Oxygen-bridged borate anions from tris(pentafluorophenyl)borane: Synthesis, NMR characterization, and reactivity

Article abstract of DOI:10.1021/ic0502168

The previously known anion [(C₆F₅) ₃B(μ-OH)B(C₆F₅)₃]^- (2) has been prepared by a two-step procedure, involving deprotonation of (C ₆F₅)₃BOH₂ to give [B(C ₆F₅)₃OH]^- (1), followed by addition of B(C₆F₅)₃. The solution structure and the dynamics of 2 have been investigated by ¹H and ¹⁹F NMR spectroscopy. The reaction of [NHEt₃]2 with NEt₃ resulted in the formation of [NHEt₃]⁺[(C₆F ₅)₃BOH]^-, [NHEt₃] ⁺[(C₆F₅)₃BH]^-, and (C₆F₅)₃B^-(CH₂CH=N ⁺Et₂). This indicates that in the presence of a nucleophile anion 2 can dissociate to B(C₆F₅)₃ and 1. The reaction of [HDMAN]2 with 1,8-bis(dimethylamino)naphthalene (DMAN) confirmed this trend. In the presence of water, 2 transformed into the adduct [(C₆F₅)₃BO(H)H...O(H)B(C₆F ₅)₃]^- (3), containing the borate 1 hydrogen-bonded to a water molecule coordinated to B(C₆F ₅)₃. The same compound is formed by treating (C ₆F₅)₃BOH₂ with 0.5 equiv of a base. A competition study established that for 1 the Lewis acid-base interaction with B(C₆F₅)₃ is about 5 times preferred over H-bonding to (C₆F₅)₃BOH₂. The X-ray single-crystal analysis of [2-methyl-3H-indolium]3 provided the first experimental observation of an asymmetric H-bond in the [H₃O ₂]^- moiety, the measured O-H and H...O bond distances being significantly different [1.14(2) vs 1.26(2) A]. The reaction of NEt₃ with an equimolar mixture of B(C₆F₅) ₃ and bis(pentafluorophenyl)borinic acid, (C₆F ₅)₂BOH, afforded the novel borinatoborate salt [NHEt ₃]⁺[(C₆F₅)₃BOB(C ₆F₅)₂]^- ([NHEt₃]4). X-ray diffraction showed that the B-O bond distances are significantly shorter than in [(C₆F₅)₃B(μ-OH)B(C₆F ₅)₃]^-. Variable-temperature ¹⁹F NMR revealed high mobility of the five aryl rings, at variance with the more crowded anion 2. 2D NMR correlation experiments showed that in CD₂Cl ₂ the two anions [(C₆F₅)₃BOH] ^- and [(C₆F₅)₃BH]^- form tight ion pairs with [NHEt₃]⁺, in which the NH proton establishes a conventional (BO...HN) or an unconventional (BH...HN), respectively, hydrogen bond with the anion. The diborate anions 2-4, on the contrary, gave loose ion pairs with the ammonium cation, due both to the delocalized anionic charge and to the more sterically encumbered position of the oxygen atoms that should act as H-bond acceptors.

Full text of DOI:10.1021/ic0502168

Inorg. Chem. 2005, 44, 5030−5041
Oxygen-Bridged Borate Anions from Tris(pentafluorophenyl)borane:
Synthesis, NMR Characterization, and Reactivity
Alessia Di Saverio,† Francesca Focante,† Isabella Camurati,† Luigi Resconi,*,† Tiziana Beringhelli,‡
Giuseppe D’Alfonso,*,‡ Daniela Donghi,‡ Daniela Maggioni,‡ Pierluigi Mercandelli,*,§ and
Angelo Sironi§
Basell Polyolefins Italia, P. le Donegani 12, 44100 Ferrara, and Dipartimento di Chimica
Inorganica, Metallorganica e Analitica, Facolta` di Farmacia, and Dipartimento di Chimica
Strutturale e Stereochimica Inorganica, UniVersita` degli Studi di Milano, Via Venezian 21,
20133 Milano, Italy
Received February 9, 2005
The previously known anion [(C6F5)3B(µ
-OH)B(C6F5)3]- (2) has been prepared by a two-step procedure, involving
deprotonation of (C6F5)3BOH2 to give [B(C6F5)3OH]- (1), followed by addition of B(C6F5)3. The solution structure
and the dynamics of 2 have been investigated by 1H and 19F NMR spectroscopy. The reaction of [NHEt3]2 with
NEt3 resulted in the formation of [NHEt3]+[(C6F5)3BOH]-, [NHEt3]+[(C6F5)3BH]-, and (C6F5)3B-(CH2CH N+Et2). This
d
indicates that in the presence of a nucleophile anion 2 can dissociate to B(C6F5)3 and 1. The reaction of [HDMAN]2
with 1,8-bis(dimethylamino)naphthalene (DMAN) confirmed this trend. In the presence of water, 2 transformed into
the adduct [(C6F5)3BO(H)H‚‚‚O(H)B(C6F5)3]- (3), containing the borate 1 hydrogen-bonded to a water molecule
coordinated to B(C6F5)3. The same compound is formed by treating (C6F5)3BOH2 with 0.5 equiv of a base. A
competition study established that for 1 the Lewis acid
over H-bonding to (C6F5)3BOH2. The X-ray single-crystal analysis of [2-methyl-3H-indolium]3 provided the first
experimental observation of an asymmetric H-bond in the [H3O2]- moiety, the measured O
H and H‚‚‚O bond
−base interaction with B(C6F5)3 is about 5 times preferred
−
distances being significantly different [1.14(2) vs 1.26(2) Å]. The reaction of NEt3 with an equimolar mixture of
B(C6F5)3andbis(pentafluorophenyl)borinicacid,(C6F5)2BOH,affordedthenovelborinatoboratesalt[NHEt3]+[(C6F5)3BOB(C6F5)2]-
([NHEt3]4). X-ray diffraction showed that the B−O bond distances are significantly shorter than in [(C6F5)3B(µ-
OH)B(C6F5)3]-. Variable-temperature 19F NMR revealed high mobility of the five aryl rings, at variance with the
more crowded anion 2. 2D NMR correlation experiments showed that in CD2Cl2 the two anions [(C6F5)3BOH]- and
[(C6F5)3BH]- form tight ion pairs with [NHEt3]+, in which the NH proton establishes a conventional (BO‚‚‚HN) or an
unconventional (BH‚‚‚HN), respectively, hydrogen bond with the anion. The diborate anions 2−4, on the contrary,
gave loose ion pairs with the ammonium cation, due both to the delocalized anionic charge and to the more
sterically encumbered position of the oxygen atoms that should act as H-bond acceptors.
Introduction
been devoted to the synthesis of delocalized polyborate
anions, where two (or more) tris(pentafluorophenyl)borane
molecules are coordinated to linking units such as pyra-
Perfluorinated aryl boron and borate compounds are being
widely studied as stoichiometric activators of single-center
olefin polymerization catalysts.1 Recently, some attention has
- 3
zolate,2 NH2 , CN-,4 or [Ni(CN)4]2-.4 In these anions the
negative charge is delocalized over more than one B atom,
* Authors to whom correspondence should be addressed. E-mail:
luigi.resconi@basell.com (L.R.); giuseppe.dalfonso@unimi.it (G.D.);
pierluigi.mercandelli@unimi.it (P.M.).
(1) (a) Chen, E. Y.-X.; Marks, T. J. Chem. Soc. ReV. 2000, 100, 1391.
(b) Krossing, I.; Raabe, I. Angew. Chem., Int. Ed. 2004, 43, 2066. (c)
Piers, W. E. AdV. Organomet. Chem. 2005, 52, 1.
(2) LaPointe, R. E.; Roof, G. R.; Abboud, K. A.; Klosin, J. J. Am. Chem.
Soc. 2000, 122, 9560.
(3) Lancaster, S. J.; Rodriguez, A.; Lara-Sanchez, A.; Hannant, M. D.;
Walker, D. A.; Hughes, D. H.; Bochmann, M. Organometallics 2002,
21, 451.
† Basell Polyolefins Italia.
‡ Dipartimento di Chimica Inorganica, Metallorganica e Analitica, Facolta`
di Farmacia, Universita` degli Studi di Milano.
§ Dipartimento di Chimica Strutturale e Stereochimica Inorganica,
Universita` degli Studi di Milano.
5030 Inorganic Chemistry, Vol. 44, No. 14, 2005
10.1021/ic0502168 CCC: $30.25
© 2005 American Chemical Society
Published on Web 06/17/2005

