The ditopic receptor L3 [1-(2-((7-(4-(tert-butyl)benzyl)-1,4,7,10-tetraazacyclododecan-1-yl)methyl)phenyl)-3-(3-nitrophenyl)urea] containing a macrocyclic cyclen unit for Cu(II)-coordination and a urea moiety for anion binding was designed for recognition of metal salts. The X-ray structure of [CuL3(SO4)] shows that the sulfate anion is involved in cooperative binding via coordination to the metal ion and hydrogen-bonding to the urea unit. This behaviour is similar to that observed for the related receptor L1 [1-(2-((bis(pyridin-2-ylmethyl)amino)methyl)phenyl)-3-(3-nitrophenyl)urea], which forms a dimeric [CuL1(μ-SO4)]2 structure in the solid state. In contrast, the single crystal X-ray structure of [ZnL3(NO3)2] contains a 1 : 2 complex (metal : anion) where one anion coordinates to the metal and the other is hydrogen-bonded to the urea group. Spectrophotometric titrations performed for the [CuL3(OSMe2)]2+ complex indicate that this system is able to bind a wide range of anions with an affinity sequence: MeCO2− > Cl− > H2PO4− > Br− > NO2− > HSO4− > NO3−. Lipophilic analogues of L1 and L3 extract CuSO4 and CuCl2 from water into chloroform with high selectivity over the corresponding Co(II), Ni(II) and Zn(II) salts.

The modes of action of the commercial solvent extractants used in extractive hydrometallurgy are classified according to whether the recovery process involves the transport of metal cations, Mn+, metalate anions, MXxn−, or metal salts, MXx into a water-immiscible solvent. Well-established principles of coordination chemistry provide an explanation for the remarkable strengths and selectivities shown by most of these extractants. Reagents which achieve high selectivity when transporting metal cations or metal salts into a water-immiscible solvent usually operate in the inner coordination sphere of the metal and provide donor atom types or dispositions which favour the formation of particularly stable neutral complexes that have high solubility in the hydrocarbons commonly used in recovery processes. In the extraction of metalates, the structures of the neutral assemblies formed in the water-immiscible phase are usually not well defined and the cationic reagents can be assumed to operate in the outer coordination spheres. The formation of secondary bonds in the outer sphere using, for example, electrostatic or H-bonding interactions are favoured by the low polarity of the water-immiscible solvents.

Interactions, particularly hydrogen bonds, between ligands in the outer coordination spheres of metal complexes have a major effect on their stabilities in the hydrocarbon solvents used in commercial solvent extraction and it is now possible to use these interactions to tune the strength and selectivity of extractants.

Eight new amido functionalized reagents, L1–L8, have been synthesized containing the sequence of atoms R2N–CH2–NR′–CO–R″, which upon protonation forms a six-membered chelate with a hydrogen bond between the tertiary ammonium N–H+ group and the amido oxygen atom. The monocationic ligands, LH+, extract tetrachloridometal(II)ates from acidic solutions containing high concentrations of chloride ions via a mechanism in which two ligands address the “outer sphere” of the [MCl4]2- unit using both N–H and C–H hydrogen bond donors to form the neutral complex as in 2L + 2HCl + MCl2 ⇌ [(LH)2MCl4]. The strengths of L1–L8 as zinc extractants in these pH-dependent equilibria have been shown to be very dependent on the number of amide groups in the R3-nN(CH2NR′COR″)n molecules, anti-intuitively decreasing with the number of strong hydrogen bond donors present and following the order monoamides > diamides > triamides. Studies of the effects of chloride concentration on extraction have demonstrated that the monoamides in particular show an unusually high selectivity for [ZnCl4]2- over [FeCl4]− and Cl–. Hybrid-DFT calculations on the tri-, di-, and monoamides, L2, L3, and L4, help to rationalize these orders of strength and selectivity. The monoamide L4 has the most favorable protonation energy because formation of the LH+ cation generates a “chelated proton” structure as described above without having to sacrifice an existing intramolecular amide–amide hydrogen bond. The selectivity of extraction of [ZnCl4]2- over Cl–, represented by the process 2[(LH)Cl] + ZnCl42- ⇌ [(LH)2ZnCl4] + 2Cl–, is most favorable for L4 because it is less effective at binding chloride as it has fewer highly polar N–H hydrogen bond donor groups to interact with this “hard” anion.

