Inorganica Chimica Acta 363 (2010) 4031–4037
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Boric acid complexes with organic biomolecules: Mono-chelate complexes
with salicylic and glucuronic acids
Dursun Ali Köse a, Birgül Zümreoglu-Karan b,⇑, Tuncer Hökelek c, Ertan Sß ahin d
a
Hitit University, Department of Chemistry, 19000 Çorum, Turkey
Hacettepe University, Department of Chemistry, Beytepe Campus, 06800 Ankara, Turkey
c
Hacettepe University, Department of Physics, 06800 Beytepe, Ankara, Turkey
d
Atatürk University, Department of Chemistry, 22240 Erzurum, Turkey
b
a r t i c l e
i n f o
Article history:
Received 12 March 2010
Received in revised form 22 July 2010
Accepted 2 August 2010
Available online 7 August 2010
Keywords:
Boric acid
Borate ester
Salicylic acid
Glucuronic acid
Crystal structure
13
C MAS NMR
a b s t r a c t
Two mono-chelate borate complexes, lithium mono-salicylatoborate and sodium mono-glucuronatoborate, are reported for the first time. The complexes were isolated from aqueous solutions and characterized by FTIR (Fourier Transform Infrared) and 13C MAS (Magic Angle Spinning) NMR techniques. Thermal
stabilities of the complexes were examined by recording their TGA (Thermogravimetric Analysis) curves.
Lithium mono-salicylatoborate, Li[B(Sal)(OH)2], was isolated in crystal form and presented as a novel
hybrid metal–organic framework possessing zeolitic structure. X-ray analysis revealed an original crystal
structure constructed with solvate-free lithium ions adopting two different types of coordination polyhedra, corner-sharing LiO4 (tetrahedral) and LiO5 (distorted square pyramidal), in the same framework.
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
There is increasing evidence that boron is nutritionally important for humans. Although this trace element is believed to be beneficial for energy and mineral metabolisms, hormone action,
immune and cognitive functions [1–3]; it does not appear to be
widely consumed in supplemental form. In most of the commercial
dietary boron supplements now available, boron is chelated with
amino acids or with polyhydroxy acids in combination with a variety of nutrients such as vitamin D, calcium, magnesium, soy isoflavones, chondroitin sulfate, glucosamine and others. However,
no information about the exact structures of these chelates are
available as they are mostly given in patent formulations.
Attempts are underway to incorporate boron into different biologically active molecules for nutritional and medicinal applications. We have recently described the in vitro complexation of
boric acid with vitamin C [4]. At intracellular pH, nearly all boron
exists as boric acid which behaves as a Lewis acid and forms
molecular addition compounds with amino- and hydroxy-acids,
carbohydrates, nucleotides and vitamins through electron
donor–acceptor interactions [5–11]. Boric acid forms complexes
with organic molecules bearing adjacent hydroxyl groups through
⇑ Corresponding author. Fax: +90 312 2992163.
E-mail address: [email protected] (B. Zümreoglu-Karan).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.08.001
esterification reactions. Partial esterification creates monoesters
(1:1 complex) that retain the planar configuration with no charge
or a tetrahedral configuration with negative charge. Complete
esterification leads to the formation of bicyclic diester (1:2 complex) structures with negatively charged tetrahedral borate anions,
as shown in Scheme 1. The role of the cation in stabilizing the ester
structures has been demonstrated [12,13]. Monoesters of boron are
quite labile and rapidly hydrolyze to their original components in
aqueous solution while diesters are thermodynamically more stable and almost undissociable in water [3,14].
