A conformational analysis of the spirocyclic quaternary ammonium cation [(CH2)4N(CH2)4]+ in its bromide and picrate salts

Article abstract of DOI:10.1515/znb-2004-0303

High-yield syntheses of the bromide (1a) and picrate salts (1b) of the 5-azonia-spiro[4]nonane cation [(CH₂)₄N(CH ₂)₄]⁺ are reported. In the single crystal X-ray diffraction analyses of the two salts

Full text of DOI:10.1515/znb-2004-0303

A Conformational Analysis of the Spirocyclic Quaternary Ammonium
Cation [(CH2)4N(CH2)4]+ in its Bromide and Picrate Salts
Uwe Monkowius, Stefan Nogai, and Hubert Schmidbaur
Anorganisch-chemisches Institut, Technische Universita¨t Mu¨nchen,
Lichtenbergstraße 4, D-85747 Garching, Germany
Reprint requests to Prof. H. Schmidbaur. E-mail: H.Schmidbaur@lrz.tum.de
Z. Naturforsch. 59b, 259 – 263 (2004); received January 22, 2004
High-yield syntheses of the bromide (1a) and picrate salts (1b) of the 5-azonia-spiro[4]nonane
cation [(CH2)4N(CH2)4]+ are reported. In the single crystal X-ray diffraction analyses of the two
salts the spirocyclic quaternary ammonium cations have their five-membered rings in envelop and
twist conformations modified by packing forces. The conformation found experimentally for 1a has
C2-symmetry as predicted for the gas phase by quantum-chemical calculations (RI-DFT, RI-MP2),
but the five-membered rings are intermediate between the expected envelop and the twist form. For
1b, both of the two independent cations can be described as a combination of rings in an envelop and
a twist conformation. According to the NMR spectra, in solution the cations are highly flexible and
pseudosymmetrical (point group D2d).
Key words: Conformational Analysis, Spirocyclic Ammonium Salts, 5-Azoniaspirononane Cation,
Heterospirononanes
Introduction
Cyclopentane (CH2)5 and the related heterocycles
(CH2)4EH2 (E = C, Si, Ge, B−, N+, P+ etc., A, B)
show a delicate equilibrium of conformations, which
can be assigned to either envelop or twist geome-
tries [1]. In spiro[4,4]nonane and its analogues with
the heteroatom in the spiro-position (C, D), i.e. where
two of these units share a common carbon or other
Group III-V heteroatom, respectively, the same con-
formational alternatives are valid for both rings which
leads to an even larger conformational variety [2].
(1)
Results and Discussion
Quaternary ammonium salts with the cation
[(CH2)4N(CH2)4]+ were first prepared by v. Braun (in
1916, [4]) and later by Blicke et al. (in 1954, [5]) and
others [6]. These authors have demonstrated that pyrro-
lidine can be quaternized with 1,4-dibromo-butane in
alkaline aqueous solution to give the bromide salt 1a.
This preparation has now been slightly modified to
give acceptable yields (86%) [eq. (1)].
In the context of preparative studies in organophos-
phorus chemistry we have recently investigated the
The product is soluble in water and in most polar
structures of several spirocyclic quaternary phos- organic solvents and highly hygroscopic, but insolu-
phonium salts based on the 5-phosphonia-spiro[4,4]- ble in solvents of low polarity. Even short exposure to
nonane cation (D, E = P+) [3]. This study has now humid air leads to the formation of a viscous yellow
been extended to the corresponding spirocyclic ammo- oil. From solutions of 1a in dry methanol the picrate
nium salts (D, E = N+), the structure of which had not 1b can be precipitated on addition of picric acid. This
previously been investigated.
salt is not hygroscopic and insoluble in water, alcohols
c
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260
U. Monkowius et al. · A Conformational Analysis of the Spirocyclic Quaternary Ammonium Cation
Fig. 2. Projection of the asymmetric unit of the spirocyclic
ammonium cation in crystals of the bromide salt 1a along
the twist axis (ORTEP, 50% probability ellipsoids, hydrogen
atoms with arbitrary radii) with atomic numbering.
