Highly selective extraction of Cu(II) and Zn(II) using [B(3-iPrpz) 4]- (iPrpz=isopropylpyrazolyl)

Article abstract of DOI:10.1016/j.poly.2003.11.017

The solvent extraction of first-series transition metal ions {Mn(II), Fe(II), Co(II), Ni(II), Cu(II) and Zn(II)} with [B(3-iPrpz)₄] ^- (iPrpz=isopropylpyrazolyl) is reported. Although the previously reported tetrakis(pyrazolyl)borates, [B(pz)₄]^- and [B(3-Mepz)₄]^-, quantitatively extracted all the studied transition metal ions, [B(3-iPrpz)₄]^- extracted only Cu(II) and Zn(II). In order to elucidate the origin of the selectivity, the structures of the extracted species, [B(3-iPrpz)₄]₂Zn (1) and [B(3-iPrpz)₄]Cu(OCOCH₃) (2), were determined by single-crystal X-ray diffraction. Complex 1 has a tetrahedral geometry with each ligand having bidentate coordination. Complex 2 has a distorted square pyramidal geometry with [B(3-iPrpz)₄]^- having tridentate coordination and the acetate anion having asymmetric bidentate coordination. These structures of the extracted species are different from those of the previously studied tetrakis(pyrazolyl)borates, which are octahedral A ₂M with each ligand having tridentate coordination (A ^-=tetrakis(pyrazolyl)borate, M²⁺=first-series transition metal ion). Because the bulky isopropyl groups at the 3-position of the pyrazolyl ring prevent octahedral geometry, the other metal ions cannot form stable and extractable complexes with [B(3-iPrpz)₄]^-.

Full text of DOI:10.1016/j.poly.2003.11.017

Polyhedron 23 (2004) 283–289
www.elsevier.com/locate/poly
Highly selective extraction of Cu(II) and Zn(II)
using [B(3-iPrpz)4]ꢀ (iPrpz ¼ isopropylpyrazolyl)
a,
a
a
b
Tsuyoshi Kitano *, Yoshiki Sohrin , Yasuo Hata , Hiroshi Mukai ,
c
Hiroki Wada , Kazumasa Ueda
c
a
Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan
Department of Chemistry, Kyoto University of Education, Fushimi, Kyoto 612-8522, Japan
b
c
Faculty of Technology, Kanazawa University, 2-40-20 Kodatsuno, Kanazawa 920-8667, Japan
Received 21 May 2003; accepted 7 August 2003
Abstract
The solvent extraction of first-series transition metal ions {Mn(II), Fe(II), Co(II), Ni(II), Cu(II) and Zn(II)} with [B(3-iPrpz)4]ꢀ
(iPrpz ¼ isopropylpyrazolyl) is reported. Although the previously reported tetrakis(pyrazolyl)borates, [B(pz)4]ꢀ and [B(3-Mepz)4]ꢀ,
quantitatively extracted all the studied transition metal ions, [B(3-iPrpz)4]ꢀ extracted only Cu(II) and Zn(II). In order to elucidate
the origin of the selectivity, the structures of the extracted species, [B(3-iPrpz)4]2Zn (1) and [B(3-iPrpz)4]Cu(OCOCH3) (2), were
determined by single-crystal X-ray diffraction. Complex 1 has a tetrahedral geometry with each ligand having bidentate coordi-
nation. Complex 2 has a distorted square pyramidal geometry with [B(3-iPrpz)4]ꢀ having tridentate coordination and the acetate
anion having asymmetric bidentate coordination. These structures of the extracted species are different from those of the previously
studied tetrakis(pyrazolyl)borates, which are octahedral A2M with each ligand having tridentate coordination (Aꢀ ¼ tetrakis(pyr-
azolyl)borate, M2þ ¼ first-series transition metal ion). Because the bulky isopropyl groups at the 3-position of the pyrazolyl ring
prevent octahedral geometry, the other metal ions cannot form stable and extractable complexes with [B(3-iPrpz)4]ꢀ.
