Fluoride ion complexation by a B2/Hg heteronuclear tridentate lewis acid

Article abstract of DOI:10.1039/b801040d

The reaction of [Li(THF)₄][1,8-μ-(Mes₂B)C ₁₀H₆] with HgCl₂ affords [1,1′-(Hg)-[8- (Mes₂B)C₁₀H₆]₂] (3) or [1-(ClHg)-8-(Mes₂B)C₁₀H₆] (4), depending on the stoichiometry of the reagents. These two new compounds have been characterized by ¹H, ¹³C, ¹¹B and ¹⁹⁹Hg NMR, elemental analysis and X-ray crystallography. The cyclic voltammogram of 3 in THF shows two distinct waves observed at E_1/2 -2.31 V and -2.61 V, corresponding to the sequential reductions of the two boron centers. Fluoride titration experiments monitored by electrochemistry suggest that 3 binds tightly to one fluoride anion and more loosely to a second one. Theses conclusions have been confirmed by a UV-vis titration experiment which indicates that the first fluoride binding constant (K₁) is greater than 10⁸ M ^-1 while the second (K₂) equals 5.2 (± 0.4) × 10³ M^-1. The fluoride binding properties of 3 have been compared to those of [1-(Me₂B)-8-(Mes₂B)C ₁₀H₆] (1) and [1-((2,6-Me₂-4-Me ₂NC₆H₂)Hg)-8-(Mes₂B)C ₁₀H₆] (2). Both experimental and computational results indicate that its affinity for fluoride anions is comparable to that of 2 but significantly lower than that of the diborane 1. In particular, the fluoride binding constants of 1, 2 and 3 in chloroform are respectively equal to 5.0 (± 0.2) × 10⁵ M^-1, 1.0 (± 0.2) × 10³ M^-1 and 1.7 (± 0.1) × 10³ M^-1. Determination of the crystal structures of the fluoride adducts [S(NMe₂)₃][1-μ₂-F] and [S(NMe ₂)₃][3-μ₂-F] along with computational results indicate that the higher fluoride binding constant of 1 arises from a strong chelate effect involving two fluorophilic boron centers.

Full text of DOI:10.1039/b801040d

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PAPER
www.rsc.org/dalton | Dalton Transactions
Fluoride ion complexation by a B2/Hg heteronuclear tridentate lewis acid†‡
Christopher L. Dorsey, Pawel Jewula, Todd W. Hudnall, James D. Hoefelmeyer, Thomas J. Taylor,
Nicole R. Honesty, Ching-Wen Chiu, Marcus Schulte and Franc¸ois P. Gabba¨ı*
Received 21st January 2008, Accepted 7th March 2008
First published as an Advance Article on the web 23rd July 2008
DOI: 10.1039/b801040d
The reaction of [Li(THF)4][1,8-l-(Mes2B)C10H6] with HgCl2 affords [1,1ꢀ-(Hg)-[8-(Mes2B)C10H6]2] (3)
or [1-(ClHg)-8-(Mes2B)C10H6] (4), depending on the stoichiometry of the reagents. These two new
compounds have been characterized by 1H, 13C, 11B and 199Hg NMR, elemental analysis and X-ray
crystallography. The cyclic voltammogram of 3 in THF shows two distinct waves observed at E1/2
−2.31 V and −2.61 V, corresponding to the sequential reductions of the two boron centers. Fluoride
titration experiments monitored by electrochemistry suggest that 3 binds tightly to one fluoride anion
and more loosely to a second one. Theses conclusions have been confirmed by a UV-vis titration
experiment which indicates that the first fluoride binding constant (K1) is greater than 108 M−1 while the
second (K2) equals 5.2 ( 0.4) × 103 M−1. The fluoride binding properties of 3 have been compared to
those of [1-(Me2B)-8-(Mes2B)C10H6] (1) and [1-((2,6-Me2-4-Me2NC6H2)Hg)-8-(Mes2B)C10H6] (2). Both
experimental and computational results indicate that its affinity for fluoride anions is comparable to
that of 2 but significantly lower than that of the diborane 1. In particular, the fluoride binding constants
of 1, 2 and 3 in chloroform are respectively equal to 5.0 ( 0.2) × 105 M−1, 1.0 ( 0.2) × 103 M−1 and 1.7
(
0.1) × 103 M−1. Determination of the crystal structures of the fluoride adducts [S(NMe2)3][1–l2-F]
and [S(NMe2)3][3–l2-F] along with computational results indicate that the higher fluoride binding
constant of 1 arises from a strong chelate effect involving two fluorophilic boron centers.
In an exploratory part of our research program, we became
Introduction
interested in the synthesis and properties of heteronuclear B/Hg
bidentate Lewis acids and synthesized compounds of type II.39,40
Although mercury is less fluorophilic than boron, these het-
eronuclear B/Hg bidentate Lewis acids are powerful fluoride
complexing agents whose properties rival those of their diboron
counterparts. Moreover, some of these B/Hg bidentate Lewis
acids39,40 capture fluoride in THF/H2O mixtures, conditions under
which diboranes of type I undergo hydrolysis.