Borate Anions from Tris(pentafluorophenyl)borane
+
Table 1. 1H NMR Data of the NHEt3 Salts of Anions 1-4 (CD2Cl2)
T (K)
δ(OH)
δ(NH)
δ(C2H5)
[NHEt3]1a
263
2.98
10.51 (s br)
2.84 (dq, CH2, 3JHH ) 7.0 Hz, 3JHH ) 4.9 Hz)
1.14 (t, CH3, 3JHH ) 7.0 Hz)
[NHEt3]2
[NHEt3]3
223
298
173
6.58 t (JHF ) 17 Hz)
8.49
17.91 (s, 1H), 4.54 (s, 2H)
5.04 (1JHN ) 51 Hz)
5.04 (1JHN ) 53 Hz, 3JHH ) 5.5 Hz)
5.14 (s br)
3.27 (dq, CH2, JHH ) 7.0 Hz) 1.34 (t CH3)
3.36 (dq, CH2, 3JHH ) 7.3 Hz, 3JHH ) 5.5 Hz)
1.47 (t, CH3, 3JHH ) 7.3 Hz)
3 0.28 (m, CH2)
1.84 (t, CH3, 3JHH ) 6.9 Hz)
[NHEt3]4
298
4.89 (1JHN ) 54 Hz, 3JHH ) 5.3 Hz)
3.36 (dq, CH2, 3JHH ) 7.3 Hz, 3JHH ) 5.3 Hz)
1.46 (t, CH3, 3JHH ) 7.3 Hz)
a Lit.28 1H NMR (CDCl3, room temperature): δ 11.04 (s, br, NH), 2.91 (q, 6H, NCH2), 2.47 (s, 1H, BOH), 1.21 (t, 9H, CH3).
+
Table 2. 19F NMR Data of the NHEt3 Salts of Anions 1, 2, 3 and 4 (CD2Cl2)
T (K)
ortho
para
meta
[B(C6F5)3OH]- (1)
223
-136.29
-160.27
-165.07
[(C6F5)3B(µ-OH)B(C6F5)3]- (2)
173a
-129.72 (A2)
-131.06 (A6)
-131.58 (B2)
-133.13 (C2)
-137.93(C6)
-140.31 (B6)
-136.30
-157.34 (A4)
-157.91 (C4)
-159.23 (B4)
-162.57 (C3)
-164.18 (C5)
-164.32 (B5)
-164.95 (A3)
165.20 (A5)
-165.57 (B3)
-164.61
[(C6F5)3BOH2‚‚‚O(H)B(C6F5)3]- (3)
[(C6F5)3BOB(C6F5)2]- (4)
173
253
-159.04
-154.37b
-161.55c
-134.11b
-163.49b
-134.68c
-166.64c
a In parentheses are given the attributions to ortho (2, 6), meta (3, 5), and para (4) positions in the C6F5 rings A-C. b Signal of the borinato moiety.
c Signal of the borate moiety.
providing improved stability and activation ability to the trityl
or ammonium salts of the complex anions. Following our
interest on the chemistry of B(C6F5)3 with donor molecules,5-7
and looking for even simpler molecules which could be
prepared from a neutral source of potentially acidic protons
(thus not requiring salt metathesis to generate the final
chemistry nor its solution structure were discussed in any
detail by the authors, the main subject of their works
concerning the chemistry of other species (the cationic
counterparts9,10 or the adducts of B(C6F5)3 with water).8
We have now prepared anion 2 as follows: the reaction
of the (C6F5)3BOH2 adduct with 1 equiv of NEt3 afforded
quantitatively the ion pair [NHEt3]+[B(C6F5)3OH]- (NHEt3‚
1), which has been spectroscopically characterized (eq 1,
L ) NEt3, Tables 1 and 2). Addition of 1 equiv of B(C6F5)3
to the borate anion 1 resulted in the formation of the
[NHEt3]+ salt of the dimeric anion [(C6F5)3B(µ-OH)B-
(C6F5)3]- (2), whose NMR data are reported in Tables 1 and
2 (eq 2). An analogous two-step procedure using the “Proton
Sponge” 1,8-bis(dimethylamino)naphthalene (DMAN) as the
base afforded the HDMAN+ salts of 1 and 2.
activator), we have investigated (or reinvestigated) the
-
synthesis and reactivity of [(C6F5)3B-O(H)x-B(C6F5)2+x
]
(x ) 0, 1) delocalized anions, and report here the first results
of such an investigation.
Results and Discussion
A. Synthesis and NMR Characterization of the Anion
[(C6F5)3B(µ-OH)B(C6F5)3]- (2). Such an anion was already
known in the literature, and it had been structurally charac-
terized with different counterions.8-10 In all cases its forma-
tion was caused by deprotonation of (C6F5)3BOH2,11 but no
synthetic procedure was reported. Moreover, neither its
(C6F5)3BOH2 + L a [HL]+[(C6F5)3BOH]-
(1)
[HL]+[B(C6F5)3OH]- + B(C6F5)3 a
[HL]+[(C6F5)3B(µ-OH)B(C6F5)3]- (2)
(4) Lancaster, S. J.; Walker, D. A.; Thornton-Pett, M.; Bochmann, M.
Chem. Commun. 1999, 1533.
(5) (a) 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. (b)
Bonazza, A.; Camurati, I.; Guidotti, S.; Mascellani, N.; Resconi, L.
Macromol. Chem. Phys. 2004, 205, 319. (c) Focante, F.; Camurati,
I.; Nanni, D.; Leardini, R.; Resconi, L. Organometallics 2004, 23,
5135.
The low-temperature 19F spectrum of anion 2 consists of
fifteen signals (three for the para and six each for the ortho
and meta fluorine atoms), indicating an idealized C2 sym-
metry. The 19F signals of the three unequivalent rings A-C
have been assigned by a 2D 19F COSY at 173 K (Figure
1a,b).
Dipolar and scalar correlations observed in 2D 19F-19F
and 1H-19F experiments led to the solution structure depicted
in Figure 2, which maintains the main features of the
previously reported solid-state structures. Relevant points for
establishing this conformation were the following. Strong
cross-peaks due to scalar “through-space” coupling between
A2 and B6, B2 and C2, and, slightly weaker, B6 and C6
(6) Beringhelli, T.; Maggioni, D.; D’Alfonso, G. Organometallics 2001,
20, 4927.
(7) (a) Maggioni, D.; Beringhelli, T.; D’Alfonso, G.; Resconi, L. J.
Organomet. Chem. 2005, 690, 640. (b) Beringhelli T.; D’Alfonso G.;
Maggioni, D.; Mercandelli P.; Sironi A. Chem.sEur. J. 2005, 11, 650.
(8) Danopoulos, A. A.; Galsworthy, J. R.; Green, M. L. H.; Cafferkey,
S.; Doerrer, L. H.; Hursthouse, M. B. Chem. Commun. 1998, 2529.
(9) Cowley, A. H.; Macdonald, C. L. B.; Silverman, J. S.; Gorden, J. D.;
Voigt, A. Chem. Commun. 2001, 175.
(10) Stender, M.; Phillips, A. D.; Power, P. P. Inorg. Chem. 2001, 40, 5314.
(11) The easy deprotonation of (C6F5)3B‚OH2 by bases is in line with the
strong acidity of the (C6F5)3B‚OH2 adduct.12
Inorganic Chemistry, Vol. 44, No. 14, 2005 5031

Di Saverio et al.
Figure 1. 2D 19F COSYGS (173 K, 7.1 T) of a CD2Cl2 solution of
[(C6F5)3BO(H)B(C6F5)3]- (2). The three aryl rings have been arbitrarily
labeled as A, B, and C. Panels a and b show the correlations for the
assignments; panel c shows the “through-space” scalar correlations in the
ortho region. The asterisks mark the signals of [(C6F5)3BO(H)H‚‚‚O(H)-
B(C6F5)3]- (3), present as a byproduct.
Figure 3. Low-field region of the variable-temperature 1H NMR spectra
of the [NHEt3]+ salt of anion 2 in CD2Cl2 solution. The room temperature
trace was acquired at 4.7 T. The multiplet at ca. 5 ppm is due to the NH
proton (see section E for the discussion of this signal).
ortho and meta positions within each ring. As a result of
this dynamics, on increasing the temperature, the OH signal
at 6.5 ppm changed its multiplicity (Figure 3) and passed
from a triplet (196 K, with JHF ) 17 Hz) to a 13-line13
multiplet, with averaged JHF ) ca. 3 Hz, indicative of the
coupling of the OH proton with all twelve ortho fluorines
in the six aryl rings. This pattern confirms that anion 2
contains a OH group bridging between two B(C6F5)3
molecules.
More detailed information on the nature of these dynamic
processes was provided by the correlations observed in the
meta region14 of a 19F NOESY/EXSY experiment at 183 K
(Figure 4), which showed the occurrence of two different
enantiomerization processes for anion 2. The first process
exchanges rings B with rings C (e.g., in the meta region
selective B3 T C3 and B5 T C5 interconversion, Figure
4b) through small oscillations occurring around their B-Cipso
interactions and simultaneous oscillations (in opposite direc-
tions) around the two B-O bonds. These small movements
do not break the four hydrogen-bond interactions C6‚‚‚HO
(C6′‚‚‚HO) and B6‚‚‚HO (B6′‚‚‚HO), but only cause their
interchange. Rings A just shift from one side to the other of
Figure 2. Solution structure of anion 2, as inferred from the NMR 2D
experiments.
were observed in the 2D 19F COSY at 173 K (Figure 1c).
Dipolar correlations between A2 and A6, A2 and C2, A6
and B2, and B6 and C6 were revealed by a 2D [19F-19F]
1
NOESY at the same temperature. Moreover, the H signal
of the BOH proton (δ 6.5) exhibited through-space coupling
with the two equivalent fluorine atoms C6 (δ -137.92) (see
Figure 3 and the 2D 19F-1H COSY shown in Figure S1 of
the Supporting Information) and dipolar correlations with
six ortho fluorine atoms, namely, A2, B6, and C6 (see the
2D 19F-1H HOESY shown in Figure S2 of the Supporting
Information).
Variable-temperature 1H and 19F spectra revealed the onset
of dynamic processes. At T > 210 K all the 19F signals
broadened and then coalesced, resulting at room temperature
in a single set of 19F signals (one ortho, one meta, and one
para resonance). This indicates the dynamic equivalence of
the three aryl rings bonded to each boron atom and of the
(12) Bergquist, C.; Bridgewater, B. M.; Harlan, C. J.; Norton, J. R. J. Am.
Chem. Soc. 2000, 122, 10581.
(13) Even if nine lines only were detectable in this multiplet, the experi-
mental intensity ratio between these components corresponded well
to the theoretical 0.015:0.18:1:3.3:7.5:12:14:12:7.5:3.3:1:0.18:0.015
ratio (Figure S3). The observed averaged JHF corresponded well to
the expected value, i.e., (17 × 2)/12.
(14) The nature of the exchange processes has been deduced mainly on
the basis of the correlations in the meta region, since the cross-peaks
in the ortho region were affected also by dipolar and scalar contribu-
tions, and the para region is intrinsically less informative.
5032 Inorganic Chemistry, Vol. 44, No. 14, 2005

