Contributions of inner and outer coordination sphere bonding in determining the strength of substituted phenolic pyrazoles as copper extractants

Article abstract of DOI:10.1039/c5dt04055h

Alkyl-substituted phenolic pyrazoles such as 4-methyl-2-[5-(n-octyl)-1H-pyrazol-3-yl]phenol (L2H) are shown to function as Cu-extractants, having similar strength and selectivity over Fe(iii) to 5-nonylsalicylaldoxime which is a component of the commercially used ACORGA solvent extraction reagents. Substitution in the phenol ring of the new extractants has a major effect on their strength, e.g. 2-nitro-4-methyl-6-[5-(2,4,4-trimethylpentyl)-1H-pyrazol-3-yl]phenol (L4H) which has a nitro group ortho to the phenolic hydroxyl group unit and has an extraction distribution coefficient for Cu nearly three orders of magnitude higher than its unsubstituted analogue 4-methyl-6-[5-(2,4,4-trimethylpentyl)-1H-pyrazol-3-yl]phenol (L8H). X-ray structure determinations and density functional theory (DFT) calculations confirm that inter-ligand hydrogen bonding between the pyrazole NH group and the phenolate oxygen atom stabilise the Cu-complexes, giving pseudomacrocyclic structures. Electron-accepting groups ortho to the phenol oxygen atoms buttress the inter-ligand H-bonding, enhancing extractant strength but the effectiveness of this is very dependent on steric factors. The correlation between the calculated energies of formation of copper complexes in the gas phase and the observed strength of comparably substituted reagents in solvent extraction experiments is remarkable. Analysis of the energies of formation suggests that big differences in strength of extractants arise principally from a combination of the effects of the substituents on the ease of deprotonation of the proligands and, for the ortho-substituted ligands, their propensity to buttress inter-ligand hydrogen bonding.

Full text of DOI:10.1039/c5dt04055h

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PAPER
Contributions of inner and outer coordination
sphere bonding in determining the strength of
substituted phenolic pyrazoles as copper
extractants†
Cite this: DOI: 10.1039/c5dt04055h
Mary R. Healy,a James W. Roebuck,a Euan D. Doidge,a Lucy C. Emeleus,a
Philip J. Bailey,a John Campbell,b Adam J. Fischmann,b Jason B. Love,a
Carole A. Morrison,*a Thomas Sassi,b David J. Whitea and Peter A. Tasker*a
Alkyl-substituted phenolic pyrazoles such as 4-methyl-2-[5-(n-octyl)-1H-pyrazol-3-yl]phenol (L2H) are
shown to function as Cu-extractants, having similar strength and selectivity over Fe(III) to 5-nonylsalicyl-
aldoxime which is a component of the commercially used ACORGA® solvent extraction reagents. Substi-
tution in the phenol ring of the new extractants has a major effect on their strength, e.g. 2-nitro-4-
methyl-6-[5-(2,4,4-trimethylpentyl)-1H-pyrazol-3-yl]phenol (L4H) which has a nitro group ortho to the
phenolic hydroxyl group unit and has an extraction distribution coefficient for Cu nearly three orders of
magnitude higher than its unsubstituted analogue 4-methyl-6-[5-(2,4,4-trimethylpentyl)-1H-pyrazol-3-
yl]phenol (L8H). X-ray structure determinations and density functional theory (DFT) calculations confirm
that inter-ligand hydrogen bonding between the pyrazole NH group and the phenolate oxygen atom
stabilise the Cu-complexes, giving pseudomacrocyclic structures. Electron-accepting groups ortho to the
phenol oxygen atoms buttress the inter-ligand H-bonding, enhancing extractant strength but the effec-
tiveness of this is very dependent on steric factors. The correlation between the calculated energies of
formation of copper complexes in the gas phase and the observed strength of comparably substituted
reagents in solvent extraction experiments is remarkable. Analysis of the energies of formation suggests
that big differences in strength of extractants arise principally from a combination of the effects of the
substituents on the ease of deprotonation of the proligands and, for the ortho-substituted ligands, their
propensity to buttress inter-ligand hydrogen bonding.
