From B(C6F5)3 to B(OC6F 5)3: Synthesis of (C6F5) 2BOC6F5 and C6F5B(OC 6F5)2 and

Article abstract of DOI:10.1021/om049091p

The pentafluorophenyl esters of bis(pentafluorophenyl)borinic acid (C ₆F₅)₂BOC₆F₅ (2) and pentafluorophenylboronic acid C₆F₆B(OC₆F ₅)₂ (3) have b

Full text of DOI:10.1021/om049091p

Organometallics 2005, 24, 1685-1691
1685
From B(C6F5)3 to B(OC6F5)3: Synthesis of (C6F5)2BOC6F5
and C6F5B(OC6F5)2 and Their Relative Lewis Acidity
George J. P. Britovsek,* Juri Ugolotti, and Andrew J. P. White
Department of Chemistry, Imperial College London, Exhibition Road, London SW7 2AY, U.K.
Received November 23, 2004
The pentafluorophenyl esters of bis(pentafluorophenyl)borinic acid (C6F5)2BOC6F5 (2) and
pentafluorophenylboronic acid C6F5B(OC6F5)2 (3) have been prepared and characterized by
multinuclear NMR and X-ray analysis. VT NMR studies have shown that restricted rotation
around the B-O bond in 2 occurs below 193 K, corresponding to ∆Gq ) 35 kJ/mol for this
process. This low barrier and the random torsion angles around the B-O bonds observed in
the solid state structures of compounds 2, 3, and B(OC6F5)3 (4) suggest that these torsion
angles are not related to pπ-pπ interactions between boron and oxygen, but more likely a
consequence of the extensive intermolecular F-π interactions seen in the solid state
structures. The Lewis acidity of 2, 3, and 4 has been compared with B(C6F5)3 (1), using
various Lewis bases. All compounds 1-4 appear to be strong Lewis acids, whereby 4 interacts
more strongly with hard bases whereas 1 binds more strongly to softer bases.
Introduction
Four decades ago, Massey, Park, and Stone1,2 reported
the synthesis of tris(pentafluorophenyl)borane, B(C6F5)3
(1, Figure 1), a powerful Lewis acid comparable in acid
strength to BF3,3,4 but much easier to handle. B(C6F5)3
has found many applications, most notably as a cocata-
lyst in the polymerization of olefins,5,6 but also in organic
synthesis.7-9 Several reactivity studies have shown that
B(C6F5)3 forms adducts with a range of Lewis bases,
including those containing soft donors such as phospho-
rus3,10,11 and sulfur12,13 as well as hard donors, for
example nitrogen2,10,14-16 and oxygen.12,17-19 The grow-
* To whom correspondence should be addressed. Tel: +44-(0)20-
75945863. E-mail: g.britovsek@imperial.ac.uk.
(1) Massey, A. G.; Park, A. J.; Stone, F. G. A. Proc. Chem. Soc. 1963,
212.
(2) Massey, A. G.; Park, A. J. J. Organomet. Chem. 1964, 2, 245-
250.
(3) Doering, S.; Erker, G.; Froehlich, R.; Meyer, O.; Bergander, K.
Organometallics 1998, 17, 2183-2187.
(4) Beckett, M. A.; Brassington, D. S.; Coles, S. J.; Hursthouse, M.
B. Inorg. Chem. Commun. 2000, 3, 530-533.
(5) Chen, Y.-X.; Marks, T. J. Chem. Rev. 2000, 100, 1391-1434.
(6) Bochmann, M.; Lancaster, S. J.; Hannant, M. D.; Rodriguez, A.;
Schormann, M.; Walker, D. A.; Woodman, T. J. Pure Appl. Chem. 2003,
75, 1183-1195.
Figure 1. Perfluorophenyl boron Lewis acids 1-4.
ing interest in B(C6F5)3 has prompted us to prepare
variations on B(C6F5)3, to gain a better understanding
and greater control over Lewis acid strength, with the
aim to design Lewis acids for specific applications.
Borinic and boronic esters of the type (C6F5)2BOR or
C6F5B(OR)2 contain both electron-withdrawing C6F5
groups and electronegative oxygen substituents. These
compounds are potentially alternative Lewis acids to
B(C6F5)3, with the advantage that the R substituents
offer the possibility of tuning the Lewis acidity at the
boron center; electron-withdrawing substituents, which
are the subject of this study, will decrease pπ-pπ
(7) Piers, W. E.; Chivers, T. Chem. Soc. Rev. 1997, 26, 345-354.
(8) Ishihara, K.; Hanaki, N.; Funahashi, M.; Miyata, M.; Yamamoto,
H. Bull. Chem. Soc. Jpn. 1995, 68, 1721-1730.
(9) Morrison, D. J.; Piers, W. E. Org. Lett. 2003, 5, 2857-2860.
(10) Jacobsen, H.; Berke, H.; Doering, S.; Kehr, G.; Erker, G.;
Froehlich, R.; Meyer, O. Organometallics 1999, 18, 1724-1735.
(11) Bradley, D. C.; Harding, I. S.; Keefe, A. D.; Motevalli, M.; Zheng,
D. H. J. Chem. Soc., Dalton Trans. 1996, 3931-3936.
(12) Drewitt, M. J.; Niedermann, M.; Kumar, R.; Baird, M. C. Inorg.
