FTIR and multinuclear magnetic resonance studies of tris(oxaalkyl) borates and their complexes with Li+ and Na+ cations

Article abstract of DOI:10.1039/a905871k

FTIR and NMR of ¹H, ¹³C, ¹¹B, ⁷Li and ²³Na nuclei were used for the study of the complexation of Li⁺ and Na⁺ cations by three tris(oxaalkyl) borates. The NMR techniques proved the formation of complexes and the fluctuation of Li⁺ and Na⁺ cations in the respective channels formed by three oxaalkyl chains. In the FTIR spectra of Li⁺ complexes with three tris(oxaalkyl) borates, continuous absorption in the far-infrared region indicates the fast fluctuations of Li⁺ ion between O atoms of the channels. The dependence of the continua shape on the length of the channels, i.e., the number of minima in the multiminima potentials, demonstrates that the fluctuation of Li⁺ ion occurs along the channel. Owing to the larger mass and diameter of the Na⁺ cation in the Na⁺-tris(oxaalkyl) borate complexes, only systems with longer channels show a large Na⁺ polarizability due to the fast fluctuation of Na⁺ cations in a multiminima potential.

Full text of DOI:10.1039/a905871k

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FTIR and multinuclear magnetic resonance studies of tris(oxaalkyl)
borates and their complexes with Li‘ and Na‘ cations
B¡az
5 ej Gierczyk,a Grzegorz Schroeder,a Grzegorz Wojciechowski,a Bartosz R o z5 alski,a
Bogumil Brzezinski*a and Georg Zundelb
a Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, PL -60 780 Poznan, Poland.
E-mail: bbrzez=main.amu.edu.pl
b Institute of Physical Chemistry, University of Munich, T heresienstr. 41, D-80333 Munich,
Germany
Received 20th July 1999, Accepted 27th August 1999
FTIR and NMR of 1H, 13C, 11B, 7Li and 23Na nuclei were used for the study of the complexation of Li` and
Na` cations by three tris(oxaalkyl) borates. The NMR techniques proved the formation of complexes and the
Ñuctuation of Li` and Na` cations in the respective channels formed by three oxaalkyl chains. In the FTIR
spectra of Li` complexes with three tris(oxaalkyl) borates, continuous absorption in the far-infrared region
indicates the fast Ñuctuations of Li` ion between O atoms of the channels. The dependence of the continua
shape on the length of the channels, i.e., the number of minima in the multiminima potentials, demonstrates
that the Ñuctuation of Li` ion occurs along the channel. Owing to the larger mass and diameter of the Na`
cation in the Na`Ètris(oxaalkyl) borate complexes, only systems with longer channels show a large Na`
polarizability due to the fast Ñuctuation of Na` cations in a multiminima potential.
acid, used for the synthesis of tris(oxaalkyl) borates, were pur-
chased from Aldrich and were used without puriÐcation.
1
Introduction
FTIR spectroscopy is a very useful method for the study of
H`, Li` and Na` complexation and motions of these cations
in the corresponding bonds.1h5 In several previous pub-
lications, multinuclear magnetic resonance was shown to be
convenient for studying complexes of macrocyclic compounds
with metal ions in solutions.6h13 The NMR techniques are
particularly sensitive to the changes in the conformation of
the ligands and the coordination sites occurring with the for-
mation of complexes with protons or metal cations.
Recently, we demonstrated that ligands including
poly(oxaethylene) groups and compounds of natural origin
such as gramicidins form channels with either protons or
metal cations, in which a large cation polarizability due to fast
Ñuctuations of the cations is observed.14,15
2.1. Syntheses of tris(oxaalkyl) borates
Tris(oxaalkyl) borates [B2, (bp 179È180 ¡C/20 mmHg, yield
6
2
5%) B3 (bp 202È203 ¡C/3 mmHg, yield 82%) and B4 (bp
56È262 ¡C/0.5 mmHg) were synthesised by the reaction of
boric(III) acid (1 mol) with the respective anhydrous alcohol
3.2 mol) (Scheme 2). The benzene solutions of the mixtures
(
were heated for about 20 h in a DeanÈStark apparatus. After
removing the solvent, the oily products were distilled under
reduced pressure.
