Multinuclear NMR studies of tris(oxaalkyl) borates and their complexes with some metal cations

Article abstract of DOI:10.1016/S0022-2860(99)00113-1

Nuclear magnetic resonance (NMR) of ¹H, ¹³C, ¹¹B and ¹⁷O nuclei has been used for the study of complexation of CoCl₂, NiCl₂ and CuCl₂ salts with three tris(oxaalkyl) borates. All NMR techniques demonstrated the formation of complexes but the most informative were the ¹³C and ¹⁷O NMR measurements. The structures of the complexes formed are discussed in this article.

Full text of DOI:10.1016/S0022-2860(99)00113-1

MOLSTR 10926
Journal of Molecular Structure 513 (1999) 149–153
www.elsevier.nl/locate/molstruc
Multinuclear NMR studies of tris(oxaalkyl) borates and their
complexes with some metal cations
*
B. Gierczyk, G. Schroeder, B. Nowak-Wydra, G. Wojciechowski, B. Brzezinski
´
Faculty of Chemistry, Adam Mickiewicz University, ul. Grunwaldzka 6, 60 780 Poznan, Poland
Received 15 February 1999; accepted 8 March 1999
Abstract
1
Nuclear magnetic resonance (NMR) of H, 13C, 11B and 17O nuclei has been used for the study of complexation of CoCl2,
NiCl2 and CuCl2 salts with three tris(oxaalkyl) borates. All NMR techniques demonstrated the formation of complexes but the
most informative were the 13C and 17O NMR measurements. The structures of the complexes formed are discussed in this
article. ᭧ 1999 Elsevier Science B.V. All rights reserved.
1
Keywords: Tris(oxaalkyl)borates; H; 13C; 11B and 17O NMR; Complexation of metal cations
1. Introduction
2. Experimental
2.1. Syntheses of tris(oxaalkyl)borates
It is well known that the multinuclear magnetic
resonance is very useful for studying the macrocyclic
complexes with metal ions in solutions [1–9]. This
technique is particularly sensitive to the conforma-
tional changes of the ligands as well as to the place
of coordination of the metal cations in complexes.
Recently we have demonstrated that ligands including
poly(oxaethylene) groups form proton channels
[10,11], in which the protons show large cation polari-
sability resulting in the so-called Zundel’s polarisa-
bility [12,13].
Tris(oxaalkyl) borates (Scheme 1; B2, B3 and B4)
were synthesized in 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 Dean–Stark
apparatus. After removing the solvent, the oily
products were distilled under reduced pressure. Yields
and boiling points of tris(oxaalkyl) borates are shown
in Scheme 1.
In this paper the formation and the structure of the
complexes of tris(oxaalkyl) borates, a new class of
boron compounds, with metal cations is studied
using various nuclear magnetic resonance (NMR)
techniques.
2.2. Preparations of complexes
The corresponding metal cation 1:1 complexes with
the tris(oxaalkyl) borates were prepared by mixing
solutions of tris(oxaalkyl) borates in acetonitrile and
anhydrous solutions of metal salts in ethanol or acet-
onitrile, respectively. The solvents were evaporated
under reduced pressure at room temperature and the
complexes were dissolved in appropriate solvents.
* Corresponding author. Tel.: ϩ 48-61-869-9181; fax: ϩ 48-61-
865-8008.
E-mail address: bbrzez@main.amu.edu.pl (B. Brzezinski)
0022-2860/99/$ - see front matter ᭧ 1999 Elsevier Science B.V. All rights reserved.
PII: S0022-2860(99)00113-1

150
B. Gierczyk et al. / Journal of Molecular Structure 513 (1999) 149–153
Scheme 1.
The concentration of the solutions was 0.1 mol dmϪ3
All preparations and transfers of the solutions were
carried out in a carefully dried glovebox under
nitrogen atmosphere.
.
For 17O NMR measurements the following para-
meters were used. Complexes with the concentration
of 0.5 mol dmϪ3 in CD3CN. D2O was used as the
external standard (d ˆ 0.00 ppm) and 2-butanon as
the internal standard (d ˆ 558.00 ppm). The operating
frequency was: sfrq ˆ 40.680 MHz; pw ˆ 85Њ; sw ˆ
100 kHz; at ˆ 0.03 s; 100 000 scans, T ˆ 348.0 K.
The spectra were recorded with sample spinning and
without lock.
2.3. NMR measurements
The NMR spectra were recorded in CD3OD using a
1
Varian Gemini spectrometer. All H NMR measure-
All 11B NMR measurements were carried out in
ments were carried out at the operating frequency:
sfrq ˆ 300.075 MHz; pw ˆ 45Њ; sw ˆ 4500 Hz;
at ˆ 2.0 s; T ˆ 293.0 K and TMS as the internal
standard.
CD3OD at the operating frequency: sfrq
ˆ
96.276 MHz; pw ˆ 45Њ; sw ˆ 100 kHz; at ˆ 0.1 s;
T ˆ 293.0 K and BF3*Et2O in CDCl3 (1 mol dmϪ3) as
the external standard.
13C NMR spectra were recorded in CD3OD at the
operating frequency: sfrq ˆ 75.454 MHz; pw ˆ 60Њ;
sw ˆ 19 000 Hz; at ˆ 1.8 s (for complexes of Co2ϩ
,
3. Results and discussion
Cu2ϩ and Ni2ϩ at ˆ 0.03 s); T ˆ 293.0 K and TMS as
the internal standard.
The 1H NMR data of tris(oxaalkyl) borates are
Scheme 2.

