Cation-Cation Pairing by Ni-Ci-H?O Hydrogen Bonds

Article abstract of DOI:10.1002/anie.201408278

The pairing of ions of opposite charge is a fundamental principle in chemistry, and is widely applied in synthesis and catalysis. In contrast, cation-cation association remains an elusive concept, lacking in supporting experimental evidence. While studying the structure and properties of 4-oxopiperidinium salts [OC₅H₈NH₂]X for a series of anions X^- of decreasing basicity, we observed a gradual self-association of the cations, concluding in the formation of an isolated dicationic pair. In 4-oxopiperidinium bis(trifluoromethylsulfonyl)amide, the cations are linked by N-H?O=C hydrogen bonds to form chains, flanked by hydrogen bonds to the anions. In the tetra(perfluoro-tert-butoxy)aluminate salt, the anions are fully separated from the cations, and the cations associate pairwise by N-C-H?O=C hydrogen bonds. The compounds represent the first genuine examples of self-association of simple organic cations based merely on hydrogen bonding as evidenced by X-ray structure analysis, and provide a paradigm for an extension of this class of compounds.

Full text of DOI:10.1002/anie.201408278

.
Angewandte
Communications
International Edition: DOI: 10.1002/anie.201408278
German Edition: DOI: 10.1002/ange.201408278
Cation–Cation Interactions
À À
Cation–Cation Pairing by N C H···O Hydrogen Bonds
Waltraud Gamrad, Angelika Dreier, Richard Goddard, and Klaus-Richard Pçrschke*
Dedicated to Professor Dr. Günther Wilke on the occasion of his 90th birthday
Abstract: The pairing of ions of opposite charge is a funda-
mental principle in chemistry, and is widely applied in synthesis
and catalysis. In contrast, cation–cation association remains an
elusive concept, lacking in supporting experimental evidence.
While studying the structure and properties of 4-oxopiperidi-
3.263(2)–3.391(2) Š) resulting in an overall neutral molecular
entity. It seems reasonable to assume that the oxygen atoms of
enolates are more basic than those of neutral ketones. We
now wish to report the apparently unprecedented self-
association of two secondary ammonium cations by NÀCÀ
H···O=C hydrogen bonding to neutral carbonyl groups with
À
nium salts [OC H NH ]X for a series of anions X of
5
8
2
decreasing basicity, we observed a gradual self-association of
the cations, concluding in the formation of an isolated
dicationic pair. In 4-oxopiperidinium bis(trifluoromethylsul-
fonyl)amide, the cations are linked by NÀH···O=C hydrogen
the formation of a dication.
The starting point of our investigation was our interest in
the structural properties of 4-piperidone hydrate hydrochlo-
ride, a widely used feedstock in pharmaceutical chemistry. We
show that the compound is actually 4,4-dihydroxypiperidi-
nium chloride, [(HO) C H NH ]Cl (1a), in which the geminal
bonds to form chains, flanked by hydrogen bonds to the anions.
In the tetra(perfluoro-tert-butoxy)aluminate salt, the anions
are fully separated from the cations, and the cations associate
pairwise by NÀCÀH···O=C hydrogen bonds. The compounds
2
5
8
2
diol is stabilized by hydrogen bonding to the chloride ion. The
[1]
chloride ion serves as a fourfold hydrogen-bond acceptor to
both hydroxy and ammonium protons in a three-dimensional
represent the first genuine examples of self-association of
simple organic cations based merely on hydrogen bonding as
evidenced by X-ray structure analysis, and provide a paradigm
for an extension of this class of compounds.
[10]
network. The bromide [(HO) C H NH ]Br (1b) displays
2
5
8
2
a similar 3D network structure. The corresponding iodide is
unstable as a solid and dehydrates upon crystallization to give
4
-oxopiperidinium iodide, [OC H NH ]I (2c). The analogous
5 8 2
I
n solution, ion pairing occurs when cations and anions
4-oxopiperidinium chloride [OC H NH ]Cl (2a) can be
5 8 2
associate in a solvent of low dielectric constant to form
a distinct chemical entity. Depending on the degree of solvent
participation, fully solvated ions, solvent-separated ion pairs,
prepared from 1a by dehydration with SOCl . Compound
2
2a reacts with HBr by halide exchange to give [OC H NH ]Br
5
8
2
(2b). The 4-oxopiperidinium halides 2a–c no longer have
a typical three-dimensional ionic structure, but rather form
parallel chains of alternating 4-oxopiperidinium cations and
halide anions connected by NH···X···HN hydrogen bonds.