Ó 2003 Elsevier Ltd. All rights reserved.
Keywords: Tetrakis(3-isopropylpyrazolyl)borate; Solvent extraction; Single-crystal X-ray diffraction; Zinc; Copper
1. Introduction
ents were determined by X-ray crystallography. As far
as the authors know, as listed in Table 1, there are many
examples of the complexes introduced phenyl, cyclo-
alkyl groups, and so on [16–33].
Poly(pyrazolyl)borates (Aꢀ) are versatile ligands for
use as metal centers of transition metal catalysts [1–10]
and for mimicking metalloenzyme systems in bioinor-
ganic chemistry [11–14]. In particular, the poly(pyraz-
olyl)borate incorporated a bulky substituent at the
3-position of the pyrazolyl ring known as a ‘‘tetrahedral
enforcer’’ [15], because the bulky substituent prevents a
second poly(pyrazolyl)borate ligand from coordinating
the metal center and other ancillary ligands (Xꢀ) coor-
dinate with ease. On the other hand, a large number of
A2M complexes of transition metal ions with the
poly(pyrazolyl)borates having various bulky substitu-
We have been interested in poly(pyrazolyl)borates as
extraction reagents for first-series transition metal ions
{M2þ: Mn(II), Fe(II), Co(II), Ni(II), Cu(II) and Zn(II)}
[34–36]. The parent poly(pyrazolyl)borate, [HB(pz)3]ꢀ,
extracted all the studied metal ions from very acidic
solution at pH <2.1 into an organic phase. [HB(pz)3]ꢀ
forms quite stable A2M complexes because the unique
trigonal structure of [HB(pz)3]ꢀ is favorable for octa-
hedral coordination. [B(pz)4]ꢀ and [B(3-Mepz)4]ꢀ also
quantitatively extracted all the metal ions as octahedral
A2M complexes. However, [B(pz)4]ꢀ extracted large
metal ions, such as Mn(II) and Cd(II), at higher pH
than [HB(pz)3]ꢀ due to the intraligand contact between
the coordinated and non-coordinated pyrazolyl rings.
* Corresponding author. Tel.: +81-774-38-3098; fax: +81-774-38-
3099.
E-mail address: kitano@inter3.kuicr.kyoto-u.ac.jp (T. Kitano).
0277-5387/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2003.11.017

284
T. Kitano et al. / Polyhedron 23 (2004) 283–289
Table 1
Structurally determined A2M complexes with bulky substituents at
3-position of pyrazolyl ringa
using the solvent chloroform-d (dH ¼ 7:24) as an inter-
1
nal standard. The reported H coupling constants are
3JHH values. The metal ion concentration was deter-
mined with a HITACHI Z-8100 polarized Zeeman
atomic absorption spectrometer or a Japan Jarrell Ash
ICAP-500 inductively coupled argon plasma emission
spectrometer. The pH was measured using a Hitachi-
Horiba M-12 pH meter equipped with a glass electrode.
The aqueous and organic phases were shaken with a
TAITEC Bio-Shaker BR-30L.
Ligand (Aꢀ)
Metal (M2þ
)
Reference
[HB(3-CF3pz)3]ꢀ
Fe
[16]
[17]
[HB(3-Phpz)3]ꢀ
Mn
Fe
[HB(3-Phpz)3]ꢀ
[17]
[HB(3-Phpz)3]ꢀ
Co
Zn
Cd
Co
Co
Co
Zn
Co
Zn
Co
Cu
Fe
[18]b
[19]c
[20]
[HB(3-Phpz)3]ꢀ
[HB(3-Phpz)3]ꢀ
[HB(3-Ph-5-Mepz)3]ꢀ
[HB(3-cumyl-5-Mepz)3]ꢀ
[HB(3-(2-thienyl)pz)3]ꢀ
[HB(3-(3-pyridyl)-5-Mepz)3]ꢀ
[H2B(3-Phpz)2]ꢀ
[21]
[22]
[23]
2.2. Hydrolysis of the ligand
[24]
[25]c
[26]c
[27]c
[27]d
[28]
[H2B(3-Phpz)2]ꢀ
An aliquot of deuterium oxide (2.0 mL) at various pH
was shaken with an equal volume of chloroform-d
containing 5.0 ꢂ 10ꢀ3 M K[B(3-iPrpz)4] at 25 ꢁ 1 °C for
10–90 min. After the two phases were separated cen-
trifugally, the pH of the aqueous phase was measured.