Received 16th October 2015,
Accepted 22nd December 2015
DOI: 10.1039/c5dt04055h
www.rsc.org/dalton
atoms have been characterised by X-ray crystallography5 and in
several cases their magnetic properties have been studied in
Introduction
Some of the earliest reports of the use of metal complexes of detail.6–8 Six different modes of coordination of the ligand have
phenolic pyrazoles with the generic structure I (Scheme 1) been identified in solid state structures (Fig. S1, ESI†).
concern their use in biomimetic investigations.1 They have also
been shown to have anti-bacterial activity2 and oxorhenium(V)
and dioxomolybdenum(VI) complexes have been used as catalysts
for epoxidation reactions.3,4 The proximity of the oxygen and the
two nitrogen donor atoms in the proligands and their ability to
lose protons from both the phenol and pyrazole moieties to
become dianionic bridging ligands favours the formation of
polynuclear complexes. Clusters containing up to 21 metal
aEaStCHEM School of Chemistry, University of Edinburgh, Edinburgh, EH9 EJJ, UK.
E-mail: c.morrison@ed.ac.uk, p.a.tasker@ed.ac.uk
bCytec Industries, 1937 West Main St, Stamford, CT 06902, USA
†Electronic supplementary information (ESI) available. CCDC 1430501 and
1430499. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c5dt04055h
Scheme 1 Pseudomacrocyclic structures of copper complexes formed
by phenolic pyrazoles I and phenolic oximes II.
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In this paper we focus on the solution chemistry of pheno-
lic pyrazoles and in particular their ability to function as
copper solvent extractants in the pH-dependent equilibrium
shown in eqn (1). Some preliminary results have been reported
in patents.9 Comparisons are made with the structurally
related phenolic oximes (II, Scheme 1) which are used in
hydrometallurgical processes to recover approximately 25% of
the world’s copper,10,11 and have large alkyl substituents to
ensure solubility in hydrocarbon solvents.
Cu2þ þ 2HLorg Ð ½CuL2ꢀorg þ 2Hþ
ð1Þ
A feature of particular interest in this equilibrium is the
extent to which the formation of hydrogen bonds between the
pyrazole N–H group and the oxygen atoms of the phenol
favour complex formation. The 14-membered pseudomacro-
cyclic structure shown in Scheme 1 is very similar to that found
in the commercial phenolic oxime reagents. Introduction of a
substituent ortho to the phenol group in these extractants has
been shown to have a major effect on the strength of copper
extraction, with substituents which can function as hydrogen
bond acceptors buttressing the intermolecular hydrogen-bond
from the oximic proton, contributing to a significant increase
in the stability of the copper complex and hence the extraction
strength.12
In this paper we examine whether similar trends result
from 2-substitution (ortho to the OH group, X in Fig. 1) in phe-
nolic pyrazoles. A comparison with the effects of 4-bromo sub-
stitution (Y = Br in Fig. 1) was undertaken to establish whether
the resulting changes in electronic properties, which influence
the ease of deprotonation of the extractants and the strength
Fig. 2 Proligands L1H–L16H reported in this work. L1H–L9H were used
in the solvent extraction experiments, L10H–L16H in the computational
study, and L10H and L12H in the X-ray diffraction study.
2−
of binding of the resulting N2O2 donor set in the inner
stituents have been used in computational work in order to
reduce the numbers of conformers with very similar energies;
L10H and L12H were also used in X-ray crystal structure
analysis.
coordination sphere, or whether the changes induced in the
hydrogen bonding in the outer coordination sphere are more
important in determining overall extractant strength.
