Preorganized tridentate analogues of mixed hydroxyoxime/carboxylate nickel extractants

Article abstract of DOI:10.1039/c5dt04288g

A series of 22 tridentate unsaturated mono-anionic ligands having the atom-sequence Y-CC-NCH-CC-Z^-1, with Y = N, O, or S and Z = O or S, has been studied to establish whether this backbone could be used to develop strong solvent extractants for nickel(ii) which will preferably also show a high selectivity over iron(iii) in the pH-dependent process: 2LH_org + NiSO₄ [(L)₂Ni]_org + H₂SO₄. All are capable of forming octahedral [(L)₂Ni] complexes with a mer-arrangement of the YNZ^-1 donor set. X-ray crystal structures of three salicylaldimine proligands derived from 3-bromo-5-t-butyl-2-hydroxybenzaldehyde show these to have pre-organised donor sets in which the three donors are held in an approximately orthogonal arrangement by intramolecular hydrogen bonds. The tautomers observed are dependent on the nature of the Y atom and the extent to which it is favourable for this to form a bonding interaction with the acidic hydrogen atom on the salicylaldimine unit. X-ray crystal structure determinations of seven of the [(L)₂Ni] complexes show these to have significantly distorted octahedral coordination geometries which partly account for the proligands proving to be fairly weak Ni-extractants. DFT calculations show that extractant strength is dependent on a combination of the binding strength of the YNZ^-1 donor set to the nickel ion and on the ease of deprotonation of the extractant. On this basis 3-nitro-4-t-octyl-6-(quinolin-8-imino)phenol is predicted, and is found, to be the strongest Ni-extractant. The extractants have low hydrolytic stability, reverting to their aldehyde precursors when solutions in water-immiscible solvents are contacted with aqueous acid, making them poor candidates for development as reagents for nickel recovery based on pH-swing processes of the type shown above.

Full text of DOI:10.1039/c5dt04288g

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Preorganized tridentate analogues of mixed
hydroxyoxime/carboxylate nickel extractants†
Cite this: DOI: 10.1039/c5dt04288g
James W. Roebuck,a Jennifer R. Turkington,a David M. Rogers,a Philip J. Bailey,a
Violina Griffin,b Adam J. Fischmann,b Gary S. Nichol,a Max Pelser,c Simon Parsonsa
and Peter A. Tasker*a
A series of 22 tridentate unsaturated mono-anionic ligands having the atom-sequence Y–CvC–NvCH–
CvC–Z−1, with Y = N, O, or S and Z = O or S, has been studied to establish whether this backbone could
be used to develop strong solvent extractants for nickel(II) which will preferably also show a high selectivity
over iron(III) in the pH-dependent process: 2LHorg + NiSO4 ⇌ [(L)2Ni]org + H2SO4. All are capable of
forming octahedral [(L)2Ni] complexes with a mer-arrangement of the YNZ−1 donor set. X-ray crystal
structures of three salicylaldimine proligands derived from 3-bromo-5-t-butyl-2-hydroxybenzaldehyde
show these to have pre-organised donor sets in which the three donors are held in an approximately
orthogonal arrangement by intramolecular hydrogen bonds. The tautomers observed are dependent on
the nature of the Y atom and the extent to which it is favourable for this to form a bonding interaction
with the acidic hydrogen atom on the salicylaldimine unit. X-ray crystal structure determinations of seven
of the [(L)2Ni] complexes show these to have significantly distorted octahedral coordination geometries
which partly account for the proligands proving to be fairly weak Ni-extractants. DFT calculations show
that extractant strength is dependent on a combination of the binding strength of the YNZ−1 donor set to
the nickel ion and on the ease of deprotonation of the extractant. On this basis 3-nitro-4-t-octyl-6-
(quinolin-8-imino)phenol is predicted, and is found, to be the strongest Ni-extractant. The extractants
have low hydrolytic stability, reverting to their aldehyde precursors when solutions in water-immiscible
solvents are contacted with aqueous acid, making them poor candidates for development as reagents for
nickel recovery based on pH-swing processes of the type shown above.
