Complementary anion binding by bidentate boron-containing Lewis acids

Article abstract of DOI:10.1016/j.jorganchem.2005.01.015

Anion binding by the pyroborates (catB)₂O (1, cat = O ₂C₆H₄-1,2) and (S,S-Ph₂C ₂H₂O₂B)₂O (2) has been investigated by spectroscopic, structural and titration methods. 1 has been shown to act as a bifunctional Lewis acid, exemplified by the complementary (1:1) binding of bidentate bases such as acetate and dihydrogen phosphate. The former complex has been characterized in the solid state by X-ray diffraction and a binding constant of 1500 ± 550 M^-1 determined in chloroform solution. The reaction of 2 with acetate, by contrast, leads to breakdown of the Lewis acid chelate and to the formation of the homochiral borate anion [(S,S-Ph ₂C₂H₂O₂)₂B]^- in good yield (84% based on the chiral component).

Full text of DOI:10.1016/j.jorganchem.2005.01.015

Journal of Organometallic Chemistry 690 (2005) 2725–2731
www.elsevier.com/locate/jorganchem
Complementary anion binding by bidentate boron-containing
Lewis acids
*
Natalie D. Coombs, Simon Aldridge , Gavin Wiltshire, Deborah L. Kays (nee Coombs),
´
Christopher Bresner, Li-ling Ooi
Centre for Fundamental and Applied Main Group Chemistry, School of Chemistry, Cardiff University, Main Building, Park Place,
Cardiff CF10 3AT, UK
Received 13 September 2004; accepted 11 January 2005
Available online 22 February 2005
Abstract
Anion binding by the pyroborates (catB)2O (1, cat = O2C6H4-1,2) and (S,S-Ph2C2H2O2B)2O (2) has been investigated by spec-
troscopic, structural and titration methods. 1 has been shown to act as a bifunctional Lewis acid, exemplified by the complementary
(1:1) binding of bidentate bases such as acetate and dihydrogen phosphate. The former complex has been characterized in the solid
state by X-ray diffraction and a binding constant of 1500 550 Mꢀ1 determined in chloroform solution. The reaction of 2 with ace-
tate, by contrast, leads to breakdown of the Lewis acid chelate and to the formation of the homochiral borate anion [(S,S-
Ph2C2H2O2)2B]ꢀ in good yield (84% based on the chiral component).
ꢀ 2005 Elsevier B.V. All rights reserved.
1. Introduction
by a single atom spacer mean that systems of the type
X2B–E0(R)n-BX2 (E0 = first row element) are unlikely
to bind a monodentate anion in chelating fashion
[2d], being more appropriate to the complementary
bidentate acid/base binding of anions, such as NOꢀ3 ,
AcOꢀ or H2POꢀ4 [3].
The binding of anions by receptors based on three
coordinate group 13 centres has been the subject of
considerable recent research effort, with applications
in anion abstraction, catalysis and sensors [1]. Within
this sphere, a number of bidentate boron-based sys-
tems have been reported, and the mode and strength
of their interactions with a variety of anions investi-
gated [2]. The possibility for stronger methide anion
binding by a chelating receptor, for example, has led
to the synthesis of a number of bifunctional Lewis
acids (featuring various spacer groups) and to their
investigation as potential alternatives to B(C6F5)3 as
olefin polymerization co-catalysts [2c,2d,2e,2f,2g,2h,2-
l,2o]. By contrast, the geometric constraints imposed
Selective binding of bidentate anions such as carb-
oxylates, RCOꢀ2 (and related anionic and neutral species
such as dihydrogenphosphates, amino acids or aromatic
nitro compounds) by hosts featuring an appropriate
arrangement of hydrogen bond donors is of consider-
able relevance to biological systems [4]. Thus, for exam-
ple, the binding properties of amides, ureas, thioureas
and guanidinium derivatives which feature geometries
appropriate for the complementary hydrogen bonding
of such anions (I, Scheme 1), have been widely investi-
gated [4]. Given the similarities in the spatial disposition
of the binding sites in bifunctional boranes II and in
hydrogen bond donors I, we were interested to investi-
gate the possibility for the binding of bidentate anions
by readily available bifunctional Lewis acids such as
*
Corresponding author. Tel.: +44 29 20875495; fax: +44 29
20874030.
