Hydrophilic interior between hydrophobic regions in inverse bilayer structures of cation-1,1′-binaphthalene-2,2′-diyl phosphate salts

Article abstract of DOI:10.1039/b510617f

A series of 1,1′-binaphthalene-2,2′-diyl phosphate (BNPPA ^-) salts have been synthesized. Their crystal packings show a separation of the hydrophobic naphthyl and hydrophilic (RO)₂PO ₂^- phosphate/cation/solvate regions. Hydrogen bonding in the latter is the driving force for "inverse bilayer" formation, with a hydrophilic interior exposing the hydrophobic binaphthyl groups to the exterior. Stacking of the inverse bilayers occurs less through π-π and more through CH...π interactions between the naphthyl groups, which correlates with the formation of thin crystal plates along the stacking direction. Cations used with R- or rac-BNPPA^- are protonated isonicotin-1-ium amide (1), isonicotin-1-ium acid (2), guanidinium (3), the metal complexes trans-tetraammine-dimethanol-copper(II) (4), trans-diaqua-tetramethanol-copper(II) (5) and cis-diaqua-bis(ethylene diamine)-nickel(II) (6). Crystallization occurs with inclusion of water and methanol solvent molecules, except in 2. Starting from R-BNPPA, inversion takes place with calcium acetate to give 1 as the racemate. 2 is crystallized as the R-BNPPA salt. The inversion-symmetrical complex trans-[Cu(H₂O) ₂(CH₃OH)₄]²⁺ in 5 has Cu-OH ₂ bond lengths of 1.937(4) A, and Cu-O(methanol) of 2.112(4) and 2.167(4) A, corresponding to a compressed tetragonal geometry. the Royal Society of Chemistry the Centre National de la Recherche Scientifique 2006.

Full text of DOI:10.1039/b510617f

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www.rsc.org/njc | New Journal of Chemistry
Hydrophilic interior between hydrophobic regions in inverse bilayer
structures of cation–1,10-binaphthalene-2,20-diyl phosphate saltsw
Thomas Dorn, Anne-Christine Chamayou and Christoph Janiak*
Received (in Durham, UK) 27th April 2005, Accepted 16th November 2005
First published as an Advance Article on the web 15th December 2005
DOI: 10.1039/b510617f
A series of 1,10-binaphthalene-2,20-diyl phosphate (BNPPAꢀ) salts have been synthesized. Tꢀheir
crystal packings show a separation of the hydrophobic naphthyl and hydrophilic (RO)2PO2
phosphate/cation/solvate regions. Hydrogen bonding in the latter is the driving force for ‘‘inverse
bilayer’’ formation, with a hydrophilic interior exposing the hydrophobic binaphthyl groups to
the exterior. Stacking of the inverse bilayers occurs less through p–p and more through CHꢁ ꢁ ꢁp
interactions between the naphthyl groups, which correlates with the formation of thin crystal
plates along the stacking direction. Cations used with R- or rac-BNPPAꢀ are protonated
isonicotin-1-ium amide (1), isonicotin-1-ium acid (2), guanidinium (3), the metal complexes trans-
tetraammine-dimethanol-copper(II) (4), trans-diaqua-tetramethanol-copper(II) (5) and cis-diaqua-
bis(ethylene diamine)-nickel(II) (6). Crystallization occurs with inclusion of water and methanol
solvent molecules, except in 2. Starting from R-BNPPA, inversion takes place with calcium
acetate to give 1 as the racemate. 2 is crystallized as the R-BNPPA salt. The inversion-
symmetrical complex trans-[Cu(H2O)2(CH3OH)4]21 in 5 has Cu–OH2 bond lengths of 1.937(4) A,
and Cu–O(methanol) of 2.112(4) and 2.167(4) A, corresponding to a compressed tetragonal
geometry.
resolution of chiral amines9 and amino acids.10 We are not
Introduction
aware, however, that BNPPAꢀ(H) has been used as a supra-
The organization and separation of hydrophobic and hydro-
molecular building block, due to its combination of polar and
philic regions is a spontaneous process in the formation of
non-polar functionalities.
monolayers, micelles, bilayers (in membranes) or liposomes
Here we report the structures of protonated isonicotinium
from molecules with a polar head and a non-polar tail, e.g.
amide and acid cations with the rac- or R-BNPPA anion. This
phospholipids or detergents (Scheme 1).1 This organization
furthers the continued interest in hydrogen bonding patterns
utilizes many non-covalent interactions, including hydrogen
together with p–p stacking of protonated, cationic nitrogen
heterocycles.11 Furthermore, guanidinium and transition
bonding.
