The basicity of unsaturated hydrocarbons as probed by hydrogen-bond- acceptor ability: Bifurcated N-H+...μ hydrogen bonding

Article abstract of DOI:10.1002/chem.200800337

The competitive substitution of the anion (A^-) in contact ion pairs of the type [Oct₃NH⁺]B(C₆F₅V by unsaturated hydrocarbons (L) in accordance with the equilibrium Oct ₃NH⁺ ...A^- + nL ? [Oct ₃NH⁺-L_n]A^- has been studied in CCl₄. On the basis of equilibrium constants, K, and shifts of vNH to low frequency, it has been established that complexed Oct₃NH ⁺-L., cations with n = 1 and 2 are formed and have unidentate and bifurcated N-H⁺...μ hydrogen bonds, respectively. Bifurcated hydrogen bonds to unsaturated hydrocarbons have not been observed previously. The unsaturated hydrocarbons studied include benzene and methylbenzenes, fused-ring aromatics, alkenes, conjugated dienes, and alkynes. From the magnitude of the redshifts in the N-H stretching frequencies, AvNH, a new scale for ranking the It basicity of unsaturated hydrocarbons is proposed: fused-ring aromatics ≤ benzene < toluene < xylene < mesitylene < durene < conjugated dienes ≈ 1 -alkynes < pentamethylbenzene < hexamethylbenzene < internal alkynes ≈ cycloalkenes < 1 -methylcycloalkenes. This scale is relevant to the discussion of It complexes for incipient protonation reactions and to understanding N-H⁺... μ hydrogen bonding in proteins and molecular crystals.

Full text of DOI:10.1002/chem.200800337

DOI: 10.1002/chem.200800337
The Basicity of Unsaturated Hydrocarbons as Probed by Hydrogen-Bond-
+
ꢀ
Acceptor Ability: Bifurcated
A
Evgenii S. Stoyanov,* Irina V. Stoyanova, and Christopher A. Reed*[a]
Abstract: The competitive substitution
rated hydrocarbons have not been ob-
served previously. The unsaturated hy-
drocarbons studied include benzene
and methylbenzenes, fused-ring aro-
matics, alkenes, conjugated dienes, and
alkynes. From the magnitude of the
ing the p basicity of unsaturated hydro-
carbons is proposed: fused-ring aro-
maticsꢁbenzene<toluene<xylene<
of the anion (Aꢀ) in contact ion pairs
ꢀ
of the type [Oct3NH+]B
A
saturated hydrocarbons (L) in accord-
ance with the equilibrium Oct3NH+
···Aꢀ + nL Q [Oct3NH+···Ln]Aꢀ has
been studied in CCl4. On the basis of
equilibrium constants, K, and shifts of
nNH to low frequency, it has been es-
tablished that complexed Oct3NH+···Ln
cations with n=1 and 2 are formed
mesitylene<durene<conjugated
di-
A
G
A
AHCTREUNG
ꢀ
redshifts in the N H stretching fre-
ACHTREUNG
quencies, DnNH, a new scale for rank-
ACHTREUNG
discussion of p complexes for incipient
protonation reactions and to under-
Keywords: basicity · bifurcated hy-
drogen bonds · IR spectroscopy ·
+
ꢀ
standing N H ···p hydrogen bonding
ꢀ
and have unidentate and bifurcated N
in proteins and molecular crystals.
hydrogen bonds
hydrocarbons
·
unsaturated
H+···p hydrogen bonds, respectively.
Bifurcated hydrogen bonds to unsatu-
ꢀ
Introduction
In the gas phase, weak X H···p hydrogen-bonding inter-
actions were identified in 1981 in complexes of HCl with al-
kynes, alkenes, and arenes.[4–6] T-shaped structures were de-
duced for the alkene and alkyne complexes (A and B) with
ꢀ
X H···p hydrogen-bonding interactions have been studied
for a long time. Following the suggestion by Dewar in 1946
that p complexes might be involved in the mechanism of
electrophilic aromatic substitution,[1] Brown and Brady
showed in 1952 that HCl formed complexes with aromatic
compounds and alkenes in hydrocarbon solutions.[2] Infor-
mation on the molecular structures of these complexes was
not available, but equilibrium binding constants were deter-
mined and, within families of structurally similar com-
pounds, a rough correlation with other measures of basicity
was found. When exceptions were noted, they were attribut-
ed to differences in p versus s basicity. Stability constants
became the primary method of studying p complexes,[3] but
a more specific measure of p basicity in condensed phases
did not develop.
ꢀ
the X H bond at approximately 908 and directed to the
center of the multiple C C bond. Recent atomic deforma-
ꢀ
tion calculations indicate that maximum electron density lies
at the mid-point of the double and triple bonds.[7] With ben-
ꢀ
zene, the X H bond is preferentially directed near the
center of the aromatic ring in a p-face manner (Scheme 1,
structure C).
Scheme 1. Structures of hydrogen-bonded alkenes (A), alkynes (B), and
benzene (C).
[a] Prof. E. S. Stoyanov, I. V. Stoyanova, Prof. C. A. Reed
Department of Chemistry
University of California
Riverside, Riverside, California 92521 (USA)
Fax : (+1)951-827-2027
E-mail: stoyanov_e@reedgroup.ucr.edu
chris.reed@ucr.edu
Stronger interactions with unsaturated hydrocarbons
occur when the complex involves positively charged proton
[8]
donors, such as the N+ H bond of the ammonium ion.
ꢀ
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FULL PAPER
Oct3NHþ Á Á Á fF20ꢀgþnL Ð ½Oct3NHþ Á Á Á nLŠfF20ꢀg
The addition of this electrostatic effect increases the binding
enthalpy of the complex by several kcalmolꢀ1 such that DH
for the gas phase NH4+···C6H6 complex is 19.3 kcalmolꢀ1.[9]
Calculations indicate that the energetics are dominated by
the charge/quadrupole interaction[10] and do not significantly
distort the p-electron distribution.
ð1Þ
Contact IP
Solvated IP
We have recently shown that nNH bands in the IR spectra
of the Oct3NH+ cation in contact IPs of the type
[Oct3NH+]Aꢀ (Aꢀ =anion) decrease in frequency as a func-
The T-shaped structure of the 2-butyne·HCl complex
tion of increasing hydrogen-bond-acceptor strength of the
anion, leading to a sensitive basicity scale for anions.[28] The
same concept is now applied to neutral molecules. In addi-
tion to changes in the frequency of the nNH band, its inten-
sity, band width, and shape give further information about
hydrogen bonding in the Oct3NH+···L interaction.