The reagents L1H–L9H in Fig. 2 that carry hydrophobic Y or
Z substituents have been used in solvent extraction experi-
ments. Analogues, L10H–L16H with smaller or no Y or Z sub-
Results and discussion
The proligands L1H–L9H and L12H were prepared from the
appropriate acetophenones (1–6) by conversion to the diketones
(7–18) and subsequent reaction with hydrazine. Details are
provided in the ESI.† The unsubstituted proligand L10H was
also synthesised by the method of Maib and Jermanowska13
via the reaction of chromone and thiosemicarbazide followed
by treatment with formic acid.
Solvent extraction
The methods employed for carrying out extractions and for
analysing results are provided in the ESI.† L1H–L3H, which
lack substituents on the phenol unit, were used to compare
the performance of the new phenolic pyrazoles as copper
extractants with phenolic oximes of the type used in commer-
cial leach/solvent extraction/electrowin operations.10,11 The
aqueous solutions employed for loading the dichloromethane
(DCM) solutions of the extractants had CuSO4, Fe2(SO4)3, and
Fig. 1 Hydrogen-bonding in proligand monomers and dimers and in
copper(II) complexes of substituted phenolic pyrazoles. Outer sphere
interactions in the complex are shown by dashed lines.
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Table 1 The metal concentrations in 0.05 M dichloromethane solutions of extractants L1H–L3H and ACORGA P50 oxime after: (i) contacting with
an equal volume of an aqueous sulphate solution containing 5.00 g L−1 Cu, pH 2.0, (ii) stripping the resulting Cu-loaded DCM solutions with an
equal volume of aqueous H2SO4 (125.0 g L−1) containing 5.00 g L−1 Cu and (iii) contacting with an equal volume of an aqueous sulphate solution
containing Cu, and Fe(III) (both 5.00 g L−1) at pH 2.0
(iii) Cu and Fe content (g L−1) after
(ii) Cu content (g L−1
after stripping with H2SO4(aq)
(125 g L−1, containing
5.00 g L−1 Cu)
)
loading from a mixed
(i) Cu content (g L−1
after loading from
5.00 g L−1 feed
)
Cu/Fe (5.00/5.00 g L−1) feed
Extractant
(substituent Z, Fig. 2)
Cu
Fe
L1H (t-Bu)
L2H (n-C8H17
L3H (C2H5OC(O))
5-Nonyl-salicylaldoxime
(ACORGA P50 oxime)
1.10
1.30
1.10
1.44
0.005
0.005
0.003
0.196
1.1
1.4
—
0.005
0.001
—
)
a
a
1.4
<1
a The stripping experiment was not undertaken for [Cu(L3)2].
H2SO4 concentrations (Table 1) similar to those used in com-
mercial feed solutions in copper recovery.14 After separation
the Cu-loaded DCM solutions were stripped with an aqueous
solution which had a composition similar to those of the
“spent electrolytes” used in industry (see ESI†). The data pre-
sented in Table 1 demonstrate that the new phenolic pyrazoles
have similar strength and selectivity over iron(III) to 5-nonyl-
salicylaldoxime (ACORGA P50 oxime solvent extraction
reagent), one of the active components used in industrial
formulations.10,11
‡
As the data presented in Table 1 indicate that phenolic pyr-
azoles could meet the fundamental requirements of commer-
cial Cu-recovery operations, a more detailed study of the
effects of substituents on extraction strength was undertaken.
The pH-dependence of copper-uptake from aqueous sulphate
solutions by chloroform solutions of L4H–L8H is given in
Fig. 3 together with pH0.5 values, i.e. the equilibrium pH at
which 50% of the available extractant is loaded as [Cu(L)2]. In
accordance with the results shown in Table 1, the pyrazole
extractant L8H having no substituent ortho to the phenol
group is a very slightly weaker extractant than the analogous
oxime extractant, 2-hydroxy-5-tert-octylbenzaldehyde oxime,
which is of the type used in commercial copper recovery
plants10,12 and shows a pH0.5 value of 1.7 under the same con-
ditions as those for Fig. 3.6
Fig. 3 The pH dependence of Cu-loading by 0.005 M chloroform solu-
tions of L4H–L8H from equal volumes of 0.005 M aqueous solution of
CuSO4 and the pH0.5 values of the extractants; 100% Cu-loading is
based on the formation of a CuL2 complex.
