Synthesis of ditopic ligands for the selective extraction of copper(II) nitrate and crystal structures of their Cu(II) complexes

Article abstract of DOI:10.1080/00958972.2016.1269894

We have synthesized two ditopic ligands for selective extraction of copper(II) nitrate. We also synthesized one cation-only binding analog for comparison. All three ligands were characterized by conventional techniques. Competitive two-phase metal ion sol

Full text of DOI:10.1080/00958972.2016.1269894

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Synthesis of ditopic ligands for the selective
extraction of copper(II) nitrate and crystal
structures of their Cu(II) complexes
Hanna-Mari Smuts & Robert C. Luckay
To cite this article: Hanna-Mari Smuts & Robert C. Luckay (2016): Synthesis of ditopic ligands
for the selective extraction of copper(II) nitrate and crystal structures of their Cu(II) complexes,
Journal of Coordination Chemistry
To link to this article: http://dx.doi.org/10.1080/00958972.2016.1269894
Accepted author version posted online: 08
Dec 2016.
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Date: 11 December 2016, At: 21:24

Publisher: Taylor & Francis
Journal: Journal of Coordination Chemistry
DOI: http://dx.doi.org/10.1080/00958972.2016.1269894
Synthesis of ditopic ligands for the selective extraction of copper(II) nitrate and
crystal structures of their Cu(II) complexes
HANNA-MARI SMUTS and ROBERT C. LUCKAY*
Department of Chemistry & Polymer Science, Stellenbosch University, Private Bag X1, Matieland,
Stellenbosch, 7602, South Africa
We have synthesized two ditopic ligands for selective extraction of copper(II) nitrate. We also
synthesized one cation-only binding analogue for comparison. All three ligands were characterized
by conventional techniques. Competitive two-phase metal ion solvent extraction experiments were
performed at 25 °C over a period of 24 h. These ligands showed significant selectivity for Cu(II)
ions, having the ditopic ligands extract 81% and 73% of the Cu(II) ions in a solution of different
metal ions {Ni(II), Co(II), Cu(II), Zn(II), Cd(II), Pb(II)} at pH 5.09. Competitive transport
experiments (water/chloroform/water) were undertaken employing each ligand separately as the
ionophore in the membrane (chloroform) phase. No metal ion transport was observed, but a large
concentration of Cu(II) was present in the membrane phase. Competitive anion extraction and
transport were carried out with the ditopic ligands, yielding selective extraction and transport of
2
nitrate. Furthermore, a pH isotherm of the best ditopic ligand (H
2
L ) with Cu(II) was determined
from pH 1.0 to 6.0, producing a pH value of approximately 2.6. Finally, crystal structures of the
½
ditopic ligands complexed with Cu(II) were determined and refined. The coordination geometry
around the metal centres are distorted square planar and the Cu(II)-donor bond lengths fall within
the normal range.
Keywords: Copper(II); Ditopic ligands; Extraction; Transport; Bulk-liquid membrane; Copper(II)-
ligand complexes
*Corresponding author. Email: rcluckay@sun.ac.za
1

1
. Introduction
Schiff-base ligands of the N
2
2
O type have many applications, including catalysis [1-3], medical
applications [3-5], corrosion inhibition [6] and electrochemistry [7, 8]. We use them for the
separation of metal ions. In industry, metal recovery and purification from their ores are mainly
performed by pyrometallurgical and hydrometallurgical processes. Pyrometallurgical processes
have environmental implications due to the emission of pollutants and toxic gases. Due to
increasingly strict environmental regulations this is a significant problem and there is an
increasing need for environmentally friendlier methods. This method is also not cost effective
when applied to low-grade ores [9].
Hydrometallurgical processes differ from pyrometallurgical processes in that the desired
metals are separated using techniques that capitalize on electrochemical properties and/or
solubility differences while in aqueous solutions. Examples of hydrometallurgical processes
include solvent extraction, membrane separation, ion exchange, leaching, precipitation and
electrolytic reduction [10].
We use membrane separation and solvent extraction for the separation of metal ions.
However, with the use of a protic extractant, the aqueous phase is not truly purified because (i) the
number of ions in solution does not change and is often increased; (ii) the pH of the stream may be
significantly lowered if there is proton exchange; and (iii) the anions are still in solution [11].
A more efficient approach would be to design an extractant that can transport metal salts in
a hydrometallurgical solvent extraction process. Salicylaldimine ligands of the H
2
salen type
bearing NR substituents could function as ditopic ligands, having two separate binding sites to
2
2
-
2 2
accommodate both the metal cation and its corresponding anion(s). The [N O ] donor set of the
salen unit will bind to the metal ion and an anion with the protonated –NR units. These features
2
create the possibility of separately recovering the metal cation and its anion and recycling the
ligand. These types of ligands have been pioneered by the Tasker group with several examples in
the literature [9, 11-13, 16]. They obtain close to 100% loading of copper(II), based on a ligand
-3
-3
concentration of 0.01 mol dm and equilibrating it with 1 mol dm copper(II) sulphate for 24 h.
The aldimine functionality is relatively resistant to hydrolysis and can therefore be used in solvent
extraction processes because it is able to withstand the relatively strong acidic media required for
efficient stripping protocols [17].
2

For our ligands, we changed the ethylenediamine linker to propylenediamine to produce
ligands consisting of three 6-membered chelate rings, which should show selectivity for smaller
metal ions Co(II), Ni(II), Cu(II) and Zn(II), relative to Cd(II) and Pb(II) [18]. We report herein the
synthesis of these ditopic ligands and a Schiff-base analogue for comparison. We subjected the
ligands to competitive metal ion extraction and competitive transport of metal ions as well as
competitive extraction and transport of anions. Because one ditopic ligand showed very high
extraction of Cu(II), we carried out a Cu(II) loading experiment from pH 1 to 6 so as to obtain
½
pH . Finally, we grew single crystals of the Cu(II) complexes of both ditopic ligands and solved
their structures.
2
2
. Experimental
.1. Materials and methods
Reagents, solvents and ICP-OES standards were acquired from Sigma-Aldrich, Merck, Alfa Aesar
or SAARChem and used without purification. Distilled water was obtained from a Milli-Q
purification system. Solvents were dried over molecular sieves or via distillation before use. NMR
1
13
3
spectra were recorded in 5 mm tubes using CDCℓ . H- and C-NMR spectra were collected on a
Bruker AVA 400 NMR spectrometer. Manipulation of spectra was carried out using Mestrec
software. Mass spectrometry was performed on a VGAutospec instrument using ionization
techniques, including electrospray ionization and electron ionization. Elemental analyses were
done on a Perkin Elmer CHNS Elemental Analyzer model 2400 at the University of KwaZulu-
Natal. ICP-OES data were gathered on a Perkin Elmer Optima 5300DV and were manipulated
using Winlab32 and Microsoft Excel.
Ligands were synthesized according to the literature [9, 11, 16] and the synthetic scheme is
depicted in scheme 1 using the piperidyl ring as an example.
O
(i)
OH
NH
N
CH OEt
2
(
ii)
O
2
OH
N
N
(
iii)
OH HO
N
3
R
R