1H NMR of the two phases was measured using the
JEOL LA-300.
[H2B(3-Ph-4-CNpz)2]ꢀ
[H2B(3-Ph-4-CNpz)2]ꢀ
[HB(3-cyclopropylpz)3]ꢀ
[B(3-cyclopropylpz)4]ꢀ
[HB(3-cyclobutylpz)3]ꢀ
[B(3-cyclobutylpz)4]ꢀ
[B(3-cyclopentylpz)4]ꢀ
[HB(3-cyclohexylpz)3]ꢀ
[HB(3-neopentylpz)3]ꢀ
[B(3-iPrpz)4]ꢀ
Fe
[28]
[29]
Co
Co
Co
Cu
Ni
[29]
[29]c
[30]d
[31]
2.3. Extraction of first-series transition metal ions
Co
Ni
[32]c
[33]c
this workc
[B(3-iPrpz)4]ꢀ
[B(3-iPrpz)4]ꢀ
An aliquot of chloroform (10 mL) containing
1.0 ꢂ 10ꢀ3 M K[B(3-iPrpz)4] was shaken with an equal
Zn
a Complexes of the rearrangement of pyrazolyl rings are omitted.
b Square pyramidal geometry.
c Tetrahedral geometry.
d Square planar geometry.
volume of an aqueous phase containing 1.0 ꢂ 10ꢀ5
M
of a metal ion and 1.0 ꢂ 10ꢀ2 M of acetic acid,
2-(N-morpholino)ethanesulfonic acid (MES), 3-(N-mor-
pholino)propanesulfonic acid (MOPS), N-tris(hydrox-
ymethyl)methyl-3-aminopropanesulfonic acid (TAPS),
2-(cyclohexylamino)ethanesulfonic acid (CHES) or 3-
(cyclohexylamino)propanesulfonic acid (CAPS) at 25 ꢁ
1 °C. After the two phases were separated centrifugally,
the pH of the aqueous phase was measured. The metal
concentration in the aqueous phase was determined
using the Z-8100 or the ICAP-500. The metal concen-
tration in the organic phase was measured after back-
extraction into 1.0 M nitric acid.
[B(3-Mepz)4]ꢀ extracted all the metal ions at higher pH
compared to [B(pz)4]ꢀ due to the interligand contact
caused by the 3-methyl groups. The selectivity of these
ligands for the metal ions generally followed the Irving–
Williams series in stability constants; the extraction pH
increased in the order of Cu(II), Ni(II), Co(II), Fe(II)
and Mn(II).
Here, we report on the solvent extraction of the first-
series transition metal ions with [B(3-iPrpz)4]ꢀ. [B(3-
iPrpz)4]ꢀ extracted only Cu(II) and Zn(II) of all the
studied metal ions. In order to examine the steric effects
of the isopropyl groups on the extraction, the structures
of the extracted species, [B(3-iPrpz)4]2Zn (1) and [B(3-
iPrpz)4]Cu(OCOCH3) (2), were examined by X-ray
crystallography.