The effect of the introduction of substituents ortho to the
phenolic hydroxyl group on the strength of copper extraction is
even greater than that found12 in the oximes, increasing the
distribution ratio by nearly three orders of magnitude for the
nitro-derivative L4H, based on its pH0.5 value being 2.7 units
lower than the unsubstituted extractant L8H. Understanding
the origins of such changes in extraction strength and its
dependence on the nature and location of substituents is
important in designing extractants to meet the requirements
of particular metal recovery processes. Whilst the introduction
of electron withdrawing groups ortho or para to the phenolic
OH group will facilitate deprotonation of the extractant to
generate the coordinating form and favour complex formation,
it will also reduce the basicity of the N2O22− donor set, thereby
diminishing the strength of the interactions with the copper
cation. H-bond acceptors ortho to the phenolic OH group (X in
Fig. 1) could buttress inter-ligand hydrogen bonding, thus
‡In commercial operations, “spent electrolyte” obtained from electrowinning
the copper (CuSO4 + H2O = Cu + H2SO4) is used to strip copper from the loaded
organic phase (CuL2org + H2SO4 = CuSO4 + 2LHorg), generating fresh electrolyte
and regenerating the extractant which is reused.10,11 In this work the sulphuric
acid concentration in the strip solution (125 g L−1) was lower than that in a
typical spent electrolyte,14 ensuring that both the phenolic oxime and the pheno-
lic pyrazole were not fully stripped, allowing a judgement to be made on their
relative ease of stripping.
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favouring the formation of copper complexes, whilst the same
Bromine substitution in [Cu(L12)2] noticeably shortens the
group para to the OH group cannot buttress the H-bonding Cu–O bond in both the experimentally and computationally
but will produce similar electronic effects. In order to evaluate determined structures. This can be attributed to the buttress-
the relative importance of these effects, DFT calculations were ing interaction, labelled ‘a’ in Fig. 4, weakening the other
carried out on L10H–L16H. These are lower mass homologues H-bond, labelled ‘b’, which in turn increases electron density
of the extractants L4H–L9H and do not carry the large solubil- on the oxygen atom, and thus strengthening the Cu–O bond.
ising alkyl groups that can adopt many similar energy confor-
The introduction of the bromine substituent also results in
mations, which would have complicated the task of locating a small lengthening of the Cu–N bond and the NH⋯O contact
the global minimum optimised structures. Details of the distances compared to the non-substituted [Cu(L10)2]
methods used in the computational work are provided in the complex, which is consistent with the bromine atom forming a
ESI.†
The energy-minimised structure of [Cu(L12)2] is compared
weak bonding interaction with the pyrazole N–H group.
There are subtle but important differences between the
with its single crystal X-ray diffraction structure in Scheme 1. inter-ligand H-bonding which is displayed by the copper
Agreement between the calculated and experimental geome- complexes of the phenolic pyrazoles and their phenolic oxime
tries of the coordination spheres is good, taking into account analogues, shown in Fig. 4. The rigidity of the pyrazole ring
that there are strong intermolecular Cu⋯O contacts in the and its internal bond angles (111.0(2)–105.7(2)° in [Cu(L12)2])
solid state structure which lead to the oxygen donor atoms constrain the pyrazole N–H group to point further away
being chemically non-equivalent.
from the phenolate oxygen atom than the oxime O–H donor,
Fig. 4 The X-ray crystal structure of [Cu(L12)2]. The bond lengths/Å and angles/° in the inner coordination sphere are compared with those in the
calculated structure and those in [Cu(L10)2] LAPXUW,15 together with some outer sphere contact distances.