Strapping two salicylaldoxime units together with aliphatic α,Ω-aminomethyl links in the 3-position gives ligands which allow the assembly of the polynuclear complexes [Fe7O2(OH)6(H2L1)3(py)6](BF4)5·6H2O·14MeOH (1·6H2O·14MeOH), [Fe6O(OH)7(H2L2)3](BF4)3·4H2O·9MeOH (2·4H2O·9MeOH) and [Mn6O2(OH)2(H2L1)3(py)4(MeCN)2](BF4)5(NO3)·3MeCN·H2O·5py (3·3MeCN·H2O·5py). In each case the metallic skeleton of the cluster is based on a trigonal prism in which two [MIII3O] triangles are tethered together via three helically twisted double-headed oximes. The latter are present as H2L2− in which the oximic and phenolic O-atoms are deprotonated and the amino N-atoms protonated, with the oxime moieties bridging across the edges of the metal triangles. Both the identity of the metal ion and the length of the straps connecting the salicylaldoxime units have a major impact on the nuclearity and topology of the resultant cluster, with, perhaps counter-intuitively, the longer straps producing the “smallest” molecules.

The use of “double-headed” phenolic oximes produces a trigonal antiprismatic [FeIII3]2 cluster with an “internal cavity” filled with an additional Fe3+ ion. Magnetic measurements reveal that the competition between different exchange interactions leads to a low-spin ground multiplet weakly separated in energy from a complex pattern of low-lying excited levels.

The syntheses, structures and magnetic properties of six iron complexes stabilised with the derivatised salicylaldoxime ligands Me-saoH2 (2-hydroxyethanone oxime) and Et-saoH2 (2-hydroxypropiophenone oxime) are discussed. The four hexanuclear and two octanuclear complexes of formulae [Fe8O2(OMe)4(Me-sao)6Br4(py)4]·2Et2O·MeOH (1·2Et2O·MeOH), [Fe8O2(OMe)3.85(N3)4.15(Me-sao)6(py)2] (2), [Fe6O2(O2CPh-4-NO2)4(Me-sao)2(OMe)4Cl2(py)2] (3), [Fe6O2(O2CPh-4-NO2)4(Et-sao)2(OMe)4Cl2(py)2]·2Et2O·MeOH (4·2Et2O·MeOH), [HNEt3]2[Fe6O2(Me-sao)4(SO4)2(OMe)4(MeOH)2] (5) and [HNEt3]2[Fe6O2(Et-sao)4(SO4)2(OMe)4(MeOH)2] (6) all are built from a series of edge-sharing [Fe4(μ4-O)]10+ tetrahedra. Complexes 1 and 2 display a new μ4-coordination mode of the oxime ligand and join a small group of Fe-phenolic oxime complexes with nuclearity greater than six.

Combining cation- and anion-binding functionalities in a salen-type extractant leads to multiple loading of ZnCl2. Zn(II) cations are bound by the salen N2O22− donor set, and chlorozincate anions are associated with protonated pendant amine groups.