(Scheme 1 structure B)[11] and the p-face structure of NH4
+
···C6H6 (Scheme 1 structure C)[12] have been confirmed in
the solid state by X-ray crystallography. In the H3O+·3C6H6
cation,[13] the O H bonds are directed more closely to
ꢀ
carbon atoms than to the centroid of benzene. More gener-
ꢀ
ally, the X H···p hydrogen-bonding interaction has become
We anticipated that these two measures of the basicity of
L would give somewhat different rankings. They measure
different properties: nNH frequencies give a measure of the
NH+···L interaction strength, whereas stability constants are
a composite of the interaction strength and the differences
in solvation energies of all reactants and products. Neverthe-
less, we did tend to choose mostly six-membered ring com-
pounds for this study so that solvation energy differences
between different compounds (or classes of compounds)
were minimized. We also anticipated that these rankings
would not correlate with existing measures of the basicity of
unsaturated hydrocarbons, for example, gas-phase ionization
energies, complexation enthalpies, and proton affinities[29,30]
or condensed-phase equilibrium constants[3] and nucleophi-
licities.[25] These scales are based on different thermodynam-
ic quantities or physical phenomena that do not readily lend
themselves to quantitative connections between gas- and
widely recognized as a weak but prevalent intermolecular
interaction in small molecule crystallography.[14–19] The range
of acceptors now includes nitrogen heterocycles[20] and p-
ligand transition-metal complexes[18] and these types of in-
teractions are beginning to be utilized in crystal engineer-
ing.[21] X H···p interactions have also been identified in pro-
ꢀ
teins[19,22,23] and N H···p interactions can be sufficiently
ꢀ
strong to affect secondary (folding) structure.[24]
ꢀ
Estimations of the strengths of X H···p interactions have
typically relied on the determination of the thermodynamics
of complexation. This can be informative in the gas phase,
but in condensed phases the difficulty of separating the in-
trinsic strength of the hydrogen-bonding interaction from
the contribution of solvation energies to the stability con-
stant has thwarted the development of useful scales of p ba-
sicity. Mayr and co-workers have recently developed a nu-
cleophilicity scale for p bases based on extensive kinetic
data for reactions with electrophiles.[25,26] In favorable cases,
distance information from crystallography might be utilized
in a comparative manner,[27] but without the precise location
of the hydrogen atom, typically achieved only in neutron
diffraction studies, this method is limited.
ꢀ
condensed-phase data. The X H···p interaction under study
is a weak, localized effect, whereas the energetics of com-
plete protonation (s basicity) involve larger, more global
molecular phenomena.
A specific point of interest is the possible existence of bi-
+
ꢀ
furcated p hydrogen bonds of the type X H ···2L, which
In the present work, with the idea of developing a ranking
system of interaction strength, we use IR spectroscopy to
explore the N H···p interaction of the trioctylammonium
are formed by a positively charged cation with two unsatu-
rated hydrocarbons (L). The formation of bi- and even tri-
furcated hydrogen bonds has occasionally been observed by
X-ray crystallography for trialkylammonium cations, when
ꢀ
ion with alkynes, alkenes, and arenes upon reaction with ion
ꢀ
pairs (IPs) of the type [Oct3NH+]{B
A
they interact with O or N-heteroatoms, such as the
Carbon tetrachloride is a weakly solvating, low dielectric
solvent that favors contact IP formation. Of commercially
available anions, the perfluorinated tetraphenylborate ion,
(C2H5)3NH+ cation with three hydroxyl oxygen atoms.[31,32]
Sometimes one of the acceptors of a bifurcated hydrogen
bond is an oxygen atom, the other an alkyne[19,33,34] or
arene.[24,35] Bi- or multifurcated hydrogen bonds are fre-
quently deduced from protein crystallography.[36] The solu-
tion-phase existence of compounds with bifurcated p hydro-
gen bonds involving only unsaturated hydrocarbons has not
been reported.
ꢀ
B
G
wards the Oct3NH+ ion, as judged by nNH stretching fre-
quencies in contact IPs.[28] This allows weak acceptors (L),
such as unsaturated hydrocarbons, to compete with the
{F20ꢀ} anion and bind to the NH+ groupof the cation, form-
ing solvated IPs of the type [Oct3NH+·L]
Further interest in X H···p interactions arises from the
ꢀ
complexes, the extent of the Oct3NH+···L interaction can be
measured by two methods: equilibrium binding constants
(K) and changes in nNH in the IR spectrum. Equilibrium
binding constants can be compared across different classes
of p bases when K values for 1:1 or potentially 1:n complex-
ation are determined from equilibria [Eq. (1)].
long-held belief that hydrogen-bonded complexes are incipi-
ent proton-transfer reactions, that is, models for the early
stages of p systems reacting with strong acids.[37] Indeed, in
the 1970s there was an intense debate on whether p com-
plexation of electrophiles could be rate-determining in the
mechanism of electrophilic aromatic substitution.[38] Current
consensus holds that s (rather than p) complexes and
Ar+Eꢀ charge-transfer IPs[39] are more important in the tran-
Chem. Eur. J. 2008, 14, 7880 – 7891
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7881

E. S. Stoyanov, I. V. Stoyanova, and C. A. Reed
sition state, nevertheless weak p complexation is widely be-
lieved to precede formation of these activated complexes.
Our interest in models for incipient proton transfer is
piqued by the recent isolation and X-ray structural charac-
terization of a fully protonated alkene (e.g., tert-butyl
cation),[40] the equivalent of a protonated alkyne (i.e., a
vinyl cation),[41] and protonated benzene (C6H7+).[42] More
information is needed on pre-protonation complexes if they
are to become widely accepted as early intermediates in
protonation reactions.
Results
IR spectra of 0.04m solutions of [Oct3NH+]{F20ꢀ} in CCl4
A
Figure 2. Representative IR spectra showing the formation of [Oct3NH+
···nL]{F20ꢀ} CIPs with their nNH frequencies for arenes L=benzene (a),
toluene (b), mesitylene (c), pentamethylbenzene (d), hexamethylbenzene
(e). Spectra, with the exception of a, are normalized to nNH of the un-
show a narrow nNH band at 3233 cmꢀ1. From studies with
other anions, we know this corresponds to a contact IP
rather than a solvent-separated IP.[28] The band frequency is
independent of concentration between 0.005 and 0.04m.[28]
When unsaturated hydrocarbons (L) are added to these sol-
utions, the intensity of this band decreases with increasing
concentration of L (cL). At the same time a new, lower fre-
quency nNH band arising from complexed ion pairs (CIP)
complexed contact IP at 3323 cmꢀ1
.
librium constant of the reaction (K) to be determined from
Equation (2):
of [Oct3NH+·nL]{F20ꢀ} appear. The intensity of the new
G
n
nNH band grows with increasing cL. An example is shown in
Figure 1 for L=benzene. Representative spectra for other
arenes, normalized to unit intensity of nNH of the uncom-
plexed contact IP (3323 cmꢀ1), are shown in Figure 2.
K ¼ cCIP=ðcLÞ cIP
ð2Þ
in which cL is equilibrium concentration of free L. Because
complexation of the p base is weak and cCIP is small com-
pared with the total concentration of L (coL), the approxima-
tion cL ꢂcoL is valid and Equation (2) becomes Equation (3):
CIP=cIP ¼ KðcoLÞ
ð3Þ
n
c
Taking logarithms of Equation (3) gives Equation (4),
logðcCIP=cIPÞ ¼ log Kþnlog cLo
ð4Þ
in which the slope of the function log
N
N
gives the number, n, of molecules L that are directly bound
to NH group. The equilibrium constant K, that is, the stabili-
ty constant for L-CIP, [Oct3NH+···nL]{F20ꢀ}, can be deter-
mined from the slope of Equation (3) for a fixed value of n.