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with the N–H⋯O contact distance (labelled ‘b’ in Fig. 4) is needed for comparison against the experimental data, the
measuring 2.25(5) Å compared with O–H⋯O 1.918(3) Å in the modelling approach adopted is sound.
analogous oxime, bis(3-bromo-5-t-butylsalicylaldimato)copper
The dependence of the calculated energies of formation
(II), ABAXEI.12 The greater flexibility of the oxime group also (ΔGform column 3, Table 2) on the nature of substituents in the
allows its OH group to make a closer contact with the buttres- ortho- and para-position in the phenyl group correlates well
sing bromine substituent than the NH group in [Cu(L12)2]), with the experimentally determined strength of the extractants,
with O⋯Br and N⋯Br being 3.666(6) and 3.769(3)
Å
which decrease in the order o-NO2 > o-Br > o-MeO > p-Br >
respectively.
H. The origins of these variations were analysed by breaking
Whilst the N–H⋯Br contacts (labelled ‘a’ in Fig. 4) are too down the complex-formation reaction into two steps: (i) de-
long to be described as conventional hydrogen bonds, they are protonation of the proligand monomer (eqn (3)) and (ii)
associated with weak bonding interactions as judged by NBO binding of the resulting anion to the Cu2+ ion (eqn (4)).
calculations (see below). Based on the structural evidence it
Cu2þ þ 2LH þ 2H2O Ð ½CuL2ꢀ þ 2H3Oþ
2LH þ 2H2O Ð 2Lꢁ þ 2H3Oþ
Cu2þ þ 2Lꢁ Ð ½CuL2ꢀ
ð2Þ
ð3Þ
ð4Þ
therefore appears that the buttressing contribution made by
single atom electronegative substituents (X in Fig. 1) that act
to stabilise the copper complexes will be smaller in the pyr-
azole extractants, L1H–L16H, than in their oxime analogues.
Free energies of complex formation were calculated in
accordance with eqn (2), using the energy-minimised forms of
Fig. 5 is a pictorial representation of the deprotonation and
the proligands (L10H–L16H) and their copper complexes (see binding free energies from Table 2 and helps to demonstrate
Table 2). Note the data presented here do not include solvation that the more favourable deprotonation energies resulting
effects. The justification for this stems from the fact that the from the introduction of electron withdrawing bromo- and
formation energies calculated from eqn (2) rely on calculating nitro-substituents are only partially offset by less favourable
differences in the absolute energies for the component binding energies of the more weakly basic donor sets to the
species. The aqueous species (H3O+ and Cu2+), are identical in Cu2+ ion. As a consequence, formation of copper complexes of
all extractions, leaving only the solvation energies of the pre- the proligands which carry these substituents in either ortho-
organised LH dimers and CuL2 complexes in the organic or para-positions (L11H, L12H, L14H and L16H) is predicted
phase for further consideration. The differences in structure to be more favourable than formation of the complex of the
between these two species are small, and so consequently the unsubstituted proligand L10H. This trend is mirrored by the
differences in their corresponding solvation energies in chloro- observed enhanced copper extraction strength shown by the
form will not be large. As a test that this reasoning is sound, bromo- and nitro-substituted reagents L4H, L5H and L7H in
we repeated the calculations for L10H, L12H and L15H using the solvent extraction experiments (see Fig. 3).
the implicit PCM solvent model.16 The results of this analysis
The more favourable complex formation energy for the
are reported in the ESI.† Absolute formation energies were ortho-bromo-substituted L12H compared with its para-isomer,
affected (notably due to the large solvation energy of the Cu2+ L15H, arises from a combination of the deprotonation energy
cation), but crucially the relative ordering of formation ener- for L12H and the binding energy of the resulting anion both
gies for the three complexes remained unchanged. Since it is being more favourable (see Table 2). The ability of the ortho-
the relative ordering, rather than the absolute energies, which substituent to form a buttressing H-bond with the pyrazole
N–H group contributes to the latter.