B. Gierczyk et al. / Journal of Molecular Structure 513 (1999) 149–153
151
Table 1
1H NMR chemical shifts (ppm) of tris(oxaalkyl) borates. ((s)-singlet, (t)-triplet, (m)-multiplet)
Borate
C2H
C3H
C5H
C6H
C8H
C9H
C11H
B2
B3
B4
3.64(t)
3.64(m)
3.64(m)
3.45(t)
3.61(m)
3.62(m)
3.35(s)
3.52(m)
3.62(m)
3.52(m)
3.62(m)
3.35(s)
3.53(m)
3.51(m)
3.34(s)
given in Table 1. The signals of the C1H protons are
shifted slightly towards lower fields due to the
deshielding effect of the boron atom. The signals of
the CH3 protons are always observed at 3.35 ppm, i.e.
they are independent of the length of the oxaalkyl
chains.
After the addition of the corresponding metal salts
to tris(oxaalkyl) borates, all C–H signals broaden
strongly indicating strong interactions between the
three oxaalkyl ligands and the cations.
Table 3
11B NMR chemical shifts (ppm) of tris(oxaalkyl) borates and their
complexes with some metal cations
Borates
Chemical shifts of complexes (ppm)
Free
CoCl2
NiCl2
CuCl2
B2
B3
B4
19.274
19.274
19.274
39.371
41.654
41.527
27.262
27.642
27.528
24.599
24.853
24.346
The 13C NMR chemical shifts of tris(oxaalkyl)
borates and their 1:1 complexes with various cations
are given in Table 2. A comparison of the chemical
shifts of the corresponding 13C NMR signals of free
borates with those of their complexes indicate a strong
interaction between the oxaalkyl ligands and the
respective cations.
In the case of 1:1 complexes of CoCl2, NiCl2 or
CuCl2 with tris(oxaalkyl) borates, the corresponding
13C NMR signals are strongly shifted towards higher
fields, indicating the formation of stable complexes.
From our previous paper, it is known that in
complexes these cations are always bonded with one
chlorine atom [11]. It is interesting to note that parti-
cularly strong complexes are formed between B4 and
Co2ϩ cations.
The 11B NMR chemical shifts of the 1:1 complexes
of CoCl2, NiCl2 or CuCl2 with tris(oxaalkyl) borates
are shown in Table 3. A comparison of the respective
signals of free borates with those of their complexes
show that for complexes the 11B NMR signals are
strongly shifted downfield. The greatest shift is
observed for the complexes with Co2ϩ cations,
much smaller for Ni2ϩ and the smallest for Cu2ϩ
cations. This result is consistent with the 13C NMR
observations.
The 17O NMR chemical shifts of free tris(oxaalkyl)
borates and their complexes with CoCl2 are given in
Table 4. The 17O NMR spectra directly reflect the
coordination sites of the cation and the O-atoms in
Table 2
13C NMR chemical shifts (ppm) of tris(oxaalkyl) borates and their
complexes with metal cations
Compound Chemical shifts (ppm)
C2
C3
C5
C6
C8
C9
C11
Table 4
17O chemical shifts (ppm) of tris(oxaalkyl) borates and its
complexes with cobalt salts
B2
62.03 75.11 59.08
B2 ϩ CoCl2 69.65 84.24 64.70
B2 ϩ NiCl2 69.67 80.40 64.79
B2 ϩ CuCl2 67.0 82.30 62.10
Borates Metal cation Oxygen-17 chemical shifts (ppm)
O1
O4
O7
O10
B3
62.16 71.15 72.95 73.67 59.11
B3 ϩ CoCl2 70.75 80.92 83.98 86.25 62.73
B3 ϩ NiCl2 66.74 70.64 78.45 81.37 59.90
B3 ϩ CuCl2 68.30 78.30 81.00 82.70 66.70
B2
B3
B4
Free
CoCl2
Free
CoCl2
Free
CoCl2
35.4
Ϫ11.2
36.0
Ϫ8.9
37.2
39.8
Ϫ29.4
Ϫ59.5
Ϫ8.4
Ϫ8.9
Ϫ6.0
B4
62.15 71.31 71.33 71.53 72.87 73.65 59.09
Ϫ27.6
Ϫ8.9
B4 ϩ CoCl2 76.64 86.80 97.70 88.38 89.50 91.32 68.22
B4 ϩ NiCl2 67.35 71.20 78.97 80.07 80.64 81.19 60.85
B4 ϩ CuCl2 67.50 78.00 78.40 80.00 82.30 65.70 58.10
Ϫ6.0
Ϫ15.9
Ϫ26.4
Ϫ15.9
Ϫ15.9

152
B. Gierczyk et al. / Journal of Molecular Structure 513 (1999) 149–153
Scheme 3.
the ligands. With increasing differences in the
chemical shifts the respective interactions also
increase. In the case of B2 the strongly upfield shift
must result from the strong interaction of the cation
with O1 and O4 atoms.
O10 atoms are in the field of the chlorine atom, as
shown in Scheme 3.
Acknowledgements
In the 17O NMR spectrum of the 1:1 complex of
CoCl2–B3 only one signal for all O-atoms is
found. For the O7 oxygen atom this means a
downfield shift from Ϫ27.6 to Ϫ8.9 ppm. This result
can be explained by the structure of the complex as
shown in Scheme 3.
The authors are grateful for financial support by
The State Committee for Scientific Research (KBN)
Research Project No 3 TO9A 03413.
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signal of O1 atoms is shifted only slightly downfield.
This complex situation demonstrates that the cation is
strongly coordinated by O4 and O7 atoms, whereas
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