Scheme 1. (i) Paraformaldehyde, distillation; (ii) MeCN, N , reflux for 90 h; (iii) diaminopropane,
2
1
2
Et
R =
2
O-EtOH (1:1), room temp. 12 h. Ligands referred to here are H
2
L : R = H, H L :
2
3
N CH
, H L : R = -CH -N(n-C H ) .
2
2
2
6
13 2
2
.2. Preparation of 5-t-butyl-salicylaldehyde
This precursor was synthesized according to the method of Levin et al. [14]. All physical
measurements correspond to those reported in the literature.
2
.3. Synthesis and characterization of 2-((E)-(3-((E)-5-tert-butyl-2-
1
hydroxybenzylideneamino)propylimino)methyl)-4-tert-butylphenol (H
2
L )
The ligand was prepared by a standard method in which two equivalents (3.00 g, 16.8 mmol) of 5-
tert-butyl-2-hydroxy-benzaldehyde and one equivalent of propylenediamine (0.620 g, 8.42 mmol)
3
were refluxed for 1.5 h in absolute ethanol (50 cm ). Upon cooling and reduction of the volume a
yellow powder (3.19 g, 96.1%) was obtained.
Mp 76.0 – 76.9 °C; IR (neat): 2953 (m, CH), 1632 (m, C=N), 1581 (m, C=C), 1486 (s, CH),
-1
1
270 cm (s, C-N);
1
H NMR (400 MHz, chloroform-d) δ ppm 1.31 (s, 18 H, C(CH ) ), 2.07 - 2.14 (quin, J=6.64 Hz, 2
3
3
H, NCH CH ), 3.69 – 3.73 (t, J=6.64 Hz, 4 H, NCH CH ), 6.92 – 6.94 (d, J=8.50 Hz, 2 H, ArH),
2
2
2
2
7
.24 – 7.25 (d, J=2.28 Hz, 2 H, ArH), 7.35 – 7.38 (dd, J=2.49 Hz, 2 H, ArH), 8.39 (s, 2 H, NCH),
1
3
1
3.23 (s, 2 H, OH); C NMR (400 MHz, chloroform-d) δ ppm 31.08 (C(CH ) ), 31.47
3
3
(
NCH CH ), 33.60 (C(CH ) ), 56.44 (NCH CH ), 116.13 (C H), 117.64 (C CN), 127.30 (C H),
2
2
3 3
2
2
Ar
Ar
Ar
1
29.21 (C H), 140.94 (C C(CH ) ), 158.40 (C OH), 165.41(ArCN);
Ar
Ar
3 3
Ar
+
+
+
Expected M : 394.55, Found: MS (ES+): m/z (%) = 395.3 (30) [M + H] , 396.3 (10) [M + 2H] ;
+
+
MS (ES-): m/z (%) 393.3 (100) [M - H] ; 394.3 (30) [M] ;
Elemental microanalysis calcd (%) for C H N O : %C: 76.11, %H: 8.69, %N: 7.10; found: %C:
2
5
34
2
2
7
6.03, %H: 8.19, %N: 7.05.
2
2
.4. Synthesis of H
2
L2
.4.1. Synthesis of 1-(ethoxymethyl)piperidine. Piperidine (21.3 g, 0.250 mol) was added
dropwise to a suspension of paraformaldehyde (30.0 g, 0.313 mol) and anhydrous potassium
4

3
carbonate (69.1 g, 0.500 mol) in absolute ethanol (150 cm ) with external cooling in an ice bath.
The mixture was then stirred vigorously for 48 h allowing the temperature to rise to ambient
3
gradually. The solid was then filtered off and washed with dried diethyl ether (2 × 35 cm ). The
filtrate was then concentrated in vacuo to give a colorless non-viscous liquid which was distilled
1
using a Kugelrohr affording the product (24.5 g, 68.1%) [15]. H NMR (300 MHz, chloroform-d)
δ ppm 1.15 – 1.20 (t, J=7.04 Hz, 3 H, OCH
2
CH
3
), 1.41 – 1.45 (m, 2 H, NCH
2
CH
2
2
CH ), 1.52 –
1
.58 (m, 4 H, NCH CH CH ), 2.61 – 2.65 (m, 4 H, NCH CH CH ), 3.44 – 3.51 (q, J=7.04 Hz,
2
2
2
2
2
2
2
H, OCH
2
CH
3
), 4.04 (s, 2 H, NCH
2
O).
2
.4.2. Synthesis of 5-tert-butyl-2-hydroxy-3-(piperidin-1-ylmethyl)benzaldehyde. A mixture
of 5-tert-butyl-2-hydroxybenzaldehyde (3.00 g, 16.8 mmol) and 1-(ethoxymethyl)-piperidine (2.68
3
g, 18.7 mmol) in acetonitrile (70 cm ) was heated to reflux under dinitrogen for 90 h. After
cooling to room temperature the solution was worked up following the method of Fenton et al.
[15], yielding the product (4.38 g, 93.6%) as a yellow powder.
1
H NMR (600 MHz, chloroform-d) δ ppm 1.26 (s, 9 H, C(CH ) ), 1.50 – 1.53 (m, 2 H,
3
3
NCH CH CH ), 1.62 – 1.66 (m, 4 H, NCH CH CH ), 2.40 – 2.69 (m, 4 H, NCH CH CH ), 3.69
2
2
2
2
2
2
2
2
2
(
s, 2 H, NCH
2
Ar), 7.23 (d, J=2.64 Hz, 1 H, ArH), 7.64 (d, J=2.64 Hz, 1 H, ArH), 10.4 (s, 1 H,
1
3
CHO), 11.39 (s, 1 H, OH); C NMR (600 MHz, chloroform-d) δ ppm 24.02 (NCH CH CH ),
2
2
2
2
5.89 (NCH
2
CH
2
CH
2
), 31.47 (C(CH
3
)
3
), 34.22 (C(CH
3
)
3
2 2 2
), 54.06 (NCH CH CH ), 61.60
(
ArNCH ), 122.55 (C ), 122.99 (C ), 123.90 (C H), 132.41 (C H), 141.60 (C C(CH ) ),
2
Ar
Ar
Ar
Ar
Ar
3 3
1
60.14 (C OH), 191.22 (ArCHO).
Ar
2
.4.3. Synthesis and characterization of 2-((E)-(3-((E)-5-tert-butyl-2-hydroxy-3-(piperidin-1-
ylmethyl)benzylideneamino)propylimino)methyl)-4-tert-butyl-6-(piperidin-1-
2
ylmethyl)phenol (H L ). The ligand was prepared as in section 2.3 by using two equivalents
2
(3.00 g, 10.9 mmol) of 5-tert-butyl-2-hydroxy-3-(piperidin-1-ylmethyl)benzaldehyde and one
equivalent of propylenediamine (0.404 g, 5.45 mol) which was refluxed for 1.5 h in absolute
3
ethanol (50 cm ). Upon cooling and reduction of the volume a sticky yellow solid (3.08 g, 92.0%)
was obtained.
Mp 72.0 – 72.9 °C; IR (neat): 2930 (m, CH), 1629 (s, C=N), 1582 (m, C=C), 1463 (s, CH),
-1
1
270 cm (s, C-N);
5