E-mail address: aldridges@cf.ac.uk (S. Aldridge).
URL: http://www.cf.ac.uk/chemy/cfamgc (S. Aldridge).
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.01.015

2726
N.D. Coombs et al. / Journal of Organometallic Chemistry 690 (2005) 2725–2731
_
Xn
E
O
O
_
Xn
O
O
1
2
=
E
O
O
B
O
B
O
O
O
O
H
N
H
N
B
B
O
O
O
Ph
Ph
R'
R'
R'
R'
R
R'
E'
(S,S)
Rn
E'
O
I
II
Scheme 1. Potential receptors for the binding of carboxylate and its isosteres using hydrogen bonding (I) or boron-based Lewis acids (II) such as 1 or 2.
(catB)2O (1, cat = O2C6H4-1,2) [5], along with analo-
gous chiral receptors such as (S,S-Ph2C2H2O2B)2O (2).
Abbreviations: st = strong, md = medium, w = weak,
s = singlet, d = doublet, t = triplet, q = quintet, sx = sex-
tet, m = multiplet, br = broad.
2. Experimental
2.2. Crystallographic method
2.1. General
Data were collected on an Enraf Nonius Kappa CCD
diffractometer equipped with a Mo Ka radiation source
(k = 0.71073 A). Data collection and cell refinement
˚
All manipulations were carried out under a nitrogen
or argon atmosphere using standard Schlenk line or
dry box techniques unless otherwise stated. Solvents
were pre-dried over sodium wire (hexanes, toluene) or
molecular sieves (dichloromethane) and purged with
nitrogen prior to distillation. Hexanes (potassium), tolu-
ene (sodium), and dichloromethane (calcium hydride)
were then distilled from the appropriate drying agent be-
fore use. Chloroform-d and dichloromethane-d2 (both
Goss) were degassed and dried over molecular sieves
prior to use. The compounds [PPN][OAc] [PPN = bis(tri-
phenylphosphoranylidene)-ammonium, Ph3PNPPh3],
[nBu4N][H2PO4], [nBu4N]F, B2cat2 (cat = catecholate,
O2C6H4-1,2) and B(OH)3 were obtained from commer-
cial sources and used as received. (catB)2O and S,S-
Ph2C2H2(OH)2 were prepared by the literature routes
[5,6].
were carried out using DENZO [7], and structure solution
and refinement (by full-matrix least-squares) using SIR-
92, DIRDIFF-99 and SHELXL-97 [8–11], respectively. In
each case, hydrogens were placed in idealised positions
and refined using a riding model with Uiso set to 1.2 or
1.5 times the Ueq of the parent atom. Details of the data
collection, structure solution and refinement for 4a, and
5 can be found in Table 1; relevant bond lengths and an-
gles are included in the figure captions. Complete details
of all structures have been deposited with the CCDC
and are included in the supporting information.
2.3. Syntheses
Spectroscopic data for (catB)2O (1). Compound 1
was prepared by the previously reported route of No¨th
and co-workers [5], and selected spectroscopic data are
reported here merely for comparison of the ꢀfreeꢁ recep-
tor with subsequently derived anionic acid/base com-
NMR spectra were measured on a Bruker AM-400
or Jeol Eclipse 300 Plus FT-NMR spectrometer.
Residual signals of solvent were used for reference
for 1H and 13C NMR, while 11B,19 F and
31
1
P
plexes. H NMR (300 MHz, CD2Cl2) d 7.13 (4H, m,
NMR spectra were referenced with respect to
Et2O Æ BF3, CFCl3 and 85% H3PO4, respectively. Infra-
red spectra were measured for each compound either
as a solution in hexanes or pressed into a disk with
an excess of dried KBr on a Nicolet 500 FT-IR spec-
trometer. Mass spectra were measured by the EPSRC
National Mass Spectrometry Service Centre, Univer-
sity of Wales Swansea and by the departmental ser-
vice. Perfluorotributylamine was used as the standard
for high-resolution EI mass spectra. Elemental analy-
ses were carried out both by the departmental service
and by Warwick Analytical Service, University of
Warwick.
aromatic CH), 7.24 (4H, m, aromatic CH). 11B NMR
(96 MHz, CD2Cl2) d 21.5.