2.2. Preparation of complexes
Mixtures (1 : 1) of tris(oxaalkyl) borates with NaBPh and
Tris(oxaalkyl) borates (Scheme 1) are a new class of boron
compounds with many acceptor oxygen atoms in oxaalkyl
chains which are able to form stable complexes with cations in
solutions. In this work, the formation and the structure of
these complexes was studied by spectroscopic methods.
4
LiClO salts for NMR measurements were prepared by dis-
4
solving equimolar amounts of the respective tris(oxaalkyl)
borate and the anhydrous salts in CD OD. The concentration
3
of the complexes was 0.1 mol dm~3.
2. Experimental
2-Methoxyethanol (97%), di(ethylene glycol) methyl ether
(
99%), triethylene glycol monomethyl ether (95%) and boric
Scheme 1
Scheme 2
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The corresponding Li` and Na` 1 : 1 complexes of
tris(oxaalkyl) borates for FTIR measurements were prepared
by mixing acetonitrile solutions of tris(oxaalkyl) borates with
LiClO or NaClO , respectively. The solvent was evaporated
4
4
under reduced pressure at room temperature and the com-
plexes were dissolved in CHCl . The concentration of the
3
solutions was 0.1 mol dm~3.
2.3. FTIR measurements
A cell with Si windows and a wedge-shaped layer was used to
avoid interferences (mean layer thickness 0.176 mm). IR
spectra were taken with an IFS 113v FTIR spectrophotometer
from Bruker, using a helium-cooled bolometer in the far-
infrared region (125 scans, resolution 1 cm~1). The concentra-
tion of the samples was 0.1 mol dm~3 in chloroform. All
preparations and transfers of solutions were carried out in a
carefully dried glove-box under a nitrogen atmosphere.
2.4. NMR measurements
NMR spectra were recorded in CD OD and CDCl using a
3
3
Varian Gemini 300 MHz spectrometer. All spectra were
locked to the deuterium resonance of CD OD or CDCl ,
3
3
respectively. The error in the ppm values was 0.01.
All 1H NMR measurements were carried out at an oper-
ating frequency of 300.075 MHz, Ñip angle pw \ 45¡, spectral
width sw \ 4500 Hz, acquisition time at \ 2.0 s, relaxation
delay d \ 1.0 s, T \ 293.0 K and TMS as the internal stan-
1
dard. No window function or zero Ðlling was used. The digital
resolution was 0.2 Hz per point.
Fig. 1 FTIR spectra of (É É É) tris(oxaalkyl) borates and (ÈÈ) their
1
: 1 LiClO complexes. For comparison, the spectrum of pure LiClO
1
3C NMR spectra were recorded at an operating frequency
4
4
solution (È È È) is given. (a) B2; (b)B3; (c) B4.
of 75.454 MHz, pw \ 60¡, sw \ 19 000 Hz, at \ 1.8 s, d \ 1.0
1
s, T \ 293.0 K and TMS as the internal standard. Line
broadening parameters were 0.5 or 1 Hz.
i.e., on the number of oxygen atoms in the three oxaalkyl
chains. This result demonstrates that the Li` cation under-
goes fast Ñuctuations in the multiminimum potential prefer-
entially along the oxygen atoms and not in the circular
arrangement of the oxygen atoms in the channel as shown in
Scheme 3.
For 7Li NMR, the following parameters were used: LiCl in
D O (1 mol dm~3) as the external standard, operating
2
frequency \ 116.621 MHz, pw \ 25¡, sw \ 20 000 Hz, at \
1
0
.0 s, d \ 0.5 s, T \ 293.0 K. The digital resolution was
1
.6 Hz per point. No window function or zero Ðlling was used.