Bis-chelate complexes of boron based on aromatic or aliphatic
diols and carboxylic acids are usually non-toxic, inexpensive, thermally and electrochemically stable. Therefore the literature covering the boron complexes of such ligands is mainly based on 1:2
complexes rather than 1:1 complexes. Salicylic acid (Fig. 1a) is a
well known complexing agent with boric acid [15–17]. Several
bis-salicylatoborate complexes have been reported in the literature
[18–26] of which lithium bis-salicylatoborates have found applications as electrolytes for Li-ion batteries. Likewise; glucuronic acid
(Fig. 1b) has a carboxylic acid function and a number of cis-OH
groups on the pyran ring available for complex formation with boron. Although there is an extensive literature on boron complexes
with sugars [11,27–31], no study has yet been reported about
the interaction of boric acid with glucuronic acid, to the authors’
knowledge. The only work that has been published is with Dxylo-5-hexulosonic acid, a molecule carrying both a sugar ring
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D.A. Köse et al. / Inorganica Chimica Acta 363 (2010) 4031–4037
O
O
OH
+
B
M (CH2)n
M
+
B
OH
O
O
(CH2)n
(CH2)n
O
O
a
b
Scheme 1. Boric acid esters (a) monoester and (b) bicyclic diester.
OH
HO
2
3
4
5
HO
1 6
7
HO
2
3
1
2.2. Instrumentation
OH
4
5
O
6
OH
O
OH
O
a
lar ratio. Because borate formation is reversible, solid H3BO3 was
added to avoid confusion between the interacting trigonal and tetrahedral boron species. Boric acid completely dissolved and the
resulting mixture was stirred for 1 h. The solution was then concentrated in vacuum and cold acetone was added into the concentrated solution. The precipitates/crystals were vacuum filtered and
kept in a dessicator over solid CaCl2 (Yield: ca. 60–70% based on
boron).
b
Fig. 1. (a) Salicylic acid, (b) glucuronic acid.
and carboxylic acid function. Boron binding in this compound was
found to occur over adjacent OH groups forming a five membered
ring and not through the carboxylate group [32].
Salicylic acid is a natural signaling molecule for activation of
plant defense mechanism and is a pharmacological agent for controlling the inflammatory response in humans. Salicylatoborate
complexes with bioactive cations might therefore be considered
as potential micronutrients and pharmacophores aiding in the
treatment of metal ion or boron deficiencies and in strengthening
the immune system. On the other hand, the human body uses glucuronic acid in a process called ‘‘glucuronidation” to make a large
variety of substances more water-soluble. Binding boron to glucuronic acid may thus open ways for the design of new drugs for easier delivery in Boron Neutron Capture Therapy and also for healthy
bones/joints as glucuronic acid is a component of hyaluronan
which is used to treat osteoarthritis of the knee. Here we report
the crystallographic characterization of a new salicylatoborate
complex, Li[B(Sal)(OH)2], and the first spectroscopic characterization of sodium glucuronatoborate complex, Na[B(Glu)(OH)2]2H2O.
2. Experimental
C, H contents were determined by an CHNS-932 LECO model
analytical instrument. Crystal water determination and thermal
analyses were performed by the Shimadzu DTG-60H system, in a
dynamic nitrogen atmosphere (100 mL/min), at a heating rate of
10 °C/min, in platinum sample vessels with reference to a-Al2O3.
Melting points were determined by an Electrothermal 9100 model
instrument. FTIR spectra were measured in the 450–4000 cm1
range with a Perkin–Elmer Spectrum One instrument, by using
the KBr pellet technique. Solution 13C NMR spectra were recorded
with a Bruker AV 400 Spectrometer in 200–0 and 8–0 ppm ranges,
respectively, at 295 K in D2O. Solid state 13C NMR spectra were recorded in 280–0 ppm with a Bruker Avance Ultrashield TM
300 MHz WB instrument, by using a 4 mm MAS prob at 5 KHz spin
rate and contact time of 2 ms. The measurement temperature was
294 K.
Crystallographic analyses were performed using a Rigaku RAXIS RAPID-S diffractometer. CrystalClear was used for data collection and cell refinement and data reduction [33], SHELXS97 [34] was
used for structure solution and SHELXL97 [34] was used for structure
refinement. Molecular drawing was performed using ORTEP3 for
Windows [35], WinGX [36] software was used to prepare the
material for publication. Crystallographic data were recorded on
a Rigaku R-AXIS RAPID-S diffractometer using Mo Ka radiation
(k = 0.71073 Å) at T = 294(2) K. No absorption correction was applied. Structure was solved by direct methods and refined by
full-matrix least squares against F2 using all data. All non-H atoms
were refined anisotropically. Only the H atoms of OH groups were
located in a difference Fourier map and refined isotropically by
keeping their positions fixed. The remaining H atom positions were
calculated geometrically at distances of 0.93 Å (CH) from the parent C atoms; a riding model was used during the refinement process and the Uiso(H) values were constrained to be 1.2Ueq(carrier
atom).