Synthesis of (S,S-Ph2C2H2O2B)2O (2). Compound 2
was prepared from B(OH)3 and S,S-Ph2C2H2(OH)2, by
an analogous condensation route to that used for 1
[5,12]. Thus a mixture of B(OH)3 (0.87 g, 1.41 mmol)
and S,S-Ph2C2H2(OH)2 (3.01 g, 1.41 mmol) in toluene
(150 cm3) was heated at reflux for 72 h with azeotropic
removal of water. Filtration, removal of volatiles in va-
cuo and recrystallization of the resulting off-white pow-
der from dichloromethane/hexane at ꢀ 30 ꢁC led to the
1
isolation of 2 in yields of ca. 60% (2.01 g). H NMR
(300 MHz, CD2Cl2) d 5.26 (4H, s, CHPh), 7.27–7.37

N.D. Coombs et al. / Journal of Organometallic Chemistry 690 (2005) 2725–2731
2727
Table 1
Crystallographic data for 4a and 5
4a
5
Empirical formula
CCDC reference
Temperature (K)
Formula weight
Crystal system
C50H41B2NO7P2 C64H54BNO4P
253468
150 (2)
851.40
253469
150(2)
486.92
Orthorhombic
C2221
Monoclinic
P21/c
Space group
Unit cell dimensions
˚
a (A)
15.0305(2)
17.8874(2)
15.9054(2)
90
11.8880(2)
17.5520(3)
24.2470(5)
90
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
94.7620(9)
90
90
90
Fig. 1. 1H NMR titration data for the addition of 1 to [PPN][OAc].
The curve was fitted to the experimental data points using WinE-
QNMR and gives a binding constant of 1500 550 Mꢀ1 [13].
˚
Volume (A3)
4261.51(9)
4
1.327
5059.34(16)
8
1.278
Z
Density (calc.) (Mg/m3)
Absorption coefficient (mmꢀ1
F(000)
)
0.158
1776
0.138
2048
the formation of [PPN][(catBOBcat) Æ (OAc)] (4a) as
large colourless crystals suitable for X-ray crystallogra-
1
phy (yield 0.25 g, 76%). H NMR (300 MHz, CDCl3)
Crystal size (mm3)
0.35 · 0.32 · 0.25 0.25 · 0.20 · 0.20
Theta range for data colln. (ꢁ) 2.95 to 27.50
Index ranges
3.26 to 27.48
d 2.09 (3H, s, CH3 of OAc), 6.51 (4H, m, aromatic
CH of cat), 6.61 (4H, m, aromatic CH of cat), 7.38–
7.45 (24H, m, ortho and meta CHs of PPN), 7.60–7.65
(6H, m para CHs of PPN).13C NMR (76 MHz, CDCl3)
d 23.1 (CH3 of AcO), 108.7, 117.4 (cat CHs), 127.0 (d,
1JPC = 108 Hz, PPN ipso C), 129.7, 132.1, 134.0 (PPN
h
ꢀ19 to 19
ꢀ23 to 23
ꢀ20 to 20
78,830
ꢀ13 to 15
ꢀ22 to 22
ꢀ31 to 31
20,796
k
l
Reflections collected
Independent reflections, Rint
Completeness to h max. (%)
Absorption correction
9791 (0.1065)
99.8
5558(0.0644)
99.3
DIFABS
Semi-empirical
from equivalents
0.9729, 0.9663
5558/0/326
1.052
CHs), 152.2 (cat quaternary), 176.5 (AcO quaternary).
31
11B NMR (96 MHz, CDCl3) d 13.4.
