Norfloxacin salts with benzenedicarboxylic acids: Charge-assisted hydrogen-bonding recognition and solubility regulation

Article abstract of DOI:10.1039/c3ce40567b

The crystallization of norfloxacin, an antibacterial fluoroquinolone compound, with different benzenedicarboxylic acids yields five novel pharmaceutical salts via molecular recognition. X-ray single-crystal diffraction analyses reveal that these pharmaceutical agents present uniform charge-assistant hydrogen-bonding networks, which are in 1:1 stoichiometry of norfloxacin and dicarboxylate therein (with the exception of that with terephthalate). Unsubstituted benzenedicarboxylates (phthalate in 1, isophthalate in 2, and terephthalate in 3, respectively) are likely to crystallize with norfloxacin without lattice solvents, while the substituted isophthalate moieties (2-aminoisophthalate in 4 and 5-aminoisophthalate in 5) tend to form supramolecular adducts with the inclusion of water. Aromatic stacking interactions are present in all these structures, occurring between the fluoroquinolone and carboxylate phenyl rings (in 1-4) as well as between the fluoroquinolone planes (in 5). Thermal stability and solubility of all crystalline binary adducts have been determined. The effect of carboxylate counterions, substituents, and hydration states on the solubility and dissolution profile of drug salts 1-5 are investigated in pure water and 0.1 M HCl. After the formation of salts, the solubility increases at near neutral pH in almost all cases (except 3), but the order is changed in acidic medium. Phthalic acid shows evidence of a good candidate to enhance the solubility of fluoroquinolone drugs for the solubility of 1 is approximately 39 times as large as that of norfloxacin in pure water and also larger than that of norfloxacin in acidic system. For the isophthalates series, 2 has very poor solubility in 0.1 M HCl (only 0.06 times as that of Nf), while amino substituted isophthalates result in the enhancement of the solubility. Remarkably, crystal packing analyses of these pharmaceutical salts allow a possible correlation between the H-bonding prototypes and the solubility to be established.

Full text of DOI:10.1039/c3ce40567b

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Norfloxacin salts with benzenedicarboxylic acids:
charge-assisted hydrogen-bonding recognition and
Cite this: CrystEngComm, 2013, 15,
6090
solubility regulation3
Xian-Feng Huang,{ab Zhi-Hui Zhang,{a Qing-Qing Zhang,a Ling-Zhu Wang,b Ming-
Yang He,a Qun Chen,*a Guo-Qiang Song,*b Lin Wei,a Fan Wangb and Miao Du*c
The crystallization of norfloxacin, an antibacterial fluoroquinolone compound, with different benzene-
dicarboxylic acids yields five novel pharmaceutical salts via molecular recognition. X-ray single-crystal
diffraction analyses reveal that these pharmaceutical agents present uniform charge-assistant hydrogen-
bonding networks, which are in 1 : 1 stoichiometry of norfloxacin and dicarboxylate therein (with the
exception of that with terephthalate). Unsubstituted benzenedicarboxylates (phthalate in 1, isophthalate
in 2, and terephthalate in 3, respectively) are likely to crystallize with norfloxacin without lattice solvents,
while the substituted isophthalate moieties (2-aminoisophthalate in 4 and 5-aminoisophthalate in 5) tend
to form supramolecular adducts with the inclusion of water. Aromatic stacking interactions are present in
all these structures, occurring between the fluoroquinolone and carboxylate phenyl rings (in 1–4) as well
as between the fluoroquinolone planes (in 5). Thermal stability and solubility of all crystalline binary
adducts have been determined. The effect of carboxylate counterions, substituents, and hydration states
on the solubility and dissolution profile of drug salts 1–5 are investigated in pure water and 0.1 M HCl.
After the formation of salts, the solubility increases at near neutral pH in almost all cases (except 3), but the
order is changed in acidic medium. Phthalic acid shows evidence of a good candidate to enhance the
solubility of fluoroquinolone drugs for the solubility of 1 is approximately 39 times as large as that of
norfloxacin in pure water and also larger than that of norfloxacin in acidic system. For the isophthalates
series, 2 has very poor solubility in 0.1 M HCl (only 0.06 times as that of Nf), while amino substituted
isophthalates result in the enhancement of the solubility. Remarkably, crystal packing analyses of these
pharmaceutical salts allow a possible correlation between the H-bonding prototypes and the solubility to
be established.
Received 31st March 2013,
Accepted 8th May 2013
DOI: 10.1039/c3ce40567b
www.rsc.org/crystengcomm
tion and biological entities as well as the pharmaceutical
industry.1–4 During the last two decades, an increasing interest
Introduction
In view of the importance of hydrogen bonds in crystal
engineering, tremendous efforts have been devoted to the
understanding of hydrogen-bonded architectures and the
exploitation of them in material science, molecular recogni-
has focused on the crystalline phases of active pharmaceutical
ingredients (APIs),5 including polymorphs,6 hydrates7, sol-
vates,8 salts,9 and cocrystals.10 These solid-state forms of APIs
are defined as structurally homogeneous crystalline materials
that are constructed from at least one active pharmaceutical
ingredient and other solvents or solid components (coformers)
with a well-defined stoichiometry. Notably, APIs and coformers
are sustained by noncovalent interactions such as hydrogen
aKey Laboratory of Fine Petrochemical Technology, Changzhou University,
Changzhou 213164, P. R. China. E-mail: chenqunjpu@yahoo.com;
Fax: +86-519-86330251; Tel: +86-519-86330251
bSchool of Pharmaceutical Engineering & Life Science, Changzhou University,
6–10
…
Changzhou 213164, P. R. China. E-mail: drugs@vip.sina.com
bond, halogen bond, and p p stacking
instead of by
cCollege of Chemistry, Tianjin Key Laboratory of Structure and Performance for
Functional Molecules, MOE Key Laboratory of Inorganic–Organic Hybrid Functional
Material Chemistry, Tianjin Normal University, Tianjin 300387, P. R. China.
