Probing the influence of steric bulk on anion binding by triarylboranes: Comparative studies of FcB(o-Tol)2, FcB(o-Xyl)2 and FcBMes2

Article abstract of DOI:10.1039/c1dt10185d

Steric crowding brought about on pyramidalization at boron has been predicted computationally to be of central importance to the strength and selectivity of anion binding by triarylboranes. The role of steric factors in systems containing a ferrocenyl reporter unit has been systematically probed in the current study by comparison of the F^-/CN^- binding properties of FcB(o-Tol)₂ (1, o-Tol = C₆H ₄Me-2), FcB(o-Xyl)₂ (2, o-Xyl = C₆H ₃Me₂-2,6) and FcBMes₂ (3, Mes = C ₆H₂Me₃-2,4,6)), both in solution and in the solid state. Somewhat surprisingly, the inclusion of an extra ortho-methyl aryl substituent (e.g. for 2/3vs.1) is found to have a relatively small effect on the binding affinities of these boranes (e.g. log₁₀K_CN = 5.94(0.02), 4.73(0.01), 5.56(0.02), for 1, 2 and 3 respectively). Consistent with this observation, the degree of pyramidalization at boron determined for the cyanide adducts [1·CN]^-, [2·CN]^- and [3·CN]^- in the solid state is also found to be essentially invariant (∠C_aryl-B-C_aryl = 338, 337, 337°, respectively), as are the B-CN and mean B-C_aryl distances. In the solid state at least, it is apparent that the adverse steric effects potentially brought about by increasing ortho substitution are mitigated by a greater degree of synchronous rotation of the aryl substituents about the B-C _aryl bonds. Thus a mean inter-plane angle of 71° is observed for [1·CN]^- while the corresponding values for [2·CN] ^- and [3·CN]^- are 78° and 79°.

Full text of DOI:10.1039/c1dt10185d

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PAPER
Probing the influence of steric bulk on anion binding by triarylboranes:
comparative studies of FcB(o-Tol)2, FcB(o-Xyl)2 and FcBMes2†
Inke Siewert,* Philip Fitzpatrick, Alexander E. J. Broomsgrove, Michael Kelly, Dragoslav Vidovic and
Simon Aldridge*
Received 2nd February 2011, Accepted 18th February 2011
DOI: 10.1039/c1dt10185d
Steric crowding brought about on pyramidalization at boron has been predicted computationally to be
of central importance to the strength and selectivity of anion binding by triarylboranes. The role of
steric factors in systems containing a ferrocenyl reporter unit has been systematically probed in the
current study by comparison of the F-/CN- binding properties of FcB(o-Tol)2 (1, o-Tol = C6H4Me-2),
FcB(o-Xyl)2 (2, o-Xyl = C6H3Me2-2,6) and FcBMes2 (3, Mes = C6H2Me3-2,4,6)), both in solution and in
the solid state. Somewhat surprisingly, the inclusion of an extra ortho-methyl aryl substituent (e.g. for
2/3 vs. 1) is found to have a relatively small effect on the binding affinities of these boranes (e.g.
log10KCN = 5.94(0.02), 4.73(0.01), 5.56(0.02), for 1, 2 and 3 respectively). Consistent with this
observation, the degree of pyramidalization at boron determined for the cyanide adducts [1·CN]-,
[2·CN]- and [3·CN]- in the solid state is also found to be essentially invariant (∠Caryl–B–Caryl = 338, 337,
337◦, respectively), as are the B–CN and mean B–Caryl distances. In the solid state at least, it is apparent
that the adverse steric effects potentially brought about by increasing ortho substitution are mitigated
by a greater degree of synchronous rotation of the aryl substituents about the B–Caryl bonds. Thus a
mean inter-plane angle of 71◦ is observed for [1·CN]- while the corresponding values for [2·CN]- and
[3·CN]- are 78◦ and 79◦.