Fig. 1. Structure of the spirocyclic ammonium cation in
crystals of the bromide salt 1a (ORTEP, 50% probabil-
ity ellipsoids; hydrogen atoms with arbitrary radii) w◦ith
N1-C1 1.474(7), N1-C4 1.512(6), C1-C2 1.523(8), C2-C3
1.447(12), C3-C4 1.559(10); C1-N1-C1’ 109.2(6), C4-N1-
C4’ 110.7(6), C1-N1-C4 105.5(4), C1-N1-C4’ 113.1(3); N1-
C1-C2 102.0(6), C1-C2-C3 110.5(9), C2-C3-C4 104.2(5),
N1-C4-C3 100.8(5).
passes through C2 and through the center of the N1-C4
bond, while the envelop is generated by a folding along
the C1–C4 line (Fig. 1).
Crystals of the picrate 1b as obtained from chlor◦o-
form solution upon addition of diethyl ether at 20 C
are monoclinic, space group P21, with Z = 4 formula
units in the unit cell. The asymmetric unit contains two
formula units, the cations of which have very similar
geometries (Fig. 3).
Each of the two cations features a combination of
five-membered rings in the envelop and twist confor-
mation. The geometry of the rings N1-C15-C16-C17-
C18 and N2-C21-C22-C23-C24 approaches the stan-
dard envelop conformation with the nitrogen atoms
folded away from the plane of the carbon atoms, while
the other two rings approach the standard twist confor-
mation (with C12-C13-C14 and C26-C27-C28 as the
reference planes). Details are given in the captions to
Fig. 3.
˚
atomic numbering. Selected bond lengths [A] and angles [ ]:
and non-polar organic solvents, but soluble in chloro-
form, dichloromethane and acetonitrile. It appears that
the spirocyclic cations are sufficiently flexible to estab-
lish an efficiently space-filling structure with less com-
pact anions such as picrate. The bromide salt 1a has
recently been used as a reagent for the precipitation
of other organic anions [7]. It appears therefore that it
shows advantages in crystallization and crystal growth
over more rigid cations like [Ph4P]+.
The salts 1a and 1b have been characterized by el-
emental analysis, mass spectrometry and NMR spec-
troscopy. The 1H and 13C resonances for the cation are
virtually identical in both cases indicating electrolytic
dissociation in the solvents employed (CDCl3, CD2Cl2
or D3CCN). The cations show equivalent CH2 groups
for the α- and β-positions suggesting a high flexibil-
ity of the five-membered rings in solution leading to
an (average) D2d symmetry. The resonances of the pi-
crate anion of 1b show no anomalies (Experimental
Section).
The picrate anions are stacked in the crystals with a
˚
plane-to-plane distance of ca. 3.4 A (Fig. 4). The nitro
groups in ortho position relative to the phenolate oxy-
gen atom are twisted disrotatorily out of the ring plane,
while the nitro group in the para position is virtually
coplanar with the reference plane. This conformation
confirms previous findings [8].
With regard to the structure of the analogous phos-
Crystals of the bromide 1a as obtained from acetoni- phonium cations [3], the geometry of the title am-
◦
trile solution upon addition of diethyl ether at 20 C are monium cations differs by small endocyclic C-N-C
trigonal, space group P321 with Z = 3 formula units angles in the range 102.38(8)– 105.5(4)◦, as com-
in the unit cell. The cation has C2 symmetry with the pared to the range of 95.7(6)– 97.9(5)◦ for the C-P-
twofold axis passing through the nitrogen atom and bi- C angles. The complementary exocyclic angles are
secting the angles C1-N1-C1’ and C4-N1-C4’ (Fig. 1). therefore larger for the phosphonium cations [C-
The conformation of each of the two equivalent five- P-C: 112.0(7)– 119.58(6)◦] than for the ammonium
membered rings is intermediate between the envelop cations [C-N-C: 109.2(6)– 113.6(2)◦]. All N-C bonds
˚
and twist reference models, but closer to the latter. The [1.474(7)– 1.514(3) A] are shorter than the P-C bonds
˚
projection chosen for Fig. 2 reveals that the twist axis [1.773(8)– 1.8145(12) A] as required by the differ-
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U. Monkowius et al. · A Conformational Analysis of the Spirocyclic Quaternary Ammonium Cation
261
Fig. 3. The two independent spirocyclic ammonium cations in crystals of the picrate salt 1b (ORTEP, 50% probabil-
◦
˚
ity ellipsoids, hydrogen atoms with arbitrary radii) with atomic numbering. Selected bond lengths [A] and angles [ ] of
the cations: N1-C11 1.499(3), N1-C14 1.517(3), N1-C15 1.509(3), N1-C18 1.509(3); N2-C21 1.509(3), N2-C24 1.508(3),
N2-C25 1.497(3), N1-C28 1.514(3); C11-C12 1.518(4), C12-C13 1.542(4), C13-C14 1.531(4); C15-C16 1.517(4), C16-
C17 1.548(4), C17-C18 1.519(4); C21-C22 1.519(4), C22-C23 1.544(4), C23-C24 1.529(4); C25-C26 1.530(5), C26-C27
1.535(6), C27-C28 1.511(4). C11-N1-C15 113.38(18), C11-N1-C14 103.3(2), C11-N1-C18 113.3(2), C14-N1-C15 113.3(2),
C14-N1-C18 110.7(2), C15-N1-C18 103.1(2); C21-N2-C24 102.4(2), C21-N2-C25 113.6(2), C21-N2-C28 111.5(2), C24-
N2-C25 113.5(2), C24-N2-C28 112.6(2), C25-N2-C28 103.5(2).