Chim. Acta 2002, 335, 43-51.
(13) Schaper, F.; Brintzinger, H. H. Acta Crystallogr. E 2002, 58,
o77-o78.
(14) Bergquist, C.; Bridgewater, B. M.; Harlan, C. J.; Norton, J. R.;
Friesner, R. A.; Parkin, G. J. Am. Chem. Soc. 2000, 122, 10581-10590.
(15) Millot, N.; Santini, C. C.; Fenet, B.; Basset, J. M. Eur. J. Inorg.
Chem. 2002, 3328-3335.
(16) Guidotti, S.; Camurati, I.; Focante, F.; Angellini, L.; Moscardi,
G.; Resconi, L.; Leardini, R.; Nanni, D.; Mercandelli, P.; Sironi, A.;
Beringhelli, T.; Maggioni, D. J. Org. Chem. 2003, 68, 5445-5465.
(17) Beringhelli, T.; Maggioni, D.; D’Alfonso, G. Organometallics
2001, 20, 4927-4938.
(18) Beckett, M. A.; Brassington, D. S.; Light, M. E.; Hursthouse,
M. B. J. Chem. Soc., Dalton Trans. 2001, 1768-1772.
(19) Vagedes, D.; Froehlich, R.; Erker, G. Angew. Chem., Int. Ed.
1999, 38, 3362-3365.
10.1021/om049091p CCC: $30.25 © 2005 American Chemical Society
Publication on Web 03/01/2005

1686 Organometallics, Vol. 24, No. 7, 2005
Table 1. 19F and 11B NMR Data for Compounds 1-4
δ(19F NMR)/ppma
Britovsek et al.
C6F5
OC6F5
compound
ortho
meta
para
ortho
meta
para
δ(11B NMR)/ppmb
1
2
3
4
-129.2
-131.1
-131.1
-160.1
-159.7
-159.0
-141.9
-144.1
-145.2
59
-157.8
-157.3
-157.7
-161.4
-161.9
-162.1
-159.3
-159.7
-159.7
41.2
26.2
14.8
a Solvent: C6D6; reference (external): CFCl3. b Solvent: C6D6; reference (external): BF3‚OEt2.
interactions between oxygen and boron,20-22 thereby
increasing the Lewis acidity, whereas electron-donating
substituents will have the opposite effect.
C6F5B(OC6F5)2, 3, is prepared by a similar procedure
from C6F5BCl2 (eq 3). The preparation of C6F5BCl2 from
Me3SnC6F5 or Me2Sn(C6F5)2 and BCl3 as previously
reported23,31 can be problematic, as the reaction products
C6F5BCl2 and Me3SnCl or Me2SnCl2 are highly volatile
and difficult to separate by distillation or sublimation.
We found that C6F5BCl2 is more conveniently prepared
from nBu3SnC6F5 (eq 2). Both compounds 2 and 3 were
purified by sublimation and have been characterized by
19F NMR and 11B NMR spectroscopy and single-crystal
X-ray analysis. For comparison, we have also deter-
mined the structure of the previously reported com-
pound B(OC6F5)3 (4).
Shortly after the first reports on B(C6F5)3, the syn-
thesis
of
bis(pentafluorophenyl)borinic
acid,
(C6F5)2BOH, and pentafluorophenyl boronic acid,
C6F5B(OH)2, were reported by Chambers and Chivers,
prepared by hydrolysis of the corresponding chloro
compounds.23 The borinic acid (C6F5)2BOH has been
reported to exist as a trimer in the solid state,22 which
is in equilibrium with the monomeric form in solution.24
Both acids have received renewed interest recently,25
including applications in organic synthesis.26,27 Only a
few esters of these acids have been reported. The ethyl
and butyl derivatives (C6F5)2BOEt and (C6F5)2BOnBu
were obtained upon thermal decomposition of
(C6F5)2BF‚OEt228 and (C6F5)2BH‚thf,29 respectively. Di-
n-propyl and di-isopropyl boronic esters C6F5B(OnPr)2
and C6F5B(OiPr)2 30 and a few cyclic esters derived from
catechol, naphthol, and pinacol have also been re-
ported.19,31 Here we report the first preparation of the
pentafluorophenyl esters of bis(pentafluorophenyl)-
borinic acid, (C6F5)2BOC6F5 (2), and pentafluorophenyl-
boronic acid, C6F5B(OC6F5)2 (3). These two compounds
form the missing links between B(C6F5)3 and tris-
(pentafluorophenyl)borate, B(OC6F5)3 (4), which was
first reported in 1992 by Naumann.32 The complete
series now in hand allows us to establish trends in the
relative Lewis acidity of 1-4.