All 11B NMR measurements were carried out an operating
The spectra of the 1 : 1 complexes of tris(oxaalkyl) borates
frequency of 96.276 MHz, pw \ 45¡, sw \ 100 kHz, at \ 0.1 s,
d \ 1.0 s, T \ 293.0 K and BF Æ Et O in CDCl (1 mol
with NaClO are given in Fig. 2(a)È(c) (solid lines). For com-
4
parison, the spectrum of an NaClO solution in acetonitrile
1
3
2
3
4
dm~3) as the external standard. The digital resolution was 6
and the spectra of the tris(oxaalkyl) borates are also shown.
Hz per point. No window function or zero Ðlling was used.
The Na` ion motion band in the spectrum of NaClO solu-
4
2
3Na NMR spectra were taken using the following param-
tion is observed at about 195 cm~1. In the spectrum of the
eters: operating frequency \ 79.373 kHz, sw \ 20 000 Hz,
1 : 1 complex of B2 with NaClO only one broad band with a
4
pw \ 70¡, at \ 1.0 s, d \ 1.0 s, T \ 293.0 K and a 1 mol
maximum slightly shifted towards smaller wavenumbers is
1
dm~3 solution of NaCl in D O as the external standard. The
found. This band is strongly broadened and shows long wings
towards higher and lower wavenumbers, which may be attrib-
uted to skeletal vibrations. The broad band indicates that the
Na` cations are relatively well localised between six oxygen
atoms and therefore the Na` polarizability is relatively low.
In the spectra of the 1 : 1 Na` complexes with B3 and B4
[Fig. 2(b) and (c)] a continuous absorption in the region
2
digital resolution was 0.7 Hz per point. No window function
or zero Ðlling was used.
3. Results and discussion
3.1. FTIR measurements
240È50 cm~1 is observed. This continuous absorption indi-
The far-FTIR spectra of the 1 : 1 complexes of tris(oxaalkyl)
borates with lithium perchlorate in chloroform are shown in
Fig. 1(a)È(c) (solid lines). For comparison, the spectrum of an
cates fast Ñuctuations of Na` cation in the multiminima
potential along the oxygen atoms in the channel as shown in
Scheme 3. Owing to these fast Ñuctuations of Na` cation
within the structure, these complexes show Na` polarizability.
The small di†erences in the spectra of Na` complexes with B3
and B4 and the di†erent spectroscopic behaviour of the Na`
complexes and the respective Li` complexes can be explained
by the di†erent molecular masses (Li` 7, Na` 23) and di†er-
ent diameters (Li` 1.36 Ó, Na` 1.94 Ó) of the two cations.16
LiClO4
solution in acetonitrile and those of the respective
borates in chloroform are also given. The Li` ion motion
band in the spectrum of LiClO is observed at 405 cm~1. In
4
the spectra of the 1 : 1 complexes of tris(oxaalkyl) borates with
lithium perchlorate this band vanishes, indicating complete
complexation of Li` by the ligand. Instead of the Li` ion
motion band, an intense continuous absorption is observed in
the far-infrared region, as expected for Li` cation undergoing
fast Ñuctuations in the channels formed by the three oxaalkyl
chains. The structure of the continuous absorption and the
wavenumber region in which the continuous absorption arises
depend strongly on the structure of the tris(oxaalkyl) borate,
3.2. NMR measurements
The 1H NMR data for tris(oxaalkyl) borates in CD OD and
3
CDCl are given in Table 1. The assignment of the proton and
3
carbon spectra of tris(oxaalkyl) borates and their complexes
4898
Phys. Chem. Chem. Phys., 1999, 1, 4897È4901

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Fig. 2 FTIR spectra of (É É É) tris(oxaalkyl) borates and (ÈÈ) their
1
: 1 NaClO complexes. For comparison, the spectrum of pure
4
Scheme 3
NaClO solution (È È È) is given. (a) B2; (b)B3; (c) B4.
4
were made with the aid of 2D experiments: COSY-45, LR
COSY (delay s \ 0.3 s was used), HETCOR and COLOC for
J
\ 3 Hz.