P NMR
Max. and min. transmission
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2r(I)]
R1
0.394, 0.128
9791/0/560
1.028
(121 MHz, CDCl3) 21.7. IR (KBr, cmꢀ1) m (OCO)
1577 st, 1460 md. Elemental analysis: calc. (for
C50H41B2 NO7 P2) C 70.54, N 1.64, H 4.85; meas.
C 70.72, N 1.57, H 4.91. 1H NMR titration experiments
to determine the equilibrium constant for the binding of
acetate to 1 were carried out by adding aliquots (typi-
cally 0.1 equiv. at a time) of 1 to 1 mmol of [PPN][OAc]
in CDCl3 (10.9 cm3). The position of the resonance due
to the acetate methyl group was monitored and data fit-
ted using WinEQNMR (see Fig. 1) [13].
0.0463
0.1054
0.0456
0.0868
wR2
R indices (all data)
R1
0.0718
0.1173
–
0.0707
0.0975
wR2
Absolute structure parameter
Largest difference peak
0.05(9)
0.246, ꢀ0.387
0.423, ꢀ0.423
ꢀ3
˚
and hole (e A
)
(ii) Reaction with [nBu4N][H2PO4]
To a solution of 1 (0.30 g, 1.18 mmol) in dichloro-
methane (5 cm3) was added a solution containing 1
equiv. of [nBu4N][H2PO4] also in dichloromethane
(5 cm3). After stirring for 24 h, monitoring by 11B
NMR revealed complete conversion of 1 (dB 21.5) to a
species giving rise to a single resonance at dB 13.1. Fil-
tration, concentration (to ca. 3 cm3) and layering with
hexanes, led to the formation of [nBu4N][(catBOB-
cat) Æ (H2PO4)] (4b) as colourless crystals (yield 0.51 g,
73%). 1H NMR (300 MHz, CDCl3) d 0.91 (12H, t,
J = 7.3 Hz, CH3 of nBu), 1.28 (8H, virtual sx,
J = 7.3 Hz, CH2 of nBu), 1.42 (8H, virtual q,
J = 8.1 Hz, CH2 of nBu), 2.97 (8H, m, CH2 of nBu),
6.17 (2H, br s, OH of H2PO4), 6.59 (8H, m, aromatic
CH of cat).13C NMR (76 MHz, CDCl3) d 13.7 (CH3
(20H, m, aromatic CH). 13C NMR (76 MHz, CD2Cl2) d
79.2 (CHPh), 125.9, 128.7, 128.9 (aromatic CH), 139.9
(aromatic quaternary). 11B NMR (96 MHz, CD2Cl2) d
21.7. IR (KBr, cmꢀ1) 2962 w, 1955 w, 1885 w, 1584 w,
1498 st, 1445 st, 1388 w, 1358 w, 1322 w, 1261 st, 1207
st, 1096 st, 1021 st. Mass spec (EI): m/z 462.2 [M+,
100%]; exact mass: calc. 462.1804, meas. 462.1802.
Interaction of 1 with bidentate anions
(i) Reaction with [PPN][OAc]
To a solution of 1 (0.10 g, 0.39 mmol) in dichloro-
methane (5 cm3) was added a solution containing 1
equiv. of [PPN][OAc] also in dichloromethane (5 cm3).
After stirring for 24 h, monitoring by 11B NMR revealed
complete conversion of 1 (dB 21.5) to a species giving
rise to a single resonance at dB 13.4. Filtration, concen-
tration (to ca. 3 cm3) and layering with hexanes, led to
n
n
of Bu), 19.6, 23.8, 57.9 (CH2 s of Bu), 108.7, 117.9
(aromatic CHs of cat), 151.8 (aromatic quaternary).

2728
N.D. Coombs et al. / Journal of Organometallic Chemistry 690 (2005) 2725–2731
11B NMR (96 MHz, CDCl3)
d
13.1. 31P NMR
(121 MHz, CDCl3) ꢀ21.5 (br). IR (KBr, cmꢀ1
)
m(OPO)1234 md, 1097 md.