Received 31st October 2015,
Accepted 11th January 2016
DOI: 10.1039/c5dt04288g
www.rsc.org/dalton
downstream recovery of metals is inefficient and is currently
preceded by raising the pH to precipitate iron(III) oxyhydroxide
Introduction
The increasing demand for nickel arising from its expanding waste.5 The ARFe (Anglo Research Iron) process2,6 generates
uses in stainless steel, electroplating and rechargeable bat- hematite as a product of commercial value and provides
teries1 is providing an incentive for the mining industry to options for recovery of cobalt and nickel from acidic sulfate
develop more efficient processes for its recovery from laterite streams. This paper considers how new solvent extractants
ores as a consequence of the declining grade and the dearth of might be designed to concentrate and separate nickel from
discoveries of sulfidic ores,2 which are processed using con- such streams, using the pH-dependent equilibrium shown in
ventional pyrometallurgy.3 Low grade nickelferrous laterite eqn (1).
ores, which contain high concentrations of water and iron, are
more abundant and can be processed by hydrometallurgical
methods following High Pressure Acid Leaching (HPAL),4 but
M
2þ þ 2LHorg Ð ½L2Mꢀorg þ 2Hþ
ð1Þ
Cyanex® 301, a dialkyldithiophosphinic acid, has been
shown to be effective for the direct recovery of nickel and
cobalt without pH adjustment.7 Due to reduced stability in the
presence of oxidizing agents, Cyanex 301 must be used under
an inert atmosphere in a closed system, achieved at Goro with
the use of very large pulsed columns.8,9 The multi-branched
decanoic acid, Versatic® 10, has been tested extensively for
nickel recovery from laterite streams but is a weak extractant,10
requiring several contacts with pH-adjustment to displace the
aEaStCHEM School of Chemistry, University of Edinburgh, David Brewster Road,
Edinburgh, Scotland EH9 3FJ, UK. E-mail: p.a.tasker@ed.ac.uk
bCytec Industries, 1937 West Main Street, Stamford, Connecticut 06902, USA
cAnglo American, 8 Schonland Street, Theta, Johannesburg, 2091, South Africa
†Electronic supplementary information (ESI) available. CCDC 1433304, 1410169,
1410168, 1448162, 1433302, 1433538, 1410171, 1430069, 1433306 and 1430068.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c5dt04288g
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equilibrium shown in eqn (1).‡ It also has a high solubility in selected for testing as these should all be readily deprotonated
aqueous streams, requiring an additional extractant-recovery to give bis mer-Ni2+-complexes with approximately planar
step.11 The use of synergistic combinations of an organic acid YNZ− donor sets favoured by the conjugation in the ligands
and a neutral extractant has been shown to enhance the (Fig. 3). If extractants of this type were to be used to recover
efficiency of nickel recovery greatly, but so far none of these nickel efficiently from laterite leach streams it would be very
have been implemented in commercial operations.12 One of beneficial if they were able to transport nickel selectively into
the best understood synergistic systems contains a mixture of the organic phase, rejecting iron, because this would remove
a carboxylic acid such as Versatic® 10 and an α-hydroxyoxime, the need for precipitation of the latter. In the presence of iron(III)
such as LIX® 63 (see Fig. 1).13,14 The extraction equilibrium is extractants should give cationic [FeL2]+ complexes, of the type
thought to involve the deprotonation of the carboxylic acid, characterised by Brashoveanu et al.,15 that will require a
rather than the hydroxyoxime as in eqn (2).
counterion to balance the charge on the complex. Such salts
are likely to have low solubility in the hydrocarbon diluents
used in solvent extraction and this will result in preferential
recovery of nickel(II) over iron(III).
Ni2þ þ 2RCOOHorg þ 2oxorg Ð ½NiðRCOOꢁoxÞ2ꢀorg þ 2Hþ ð2Þ
An X-ray structure of the model system shown in Fig. 1b,13
contains a pseudooctahedral, charge-neutral, nickel complex
with two carboxylate and two neutral hydroxyoxime ligands.
Hydrogen bonding between the ligands generates a planar
pseudo-tridentate [NO2]− donor set which forms a sequence of
5/7-membered chelate rings (Fig. 1c). The N–OH group of the
oxime unit acts as a hydrogen bond donor to the carboxylate
rather than the α-hydroxy (C–OH) group which would give the
5/6-membered chelate ring sequence shown in Fig. 1d.
This paper considers whether single hydrophobic ligand
molecules with comparable tridentate donor sets can generate
sufficiently stable and soluble complexes to extract nickel from
more acidic solutions than the LIX 63/Versatic system. The
salicylaldimines, L1H–L19H, and the acylpyrazolone- and
acylthiopyrazolone-imines L20H–L22H shown in Fig. 2 were
Results and discussion
Synthesis and characterization of the proligands
Three 3-substituted-5-tert-butyl-salicylaldyde precursors16 (2–4;
for structures see ESI†) for L1H–L3H, L8H–L10H and L12H–
L14H were condensed with 8-aminoquinoline or with 2-meth-
oxyaniline or 2-thiomethoxyaniline to yield the proligands.