E-mail: dumiao@public.tpt.tj.cn; Fax: +86-22-23766556; Tel: +86-22-23766556
covalent forces.
Meantime, the ‘‘supramolecular synthons’’11 strategy has
become a promising tool to design new solid forms and
generate novel structures of APIs. A supramolecular synthon is
‘‘… a structural unit within supermolecules, which can be
formed and/or assembled by known or conceivable synthetic
Electronic supplementary information (ESI) available: PXRD patterns, IR
3
spectra, DSC curves, H-bonding metrics, molar extinction coefficient and
crystallographic data in CIF format for 1–5 are included. CCDC 926036–926040.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3ce40567b
operations
involving
intermolecular
interactions’’.11
Significant numbers of documents have indicated that the
{ Both authors contributed equally to this work.
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physical and chemical properties of the APIs can be altered by
the formation of suitable salts.12 Enhancing the poor solubility
of active drugs is primarily important for optimal delivery
when the APIs serve in bioavailability. One effective approach
to improve the dissolution and solubility of an API candidate
is to prepare its cocrystal or salt forms driven by neutral or
charge-assisted hydrogen-bonding interactions.12–14
speculate that the higher solubility of the salts could translate
into higher bioavailability.20
Taking all these points into account, we would like to
investigate the supramolecular assemblies of molecular
adducts with norfloxacin and various benzenedicarboxylic
acids. In this contribution, we demonstrate the syntheses,
characteristics, and solubility of pharmaceutical salts
assembled by zwitterionic hydrogen-bonding interactions
between norfloxacin and diverse aromatic dicarboxylic acids
(see Scheme 1), namely phthalic acid (H2pa), isophthalic acid
(H2ip), terephthalic acid (H2tp),21 5-aminoisophthalic acid (5-
NH2-H2ip), and 2-aminoisophthalic acid (2-NH2-H2ip). In all
As a synthetic broad antibacterial compound, norfloxacin
(Nf, 1-ethyl-6-fluoro-1,4-dihydro-4-oxo-7-(1-piperazinyl)-3-qui-
nolinecarboxylic acid, Scheme 1), is a poorly soluble zwitter-
ionic fluoroquinolone marketed as Noroxin1 by Merck, which
has been widely used in clinical practice for the treatment of
various infections. The reduced dissolution behavior in water
+
2
…
the resulting compounds, ion-pair N –H
O
hydrogen bonds
(0.28–0.40 mg mL21 15
of norfloxacin counters significant
)
+
2
…
are observed with N
O
type H-bond donor and acceptor
groups, which can be considered to be salt bridges22 and are
stronger than those involving neutral acceptors and donors.
These binary adducts exhibit different types of H-bonding
patterns with diversiform structural features and stabilities,
influenced by the geometric effect or the H-bonding competi-
tion of involved water solvents (in 4 and 5).
challenges in the formulation of conventional dosage forms,
e.g. tablets, and obstructs the design of liquid dosage forms,
such as parenteral and ophthalmic solutions. Therefore, a
number of efforts have been devoted to the improvement of
the aqueous solubility of norfloxacin, mainly through the
preparation of different solid pharmaceutical forms of
norfloxacin such as polymorphs, salts, cocrystals and solvates
for the dosage forms.16–18 For example, norfloxacin is known
to exhibit polymorphism in many hydrates of changeable
stoichiometry water molecules19 and a varied degree of
bioavailability including its anhydrate and hydrates.19b
Notably, norfloxacin has a fluorine atom and piperazine ring,
which is likely to generate complex adducts in salt-form with
acidic moieties. In recent cases, proton transferring from
carboxylic acid to piperazine base was confirmed by the crystal
structure of such fluoroquinolone drugs.17 Among all the
binary charge-transferred crystalline products of norfloxacin,
improvement of the solubility based on benzoic acid is the
highest with about 40-fold improvement in pure water and
phosphate buffer media. It could be anticipated that norflox-
acin drugs with aromatic acids may have a considerable
improvement in solubility and it is also reasonable to
Experimental section
General materials and methods
All the chemicals and solvents were commercially available
and used as received. Crystalline form salts based on both
norfloxacin and various aromatic dicarboxylic acids were
generally obtained by slow evaporation of water–CH3OH or
water–DMF (in the case of 3) solution of the reactants in a 1 : 1
ratio. Fourier transform (FT) infrared data were collected on an
AVATAR-370 (Nicolet) spectrometer by transmission through
the sample deposited on a KBr pellet. Elemental analyses were
performed
on
a
CE-440
(Leemanlabs)
analyzer.
Thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) experiments were performed on a TG/DTA
6300 thermoanalyzer from room temperature to 800 uC under
nitrogen atmosphere at a heating rate of 10 uC min21. Powder
X-ray diffraction (PXRD) of bulk samples were recorded on a
SMART Bruker D8 Advanced powder X-ray diffractometer
using Cu Ka radiation (l = 1.5406 Å) at 40 kV and 30 mA.
Each sample was scanned in the 2h range 5–50u with a step
size of 0.02u.
The following procedure for the preparation of [HNf]?[Hpa]
(1) is typical.
Synthesis of [HNf]?[Hpa] (1)
Phthalic acid (16.6 mg, 0.1 mmol) was dissolved in a methanol
solution (6 mL), to which an aqua solution (3 mL) of
norfloxacin (Nf) (31.9 mg, 0.1 mmol) was added with stirring.
The mixture was then filtered and left to slow evaporation.
Colorless block single crystals suitable for X-ray analysis were
afforded after several weeks. Yield: 44% (21.3 mg, based on
Nf). Anal. calcd for C25H24FN2O7: C, 62.11; H, 5.00; N, 5.79%.
Found: C, 62.44; H, 5.10; N, 5.61%. FT-IR (cm21): 3438b,
2728w, 2642w, 2490b, 1728vs, 1627s, 1476s, 1377s, 1268vs,
1032s, 934m, 787m, 737s, 621m, 572s, 444m, 410s.
Scheme 1 Structures of norfloxacin and benzenedicarboxylic acids.
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was no evidence of crystal decay during data collection. A
semiempirical absorption correction was applied (SADABS),
and the program SAINT was used for integration of the
diffraction profiles.23 All structures were solved by direct
methods with SHELXS and refined by full-matrix least-squares
on F2 with SHELXL program of the SHELXTL package.24
Notably, in the structure of 5, the disorder lattice water O
atoms (O10, O11, and O12) are assigned to partial site-
occupancy of 0.50, and the amount of water molecules (3.5
H2O in the asymmetric unit) is also consistent with the results
of TG and elemental analyses. All H atoms were first found in
difference electron density maps, and then placed in the
calculated sites and included in the final refinement with fixed
thermal factors. Crystallographic data and structural refine-
ment parameters are summarized in Table 1. Hydrogen-
[HNf]?[Hip] (2)
Yield: 90% (43.5 mg, based on Nf). Anal. calcd for
C25H24FN2O7: C, 62.11; H, 5.00; N, 5.79%. Found: C, 62.40;
H, 5.11; N, 5.71%. FT-IR (cm21): 3425b, 2997s, 2840s, 2665w,
2553b, 1693vs, 1631w, 1479s, 1273vs, 1220m, 1145w, 937s,
837w, 746s, 730m, 690s, 574s, 528w, 410s.
[HNf]?[Htp]?[H2tp]0.5 (3)
The synthetic media are water–DMF (3 mL : 3 mL) system.
Yield: 88% (33.4 mg, based on H2tp). Anal. calcd for
C28H27FN3O9: C, 59.15; H, 4.79; N, 7.39%. Found: C, 59.12;
H, 4.67; N, 7.43%. FT-IR (cm21): 3404b, 3065w, 2855w, 2750w,
2593w, 2453b, 1710vs, 1631s, 1584w, 1482s, 1362m, 1325w,
1273vs, 1223s, 932m, 766s, 739s, 508m, 416w.
[HNf]?[2-NH2-Hip]?H2O (4)
bonding geometries are listed in Tables S1 and S2, ESI.
3
Yield: 85% (43.9 mg, based on Nf). Anal. calcd for
C25H27FN3O8: C, 58.14; H, 5.27; N, 8.14%. Found: C, 58.12;
H, 5.33; N, 8.17%. FT-IR (cm21): 3435b, 2995w, 2857w, 2770w,
2505b, 1918w, 1699s, 1631vs, 1548s, 1479vs, 1380s, 1270vs,
1205m, 1109w, 1037w, 928w, 893w, 809m, 773m, 755m, 681m,
569w.
Solubility studies
The solubility studies were performed by a UV-1201 spectro-
photometer. The bulk phase of each salt was washed by water
and methanol 3 times before the solubility and dissolution
experiment and each sample matches well with its single
crystal form as revealed by the X-ray powder diffraction (PXRD)
[HNf]2?[5-NH2-Hip]2?7H2O (5)
pattern (Fig. S1, ESI ). The concentrations of norfloxacin for 1–
3
5 were calculated by means of a standard graph, which was
made by measuring the absorbance of varied concentrations of
norfloxacin at their respective lmax. These absorbance values
were plotted against the concentration to determine the
calibration curve. From the slope of the calibration curves,
Yield: 70% (39.3 mg, based on Nf). Anal. calcd for
C50H64F2N6O21: C, 53.47; H, 5.74; N, 7.48%. Found: C, 53.44;
H, 5.70; N, 7.43%. FT-IR (cm21): 3434vs, 3308s, 3144w, 3046w,
2968w, 2834w, 2458b, 1912w, 1687vs, 1626s, 1433s, 1366m,
1300w, 1268s, 1192s, 1137s, 1039m, 930w, 873m, 753s, 685s,
599m, 492s, 404m.