As might be expected, the introduction of electron-donating
substituents such as the ortho- and para-methoxy groups in
L13H and L16H, disfavours deprotonation. This effect is com-
pensated in both cases by increased binding energies (see
Table 2 The free energies (all in kJ mol−1) of deprotonation (ΔGdeprot
,
column 5, Table 2) and is predicted to lead to formation of
copper complexes that are slightly more favourable than for
the unsubstituted L10H.
eqn (3)) of binding to Cu2+ (ΔGbind, eqn (4)) and of complex formation
(ΔGform, eqn (2)) of the substituted proligands L8H–L16H, together with
the energies of dimerization (ΔUdimer) of the proligands, 2LH → (LH)2. All
values are expressed relative to those of the unsubstituted proligand
L10H
The natural bond orbital method NBO6 17 was used to
evaluate the relative strengths of the inner sphere Cu–O and
Cu–N bonds in complexes of L10–L16 (Table 3). The Cu–N
bond strength shows only a small dependence on the nature
and position of substitution on the phenol unit. Larger vari-
ations are observed for the Cu–O bond, with a stronger inter-
action observed in complexes of the ortho-substituted ligands
[L11]−, [L12]− and [L13]−. This is consistent with the substitu-
ent forming a bonding interaction with the pyrazole N–H
groups (see Fig. 4) which weakens its interaction with the
phenolate oxygen atom thus rendering the pyrazole N atom a
ΔGdeprot
cf. L10H
ΔGform
ΔGbind
ΔUdimer
cf. L10H cf. L10H cf. L10H
Substituent Proligand kJ mol−1
kJ mol−1 kJ mol−1 kJ mol−1
H
o-NO2
o-Br
o-MeO
p-NO2
p-Br
L10H
L11H
L12H
L13H
L14H
L15H
L16H
0.0
−130.3
−56.7
4.9
−144.8
−46.5
11.7
0.0
−70.7
−22.9
−19.9
−9.6
0.0
59.6
33.8
−24.8
135.2
45.9
0.0
−34.8
−9.3
−11.1
8.0
2.0
−1.5
−0.5
p-MeO
5.2
−6.5
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Fig. 5 Gibbs free energies of deprotonation of two proligand molecules (ΔGdepro, eqn (3)), as shown in the upper chart, and those of binding of
two ligand anions to Cu2+ (ΔGbind, eqn (4)), lower chart, together with the structure of the most stable tautomer of the ligand anions used in
calculations.
Table 3 Strengths (in kJ mol−1) of selected interactions in the proligand dimers, [LH]2, and in the inner and outer coordination spheres of the [CuL2]
complexes from NBO6 17 calculations
L10
none
L11
o-NO2
L12
o-Br
L13
o-MeO
L14
p-NO2
L15
p-Br
L16
p-MeO
Ligand substituent
[CuL2] complexes
Cu–N
Cu–O
NH⋯O
NH⋯X
212
221
16
207
234
12
200
228
13
204
238
15
211
203
14
209
216
15
212
208
19
—
37
11
2
—
—
—
L10H
none
L11H
o-NO2
L12H
o-Br
L13H
o-MeO
L14H
p-NO2
L15H
p-Br
L16H
p-MeO
[LH]2 dimers
NH⋯O
NH⋯X
40
—
25
38
37
5
41
1
35
—
39
—
41
—
better electron donor. The data in Table 3 suggest that this bromo and methoxy substituents ( just 11 and 2 kJ mol−1
,
effect is much smaller in the complex of [L13]− for which the respectively).
strengthening of the Cu–O bond arises principally from the
electron-donating properties of the methoxy group.
Similar trends are observed in the proligand dimers [LH]2,
the structures of which are shown in the top right in Fig. 1.