Borate Anions from Tris(pentafluorophenyl)borane
Figure 4. Para and meta regions of a [19F-19F] EXSY experiment on [NHEt3]2 (CD2Cl2, 183 K, mixing time 0.1 s).
the BO(H)B plane (see A3 T A5 exchange), without any
oscillation around their B-Cipso bond.
Scheme 1
The second process exchanges rings A and B (selectively
A5 T B3 and A3 T B5, Figure 4b) by wider simultaneous
oscillations around the B-O bond and around all six B-Cipso
interactions, to restore the propeller-like conformation. These
oscillations cause the B6‚‚‚HO and B6′‚‚‚HO interactions
to be broken, while C6‚‚‚HO and C6′‚‚‚HO interactions are
maintained. Indeed, rings C just move from one side to the
other of the BO(H)B plane, maintaining their identity.
The A/C exchange peaks (A3 T C5, A5 T C3, the first
one partially overlapped with the A3 T B5 exchange cross-
peak, Figure 4b) are a consequence of the two above
processes (A T B T C). Indeed, the volume analysis of the
cross-peaks in the para and meta regions indicated that, while
the two processes exchanging B/C and A/B rings have similar
rates [kBC ) 0.71 s-1 from the para cross-peaks of Figure
4a and 0.73 s-1 from the meta cross-peaks; kAB ) 0.65 s-1
(para) and 0.64 s-1 (meta)], the indirect A/C exchange was
about half those concerning the B/C and A/B exchanges
[kAC ) 0.38 s-1 (para) and 0.34 s-1 (meta)], in agreement
with the kinetic theory for this particular exchange.15
B. Reactions of 2 with NEt3 and DMAN. We reasoned
that B(C6F5)3 coordination to the borate oxygen atom might
provide some acidic character to the oxydrilic hydrogen atom,
allowing its abstraction by a suitable base. However, attempts
to deprotonate 2 with NEt3 resulted in the reaction process
depicted in Scheme 1.
The identification of the reaction products (see the
Experimental Section) has been greatly facilitated by a recent
study concerning the reaction between B(C6F5)3 and the
tertiary amine NEt2Ph,16 which afforded the zwitterionic
adduct (C6F5)3B-(CH2CHdN+EtPh) and the ion pair
[NHEt2Ph]+[(C6F5)3BH]-. In our case, the B(C6F5)3 source
is anion 2, as depicted in Scheme 1. Indeed, we have verified
that the reaction of B(C6F5)3 with stoichiometric NEt3 resulted
in the instantaneous formation (even at low temperature, 193
K) of the same products as those obtained from anion 2 (eq
3).
(15) This system can be assimilated to a reversible two-step A T B T C
reaction, in which the direct and the reverse kinetic constants in each
step are equal. Moreover, in the present case, the kinetic constant from
A to B is equal to that from C to B. In these hypotheses the kinetic
constant for the A-C conversion should be half that for the A-B
conversion: Bernasconi, C. F. Investigation of rates and Mechanisms
of Reactions. In Techniques of Chemistry, 4th ed.; Weissemberger,
A., Ed.; Wiley-Interscience Publishers: New York, 1986; Vol. VI,
Part I, Chapter VI, p 449.
2B(C6F5)3 + 2NEt3 a [NHEt3]+[(C6F5)3BH]- +
(C6F5)3B-(CH2CHdN+Et2) (3)
Inorganic Chemistry, Vol. 44, No. 14, 2005 5033

Di Saverio et al.
Scheme 2
Scheme 3
So the B(C6F5)3-NEt3 interaction does not afford a stable
Lewis acid-base adduct,17 but rather results in a formal
hydride abstraction. Actually, even if a number of stable
adducts are known between B(C6F5)3 and neutral bases
containing sp2 or sp nitrogen atoms (such as imines,18
aromatic heterocycles,5,19 or cyano derivatives19c), the reports
concerning adducts with sp3 amines are much more scanty.
It has recently been shown that B(C6F5)3 adducts with the
sec-amines pyrrolidine and piperidine are stable,5,20 while
the corresponding adduct with the tertiary amine NEt2Me is
stable only at low temperature, because at room temperature
significant generation of an iminium salt of [B(C6F5)3H]-
was observed, as mentioned above.16
The failure of NEt3 to deprotonate anion 2 prompted us
to try the use of a stronger Brønsted base, such as the Proton
Sponge DMAN. However, no deprotonation of 2 was
observed, but rather reaction 4, in which B(C6F5)3 reacts with
DMAN (Scheme 2) to give the 1,1,3-trimethyl-2,3-dihydro-
perimidin-1-ium cation, [(C10H6)(CH3)2N(CH2)N(CH3)]+
(TMP+), previously structurally characterized.21
reaction mixtures arising from B(C6F5)3 addition to the
NHEt3+ salt of the anion [B(C6F5)3OH]- (1) always showed,
besides anion 2, also another minor component. Such a
species has been identified as the ion pair [NHEt3]+[(C6F5)3-
BO(H)H‚‚‚O(H)B(C6F5)3]- ([NHEt3]3), containing the borate
anion 1 hydrogen-bonded to a proton of one water molecule
coordinated to B(C6F5)3, as shown in Scheme 3. The
formation of this species is due to the presence of traces of
water, both in the solvent and/or in the reactants, which
transforms part of B(C6F5)3 into its stable water adduct,
(C6F5)3BOH2. This has been confirmed by the observation
that water addition to the reaction mixtures increased the
amount of 3 at the expense of 2 (see Scheme 3, top).
Anion 3 has been obtained in high yields also by treating
the (C6F5)3BOH2 adduct with 0.5 equiv of NEt3, according
to reaction 1 (L ) NEt3) plus reaction 5. The process has
[HDMAN]+[(C6F5)3B(µ-OH)B(C6F5)3]- + DMAN a
[HDMAN]+[(C6F5)3BOH]- + [TMP]+[(C6F5)3BH]- (4)
[NHEt3]+[(C6F5)3BOH]- + B(C6F5)3OH2 a
C. Reaction of 2 with Water: The Anion [(C6F5)3BO-
(H)H‚‚‚O(H)B(C6F5)3]- (3). The NMR spectra of the
[NHEt3]+[(C6F5)3BO(H)H‚‚‚O(H)B(C6F5)3]- (5)
been monitored in an NMR tube and afforded spectroscopi-
cally pure [NHEt3]3. Reaction 5 is reminiscent of the reaction
of anion 1 with the [Tp′Zn(OH2)]+ cation (Tp′ ) substituted
trispyrazolylborate), where 1 did not displace the coordinated
water molecule, but rather gave a hydrogen bond with the
(16) Millot, N.; Santini, C. C.; Fenet, B.; Basset, J. M. Eur. J. Inorg. Chem.
2002, 3328.
(17) A (C6F5)3B‚NEt3 adduct has been previously briefly mentioned, but
no detail of its behavior was given: (a) Doerrer, L. H.; Graham, A.
J.; Haussinger, D.; Green, M. L. H. Dalton Trans. 2000, 813. (b)
Doerrer, L. H.; Graham, A. J., Green, M. L. H. Dalton Trans. 1998,
3941.
-
water protons, affording a B-bound H3O2 anion.23
(18) Blackwell, J. M.; Piers, W. E.; Parvez, M.; McDonald, R. Organo-
metallics 2002, 21, 1400.
Interestingly, also the reaction of (C6F5)3BOH2 with 0.5
equiv of 2-methylindole afforded anion 3 in high yields,
according to eq 6. A solid-state X-ray diffractometric analysis
(19) (a) Lesley, M. J. G.; Woodward, A.; Taylor, N. J.; Marder, T. B.;
Cazenobe, I.; Ledoux, I.; Zyss, J.; Thornton, A.; Bruce, D.; Kakkar,
K. Chem. Mater. 1998, 10, 1355. (b) Vagedes, D.; Erker, G.; Kehr,
G.; Bergander, K.; Kataeva, O.; Fro¨hlich, R.; Grimme, S.; Mu¨ck-
Lichtenfeld, C. Dalton Trans. 2003, 1337. (c) Fraenk, W.; Klapoetke,
T. M.; Krumm, B.; Mayer, P.; Piotrowski, H.; Vogt, M. Z. Anorg.
Allg. Chem. 2002, 628, 745.
C9H9N + 2B(C6F5)3OH2 a
[C9H10N]+[(C6F5)3BO(H)H‚‚‚O(H)B(C6F5)3]- (6)
(20) Mountford, A. J.; Hughes, D. L.; Lancaster, S. J. Chem. Commun.
2003, 2148.
of the reaction product has been performed, providing a
detailed characterization of the H3O2- moiety in the adduct
(see section F). The 1:1 reaction between the same reactants
(21) Gamage, S. N.; Morris, R. H.; Rettig, S. J.; Thackray, D. C.; Thorburn,
I. S.; James, B. R. J. Chem. Soc., Chem. Commun. 1987, 894. In this
work DMAN dehydrogenation was promoted by the complex mer-
RhCl3(Me2SO)3. The same Rh complex had been previously shown
to easily dehydrogenate triethylamine with resulting formation of the
ylidic imine complex RhCl3(Me2SO)2(CH2CHdNEt2).22
(22) Gamage, S. N.; Morris, R. H.; Rettig, S. J.; James, B. R. J. Organomet.
Chem. 1986, 309, C59.
(23) (a) C. Bergquist G. Parkin, J. Am. Chem. Soc. 1999, 121, 6322. (b)
Bergquist, C.; Fillebeen, T.; Morlok, M. M.; Parkin, G. J. Am. Chem.
Soc. 2003, 125, 6189.
5034 Inorganic Chemistry, Vol. 44, No. 14, 2005