the molar extinction coefficient (Table S3, ESI ) for each salt/
3
drug was calculated in different media. An excess amount of
all the samples was added to the solvent of interest (pH 1.2, 0.1
mol L21 HCl and pH 6.4, distilled water). The solutions were
stirred at 300 rpm using a magnetic stirrer at 25 uC (¡1). After
72h, the suspensions were filtered through filter paper. The
X-Ray crystallography
X-Ray single-crystal diffraction data for 1–5 were collected on a
Bruker Apex II CCD diffractometer at room temperature by
using a fine-focus molybdenum Ka tube (l = 0.71073 Å). There
Table 1 Crystal data and structure refinement parameters for compounds 1–5
1
2
3
4
5
Formula
Formula weight
Crystal system
Space group
C24H24FN3O7
485.46
Triclinic
¯
P1
C24H24FN3O7
485.46
Orthorhombic
Pbca
C28H27FN3O9
568.53
Triclinic
¯
P1
C24H27FN4O8
518.50
Monoclinic
P21/n
C48H64F2N8O21
1127.07
Monoclinic
P21/c
a/Å
b/Å
c/Å
8.998(12)
9.624(13)
13.044(18)
94.09(3)
106.38(3)
91.93(3)
1079(3)
2
13.9940(17)
13.6288(18)
22.537(3)
90
90
90
4298.3(9)
8
1.500
9.8768(19)
10.2489(19)
13.674(3)
89.319(4)
74.752(4)
71.679(4)
1263.9(4)
2
9.5670(8)
8.1152(6)
30.906(3)
90
92.476(2)
90
2397.3(3)
4
1.437
9.964(3)
14.760(4)
18.275(5)
90
90.283(5)
90
2687.6(13)
2
1.393
a/u
b/u
c/u
V/Å3
Z
rcalcd/g cm23
1.494
1.494
F(000)
508
0.117
2032
0.117
594
0.117
1088
0.114
1188
0.114
m/cm21
Measured reflections
Independent reflections
6088
3768
26 842
4914
7289
4654
13 418
4437
14 525
4888
Rint
0.0264
0.0549
0.1769
1.087
0.1042
0.0592
0.1332
0.995
0.0214
0.0585
0.1782
1.079
0.0250
0.0472
0.1507
1.102
0.0528
0.0797
0.2345
1.039
Ra
b
Rw
GOF
R = S||Fo| 2 |Fc||/S|Fo|. Rw = [S[w(Fo 2 Fc2)2]/Sw(Fo2)2]1/2
.
a
b
2
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filtered solutions were diluted sufficiently and the absorbance
was measured at their respective lmax (276 nm for 1–3 and 274
nm for 4–5). Finally, the concentrations of the norfloxacin salts
after 72h were calculated using their respective calibration
curves.
For the powder dissolution experiments, the solids of
norfloxacin and 1–5 were milled to powder and sieved using
standard mesh sieves to provide samples with approximate
particle size ranges of 75–150 mm. Excess amounts (500 mg) of
the samples were suspended in 10 mL of water or 0.1 mol L21
HCl in flasks. These flasks were maintained at 25 uC (¡1) and
were stirred at 300 rpm using a magnetic stirrer. At each time,
the solution was withdrawn from the flask and filtered
through a 0.2 mm nylon filter. A 0.1 mL portion of the filtered
aliquot was diluted to 10 mL with different media and was
measured with UV-vis spectrophotometry.
Results and discussion
The crystallization of all the binary organic salts can be
achieved in the water and methanol solvents, with the
exception of compound
3 where water–DMF solvent is
propitious to the formation of suitable single crystals for
X-ray diffraction. All the crystalline materials were prepared by
using 1 : 1 norfloxacin : acid molar ratio for the reaction,
while the final charge-assisted hydrogen-bonding products
were obtained as 1 : 1 or 1 : 1.5 (3) norfloxacin : acid binary
compounds. The purities of these were confirmed by PXRD
analyses, in which the experimental data are consistent with
the single-crystal-simulated spectra (Fig. S1 in the ESI ).
3
Generally, the final molecular molar ratio is consistent with
the amido : carboxylate ratio and independent of the original
agent ratio. Unexpectedly, the 1 : 1 ratio of hydrogen-bonding
donor and acceptor was destroyed in
3
for the
[HNf]?[Htp]?[Htp]0.5 system, despite no metal salt being been
added in the synthetic procedure comparing with the formerly
published experiment.21 The intense infrared (IR) peaks at
about 1700 and 1270 cm21 indicate that the COOH group of
norfloxacin is unionized in all these cases (Fig. S4, ESI ). The
3
two characteristic peaks at 1584 and 1339 cm21 of carboxylic
acid are absent in all salts with the exception of 3, which has a
carboxylic acid moiety. Moreover, the broad peaks at 2450–
2553 cm21 in 1–5 should be ascribed to the protonated
piperazine nitrogen (NH2+) via proton transfer from carboxylic
acid, which is absent in norfloxacin anhydrate.16
Fig. 1 (a) Molecular structure of 1 with atom labeling of the asymmetric unit. (b)
…
A chain structure of 1 in which norfloxacin molecules are connected by N–H
O
…
hydrogen bonds. The intramolecular O–H O hydrogen bonds are also
Molecular and supramolecular structural description of
[HNf]?[Hpa] (1)
illustrated. (c) A 3D supramolecular network constructed by p–p stacking
interactions (highlighted in green dashes) between the acid and the piperazine
components.