The use of supramolecular interactions to mimic such a
natural organization process in molecular crystals is a chal-
lenge in crystal engineering.2,3 In this respect, we have synthe-
sized and structurally characterized salts 1–6 of the 1,10-
binaphthalene-2,20-diyl phosphate (BNPPA) anion, which
has a polar phosphate group and a non-polar binaphtha-
lene-diyl moiety (Scheme 2). The phosphate group is an
excellent acceptor of classical O/N–Hꢁ ꢁ ꢁO hydrogen bonds,2,4
whereas the naphthyl group can interact through pꢁ ꢁ ꢁp stack-
ing5,6 and C–Hꢁ ꢁ ꢁp bonding.7 Our work was influenced by a
recent report from S. K. Das et al. on the bilayer structure of
ortho-phenylene diammonium perchlorate2 and by the seminal
work of M. D. Ward et al. on layered materials of guanidi-
nium alkane and arenesulfonates.8 The chiral compound 1,10-
binaphthalene-2,20-diyl phosphoric acid (BNPPAH) in one of
its enantiomeric forms has been employed for the optical
Institut fur Anorganische und Analytische Chemie, Universitat
¨
¨
Freiburg, Albertstraße 21, D-79104 Freiburg, Germany.
E-mail: janiak@uni-freiburg.de; Fax: (þ49) 761 2036147;
Tel: (þ49) 761 2036127
w Electronic supplementary information (ESI) available: Annotated
pictures of the hydrogen bonding interactions. See DOI: 10.1039/
b510617f.
Scheme 1 Molecules with a polar head (J) and a non-polar tail (Q)
can spontaneously form (a) monolayers, (b) micelles, (c) bilayers and
(d) liposome vesicles.
ꢂc
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Scheme 3 Suggested relative rates for racemization and crystalliza-
tion during the formation of 1.
solution in methanol/water, from which, under otherwise
identical conditions, the solvent was evaporated. In both
solutions, the optical rotation increased with concentration,
and within experimental error, these two values were identical.
This excludes the possibility of a fast racemization in solution.
Yet when the crystals of 1 were dissolved again, they did not
show an optical rotation. From the above observations we
conclude that calcium acetate slowly catalyzes the racemiza-
tion. With both enantiomers available, crystallization pro-
ceeds in a quick yet controlled manner through the relatively
slow supply of S-BNPPAꢀ (Scheme 3). Further investigations
on the process of racemization in R-BNPPAH will be carried
out.
From isonicotinic acid and R-BNPPAH, crystals of 2 could
be obtained in the presence of guanidinium chloride without
racemization. Many more attempts were made to crystallize R-
BNPPAꢀ with the various cations found in 1–6, but all failed.
Compared to the facile crystallization from rac-BNPPAH, we
encountered difficulties when trying to crystallize from R-
BNPPAH12 that were consistent with the difficulties we ex-
perienced when trying to crystallize related R- or S-1,10-bi-2-
naphtholate metal salts.13 This failure is also contrary to the
use of S-BNPPAH as a good optical resolution agent for
amines and its application in the resolution of pharmaceutical
intermediates by fractional crystallization.9,10
Scheme 2 Molecular composition of the prepared 1,10-binaphtha-
lene-2,20-diyl phosphate salts 1–6 with the nomenclature numbering
scheme.
metal cations were crystallized with rac-BNPPAꢀ (Scheme 2).
The supramolecular inter-cation–anion and inter-anion inter-
actions, such as N/O–Hꢁ ꢁ ꢁO, p–p and C–Hꢁ ꢁ ꢁp, are analyzed.
Results and discussion
The BNPPA salts 1–6 (Scheme 2) are reproducibly prepared as
crystals in good yields within a week through slow solvent
evaporation of methanol/water solutions or by a diffusion
method.
The phosphate salts 1 and 3–6 form as dihydrates, the latter
also as methanol solvates. We trace this finding to the good
hydrogen acceptor properties of the polar phosphate group. A
Cambridge Structure Database study has recently shown that
hydrate formation is above average (B40% and 36%) for
pyridinium chloride and phosphate salt structures, respec-
tively, when compared to the total of pyridinium salts (22%
hydrate formation).14 In the following structures, we will
compare and interpret the crystal packing based on the
supramolecular interactions found.
In the crystallization with isonicotinamide and isonicotinic
acid, the R-enantiomer of BNPPAH was employed. Crystals
of the isonicotinium amide BNPPAꢀ salt (1) did, however,
consist of the BNPPA racemate, both according to the X-ray
crystallographic refinement and from the absence of a measur-
able optical rotation. Crystallization of 1 was only successful
when calcium acetate was added to the solution and when
starting from R-BNPPAH. The use of rac-BNPPAH yielded
an immediate precipitate and no crystals could be obtained.