Results and Discussion
The reaction of (C6F5)2BCl23,29 with pentafluorophenol
in dichloromethane leads to the clean formation
NMR Spectroscopy. 19F NMR and 11B NMR spec-
troscopic data for compounds 1-4 have been collected
in Table 1. The 11B NMR chemical shift values decrease
in the order from 1 to 4 with values in the typical range
expected for compounds of the type BAr3, Ar2BOAr,
ArB(OAr)2, and B(OAr)3.33 This trend indicates an
increased shielding of the boron center going from 1 to
4. However, the 11B chemical shift is not a direct
measure for Lewis acidity. For example, the 11B chemi-
cal shift values of B(C6H5)3 and B(OC6H5)3 have been
reported as 68 and 16.5 ppm, respectively,33 which are
comparable to the values of 59 and 15 ppm reported for
the much stronger Lewis acids B(C6F5)3 and B(OC6F5)3,
respectively.32,34 Moreover, BF3 and B(C6F5)3 have been
reported to have similar Lewis acidities3,4 but have
completely different 11B NMR values of 10 and 59 ppm,
respectively.
of (C6F5)2BOC6F5,
2 (eq 1). The boronic ester
(20) Finocchiaro, P.; Gust, D.; Mislow, K. J. Am. Chem. Soc. 1973,
95, 7029-7036.
(21) Ashby, M. T.; Sheshtawy, N. A. Organometallics 1994, 13, 236-
243.
(22) Beringhelli, T.; D’Alfonso, G.; Donghi, D.; Maggioni, D.; Mer-
candelli, P.; Sironi, A. Organometallics 2003, 22, 1588-1590.
(23) Chambers, R. D.; Chivers, T. J. Chem. Soc. 1965, 3933-3939.
(24) Beringhelli, T.; D’Alfonso, G.; Donghi, D.; Maggioni, D.; Mer-
candelli, P.; Sironi, A. Organometallics 2004, 23, 5493-5502.
(25) Frohn, H.-J.; Adonin, N. Y.; Bardin, V. V.; Starichenko, V. F.
Z. Anorg. Allg. Chem. 2002, 628, 2827-2833.
(26) Ishihara, K.; Kurihara, H.; Yamamoto, H. J. Org. Chem. 1997,
62, 5664-5665.
(27) Ishihara, K.; Kurihara, H.; Yamamoto, H. Synlett 1997, 597-
599.
(28) Duchateau, R.; Lancaster, S. J.; Thornton-Pett, M.; Bochmann,
M. Organometallics 1997, 16, 4995-5005.
(29) Parks, D. J.; Piers, W. E.; Yap, G. P. A. Organometallics 1998,
17, 5492-5503.
(30) Frohn, H.-J.; Franke, H.; Fritzen, P.; Bardin, V. V. J. Orga-
nomet. Chem. 2000, 598, 127-135.
(31) Lee, H. S.; Sun, X.; Yang, X. Q.; McBreen, J. J. Electrochem.
Soc. 2002, 149, A1460-A1465.
(33) Wrackmeyer, B. Annual Reports on NMR Spectroscopy; Webb,
G. A., Ed.; Academic Press: London, UK, 1988; Vol. 20.
(34) Sundararaman, A.; Jaekle, F. J. Organomet. Chem. 2003, 681,
134-142.
(32) Naumann, D.; Butler, H.; Gnann, R. Z. Anorg. Allg. Chem. 1992,
618, 74-76.

From B(C6F5)3 to B(OC6F5)3
Organometallics, Vol. 24, No. 7, 2005 1687
Figure 2. VT-19F NMR of (C6F5)2BOC6F5 (2) in CD2Cl2.
The 19F NMR chemical shift values for the C6F5
fluorine atoms are generally found at higher frequency
compared to the OC6F5 fluorine atoms and cover a larger
shift range (ca. 30 ppm versus 5 ppm). For compounds
2 and 3, two sets of ortho-, meta-, and para-fluorine
signals for the C6F5 and OC6F5 groups are observed,
indicating free rotation around all B-C and B-O bonds
at room temperature. Restricted rotation around B-O
bonds has been observed previously, and activation
barriers have been determined using a variety of
cence temperature Tc results in freezing of the B-O
bond rotation, and the rate of the interconversion of the
two isomers, as shown in eq 4, will be slower than the
NMR time scale. The two C6F5 groups become non-
equivalent, giving rise to separate chemical shifts for
the ortho, meta, and para fluorine atoms of these groups.
The activation barrier for this process ∆Gq ) 35 kJ/mol
(calculated from Tc ) 193 K and δν(ortho-F) ) 450 Hz
at 183 K)35 is the lowest value observed to date for a
B-O bond rotation, indicating a rather weak pπ-pπ
interaction and negligible B-O double-bond character
in 2. A VT 19F NMR investigation of C6F5B(OC6F5)2, 3,
did not show any coalescence down to 183 K in CD2Cl2.