Aromatic hydrocarbons: As illustrated for benzene and
hexamethylbenzene in Figure 3, the slope of Equation (4) is
G
Figure 1. Evolution of the IR spectrum in the nNH region of 0.4m
[Oct3NH+]{F20ꢀ} in CCl4 as benzene is added. Benzene concentrations in-
crease from zero (a) to 100% (b).
equal to one for all aromatic hydrocarbons studied (L=ben-
zene; toluene; o- and p-xylene; 2,4,6-mesitylene; tetra-,
penta- and hexamethylbenzenes; naphthalene, and phenan-
threne; see Table 1). In other words, only 1:1 adducts with
the NH+ groupare formed. The frequencies of the nNH
band for all compounds are practically independent of the
The concentration of the [Oct3NH+]{F20ꢀ} contact IP (cIP)
A
1
was determined from the decreasing intensity of its nNH
band at 3233 cmꢀ1 as cIP =0.04f, in which f is the scaling
factor with which the nNH band is fully subtracted from the
concentration of L, although the width of this band (S= )
2
shows mild concentration dependence in the case of ben-
zene and the methylbenzenes. As illustrated in Figure 4 for
the representative case of benzene, S = increases with in-
creasing cL. There is a break at approximately 6m benzene
spectrum of the 0.04m solution of [Oct3NH+]
A
1
2
centration of the [Oct3NH+···nL]
A
1
and S = increases more rapidly with increasing concentra-
2
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Chem. Eur. J. 2008, 14, 7880 – 7891

+
ꢀ
Bifurcated N H ···p Hydrogen Bonding
FULL PAPER
Table 1. Characteristics of [Oct3NH+···nL]{F20ꢀ} complexes in CCl4.
nNH[a] [cmꢀ1
]
DnNH[b] [cmꢀ1
]
S = [d] [cmꢀ1
]
1
L
K
n
2
C6H6
0.085
0.114
0.139
0.142
0.148
0.160
0.154
0.99
1.05
1.06
0.97
3156.5
3151
3144
3140
3130
3120
3114
75
27
34
36
38
46
39
35
C6H5CH3
C6H4(CH3)2
C6H3(CH3)3
C6H3(CH3)4
C6H(CH3)5
C6(CH3)6
82–83[c]
88–91[c]
94.5–98[c]
103.5–104.5
113
0.997
1.09
118–119[c]
0.260
1.06
3136
70
67
30
0.458
1.04
3166
39
<0.002
0.99
0.99
ꢂ3075
ꢂ3075
ꢂ158
ꢂ158
155ꢃ5
155ꢃ5
0.084
[f]
0.0725
1.04
ꢂ3072[g]
ꢂ160[g]
0.41
0.257
1.14
1.97
3125
3084
108
149
76
0.216
0.722
0.496
0.98
3125
3084
108
149
49
131
[f]
[f]
[f]
[f]
0.025
0.094
0.998
3100ꢃ5
133ꢃ5
150ꢃ10
0.215
1.15
1.00
1.04
3063
170
185
150
3048, ꢂ2880[e]
[f]
0.254
1.01
(2)[f]
1.08
1.94
0.94
3123
3086
109–117
147
150ꢃ5
[f]
115
2.02
3.65
174
3094
139
161
143
163
3072, ꢂ2880[c]
3090
[f]
water
61
[a] For 1:1 arene complexes at low concentrations of L. [b] The scale is anchored to the contact IP (DnNH=0 at 3233 cmꢀ1), for 1:1 complexes data are
given for low concentrations of L (except for arenes). [c] Frequency interval for solutions of arenes from low to high concentrations. [d] For 1:1 com-
plexes at low concentration of L. [e] nNH band is distorted by Fermi resonance; the true nNH frequency cannot be obtained because both components
are partially overlapped with intense absorptions from the octyl chains of the cation and CH vibrations from the unsaturated hydrocarbon. [f] Cannot be
determined. [g] Calculated from nND frequency (see Table 2).
tion. With the methylbenzenes, the break is shifted to lower
concentrations of L as the number of methyl groups increas-
es, and is approximately 0.32m for hexamethylbenzene. This
indicates some change in the character of the [Oct3NH+···L]
groupat high cL concentrations, presumably due to outer-
sphere solvation of the cation by additionally associated L
[Eq. (5)], that is, the formation of solvent separated com-
plexed ion pairs (SSCIPS) that nevertheless remain specified
as 1:1 hydrogen-bonded complexes.
½Oct3NHþ Á Á Á LŠfF20ꢀgþmL Ð ½Oct3NHþ Á Á Á LŠLmfF20ꢀg
CIP
SSCIP
ð5Þ
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7883

E. S. Stoyanov, I. V. Stoyanova, and C. A. Reed
tene, and two cyclic dienes (1,4-cyclohexadiene and 1,5-
cyclooctadiene) with isolated (nonconjugated) C=C bonds,
A
are able to displace the {F20ꢀ} ion and bind to the NH+
group. The slope of Equation (3) indicates that these com-
plexes have a 1:1 composition with common T-shaped p hy-
drogen bonds of type A (Scheme 1). The K values are given
in Table 1. Because the K value for 1,5-cyclooctadiene is
small, it was only possible to evaluate a 50 vol% solution.
The nNH bands of all of these complexes have low intensity,
are strongly broadened, and are redshifted (Table 1).
ꢀ
Methylcycloalkenes with the unsymmetrical C(CH3)=
ꢀ
CH chromophore (1-methyl-1-cyclopentene and 1-methyl-
1-cyclohexene) in the concentration range below 1m form
1:1 compounds with larger K values, especially for 1-methyl-
1-cyclopentene. Similar to symmetrical cycloalkenes, their
nNH bands show large redshifts and very strong broadening
(Table 1). The redshift of nNH for unsymmetrical alkenes is
even greater than that for symmetrical alkenes. At concen-
trations higher than 1m, the slope of Equation (4) starts to
exceed one and, as the intensity of nNH of the 1:1 com-
pounds decreases, a new, very broad, and greatly redshifted
nNH band with maximum at approximately 2880 cmꢀ1 ap-
pears, increasing in intensity
Figure 3. Slopes of Equation (4) for benzene (1, n=0.99), hexamethyl-
benzene (2, n=0.98), and 1,3,5,7-cyclooctatetraene (3, n=0.50 and 0.98).
with increasing alkene concen-
tration. The higher stoichiome-
try (presumably 1:2), and the
very low nNH value, suggests
the formation of bifurcated p
hydrogen bond (D).
Cycloalkenes with conjugated double bonds, 1,3-cyclo-
A
ability to solvate the NH+ groupthan alkenes with symmet-
rical isolated C=C bonds. For 1,3-cyclohexadiene, the slope
of Equation (4) is close to one at low alkene concentrations
(CL <1m), but at higher concentrations is close to two
(Figure 5). Thus, 1,3-cyclohexadiene forms both 1:1 and 1:2
compounds. In the ranges of cL concentrations at which only
one compound is formed, the dependence of Equation (3)
increases proportionally with concentration, allowing both
K values to be determined (Table 1). The complexes differ
significantly in their nNH frequencies, 3125 cmꢀ1 for the 1:1
complex and 3084 cmꢀ1 for 1:2, indicating distinctly different
types of p hydrogen bonding. If the 1:1 complex has a
normal p hydrogen bond (I), then the 1:2 complex (with the
much lower nNH frequency) must have a bifurcated hydro-
gen bond (II).
