortho-lithiation protocols. While none of these systems are competent for the Lewis acid chelation of fluoride, related systems featuring a mixed B/Si acceptor set capture 1 equiv of fluoride via a Si-F-B bridging motif.

Full text of DOI:10.1021/om400152x

Article
pubs.acs.org/Organometallics
Syntheses and Anion Binding Capabilities of Bis(diarylboryl)
Ferrocenes and Related Systems
Michael J. Kelly, Alexander E.J. Broomsgrove, Ian R. Morgan, Inke Siewert,† Philip Fitzpatrick,
Jessica Smart, Dragoslav Vidovic,‡ and Simon Aldridge*
Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QR, U.K.
S
* Supporting Information
ABSTRACT: Isomeric diborylated ferrocenes featuring 1,1′-,
1,2-, and 1,3-substitution patterns have been targeted via a
combination of electrophilic aromatic substitution and directed
ortho-lithiation protocols. While none of these systems are
competent for the Lewis acid chelation of fluoride, related
systems featuring a mixed B/Si acceptor set capture 1 equiv of
fluoride via a Si−F−B bridging motif.
INTRODUCTION
RESULTS AND DISCUSSION
Synthetic Chemistry. Synthetically, the 1,1′-ferrocenediyl
■
■
The selective detection of CN− and F− represent significant
chemical challenges with applications, for example, in environ-
mental and medical monitoring/remediation.1−3 The affinity of
cyanide for tricoordinate boranes has been known for more
than 50 years,4 and recent studies have demonstrated the use of
Lewis acid receptors containing the related −BMes2 (Mes =
2,4,6-Me3C6H2) function to detect cyanide,5 in one case
offering selective binding in aqueous media.5d In this respect, a
major challenge in sensor design stems from the potential for
competitive binding of fluoride at −BMes2 and related Lewis
acid functions.6,7 Interestingly, recent reports emphasize the
importance not only of electronic factors (i.e. borane Lewis
acidity) but also of steric factors in determining the relative
binding affinities for cyanide and fluoride.5d,n
disubstitution pattern is easiest to achieve, relying on simple
electrophilic substitution chemistry to deliver the known 1,1′-
bis(dibromoboryl) precursor in a single step from ferrocene.11
Subsequent reaction with excess mesityllithium (although not
with the corresponding Grignard reagent)5k generates 1,1′-
fc(BMes2)2 (1a; fc = ferrocenediyl) as a red microcrystalline
material in 60−70% yield (Scheme 1). Particularly diagnostic of
Scheme 1. Syntheses of 1,1′-Bis(diarylboryl)ferrocenes 1a−c
a
in Two Steps from Ferrocene
Lewis acids featuring a ferrocenyl substituent offer the
additional possibility for an electrochemical (or even
colorimetric) reporter response.8 Moreover, multifunctional
systems of this type offer the potential for the complexation of
more than 1 equiv of substrate and an enhanced magnitude of
the binding-induced electrochemical shift.8d Alternatively,
convergent bifunctional systems offer the possibility for the
chelation of a single (appropriately shaped) anion, with benefits
in terms of enhanced binding affinity and potential selectivity.9
Disubstituted ferrocenes offer three distinct regiochemistries,
and in the current paper we report on synthetic routes to
diborylated systems with 1,1′-, 1,2-, and 1,3-dispositions,10
together with an investigation of their anion binding capabilities
with respect to F− and CN−. One aim was to explore strategies
for the binding of fluoride over cyanide via a preorganized
chelating motif. While 1,2-diborylated ferrocenes offer super-
ficial promise in this regard, consideration of the size of the
anion binding cavity led us also to evaluate Lewis acids
featuring larger, second-row elements (e.g. silanes). In the
event, among the receptor systems studied, mixed borane/
silane receptors represent the most promising candidates for
the Lewis acid chelation of F−.
a
Key reagents and conditions: (i) BBr3, as per ref 11; (ii) ArLi (> 4
equiv), diethyl ether, 18 h, 20 °C, 39−66%.
this transformation is the conversion of the 11B NMR
resonance at δB 50 due to 1,1′-fc(BBr2)2 to a broad peak at
δB 76 ppm (cf. δB 76 ppm for FcBMes2).5e 1a has been further
characterized by standard spectroscopic and analytical methods
but has thus far proved impossible to obtain as single crystals
suitable for X-ray crystallography, primarily due to its extremely
high solubility even in nonpolar organic media. As a
consequence, the closely related derivatives 1,1′-fc(BXylF )2
2
(1b) and 1,1′-fc(BXyl2)2 (1c) were synthesized by analogous
chemistry (XylF = 2,6-Me2-4-F-C6H2; Xyl = 2,6-Me2C6H3).
These compounds were characterized by similar 11B NMR
Received: February 25, 2013
Published: April 18, 2013
© 2013 American Chemical Society
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shifts (δB 78 and 77 ppm, respectively), and in the case of 1c,
suitable single crystals can be obtained by slow evaporation of a
solution in diethyl ether.
Scheme 2. Synthetic Strategy for the Synthesis of 1,3-
Bis(diarylboryl) Systems Featuring a Pentamethylferrocene
a
Skeleton
The molecular structure of 1c (Figure 1) confirms the
expected planar three-coordinate geometry at each borane
a
Key reagents and conditions: (i) BBr3 (4.5 equiv), as per ref 13; (ii)
MesLi (> 6 equiv), diethyl ether, −78 to +20 °C then 12 h at +20 °C,
43% (as acetone disolvate) after recrystallization from acetone.
protons), reflecting hindered rotation around both the B−
Cipso and B−CCp bonds. At −50 °C the limiting low-
temperature spectrum is obtained, featuring four ortho and
two para methyl signals. Such a spectrum is consistent with the
two −BMes2 groups being related by symmetry, but with slow
exchange on the NMR time scale within each −BMes2 unit. At
+50 °C, in contrast, two sharp mesityl methyl signals are
observed (in the ratio 2:1) corresponding to all eight ortho and
all four para methyl groups, respectively. Interestingly, at −10
°C the spectrum shows one signal for the para methyl groups
and two signals for the ortho methyls, implying that rotation
about the B−CCp,ipso linkage is facile (rendering equivalent the
two Mes substituents within each −BMes2 unit) but that
rotation about each B−CMes,ipso bond (which would inter-
convert the ortho methyl groups within each Mes substituent)
is not.
Crystallographic studies also provide evidence of the highly
strained nature of 2 (Figure 1). Thus the angle α between the
least-squares planes of the Cp and Cp* rings is 17.9°, a figure
which can be put into context by comparison with 0.8° for
pentamethylferrocene itself and 16.1° for the single-atom ansa-
bridged system {Me2Si(C5Me4)2}Fe.14 Green has calculated the
energetic profile for ferrocene associated with distorting the Cp
rings away from coplanarity, and on that basis a penalty of ca.
40−50 kJ mol−1 can be estimated for the conformation adopted
by 2 (in the solid state at least).15 In addition, as in the case of
1c, the −BAr2 units in 2 are bent away from the iron center,
such that the two ∠(Cp centroid−Cipso−B) angles are 168.9
and 171.1°.
Figure 1. Molecular structures of 1,1′-fc(BXyl2)2 (1c, upper) and 1,3-
fc*(BMes2)2 (2, lower) as determined by X-ray crystallography.
Hydrogen atoms are omitted and Cp* methyl groups (for 2) shown in
wireframe format for clarity; thermal ellipsoids are set at the 40%
probability level. Key bond lengths (Å) and angles (deg): for 1c,
C(6)−B(7) = 1.546(3), B(7)−C(8) = 1.602(3), B(7)−C(16) =
1.594(3), C(25)−B(29) = 1.552(3), C(6)−B(7)−C(8) = 122.5(2),
C(6)−B(7)−C(16) = 119.0(2), C(8)−B(7)−C(16) = 118.3(2); for 2,
C(3)−B(26) = 1.558(3), C(6)−B(7) = 1.549(3), C(6)−B(7)−C(8)
= 120.1(2), C(6)−B(7)−C(17) = 119.3(2), C(8)−B(7)−C(17) =
120.4(2).
function (with angles at boron summing to 359.8 and 359.6°);
the xylyl substituents are aligned in a propeller-like arrange-
ment, with angles between the least-squares BC3 plane and the
aryl rings of 61.7, 64.0, 64.3, and 71.1°. Additionally, the
bending toward the iron center typically observed in ferrocenes
featuring a Lewis acidic substituent is not seen in 1c;12 instead,
a contrasting effect is observed whereby the BXyl2 groups both
bend away from Fe(1) (∠(Cp centroid−Cipso−B) = 173.0,
171.4°), presumably on steric grounds.
Electrophilic aromatic substitution chemistry also provides
the underpinning synthetic entry point into one class of 1,3-
disubstituted ferrocene. Thus, the ability to introduce two BBr2
groups onto a single cyclopentadienyl ring under more forcing
conditions can be exploited in the case of pentamethylferrocene
to generate 1′,2′,3′,4′,5′-pentamethyl-1,3-bis(dibromoboryl)-
ferrocene, 1,3-fc*(BBr2)2;13 subsequent conversion into the
bis(dimesitylboryl) compound 1,3-fc*(BMes2)2 (2; Scheme 2)
can be effected by employing the same methodology as used for
1a. 2 has been characterized by standard spectroscopic and
analytical techniques and by single-crystal X-ray diffraction
(Figure 1).
While the “blocking” of one cyclopentadienyl ring to
electrophilic attack by permethylation allows a 1,3-disposition
of boryl functions to be realized, achieving the same
substitution pattern for the parent ferrocene skeleton proves
to be more difficult. Perhaps the most promising approach
(Scheme 3) utilizes the known intermediate rac-1,2-fc(BMes2)
Br (3a), itself produced via a convenient isomerization step
from readily available 1,1′-fcBr2.10a,c,16 Reaction of 3a with the
hindered amide base lithium 2,2,6,6-tetramethylpiperidide,
Li(tmp), followed by FBMes2 leads to the formation of the
2,5-bis(dimesitylboryl)-1-bromo derivative 4 in ca. 50% isolated
yield via bromide-directed ortho lithiation. Debromination of 4,
however, proves remarkably difficult to drive to completion,
presumably due to the highly sterically shielded nature of the Br
substituent located between two o-BMes2 groups. Thus, even in
t
the case of BuLi, the resulting debrominated product 5 could
Both NMR and crystallographic studies are consistent with a
very high degree of steric crowding in this system. Thus, the 1H
NMR spectrum of 2 at 20 °C shows a single broad resonance
for the ortho methyl groups of the mesityl substituents (and a
corresponding broad singlet for the meta aromatic CH
only be obtained in 95% purity, as an intractable mixture with
5% of unreacted 4.
Intermediate 3a, used in conjunction with alkyllithium bases,
also provides access to the final (1,2) disubstitution pattern
n
(Scheme 3). Thus, lithiation with BuLi followed by a FBMes2
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Scheme 3. Syntheses of Symmetrical and (Racemic)
Unsymmetrical 1,2-Bis(diarylboryl)ferrocenes and of 1,3-
Disubstituted Systems through the Intermediacy of rac-1,2-
a
fc(BMes2)Br (3a)
a
n
Key reagents and conditions: (i) BuLi (1 equiv), tetramethylpiper-
idine (1 equiv), thf, −40 °C, 2 h, then FBAr2 (>3 equiv), thf, −78 to
+20 °C then 15 h at +20 °C, 51−76% yield after chromatography and
recrystallization; (ii) Li(tmp) (0.94 equiv), thf, −78 °C, 2 h, then
FBMes2 (1.21 equiv), thf, −78 to +20 °C, then 12 h at 20 °C, 52%;
(iii) tBuLi (2.2 equiv), thf, −78 °C, 2 h, aqueous workup, then
Figure 2. Molecular structures of one of the two components of the
asymmetric unit of 1,2-fc(BMes2)2 (6a, upper) and rac-1,2-fc(BMes2)-
(SiMe2OMe) (9, lower) as determined by X-ray crystallography.