Organoboranes have received a great deal of attention as re-
ceptors for fluoride anions.1–22 Simple triarylboranes such as
tris(9-anthryl)borane complex fluoride in aprotic organic solvents
to form the corresponding fluoroborate anions whose stability
constants range from 105 to 106 M−1.23–38 In order to increase the
compatibility of these fluoride receptors with protic environments,
various strategies have been recently considered.39–44 Once such
strategy involves the use of bidentate diboranes45–47 of type I27,48–53
whose fluoride ion affinity exceeds that of their monofunctional
analogs by at least three orders of magnitude.27,53 The increased
fluoride ion affinity of these derivatives results at least in part from
a favorable chelate effect which leads to the formation of a very
stable fluoride complex.
In this paper, we present the results of a study in which we have
compared the fluoride binding properties of the neutral diborane
154 to that of the neutral B/Hg bidentate Lewis acids 2.39 We
also describe the synthesis, characterization and fluoride binding
properties of a novel B/Hg/B trifunctional Lewis acid.
Results and discussions
Chemistry TAMU 3255, Texas A & M University, College Station, Texas,
77843-3255, USA. E-mail: francois@tamu.edu
Synthesis and spectroscopic characterization
† Based on the presentation given at Dalton Discussion No. 11, 23–25 June
2008, University of California, Berkeley, USA.
‡ Electronic supplementary information (ESI) available: Data for the
titration of 1, 2 and 3 in CHCl3 and computational details. CCDC reference
numbers 675714–675717. For ESI or crystallographic data in CIF or other
electronic format see DOI: 10.1039/b801040d
We have previously described the synthesis of [1-(Me2B)-
8-(Mes2B)C10H6] (1)54 and [1-((2,6-Me2-4-Me2NC6H2)Hg)-8-
(Mes2B)C10H6] (2)39 which were obtained by reaction of
[Li(THF)4][1,8-l-(Mes2B)C10H6]54 with Me2BBr and (2,6-Me2-
4-Me2NC6H2)HgCl, respectively. We have now carried out the
4442 | Dalton Trans., 2008, 4442–4450
This journal is
The Royal Society of Chemistry 2008
©

reaction of this borate with half an equivalent of HgCl2 and found
that it affords the B/Hg/B trifunctional Lewis acid 3 in 70% yield
(Scheme 1). When the same reaction is carried out in a 1 : 1 stoi-
chiometry, 1-(chloromercury)-8-(dimesitylboron)naphthalenediyl
(4) is obtained in 53% yield (Scheme 1). This compound can be
obtained in a substantially higher yield (94%) by reaction of 1-
(dimesitylboron)-8-(trimethyltin)naphthalenediyl with HgCl2.
Fig. 1 ORTEP view of 3 (50% ellipsoid), H atoms are omitted and
˚
the mesityl groups are represented by thin lines. Selected distances [A]
and angles [◦]: Hg(1)–C(31) 2.039(10), Hg(1)–C(1) 2.176(12), B(1)–C(21)
1.546(18), B(1)–C(8) 1.617(18), B(1)–C(11) 1.646(19), B(2)–C(51)
1.560(19), B(2)–C(38) 1.571(17), B(2)–C(41) 1.603(18); C(31)–Hg(1)–C(1)
167.2(4), C(21)–B(1)–C(8) 126.1(12), C(21)–B(1)–C(11) 119.4(11),
C(8)–B(1)–C(11) 113.7(11), C(51)–B(2)–C(38) 114.1(11), C(51)–B(2)–
C(41) 120.8(11), C(38)–B(2)–C(41) 124.8(11).
Scheme 1
the rigid naphthalene backbone. The two trigonal planar boryl
˚
moieties which are separated by a B–B distance of 6.14 A; form a
Compounds 3 and 4 have been characterized by multinuclear
NMR spectroscopy and elemental analysis. In both cases, the 1H
NMR spectrum exhibits six distinct resonances that correspond
to the aromatic CH groups of the unsymmetrically substituted
naphthalene backbone. In 4, the 1H and 13C NMR spectra reveal
the existence of two chemically distinct mesityl groups. One of
these mesityl groups gives rise to one aromatic C–H resonance
thus indicating that it can undergo free reorientation about the
Cipso–boron bond. The second mesityl substituent gives rise to
two distinct C–H resonances indicating that reorientation about
the Cipso–boron bond is restricted. In the case of 3, the aryl
and methyl proton resonances of the mesityl groups are split
into broad multiple signals thus indicating the existence of a
congested structure. The 11B NMR signal detected at 72.3 ppm
for 3 and 72.1 ppm for 4 confirms the presence of a base-
free trigonal planar boron center.39,40 The 199Hg resonance of 3
(−800 ppm, CDCl3) and 4 (−1402 ppm, CDCl3) are respectively
comparable to that of diphenylmercury (−820 ppm, d6-DMSO),55
and mesitylmercurychloride (−1123 ppm in CDCl3).56
dihedral angle of only 29.4◦ and are oriented in a co-facial fashion
with respect to one another.
Compound 4 crystallizes as a benzene solvate in the monoclinic
P21/n space group with two independent molecules in the
asymmetric unit. These two molecules form a dimer which is held
together by secondary Hg–Cl interactions of 3.1925(18) (Hg(1)–
Cl(2)) and 3.3733(17) A (Hg(2)–Cl(1)) (Fig. 2, Table 1). Both
independent molecules, arbitrarily denoted as molecule A and
˚
molecule B, feature very similar structures. The boron atom of each
ꢀ
molecule adopts a trigonal planar geometry (
= 359.6◦,
(C–B–C)
ꢀ
molecule A;
= 359.8◦, molecule B) and are separated
(C–B–C)
from the mercury atom by only 3.190(7) (Hg(1)–B(1), molecule
˚
A) and 3.290(7) A (Hg(2)–B(2), molecule B). These distances are
slightly shorter than those measured in 3 which can be assigned
to the smaller steric crowding present in 4. Further inspection of
the structure reveals that the mercury centers of both molecules
form short contacts with the ipso-carbon atom of one of the
mesityl groups. As in 3, these contacts of 3.039(6) A (Hg(1)–C(11),
molecule A) and 3.037(7) A (Hg(2)–C(41), molecule B) indicate
˚
˚
the presence of an intramolecular Hg–p interaction.