1
3 3
H NMR (300 MHz, CHLOROFORM-d) δ ppm 1.34 (s, 18 H, C(CH ) ), 1.42 – 1.50 (m, 4 H,
NCH CH CH ), 1.60 – 1.67 (m, 8 H, NCH CH CH ), 2.08 – 2.13 (m, 2 H, NCH CH ), 2.50 (m,
2
2
2
2
2
2
2
2
8
7
2
H, NCH
2
CH
2
CH
2
), 3.63 (s, 4 H, (NCH
2
Ar), 3.69 – 3.74 (t, J=6.54 Hz, 4 H, NCH
2
2
CH ), 7.23 –
.24 (d, J=2.34 Hz, 2 H, ArH), 7.42 – 7.43 (d, J=2.49 Hz, 2 H, ArH), 8.44 (s, 2 H, NCH), 13.27 (s,
H, OH); C NMR (300 MHz, chloroform-d) δ ppm 24.32 (NCH CH CH ), 25.95
1
3
2
2
2
(
NCH
2
CH
2
CH
2
), 31.42 (C(CH
3
)
3
), 31.81(C(CH
3
)
3
), 33.89 (NCH
2
CH
2
), 54.41 (NCH
2
2 2
CH CH ),
5
1
6.98 (NCH CH ), 57.27 (ArCH N), 117.99 (C ), 124.78 (C ), 125.89 (C H), 130.57 (C H),
2
2
2
Ar
Ar
Ar
Ar
40.51 (CArC(CH
3
)
3
), 157.21 (CArOH), 165.01 (ArCN);
+
+
Expected M : 588.8, Found: MS (ES+): m/z (%) = 589.4 (12) [M + H] ; MS (ES-): m/z (%) 587.4
+
+
(
100) [M - H] , 588.4 (43) [M] ;
Elemental microanalysis calcd (%) for C H N O : %C: 75.47, %H: 9.59%, %N: 9.51; found:
3
7
56
4
2
%
C: 75.20, %H: 10.08, %N: 9.41.
2
2
0
.5. Synthesis of H L3
2
.5.1. Synthesis of N-(ethoxymethyl)-N-hexylhexan-1-amine. Dihexylamine (23.2 g,
.125 mol) was added dropwise to a suspension of paraformaldehyde (4.69 g, 0.156 mol) and
3
anhydrous potassium carbonate (34.6 g, 0.250 mol) in absolute ethanol (75 cm ) with external
cooling in an ice bath. The mixture was then stirred vigorously for 48 h allowing the temperature
to reach ambient gradually. The solid was then filtered off and washed with dried diethyl ether
3
(
2 × 25 cm ). The filtrate was then concentrated in vacuo to give a colorless oil which was
fractionally distilled through a Kugelrohr vacuum distillation apparatus affording the product
(18.9 g, 62.2%) [15].
1
3 2 2 2 2 2
H NMR (300 MHz, chloroform-d) δ ppm 0.86 – 0.91 (m, 6 H, CH CH CH CH CH CH N), 1.16
–
1
–
1.20 (t, J=7.16 Hz, 3 H, OCH CH ), 1.26 – 1.34 (m, 12 H, CH CH CH CH CH CH N), 1.40 –
2
3
3
2
2
2
2
2
.47 (m, 4 H, CH CH CH CH CH CH N), 2.59 – 2.64 (m, 4 H, CH CH CH CH CH CH N), 3.41
3
2
2
2
2
2
3
2
2
2
2
2
3.48 (q, J=7.01 Hz, 2 H, OCH
2
CH
3
), 4.15 (s, 2 H, OCH
2
N).
2
.5.2. Synthesis of 5-tert-butyl-3-[(dihexylamino)methyl]-2-hydroxybenzaldehyde. A mixture
of 5-tert-butyl-2-hydroxybenzaldehyde (3.49 g, 19.6 mmol) and N-(ethoxymethyl)-N-hexylhexan-
3
1
-amine (5.00 g, 20.5 mmol) in acetonitrile (75 cm ) was heated to reflux under dinitrogen for 90
6