These chains are closely packed, with the very weakly basic
keto functions being barely involved in any hydrogen bond-
[1]
and contact ion pairs are usually distinguished. In contrast,
cation–cation interactions of organic species appear as a rare
and counterintuitive phenomenon which has been spectro-
scopically detected, for example, for NMe ions in aqueous
K/CsBr solutions, for guanidinium ions in water, certain
ionic liquids, and in the micellization of tetraalkylammo-
nium surfactants, although there is no direct structural
evidence to support this.
+
4
[
2]
[3]
[4]
[10]
ing. For the halide salts the carbonyl IR resonance occurs at
[
5]
À1
about 1720 cm , which corresponds to that of a typical
À1
neutral ketone (cyclohexanone, n˜ CO = 1717 cm ).
In quaternary tetraalkylammonium ions the positive
charge is significantly delocalized onto the protons attached
to the a-C atoms of the hydrocarbon chains, rendering these
When 2a is reacted with silver reagents AgX with
À
À
a weakly or noncoordinating anion X such as MeCO2
,
[
6]
À
À
À
À
À
BF4 , ClO4 , OSO CF3 (OTf , triflate), N(SO CF )
2 2 3
2
protons sufficiently acidic to engage in hydrogen bonds to
[
7a]
[7b]
anions. In N(C H ) Br and N(C H ) I, for example, most
2
5
4
2
5 4
protons of the a- and b-C atoms form CÀH···X hydrogen
bonds to the halide X, which in turn functions as a hydrogen-
[
8]
bond acceptor. Reetz et al. have shown that the a protons of
[
9]
the NBu cation form NÀCÀH···O=C hydrogen bonds to the
4
enolate oxygen atoms of a malonic acid diester anion (C···O
[
*] W. Gamrad, A. Dreier, Dr. R. Goddard, Prof. Dr. K.-R. Pçrschke
Max-Planck-Institut für Kohlenforschung
Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr (Germany)
E-mail: poerschke@kofo.mpg.de
À
À
À
4
(
NTf ), and [Al{OC(CF ) } ] (= [Al(PFTB) ] , Krossingꢀs
2 3 3 4
[
11a]
anion)
in diethyl ether or CH Cl solution, chloride is
2 2
replaced by these anions to give the [OC H NH ]X salts 2d–
5
8
2
Homepage: http://www.kofo.mpg.de/poerschke.html
i [Eq. (1)]. While 2d–h crystallize without the inclusion of
solvent, the aluminate 2i separates from such solutions in the
form of the solute compounds [OC H NH (OEt ) ][Al-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201408278.
5
8
2
2 2
4
482
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 4482 –4487

Angewandte
Chemie
(
PFTB) ] (2i·2Et O) and [OC H NH ][Al(PFTB) ]·CH Cl
decreasing basicity of the anions X in the sequence Cl > Br>
4
2
5
8
2
4
2
2
(
2i·CH Cl ).
I ꢀ MeCO > BF > ClO > OTf ꢁ NTf @ Al(PFTB) hydro-
2
2
2
4
4
2
4
In the solid state, the CO stretching frequency (wave-
gen bonds NH···X···HN to the anion X would become less
structure-determining and the weakly basic carbonyl group of
the 4-oxopiperidinium ion should become increasingly
involved in the bonding. Herein we describe essential features
number) of the 4-oxopiperidinium cation in compounds 2
depends on the counterion X and increases with the
diminishing basicity of the anion in the series 2a–e (ca.
À1
À1
À1
À
À
À
1
720 cm ), 2 f (1725 cm ), 2g (1741 cm ), and 2i·2Et O
through the crystal structures of 2 f (X = ClO ), 2g (X =
2
4
À1
À
À
À
À
À
(
1744 cm ). Compound 2h (X = NTf ) appears to be an
OTf ), 2h (X = NTf ), and 2i·2Et O (X = [Al(PFTB) ] ).
2 2
2
4
exception, since it exhibits two n˜ CO bands with frequencies
The 4-oxopiperidinium salts 2 f, 2g, and 2i·2Et O crystal-
lize in the triclinic space group P1 (no. 2). Each unit cell
2
À1
ꢀ
expected for a typical ketone (1720, 1728 cm ) despite the
[
12]
low basicity of the anion. In analogy to related systems, we
assume that a positive charge on the 4-oxopiperidinium ring
results in a higher C=O bond strength and, consequently, an
contains two cations and anions related by inversion centers.
Compound 2h crystallizes in the monoclinic space group P21/
c (no. 14). The main structural features of the 4-oxopiper-
idinium cations correspond to those of most other salts of this
[13]
increase in the value of n˜ CO
largest for 2i, which has the least basic anion (X = Al-
PFTB) ). However, this explanation appears no longer
.