affinities.3s This selectivity is thought to reflect the pyramidaliza-
Introduction
tion inherent at boron on formation of the host/guest complex,
In landmark recent studies triarylboranes have been successfully
and the consequent increase in steric repulsion between the aryl
substituents. Thus, only small anions (F-, CN-, OH-, N3 ) which
limit the extent to which such interactions can develop, typically
bind to ArBMes2 derivatives.3s
-
exploitedas sensor systems for the environmentallyrelevantanions
fluoride and cyanide.1–9 In part, such applications rely on the
inherent strength of the host/guest interaction (e.g. 345 kJ mol-1
for the binding of F- to Ph3B),10 which offers the basis for
overcoming fundamental challenges in supramolecular chemistry,
such as the detection of the highly solvated fluoride ion in water.3s
Thus, borane-based sensors have been reported which capture
fluoride from protic media, with a number of these systems even
being capable of workable sensing in water itself.4z Typically,
such receptors are of the form ArBMes2 (Mes = C6H2Me3-2,4,6)
reflecting not only their high binding affinities for F-/CN-, but
also their stability to air and moisture.4,7 Moreover, even simple
systems of this type (i.e. those lacking in further functionalization)
have been shown to display a degree of selectivity in their binding
properties, with larger, less basic anions exhibiting very low
From the point of view of the receptor itself, the extent to
which steric factors can influence binding constants has been
predicted computationally, e.g. by calculation of the gas-phase
fluoride binding enthalpies of Ph3B and PhBMes2 (DH = -345 and
-269 kJ mol-1, respectively),10,5x and the geometric parameters for
the adducts [Ph3B·F]- and [PhBMes2·F]-. If entropic factors are
assumed to be comparable, the difference (DDH) of -76 kJ mol-1
in the fluoride ion affinity for the two compounds points to a
difference in log10KF of ca. 13 orders of magnitude (albeit in the
gas phase). Moreover the lower binding affinity of PhBMes2 is also
reflected in calculated structural parameters for the F- adducts
which reveal (i) a smaller degree of pyramidalization at the boron
centre and (ii) a slightly extended B–F bond in [PhBMes2·F]-
compared to [Ph3B·F]- [∠C–B–C = 341, 333◦, d(B–F) = 1.491,
Inorganic Chemistry Laboratory, Department of Chemistry, University of
Oxford, South Parks Road, Oxford, U.K., OX1 3QR. E-mail: simon.
aldridge@chem.ox.ac.uk; Fax: +44 (0)1865 272690; Tel: +44 (0)1865
285201
† Electronic supplementary information (ESI) available: Details of binding
constant determinations for 1, 2 and 3. CCDC reference numbers 809729–
80973. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c1dt10185d
10a,5x
˚
1.465 A, respectively].
With sensing applications in mind, related derivatives in which
one of the aryl groups is a ferrocenyl function (e.g. FcBMes2)7f,7j
offer additional benefits relating to electrochemical or colorimetric
reporting of the anion binding event.11 As such, we have sought
to probe experimentally the influence of steric factors on anion
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 10345–10350 | 10345
©

binding by systems of the type FcBAr2. In particular, we have set
out to evaluate the consequences of variation in the number of
para- and more importantly ortho- substituents in determining the
steric profile of the aryl groups. Thus, in the current manuscript
we report comparative studies of fluoride and cyanide binding
by FcB(o-Tol)2 (1; o-Tol = C6H4Me-2), FcB(o-Xyl)2 (2, o-Xyl =
C6H3Me2-2,6) and FcBMes2 (3) in solution and in the solid state.
For these systems at least, the effect of increasing ortho substitution
is shown to be rather smaller, reflecting conformational flexibility
about the B–Caryl bonds, and a consequent ability to minimize
adverse steric effects.