Two 3-substituted-5-tert-octyl-salicylaldyde precursors16 (5 and
6) were used to prepare L4H, L5H, L11H and L15H in an
attempt to enhance the solubility of the extractants and their
Fig. 2 Tridentate salicylaldimine (L1H–L19H), acylpyrazoloneimine
(L20H and L21H) and acyl thiopyrazoloneimine (L22H) proligands.
Fig. 1 The structure of LIX63 (a); the X-ray crystal structure of [Ni(nPr–
hydroxyoxime)2(iPr–CO2)2] (b);13 the 7/5 membered ring H-bonding
arrangement observed in [Ni(nPr–hydroxyoxime)2(iPr–CO2)2] (c); and the
alternative 5/6 membered ring arrangement (d).
‡Metal extraction by hydrophobic carboxylic acids often involves more compli-
cated stoichiometries than those implied in eqn (1). When the extractant is
present in excess, usually both carboxylate anions and neutral carboxylic acid
molecules are coordinated to the metal ion. These arrangements allow the
hydrogen bonds, which are favoured in the metal free extractant, to be retained
in the supramolecular configuration of the extracted.11
Fig. 3 The formation of a neutral octahedral nickel(II) complex by mer-
forms of the monoanionic tridentate ligands formed by deprotonation
of L1H–L22H.
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nickel(II) complex. The acylpyrazolone imines L20H and L21H the pre-organisation of the donor atoms to generate a planar
were obtained from similar Schiff base condensations using 6/5 chelate sequence. The extent to which the resulting donor
1-phenyl-3-methyl-5-oxo-4,5-dihydro-1H-pyrazole-4-carbaldehyde set is orthogonal is important if the ligands are to define a
(1) or 4-hexanoyl-3-methyl-1-phenyl-4,5-dihydro-1H-pyrazol-5- regular octahedral geometry about a nickel(II) ion.
one (7). L22H was obtained by the two step, one pot, reaction
Data presented in Table 1 define the geometries of the pre-
of 1-(5-chloro-3-methyl-1-phenyl-4,5-dihydro-1H-pyrazol-4-yl) organised donor atoms in the crystal structures of L2H, L9H
ethan-1-one shown in Scheme 1 using conditions developed by and L13H. The structure of L9H has three crystallographically
Smith et al.17
independent molecules per asymmetric unit (Fig. 5). Molecule
b is the most nearly planar with the linking carbon atoms
showing ≤0.04 Å deviations from the plane defined by the Y1–
N4–Z8 donor set of the proligand. Molecules a and c are less
planar with larger (maximum) deviations of the linking carbon
atoms from the Y1–N4–Z8 plane of 0.85 and 0.65 Å respecti-
vely, and have similar conformations with the five- and six-
membered potential chelating units being displaced to oppo-
site sides of the Y1–N4–Z8 plane. The phenolic imine N4–C5–
C6–C7–Z8 units show a larger inclination from this plane (25
and 40° in molecules a and c) than the methoxy imine N4–C3–
C2–Y1 units for which the least squares planes are inclined by
16 and 27°. A difference map calculated during the solution of
the structure of L9H indicated that the acidic hydrogen atom
in molecule b is disordered between the phenolic oxygen,
O(8b), and the imine nitrogen, N(4b), and refinement showed
a 7 : 3 occupancy of these sites which correspond to the tauto-
mers shown at the bottom of Fig. 5. The solid state structures
of L2H and L13H both contain two crystallographically inde-
pendent molecules per unit cell. Only molecules a are shown
in Fig. 6. Inter-molecular contact distances are listed in
Table 2. The thioether-substituted proligand, L13H, exists
exclusively as the conventional phenolic imine whereas L2H
has the acidic hydrogen atom attached to N4 where it can form
comparably strong hydrogen bonds to the quinone oxygen and
quinoline nitrogen atoms, with contact distances of 2.6 and
2.7 Å respectively. This bifurcated hydrogen bonding relation-
ship favours a highly planar conformation of the proligand.
Proligand structures
The conjugation through the imine moiety and intramolecular
hydrogen bonding in the salicylaldimine (Fig. 4a) and the acyl-
pyrazoloneimine proligands (Fig. 4b) was expected to favour
Scheme 1 The one-pot synthesis of L22H.
Fig. 4 Salicylaldimine and acylpyrazolone imine proligand structures
(a and b) with approximately orthogonal Z8, N4 and Y1 donor atoms,
using the atom labelling in X-ray structure determinations below and (c) the
Y1–N4–Z8 plane, and the Y1–C2–C3–N4 and N4–C5–C6–C7–Z8 least
squares planes used in structure analysis (Table 1).