Structures
Compound 3 crystallizes in the monoclinic space group P21/c
with half a molecule of interstitial chloroform (Fig. 1, Table 1).
Calculations
The boron atoms B(1) and B(2) adopt a trigonal planar geometry
The structure of 1, 2 and 3 have been optimized computationally
using density functional theory (DFT) methods at the B3LYP level
of theory with mixed basis sets. The fully optimized geometry of 1
and 3 are close to that observed in the solid state. In particular, the
ꢀ
(
= 359.1◦, 359.4◦) and are separated from the mercury
(C–B–C)
˚
˚
atom Hg(1) by 3.46(2) A and 3.47(2) A, respectively. These
˚
distances are slightly longer than the Hg–B separation of 3.3 A
measured in compounds of type II39,40 which probably results from
the greater degree of steric crowding present in 3. The mercury cen-
ter Hg(1) adopts a distorted linear geometry (C(1)–Hg(1)–C(31)
calculated B–B separation of 3.31 A in 1 and the B–Hg separation
˚
˚
of 3.51 A in 3 are comparable to those observed in the crystal
54
39
˚
˚
structure (B–B = 3.206 A for 1; av. B–Hg = 3.46 A for 3).
◦
˚
˚
168.1(4) ) and forms short interactions of 3.15(2) A and 3.13(2) A
with the mesityl ipso-carbon atoms C(21) and C(41). These short
distances indicate the presence of a secondary Hg–p interaction.
When compared to intermolecular Hg–p interactions,57,58 these
distances appear relatively short and might be partly enforced by
Examination of the orbitals indicates that the LUMO in 1, 2, and 3
bears a large contribution from the 2p-orbitals of the boron centers
of the dimesityl boryl moieties with essentially no participation
from the other Lewis acidic centers (Fig. 3). In the case of 1, the +I
effect of the methyl group as well as hyperconjugation59 probably
This journal is
The Royal Society of Chemistry 2008
Dalton Trans., 2008, 4442–4450 | 4443
©

Table 1 Crystal data and structure refinement for 3, 4, [S(NMe2)3][1–l2-F], and [S(NMe2)3][3–l2-F]
Crystal data
3·0.5(CHCl3)
4·0.5(C6H6)
[S(NMe2)3][1–l2-F]
[S(NMe2)3][3–l2-F]
Formula
Mr
C56.50H56.50B2Cl1.50Hg
1010.90
C34H34BClHg
689.46
C36H52B2FN3S
599.49
C62H74B2FHgN3S
1134.51
Crystal size/mm
Crystal system
Space group
0.08 × 0.05 × 0.015
Monoclinic
P21/c
12.1981(19)
27.747(4)
14.241(3)
90.0000
98.204(7)
90.0000
4770.6(15)
4
0.41 × 0.29 × 0.24
Monoclinic
P21/n
13.9211(17)
18.702(2)
22.471(3)
90.0000
97.443(2)
90.0000
5801.0(12)
8
0.23 × 0.20 × 0.20
Monoclinic
P21/c
10.5814(15)
17.441(2)
20.6449(19)
90.0000
114.742(5)
90.0000
3460.3(7)
4
.014 × .01 × .01
Monoclinic
Cc
11.9011(9)
11.9011(9)
23.5739(17)
90.0000
95.015(2)
90.0000
5382.1(7)
4
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
V/A
Z
qcalc/g cm−3
l/mm−1
F(000)
1.407
6.822
2044
110(2)
1.579
5.419
2720
110(2)
1.151
0.127
1296
110(2)
1.400
2.944
2328
110(2)
T/K
Scan mode
hkl Range
x
x
x
x
−13 → +13,
−31 → +31
−15 → +15
31362
−16 → +16,
−22 → +18
−26 → +23
39001
−9 → +12,
−19 → +19
−23 → +17
15743
−13 → +13,
−22 → +22
−26 → +26
26547
Measd reflns
Unique reflns [Rint
]
6768 [0.1001]
6768
582
1.012
0.1167, 0.1860
10220 [0.0472]
10220
667
1.012
0.0675, 0.1151
5424 [0.0460]
5424
388
1.093
0.1346, 0.2574
8402 [0.0476]
8402
638
1.035
0.0483, 0.0952
Reflns used for refinement
Refined parameters
GooF
R1,a wR2b (all data)
−3
˚
qfin (max., min.)/e A
3.103, −1.782
3.597, −1.521
1.084, −0.470
1.705, −0.544
ꢀ
ꢀ
ꢀ
ꢀ
a R1 = ꢁFo| − |Fcꢁ/ |Fo|. b wR2 = ([ w(Fo − Fc2)2]/[ w(Fo2)2])1/2; w = 1/[r2(Fo2) + (ap)2 + bp]; p = (Fo + 2Fc2)/3.
