Anion recognition by highly sterically encumbered 1,2-diborylferrocenes

Article abstract of DOI:10.1021/om1005337

The syntheses and structural characterization of sterically encumbered 1,2-diborylferrocenes are reported, together with an investigation of their anion recognition capabilities with respect to fluoride and cyanide. Surprisingly, 1,2-fc(BMes₂)<

Full text of DOI:10.1021/om1005337

4762 Organometallics 2010, 29, 4762–4765
DOI: 10.1021/om1005337
Anion Recognition by Highly Sterically Encumbered
1,2-Diborylferrocenes§
Ian R. Morgan,†,‡ Alexander E. J. Broomsgrove,† Philip Fitzpatrick,† Dragoslav Vidovic,†
Amber L. Thompson,† Ian A. Fallis,‡ and Simon Aldridge*,†
†Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford,
South Parks Road, Oxford, U.K., OX1 3QR, and School of Chemistry, Cardiff University,
‡
Main Building, Park Place, Cardiff, U.K., CF10 3AT
Received May 30, 2010
Summary: The syntheses and structural characterization of
sterically encumbered 1,2-diborylferrocenes are reported, together
with an investigation of their anion recognition capabilities with
respect to fluoride and cyanide. Surprisingly, 1,2-fc(BMes2)2 is
found to be highly selective for CN-, with the uptake of F-
being shown to be not only thermodynamically less favorable
but also kinetically much slower.
cyanide for three-coordinate boranes has been known for
more than 45 years;4 recent studies have demonstrated the
use of Lewis acid receptors containing the -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-derived recep-
tors.6 Recent reports, for example, 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 In this respect, the use
of the proton affinities (or even the pKas) of F-/CN- in a
predictive capacity is fraught with difficulty, as reflected by
The selective detection of CN- and F- constitute signifi-
cant chemical challenges both from a supramolecular view-
point and from the perspective of potential applications in
environmental and medical monitoring.1-3 The affinity of
§ Part of the Dietmar Seyferth Festschrift.
*Contact details for corresponding author. Tel: 44 (0)1865 285201.
Fax: þ44 (0)1865 272690. E-mail: simon.aldridge@chem.ox.ac.uk.
URL: http://users.ox.ac.uk/∼quee1989.
(6) For a very recent review of fluoride binding by triarylboranes and
boronic acid/esters see, for example: Wade, C. R.; Broomsgrove,
(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
A. E. J.; Aldridge, S.; Gabbaı, F. P. Chem. Rev. 2010, 110, 3958.
¨
(7) See, for example: http://www2.lsdiv.harvard.edu/labs/evans/pdf/
evans_pKa_table.pdf.
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Publications: New York, 1997; pp 271-286. (c) Sohn, H.; Letant, S.; Sailor,
(8) For bidentate boron-centered 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. Organometallics 1994, 13,
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r
pubs.acs.org/Organometallics
Published on Web 08/12/2010
2010 American Chemical Society

Communication
Organometallics, Vol. 29, No. 21, 2010 4763
Chart 1
Scheme 1. Syntheses of Symmetric and Asymmetric
1,2-Bis(diarylboryl)ferrocenes from 1,10-Dibromoferrocene
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)].7 With selective anion binding in
mind we have been exploring the interaction of convergent
bifunctional Lewis acids with fluoride and cyanide.8 Systems
based on 1,2-diborylbenzenes (I) or 1,8-diborylnaphthalenes
(II) have been known for some time, but despite the addi-
tional benefits of the ferrocenediyl unit for electrochemical
or colorimetric reporting,9 1,2-diborylferrocenes are essen-
tially unknown.10,11 In this communication we report on the
synthesis of systems of type III, together with the somewhat
surprising selectivity of 1,2-fc(BMes2)2 for cyanide (over
fluoride), based on both thermodynamic and kinetic factors.
The syntheses of 1,2-bis(diarylboryl)ferrocenes 3, 30, and
300 can be accomplished in two steps from commercially avail-
able 1,10-dibromoferrocene, 1 (or three steps from ferrocene),
according to the methodology outlined in Scheme 1. Synthe-
tically, the key step involves the isomerization of 1,10-lithio-
(bromo)ferrocene in the presence of 2,2,6,6-tetramethylpiper-
idine (tmp) to the corresponding 1,2-isomer;9e,12,13 the inter-
mediate species 1,2-fc(Br)BMes2 (2) and 1,2-fc(Br)BXyl2 (20)
are thus accessible in yields of 50-75%. Subsequent reaction
compounds but of ferrocenyldiarylboranes in general.5k,15
Moreover, variable-temperature NMR studies, along with
the crystallographically determined structure of 3 in the solid
state (Figure 1), are consistent with an extremely high degree
of steric loading at the Lewis acidic boron centers. This is
manifested in considerable distortion of one of the two
-BMes2 units in each of the two 1,2-fc(BMes2)2 molecules
making up the asymmetric unit. Thus, the -BMes2 groups
in question feature boron centers [B(7) and B(76)] that lie ca.