Reaction of B2cat2 (3) with [PPN][OAc]
To a solution of 3 (0.05 g, 0.21 mmol) in dichloro-
methane (5 cm3) was added a solution containing 1
equiv. of commercial [PPN][OAc] (Aldrich) also in
dichloromethane (5 cm3). After stirring for 72 h, moni-
toring by 11B NMR revealed complete conversion of 3
(dB 30.7) to a species giving rise to a single resonance
at dB 13.4. Filtration, concentration (to ca. 1.5 cm3)
and layering with hexanes, led to the formation of large
colourless crystals of 4a having identical spectroscopic
properties to samples prepared from 1.
Fig. 2. Structure of the anionic component of [PPN][(catBOB-
cat) Æ (OAc)], 4a; hydrogen atoms omitted for clarity. Important bond
lengths (A) and angles (ꢁ): B(1)–O(2) 1.574(2), O(2)–C(1) 1.265(2),
C(1)–O(1) 1.265(2), O(1)–B(2) 1.617(3), B(2)–O(3) 1.375(2), O(3)–B(1)
1.393(2), B(1)–O(4) 1.470(2), O(1)–C(1)–O(2) 124.15(17), B(1)–O(3)–
B(2) 128.90(16), O(3)–B(1)–O(4) 114.46(16), O(3)–B(1)–O(5)
115.15(16), O(4)–B(1)–O(5) 105.59(15).
˚
Reaction of 2 with [PPN][OAc] – synthesis of
[PPN][(S,S-Ph2C2H2O2)2B] (5)
To a solution of 2 (0.15 g, 0.32 mmol) in dichloro-
methane (5 cm3) was added a solution containing 1
equiv. of [PPN][OAc] also in dichloromethane (5 cm3).
After stirring for 3 h, monitoring by 11B NMR revealed
complete consumption of 2 (dB 21.7) and the appearance
of two new resonances at dB 19.6 and 10.6. Filtration,
concentration (to ca. 2 cm3) and layering with hexanes,
led to the formation of [PPN][(S,S-Ph2C2H2O2)2B] (5)
as colourless crystals suitable for X-ray crystallography
(yield 0.13 g, 42% based on boron, 84% based on chiral
component). 1H NMR (300 MHz, CDCl3) d 4.70 (4H, s,
CHPh), 7.02–7.60 (50H, m, aromatic CH of PPN and of
CHPh).13C NMR (76 MHz, CDCl3) d 84.9 (CHPh),
in the 11B NMR resonance (dB 21.5–13.1) and by a
downfield shift (dH 1.91–2.09) in the acetate CH3 1H
NMR signal. The former shift is characteristic of quat-
ernization of the boron centre on anion binding [14],
and is consistent with a symmetrically bound g2(O,O)
acetate ligand, or with rapidly fluxional g1 coordination.
Proton NMR titration measurements (Fig. 1) are consis-
tent with a 1:1 binding stoichiometry, and yield a bind-
ing constant of 1500 550 Mꢀ1. This value is similar to
that determined for the binding of carboxylate anions to
simple urea-based receptors in chloroform solution (e.g.,
1300 200 Mꢀ1 [4d]), but about one order of magnitude
less than those reported, for example, by Beer and by
Smith for the binding of AcOꢀ to receptors featuring
either macrocyclic or Lewis acid assisted binding do-
mains [4f,4g,4h]. Our results are also indicative of a sig-
nificantly stronger acetate/Lewis acid interaction than
has been reported for boronate ester systems of the type
ArBpin (pin = OCMe2CMe2O). Based on 11B NMR
data, such systems have been reported to show ꢀno affin-
ityꢁ for acetate [4f].
1
125.7 (aromatic CH of anion), 127.0 (d, JPC = 108 Hz,
PPN ipso C), 127.3, 127.4 (aromatic CHs of anion),
129.7, 132.1, 134.0 (PPN CHs), 139.9 (aromatic quater-
nary of anion). 11B NMR (96 MHz, CDCl3) d 10.6. 31P
NMR (121 MHz, CDCl3) 21.7. IR (KBr, cmꢀ1) 3154 w,
2962 w, 1815 w, 1793 w, 1646 w, 1560 w, 1439 st, 1381
st, 1261 st, 1094 st, 1016 md.