Borate Anions from Tris(pentafluorophenyl)borane
Scheme 4
The borate anion 1 is therefore able to give adducts both
with B(C6F5)3 (eq 2) and with (C6F5)3BOH2 (eq 5). A
competition study has been performed to establish the
preferred reaction path. Mixtures containing B(C6F5)3/(C6F5)3-
BOH2 in different ratios have been treated with NEt3 (less
than 1 equiv with respect to (C6F5)3BOH2), and the resulting
equilibrium mixtures analyzed by 19F NMR, at 173 K. In
Figure 5. Low-field region of the variable-temperature 1H NMR spectra
of [NHEt3]3 in CD2Cl2. The asterisk marks the signal of the NH proton,
while the pound sign indicates unidentified byproducts.
afforded the indolium salt of anion 1, as described in the
Experimental Section.
In CD2Cl2 solution, the room temperature 1H spectrum of
anion 3 consisted of a unique resonance, at δ 8.55. This
indicates fast exchange among the three different protonic
1
these systems, H and 19F averaged signals were observed
for 3 and (C6F5)3BOH2, indicating fast exchange between
the two species. Therefore, the relative amounts of 3 and of
(C6F5)3BOH2 in the equilibrium systems had to be evaluated
from the chemical shift of their molar fraction weighted
averaged 19F signals (while the amounts of 2 and of B(C6F5)3
were estimated directly from the integrated intensities of their
signals). From these computations a value of 0.20(3) was
estimated for the constant of equilibrium 7. Such a value
-
sites of the H3O2 moiety, i.e., the two water protons of
(C6F5)3BOH2 and the proton of (C6F5)3BOH-. On lowering
the temperature, the exchange slowed, causing broadening
and collapse of the protonic resonance, eventually resulting
in the appearance of two resonances at 17.9 and 4.5 ppm
(ratio 1:2, Figure 5). The downfield position of the signal of
intensity 1 indicates that this proton is involved in a very
strong H-bonding interaction, and the 1:2 ratio indicates that
such an interaction is (by itself or dynamically; see Scheme
4, top) symmetric. A possible mechanism for the hydrogen
exchange could imply a partial breaking of the H-bond
interaction, most likely with the intermediate formation of a
bifurcated interaction, as shown in Scheme 4.
[(C6F5)3B(µ-OH)B(C6F5)3]- + B(C6F5)3OH2 T
[(C6F5)3BO(H)H‚‚‚O(H)B(C6F5)3]- + B(C6F5)3 (7)
corresponds to the ratio between the constants of equilibria
5 and 2 (with L ) NEt3). This indicates that for the borate
1 the Lewis acid-base interaction with B(C6F5)3 (resulting
in anion 2) is about 5 times preferred over H-bonding to
(C6F5)3BOH2 (resulting in 3).
D. Synthesis and NMR Characterization of the Anion
[(C6F5)3BOB(C6F5)2]- (4). As described in section B, it was
not possible to obtain the dianion [(C6F5)3BOB(C6F5)3]2- by
treating the salts of 2 with bases such as NEt3 or DMAN.
Nonetheless, we have been able to obtain a species containing
the nonprotonated B-O-B moiety, by the reaction of NEt3
with an equimolar mixture of B(C6F5)3 and bis(pentafluo-
rophenyl)borinic acid, (C6F5)2BOH.24,25 In this way the
The 19F NMR spectra showed one resonance only, for each
of the ortho, meta, and para positions of the aryl rings
(Figure 1). At T < 223 K, some broadening of the signals
was observed, particularly for the para one, but no collapse
was observed down to 166 K. This indicates that the
fluxionality of the C6F5 rings, which equalizes the three aryls
on each B atom and their ortho and meta positions, is fast
on the NMR time scale even at 166 K, differently from what
was observed for anion 2. Therefore, the aryl rings in anion
3 enjoy a conformational freedom higher than that in 2, due
to the longer distance between the two B(C6F5)3 moieties,
which relieves inter-ring steric crowding and hinders the
H‚‚‚F hydrogen-bond interaction between the OH proton and
the aryl substituents.
(24) Chambers, R. D.; Chivers, T. J. Chem. Soc. 1965, 3933.
(25) Beringhelli, T.; D’Alfonso, G.; Donghi, D.; Maggioni, D.; Mercandelli,
P.; Sironi, A. Organometallics 2003, 22, 1588.
Inorganic Chemistry, Vol. 44, No. 14, 2005 5035

Di Saverio et al.
In the case of [NHEt3]4 a low-temperature 2D 19F-1H
HOESY (Figure S6) showed weak correlations between the
ortho fluorine atoms of the anion and the C2H5 resonances
of the cation (interestingly enough, stronger correlations were
observed for the resonances of the B(C6F5)3 moiety, in
agreement with the electron density map described in Section
F, Figure 9). Moreover, the N-H proton was observed at
the usual chemical shift (δ 5.1), indicating the absence of
significant (C6F5)3BO(R)‚‚‚HN hydrogen bonding. This is
attributable to the higher steric crowding in 4 [R ) B(C6F5)2]
than in 1 (R ) H): the aryl rings effectively shield the
Scheme 5
borinatoborate salt [NHEt3]+[(C6F5)3BOB(C6F5)2]- ([NHEt3]-
4) has been obtained in quantitative yield (Scheme 5). The
solid-state structure is described in section F.
The 19F NMR spectrum of 4 showed at all the temperatures
two sets of resonances (of relative intensity 3:2) attributable
to the B(C6F5)3 and B(C6F5)2 moieties of the anion, respec-
tively. All the signals of the B(C6F5)3 fragment were upfield
shifted with respect to those of the B(C6F5)2 fragment, in
line with a higher electron density on tetracoordinated boron
centers. On lowering the temperature, the resonances lost
their fine structure and broadened, but even at 178 K no
collapse was observed. This indicates higher mobility of the
five aryl rings, compared to the more crowded anion 2.
The sequence of reactant addition is crucial for the reaction
depicted in Scheme 5. Indeed, NEt3 reacts with B(C6F5)3,
according to reaction 3, and reactions occur also between
NEt3 and (C6F5)2BOH alone, affording complex mixtures that
are presently under investigation. Therefore, we added NEt3
to an equimolar mixture of B(C6F5)3 and borinic acid. We
have clear evidence that no fast reaction takes place upon
mixing B(C6F5)3 and (C6F5)2BOH (the only reaction that
occurs is the very slow formation of the(C6F5)2BOB(C6F5)2
anhydride, by water removal due to the formation of the
adduct of B(C6F5)3 with water).26 Therefore, we may suppose
that the formation of 4 occurs by deprotonation of borinic
acid and coordination of the (C6F5)2BO- anion to the acidic
boron atom of B(C6F5)3, these steps having higher rates than
reaction 3.
+
oxygen atom toward the approach of the NHEt3 cation.
In the case of ion pairs [NHEt3]2 and [NHEt3]3 low-
temperature 2D experiments did not show any correlation
between the cationic protons and the hydrogen or fluorine
atoms of anions 2 and 3. Moreover, a unique set of cationic
resonances was observed, even at the lowest temperatures,
in mixtures containing both [NHEt3]2 and [NHEt3]3, indicat-
ing fast exchange between the cationic part of the ion pairs.
These ion pairs should therefore be considered loose, due
both to the higher delocalization of the anionic charge and
to the lower propensity of the oxygen atoms to engage in a
H-bond with the cation.
Finally, we have also investigated the nature of the ion
pair [NHEt3]+[(C6F5)3BH]-, which is formed in the reaction
of B(C6F5)3 with NEt3 (eq 3). In this case a 2D 19F-1H
HOESY showed close proximity between all the cationic
protons and the ortho fluorine atoms (see Figure S6 of the
Supporting Information). This tight interaction is attributable
to an unconventional hydrogen bond (a “dihydrogen bond”)31
involving the ammonium proton and the boron hydride, as
revealed by the strong cross-peak between the corresponding
resonances (at 6.2 and 3.4 ppm, respectively), observed in a
11B-decoupled 2D NOESY (Figure 6).
The cationic NH resonance of the ion pairs with anions
2-4 (δ 5.1 ppm) shows the fine structure (pseudo 1:1:1
triplet, JHN ) 51 Hz) arising from the coupling with 14N (in
the spectra with the best resolution each triplet component
is further split by the coupling with the six CH2 protons).
E. Different Nature of the Ion Pairs Containing
+
NHEt3 , in CD2Cl2 Solution. In the 1H spectrum the N-H
signal of [NHEt3]1 was well downfield shifted with respect
to that of the other ion pairs presented here (δ 10.5 vs 5.1).
This is attributable to the occurrence of hydrogen bonding
between the NH proton and the oxygen atom of the borate
anion 1 (Scheme 1), as observed in the solid-state structure
of the same ion pair.27 Such a hydrogen bond makes [NHEt3]-
1 a tight ion pair, and all the cationic signals of this ion pair
were always separated from those of the other ion pairs, if
present in the same reaction mixture.30 The cation-anion
spatial proximity was proved also by the dipolar correlations
between the ortho fluorines and the cationic C2H5 resonances
observed in a 2D 19F-1H HOESY at 173 K (Figure S5 of
the Supporting Information).
1
The observation of H-14N coupling is rather unusual and
requires both the absence of proton exchange (typically with
water traces) and relatively long quadrupolar 14N relaxation
times (at low temperatures the coupling disappeared, see
Figures 3 and 5, due to the increase of the quadrupolar
relaxation rate). This observation confirms that in these
samples water is effectively trapped by B(C6F5)3: indeed,
the presence of traces of water destroyed the fine structure
and modified the position of the signal, due to proton
exchange.
F. Crystal and Molecular Structures of Anions 3 and
4 and Electrostatic Potential Maps for Anions 1-4. The
structures of [2-methyl-3H-indolium]3 and [NHEt3]4 have
been determined in the solid state by X-ray diffraction
analysis. Figures 7 and 8 show ORTEP drawings of anions
(26) Beringhelli, T.; D’Alfonso, G.; Donghi, D.; Maggioni, D.; Mercandelli,
P.; Sironi, A. Organometallics 2004, 23, 5493.
(27) The solid-state X-ray structure of the [NHEt3]+ 28 (and also of the
[NHMe2Ar]+)29 salt of 1 showed a short N-O contact of ca. 273 pm.
(28) Duchateau, R.; Van Santen, R. A.; Yap, G. P. A. Organometallics
2000, 19, 809.
(29) Stibrany, R. T.; Brant, P. Acta Crystallogr. 2001, C57, 644.
(30) Exchange with the [NHEt3]+ cation of the anions 2 and/or 3 does
occur (as revealed by a cross-peak in the 2D EXSY map of Figure
S4), but it is too slow to lead to averaged signals.
(31) (a) Custelcean, R.; Jacson, J. E. Chem. ReV. 2001, 101, 1963. (b)
Crabtree, R. H.; Siegbahn, P. E. M.; Eisenstein, O.; Rheingold, A. L.;
Koetzle, T. F. Acc. Chem. Res. 1996, 29, 348.
5036 Inorganic Chemistry, Vol. 44, No. 14, 2005