h. After cooling to room temperature the solution was worked up following the method of Fenton
et al. [15] to yield the product (7.35 g, 99.1%) as a dark yellow oil.
1
H NMR (400 MHz, chloroform-d) δ ppm 0.83 – 0.86 (m, 6 H, CH CH CH CH CH CH N), 1.26
3
2
2
2
2
2
(
m, 20 H, CH
3
CH
2
CH
2
CH
2
CH
2
CH
2
3 3
N and C(CH ) ), 1.50 – 1.54 (m, 4 H,
CH CH CH CH CH CH N) 2.48 – 2.52 (m, 4 H, CH CH CH CH CH CH N), 3.77 (s, 2 H,
3
2
2
2
2
2
3
2
2
2
2
2
NCH Ar), 7.24 – 7.25 (d, J=2.49 Hz, 1 H, ArH), 7.62 – 7.63 (d, J=2.49 Hz, 1 H, ArH), 10.40
2
1
3
(
(
(
(
(
1
s, 1 H, CHO), 11.78 (s, 1 H, OH); C NMR (400 MHz, CHLOROFORM-d) δ ppm 13.91
CH CH CH CH CH CH N), 22.45 (CH CH CH CH CH CH N), 26.14
3
3
2
2
2
2
2
2
2
2
2
2
3
3
2
2
2
2
2
2
2
2
2
2
CH
CH
CH
CH
CH
CH
N), 26.91 (CH
CH
CH
CH
CH
CH
3 3
N), 31.24 (C(CH ) ), 31.53
CH CH CH CH CH CH N), 33.98 (C(CH ) ), 53.43 (CH CH CH CH CH CH N), 57.31
ArCH N), 122.28 (C CH ), 123.42 (C ), 123.58 (C ), 132.06 (C ), 141.27 (C C(CH ) ),
3
2
2
2
2
2
3 3
3
2
2
2
2
2
2
Ar
2
Ar
Ar
Ar
Ar
3 3
60.06 (CArOH), 191.01 (ArCHO).
2
2
.5.3. Synthesis and characterization of 2-((E)-(3-((E)-5-tert-butyl-3-((dihexylamino)methyl)-
-hydroxybenzylideneamino)propylimino)methyl)-4-tert-butyl-6-
3
(
(dihexylamino)methyl)phenol (H L ). The ligand was prepared as in section 2.4.3 by using two
2
equivalents (3.50 g, 9.32 mmol) of 5-tert-butyl-3-[(dihexylamino)-methyl]-2-hydroxy-
benzaldehyde and one equivalent of propylenediamine (0.347 g, 4.71 mmol) which was refluxed
3
3
for 1.5 h in absolute ethanol (50 cm ). The solvent was removed on a rotary evaporator and 30 cm
3
of chloroform was added. This solution was washed twice with 20 cm distilled water. Anhydrous
magnesium sulphate was added to the chloroform solution to remove water. An orange oil was
produced upon reduction of the volume (3.42 g, 92.9%). IR (neat): 2855 - 2953 (s, CH), 1632 (s,
-1
C=N), 1588 (m, C=C), 1462 (s, CH), 1268 cm (s, C-N);
1
H NMR (400 MHz, chloroform-d) δ ppm 0.85 – 0.88 (m, 12 H, CH CH CH CH CH CH N),
3
2
2
2
2
2
1
.27 – 1.31 (m, 24 H, CH
3
CH
2
CH
2
CH
2
CH
2
CH
2
3 3
N and C(CH ) ), 1.46 – 1.51(m, 8 H,
CH CH CH CH CH CH N), 2.08 (m, 2 H, NCH CH ), 2.45 – 2.48 (m, 8 H,
3
2
2
2
2
2
2
2
CH CH CH CH CH CH N), 3.65 (s, 4 H, NCH C ), 3.68 – 3.71 (m, 4 H, NCH CH ), 7.19 (s,
3
2
2
2
2
2
2
Ar
2
2
1
3
2
H, ArH), 7.52 – 7.53 (m, 2 H, ArH), 8.42 (s, 2 H, NCH), 13.25 (s, 2 H, OH); C NMR
(
(
(
400 MHz, chloroform-d) δ ppm 14.05 (CH CH CH CH CH CH N), 22.66
3
2
2
2
2
2
CH
3
CH
2
CH
2
CH
2
CH
2
CH
2
N), 27.05 (CH
3
CH
2
CH
2
CH
2
CH
2
CH
2
N), 27.29
CH CH CH CH CH CH N), 30.13 (C(CH ) ), 31.44 (C(CH ) ), 31.85 (NCH CH ), 33.98
3
2
2
2
2
2
3 3
3 3
2
2
7

(CH
3
CH
2
CH
2
CH
2
CH
2
CH
2
N), 50.14 (ArCH
2
N), 54.28 (CH
3
CH
2
CH
2
CH
2
CH
2
2
CH N), 57.01
(
NCH CH ), 116.46 (C ), 125.39 (C ), 127.60 (C H), 129.96 (C H), 140.62 (C C(CH ) ),
2
2
Ar
Ar
Ar
Ar
Ar
3 3
1
56.98 (CArOH), 165.68 (ArCN);
+
+
+
Expected M : 789.28, Found: MS (ES+): m/z (%) = 789.7 (100) [M] ; 790.7 (50) [M + H] ;
88 4 2
Elemental microanalysis calcd for C51H N O : %C: 77.61, %H: 11.24, %N: 7.10; found: %C:
7
7.23, %H: 10.59, %N: 6.68.
2
.6. [Copper-bis-2-((E)-(3-((E)-5-tert-butyl-2-hydroxy-3-(piperidin-1-
ylmethyl)benzylideneamino)propylimino)methyl)-4-tert-butyl-6-(piperidin-1-ylmethyl)phenoxy],
2
pentahydrate [Cu(L )] ·5 H
2
O
2
3
A solution of H L (0.177 g, 0.300 mmol) in CHCl (15 cm ) and Cu(CH COO) ·H O (59.9 mg,
2
3
3
2
2
3
0
.300 mmol) in methanol (50 cm ) was mixed and stirred overnight. The solvent was removed
3
in vacuo to yield a dark green oil which was dissolved in CHCl (30 cm ) and washed with a pH 9
3
3
ammonia solution (2 × 15 cm ). The resulting organic phase was dried with anhydrous MgSO ,
4
filtered and then concentrated in vacuo to yield the metal complex which was used without
-1
purification. IR (neat): 2939 (m, CH), 1615 (s, C=N), 1261 cm (s, C-N). Single crystals for
2
[
Cu(L )] was obtained from a solution of the complex dissolved in methanol, enclosed by
diethylether in a sealed container, allowing the diethyl ether to diffuse slowly into the methanol.
2
.7. [Copper-2-((E)-(3-((E)-5-tert-butyl-3-((dihexylamino)methyl)-2-
hydroxybenzylideneamino)propylimino)methyl)-4-tert-butyl-6-((dihexylamino)methyl)phenoxy],
3
[
Cu(L )]
3
3
A solution of H L (0.237 g, 0.300 mmol) in CHCl (15 cm ) and Cu(CH COO) ·H O (59.9 mg,
2
3
3
2
2
3
0
.300 mmol) in methanol (50 cm ) was mixed and stirred overnight. The solvent was removed
3
in vacuo to yield a dark green oil which was dissolved in CHCl
3
(30 cm ) and washed with a pH 9
3
ammonia solution (2 × 15 cm ). The resulting organic phase was dried with anhydrous MgSO ,
4
filtered and then concentrated in vacuo to yield the metal complex which was used without
purification. IR (neat): 2857 - 2955 (s, CH), 1616 (s, C=N), 1597 (m, C=C), 1456 (s, CH). Single
3
crystals of [Cu(L )] were obtained by vapor diffusion as in the previous complex.
8

