Zwitterionic macrocyclic metal sulfate extractants containing 3-dialkylaminomethylsalicylaldimine units

Article abstract of DOI:10.1039/b313078a

10,25-Di-tert-alkyl-14,21-dimethyl-3,6,14,21-tetraazatricyclo[21.3.1.18, 12]octacosa-1(27),2,6,8,10,12(28),23,25-octaene-27,28-diol macrocycles form Ni(II) and Cu(II) complexes in which the metal cation and the sulfate anion are bound in separated sites in a zwitterionic form of the ligand. The nonyl-substituted macrocycle shows a higher affinity for SO₄^2- and a lower binding strength for Cu²⁺ than open chain analogues, the pH-dependences for which fall in ranges which allow loading of CuSO₄ at pH ≈4 and easy stripping to recycle the ligand. X-Ray structure determinations of the Cu(II) and Ni(II) sulfate complexes of the tert-butyl substituted ligand suggest that the de-tuning of M²⁺-binding results from a distortion from planarity of the "salen" N₂O₂^2- donor set imposed by the incorporation of the hexamethylene strap in the ligand and reveal that the sulfate is bound as a hydrate in a 2:2:2:2, ligand-M²⁺-SO₄^2--H₂O, assembly.

Full text of DOI:10.1039/b313078a

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Zwitterionic macrocyclic metal sulfate extractants containing
-dialkylaminomethylsalicylaldimine units
3
Paul G. Plieger, Peter A. Tasker* and Stuart G. Galbraith
School of Chemistry, The University of Edinburgh, Edinburgh, UK EH9 3JJ.
E-mail: p.a.tasker@ed.ac.uk
Received 17th October 2003, Accepted 19th November 2003
First published as an Advance Article on the web 5th December 2003
1
0,25-Di-tert-alkyl-14,21-dimethyl-3,6,14,21-tetraazatricyclo[21.3.1.18,12]octacosa-1(27),2,6,8,10,12(28),23,25-
octaene-27,28-diol macrocycles form Ni() and Cu() complexes in which the metal cation and the sulfate anion
are bound in separated sites in a zwitterionic form of the ligand. The nonyl-substituted macrocycle shows a higher
2Ϫ
2ϩ
affinity for SO4 and a lower binding strength for Cu than open chain analogues, the pH-dependences for which
fall in ranges which allow loading of CuSO at pH ≈ 4 and easy stripping to recycle the ligand. X-Ray structure
4
determinations of the Cu() and Ni() sulfate complexes of the tert-butyl substituted ligand suggest that the
2ϩ
2Ϫ
2
de-tuning of M -binding results from a distortion from planarity of the “salen” N O donor set imposed by
2
the incorporation of the hexamethylene strap in the ligand and reveal that the sulfate is bound as a hydrate in a
2ϩ
2Ϫ
2
: 2 : 2 : 2, ligand–M –SO –H O, assembly.
4 2
Introduction
function as strong sulfate extractants represents a challenge
because the hydrophilicity of sulfate makes extraction into low
polarity organic solvents difficult and obtaining selectivity over
other anions such as chloride is especially challenging, given
Solvent extraction has proven to be a robust and efficient tech-
nology to achieve the concentration and separations in hydro-
1
metallurgy. Whilst ion exchange based processes, typically
Ϫ
2Ϫ
the well established order of hydrophobicity Cl ӷ SO (the
4
using kerosene-soluble acidic reagents to transport metal ions,
5
Hofmeister order).
Sulfate binding in the prototype open chain reagents (Scheme
2
ϩ
(aq)
ϩ
(aq)
2
LH(org) ϩ M
[ML2](org) ϩ 2H
(1)
1
) has been shown to be templated by incorporation of the
2Ϫ
6
metal dication into the “salen” N O2 donor set. In this paper
2
give very good materials balances in certain processes, there are
flowsheets which require transport of metal salts to achieve an
we present work on macrocyclic analogues in which a hexa-
methylene strap has been inserted between the pendant amine
groups in order to preorganise the sulfate binding site. It was
also hoped that this strap would screen the complexed ion,
increasing solubility in non polar, water immiscible solvents.
2
efficient process
L(org) ϩ MX(aq)
[MLX](org)
(2)
When processing aqueous streams containing coordinating
anions it is possible to generate neutral, hydrocarbon soluble,
complexes with the anion in the inner coordination sphere. This
strategy has been deployed extensively in chloride hydrometal-
Experimental
Instrumentation
3
lurgy, but is less likely to succeed for processing streams con-
Nuclear magnetic resonance spectra were obtained on Bruker
AC 250 and 300 instruments, FAB mass spectra on a Kratos
MS 50 machine, EI mass spectra on a Kratos Profile instru-
ment, FTIR spectra on a Perkin-Elmer paragon 1000 spectro-
meter as KBR discs and electronic absorption spectra on a
Perkin-Elmer λ-900 spectrometer. Inductively coupled plasma
optical emission spectroscopy (ICP-OES) analysis was per-
formed on a Thermo Jarrell Ash IRIS ICP-OES spectrometer.
The measurement of pH was carried out using a Fisher
Scientific AR50 pH meter.
taining weakly coordinating anions such as sulfate. This is
unfortunate because new leaching technology for sulfidic ores
generates such streams and there is also a need to remove metal
sulfates from acid mine drainage streams. These requirements
have led us to consider the development of reagents such as 1
4
(
Scheme 1) to transport metal sulfates. In these reagents the
base metal dication and its sulfate counter-ion are bound in
separated charged sites generated in a zwitterionic form of the
ligand. The zwitterionic nature of the carrier allows the trans-
ported ions to be stripped by pH adjustment and the free ligand
to be regenerated and recycled. The design of ditopic ligands to
Ligand synthesis
N,NЈ-Bis(ethoxymethyl)-N,NЈ-dimethylhexane-1,6-diamine.