solution was layered with hexane to yield K(18-crown-6)][1·CN] as
single crystals suitable for X-ray diffraction. Although 11B and 1H
NMR monitoring of the reaction reveals quantitative conversion
of 1 to its cyanide adduct, the much lower isolated yield of single
crystals reflects its intrinsic solubility. Yield 0.016 g (0.023 mmol,
1
6%◦). Spectroscopic data: H NMR (300 MHz, [D]chloroform,
20 C, ppm) dH = 7.64 (d, 2H, J = 7.2 Hz, CH6 of Tol), 6.92 (m,
6H, CH3, CH4 and CH5 of Tol), 4.16 (s br, 2H, C5H4), 4.11 (s, 5H,
C5H5), 4.05 (s br, 2H, C5H4), 3.56 (s, 24H, OCH2), 2.91 (s, 6H, CH3
of Tol). 13C NMR (75 MHz, [D]chloroform, 20 ◦C, ppm) dC = 154.2
(br, C1 of Tol), 141.8 (C2 of Tol), 135.0 (C6 of Tol), 128.6, 123.6,
123.4 (CH of Tol), 73.8 (C5H4), 69.9 (OCH2), 67.9 (C5H5), 67.2
(C5H4), 23.3 (CH3 of Tol). The boron bound carbon atom of C5H4
could not be observed. 11B (96 MHz, [D]chloroform, 20 ◦C, ppm):
dB = -14. UV/Vis: lmax = 445 nm, e = 1500 dm2 mol-1. MS (ESI+):
exact mass (calc. for M+) 303.1204, (meas.) 303.1201 (100%). MS
(ESI-): M- exact mass (calc. for M-, 10B isotopomer) 401.1226,
(meas.) 401.1365. Spectroscopic data for [K(18-crown-6)][1·F]: 1H
NMR (300 MHz, [D]chloroform, 20 ◦C, ppm) dH = 7.58 (dd, 2H,
J = 1.3, 7.2 Hz, CH6), 6.92 (m, 6H, CH3, CH4 and CH5 of Tol),
4.08 (s br, 2H, C5H4), 4.00 (s br, 2H, C5H4), 3.90 (s, 5H, C5H5),
3.56 (s, 24H, OCH2), 1.99 (s, 6H, CH3 of Tol). 13C NMR (75 MHz,
[D]chloroform, 20 ◦C, ppm) dC = 158.2 (br., C1 of Tol), 140.6 (d,
J = 1.4 Hz, C2 of Tol), 133.6 (d, J = 7.3 Hz, C6), 128.4, 123.4, 123.2
(CH of Tol), 72.5 (d, J = 3.5, C5H4), 70.1 (OCH2), 67.3 (C5H5), 66.8
(C5H4), 23.3 (CH3). The boron bound carbon atom of C5H4 could
not be observed. 11B (96 MHz, [D]chloroform, 20 ◦C, ppm): dB = 5.
19F NMR (282 MHz, [D]chloroform, 20 ◦C, ppm) dF = -185 (br).
UV/Vis: lmax = 448 nm, e = 1000 dm2 mol-1. MS (ESI+): M+ exact
mass (calc. for M+, 10B isotopomer) 303.1204, (meas.) 303.1202
(100%). MS (ESI-): M- exact mass (calc. for M-, 10B isotopomer)
397.1226, (meas.) 397.1232.
Experimental
General considerations and physical methods
Manipulations of air-sensitive reagents were carried out in a glove-
box, 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 commercial
available Braun Solvent Purification System (SPS 500). The known
compounds FcBBr2 and o-TolLi13 were prepared according to
12
1
literature procedures. H and 13C NMR spectra were measured
on a Bruker AVII 500 FT-NMR or Varian Mercury VX-300
spectrometer with [D]chloroform as the solvent. 1H and 13C
NMR spectra were calibrated using the residual proton or natural
abundance 13C resonances of [D]chloroform (dH = 7.26 ppm, dC =
77.0 ppm). Mass spectra were measured by the EPSRC National
Mass Spectrometry Service Centre, Swansea University. Elemental
microanalysis was carried out at London Metropolitan University.
Synthesis
1: A solution of o-TolLi (1.00 g, 10 mmol) in diethyl ether was
added to a solution of FcBBr2 (1.45 g, 4 mmol) also in diethyl
ether, and the reaction mixture stirred at room temperature for
15 h. Subsequently, volatiles were removed in vacuo, the product
extracted into hexane, and single crystals of 1 obtained after
concentration to near saturation and cooling to -35 ◦C. Yield
1.17 g (3.1 mmol, 76%).