2
2
Fig. 3 Optimized geometry and LUMO of 1, 2 and 3 (isodensity value =
0.04).
Electrochemistry
Fig. 2 ORTEP view of 4 (50% ellipsoid), H atoms and benzene solvate
◦
˚
molecules are omitted for clarity. Selected distances [A] and angles [ ]:
Hg(1)–C(1) 2.065(7), Hg(1)–Cl(1) 2.3411(19), Hg(1)–C(11) 3.039(6),
Hg(1)–B(1) 3.190(7), Hg(1)–Cl(2) 3.1925(18), Hg(2)–C(31) 2.060(7),
Hg(2)–Cl(2) 2.3207(19), Hg(2)–C(41) 3.037(7), Hg(2)–B(2) 3.290(7),
Hg(2)–Cl(1) 3.3733(17), B(1)–C(11) 1.565(11), B(1)–C(8) 1.583(11), B(1)–
C(21) 1.613(10), B(2)–C(38) 1.564(11), B(2)–C(41) 1.579(11), B(2)–C(51)
1.593(11); C(1)–Hg(1)–Cl(1) 174.8(2), C(31)–Hg(2)–Cl(2) 174.46(19),
C(11)–B(1)–C(8) 125.5(6), C(11)–B(1)–C(21) 120.5(7), C(8)–B(1)–C(21)
113.6(6), C(38)–B(2)–C(41) 124.6(7), C(38)–B(2)–C(51) 115.9(6),
C(41)–B(2)–C(51) 119.3(6).
The cyclic voltammogram of 1 shows a reversible reduction wave
at E1/2 −2.31 V (vs. Fc/Fc+) which is followed by an irreversible
one at Epeak −2.93 V (Fig. 4). As in the case of simple triarylboranes
such as Mes3B,60–69 these two waves are respectively assigned to the
reversible one-electron and irreversible two-electron reduction of
the dimesitylboryl moiety. This electrochemical study also suggests
that the dimethylboryl moiety is not electroactive in the potential
window studied. In the case of 2, the cyclic voltammogram in
THF shows a reversible reduction wave at E1/2 −2.26 V (vs.
Fc/Fc+) which is followed by undefined irreversible processes,
again in agreement with the reversible one-electron reduction of
the dimesitylboryl moiety (Fig. 4). Although the electrochemical
behavior of 1 and 2 is similar to that of triarylboranes, we note that
their reduction potentials are markedly more positive than those of
other triaryl boranes such as dimesityl-1-naphthylborane which is
raise the energy of the empty p-orbital of the Me2B moiety thus
precluding its participation in the LUMO. For 2 and 3, the vacant
mercury atom orbitals might be intrinsically too high in energy to
participate in the LUMO.
4444 | Dalton Trans., 2008, 4442–4450
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The Royal Society of Chemistry 2008
©

Fig. 4 Cyclic voltammograms of 1 (top), 2 (middle) and 3 (bottom) in
THF with a glassy-carbon working electrode (0.3 M nBu4NPF6). Scan
rates: m = 100 mV s−1
.
reduced at −2.41 V (vs. Fc/Fc+).50 This difference suggests that the
Me2B moiety in 154 and the (2,6-Me2-4-Me2NC6H2)Hg)39 moiety
in 2 increases the electrophilicity of the derivatives. The cyclic
voltammogram of 3 in THF shows two reversible reduction waves
at E1/2 −2.31 V and −2.61 V corresponding to the reduction of the
two boron centers. As expected, the potential of the first reduction
wave is close to that measured for 1 and 2 and can be regarded
as corresponding to the reduction of the first boron center. The
relatively large DE1/2 observed between these two waves indicate
substantial coupling of the two electroactive boryl moieties of 3.
Because mercury should not efficiently mediate p-conjugation, we
propose that this coupling has electrostatic origins and is magnified
by the proximity of the two boryl units.
Fig. 5 Changes in the differential pulsed voltammogram of 2 (top) and
3 (bottom) observed upon the addition of nBu4NF to a THF solution
(0.3 M).
Fluoride anion binding
In previous reports we have shown that bidentate diboranes of type
I are well adapted for the complexation of fluoride anions.2,27,53
We have also shown that the reduction waves present in the
voltammogram of such diboranes could serve to monitor fluoride
binding in solution.53 As part of the present studies, we decided
to determine whether a similar behavior would be observed for
heteronuclear B/Hg derivatives such as 2 and 3. Addition of
fluoride to a THF solution of 2 or 3 containing nBuN4PF6 as a
supporting electrolyte results in the progressive disappearance of
the reduction wave, which is no longer detected after the addition
of 1 equiv. of fluoride (Fig. 5). The disappearance of this wave
results from coordination of a fluoride anion to the dimesitylboron
center which can no longer be reduced because of coordinative
saturation (Schemes 2 and 3). In the case of 3, addition of fluoride
results in the rapid disappearance of the first reduction wave at
−2.26 V which is no longer visible after the addition of 1 equiv.
of the anion (Scheme 3). Interestingly, the potential of the second
wave undergoes an anodic shift of 50–60 mV. These observations
can be interpreted by invoking the formation of a 1 : 1 complex [3–
l2-F]− in which the fluoride binds to only one boron center. The
reduction observed at −2.66 V thus corresponds to the reduction
of the remaining tricoordinate boron center (Fig. 5). Further
addition of fluoride only leads to a more progressive decrease of
Scheme 2
the remaining reduction wave suggesting that binding of a second
fluoride anion is less favorable than the first one.