2.4. Syntheses of the complexes
2.4.1. Preparation of [B(3-iPrpz)4]2Zn (1)
Ten milliliters of chloroform containing 2.0 ꢂ 10ꢀ3
M
K[B(3-iPrpz)4] was shaken for 1 h with an aqueous so-
lution (10 mL) at pH 8.0 containing 1.0 ꢂ 10ꢀ3
M
ZnSO4 and 1.0 ꢂ 10ꢀ2 M acetic acid. After centrifuga-
tion, the organic phase was evaporated to precipitate the
complex. Recrystallization from a mixture of chloro-
form and acetonitrile in a 2:1 volume ratio gave color-
less crystals suitable for X-ray diffraction. Anal. Calc.
for C48H72N16B2Zn: C, 60.40; H, 7.52; N, 22.90. Found:
C, 60.15; H, 7.64; N, 23.52%; m.p. 263–264 °C. 1H
NMR (CDC13, d): 0.74, 1.20 (24, 24, d, d, J ¼ 6:8, 7.0
Hz, Me(iPr)), 2.00, 2.95 (4, 4, m, m, CH(iPr)), 5.98, 6.09,
6.42, 7.13 (4, 4, 4, 4, d, d, d, d, J ¼ 2:2, 2.6, 2.2, 2.4 Hz,
4-H (pz), 5-H(pz)). Although Trofimenko et al. [32]
2. Experimental
2.1. General procedure
K[B(3-iPrpz)4] was synthesized as previously reported
[32] and recrystallized from hot ethanol–water (1:5). All
other chemicals were of reagent grade and distilled water
was used throughout. Proton NMR spectra were ob-
tained at 25 ꢁ 1 °C with a JEOL LA-300 spectrometer.
Chemical shifts (ppm) are reported downfield from TMS

T. Kitano et al. / Polyhedron 23 (2004) 283–289
285
prepared and characterized 1, its X-ray structure has not
been reported.
structures were solved by direct methods [38] and ex-
panded using a Fourier technique [39]. The non-hydro-
gen atoms of 1 were refined anisotropically. For 2,
isotropic temperature factors were used for the nitrogen
and carbon atoms of the non-coordinated pyrazolyl ring
2.4.2. Preparation of [B(3-iPrpz)4]Cu(OCOCH3) (2)
Ten milliliters of chloroform containing 1.0 ꢂ 10ꢀ3
M
K[B(3-iPrpz)4] was shaken for 1 h with an aqueous so-
lution (10 mL) at pH 3.0 containing 1.0 ꢂ 10ꢀ3
M
Table 3
Selected bond distances and angles of [B(3-iPrpz)4]2Zn (1) and [B(3-
iPrpz)4]Cu(OCOCH3) (2)
CuSO4 and 1.0 ꢂ 10ꢀ2 M acetic acid. After centrifuga-
tion, the organic phase was evaporated to precipitate the
complex. Recrystallization from methanol gave blue
crystals suitable for X-ray diffraction. Anal. Calc. for
C25H39N8BO2Cu: C, 54.82; H, 6.80; N, 19.67. Found: C,
54.73; H, 7.00; N, 19.71%; m.p. 145–146 °C.
ꢀ
Distances (A)
Compound
Angles (°)
1
Zn(1)–N(1) 1.956(2)
Zn(1)–N(3) 1.956(2)
N(1)–Zn(1)–N(1) 125.7(1)
N(1)–Zn(1)–N(3) 100.03(9)
N(1)–Zn(1)–N(3) 113.0(1)
N(1)–Zn(1)–N(3) 103.5(1)
O(1)–Cu(1)–O(2) 63.0(2)
O(1)–Cu(1)–N(1) 162.3(2)
O(1)–Cu(1)–N(3) 100.2(2)
O(1)–Cu(1)–N(5) 104.3(2)
O(2)–Cu(1)–N(1) 102.1(2)
O(2)–Cu(1)–N(3) 155.5(2)
O(2)–Cu(1)–N(5) 109.6(2)
N(1)–Cu(1)–N(3) 90.6(2)
N(1)–Cu(1)–N(5) 89.3(2)
N(3)–Cu(1)–N(5) 91.2(2)
2
Cu(1)–O(1) 2.107(4)
Cu(1)–O(2) 2.012(4)
Cu(1)–N(1) 1.996(4)
Cu(1)–N(3) l.995(4)
Cu(1)–N(5) 2.155(4)
2.5. X-ray crystal structure determination
Crystallographic data are summarized in Table 2.