Hydrogen atoms are omitted for clarity, and thermal ellipsoids are set
at the 40% probability level. Key bond lengths (Å) and angles (deg):
for 6a, C(6)−B(7) = 1.574(7), C(5)−B(26) = 1.569(7), B(7)···B(26)
= 3.702, C(5)−C(6)−B(7) = 135.5(4), C(6)−C(5)−B(26) =
133.7(4), C(2)−C(6)−B(7) = 114.4(4), C(4)−C(5)−B(26) =
120.6(4); for 9, C(2)−Si(26) = 1.844(2), C(3)−B(7) = 1.572(3),
Si(26)−O(27) = 1.640(2), C(2)−C(3)−B(7) = 132.4(2), C(4)−
C(3)−B(7) = 119.4(2), C(3)−C(2)−Si(26) = 127.4(1), C(6)−
C(2)−Si(26) = 125.6(2).
n
repetition, up to 90−95% purity (remainder 4); (iv) BuLi (1 equiv),
tmeda (1 equiv), diethyl ether, −78 °C, 1 h, then FBAr2 (> 4 equiv),
diethyl ether, −78 to +20 °C then 12 h at +20 °C, 17−83% yield after
chromatography and recrystallization.
quench provides access to the 1,2-bis(dimesitylboryl) species
6a, while analogous chemistry utilizing rac-1,2-fc(BXyl2)Br
(3b) and either FBMes2 or FBXyl2 generates rac-1,2-fc(BXyl2)-
(BMes2) (6b) and 1,2-fc(BXyl2)2 (6c), respectively (Scheme
3). 6a−c have been characterized by standard spectroscopic
and analytical techniques, with the 11B NMR shift in each case
(δB ∼80 ppm) again being diagnostic.5e Here too, variable-
temperature NMR studies, allied to crystallography (Figure 2),
reveal very high steric loading at the Lewis acidic boron centers.
In this case steric repulsion between the two −BMes2 units
presumably contributes (i) to the wide B···B separation (3.684
Å (mean), cf. 3.263 Å for 1,2-C6H4[B(C6F5)2]2 and 3.353 Å for
1,8-C10H6(BMes2)(BPh2)),9g,j and (ii) to considerable dis-
tortion of one of the two −BMes2 units. Thus, in each of the
two 1,2-fc(BMes2)2 molecules making up the asymmetric unit,
one −BMes2 group features a boron center (i.e. B(7) or B(76))
which lies ca. 0.5 Å out of the plane of the C5H3 ring (cf. <0.06
Å for the other boron center in each molecule); the associated
BC2 least-squares plane is canted at an angle of ca. 45° with
respect to that of the Cp ring (cf. ∼5° for the other). In
addition, there is a marked difference between the two B−
CCp,ipso−CCp,ortho angles for both B(7) and B(76) (135.2°
(mean) and 114.6° (mean)), reflecting the need to minimize
steric repulsions between mesityl ring systems by “bending
back” the −BMes2 units. These geometric factors presumably
contribute to a redox potential for 6a (+0.177 V vs FcH/FcH+
in thf) that while shifted anodically with respect to FcBMes2
(0.131 V in the same solvent) is less positive than that for the
1,1′-isomer 1a (+0.197 V).
signal is observed at 120 °C, while eight distinct o-CH3 signals
can be identified at −60 °C. In addition, at −60 °C the 2:1
pattern for the protons of the C5H3 group is split into three
distinct signals (with relative intensities 1:1:1), consistent with
slowing down of the synchronized “windshield wiper” motion
of the two −BMes2 units. Consideration of the coalescence
behavior of the respective signals yields an activation barrier,
ΔG⧧, of 48 kJ mol−1.10c A further fluxional process involving
completely free rotation about both B−CCp,ipso and B−CMes,ipso
bonds on the NMR time scale (and thus giving rise to the
1
simple H spectrum observed at +120 °C) can be identified
with a coalescence temperature of +45 °C.10c
Finally, in order to explore the broader possibilities for anion
chelation by 1,2-disubstituted ferrocenes we have additionally
prepared the (racemic) mixed dimesitylboryl/dimethylfluor-
osilyl species 8. The synthesis of 8 also relies on the key
intermediate 3a, being accomplished via the corresponding
chlorosilyl derivative 7 (Scheme 4).10a,c,17,18 Characterization
was achieved by standard spectroscopic techniques, and
although 8 itself proved unsuitable for crystallographic study,
the related methoxysilyl derivative 9 could be structurally
authenticated (Figure 2). The expected sterically crowded 1,2-
disposition of the boryl and silyl functions is thus confirmed. As
with the corresponding bis(boryl) system 6a, comparison of the
two CCp,ortho−CCp,ipso−B angles (119.4(2) and 132.4(2)°) is
consistent with bending of the boryl function away from the
ortho substituent, presumably on steric grounds. Despite the
presence of the silyl ether oxygen donor and borane Lewis acid,
As with complex 2, the influence of steric factors is also
apparent in the 1H NMR spectrum of 6a, which features broad
signals for both the mesityl ortho methyl and meta CH protons
at 20 °C. A spectrum featuring only one o-CH3 and one m-CH
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Scheme 4. Syntheses of Racemic 1,2-Disubstituted
Ferrocenes Featuring Dimesitylboryl and Silyl Substituents
a
a
n
Key reagents and conditions: (i) BuLi (2.0 equiv), Et2O/pentane,
−78 °C, 30 min, then Me2SiCl2 (ca. 10 equiv), −78 °C to room
temperature, 15 h at room temperature, 91%; (ii) for 8, CuF2 (1.0
equiv.), CH2Cl2, room temperature, 3 h, 93% (of 98% pure material),
for 9, stirring in MeOH, 1 h, room temperature, then MeOH/Et2O
(1/1), 3 h, room temperature, 82%.
the B···O distance (3.352(3) Å) is comfortably outside the sum
of the respective covalent radii.
Fluoride and Cyanide Binding. The reaction of 1a with
excess KCN/18-crown-6 in acetonitrile is characterized by a
shift in the 11B NMR spectrum from a broad resonance at δB 76
to a sharp peak at δB −16 ppm and is accompanied by a color
change from dark red to yellow. Both observations are
consistent with the behavior of the parent compound FcBMes2
in the presence of cyanide.5e With regard to binding
stoichiometry, single crystals of the bis(cyanide) adduct
[K(18-crown-6)]2[1a·(CN)2] suitable for crystallography
could subsequently be obtained by layering the acetonitrile
reaction mixture with diethyl ether. The structure of the
dianionic component so determined is centrosymmetric,
featuring equivalent binding of cyanide at both receptor sites,
which are then oriented so as to be optimally distant from one
another. While the quality of the structure solution is not
optimal for discussion of the metrical parameters, a very closely
related structure has been determined for the corresponding
XylF system [K(18-crown-6)]2[1b·(CN)2] (Figure 3). In this
case the metrics of the B−C−N binding motif are more reliable
and are in fact very similar to those determined for the parent
complex [FcBMes2·CN]−: e.g., d(B(1)−C(6)) = 1.630(4) Å
and ∠(B(1)−C(6)−N(1)) = 171.0(3)° (cf. 1.621(3) Å and
169.8(3)°5e). A secondary interaction between the nitrogen
atom of the cyanide guest and the potassium center of the
[K(18-crown-6)]+ counterion (d(K···N) = 2.780(3) Å) also
finds precedent in previously reported cyanide adducts of
simple monofunctional ferrocenyl boranes.5e,k,m
The mode of interaction of cyanide with 1a can be shown to
be solvent dependent. Thus, in much more weakly donating
chloroform, the reaction of 1a with KCN/18-crown-6 generates
the monoanionic monocyanide adduct [K(18-crown-6)]
[1a·CN]. The progress of this reaction is difficult to assess
unambiguously by 11B NMR, insofar as the resonance
associated with the remaining uncomplexed (three-coordinate)
borane is extremely broad and difficult to detect. The four-
coordinate borate center gives rise to a sharp signal at δB −13
ppm, similar to those measured for [1a·(CN)2]2− and
[FcBMes2·CN]−.5e Interestingly, the marked color change
from deep red to yellow observed on formation of
[1a·(CN)2]2− from 1a (and indeed on the complexation of
CN− to the parent system FcBMes2) is not observed on
formation of [1a·CN]−, consistent with the idea that the Fe(η5-
C5H4BMes2) unit is a key chromophore. The formation of
[1a·CN]− in chloroform solution is supported by the results of
negative ion ESI-MS and 1H/13C NMR spectroscopy, the latter
Figure 3. Molecular structures of [K(18-crown-6)]2[1b·(CN)2]
(upper) and [K(18-crown-6)][1a·CN] (lower) as determined by X-
ray crystallography. Hydrogen atoms and second disorder component
(for [K(18-crown-6)][1a·CN]) are omitted and crown ether
molecules shown in wireframe format for clarity; thermal ellipsoids
are set at the 40% probability level. Key bond lengths (Å) and angles
(deg) for [K(18-crown-6)]2[1b·(CN)2]: B(1)−C(6) = 1.630(4),
C(6)−N(1) = 1.151(4), N(1)−K(1) = 2.780(3), B(1)−C(1) =
1.636(4), B(1)−C(6)−N(1) = 171.0(3), C(6)−N(1)−K(1) =
142.3(2).
revealing distinct sets of resonances due to the inequivalent
mesityl and C5H4 moieties associated with the boryl/borate
functionalized Cp rings. The structure of [K(18-crown-
6)][1a·CN] in the solid state was also confirmed crystallo-
graphically (Figure 3), although here the presence of pernicious
disorder rules out an in-depth analysis of metrical data.
UV/vis titrations were carried out in order to estimate the
binding affinities of 1a associated with both the first and second
cyanide complexation events; values of log K1 = 8.88(0.20) and
log K2 = 5.90(0.11) were determined in thf solution, the solvent
system being chosen to provide compatibility with measure-
ments carried out on 1,2-bifunctional systems (vide infra). The
value determined for log K1 is significantly greater than that for
the related monofunctional system FcBMes2 in thf (log K =
6.55(0.13)), consistent with the (known) electron-withdrawing
effects of the additional −BMes2 group.5e The negative
cooperativity implied by the value determined for log K2
reflects obvious electrostatic factors, albeit mitigated somewhat
by the conformational flexibility of the ferrocenediyl core.
The parent system FcBMes2 shows a competing response in
the presence of fluoride,5e and 1a was found to be no different
in this respect. Reaction with either [nBu4N]F or KF/18-crown-
6 in acetonitrile or chloroform results in a shift in the 11B NMR
spectrum from δB 76 to ca. 6 ppm, consistent with previously
reported fluoride adducts of ferrocenylboranes.5e Moreover, the
corresponding 19F NMR spectrum reveals a broad peak at δF
−181 ppm, also indicative of fluoride complexation. This
competing affinity for fluoride can be attributed to the known
strength of the B−F bond in such systems;6 UV/vis titrations
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were carried out to determine fluoride binding constants in thf
solution, which (as with FcBMes2) were found to be very
similar to the corresponding cyanide affinities (log K1
8.26(0.09), log K2 = 5.60(0.06)).
=
Investigation of 1,1′-disubstituted ferrocenes therefore
reveals that the two binding sites in these systems act more
or less independently of one another. While alternative
convergent binding motifs have been established for more
strongly Lewis acidic 1,1′-diborylated coboltocenium systems,19
we sought in the current study to encourage chelation by using
ferrocene-based frameworks with reduced conformation
flexibility. With this in mind, the anion binding capabilities of
other diborylated ferrocenes have been investigated, with the
ultimate aim of achieving selectivity for anionic guest species
based on a chelating mode of interaction.
