Peter A. Tasker

The structures of phenolic oximes and their complexes

Abstract This review article describes the rich coordination chemistry of phenolic oxime ligands based on solid state structures in the October 2000 release of the Cambridge Structural Database. The phenolic oxime ligands discussed have the general formula n-(R1)x-2-OHC6H4−xC(R2)NOH. Whilst their classical mode of coordination via the deprotonation of the phenol to bind to a the metal centre as a mono-anionic bidentate ligand is frequently observed, the structural survey reveals a plethora of less common binding motifs that can be attained via the deprotonation of both the phenolic and oximic hydrogens resulting in the formation of tridentate ligands and polynuclear bridged complexes. The formation of such polynuclear complexes at surface sites may account for their efficacy as corrosion inhibitors. Also highlighted is the occurrence of hydrogen bonding between oximic hydrogen and phenolic oxygen atoms and its implications on the thermodynamic stability of metal complexes via the formation of pseudo-macrocyclic motifs and on the pre-organisation of dimeric metal-free ligand species. The formation of ligand–ligand hydrogen bonds to define donor cavities of a well defined size accounts for their remarkable selectivity when used in commercial metal recovery processes based on solvent extraction.

Surface coordination chemistry: corrosion inhibition by tetranuclear cluster formation of iron with salicylaldoxime.

A tetranuclear iron cluster is the principal component of the purple coatings produced by treating a mild steel surface with a salicylaldoxime corrosion inhibitor. This was shown by comparison of the spectroscopic data with those of the cluster [{Fe(salH)(HsalH)}4 ], which was obtained from FeCl3 and salicylaldoxime (H2 salH) and has a distorted tetrahedral arrangement of Fe(III) atoms coordinated by terminal (1-) and bridging (2-) salicylaldoximate ligands (the central core of the cluster is depicted).

Inter-ligand reactions: in situ formation of new polydentate ligands

Two ligands have been synthesized by derivatisation of cyanuric chloride: 6-(diethylamino)-2,4-disulfanyl-1,3,5-triazine (H2SSta) 1 and 6-(diethylamino)-2-hydroxo-4-sulfanyl-1,3,5-triazine (H2OSta) 2 have been characterised by X-ray crystallography, which shows intermolecular hydrogen bonding in the solid state, leading to dimers of 1 and ribbons of 2. On reaction with metal salts both ligands undergo oligomerisation reactions. Compound 1 reacts with nickel chloride to form a mononuclear complex, [Ni{(Sta)S(S2ta)}] 3. In 3 two triazine ligands have reacted, to form a tetradentate ligand in which two triazine rings are bridged by a sulfur group, with a co-ordinated disulfide group present on one ring and a co-ordinated thiolate on the second. Compound 2 reacts with cobalt(II) chloride to form a cage complex, [Co6NaO(OStaH)7{S(Ota)2}2(O2CPh)2(H2O)2] 4. This complicated structure contains two polydentate ligands formed by linking triazine groups through a bridging sulfur. The cage contains four cobalt(II) and two cobalt(III) sites which are assigned by bond length considerations. The compound [Co(OSta)3] 5 co-crystallises with 4, and its structure has also been determined.

Selective Extraction and Transport of the [PtCl6]2− Anion through Outer-Sphere Coordination Chemistry

A series of tripodal receptors designed to recognise the outer coordination sphere of the hexachlorometallate anion [PtCl(6)](2-), and thus show selectivity for ion-pair formation over chloride binding, has been synthesised and characterised. The tripodal ligands contain urea, amido or sulfonamido hydrogen-bond donors, which are aligned to bind to the regions of greatest electron density along the faces and edges of the octahedral anion. The ligand structure incorporates a protonatable bridgehead nitrogen centre that provides a positive charge to ensure the solubility of a neutral 2:1 [LH](+)/[PtCl(6)](2-) complex in water-immiscible solvents. The extraction of [PtCl(6)](2-) from acidic chloride solutions was evaluated by using a pH-swing mechanism to control the loading and stripping of the metallate anion. The ligands L(1)-L(3), L(5)-L(9), L(11)-L(13) and L(15) showed extremely high loading (up to 95% in some cases) and high selectivity for [PtCl(6)](2-) over chloride ions (present in a 60-fold excess) compared with trioctylamine, a model Alamine reagent, which, under identical conditions, only extracted 10% of the Pt(IV) anions. Generally, extraction was observed to be greater for urea-containing ligands than their amido analogues, and a quantitative recovery of platinum from feed solutions was achieved. The formation of neutral ([LH](+))(2)[PtCl(6)](2-) packages in organic media is supported by single-crystal X-ray structure determinations of [(L(2)H)(2)PtCl(6)] x 2 CH(3)CN, [(L(8)H)(2)PtCl(6)(MeOH)(2)], [(L(12)H)(2)PtCl(6)] x 2 CH(3)CN and [(L(14)H)(2)PtCl(6)], which confirm the presence of significant hydrogen bonding between the anion and urea or amido moieties of the protonated ligand and the anion. The structure of [(L(1)H)(H(3)O)PtCl(6)] x C(6)H(6) x CH(3)CN shows hydrogen bonding of a H(3)O(+) cation to the receptor and confirms that other stoichiometries are also possible, indicating that speciation in solution may be more complex.