2.1. Preparations
3. Results and discussion
The reactions were performed in aqueous solutions prepared
with deionized water. The reagents, salicylic acid (Merck) and glucuronic acid (Merck) were used as received. Monoanionic forms of
salicylic and glucuronic acids were prepared by reacting the acid
solutions with either LiOH or NaHCO3 in appropriate molar ratio,
to produce the respective salts in the solution phase.
The complexes were prepared by adding solid H3BO3 into the
solutions containing the metal salt of the respective acid in 1:1 mo-
HO
OH
OM
O
OH
OH
B
R
Our studies and those reported elsewhere [17,23,37] suggest
that boric acid first forms the 1:1 mono-chelate complex with
the acid anion and then the 1:1 complex undergoes condensation
reaction with the fully protonated acid to yield the 1:2 bis-chelate
complex. Following the route given in Scheme 2, and by controlling
the stochiometries of the reactants, the complexes were isolated
from aqueous solutions as white powders with Li+ and Na+ cations.
OH2
OH
R
O
OH
O
B
HO
R
OH
M
O
O
O
O
B
HO
OH
M
M R
OH
B
O
O
Scheme 2. Proposed mechanism for the reaction of boric acid with salicylic acid and glucuronic acid to yield 1:1 complexes.
OH
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D.A. Köse et al. / Inorganica Chimica Acta 363 (2010) 4031–4037
Table 1
Chemical compositionsa and melting points of salicylato- and glucuronatoborate
complexes.
a
Compound
m.p.
(°C)
C (%)
H (%)
H2O (%)
Li[B(Sal)(OH)2]
Na[B(Glu)(OH)2]2H2O
162
125
43.98(44.68)
25.10(24.74)
3.77(3.19)
3.61(4.72)
–
12.20(12.75)
Calculated in parentheses (Sal = C7H4O3, Glu = C6H3O7).
The purities of the compounds were confirmed by elemental analyses (Table 1).
3.1. FTIR spectra
Fig. 2.
A summary of the FTIR spectral data of the compounds and the
precursor acid salts are given in Table 2, with their assignments for
comparison. The spectrum of Li[B(Sal)(OH)2] displayed two sharp
peaks at 3576 and 3505 cm1 in the O–H stretching region.
Complexation with boron resulted in shifting to high frequency
direction for the m(CO) and ma(COO) bands and to opposite direction
for the ms(COO) band, with respect to sodium salicylate. ms(COO)
was splitted into two sharp peaks at 1281 and 1251 cm1 for Li[B(Sal)(OH)2]. ma(B–O) vibration overlapped with the m(C–O–) band in
the range 1200–900 cm1. Two signals for ms(B–O) vibrations were
observed around 750 and 700 cm-1, as diagnostic for the tetrahedral borate anion [39,40].
The FTIR spectrum of sodium glucuronate displayed a
complicated nature in the OH stretching region and also in the
1500–1000 cm-1 region, where characteristic (CC)ring, m(COC)ring,
ma(COO) and ms(COO) vibrations appear, due to the presence of
several hydroxo groups on the pyran ring. Resolution was considerably lost on complexation with boron, nevertheless, some pronounced spectral changes were observed. The pattern in the
1200–800 cm1 region was consistent with the data reported by
Davis and Mott [40] for several carbohydrate borate complexes.
In this region, the individual assignment of bands was difficult
due to the overlap of numerous vibrations of glucuronate and in
plane B–O–H bending vibrations of tetrahedral borate. The tetrahedral B–O symmetrical stretching band appeared as a doublet at 815
and 783 cm1. Out-of-plane C–H bending peaks, characteristic
for heterocyclic molecules, were observed in the 700–500 cm1
region.
3.2. NMR spectra
Solution 13C NMR spectra of the complexes in D2O yielded
signals identical to those present in the spectra of the precursor
acid salts. It appears that hydrolysis reaction restored the corresponding organic acid anions and boric acid species during the
recording of the spectrum. The structural analyses were therefore
continued with solid state NMR studies.