As expected, the effect is
À
À
À
À
À
À
cation in our study (Cl , Br , I , ClO , OTf , NTf , and
4
2
À
(
[Al(PFTB) ] ; see TablesS1 and S2 in the Supporting
4
4
applicable when the carbonyl lone electron pairs are
bonded to a Lewis acid, as in the case for 2h, where the
carbonyl group is hydrogen-bonded to acidic ammonium
protons (see below).
Information). Based on the ring torsional angles, the cation
adopts a chair conformation that is relatively steep around
nitrogen and quite flat near the carbonyl group. As a result of
this geometry, the axial and equatorial protons of the 4-
oxopiperidinium cation are electronically inequivalent. Pro-
tons on the nitrogen atoms were all located and refined.
The crystal structures of the perchlorate (2 f) and trifluor-
omethylsulfonate salts (2g) are remarkably similar and
indicative of low but distinct basicity of the anions. In the
crystal of 2 f eclipsed 4-oxopiperidinium cations alternate
with perchlorate anions to form infinite chains along the b
axis (Figure 1). The ammonium proton N1-H6ax engages in
When acetone was used as a solvent, variable amounts of
the Schiff base [OC H N=CMe ]X (A) were formed as
5
8
2
a byproduct, as a result of partial condensation. In addition,
Figure 1. Crystal structure of 2 f showing parallel chains of alternating
4-oxopiperidinium cations and perchlorate anions along the b axis
(horizontal).
a single hydrogen bond with the O2 atom of a perchlorate
anion (N1···O2 2.883(3) Š, N1ÀH6 ···O2 160 88 ), whereas the
ax
ammonium proton N1-H7 appears to participate in twofold
eq
the synthesis of 2i was frequently accompanied by the
formation of some viscous brownish byproducts which,
according to ESI mass spectrometry, are polycation salts B
formed by self-condensation of the 4-oxopiperidinium cation
hydrogen bonding to the O3 and O4 atoms of a neighboring
anion (N1···O3 3.007(3) Š, N1ÀH7 ···O3 137(1)8 and N1···O4
eq
3.107(3) Š, N1ÀH7 ···O4 138(1)8). There is evidence of
eq
additional short NÀCÀH···O=C hydrogen-bonding interac-
[
14,15]
by water elimination.
Such a condensation reaction
tions between the carbonyl O1 atom in one chain and the
appears to result from a linear head-to-tail association of 4-
equatorial proton on C1, H1B , of a neighboring chain
eq
oxopiperidinium cations by NÀH···O=C bonding in the
(C1···O1 3.125(3) Š, C1ÀH1B ···O1 125(1)8). Otherwise,
eq
absence of stabilization by the anions (C). This observation
appears to verify the importance of the anions in the other
compounds, such as 2h (see below).
association of the chains appears to be mainly governed by
close packing, since each parallel chain is surrounded by six
nearest neighbors.
In order to investigate the possible role of the anion in the
formation of hydrogen bonds involving the 4-oxopiperidi-
nium cation in more detail, we studied the hydrogen bonding
in the ammonium ketones 2a–i. We reasoned that with
In the trifluoromethylsulfonate salt 2g, the 4-oxopiper-
idinium cations are similarly linked by NÀH···O hydrogen
bonds to the O atoms of triflate anions to form chains
extending along the b axis (Figure 2). Thus, the axial
Angew. Chem. Int. Ed. 2015, 54, 4482 –4487
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
4483

.
Angewandte
Communications
each cation forms a hydrogen bond between its
axial H atom of the ammonium group and a NTf2
anion. Whereas N1 forms a hydrogen bond to the
sulfonyl O3 (N1···O3 2.765(3) Š, N1ÀH6 ···O3
ax
1
61(1)8), N2 hydrogen-bonds to the imide N4
(
N2···N4 2.966(3) Š, N2ÀH11 ···N4 166(1)8).
ax
Clearly, the use of an exceedingly weakly basic
anion promotes hydrogen-bonding interactions
between the cations.
Inspired by these results, we decided to inves-
Figure 2. Crystal structure of 2g, showing parallel chains of alternating 4-oxopiper- tigate a 4-oxopiperidinium salt of a still less basic
À
idinium cations and trifluoromethylsulfonate anions along the b axis (horizontal).
anion and selected the [Al(PFTB)4] anion in this
case, since it is well known as a noncoordinating
ion.