Borate Anions from Tris(pentafluorophenyl)borane
significantly different and in fair agreement with the
computed one for the free anion.34
Hydrogen atoms H(1A) and H(1B) are involved in
bifurcated hydrogen bonds with two fluorine atoms each. In
particular, H(1A) interacts with F(22A) of the same anion
and with F(33B) of a symmetry-related anion (-x, 1 - y,
-z), while H(1B) interacts with F(22B) of the same anion
and with F(33A) of another symmetry-related anion (-x,
1 - y, 1 - z). Geometric parameters of these interactions
are reported in Table 4.
The cation in [C9H10N]3 is a rare example of the 3H-indole
tautomeric form. Its structure compares well with the few
previously reported: (3H-indole)tris(pentafluorophenyl)-
boron5 and [PdCl2(2-methyl-3H-indole)2].37 The observed
bond distances are in accordance with a localization of the
double bond between N(1) and C(2) [1.271(3) Å] in the five-
membered ring.
Anion [(C6F5)3B-O-B(C6F5)2]- (4) is the first borina-
toborate structurally characterized. As expected, both the
B-O bond distances are shorter in 4 than in the previously
reported structures of [(C6F5)3B-OH-B(C6F5)3] (2) (mean
value 1.56 Å).8-10 The observed B(2)-O(1) bond distance
[1.306(2) Å] compares well with the values found in other
borinic acid derivatives: 1.348 and 1.350 Å in [(C6H5)2BOH]
and [(2,4,6-(CF3)3C6H2)2BOH], respectively.38 A direct com-
parison with [(C6F5)2BOH] is hampered by the trimerization
of this species in the solid state.25 The B(1)-O(1) bond
distance [1.496(2) Å] is unexceptional for [(C6F5)3BOX]
derivatives; in particular it is similar to the value found in 3
[1.514(2) Å].
Figure 6. {11B}1H-1H NOESY experiment on a mixture containing the
ion pair [NHEt3]+[(C6F5)3BH]- and the zwitterionic imine (C6F5)3B-(CH2-
CHdN+Et2), showing the proton-hydride NH‚‚‚HB interaction (298 K,
9.4 T, mixing time 0.26 s). The signals of the imine are marked with the
subscript “im”.
3 and 4, respectively. Tables 3 and 4 contain the most
relevant bonding parameters for the two compounds.
Anion 3 consists of two triarylborane moieties bridged by
a [H3O2]- anion, bound through the two oxygen atoms.
Anion [H3O2]- has been extensively investigated theoretically
to study its proton-transfer properties and as a prototypical
strongly hydrogen bonded charged species.32 Its computed
geometry is strongly dependent on the model chemistry
employed;33 however, high-level correlated computations
undoubtedly assign an asymmetric structure to the hydrogen
bond.34
The solid-state structure of anion 3 can contribute to
support these theoretical findings. Anion 3 has been previ-
ously structurally characterized as a salt of different cations;
however, either it was impossible to properly locate hydrogen
atoms8 or the anion was constrained to lie on a crystal-
lographic C2 axis.35 An accurate structural determination of
[HDMAN]3 has been recently published.36 A distinct asym-
metry of the overall anion was observed; however, the large
estimated standard uncertainties affecting the OH bond
distances hampered the direct assessment of the symmetric
or asymmetric nature of the [H3O2]- moiety.
As can be seen from Figures 7 and 8 and from the torsional
angles reported in Table 3, in the two anions the B(C6F5)3
moiety adopts a very similar conformation in which one of
the three phenyl rings [the one labeled C(11)-C(16)] eclipses
the B-O bond, whereas the other two exhibit a chiral two-
bladed propeller-like conformation. This very same confor-
mation can be found in most of the crystal structures of
tris(pentafluorphenyl)borane derivatives collected in the
Cambridge Structural Database:39 from this evidence it can
be inferred that this molecular fragment is a very rigid one,
so that the presence of a somehow high-energy barrier to
the enantiomerization process of the chiral propeller can be
expected.
The geometry of anion 3 as determined in its 2-methyl-
3H-indolium salt shows a clear deviation from the idealized
C2 symmetry, the observed bond distances and angles listed
in Table 3 being consistent with an asymmetric structure for
this species. Moreover, the measured O(1B)-H(1) and
H(1)‚‚‚O(1A) bond distances [1.14(2) and 1.26(2) Å] are
As can be seen from the data in Table 4, the oxygen atoms
in anions 3 and 4 are not involved in hydrogen bonding, the
cation protons mainly interacting with the ortho fluorine
atoms of the perfluorophenyl moieties. The very same
situation has been found for the previously known structures
of anions 2 and 3 with different counterions.8-10,35,36 At
variance, in the structure of [NHEt3]1 the oxygen atom acts
as a hydrogen-bond acceptor for the cation.28 These findings
(32) Tuckerman, M. E.; Marx, D.; Klein, M. L.; Parrinello, M. Science
1997, 275, 817.
(33) Wei, D.; Proynov, E. I.; Milet, A.; Salahub, D. R. J. Phys. Chem. A
2000, 104, 2384 and references therein.
(34) At the MP2(fc)/aug-cc-pVQZ level O-H and H‚‚‚O bond distances
are equal to 1.125 and 1.342 Å. Samson, C. C. M.; Klopper, W. J.
Mol. Struct.: THEOCHEM 2002, 586, 201.
(35) Doerrer, L. H.; Green, M. L. H. J. Chem. Soc., Dalton Trans. 1999,
4325.
(37) Yamauchi, O.; Takani, M.; Toyada, K.; Masuda, H. Inorg. Chem. 1990,
29, 1856.
(38) (a) Retting, S. J.; Trotter, J. Can. J. Chem. 1983, 61, 2334. (b) Fraenk,
W.; Klapotke, T. M.; Krumm, B.; Mayer, P.; Noth, H.; Piotrowski,
H.; Suter, M. J. Flourine Chem. 2001, 112, 73.
(36) Drewitt, M. J.; Niedermann, M.; Baird, M. C. Inorg. Chim. Acta 2002,
340, 207.
(39) CSD (version 5.24, November 2002). Allen, F. H. Acta Crystallogr.,
Sect. B: Struct. Sci. 2002, 58, 380.
Inorganic Chemistry, Vol. 44, No. 14, 2005 5037