The strength of the buttressing interaction NH⋯X in the Formation of these dimers is favourable for all the proligands
copper complexes depends greatly on steric factors. The nitro L10H–L16H (see ESI Table S4†), but particularly so for those
group, when utilising one of its oxygen atoms as an H-bond containing a buttressing NO2, Br or MeO group ortho to the
acceptor, forms a sterically-favourable six-membered ring, phenol group, i.e. L11H, L12H and L13H. For the o-nitro-sub-
accounting for the much stronger bonding interaction (37 stituted proligand [L11H]2, the buttressing H-bond (NH⋯X in
kJ mol−1) with the pyrazole N–H group than those shown by Table 3) is stronger than the H-bond to the phenolic oxygen
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atom, NH⋯O in Table 3, (38 cf. 25 kJ mol−1, respectively). In for the ortho-substituted ligands, their propensity to buttress
contrast, for the ortho-bromo and methoxy substituted pro- inter-ligand hydrogen bonding.
ligand dimers, [L12H]2 and [L13H]2, in which the H-bond
The pyrazole N–H donor is less accessible to ortho buttress-
accepting atom is constrained to be well separated from the ing groups than the oxime O–H group in the structurally
N–H group, the buttressing interaction is very weak. As a con- related phenolic oxime reagents and only in the case of the
sequence it is largely the inter-ligand NH⋯O interaction which o-nitro derivatives is a strong, conventional inter-ligand hydro-
favours dimer formation, with energies comparable to those gen bond formed. This extractant (L4H) is stronger than its
found for the para-substituted analogues [L15H]2 and [L16H]2. oxime analogue,12 whilst the unsubstituted (L5H) is slightly
Only in the case of the ortho nitro-substituted proligand weaker. The comparable strength of the pyrazoles suggests
L8H can the buttressing interactions in the Cu-complexes and that they might function as alternatives to the oximes in com-
the proligand dimers be described by a conventional hydrogen mercial operations in which the latter are used.
bonding model. The much weaker interactions found in the
bromo and methoxy analogues L12H and L13H are associated
with a small electrostatic contribution which arises from the
electronegativity of the ortho substituent.
Acknowledgements
We thank the EPSRC, Cytec Industries and Anglo American for
the funding of PhD studentships for MRH and JWR, Zeneca
for PDRA funding (DJW), the EaStCHEM Research Computing
Facility for access to software and the Edinburgh Computer
and Data Facilities (ECDF) for access to hardware.
Conclusions
The very good correlation between the calculated energies of
formation of the copper complexes in the gas phase and the
observed relative strengths of comparably substituted reagents
in solvent extraction experiments may at first sight be surpris-
ing. The rationale for it, however, is sound. It arises for the
reported extraction equilibria because the solvation/desolva-
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only H+ and Cu2+ in every case. Consequently only the
aq
aq
differences in the solvation energies of the proligands and
their copper complexes in the organic phase (chloroform),
arising from variations of the X and Y substituents, will influ-
ence the correlation between calculated formation energies
and the observed strength of extraction. Variations in the
nature of X and Y are likely to lead to only minor differences in
the energies of solvation by chloroform because these substitu-
ents are small in comparison with the solubilising 5-(2,4,4-tri-
methylpentyl)-group used in the extractants, L4H–L9H. Also
the structures of the preorganised proligand dimers [(LH)2] are
similar to those of their copper complexes [Cu(L)2] and conse-
quently the differences in their solvation energies will not be
large. Under these circumstances, as demonstrated above,
computational methods can be very helpful in providing an
insight into the origins of variation strength of extractants and
in principle could be used to guide the design of new extrac-
tants of the same chemical type. We have also reported similar
excellent correlations between energies of formation of metal
assemblies in the gas phase and the observed strength of
extraction in very different classes of process, for example the
recovery of their chloridometalates as in eqn (5).18–20
yꢁ
yLorg þ yHþaq þ ½MClxꢀaq Ð ½ðLHÞyMClxꢀorg
ð5Þ
An analysis of the energies of formation of the copper com-
plexes of the phenolic pyrazoles, as summarised in Table 2,
suggests that big differences in strength of the extractants 12 R. S. Forgan, B. D. Roach, P. A. Wood, F. J. White,
arise principally from a combination of the effects of the sub-
stituents on the ease of deprotonation of the proligands and,
J. Campbell, D. K. Henderson, E. Kamenetzky,
F. E. McAllister, S. Parsons, E. Pidcock, P. Richardson,
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