Table 1 A comparison of the geometries of the Br-substituted salicylaldimine proligands, L2H, L9H and L13H
L2H (YvN, ZvO)
L9H (YvO, ZvO)
L13H (YvS, ZvO)
Ligand
Molecule
a
b
a
b
c
a
b
Angles/° between donor atoms and Z8–Y1 midpoint (H)a
Y1–M–N8
Z8–M–N4
Y1–N4–Z8
91.84
88.16
84.70
92.12
87.88
83.30
91.36
88.64
95.34
91.12
88.88
79.79
91.36
88.64
95.84
95.16
84.39
77.61
95.86
84.14
78.17
Angle/° between the Z, N8, Y and the N8, C7, C6, C1, Z least squares planesa
10.37(5) 9.51(8) 25.07(7)
γ
0.96(8)
0.04(1)
39.89(8)
26.62(1)
13.08(2)
4.97(2)
13.93(2)
2.64(2)
Angle/° between the Z, N8, Y and N8, C9, C14, Y least squares planesa
δ
3.57(6)
3.68(8)
15.85(9)
Displacement/Å from the Z, N8, Y least squares planea
C7
C6
C5
C3
C2
0.255(1)
0.357(1)
0.220(1)
0.052(1)
0.102(1)
0.198(1)
0.335(1)
0.218(1)
0.001(2)
−0.002(2)
0.600(2)
0.858(2)
0.511(2)
−0.328(2)
−0.350(2)
−0.003(1)
0.571(2)
0.326(2)
−0.050(2)
0.002(2)
−0.658(2)
−0.880(2)
−0.470(2)
0.443(2)
0.294(3)
0.451(3)
0.327(3)
−0.120(4)
−0.126(4)
0.358(3)
0.479(3)
0.303(3)
−0.062(3)
−0.068(3)
0.475(2)
a Atom labelling scheme is provided in Fig. 4.
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The linking carbon atoms show smaller deviations from the
Y1–N4–Z8 plane (≤0.35 Å) than in L9H. L13H shows very weak
hydrogen bonding between the phenolic protons and the
sulfur atoms of the thioether groups with long contacts
(ca. 3.4 Å). The crystal structures of the proligands suggest that
the quinoline-containing L2H has the most nearly planar pre-
organised (mer) arrangement of donor atoms as judged by the
angles and distances listed in Table 1.
Nickel complex structures
The atom labelling scheme for the X-ray crystal structures of
[(L2)2Ni], [(L3)2Ni], [(L9)2Ni], [(L10)2Ni], [(L13)2Ni], [(L14)2Ni]
and [(L22)2Ni] is shown in Fig. 4 and 7. All contain two mono-
anionic ligands with mer arrangements of the tridentate [Y1–
N4–Z8]− donor sets, where Z = O or S, and Y = O or N or S. A
comparison of the distorted octahedral geometries is provided
in Table 3. The salicylaldimine nickel complexes (see for
example [(L2)2Ni] and [(L9)2Ni] in Fig. 7) have intra-ligand cis-
angles in the range 75.05 to 83.76° for their 5-membered che-
lating units and 87.76 to 96.54° for their 6-membered chelates.
The trans-angles range between 161.02 and 171.30° and the
angles (α) between the Y1–N4–Z8 planes vary between 87.47
and 89.21°. Intra-ligand trans-angles in the thiopyrazolone
complex [(L22)2Ni] (see Fig. 7) and the pseudo-tridentate
complex, [Ni(nPr–hydroxyoxime)2(iPr–COOH)2] b) in Fig. 1, are
closer to 180° (between 176.55 and 177.71°).
Fig. 5 The structures of the three crystallographically independent
molecules of L9H, (a–c). The disorder of the acidic protons H(8) and H
(4) in the molecule b corresponds to a 7 : 3 occupancy of the phenol/
quinone tautomers shown.
Fig. 6 The X-ray crystal structures of L2H and L13H.
Table 2 Intra-molecular contact distances for the Br-substituted sali-
cylaldimine proligands, L2H, L9H and L13
L2H hydrogen bond (D⋯A) distances/Å
N(4a)–H(4a)⋯O(8a)
N(4a)–H(4a)⋯N(1a)
2.603(1)
2.688(1)
N(4b)–H(4b)⋯O(8b)
N(4b)–H(4b)⋯N(1b)
2.579(1)
2.676(1)
L9H hydrogen bond (D⋯A) distances/Å
O(8a)–H(8a)⋯N(4a)
O(8b)–H(8b)⋯O(1b)
O(8a)–H(8a)⋯O(1a)
2.579(2)
3.280(2)
3.708(2)
O(8b)–H(8b)⋯N(4b)
O(8c)–H(8c)⋯N(4c)
O(8c)–H(8c)⋯O(1c)
2.532(2)
2.567(2)
3.856(2)
Fig. 7 The atom labelling system for nickel(II) complexes with two
monoanionic ligands a and b coordinating via donor atoms Y1, N4 and
Z8 which have linking carbon atoms (C2 and C3 and C5, C6 and C7) and
the X-ray crystal structures of [(L22)2Ni] (top right), [(L2)2Ni] (bottom left)
and [(L9)2Ni] (bottom right).