2
.8. Competitive extraction
To investigate the selectivities of the ligands for different metal ions, competitive two-phase
solvent extraction studies were carried out with all three ligands. The metal ions were Co(II),
-3
Ni(II), Cu(II), Zn(II), Cd(II) and Pb(II) at 0.01 mol dm made up in an acetic acid / acetate buffer
-3
at pH 5.09. The ligand concentration was 0.01 mol dm and made up in chloroform which was
presaturated with distilled water. 10 ml of the mixed metal ion solution and 10 ml of the ligand
solution were placed in polytops (duplicate) and shaken at 220 rpm on a labcon-oscillating shaker
for 24 h at 25 °C. We chose 24 h since it has previously been shown that equilibration has been
reached after this time period for these types of systems [16, 19]. One ml samples of the aqueous
-3
phase were removed, diluted to 10 ml with 0.1 mol dm HNO and analyzed using ICP-OES.
3
More than one analyzing wavelength was initially selected for each metal ion to confirm the
absence of interference from other elements on the selected analyte wavelength, with the
interferent free wavelength consequently used for quantification.
2
.9. Competitive transport studies
-3
For the competitive transport experiments, each ligand at a concentration of 0.002 mol dm was
3
used as a carrier for the transport of metal ions across a CHCl
3
(50 cm ) bulk liquid membrane.
3
Details of the cell design have been reported elsewhere [19]. The source phase consisted of 10 cm
-3
of a 0.01 mol dm solution of Cu(II), Zn(II), Co(II), Ni(II), Pb(II) and Cd(II) metal ions in a
3
CH COONa/CH COOH buffer at pH 5.09. The receiving phase consisted of 30 cm of 0.1 mol
3
3
-3
dm HNO at pH 1.0. All three phases in the transport cell were stirred at 10 rpm at 25 °C. Under
3
these conditions, not only was the stirring process consistent, but also the interface between the
organic membrane and the two aqueous phases remained flat and well defined. The cells were
covered to prevent evaporation of solvents over the 24 h period. Samples of the source and
receiving phases were used for metal analysis by ICP-OES.
Competitive two-phase solvent extraction experiments were also performed using the
-
copper(II)-ligand complexes in solution to determine the selectivity of the ditopic ligands for Cl ,
-
2-
2
3
NO and SO . For the competitive transport of anions, the Cu(II) complexes of L and L at
3
4
-3
3
0
.002 mol dm were used as a carrier across a CHCl
3
(50 cm ) bulk liquid membrane. The source
3
-3
phase contained 10 cm of a 0.01 mol dm solution of CuSO , HNO and HCl at pH 1.91. The
4
3
3
receiving phase contained 30 cm NaH PO /Na HPO buffer at pH 6.95. All three phases in the
2
4
2
4
9

transport cell were stirred at 10 rpm at 25 °C. Other experimental details were the same as that
with the transport of the cations. Samples of the source phase and the receiving phase were
analyzed using ion chromatography. The results are reported in table 1 and quoted as the average
value obtained from duplicate runs. Any apparent extraction of an anion of less than 2.0% was
assumed to be within the experimental error of zero and hence ignored in the treatment of results.
In all cases the values between any two duplicate runs did not differ by more than 5%.
2
.10. pH Isotherms
3
Experiments were carried out by contacting chloroform solutions (5 cm ) which contained the
-3
3
ligand at 0.01 mol dm with aqueous solutions (5 cm ) which contained copper(II) nitrate at a
-3
concentration of 0.01 mol dm . After adjusting the pH using a few drops of either concentrated
HNO or concentrated NaOH the chloroform solution was added and placed on a shaker, shaking
3
3
at 220 rpm for 24 h at room temperature. Aliquots (1 cm ) of the aqueous phase were removed and
diluted to a volume of 10 ml with 0.1M HNO , and copper was analyzed by ICP-OES. The
3
calculated percentage of copper(II) uptake into the organic phase was plotted against the measured
initial pH of the aqueous phase to give an isotherm.
2
.11. Crystallographic data collection and refinement
Single crystal X-ray diffraction data were collected using a Bruker SMART APEX diffractometer
equipped with a Bruker APEX-II CCD area detector and an Oxford Cryosystems 700Plus cryostat
for temperature control. A multilayer monochromator with Mo Kα radiation (λ = 0.71073 Å) from
a fine-focus sealed tube source was used.
Data reduction was carried out by standard procedure using the Bruker software package
SAINT [20] and absorption corrections and correction of other systematic errors were performed
using SADABS [21, 22]. The structures were solved by direct methods using SHELXS-97 and
refined using SHELXH-97 [23]. X-Seed was used as the graphical interface for the SHELX
program suite [24]. Hydrogens were placed in calculated positions using riding models.
10