The activation barrier is likely to be even lower in this
case, as the two oxygen donors, already weakened by
the electron-withdrawing pentafluorophenyl substitu-
ents, will now have to compete for donation to the boron
center, resulting in an even further weakening of the
B-O double-bond character. A much lower barrier was
also observed for MesB(OMe)2, compared to Mes2-
BOMe.20
1
techniques.35 Mislow and co-workers used dynamic H
NMR to determine activation barriers of ∆Gq ) 54-58
kJ/mol for alkyl esters of diarylborinic acids Aryl2BOR
[Aryl ) Mes (2,4,6-trimethylphenyl) or anthracenyl].20
These values have been supported by theoretical cal-
culations on H2BOH21,36,37 and by dynamic 13C NMR
spectroscopy studies on Aryl2BOR (where R ) alkyl and
aryl).38 More recently, dynamic 19F NMR has been used
to measure the activation barrier in (C6F5)2BOH, and a
barrier for rotation around the B-O bond of ∆Gq ) 39
kJ/mol was determined.22
The Lewis acidity of the boron center in compounds
2-4 will be affected by pπ-pπ interactions between the
lone pairs on the oxygen atoms and the empty pz orbital
on boron. On the other hand, the electron-withdrawing
C6F5 substituents on oxygen will decrease the electron
density at oxygen. To determine the B-O bond order
in (C6F5)2BOC6F5, 2, we have carried out a VT 19F NMR
study in CD2Cl2, which is shown in Figure 2. It can be
seen that free rotation around the B-O bond occurs
down to 193 K. Cooling the sample beyond this coales-
Solid State Structures. Single-crystal X-ray struc-
ture determinations of compounds 2, 3, and 4 (Figures
3, 5, and 7, respectively) showed in each case the central
boron atom to have a slightly distorted trigonal planar
geometry, the boron atom lying only ca. 0.03, 0.01, and
0.03 Å out of the plane of its substituents in 2, 3, and
4, respectively. As discussed in the previous section, the
(35) Oki, M. Applications of Dynamic NMR Spectroscopy to Organic
Chemistry; VCH: Deerfield Beach, FL, 1985; Vol. 4.
(36) Gropen, O.; Johansen, R. J. Mol. Struct. 1975, 25, 161-167.
(37) Dill, J. D.; Schleyer, P. v. R.; Pople, J. A. J. Am. Chem. Soc.
1975, 97, 3402-3409.
(38) Brown, N. M. D.; Davidson, F.; Wilson, J. W. J. Organomet.
Chem. 1981, 210, 1-8.

1688 Organometallics, Vol. 24, No. 7, 2005
Britovsek et al.
Figure 3. Molecular structure of (C6F5)2BOC6F5 (2).
Selected bond lengths (Å) and angles (deg): B-O
1.358(6), B-C(7) 1.587(7), B-C(13) 1.573(7), O-C(1)
1.392(5), O-B-C(7) 123.4(4), O-B-C(13) 113.9(4), C(7)-
B-C(13) 122.6(4), B-O-C(1) 126.6(4).
Figure 4. Part of the 2-D sheet of molecules linked by
F‚‚‚π interactions present in the crystals of (C6F5)2BOC6F5
(2).
Lewis acidity of boron compounds containing B-O
bonds can be reduced due to pπ-pπ donation from
oxygen to boron facilitated by a coplanar arrangement
of the substituents at the two centers. In 2, however,
the B-O torsion angle of ca. 19° is relatively large,
indicating a lesser degree of pπ-pπ donation, probably
due to the electron-withdrawing effects of the penta-
fluorophenyl substituents. In 3, while the B-O(1)
torsion angle is small (ca. 5°), that for the B-O(2) bond
is ca. 10°. In 4, with three OC6F5 units, one of the B-O
torsion angles [B-O(2) ca. 21°] is markedly larger than
the other two [B-O(1) ca. 5°, B-O(3) ca. 7°]. It is
noticeable that this correlates with an apparently longer
B-O(2) bond length [1.371(9) Å], cf. those to O(1) and
O(3) [1.357(10) and 1.346(9) Å, respectively], but this
must be treated with caution as the relatively high esd’s
mean that the differences are not statistically signifi-
cant. In the related complexes (Mes)2BOMe and
(Mes)2BSMe, the B-O and B-S torsion angles are ca.
5° and 4°, respectively.21 In both 2 and 3, all three C6F5
ring systems are inclined in the same sense (i.e.,
propeller-like) with respect to the BX3 plane (by ca. 65°,
40°, 38° and ca. 63°, 75°, 49° for the C(1)-, C(7)-, and
C(13)-based rings in 2 and 3, respectively). In 4,
however, the C(1)-based ring is inclined in the opposite
sense to the other two, the inclination angles being ca.
104°, 77°, and 52° for the C(1)-, C(7)-, and C(13)-based
rings, respectively. In (Mes)2BOMe and (Mes)2BSMe
the inclinations of the mesityl rings range between ca.
50° and 68°.21
Figure 5. Molecular structure of C6F5B(OC6F5)2 (3).
Selected bond lengths (Å) and angles (deg): B-O(1)
1.371(4), B-O(2) 1.367(4), B-C(13) 1.561(4), O(1)-C(1)
1.379(3), O(2)-C(7) 1.381(4), O(1)-B-O(2) 116.9(3), O(1)-
B-C(13) 126.0(3), O(2)-B-C(13) 117.1(3), B-O(1)-C(1)
125.0(2), B-O(2)-C(7) 123.0(2).
Adjacent molecules along the a and b axis directions
in the crystals of 2 are linked to form a sheet by F‚‚‚π
interactions between ortho and meta fluorines on the
C(1) ring and the π-system of the C(7) ring (Figure 4).
Such F‚‚‚π interactions are not uncommon, and their
occurrence has been recently reviewed.39 The F‚‚‚ring
centroid separations are ca. 3.16 and 3.11 Å for interac-
tions a and b, respectively, with associated C-F‚‚‚π
angles of ca. 155° and 130°. The F‚‚‚π vectors are
inclined by ca. 77° and 76° to the plane of the aromatic
ring (for a and b, respectively), and the two vectors
subtend an angle of ca. 154° at the ring centroid.