Crystals of 1 and 2 were each mounted onto a fine glass
fiber with epoxy cement. The lattice parameters and
intensity data were measured on a Rigaku AFC7R four-
circle diffractometer with graphite monochromated Cu
ꢀ
Ka radiation (k ¼ 1:54178 A) at 20 ꢁ 1 °C. An x ꢀ 2h
scan technique to a maximum 2h value of 140° was used.
An empirical absorption correction based on azimuthal
scans of several reflections was applied [37]. The data
were corrected for Lorentz and polarization effects, and
a correction for secondary extinction was applied. The
Table 2
Crystal data and structure refinement
Compound
1
2
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
C
48H72Nl6B2Zn
C26H39N8O2BCu
570.00
960.20
293
monoclinic
293
orthorhombic
C2=c
Pbca
18.867(4)
ꢀ
a (A)
19.423(3)
16.257(3)
19.160(4)
90
Fig. 1. ORTEP view of [B(3-iPrpz)4]2Zn (1) (20% probability). Hy-
drogen atoms are omitted for clarity.
ꢀ
b (A)
20.611(5)
15.657(2)
ꢀ
c (A)
a (°)
b (°)
c (°)
90
90
109.89(1)
90
90
6088(1)
3
ꢀ
V (A )
5689(1)
4
1.121
Z
8
1.244
Dcalc (Mg mꢀ3
)
l (mmꢀ1
F ð000Þ
)
0.949
2048
1.296
2408
Crystal size (mm)
Color
0.70 ꢂ 0.40 ꢂ 0.20
colorless
0.80 ꢂ 0.40 ꢂ 0.30
blue
Ranges of h, k, l
Reflections collected
Independent reflections 5527 (0.022)
)21/23, 0/19, )21/0 0/22, 0/24, )18/0
5699
6173
6040 (0.022)
(Rint
)
Data/parameters
Goodness-of-fit on F
Final R1=wR2 indices
ðI > 3rðIÞÞ
4336/304
1.80
0.061/0.094
4180/336
2.54
0.082/0.128
Extinction coefficient
9.9057 ꢂ l0ꢀ7
2.0541 ꢂ 10ꢀ6
Fig. 2. ORTEP view of [B(3-iPrpz)4]Cu(OCOCH3) (2) (20% proba-
bility). Hydrogen atoms are omitted for clarity except for the hydrogen
atoms bonded to the carbon atom (C31).
Largest difference peak 0.69 and )0.35
0.73 and )0.37
ꢀ3
ꢀ
and hole (e A
)

286
T. Kitano et al. / Polyhedron 23 (2004) 283–289
because of disorder. Hydrogen atoms were fixed at the
positions generated by calculation, while those on the
non-coordinated pyrazolyl ring of 2 were not incorpo-
rated in the refinement. All calculations were performed
using the TEXSAN crystallographic software package
developed by Molecular Structure Corporation.
Selected bond distances and angles of 1 and 2 are
shown in Table 3. Figs. 1 and 2 show ORTEP drawings
of 1 and 2, respectively.
3. Results and discussion
3.1. Hydrolysis and distribution of the ligands
It was previously reported that poly(pyrazolyl)bo-
rates hydrolyzed into pyrazoles and boric acid [34–36].