[11c]
Its salt crystallizes from diethyl ether with
ammonium proton H6ax attached to N1 of a 4-oxopiperidi-
nium cation is hydrogen-bonded to O2 of a triflate anion in
two solute molecules: 2i·2Et O. The compound comprises
2
discrete perfluoroaluminate anions and 4-oxopiperidinium
cations, whose ammonium protons are engaged in hydrogen
bonds to the oxygen atoms of two ether molecules (N1···O2
the chain (N1···O2 2.917(3) Š, N1ÀH6 ···O2 170(1)8),
ax
whereas the equatorial ammonium proton, H7 , is hydro-
eq
gen-bonded to O3 of a neighboring triflate anion (N1···O3
2.776(2) Š, N1ÀH1C ···O2 168(1)8 and N1···O3 2.815(2) Š,
eq
2
.964(3) Š, N1ÀH7 ···O3 174(1)8). As in 2 f, in 2g parallel
N1ÀH1D ···O3 168(1)8; Figure 4a). There are no obvious
eq
ax
chains of NÀH···O bonded anions and cations are each
hydrogen-bonding interactions between the cations and the
anions. Instead, pairs of solvated cations self-assemble around
centers of symmetry. Association is based on NÀCÀH···O=C
surrounded by six nearest neighbors in a close packing
arrangement. Analogously, the carbonyl O1 atom appears to
undergo an inter-chain NÀCÀH···O=C hydrogen-bonding
hydrogen bonding between the axial protons H1A and
ax
interaction with the equatorial proton on C1, H1B , of
H5Bax on the a-C atoms (C1 and C5, respectively) of the
ammonium group of one cation and the oxygen atom of the
carbonyl group of a neighboring cation (C5···O1 3.142(2) Š,
C5ÀH5B ···O1 1348 and C1···O1 3.392(2) Š, C1ÀH1A ···O1
eq
a neighboring chain (C1···O1 3.049(3) Š, C1ÀH1B ···O1
eq
1
25(1)8).
+
À
Thus, all 4-oxopiperidinium salts 2a–g exhibit N ÀH···X
ax
ax
hydrogen-bonded chains of alternating ammonium centers
and anions in the solid state. First indications of a change in
the cation–anion bonding pattern occurred when we looked at
the bis(trifluoromethylsulfonyl)imide salt. We chose the NTf2
anion because of its low basicity and similarity to the triflate
anion. 4-Oxopiperidinium bis(trifluoromethylsulfonyl)imide
131 88 ; Figure 4b). The C···O distances of the NÀCÀH···O=C
bonds in 2i·2Et O (and also 2 f,g) compare favorably with
2
those of the bonds between ammonium groups and anionic
[
8]
carbonyl groups mentioned in the introduction and other
[
9]
literature data. In the crystal structure one can view
2i·2Et O as an AB -type salt whose molecular volume
2
2
3
2
h was synthesized as described above and crystals of the salt
corresponds to the volume of the unit cell (2.241 nm ).
From this and considering the known large thermochemical
volume
were grown. Figure 3 summarizes the results of the crystal
structure analysis of 2h. There are two independent cations
and two independent anions in the asymmetric unit, as
expected from the presence of two n˜ CO absorptions. In
contrast to 2a–g, in 2h both equatorial ammonium H atoms
on the two 4-oxopiperidinium cations form short NÀH···O=C
[17a]
À
3
[11b]
of the [Al(PFTB) ] anion (V
= 0.758 nm ),
4
therm
we calculated the thermochemical volume of the tetrakis
(diethylether)-di(4-oxopiperidinium) dication in 2i·2Et O at
2
3
Vtherm = 0.725 nm . The comparable size of the dication and
the anion suggests that a treatment of 2i·2Et O as an AB salt
2
2
[16]
hydrogen bonds to the carbonyl O atoms of the neighboring
of large close-packed ions is reasonable.
In addition to its size, the nature of the [Al(PFTB)4]
anion as an exceedingly weak base and weakest hydrogen-
À
cations (N1···O2 2.783(3) Š, N1ÀH7 ···O2 161(1)8, and
eq
N2···O1 2.807(3) Š, N2ÀH12 ···O1 153(1)8). In addition,
eq
[
11d,18]
bond acceptor
of the system appears critical
for the formation of the dication in 2i·2Et O.
2
Unlike the other anions described in this paper, the
perfluoroaluminate anion does not function as
a hydrogen-bond acceptor for the 4-oxopiperidi-
nium cation. Consistent with the rules put forward
[19b]
by Etter
various types of hydrogen bonding,
that in complex systems involving
[
19]
the most
acidic hydrogen atoms form H-bonds to the most
basic donor atoms, the ammonium protons in
2
i·2Et O hydrogen-bond to the oxygen atoms of
2
solvent diethyl ether. Such NÀH···O(ether) hydro-
gen bonds appear unprecedented for diethyl
[
20]
ÀH···O hydrogen-bonding ether, and even for the more basic THF only
Figure 3. Crystal structure of 2h, showing cation–cation N
and cation–anion NÀH···N and NÀH···O hydrogen-bonding interactions.