Di Saverio et al.
Figure 7. ORTEP view of the hydrogen-bonded adduct 3 between aquatris(pentafluorophenyl)borane and hydroxotris(pentafluorophenyl)borate, as determined
in the crystal of its 2-methyl-3H-indolium salt. Thermal ellipsoids are drawn at the 30% probability level; hydrogen atoms are given arbitrary radii.
atoms: the electrostatic potential maps represented in Figure
940 show a considerable reduction of the negative potential
on the oxygen atom and on the whole accessible surface of
anions 2-4, the charge being delocalized on all the fluorine
atoms. Between the three diborate anions, the borinatoborate
species 4 shows the largest local negative potential, due to
the lower number of fluorine atoms among which the charge
has to be distributed: the NMR evidence of a partial
interaction of this species with the proton of the triethyl-
ammonium cation in solution agrees with these findings.
Conclusions
We have pursued the synthesis of diborate anions in the
hypothesis that the delocalization of the negative charge over
more than one boron atom might improve the stability of
the anion and provide higher acidity to the ammonium salts
of the complex anions. The results reported here give some
support to this idea. Indeed, the dinuclear anions [(C6F5)3B-
(µ-OH)B(C6F5)3]- (2) and [(C6F5)3B(µ-O)B(C6F5)2]- (4) form
Figure 8. ORTEP view of the [bis(pentafluorophenyl)borinato]tris-
(pentafluorophenyl)borate anion (4), as determined in the crystal of its
loose ion pairs and no evidence of the involvement of their
triethylammonium salt. Thermal ellipsoids are drawn at the 30% probability
ammonium cations in hydrogen bonds has been found,
differently from the mononuclear anions [(C6F5)3BOH]- (1)
and [(C6F5)3BH]-. Therefore, the cations in the dinuclear ion
pairs are expected to be more active toward nucleophiles.
On the other hand, the reaction of 2 with NEt3 and DMAN
indicates that reaction 2 is reversible, so that, in the presence
of competing bases, 2 can dissociate, generating B(C6F5)3
and [(C6F5)3BOH]-. Then anion 2 does not seem to be the
ideal partner of the Brønsted acid [NHEt3]+, because it can
behave as a source both of the competing Lewis acid
B(C6F5)3 and of the coordinating41 anion [(C6F5)3BOH]-.
level.
closely parallel the NMR observations concerning the ion
pair interactions in dichloromethane solution (see section E).
The limited accessibility of the oxygen atom in anions 2-4
with respect to 1 is clearly evidenced in Figure 9, in which
the space-filling models of these species are depicted. In all
cases, and particularly for anion 2, the oxygen atom is buried
inside the entangled arrangement of the perfluorophenyl
rings. The situation is less severely hindered for anions 3
and 4, this being reflected by the greater rotational freedom
noticed in solution for these two anions.
However, in addition to these steric considerations, the
tendency of diborates 2-4 to give looser ion pairs with
respect to the borate 1 is attributable to the spreading of the
anionic charge on a greater number of electronegative
(40) The electrostatic potential was computed at the PM3 semiempirical
level, employing the solid-state geometries. The calculations were
performed with SPARTAN 02; Wavefunction Inc.: Irvine, CA, 2002.
(41) Hill, G. S.; Manojlovic-Muir, L.; Muir, K. W.; Puddephatt, R. J.
Organometallics 1997, 16, 525
5038 Inorganic Chemistry, Vol. 44, No. 14, 2005

Borate Anions from Tris(pentafluorophenyl)borane
Table 3. Selected Geometric Parameters (Å,deg) for [2-Methyl-3H-indolium]3 and [NHEt3]4
[2-methyl-3H-indolium]3
[NHEt3]4
B(1A)-O(1A)
1.514(2)
1.648(2)
1.658(2)
1.640(2)
1.527(2)
1.651(2)
1.648(2)
1.632(2)
1.271(3)
1.430(3)
1.467(3)
1.479(4)
1.495(3)
1.379(3)
1.368(3)
1.363(5)
1.384(6)
1.377(6)
1.346(4)
112.50(12)
106.75(12)
102.83(12)
105.55(12)
113.13(12)
116.07(13)
112.46(12)
106.80(12)
102.64(12)
104.48(12)
115.08(12)
115.32(13)
113.2(2)
108.7(2)
123.3(3)
128.0(3)
103.89(18)
134.0(2)
107.3(2)
118.8(3)
118.3(3)
121.8(3)
119.9(3)
117.1(3)
106.9(2)
129.0(3)
124.1(3)
-3.9(2)
B(1)-C(11)
1.652(2)
1.661(2)
1.650(2)
1.496(2)
1.306(2)
1.615(2)
1.600(2)
103.92(12)
116.57(13)
110.54(13)
111.04(13)
107.18(12)
107.25(13)
126.53(15)
118.46(15)
114.96(13)
134.07(13)
-6.9(2)
B(1A)-C(11A)
B(1)-C(21)
B(1A)-C(21A)
B(1)-C(31)
B(1A)-C(31A)
B(1)-O(1)
B(1B)-O(1B)
B(2)-O(1)
B(1B)-C(11B)
B(2)-C(41)
B(1B)-C(21B)
B(2)-C(51)
B(1B)-C(31B)
C(11)-B(1)-C(21)
C(11)-B(1)-C(31)
C(11)-B(1)-O(1)
C(21)-B(1)-C(31)
C(21)-B(1)-O(1)
C(31)-B(1)-O(1)
O(1)-B(2)-C(41)
O(1)-B(2)-C(51)
C(41)-B(2)-C(51)
B(1)-O(1)-B(2)
N(1)-C(2)
N(1)-C(7a)
C(2)-C(3)
C(2)-C(8)
C(3)-C(3a)
C(3a)-C(4)
C(3a)-C(7a)
C(4)-C(5)
C(5)-C(6)
C(6)-C(7)
O(1)-B(1)-C(11)-C(12)
O(1)-B(1)-C(21)-C(22)
O(1)-B(1)-C(31)-C(32)
O(1)-B(2)-C(41)-C(42)
O(1)-B(2)-C(51)-C(52)
C(7)-C(7a)
44.93(19)
-119.37(17)
78.1(2)
O(1A)-B(1A)-C(11A)
O(1A)-B(1A)-C(21A)
O(1A)-B(1A)-C(31A)
C(11A)-B(1A)-C(21A)
C(11A)-B(1A)-C(31A)
C(21A)-B(1A)-C(31A)
O(1B)-B(1B)-C(11B)
O(1B)-B(1B)-C(21B)
O(1B)-B(1B)-C(31B)
C(11B)-B(1B)-C(21B)
C(11B)-B(1B)-C(31B)
C(21B)-B(1B)-C(31B)
C(2)-N(1)-C(7a)
N(1)-C(2)-C(3)
-134.08(16)
N(1)-C(2)-C(8)
C(3)-C(2)-C(8)
C(2)-C(3)-C(3a)
C(3)-C(3a)-C(4)
C(3)-C(3a)-C(7a)
C(4)-C(3a)-C(7a)
C(3a)-C(4)-C(5)
C(4)-C(5)-C(6)
C(5)-C(6)-C(7)
C(6)-C(7)-C(7a)
N(1)-C(7a)-C(3a)
N(1)-C(7a)-C(7)
C(3a)-C(7a)-C(7)
O(1A)-B(1A)-C(11A)-C(12A)
O(1A)-B(1A)-C(21A)-C(22A)
O(1A)-B(1A)-C(31A)-C(32A)
O(1B)-B(1B)-C(11B)-C(12B)
O(1B)-B(1B)-C(21B)-C(22B)
O(1B)-B(1B)-C(31B)-C(32B)
48.36(19)
-104.38(17)
-8.5(2)
48.18(18)
-108.17(17)
Table 4. Hydrogen-Bond Geometry (Å,deg) for
[2-Methyl-3H-indolium]3 and [NHEt3]4a
due to its higher stability toward dissociation. Studies aimed
at further investigating its properties are in progress.
D-H‚‚‚A
D-Ha
[2-Methyl-3H-indolium]3
1.14(2) 1.26(2) 2.4040(16)
H‚‚‚A
D‚‚‚A
-D-H‚‚‚A
Experimental Section
O(1B)-H(1)‚‚‚O(1A)
173(2)
128(2)
140(3)
128(2)
140(2)
157.80
General Procedures. All the manipulations were performed
under nitrogen using oven dried Schlenk-type glassware. Deuterated
solvents (CIL or Isotec Inc.) were anhydrified on activated
molecular sieves. B(C6F5)3 (Boulder Scientific Co.) and DMAN
(Aldrich) were used as received. (C6F5)2BOH was provided by
Basell GmbH. Triethylamine (Aldrich) was dried over KOH pellets
and stored over activated 4 Å molecular sieves before use. (C6F5)3-
BOH2 was synthesized as reported in a previous paper.5
O(1A)-H(1A)‚‚‚F(22A) 0.80(3) 2.16(3) 2.7401(18)
O(1A)-H(1A)‚‚‚F(33B)i 0.80(3) 2.23(3) 2.8977(17)
O(1B)-H(1B)‚‚‚F(22B)
0.78(3) 2.16(3) 2.7058(18)
O(1B)-H(1B)‚‚‚F(33A)ii 0.78(3) 2.42(3) 3.0601(19)
N(1)-H(N1)‚‚‚F(12A)
[NHEt3]4
0.991
1.998
2.940(2)
N(1)-H(1)‚‚‚F(56)
0.958
2.126
3.0246(19)
155.63
a Symmetry-equivalent positions: (i) -x, 1 - y, -z, (ii) -x, 1 - y,
1 - z.
NMR spectra were acquired on Bruker spectrometers: DPX 200
for 13C and 11B spectra at room temperature, AVANCE DRX-300
1
equipped with a 5 mm QNP probe for H and 19F spectra, and
Therefore, the novel borinatoborate anion 4, despite its
ion pair with [NHEt3]+ being slightly tighter than that of 2,
appears the most promising delocalized anion in this series,
AVANCE DRX 400 for 1H and 11B spectra. 19F NMR spectra were
referenced to external CFCl3 (δ ) 0 ppm) and 11B NMR spectra to
Inorganic Chemistry, Vol. 44, No. 14, 2005 5039

