Anthony J. Leong

Studies of metal-ion recognition. The interaction of COII, NiII, and CuII with new oxygen–nitrogen donor macrocycles; X-ray structures of complexes of CuII and NiII with a 15-membered O2N3 derivative

Tetrahydroborate reduction of a range of Schiff-base di-imine macrocycles, prepared by template cyclization reactions on manganese(II) or lead(II), has afforded a series of structurally related metal-free macrocycles incorporating both oxygen and nitrogen donor atoms. In all, ten new rings are reported. Physical measurements indicate that many of these rings form 1 : 1 complexes with CoII, NiII, and/or CuII. The stabilities of selected complexes have been determined using the potentiometric (pH) titration technique. In all cases much greater stability for the complexes of CuII relative to those of CoII or NiII was observed; the structural origin of this selectivity has been investigated by X-ray diffraction studies. These indicate that the complexes of NiII and CuII have markedly different structures for one of the O2N3 ligands. In the nickel(II) species the geometry is distorted octahedral with the macrocycle occupying four co-ordination positions with one ether oxygen not co-ordinated. The remaining (cis) co-ordination positions are filled by nitrate ions. In the copper(II) complex the macrocycle folds such that an unsymmetric but apparently favourable cavity is formed and all five donor atoms co-ordinate to the metal. A sixth co-ordination position is filled by a nitrate ion.

On the nature of the host–guest interaction between cyclam and 4-tert-butylbenzoic acid—a system pre-assembled for metal complex formation

The host–guest interaction between cyclam and 4-tert-butylbenzoic acid has been investigated by NMR titration, X-ray diffraction, neutron diffraction and semi-empirical MO calculation; the product represents a system preassembled for metal-ion complexation.

Host–guest assembly of ligand systems for metal ion complexation; synergistic solvent extraction of copper(II) ions by N3O2-donor macrocycles and carboxylic or phosphinic acids

Host–guest formation between N3O2-donor macrocyclic hosts and lipophilic organic acid guests (that are themselves potential metal-ion ligands/extractants) has been investigated and the implications of formation of such ligand assemblies for the binding of metal ions have been probed. The formation of 2 ∶ 1 (organic acid ∶ macrocycle) assemblies has been demonstrated using NMR titration experiments in deuterochloroform for 4-tert-butylbenzoic, palmitic, phenylphosphinic, diphenylphosphinic and salicylic acids, with evidence for the additional formation of 1 ∶ 1 intermediates in some cases. Molecular adducts of this type have been used in solvent extraction (water/chloroform) experiments to define the effect of assembly formation on metal ion binding. From entropy considerations it was anticipated that the formation of particular host–guest species of the above type might result in enhanced metal ion binding (and hence enhanced metal ion extraction) – a consequence of the fact that the components of the coordination sphere are, at least in part, assembled for complex formation. Using a range of assemblies of the above type, enhanced (synergistic) extraction of Cu(II) ions was confirmed in all cases. Adducts of 1,12,15-triaza-3,4:9,10-dibenzo-5,8-dioxacycloheptadecane and 1,12,15-triaza-3,4:9,10-bis(4′-tert-butylbenzo)-5,8-dioxacycloheptadecane with 4-tert-butylbenzoic acid and salicylic acid have been isolated and their solid-state structures (and 1 ∶ 2 stoichiometries) confirmed by X-ray diffraction.

The interaction of transition and post-transition metal ions with 14- and 16-membered trans N2X2-donor macrocycles (X = NH, O, S): stabilities and X-ray structures

Abstract A comparative investigation of the 1:1 (metal/ligand) complex stabilities of 14- and 16-membered derivatives of dibenzo-substituted macrocycles incorporating trans -N 2 O 2 , -N 2 S 2 and -N4 donor sets towards Co(H), Ni(H), Cu(H), Zn(II), Cd(II), Ag(I) and Pb(II) ions has been carried out with emphasis on the influence of systematic donor atom variation on the respective 1:1 stability constants. For comparison, previously determined log K values for corresponding cis -N 2 O 2 and -N4 donor ligand systems are also discussed. The crystal structures of the nickel(II) thiocyanate complexes of the 14-membered N 4 (diamine) and precursor N 4 (diimine) macrocycles are also presented. C 20 H 24 N 6 NiS 2 crystallised in the triclinic space group P 1 (No. 2) with a = 8.913(2) A , b = 9.441(2)1 , c = 13.583(3) A , α = 94.29(2)°, β = 107.48(2)°, γ = 90.90(2)° and Z = 2 . C 20 H 20 N 6 NiS 2 ·CH 3 CN crystallised in the monoclinic space group P 2 1 n (No. 14) with a = 10.321(2) A , b = 13.855(8) A , c = 17.435(1) A , β = 102.59(10)° and Z = 4 .