+
1
Figure 4. Plot of band width at half height (S = ) for nNH of [Oct3NH
2
···C6H6]{F20ꢀ] with increasing concentration of benzene (2.24–11.2m).
Later we provide evidence to support this proposal. As in-
dicated by the essential constancy of the nNH frequency,
outer-sphere solvation of the CIP by aromatic hydrocarbons
does not change the strength of the NH+···L interaction.
Thus, outer-sphere solvation does not affect the determina-
tion of n or K. In the case of condensed aromatics, naphtha-
lene and phenanthrene, there is no concentration depen-
1
dence of S = of the nNH band, presumably because outer-
sphere association is absent or it is present to a similar
extent over the entire concentration range studied.
2
For the methylbenzenes, C6H6ꢀy(CH3)y, the nNH frequen-
cy decreases regularly with increasing number of methyl
groups (y), which indicates that the strength of
+
ꢀ
N H ···L hydrogen bonding increases with increasing arene
basicity (Figure 2, Table 1). K values increase in a roughly
parallel manner.
Alkenes: From the lack of changes in the IR spectrum, 1-
hexene is unable to displace the {F20ꢀ} anion in the
[Oct3NH+]{F20ꢀ} contact IP, even when used instead of CCl4
as a solvent. On the other hand, cyclohexene and cyclopen-
1
For 1,3,5,7-cyclooctatetraene, the value of n was = at low
2
cL and transitioned to one at higher cL (Figure 3). This indi-
cates that at low cL, 1,3,5,7-cyclooctatetraene acts as two
7884
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Chem. Eur. J. 2008, 14, 7880 – 7891

+
ꢀ
Bifurcated N H ···p Hydrogen Bonding
FULL PAPER
separate diene units bonded to two Oct3NH+ cations, result-
ing in the formation of 2:1 complex III.
slope (n) of Equation (4) increases, indicating formation of
a 1:2 complex with a bifurcated p hydrogen bond (VI).
In the case of 3-hexyne, the nNH frequencies are more
strongly redshifted than for 1-hexyne and the transition
from a 1:1 to a 1:2 complex begins to take place at a lower
alkyne concentration (cL ꢂ0.88) (Figure 6). In the range of
1:1 complex formation, the slope of function cCIP/cCIP =f(cL)
gives a value of K=2.02 (Figure 7) and in the range of 1:2
complex formation, the slope of the function cCIP/cCIP =f(cL)2
gives a value of K=3.65 (Figure 8).
Outer-sphere solvation: The existence of outer-sphere solva-
tion of the complexed Oct3NH···L+ cation by L for arenes
and dienes is indicated by broadening of the nNH band at
Figure 5. The slope of Equation (4) with 1,3-cyclohexadiene concentra-
tion.
Indeed, the exact coincidence of the nNH frequencies of
complexes
I
and III (3125 cmꢀ1) indicates that both
ꢀ
C=C C=C chromophores in 1,3,5,7-cyclooctatetraene are
acting independently, uninfluenced by each another. On the
other hand, the 1,3,5,7-cyclooctatetraene complex with n=1
has the same nNH frequency (3084 cmꢀ1) as complex II
(that is, 1:2 with 1,3-cyclohexadiene). This indicates a 2:2
composition with two bifurcated hydrogen bonds (IV).
The K values of all compounds are given in Table 1.
Figure 6. The slope of Equation (4) with 3-hexyne concentration.
Alkynes: Whereas linear alkenes, such as 1-hexene, do not
complex the NH+ group, linear alkynes do. IR spectra of
the complexes formed by 1- and 3-hexynes are concentra-
tion dependent. In the case of 1-hexyne in the range of cL =
0.26–1.73m, the maximum of the nNH band varies somewhat
from 3124 to 3116 cmꢀ1. This is probably due to a greater
role for alkynes in outer-sphere solvation of the Oct3NH+L
cation, that is, the formation of the solvent-separated IPs
Oct3NH+···L·(Lm){F20ꢀ} with variable m. In this concentra-
tion region, Equation (4) has a linear dependence on cL with
slope n=1 and Equation (3) leads to K=0.254. Therefore,
this complex has a 1:1 composition with common p hydro-
gen bonding (V).
At higher cL (50–100 vol% of 1-hexyne), a new nNH
band from a second complex appears at 3087 cmꢀ1 and the
Figure 7. The slope of Equation (3) with n=1 for 3-hexyne.
Chem. Eur. J. 2008, 14, 7880 – 7891
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7885

E. S. Stoyanov, I. V. Stoyanova, and C. A. Reed
termined as cfLree =f”csLt, in which f is the coefficient with
which the band at 1680 cmꢀ1 in the spectrum of the studied
solution is fully subtracted by using the spectrum of stan-
dard 3-hexyne solution, and csLt is the concentration of L in
the standard solution (in Figure 9 f=0.755, csLt =0.876m or
10 vol%). The concentration of 3-hexyne molecules experi-
encing the conformation change in the solvation shell of the
NH+ groupwas determined from csLolv =cLoꢀcfLree. Figure 10
Figure 8. The slope of Equation (3) with n=2 for 3-hexyne.
high cL. In the case of 1- and 3-alkynes there is also a gradu-
al shift in the nNH frequency (ꢂ8 cmꢀ1). In general, we find
no evidence that the IR spectra of the unsaturated hydrocar-
bon molecules forming outer-sphere solvation shells are al-
tered. Indeed, even those directly p hydrogen bonded to the
NH+ groupare practically unchanged. However, 3-hexyne
is an exception. Even though the frequencies and intensities
of the fundamental vibrations of 3-hexyne are scarcely
changed, the frequency of one of its combination bands at
1680 cmꢀ1 decreases significantly (by 25 cmꢀ1) to 1655 cmꢀ1
(Figure 9). This combination band may include one of the
Figure 10. Dependence of m on 3-hexyne concentration.
shows the dependence of the number of conformationally
affected 3-hexyne molecules per NH+ group( m) as a func-
tion of coL. The composition of the solvating shell is unex-
pectedly large. At coL =0.088m (1 vol%), 92.5% of the
alkyne is absorbed into the solvation shell, bringing the
value of m to approximately 13. At coL =4.38m (50 vol%),
this increases to about 30. Thus, substantial clustering of the
alkyne occurs around the CIP. Most of the alkyne is not ran-
domly mixed in the solution in CCl4.
To rule out the possibility that this alkyne solvation might
ꢀ
originate from solvation of the B(C6F5)4 anion or the octyl
ꢀ
chains of cation, as opposed to that from the N H···alkyne
interaction, the solvation of the tetraoctylammonium salt of
ꢀ
B(C6F5)4 in 3-hexyne was studied under identical condi-
tions. In the IR spectra of these solutions, the intensity of
the band at 1680 cmꢀ1 did not significantly decrease as 3-
hexyne was added. A weak band grew in at 1671 cmꢀ1 and
the small redshift of this band relative to free 3-hexyne
(1680 cmꢀ1, D=9 cmꢀ1) suggests a minor association with
the IP. The stoichiometric ratio determined as csLolv/co is
around 2.4 for 10 vol% of 3-hexyne and increases to around
3.1 for 20 vol% solution.