N,NЈ-Dimethyl-1,6-hexanediamine (30.0 g, 0.21 mol) was
added dropwise to a stirred suspension of paraformaldehyde
(
12.5 g, 0.42 mol) and K CO (57.5 g, 0.42 mol) in ethanol
2 3
(
72 ml) at 0 ЊC. The mixture was allowed to warm to room
temperature and stirred for 48 h. K CO was removed by fil-
2
3
tration, the solvent was evaporated in vacuo and the resulting oil
was purified by vacuum distillation to give 1 as a colourless oil
(
1
125 ЊC, 1.5 mmHg) (39.3 g, 72%) (Found: C, 64.3; H, 12.3; N,
0.8. C H N O requires C, 64.6; H, 12.4; N, 10.8%); δ (360
14
32
2
2
H
MHz; solvent CDCl ; reference CDCl ) 3.89 (4H, s, NCH O),
3
3
2
Scheme 1 Metal() sulfate complexes of open chain and macrocyclic
zwitterionic ligands containing “salen” metal-binding sites. 1a, R =
t-Bu, RЈ = n-C H , 1b, has the 1,2-ethane link between the imine
3
.28 (4H, q, OCH Me), 2.36 (4H, t, NCH C H ), 2.20 (6H, s,
2
2
2
4
NCH ), 1.29 (4H, t, NCH CH CH ), 1.14 (4H, s, NCH -
3
2
2
2
2
2
13
nitrogen atoms replaced by an 1,2-phenylene unit.
CH
2
CH
2
), 1.00 (6H, t, OCH
2
CH
3
); δ
C
(360 MHz; solvent
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
D a l t o n T r a n s . , 2 0 0 4 , 3 1 3 – 3 1 8
313

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CDCl ; reference CDCl ) 87.6 (NCH O), 63.3 (OCH Me), 53.4
mmol) in chloroform (200 ml). After stirring for 24 h the
3
3
2
2
(
1
NCH ), 39.1 (NMe), 27.5 (NCH CH ), 27.0 (NCH CH CH ),
solvent was removed in vacuo to yield a yellow oil which was
dissolved in chloroform (50 ml) and washed with water (2 ×
2
2
2
2
2
2
4.9 (OCH Me); m/z 189, 173, 157.
2
3
0 ml). The chloroform solution was dried over MgSO , filtered
4
N,NЈ-Dimethyl-N,NЈ-hexamethylenedi(3-formyl-2-hydroxy-
-tert-butylbenzylamine) (2a). A solution of 5-tert-butyl-2-
and evaporated in vacuo to yield a viscous yellow oil (1.24 g,
5
98%) (Found: C, 77.3; H, 10.5; N, 8.4%;. C H N O requires
44
72
4
2
Ϫ1
hydroxybenzaldehyde (35 g, 0.2 mol) and 1 (26 g, 0.1 mol) in
acetonitrile (400 ml) was refluxed under dinitrogen for 72 h.
After cooling to room temperature the solvent was removed
in vacuo to give a pale yellow oil which was triturated with
pentane to give 2a as a pale yellow powder (41.7 g, 81%),
mp 90 ЊC (from hexane) (Found: C, 73.3; H, 9.3; N, 5.4.
C, 76.7; H, 10.5; N, 8.1%); νmax/cm 2931s, 2791s, 1632s (C᎐N),
1466s, 1264, 1033, 740; δ (360 MHz; solvent CDCl ; reference
H
3
CDCl ) 8.39 (2H, s, N᎐CH), 7.15–7.07 (4H, m, Ph), 3.88 (4H, s,
3
NCH CH N), 3.49 (4H, s, PhCH N), 2.31 (4H, t, J 7.2 Hz,
2
2
2
NCH ), 2.23 (6H, s, NCH ), 1.60–1.37 (8H, m, NCH CH CH ),
2
3
2
2
2
1.25–0.55 (38H, s, C H ); δ (360 MHz; solvent CDCl ; refer-
9
19
C
3
Ϫ1
C H N O requires C, 73.2; H, 9.2; N, 5.3%); νmax/cm 2962s,
ence CDCl ) 166.4 (N᎐CH), 157.3 (Ph), 137.0 (Ph), 131.7 (Ph
32
48
2
4
3
2
862, 2846, 1678s (C᎐O), 1233s, 978, 889, 826 (KBr); δH (360
CH), 127.8 (Ph CH), 125.0 (Ph), 118.1 (Ph), 60.0 (NCH2-
᎐
MHz; solvent CDCl ; reference CDCl ) 11.95 (2H, s, br) 10.35
CH N), 57.7 (NCH Ph), 55.9 (CH N) 45–8 (C mixed isomer
3
3
2
2
2
9
(
2H, s, CHO), 7.61 (2H, d, J 2.6 Hz, Ph), 7.24 (2H, d, J 2.6 Hz,
chain); λmaxϩ/nm (MeOH) 260 (13120), 328 (4732), 414 (1462);
m/z 689 (M ).