3
3
. Results and discussion
.1. Synthesis
1
13
All three ligands were characterized by H and C NMR spectroscopy, FTIR spectroscopy, mass
1
spectrometry, melting point determination (where possible) and elemental analysis. H L and
2
2
H L are both yellow solids, having melting points 76.0 – 76.9 °C and 72.0 – 72.9 °C,
2
1
2
3
1
respectively. The overall yield of H
2
L was 96.1%, H
2
L was 92.0% and H
2
L 92.9%. H
2
L had a
2
slightly higher yield because its synthesis involves only two steps, whereas the synthesis of H L
2
3
and H
2
L involved several steps.
Evidence for complete formation of products was confirmed by the presence of
1
characteristic resonances for the imine proton at δ = 8.3-8.5 ppm in the H NMR spectrum and of
1
3
the imine C centre at δ = 165-166 ppm in the C NMR spectrum, together with the absence of
aldehydic C and H resonances. In the IR spectrum, the aldehyde functionality at approximately
-1
-1
1
700 cm disappeared with the appearance of the imine functionality at 1629 - 1632 cm . The
elemental analyses of the two solids were satisfactory, but the viscous oil still contained some
residual solvent which was difficult to remove. This produced a H % which was out by 0.6%. The
free ligands as well as complexes of the ligands with various metal salts all showed sufficient
solubility in CHCl to allow solvent extraction and transport experiments to be carried out. In IR
3
2
3
-1
spectra of the Cu(II) complexes with H
2
L and H L , the imine peak shifts from ~1630 cm to
2
-1
1
615 cm which is indicative of complex formation.
3
.2. Extraction and transport
To investigate selectivities of the ligands for different metal ions, competitive two-phase metal ion
solvent extraction experiments were performed (seen macroscopically in figure 1; see figure 2 for
1
2
results). All three ligands showed significant selectivity for Cu(II) (H L -71%, H L -81%,
2
2
3
H L -73%) in a solution of different metal ions at pH 5.09. This selectivity can be explained by
2
formation of three six-membered chelate rings upon metal complexation which are more selective
for smaller metal ions and have a good fit for Cu(II) [16]. Also, the cleft size of the N O -donor
2
2
2
set fits the Cu(II) ideally [9]. Why H L extracts Cu(II) a little more compared to the other two
2
ligands is possibly due to the electronic effect of the different pendant arms present on this ligand.
However, this needs to be explored further. All three ligands extracted less than 10% of any of the
other metal ions as can be seen in figure 2. Furthermore, based on the Irving-Williams series,
11

Cu(II) would be the metal ion which would form the most stable complex with these ligands
compared to the other metal ions [25].
Competitive transport experiments (water/chloroform/water) have been undertaken
employing the three ligands separately as the ionophores in the membrane (chloroform) phase.
The respective pH values used for the aqueous source (pH 5.09) and receiving (pH 1.0) phases
were chosen such that under these conditions it was assumed that the resulting pH gradient across
the cell will be favorable for uptake of metal ions from the source phase into the membrane phase
while promoting its loss from the membrane phase into the receiving phase. Under the conditions
employed, no transport was observed for the six metal ions present in the source phase. There was
however a large amount of copper(II) transported from the source phase into the membrane phase,
indicative that a pH of 1.0 is not the optimum pH for stripping into the receiving phase. It is also
likely that the formation constants of Cu(II) with the ligands are high and it is difficult to release
Cu(II) into the receiving phase.
Competitive two-phase solvent extraction experiments were also performed using the
2
3
copper(II)-complexes of H L and H L in solution to determine the selectivity of the ligand
2
2
-
-
2-
-
complex for Cl , NO
3
and SO
4
3
. The ligands showed a high selectivity for NO anions (100%
-
2
-
3
2-
extraction), 74% Cl (H L ) and 76% Cl (H L ) were extracted and SO anions were extracted in
2
2
4
2
3
the least amount (H
2
L – 53% and H L – 18%) (see table 1). Competitive anion transport
2
experiments with the same ligands and anions as above were also studied. The respective pH
values used were pH 1.91 for the aqueous source phase and pH 6.95 for the receiving phase. As
-
-
indicated in table 2, the ligands showed selectivity for NO
3 3
and transported all of the NO anions
out of the source phase. The remainder of the anions that were not transported into the receiving
2
3
phase must be present in the organic membrane phase. The tertiary amines of CuL and CuL are
protonated and positively charged and there is an electrostatic attraction towards the anion
resulting in ion pairing. Since there are two tertiary amines on each ligand which produce metal-
ligand complexes which are doubly positively charged, this then results in substantial quantities of
-
-
NO and Cl extracted.
3
The above mentioned selectivities associated with anion receptors are strongly linked to a
phenomenon known as the Hofmeister bias (figure 3). The term ‘Hofmeister bias’ has its origins
in protein chemistry and originally defined the extent to which soluble anions salted out proteins
-
-
2-
[
3 4
26]. This can be used to predict why NO and Cl were extracted in preference to SO . This is
12

2
-
-
-
because SO
4 3
is more heavily hydrated than Cl and NO and therefore more hydrophilic and so
prefers to stay in an aqueous environment.
3
.3. pH Isotherm
In hydrometallurgical processes, the strengths of organic acid extractants are often compared by
measuring the pH1/2 value, which is the pH associated with 50% loading of the extractant [27]. We
2
carried out this measurement for H L which produced the highest Cu(II) extraction. The metal
2
content in the organic phase was determined over a pH 1-6 range to obtain a pH1/2 value. The
results of the experiment are indicated in figure 4. The pH1/2 value is estimated at pH 2.6. In metal
ion extraction, a lower pH1/2 value indicates stronger metal complexation [28]. A commercial
extractant used for Cu(II) extraction, LIX 84-I, has a pH1/2 value of 1.0 [29], indicating that this
extractant loads 50% Cu(II) ions at pH 1.0. This puts into perspective how our ligand compares to
commercial ligands.
3
.4. Crystal structures
2
[Cu(L )]·5H
2
O
2
The molecular structure of [Cu(L )]·5H O is shown in figure 5 with selected atoms numbered.
2
Bond lengths and angles are shown in tables 3 and 4, respectively. The complex crystallizes in a
monoclinic space group P2 /c and the crystal data parameters can be found in table 5. The
1
program SQUEEZE [30] was used to remove un-modelled electron density owing to badly
disordered guest solvent. After the data were squeezed the .res file was refined against the new
HKL file generated by SQUEEZE. Details of the program SQUEEZE have been appended to the
cif file. There is disorder in the structure – at one of the t-butyl groups and the associated phenyl
ring and at both piperidyl rings. Two positions were determined for both piperidyl and one phenyl
moiety. The two positions were labelled A and B. The site-occupancies of the major components
of disorder (A) were refined as x while the minor components (B) were refined as 1-x. An initial
value of x = 0.5 was set, eventually refining to 0.65. The temperature factors for the atoms of the
major and minor components were constrained with an EADP command with the values ranging
2
from 0.03 to 0.07 Å .
The metal centre is coordinated to two nitrogen and two oxygen donors in a distorted
square planar mode. This distortion helps to separate the bulkier piperidyl groups from each other.
13