The syntheses, structures and magnetic properties of nine new iron complexes containing salicylaldoxime (saoH2) or derivatised salicylaldoximes (R-saoH2), [Fe3O(OMe)(Ph-sao)2 Cl2(py)3]·2MeOH (1·2MeOH), [Fe3O(OMe)(Ph-sao)2Br2(py)3]·Et2O (2·Et2O), [Fe4(Ph-sao)4F4(py)4]·1.5MeOH (3·1.5MeOH), [Fe6O2(OH)2(Et-sao)2(Et-saoH)2(O2CPh)6] (4), [HNEt3]2[Fe6O2(OH)2(Et-sao)4(O2CPh(Me)2)6]·2MeCN (5·2MeCN), [Fe6O2(O2CPh)10(3-tBut-5-NO2-sao)2(H2O)2]·2MeCN (6·2MeCN), [Fe6O2(O2CCH2Ph)10(3-tBut-sao)2(H2O)2]·5MeCN (7·5MeCN), {[Fe6Na3O(OH)4(Me-sao)6(OMe)3(H2O)3(MeOH)6]·MeOH}n (8·MeOH) and [HNEt3]2[Fe12Na4O2(OH)8(sao)12(OMe)6(MeOH)10] (9) are discussed. The predominant building block appears to be the triangular [Fe3O(R-sao)3]+ species which can self-assemble into more elaborate arrays depending on reaction conditions. An interesting observation is that the R-saoH−/R-sao2− ligand system tends to adopt coordination modes similar to carboxylates. The most unusual molecule is the [Fe4F4] molecular square, 3. While Cl− and Br− appear to act only as terminal ligands, the F− ions bridge making a telling impact on molecular structure and topology.

3-Dialkylaminomethyl substituted salicylaldoximes are efficient metal salt extractants, and, in contrast to related “salen”-based reagents, are sufficiently stable to acid hydrolysis to allow commercial application in base metal recovery. Crystal structures show that metal salts are bound by a zwitterionic form of the reagents, with copper(II) nitrate, tetrafluoroborate and trifluoroacetate forming [Cu(L)2X2] assemblies in a tritopic arrangement with a trans-disposition of the anions outwith the coordination sphere. Copper(II) chloride, bromide and zinc(II) chloride form 1:1 assemblies, [Cu(L)X2], with the halides in the inner coordination sphere of the metal, leading to high chloride selectivity and very good mass transport efficiencies of CuCl2. Introduction of the anion-binding sites into the salicylaldoxime extractants changes their cation selectivities; the ligands co-extract small amounts of FeIII along with CuII from mixed metal aqueous feed solutions, an issue which will need to be addressed prior to industrial application.

Protonated amidopyridyl ligands show high selectivity for the extraction of [CoCl4]2− or [ZnCl4]2− over chloride ion into organic media via the formation of N–H and C–H hydrogen-bond donors to the outer coordination spheres of the chlorometallates.

Novel polynucleating, di- and tri-acidic ligands have been designed to increase the molar and mass transport efficiencies for the recovery of base metals by solvent extraction.

Attaching dialkylaminomethyl arms to commercial phenolic oxime copper extractants yields reagents which transport base metal salts very efficiently by forming neutral 1 ∶ 1 or 1 ∶ 2 complexes with zwitterionic forms of the ligands.

Benzohydroxamic acid is shown to be an unexpectedly good ligand for iron(III) oxides, favouring surface attachment to the formation of trisbenzohydroxamato complexes, which are known to have very high thermodynamic stability in solution.

3-Substitution of salicylaldoximes alters their copper(II) binding strengths by buttressing stabilising hydrogen bonding.

3-Substitution of salicylaldoximes alters their copper(II) binding strengths by buttressing stabilising hydrogen bonding.

Interactions, particularly hydrogen bonds, between ligands in the outer coordination spheres of metal complexes have a major effect on their stabilities in the hydrocarbon solvents used in commercial solvent extraction and it is now possible to use these interactions to tune the strength and selectivity of extractants.