The dominant feature of the packing in the solid state
structure of 3 is a π‚‚‚π stacking interaction (or a double
Figure 6. Portion of the 3-D network of F‚‚‚π and π‚‚‚π
linked molecules present in the crystals of C6F5B(OC6F5)2
(3).
F‚‚‚π interaction) between the C(1)-based ring and its
centrosymmetrically related counterpart (interaction a
in Figure 6). The centroid‚‚‚centroid and mean inter-
planar separations are ca. 4.15 and 3.33 Å, respectively,
(39) Prasanna, M. D.; Guru Row, T. N. Cryst. Eng. 2000, 3, 135-
154.

From B(C6F5)3 to B(OC6F5)3
Organometallics, Vol. 24, No. 7, 2005 1689
and, as was seen for the π‚‚‚π interaction in 3, one of
the fluorine atoms [F(6)] on one ring is placed almost
directly above the centroid of the other ring, and vice
versa (F‚‚‚centroid 3.50 Å, vector inclined by ca. 84° to
the ring plane).
The low barrier for rotation around the B-O bond
determined by 19F NMR for compounds 2 and 3,
combined with the extensive F‚‚‚π interactions seen in
the solid state structures, indicates that the observed
and rather random torsion angles in compounds 2-4
are probably not related to pπ-pπ interactions between
B and O, but simply a result of crystal packing.39
Trends in Relative Lewis Acidity. Since Lewis
first proposed his acid-base theory in 1923, many
attempts have been made to quantify Lewis acidity, for
example by correlating Lewis acid strength with ther-
modynamic data,40-43 chemical reactivity,44,45 or spec-
troscopic data.46,47 To evaluate the relative Lewis acid
strength of the series of pentafluorophenyl boron com-
pounds 1-4, we have chosen two NMR spectroscopic
methods, which have been previously employed by
others. The first method is based on Gutmann’s clas-
sification of solvents by acceptor numbers,48,49 which
measures the change in 31P NMR chemical shift (∆δ)
between Et3PO and its adduct. This method has been
modified by Beckett for Lewis acids.4,18,50 We have
changed the solvent in this method from thf to benzene.
The same changes in chemical shift (∆δ) were observed
in both solvents and also in CDCl3. Thf was avoided
because slow polymerization of thf was observed with
some Lewis acids.51 The second method, developed by
Childs, measures the change in 1H NMR chemical shift
of the proton H3 in crotonaldehyde (CA) upon binding
of a Lewis acid to the carbonyl oxygen atom.52 Our
results are summarized in Table 2 and Figure 9. For
comparison, we have also included two nonfluorinated
boron compounds, triphenylborane B(C6H5)3 and triph-
enylborate B(OC6H5)3, in our evaluation.
Figure 7. Molecular structure of B(OC6F5)3 (4). Selected
bond lengths (Å) and angles (deg): B-O(1) 1.357(10),
B-O(2) 1.371(9), B-O(3) 1.346(9), O(1)-C(1) 1.386(7),
O(2)-C(7) 1.372(8), O(3)-C(13) 1.350(8), O(1)-B-O(2)
118.7(6), O(1)-B-O(3) 120.7(6), O(2)-B-O(3) 120.5(7),
B-O(1)-C(1) 122.1(5), B-O(2)-C(7) 122.4(5), B-O(3)-
C(13) 126.3(5).
The difference in chemical shift (∆δ 31P NMR) ob-
served upon reaction of Et3PO with a Lewis acid (Figure
9, left-hand Y-axis) increases in the order 1 < 2 < 3 <
4. The strongest interaction is observed for B(OC6F5)3
(4), indicating that this is the strongest Lewis acid in
the case of Et3PO. A similar trend is observed for
Ph3PO as the reference base,53 albeit with an overall
Figure 8. F‚‚‚π and π‚‚‚π interactions linking to neighbor-
ing molecules in the crystals of B(OC6F5)3 (4).
and the overlap is such that the F(6) fluorine atom of
one ring is placed almost directly above the centroid of
the other and vice versa (F‚‚‚centroid 3.30 Å, vector
inclined by ca. 86° to the ring plane). This interaction
is supplemented by three relatively weak F‚‚‚π contacts
(b, c, and d in Figure 6) with F‚‚‚π distances (Å) and
C-F‚‚‚π angles (deg) of b 3.25, 130; c 3.35, 163; d 3.31,
145. The F‚‚‚π vectors are inclined by ca. 74°, 68°, and
77° to their associated ring planes for b, c, and d,
respectively. Interactions a and c subtended an angle
of ca. 144° at the C(1) ring centroid.
In 4, neighboring molecules are linked by a combina-
tion of F‚‚‚π and π‚‚‚π interactions (Figure 8) to form a
complex 3-D network. The F‚‚‚π interaction a has an
F‚‚‚ring centroid separation of ca. 3.17 Å and a
C-F‚‚‚π angle of ca. 164°; the F‚‚‚π vector is inclined
by ca. 72° to the plane of the π system. For interaction
b the respective values are 3.23 Å, 143°, and 82°. The
π‚‚‚π or double F‚‚‚π contact c has a centroid‚‚‚centroid
distance of ca. 4.40 Å with a mean interplanar separa-
tion of ca. 3.49 Å. Being related to each other by a center
of symmetry, the two ring systems are perfectly parallel
(40) Childs, R. F.; Mulholland, D. L.; Nixon, A. Can. J. Chem. 1982,
60, 809-812.