0.5 A out of the plane of the difunctionalized Cp ring
[cf. <0.06 A for the other boron center in each molecule,
B(26) and B(57)]; the associated BC2 least-squares plane is
canted at an angle of ca. 45ꢀ with respect to that of the Cp
ring (cf. ∼5ꢀ 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) [135.2ꢀ (mean) and 114.6 (mean)o],
reflecting the need to minimize steric repulsions between
mesityl ring systems by “bending back” the -BMes2 units.
Such interactions presumably contribute to the relatively wide
(15) Data for 3: To a stirred solution of 2 (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 warming to 20 ꢀC
over 2 h and stirring 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 (hexane to 10% ethyl
acetate/hexane) yielded 3 as a dark red amorphous solid still contami-
nated with Mes2BOBMes2. Fractional crystallization from hexane (to
remove the borinic anhydride) and finally from MeOH/Et2O yielded the
product as dark red crystals (0.24 g, 83%). Single crystals suitable for
X-ray analysis were obtained by slow evaporation of a concentrated
solution in diethyl ether. 1H NMR (300 MHz, [D]chloroform, 20 ꢀC): δ
1.55 (s, 12H, para-CH3 of Mes), 2.00-2.70 (br overlapping signals, 24H,
ortho-CH3 of Mes), 4.15 (s, 5H, Cp), 4.87 (t, J=2.5 Hz, 1H, C5H3), 5.07
(d, J=2.5 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, [D]toluene, 120 ꢀC): δ 2.07
(s, 12H, para-CH3 of Mes), 2.25 (s, 24H, ortho-CH3 of Mes), 3.92 (s, 5H,
Cp), 4.52 (m, 1H, C5H3), 5.02 (d, J = 2.0 Hz, 2H, C5H3), 6.50 (s, 8H,
aromatic CH of Mes). 1H NMR (300 MHz, [D]toluene, -60 ꢀC): δ 1.70,
1.77, 2.08, 2.19, 2.25, 2.39, 2.58, 2.97, 3.21 (s, each 3H, ortho-CH3 of
Mes), 2.02 (s, 12H, para-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, [D]toluene, 120
ꢀC): δ 20.3 (para-CH3 of Mes), 24.5 (ortho-CH3 of Mes), 71.2 (Cp), 73.5,
90.1 (C5H3), 128.1 (aromatic CH of Mes), 136.6 (para-quaternary C of
Mes), 139.0 (ortho-quaternary C of Mes), boron-bound quaternary
carbons not observed. 11B NMR (96 MHz, [D]chloroform, 20 ꢀC): δ
80. UV-vis (dichloromethane): λmax = 510 nm, ε = 1903 mol-1 cm-1
dm3. E1/2(CH2Cl2) = þ0.177 V (with respect to FcH/FcHþ); peak-to-
peak separation=65 mV. MS (EIþ): 682 (100%) Mþ; exact mass (calcd
for Mþ, 10B isotopomer) 680.3677, (meas.) 680.3657. Microanalysis
(calcd for C46H52B2Fe) C 80.91, H 7.68; (measd) C 80.66, H 7.44.
Crystallographic data for 3: Mr 682.39, orthorhombic, Pca 21, a =
22.3532(3) A, b=11.3185(1) A, c=29.4442(4) A, V=7447.7(2) A3, Z=
8, Fc = 1.217 Mg m-3, T = 150 K, λ = 0.71073 A; 42 541 reflections
collected, 7629 independent [R(int) = 0.000], which were used in all
calculations. R1=0.0459, wR2=0.1029 for observed unique reflections
[I > 2σ(I)] and R1 = 0.0679, wR2 = 0.1124 for all unique reflections.
Max. and min. residual electron densities 0.45 and -0.48 e A-3. CSD
reference: 778112.
n
of 2 with BuLi/tmeda and FBMes2 gives access to 3 in ca.
38% yield; related chemistry utilizing 20 and either FBMes2
or FBXyl2 gives access to 30 and 300, respectively.14
3, 30, and 300 have been characterized by standard analy-
tical and spectroscopic techniques, with the 11B NMR shift
measured for 3 (δB 80 ppm) being typical not only of the three
(9) (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.; Coombs, N. D.;
Fallis, I. A.; Ooi, L.-L.; Aldridge, S. Inorg. Chem. 2008, 47, 793. (e) Morgan,
I. R.; Di Paolo, A.; Vidovic, D.; Fallis, I. A.; Aldridge, S. Chem. Commun.
2009, 7288.
