Mutsuo Kodama

Complexation of the macrocyclic hexa-amine ligand 1,4,7,10,13,16-hexa-azacyclo-octadecane(‘18-azacrown-6’)

Complexation constants in aqueous solutions of the title compound with various metal ions are presented, together with ΔH and ΔS parameters. Unlike lower polyamine macrocyclic homologues, it has appreciable affinity to non-polarizable cations.

Equilibria and kinetics of copper(II) complex formation of a linear and of 13–15-membered macrocyclic dioxo-tetra-amines

Ligating properties of a linear (L8) and of 13–15-membered cyclic dioxo-tetra-amines (L5–L7)(which possess two internal amide bonds) have been investigated with reference to structurally relevant tetra-amines (L1–L4) and the tripeptides glycylglycylglycine (L9) and glycylglycylhistidine (L10). The pH-metric titrations show that ionization of the two hydrogens (most likely at the two amide nitrogens) with concurrent 1 : 1 complex ([MH–2L]) formation with CuII occurs at pH < 7. The complex species [ML]2+ and [MH–1L]+ are not found in the equilibria or kinetic studies. The macrocyclic effect and the cation–ring size selectivity observed for tetra-amines L1–L4 are retained for the dioxo-tetra-amines, as illustrated by logarithmic values of the cumulative formation constants KCuH–2L(=[CuH –2L][H+]2/[CuII][L]) of –2.2, 1.0, –4.5, and –5.1 (at I 0.2 mol dm–3 and 25 °C) for L5, L6, L7 and L8, respectively. The stability of the 14-membered L6 complex surpasses that of the L10 complex. The kinetics have been measured for the [CuH–2L] formation (L = L5 and L6) in acetate buffers (4.8 < pH < 5.9 at I 0.2 mol dm–3 and 25 °C). The rate of reaction is expressed as kL[Cu(O2CMe)+][L]+kHL[Cu(O2CMe)+][HL+] for L5(where kL= 2.0 × 107 and kHL= 9.5 × 102 dm3 mol–1s–1) and as kHL[Cu(O2CMe)–][HL+] for L6(where kHL= 3.1 × 103 dm3 mol–1s–1).

The rapidly dropping mercury electrode in direct current and alternating current polarography

Abstract The characteristics of the rapidly dropping mercury electrode (RADME) in direct current and alternating current polarography have been investigated. The RADME gives a maximum of the second kind on the D.C. current-voltage curve because of the “Spuleffekt”. This maximun is suppressed upon the addition of a large amount of surface-active substance, and typical currentvoltage curves similar to those at the DME are obtained. The limiting current at the RADME is proportional to the concentration of the electroactive substance. In A.C. polarography the RADME gives the peak current more accurately than the conventional DME. The height of the peak current is not affected by the “Spuleffekt”, and is proportional to the concentration of the electroactive species.

Equilibria of complex formation between several bivalent metal ions and macrocyclic tri- and penta-amines

Values of log KML, ΔH, and ΔS(at / 0.2 mol dm–3 and 25 °C) have been determined potentiometrically or polarographically for the 1 : 1 reactions between ZnII, CdII, HgII, or PbII and macrocyclic polyamine ligands including 9- and 10-membered triamines and 15-, 16-, and 17-membered penta-amines. The resulting data are compared with those for the relevant linear polyamines. Whether in tri- or penta-amines, ligand cyclization has minor effects on the enthalpy change but has major effects on the entropy change on complex formation with these metal ions. The latter term is responsible for the increased stability of the macrocyclic complexes. This is opposite to the enthalpy-governed macrocyclic effect earlier reported for copper(II)–penta-amine complexes. Among the homologous macrocyclic ligands, the smallest ligands consistently show the highest affinity towards any of the metal ions tested, and this originates from the entropy term. A survey of the tri-, tetra-, and penta-amine systems indicates some correlation between large macrocyclic effects and the ratio of the size of the metal ion to the size of the macrocyclic cavity.

The use of the rotated dropping mercury electrode in polarographic analyses and amperometric titrations of micromolar solution

Abstract The applicability of the rotated dropping mercury electrode in conventional direct current polarography, alternate current polarography, and amperometric titrations, has been investigated. The direct current polarographic method was applied to the determination of minute quantities of lead contained in high-purity electrolytic zinc, the lowest content of lead determined being 0.0001 %. It was confirmed that the rotation of the electrode has no advantage in alternating current polarography. Some characteristics of the electrode, revealed by the measurement of current-time curves, are also presented and discussed.

Square planar co-ordination of the 12-membered macrocyclic tetraamine ligand 1,4,7,10-tetra-azacyclododecane-2,6-dione

A novel example of square-planar N4 co-ordination of a 12-membered macrocycle has been found with the title compound, L5. Rigid planarity caused by the dissociation of two protons from the amide groups at pH ca. 8 stabilizes the 1 : 1 complex [M(H–2L)], with M = Cu2+ and Ni2+, as shown by pH-metric titrations. The conceivable bond strain around Cu2+ is manifested in the visible spectrum exhibiting an unusual blue shift and also in the cumulative formation constant KCuH–2L{=[Cu(H–2L)][H+]2/[Cu2+][L]} which is much smaller than the reported values for the square-planar complexes of 13–15-membered homologous macrocycles, L6–L8, or glycylglycylglycine, L11. The advantages of the planar ligand field and the small cavity size are reflected in the ready attainment of a d8 Cu3+ complex as demonstrated by the extremely low electrode potential EΘ measured using cyclic voltammetry. In contrast, Ni2+ with a smaller cation size seems to fit more easily in the 12-membered cavity. The KNiH–2L value is comparable to that for L11. The yellow visible spectrum is indicative of low-spin square-planar complex geometry.