[
21]
a few examples have been recorded. Thus, both
4
484 www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 4482 –4487

Angewandte
Chemie
enthalpies DlattH of the dicationic salt AB and the monocat-
2
ionic salt AB were estimated to amount to DlattH298
=
À1
À1
+
967 kJmol
and DlattH298 =+ 323 kJmol , respectively
(see the Supporting Information). Coulomb explosion of the
gaseous dication was calculated by DFT methods to be
À1 [24]
exothermic by DH298 = À117 kJmol . As a result, dication
formation in the crystal lattice is exothermic by DH
=
98
2
À1
À1
À204 kJmol (DH200 = À214 kJmol ). We have refrained
from calculating the Gibbs energies, since entropic contribu-
tions can be assessed less precisely and for the solid state
largely cancel each other out in the cycle. It becomes clear
from the BFH cycle that the strong dication–anion attraction
associated with the dication in the AB lattice represents the
2
driving force for the dication formation, overcompensating
Coulomb repulsion.
There are various interesting aspects associated with the
formation of the dicationic 2i·2Et O. 1) Cation–cation inter-
2
actions in the solid state have been frequently observed for
metal complexes, usually involving large metal ions such as
[25]
the actinides (which provide ample coordination sites), but
[26]
[27]
also for rhodium
and silver
complexes with smaller
anions. 2) Of particular interest in the context of our results is
the cation–cation binding observed in metal complexes of
[
28a,29]
[28b]
[30]
Ag,
Mo,
and Tl involving large and noncoordinat-
À
3 2 4
[30]
ing counterions such as [B{C H (CF ) } ] ,
PFTB) ] ,
part have precedent in the “lattice stabilization”
unusual dications such as E (E = S, Se, Te)
[Al-
6
3
[29]
À
[28]
À
(
and [(m-F)Al {(PFTB) ] .
2 6
These for their
4
[
17a,b]
of
2+
[17b,c]
2+
and E8
4
[31]
(
E = S, Se) by complex anions. BFH studies on the isolated
AB salts revealed stabilization of the dications by lattice
2
forces, and a very similar situation appears to be the case for
Figure 4. Crystal and molecular structure of 2i·2Et O. a) View of the
2
the organic dication salt 2i·2Et O. 3) Organic dications held
2
unit cell, showing the solvated self-associated cations and the sur-
together solely by hydrogen bonds have been detected in gas-
À
rounding [Al(PFTB)4] anions. b) Detail of the pairwise association of
[32]
phase studies
investigations.
and were also the topic of theoretical
According to these results, metastable
the 4-oxopiperidinium cations, solvated by diethyl ether molecules.
[
33,34]
cation–cation complexes are viable species in the gas phase
and show, although their dissociation enthalpies are exother-
mic due to Coulomb repulsion, distinct fragmentation bar-
riers. Such organic dications have been suggested to become
stabilized by “kinetic trapping in a sufficiently deep H-
bonded well [which] should allow such species to be
the cation–cation interaction and the ability of the cation to
bond to ether molecules makes the structure of 2i·2Et O
2
exceptional.
To shed light on the thermodynamics of the dication
[22]
formation we carried out a Born–Fajans–Haber
(BFH)
[
34a]
cycle study on the dication salt as compared to the monocat-
ion salt at 298 K and 200 K in order to assess the stabilization
experimentally detected”.
In the case of 2i·2Et O we
2
have isolated such a metastable H-bonded cation–cation
[
23]
[29a,35]
of the dication by lattice effects (Scheme 1).
Lattice
species in a crystal lattice, thus verifying the perception
that gas-phase cations can be effectively trapped in
the solid state when combined with large non-
coordinating anions.
To summarize, we have studied the synthesis
and solid-state structural properties of 4-oxopiper-
idinium salts [OC H NH ]X for a series of anions X
5
8
2
of decreasing basicity. In these salts both the NH
and NCH protons of the 4-oxopiperidinium cation
engage in various forms of hydrogen bonding to
available hydrogen-bond acceptors. For basic
À
À
anions such as Cl and Br strong NH···X···HN
hydrogen bonds are formed. These attenuate the
positive charge on the 4-oxopiperidinium ring, and
such salts exhibit an infrared CO stretching band of
Scheme 1. BFH study of 2i·2Et O (diethyl ether molecules are omitted from the
2
formulae). The lattice enthalpies DlattH298 of both salts were calculated by using
[
17]
volume-based thermodynamics, and the gas-phase dissociation enthalpy of the
dication was determined by DFT calculations at the dispersion-corrected
B3LYP/aug-cc-pVTZ and B3LYP/cc-pVQZ levels of theory.