The chloroform-d solution containing K[B(3-iPrpz)4]
was shaken with deuterium oxide to study the hydrolysis
1
of [(3-iPrpz)4]ꢀ. On the basis of H NMR, 5% of [B(3-
iPrpz)4]ꢀ in chloroform-d hydrolyzed into isopropylpy-
razole after 10 min of shaking with an aqueous phase at
pH 2.5–13. Twenty five percent of [B(3-iPrpz)4]ꢀ hy-
drolyzed after 90 min of shaking. Although no peaks of
[B(3-iPrpz)4]ꢀ and isopropylpyrazole were observed in
deuterium oxide after 10 min of shaking, one set of
peaks of isopropylpyrazole was observed after 90 min.
We also confirmed that the concentration of Kþ in the
aqueous phase does not significantly increase after
shaking. These data suggest that K[B(3-iPrpz)4] does not
distribute into the aqueous phase because of the high
hydrophobicity of [B(3-iPrpz)4]ꢀ, and some isopro-
pylpyrazole distributes into the aqueous phase after
hydrolysis of [B(3-iPrpz)4]ꢀ.
Fig. 3. Effect of pH on the percentage extraction of metal ions. The
extraction time was 10 min. Key: ꢀ, Mn(II); M, Fe(II); +, Co(II); ꢂ,
Ni(II); r, Cu(II);
, Zn(II). (a) K[B(pz) ]; (b) K[B(3-Mepz) ]; (c)
4
ꢄ
K[B(3-iPrpz)4]. (a,b) Aqueous phase: 1.0 ꢂ410ꢀ2 M buffer, 1.0 ꢂ 10ꢀ4
M M2þ, 1.0 ꢂ 10ꢀ2 M KA (10 mL). Organic phase: chloroform 10 mL;
3.2. Extraction of first-series transition metal ions
Data taken from [34,35], respectively. (c) Aqueous phase: 1.0 ꢂ 10ꢀ2
M
buffer, 1.0 ꢂ 10ꢀ5 M M2þ (10 mL). Organic phase: 1.0 ꢂ 10ꢀ3 M KA
Fig. 3 shows the relationship between the percent
extraction (%E) of metal ions and the pH of the aqueous
phase using [B(pz)4]ꢀ, [B(3-Mepz)4]ꢀ and [B(3-iPrpz)4]ꢀ
as a ligand. K[B(pz)4] and K[B(3-Mepz)4] were initially
dissolved in the aqueous phase, since they were not
soluble in chloroform. Ten minutes of shaking was
adopted for a comparison with [B(pz)4]ꢀ and [B(3-
Mepz)4]ꢀ, although they were not sufficient for equi-
librium for [B(3-iPrpz)4]ꢀ. The %E values were
independent of the metal ion concentrations of
3.0 ꢂ 10ꢀ5–3.0 ꢂ 10ꢀ3 M. [B(3-iPrpz)4]ꢀ quantitatively
extracted only Cu(II) and Zn(II) ions. It should be noted
that the extraction with [B(3-iPrpz)4]ꢀ was very different
from that with [B(pz)4]ꢀ and [B(3-Mepz)4]ꢀ.
(chloroform 10 mL).
where CM signifies the analytical molar concentration of
the metal ion (M2þ) and the subscript o denotes the
species in the organic phase. When an A2M complex is
formed and extracted, the overall extraction equilibrium
and extraction constant (Kex) of M2þ are described as
follows:
M
2þ þ 2KAo $ A2Mo þ 2Kþ
ð2Þ
ð3Þ
ð4Þ
2
2
o
Kex ¼ ½A2Mꢃ ½Kþꢃ =½M2þꢃ½KAꢃ
log D ¼ log Kex ꢀ 2 log½Kþꢃ þ 2 log½KAꢃ
o
o
The %E reached a maximum value after 50 and 90
min of shaking for Cu(II) and Zn(II), respectively. In
order to elucidate the extracted species, the distribution
ratio (D) under these conditions was measured using
To analyze the extraction data graphically, the hydro-
lysis of the ligand must be taken into consideration.