The 13C NMR chemical shifts of tris(oxaalkyl) borates and
CH
The signals of the C2H protons in both solutions are shifted
their 1 ] 1 mixtures with various cations in CDCl and
3
CD OD are given in Table 2. A comparison of the chemical
3
slightly towards higher frequency owing to the deshielding
e†ect of the boron atom. A comparison of 1H NMR chemical
shifts in both solutions indicates that in particular the C2ÈH
shifts of the corresponding 13C NMR signals of free borates in
the two solutions with those of their complexes indicates a
strong interaction between the oxaalkyl ligands and the
cations. In the case of complexes with Li` or Na` cations, all
signals are slightly shifted towards lower frequency, pointing
to strong interactions of Li` and Na` cations with all oxygen
atoms of the tris(oxaalkyl) borates. Such interactions can be
only realised thanks to the fast Ñuctuations of Li` or Na`
cations in a channel formed by the three oxaalkyl chains. The
7Li and 23Na NMR and FTIR studies conÐrm the suggestions
about the occurrence of fast Ñuctuations of the Li` and Na`
cations in the channels.
signals in CDCl solution are shifted towards higher ppm
3
values, additionally probably because of the interaction of the
boron atom with chlorine atoms from chloroform. This obser-
vation is also reÑected by the C2ÈH chemical shifts in the 13C
NMR spectra. The position of the signals of other protons is
insigniÐcantly di†erent in the two solutions. The proton
signals of oxaalkyl chains in CD OD are observed in a short
3
3
region at 3.64È3.35 ppm. For CD Cl solution these signals are
observed in the region 3.30È3.39 ppm and are slightly depen-
dent on the length of the oxaalkyl chains.
The chemical shift of the signals of the CH protons in
The 23Na NMR chemical shifts of the signals in the spectra
of 1 ] 1 and 2 ] 1 mixtures of tris(oxaalkyl) borates with
sodium tetraphenyl borate are given in Table 3. The 23Na
NMR signal of the NaBPh in CD OD solution is observed
3
CD OD is always observed at 3.35 ppm, i.e., they are indepen-
3
dent of the length of the oxaalkyl chains. For CDCl solutions
3
these signals are observed in the region 3.30È3.39 ppm and are
4
3
slightly dependent on the length of the oxaalkyl chains. After
addition of the corresponding salts to tris(oxaalkyl) borates,
at [2.99 ppm and in CDCl at 1.87 ppm. In the spectra of the
3
1 : 1 complexes the chemical shifts of the corresponding 23Na
all proton signals of the CH groups become strongly
signals become more and more negative depending on the
length of the oxaalkyl chains. Furthermore, these signals
2
broadened (6È15 Hz), indicating strong interactions between
the oxaalkyl ligands and the cations.
become at least twice as broad as that of NaBPh . It is worth
4
Table 1 1H NMR chemical shifts (ppm) of tris(oxaalkyl) borates in CD OD and CDCL
3
3
Borate
B2
Solvent
C2H
C3H
C5H
C6H
C8H
C9H
C11H
CD OD
3.64(t)
3.86(t)
3.64(m)
3.95(t)
3.64(m)
3.93(t)
3.45(t)
3.44(t)
3.61(m)
3.63(m)
3.62(m)
3.65(m)
3.35(s)
3.30(s)
3.52(m)
3.55(m)
3.62(m)
3.65(m)
3
CDCl
3
B3
CD OD
3.52(m)
3.55(m)
3.62(m)
3.65(m)
3.35(s)
3.39(s)
3.53(m)
3.58(m)
3
CDCl
3
B4
CD OD
3.51(m)
3.56(m)
3.34(s)
3.38(s)
3
CDCl
3
s \ Singlet, t \ triplet, m \ multiplet.