Ph), 3.70 (4H, s, PhCH N), 2.48 (4H, t, J 7.2 Hz, NCH ), 2.28
2
2
(
6H, s, NCH ), 1.57 (4H, m, J 7.2 Hz, NCH CH CH ), 1.34
3 2 2 2
(
4H, m, J 7.2 Hz, NCH CH CH ), 1.26 (18H, s, t-Bu); δ (360
Preparation of complexes of metal sulfates
2
2
2
C
MHz; solvent CDCl ; reference CDCl ) 191.3 (CHO), 159.8
3
3
Solutions of ligand (0.182 mmol) in methanol (25 ml) and
the appropriate metal sulfate (1 mol equivalent) in methanol
(
CHOC), 141.5 (COH), 132.4 (Ph CH), 124.0 (Ph CH), 123.3
(
Ph), 122.2 (Ph), 60.3 (PhCH N), 57.0 (NCH ), 41.3 (NMe),
2
2
(
20 ml) were mixed and stirred for 24 h. After removal of most
3
4.0 (CMe ), 31.3 (Me), 26.9 (NCH CH CH ), 26.8 (NCH -
3 2 2 2 2
ϩ
of the solvent in vacuo the precipitates were purified as
indicated.
CH CH ); m/z 525 (M ).
2
2
N,NЈ-Dimethyl-N,NЈ-hexamethylenedi(3-formyl-2-hydroxy-
-nonylbenzylamine) (2b). A solution of 5-nonyl-2-hydroxy-
benzaldehyde (3 g, 12.1 mmol) and 1 (1.57 g, 6 mmol) in
acetonitrile (150 ml) was refluxed under dinitrogen for 72 h. The
solution was cooled to room temperature, concentrated to
Nickel[10,25-di-tert-butyl-14,21-dimethyl-3,6-diaza-14,21-
diazoniatricyclo[21.3.1.18,12]octacosa-(27),2,6,8,10,12(28),
5
3
6
27
28
2
3,25-octaene-27,28-diolato[ꢀN ,ꢀN ,ꢀO ,ꢀO ] sulfate mono-
hydrate, [Ni(3a)(SO )]ؒH O. Recrystallisation from methanol–
4
2
diisopropyl ether, followed by crushing and drying gave
Ni(3a)(SO )]ؒH O (0.074 g, 57%) (Found: C, 58.2; H, 8.0;
3
0 ml in vacuo and then heated to 60 ЊC. An oil separated
[
4
2
from a yellow solution on cooling which was collected, washed
with acetonitrile (2 × 20 ml) and dried in vacuo to give 2b
as a light brown oil (2.3 g, 57%) (Found: C, 74.4; H, 10.0; N,
N 8.0. C H N O NiSO requires C, 58.0; H, 7.5; N, 8.0%);
3
4
52
4
2
4
Ϫ1
νmax/cm 3409, 2957s, 2867, 1625s (C᎐N), 1548, 1455s, 1245s,
1
3
128s, 609 (KBr); λmax/nm (MeOH) 435sh, 406 (8889), 334sh,
4
.4. C H N O ؒ½H O requires C, 74.8; H, 10.3; N, 4.2%);
42 68 2 4 2
Ϫ1
ϩ ϩ ϩ
26 (11187); m/z 704 (M ), 606 (M Ϫ SO ), 1403 (2M ), 1309
4
νmax/cm 2957s, 2859, 1681s (C᎐O), 1233s, 1014, 975, 747;
δH (250 MHz; solvent CDCl ; reference CDCl ) 10.36 (2H, s,
ϩ
(
2M Ϫ SO ). Crystals of the monohydrate for X-ray structure
4
3
3
determination were grown by the diffusion of diisopropyl ether
into a methanolic solution and were found also to contain
diisopropyl ether of solvation.
CHO), 7.56 (2H, m, Ph), 7.20 (2H, m, Ph), 3.70 (4H, s,
PhCH N), 2.47 (4H, t, J 7.2 Hz, NCH ), 2.28 (6H, s, NCH ),
2
2
3
1
.56–1.38 (8H, m, NCH CH CH ), 1.25–0.86 (38H, s, C H );
2 2 2 9 19
δ (250 MHz; solvent CDCl ; reference CDCl ) 191.4 (CHO),
C
3
3
Copper[10,25-di-tert-butyl-14,21-dimethyl-3,6-diaza-14,21-
diazoniatricyclo[21.3.1.18,12]octacosa-(27),2,6,8,10,12(28),
1
1
59.4 (CHOC), 138.2 (COH), 133.0 (Ph CH), 124.7 (Ph CH),
23.2 (Ph), 122.0 (Ph), 60.1 (PhCH N), 56.9 (NCH ), 41.2
2
2
3
6
27
28
2
3,25-octaene-27,28-diolato[ꢀN ,ꢀN ,ꢀO ,ꢀO ] sulfate mono-
hydrate, [Cu(3a)(SO )]ؒH O. Recrystallisation from ethanol–
ϩ
(
NMe), 40–8 (C mixed isomer chain); m/z 665 (M ).
9
4
2
diethyl ether, followed by crushing and drying gave [Cu(3a)-
SO )]ؒH O (0.050 g, 39%) (Found: C, 57.7; H, 7.7; N 7.9.
1
0,25-Di-tert-butyl-14,21-dimethyl-3,6,14,21-tetraaza-
(
4
2
tricyclo[21.3.1.18,12]octacosa-1(27),2,6,8,10,12(28),23,25-
octaene-27,28-diol (3a). 1,2-Ethanediamine (130 µl, 1.94 mmol)
in ethanol (20 ml) was added dropwise to 2a (1.00 g, 1.94 mmol)
in diethyl ether (200 ml). After stirring for 24 hr the solvent was
removed in vacuo and the resulting orange residue was tritur-
ated with pentane (10 ml) to give 3a as a pale yellow powder
Ϫ1
C H N O CuSO requires C, 57.6; H, 7.4; N, 7.9%); νmax/cm
34
52
4
2
4
3
1
7
418, 2954s, 2866, 2664, 1633s (C᎐N), 1546, 1452s, 1121s, 1089,
᎐
042, 619 (KBr); λ /nm (MeOH) 363 (22040), 591, (680); m/z
max
ϩ
ϩ
ϩ
09 (M ), 611 (M Ϫ SO ), 1414 (2M ), 1318 (2M Ϫ SO ).