Di Saverio et al.
variations nor exothermicity was observed. After 15 min a sample
1
was evaporated to dryness and analyzed by H NMR, showing
quantitative formation of [NHEt3]2. 1H and 19F NMR data are
reported in Tables 1 and 2. 11B NMR (CD2Cl2, 298 K): δ -0.63.
Reaction of B(C6F5)3 with NEt3. A CD2Cl2 solution of B(C6F5)3
(30.6 mg, 0.0598 mmol) was treated directly in an NMR tube with
1
0.95 equiv of NEt3 (7.9 µL) at 193 K. H and 19F NMR spectra
were then acquired at different temperatures, showing the quantita-
tive formation of an equimolar mixture of [NHEt3]+[(C6F5)3BH]-
and (C6F5)3B-(CH2CHdN+Et2). 19F and 1H assignments have been
1
performed through 19F-1H HOESY and H-1H NOESY experi-
ments, respectively. The hydrogen of the anion [(C6F5)3BH]- has
been detected in a {11B}1H spectrum (3.33 ppm), the four lines of
its multiplet (JBH ) 89 Hz) being otherwise largely buried under
the methylene resonances. The following are data for (C6F5)3B--
(CH2CHdN+Et2), in CD2Cl2 at 298 K. 1H NMR (CD2Cl2, 298 K):
3
δ 1.23 (t, 3H, CH3 anti with respect to CH, JHH ) 7.2 Hz), 1.25
3
(t, 3H, CH3 syn with respect to CH, JHH ) 7.35 Hz), 3.15 (br m,
3
1H, CH2; d in {11B}1H, JHH ) 8.0 Hz), 3.47 (q, 2H, CH2 anti
3
with respect to CH, JHH ) 7.2 Hz), 3.63 (q, 2H, CH2 syn with
3
3
respect to CH, JHH ) 7.35 Hz), 8.05 (t, 1H, NdCH, JHH ) 8.0
Hz). 19F NMR: δ -132.84 (m, 6Fortho), -160.65 (m, 3Fpara),
-165.29 (m, 6Fmeta). 11B NMR (CD2Cl2, 298 K): δ -14.04 (pseudo
s). The following are data for [NHEt3]+[(C6F5)3BH]-, in CD2Cl2
at 298 K. {11B}1H NMR: δ 1.35 (t, 9H, CH3, JHH ) 7.3 Hz),
3
3.26 (q, 6H, CH2, 3JHH ) 7.4 Hz), 3.33 (br s, 1H, BH), 6.48 (br s,
1H, NH). 19F NMR: δ -134.76 (m, 6Fortho), -162.79 (m, 3Fpara),
-166.48 (m, 6Fmeta). 11B NMR: δ -24.40 (d, JBH ) 89 Hz).
Reaction of [NHEt3]2 with NEt3. A CD2Cl2 solution of [NHEt3]-
2 (0.599 g, 5.24 mmol, 0.105 M) was treated with 1 equiv of NEt3
(3.5 mL of a 1.5 M solution in CD2Cl2). Neither color variations
nor exothermicity was observed. A sample was evaporated to
dryness and analyzed by 1H NMR, which showed the presence of
[NHEt3]+[(C6F5)3BOH]- as the main product (ca. 60%) together
with an equimolar mixture of [NHEt3]+[(C6F5)3BH]- and (C6F5)3B--
(CH2CHdN+Et2).
Figure 9. Space-filling models (left) and electrostatic potential mapped
onto the electron density isosurfaces (0.002 au, right) for anions 1-4. Dark
red (blue) corresponds to -0.145 (-0.020) au.
Preparation of [1-(Me2NH)-8-(Me2N)C10H6]+[(C6F5)3BOH]-
([HDMAN]1). A CH2Cl2 (6 mL) suspension of (C6F5)3BOH2 (0.605
g, 1.14 mmol) was treated with 6 mL of a CH2Cl2 solution of
DMAN (0.244 g, 1.14 mmol). A sample was evaporated to dryness
external Et2O‚BF3 (δ ) 0 ppm). The temperature was calibrated
1
with a standard CH3OH/CD3OD solution.42
and analyzed by H NMR, showing the quantitative formation of
1
[HDMAN]1. H NMR (CD2Cl2, 298 K): δ 1.99 (br s, OH), 3.15
Preparation of [NHEt3]+[B(C6F5)3OH]- ([NHEt3]1). A CD2-
Cl2 solution of B(C6F5)3 (26.7 mg, 0.0522 mmol) was treated
directly in an NMR tube with H2O up to exactly 1 equiv (as checked
by low-temperature 19F NMR) and then with 1 equiv (7.2 µL) of
(d, 12H, CH3, JHH ) 2.5 Hz), 7.7-8.1 (m, 6H, CH), 19.4 (pseudo
br s, 1H, NH). 13C NMR (CD2Cl2, 298 K, 4.7 T): δ 46.78 (CH3),
121.53 (CH), 127.67 (CH), 130.45 (CH), 143.48 (Cipso). COSY
(CD2Cl2): δ(1H)/δ(1H) 3.15/19.3 (CH3/NH).
1
NEt3. The low-temperature H and 19F data are reported in Table
Preparation of [1-(Me2NH)-8-(Me2N)C10H6]+[(C6F5)3B(µ-
OH)B(C6F5)3]- ([HDMAN]2). A CH2Cl2 solution of [HDMAN]1
(11.8 mL, 0.095 M) was treated with 1 equiv of B(C6F5)3 (4.0 mL
of a 0.28 M solution in CH2Cl2). A sample was evaporated to
dryness and analyzed by 1H NMR, showing the quantitative
1. The attribution to the NH proton of the signal at 10.51 ppm was
confirmed by a COSY experiment (263 K). The identification of
the OH signal was provided by a 2D NOESY (173 K), which
showed correlation between the NH signal and a broad resonance
almost overlapped with the CH2 resonance, at 2.59 ppm. 11B NMR
(CD2Cl2, 298 K): δ -3.79 (br s) (lit.43 δ -3.8 in CDCl3, with the
same counterion; lit.41 δ -4.4, in CD2Cl2, with a different cation).
Preparation of [NHEt3]+[(F5C6)3B(µ-OH)B(C6F5)3]- ([NHEt3]2).
A slightly beige and cloudy CH2Cl2 solution of (C6F5)3BOH2
(0.3626 g, 0.684 mmol) was treated with a CH2Cl2 solution of NEt3
(0.0693 g, 99.5%, 0.685 mmol), reaching a total volume of 8 mL
(0.085 M). Then 7 mL of a CH2Cl2 solution of B(C6F5)3 (0.3499
g, 0.683 mmol) was added to the reaction mixture. Neither color
1
formation of [HDMAN]2. H NMR (CD2Cl2, 298 K): δ 6.67 (m
with 13 lines, OH, averaged JHF ) 3.1 Hz), 3.18 (d, 12H, CH3,
JHH ) 2.7 Hz), 7.7-8.1 (m, 6H, CH), 19.4 (pseudo br s, 1H, NH).
Reaction of B(C6F5)3 with DMAN. A slightly beige suspension
of B(C6F5)3 (0.406 g, 0.793 mmol) in CH2Cl2 (5 mL) was treated
with 5 mL of a CH2Cl2 solution of DMAN (0.170 g, 0.793 mmol).
The reaction was instantaneous, producing a slightly yellow and
clear solution containing [(C10H6)(CH3)2N(CH2)N(CH3)]+-
1
[(C6F5)3BH]-. H NMR (CD2Cl2, 298 K): δ 3.36 (s, 3H, CH3),
3.57 (br s, 6H, CH3), 3.68 (br q, 1H, BH, JBH ) 89 Hz), 4.76 (s,
2H, CH2), 7.08 (dd, 1H, CH, JHH ) 1.76, 6.85 Hz), 7.6-7.8 (m,
4H, CH), 8.0-8.1 (m, 1H, CH). 11B NMR (CD2Cl2, 298 K): δ
(42) Van Geet, A. L. Anal. Chem. 1970, 42, 679.
(43) Siedle, A. R.; Newmark, R. A.; Lamanna, W. M.; Huffman, J. C.
Organometallics 1993, 12, 1491.
5040 Inorganic Chemistry, Vol. 44, No. 14, 2005