3. Results and discussion
The preceding spectroscopic inferences were given
further credence by the results of an X-ray diffraction
study undertaken on crystals obtained by diffusion of
hexane into a dichloromethane solution. The structure
of the adduct [(catBOBcat) Æ (OAc)]ꢀ (4a) so obtained
(as the PPN salt, Fig. 2 and Table 1) confirms its 1:1
stoichiometry and the complementary nature of the
bidentate anion/bidentate Lewis acid interaction. This
mode of binding and the six-membered chelate ring so
formed are similar to those observed by Uhl and co-
workers [3,15] for the adduct [(R2AlCH2AlR2) Æ (NO3)]ꢀ
[R = CH(SiMe3)2]; a similar structural motif is also
found in a number of neutral boron-containing species.
In the case of 4a there is noticeable puckering of the six-
membered ring due to the fact that the CO2 unit of the
acetate ꢀligandꢁ and the BOB bridge of the Lewis acid
chelate are not co-planar. Thus the C(1)–O(2)–B(1)–
Given significant recent interest in anion binding by
boron-centred Lewis acids and in particular the develop-
ment of bifunctional or chelating systems, we were inter-
ested in examining the anion binding properties of the
pyroborate 1. In addition, given that chiral Lewis acids
offer the potential for enantioselective anion recognition
and/or chiral delivery of achiral anions, we were inter-
ested in extending this investigation to homochiral
receptors such as (S,S-Ph2C2H2O2B)2O (2). In practice,
the chemistry reported by No¨th and co-workers for 1 [5]
is readily extended to the synthesis of 2 from S,S-stil-
benediol (Scheme 1), and 2 has been characterized by
IR and multinuclear NMR spectroscopies and by mass
spectrometry (including exact mass determination).
The interaction of 1 with one equivalent of acetate in
chloroform solution is characterized by an upfield shift

N.D. Coombs et al. / Journal of Organometallic Chemistry 690 (2005) 2725–2731
2729
O(3) and C(1)–O(1)–B(2)–O(3) torsion angles are
20.9(2) and 17.5(2)ꢁ, respectively. Further asymmetry
within the chelate ring reflects small but significant dif-
ferences in the binding of the two acetate oxygen do-
nors; thus, disparate B–O(acetate) distances of 1.574(2)
consistent with the crystallographically determined
structure. Thus, acetate OCO stretching vibrations at
1577 (mas) and 1460 cmꢀ1 (ms) are consistent with previ-
ously reported examples of this type of coordination
(c.f. values of 1588, 1426 and 1585, 1428 cmꢀ1 for ace-
tate ligands bridging dinickel and dizinc systems, respec-
tively [18]). Similar analyses of the spectroscopic data
obtained for the species formed on exposure of 1 to
[H2PO4]ꢀ or to Fꢀ allow some comments to be made
concerning the structures of these adducts. Thus the
11B resonance for 4b, the 1:1 adduct isolated from 1
and [H2PO4]ꢀ, has a very similar chemical shift (dB
13.1) to that for observed for 4a, and does not vary sig-
nificantly with temperature. Furthermore, OPO stretch-
ing vibrations at 1234 and 1097 cmꢀ1 are similar to
values of 1200 and 1130 cmꢀ1 for previously reported
examples of g2(O,O) bound [H2PO4]ꢀ ligands [18].
Although single crystals suitable for X-ray diffraction
were not forthcoming a structure analogous to 4a is
therefore conceivable for the complex [(catBOB-
cat) Æ (H2PO4)]ꢀ (Scheme 2).
Not unexpectedly, given the constraints of B–O–B
bridge, the interaction 1 with one equivalent of fluoride
in chloroform solution appears to involve binding to a
single boron centre and fluoride exchange between the
two Lewis acidic sites. Hence at room temperature,
two broad overlapping 11B resonances are observed
which coalesce at 50ꢁ to a single signal (dB 18.0), and
which can be resolved into two distinct signals at ꢀ20
ꢁC. The positions of the latter two resonances (dB 13.8
and 21.0) are consistent, respectively, with a four-
coordinate fluoride-complexed boron and a vacant
three-coordinate site.