The two piperidyl rings are on opposite sides of this distorted plane, most likely due to steric
repulsion. The average copper(II) – oxygen and copper(II) - nitrogen bond lengths are 1.905 Å and
1
–
.937 Å, respectively, with copper(II) – nitrogen bond lengths being longer than that of copper(II)
oxygen bond lengths as expected. The four bond angles around copper are: O1–Cu1–O2, 86.9°;
O1–Cu1–N1, 93.6°; O2–Cu1–N2, 94.0° and N2–Cu1–N1, 91.6°. The bond angles O1-Cu1-N2,
61.22° and O2-Cu1-N1, 160.98° are not 180°, with both sets of bond angles confirming the
1
distortion from a perfect square plane.
3
[
Cu(L )]
3
The molecular structure of [Cu(L )] is shown in figure 6 with selected atoms numbered. Bond
lengths and angles are shown in tables 3 and 4, respectively. The complex crystallizes in a
monoclinic space group C2/c and the crystal data parameters can be found in table 5. There is
disorder in the structure around the propyl bridge, however, in spite of this the R-factor is still
below 5%. The central carbon of the diaminopropyl moiety is disordered over two positions. The
major component of disorder (C60) was refined as x while the minor component (C59) was
refined as 1-x. The initial setting of 0.5 refined to 0.53 while the temperature factors were very
2
similar (~ 0.02 Å ).
The metal centre is coordinated to two nitrogen and two oxygen donors in a distorted
square planar mode. This distortion helps to separate the long alkyl chains from each other. Figure
6
is slightly deceptive, since it does not show how these long chains are arranged away from each
other, chains being on opposite sides of this distorted plane, very likely due to steric repulsion.
The average copper(II) – oxygen and copper-nitrogen bond lengths are 1.904 Å and 1.959 Å,
respectively. The four bond angles around the metal centre are: O1–Cu1–O2, 85.11°; O1–Cu1–
N9, 93.30°; N9–Cu1–N5, 96.49° and N5–Cu1–O2, 93.19°. The bond angles O1–Cu1–N5, 157.65°
and O2–Cu1–N9, 157.60° are closer to 180° compared to 109°, hence confirming a distorted
square planar coordination geometry.
Both structures have four-coordinate Cu(II) in a distorted square planar environment. The
Cu(II)-O bond lengths are virtually the same in both structures, but the Cu(II)-N bond lengths are
slightly longer with the long alkyl chains. It is most likely the disorder in the propyl bridge which
causes these bonds to be slightly longer.
14

There are three literature references which have Cu(II)-L crystal structures similar to our
Schiff-base ligand system [31-33]. In two cases, Cu(II) is in an octahedral environment and the
two axial bond lengths are longer than the equatorial bond lengths due to Jahn-Teller distortion
[31, 32]. In the third structure, Cu(II) is five-coordinate and there is one bond length which is
longer than the other bond lengths [33]. All our bond lengths around the Cu(II) centre are within
the normal range of copper(II) – donor bond lengths. Packing diagrams for both structures show
extensive intermolecular hydrogen bonding between the solvent molecules and the copper(II)-
ligand complexes.
The stoichiometries found in the structures are consistent with those found by solvent
extraction.
4
. Conclusion
Relatively simple ditopic ligands related to H
2
salen were used to establish selective extraction and
transport of copper(II) and nitrate, respectively. The co-extraction of a metal cation with its
concomitant anion(s) in the form of a metal salt by these types of ligands provide an advantage
2
over conventional reagents which add components to the aqueous stream. The pH
½
for H
2
L is
2
.6, which is reasonable in comparison to the oxime ligands which are used in industry. Both
metal-ligand crystal structures are four-coordinate and accommodate Cu(II) in a distorted square
planar environment. This distorted plane ensures that the bulky groups are accommodated on
either side of the distorted plane.
Supplementary data
2
Crystallographic data including atomic coordinates, bond distances and angles for [Cu(L )](H
2
O)
5
3
and [Cu(L )] have been deposited with the CCDC (Cambridge Crystallographic Data Centre,
1
2 Union Road, Cambridge CB2 1EZ, UK; Fax: +44 1223 336 033; E-mail:
deposit@ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk) and may be obtained free of charge
by quoting the deposition numbers CCDC 1020794 and CCDC 1435565.
15

Acknowledgements
We thank the National Research Foundation (NRF) of South Africa, grant number 92777, and
Stellenbosch University for their financial assistance in this work. We wish to thank Dr. Vincent J.
Smith for valuable discussions with the crystal structures.
Disclosure statement
No potential conflict of interest was reported by the authors.
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[
11] D.J. White, N. Laing, H. Miller, S. Parsons, P.A. Tasker, S. Coles. Chem. Commun., 2077
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(
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Dalton Trans., 55 (2002).
[
14] R. Aldred, R. Johnston, D. Levin, J. Neilan. J. Chem. Soc., Perkin Trans., 1, 1823 (1994).
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[
16

[
16] S.G. Galbraith, Q. Wang, L. Li, A.J. Blake, C. Wilson, S.R. Collinson, L.F. Lindoy, P.G.
Plieger, M. Schroder, P.A. Tasker. Chem. Eur. J., 13, 6091 (2007).
[
17] J. Szymanowski, in Hydroxyoximes and Copper Hydrometallurgy, CRC Press, Boca
Raton, FL, p. 62 (1993).
[
[
[
[
[
[
[
[
[
18] R.D. Hancock. J. Chem. Ed., 69, 615 (1992).
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20] SAINT Data Reduction Software, Version 6.45; Bruker AXS Inc.: Madison, WI (2003).
21] SADABS, Version 2.05; Bruker AXS Inc.: Madison, WI (2002).
22] R.H. Blessing. Acta Crystallogr., Sect. A: Found. Crystallogr., 51, 33 (1995).
23] G.M. Sheldrick. Acta Crystallogr., Sect. A: Found. Crystallogr., 64, 112 (2008).
24] L.J. Barbour. J. Supramol. Chem., 1, 189 (2001).
25] H.M. Irving, R.J.P. Williams. J. Chem. Soc., Dalton Trans., 3192 (1953).
26] (a) F. Hofmeister. Arch. Exp. Pathol. Pharmakol., 247 (1888). (b) B.A. Moyer, P.V.
Bonnesen, R. Custelcean, L.H. Delmau, B.P. Hay. Kem. Ind., 54, 65 (2005).
27] P.A. Tasker, P.G. Plieger, L.C. West, In Comprehensive Coordination Chemistry, J.A.
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Levitskaia, International Solvent Extraction Conference Cape Town, South Africa,
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[
[
[29] G.A. Kordosky, J.M. Sierakoski, J.E. House, International Solvent Extraction Conference,
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[
32] F. Thomas, H. Arora, C. Philouze, O. Jarjayes. Inorg. Chim. Acta, 363, 3122 (2010).
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[
17