Borate Anions from Tris(pentafluorophenyl)borane
-25.25 (d, JBH ) 89 Hz). 13C NMR (C2D2Cl4, 298 K): δ 37.83
(NCH3), 52.55 (N(CH3)2), 80.68 (CH2), 110.9 (CH), 114.2 (CH),
121.2 (CH), 126.2 (CH), 129.4 (CH), 132.1 (CH), 135.3 (q), 138.8
(q).
Reaction of [HDMAN]2 with DMAN. A CH2Cl2 (16 mL)
solution of [HDMAN]2 (1.11 mmol) was treated with 4 mL of a
CH2Cl2 solution of DMAN (0.237 g, 1.11 mmol). Neither color
variations nor exothermicity was observed. After 3 days 1H NMR
analysis showed the presence mainly of [HDMAN]1 (49.3%) and
[1,1,3-trimethyl-2,3-dihydro-1H-perimidin-1-ium]+[HB(C6F5)3]-
(32.8%), together with some unreacted [HDMAN]2 (9.4%) and
DMAN (8.5%).
0.44 × 0.26 mm × 0.26 mm3, µ(Mo KR) ) 0.197 mm-1, empirical
absorption correction (SADABS,44 16298 symmetry-equivalent
reflections, effective data-to-parameters ratio 9.1), minimum/
maximum transmission factors 0.930/0.950, Bruker SMART dif-
fractometer, ω scans (∆ω ) 0.3°), 2400 frames each at 30 s of
exposure, keeping the detector at 5.0 cm from the sample, 1.5° e
θ e 29.0°, index ranges h ) -17 f +17, k ) -17 f +17, l )
-19 f +19, 37730 reflections, 11650 of which are unique (Rint
)
0.0290, Rσ ) 0.0176), 7907 reflections with I > 2σ(I), intensity
decay 7%, solution by direct methods (SIR9745) and subsequent
Fourier synthesis, anisotropic full-matrix least-squares refinement
on F2 using all reflections (SHELX9746), hydrogen atoms of
hydroxo and aqua ligands evidenced in a difference Fourier map
and refined isotropically without imposing any constraints, all other
hydrogen atoms placed in idealized positions, data/parameters
11650/741, S(F2) ) 1.076, R[F, I > 2σ(I)] ) 0.0456, Rw(F2, all
Reaction of 2-methylindole with (C6F5)3BOH2. a. Reaction
in the Ratio 1:1. Synthesis of [2-Methyl-3H-indolium]1. 2-Me-
thylindole (0.419 g, 3.13 mmol) was dissolved in 12 mL of CH2-
Cl2 and added at room temperature to a suspension of (C6F5)3BOH2
(1.66 g, 3.13 mmol) in 12 mL of pentane. The resulting light pink
suspension was stirred for 6 days and constantly monitored via 1H
NMR, but the spectra did not change with time, showing that the
reaction had occurred in a few minutes, affording stable products.
The main component of the reaction mixture was [2-methyl-3H-
indolium]1, together with a small amount (ca. 10%) of N-[B(C6F5)3]-
2-methyl-3H-indole,5a possibly due to the presence of some free
2
data) ) 0.1461, weighting scheme w ) 1/[σ2(Fo ) + (0.085P)2],
2
where P ) (Fo + 2Fc2)/3, maximum/minimum residual electron
density 0.333/-0.278 e Å-3
.
Crystal data for [NHEt3]4: C36H16B2F25NO, Mr ) 975.12,
monoclinic, space group P21/c (No. 14), a ) 14.411(4) Å, b )
11.741(3) Å, c ) 22.080(7) Å, â ) 104.44(1)°, V ) 3617.9(18)
Å3, Z ) 4, T ) 90(2) K, graphite-monochromated Mo KR radiation
(λ ) 0.71073 Å), Fcalcd ) 1.790 g cm-3, F(000) ) 1928, colorless
1
B(C6F5)3 in the starting (C6F5)3BOH2. H NMR (CD2Cl2, 298 K):
crystal, 0.24 × 0.22 mm × 0.14 mm3, µ(Mo KR) ) 0.196 mm-1
,
δ 2.66 (s, 3H, CH3), 4.90 (br s, 4H, NH, OH and CH2), 7.27-7.70
(m, 4H, Ar). 1H NMR (CD2Cl2, 215 K): δ 2.42 (s, 1H, OH), 2.70
(s, 3H, CH3), 4.17 (s, 2H, CH2), 7.03 (m, 1H, NH), 7.28-7.54 (m,
3H, H5, H6, and H7), 7.57-7.71 (m, 1H, H4). COSY (CD2Cl2,
215 K): δ(1H)/δ(1H) 7.57-7.71/4.17 (H4/CH2), 4.17/2.70 (CH2/
CH3), 7.03/7.28-7.54 (NH/Ar). NOESY (CD2Cl2, 215 K): δ(1H)/
δ(1H) 4.17/2.70 (CH2/CH3), 2.42/4.17 (OH/CH2), 4.17/7.57-7.71
(CH2/H4). Colorless crystals suitable for X-ray diffraction were
obtained from a solution in CH2Cl2/pentane (9:1) stored at -20 °C
for 5 days, which unexpectedly resulted to be the [2-methyl-3H-
indolium]3 salt.
empirical absorption correction (SADABS,44 30754 symmetry-
equivalent reflections, effective data-to-parameters ratio 10.2),
minimum/maximum transmission factors 0.945/0.973, Bruker
SMART diffractometer, ω scans (∆ω ) 0.3°), 2400 frames each
at 20 s of exposure, keeping the detector at 5.0 cm from the sample,
1.5° e θ e 25.0°, index ranges h ) -16 f +16, k ) -13 f
+13, l ) -26 f +26, 41262 reflections, 6359 of which are unique
(Rint ) 0.0239, Rσ ) 0.0149), 5836 reflections with I > 2σ(I), no
intensity decay, solution by direct methods (SIR9745) and subse-
quent Fourier synthesis, anisotropic full-matrix least-squares refine-
ment on F2 using all reflections (SHELX9746), hydrogen atoms
placed in idealized positions, data/parameters 6359/596, S(F2) )
1.036, R[F, I > 2σ(I)] ) 0.0315, Rw(F2, all data) ) 0.0873,
b. Reaction in the ratio 1:2. Synthesis of [2-Methyl-3H-
indolium]3. A solution of 2-methylindole (63.4 mg, 0.474 mmol,
in 3 mL of CH2Cl2) was added at room temperature to a suspension
of (C6F5)3BOH2 (507 mg, 0.957 mmol, in 5 mL of CH2Cl2) to give
2
weighting scheme w ) 1/[σ2(Fo ) + (0.08P)2 + 2P], where P )
1
2
(Fo + 2Fc2)/3, maximum/minimum residual electron density
a pink suspension. A H NMR analysis after 10 min of stirring
0.396/-0.213 e Å-3
.
showed the almost complete conversion to [2-methyl-3H-indolium]-
3, together with traces of N-[B(C6F5)3]-2-methyl-3H-indole.5a 1H
NMR (CD2Cl2, 298 K): δ 2.95 (s, 3H, CH3), 4.49 (br s, 2H, CH2),
6.86 (br s, 4H, NH, and OH’s), 7.53-7.79 (m, 4H, Ar).
Acknowledgment. T.B. and G.D. thank Basell Poly-
olefins for partial funding of this work. Thanks are also due
to the Italian CNR (ISTM) for providing facilities for inert
atmosphere and low-temperature experiments.
Preparation of [NHEt3]+[(F5C6)3BOB(C6F5)2]- ([NHEt3]4). A
CD2Cl2 solution of (C6F5)2BOH (413 mg, 1.14 mmol) was added
at 0 °C to a suspension of B(C6F5)3 (584 mg, 1.14 mmol) in CD2-
Cl2, the mixture was allowed to reach room temperature, and then
1 equiv of NEt3 was added. Removal of the solvent at reduced
pressure afforded a white solid characterized as the novel species
[NHEt3]4 formed quantitatively. Colorless crystals suitable for X-ray
analysis were obtained by cooling at -20 °C a solution of [NHEt3]-
4 in a 2:1 mixture of pentane/dichloromethane. 1H NMR (CD2Cl2,
Supporting Information Available: Seven figures showing
details of the NMR experiments. This material is available free of
charge via the Internet at http://pubs.acs.org. CCDC 251191 and
251192 contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12
Union Rd., Cambridge CB2 1EZ, U.K., fax +44-1223-336033,
e-mail deposit@ccdc.cam.ac.uk).
3
298 K): δ 1.44 (t, 9H, CH3, JHH ) 7.2 Hz), 3.33 (q, 6H, CH2,
3JHH ) 7.4 Hz), 5.14 (br t, 1H, NH). 19F NMR (CD2Cl2, 253 K):
δ -134.11 (m, 4Fortho), -134.68 (m, 6Fortho), -154.37 (m, 2Fpara),
-161.55 (m, 3Fpara), -163.49 (m, 4Fmeta), -166.64 (m, 6Fmeta).
X-ray Diffraction Structural Analysis. Crystal data for [C9H10N]-
3: C45H13B2F30NO2, Mr ) 1191.18, triclinic, space group P1h (No.
2), a ) 12.606(4) Å, b ) 12.910(4) Å, c ) 14.040(4) Å, R )
85.99(1)°, â ) 85.17(1)°, γ ) 74.96(1)°, V ) 2196.1(12) Å3, Z )
2, T ) 295(2) K, graphite-monochromated Mo KR radiation (λ )
0.71073 Å), Fcalcd ) 1.801 g cm-3, F(000) ) 1172, colorless crystal,
IC0502168
(44) Sheldrick, G. M. SADABS; Universita¨t Go¨ttingen, Go¨ttingen, Germany,
1996.
(45) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giaco-
vazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna,
R. J. Appl. Crystallogr. 1999, 32, 115.
(46) Sheldrick, G. M. SHELX97; Universita¨t Go¨ttingen, Go¨ttingen, Ger-
many, 1997.
Inorganic Chemistry, Vol. 44, No. 14, 2005 5041