(10) Reports of ferrocenes with boron substituents in the 1 and 2
positions are confined to a preliminary report of 1,2-fc(BMes2)[B(OH)2]9e
€
and to the diboryldiferrocenes reported by Jakle, which by virtue of
geometric constraints do not offer the possibilty for chelation of mono-
dentate anions: (a) Venkatasubbaiah, K.; Zakharov, L. N.; Kassel, W. S.;
€
Rheingold, A. L.; Jakle, F. Angew. Chem., Int. Ed. 2005, 44, 5428.
€
(b) Venkatasubbaiah, K.; Nowik, I.; Herber, R. H.; Jakle, F. Chem. Commun.
2007, 2154. (c) Venkatasubbaiah, K.; Doshi, A.; Nowik, I.; Herber, R. H.;
€
Rheingold, A. L.; Jakle, F. Chem.;Eur. J. 2008, 14, 44. (d) Pakkirisamy, T.;
€
Venkatasubbaiah, K.; Kassel, W. S.; Rheingold, A. L.; Jakle, F. Organometallics
2008, 27, 3056. (e) Venkatasubbaiah, K.; Pakkirisamy, T.; Lalancette, R. A.;
€
Jakle, F. Dalton Trans. 2008, 4507.
(11) For a chelating ferrocene-based B/Sn Lewis acid see, for exam-
ple: Boshra, R.; Venkatasubbaiah, K.; Doshi, A.; Lalancette, R. A.;
€
Kakalis, L.; Jakle, F. Inorg. Chem. 2007, 46, 10174.
(12) Butler, I. R. Inorg. Chem. Commun. 2008, 11, 15.
(13) Preliminary DFT calculations imply that 1,2-fc(Li)Br is thermo-
dynamically more stable than the corresponding 1,10-isomer by ca. 15 kJ
mol-1 in thf solution.
(14) The monofunctional boranes FcBMes2 and FcBXyl2 are invari-
ably encountered as minor side products in the final synthetic step
(Scheme 1), resulting from protonation of 1,2-fc(Li)BMes2 [or 1,2-
fc(Li)BXyl2] in competition with electrophilic quenching by the steri-
cally encumbered electrophile FBMes2/FBXyl2; separation is easily
achieved by fractional crystallization from hexanes.

4764 Organometallics, Vol. 29, No. 21, 2010
Morgan et al.
Figure 3. Molecular structures of (a) [K(18-crown-6)][3 CN]
3
3
CHCl3 and (b) the anionic component of [K(18-crown-6)][3 F].
3
Hydrogen atoms, solvate molecule [for (a)], and counterion
[for (b)] are omitted for clarity, and thermal ellipsoids are set
at the 40% probability level. Key bond lengths (A) and angles
Figure 1. One of the two independent molecules in the asym-
metric unit of 1,2-fc(BMes2)2 (3). Hydrogen atoms are omitted
for clarity, and thermal ellipsoids are set at the 40% probability
level. Key bond lengths (A) and angles (deg): C(6)-B(7) 1.574(7),
(deg): (for [K(18-crown-6)][3 CN]) B(7)-C(8) 1.648(4), C(8)-
3
N(9) 1.146(3), C(6)-B(7) 1.672(4), C(2)-B(28) 1.568(4), B(7)
C(5)-B(26) 1.569(7), B(7) B(26) 3.702, C(5)-C(6)-B(7)
3 3 3
3 3 3
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),
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).
B(7)-C(8)-N(9) 173.2(3); (for [K(18-crown-6)][3 F]) B(31)-
3
F(50) 1.471(5), B(12) B(31) 3.602, C(10)-B(31) 1.635(8),
3 3 3
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).
Scheme 2. Interaction of Cyanide and Fluoride Ions with 3
Figure 2. Interconversion of the enantiomeric conformations
of 3 by synchronized motion of the -BMes2 units.