Figure 4. Plots of (left) (A − A∞)/(A0 − A∞) vs time and (right) ln(A
− A∞) vs time (both measured at λ 570 nm) for the reactions in thf of
n
⧫
■
▲
[ Bu4N]F with 6a ( ), 6b ( ), and 6c ( ). For the logarithmic plot,
the solid lines give a least-squares (linear) fit (with R2 = 0.998, 0.996,
and 0.993, respectively), from which pseudo-first-order rate constants
(kobs) of 1.64 × 10−2, 2.08 × 10−2, and 2.79 × 10−2 s−1 can be derived.
The 1,3-disubstituted system 1,3-fc*(BMes2)2 (2) is clearly
not optimized for a chelating mode of anion binding, and
(although preliminary experiments are consistent with the
binding of only 1 equiv of either fluoride or cyanide) precise
structural and thermodynamic data for the respective adducts
proved impossible to obtain in our hands due to their
unworkable sensitivity toward oxidation.20 The electronic
effects of the Cp* ligand (estimated to give rise to a cathodic
redox shift of ca. −300 mV) render this system markedly more
susceptible to electron loss.8d,21 Superficially, the 1,2-disub-
stituted systems, although synthetically the most challenging,
offer the most favorable degree of preorganization with respect
to the chelation of monatomic anions; similar bis(borylated)
benzene frameworks, for example, have previously been shown
to be capable of the chelation of fluoride (Chart 1).9z
very slow. Monitoring of the intensity of the band at 570 nm
due to the free receptor as a function of time under pseudo-
first-order conditions (i.e. in the presence of an 20-fold excess
of fluoride) gives a linear plot of ln(A − A∞) vs. time, from
which a rate constant, kobs, of 1.64 × 10−2 s−1 can be
determined. In an attempt to understand the structural origins
of the slow fluoride uptake by 6a and the essentially
instantaneous binding of cyanide, X-ray diffraction studies
were carried out on single crystals of both [K(18-crown-
6)][6a·F] and [K(18-crown-6)][6a·CN] (Figure 5). Notably,
the F− anion interacts with only one of the two boron centers,
presumably reflecting the wide B···B separation determined for
the free receptor 6a (mean 3.684 Å for the two molecules in the
asymmetric unit), in comparison to typical B−μ-F bond lengths
(e.g. 1.585(5) and 1.633(5) Å for [1,8-C10H8(BMes2)
(BC12H8S)(μ-F)]−).8 22 Nevertheless, the fluoride ion is
s,
bound endo to the B...B cavity and features no significant
secondary interactions with the [K(18-crown-6)]+ counterion.
In contrast, the structure of [K(18-crown-6)][6a·CN], while
revealing that the anion is also bound in a non-chelating
fashion, implies that cyanide is situated exo to the B...B cavity
(in the solid state at least) and additionally interacts with the
potassium center of the [K(18-crown-6)]+ unit via N(9).
To determine whether a similar mode of fluoride binding
within the B···B binding domain could persist in solution and,
given the steric demands of the flanking mesityl substituents,
could therefore be implicated in the slow uptake of F−, we
examined the corresponding rates of reaction for 1,2-
fc(BMes2)(BXyl2) (6b) and 1,2-fc(BXyl2)2 (6c), which feature
successively reduced steric loading on the periphery of the
binding cavity. The results of these experiments are shown in
Figure 4, from which pseudo-first-order rate constants of 2.08 ×
10−2 and 2.79 × 10−2 s−1 can be determined for 6b,c,
respectively. These (reproducible) differences in the rate of
fluoride uptake and the more marked difference in the binding
kinetics for fluoride and cyanide are consistent with binding of
F− within the B···B cavity in thf solution.
In contrast to related systems based on a benzene
backbone,9z the dimensions of the B···B cavity in 1,2-
diborylated ferrocenes therefore appear to preclude fluoride
chelation (Chart 1); the constraints of a five- (vs six-)
membered carbocyclic ring force the two borane binding sites
to lie too far apart, an effect which is exacerbated for the 1,2-
bis(dimesitylboryl) system due to steric factors. However, the
synthetic chemistry outlined in Scheme 3 offers wide
applicability in terms of the range of 1,2-difunctional ferrocenes
that are potentially accessible. Lithiation of the intermediate
rac-fc(BMes2)Br followed by an electrophilic quench offers
Chart 1. Interaction of Fluoride with 1,2-Diborylated
Receptors Based on Six (I)- and Five-Membered Carbocyclic
Skeletons (II) and Potential Interaction with a Heterotopic
Lewis Acid Featuring the Second-Row Lewis Acidic
Function ERn (III)
Somewhat surprisingly, however, a number of empirical
observations indicate that the binding of fluoride by 6a is in
fact very weak. Thus, no evidence for the formation of [6a·F]−
could be obtained from NMR measurements in either
dichloromethane or chloroform solution, and while single
crystals of [K(18-crown-6)][6a·F] could be obtained from thf
saturated with fluoride, solutions of the crystalline adduct in thf-
d8 rapidly regenerate the free receptor by fluoride loss in the
absence of excess anion. Electrochemical studies of 6a in thf
solution are also consistent with a small (≪100 mV) shift even
in the presence of excess fluoride. In addition, fluoride binding
by 6a can be shown by simple competition experiments to be
weaker than cyanide binding: e.g., by the quantitative formation
(by 11B NMR) of [6a·CN]− on addition to the free receptor of
a solution equimolar in both F− and CN−.
Interestingly, even in the presence of a vast excess of fluoride,
the rate of formation of the host/guest complex [6a·F]− (as
appraised by UV−vis spectroscopy; Figure 4) appears to be
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1
doublet of doublets (δSi 25.2 ppm, JSiF = 70, 190 Hz).
1
Additional evidence for a B−F−Si motif comes from the H
NMR signals associated with the diastereotopic Si−Me groups.
Thus, in the free receptor these give rise to two doublets (at δH
3
0.11 and 0.15 ppm, each with JHF = ca. 8 Hz), while in the
fluoride adduct these resonances are not only shifted apart (δH
−0.07 and 0.50 ppm) but also further split in each case by an
additional coupling (3−4 Hz) to the bridging fluorine (Figure
6).23 Precedent for this chelating mode of fluoride capture in
ferrocene systems comes from a B−μ-F−SnMe2F motif
reported by Jakle and co-workers.18d
̈
Figure 5. Molecular structures of (upper) the anionic component of
[K(18-crown-6)][6a·F] and (lower) [K(18-crown-6)][6a·CN]·CHCl3
as determined by X-ray crystallography. Hydrogen atoms, counterion
(for F− adduct), and solvate molecule (for CN− adduct) are omitted,
and the crown ether is shown in wireframe format for clarity; thermal
ellipsoids are set at the 40% probability level. Key bond lengths (Å)
and angles (deg): for [K(18-crown-6)][6a·F], B(31)−F(50) =
1.471(5), B(12)···B(31) = 3.602, C(10)−B(31) = 1.635(8), C(9)−
B(12) = 1.548(7), C(8)−C(9)−B(12) = 120.9(4), C(11)−C(10)−
B(31) = 123.2(4), C(10)−C(9)−B(12) = 133.0(4), C(9)−C(10)−
B(31) = 130.6(4); for [K(18-crown-6)][6a·CN], B(7)−C(8) =
1.648(4), C(8)−N(9) = 1.146(3), C(6)−B(7) = 1.672(4), C(2)−
B(28) = 1.568(4), B(7)···B(28) = 3.887, C(3)−C(2)−B(28) =
116.5(2), C(5)−C(6)−B(7) = 115.0(2), C(2)−C(6)−B(7) =
138.5(2), C(6)−C(2)−B(28) = 137.2(2), B(7)−C(8)−N(9) =
173.2(3).
1
Figure 6. Doublet of doublets H NMR resonances for each of the
diastereotopic methyl groups in [8·F]−.
CONCLUSIONS
■
Isomeric diborylated ferrocenes featuring all three potential
substitution patterns have been synthesized via a combination
of electrophilic aromatic substitution and ortho-lithiation
protocols. Of these, 1,1′-fc(BMes2)2 displays first binding
affinities (K1) for both F− and CN− in excess of those of the
parent monofunctional system FcBMes2, consistent with the
electron-withdrawing properties of the additional −BMes2
group. The values of K2 determined for both anions reflect a
negative cooperativity of between 2 and 3 orders of magnitude.
By comparison, 1,2-fc(BMes2)2 features a more rigid bifunc-
tional motif and, although only 1 equiv of either fluoride or
cyanide is taken up, binding in each case occurs via a single B−
X bond. Moreover, the diminished binding constants
determined for both F− and CN− in comparison to FcBMes2
reflect the unfavorable steric effects of the additional o-BMes2
group. While the size of the binding cavity (notably a B···B
separation on the order of 3.7 Å) precludes the chelation of
fluoride, the broad versatility of the synthetic methodology
employed allows access to a related B/Si heterotopic Lewis acid
which captures 1 equiv of fluoride via a Si−F−B bridging motif.
access to a wide variety of systems of the type rac-1,2-
fc(BMes2)(ERn). With this in mind, we hypothesized that an
alternative Lewis acid group, ERn, centered on a second-row
element E (such as silicon) might be consistent with fluoride
chelation, on the basis of the expectation of a longer E−μ-F
bond.17,18 Consistently, ESI-MS measurements reveal the
formation of a 1:1 adduct between fluoride and dimethyl-
fluorosilyl system 8. Moreover, data obtained from multinuclear
NMR measurements (1H, 11B, 13C, 19F, and 29Si) in
chloroform-d provide strong evidence for a chelating mode of
interaction in solution. Thus, the formation of a B−F bond is
consistent with the appearance of a doublet (1JBF = 85 Hz) at δB
9 ppm, which compares well with corresponding shift reported
by Kawachi and co-workers for [1,2-C6H4(BMes2)(SiMe2F)(μ-
F)]− (δB 6.5 ppm).17a The 19F NMR spectrum is characterized
by a small shift from δF −152 to −153 ppm in the resonance
associated with the terminal Si−F linkage and by the
appearance of a broad unresolved multiplet at δF −165 ppm
for the Si−F−B unit (cf. δF −154 ppm for Kawachi’s Si−F−B
system but a higher field signal at δF −181 ppm for the
terminally bound fluoride adduct of 1a).17a The formation of an
additional Si−F linkage on addition of fluoride is also
supported by the change in the 29Si spectrum from a simple
doublet (δSi 23.4 ppm, 1JSiF = 275 Hz) for the free receptor to a
EXPERIMENTAL SECTION
■
General Procedures. Manipulations of air-sensitive reagents were
carried out in a glovebox or by means of Schlenk-type techniques
involving the use of a dry argon or nitrogen atmosphere. HPLC grade
solvents were purified, dried, and degassed prior to use by a
commercially available Braun Solvent Purification System (SPS 500).
NMR solvents CDCl3 (molecular sieves) and C6D6 and C6D5CD3
(both potassium) were predried before use. The known compounds
1,1′-fc(BBr2)2, 1,3-fc*(BBr2)2, 1,1′-fcBr2, MesLi, 1-iodo-2,6-dimethyl-
4-fluorobenzene, and FBXyl2 were prepared according to literature
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procedures; o-XylLi was prepared by minor modification of the
method used for MesLi.11,13,24−27 The syntheses of compounds 3b
of C5H3), 95.3 (CH of C5H3), 129.3 (aromatic CH of Mes), 137.8 (p-
quaternary C of Mes), 139.8 (o-quaternary C of Mes), 144.5 (ipso-
quaternary C of Mes); ipso-quaternary C of Cp not observed;, acetone
solvate not included. 11B NMR (96 MHz, C6D6, 20 °C): δ 79 (br s).
UV/vis (CH2Cl2): λmax 313 nm, ε = 3940 mol−1 cm−1 dm3. MS (EI+):
753 (100%), M+; exact mass (calcd for M+, 10B isotopomer) 750.4428,
(measd) 750.4428. Anal. Calcd for C57H74B2FeO2: C, 78.76; H, 8.59.