Further insights into the fluoride binding behavior of 3 were
gained from UV-vis titration studies in THF. The addition of
TBAF to a THF solution of 3 (e360 = 17 400 M−1 cm−1) results in
a decrease of the absorbance at 360 nm caused by coordinative
saturation of the boron center (Fig. 6). Interestingly, addition of
up to one equivalent of fluoride anion leads to a linear decrease of
the absorbance. At exactly one equivalent of added fluoride, the
isotherm shows a distinct break and addition of over 1 equiv.
of fluoride leads to a much more progressive decrease of the
This journal is
The Royal Society of Chemistry 2008
Dalton Trans., 2008, 4442–4450 | 4445
©

Scheme 3
can be easily understood by considering the fact that the binding
of the second fluoride anion would lead to the formation of a
dianion and is therefore electrostatically and possibly sterically
disfavored.
Comparative studies
Having established that compound 3 complexes fluoride, we
decided to compare its fluoride affinity to that of 1 and 2. Because
the fluoride binding constants measured in THF are too elevated
to be accurately compared, we decided to carry out these studies
in CHCl3 which, we have shown,42 is a more competitive medium.
Spectrophotometric titrations carried out in this solvent indicate
that the fluoride binding constants of 1, 2 and 3 are respectively
equal to 5.0 ( 0.2) × 105 M−1, 1.0 ( 0.2) × 103 M−1 and 1.7 (
0.1) × 103 M−1. These results show that diboranes of type I have a
higher fluoride affinity than heteronuclear B/Hg compounds such
as 2 and 3. It is also interesting to note that the fluoride binding
constants of 2 and 3 are slightly lower than that measured for IIa in
the same solvent (2.1 ( 0.2) × 104 M−1).42 This latter observation
indicates that electron-withdrawing pentafluorophenyl group of
IIa effectively increased the fluoride affinity of this type of receptor.
In order to further rationalize these results, we decided to
investigate the structures of the fluoride complexes. We were able
to obtain single crystals of [S(NMe2)3][1–l2-F] and [S(NMe2)3][3–
l2-F]. The salt [S(NMe2)3][1–l2-F] crystallizes in the P21/c mon-
oclinic space group with four molecules in the unit cell (Fig. 7,
Table 1). The fluorine atom F(1) is bound to both boron centers
Fig. 6 Top: Changes in the UV-vis absorption spectra of a solution of 3
(3 mL, 5.5 × 10−5 M in THF) upon addition of a TBAF solution (4.7 ×
10−3 M in THF). Bottom; Binding isotherm obtained by monitoring the
absorbance at 360 nm.
absorbance. These observations can be rationalized based on the
following argument. Addition of the first equivalent essentially
leads to the quantitative formation of the 1 : 1 addduct [3–l2-
F]− (e360 = 8200 M−1 cm−1). Addition of a second fluoride anion
to produce [3–(l2-F)2]2− is much less favorable explaining the
more progressive decrease of the absorbance (Scheme 3). Because
fluoride binding to 3 occurs in two very distinct regimes, the
fluoride binding constants can be evaluated independently. The
first fluoride binding constant K1 exceeds the value of 108 M−1
measurable by a UV-vis titration. Fitting of the data obtained
above 1 equiv. of added fluoride affords K2 = 5.2 ( 0.4) × 103 M−1.
These results are in agreement with the conclusions derived from
the fluoride binding experiments monitored by differential pulsed
voltammetry which also showed that binding of the first fluoride
anion is much more favorable than the second one. These results
Fig. 7 ORTEP view of [1–l2-F]− in [S(NMe2)3][1–l2-F] (50% ellip-
soid). H atoms are omitted and the mesityl groups are represented
◦
˚
by thin lines. Selected distances [A] and angles [ ]: B(1)–F(1) 1.600(5),
B(2)–F(1) 1.606(5), B(1)–C(1) 1.611(6), B(1)–C(11) 1.633(6), B(1)–C(21)
1.656(6), B(2)–C(8) 1.608(7), B(2)–C(31) 1.585(7), B(2)–C(32) 1.606(7);
B(1)–F(1)-B(2) 125.7(3), F(1)–B(1)–C(1) 103.9(3), F(1)–B(2)–C(8)
105.2(3), C(1)–B(1)–C(11) 110.1(3), C(1)–B(1)–C(21) 117.7(3), C(11)–
B(1)–C(21) 117.5(3), C(31)–B(2)–C(32) 113.9(4), C(31)–B(2)–C(8)
112.7(4), C(32)–B(2)–C(8) 113.4(4).
4446 | Dalton Trans., 2008, 4442–4450
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The Royal Society of Chemistry 2008
©

˚
fluorophilicity of boron when compared to mercury. This greater
fluorophilicity leads to a more efficient chelation of the fluoride
anion as indicated by the formation of two almost equal B–F
bonds in [1–l2-F]−. In order to provide additional support for
this interpretation, we computed the fluoride ion affinity (FIA) of
the boranes based on a method that we have described earlier.53
According to these calculations, the FIA of 1 (75.1 kcal mol−1) is
greater than that of 2 (68.1 kcal mol−1) and 3 (71.5 kcal mol−1)
which suggest that the higher fluoride binding constant observed
for 1 is, at least partly, of enthalpic origin. The FIAs of 2 and 3
are greater than that computed for BPhMes2 (64.18 kcal mol−1)53
which indicates that the mercury atom of these heteronuclear
Lewis acids is non-innocent and effectively increases their fluoride
affinity by interacting with the fluoride anion. In agreement with
this view, simple triaryl boranes such as BMes3 do not bind fluoride
in CHCl3.42
via B–F bonds of 1.600(5) (B(1)–F(1)) and 1.606(5) A (B(2)–F(1)).