(41) Fenwick, J. T. F.; Wilson, J. W. Inorg. Chem. 1975, 14, 1602-
1604.
(42) Luo, L.; Marks, T. J. Top. Catal. 1999, 7, 97-106.
(43) Metz, M. V.; Schwartz, D. J.; Stern, C.; Marks, T. J.; Nickias,
P. N. Organometallics 2002, 21, 4159-4168.
(44) Olah, G. A.; Kobayashi, S.; Tashiro, M. J. Am. Chem. Soc. 1972,
94, 7448-7461.
(45) Kobayashi, S.; Busujima, T.; Nagayama, S. Chem. Eur. J. 2000,
6, 3491-3494.
(46) Lappert, M. F. J. Chem. Soc. 1962, 542-548.
(47) Branch, C. S.; Bott, S. G.; Barron, A. R. J. Organomet. Chem.
2003, 666, 23-34.
(48) Mayer, U.; Gutmann, V.; Gerger, W. Monatsh. Chem. 1975, 106,
1235-1257.
(49) Gutmann, V. Coord. Chem. Rev. 1976, 18, 225-255.
(50) Beckett, M. A.; Strickland, G. C.; Holland, J. R.; Varma, K. S.
Polym. Commun. 1996, 37, 4629-4631.
(51) Olah, G. A.; Farooq, O.; Li, C. X.; Farnia, M. A. M. F.; Aklonis,
J. J. J. Appl. Polym. Sci. 1992, 45, 1355-1360.
(52) Childs, R. F.; Mulholland, D. L.; Nixon, A. Can. J. Chem. 1982,
60, 801-808.
(53) Burford, N.; Royan, B. W.; Spence, R. E. v. H.; Cameron, S.;
Linden, A.; Rogers, R. D. J. Chem. Soc., Dalton Trans. 1990, 1521-
1528.

1690 Organometallics, Vol. 24, No. 7, 2005
Britovsek et al.
Table 2. NMR Data for Lewis Acid-Base Adducts between 1-4 and R3PX (R ) Et, Ph; X ) O, S)
Et3PO
Et3PO
Et3PO
Ph3PO
Ph3PO
Et3PS
Et3PS
Et3PS
CA
CA
Lewis acid
31P ppma
∆δ ppma
11B ppma
31P ppma
∆δ ppma
31P ppma
∆δ ppma
11B ppma
1H ppmb
∆δ ppmb
none
46.4
65.9
69.4
76.6
80.0
80.5
80.9
25.5
25.5
27.0
45.8
46.1
46.2
46.6
53.6
53.6
53.6
57.8
53.6
53.6
53.6
6.85
6.90
6.88
7.90
7.85
7.35
7.25
BPh3
19.6
23.0
30.2
33.6
34.1
34.5
0
0
0.05
0.03
1.05
1.00
0.50
0.40
B(OPh)3
1.5
0
1
2
3
4
-2.8
3.5
20.3
20.6
20.7
21.1
4.2
0
0
0
0.0
41.4
2.6
0.0
a In C6D6 at room temperature. b In CD2Cl2 at room temperature.
reference bases were also seen by Graham and Stone55
and more recently by Marks and Luo.42 The observation
of different orders can be rationalized in terms of
Pearson’s hard soft acid base (HSAB) classification,56
whereby the hardness or softness of a particular Lewis
acidic or Lewis basic center is affected by the nature of
the other atoms attached to it and the type of bonding
between them. BF3 is generally regarded as a harder
acid than BH3, because the B-F bond is more ionic in
character, whereas the B-H bond is more covalent. For
the same reasons, B-O bonds are more ionic than B-C
bonds, and therefore B(OC6F5)3 will be a harder Lewis
acid than B(C6F5)3. The oxygen atom in crotonaldehyde
is attached to a carbon atom, and the CdO pπ-pπ
double bond is largely covalent in character. The PdO
double bond in R3PdO is a pπ-dπ bond, which is much
more ionic in character, making the oxygen atom harder
in this case. The combination of a hard Lewis acid with
a hard Lewis base, or a soft Lewis acid with a soft Lewis
base, results in a stronger interaction than the mixed
hard-soft interaction.
In conclusion, compared to B(C6F5)3 (1), the penta-
fluorophenyl boron compounds 2, 3, and 4 are progres-
sively harder Lewis acids, which form increasingly
stronger interactions with a hard Lewis base such as
Et3PO, whereas the interaction with softer Lewis bases
such as Et3PS or crotonaldehyde is strongest in the case
of B(C6F5)3 (1). VT NMR studies have shown that there
is no significant pπ-pπ interaction between B and O in
compounds 2-4, resulting in free rotation around the
B-O bond at room temperature. In the solid state
structures of 2-4 extensive intermolecular F‚‚‚π inter-
actions are observed. We are currently exploring ap-
plications of these new Lewis acids in synthesis and
catalysis.