The syntheses, structures and magnetic properties of nine new iron complexes containing salicylaldoxime (saoH2) or derivatised salicylaldoximes (R-saoH2), [Fe3O(OMe)(Ph-sao)2 Cl2(py)3]·2MeOH (1·2MeOH), [Fe3O(OMe)(Ph-sao)2Br2(py)3]·Et2O (2·Et2O), [Fe4(Ph-sao)4F4(py)4]·1.5MeOH (3·1.5MeOH), [Fe6O2(OH)2(Et-sao)2(Et-saoH)2(O2CPh)6] (4), [HNEt3]2[Fe6O2(OH)2(Et-sao)4(O2CPh(Me)2)6]·2MeCN (5·2MeCN), [Fe6O2(O2CPh)10(3-tBut-5-NO2-sao)2(H2O)2]·2MeCN (6·2MeCN), [Fe6O2(O2CCH2Ph)10(3-tBut-sao)2(H2O)2]·5MeCN (7·5MeCN), {[Fe6Na3O(OH)4(Me-sao)6(OMe)3(H2O)3(MeOH)6]·MeOH}n (8·MeOH) and [HNEt3]2[Fe12Na4O2(OH)8(sao)12(OMe)6(MeOH)10] (9) are discussed. The predominant building block appears to be the triangular [Fe3O(R-sao)3]+ species which can self-assemble into more elaborate arrays depending on reaction conditions. An interesting observation is that the R-saoH−/R-sao2− ligand system tends to adopt coordination modes similar to carboxylates. The most unusual molecule is the [Fe4F4] molecular square, 3. While Cl− and Br− appear to act only as terminal ligands, the F− ions bridge making a telling impact on molecular structure and topology.

Attaching dialkylaminomethyl arms to commercial phenolic oxime copper extractants yields reagents which transport base metal salts very efficiently by forming neutral 1 ∶ 1 or 1 ∶ 2 complexes with zwitterionic forms of the ligands.

The ditopic receptor L3 [1-(2-((7-(4-(tert-butyl)benzyl)-1,4,7,10-tetraazacyclododecan-1-yl)methyl)phenyl)-3-(3-nitrophenyl)urea] containing a macrocyclic cyclen unit for Cu(II)-coordination and a urea moiety for anion binding was designed for recognition of metal salts. The X-ray structure of [CuL3(SO4)] shows that the sulfate anion is involved in cooperative binding via coordination to the metal ion and hydrogen-bonding to the urea unit. This behaviour is similar to that observed for the related receptor L1 [1-(2-((bis(pyridin-2-ylmethyl)amino)methyl)phenyl)-3-(3-nitrophenyl)urea], which forms a dimeric [CuL1(μ-SO4)]2 structure in the solid state. In contrast, the single crystal X-ray structure of [ZnL3(NO3)2] contains a 1:2 complex (metal:anion) where one anion coordinates to the metal and the other is hydrogen-bonded to the urea group. Spectrophotometric titrations performed for the [CuL3(OSMe2)]2+ complex indicate that this system is able to bind a wide range of anions with an affinity sequence: MeCO2− > Cl− > H2PO4− > Br− > NO2− > HSO4− > NO3−. Lipophilic analogues of L1 and L3 extract CuSO4 and CuCl2 from water into chloroform with high selectivity over the corresponding Co(II), Ni(II) and Zn(II) salts.

The use of “double-headed” phenolic oximes produces a trigonal antiprismatic [FeIII3]2 cluster with an “internal cavity” filled with an additional Fe3+ ion. Magnetic measurements reveal that the competition between different exchange interactions leads to a low-spin ground multiplet weakly separated in energy from a complex pattern of low-lying excited levels.

The modes of action of the commercial solvent extractants used in extractive hydrometallurgy are classified according to whether the recovery process involves the transport of metal cations, Mn+, metalate anions, MXxn−, or metal salts, MXx into a water-immiscible solvent. Well-established principles of coordination chemistry provide an explanation for the remarkable strengths and selectivities shown by most of these extractants. Reagents which achieve high selectivity when transporting metal cations or metal salts into a water-immiscible solvent usually operate in the inner coordination sphere of the metal and provide donor atom types or dispositions which favour the formation of particularly stable neutral complexes that have high solubility in the hydrocarbons commonly used in recovery processes. In the extraction of metalates, the structures of the neutral assemblies formed in the water-immiscible phase are usually not well defined and the cationic reagents can be assumed to operate in the outer coordination spheres. The formation of secondary bonds in the outer sphere using, for example, electrostatic or H-bonding interactions are favoured by the low polarity of the water-immiscible solvents.

Novel polynucleating, di- and tri-acidic ligands have been designed to increase the molar and mass transport efficiencies for the recovery of base metals by solvent extraction.