These bonds are comparable to or slightly shorter than those
measured in the fluoride adduct of 1-(dimesitylboryl)-8-(10ꢀ-bora-
ꢀ
53
˚
9 -oxaanthryl)naphthalene (av. 1.63 A) and 1-(dimesitylboryl)-
ꢀ
ꢀ
27
˚
8-(10 -bora-9 -thiaanthryl)naphthalene (av. 1.61 A) which may
result from the lower steric requirement of the dimethylboryl
moiety present in 1. It remains that these bonds are longer
than those formed in the fluoride adduct of [C6F4-1,2-[(BF2)2]]
which possesses more Lewis acidic and accessible difluoroboryl
moieties.46 They are also longer than those in triarylfluoroborate
24,42,43
˚
anions (∼1.48 A)
as expected from the bridging location of
the fluorine atom. The cooperative binding of the fluoride anion
leads to pyramidalization of both boron centers as indicated by
ꢀ
ꢀ
the sum of the C–B–C angles ( C–B1-C = 345.2◦; C–B2-C = 344.8◦).
Salt [S(NMe2)3][3–l2-F] belongs to the monoclinic Cc space
group with four molecules in the unit cell (Fig. 8, Table 1). The
fluorine atom F(1) bridges the mercury center (Hg1) and one of
the two boron centers (B1). The resulting B(1)–F(1) bond length
Conclusion
−
40
˚
˚
of 1.487(10) A is close to those found in [IIa–l2-F] (1.483(4) A)
and [IIb–l2-F] (av. 1.48 A)39 and does not appear lengthened when
compared to that found in simple triarylfluoroborates.24,42,43 As
in [IIa–l2-F]− and [IIb–l2-F]− which possess Hg–F bond lengths
The results presented in this paper further document the affinity of
naphthalene-based multidendate Lewis acids for fluoride anions.
The trinuclear Lewis acid 3 is able to bind two fluoride anions.
However, binding of the second fluoride anion is much less
favorable than that of the first because of unfavorable Coulombic
and steric effects. The comparison of 1, 2 and 3 indicates that the
fluoride binding constant of 3 is comparable to that of simple
B/Hg bidentate Lewis acids such as 2 but significantly lower
than that of bidentate diboranes such as 1. From an analytical
perspective, this work indicates that the electrochemical properties
of heteronuclear B/Hg derivatives such as 2 and 3 can also be used
to signal fluoride binding.
˚
˚
˚
in the 2.59–2.63 A range, the Hg(1)–F(1) bond of 2.576(4) A
measured in [3–l2-F]− is well within the sum of the van der
Waals radii of mercury (1.75 A) and fluorine (1.30–1.38 A)
thus indicating the presence of a strong secondary interaction.71–73
70
˚
˚
Coordination of the fluoride results in a pyramidalization of the
ꢀ
boron centers (
= 338.9◦) but does not noticeably affect
(C–B(1)–C)
the C(1)–Hg(1)–C(31) angle which is equal to 169.0(4)◦. The
coordination geometry of the remaining boron center B(2) is not
affected by the chelation of a fluoride ion. It remains trigonal
˚
planar and the B(2)–Hg(1) distance of 3.445(10) A is essentially
identical to that observed in 3.
Experimental
General
Extra care was taken at all times to avoid contact with solid, solu-
tion, and airborne particulate mercury compounds. Compounds
1, 2 and [Li(THF)4][1,8-l-(Mes2B)C10H6] were prepared according
to the reported procedures. [S(NMe2)3][Me3SiF2] (TASF) was
purchased from Aldrich and used as provided. Chloroform was
distilled over CaH2; THF over Na/K amalgam. Air-sensitive
compounds were handled under N2 atmosphere using standard
Schlenk and glovebox techniques. UV-vis spectra were recorded on
a HP8453 or Ocean Optics USB2000 spectrophotometer. Elemen-
tal analyses were performed at Atlantic Microlab (Norcross, GA).
NMR spectra were recorded on Inova-400 FT NMR (broadband)
(399.63 MHz for 1H, 376.03 MHz for 19F, 128.22 MHz for
11B, 100.50 MHz for 13C, 75.52 for 199Hg) by using an internal
deuterium lock. Chemical shifts d are given in ppm. Spectra are
internally referenced to Me4Si (1H, 13C, d = 0 ppm), and externally
referenced to BF3·OEt2 (11B, d = 0 ppm), CFCl3 (19F, d = 0 ppm)
and HgCl2 in DMSO (199Hg, d = −1501.6 ppm).
Fig. 8 ORTEP view of [3–l2-F]− in [S(NMe2)3][3–l2-F] (50% ellipsoid).
H atoms are omitted and the mesit◦yl groups are represented by thin lines.