NMR spectrum and of
(i)
(ii)
Figure 1. (i) Before and (ii) after images of competitive two-phase metal ion solvent extraction
experiments at pH 5.09.
18

9
8
7
6
5
4
3
2
1
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
Cd
Co
Cu
Ni
Pb
Zn
0
.0
L1
L2
L3
Ligand
Figure 2. Percentage metal ion extraction for the three ligands. pH of the aqueous phase = 5.09
and concentration of the metal ions = 0.01 mol dm . Concentration of the ligand = 0.01 mol dm .
-3
-3
19

←strongly hydrated anions
weakly hydrated anions→
3
-
2-
2-
2-
-
-
-
-
-
-
Citrate > SO
4 4 4 3 3 4
= tartrate > HPO > CrO > acetate > HCO > Cl > NO >ClO
Figure 3. The original Hofmeister bias [26].
20

80
70
60
50
40
30
20
10
0
0
1
2
3
4
5
6
7
pH
2
Figure 4. Copper(II) extraction of H L as a function of pH.
2
21

2
Figure 5. The structure of the Cu(II) complex with L , with ellipsoids at the 50% probability level.
Some atoms are numbered and hydrogens are omitted for clarity.
22

3
Figure 6. The structure of [Cu(L )]. Displacement ellipsoids are drawn at 50% probability level
and hydrogens are omitted for clarity.
23

Table 1. Results of the extraction experiments indicating the
percentage of anions extracted from the aqueous phase.
-
-
2-
Ligand
Phase
Cl
NO3
SO4
2
H L
Aqueous
Aqueous
74.0
75.7
100.0
100.0
52.6
18.1
2
3
H L
2
24

Table 2. Results of transport experiments indicating the percentage of
2
3
anions present in the source and receiving phases for CuL and CuL .
-
-
2-
Ligand
Phase
Source
Receiving
Source
Receiving
Cl
NO3
0.0
24.0
0.0
SO4
2
CuL
CuL
CuL
26.0
26.9
24.3
22.8
47.4
0.0
81.9
0.0
2
3
3
CuL
26.6
25

2
3
Table 3. Selected both lengths for [Cu(L )] and [Cu(L )].
2
3
[
Cu(L )]
[Cu(L )]
Atom Atom Length/Å
Atom
Atom Length/Å
Cu1
Cu1
Cu1
Cu1
O1
O2
N2
N1
1.904(2)
1.905(3)
1.934(3)
1.944(3)
Cu(1)
Cu(1)
O(1)
O(2)
1.902(18)
1.905(17)
Cu(1)
Cu(1)
N(9)
N(5)
1.961(2)
1.958(2)
26

2
3
Table 4. Selected bond angles for [Cu(L )] and [Cu(L )].
2
3
[
Cu(L )]
[Cu(L )]
Atom Atom Atom Angle/˚
Atom Atom Atom Angle/˚
O1
O1
O2
O1
O2
N2
Cu1
Cu1
Cu1
Cu1
Cu1
Cu1
O2
N2
N2
N1
N1
N1
86.94(11)
161.22(13)
94.01(13)
93.56(12)
160.98(12)
91.58(14)
O(1)
O(1)
N(9)
N(5)
O(1)
O(2)
Cu(1) O(2)
Cu(1) N(9)
Cu(1) N(5)
Cu(1) O(2)
Cu(1) N(5)
Cu(1) N(9)
85.11(7)
93.30(8)
96.49(9)
93.19(8)
157.65(9)
157.60(9)
27

2
3
Table 5. Crystallographic data and refinement parameters for [Cu(L )] and [Cu(L )].
2
3
Identification code
[Cu(L )]
[Cu(L )]
Empirical formula
C H Cu N O
C25.50H42.12N OCu
3
7
54
4
4.16
2
0.5
Formula weight
684.98
424.50
Temperature/K
100(2)
100(2)
Crystal system
Monoclinic
Monoclinic
C2/c
42.8482(14)
9.1372(3)
31.8323(12)
90.00
Space group
P2 /c
1
a/Å
b/Å
c/Å
α/°
β/°
13.7015(8)
13.1356(8)
23.3054(14)
90.00
102.442(3)
90.00
128.170(4)
90.00
γ/°
3
Volume/Å
4095.9(4)
4
9798.0(6)
16
Z
3
ρcalcg/cm
1.111
1.151
-1
μ/mm
0.572
0.487
F(000)
1465.0
3706
3
Crystal size/mm
Radiation
0.10 × 0.08 × 0.06
MoKα (λ = 0.71073)
3.04 to 56.56
0.10 × 0.10 × 0.03
MoKα (λ = 0.71073)
1.21 to 26.36
2
Θ range for data collection/°
Index ranges
-18 ≤ h ≤ 18, -17 ≤ k ≤ 17, -53 ≤ h ≤ 53, -11 ≤ k ≤ 11,
-30 ≤ l ≤ 31
-39 ≤ l ≤ 39
Reflections collected
100325
114671
Independent reflections
Data / restraints / parameter
10150 [Rint = 0.0801]
10009 [Rint = 0.1253]
10009 / 0 / 543
1.016
10150 / 1364 / 396
2
Goodness-of-fit on F
1.074
Final R indexes [I>=2σ (I)]
Final R indexes [all data]
R = 0.0821,
R = 0.0480,
1
1
wR = 0.2127
wR = 0.1235
2
2
1
R = 0.1137,
1
R = 0.0909,
wR
2
= 0.2301
wR
2
= 0.1453
-3
Largest diff. peak and hole/e Å
0.90 and -0.75
0.358 and -0.469
28

Graphical abstract
80
70
60
50
40
30
20
10
0
0
1
2
3
4
5
6
7
pH
29