Protonated amidopyridyl ligands show high selectivity for the extraction of [CoCl4]2− or [ZnCl4]2− over chloride ion into organic media via the formation of N–H and C–H hydrogen-bond donors to the outer coordination spheres of the chlorometallates.

Benzohydroxamic acid is shown to be an unexpectedly good ligand for iron(III) oxides, favouring surface attachment to the formation of trisbenzohydroxamato complexes, which are known to have very high thermodynamic stability in solution.

3-Dialkylaminomethyl substituted salicylaldoximes are efficient metal salt extractants, and, in contrast to related “salen”-based reagents, are sufficiently stable to acid hydrolysis to allow commercial application in base metal recovery. Crystal structures show that metal salts are bound by a zwitterionic form of the reagents, with copper(II) nitrate, tetrafluoroborate and trifluoroacetate forming [Cu(L)2X2] assemblies in a tritopic arrangement with a trans-disposition of the anions outwith the coordination sphere. Copper(II) chloride, bromide and zinc(II) chloride form 1:1 assemblies, [Cu(L)X2], with the halides in the inner coordination sphere of the metal, leading to high chloride selectivity and very good mass transport efficiencies of CuCl2. Introduction of the anion-binding sites into the salicylaldoxime extractants changes their cation selectivities; the ligands co-extract small amounts of FeIII along with CuII from mixed metal aqueous feed solutions, an issue which will need to be addressed prior to industrial application.

The syntheses, structures and magnetic properties of six iron complexes stabilised with the derivatised salicylaldoxime ligands Me-saoH2 (2-hydroxyethanone oxime) and Et-saoH2 (2-hydroxypropiophenone oxime) are discussed. The four hexanuclear and two octanuclear complexes of formulae [Fe8O2(OMe)4(Me-sao)6Br4(py)4]·2Et2O·MeOH (1·2Et2O·MeOH), [Fe8O2(OMe)3.85(N3)4.15(Me-sao)6(py)2] (2), [Fe6O2(O2CPh-4-NO2)4(Me-sao)2(OMe)4Cl2(py)2] (3), [Fe6O2(O2CPh-4-NO2)4(Et-sao)2(OMe)4Cl2(py)2]·2Et2O·MeOH (4·2Et2O·MeOH), [HNEt3]2[Fe6O2(Me-sao)4(SO4)2(OMe)4(MeOH)2] (5) and [HNEt3]2[Fe6O2(Et-sao)4(SO4)2(OMe)4(MeOH)2] (6) all are built from a series of edge-sharing [Fe4(μ4-O)]10+ tetrahedra. Complexes 1 and 2 display a new μ4-coordination mode of the oxime ligand and join a small group of Fe-phenolic oxime complexes with nuclearity greater than six.

Combining cation- and anion-binding functionalities in a salen-type extractant leads to multiple loading of ZnCl2. Zn(II) cations are bound by the salen N2O22− donor set, and chlorozincate anions are associated with protonated pendant amine groups.

Strapping two salicylaldoxime units together with aliphatic α,Ω-aminomethyl links in the 3-position gives ligands which allow the assembly of the polynuclear complexes [Fe7O2(OH)6(H2L1)3(py)6](BF4)5·6H2O·14MeOH (1·6H2O·14MeOH), [Fe6O(OH)7(H2L2)3](BF4)3·4H2O·9MeOH (2·4H2O·9MeOH) and [Mn6O2(OH)2(H2L1)3(py)4(MeCN)2](BF4)5(NO3)·3MeCN·H2O·5py (3·3MeCN·H2O·5py). In each case the metallic skeleton of the cluster is based on a trigonal prism in which two [MIII3O] triangles are tethered together via three helically twisted double-headed oximes. The latter are present as H2L2− in which the oximic and phenolic O-atoms are deprotonated and the amino N-atoms protonated, with the oxime moieties bridging across the edges of the metal triangles. Both the identity of the metal ion and the length of the straps connecting the salicylaldoxime units have a major impact on the nuclearity and topology of the resultant cluster, with, perhaps counter-intuitively, the longer straps producing the “smallest” molecules.