L13H hydrogen bond (D⋯A) distances/Å
O(8a)–H(8a)⋯N(4a)
O(8b)–H(8b)⋯N(4b)
2.566(4)
2.575(4)
O(8a)–H(8a)⋯S(1a)
O(8b)–H(8b)⋯S(1b)
3.384(3)
3.444(3)
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Table 3 A comparison of the coordination geometries of the salicylyaldimine and pyrazolone imine nickel complexes
[Ni(nPr–
hydroxyoxime)2-
(iPr–CO2)2]13
(YvO, ZvO)
Structure [(L2)2Ni]
(YvN, ZvO)
[(L3)2Ni]
(YvN, ZvO)
[(L9)2Ni]
(YvO, ZvO) (YvO, ZvO)
[(L10)2Ni]
[(L13)2Ni]
(YvS, ZvO)
[(L14)2Ni]
(YvS, ZvO)
[(L22)2Ni]
(YvN, ZvS)
Ligand
a
b
ab
bb
a
b
aubc
a
b
a
b
a
b
a
b
a
b
Bond lengths/Å
Ni–Y
Ni–N8
Ni–Z
2.089(2) 2.114(2) 2.107(2) 2.146(2) 2.091(1) 2.091(1) 2.216(2)
2.030(2) 2.040(2) 2.030(2) 2.027(2) 2.038(2) 2.041(2) 2.015(2)
2.024(1) 2.047(1) 2.046(1) 2.068(1) 2.054(1) 2.053(1) 1.964(1)
2.181(2) 2.177(2) 2.491(1) 2.412(1) 2.453(1) 2.425(1) 2.091(2) 2.084(2) 2.032(2) 2.026(1)
2.004(2) 2.006(3) 2.031(2) 2.041(2) 2.027(2) 2.025(2) 2.080(2) 2.079(2) 2.061(2) 2.057(2)
1.988(2) 1.984(2) 2.017(2) 1.988(2) 2.020(3) 1.989(2) 2.385(1) 2.370(2) 2.076(2) 2.083(2)
Interatomic bond angles/°
Y–Ni–N8 80.84(7) 80.23(7) 79.97(7) 79.55(7) 80.47(6) 80.49(6) 75.05(7)
N8–Ni–Z 90.45(7) 90.06(7) 90.04(6) 90.28(6) 89.97(6) 89.98(6) 90.92(7)
77.89(10) 77.89(10) 81.95(7) 83.76(7) 82.90(9) 83.52(6) 80.29(6) 80.72(6) 104.65(7) 103.93(7)
92.93(10) 95.95(10) 87.83(9) 88.76(9) 87.76(10) 87.96(8) 96.32(4) 96.54(5) 76.96(7) 77.04(7)
166.48(9) 164.46(9) 169.69(6) 171.02(7) 170.29(5) 170.65(6) 176.55(5) 177.20(5) 177.71(7) 177.15(7)
Y–Ni–Z
171.30(7) 167.66(7) 169.86(6) 169.76(6) 170.43(6) 170.46(6) 161.02(6)
Octahedral distortion factora
σoct 6.1
5.5
4.8
9.6
7.6
5.1
5.0
5.8
7.7
Angle/° between the two Y1–N4–Z8 planes
89.19(11) 88.35(11)
α
89.10(9)
89.21(11)
87.47(16)
88.23(13)
87.52(13)
88.02(8)
89.03(9)
Angle/° between the Y1–C2–C3–N4–Ni and N4–C5–C6–C7–Z8–Ni least squares planes
9.48(9) 9.02(8) 8.14(8) 11.14(8) 6.90(7) 6.92(7) 21.95(8)
β
7.85(12) 16.50(12) 27.52(10) 23.01(10) 26.29(8) 24.86(12) 5.24(6)
10.33(7) 3.6(2)
8.4(2)
Angle/° between the Y1–N4–Z8 plane and the N4–C5–C6–C7–Z8 least squares plane
9.21(11) 14.69(11) 12.32(11) 16.46(11) 10.57(9) 10.48(9) 26.59(11)
8.12(16) 19.95(16) 36.93(13) 31.08(14) 36.24(12) 32.26(16) 13.83(8) 1.39(8)
1.69(12) 9.12(13)
Angle/° between the Y1–N4–Z8 plane and the Y1–C2–C3–N4 least squares plane
δ
4.52(13) 3.49(14) 2.63(13) 1.33(12) 1.40(11) 1.59(11) 21.17(13)
6.60(19) 13.27(19) 23.25(14) 21.25(15) 20.92(12) 24.29(18) 8.85(11) 7.87(10) 6.1(3)
12.3(3)
a The bond angle variance, σoct 18 is the average difference of the twelve cis-angles from the idealised value of 90° as shown in eqn (3). b This complex has two crystallographically independent molecules per
,
asymmetric unit. c The two ligands in [(L9)2Ni] are related by a crystallographic C2 axis through the Ni atom.