[KA]o is expressed as
½KAꢃ ¼ CKAa;
ð5Þ
D ¼ CM;o=CM
ð1Þ
o

T. Kitano et al. / Polyhedron 23 (2004) 283–289
287
where CKA and a mean the initial concentration of the
ligand in the organic phase and the remaining ratio of
the ligand after hydrolysis, respectively. The substitution
of Eq. (5) into Eq. (4) and a rearrangement result in
to form the complexes according to Eq. (2) or Eq. (7).
The pH was appropriately adjusted to produce
complexes. The complexes, [B(3-iPrpz)4]2Zn (1) and
[B(3-iPrpz)4] Cu(OCOCH3) (2), were obtained after
evaporation of the organic phases. They dissolved into
chloroform, dichloromethane and benzene, and were
stable in air.
log D ꢀ 2 log a þ 2 log½Kþꢃ ¼ 2 log CKA þ log Kex:
ð6Þ
The plot of log D ꢀ 2 log a þ 2 log½Kþꢃ versus log CKA
for Zn(II) gave a straight line with a slope close to 2
(Fig. 4). The log D of Zn(II) did not depend on the initial
concentration of Zn(II) [C(Zn2þ)]. These results suggest
the validity of Eq. (2) for the extraction of Zn(II).
However, the log D ꢀ 2 log a þ 2 log½Kþꢃ versus log CKA
plot gave a slope close to 1 for Cu(II) (Fig. 5). The log D
of Cu(II) did not depend on the initial concentration of
Cu(II) [C(Cu2þ)]. These results suggest that the extracted
species was ACuX, where Xꢀ denotes an ancillary
ligand. The extraction equation and constant are ex-
pressed as
3.4. Structure of the Zn(II) complex
The zinc atom sits on a center of inversion for 1 as
seen in Fig. 1. Both ligands are bidentate and the ge-
ometry about the zinc atom is a tetrahedron. The li-
gands coordinate to the zinc atom through the nitrogen
atom in the 2-position of two of the 3-isopropylpyraz-
olyl rings, with the two additional rings of the ligand not
coordinated to the metal atom. The metal-coordinated
rings have their isopropyl groups bisecting the other
chelate ring and the C–H bond oriented toward the
metal ion, whereas the isopropyl groups of the non-
coordinated groups are in an intermediate position. The
structure of 1 resembles that of [B(3-iPrpz)4]2Co [32] and
[B(3-iPrpz)4]2Ni [33]. The distances and bond angles
between the zinc ion and the nitrogen atoms are nearly
identical with those observed for [H2B(pz)2]2Zn [40],
which also has the Zn(II) ion coordinated by four pyr-
azolyl groups from two ligands. These bond distances
are also typical of Zn–N distances in poly(pyraz-
olyl)borate complexes.
Cu2þ þ KAo þ Xꢀ $ ACuXo þ Kþ
Kex ¼ ½ACuXꢃ ½Kþꢃ=½Cu2þꢃ½KAꢃ ½Xꢀꢃ
ð7Þ
ð8Þ
o
o
Since we used acetate as buffer in the pH range where
Cu(II) was extracted, acetate is an ancillary ligand
candidate.
3.3. Syntheses of the complexes
An aqueous phase containing Zn(II) or Cu(II) and
an organic phase containing K[B(3-iPrpz)4] were shaken
[B(pz)4]2Zn (3) and [B(3-Mepz)4]2Zn (4) are octahe-
dral with [B(pz)4]ꢀ and [B(3-Mepz)4]ꢀ coordinating in a
tridentate fashion by X-ray diffraction [34,35]. Only one
kind of pyrazolyl ring of 3 and 4 in chloroform-d was
1
observed with H NMR, because the exchange of the
coordinated and non-coordinated pyrazolyl rings in 3
and 4 was fast on the NMR time scale at ambient
temperature. However, the structure observed by 1H
NMR for 1 indicates that [B(3-iPrpz)4]ꢀ coordinates in a
bidentate fashion, probably with tetrahedral geometry
around the zinc atom. These facts suggest that the iso-
propyl group makes the exchange of pyrazolyl rings
slower than the hydrogen atom and the methyl group,
and that the solution structure of 1 is similar to that in
the solid state.