4
4
Crystals of the monohydrate for X-ray structure determination
were grown by the diffusion of diethyl ether into an ethanolic
solution and were found also to contain ether and alcohol of
solvation.
(
0.46 g, 44%), mp 94–96 ЊC (Found: C, 73.8; H, 9.4; N 10.2.
Ϫ1
C H N O ؒ¼H O requires C, 73.8; H 9.6; N 10.1%); νmax/cm
34
52
4
2
2
2
952s, 2859s, 2788s, 1636s (C᎐N), 1466s, 1271, 1034, 823 (KBr);
᎐
δ (360 MHz; solvent CDCl ; reference CDCl ) 13.25 (2H, s-br,
H
3
3
Preparation of “metal-only” complexes
Ph OH), 8.38 (2H, s, N᎐CH), 7.36 (2H, d, J 2.5 Hz, Ph), 7.16
᎐
(
2H, d, J 2.5 Hz, Ph), 3.89 (4H, s, NCH CH N), 3.55 (4H, s,
Copper[10,25-dinonyl-14,21-dimethyl-3,6,14,21-tetraaza-
2
2
PhCH N), 2.49 (4H, t, CH N), 2.22 (6H, s, NCH ), 1.55 (4H,
tricyclo[21.3.1.18,12]octacosa-1(27),2,6,8,10,12(28),23,25-octa-
2
2
3
3
6
27
28
m, CH CH N), 1.32 (4H, m, CH CH CH N), 1.28 (18H, s,
ene-27,28,diolato(2؊)[ꢀN ,ꢀN ,ꢀO ,ꢀO ], [Cu(3b ؊ 2H)].
Using the ligand 3b and Cu(CH CO ) ؒH O as the source of
2
2
2
2
2
t-Bu); δC (360 MHz; solvent CDCl ; reference CDCl ) 166.3
3
3
3
2
2
2
(
N᎐CH), 157.2 (Ph CH), 140.7 (Ph CH), 130.6 (Ph), 126.3 (Ph),
᎐
copper in the procedure outlined above for the preparation of
1
25.5 (Ph), 118.0 (Ph), 60.0 (NCH CH N), 57.7 (NCH Ph),
[Cu(3a)(SO )]ؒH O gave a black oil which was dissolved in
2
2
2
4
2
5
5.9 (CH N), 42.3 (NMe), 33.9 (CMe ), 31.3 (Me), 27.5
chloroform (30 ml) and washed with pH 9 ammonia solution
(2 × 20 ml). The chloroform solution was dried over MgSO4,
filtered, and on evaporation in vacuo gave a viscous black oil
(0.109 g, 80%) which was used without further purification in
solvent extraction experiments (copper content determined by
ICP-OES for a 0.001 M solution in butanol: found 77.5 ppm,
2
3
ϩ
ϩ
(
NCH CH ), 27.2 (NCH CH CH ); m/z 549 (M ), 1092 (2M ).
2 2 2 2 2
1
0,25-Dinonyl-14,21-dimethyl-3,6,14,21-tetraazatricyclo-
21.3.1.18,12]octacosa-1(27),2,6,8,10,12(28),23,25-octaene-
7,28-diol (3b). 1,2-Ethanediamine (122 µl, 1.83 mmol) in
[
2
Ϫ1
methanol (100 ml) was added dropwise to 2b (1.22 g, 1.83
calc. for C H N O Cu 78.9 ppm). νmax/cm 3234, 2958s, 2870,
44
70
4
2
3
14
D a l t o n T r a n s . , 2 0 0 4 , 3 1 3 – 3 1 8

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Table 1 Crystallographic data
Cu(3a)(SO )]ؒH O
[
[Ni(3a)(SO )]ؒH O
4
2
4
2
Emp. formula
M
CuC42.5H78.5N O10.75S
913.19
NiC H N O S
721.58
4
34 54
4
7
Crystal system
Space group
Monoclinic
Monoclinic
P2 /c
P2 /n
1
1
a/Å
b/Å
c/Å
β/Њ
V/Å
Z
18.500(3)
18.493(3)
14.850(3)
104.677(2)
4914.5(14)
4
14.447(3)
18.992(4)
33.985(7)
96.60(3)
9263(3)
8
3
Ϫ3
D /g cm
1.234
0.543
1.035
0.504
c
Ϫ1
µ/mm
T /K
Unique data (Rint
Unique data with
150
150
)
12084 (0.0709)
10422
12291 (0.0921)
10173
F > 4σ(F )
o
o
R , wR2
0.1205, 0.2929
0.1268, 0.2669
1
1
625s (C᎐N), 1540, 1446s, 755 (KBr); λmax/nm (MeOH) 364
᎐
ϩ
(
7840), 585 (260); m/z 750 (M ).
Solvent extraction studies
Scheme 2 Ligand synthesis. Reagents and conditions: (i) para-
formaldehyde, ethanol, distillation; () MgOMe, MeOH–toluene, reflux
3 h; (iii) MeCN, N , reflux 66 h; (iv) 1,2-ethanediamine, Et O–EtOH
10 : 1), r.t. 24 h.