Fig. 2 shows the 13C MAS NMR spectrum of Li[B(Sal)(OH)2]. The
signals for the seven carbon atoms are seen at 192, 184, 181, 160,
154, 141 and 136 ppm together with spinning sidebands. Since
13
C MAS NMR spectrum of Li[B(Sal)(OH)2].
there is no available reference data for this compound in the literature, the chemical shifts were compared with those of salicylic
acid. In the solid state, the chemical shifts of salicylic acid carbons
are C-1 118, C-2 162, C-3 112, C-4 138, C-5 121, C-6 133 and C-7
176 ppm [41]. On chelation to the NMR-active boron atom, all
the signals were shifted and broadened due to the quadrupole
relaxation of the boron nucleus. Although the carbon atoms C-7
and C-2 are normally expected to be the most deshielded [42],
the electronic effect of boronate substitution on aromatic ring carbons is more strongly felt at b-carbons. The a-effect is generally
slight while a wide range of NMR chemical shifts are observed
for b-carbons [43]. By analogy with these observations, the assignments for the 13C chemical shifts of Li[B(Sal)(OH)2] were tentatively assigned in Table 3. The observed splitted pattern for
almost all carbon resonances is probably due to crystal symmetry
effects. Exact structural description of the complex was made possible by single crystal X-ray analysis.
Fig. 3 displays the 13C MAS NMR spectra of sodium glucuronatoborate and its parent compound sodium glucuronate. The interpretation of the 13C MAS NMR data (Table 4) was based on the
solution NMR data reported by Napier and Hadler [44]. Glucuronic
acid has both a- and b-anomeric forms and the population of various anomeric forms in aqueous solutions made individual assignments for carbon chemical shifts difficult. This was also the case for
the solid state NMR analysis, as the complex was flash precipitated
from aqueous solution. Nevertheless, significant upfield shifts were
observed on complexing with boron for almost all ring carbons (C1,
C2, C3, C4 and C5) and C6, indicating that not only those in favorable disposition to form complex, but also all the OH groups on the
sugar acid sense the oxygen–boron and hydrogen-bonding interactions in the solid state.
Table 3
Suggested
13
C MAS NMR chemical shifts of Li[B(Sal)(OH)2] (ppm).
Compound
C-1
C-2
C-3
C-4
C-5
C-6
C-7
Salicylic acid [41]
Li[B(Sal)(OH)2]
Dd
118
192
+74
162
184
+22
112
160
+48
138
154
+16
121
136
+15
133
141
+8
176
181
+5
Table 2
A summary of the FT-IR spectral data of salicylato- and glucuronatoborate complexes.
Compound
m(OH)
m(CO)
ma(COO), ms(COO)
m(C–O–) and ma(BO)/BO4
ms(BO)/BO4
Na-salicylate [38]
Borax [39]
Li[B(Sal)(OH)2]
Na-glucuronate
Na[B(Glu)(OH)2]2H2O
3100–2600
3300b
3576shp, 3505shp, 3300sh
3600–2500b
30615, 3266
1597s
_
1662vs
1643s
1732vs
1583vs, 1376vs
_
1619vs, 1281s, 1251s
1595, 1382
1657, 1386
1250w
1220s
1280–940,b (1142shp)
1238–901
1174–936
–
(834 + 815)d
748, 697
–
815, 783
d, doublet; s, strong; vs, very strong; m, medium; sh, shoulder; shp, sharp.
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D.A. Köse et al. / Inorganica Chimica Acta 363 (2010) 4031–4037
Table 5
Crystallographic data and structure refinement parameters for Li[B(Sal)(OH)2].
Emprical formula
Mr
T (K)
System
Space group
a (Å)
b (Å)
c (Å)
a (°)
b (°)
c (°)
V (A3)
Z
Dcalc (g cm3)
l (Mo Ka) (mm1)
F(0 0 0)
H (range) (°)
Reflections collected/independent/
observations
Rint
Number of variables
Goodness-of-fit (GOF) on F2
R1 (I > 2r(I))
wR2 (all data)=
Fig. 3. 13C MAS NMR spectra of sodium glucuronate (top) and Na[B(Glu)(OH)2]2H2O (bottom).