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Distortion of the ligands from planarity will adversely affect
conjugation and decrease the stability of their nickel com-
plexes. Planarity can be compared using the angle, β, between
the least squares planes of the 5-membered (Ni–Y1–C2–C3–N4)
and 6-membered (Ni–N4–C5–C6–C7–Z8) chelate rings. The
complexes of L2H, L3H and L22H, which contain iminoquino-
line units, have the most nearly flat mer-planes with β values
between 5.24 and 11.14°. In Table 3 the angle γ, between the
least squares plane of N4–C5–C6–C7–Z8 and the plane of the
three donor atoms, measures the deviation of the phenolic/
thiopyrazolone moiety from the donor plane and δ is the angle
between the least squares planes of the imine moiety, Y1–C2–
C3–N4, and the donor atoms, Y1–N4–Z8.
Deviation from regular octahedral geometry will also result
from variation of the donor atoms Y and Z. The thioether
(Y1 = S) bonds to nickel are longer (2.41–2.49 Å) than those
from the quinoline (Y1 = N) and methoxy (Y1 = O) groups
(2.08–2.14 and 2.17–2.21 Å respectively), leading to [(L13)2Ni]
and [(L14)2Ni] having smaller bite angles in their 6-membered
chelate rings (N4–Ni1–Z8 falls in the range 87.76–88.76°)
than in the other salicylaldimine complexes. This forces the
phenolic moiety to twist out of the plane of the donor atoms
(γ is between 31.08 and 36.93°). There are no unusual
features associated with the packing of the complexes in the
solid state structures or with the inclusion of solvent
molecules in the lattices. Further information is provided in
the ESI.†
The crystal structures of the nickel(II) complexes provide
information on the geometry around the nickel(II) cation and
conformation of the ligands. Deviations from regular octa-
hedral geometry, as determined by the bond angle variance
(σoct)18 according to eqn (3), and distortion of the ligands
from a planar arrangement with the consequent loss of conju-
gation are expected to lower the stability of the complexes of
the different extractants.
Fig. 8 The pH-dependence of nickel-loading by 0.005 M CHCl3 solu-
tions of L4H, L5H, L11H and L15H from an equal volume of aqueous
0.01 M NiSO4. 100% loading is equivalent to that expected for formation
of a 1 : 2 complex [Ni(L)2].
theoretical nickel maximum value, assuming the 1 : 2 (metal :
ligand) ratio in the extracted [L2Ni] species, could not be
achieved at pH ≥ 4.5 because nickel hydroxides were precipi-
tated. L15H and L11H are too weak to obtain a value for pH0.5
(the pH at which 50% of the theoretical loading is achieved),
but it is possible to define the order of extractant strength as:
L5H > L4H > L15H > L11H. L5H and L4H have identical N2O−
donor sets but the incorporation of a nitro group ortho to the
phenol oxygen atom in L5H increases the extractant strength
by one pH unit based on pH0.5 values, which correspond to
approximately to an order of magnitude increase in the extrac-
tion distribution coefficient. The origin of this is discussed
below. L15H and L11H also have nitro groups ortho to the
phenol OH group but different Y donor atoms, a thioether and
an ether. The NSO− donor set of L15H can load nickel into the
−
organic phase at a lower pH than L11H, which has a NO2
12
X
2
donor set, but both are significantly weaker than the quinoline
containing reagents, L4H and L5H.