range between 102.4 and 105.1◦ and exocyclic C-N-
C angles in the range between 111.0 and 118.6◦ de-
pending on the conformational model (envelop / twist)
and the computational method (DFT / MP). In the gas
phase, the spirocyclic ammonium cation has an energy-
minimum geometry of C2 symmetry with both rings
in an envelop conformation. A second conformation
of D2 symmetry is ca. 13 kJmol−1 higher in energy
(Fig. 5). It therefore appears that the conformations of
lower symmetry observed in the crystals are strongly
influenced by packing forces. Details of the compu-
tational results have been documented elsewhere to-
gether with data for the spirocyclic compounds of type
D with E = C, Si, P+, or B− [10].
Experimental Section
Preparations
5-Azonia-spiro[4.4]nonane bromide, (1a)
Fig. 4. Stacking of the picrate anions (central column) in
A mixture of pyrrolidine (71.0 g, 1 mol), 1,4-dibromo-
crystals of the picrate salt 1b (ORTEP, 50% probability el-
butane (135.9 g, 1 mol) and a solution of potassium hy-
lipsoids, hydrogen atoms omitted).
droxide (56.0 g, 1 mol) in water (760 ml) is heated un-
der reflux for 2 h. Subsequently, most of the water and re-
maining other volatiles are removed by vacuum distillation.
ence in the colvalent radii of nitrogen and phospho-
rus [ca. 0.30 A]. All C-C bonds and all C-C-C an-
˚
The residue is filtered and KOH is added to the filtrate un-
til two liquid phases are formed which are separated. The
lower phase is taken up with chloroform and dried over an-
hydrous magnesium sulfate. The mixture is filtered and the
product (1a) precipitated from the filtrate by addition of di-
ethyl ether. The colourless, hygroscopic solid is filtered off,
washed with diethyl ether and dried in a vacuum. (Single
gles in 1a and 1b are similar to the gas phase data
of cyclopentane [1d] and X-ray data of its substituted
derivatives [2b, 9].
The experimental structural data for the title cations
are in good agreement with the results of quantum-
chemical calculations (RI-DFT and RI-MP2). The the-
oretical data call for endocyclic C-N-C angles in the crystals can be grown from acetonitrile / diethyl ether [2:1,
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262
U. Monkowius et al. · A Conformational Analysis of the Spirocyclic Quaternary Ammonium Cation
Fig. 5. Calculated energy minimum geometries of the spirocyclic ammonium cation [(CH2)4N(CH2)4]+ (RI-MP2/SV(P)).
The conformation with C2 symmetry is lower in energy by 13 kJmol−1 than the D2 conformation. (The atomic numbering is
different from that used in the crystal structure determination.)
v/v]). 177.3 g (86% yield), m.p. 273 ◦C. NMR (CDCl3,
Table 1. Crystal data, data collection, and structure refine-
ment of the compounds 1a and 1b.
◦
20 C), 1H: 2.22 (m, CH2, 8H) and 3.79 (m, CH2N, 8H),
1
AA’BB’. 13C{ H}: 22.11 (s, CH2), 62.92 (s, CH2N). MS
1a
1b
(FAB): m/z 126.1 (100%) [N(CH2)8]+. Elemental analysis
for C8H16NBr (206.13): calcd. C 46.62, H 7.82, N 6.80,
Br 38.76; found C 47.04, H 7.92, N 6.96, Br 37.87.