1
0
0 ml aliquots of H SO /Na SO with pH values in the range
2 4 2 4
2Ϫ Ϫ3
2
2
–7 and a constant SO4 concentration (0.8 mol dm ) were
(
intimately mixed for 16 h with CHCl solutions (containing
3
5
% butan-1-ol) of the “copper-only” complex [Cu(3b Ϫ 2H)]
resulting from a 2 ϩ 2 condensation of 2a and 1,2-ethane-
diamine had been formed. No attempt was made to remove this
component before the preparation of the metal complexes of 3a
(
10 ml, 0.005 M). The organic layer was separated, a 2 ml
aliquot removed, evaporated in vacuo, and then redissolved in
butan-1-ol (10 ml). The copper and sulfur content was sub-
sequently analysed using ICP-OES. Solutions of the free ligand
(
see below). The related macrocycle (3b) with two multiply
branched, mixed isomer, nonyl substituents and its copper
sulfate complexes have sufficient solubility in chloroform ϩ 5%
butan-1-ol to allow copper and sulfate loadings from aqueous
solutions to be investigated. Whilst FAB mass spectra showed
only very weak peaks with m/z greater than that of 3b, it
is probable that, as in the case of the tert-butyl-substituted
macrocycle 3a, some large-ring 2 ϩ 2 condensation product
is present in the material used in the solvent extraction
experiments (see below).
3
b (10 ml, 0.005M) were treated similarly and the sulfur content
was determined.
X-Ray structure determinations of [Ni(3a)(SO )]ؒH O and
4
2
[
Cu(3a)(SO )]ؒH O
4 2
Data were collected at 150 K on a Bruker SMART APEX dif-
fractometer using Mo-Kα radiation (λ = 0.71073 Å). The struc-
tures were solved by direct methods (SHELXS 97) and refined
on F by full-matrix least squares (SHELXL 97). All hydrogen
atoms were placed at calculated positions and refined using a
riding model. For [Cu(3a)(SO )]ؒH O an anisotropic model was
used for all non-H atoms except S1. A 0.75/0.25 disorder model
was applied to one tert-butyl group and one ethanol solvent
molecule was refined with a partial occupancy of 0.75. The
C–C bond distance in ethanol was restrained. The final refine-
7
2
7
Formation of metal salt complexes
4
2
Ni() and Cu() sulfate complexes [M(L)(SO )] of the ligand 3a
4
were readily isolated from methanol. Microanalysis and FAB
mass spectrometry and X-ray structure determination (see
below) confirmed the formation of complexes containing a
zwitterionic form of the ligand in which the phenolic protons
have been transferred to the nitrogen atoms in hexamethylene
diamine strap, and that the sulfate ion is “complexed” to these
cationic sites rather than the metal centre.
A practicable solvent extraction system to transport metal()
sulfates should be capable of loading both metal dications and
sulfate (or bisulfate) anions from an aqueous solution with pH
values typical of pregnant leach solutions or other process
streams or effluents. Whilst a complex series of equilibria is
needed to define the relative concentrations of species involved
in transport, titration of an organic solution of the “metal
only” complex, [M(L Ϫ 2H)] (top right, Scheme 3), with aque-
ous sulfuric acid and monitoring the metal and sulfate content
in the organic phase provides useful information on the
“strengths” of metal and sulfate binding.
ment statistics are presented in Table 1. For [Ni(3a)(SO )] an
4
isotropic model was applied for all C, H and N atoms and
diffuse solvent equivalent to eight diisopropyl ether molecules
per unit cell was treated in the manner described by van der
Sluis and Spek. The final refinement statistics are presented in
Table 1.
CCDC reference numbers 222180 and 222181.
See http://www.rsc.org/suppdata/dt/b3/b313078a/ for crystal-
lographic data in CIF or other electronic format.
8
Results and discussion
Synthesis of new Schiff base ligands
The 22-membered macrocycle 3a was prepared from 2-hydroxy-
9
5
1
-tert-butylbenzaldehyde, paraformaldehyde, N,N-dimethyl-
,6-hexanediamine and 1,2-ethanediamine as outlined in
The pH-dependence of sulfate loading of the “copper-only”
complex [Cu(3b Ϫ 2H)] and the free ligand 3b is compared in
Fig. 1. The copper complex binds sulfate when contacted with
aqueous solutions of pH < 7, and the plateau region pH 3–5 is
consistent with more than 90% of the copper complex being
10
Scheme 2. A two-step Mannich reaction gave the parent
dialdehyde 2a in 81% yield which was purified by first soni-
cating the oil in pentane and recrystallising the resulting pale
yellow solid from hexane. The macrocycle was characterised by
present in the form [Cu(3b)(SO )] in the organic phase. In con-
4
1
13
H and C NMR and elemental analysis. Mass spectrometry
indicated that some of the related 44-membered macrocycle
trast, the free ligand is less than 10% sulfate-loaded, as [(3b ϩ
2H)(SO )] or [(3b ϩ H)(HSO )], in this region, indicating that
4
4
D a l t o n T r a n s . , 2 0 0 4 , 3 1 3 – 3 1 8
315

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[
Cu(3b)(SO )] is contacted with dilute sulfuric acid of pH 1.2.
4
The pH0.5 for the copper stripping in this macrocycle is 1.9.‡
Cu(1a)(SO )] under the same conditions has a pH value for
[
4
0.5
copper-stripping of 1.4 and at pH 1.2 less than 70% of the
copper had been stripped. This slight de-tuning of the metal
binding site may arise from the hexamethylene strap imposing
2Ϫ
a distortion from a square planar arrangement of the N O2
2
“
salen” donor set preferred by Cu(). This is suggested by the
X-ray structure determinations (see below).