In the molecular structure of 1 (Fig. 1a), the asymmetric unit
contains one protonated HNf+ cation and one phthalate anion,
resulting from the proton transformation of phthalic acid to
the piperazine ring of norfloxacin. Within each HNf+ cation,
the carboxylic acid group is coplanar with the quinolone
moiety (torsion angle O2–C15–C13–C14 0.17(4)u) and partici-
intramolecular interactions based on fluorine atoms are found
within the cationic norfloxacin. Meanwhile, the intramolecular
…
S(7) hydrogen bond motif (O6–H6 O5, synthon II, Scheme 2),
…
pates in intramolecular hydrogen bonding (O2–H2 O3,
which is a common occurrence in the 1,2-disubstituted
dicarboxylic acids form,26 is found in the phthalate moiety.
known as S(6) graphic set,25 synthon I, Scheme 2) with the
carbonyl oxygen atom of the quinolone moiety. Multiple weak
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Scheme 2 Possible hydrogen-bonding synthons of norfloxacin cation with
coformers.
Each phthalate forms intermolecular interactions with two
adjacent norfloxacin cations through the connection between
carboxylate oxygen atoms and protonated piperazine nitrogen
atoms.
A herring skeleton-like supramolecular motif is
afforded to run along the crystallographic [100] direction, as
shown in Fig. 1b. Meanwhile, the piperazine ring takes a chair
conformation with the exposed protonated nitrogen, facilitat-
ing the formation of H-bonding with the carboxylate group.
These 1D motifs arrange in a parallel fashion along the b axis
with p–p stacking interactions (the center to center distance of
3.64 Å) between the adjacent fluoroquinolone and phthalate
benzene rings, which fulfill the 2D supramolecular network.
Examination of this structure with PLATON27 shows that there
is no solvent-accessible void in the crystal lattice.
Fig. 2 (a) Molecular structure of 2 with atom labeling of the asymmetric unit,
showing the hydrogen-bonding synthon R22(7). (b) A double-chain motif of 2
running along the [001] direction, in which a synthon R44(8) of piperazine amino
and carboxylate groups joins two adjacent acid–base alternating chains. (c) A
2D supramolecular network built by p–p stacking interactions between the acid
and the piperazine components.
Molecular and supramolecular structural description of
[HNf]?[Hip] (2)
The molecular structure of 2 is composed by one proton
transferred HNf cation and one anionic isophthalate (Hip)
…
component (Fig. 2a). Similar intramolecular O2–H2 O1 hydro-
gen bonds are found to connect the carboxylic acid group with
the carbonyl oxygen atom of the quinolone moiety within the
determined this structure. The asymmetric unit contains a
cationic HNf molecule, an anionic Htp component and a
centrosymmetric H2tp molecule, as indicated in Fig. 3a. Each
HNf+ cation. Unlike in compound 1, the weak C1–H1B O6
…
…
interactions provide support for the directional N1–H1D O7
piperazine moiety adopts the familiar chair-conformation and
…
a charge-assistant N–H O interaction with the
hydrogen bonding between the two components [synthon III,
builds
Scheme 2, R22(7)].25 A pair of inversion-related isophthalate and
carboxylic moiety from another cationic HNf molecule. In this
way, the HNf molecules are head-to-tail joined into a 1D
hydrogen-bonding array running along the crystallographic
[001] direction. The hydrogen-bonding chains are further
penetrating the supramolecular ladder constructed by the
piperazinyl arrange alternately along the [001] direction to
i
…
…
generate a double-chain motif [N1–H1C O7 /N1–H1D O7,
symmetry code: i = 2x + 1, 2y + 2, 2z + 1, synthon IV R24(8),
see Scheme 2], as indicated in Fig. 2b. Interestingly, p–p
interactions (the center to center distance of 3.82 Å) between
fluoroquinolone and isophthalate rings extend the 1D arrays to
form a 2D network parallel to the bc plane (Fig. 2c).
…
linkage of multiple O–H O interactions between Htp and
…
H2tp units (Fig. 3b). An additional N–H O hydrogen bond
(between one of the Htp carboxylate O atoms and a piperazine
nitrogen atom of HNf) completes the final 3D H-bonding
network, as depicted in Fig. 3c. Notably, there are significant
p–p interactions (center to center distance: 3.41 Å) between the
Molecular and supramolecular structural description of
[HNf]?[Htp]?[H2tp]0.5 (3)
The structure of 3 has been reported previously,21 but for the
consistency of the systematic investigation, we have re-
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One of the two carboxylic acid groups of 2-aminoisophthalic
acid transfer one proton to the piperazinyl ring N atom of the
norfloxacin molecule, thereby forming a 1D hydrogen-bonding
motif that is an acidic chain decorated by norfloxacin cations,
in which two hydrogen-bonding patterns are formed as the
graphic set S(6) within an acid anion and R33(10) (synthon V,
Scheme 2) among two acid and one norfloxacin molecules. The
two norfloxacin cations and two lattice water molecules form a
cyclic tetramer synthon R44(12) (VI, Scheme 2) and connect the
adjacent two inversion-related 1D hydrogen-bonding motifs
into a 1D supramolecular ladder along the [100] direction
…
(Fig. 4b). An additional N–H O interaction joins the neighbor-
ing ladder motifs into a complicated 3D hydrogen-bonding
arrangement, as depicted in Fig. 4c. Within this 3D network,
the quinolone rings are stacked in a parallel fashion as that in
4 with the center-to-center separations of 3.66(2) and 3.75(2) Å,
respectively, which will further strengthen the extended 3D
host–guest network.