Crystallographic data†
˚
1: Mr = 378.10, Orthorhombic, Pna21, a = 15.3409(6) A, b =
3
˚
˚
˚
7.4354(3) A, c = 16.5260(7) A, V = 1885.05(13) A , Z = 2, rc = 1.322
-3
˚
Mg m , T = 150 K, l = 0.71073 A, 13079 reflections collected,
2211 independent [R(int) = 0.070]. R1 = 0.0425, wR2 = 0.1130 for
observed unique reflections [F2 > 2s (F2)] and R1 = 0.0488, wR2 =
0.1183 for all unique reflections. Max. and min. residual electron
1H NMR (500 MHz, [D]chloroform, 20 ◦C, ppm) dH = 7.75 (dd,
2H, J = 1.3, 7.2 Hz, CH6), 7.27 (dt, 2H, J = 1.6, 7.4 Hz, CH4), 7.22
(t, 2H, J = 6.8 Hz, CH5), 7.13 (d, 2H, J = 7.5 Hz, CH3), 4.77 (t, 2H,
J = 1.9, C5H4), 4.36 (t, 2H, J = 1.8, C5H4), 4.23 (s, 5H, C5H5), 2.18
-3
˚
densities 0.56 and -0.46 e A . CCDC reference: 809729.
¯
[K(18-crown-6)][1·CN]: Mr
=
707.54, Triclinic, P1,
a
=
◦
˚
˚
˚
11.5278(2) A, b◦= 13.2927(2) A, c = 13.7154(2) A, a = 84.1631(7) ,
◦
◦
(s, 6H, CH3 of Tol). 13C NMR (75 MHz, [D]chloroform, 20 C,
b = 71.6023(7) , g = 65.2619(7) , V = 1810.13(5) A , Z = 2,
3
˚
ppm) dC = 145.5 (br, C1 of Tol), 140.0 (C2 of Tol), 132.4 (C6 of
Tol), 129.6 (C3 of Tol), 128.3 (C4 of Tol), 124.2 (C5 of Tol), 78.0
(C5H4), 75.5 (C5H4), 69.3 (C5H5), 23.1 (CH3 of Tol). The boron
bound carbon atom of C5H4 could not be observed. 11B (96 MHz,
[D]chloroform, 20 ◦C, ppm): dB = 70. UV/Vis: lmax = 480 nm, e =
12200 dm2 mol-1. MS (EI-): M+ mass (calc. for M+, 10B isotopomer)
378.1, (meas.) 378.1 (100%). Elemental microanalysis (%): (calc.
for C24H23BFe) C 76.24, H 6.13; (meas.) C 76.18 H 6.09.
rc = 1.298 Mg m , T = 150 K, l = 0.71073 A, 25142 reflections
-3
˚
collected, 8175 independent, [R(int) = 0.032]. R1 = 0.0369, wR2 =
0.0820 for observed unique reflections [F2 > 2s (F2)] and R1 =
0.0536, wR2 = 0.0897 for all unique reflections. Max. and min.
residual electron densities 0.52 and -0.45 e A . CCDC reference:
809730.
-3
˚
[K(18-crown-6)][1·F]: Mr = 819.90, Monoclinic, P21, a =
˚
˚
˚
8.55170(10)◦ A, b = 22.6990(3) A, c = 10.3460(2) A, b =
3
-3
˚
[K(18-crown-6)][1·CN] and [K(18-crown-6)][1·F]: The two com-
pounds were prepared by analogous methods from KCN and KF,
exemplified here for [K(18-crown-6)][1·CN]. To a solution of 1
(0.140 g, 0.37 mmol) in chloroform was added KCN (0.048 g, 0.74
mmol) and 18-crown-6 (0.098 g, 0.37 mmol), and the reaction
mixture stirred at room temperature for 15 h. Subsequently the
101.3943(5) , V = 1968.73(5) A , Z = 2, rc = 1.383 Mg m , T = 150
˚
K, l = 0.71073 A, 27162 reflections collected, 8428 independent
[R(int) = 0.050]. R1 = 0.0361, wR2 = 0.0719 for observed unique
reflections [F2 > 2s (F2)] and R1 = 0.0492, wR2 = 0.0780 for all
unique reflections. Max. and min. residual electron densities 0.65
-3
˚
and -0.65 e A . CCDC reference: 809731.