Found: C, 79.07; H, 8.57.
and 6b,c have been reported by us previously in communication
format.6 10a FBMes2 was used as supplied (Sigma Aldrich). 1H and 13
C
c,
NMR spectra were measured on a Bruker AVII 500 FT-NMR or
Varian Mercury VX-300 spectrometer; 1H and 13C NMR spectra were
calibrated using the residual proton or natural-abundance 13C
resonances of the solvent, and 11B and 19F spectra were referenced
with respect to Et2O·BF3 and CFCl3, respectively. Mass spectra were
measured by the EPSRC National Mass Spectrometry Service,
Swansea University, and elemental microanalyses by London
Metropolitan University.
rac-1,2-fc(BMes2)Br (3a). To a solution of 1,1′-fcBr2 (1.09 g, 3.16
n
mmol) in thf (18 mL) at −78 °C was added BuLi (1.98 mL of a 1.6
M solution in hexanes, 3.16 mmol). After 30 min tetramethylpiper-
idine (0.53 mL, 3.16 mmol) was added and the resulting mixture
warmed to −40 °C. The temperature of the reaction was then carefully
maintained between −40 and −30 °C for 2 h. After this time, the
reaction mixture was again cooled to −78 °C and FBMes2 (1.13 g,
4.21 mmol, 90% purity as received from Sigma Aldrich) added as a
solution in thf (10 mL); the reaction mixture was slowly warmed to
room temperature and stirred for a further 15 h. The resulting red
solution was diluted with ether (100 mL) and washed with water (50
mL) and brine (50 mL), before being dried in vacuo, yielding the
crude product as a dark red solid. Purification using column
chromatography (hexane to 10% ethyl acetate/hexanes) yielded 3a
as a dark red amorphous solid. Yield: 1.23 g, 76%. Single crystals
suitable for X-ray crystallography were obtained by slow evaporation of
a concentrated solution in Et2O. 1H NMR (300 MHz, CDCl3, 20 °C):
δ 6.75 (s, 4H, aromatic CH of Mes), 4.98 (m, 1H, C5H3), 4.51 (virtual
t, J = 3 Hz, 1H, C5H3), 4.38 (m, 1H, C5H3), 4.25 (s, 5H, Cp), 2.30 (s,
12H, o-CH3 of Mes), 2.26 (s, 6H, p-CH3 of Mes). 13C NMR (75 MHz,
CDCl3, 20 °C): δ 139.5 (o-quaternary C of Mes), 137.4 (p-quaternary
C of Mes), 127.9 (aromatic CH of Mes), 78.7, 77.0 (C5H3), 71.9
(Cp), 70.5 (C5H3), 24.3 (o-CH3 of Mes), 21.1 (p-CH3 of Mes); B-
bound quaternary carbons not observed. 11B NMR (96 MHz,
[D]chloroform, 20 °C): δ 78. UV/vis (CH2Cl2): λmax 502 nm, ε =
1028 mol−1 cm−1 dm3. MS (EI+): 512 (100%) M+; exact mass (calcd
for M+, 11B isotopomer) 512.0973, (measd) 512.0981. Anal. Calcd for
C28H30BBrFe: C, 65.54; H, 5.89. Found: C, 65.50; H, 5.92.
Syntheses of New Compounds. 1,1′-fc(BMes2)2 (1a), 1,1′-
fc(BXylF ) (1b), and 1,1′-fc(BXyl2)2 (1c). To a suspension of
2 2
mesitylithium (0.65 g, 5.14 mmol) in Et2O (50 mL) was added
1,1′-fc(BBr2)2 (0.19 equiv) in Et2O (50 mL), and the reaction mixture
was stirred for 18 h. After which time the reaction was judged to be
complete by 11B NMR spectroscopy. Removal of volatiles in vacuo,
extraction into hexanes, concentration, and cooling to −30 °C led to
the isolation of 1a as a red microcrystalline material. Yield: 0.43 g,
1
66%. H NMR (300 MHz, CDCl3, 20 °C): δ 2.26 (s, 36H, o- and
para-CH3 of Mes), 4.35 (m, 4H, C5H4), 4.72 (m, 4H, C5H4), 6.74 (s,
8H, aromatic CH of Mes). 13C NMR (126 MHz, CDCl3, 20 °C): δ
21.2 (p-CH3 of Mes), 24.5 (o-CH3 of Mes), 75.0, 80.6 (C5H4), 128.4
(o-quaternary C of Mes), 137.3 (p-quaternary C of Mes), 139.2
(aromatic CH of Mes), 142.6 (B-bound quaternary C of Mes). 11B
NMR (96 MHz, CDCl3, 20 °C): δ 76. UV/vis (thf): λmax 542 nm, ε =
1420 mol−1 cm−1 dm3. MS (EI): 682.3 (100%) M+, exact mass (calcd
for 11B, 56Fe isotopomer) 682.3599, (measd) 682.359. E1/2(CH2Cl2) =
+0.197 V (with respect to FcH/FcH+). The syntheses of 1b,c were
carried out in a manner similar to that for 1a via the reaction of in situ
lithiated 1-iodo-2,6-dimethyl-4-fluorobenzene or o-xylyllithium with
1,1′-fc(BBr2)2 (0.22 equiv) in diethyl ether, with the reaction followed
1
by 11B NMR spectroscopy. Yield of 1b: 0.39 g, 39%. H NMR (300
MHz, CDCl3, 20 °C): δ 2.26 (s, 24H, o-CH3 of XylF), 4.45 (m, 4H,
3
C5H4), 4.77 (m, 4H, C5H4), 6.66 (d, JHF = 12 Hz, 8H, aromatic CH
of XylF). 13C NMR (126 MHz, CDCl3, 20 °C): δ 24.4 (o-CH3 of
1,2,5-fcBr(BMes2)2 (4). A solution of nBuLi in pentane (1.31 mL of a
1.6 M solution, 2.1 mmol) was added to a solution of
tetramethylpiperidine (0.28 mL, 1.66 mmol) in thf (10 mL) at 0
°C. After it was stirred at 0 °C for 45 min, the reaction mixture was
cooled to −78 °C and a solution of 3a (0.92 g, 1.79 mmol) in thf (10
mL) added. After the mixture was stirred for a further 2 h at −78 °C, a
solution of FBMes2 (0.58 g, 2.16 mmol) in thf (10 mL) was added,
and the reaction mixture was warmed slowly to room temperature.
After this mixture was stirred for 12 h, diluted with Et2O (50 mL), and
quenched with water, the aqueous layer was extracted with
dichloromethane and the combined organic layers were washed with
water and brine prior to drying over MgSO4. Volatiles were then
removed in vacuo, and the crude material was washed with acetone (2
× 10 mL). Yield: 0.71 g, 52%. 1H NMR (500 MHz, CDCl3, 20 °C): δ
2.27 (s, 12H, p-CH3 of Mes), 2.34 (s, 24H, o-CH3 of Mes), 4.23 (s,
5H, Cp), 4.78 (s, 2H, C5H2), 6.75 (s, 8H, aromatic CH of Mes). 13C
NMR (126 MHz, CDCl3, 20 °C): δ 21.0 (p-CH3 of Mes), 24.4 (o-
CH3 of Mes), 72.0 (Cp), 80.4 (C5H2), 85.0 (B-bound quaternary C of
C5H2), 94.0 (Br-bound quaternary C of C5H2), 128.0 (aromatic CH of
Mes), 137.6 (p-quaternary C of Mes), 139.7 (o-quaternary C of Mes),
142.6 (B-bound quaternary C of Mes). 11B NMR (96 MHz, CDCl3, 20
°C): δ 89. MS (EI): 760.0 (100%) M+, exact mass (calcd for M+, 11B,
56Fe isotopomer) 760.2710, (measd) 760.2709.
2
XylF), 75.3, 80.4 (C5H4), 114.1 (d, JCF = 19 Hz, aromatic CH of
3
XylF), 140.7 (B-bound quarternary of XylF), 141.6 (d, JCF = 8 Hz, o-
quaternary C of XylF), 162.5 (d, 1JCF = 247 Hz, CF of XylF). 11B NMR
(96 MHz, CDCl3, 20 °C): δ 78.0. 19F NMR (282 MHz, CDCl3, 20
3
°C): δ −116.3 (t, JFH = 12 Hz, p-F of XylF). MS (EI): 698.4 (100%)
M+, exact mass (calcd for M+, 10B isotopomer) 698.2596, (measd)
698.2591. Single crystals of 1c suitable for X-ray crystallography were
obtained by slow evaporation of a concentrated diethyl ether solution.
1
Yield: 0.35 g, 59%. H NMR (300 MHz, CDCl3, 20 °C): δ 2.30 (s,
24H, o-CH3 of Xyl), 4.44 (m, 4H, C5H4), 4.76 (m, 4H, C5H4), 6.93
(AB m, 8H, m-CH of Xyl), 7.10 (AB m, 4H, p-CH of Xyl). 13C NMR
(126 MHz, CDCl3, 20 °C): δ 24.5 (o-CH3 of Xyl), 75.1, 80.5 (C5H4),
127.3 (m-CH of Xyl), 127.7 (p-CH of Xyl), 138.9 (o-quaternary C of
Xyl), 145.2 (B-bound quarternary of Xyl). 11B NMR (96 MHz, CDCl3,
20 °C): δ 77. MS (EI): 626.2 (100%) M+, exact mass (calcd for M+,
10B isotopomer) 626.2973, (measd) 626.2978. Anal. Calcd for
C42H44B2Fe: C, 80.49; H, 7.08. Found: C, 80.44; H, 7.00.
1,3-fc*(BMes2)2 (2). To a rapidly stirred solution of 1,3-fc*(BBr2)2
(0.65 g, 1.09 mmol) in Et2O (60 mL) was added mesityllithium (0.83
g, 6.6 mmol) also in Et2O (40 mL). The reaction mixture was warmed
to 20 °C and stirred for a further 12 h. Volatiles were then removed in
vacuo, and the residual solid was extracted into pentane. After
concentration and cooling to −35 °C the precipitated solids were
removed by filtration, and the solvent was removed from the filtrate in
vacuo. Recrystallization of the resulting solid from acetone (25 mL)
yielded dark purple crystals of the acetone bis(solvate) 2·2(OCMe2)
1,3-fc(BMes2)2 (5). To a solution of 4 (0.35 g, 0.45 mmol) in thf at
−78 °C was added tBuLi (0.63 mL of a 1.6 M solution in hexanes, 1.0
mmol, 2.2 equiv), and the reaction mixture was stirred for 2 h at that
temperature. Excess propan-2-ol was then added at −78 °C, the
mixture was warmed to room temperature, and water was added. After
extraction with Et2O and removal of volatiles in vacuo the process was
1
suitable for X-ray crystallography. Yield: 0.35 g, 43%. H NMR (300
MHz, C6D6, 20 °C): δ 1.48 (s, 15H, CH3 of Cp*), 1.55 (s, CH3 of
acetone), 2.12 (s, 12H, p-CH3 of Mes), 2.54 (br s, 24H, o-CH3 of
Mes), 4.88 (s, 2H, C5H3), 5.08 (s, 1H, C5H3), 6.79 (br s, 8H, aromatic
CH of Mes), acetone solvate not included. 13C NMR (75 MHz, C6D6,
20 °C): δ 11.6 (CH3 of Cp*), 21.5 (p-CH3 of Mes), 23.6 (o-CH3 of
Mes), 30.4 (CH3 of acetone), 81.9 (quaternary C of Cp*), 89.9 (CH
t
repeated using a further 2.2 equiv of BuLi. Following a second
1
aqueous workup and removal of volatiles in vacuo, analysis by H
NMR indicated a crude yield of 5 of 80% contaminated by ca. 20% of
unreacted 4. Repeated washing of the crude product with acetone led
to the formation of a 95% pure material in low (ca. 5%) overall yield.