σoct ¼
ðσi ꢂ 90Þ
ð3Þ
i¼1
Imine extractants have low stability under acidic conditions
and readily hydrolyse to form their component aldehydes and
amines.19 The low hydrolytic stability of the pyrazolone imine
reagents L20H–L22H prohibited conventional studies of the
pH dependence of metal-loading as in Fig. 8. Nevertheless, the
binding preference for uptake of nickel(II) or iron(III) by the
pyrazolone imine reagents could be investigated by mixing a
toluene solution of preformed [(L)2Ni] with aqueous Fe2(SO4)3
(see Fig. 9). ICP-OES analysis of [(L20)2Ni] solutions showed
that nickel transferred to the aqueous phase and iron was
taken up by the toluene solution. 13C NMR analysis of the con-
centrated organic phase showed that extractant L20H had
been hydrolysed to the parent acylpyrazolone. Formation of
the charge neutral [(acypyrazolonate)3Fe] complex is presum-
ably favoured because the hard iron(III) ion is provided with an
The quinoline-containing complexes [(L2)2Ni], [(L9)2Ni] and
[(L22)2Ni] have the most nearly planar ligands and the most
nearly regular octahedral geometry around the nickel(II) centre
as judged by the values listed in Table 3. On this basis the
reagents containing this moiety and solubilising alkyl groups,
L4H, L5H and L20H, may be expected to extractant nickel from
more acidic solutions because they are likely to generate more
stable complexes.
Solvent extraction studies
A strong extractant will displace the pH-dependent equilibrium
in eqn (1) to favour the formation [L2Ni], leading to nickel(II)
extraction into the organic phase at a lower pH. Plots showing
the pH-dependence of nickel(II) uptake from aqueous sulfate
solutions by L4H, L5H, L11H and L15H into chloroform are
presented in Fig. 8. No appreciable nickel extraction was
detected below pH 2 and loading of the organic phase to a
3−
O6 donor set. The thiopyrazolone complex [(L22)2Ni] was
stable under similar conditions and no iron was transported
into the organic phase (Fig. 9). Soft sulfur donor atoms
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ΔEb ¼ ½NicomplexðEtotÞꢀ ꢂ ½2LꢂðEtotÞ þ NicationðEtotÞꢀ þ BSSE
þ 2RT
ð7Þ
In eqn (6) and (7) Etot is made up of the sum of the elec-
tronic (Eel), vibrational (Evib), rotational (Erot) and translational
(Etrans) energies as shown in eqn (8).
Etot ¼ Eel þ Evib þ Erot þ Etrans
ð8Þ
The phenolic tautomer of L6H was shown to be 13 kJ mol−1
lower in energy than the quinone in the gas-phase, and a
similar trend was established for the other salicylaldimines.
The nickel(II) binding energies (ΔEb) and proligand deprotona-
tion energies (ΔEd) were separately calculated, according to
eqn (6) and (7), to determine how structural variations of the
ligand affect these processes. An analysis of the energy terms
in Table 4 provides an insight into the origins of the variations
in formation energies of complexes in the gas phase which cor-
relate with measured strengths of the extractant analogues (see
below).
For the nitro-substituted ligands L7H, L18H and L19H,
values of the calculated formation energies, ΔEf, follow the
trend of measured extractant strengths of the t-octyl homo-
logues (see Fig. 8). Extractant strength varies with the nature of
the donor set in the order N2O− > NSO− > NO2−. The calculated
deprotonation energy (ΔEd) is more favourable for the nitro-
substituted proligand, L7H, than for the unsubstituted ana-
logue, L6H, by 61 kJ mol−1. A similar trend was found for the
other nitro-substituted proligands, L18H and L19H. Conver-
sely, the nickel(II)-binding energies, ΔEb, of the anionic forms
of L6H, L16H and L17H are more favourable than their nitro-
analogues (Table 4). This is consistent with the electron with-
drawing nitro group increasing the acidity of the reagents
whilst decreasing the basicity of their conjugate anions.
The low deprotonation energy of the thioether appears to
arise from the sulfur not being able to form an effective intra-
molecular hydrogen bond with the phenolic hydrogen, making
it easier for the latter to be released. The absence of an intra-
molecular thioether to phenolic hydrogen interaction is
observed in the solid state structure of L13H, contrasting with
the structure of the ether analogue L9H (molecule b) and the
quinoline analogue L2H. Despite the thioether L19H having the
lowest deprotonation energy, the formation energy of its nickel
complex is much less favourable than that of the quinoline ana-
Fig. 9 Loading of nickel(II) and iron(III) when 1.8 mM solutions of [NiL2]
in toluene were contacted with a 1.8 mM aqueous solution of Fe2(SO4)3
(L20 top, L22 bottom). The % metal loading is based on the ligand avail-
able in toluene forming [Ni(L)2] or [Fe(L3)] complexes.
provide a less favourable donor set for iron(III) and the for-
mation of [(L)3Fe] via hydrolysis of the imine can be assumed
to be less thermodynamically favourable.