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Table 2 13C NMR chemical shifts (ppm) of tris (oxaalkyl) borates and their complexes with Li` and Na` cations in CD OD and CDCl
3
3
Chemical shift (ppm)
Borate
Cation
Solvent
C2
C3
C5
C6
C8
C9
C11
B2
È
NaClO
LiCiO
È
NaClO
LiClO
È
NaClO
LiClO
CD OD
62.03
61.70
60.96
62.16
61.83
60.41
62.15
61.67
62.02
75.11
74.80
74.10
71.15
70.69
70.02
71.31
70.66
70.13
59.08
58.90
58.83
72.95
72.57
72.12
71.33
70.71
69.94
3
CD OD
4
3
CD OD
4
3
B3
B4
CD OD
73.67
73.18
72.83
71.53
70.83
69.99
59.11
59.06
59.02
72.87
72.37
72.18
3
CD OD
4
3
CD OD
4
3
CD OD
73.65
73.07
72.58
59.09
59.09
59.01
3
CD OD
4
3
CD OD
3
4
B2
B3
B4
È
NaClO
LiClO
È
NaClO
LiClO
È
NaClO
LiClO4
CDCl
CDCl
CDCl
CDCl
63.42
62.63
62.13
62.58
62.04
61.54
62.55
62.02
61.95
73.54
73.02
72.91
70.28
70.01
69.82
70.37
69.87
69.80
58.85
58.83
58.81
71.53
71.19
71.05
70.41
69.96
69.13
3
4
3
4
3
71.87
71.42
71.20
70.53
70.08
69.48
58.89
58.84
58.75
71.47
71.46
71.46
3
CDCl
4
3
CDCl
4
3
CDCl
3
71.81
71.54
71.39
58.86
58.83
58.82
CDCl
3
4
CDCl
3
emphasising that the data in Table 3 for 2 ] 1 mixtures indi-
cate that a doubled concentration of the ligand causes no sig-
niÐcant changes in the chemical shift or *l . These results
signals in the spectra of LiCl and LiClO solution are
observed at 0.10 and 2.12 ppm, respectively. In the spectra of
4
the 1 ] 1 mixtures of tris(oxaalkyl) borates and Li` salts in
both solvents, the corresponding signals are shifted towards
higher Ðeld and become broadened, indicating the complex
formation of Li` cations in the channels formed by the three
oxaalkyl chains. The broadening of the signal indicates also
fast Ñuctuations in the channels because in the 2 ] 1 mixtures
1
@2
indicate unambiguously that the complexation of Na` in the
corresponding 1 : 1 complexes is quantitative on a spectro-
scopic scale. This means that in the studied CD OD solution
3
an equilibrium between complexed and free cations can be
neglected. This conclusion is consistent with the FTIR studies
discussed above.
no changes in the chemical shifts and *l
are observed
1
@2
The 7Li NMR data for the 1 ] 1 and 2 ] 1 mixtures of
(Table 4). In this case, as for Na` complexes, an equilibrium
between complexed and free cations can be neglected.
The 11B NMR data for tris(oxaalkyl) borates and their
tris(oxaalkyl) borates with LiCl salt in CD OD and LiClO in
3
4
CDCl are given in Table 4. For comparison, the 7Li NMR
3
signal of LiCl in CD OD solution and LiClO in 1 ] 4
acetonitrileÈchloroform are also given. Sharp 7Li NMR
complexes with LiCl and NaBPh are summarised in Table 5.