2ϩ
2Ϫ
The pH profiles for Cu and SO4 loading, and in particu-
lar the overlapping plateaux in the region pH 3–5 are close to
ideal for base metal recovery from sulfate feeds. At pH ≈ 4,
>
90% loading of CuSO is possible without pH adjustment,
4
L(org) ϩ CuSO4(aq)
[CuL(SO )]
(3)
4
(org)
and an efficient materials balance for a circuit will result if the
copper is stripped with sulfuric acid at pH < 1.0,
[
CuL(SO )]
ϩ H SO
4
(org)
2
4(aq)
[
(L ϩ 2H)(SO )]
ϩ CuSO4(aq)
,
(4)
4
(org)
followed by reagent regeneration with base to pH ≈ 5.0,
(Lϩ2H)(SO )] ϩ 2 NH3(aq)
[
4
(org)
L(org) ϩ [NH ] SO4(aq), (5)
4 2
and electrolytic recovery of the metal,
CuSO ϩ H O Cu ϩ H SO ϩ 0.5O2,
4(aq)
(6)
which will regenerate the acid for stripping and gives overall:
CuSO ϩ H O ϩ 2NH Cu ϩ 0.5O ϩ [NH ] SO (7)
4
2
2
Scheme 3 Some of the equilibria involved in transporting metal()
sulfates in zwitterionic forms of ligands 3.
4
2
3
2
4 2
4
The slow rise of copper loading with increase of pH and the
failure to load fully until the pH is > 5.5 may be a feature of
the reagent containing a significant quantity of the larger ring
2
ϩ 2 condensation product. It is quite probable that this will
not so readily form the desired CuSO complex, and 100%
4
metal loading can only be achieved in the “metal only” form by
deprotonation of the ligand at pH > 5.5.
X-Ray structure determination
[
Ni(3a)(SO )]ؒH O and [Cu(3a)(SO )]ؒH O are solvated differ-
4 2 4 2
ently in the solid state and the copper complex shows some
minor disorder of one of the tert-butyl substituent. Apart from
this, structural features associated with the assembly of sulfate
Fig. 1 The pH dependence of H SO and CuSO uptake by ligand 3b
2
4
4
in a chloroform–5% butanol solvent system.
complexes are remarkably similar. Both are dimers [M(3a)(SO )-
4
(
H O)] , Figs. 2–4, with approximately planar M()(salen)
2
2
the copper centre is needed to “template” the tertiary amine
2Ϫ
units. The N O2 donor set is more distorted from planarity
2
groups to provide an effective receptor for H SO in chloro-
2
4
2Ϫ
and has a larger O–M–O angle (see Table 2) than the related
open-chain ligands. This is more pronounced in the nickel com-
plex [Ni(3a)(SO )]ؒH O with O–Ni–O values of 88.5(3) and
2ϩ
form–butanol. Such cooperativity of M and SO4 binding is
potentially of great significance for tuning reagents to show
high selectivities of metal sulfate transport.
4
2
8
8.8(3)Њ compared with 83.2(1) to 86.4(1)Њ for a range of open
11
Sulfate-loading corresponding to formation of the mono-
chain analogues and 82.7(1)Њ for the related mononickel()
macrocycle complex of Okawa and co-workers. These distor-
tions may arise in part from the incorporation of the hexa-
methylene strap and account for the weaker binding of Cu
sulfato complexes [Cu(L)(SO )] occurs at higher pH values for
12
4
the macrocyclic system 3b than for linear analogues such as 1a
and 1b (Scheme 1). The pH values for 50% sulfate uptake by
2ϩ
[Cu(3b Ϫ 2H)] and of [Cu(1b Ϫ 2H)] under identical conditions†
shown in the solvent extraction experiments (above).
are 5.9 and 4.2, respectively.
Whilst the incorporation of the hexamethylene strap
enhances sulfate binding cf. the linear analogues 1, the
The centres of the dimeric units are composed of two sulfate
anions bridged by H-bonded water molecules, see Fig. 5,
distances i and j. This tetranionic unit interacts strongly with
the protonated alkylammonium N(6) groups via a combination
of electrostatic bonding (distances c and d in Fig. 5) and
“
strength” of metal-binding is decreased slightly. More than
8
5% of the copper is stripped when a chloroform solution of
†
These values are for 0.01 M solutions in neat CHCl . Data for 1 were
‡ This value for [Cu(3b)(SO )] is for a 0.01 M solution in chloro-
3
4
generated using a 0.005 M solution in CHCl containing 5% butan-1-ol
because solid material separated from a 0.01 M solution in neat CHCl3
in the pH region 2–5.5. The pH in Fig. 1 is 6.2 which is slightly higher.
form, under the same conditions used for [Cu(1a)(SO )], whilst the
3
4
value calculated from Fig. 1 of 1.8 is for chloroform containing 5%
butan-1-ol.