10346 | Dalton Trans., 2011, 40, 10345–10350
This journal is
The Royal Society of Chemistry 2011
©

Binding constant determination
at boron. Notably, there are no significant differences between
the solid state structure of 1 and that of 3; in particular the B–
CCp [1.522(8) and 1.546(7) A, respectively] and B–Caryl distances
Typical protocol: 3 mL of a solution of 1 in dichloromethane
(typically 0.4–0.6 mM) were placed in the cell and aliquots of
[nBu4N]F·4H2O or [nBu4N]CN·2H2O in dichloromethane added
(typically 10–20 mL of a 10–15 mM solution). The solution was
stirred for 1 min after each addition and the UV-vis spectrum then
measured. The program ReactLab Equilibria was subsequently
used to determine the binding constants, with data being fitted
over the wavelength range 430–530 nm.14 Full details are available
in the ESI.
˚
˚
[1.577(8), 1.582(9) and 1.581(7), 1.597(7) A] and the inter-plane
torsion angles defining the propeller conformation [63, 62 and 62,
62◦] are essentially identical for both receptor compounds.7j
The response of 1 to the presence of cyanide or fluoride
was investigated by a combination of spectroscopic techniques.
Exposure to either anion leads to a marked shift of the 11B NMR
signal to 5 ppm (fluoride) and -14 ppm (cyanide), respectively. In
each case, 1H NMR spectroscopy reveals complete conversion to
the borane/anion host/guest complex in presence of 2 equiv. of
the anion and 1 equiv. of 18-crown-6. Additional confirmation of
anion binding was obtained in the case of the fluoride adduct
through the presence in the 13C NMR spectrum of coupling
between the 19F nucleus and the C6 and C2 carbon atoms of the
o-tolyl substituents, as well as by the appearance of a broad signal
at dF = -185 ppm, which is typical for a fluoride anion bound
to a triarylborane moiety.3s In addition, the structures of [K(18-
crown-6)][1·F] and [K(18-crown-6)][1·CN] were obtained by X-ray
crystallography and are shown in Fig. 2 and 3, respectively.
Results and discussion
The synthesis of 1 can be accomplished in a manner similar to
that previously described for 2 and 3.7j Thus, the reaction between
FcBBr2 and a slight excess of o-tolyl lithium leads to the formation
of 1 in reasonable (76%) yield after recrystallisation from hexane
(Scheme 1).
Scheme 1 Synthesis of 1; key reagents and conditions: (a) o-TolLi (2.5
equiv.), Et2O, room temperature, 15 h, 76% after recrystallization from
hexane.
Characterisation of the product by multinuclear NMR, mass
spectrometry, elemental microanalysis and single crystal X-ray
diffraction is consistent with its formulation as 1. The 11B NMR
spectrum, for example, shows a broad peak at dB = 70 ppm, in the
region expected for tri-coordinate boron species featuring two aryl
and one ferrocenyl substituents.7j,7f In contrast to 2 and 3, however,
1 is slowly decomposed by moisture in solution, presumably due
to the lack of steric protection afforded by a second methyl
substituent in the ortho position.
Fig. 2 Molecular structure of [K(18-crown-6)][1·F]·CHCl3 with hy-
drogen atoms (except the chloroform hydrogen) omitted for clarity
and thermal ellipsoids set at the 40% level. Selected bond lengths
˚
= 1.622(4), B(7)–C(9) = 1.651(4),
(A) and angles (◦): C(2)–B(7)
B(7)–C(16) = 1.644(3), B(7)–F(8) = 1.481(3), F(8) ◊ ◊ ◊ K(28) = 2.647(2),
C(16)–B(7)–C(9) = 113.6(2), C(9)–B(7)–C(2) = 112.1(2), C(2)–B(7)–C(16) =
110.8(2), K(28)–F(8)–B(7) = 134.0(1).