Figure 9. Evaluation of relative Lewis acidities.
lowermagnitude of ∆δ, which could be due to steric
factors or to the difference in basicity. The much softer
base Et3PS, which is sterically comparable to Et3PO,
shows only a weak interaction with B(C6F5)3 (1) and no
interaction with the Lewis acids 2-4. Noteworthy, it
has been reported that the soft base PH3 interacts
strongly with B(C6F5)3, but not with B(OC6F5)3.11 The
formation of Lewis acid-base adducts between Et3PO
and compounds 1-4 and between Et3PS and 1 is
confirmed by the changes in the 11B NMR spectra. In
all cases a chemical shift value around 0 ppm is
observed, characteristic for tetracoordinate boron com-
pounds of this type.4,10,33
The evaluation of the Lewis acidity by Childs’ method,
using crotonaldehyde as the reference base, resulted in
a different trend (Figure 9, right-hand Y-axis). In this
case, the largest difference in chemical shift (∆δ 1H
NMR) is seen for B(C6F5)3 (1) and the relative order
is reversed: 1 > 2 > 3 > 4. As might be expected,
the nonfluorinated boron compounds B(C6H5)3 and
B(OC6H5)3, which are much less Lewis acidic, show
much weaker interactions. Although analogous trends
are seen for these Lewis acids in both methods, the
observed changes in chemical shift are too small to draw
any firm conclusions.
A linear correlation between Gutmann’s method and
Child’s method was previously observed for a selection
of Lewis acids.4 From our results, we suggest that such
a linear correlation does not strictly apply to all Lewis
acids, but that different bases can give different trends.
Drago and Matwiyoff stated already in 1968 that, “Any
order of donor or acceptor strengths must be established
relative to a given donor or acceptor. Reversals may be
expected when orders toward different donors (or ac-
ceptors) are compared.”54 The results obtained here for
the Lewis acid series 1-4, when compared with differ-
ent Lewis bases, show that this is indeed the case.
Different trends in Lewis acidity when using different
Experimental Section
General Procedures. All moisture-sensitive compounds
were manipulated using standard vacuum line, Schlenk, or
cannula techniques, or in a conventional nitrogen-filled glove-
box. 1H, 19F, 31P, and 11B NMR spectra were recorded on a
Bruker AC-250 or a JEOL JNM-EX270 spectrometer; chemical
shifts for 1H NMR are referenced to the residual protio
impurity of the deuterated solvent. 19F, 31P, and 11B chemical
shifts are reported relative to CFCl3, H3PO4 (85%), and BF3‚
OEt2, respectively.
Solvents and Reagents. Pentane was dried by passing
through a column, filled with commercially available Q-5
reagent (13 wt % CuO on alumina) and activated alumina
(pellets, 3 mm). Diethyl ether and tetrahydrofuran were dried
over sodium metal with a benzophenone ketyl indicator,
(55) Graham, W. A. G.; Stone, F. G. A. J. Inorg. Nucl. Chem. 1956,
3, 164-177.
(56) Pearson, R. G. J. Am. Chem. Soc. 1963, 85, 3533-3539.
(54) Drago, R. S.; Matwiyoff, N. A. Acids and Bases; Ratheon
Education: Lexington, MA, 1968; p 64.

From B(C6F5)3 to B(OC6F5)3
Organometallics, Vol. 24, No. 7, 2005 1691
group -157.3 (4F, o), -159.7 (2F, p), -161.9 (4F, m) ppm. 11
NMR (C6D6, 83 MHz): 26.2 ppm. Anal. Calcd for C18BF15O2:
C, 39.74. Found: C, 39.84.
whereas dichloromethane and acetonitrile were dried over
CaH2. Pentafluorophenol was dried over molecular sieves (4
Å) and distilled prior to use. The syntheses of (C6F5)2BCl,29
nBu3SnC6F5,57 Et3PS,58 and B(OC6F5)3 (4)32 have been reported
previously. Crystals of 4 suitable for X-ray diffraction were
grown from a pentane solution at -20 °C. All other chemicals
and NMR solvents were obtained commercially and used as
received. Elemental analyses were performed by the Science
Technical Support Unit at London Metropolitan University.
Pentafluorophenyl Boron Dichloride, C6F5BCl2.
nBu3SnC6F5 (17.56 g, 38.42 mmol) was placed in a 50 mL glass
ampule containing a magnetic stirrer bar. The ampule was
evacuated and sealed, and it was placed in a NaCl/ice bath at
-10 °C. After 30 min, BCl3 (6.75 g, 57.62 mmol) was condensed
into the flask. The clear yellow solution was left stirring at
-10 °C for 1 h and then at room temperature overnight. The
next day, the ampule was put again in a NaCl/ice bath at -10
°C and opened to dynamic vacuum for 30 min, to remove the
excess BCl3. The product was subsequently distilled at 40 °C/
0.05 mbar. Yield ) 2.6 g (27%). Spectroscopic data were
identical to the reported data.34
Bis(pentafluorophenyl)borinic Acid Pentafluoro-
phenyl Ester, (C6F5)2BOC6F5 (2). C6F5OH (0.63 g, 3.4
mmol) and (C6F5)2BCl (1.31 g, 3.4 mmol) were placed in two
separate Schlenk flasks and dissolved in 15 mL of di-
chloromethane. The flask containing the (C6F5)2BCl solution
was placed in an ice bath at 0 °C. The C6F5OH solution was
added dropwise via cannula. When the addition was complete,
the mixture was left at 0 °C for 30 min and then allowed to
warm to room temperature, at which it was left stirring for 2
h. The solvent was removed by vacuum, leaving an off-white
solid. The compound was purified by sublimation at 90 °C/0.1
mbar. Yield ) 64%. Crystals suitable for X-ray diffraction were
grown from a pentane solution at -20 °C. 19F NMR (C6D6, 235
MHz): C6F5 group -131.1 (o, 4F) -144.1 (p, 2F) -159.7 (m,
4F) ppm; OC6F5 group -157.8 (o, 2F) -159.3 (p, F) -161.4
(m, 2F) ppm. 11B NMR (C6D6, 83 MHz): 41.2 ppm. Anal. Calcd
for C18BF15O: C, 40.95. Found: C, 40.91.