Figure 9. IR spectra of a solution of 0.04m Oct3NH+{F20} in CCl4 with
10 vol% of 3-hexyne (c) and a solution of 10 vol% 3-hexyne in CCl4
(g). The dashed spectrum shows the band at 1655 cmꢀ1, which was iso-
lated by sequential subtraction of the spectrum of free hexyne and the
band at 1642 cm of {F20}ꢀ.
skeletal bending vibrations in the 200–550 cmꢀ1 region that
are conformationally dependent.[43] Most probably, the for-
mation of an outer-sphere shell causes the 3-hexyne mole-
cules to change their conformation. The relative band inten-
sities at 1680 and 1655 cmꢀ1 may therefore be used to inves-
tigate outer-sphere solvation of the Oct3NH+ cation by 3-
hexyne.
Finally, for the purposes of comparison of unsaturated hy-
drocarbons to a traditional hydrogen-bond acceptor, we
have investigated the interaction of H2O with the
[Oct3NH+]
C
ꢀ
(0.0055m H2O) shows nOH bands from water bound to the
ꢀ1
ꢀ
N H groupat 3674 and 3596 cm
and a new nNH band at
From the intensity of the band of free 3-hexyne at
3090 cmꢀ1 (Figure 11). From the concentrations of the com-
plexed water and the Oct3NH+ cation, it was determined
1680 cmꢀ1, the equilibrium concentration in solution was de-
7886
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Chem. Eur. J. 2008, 14, 7880 – 7891

+
ꢀ
Bifurcated N H ···p Hydrogen Bonding
FULL PAPER
This phenomenon, in varying degrees, may explain the
change in the nNH band width as a function of progressive
methylation of benzene. As shown in Figure 12, the band
Figure 11. IR spectrum of 0.0156m (Oct)3NH+{F20ꢀ} in CCl4 with 0.0055m
water in the frequency region of nOH (a) and nNH (b). c: measured
spectrum. a: spectrum of 0.0055m of water in CCl4. b: spectrum of
[(Oct)3NH+···OH2]{F20ꢀ} IP obtained by subtraction of a from c.
Figure 12. Dependence of the width of the nNH band on the DnNH fre-
quency for a set of arenes C6H6ꢀy(CH3)y in which y=0–6.
that the molar ratio was close to one. The K value is given
in Table 1.
width reaches a maximum at durene. It is smallest for sym-
metrical benzene and hexamethylbenzene. Possibly, in the
most symmetrical arenes, the proton is directed at the exact
center of the aromatic ring and this minimizes the angular
Discussion
In the solutions under study, three types of IPs are formed
that differ in the character of the solvation of the NH+
group: 1) 1:1 mono-CIP, [Oct3NH+···L]{F20ꢀ}, with the cation
and anion separated by one unsaturated hydrocarbon mole-
cule L, 2) SSCIPs with some intervening unsaturated hydro-
carbon molecules Lm, [Oct3NH+···L]·Lm{F20ꢀ}, in which the
cation retains the essential character of mono-CIPs, and
3) 1:2 di-SSCIPs [Oct3NH+···2L] ·Lm{F20ꢀ}, with two mole-
cules of L directly bonded to the NH+ group, that is, with
bifurcated hydrogen bonds. The transition from CIP to
SSCIP, caused by outer-sphere solvation of the NH+ group
by excess L, has practically no effect on the experimentally
determined values of n and K. Therefore, both CIP and
SSCIP formulations are attributed to 1:1 compounds.
fluctuation of the NH bond. This is in line with neutron dif-
+
ꢀ
ꢀ
fraction data in which N H vectors of the N H···phenyl
+
interactions between an NH4 ion and phenyl groups are
time-averaged towards the centroid of the phenyl rings,
ꢀ
whereas the 50% probability ellipsoids of the N H vector
samples the entire face of the aromatic ring.[12]
In the 1:1 complexes formed by cyclic molecules with a
pair of conjugated C=C bonds, the functionality acts as a
ꢀ
single C=C C=C chromophore. The absorption of this chro-
mophore for the free (noncomplexed) molecule of L has
two nC=C bands (1603 and
1578 cmꢀ1) of medium to low
intensity. Upon complex forma-
tion in I and III the nC=C ab-
sorption does not change, which
indicates that this type of inter-
action is symmetrical (F).
p Hydrogen bonding: The nNH band is exceedingly broad
(ꢂ150 cmꢀ1) for both alkene and alkyne complexes
(Table 1). For example, comparing the 1-hexyne complex
with the 1,3-cyclohexadiene, 1,3,5,7-cyclooctatetraene, and
tetra- and pentabenzene complexes, all with similar nNH
frequencies, the nNH band width (S = ) is two to four times
higher. Some contribution to the broadness that develops in
the nNH band in all complexes may arise from outer-sphere
solvation effects. Another, perhaps larger, effect may be be-
cause the p hydrogen bond directed towards the center of
The four conjugated C=C groups of 1,3,5,7-cycloocta-
ꢀ
A
1
chromophores that do not influence one another. This case
is distinguished from 1:1 complexes formed by arenes with
formally three conjugated C=C bonds. These act as a single
aromatic chromophore with an interaction strength lower
2
ꢀ
than that of the C=C C=C chromophore (Table 1).
ꢄ
the C=C or C C bond is not
Conjugated dienes and alkynes show an unexpectedly
high capacity for the formation of 1:2 complexes with bifur-
cated p hydrogen bonds (such as in complexes II, IV, and
VI). The bonding of the second base is indicated by greater
DnNH shifts. This phenomenon may be much more preva-
lent than previously realized and may play important role in
synthetic organic chemistry and biochemistry.
fixed, but fluctuates in the
limits of some angle (a) be-
cause the maximum of the p-
electron density centered be-
tween two carbon atoms is very
flat (E).
Chem. Eur. J. 2008, 14, 7880 – 7891
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7887

E. S. Stoyanov, I. V. Stoyanova, and C. A. Reed
Trends in nNH and K: Application of the nNH frequency as
nNH for the 1,4-cyclohexadiene complex from its nND
value (Table 2). The nND frequencies for other hydrocar-
bons with higher K values and faster H/D exchange could
not be determined with reliable accuracy.
Only two complexes show definite distortion of the nNH
band from Fermi resonance, the 1:1 complex with 1-methyl-
1-cyclopentene and the 1:2 complex with 3-hexyne. An addi-
tional broad component appears at around 2880 cmꢀ1. The
true nNH frequency is the center of gravity of the whole ab-
sorption, but this cannot be determined because both com-
ponents are partially overlapped with intense absorptions
from the octyl chains of the cation and CH vibrations from
the unsaturated hydrocarbon.