B
B separation {3.684 A (mean) cf. 3.263 A for 1,2-C6H4-
3 3 3
[B(C6F5)2]2 and 3.353 A for 1,8-C10H6(BMes2)(BPh2)}.8g,j
1
The influence of steric factors is also apparent in the H
NMR spectrum of 3, which features broad signals for both
the mesityl ortho-CH3 and meta-CH protons at 20 ꢀC,
characterizing slow rotation about the B-CCp,ipso bond; a
spectrum featuring only one ortho-CH3 and one meta-CH
signal is observed at 120 ꢀC, while eight distinct ortho-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 (relative intensities: 1:1:1), consistent
with slowing of the synchronized “windshield wiper” motion
of the two -BMes2 units (Figure 2). Consideration of the
coalescence behavior of the respective signals yields an
ments imply that a single equivalent of cyanide is taken up,
and single-crystal X-ray diffraction studies not only confirm
this binding stoichiometry but also reveal that the anion is
bound in a nonchelating fashion, exo to the B B cavity and
3 3 3
(in the solid state at least) to the potassium center of the
[K(18-crown-6)]þ unit via N(9) (Figure 3a).17
The thermodynamics of cyanide binding by 3 can be
assessed by UV-vis titration experiments (see Supporting
Information) and a binding constant, KCN(thf), of 3.7(0.6) ꢀ
104 M-1 determined in thf solution (Scheme 2).18 This can be
put into context by the corresponding value for the closely
related monofunctional system FcBMes2, which is indicative
of stronger binding of the CN- anion [8.3(2.0) ꢀ 104 M-1 in the
more strongly competitive solvent dichloromethane (Gutmann’s
acceptor number, AN=20.8, cf. 8 for thf)].5k,19 Presumably
the presence of a second, highly sterically demanding,
-BMes2 group presents a thermodynamic disincentive for
the binding of cyanide, involving as it does an increase in
steric bulk on formation of the -BMes2CN- function.
activation barrier, ΔGq, of 48 kJ mol-1 16
. 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 simple 1H spectrum observed at
þ120 ꢀC) can be identified with a coalescence temperature of
ca. þ45 ꢀC (see Supporting Information for details).
The interaction of 3 with cyanide can readily be monitored
by multinuclear NMR (e.g., δB 80 to -15 ppm for the CN--
bound boron center) and IR spectroscopies [ν(CN) shift 2080
to 2162 cm-1] and by cyclic voltammetry (cathodic shift of
-430 mV in thf).5k Electrospray mass spectrometry experi-
Consistent with this assertion, the B B separation in-
3 3 3
creases from 3.684 A (mean) for 3 to 3.883 A for [3 CN]-.
Interestingly, if crystals of [K(18-crown-6)][3 CN] are redis-
3
3
(16) Coalescence of the two signals is observed at -35 ꢀC; using Δν
determined from the low-temperature limit (0.18 ppm, 54 Hz), k at this
temperature can thus be evaluated as 120 Hz. From the Eyring equation,
solved in chloroform-d rapid re-equilibration takes place,
leading to loss of cyanide and to the formation of a mixture
therefore, ΔGq =(24.4)RT=48 kJ mol-1
.
(17) Crystallographic data for [K(18-crown-6)][3 CN] CHCl3: Mr
1131.20, monoclinic, P21/c, a = 16.0148(2) A, b = 16.4211(2) A, c =
of [3 CN]- and 3. Integration of the respective NMR sig-
nals with respect to a tetramethylsilane standard allows an
3
3
3
22.8565(3) A, β = 106.626(1)o, V = 7447.7(2) A3, Z = 4, Fc = 1.304 Mg
m
-3, T = 150 K, λ = 0.71073 A; 65 506 reflections collected, 13 125
independent [R(int) = 0.050], which were used in all calculations. R1 =
0.0547, wR2=0.1163 for observed unique reflections [I > 2σ(I)] and R1=
0.1062, wR2 = 0.1359 for all unique reflections. Max. and min. residual
electron densities 1.11 and -0.95 e A-3. CSD reference: 778113.
(18) For a description of the method used for binding constant
determination see refs 8s and 9e.
(19) Definition of AN scale: Mayer, U.; Gutmann, V.; Gerger, W.
Monatsh. Chem. 1975, 106, 1235.

Communication
Organometallics, Vol. 29, No. 21, 2010 4765
of ln(A - A¥) versus time, from which a rate constant, kobs
of 1.64 ꢀ 10-2 s-1 can be determined.