Table 4
Suggested
13
C MAS NMR chemical shifts of Na[B(Glu)(OH)2]2H2O.
Compound
C-1(C10 )
C-2 ? C5
C6(C60 )
Na-glucuronate
Na[B(Glu)(OH)2]2H2O
Dd
86
151(152)
+65
69–63
127–104
+(40–60)
87
163(159)
+76
3.3. Thermal stabilities of the complexes
Thermal stabilities of the complexes were examined by recording their TGA (Thermogravimetric Analysis) and DTA (Differential
Analysis) curves. Li[B(Sal)(OH)2] decomposed in two main steps
C14H12B2Li2O11
391.74
293
triclinic
P1
6.943(5)
7.647(5)
16.577(5)
85.734(5)
79.057(5)
80.245(4)
3536.7(2)
2
1.409
0.128
400
2.51–30.54
24095 / 5155 / 4163
0.0427
266
1.053
0.0514
0.1615
(Fig. 4a) with a complicated mechanism as the lithium ion is covalently bonded to the salicylatoborate structure. Though it was difficult to clearly define the steps, the first mass loss at 160 °C and
the following one around 270 °C are possibly due to elimination
of water from B–OH and CO from salicylate moieties, respectively.
The greatest mass loss between 375 and 425 °C refers to the degradation of the aromatic ring. The decomposition behavior of
Na[B(Glu)(OH)2]2H2O agreed with the previous studies reported
for analogous compounds [45]. At low temperatures (50–
175 °C), the crystal water molecules removed first, followed by
dehydroxylation from B–OH moieties up to ca. 225 °C (observed
mass loss at 225 °C: 18.2%, calculated for the removal of three
water molecules: 18.3%). Degradation of the organic ligand
proceeded after 225 °C. DTA curve displayed a multi-step pattern
(Fig. 4b). Both complexes left behind some greyish-black pyrolytic
carbon residue in the crucibles deposited on the expected metal
borate end product (M2OB2O3). The observed total mass losses
at 800 °C (56% and 62%) were therefore less than the expected
values (73% and 77%), respectively, for Li[B(Sal)(OH)2] and
Na[B(Glu)(OH)2]2H2O. The complexes displayed higher thermal
stabilities with regard to their parent acids for which the decomposition onset temperatures were recorded as 140 °C (glucuronic
acid) and 110 °C (salicylic acid).
Fig. 4. (a) TGA, (b) derivative TGA and (c) DTA curves of (A) Na[B(Glu)(OH)2].2H2O and (B) Li[B(Sal)(OH)2].
D.A. Köse et al. / Inorganica Chimica Acta 363 (2010) 4031–4037
Table 6
Selected bond lengths (Å) and angles (°) for Li[B(Sal)(OH)2].
O6–B2
1.473(18)
C9–O6–B2
121.69(11)
O10–B2
O7–B2
O4–B2
1.428(2)
1.441(19)
1.525(2)
O9–B1
O2–B1
O3–B1
O8–B1
1.325(19)
1.395(2)
1.339(19)
1.448(2)
O6–Li1
O8–Li1
O7–Li1
2.272(4)
1.917(3)
2.000(3)
O5–Li1
O11–
Li1
Li1–Li2
O10–
Li2
O9–Li2
O1–Li2
1.925(3)
2.289(4)
C9–O6–Li1
B2–O6–Li1
B2–O10–
Li2
B1–O9–Li2
C7–O2–B1
C7–O1–Li2
C14–O4–
B2
B1–O8–Li1
B2–O7–Li1
C14–O5–
Li1
C2–O3–B1
O10–B2–
O7
O7–B2–O6
O10–B2–
O4
O7–B2–O4
O6–B2–O4
3.474(5)
1.898(3)
1.994(3)
1.948(3)
127.92(12)
130.79(12)
88.23(11)
130.22(13)
O10–B2–
Li1
O7–B2–Li1
O6–B2–Li1
O4–B2–Li1
114.68(12)
122.64(12)
129.10(13)
125.17(11)
O8–B1–O9
O8–B1–O3
O9–B1–O3
O8–B1–O2
108.95(13)
112.55(13)
109.58(12)
105.77(12)
129.25(15)
100.48(14)
150.28(16)
O9–B1–O2
O3–B1–O2
O8–Li1–O7
108.84(12)
111.03(12)
108.57(15)
118.22(12)
115.43(12)
O5–Li1–O6
O7–Li1–B2
92.58(15)
32.07(7)
105.73(11)
109.72(12)
O1–Li2–O9
O9–Li2–
O11
O1–Li2–Li1
Li2–O11–
Li1
107.92(14)
117.82(15)
106.72(12)
110.28(11)
47.45(10)
58.29(10)
122.18(12)
148.82(14)
107.60(12)
Table 7
Hydrogen-bond geometry (Å, °).