Also, without the addition of calcium acetate, no crystalline
product could be obtained from R-BNPPAH. The angle of
optical rotation of the solution of R-BNPPAH/isonicotina-
mide/calcium acetate was measured during evaporation of the
methanol/water mixture until crystallization started. This
value was compared to the optical rotation of a R-BNPPAH
Crystal packing and hydrogen bonding
Generally, the packing arrangements found in 1–6 can be
rationalized by a separation of the hydrophobic binꢀaphthyl
backbone of BNPPA from the hydrophilic (RO)2PO2 phos-
phate groups, cation and solvate components.2,12
Figs. 1–6 show projections of the unit cell crystal packings
of compounds 1–6, respectively, to illustrate the lamellar or
ꢂc
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Fig. 1 Projection of the crystal packing in 1 onto the (0 1 0) plane.
Hydrogen bonds are indicated with red dashes.
Fig. 3 Projection of the crystal packing in 3 onto the (1 0 0) plane.
Hydrogen bonds are indicated with red dashes.
there is only one hydrogen bond to each terminal oxygen
atom.
A brief statistical analysis of the unambiguously determined
N/O(–H)-donorꢁ ꢁ ꢁO-acceptor distances in 1–6, shown in
Fig. 7, indicates the higher number of hydrogen bonds to
phosphate, as well as their shorter donorꢁ ꢁ ꢁacceptor contacts.
We trace this to the fact that the hydrogen bond acceptor
strength of oxygen follows the sequence Oꢀ 4 O.12,16 The
additional electrostatic attraction leads to shorter Hꢁ ꢁ ꢁOꢀ
than Hꢁ ꢁ ꢁO contacts.
Compound 5 contains the trans-[Cu(H2O)2(CH3OH)4]21
cation (Fig. 8). For Cu(II), a Jahn–Teller distortion is to be
expected, which generally manifests itself in a tetragonal
elongated octahedron. The elongated axial bonds are typically
longer by 0.3–0.6 A over the equatorial Cu–ligand bonds.17 In
the complex cation [Cu(H2O)2(CH3OH)4]21, the three crystal-
lographically different Cu–O bond lengths differ somewhat
less with Cu–OH O = 1.937(4) A, and Cu–OCH OH = 2.112(4)
Fig. 2 Projection of the crystal packing in 2 onto the (1 0 0) plane.
Hydrogen bonds are indicated with red dashes.
layer-like packing of the hydrophobic and hydrophilic regions.
The latter are also highlighted by the hydrogen bonding
network shown as red dashes. Details of the hydrogen bonding
in 1–6 are listed in Tables 1–4. Annotated pictures of the
hydrogen bonding interactions can be found in the supple-
mentary material.w Other bond lengths and angles can also be
found in the CIF files, which are also part of the supplemen-
tary material.z We note that in 1–6, the P–O(C) bonds average
1.614 A (with a maximum of 1.627 A and a minimum of 1.607
A), while the terminal P–O(ꢀ) bonds, with their partial double
bond character, average 1.484 A (with a maximum of 1.494 A
and a minimum of 1.472 A). Hydrogen bonding to phosphate
occurs essentially only to the terminal P–O(ꢀ) bonds, with each
oxygen atom typically accepting 2–3, and rarely 4, hydrogen
bonds.15 The only exception is the isonicotinium acid structure
2, where, in the absence of solvent molecules of crystallization,
2
3
and 2.167(4) A. Furthermore, the variation is such that a
tetragonal compressed octahedron results, which is a rare
example for Cu(II) complexes.18–22 Together with regular ꢀ3-
symmetrical Cu(II) structures,23,24 a tetragonally-distorted
z CCDC reference numbers: 286354 for 1, 286355 for 2, 286356 for 3,
286357 for 4, 286358 for 5, 286359 for 6 at 100 K and 286360 for 6 at
203 K. For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b510617f
Fig. 4 Projection of the crystal packing in 4 onto the (0 0 1) plane.
Hydrogen bonds are indicated with red dashes.