,
To understand the structural origins of the slow fluoride
uptake by 3 (cf. essentially instantaneous binding of cya-
nide), X-ray diffraction studies were carried out on single
crystals of [K(18-crown-6)][3 F] (Figure 3b). Notably, the
3
F- anion interacts with only one of the two boron centers, as
might have been expected given the wide B B separation
3 3 3
determined for the free receptor 3 (3.684 A), compared to
typical B-μF bond lengths {e.g., 1.585(5) and 1.633(5) A for
[1,8-C10H8(BMes2)(BC12H8S)(μ-F)]-}.8s Nevertheless, the
Figure 4. Plots of (a) (A - A¥)/(Ao - A¥) vs time and (b) ln(A -
A¥) versus time (both measured at λ = 570 nm) for the reactions
in thf of [nBu4N]F with 1,2-fc(BMes2)2 ((), 1,2-fc(BMes2)-
(BXyl2) (2), and 1,2-fc(BXyl2)2 (9). 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) of1.64ꢀ 10-2,2.08ꢀ 10-2, and 2.79 ꢀ 10-2 s-1
can be derived.
fluoride ion is bound endo to the B B cavity22 and, in
3 3 3
further contrast to [3 CN]-, features no secondary inter-
3
actions between the anion and the [K(18-crown-6)]þ counter-
ion. In an attempt to determine whether similar binding
within the B B binding domain might be occurring in
3 3 3
solution and;given the steric demands of the flanking
mesityl substituents;might therefore be implicated in the
slow uptake of F-, we have examined the corresponding
rates of reaction for 1,2-fc(BMes2)(BXyl2) (30) and 1,2-fc-
(BXyl2)2 (300), which feature successively reduced steric
loading on the periphery of the binding cavity. The results
of these experiments are illustrated in Figure 4, from which
pseudo-first-order rate constants of 2.08 ꢀ 10-2 and 2.79 ꢀ
10-2 s-1 were calculated for 30 and 300, respectively. These
(reproducible) differences in the rate of fluoride uptake,
together with the more marked difference in the binding
kinetics for fluoride and cyanide, are consistent with binding
equilibrium constant KCN(CDCl3) of 1.5 ((0.5) M-1 to be
estimated; the significantly weaker binding in this solvent
(cf. thf) is consistent with its much more strongly competitive
nature (AN = 23.1).19
Fluoride binding by 3 can be shown by simple competition
experiments to be weaker than cyanide binding, for example,
by the quantitative formation (by 11B NMR) of [3 CN]- on
3
addition to 3 of a solution equimolar in both F- and CN-.
Moreover, further experiments reveal that the binding of
fluoride is very weak indeed; no evidence for the formation of
[3 F]- could be obtained from NMR measurements in either
3
of F- within the B B cavity in thf solution.
dichloromethane or chloroform. Single crystals of [K(18-
3 3 3
In summary, bifunctional Lewis acid 3 can discriminate
both thermodynamically and kinetically between fluoride
and cyanide anions, the latter being attributed to the differ-
ential binding modes of cyanide (exo) and fluoride (endo)
within the receptor cavity. Further studies aimed at devel-
oping the chemistry of 3 and related derivatives, e.g., for the
chelation of chloride, are in progress and will be reported in
due course.
crown-6)][3 F] can be grown from thf solutions saturated
3
with fluoride,20 although in the absence of excess anion,
solutions in thf-d8 rapidly regenerate the free receptor by
fluoride loss. Interestingly, even in the presence of a vast
excess of fluoride, the rate of formation of the host/guest
complex (as appraised by UV-vis spectroscopy; Figure 4)
appears to be 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 [nBu4N]þF- 4H2O),21 gives a linear plot
Acknowledgment. We thank the EPSRC for funding
(EP/G006156/1) and for access to the National Mass
Spectrometry Service Centre, Swansea University.
3
(20) Crystallographic data for [K(18-crown-6)][3 F]: Mr 1039.91,
3
triclinic, P1, a = 10.2123(1) A, b = 12.6792(2) A, c = 26.4929(5) A,
R= 81.506(1)ꢀ, β=83.000(1)ꢀ, γ= 74.591(1)o, V=3258.5(1) A3, Z=2,
Fc=1.060 Mg m-3, T=150 K, λ=0.71073 A; 52 098 reflections collected,
14 705 independent [R(int) = 0.0506], which were used in all calculations.
R1=0.1229, wR2=0.2342 for observed unique reflections [I > 2σ(I)] and
R1=0.1496, wR2=0.2486 for all unique reflections. Max. and min. residual
electron densities 1.43 and -1.15 e A-3. CSD reference: 778114.
Supporting Information Available: Synthetic and spectro-
scopic data for 30 and 300; details of binding constant determina-
tions for 3; synthetic, spectroscopic, and crystallographic data
for 20. This material is available free of charge via the Internet at
http://pubs.acs.org.
(21) The compositions of the tetrabutylammonium fluoride and
cyanide hydrates used in anion binding studies (and prepared by
prolonged drying in vacuo) were determined to be [nBu4N]F 4H2O
(22) For a previous analysis of the factors influencing endo/exo
coordination of Lewis bases to bifunctional boranes, see ref 8z.
3
and [nBu4N]CN 2H2O by elemental microanalysis.
3