½
3
16
D a l t o n T r a n s . , 2 0 0 4 , 3 1 3 – 3 1 8

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Table 2 Selected bond lengths (Å) and angles (Њ) for [Cu(3a)(SO )]ؒH O and [Ni(3a)(SO )]ؒH O
4
2
4
2
Cu(1)–O(1B)
Cu(1)–O(1)
1.926(4)
1.912(4)
Cu(1)–N(2)
Cu(1)–N(2B)
1.947(4)
1.950(4)
O(1)–Cu(1)–O(1B)
O(1)–Cu(1)–N(2)
O(1B)–Cu(1)–N(2)
O(1)–Cu(1)–N(2B)
O(1B)–Cu(1)–N(2B)
N(2)–Cu(1)–N(2B)
O(11)–S(1)–O(13)
92.94(15)
91.66(17)
173.75(17)
174.71(17)
92.04(17)
83.50(18)
111.8(3)
O(11)–S(1)–O(14)
O(13)–S(1)–O(14)
O(11)–S(1)–O(12)
O(13)–S(1)–O(12)
O(14)–S(1)–O(12)
N(6B)_$2–S(1)_$1–N(6)
110.0(3)
108.1(3)
109.5(3)
109.0(2)
108.3(2)
171.28(11)
Ni(1)–O(1)
Ni(1)–O(1C)
Ni(1)–N(2)
Ni(1)–N(2C)
1.862(6)
1.881(6)
1.877(7)
1.864(7)
Ni(2)–O(1B)
Ni(2)–O(1D)
Ni(2)–N(2B)
Ni(2)–N(2D)
1.870(6)
1.849(5)
1.845(7)
1.846(7)
O(1)–Ni(1)–N(2C)
O(1)–Ni(1)–N(2)
178.2(3)
92.7(3)
85.5(3)
88.5(3)
93.3(3)
177.9(3)
86.0(3)
176.4(3)
92.9(3)
92.3(3)
178.3(3)
88.8(3)
109.0(4)
O(11)–S(1)–O(14)
O(13)–S(1)–O(14)
O(11)–S(1)–O(12)
O(13)–S(1)–O(12)
O(14)–S(1)–O(12)
O(24)–S(2)–O(23)
O(24)–S(2)–O(22)
O(23)–S(2)–O(22)
O(24)–S(2)–O(21)
O(23)–S(2)–O(21)
O(22)–S(2)–O(21)
N(6)–S(2)–N(6B)_$1
N(6B)–S(1)–N(6D)_$1
109.1(4)
112.7(5)
107.7(4)
109.3(4)
108.9(5)
112.3(5)
110.2(4)
108.8(4)
107.4(4)
109.3(4)
108.8(4)
170.3(2)
177.5(2)
N(2C)–Ni(1)–N(2)
O(1)–Ni(1)–O(1C)
N(2C)–Ni(1)–O(1C)
N(2)–Ni(1)–O(1C)
N(2B)–Ni(2)–N(2D)
N(2B)–Ni(2)–O(1D)
N(2D)–Ni(2)–O(1D)
N(2B)–Ni(2)–O(1B)
N(2D)–Ni(2)–O(1B)
O(1D)–Ni(2)–O(1B)
O(11)–S(1)–O(13)
Symm. equiv. Cu structure: $1 = x, y Ϫ 1, z; $2 = 1 Ϫx, Ϫy, Ϫz; $3 = 1 Ϫ x, 1 Ϫ y, Ϫz; $4 = 1 Ϫ x, 2 Ϫ y, Ϫ z. Symm. equiv. Ni structure: $1 = x Ϫ 1,
y, z.
Fig. 2 The structure of the centrosymmetric [Cu(3a)(SO )(H O)]
2
4
2
unit. Molecules of solvation and H-atoms attached to C-atoms have
been removed for clarity.
Fig. 4 Side-on and end-on views of the [Cu(3a)(SO )(H O)] unit.
4
2
2
Note the hydrogen boding arrangement of the sulfate and water
molecules with the ammonium nitrogens, and the long alkyl chains
above and below the macrocyclic plane. The coordinated nickel
structure is very similar. Molecules of solvation and H-atoms attached
to C-atoms have been removed for clarity.
H-bonding (distances g and h) between the ammonium nitro-
gen atoms and the sulfate ions and also weak H-bonding from
the water molecules to the ammonium nitrogen atoms (dis-
tances e and f). On the basis of the distances shown in Fig. 5 the
sulfate ion appears to be equally strongly bonded to each of its
neighbouring alkylammonium groups and water molecules.
Fig. 3 The structure of the pseudo-centrosymmetric [Ni(3a)(SO )-
2 2
4
(
H O)] unit. Molecules of solvation and H-atoms attached to C-atoms
have been removed for clarity.
D a l t o n T r a n s . , 2 0 0 4 , 3 1 3 – 3 1 8
317

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dimers opens up the possibility of transporting the very hydro-
2
Ϫ
Ϫ
philic SO4 and HSO4 ions as dimers, the geometries of which
13
are well defined, see for example the recently published studies
of [Et N][HSO ] and references therein.
4
4
Acknowledgements
We thank Mr J. Millar and Mr W. Kerr for obtaining
NMR spectra, Mr A. Taylor and Mr H. Mackenzie for mass
spectra and Ms S Djurdjevic for elemental analysis. We acknow-
ledge the EPSRC for support under grant GR/R21653 and a
studentship (SGG).
Fig. 5 Schematic diagram showing the hydrogen bonding interactions
of the macrocycle dimers with corresponding contact distances.