Empirical formula
M
C8H16NBr
618.38
trigonal
P321
11.7729(1)
a
5.8447(1)
90
701.55(1)
1.464
C14H18N4O7
354.32
monoclinic
P21
7.0170(1)
11.1590(2)
20.4295(5)
97.2986(8)
1586.72(5)
1.483
Crystal system
Space group
˚
a/A
5-Azonia-spiro[4.4]nonane picrate, (1b)
˚
b/A
˚
A solution of picric acid (2.5 g, 11 mmol) in 10 ml
c/A
of ethanol is added to a solution of compound 1a (1.0 g, β/◦
3
˚
V/A
ρ
Z
5.0 mmol) in ethanol (10 ml). Immediately a yellow solid
precipitates which is filtered off, washed with ethanol and
redissolved in chloroform. Careful layering of this solution
with diethyl ether induces the formation of single crystals
of the product 1b, which are not hygroscopic. 1.63 g (92%
yield), m.p. 230 ◦C with decomposition. 1H NMR (CD2Cl2,
20◦C): 2.26 (m, CH2, 8H), 3.79 (m, CH2N, 8H,), AA’BB’;
calc/g cm−3
3
4
F(000)
µ(Mo-Kα ) (cm−1
T/K
318
43.28
143
744
1.21
143
)
Refls. measured
Refls. unique
19858
1010
40658
6270
1
8.76 (s, CH, 2H). 13C{ H} NMR (acetone-d6, 20 ◦C): 22.87
[Rint = 0.041]
48 / 0
[Rint = 0.049]
596 / 1
0.0431
Refined parameters /restraints
R1[I ≥ 2σ(I)]
(s, CH2), 63.68 (s, CH2N), 125.85 (s, C3), 143.48 (s, C2),
162.21 (s, C1), C4 not detected. MS (FAB): m/z 126.1
(100%) [N(CH2)8]+. Elemental analysis for C14H18N4O7
(354.32): calcd. C 47.46, H 5.08, N 15.82; found C 46.80,
H 5.21, N 15.27%.
0.0395
0.0975
A = 0.0303
B = 2.0240
0.939 / −0.604
wR2a
0.1031
Weighting scheme
a = 0.0403
b = 0.5017
0.428 / −0.207
−3
˚
fin(max/min)/eA
σ
Crystal structure determinations
on crystal data, data collection and structure refinement are
summarized in Table 1. Absorption correction was carried
out for 1a using DELABS as part of the PLATON suite of
programs [12], for 1b no absorption correction was applied.
Displacement parameters and tables of interatomic distances
and angles have been deposited with the Cambridge Crys-
tallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK. The data are available on request quoting CCDC-
229982 and 229981.
Specimens of suitable quality and size of compounds 1a
and 1b were mounted on the ends of quartz fibers in in-
ert perfluoropolyalkylether and used for intensity data col-
lection on a DIP2020 diffractometer, employing graphite-
monochromated Mo-Kα radiation. The structures were
solved by a combination of direct methods (SHELXS-97)
and difference-Fourier syntheses and refined by full ma-
trix least-squares calculations on F2 (SHELXL-97) [11].
The thermal motion was treated anisotropically for all non-
hydrogen atoms. The hydrogen atoms of 1a were calcu-
lated in ideal positions and allowed to ride on their parent
Quantum-chemical calculations
All calculations were carried out with the TURBOMOLE
atoms with fixed isotropic contributions. Further information package of programs (version 5.3, 5.4 and 5.6) on the
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U. Monkowius et al. · A Conformational Analysis of the Spirocyclic Quaternary Ammonium Cation
263
RI-MP2 and RI-DFT level of theory [13]. RI-MP2 desig- a polarization function for all atoms) and TZVPP type (cc-
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Resolution-of-the-Identity method [14], RI-DFT the density
functional method with the Resolution-of-the-Identity ap-
proach of the Coulomb interactions [15]. For numerical in-
Acknowledgements
tegrations the “standard lattice” m3 was employed as im-
This work was generously supported by Fonds der
plemented in TURBOMOLE. Basis sets were of the SV(P) Chemischen Industrie. We thank Prof. P. Pyykko¨, University
(double zeta quality, split level, with a polarization function of Helsinki, for assistance with the quantum-chemical calcu-
P for non-hydrogen atoms), TZVP (triple zeta basis set with lations.
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