D–HA
D–H
HA
DA
D–HA
O7–H7O9i
O8–H8O1ii
O9–H9O5
O9–H9O6ii
O10–H10O4i
0.90
0.88
0.89
0.89
0.93
1.90
2.22
2.49
2.31
1.93
2.738(2)
2.966(3)
3.262(3)
2.963(3)
2.824(3)
169
142
146
131
161
Symmetry codes: (i) 2 x, y, 1 z, (ii) 1 + x, y, z.
3.4. X-ray crystallography
With respect to inorganic borates, the crystallographic investigations on organic borate esters are limited. The difficulty lies in
the isolation of the complexes which undergo hydrolysis very
readily. The highest stability is achieved in the solid state. Those
salicylatoborate complexes that have been studied by X-ray crystallography so far are the bis-chelate complexes [18–22]. In each
4035
of these complex structures, two salicylic acid molecules are
coordinated to boron via adjacent carboxylate and hydroxo
groups forming BO4 tetrahedra with the planes of salicylate moieties nearly perpendicular to each other. In some cases, organic
solvent molecules are incorporated into the structure, particularly
with lithium counter ions [18,20]. Table 5 summarizes the
crystallographic data and structure refinement parameters for
Li[B(Sal)(OH)2]. The selected bond lengths and angles are given in
Table 6 and hydrogen bond geometry is given in Table 7.
The molecular conformation of Li[B(Sal)(OH)2] along with the
numbering scheme is shown in Fig. 5a. It is clearly seen that salicylate ligand binds to boron via its hydroxo and carboxylate oxygens forming a six-membered chelate ring with boron. The
tetrahedral coordination about each boron atom is satisfied with
two OH groups. The negatively charged tetrahedral boron atoms
are counterbalanced by Li+ ions. The asymmetric unit includes
two lithium ions and two salicylatoborate moieties. Li1 adopts a
distorted square pyramidal geometry to coordinate to carbonyl
oxygen, three hydroxyl oxygens and the fifth coordination is completed by sharing an oxygen corner (O11) with Li2 which in turn is
tetrahedrally linked to one carbonyl oxygen and two hydroxyl oxygens (Fig. 5b). Among the salicylatoborate complexes, to our
knowledge, this structural feature is unique as the solvate-free
lithium ions have variable coordination modes. Rings A (C1–C6)
and C (C8–C13) are, of course, planar and they are oriented at a
dihedral angle of 8.71(6)°. Rings B (B1/O2/O3/C1/C2/C7) and D
(B2/O4/O6/C8/C9/C14) adopt envelope conformations with atoms
B1 and B2 displaced by 0.407(2) and 0.261(2) Å from the planes
of the other rings atoms, respectively.
Examination of the unit cell packing diagram shows no disorder
suggesting that packing is dominated by the salicylato ligands connected by Li2–O–Li1 bridges forming 2D sheets parallel to the ac
plane. The sheets in turn connect to each other in the ab crystal
plane via OH groups bridging between the Li atoms of one layer
and boron atoms of the other layer forming 16-membered cavities
which constitute a tunnel structure on stacking parallel to the ac
plane (Fig. 6a and b). With its porous structure (Fig. 7), the compound resembles lithium containing zeolite-type frameworks, e.g.
lithium boron imidazolates [46].