Molecular and supramolecular structural description of
[HNf]2?[5-NH2-Hip]2?7H2O (5)
The structure of 5 (Fig. 5a) contains a cationic pharmaceutical
agent norfloxacin, a mono-deprotonated 5-aminoisophthalate
and 3.5 lattice water molecules, including two crystallographi-
cally independent O8 and O9 molecules and three water sites
(O10, O11 and O12) with half-occupancy. The dihedral angle
between the substituent phenyl ring of 5-NH2-Hip and that of
HNf is 76.2(2)u. A hydrogen-bonding motif of herring skeleton
is shaped along the [010] direction, linked by the interactions
i
iv
…
…
of N1–H1C O5 and O7–H7A O5 (symmetry code: i = x + 1,
y, z; iv = 2x, y 2 1/2, 2z + 1/2), where a head-to-tail chain of
acid moieties and the decorated norfloxacin molecules are
involved (Fig. 5b).
Calculation with the PLATON27 program shows that the
solvent-accessible-void in 5 (438.9 Å, 16.3% of the unit cell) is
occupied by lattice water moieties. Analysis of the crystal
packing indicates a distinct channel residing along the [100]
direction. Two fully occupied water sites (O8 and O9) are
…
…
anchored to the host chains through N–H O and O–H
O
interactions (Table S2, ESI ) accommodated outside the
3
channels (Fig. 5c). While the partially-occupied O10, O11 and
O12 are hydrogen bonded to each other to form a water chain
running along the straight channel centered at (x, 1/2, 1/2) (see
Fig. 5c and 5d). With respect to the channel, a water tetramer
…
…
Fig. 3 (a) Molecular structure of 3 with atom labeling of the asymmetric unit. (b)
[O10–H10A O11 and O11–H11A O10] with the graphic set
R44(8) (synthon VII, Scheme 2) and two sets of double hydrogen-
bonded bridges (synthon VIII, Scheme 2) [R22(4) between the
O11 and O12 water molecules, and between each two O12
water molecules] are found to have a planar configuration
because of their crystallographic symmetry. The hydrogen-
bonding behavior within the channel spaces must have a
considerable effect on the dehydration behavior of 5, although
the H atoms of the water molecules could not be reliably
placed. It is reasonable to suggest that the loss of the channel
water molecules should be favored in such a channel hydrate
form.28
…
A norfloxacin chain of 3 through the N–H O interactions running along the
[001] direction. (c) A supramolecular ladder of the terephthalate moieties with
the penetration of the norfloxacin chain. (d) A portion view of the 3D hydrogen-
bonding network in 3.
fluoroquinolone and terephthalate benzene rings, which
further stabilize the resultant 3D network.
Molecular and supramolecular structural description of
[HNf]?[2-NH2-Hip]?H2O (4)
The structure of 4 has one norfloxacin cation, one mono-
deprotonated 2-aminoisophthalate anion, and one H2O
molecule in the asymmetric unit, as indicated in Fig. 4a.
An extended 3D network is eventually constructed via
…
…
multiple O–H O and N–H O hydrogen bonds involving the
lattice water and the amido of 5-NH2-Hip, which is also
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Fig. 4 (a) Molecular structure of 4 with atom labeling of the asymmetric unit. (b)
A supramolecular ladder involving guest water molecules running along the
[100] direction. (c) A portion view of the H-bonding interactions between acid
chains and norfloxacin with only the piperazine rings shown for clarity.
Fig. 5 (a) Molecular structure of 5 with atom labeling of the asymmetric unit. (b)
An aminoisophthalate chain running along the [010] direction with decoration
consolidated by the aromatic stacking forces with the center-
to-center separations of 3.58(2) and 3.70(2) Å, between both of
the fluoroquinolone benzene rings or between one of the
…
of norfloxacin molecules through the N–H O interactions. (c) A supramolecular
host–guest system of 5 with a hydrogen-bonding water chain highlighted in the
space-filling mode. (d) A portion view of the hydrogen-bonding array including
water moieties in 5.
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fluoroquinolone benzene rings and the phenyl rings of 5-NH2-
Hip molecules (Fig. 5c). Meanwhile, the aqua molecule O10
also forms a similar double hydrogen-bonded bridge (O10–
(Fig. 5d) in 5. For 1 and 2, similar hydrogen-bonding chains
are observed with the acid and piperazine base arranging
alternately. For 3–5, the lack of acid–base alternating chains
may be due to the unexpected components ratio in 3 and the
inclusion of water guests in 4 and 5, respectively. Additionally,
p-stacking interactions are found to be the most prominent in
all the five novel pharmaceutical salts.
…
…
H10B O8 and O8–H8A O10) with the O8 water molecule
(Fig. 5d). Moreover, this hydrogen-bonded bridge combines
with another bridge of the 1D water chain motif to connect the
same amino group of the 5-NH2-Hip moiety (Fig. 5d). In this
way, a novel H-bonded pattern notated as the graphic set
R55(10) (synthon IX, Scheme 2)25 is afforded to further
consolidate such 1D water motifs within the extended 3-D
host network.