The structure of 1 determined by X-ray crystallography is
shown in Fig. 1; the aryl groups are aligned in a propeller
type arrangement, with the angles between the least squares
5
planes defined by the o-tolyl groups and that of the h -C5H4
substituent being 63◦ and 62◦. The sum of the CBC angles is
359◦, consistent with the expected planar tri-coordinate geometry
Fig. 3 Molecular structure of [K(18-crown-6)][1·CN] with hydrogen
atoms omitted for clarity and thermal ell◦ipsoids set at the 40%
˚
Fig. 1 Molecular structure of 1 with hydrogen atoms omitted for clarity
level. Selected bond length (A) and angles ( ): C(2)–B(7) = 1.627(3),
˚
and thermal ellipsoids set at the 40% level. Selected bond length (A) and
C(15)–B(7) = 1.660(2), C(8)–B(7) = 1.644(2), B(7)–C(22) = 1.622(3),
C(22)–N(23) = 1.150(3), N(23) ◊ ◊ ◊ K(29) = 2.769(2), C(15)–B(7)–C(2) =
angles (◦): C(20)–B(12) = 1.582(9), B(12)C(13) = 1.577(8), B(12)–C(10) =
1.522(8), C(20)–B(12)–C(13) = 119.6(5), C(13)–B(12)–C(10) = 122.2(5),
C(10)–B(12)–C(20) = 117.7(5).
111.0(1), C(2)–B(7)–C(8)
= 110.5(1), C(8)–B(7)–C(15) = 116.5(1),
B(7)–C(22)–N(23) = 179.3(2).
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 10345–10350 | 10347
©

Not unexpectedly, the solid state structure of [K(18-crown-
6)][1·F] reveals that the coordination sphere around the boron
ꢀ
centre has become pyramidalized [ (∠CBC) = 337◦ for the ferro-
cenyl and o-tolyl substituents] and that the distances between the
boron atom and ipso carbon atoms of the cyclopentadienyl and o-
tolyl substituents are elongated compared to 1 [d(B–C) = 1.522(8),
˚
1.577(8), 1.582(9) A for 1 cf. d(B–C) = 1.622(4), 1.644(3), 1.651(4)
˚
A for [K(18-crown-6)][1·F]]. The B–F distance is similar to that in
[nBu4N][PhBMes2·F] but slightly longer than in [K(2.2.2crypt)][(9-
˚
anthryl)3B·F] [d(B–F) = 1.481(3), cf. 1.481(2) and 1.466(5) A,
respectively].4ff,4a In addition, the solid state structure reveals a
weak interaction between the fluoride anion and the potassium
centre of the [K(18-crown-6]+ counter-ion [d(K ◊ ◊ ◊ F) = 2.647(2)
˚
˚
A, cf. 4.22 A for the sum of the van der Waals’ radii of potassium
and fluorine].15 This kind of secondary interaction is unusual for
fluoride bound FcBAr2 compounds, but finds precedent in related
cyanide adducts,7j and in the fluoride adduct of a catecholboryl-
functionalized crown ether,16 as well as in [K(18-crown-6)(thf)][1,2-
fc(BMeF)(SnMe2X)·F] (X = Cl or F).17 In similar fashion, the
solid state structure of [K(18-crown-6)][1·CN] features a linear
boron-bound cyanide unit [∠BCN = 179.3(2)], and an additional
secondary interaction between the peripheral nitrogen atom and
+
˚
˚
[K(18-crown-6] [d(K ◊ ◊ ◊ N) = 2.769(2) A, cf. 4.30 A for the sum of
the van der Waals’ radii of potassium and nitrogen].15 The boron
centre, B(7), is quaternized in a similar manner to that in [K(18-
ꢀ
crown-6)][1·F] [ (∠CBC) = 338◦ for the ferrocenyl and o-tolyl
substituents] and a similar degree of elongation of the aryl B–C
bonds is also observed for the two adducts (d(B–C) = 1.644(2),
1.627(3), 1.660(2) for [1·CN]-, cf. 1.644(3), 1.622(4), 1.651(4) for
[1·F]-).