The Interaction of Co(II), Ni(II), Cu(II), Zn(II), and Cd(II) with Mixed Donor Macrocycles Containing Pendant Carboxylic Acid Functions

The syntheses of mixed oxygen-nitrogen donor macrocycles incorporating two or three pendant carboxylic acid groups are described. Potentiometric titrations in water (I = 0.1; KNO3) at 25°C have been used to determine the stability constants for the 1: 1 (metal:ligand) complexes of Co(II). Ni(II), Cu(II), Zn(II), and Cd(II). The constants obtained are compared with the previously determined values for the corresponding complexes of the unsubstituted macrocyclic precursors. The results of these studies indicate that each carboxylate function participates in binding to the central metal. For some metal-ion/ligand systems there is evidence that ring size effects influence the overall stability patterns and that, in such cases, both the ether oxygens as well as the tertiary amines of the macrocyclic rings appear to bind to the metal.

Macrocyclic ligand design: Structure—function relationships involving the interaction of cobalt(II), nickel(II) and copper(II) with mixed donor macrocyclic ligands

Abstract A comparative investigation of the interaction of Co(II), Ni(II) and Cu(II) with a series of mixed-donor dibenzo-substituted macrocyclic ligands, incorporating O 2 N 3 -, O 2 N 4 -, O 3 N 2 -, O 3 N 3 - and O 4 N 2 -donor sets, has been carried out. The thermodynamic stabilities of the respective complexes in 95% methanol ( I = 0.1; Et 4 NClO 4 , 25°C) have been determined. All ligands form 1:1 (metal:ligand) species with the above metal ions although, in a few instances, species of type MLH 2+ were also detected. In an attempt to elucidate ligand structure/function relationships, an analysis of the effect of variation of ligand backbone and/or donor atom set on the respective complex stabilities has been carried out; the results are compared with those obtained previously for related mixed-donor (cyclic) systems. The X-ray structure of the nickel(II) nitrate complex of the O 3 N 3 -donor macrocycle (V) is presented. C 23 H 35 N 5 NiO 10 crystallised in the monoclinic space group P 2 1 / n , with a = 17.201(3), b = 11.341(2), c = 14.742(3) A, β = 104.34(2)° and Z = 4.

Interaction of transition and post-transition metal ions with oxygen–nitrogen donor macrocycles incorporating pendant hydroxyethyl and carbarnoylethyl groups

The interaction of new derivatives of O2N2-donor macrocycles incorporating pendant hydroxyethyl or carbamoylethyl arms with cobalt(II), nickel(II), copper(II), zinc(II) and cadmium(II) has been investigated. The thermodynamic stabilities of a range of 1 : 1 (metal:ligand) complexes of these macrocycles have been determined potentiometrically in 95% methanol (I= 0.1 mol dm–3, NEt4ClO4 or NMe4Cl; at 25 °C). Comparison of selected complexes with those of the corresponding parent (non-pendant arm) macrocycles indicates that the former derivatives show more varied co-ordination behaviour than the latter. X-Ray diffraction studies of five nickel(II) complexes have been made. The complexes containing respectively 14-, 15- and 16-membered rings and two pendant alcohol arms have similar distorted-octahedral structures. In each case the nickel ion is situated outside the macrocyclic ring with only the ring nitrogen atoms and the pendant alcohol oxygen atoms co-ordinating; the remaining sites are occupied by the oxygen atoms of a bidentate nitrate ion. In contrast, the structure of the related 15-membered, mono-pendant arm (alcohol) derivative has the nickel in the macrocyclic cavity; the near-octahedral co-ordination sphere consists of two ether oxygens and two nitrogens from the macrocyclic ring in the equatorial plane together with oxygen atoms from the pendant alcohol arm and a water molecule in the axial sites. The structure of a pendant bis(amide) derivative also incorporates the nickel in the macrocyclic cavity with the pendant amide groups co-ordinated in axial sites to yield an overall distorted-octahedral geometry.