3
4
4
The 11B NMR signals in the spectra of all borates are found
Table 3 23Na NMR chemical shifts (ppm) of 1 : 1 and 2 : 1 complexes of tris(oxaalkyl) borates with sodium tetraphenylborate and of sodium
tetraphenylborate in CD OD and CDCl
3
3
Chemical shift (ppm)
*l
@2/Hz
1
Compound
Complex
CD OD
CDCl3
CD OD
CDCl3
3
3
NaBPh4
È
1 : 1
[2.99
[3.81
[3.82
[4.22
[4.22
[5.77
[5.78
1.87
22
45
45
48
48
56
56
3.1
7.2
7.2
11.8
11.8
15.1
15.1
B2]NaBPh
[2.71
[2.71
[4.07
[4.08
[6.27
[6.27
4
4
4
2
: 1
1 : 1
: 1
1 : 1
: 1
B3]NaBPh
B4]NaBPh
2
2
Table 4 7Li NMR chemical shifts (ppm) of 1 : 1 and 2 : 1 complexes of tris(oxaalkyl) borates with LiCl and of LiCl in CD OD and CDCl
3 or
3
LiClO4
Chemical shift
(ppm)
Compound
Complex
Solvent
*l
@2/Hz
1
LiCl
B2]LiCl
È
1 : 1
CD OD
0.10
[0.03
[0.03
[0.05
[0.06
[0.01
[0.02
2.12
[0.99
[1.00
[1.03
[1.03
[0.99
[0.98
0.80
3.55
3.56
5.84
5.84
6.80
6.81
0.80
2.1
2.1
2.3
2.3
2.4
3
CD OD
3
2
: 1
1 : 1
: 1
1 : 1
: 1
CD OD
3
B3]LiCl
B4]LiCl
CD OD
3
2
CD OD
3
CD OD
3
2
CD OD
3
LiClO4
È
1 : 1
CD CNÈCDCl
3
3
B2]LiClO
CDCl
4
4
4
3
2
: 1
1 : 1
: 1
1 : 1
: 1
CDCl
3
B3]LiClO
B4]LiClO
CDCl
3
CDCl
3
2
CDCl
3
2
CDCl
2.4
3
4900
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Table 5 11B NMR chemical shifts (d, ppm) of tris(oxaalkyl) borates and their complexes with Li` and Na` cations and of the BPh ~ anion in
4
CD OD
3
Compound
Free d
*l
@2/Hz
LiCl d
*l1@2/Hz
NaBPh4 d
*l1@2/Hz
BPh ~ d
1
4
NaBPh4
B2
[6.28
[6.03
[5.90
[5.90
19.27
19.27
19.27
34
34
31
19.15
19.15
19.15
50
67
77
19.02
19.02
19.02
60
69
74
B3
B4
at 19.27 ppm. In the spectra of the 1 ] 1 mixtures of borates
4
5
6
7
B. Brzezinski and G. Zundel, J. Chem. Soc., Faraday T rans. 1,
1
985, 81, 2375.
with LiCl, the signals are slightly shifted towards 19.15 ppm
B. Brzezinski, G. Zundel and J. Olejnik, J. Mol. Struct., 1993, 300,
573È592.
G. A. Melson, Coordination Chemistry of Macrocyclic Com-
pounds, Plenum Press, New York, 1979.
M. Shamsipur and A. I. Popov, J. Am. Chem. Soc., 1979, 101,
and in the case of the 1 ] 1 mixtures of borates with NaBPh
4
towards 19.02 ppm. In both cases the chemical shifts are inde-
pendent of the nature of the ligands. These results prove that
the Li` and Na` cations Ñuctuating between the oxygen
atoms in the channels have an insigniÐcant e†ect on the boron
atoms.
4
051.
8
9
0
H. Saito and R. Tabeta, Bull. Chem. Soc. Jpn., 1987, 60, 61.
J.-S. Shmih and A. I. Popov, Inorg. Chem. 1980, 19, 1689.
Y. Li, G. Gokel, J. Hernandez and L. Echegoyen, J. Am. Chem.
Soc., 1994, 116, 3087.
1
Acknowledgements
The authors are grateful for Ðnancial support from the Polish
Committee for ScientiÐc Research (KBN), Research Projects
1
1
1
2
R. T. Streeper and S. Khazaeli, Polyhedron, 1991, 10, 221.
J. A. Semlyen, L arge Ring Molecules, John Wiley and Sons, Chi-
chester, 1996.
3
TO9A 01817 and 3 TO9A 03413. B. Gierczyk thanks the
13 G. Schroeder, B. Gierczyk and B. ¢Íska, J. Inclusion Phenom., in
Foundation of Polish Science for a fellowship.
the press.
1
4
5
B. Brzezinski, B. Ro
J. Chem. Soc., Faraday T rans., 1998, 94, 2093.
F. Bartl, B. Brzezinski, B. Ro alski and G. Zundel, J. Phys. Chem.
 z5 alski, G. Schroeder, F. Bartl and G. Zundel,
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