˚
Selected distances [A] and angles [ ]: Hg(1)–C(1) 2.079(14), Hg(1)–C(31)
2.119(12), Hg(1)–F(1) 2.576(4), B(1)–F(1) 1.487(10), B(1)–C(11)
1.672(14), B(1)–C(8) 1.655(12), B(1)–C(21) 1.685(16), B(2)–C(41)
1.567(14), B(2)–C(38) 1.592(13), B(2)–C(51) 1.576(16); C(1)–Hg(1)–C(31)
169.0(4), C(1)–Hg(1)–F(1) 86.4(3), C(31)–Hg(1)–F(1) 104.3(3),
F(1)–B(1)–C(11) 104.8(6), F(1)–B(1)–C(8) 106.4(6), C(11)–B(1)–C(8)
116.1(7), F(1)–B(1)–C(21) 105.7(7), C(11)–B(1)–C(21) 113.2(8),
C(8)–B(1)–C(21) 109.6(7), C(41)–B(2)–C(38) 116.6(9), C(41)–B(2)–C(51)
119.5(9), C(38)–B(2)–C(51) 122.9(8), B(1)–F(1)–Hg(1) 104.7(4).
Crystallography
Single crystals of 3·0.5(CHCl3) and 4·(C6H6) were obtained from
chloroform and benzene, respectively, upon slow evaporation
of the solvent. Colorless single crystals of [S(NMe2)3][1–l2-F]
were obtained by vapor diffusion of hexane into a concentrated
Based on these structural features, we propose that the high
fluoride binding constant measured for 1 results from the higher
This journal is
The Royal Society of Chemistry 2008
Dalton Trans., 2008, 4442–4450 | 4447
©

chloroform solution containing an equimolar amount of 1 and
TASF. Single crystals of [S(NMe2)3][3–l2-F] were obtained from
a THF solution containing an equimolar amount of 3 and
TASF upon slow evaporation of the solvent. The crystallographic
measurements were performed using a Bruker SMART-CCD
(for 4 and [S(NMe2)3][1–l2-F]) or a Bruker APEX-II CCD (for
[S(NMe2)3][3–l2-F]) area detector diffractometer with graphite-
The reaction enthalpies DH were derived from the energy of each
molecule (from the single point calculation) and corrected to
enthalpy by the “thermal correction to enthalpy term” obtained
in the frequency calculations.
Synthesis of [1,1ꢀ-(Hg)-[8-(Mes2B)C10H6]2] (3)
˚
monochromated Mo-Ka radiation (k = 0.71073 A). The crystal-
To a solution of [Li(THF)4][1,8-l-(Mes2B)C10H6] (294 mg,
0.44 mmol) in THF (3 mL) was added a solution of mercury
dichloride (60 mg, 0.22 mmol) in THF (2 mL) at −40 ◦C. The
resulting mixture was stirred at room temperature for 18 h. During
this time, the compound precipitated. It was isolated by filtration
after cooling the reaction mixture in an ice bath. Further washing
with two 1 mL portions of ice cold THF followed by drying in vacuo
afforded compound 3 in 70% yield (146 mg); mp 305 ◦C (decomp.).
This compound could be further purified by recrystallization from
a concentrated chloroform solution from which it crystallizes with
half a molecule of interstitial chloroform. Elemental analysis of a
dry sample indicated partial loss of the interstitial solvent. Anal.
(found) for C56H56B2Hg·0.22(CHCl3): C, 69.08 (69.08); H, 5.80
lographic measurements for 3·0.5(CHCl3) were performed using
a Bruker AXS GADDS MWPC area detector diffractometer
˚
with graphite-monochromated Cu-Ka radiation (k = 1.54178 A).
In each case, the crystal was mounted onto a nylon loop with
Apiezon grease. The structure was solved by direct methods, which
successfully located most of the non-hydrogen atoms. Subsequent
refinement on F2 with the SHELXTL/PC package (version 5.1)
allowed location of the remaining non-hydrogen atoms.
Electrochemistry
Electrochemical experiments were performed with an electro-
chemical analyzer from CH Instruments (Model 610A) with
a glassy carbon working electrode and a platinum auxiliary
electrode. The reference electrode was built from a silver wire
inserted into a small glass tube fitted with a porous vycor frit
at the tip and filled with a THF solution containing (n-Bu)4NPF6
(0.1 M) and AgNO3 (0.005 M). All three electrodes were immersed
in a THF solution (2 mL) containing (n-Bu)4NPF6 (0.3 M) as a
supporting electrolyte and the analyte. The electrolyte was dried
under vacuum prior to use. In all cases, ferrocene was used as an
internal standard, and all reduction potentials are reported with
respect to the E1/2 of the Fc/Fc+ redox couple. The differential
pulsed voltammograms of 2 and 3 were recorded in THF (2.5 ml)
with (n-Bu4)NPF6 (0.3 M) as supporting electrolyte. For 2, 2.5 ml
of the borane was titrated with 5 ll, 10 ll, 15 ll, 20 ll, 25 ll,
respectively, of a 0.32 M solution of TBAF in THF. For 3, 2.5 ml
of the borane was titrated with 15 ll, 30 ll, 45 ll, 60 ll, 75 ll,
90 ll, 100 ll, 125 ll, 135 ll respectively, of a 0.14 M solution of
TBAF in THF.