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1H NMR (500 MHz, CDCl3, 20 °C): δ 2.26 (s, 12H, p-CH3 of Mes),
2.40 (s, 24H, o-CH3 of Mes), 4.10 (s, 5H, Cp), 4.65 (s, 1H, C5H3),
4.95 (s, 1H, C5H3), 6.79 (s, 8H, aromatic CH of Mes). 13C NMR (126
MHz, CDCl3, 20 °C): δ 21.0 (p-CH3 of Mes), 24.5 (o-CH3 of Mes),
70.5 (Cp), 82.6, 92.3 (C5H3), 128.3 (aromatic CH of Mes), 137.2 (p-
quaternary C of Mes), 139.2 (ortho-quaternary C of Mes), 142.7 (B-
bound quaternary C of Mes). B-bound quaternary C of C5H3 not
observed. 11B NMR (96 MHz, CDCl3, 20 °C): δ 89.
rac-1,2-fc(BMes2)(SiMe2F) (8) and rac-1,2-fc(BMes2)(SiMe2OMe)
(9). 8 and 9 are most easily prepared from the common precursor 7 by
the use of either CuF2 or MeOH, respectively. For 8: to CuF2 (0.20 g,
1.00 mmol) was added a solution of 7 (0.50, 1.00 mmol) in
dichloromethane (10 mL) at room temperature. After the dark
suspension was stirred for 3 h, volatiles were removed in vacuo, the
residual solid was extracted with hexanes (3 × 20 mL), and the
volatiles were removed in vacuo from the combined extracts to afford
the target material as a red tar. Yield: 0.45 g, 93% (98% pure with 2%
1,2-fc(BMes2)2 (6a). To a stirred solution of 3a (0.22 g, 0.43 mmol)
in Et2O (15 mL) at −78 °C were added nBuLi and tmeda (1.0 equiv of
each) and (after 1 h) a solution of FBMes2 (1.13 g, 4.21 mmol, 90%
purity as received from Sigma Aldrich) in Et2O (5 mL). After it was
warmed to room temperature over 2 h and stirred for a further 12 h,
the reaction mixture was diluted with Et2O (100 mL) and washed with
water (50 mL) and brine (50 mL). Removal of volatiles in vacuo
yielded the crude product as a dark red solid. Purification using
column chromatography (hexanes to 10% ethyl acetate/hexanes)
yielded a dark red amorphous solid still contaminated with
Mes2BOBMes2. Fractional crystallization from hexanes (to remove
the borinic anhydride) and finally from MeOH/Et2O yielded 6a as
dark red crystals. Yield: 0.24 g, 83%. Single crystals suitable for X-ray
analysis were obtained by slow evaporation of a concentrated solution
1
3
7). H NMR (500 MHz, CDCl3, 20 °C): δ 0.11 (d, JHF = 8 Hz, 3H,
CH3 of SiMe2F), 0.15 (d, 3JHF = 8 Hz, 3H, CH3 of SiMe2F), 2.15 (s, 12
H, o-CH3 of Mes), 2.28 (s, 6H, p-CH3 of Mes), 4.21 (s, 5H, Cp), 4.61
(m, 1H, C5H3BSi), 4.77 (m, 1H, C5H3BSi), 4.78 (m, 1H, C5H3BSi),
6.76 (s, 4H, aromatic CH of Mes). 13C NMR (125 MHz, CDCl3, 20
2
2
°C): δ −0.1 (d, JCF = 16 Hz, CH3 of SiMe2F), 0.2 (d, JCF = 16 Hz,
CH3 of SiMe2F), 21.0 (p-CH3 of Mes), 23.7 (o-CH3 of Mes), 69.4
(CSi of C5H4BSi), 70.4 (Cp), 73.6 (CH of C5H3BSi), 80.0 (CH of
C5H3BSi), 86.8 (CH of C5H3BSi), 92.2 (br, CB of C5H3BSi), 128.0
(aromatic CH of Mes), 137.7 (p-quaternary C of Mes), 139.6 (s, o-
quaternary C of Mes), 142.8 (br, ipso-quaternary C of Mes). 11B NMR
(96 MHz, CDCl3, 20 °C): δ 83. 19F NMR (282 MHz, CDCl3, 20 °C):
1
3
δ −152.0 (sept with 29Si satellites, JFSi = 275 Hz, JHF = 7 Hz). 29Si
1
1
NMR (60 MHz, CDCl3, 20 °C): δ 23.4 (d, JSiF = 275 Hz). For 9: a
in Et2O. H NMR (300 MHz, CDCl3, 20 °C): δ 1.55 (s, 12H, p-CH3
suspension of 7 (0.30 g, 0.57 mmol) in methanol (50 mL) was stirred
for 1 h, at which point Et2O (50 mL) was added and the reaction
mixture stirred for a further 3 h. Volatiles were then removed in vacuo,
and the residual solid was extracted with hexane (3 × 20 mL).
Volatiles were removed in vacuo from the combined extracts to afford
the target material as a red solid. Yield: 0.24 g, 82%. Single crystals
suitable for X-ray diffraction were obtained by slow evaporation from
hexamethyldisiloxane. 1H NMR (300 MHz, CDCl3, 20 °C): δ 0.02 (s,
3H, CH3 of SiMe2), 0.03 (s, 3H, CH3 of SiMe2), 2.12 (s, 12 H, o-CH3
of Mes), 2.25 (s, 6H, p-CH3 of Mes), 3.31 (s, 3H, CH3 of SiOMe),
4.15 (s, 5H, Cp), 4.51 (m, 1H, C5H3BSi), 4.65 (m, 1H, C5H3BSi), 4.70
(m, 1H, C5H3BSi), 6.73 (s, 4H, aromatic CH of Mes). 13C NMR (75
MHz, CDCl3, 20 °C): δ −1.2 (SiMe2), −1.0 (SiMe2), 21.1 (p-CH3 of
Mes), 23.6 (o-CH3 of Mes), 50.2 (SiOMe), 70.1 (Cp), 71.7 (CSi of
C5H3BSi), 72.7 (CH of C5H3BSi), 80.1 (CH of C5H3BSi), 85.9 (CH
of C5H3BSi), 128.0 (aromatic CH of Mes), 137.7 (p-quaternary C of
Mes), 139.7 (o-quaternary C of Mes), 143.0 (br, ipso-quaternary C of
Mes); the signal for the boron bound carbon atom of the C5H3BSi unit
was not observed. Assignments were assisted by gCOSY, HMQC, and
HSQC NMR spectra. 11B NMR (96 MHz, CDCl3, 20 °C): δ 87 (br).
29Si NMR (60 MHz, CDCl3, 20 °C): δ 11. MS (EI+): 522.2 (100%)
M+; exact mass (calcd for M+, 10B, 28Si isotopomer) 519.2290,
(measd) 519.2288. E1/2(CH2Cl2) = +0.047 V (with respect to FcH/
FcH+).
[K(18-crown-6)][1a·CN]. To a solution of 1a (0.040 g, 0.06 mmol)
in chloroform (5 cm3) were added KCN and 18-crown-6 (1.0 equiv of
each), and the reaction mixture was stirred for 1 h, at which point the
reaction was judged complete by quantitative conversion to a single
11B NMR resonance at δB −13. Layering of the reaction mixture with
diethyl ether led to the isolation of [K(18-crown-6)][1a·CN]− as
single crystals suitable for X-ray crystallography. Yield: 0.034 g, 81%.
1H NMR (300 MHz, CDCl3, 20 °C): δ 2.14 (s, 12H, o-CH3 of Mes),
2.22 (s, 6H, p-CH3 of Mes), 2.26 (s, 6H, p-CH3 of Mes) 2.29 (s, 12H,
o-CH3 of Mes), 3.48 (s, 24H, 18-crown-6), 4.04 (br s, 2H, C5H4),
4.21, 4.26, 4.68 (each br m, 2H, C5H4), 6.52, 6.69 (each s, 4H,
aromatic CH of Mes). 13C NMR (75 MHz, CDCl3, 20 °C): δ 20.7,
21.0 (p-CH3 of Mes), 24.6, 25.2 (o-CH3 of Mes), 69.9 (18-crown-6),
76.0, 78.9 (C5H4), 127.8, 128.8 (o-quarternary of Mes), 131.6, 136.0
(p-quaternary C of Mes), 139.1, 141.6 (aromatic CH of Mes), boron-
bound quaternary carbons not observed. 11B NMR (96 MHz, CDCl3,
20 °C): δ −13. MS (EI): 708.3 (100%) M+; exact mass (calcd for 56Fe
isotopomer) 706.3687, (measd) 706.3687. Reproducible elemental
microanalyses for crystalline samples of [K(18-crown-6)][1a·CN]
proved difficult to obtain due to the presence of both chloroform and
diethyl ether in the crystal lattice (as shown by X-ray crystallography).
[K(18-crown-6)]2[1a·(CN)2] and [K(18-crown-6)]2[1b·(CN)2]. The
two adducts were prepared by a common method exemplified for
of Mes), 2.00−2.70 (br overlapping signals, 24H, o-CH3 of Mes), 4.15
(s, 5H, Cp), 4.87 (t, 3JHH = 3 Hz, 1H, C5H3), 5.07 (d, 3JHH = 3 Hz, 2H,
C5H3), 6.22 (s, 2H, aromatic CH of Mes), 6.36 (s, 2H, aromatic CH of
Mes), 6.74 (s, 2H, aromatic CH of Mes), 6.81 (s, 2H, aromatic CH of
Mes). 1H NMR (300 MHz, C6D5CD3, 120 °C): δ 2.07 (s, 12H, p-CH3
of Mes), 2.25 (s, 24H, o-CH3 of Mes), 3.92 (s, 5H, Cp), 4.52 (m, 1H,
C5H3), 5.02 (d, 3JHH = 2 Hz, 2H, C5H3), 6.50 (s, 8H, aromatic CH of
1
Mes). H NMR (300 MHz, C6D5CD3, −60 °C): δ 1.70, 1.77, 2.08,
2.19, 2.25, 2.39, 2.58, 2.97, 3.21 (s, each 3H, o-CH3 of Mes), 2.02 (s,
12H, p-CH3 of Mes), 3.78 (s, 5H, Cp), 4.26, 4.84, 5.02 (m, each 1H,
C5H3), 6.02, 6.06, 6.30, 6.58 (2 signals), 6.61 (2 signals), 6.68 (s, each
1H, aromatic CH of Mes). 13C NMR (75 MHz, C6D5CD3, 120 °C): δ
20.3 (p-CH3 of Mes), 24.5 (o-CH3 of Mes), 71.2 (Cp), 73.5, 90.1
(C5H3), 128.1 (aromatic CH of Mes), 136.6 (ipso-quaternary C of
Mes), 139.0 (o-quaternary C of Mes); B-bound quaternary carbons
not observed. 11B NMR (96 MHz, CDCl3, 20 °C): δ 80. UV−vis
(dichloromethane): λmax 510 nm, ε = 1903 mol−1 cm−1 dm3. MS
(EI+): 682 (100%) M+; exact mass (calcd for M+, 10B isotopomer)
680.3677, (measd) 680.3657. Anal. Calcd for C46H52B2Fe): C, 80.91;
H, 7.68. Found: C, 80.66; H, 7.44. E1/2(CH2Cl2) = +0.177 V (with
respect to FcH/FcH+).
rac-1,2-fc(BMes2)(SiMe2Cl) (7). To a stirred solution of 3a (0.84 g,
t
1.64 mmol) in Et2O (100 mL) at −78 °C was added dropwise BuLi
(1.72 mL of a 1.9 M solution in pentane, 3.27 mmol). After the
mixture was stirred at −78 °C for 30 min, Me2SiCl2 (2 mL, 16.6
mmol) was added to the purple solution at −78 °C. The reaction
mixture was warmed to room temperature and stirred for 15 h.