À1
a typical neutral ketone at n˜ CO ꢀ 1720 cm . As the
Angew. Chem. Int. Ed. 2015, 54, 4482 –4487
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
4485

.
Angewandte
Communications
basicity of the anion diminishes, the positive charge of the 4-
oxopiperidinium cation increases and concurrently the car-
bonyl oxygen atom of the 4-oxopiperidinium cation gains in
importance as a hydrogen-bond acceptor toward either the
[13] The effect may also be compared to the observed change in IR
frequencies for the CꢂO stretch in nonclassical metal carbonyl
complexes: a) A. S. Goldman, K. Krogh-Jespersen, J. Am.
Chem. Soc. 1996, 118, 12159 – 12166; b) H. Willner, F. Aubke,
Organometallics 2003, 22, 3612 – 3633.
NH or NCH protons of a neighboring cation. For the NTf salt
2
[
14] Organic polycations are relatively rare and have been realized in
2
h the 4-oxopiperidinium cations linearly associate by head-
[14a,c]
[14b]
particular for dendrimeric
and cyclic
compounds in which
to-tail NÀH···O=C bonding to form oligomeric chains flanked
the multiple onio centers are perforce confined to a single
molecular entity: a) R. Weiss, B. Pomrehn, F. Hampel, W. Bauer,
Angew. Chem. Int. Ed. Engl. 1995, 34, 1319 – 1321; Angew.
Chem. 1995, 107, 1446 – 1448; b) A. Ito, Y. Ono, K. Tanaka,
Angew. Chem. Int. Ed. 2000, 39, 1072 – 1075; Angew. Chem.
À
by interactions with the NTf2 anions, with an associated
À
almost unperturbed n˜ CO frequency. In the [Al(PFTB) ] salt
4
2
i·2Et O the anions are no longer able to hydrogen-bond
2
with the cations. Here, the 4-oxopiperidinium cations not only
2000, 112, 1114 – 1117; c) A. W. Kleij, R. van de Coevering,
bond to diethyl ether molecules by NÀH···O bonding, but two
R. J. M. Klein Gebbink, A.-M. Noordman, A. L. Spek, G.
van Koten, Chem. Eur. J. 2001, 7, 181 – 192.
such solvated 4-oxopiperidinium cations self-associate in
cyclic head-to-tail fashion by NÀCÀH···O=C bonding to
[15] For a polycationic ammonium – ammonia structure, see: T.
afford a dimeric dication, exhibiting an IR CO stretching
Roßmeier, N. Korber, Z. Anorg. Allg. Chem. 2004, 630, 2665 –
À1
band at a remarkable n˜ CO = 1744 cm . The compound
2668.
+
[
16] Intermolecular NÀH ···O=C hydrogen bonding between ammo-
nium and carbonyl groups of neutral molecules or anions is not
uncommon and typical N···O distances are in the 2.69–3.00 Š
range. For some relatively short distances (2.69–2.86 Š), see:
a) J.-J. Brunet, R. Chauvin, J. Chiffre, B. Donnadieu, S. Huguet,
P. Leglaye, E. Mothes, Inorg. Chim. Acta 1999, 291, 300 – 310;
b) F. A. R. Kaul, G. T. Puchta, H. Schneider, M. Grosche, D.
Mihalios, W. A. Herrmann, J. Organomet. Chem. 2001, 621, 177 –
2
i·2Et O appears to be the first authentic example of the
2
self-association of two organic cations to form a dication held
together merely by hydrogen bonds. The trapping of the di(4-
oxopiperidinium) dication in an AB -type crystal lattice
2
containing large noncoordinating anions provides a paradigm
for the creation of further elusive dications that are based on
[36]
weak attractive forces.
183; c) S. Amatori, G. Ambrosi, M. Fanelli, M. Formica, V. Fusi,
L. Giorgi, E. Macedi, M. Micheloni, P. Paoli, R. Pontellini, P.
Rossi, J. Org. Chem. 2012, 77, 2207 – 2218; d) S. Ahmed, L. A.
Baker, R. S. Grainger, P. Innocenti, C. E. Quevedo, J. Org.
Chem. 2008, 73, 8116 – 8119; e) A. R. Todorov, M. Nieger, J.
Helaja, Chem. Eur. J. 2012, 18, 7269 – 7277; f) J. Zhou, S. J.
Lancaster, D. A. Walker, S. Beck, M. Thornton-Pett, M.