Application of the K value as a probe of the complexing
ability of unsaturated hydrocarbons in 1:1 compounds with
the Oct3NH+ cation leads to the following order: 1-al-
kenes !cycloalkenes<benzene<methylbenzenes<1-al-
kynesꢁconjugated dienesꢁ1-methylcycloaklenes<internal
alkynes<condensed aromatics. While there is some correla-
tion with the nNH scale ordering, a close correspondence is
not expected because K values are influenced not only by
hydrogen-bonding strength, but also by solvation energy
changes of both reactants and products. It is not possible to
an IR probe of the relative interaction strength of N+
ꢀ
H
with unsaturated hydrocarbons can be made for all 1:1 com-
pounds. This indicates that p basicity increases phenanthra-
cene<naphthalene<benzene<toluene<xylene<mesityl-
ACHTREUNG
ACHTREUNG
AHCTREUNG
aromatics show slightly lower p basicities than single ring
benzenes. This is a reversal from that observed in gas-phase
proton affinities[29] and can be understood in terms of the
importance of large size in stabilizing gas-phase cations. The
ranking for increasingly methylated benzenes correlates
with expectations based on traditional measures of basicity,
showing that the inductive effects of methyl groups do affect
p basicity. Alkenes, especially 1-methylcycloalkenes, register
more basic than alkynes. This is a counterintuitive result in-
asmuch as alkynes might be considered more p-electron rich
than alkenes.
The IR spectra of the 1:2 complexes with bifurcated hy-
drogen bonds are less suitable for ranking interaction
strength because they are formed more rarely than 1:1 com-
plexes and their nNH frequencies cannot be determined
with the same accuracy as for 1:1 complexes.
ꢀ
extract the N H···L interaction strength from the composite
When the nNH frequency is less than 3054 cmꢀ1, this band
becomes subject to Fermi resonance interactions with over-
tones of low frequency vibrations.[28] If this takes place, the
band becomes distorted and a broad asymmetric component
appears in the low frequency region (2700–2950 cmꢀ1). The
best way to determine the presence of the Fermi resonance
is to study deuterated analogues because the nND band of
Oct3ND+{F20}ꢀ lies in a frequency range that is not coupled
with overtones of low-frequency vibrations.[28] However,
these experiments can be thwarted by H/D exchange reac-
tions. For example, the D atom of the Oct3ND+ cation read-
ily exchanges with H atoms of alkynes, such that the IR
spectra of [Oct3ND+···L]{F20} IPs with L=1-hexyne or 3-
hexyne developed only the nNH band. Similar fast H/D ex-
change was observed in the D3O+·(C6H6)3 cation in solution
in benzene, the spectrum of which showed only bands due
to the H3O+ cation and C6H5D.[44] H/D exchange is much
slower between the Oct3ND+ cation and cyclohexene and
cyclohexadienes. Thus, IR spectra of these showed a single
symmetric nND band with nNH/nND isotope ratios close to
the ideal harmonic oscillator value of 1.34 (Table 2). This
means that their nNH bands are not disturbed by Fermi res-
onance. Using this isotope ratio, it is possible to calculate
of enthalpic and entropic terms that make up the observed
K value. Nevertheless, for a family of structurally very simi-
lar compounds, such as methylbenzenes or, with more cau-
tion, hexacyclic mono- and dienes, the solvation energy ef-
fects may be approximately leveled and K may reflect hy-
drogen-bond interaction strengths.
To illustrate this point, consider the correlation between
K and DnNH for progressively methylated benzenes. For 1:1
[Oct3NH+···L]{F20ꢀ} CIPs with C6H6ꢀy(CH3)y, the low fre-
quency shift relative to the uncomplexed contact IP, DnNH,
increases proportionally with increasing number of CH3
groups y (Figure 13). This mirrors the well known increasing
s basicity of progressively methylated benzenes towards
protonation[45] and suggests that p basicity follows the same
qualitative trend. Each additional CH3 groupincreases
DnNH an average increment of 7.2 cmꢀ1. The increment is
Table 2. The nNH/D [cmꢀ1] frequencies of [Oct3NH+···nL]{F20ꢀ} com-
plexes.
L
Complex nNH
nND nNH/nND
2260 ꢂ1.36
1-cyclohexene
1,4-cyclohexadiene
1:1
1:1
1:2
ꢂ3075
ꢂ3072[a] 2301
-
1,3-cyclohexadiene
3084
3233
2321 1.33
2407 1.34
noncomplexed Oct3NH/D+{F20}
[a] Calculated from nND. Unavailable experimentally due to overlap
with strong nCH bands from the CH=CH group.
Figure 13. Dependence of DnNH shifts on the number of CH3 groups in
methylbenzenes.
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Chem. Eur. J. 2008, 14, 7880 – 7891

+
ꢀ
Bifurcated N H ···p Hydrogen Bonding
FULL PAPER
very similar for the first three methyl groups (ꢂ6.5 cmꢀ1),
larger for the fourth (9.2 cmꢀ1) and fifth (9.4 cmꢀ1), and
lower for the sixth (5 cmꢀ1). This may reflect a steric effect
and changes in the p-electron density as a function of the lo-
gen-bonding acceptor. Their nNH frequencies are similar as
well. Probably because these p donors are all unsubstituted
six-membered rings, the solvation energy differences are lev-
eled and their comparable binding constants are in line with
their comparable basicity on the NH scale.
ꢀ
cation of the N H···p bond, that is, being closer to a CH
rather than C(CH3) carbon atom of the aromatic ring, inas-
much as the smallest increment occurs at hexamethylben-
zene in which no unmethylated C atoms remain. For ortho-
and para-xylenes (y=2), no difference in their p basicities
was discernable by the nNH probe (Table 1).
Some comments can be made about the p basicity of the
ꢀ
conjugated diene C=C C=C chromophore. The nNH fre-
quencies of complexes formed by both 1,3-cyclohexadiene
and 1,3,5,7-cyclooctatetraene coincide when they have uni-
dentate p hydrogen bonds (structures I and III respectively)
The K values for progressively methylated benzenes also
increase with an increasing number of methyl groups
(Figure 14), but the trend is not as smooth as with nNH and
or bifurcated p hydrogen bonds (structures II and IV)
ꢀ
(Table 1). Therefore, the basicity of the C=C C=C chromo-
phore apparently does not depend on the size of the cycle
or on joining two of the same chromophores. Coincidentally,
the NH scale indicates similar basicity for conjugated dienes
with 1-hexyne (DnNHꢂ109 cmꢀ1). Movement of the C=C
bond to an internal part of the hydrocarbon chain in 3-
hexyne results in
a significant increase in p basicity
(DnNH=139 cmꢀ1), reflecting the stronger electron dona-
tion of an alkyl substituent versus H.
The lack of a broad correlation between nNH and K,
except perhaps in a closely related family of compounds, is
also illustrated by the comparison of unsaturated hydrocar-
bons with a common hydrogen-bonding base, water. The
nNH shift for [Oct3NH+···OH2]{F20ꢀ} is about the same as
for an internal alkyne, but the K value is about two orders
of magnitude higher. This very large difference in complex
ability can be understood in terms of solvation energy differ-
ences between the polar water molecule and the less-polar
alkyne. The NH+···L interaction strength may be very simi-
lar, but complexed water will not be as well solvated as the
alkyne in nonpolar CCl4, thereby driving the equilibrium to-
wards product formation.