Computational studies
Previous work has shown that calculated (gas phase) enthal-
pies of formation, ΔEf, for copper complexes formed by a
series of phenolic oximes correlate with their measured
strengths as solvent extractants.20 The energies of the energy-
minimised structures of the proligands L6H, L7H, L16H–
L19H, the related ligands (L−) and their complexes, [L2Ni],
were calculated (see ESI†) and used to evaluate the formation
enthalpy (ΔEf) associated with the process in eqn (4).
2LH þ 2H2O þ Ni2þ Ð ½ðLÞ2Niꢀ þ 2H3Oþ
ð4Þ
To define the origins of the dependence of the variations of
formation enthalpies on the nature of the proligands, the
deprotonation enthalpies (ΔEd) for the reaction in eqn (5) were
calculated using eqn (6),
Table 4 Formation energies, ΔEf, binding energies, ΔEb, and deproto-
nation energies, ΔEd, for proligands L4H, L5H and L16–19H calculated
using B3LYP/6-31G(d,p)
ΔEf
ΔEd
ΔEb
Proligand
(kJ mol−1
)
(kJ mol−1
)
(kJ mol−1
)
LH þ H2O Ð Lꢂ þ H3Oþ
ð5Þ
ΔEd ¼ ½LꢂðEtotÞ þ H3OþðEtotÞꢀ ꢂ ½LHðEtotÞ þ H2OðEtotÞꢀ ð6Þ
L6H (X = H and Y = N)
L16H (X = H and Y = O)
L17H (X = H and Y = S)
L7H (X = NO2 and Y = N)
L18H (X = NO2 and Y = O)
L19H (X = NO2 and Y = S)
−1279
−1219
−1209
−1315
−1243
−1259
784
793
778
723
729
715
−3066
−3028
−2773
−2769
−2710
−2696
and binding enthalpies (ΔEb), the energy released in bringing
together two preformed anions, L−, and a Ni2+ cation in the
gas phase were calculated using the terms in eqn (7);
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logue L7H. This arises because the binding energy of the conju- ditions to synergistic mixtures of Versatic® 10 and
gate anion to nickel(II) is much more favourable for the latter.
α-hydroxyoximes, i.e. at pH > 3, after iron(III) has been precipi-
tated as in the Goro and Bulong processes.5,23 However, the
currently tested versions do not have very high solubility in
hydrocarbon solvents which will limit mass-transport
efficiency. Imine linkages are susceptible to hydrolysis and
Conclusion
Despite being pre-organised by intramolecular hydrogen consequently they would decompose in continuous operations
bonding to provide nearly orthogonal YNZ− donor sets, the under conditions similar to those used for Ni-recovery by Ver-
new tridentate reagents are not strong nickel(II) extractants. satic 10/LIX 63 mixtures.5
X-ray structure determination has shown that they form the
expected 2 : 1 mer-complexes, but with significant deviations
from regular octahedral geometry. Extraction strength is very
dependent on the nature of the donor set, with N-donor quino-
Acknowledgements
We thank the EPSRC, Anglo American and Cytec Industries for
funding PhD studentships for JWR and JRT and EaStCHEM
for access to the Research Computing Facility.
line reagents recovering nickel at a lower pH than their
o-methoxyphenyl and o-thiomethoxyphenyl analogues. The
introduction of electron-withdrawing substituents onto the
chelating unit containing the ionisable proton increases extrac-
tant strength, e.g. the nickel(II)-extraction coefficient of the
o-nitro-substituted reagent, L5H, is more than an order of mag-
nitude greater than that of the unsubstituted analogue L4H.
The donor sets provided by the quinoline-containing
reagents have slightly smaller angular distortions from regular
octahedral geometry than those in the model complex [Ni(nPr–
hydroxyoxime)2(iPr–CO2)2], but the bond lengths to nickel(II)
fall in a slightly wider range. If the regularity of the coordi-
nation geometry assumed by the nickel ion is an important cri-
terion in defining the stability of complexes, favouring their
formation in the extraction equilibrium, there are no obvious
benefits provided by the new tridentate salicylaldimine
reagents. The energies of formation of the nickel complexes in
the gas phase, ΔEf, evaluated by DFT calculations, show a
good correlation with the measured pH0.5 of the extractant
analogues, providing further evidence that this modelling
strategy is an effective tool for predicting the strength of cation
exchange extractants and assessing the different effects of
varying a substituent in a series of analogous compounds.
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Dalton Trans.