Fig. 4 The response of 1 to exposure to [nBu4N]CN·2H2O in CH2Cl2
as monitored by UV-vis spectroscopy (top); experimental data (points)
obtained at l = 490 nm as a function of added cyanide, and the best-fit line
obtained using ReactLabTM Equilibria (bottom).14
In order to determine quantitatively the affinity of 1 for fluoride
and cyanide, UV-vis titrations were carried out, yielding the
binding constants KF and KCN. The absorption maximum of
1 in dichloromethane is slightly blue shifted and the extinction
coefficient slightly lower than the analogous values for 3 (lmax
=
Information; the small shift in l reflects the slightly shifted
absorption maxima for 2/3 compared to 1). These three cyanide
adducts offer a basis for probing the underlying structural factors
influencing binding affinities, since crystallographic data are also
available for [2·CN]- and [3·CN]-.7j Somewhat surprisingly, the
480 nm, e = 12200 dm2 mol-1 for 1, lmax = 510 nm, e = 13100
dm2 mol-1 for 3).7j Fig. 4 shows the UV-vis spectra of 1 on
exposure to successive aliquots of [nBu4N]CN·2H2O, and Fig. 5
the corresponding spectra with [nBu4N]F·4H2O.18
These spectra show that 1 is able to act as a colorimetric sensor
for fluoride and cyanide anions, with a colour change from deep
orange to light yellow effected on addition of either anion. The
binding constants were calculated in the wavelength range 430 nm
to 530 nm by fitting the absorbance vs. anion concentration data
(in dichloromethane solution) with ReactLabTM Equilibria.14 This
wavelength range covers the absorption maxima both of 1 and the
bound species [1·F]-/[1·CN]-. As an indication of the fit obtained,
the experimental data and the best-fit line at l = 490 nm are shown
in Fig. 4 and 5 (for F- and CN-, respectively).
Derivation of these isotherms reveals that log10KCN for 1 is
5.94(0.02),19 and log10KF = 5.82(0.02).20 The small difference
between these binding constants is consistent with the very
similar structural features noted above for [K(18-crown-6)][1·F]
and [K(18-crown-6)][1·CN]. While there is therefore little discrim-
ination between the two analytes, competition experiments are
consistent with the stronger binding of CN-.7j
cyanide binding constants determined for 2 and 3 [log10KCN =
4.73(0.01) and 5.56(0.02), respectively] are actually very similar to
that measured for 1.
Structurally (on the basis of quantum chemical studies carried
out on the gas phase [Ph3B·F]- and [PhMes2B·F]- ions) a
greater degree of pyramidalization of the B(Caryl)3 unit might be
expected at the boron centre in [1·CN]- compared to [2·CN]- and
[3·CN]-.10,5x Interestingly, however, the sum of the aryl/ferrocenyl
CBC angles shows virtually no difference between the three struc-
tures ([1·CN]-: 338◦; [2·CN]-: 337◦; [3·CN]- 337◦). Furthermore,
the B–CN bond lengths are also statistically identical (d(B–
-
-
˚
CN) = 1.622(3), 1,624(3), 1.621(3) A for [1·CN] , [2·CN] and
[3·CN]-, respectively), and the same is true for the distances
between the boron atoms and the ipso carbon atoms of the aryl
and cyclopentadienyl substituents.7j As such, the steric demands
of the additional ortho methyl groups do not appear to alter
either the degree of pyramidalization at the boron centre or the
distance between the anion and the boron atom-findings which are
consistent with the similar cyanide binding constants determined
for the three receptors.
In order to put these figures in broader context, the corre-
sponding values of KCN have been re-determined for 2 and 3,7j
using a similar wavelength range, 450–550 nm (see Supporting
10348 | Dalton Trans., 2011, 40, 10345–10350
This journal is
The Royal Society of Chemistry 2011
©

Conclusions
The current investigation shows that FcB(o-Tol)2 (1) is easy
accessible via the reaction of FcBBr2 with o-tolyl lithium and
that it acts as a colorimetric anion sensor for cyanide and
fluoride anions. Interestingly, the binding affinity of 1 for cyanide
is shown to be only slightly enhanced compared to the more
sterically encumbered derivatives FcB(o-Xyl)2 and FcBMes2, a
phenomenon attributed to conformational flexibility about the
B–Cipso bonds which mitigates the effects on increasing ortho
substitution.
Acknowledgements
We thank the EPSRC for funding and for access to the National
Mass Spectrometry Service Centre, Swansea University. IS is
grateful to the DFG for financial support.
Notes and references
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Further consideration of the conformations adopted by the
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19 Error: ssq = 0.07, sr = 0.006.
20 Error: ssq = 0.08, sr = 0.006.
10350 | Dalton Trans., 2011, 40, 10345–10350
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The Royal Society of Chemistry 2011
©