Figure 14. Dependence of K on the number of CH3 groups in methylben-
zenes.
there is a reversal at hexamethylbenzene. For toluene and
xylenes, the increase from benzene is significant, but for the
higher methylated benzenes the incremental increase is less.
The origins of the discontinuities presumably lie in the sub-
tleties of the solvation energy contributions to K, suggesting
that nNH is a better probe of the p basicity of arenes.
Proceeding to the condensed aromatics, naphthalene and
phenanthrene, the lack of a correlation between nNH and K
becomes more obvious. Relative to benzene, the DnNH
values actually decrease: 5 cmꢀ1 for naphthalene and a few
more cmꢀ1 for phenanthrene (Table 1) even though the K
values increase significantly. Therefore, by the nNH measure
of arene p basicity, condensed aromatics are somewhat
weaker p bases as the number of fused rings increases. The
K values, however, increase significantly as the number of
fused rings increases.
The role of solvation energy is well illustrated by the ob-
servation that linear alkenes, such as 1-hexene, are unable
to compete with the anion in the Oct3NH+{F20ꢀ} IP, whereas
cycloalkenes can. There is no reason to believe that the in-
trinsic basicity of these C=C bonds towards hydrogen bond-
ing are very different and yet the binding constants differ
significantly. On the other hand, the K values for 1:1 com-
plexes of cyclohexene, 1,4-cyclohexadiene and benzene are
all similar despite the different functionality of the hydro-
The most p-basic chromophores, that is, internal alkynes,
ꢀ
conjugated dienes, and the asymmetric cyclic C(CH3)=
CH group, can bind to the NH+ moiety in a bifurcated
ꢀ
manner forming 1:2 and 2:2 complexes (structures IV, VI
and VIII). To the best of our knowledge, the existence of
this type of bifurcated p hydrogen bond has not been pre-
dicted theoretically nor determined experimentally before.
Relative strength of the N+ H Proton Donor: To get an in-
ꢀ
dication of the strength of the Oct3NH+ ion as a hydrogen-
bond donor, it is instructive to compare the NH+ interaction
with those of the hydrated proton. Such an evaluation can
be made by comparing the values of the redshift of the
nP=O band in tributylphosphate (TBP) in Oct3NH+···TBP
complex with those in the complexes with H+(H2O)nTBPm.
As shown in Table 3, the proton-donor ability of Oct3NH+
cation on the nPO scale is roughly comparable to that of the
+
tetra-solvated H5O2 cation. It is considerably weaker than
that of the H3O+ ion, but much stronger than that of a free
H2O molecule. The basicity of the TBP molecule and that of
self-associated water molecule are practically the same.
Thus, the interaction of the hydrated proton, H+(H2O)n (n>
2), with unsaturated hydrocarbons is expected to be very
similar to those found for (Oct)3NH+ cation.
Chem. Eur. J. 2008, 14, 7880 – 7891
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7889

E. S. Stoyanov, I. V. Stoyanova, and C. A. Reed
Table 3. Comparison of proton-donor abilities of the Oct3NH+ cation
and proton hydrates through low frequency shifts of the nP=O band in
TBP.
All unsaturated hydrocarbons were purchased from Acros and used with-
out additional purification. Solutions were prepared by dissolving a
weighed quantity of [Ph3C][B(C6F5)4] and the solid hydrocarbon or a
measured volume of the liquid hydrocarbon in CCl4. The [Oct3NH]
[B(C6F5)4] concentration in all solutions under study was constant at
0.04m. IR spectra were recorded by using a Shimadzu-8300 FTIR spec-
trometer in the n˜ =4000–450 cmꢀ1 range. A cell with Si windows with a
0.036 mm separation at the beam transmission point was used. To avoid
interference effects, the cell configuration was slightly wedge-shaped. IR
data were manipulated using GRAMS/AI software (Galactic Industries,
Salem, NH).
Compound
nP=O
DnP=O
Reference
[cmꢀ1
]
[cmꢀ1
]
H3O+(TBP)3
H5O2+(TBP)4
H5O2+(TBP)2(H2O)2
Oct3NH+···TBP
TBP···H2O
1187
1222
1222
1228
1248
1261
74
39
39
33
13
0
[46]
[46]
[46]
present work
[47]
TBPfree
present work
Conclusion
Acknowledgements
We have developed a nNH scale for ranking the localized
basicity of unsaturated hydrocarbons in their interactions
with hydrogen-bond donors. The scale indicates that p basic-
ity increases in the following order: fused-ring aromaticsꢁ
benzene<toluene<xylene<mesitylene<durene<conjugat-
ed dienesꢂ1-alkynes<pentamethylbenzene<hexamethyl-
benzene<internal alkynesꢂcycloalkenes<1-methylcycloal-
kenes. Alkenes are unexpectedly strong p bases. As expect-
ed for a localized weak interaction, there is no overall corre-
lation with molecular basicity as measured by gas-phase
proton affinities. The utility of the scale is that it offers one
We thank the National Institutes of Health (GM 23851) and the National
Science Foundation (CHE-0349878) for support.
[1] M. J. S. Dewar, J. Chem. Soc. 1946, 406, 777–781.
[2] H. C. Brown, J. D. Brady, J. Am. Chem. Soc. 1952, 74, 3570–3582.
[3] L. J. Andrews, R. M. Keefer, Molecular Complexesin Organic
Chemistry, Holden-Gay, San Francisco, CA, 1964.
[4] A. C. Legon, P. D. Aldrich, W. H. Flygare, J. Chem. Phys. 1981, 75,
625–630.
[5] P. D. Aldrich, A. C. Legon, W. H. Flygare, J. Chem. Phys. 1981, 75,
2126–2134.
[6] W. G. Read, E. J. Campbell, G. Henderson, W. H. Flygare, J. Am.
Chem. Soc. 1981, 103, 7670–7672.
[7] J. F. Rico, R. López, I. Ema, G. Ramírez, J. Chem. Theory Comput.
2005, 1, 1083–1095.
[8] M. Meot-Ner, C. A. Deakyne, J. Am. Chem. Soc. 1985, 107, 474–
479.
[9] J. C. Ma, D. A. Dougherty, Chem. Rev. 1997, 97, 1303–1324.
[10] W. L. Zhu, X. J. Tan, C. M. Puah, J. D. Gu, H. L. Jiang, K. X. Chen,
C. E. Felder, I. Silman, J. L. Sussman, J. Phys. Chem. A 2000, 104,
9573–9580.
[11] D. Mootz, A. Deeg, J. Am. Chem. Soc. 1992, 114, 5887–5888.
[12] T. Steiner, S. A. Mason, Acta Crystallogr. Sect. A 2000, 56, 254–260.
[13] E. S. Stoyanov, S. P. Hoffmann, K.-C. Kim, F. S. Tham, C. A. Reed,
J. Am. Chem. Soc. 2005, 127, 7664–7665.
ꢀ
of the first ways to gauge the relative strength of the N
H···p interactions in crystals, proteins, and pre-protonation
complexes.
Stability constants (K) have been measured for the bind-
ing of unsaturated hydrocarbons to the [Oct3NH+]{F20ꢀ} IP
and these have led to the discovery of the first examples of
bifurcated hydrogen bonding to unsaturated hydrocarbons
in solution. Bifurcated hydrogen bonds occur only with the
more strongly complexing substrates. This structural motif is
likely to be found in proteins and small-molecule crystals.