ꢂc
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unresolved static disorder or a dynamic Jahn–Teller dis-
order.23–25 For the cation trans-[Cu(H2O)2(CH3OH)4]21 in 5,
a visual inspection of Fig. 8 has to be augmented by a
comparison (DA,B) of the mean square vibration amplitudes
z2A,B and z2B,A for the bonded pair of atoms A and B (with zA,B
as the radius of the vibration ellipsoid of atom A along the
vector from A to B). This is known as the ‘‘Hirshfeld Rigid-
Bond’’ test, and with true vibrational ellipsoids should give
DA,B = z2A,B ꢀ z2B,A D 0, since from the ‘‘rigid-bond postu-
late’’, the relative vibrational motion of a pair of bonded
atoms has an effectively vanishing component in the direction
of the bond. Hence, the mean square vibrational amplitudes of
a pair of bonded atoms should be equal along the bond
direction, but can differ in other directions due to bond
bending or torsional vibrations.27 For the three crystallogra-
phically different Cu–O bond lengths in the [Cu(H2O)2
(CH3OH)4]21 component of 5, DA,B is 24(21) ꢃ 10ꢀ4 A2 for
Fig. 5 Projection of the crystal packing in 5 onto the (0 1 0) plane.
Hydrogen bonds are indicated with red dashes.
Cu–OH O, and is 136(24) and 205(30) ꢃ 10ꢀ4 A2 for the two
2
Cu–O
bonds. The large DA,B values for Cu–O
CH3OH
CH3OH
indicates either a considerable relative motion (signifying the
presence of a dynamic Jahn–Teller effect) or a static disorder
between the atoms involved.25 Further investigations on this
matter in compound 5 will be carried out.
The crystal structure of compound 6 is noteworthy because
with independent crystallographic data sets collected at 100(2)
and 203(2) K, refinement was only successful as a racemic twin
in the non-centrosymmetric polar space group Cc (see Experi-
mental section—X-Ray crystallography).28 The unit cell con-
tains both enantiomers R- and S-BNPPAꢀ, as well as D- and
L-cis-[Ni(en)2(H2O)2]21 as independent molecules. Both re-
spective configurations alternate along the c-axis, as shown in
Fig. 9. Thus, the structure does not present an example of
spontaneous resolution from a racemic mixture. The non-
centrosymmetric polar packing is due to the identical orienta-
tion of the [Ni(en)2(H2O)2]21 cations with their aqua ligands
along the polar c-axis (see Fig. 6). This identical alignment can
be traced to the hydrogen bonding interactions between
adjacent complexes, either direct from an NH2 group to the
aqua ligand or transmitted via a phosphate group or a solvate
water molecule (Fig. 10).
Fig. 6 Projection of the crystal packing in 6 onto the (0 1 0) plane.
Hydrogen bonds are indicated with red dashes.
compressed octahedron is usually evidence of a dynamic Jahn–
Teller distortion.25,26 Variable temperature single crystal or
EPR studies then show an interchange to a second distortion.
When the compressed octahedron is observed at room tem-
perature, the elongated octahedron is found at lower tempera-
ture.18,22
If the atom ligated to a Jahn–Teller active metal center is
oscillating between short and long distances, then the aniso-
tropic displacement parameters of the ligated atom along the
metal–ligand bond should show considerably more displace-
ment than the Cu atom. An elongated thermal ellipsoid for the
ligated atom towards the metal center would indicate either an
Table 1 Hydrogen bonding interactions in 1 and 2a
D–Hꢁ ꢁ ꢁA
D–H/A
Hꢁ ꢁ ꢁA/A
Dꢁ ꢁ ꢁA/A
D–Hꢁ ꢁ ꢁA/1
1, N/O–Hꢁ ꢁ ꢁO(phosphate):
N1–H1ꢁ ꢁ ꢁO4
0.90(3)
0.90(3)
0.88(3)
0.88(3)
2.14(3)
2.39(3)
2.19(3)
1.92(3)
2.917(2)
3.027(3)
3.058(2)
2.775(2)
144(2)
128(2)
168(2)
164(3)
N1–H1ꢁ ꢁ ꢁO45
N2–H2Bꢁ ꢁ ꢁO36
O6–H6Aꢁ ꢁ ꢁO36
1, N/O–Hꢁ ꢁ ꢁO(water, amide):
N2–H2Aꢁ ꢁ ꢁO61
0.91(3)
0.92(3)
2.20(3)
1.90(3)
3.105(3)
2.815(2)
179(2)
174(3)
O6–H6Bꢁ ꢁ ꢁO5
1
2
Symmetry transformations: 1 = x, y ꢀ 1, z; 5 = ꢀx, ꢀy, ꢀ z; 6 = x, 1 ꢀ y, z ꢀ
2, N/O–Hꢁ ꢁ ꢁO(phosphate):
N–Hꢁ ꢁ ꢁO3
0.92(6)
0.90
1.77(6)
1.84
2.640(5)
2.744(6)
156(6)
173
O5–H51ꢁ ꢁ ꢁO41
Symmetry transformation: 1 = x, y ꢀ 1, z
D = donor, A = acceptor. For found and refined atoms the standard deviations are given.
a
ꢂc
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