Volatiles were then removed in vacuo, the residual solid was extracted
with pentane (3 × 20 mL), and the volatiles were removed in vacuo
from the combined extracts to afford the target material as an orange
solid. Yield: 0.80 g, 91%. 1H NMR (500 MHz, CDCl3, 20 °C): δ 0.21
(s, 3H, CH3 of SiMe2Cl), 0.36 (s, 3H, CH3 of SiMe2Cl), 2.12 (br s, 12
H, o-CH3 of Mes), 2.26 (s, 6H, a-CH3 of Mes), 4.22 (s, 5H, Cp), 4.58
(m, 1H, C5H3BSi), 4.73 (m, 1H, C5H3BSi), 4.83 (m, 1H, C5H3BSi),
6.74 (s, 4H, aromatic CH of Mes). 13C NMR (125 MHz, CDCl3, 20
°C): δ 3.6 (CH3 of SiMe2Cl), 4.0 (CH3 of SiMe2Cl), 21.1 (o-CH3 of
Mes), 23.6 (p-CH3 of Mes), 70.7 (Cp), 71.7 (CSi of C5H3BSi), 73.1
(CH of C5H3BSi), 80.8 (CH of C5H3BSi), 87.2 (CH of C5H3BSi),
94.1 (CB of C5H3BSi), 128.2 (aromatic CH of Mes), 138.0 (p-
quaternary C of Mes), 139.7 (o-quaternary C of Mes), 142.7 (ipso-
quaternary C of Mes). Assignments wree assisted by gCOSY, HSQC,
and HMBC NMR (500 MHz (1H)/125 MHz (13C), CDCl3, 20 °C).
11B NMR (96 MHz, CDCl3, 20 °C): δ 82. 29Si NMR (60 MHz,
CDCl3, 20 °C): δ 24.5 (SiMe2Cl). MS (EI+): 526.2 (100%) M+; exact
mass (calcd for M+, 10B, 35Cl, 28Si isotopomer) 523.1795, (measd)
523.1794. Isotopic profile consistent with assignment. Anal. Calcd for
C30H36BClFeSi: C, 68.40; H, 6.89. Found: C, 68.42; H, 6.84.
2681
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Organometallics
Article
[K(18-crown-6)]2[1a·(CN)2]. To a solution of 1a (0.040 g, 0.06
mmol) in acetonitrile (5 cm3) were added KCN/ and 18-crown-6 (2.0
equiv of each), and the reaction mixture was stirred for 1 h. At this
point the reaction was judged to be complete by quantitative
conversion to a single 11B NMR resonance at δB −16 ppm. An
accompanying color change from deep red to yellow was observed for
the solution together with the precipitation of a yellow-orange solid.
Layering of the supernatant solution with Et2O led to the formation of
[K(18-crown-6)]2[1a·(CN)2]·OEt2 as orange blocks suitable for X-ray
crystallography. Yield: 0.023 g, 27%. For [K(18-crown-
6)]2[1b·(CN)2]: δB −15 ppm, yield 0.022 g, 25%. Attempts to fully
characterize [K(18-crown-6)]2[1a·(CN)2] or [K(18-crown-
6)]2[1b·(CN)2] proved unsuccessful, as crystalline samples of both
adduct,s once formed, proved to be resolutely insoluble in compatible
solvents. Anal. Calcd for C76H110B2FeK2N2O13: C, 64.46; H, 7.84; N,
1.98. Found: C, 64.33; H, 7.56,; N, 2.18. Calcd for
C66H88B2F4FeK2N2O12: C, 59.43; H, 6.65; N, 2.10. Found: C, 59.00;
H, 6.42; N, 2.38.
[K(18-crown-6)][6a·CN]. Potassium cyanide (3.8 mg, 0.059 mmol),
18-crown-6 (1.0 equiv), 6a (1.0 equiv), and thf-d8 (1 mL) were mixed
in a J. Young NMR tube, and the resulting mixture was sonicated until
the reaction was judged complete by 11B NMR spectroscopy
(quantitative conversion of the signal at δB 80 to a signal at −15
ppm). Layering the resulting solution with pentane led to growth of
single red crystals of the chloroform solvate suitable for X-ray
diffraction. Yield: 0.035 g, 59%. 1H NMR (500 MHz, thf-d8, 20 °C): δ
1.76, 1.84, 2.04, 2.09, 2.18, 2.22 (each 6H, o- and p-CH3 of Mes), 3.58
(s, 24H, 18-crown-6), 4.20 (s, 5H, Cp), 4.45, 4.53, 5.56 (s, each 1H,
C5H4), 5.58, 6.08 (s, each 1H, aromatic CH of Mes), 6.30
(overlapping s, 3H, aromatic CH of Mes), 6.51, 6.58, 6.64 (each s,
1H, aromatic CH of Mes). 13C NMR (126 MHz, thf-d8, 20 °C): δ
13.4, 19.9, 20.0, 20.2, 20.4, 20.5, 22.2, 23.8, 25.5, 27.1, 27.9, 34.1 (o-
and p- CH3 of Mes), 67.4, 67.5, 68.6 (C5H3), 69.7 (Cp), 70.1 (18-
crown-6), 126.7, 126.8, 126.9, 127.4, 127.8, 128.2, 128.5, 128.6
(aromatic CH of Mes), 129.0, 129.7, 130.3, 130.8 (p-quaternary C of
Mes), 134.8, 135.0, 138.9, 140.4, 140.9, 141.4, 143.0, 143.6 (o-
quaternary C of Mes); boron-bound quaternary carbons not observed.
11B NMR (96 MHz, d8-THF, 20 °C): δ −15, 78 (v br). MS (ES
negative ion): 708.3635 (100%) [1,2-fc(BMes2)2CN]−; exact mass
(calcd for M+, 54Fe isotopomer) 705.3735, (measd) 705.3724. Anal.
Calcd for C60H77B2Cl3FeKNO6: C, 63.37; H, 6.86; N, 1.24. Found: C,
63.37; H, 6.84; N, 1.22.
Determination of Binding Constants: Typical Protocol. A 3
mL portion of a solution of the receptor in thf (typically 1.0−2.0 μM)
was placed in the cell, and aliquots of [nBu4N]F·3H2O or [nBu4N]CN
in dichloromethane were added (typically 10−25 μL of a 10−15 μM
solution). The solution was stirred for 1 min after each addition and
the UV−vis spectrum then measured. The program ReactLab
Equlibria was subsequently used to determine the binding constants,
with data being fitted over the wavelength range 430−530 nm.28 See
the Supporting Information for fuller details.
Crystallography. Included in this paper are the structures of
compounds 1c, 2, 9, [K(18-crown-6)][1a·CN], [K(18-crown-
6)]2[1a·(CN)2], [K(18-crown-6)]2[1a·(CN)2]·OEt2, [K(18-crown-
6)]2[1b·(CN)2], and [nBu4N][8·CN]; those of the hydrolysis
products [K(18-crown-6)][rac-1,2-fc(BMes2F)(SiMe2OH)] and
[nBu4N][rac-1,2-fc(BMes2CN) (SiMe2OH)] have been included in
the Supporting Information only (CCDC references 923316 and
924826),23 and those of 6a, [K(18-crown-6)][6a·F], and [K(18-
crown-6)][6a·CN] have been reported previously by us in
communication format (CCDC references: 778111−778113).10a,c
With the exception of [nBu4N][8·CN], data were collected on a
NoniusKappa CCD diffractometer (Mo Kα radiation, λ = 0.71073 Å)
at 150 K. Data for [nBu4N][8·CN] were collected on an Oxford
Diffraction Supernova diffractometer (Cu Kα radiation, λ = 1.54184
Å) at 150 K. Data were processed using the DENZO-SMN package,
and structures were solved using SHELXS, Superflip, or SIR92.
Refinement was carried out using full-matrix least squares within the
CRYSTALS suite or with SHELXTL.29 Full crystallographic data for
all structures have been deposited with the Cambridge Crystallo-
graphic Data Centre (CCDC references 923310−923316). Copies of
these data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_ request/
cif.
Crystallographic data for 1c: Mr = 626.28, monoclinic, P21/c, a =
9.7284(1) Å, b = 30.8099(4) Å, c = 11.1839(2) Å, β = 96.598(1)°, V =
3330.1(1) Å3, Z = 4, ρc = 1.249 Mg m−3, T = 150 K, 27276 reflections
collected, 7541 independent (R(int) = 0.048). R1 = 0.0428, wR2 =
0.0927 for observed unique reflections (F2 > 2σ(F residual electron
densities 0.68 and −0.48 e Å−3. CSD reference: 923313.
Crystallographic data for 2: Mr = 752.52, triclinic, P1, a =
̅
12.8992(2) Å, b = 13.3331(2) Å, c = 15.6365(2) Å, α = 96.926(1)°, β
= 112.5314(6)°, γ = 90.6447(6)°, V = 2461.0(1) Å3, Z = 2, ρc = 1.015
Mg m−3, T = 150 K, 11163 reflections collected, 11163 independent
(R(int) = 0.026). R1 = 0.0531, wR2 = 0.1537 for observed unique
reflections (F2 > 2σ(F2)) and R1 = 0.0706, wR2 = 0.1663 for all
unique reflections. Maximum and minimum residual electron densities
0.46 and −0.52 e Å−3. CSD reference: 923314.
[K(18-crown-6)][8·F]. A solution of 8 (0.29 g, 0.57 mmol) in
toluene (10 mL) was added to KF (0.033 g, 0.57 mmol) and 18-
crown-6 (0.15 g, 0.57 mmol) and the reaction mixture stirred for 5 h. |
Volatiles were removed in vacuo; the residual solid was washed with
hexane (10 mL) and then toluene (10 mL) and dried in vacuo to give
Crystallographic data for 9: Mr = 522.39, monoclinic, P21/c, a =
14.9853(2) Å, b = 9.3111(1) Å, c = 20.9000(3) Å, β = 96.469(1)°, V =
2773.1(1) Å3, Z = 4, ρc = 1.251 Mg m−3, T = 150 K, 92997 reflections
collected, 6283 independent (R(int) = 0.024). R1 = 0.0434, wR2 =
0.0902 for observed unique reflections (F2 > 2σ(F2)) and R1 = 0.0556,
wR2 = 0.0981 for all unique reflections. Maximum and minimum
residual electron densities 0.47 and −0.45 e Å−3. CSD reference:
923315.
Crystallographic data for [K(18-crown-6)][1a·CN]: Mr = 1179.52,
monoclinic, P21/c, a = 21.6605(2) Å, b = 13.7952(1) Å, c =
22.3225(1) Å, β = 109.212(1)°, V = 6299.6(1) Å3, Z = 4, ρc = 1.244
Mg m−3, T = 150 K, 93176 reflections collected, 14307 independent
(R(int) = 0.083). R1 = 0.093, wR2 = 0.252 for observed unique
reflections (F2 > 2σ(F2)) and R1 = 0.1375, wR2 = 0.2875 for all
unique reflections. Maximum and minimum residual electron densities
1.52 and −1.12 e Å−3. CSD reference: 923311.
Crystallographic data for [K(18-crown-6)]2[1a·(CN)2]·OEt2: Mr =
1415.33, monoclinic, C2/c, a = 23.3200(4) Å, b = 16.0340(3) Å, c =
22.3901(4) Å, β = 113.478(1)°, V = 7678.8(2) Å3, Z = 4, ρc = 1.224
Mg m−3, T = 150 K, 16489 reflections collected, 8649 independent
(R(int) = 0.034). R1 = 0.140, wR2 = 0.3115 for observed unique
reflections (F2 > 2σ(F2)) and R1 = 0.1618, wR2 = 0.3200 for all
unique reflections. Maximum and minimum residual electron densities
0.54 and −0.72 e Å−3. CSD reference: 923310.