Metal ion recognition. The interaction of cobalt(II), nickel(II), copper(II), zinc(II), cadmium(II), silver(I) and lead(II) with N-benzylated macrocycles incorporating O2N2-, O3N2- and O2N3-donor sets

The interaction of cobalt(II), nickel(II), copper(II), zinc(II), cadmium(II), silver(I) and lead(II) with a series of mixed-donor, N-benzylated macrocyclic ligands incorporating O2N2-, O3N2- and O2N3-donor sets has been investigated. The log K values for the respective 1 ∶ 1 complexes in 95% methanol (I = 0.1 mol dm−3; Et4NClO4, 25 °C) have been determined potentiometrically and the results compared with the values obtained previously for the parent (non-benzylated) macrocyclic systems. Examples of N-benzylation of individual parent macrocycles leading to enhanced discrimination for silver(I) are presented. In the case of a dibenzylated O3N2-ring and a tribenzylated O2N3-ring, both 17-membered, high selectivity for this ion was observed relative to the other metal ions investigated. Competitive mixed-metal transport experiments across a bulk chloroform membrane have been performed using the benzylated derivatives as ionophores. For each experiment the source contained equimolar concentrations of the above metal ions and transport was performed against a pH gradient; the aqueous source and receiving phases were buffered at pH 4.9 and 3.0, respectively. In parallel to the log K results, transport selectivity for silver(I) was exhibited by the di- and tri-benzylated ligands mentioned above. The remaining ligands proved to be poor ionophores, showing no significant metal ion transport under the conditions employed. In a further study, high silver ion (transport) selectivity was maintained when the above tribenzylated ligand was incorporated as the ionophore in a polymer inclusion membrane system. Single metal and mixed (seven-metal) solvent extraction experiments employing similar aqueous source and chloroform phases to those used in the bulk membrane transport runs have also been performed for the tribenzylated ligand derivative; significant selectivity for silver was again maintained in these solvent extraction experiments. An X-ray study of [AgL]ClO4 (where L is the above-mentioned dibenzylated derivative) confirms that all donor atoms of the macrocycle coordinate to the silver ion; the latter is five-coordinate with the complex cation exhibiting a highly distorted trigonal bipyramidal geometry.

Metal-ion recognition. Competitive bulk membrane transport of transition and post transition metal ions using oxygen–nitrogen donor macrocycles as ionophores

A series of competitive metal ion transport experiments have been performed. Each involved transport from an aqueous source phase across a chloroform membrane phase into an aqueous receiving phase. The source phase contained equimolar concentrations of cobalt(II), nickel(II), copper(II), zinc(II), cadmium(II), silver(I) and lead(II) while the membrane phase incorporated an ionophore chosen from a series of single-ring and double-ring macrocyclic ligands (incorporating mixed oxygen–nitrogen donor sets) together with hexadecanoic acid. The transport process was ‘driven’ by a back flux of protons, maintained by buffering the source and receiving phases at pH 4.9 and 3.0, respectively. Transport selectivity for copper(II) was observed in all cases. The results confirm that consideration of the mass balance of the metal ions present across all three transport phases is important for a fuller understanding of the nature of the discrimination process for the present systems.

Ligand design and metal-ion recognition. The interaction of copper(II) with a range of 16- to 19-membered macrocycles incorporating oxygen, nitrogen and sulfur donor atoms

The interaction of copper(II) with a series of quinquedentate macrocycles incorporating nitrogen, oxygen, and/or sulfur donor atoms is reported. These ligands form part of a structural matrix in which a systematic variation in donor atom pattern and ring size is present (and against which structure–function relationships are more clearly seen). Stability constants for the 1 : 1 copper complexes have been determined in 95% methanol (I= 0.1 mol dm–3, NEt4ClO4) at 25 °C. The study demonstrates the extremely large stability differentials [up to 1010 for copper(II)] that may be achieved through structural variation within the ligand framework employed for this study. The contribution of the respective donor atom types to overall complex stability was found to follow the expected order of NH(aliphatic) > NH(anilino) > S > O. Based on earlier X-ray studies, the molecular mechanics modelling of two representative examples is also reported.