Thermogravimetric (TG) and differential scanning calorimetry
(DSC) analyses
These binary pharmaceutical salt materials are air stable at
ambient conditions and thermogravimetric experiments were
implemented to investigate their thermal stabilities. Thermal
Structural comparison of norfloxacin salts
analysis (Fig. 6 and S5, ESI ) shows that the TGA and DSC
3
As described above, a similar components ratio has been used
for the crystallization of norfloxacin and the five aromatic
acids. With the exception of 3 (norfloxacin : acid molar ratio =
1 : 1.5), all these crystalline products were yielded in a 1 : 1
ratio with the salt forms. Several attempts have been carried
out to obtain a 1 : 1 ratio salt of norfloxacin terephthalate in
the water–DMF or water–methanol system with the norflox-
acin : acid molar ratio of 2 : 1, 1 : 1 and 1 : 2, respectively.
X-ray single-crystal diffraction of the resultant crystalline
products indicates that all the attempts failed, which may
partly arise from the structural nature of terephthalic acid for
the tendency to crystallize with more than one molecule in the
asymmetric unit, with slight conformational dissimilarity.29 It
is generally known as the Rule of Three that the reaction of an
acid with a base will be expected to form a salt if DpKa (DpKa =
pKa(base) 2 pKa(acid)) is greater than 3, while either a
cocrystal or a salt will form when DpKa is between 0 and
3.30,31 Estimated DpKa values between the piperazine base32
and carboxylic acids33 (pKa1) suggested salt formation
(Table 2). Also, X-ray single-crystal diffraction gives evidence
that all the crystalline compounds are ionized, with proton
transfer occurring from the carboxylic acid to the chair-
conformational piperazine NH group.
curves are consistent with the salt composition. The thermo-
gravimetric (TG) curves of 1–3 reveal a first weight loss of
33.6%, 34.7% and 28.9% in the temperature range from 201–
367 uC, 207–367 uC, and 188–300 uC, respectively, which can be
ascribed to decomposition of the crystalline samples and
elimination of a benzene dicarboxylate molecule. While for 4
and 5, the first weight losses in the region from 85 to 160 uC
and 40 to 120 uC, respectively, correspond to the removal of the
guest water molecules (for 4, calculated: 3.5%; observed: 4.3%;
for 5, calculated: 11.2%; observed: 12.1%). The decomposi-
tions of the retained hydrogen-bonding networks start at 245
and 230 uC with different speeds of the weight losses for 4 and
5, respectively. The data of DSC analyses show the endother-
mic melting peaks at 230.8, 282.6, 312.9 and 269.1 uC for 1–4,
respectively, which have lower melting points and demon-
strate the same trend as those of acidic coformers. The
endothermic peak at 64.3 uC for 5 is consistent with the release
of lattice water molecules followed by a blurred endothermic
melting transition at around 195 uC, while no significant
endothermic transition has been found to indicate the loss of
guest water for 4.
Solubility of norfloxacin salts 1–5
In all cases, the carboxylic acid group of norfloxacin is in the
neutral state for C–O and CLO distances differ by ca. 0.1 Å and
the intramolecular hydrogen bond with the quinolone CLO
presents as S(6) graphic set. The C–O distances of the
carboxylate moiety in mono-deprotonated dicarboxylate are
near-equal (difference , 0.03 Å), which can distinguish the
carboxylate salts from the carboxylic acids of cocrystals or
solvates. The inclusion of water was found in two substituted
isophthalate cases, leading to a cyclic tetramer synthon R44(12)
(VI in Scheme 2) in 4 and a supramolecular water chain
The solubility data for norfloxacin salts 1–5 were measured by
estimating the drug concentration in pure water (pH 6.4) and
0.1 M HCl (pH 1.2). All salt samples used for the powder
dissolution profiles are shown in Fig. 7 and the apparent
solubility data are listed in Table 3. The undissolved salt
materials analyzed by PXRD after the dissolution experiment
show that all the carboxylate salts are stable to the dissolution
conditions in pure water (Fig. S2, ESI ). Partial transformation
3
to the norfloxacin dihydrate form may occur in salts 1 and 5
after the dissolution experiment in 0.1 M HCl solution, but salt
2 is stable. However, the PXRD patterns of salts 3 and 4 are
different from the original patterns after the dissolution
experiment in acidic medium.
Table 2 pKa values of the norfloxacin and benzenecarboxylic acids used
The solubility profile in pure water medium shows a clear
increase in the solubility of the drug when in the form of most
of the carboxylate salts. From Fig. 7a, it can be found that both
the dissolution rate and solubility values of the salts (with the
exception of 3) are larger than those of norfloxacin in pure
water medium, indicating a clear increase in the solubility of
norfloxacin as carboxylate salts. Especially for 1, the solubility
value is approximately 39.04 times as large as that of
norfloxacin. The very poor solubility of 3 may be due to the
Norfloxacina
Carboxylic acidb
DpKa
c
6.30 (COOH)
8.38 (piperazine)
2.98 (phthalic acid)
3.46 (isophthalic acid)
3.51 (terephthalic acid)
3.69 (5-aminoisophthalic acid)
4.27 (2-aminoisophthalic acid)
5.40
4.92
4.87
4.69
4.11
a
b
c
Ref. 32. First ionization constant for diacids. DpKa is calculated
as pKa (piperazine NH+) 2 pKa1 (carboxylic acid).
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Fig. 7 Powder dissolution profiles of norfloxacin salts in (a) pure water and (b)
0.1 M HCl.
Fig. 6 TG curves for norfloxacin salts 1–3 (a) and 4–5 (b).
group not being involved in intermolecular hydrogen-bonding.