In the crystal structure, intra- and intermolecular O–HO
hydrogen bonds (Table 7) link the molecules into a supramolecular
structure, in which they may be effective in the stabilization of the
structure. The pp contacts between rings A and C, Cg1–Cg2
[where Cg1 and Cg2 are centroids of the rings A and C] with
Fig. 5. (a) ORTEP diagram of Li[B(Sal)(OH)2], (b) representation of the title molecule within the framework.
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D.A. Köse et al. / Inorganica Chimica Acta 363 (2010) 4031–4037
Fig. 6. (a) Packing in the crystal structure of Li[B(Sal)(OH)2], (b) representation of the 16-membered ring.
Fig. 7. Representation of the open-framework structure of Li[B(Sal)(OH)2] and alignment of tetrahedral and square pyramidal Li+ ions through the a axis.
centroid–centroid distance of 3.841(3) Å may further stabilize the
structure. The short B–O distances (Table 6) are consistent with
the relatively higher covalent character of the B–O bonds. The
higher Lewis acidity of the boron atoms caused changes in the
NMR chemical shifts of almost all salicylate carbons. Although a
single type of boron polyhedra participates in the crystal structure,
the average B–O distances in the B1 and B2 tetrahedra are different. In the B2 tetrahedron, the B–O distances vary from 1.445 to
1.511 Å with an average value of 1.469 Å, agreeing with the values
reported for 4-coordinate boron [47,48]. On the other hand, the B–
O distances are smaller in the B1 tetrahedron ranging in 1.325–
1.448 Å. Analysing the cation–oxygen interactions reveals that B2
is associated with three oxygen–lithium interactions (O7–Li1,
O6–Li1 and O10–Li2) while B1 is coordinated to Li1 through O8
and to Li2 through O9. The weaker oxygen–lithium interactions
lead to stronger B–O interactions and thus shorter B–O bonds in
the B1 tetrahedron. Accordingly, the O–H bond distances associated with Li2 are longer and those with Li1 are shorter (Table 7).
These differences correlate with the existence of two distinct OH
peaks in the FTIR spectra.
4. Conclusions
Lithium salicylatoborate and sodium glucuronatoborate were
prepared as mono-chelate borate esters and isolated in salt form
where negatively charged tetrahedral borate anions were counterbalanced by the bio-active lithium and sodium cations. The
complexes were readily soluble in water but slowly underwent
hydrolytic dissociation to boric acid and organic ligand as indicated
by solution NMR studies. These high water solubility and slow
hydrolysis properties may allow them use in the cumulative treatment of metal ion, boron and biomolecule deficiencies or aid in the
metabolic processes where boron is claimed to be active [49]. The
interaction of boric acid with glucuronic acid has been investigated
for the first time. The structural information achieved might be
helpful in the appropriate design of novel boron based drugs.
Li[B(Sal)(OH)2], has been introduced as a new type of organic
borate ester. The arrangement adopted by Li differs from those observed for bis-chelate complexes. For the first time, a lithium salicylatoborate complex exhibits solvate-free lithium ions with
variable coordination modes. The crystal structure is unique such
that corner-sharing LiO4 and LiO5 polyhedra are interconnected
through salicylatoborate groups forming tunnels as in the zeolite-type metal–organic frameworks. In addition to the expected
nutritional and/or pharmacological applications, this lithiumbased open-framework material may well serve in gas-sorption,
separation and catalysis areas. The compound may be a potential
substitute for traditional lithium battery electrodes due to interesting conductivity properties that might arise from the alternating
fourfold and fivefold Li+ ions aligning through the a axis.
D.A. Köse et al. / Inorganica Chimica Acta 363 (2010) 4031–4037
Acknowledgements
The authors are indebted to the Department of Chemistry, Ataturk University, Erzurum, Turkey, for the use of X-ray diffractometer purchased under grant No. 2003/219 of the University Research
Fund. This work has been supported by Hacettepe University
Research Center (Project 06 D 02 1002).
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
Appendix A. Supplementary material
[26]
CCDC 766429 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via http://www.ccdc.cam.
ac.uk/data_request/cif.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2010.08.001.
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