Bochmann, J. Am. Chem. Soc. 2001, 123, 223 – 237.
Keywords: hydrogen bonds · piperidine derivatives ·
polycations · structure elucidation
How to cite: Angew. Chem. Int. Ed. 2015, 54, 4482–4487
Angew. Chem. 2015, 127, 4564–4569
[
17] a) H. D. B. Jenkins, H. K. Roobottom, J. Passmore, L. Glasser,
Inorg. Chem. 1999, 38, 3609 – 3620; b) T. S. Cameron, I. Dionne,
H. D. B. Jenkins, S. Parsons, J. Passmore, H. K. Roobottom,
Inorg. Chem. 2000, 39, 2042 – 2052; c) S. Brownridge, I. Krossing,
J. Passmore, H. D. B. Jenkins, H. K. Roobottom, Coord. Chem.
Rev. 2000, 197, 397 – 481; d) L. Glasser, H. D. B. Jenkins, Chem.
Soc. Rev. 2005, 34, 866 – 874; e) L. Glasser, H. D. B. Jenkins, J.
Chem. Eng. Data 2011, 56, 874 – 880.
[
1] C. Reichardt, T. Welton, Solvents and Solvent Effects in Organic
Chemistry, 4th ed., Wiley-VCH, Weinheim, 2011.
[
[
2] M. Holz, K. J. Patil, Ber. Bunsen-Ges. 1991, 95, 107 – 113.
3] O. Shih, A. H. England, G. C. Dallinger, J. W. Smith, K. C.
Duffey, R. C. Cohen, D. Prendergast, R. J. Saykally, J. Chem.
Phys. 2013, 139, 035104.
[
4] A. Mele, G. Romanò, M. Giannone, E. Ragg, G. Fronza, G.
Raos, V. Marcon, Angew. Chem. Int. Ed. 2006, 45, 1123 – 1126;
Angew. Chem. 2006, 118, 1141 – 1144.
[
18] For the protonated anion, H[Al(PFTB) ], see: A. Kraft, J. Beck,
4
G. Steinfeld, H. Scherer, D. Himmel, I. Krossing, Organo-
metallics 2012, 31, 7485 – 7491.
[
[
5] M. Benrraou, B. L. Bales, R. Zana, J. Phys. Chem. B 2003, 107,
13432 – 13440, and references therein.
[
19] a) A. C. Legon, D. J. Millen, Chem. Soc. Rev. 1987, 16, 467 – 498;
b) M. C. Etter, Acc. Chem. Res. 1990, 23, 120 – 126; c) G. Gilli, P.
Gilli, The Nature of the Hydrogen Bond, Oxford University
Press, Oxford, UK, 2009; d) P. Gilli, L. Pretto, V. Bertolasi, G.
Gilli, Acc. Chem. Res. 2009, 42, 33 – 44.
6] a) M. T. Reetz, Angew. Chem. Int. Ed. Engl. 1988, 27, 994 – 998;
Angew. Chem. 1988, 100, 1026 – 1030; b) V. B. Luzhkov, F.
Österberg, P. Acharya, J. Chattopadhyaya, J. Šqvist, Phys.
Chem. Chem. Phys. 2002, 4, 4640 – 4647.
[
7] a) M. Ralle, J. C. Bryan, A. Habenschuss, B. Wunderlich, Acta
Crystallogr. Sect. C 1997, 53, 488 – 490; b) B. R. Vincent, O.
Knop, A. Linden, T. S. Cameron, K. N. Robertson, Can. J. Chem.
[
[
20] F. H. Allen, Acta Crystallogr. Sect. B 2002, 58, 380 – 388.
21] a) W. J. Evans, M. A. Ansari, J. W. Ziller, Inorg. Chem. 1999, 38,
1160 – 1164; b) M. K. Krepps, S. Parkin, D. A. Atwood, Cryst.
1
988, 66, 3060 – 3069.
8] M. T. Reetz, S. Hütte, R. Goddard, J. Am. Chem. Soc. 1993, 115,
339 – 9340.
9] G. R. Desiraju, Acc. Chem. Res. 1991, 24, 290 – 296.
Growth Des. 2001, 1, 291 – 297; c) H. Kumagai, S. Kitagawa, M.
Maekawa, S. Kawata, H. Kiso, M. Munakata, J. Chem. Soc.
Dalton Trans. 2002, 2390 – 2396; d) T. C. Rosen, K. Kirschbaum,
D. M. Giolando, Dalton Trans. 2003, 120 – 125; e) B. Kurscheid,
B. Neumann, H.-G. Stammler, B. Hoge, Chem. Eur. J. 2011, 17,
14935 – 14941.