Finally, this work has revealed how solutions containing
ion-pair solutes can be quite nonideal. For example, with 3-
hexyne in <0.1m solution, >90% of the alkyne is absorbed
into the second solvation shell of the IP, bringing the
number of molecules associated with it to around 13.
[14] E. Steinwender, E. T. G. Lutz, J. H. Maas, J. A. Kanters, Vib. Spec-
trosc. 1993, 4, 217–229.
[15] M. A. Viswamitra, R. Radhakrishnan, J. Bandekar, G. R. Desiraju,
J. Am. Chem. Soc. 1993, 115, 4868–4869.
[16] H. S. Rzepa, M. H. Smith, M. L. Webb, J. Chem. Soc. Perkin Trans. 2
1994, 703–707.
[17] T. Steiner, E. B. Starikov, A. M. Amado, J. J. C. Teixeira-Dias, J.
Chem. Soc. Perkin Trans. 2 1995, 1321–1326.
[18] D. Braga, F. Grepioni, E. Tedesco, Organometallics 1998, 17, 2669–
2672.
Experimental Section
[19] G. R. Desiraju, T. Steiner, The Weak Hydrogen Bond in Structural
Chemistry and Biology, Oxford University Press, New York, 1999.
[20] E. B. Starikov, T. Steiner, Acta Crystallogr. Sect. B 1998, 54, 94–96.
[21] C. M. Reddy, L. S. Reddy, S. Aitipamula, A. Nangia, C.-K. Lam,
T. C. W. Mak, CrystEngComm 2005, 7, 44–52.
[22] J. F. Malone, C. M. Murray, M. H. Charlton, R. Docherty, A. J.
Lavery, J. Chem. Soc. Faraday Trans. 1997, 93, 3429–3436.
[23] M. F. Perutz, Philos. Trans. R. Soc. A 1993, 345,105–112.
[24] T. Steiner, G. Koellner, J. Mol. Biol. 2001, 305, 535–557.
[25] H. Mayr, T. Bug, M. F. Gotta, N. Hering, B. Irrgang, B. Janker, K.
Bernhard, R. Loos, A. R. Ofial, G. Remennikov, H. Schimmel, J.
Am. Chem. Soc. 2001, 123, 9500–9512.
All manipulations were carried out under dry conditions in an inert at-
mosphere in a glove box (H2O, O2 <0.5 ppm). Solvents and Et3SiH (Al-
drich, 99% purity) were dried by using standard methods.[48] [Ph3C]
[B(C6F5)4] was a gift from Albemarle Corporation. Et3Si{B(C6F5)4}[49] was
prepared by adding Et3SiH (1.5 mL) to [Ph3C][B(C6F5)4] (1 g) in benzene
(10 mL). The mixture was stirred for 1–2 d until the solid changed com-
pletely from yellow to colorless. The volatile compounds (benzene and
Et3SiCl) were removed under reduced pressure and the solid was washed
with several aliquots of hexane. [Oct3NH]Cl was prepared by passing gas-
eous anhydrous HCl through a solution of Oct3N in hexane at 08C. The
white crystalline product was collected by filtration. [Oct3NH][B(C6F5)4]
was prepared from 1:1 molar ratios of [Oct3NH]Cl and Et3Si(B(C6F5)4) in
benzene. The solution was stirred for 30 min. and the volatile compounds
(benzene and Et3SiCl) were removed under reduced pressure. The light-
yellow oil product crystallized over time.
[26] S. Minegishi, H. Mayr, J. Am. Chem. Soc. 2003, 125, 286–295.
[27] T. Steiner, Biophys. Chem. 2002, 95,195–210.
[28] E. S. Stoyanov, K.-C. Kim, C. A. Reed, J. Am. Chem. Soc. 2006, 128,
8500–8508.
7890
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 7880 – 7891

+
ꢀ
Bifurcated N H ···p Hydrogen Bonding
FULL PAPER
[29] E. P. L. Hunter, S. G. Lias, J. Phys. Chem. Ref. Data 1998, 27, 413–
656.
[30] M. Meot-Ner, Int. J. Mass Spectrom. 2003, 227, 525–554.
[31] T. Steiner, A. M. M. Schreurs, M. Lutz, J. Kroon, New J. Chem.
2001, 25, 174–178.
[32] T. Steiner, S. A. Mason, Z. Kristallogr. 2001, 18, 93.
[33] H. S. Rzepa, M. H. Smith, M. L. Webb, J. Chem. Soc. Perkin Trans. 2
1994, 703–707;
[34] M. Pilkington, J. D. Wallis, S. Larsen, J. Chem. Soc. Chem. Commun.
1995, 1499–1500.
[35] P. K. Bakshi, A. Linden, B. R. Vincent, S. P. Roe, D. Adhikesavalu,
T. S. Cameron, O. Knop, Can. J. Chem. 1994, 72, 1273–1293.
[36] R. Preissner, U. Egner, W. Saenger, FEBS Lett. 1991, 288, 192–196.
[37] T. Steiner, Angew. Chem. 2002, 114, 50–80; Angew. Chem. Int. Ed.
2002, 41, 48–76.
[41] T. Mꢁller, M. Juhasz, C. A. Reed, Angew. Chem. 2004, 116, 1569–
1572; Angew. Chem. Int. Ed. 2004, 43, 1543–1546.
[42] C. A. Reed, K.-C. Kim, E. S. Stoyanov, D. Stasko, F. S. Tham, L. J.
Mueller, P. D. Boyd, J. Am. Chem. Soc. 2003, 125, 1796–1804.
[43] G. A. Crowder, P. Blankenship, J. Mol. Struct. 1987, 156, 147–150.
[44] E. S. Stoyanov, K.-C. Kim, C. A. Reed, J. Am. Chem. Soc. 2006, 128,
1948–1958.
[45] E. L. Mackor, A. Hofstra, J. H. Waals, Trans. Faraday Soc. 1958, 54,
186–194.
[46] E. S. Stoyanov, J. Chem. Soc. Faraday Trans. 1997, 93, 4165–4175.
[47] E. S. Stoyanov, L. V. Lastovka, Zh. Neorg. Khim. 1981, 26, 1613–
1619.
[48] D. D. Perrin, W. L. F. Armarego, D. R. Perrin, Purification of Labo-
ratory Chemicals, 2nd ed., Pergamon Press, Sydney, 1980.
[49] J. B. Lambert, S. Z. Zhang, S. M. Ciro, Organometallics 1994, 13,
2430–2443.
[38] R. Taylor, Electrophilic Aromatic Substitution, Wiley, New York,
1990.
[39] S. Fukuzumi, J. K. Kochi, J. Am. Chem. Soc. 1981, 103, 7240–7252.
[40] T. Kato, C. A. Reed, Angew. Chem. 2004, 116, 2968–2971; Angew.
Chem. Int. Ed. 2004, 43, 2908–2911.
Received: February 22, 2008
Published online: July 21, 2008
Chem. Eur. J. 2008, 14, 7880 – 7891
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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7891