1
an orange solid. Yield: 0.25 g, 53%. H NMR (500 MHz, CD3CN, 20
°C): δ −0.07 (dd, 3JHF = 3, 8 Hz, 3H, CH3 of SiMe2F), 0.50 (dd, 3JHF
= 4, 8 Hz, 3H, CH3 of SiMe2F), 1.55 (s, 3H, o-CH3 of Mes), 1.79 (s,
3H, o-CH3 of Mes), 2.05 (m, 6H, o-CH3 and p-CH3 of Mes), 2.19 (s,
3H, p-CH3 of Mes), 2.81 (s, 3H, o-CH3 of Mes), 3.57 (s, 24H, CH2 of
18-crown-6), 3.81 (s, 5H, Cp), 3.99 (m, 1H, C5H3BSi), 4.18 (m, 1H,
C5H3BSi), 4.22 (m, 1H, C5H3BSi), 6.27 (br s, 1H, aromatic CH of
Mes), 6.42 (br s, 1H, aromatic CH of Mes), 6.49 (br s, 1H, aromatic
CH of Mes), 6.63 (br s, 1H, aromatic CH of Mes). 13C NMR (125
MHz, CD3CN, 20 °C): δ 1.0 (CH3 of SiMe2F), 2.2 (CH3 of SiMe2F),
19.8 (p-CH3 of Mes), 20.0 (p-CH3 of Mes), 23.0 (o-CH3 of Mes), 24.3
(o-CH3 of Mes), 25.1 (two o-CH3 of Mes), 70.1 (CH2 of 18-crown-6),
68.8 (Cp), 69.9 (CH of C5H3BSi), 73.3 (CH of C5H3BSi), 76.5 (CH
of C5H3BSi), 127.6 (aromatic CH of Mes), 127.7 (aromatic CH of
Mes), 131.4 (aromatic CH of Mes), 131.8 (aromatic CH of Mes). The
signals for the quaternary carbons of Mes and the boron- and silicon-
bound carbon atoms in the C5H3BSi unit were not observed.
Assignments were assisted by gCOSY, HSQC, and HMBC spectra. 11B
1
NMR (96 MHz, CD3CN, 20 °C): δ 5 (d, JBF = 85 Hz). 19F NMR
(282 MHz, CD3CN, 20 °C): δ −153.3 (br, SiMe2F), −164.6 (br,
BFSi). 29Si NMR (60 MHz, CD3CN, 20 °C): 25.2 (dd, 1JSiF = 70, 190
Hz). MS (ES−): 529.2 (M−, 100%).
2682
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Organometallics
Article
Crystallographic data for [K(18-crown-6)]2[1b·(CN)2]: Mr
=
(4) Havir, J. Collect. Czech. Chem. Commun. 1961, 26, 1775.
1357.07, triclinic, P1, a = 11.0600(3) Å, b = 12.3916(2) Å, c =
(5) For examples of cyanide binding by triarylboranes, see for
example: (a) Zhou, J.; Lancaster, S. J.; Walker, D. A.; Beck, S.;
Thornton-Pett, M.; Bochmann, M. J. Am. Chem. Soc. 2001, 123, 223.
(b) Vei, I. C.; Pascu, S. I.; Green, M. L. H.; Green, J. C.; Schilling, R.
E.; Anderson, G. D. W.; Rees, L. H. Dalton Trans. 2003, 2550.
̅
14.0340(3) Å, α = 64.573(1)°, β = 75.472(1)°, γ = 79.957(1)°, V =
1676.6(1) Å3, Z = 1, ρc = 1.344 Mg m−3, T = 120 K, 30041 reflections
collected, 7649 independent (R(int) = 0.053). R1 = 0.0640, wR2 =
0.1554 for observed unique reflections (F2 > 2σ(F2)) and R1 = 0.0816,
wR2 = 0.1666 for all unique reflections. Maximum and minimum
residual electron densities 1.16 and −0.65 e Å−3. CSD reference:
923312.
(c) Parab, K.; Venkatasubbaiah, K.; Jakle, F. J. Am. Chem. Soc. 2006,
̈
128, 12879. (d) Hudnall, T. W.; Gabbaï, F. P. J. Am. Chem. Soc. 2007,
129, 11978. (e) Broomsgrove, A. E. J.; Addy, D. A.; Bresner, C.; Fallis,
I. A.; Thompson, A. L.; Aldridge, S. Chem. Eur. J. 2008, 14, 7525.
(f) Chiu, C.-W.; Gabbaï, F. P. Dalton Trans. 2008, 814. (g) Huh, J.-O.;
Do, Y.; Lee, M. H. Organometallics 2008, 27, 1022. (h) Chiu, C. W.;
Kim, Y.; Gabbaï, F. P. J. Am. Chem. Soc. 2009, 131, 60. (i) Kim, Y.;
Zhao, H.; Gabbaï, F. P. Angew. Chem., Int. Ed. 2009, 48, 4957.
(j) Agou, T.; Sekine, M.; Kobayashi, J.; Kawashima, T. J. Organomet.
Chem. 2009, 694, 3833. (k) Broomsgrove, A. E. J.; Addy, D.; Di Paolo,
A.; Morgan, I. R.; Bresner, C.; Chislett, V.; Fallis, I. A.; Thompson, A.
L.; Vidovic, D.; Aldridge, S. Inorg. Chem. 2010, 49, 157. (l) Wade, C.
R.; Gabbaï, F. P. Inorg. Chem. 2010, 49, 714. (m) Bresner, C.; Haynes,
C. J. E.; Addy, D. A.; Broomsgrove, A. E. J.; Fitzpatrick, P.; Vidovic, D.;
Thompson, A. L.; Fallis, I. A.; Aldridge, S. New J. Chem. 2010, 34,
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, figures, and CIF files giving details of the crystal structures
of products [K(18-crown-6)][rac-1,2-fc(BMes2F)(SiMe2OH)]
and [nBu4N][rac-1,2-fc(BMes2CN)(SiMe2OH)], crystallo-
graphic data for all structures given in this paper, and fluoride
binding isotherms. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*S.A.: email, Simon.Aldridge@chem.ox.ac.uk; tel, +44 (0)1865
285201; web, http://users.ox.ac.uk/∼quee1989/.
■
̈
1652. (n) Matsumoto, T.; Wade, C. R.; Gabbaı, F. P. Organometallics
2010, 29, 5490. (o) Kim, Y.; Huh, H.-S.; Lee, M. H.; Lenov, I. L.;
̈
Zhao, H.; Gabbaı, F. P. Chem. Eur. J. 2011, 17, 2057. (p) Cao, D.;
Zhao, H.; Gabbaï, F. P. New J. Chem. 2011, 35, 2299. (q) Siewert, I.;
Fitzpatrick, P.; Broomsgrove, A. E. J.; Kelly, M.; Vidovic, D.; Aldridge,
S. Dalton Trans. 2011, 40, 10345. (r) Zhao, H.; Reibenspies, J. H.;
Gabbaï, F. P. Dalton Trans. 2013, 42, 603.
Present Addresses
†Institut fur Anorganische Chemie, Georg-August-Universitat
̈
̈
Gottingen, Tammannstrasse 4, 37077 Gottingen, Germany.
̈
̈
‡Division of Chemistry and Biological Chemistry, School of
Physical and Mathematical Sciences, Nanyang Technological
University, 21 Nanyang Link, 637371 Singapore.
(6) For a recent review of fluoride binding by triarylboranes and
boronic acid/esters see: Wade, C. R.; Broomsgrove, A. E. J.; Aldridge,
S.; Gabbaï, F. P. Chem. Rev. 2010, 110, 3958.
(7) Note the very strong solvent dependence of the relative basicities
of F− and CN−. HF: pKa 3 (in H2O), 15 (in DMSO). HCN: pKa 9 (in
H2O), 13 (in DMSO). See, for example: http://www2.lsdiv.harvard.
edu/labs/evans/pdf/evans_pKa_table.pdf.
(8) (a) Dusemund, C.; Sandanayake, K. R. A. S.; Shinkai, S. J. Chem.
Soc., Chem. Commun. 1995, 333. (b) Yamamoto, H.; Ori, A.; Ueda, K.;
Dusemund, C.; Shinkai, S. Chem. Commun. 1996, 407. (c) Bresner, C.;
Day, J. K.; Coombs, N. D.; Fallis, I. A.; Aldridge, S.; Coles, S. J.;
Hursthouse, M. B. Dalton Trans. 2006, 3660. (d) Day, J. K.; Bresner,
C.; Fallis, I. A.; Ooi, L.-L.; Watkin, D. J.; Coles, S. J.; Male, L.;
Hursthouse, M. B.; Aldridge, S. Dalton Trans. 2007, 3486. (e) Day, J.
K.; Bresner, C.; Coombs, N. D.; Fallis, I. A.; Ooi, L.-L.; Aldridge, S.
Inorg. Chem. 2008, 47, 793.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge the EPSRC (studentship for M.J.K.; PDRAs
for I.R.M. and D.V.; access to the National Mass Spectrometry
Centre, Swansea University; National Crystallography Service,
University of Southampton, for the structure of [K(18-crown-
6)]2[1b·(CN)2]), the DFG (postdoctoral fellowship for I.S.),
Oxford University (studentship for A.E.J.B.), and Amber
Thompson (Oxford departmental crystallography service) for
additional crystallography.
(9) For convergent bifunctional borane Lewis acids see, for example:
(a) Katz, H. E. J. Am. Chem. Soc. 1985, 107, 1420. (b) Katz, H. E. J.
Org. Chem. 1985, 50, 5027. (c) Jia, L.; Yang, X.; Stern, C.; Marks, T. J.
REFERENCES
■
Organometallics 1994, 13, 3755. (d) Kohler, K.; Piers, W. E.; Sin, X.;
̈
(1) (a) Yang, Y.-C.; Baker, J. A.; Ward, J. R. Chem. Rev. 1992, 92,
1729. (b) Baskin, S. I.; Brewer, T. G. Medical Aspects of Chemical and
Biological Warfare; Sidell, F. R., Takafuji, E. T., Franz, D. R., Eds.;
TMM Publications: New York, 1997; pp 271−286. (c) Sohn, H.;
Feng, Y.; Bravakis, A. M.; Jarvis, A. P.; Collins, S.; Clegg, W.; Yap, G. P.
A.; Marder, T. B. Organometallics 1998, 17, 3557. (e) Williams, V. C.;
Piers, W. E.; Clegg, W.; Elsegood, M. R. J.; Collins, S.; Marder, T. B. J.
Am. Chem. Soc. 1999, 121, 3244. (f) Williams, V. C.; Dai, C.; Li, Z.;
Collins, S.; Piers, W. E.; Clegg, W.; Elsegood, M. R. J.; Marder, T. B.
Angew. Chem., Int. Ed. 1999, 38, 3695. (g) Metz, M. V.; Schwartz, D. J.;
Stern, C. L.; Nickias, P. N.; Marks, T. J. Angew. Chem., Int. Ed. 2000,
39, 1312. (h) Williams, V. C.; Irvine, G. J.; Piers, W. E.; Li, Z. M.;
Collins, S.; Clegg, W.; Elsegood, M. R. J.; Marder, T. B.
Organometallics 2000, 19, 1619. (i) Piers, W. E.; Irvine, G.;
Williams, V. C. Eur. J. Inorg. Chem. 2000, 2131. (j) Hoefelmeyer, J.
D.; Gabbaï, F. P. Organometallics 2002, 21, 982. (k) Hoefelmeyer, J.
D.; Schulte, M.; Tschinkl, M.; Gabbaï, F. P. Coord. Chem. Rev. 2002,
235, 93. (l) Henderson, L. D.; Piers, W. E.; Irvine, G. J.; McDonald, R.
Organometallics 2002, 21, 340. (m) Metz, M. V.; Schwartz, D. J.; Stern,
C. L.; Marks, T. J.; Nickias, P. N. Organometallics 2002, 21, 4159.
(n) Emslie, D. J. M.; Piers, W. E.; Parvez, M. Angew. Chem., Int. Ed.
2003, 42, 1252. (o) Gabbaï, F. P. Angew. Chem., Int. Ed. 2003, 42,
2218. (p) Lewis, S. P.; Taylor, N. J.; Piers, W. E.; Collins, S. J. Am.