[
9
[
[
[
10] K.-R. Pçrschke, W. Gamrad, R. Goddard, J. Breidung, H. Cui, in
preparation.
11] a) I. Krossing, Chem. Eur. J. 2001, 7, 490 – 502; b) I. Krossing, H.
Brands, R. Feuerhake, S. Koenig, J. Fluorine Chem. 2001, 112,
[22] a) M. Born, Verh. Dtsch. Phys. Ges. 1919, 21, 679 – 685; b) K.
Fajans, Verh. Dtsch. Phys. Ges. 1919, 21, 714 – 722; c) F. Haber,
Verh. Dtsch. Phys. Ges. 1919, 21, 750 – 768; d) D. F. C. Morris,
E. L. Short, Nature 1969, 224, 950 – 952.
8
2
3 – 90; c) I. Krossing, I. Raabe, Angew. Chem. Int. Ed. 2004, 43,
066 – 2090; Angew. Chem. 2004, 116, 2116 – 2142; d) I. Krossing,
A. Reisinger, Eur. J. Inorg. Chem. 2005, 1979 – 1989.
[
12] Various effects may increase the carbonyl IR frequency, such as
[23] We thank Professor Ingo Krossing, Freiburg, for suggesting the
BFH study to us.
[24] We would like to thank Dr. Jürgen Breidung of our Max-Planck-
Institute for performing the DFT studies.
ring strain in cyclic compounds (e.g., cyclopentanone, n˜
=
CO
À1
1
745 cm ) and electron-withdrawing substituents (e.g., hexa-
À1
chloroacetone, n˜ CO = 1780, 1752 cm ).
4
486 www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 4482 –4487

Angewandte
Chemie
[
[
[
25] N. N. Krot, M. S. Grigoriev, Russ. Chem. Rev. 2004, 73, 89 – 100.
26] S. Grimme, J.-P. Djukic, Inorg. Chem. 2011, 50, 2619 – 2628.
27] A. VellØ, A. Cebollada, M. Iglesias, P. J. Sanz Miguel, Inorg.
Chem. 2014, 53, 10654 – 10659.
[33] A. Shokri, M. Ramezani, A. Fattahi, S. R. Kass, J. Phys. Chem. A
2013, 117, 9252 – 9258.
[34] a) F. Weinhold, R. A. Klein, Angew. Chem. Int. Ed. 2014, 53,
11214 – 11217; Angew. Chem. 2014, 126, 11396 – 11399; b) Corri-
gendum: F. Weinhold, R. A. Klein, Angew. Chem. Int. Ed. 2014,
53, 12992; Angew. Chem. 2014, 126, 13207.
[
28] a) M. Scheer, L. J. Gregoriades, A. V. Virovets, W. Kunz, R.
Neueder, I. Krossing, Angew. Chem. Int. Ed. 2006, 45, 5689 –
5
693; Angew. Chem. 2006, 118, 5818 – 5822; b) M. Scheer, L. J.
[35] T. S. Cameron, A. Decken, I. Dionne, M. Fang, I. Krossing, J.
Passmore, Chem. Eur. J. 2002, 8, 3386 – 3401.
Gregoriades, M. Zabel, J. Bai, I. Krossing, G. Brunklaus, H.
Eckert, Chem. Eur. J. 2008, 14, 282 – 295.
[36] Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/. Data for the single-crystal
structure determinations of the compounds, including the
CCDC deposition numbers, are contained in Tables S1 and S2
of the Supporting information. The detailled crystallographic
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
[
29] a) I. Krossing, A. Reisinger, Coord. Chem. Rev. 2006, 250, 2721 –
2
1
744; b) I. Raabe, S. Atonijevic, I. Krossing, Chem. Eur. J. 2007,
3, 7510 – 7522.
[
[
30] J. Cullinane, A. Jolleys, F. S. Mair, Dalton Trans. 2013, 42, 11971 –
1975.
1
31] T. S. Cameron, R. J. Deeth, I. Dionne, H. Du, H. D. B. Jenkins, I.
Krossing, J. Passmore, H. K. Roobottom, Inorg. Chem. 2000, 39,
5614 – 5631.
[
32] a) S.-W. Lee, J. L. Beauchamp, J. Am. Soc. Mass Spectrom. 1999,
Received: August 15, 2014
1
0, 347 – 351; b) L. Feketeovµ, R. A. J. OꢀHair, Chem. Commun.
Revised: December 2, 2014
Published online: February 25, 2015
2
008, 4942 – 4944.
Angew. Chem. Int. Ed. 2015, 54, 4482 –4487
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 4487