Fig. 4. Effect of log CKA and log C(Zn2þ) on log D for the extraction of
Zn2þ with [B(3-iPrpz)4]ꢀ. The extraction time was 90 min. (a) pH 4.80,
Zn2þ; 1.0 ꢂ 10ꢀ5 M, (b) CKA; 1.0 ꢂ 10ꢀ3 M, pH 4.85. Solid lines, of
which the slopes are 2 and 0, respectively, were obtained by a least-
squares fit.
3.5. Structure of the Cu(II) complex
As shown in Fig. 2 the [B(3-iPrpz)4]ꢀ ligand and
acetate anion are bonded to the copper atom in a
tridentate and an asymmetric bidentate mode, respec-
tively. The non-coordinated pyrazolyl ring stands out
in the opposite direction to the copper atom. The ge-
ometry about the copper atom is best described as
square pyramidal by the structural parameter s ¼ 0:11;
s ¼ ða ꢀ bÞ=60°, where a and b are the largest angles
(a > b) around a five-coordinate metal center, as
Fig. 5. Effect of log CKA and log C(Cu2þ) on log D for the extraction of
Cu2þ with [B(3-iPrpz)4]ꢀ. The extraction time was 50 min. (a) pH 3.46,
Cu2þ;1.0 ꢂ 10ꢀ5 M, (b)CKA;1.0 ꢂ 10ꢀ3 M, pH3.51. Solidlines, ofwhich
the slopes are 1 and 0, respectively, were obtained by a least-squares fit.

288
T. Kitano et al. / Polyhedron 23 (2004) 283–289
reported by Addison et al. [41]. The square plane con-
sists of two nitrogen atoms from the [B(3-iPrpz)4]ꢀ li-
gand and two oxygen atoms from the acetate. This
structure is similar to that of [HB(3,5-iPr2pz)3]
Cu(O2NO) [42] and [HB(3,5-iPr2pz)3]Fe(OCOCH3) [43].
(fax: +44-1223-336033; e-mail: deposit@ccdc.cam.ac.uk
or www: http://www.ccdc.cam.ac.uk).
Acknowledgements
The authors are grateful to Assistant Professors
Yasushi Kawai and Tomomi Fujii (Kyoto University)
for their helpful discussion on X-ray crystallography.
3.6. Steric effects of 3-isopropyl group
When two [B(3-iPrpz)4]ꢀ ligands coordinate a metal
ion in a trigonal tridentate fashion, the steric repulsion
around the metal ion increases by the interligand contact
between the isopropyl groups, and the stability of the
complexes becomes too low to be extracted into the
organic phase. Thus, in order to avoid this steric repul-
sion, two [B(3-iPrpz)4]ꢀ ligands coordinate the Zn(II)
ion in a bidentate fashion and one [B(3-iPrpz)4]ꢀ coor-
dinates the Cu(II) ion in a tridentate fashion with the
acetate anion having a bidentate configuration. Since the
stabilities of 1 and 2 are much lower than the octahedral
A2M complexes with [B(pz)4]ꢀ and [B(3-Mepz)4]ꢀ, the
extraction pH for 1 and 2 are high. Trofimenko et al.
prepared the A2M complexes of Fe(II), Co(II), Ni(II),
Cu(II) and Zn(II) with [B(3-iPrpz)4]ꢀ. They were tetra-
hedral in the solid state on the basis of infrared spec-
troscopy. It is likely that the stabilities of the A2M
complexes of Fe(II), Co(II) and Ni(II) are not suffi-
ciently high to be extracted.
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