Chem. Soc. 2003, 125, 14686. (q) Ghesner, I.; Piers, W. E.; Parvez, M.;
McDonald, R. Organometallics 2004, 23, 3085. (r) Morrison, D. J.;
́
Letant, S.; Sailor, M. J.; Trogler, W. C. J. Am. Chem. Soc. 2000, 122,
5399. (d) Bresner, C.; Aldridge, S.; Fallis, I. A.; Jones, C.; Ooi, L.-L.
Angew. Chem., Int. Ed. 2005, 44, 3606. (e) Colquhoun, J. Perspect. Biol.
Med. 1997, 41, 29. (f) Carton, R. J. Fluoride 2006, 39, 163.
(2) (a) Holland, M. A.; Kozlowski, L. M. Clin. Pharm. 1986, 5, 737.
(b) Baud, F. Hum. Exp. Toxicol. 2007, 26, 191.
(3) For selected reviews of anion binding, see Coord. Chem, Rev.
2003, 240, issues 1 and 2, Coord. Chem, Rev. 2006, 250, issues 23 and
24, and: (a) de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.;
Huxley, A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem.
Rev. 1997, 97, 1515. (b) Schmidtchen, F. P.; Berger, M. Chem. Rev.
1997, 97, 1609. (c) Gale, P. A.; Sessler, J. A.; Kral, V. Chem. Commun.
1998, 1. (d) Beer, P. D.; Gale, P. A. Angew. Chem., Int. Ed. 2001, 40,
486. (e) Lavigne, J. J.; Anslyn, E. V. Angew. Chem., Int. Ed. 2001, 40,
́
́ ́
3118. (f) Martınez-Manez, R.; Sancernon, F. Chem. Rev. 2003, 103,
4419. (g) Hudnall, T. W.; Chiu, C. W.; Gabbaï, F. P. Acc. Chem. Res.
2009, 42, 388. (h) Hudson, Z. M.; Wang, S. Acc. Chem. Res. 2009, 42,
1584.
2683
dx.doi.org/10.1021/om400152x | Organometallics 2013, 32, 2674−2684

Organometallics
Article
̈
Piers, W. E.; Parvez, M. Synlett 2004, 13, 2429. (s) Sole,
P. Chem. Commun. 2004, 1284. (t) Piers, W. E. Adv. Organomet. Chem.
́
S.; Gabbaı, F.
(20) Preliminary binding studies carried out on samples of 5 (albeit
contaminated with ca. 5% unreacted 4) imply a 1:1 binding
stoichiometry with both cyanide and fluoride.
̈
2005, 52, 1. (u) Melaimi, M.; Gabbaı, F. P. Adv. Organomet. Chem.
(21) Connelly, N.; Geiger, W. E. Chem. Rev. 1996, 96, 877.
(22) For a previous analysis of the factors influencing endo/exo
coordination of Lewis bases to bifunctional boranes, see ref 9z.
(23) Attempts to obtain reliable fluoride and cyanide binding
constants for 8 were frustrated by its moisture sensitivity, which leads
to the isolation of the (structurally characterized) hydroxysilyl species
[rac-1,2-fc(BMes2X)(SiMe2OH)]− (X− = F−, CN−; see the
Supporting Information). The fluoride binding constant determined
in thf for the less sensitive methoxysilyl species 9 (log K = 5.42(0.08))
is actually less than that determined for both FcBMes2 (log K =
6.06(0.04)) and 9/CN− (log K = 6.36(0.24)) in the same solvent.
Interpretation of these dataand comparison with anion binding by
8is complicated, however, by the observation through multinuclear
NMR of a nonchelating mode of interaction of 9 with F−.
(24) Shafir, A.; Power, M. P.; Whitener, G. D.; Arnold, J.
Organometallics 2000, 19, 3978.
(25) Rybinskaya, M. I.; Kreindlin, A. Z.; Fadeeva, S. S.; Petrovskii, P.
V. J. Organomet. Chem. 1988, 345, 341.
(26) Dewar, M. S.; Takeuchi, Y. J. Am. Chem. Soc. 1967, 89, 390.
(27) Hong, C.; Bartlett, R. A.; Olmstead, M. M.; Power, P. P.;
Shoner, S. C. J. Am. Chem. Soc. 1990, 112, 1048.
(28) ReactLab equlibria, Version 1.0; JPlus Consulting Pty Ltd., 2010.
(29) (a) Cosier, J.; Glazer, A. M. J. Appl. Crystallogr. 1986, 19, 105.
(b) Denzo: Otwinowski, Z.; Minor, W. Methods in Enzymology;
Carter, C. W., Sweet, R. M., Eds.; Academic Press: New York, 1996;
Vol. 276, p 307. (c) Sir-92: Altomare, A.; Cascarano, G.; Giacovazzo,
C.; Guagliardi, A.; Burla, M. C.; Polidori, G.; Camalli, M. J. Appl.
Crystallogr. 1994, 27, 435. (d) Sheldrick, G. M. Acta Crystallogr., Sect. A
2008, 64, 112. (e) CRYSTALS: Betteridge, P. W.; Carruthers, J. R.;
Cooper, R. I.; Prout, J.; Watkin, D. J. J. Appl. Crystallogr. 2003, 36,
1487.
2005, 52, 61. (v) Scheibitz, M.; Heilmann, J. B.; Winter, R. F.; Bolte,
M.; Bats, J. W.; Wagner, M. Dalton Trans. 2005, 159. (w) Lorbach, A.;
Bolte, M.; Lerner, H.-W.; Wagner, M. Chem. Commun. 2010, 46, 3592.
̈
(x) Gabbaı, F. P. ACS Symp. Ser. 2006, 917, 208. (y) Melaïmi, M.;
̈
́
Sole, S.; Chiu, C.-W.; Wang, H.; Gabbaı, F. P. Inorg. Chem. 2006, 45,
8136. (z) Chase, P. A.; Henderson, L. D.; Piers, W. E.; Parvez, M.;
Clegg, W.; Elsegood, M. R. J. Organometallics 2006, 25, 349. (aa) Lee,
̈
M. H.; Gabbaı, F. P. Inorg. Chem. 2007, 46, 8132. (ab) Chai, J. F.;
Lewis, S. P.; Collins, S.; Sciarone, T. J. J.; Henderson, L. D.; Chase, P.
A.; Irvine, G. J.; Piers, W. E.; Elsegood, M. R. J.; Clegg, W.
Organometallics 2007, 26, 5667. (ac) Lorbach, A.; Bolte, M.; Li, H. Y.;
Lerner, H. W.; Holthausen, M. C.; Jakle, F.; Wagner, M. Angew. Chem.,
̈
Int. Ed. 2009, 48, 4584. (ad) Jiang, C.; Blacque, O.; Berke, H. Chem.
Commun. 2009, 5518. (ae) Chem. Commun. 2011, 47, 1833; (af) Zhao,
H.; Gabbaï, F. P. Organometallics 2012, 31, 2327. (ag) Seven, O.; Qu,
Z.-W.; Zhu, H.; Bolte, M.; Lerner, H.-W.; Holthausen, M. C.; Wagner,
M. Chem. Eur. J. 2012, 18, 11284.
(10) Reports of ferrocenes with boron substituents in the 1- and 2-
positions are confined to 1,2-fc(BMes2)[B(OH)2]: (a) Morgan, I. R.;
Di Paolo, A.; Vidovic, D.; Fallis, I. A.; Aldridge, S. Chem. Commun.
2009, 7288. (b) Siewert, I.; Vidovic, D.; Aldridge, S. J. Organomet.
Chem. 2011, 696, 2528. There is also a preliminary report by us of
compounds 6a−c: (c) Morgan, I. R.; Broomsgrove, A. E. J.;
Fitzpatrick, P.; Vidovic, D.; Thompson, A. L.; Fallis, I. A.; Aldridge,
S. Organometallics 2010, 29, 4762. In addition, diboryldiferrocenes have
been reported by Jakle, which by virtue of geometric constraints offer
̈
reduced possibilities for chelation of monodentate anions: (d) Ven-
katasubbaiah, K.; Zakharov, L. N.; Kassel, W. S.; Rheingold, A. L.;
Jakle, F. Angew. Chem., Int. Ed. 2005, 44, 5428. (e) Venkatasubbaiah,
̈
K.; Nowik, I.; Herber, R. H.; Jakle, F. Chem. Commun. 2007, 2154.
̈
(f) Venkatasubbaiah, K.; Doshi, A.; Nowik, I.; Herber, R. H.;
Rheingold, A. L.; Jakle, F. Chem. Eur. J. 2008, 14, 44. (g) Pakkirisamy,
̈
T.; Venkatasubbaiah, K.; Kassel, W. S.; Rheingold, A. L.; Jakle, F.
̈
Organometallics 2008, 27, 3056. (h) Venkatasubbaiah, K.; Pakkirisamy,
T.; Lalancette, R. A.; Jakle, F. Dalton Trans. 2008, 4507. (i) Thilagar,
̈
P.; Murillo, D.; Chen, J.; Jakle, F. Dalton Trans. 2013, 42, 665.
̈
(11) Ruf, W.; Renk, T.; Siebert, W. Z. Naturforsch. 1976, 31b, 1028.
(12) Scheibitz, M.; Bolte, M.; Bats, J. W.; Lerner, H.-W.; Nowik, I.;
Herber, R. H.; Krapp, A.; Lein, M.; Holthausen, M. C.; Wagner, M.
Chem. Eur. J. 2005, 11, 584.
(13) Eckensberger, U. D.; Kunz, K.; Bolte, M.; Lerner, H.-W.;
Wagner, M. Organometallics 2008, 27, 764.
(14) (a) Bildstein, B.; Hradsky, A.; Kopacka, H.; Malleier, R.;
Ongania, K. H. J. Organomet. Chem. 1997, 540, 127. (b) Mayer, U.;
Gutmann, V.; Gerger, W. Monatsh. Chem. 1975, 106, 1235.
(15) Green, J. C. Chem. Soc. Rev. 1998, 27, 263.
(16) Butler, I. R. Inorg. Chem. Commun. 2008, 11, 15.
(17) (a) Kawachi, A.; Tani, A.; Shimada, J.; Yamamoto, Y. J. Am.
Chem. Soc. 2008, 130, 4222. See also: (b) Katz, H. E. J. Am. Chem. Soc.
1986, 108, 7640.
(18) For previous reports of ferrocene derivatives featuring boron
and heavier group 14 element substituents in the 1- and 2-positions,
see: (a) Jakle, F.; Lough, A. J.; Manners, I. Chem. Commun. 1999, 453.
̈
(b) Gamboa, J.; Sundaraman, A.; Kakalis, L.; Lough, A. J.; Jakle, F.
̈
Organometallics 2002, 21, 4169. (c) Boshra, R.; Sundaraman, A.;
Zakharov, L. N.; Incarvito, C. D.; Rheingold, A. L.; Jakle, F. Chem. Eur.
̈
J. 2005, 11, 2810. (d) Boshra, R.; Venkatasubbaiah, K.; Doshi, A.;
Lalancette, R. A.; Kakalis, L.; Jakle, F. Inorg. Chem. 2007, 46, 10174.
̈
(e) Boshra, R.; Doshi, A.; Jakle, F. Organometallics 2008, 27, 1534;
̈
Angew. Chem., Int. Ed. 2008, 47, 1134. (f) Boshra, R.; Venkatasubbaiah,
K.; Doshi, A.; Jakle, F. Organometallics 2009, 28, 4141. (g) Boshra, R.;
̈
Doshi, A.; Venkatasubbaiah, K.; Jakle, F. Inorg. Chim. Acta 2010, 364,
162.
̈
(19) Herberich, G. E.; Englert, U.; Fischer, A.; Wiebelhaus, D. Eur. J.
Inorg. Chem. 2004, 4011.
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