1
(5.75%). H NMR (CDCl3): d 0.90–1.85 (sh 1.10) (br s, 24H, o-
CH3), 2.26 (s, 12H, p-CH3), 6.42 (br s, 2H, naph-C(2)-H), 6.60
(br s, 8H, Mes-C-H), 7.26–7.35 (m, 4H, naph-C(3,6)-H), 7.43 (d,
2H, naph-C(4/5/7)-H), 7.69 (d, 2H, naph-C(4/5/7)-H), 7.89 (d,
2H, naph-C(4/5/7)-H). 11B (CDCl3): d 72. 199Hg (CDCl3): d −800.
Synthesis of [1-(ClHg)-8-(Mes2B)C10H6] (4)
Method 1. To a solution of 1-(dimesitylboron)-8-(trimethyl-
tin)naphthalenediyl (243 mg, 0.45 mmol) in THF (2 mL) was
added a solution of mercury dichloride (122 mg, 0.45 mmol) in
THF (2 mL) at −40 ◦C and the resulting mixture stirred at room
temperature for 12 h. The reaction mixture was filtered, and the
solvent evaporated in vacuo. The residue was washed with three
1 mL portions of pentane, and dried in vacuo, yielding a colorless
solid. Compound 4 was isolated in 94% yield (250 mg).
Method 2. A solution of mercury dichloride (178 mg,
0.66 mmol) in THF (3 mL) was added to a solution of
[Li(THF)4][1,8-l-(Mes2B)C10H6] (250 mg, 0.68 mmol) in THF
(3 mL). After stirring for 24 h at 25 ◦C, a cloudy brown material
formed. The solution was filtered, and evaporated by vacuum.
The resulting brown oil was extracted with hot toluene, and the
solution filtered. Evaporation of the toluene afforded a tan powder.
Compound 4 was isolated in 53% yield (213 mg). Crystals suitable
for X-ray diffraction were obtained by evaporation of a pyridine
solution. Anal. (found) for C28H28ClHgB: C, 55.01 (55.39); H, 4.62
(4.65%); mp 220 ◦C. 1H NMR (CDCl3): d 1.30 (br s, 3H), 2.00 (br s,
3H), 2.06 (br s, 3H), 2.21 (br s, 3H), 2.35 (br s, 3H), 2.43 (br s, 3H),
6.38 (d, 1H), 6.54 (br s, 1H), 6.77 (br s, 2H), 6.94 (br s, 1H), 7.05 (dt,
2H), 7.53 (d, 1H), 7.61 (d, 1H), 7.63 (d, 1H). 13C NMR (CDCl3):
d 21.81 (s, p-CH3), 23.16 (br s, o-CH3), 24.09 (br s, o-CH3), 25.02
(br s, o-CH3), 26.85 (br s, o-CH3), 126.21 (s, naph C-H), 126.41
(s, naph C-H), 129.76 (br s, B–C), 130.98 (s, naph C-H), 131.27
(br s, B–C), 132.17 (br s, B-C), 134.24 (s, naph C-H), 135.47 (s,
mes C-H), 136.04 (s, naph C-H), 137.37 (s, naph C-H), 142.41 (s,
mes C-H), 152.14 (s, mes C-H); mes C-H, C(9/10), C-CH3, C-Hg
were not located. 11B (CDCl3): d 72. 199Hg (CDCl3): d −1402.
Theoretical calculations
DFT calculations (full geometry optimization) were carried out
with Gaussian 0374 using the B3LYP functional with the following
basis sets: 6-31g for all carbon and hydrogen atoms,75 6-31+g(dꢀ)
for the boron and nitrogen atoms,76–78 and Stuttgart RSC 1997
ECP for the mercury centers.79 Frequency calculations, which
were carried out on the optimized structure of each compound,
confirmed the absence of imaginary frequencies. Frontier orbitals
were obtained from the optimized geometry. In order to calculate
the fluoride ion affinity of the boranes, the optimized geometries
of the boranes and fluoroborates were subjected to a single point
energy calculation using the gradient-corrected B3LYP functional
and the 6-311+g(2d,p) basis set for all atoms80,81 except for mercury
for which the Stuttgart RSC 1997 ECP was used. The fluoride ion
affinities were calculated as per eqn (1) and (2).53
borane + F- ⎯Δ⎯H → fluoroborate
(1)
fluoride ion affinity = −DH
(2)
4448 | Dalton Trans., 2008, 4442–4450
This journal is
The Royal Society of Chemistry 2008
©

36 Z. Zhou, H. Yang, M. Shi, S. Xiao, F. Li, T. Yi and C. Huang,
ChemPhysChem, 2007, 8, 1289–1292.
Acknowledgements
37 Z. Zhou, F. Li, T. Yi and C. Huang, Tetrahedron Lett., 2007, 48, 6633–
6636.
This work was supported by NSF (CHE-0646916), the Welch
Foundation (A-1423), the Petroleum Research Funds (Grant
44832-AC4) and the US Army Medical Research Institute of
Chemical Defense.
38 S.-B. Zhao, T. McCormick and S. Wang, Inorg. Chem., 2007, 46, 10965–
10967.
39 M. H. Lee and F. P. Gabba¨ı, Inorg. Chem., 2007, 46, 8132–8138.
40 M. Mela¨ımi and F. P. Gabba¨ı, J. Am. Chem. Soc., 2005, 127, 9680–9681.
41 C.-W. Chiu and F. P. Gabba¨ı, J. Am. Chem. Soc., 2006, 128, 14248–
14249.
42 M. H. Lee, T. Agou, J. Kobayashi, T. Kawashima and F. P. Gabba¨ı,
Chem. Commun., 2007, 1133–1135.
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