Pentafluorophenylboronic Acid Bis(pentafluoro-
phenyl) Ester, C6F5B(OC6F5)2 (3). C6F5BCl2 (2.6 g, 10.45
mmol) and C6F5OH (3.85 g, 20.9 mmol) were placed under N2
in two Schlenk flasks and dissolved in 20 mL of dry CH2Cl2.
The C6F5BCl2 solution was placed in an ice bath at 0 °C, and
the pentafluorophenol solution was added dropwise via can-
nula. The clear, colorless solution was left stirring overnight
at room temperature. The day after, the solvent was removed
by high vacuum, leaving an off-white solid, which was purified
by sublimation at 90 °C/0.1 mbar. Yield ) 62%. Crystals
suitable for X-ray diffraction were grown from a pentane
solution at -20 °C. 19F NMR (C6D6, 235 MHz): C6F5 group
-131.1 (2F, o), -145.2 (1F, p), -159.0 (2F, m) ppm; OC6F5
B
Structure Determinations. Crystal data for 2:
C18BF15O, M ) 527.99, triclinic, P1h (no. 2), a ) 6.6111(16) Å,
b ) 8.6330(19) Å, c ) 15.917(4) Å, R ) 83.18(2)°, â ) 78.31-
(3)°, γ ) 89.39(2)°, V ) 883.2(3) Å3, Z ) 2, Dc ) 1.985 g cm-3
,
µ(Mo KR) ) 0.231 mm-1, T ) 173 K, colorless thin plates; 3071
independent measured reflections, F2 refinement, R1 ) 0.067,
wR2 ) 0.161, 1881 independent observed absorption-corrected
reflections [|Fo| > 4σ(|Fo|), 2θmax ) 50°], 317 parameters. CCDC
255626.
Crystal data for 3: C18BF15O2, M ) 543.99, triclinic, P1h
(no. 2), a ) 6.0687(13) Å, b ) 10.5527(19) Å, c ) 14.722(3) Å,
R ) 83.699(15)°, â ) 84.364(18)°, γ ) 79.853(11)°, V ) 919.5-
(3) Å3, Z ) 2, Dc ) 1.965 g cm-3, µ(Mo KR) ) 0.229 mm-1, T )
203 K, colorless plates; 3249 independent measured reflections,
F2 refinement, R1 ) 0.044, wR2 ) 0.090, 2043 independent
observed absorption-corrected reflections [|Fo| > 4σ(|Fo|), 2θmax
) 50°], 326 parameters. CCDC 255627.
Crystal data for 4: C18BF15O3, M ) 559.99, monoclinic,
P21/c (no. 14), a ) 16.611(5) Å, b ) 5.618(6) Å, c ) 20.090(7)
Å, â ) 93.620(19)°, V ) 1871(2) Å3, Z ) 4, Dc ) 1.988 g cm-3
,
µ(Mo KR) ) 0.232 mm-1, T ) 203 K, colorless needles; 3276
independent measured reflections, F2 refinement, R1 ) 0.087,
wR2 ) 0.224, 1645 independent observed absorption-corrected
reflections [|Fo| > 4σ(|Fo|), 2θmax ) 50°], 335 parameters. CCDC
255628.
Determination of Lewis Acidity. Gutmann’s Method.
This method was performed as described by Beckett et al.,50
except that C6D6 was used as the solvent rather than tetra-
hydrofuran, as slow polymerization of thf was observed with
certain Lewis acids. The Lewis acid and the phosphine oxide
(or phosphine sulfide) were placed together in 1:1 ratio in a
small glass vial and dissolved in the minimum amount of dry
C6D6. The solution was placed in an NMR tube, and the 31P
NMR chemical shift was recorded at room temperature.
Childs’ Method. This method was performed as described
by Childs et al.,52 except that the NMR measurement was
carried out at room temperature rather than at -20 °C. The
Lewis acid and crotonaldehyde were mixed together in a glass
vial and dissolved in CD2Cl2. The resulting mixture was then
1
placed into an NMR tube, and the H NMR chemical shift of
the H3 proton of crotonaldehyde was recorded.
Acknowledgment. We thank the EPSRC for fund-
ing (GR/R92042/01). Mr. Richard Sheppard and Mr.
Peter Haycock are thanked for VT-NMR measurements.
Supporting Information Available: For 2-4, figures
giving molecular structures and ClF files giving crystal-
lographic data. This material is available free of charge via
the Internet at http://pubs.acs.org.
(57) Deacon, G. B.; Gatehouse, B. M.; Nelson-Reed, K. T. J.
Organomet. Chem. 1989, 359, 267-283.
(58) Issleib, K.; Brack, A. Z. Anorg. Allg. Chem. 1954, 277, 258-
270.
OM049091P