˚
and 1.617(3) A are observed, presumably reflecting a
marginally stronger O(2)–B(1) interaction [over O(1)–
B(2)] in the solid state. Consistent with this, the length-
˚
ening of the B(1)–O(3) bond [1.393(2) A] with respect to
˚
that observed in the free (catB)2O receptor [1.346(2) A]
is significantly greater than that found for B(2)–O(3)
˚
[1.375(2) A] [5]. Further structural differences between
the acetate complex 4a and the free receptor 1 include
the increased B–O bond lengths and decreased OBO an-
gles at the boron centre expected on quaternization.
Interestingly, the increased bond lengths for the B–O–
B fragment found in the complex 4a are almost entirely
˚
offset by a narrowing of the B–O–B angle [128.90(16) A
for 4a, 135.9(2) for 1], such that the Bꢁ ꢁ ꢁB separation is
˚
essentially unchanged [2.498(2) and 2.495 A, respec-
tively] [5]. This observation implies a near ideal inter-
boron separation in the B–O–B unit of 1 with respect
to the complementary binding of acetate.
Interestingly complex 4a is also the product isolated
from the reaction of B2cat2 with one equivalent of com-
mercial (i.e., wet) [PPN][OAc]. At short reaction times,
11B monitoring is consistent with fluxional g1 binding
of AcOꢀ to B2cat2 [16], but after 72 h the predominant
product is 4a, possibly formed by insertion of a water-
derived oxygen atom into the B–B bond of 3. Such a
reaction presumably reflects the thermodynamic stabil-
ity of the complementary AcOꢀ binding in 4a, and in
turn the better match of acetate Oꢁ ꢁ ꢁO separation
˚
˚
(2.236 A in 4a) to the Bꢁ ꢁ ꢁB separation in 1 (2.495 A
Given that pyroborate 1 has been shown to act as a
chelating Lewis acid in the complementary binding of
acetate and dihydrogen phosphate, we were interested
to determine whether similar binding properties for
homochiral analogue 2 might be exploited in the enan-
tioselective recognition (or delivery) of carboxylate or
phosphate derived anions [19]. In the event, the reaction
˚
vs. 1.678 A in B2cat2 [17]).
1H and 11B NMR data obtained from isolated crys-
talline samples of 4a are identical to those obtained
for in situ acetate binding experiments. In addition, IR
measurements for both isolated crystalline samples of
4a and for the adduct in dichloromethane solution are
O
B
O
_
Xn
E
M+
(i)
O
2
O
B
O
B
O
O
O
O
1a
=
O
O
(ii)
O
O
O
O
O
O
B
B
4a M = PPN, EXn = CCH3
4b M = nBu4N, EXn = P(OH)2
O
3
Scheme 2. Complementary binding of bifunctional Lewis bases by pyroborate 1a. Conditions: (i) [M][Xn E(@O)O] (1 equiv.), dichloromethane,
20 ꢁC, 24 h, 76%; (ii) commercial [PPN][OAc] (1 equiv.), dichloromethane, 20 ꢁC, 96 h, 68%.

2730
N.D. Coombs et al. / Journal of Organometallic Chemistry 690 (2005) 2725–2731
_
[PPN]+
Acknowledgements
Ph
Ph
O
O
O
O
O
O
O
O
O
O
(i)
(S,S)
=
B
B
O
2
S.A. thanks the EPSRC for funding (GR/S98771/01)
andfor access tothe National Mass Spectrometry Service.
2
5
Scheme 3. Reaction of pyroborate 2 with [PPN][OAc] – synthesis of
homo-chiral borate anion [(S,S-Ph2C2H2O2)2B] (5). Conditions: (i)
[PPN][OAc] (1 equiv.), dichloromethane, 20 ꢁC, 3 h, 84% (based on
chiral component).
Appendix A. Supplementary data
Supplementary data associated with this article can
be found, in the online version, at doi:10.1016/
j.jorganchem.2005.01.015.
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