Metal Complexes for Hydrometallurgy and Extraction

9.17.

Effect of pressure on the crystal structure of salicylaldoxime‐I, and the structure of salicylaldoxime‐II at 5.93 GPa

The effect of pressure on the crystal structure of salicylaldoxime has been investigated. The ambient-pressure phase (salicylaldoxime-I) consists of pairs of molecules interacting through oximic OH...O hydrogen bonds; taken with phenolic OH...N intramolecular hydrogen bonds, these dimers form a pseudo-macrocycle bounded by an R4 4(10) motif. The dimers interact principally via pi...pi stacking contacts. Salicylaldoxime derivatives are used industrially as selective solvent extractants for copper; the selectivity reflects the compatibility of the metal ion with the pseudo-macrocycle cavity size. On increasing the pressure to 5.28 GPa the size of the cavity was found to decrease by an amount comparable to the difference in hole sizes in the structures of the Cu2+ salicylaldoximato complex and its Ni2+ equivalent. On increasing the pressure to 5.93 GPa a new polymorph, salicylaldoxime-II, was obtained in a single-crystal to single-crystal phase transition. PIXEL calculations show that the phase transition is driven in part by relief of intermolecular repulsions in the dimer-forming OH...O-bonded ring motif, and the ten-centre hydrogen-bonding ring motif of the phase I structure is replaced in phase II by a six-centre ring formed by oximic OH...N hydrogen bonds. The transition also relieves repulsions in the pi...pi stacking contacts. The intramolecular OH...N hydrogen bond of phase I is replaced in phase II by a intermolecular phenolic OH...O hydrogen bond, but the total interaction energy of the pairs of molecules connected by this new contact is very slightly repulsive because the electrostatic hydrogen-bond energy is cancelled by the repulsion term. The intra- to intermolecular hydrogen-bond conversion simply promotes efficient packing rather than contributing to the overall lattice energy.

Exploiting outer-sphere interactions to enhance metal recovery by solvent extraction

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.

Elucidating the mode of action of a corrosion inhibitor for iron

Two polymetallic iron(III) complexes 1 and 2 have been synthesised from the known corrosion inhibitor 3-(4-methylbenzoyl)-propionic acid HL1 and their crystal structures determined. Coordination geometries extracted from these structures have been used as the basis for molecular modelling onto idealised iron(III) oxide surfaces as an aid to understanding the efficacy of inhibitors of the 4-keto acid type. The proposed mode of action involves 1,3-bridging didentate coordination of the carboxylate function of L1 to two FeIII ions, hydrogen-bond formation between the 4-keto group of L1 and a bridging surface hydroxy group, as well as close packing of the aromatic end groups, which should generate a hydrophobic barrier on the surface. Adsorption isotherm experiments have been used to compare the strengths of binding of related carboxylic acids onto iron(III) oxide surfaces and indicate that the presence of the 4-keto function leads to the formation of significantly more stable surface complexes.

Review: The Design of Ligands for the Transport of Metal Salts in Extractive Metallurgy

Anion recognition is key to the development of reagents that can selectively transport metal salts in extractive metallurgy. This review outlines flowsheets that require such reagents and considers how anion receptors could be used to transfer base metal salts into water-immiscible media in solvent extraction processes. Particular attention is paid to the extraction of sulfate because there is a need to develop systems for the unit operations of concentration and separation to process sulfate streams.

Ditopic ligands for the simultaneous solvent extraction of cations and anions

Incorporation of a dianionic binding site for transition metal cations and a dicationic binding site for anions into a lipophilic molecule has produced a ligand with a high efficacy for the solvent extraction of a transition metal salt [M2+X2–].