It seems that more structural data are needed in order to find a
plausible explanation for these differences. Revisiting the
previously published structures and solubility of Nf salts
reveals that the carboxylic group of norfloxacin is absent in
intermolecular H-bonding interaction in the solid states of
norfloxacin maleate hydrate16 and norfloxacin benzoate
monohydrate,17 both of which show the highest solubility
(Table 3) in the respective work.16,17
In the acidic medium (0.1 M HCl, pH 1.2) the dissolution
rate order changed, following the order 1 . Nf . 5 . 3 . 4 . 2
(Fig. 7b), with the salts showing faster dissolution at this pH
value because of the protonated feature. For the normally less
soluble norfloxacin, the higher dissolution is due to protona-
tion of the piperazine ring in acidic medium. Meaningfully,
most of the salts have generally lower solubility in relation to
undeprotonated H2tp moiety in the molecule and the resultant
H-bonding assembly. The solubility of 1–5 increases as the pH
decreases (from 6.4 to 1.2) but the values for the salts are
higher than that for the reference drug at near neutral pH,
which is expected because the counterions in the salts are
conjugate bases. A further understanding of the crystal
packing for all Nf salts indicates a reliable correlation between
the hydrogen-bonding networks and the solubility. In compar-
ison with the anhydrous analogues (2 and 3), the highest
solubility of 1 may be caused by the absence in intermolecular
hydrogen-bonding for the carboxylic group of norfloxacin.
Correspondingly, the highest solubility of 5 comparing with 2
and 4 could also be an effect of the norfloxacin carboxylic
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Table 3 Solubility (mg mL21) of norfloxacin and norfloxacin salts measured at 72 h in different pH medium (25 ¡ 1 uC)
Number that Nf is multiplied
by to give the compound
solubility value in pure H2O
Number that Nf is
multiplied by to give
the compound solubility value in 0.1 M HCl
Compound
Pure H2O
0.1 M HCl
Nf
1
2
3
0.38
14.84
0.71
0.11
1.02
2.48
2.93
5.50
12.80
6.60
3.90
9.80
—
28.19
38.28
1.65
3.32
3.06
17.17
21.84
14.53
29.97
—
—
39.04
1.87
0.28
2.68
6.53
—
—
—
—
—
1.36
0.06
0.12
0.11
0.61
—
—
—
—
—
4
5
Norfloxacin oxalate dihydratea
Norfloxacin tartarate dihydratea
Norfloxacin benzoate monohydratea
Norfloxacin 0.5 (succinate) hydrateb
Norfloxacin malonate dihydrateb
Norfloxacin maleate hydrateb
—
—
—
—
a
b
Ref. 17. Ref. 16.
norfloxacin with the exception of 1, which has a solubility
approximately 1.36 times that of the parent drug in the 0.1 M
HCl system. Comparing with the norfloxacin salt of benzoate
(lower solubility in the acidic system),17 1 demonstrates a
higher solubility than that of norfloxacin either in pure water
or acidic systems, indicating phthalic acid is a good candidate
to enhance the solubility of fluoroquinolone drugs in acidic
environment. For the isophthalates series, norfloxacin salts
with amino substituted isophthalates have the enhanced
solubility, while norfloxacin isophthalate shows very poor
solubility in 0.1 M HCl (only 0.06 times that of Nf).
regarded as safe) compounds. As solubility and bioavailability
are often correlated, it is anticipated that the bioavailability of
norfloxacin may also be increased after the formation of such
salts, and further studies on other fluoroquinolone salts are
encouraged.
Acknowledgements
We gratefully acknowledge the financial support from the
National Natural Science Foundation of China (Grant
21201026),
A Project Funded by the Priority Academic
Program Development of Jiangsu Higher Education
Institutions (PAPD), Jiangsu Key Laboratory of Advanced
Catalytic Materials and Technology (Grant BM2012110), and
the Natural Science Fund for Colleges and Universities in the
Jiangsu Province (Grant 12KJB150002).
Conclusions and perspectives
A series of norfloxacin salts of benzenedicarboxylates were
synthesized to confirm the proton transfer of hydrogen-
bonding and control their solubilities. Pharmaceutical agents
1–5 were obtained and their structures were determined by
single crystal X-ray diffraction, in which 1 and 2 form similar
1D H-bonding chains with the acid and piperazine base
alternating and a further 2D supramolecular network via p–p
interactions, while for 3–5, the lack of acid–base alternating
chains leads to very complicated 3D H-bonding architectures
stabilized by p-stacking. In summary, norfloxacin turns out to
be a reliable H-bonded agent capable of forming charge-
assisted H-bonded networks by recognizing various benzene-
carboxylates. The aqueous solubility of norfloxacin is
increased after the formations of all salts except 3. To establish
how the hydrogen-bonding interactions influence the solubi-
lity, crystal packing analyses were carried out. These indicate
that the absence of intermolecular H-bonding for the
carboxylic group of norfloxacin is helpful to improve the
solubility of related salts. Consequently, 1 shows considerable
solubility either in pure water or acidic medium, indicating the
solubility of Nf can be enhanced via the formation of salts with
suitable carboxylate agents. Although the preliminary investi-
gations show that these norfloxacin salts have a certain
security, further study to improve their safety are in progress
for the coformers do not belong to the GRAS (generally
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