Notes and references
Cu
Ni
1
W. G. Davenport, Copper 99–Cobre 99 International Conference,
Pheonix, Arizona, 1999, p. 55; G. A. Kordosky, International Solvent
Extraction Conference, Cape Town, South Africa, Mar. 17–21, 2002,
p. 853; H. Singh and C. K. Gupta, Miner. Process. Extr. Metall. Rev.,
2000, 21, 307; J. Szymanowski, Hydroxyoximes and Copper
Hydrometallurgy, CRC Press, Boca Raton, FL, 1993.
a
b
c
d
e
f
g
h
i
5.909(7)
7.364(6)
3.764(5)
3.622(4)
3.518(7)
3.221(7)
2.695(6)
2.751(6)
2.739(7)
2.786(7)
6.057(11)
7.292(11)
3.685(7)
6.009(10)
7.299(10)
3.587(8)
3.706(8)
3.640(8)
3.170(11)
3.783(12)
2.776(10)
2.694(10)
2.720(12)
2.765(10)
3.641(11)
3.220(11)
2.701(10)
2.747(10)
2.785(10)
2.717(10)
2 N. Akkus, J. C. Campbell, J. Davidson, D. K. Henderson, H. A.
Miller, A. Parkin, S. Parsons, P. G. Plieger, R. M. Swart, P. A. Tasker
and L. C. West, Dalton Trans., 2003, 1932.
3 K. H. Soldenhoff, Solvent Extr. Ion Exch., 1987, 5, 833; R. F.
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Mater. Soc., 1991, 43, 51; G. Cote, A. Jakubiak, D. Bauer,
J. Szymanowski, B. Mokili and C. Poitrenaud, Solvent Extr. Ion
Exch., 1994, 12, 99; J. Szymanowski, J. Radioanal. Nucl. Chem. Art.,
j
These interactions lead to an approximately linear inter-
molecular N(6) ؒ ؒ ؒ S(1) ؒ ؒ ؒ N(6) arrangement optimising the
electrostatic component to bonding. The short intra-molecular
N(6) ؒ ؒ ؒ N(6) separations (distances a in Fig. 5 of 5.909–6.067
Å) make it impossible for a sulfate ion to be included within the
macrocyclic cavity defined by the hexamethylene strap as
implied in Fig. 1, but point the way towards the design of
systems which could incorporate sulfate dimers within the
cavity of a related 2 ϩ 2 macrocycle with longer, functionalised,
straps between the benzylamino nitrogen atoms (see Fig. 6).
1
996, 208, 183; A. Jakubiak, A. Borowiak-Resterna and
J. Szymanowski, Hydrometallurgy, Proceedings of the International
Conference, 3rd, Kunming, China, Nov. 3–5, 1998, p. 430.
4
V. G. Papangelakis, D. Georgiou and D. H. Rubisov, Iron Control
and Disposal, Proceedings of the International Symposium on Iron
Control in Hydrometallurgy, 2nd, Ottawa, Oct. 20–23, 1996, p. 263;
E. Krause, A. Singhal, B. C. Blakey, V. G. Papangelakis and
D. Georgiou, Hydrometallurgy and Refining of Nickel and Cobalt,
Annual Hydrometallurgy Meeting of CIM, 27th, Sudbury, Ont., Aug.
17–20, 1997, p. 441; D. Georgiou and V. G. Papangelakis,
Hydrometallurgy, 1998, 49, 23; D. H. Rubisov and V. G.
Papangelakis, EPD Congress 1999, Proceedings of Sessions and
Symposia held at the TMS Annual Meeting, San Diego, Feb. 28–Mar.
4
, 1999, p. 535; D. H. Rubisov and V. G. Papangelakis,
Hydrometallurgy, 2000, 58, 89; D. H. Rubisov and V. G.
Papangelakis, Hydrometallurgy, 2000, 58, 13; R. Mayze, Copper,
Cobalt, Nickel and Zinc Recovery, Conference, Victoria Falls,
Zimbabwe, July 16–18, 2001, F1.
5
F. Hofmeister, Arch. Exp. Pathol. Pharmakol. (Leipzig), 1888, 24,
247; K. D. Collins and M. W. Washabaugh, Quart. Rev. Biophys.,
1985, 18, 323; K. Kavallieratos and B. A. Moyer, Chem. Commun.,
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T. Vercouter and B. A. Moyer, Chem. Commun., 2003, 2248.
S. G. Galbraith, P. G. Plieger and P. A. Tasker, Chem. Commun.,
6
7
8
9
2
002, 2662.
G. M. Sheldrick, SHELXTL Programs for Crystal Structure
Analysis (Release 97-2), University of Göttingen, Germany, 1998.
P. Van der Sluis and A. L. Spek, Acta Crystallogr., Sect. A, 1990, 46,
1
94.
R. Aldred, R. Johnston, D. Levin and J. Neilan, J. Chem. Soc.,
Perkin Trans. 1, 1994, 1823.
Fig. 6 Inclusion of sulfate dimers within a tetracationic macrocycle
cavity.
1
0 H. Adams, N. A. Bailey, D. E. Fenton and G. Papageorgiou,
J. Chem. Soc., Dalton Trans., 1995, 1883.
1
1 H. A. Miller, N. Laing, S. Parsons, A. Parkin, P. A. Tasker and
D. J. White, J. Chem. Soc., Dalton Trans., 2000, 3773; M. Kondo,
K. Nabari, T. Horiba, Y. Irie, M. K. Kabir, R. P. Sarker, E. Shimizu,
Y. Shimizu and Y. Fuwa, Inorg. Chem. Commun., 2003, 6, 154.
2 K. Inoue, M. Ohba and H. Okawa, Bull. Chem. Soc. Jpn., 2002, 75,
Conclusion
Formation of metal sulfate complexes in the zwitterionic forms
of the macrocyclic ligands 3a and 3b occurs in pH regions
which allow loading and stripping under convenient operating
conditions. The formation of assemblies with water-bridged
1
1
9
9.
3 M. E. Light, P. A. Gale and M. B. Hursthouse, Acta Crystallogr.,
Sect. E, 2001, 57, o705.
3
18
D a l t o n T r a n s . , 2 0 0 4 , 3 1 3 – 3 1 8