Equilibria and kinetics of complex formation between zinc(II), lead(II), and cadmium(II), and 12-, 13-, 14-, and 15-membered macrocyclic tetra-amines

Polarographic methods have been used to determine the equilibria and kinetics of reaction of ZnII, PbII, and CdII with 12- to 15-membered macrocyclic tetra-amines including 1,4,7,10-tetra-azacyclododecane (L1), 1,4,7,10-tetra-azacyclotridecane (L2), 1,4,8,11-tetra-azacyclotetradecane (L3), and 1,4,8,12-tetra-azacyciopentadecane (L4) in acetate buffer solutions. Unlike the copper(II) system. the stability constants of the zinc complexes hardly change with macrocyclic ring size : log KZnL= 16.2, 15.6, 15.5, and 15.0 for L1, L2, L3, and L4, respectively (I0.20 mol dm–3 and 25 °C). The much larger CdII and PbII form complexes only with L1: log KCdL 14.3 and log KPbL, 15.9. The 103–105 times greater stabilities of the macrocyclic complexes compared with the related complexes of linear terra-amines are all due to favourable entropy changes, regardless of the metal-ion size. The rate law for complex formation in acetate buffers is d[ML]/dt=kH[M(O2CMe)+][HL+]+k2H[M(O2CMe)+][H2L2+] for all the metal ions. A comparison with reactions of CuII and L1–L4 shows that the values of kH parallel the well established rates of water exchange of the aquametal ions.

Kinetic and equilibrium studies on copper(II) complexes of a 9-membered macrocyclic triamine, 1,4,7-triazacyclononane, and a 14-membered macrocyclic tetra-amine, 1,4,8,11-tetra-azacyclotetradecane

The thermodynamics and kinetics of formation of copper(II) complexes of 1,4,7-triazacyclononane (L1) and 1,4,8,11-tetra-azacyclotetradecane (L2) have been studied. The stability constants and thermodynamic parameters have been determined by a polarographic method for L1 and by the potentiometric method of Bjerrum and Nielson for L2. The complex [CuL2]2+ has log KCuL 27.2 (ΔH–30.4 kcal mol–1 and ΔS 22.4 cat K–1 mol–1) at I 0.20 mol dm–3 and 25 °C, which represents stability enhancement over the complex of the corresponding linear tetra-amine by three orders of magnitude due to more favourable entropy and enthalpy contributions. The higher frequency of the maximum of the visible absorption band indicates that the favourable ΔH term derives from an increase in Cu–N bond energy. On the other hand, the cyclization poses severe steric constraints on the complex of the triamine ligand, as manifested by a large enthalpy loss. This destabilization is barely compensated by the favourable entropy term: log KCuL 16.2, ΔH–13.0 kcal mol–1, and ΔS 30.5 cal K –1 mol–1. Kinetic studies in acetate buffers using the stopped-flow method have given the following rate laws for the complex formation at 25 °C and I 0.20 mol dm–3: for L1, d[CuL2+]/dt=kH[Cu(O2CMe)+][HL+], where kH= 106.8 dm3 mol–1 s–1; for L2, d[CuL2+]/dt=kH[Cu(O2CMe)+][HL+]+ k2H[Cu(O2CMe)+][H2L2+], where kH= 106.7 and k2H= 100.9dm3 mol–1 s–1. The rate law for the formation of [CuL2]2+ at pH 1.8–2.5 (unbuffered) and 25 °C has been determined by the polarographic method: d[CuL2+]/dt=kH′[Cu2+][HL+]+k2H′[Cu2+][H2L2+], where kH′= 106.9 and k2H′= 10–1-1 dm3 mol–1 s–1.

Metal chelates of sulphur-containing polyamine macrocycles and oxygenation of the corresponding cobalt(II) complexes

The protonation and complex formation of macrocyclic polyamines containing a sulphur donor atom, 1-thia-4,7,10-triazacyclododecane (L4) and 1-thia-4,7,11,14-tetra-azacyclohexadecane (L5), have been investigated with the metal ions CoII, NiII, CuII, HgII, and FeII. The equilibrium and kinetic constants for formation of the dioxygen adducts of the cobalt(II) chelates in aqueous solutions are also reported, and compared with previous data for the analogous macrocycles containing nitrogen, oxygen, and pyridine instead of sulphur. The values of log KML for the macrocyclic polyamines follow the trend N > S > O. Oxygenation of the cobalt(II) complexes of L4 and L5 yields µ-peroxo-adducts as with the nitrogen and oxygen analogues, and the O2-affinity order is N S > O.

Reaction of cobalt(II) macrocyclic tetra-amine complexes with dioxygen

Low-pH equilibrium and kinetic studies on the oxygenation of [CoL]2+ complexes containing 12–(L2), 13–(L3), and 14-membered (isomeric, L4 and L5) fully saturated macrocyclic tetra-amines are reported. The ring size strongly influences the stability of the oxygenated products. The relative stability is connected with the stoicheiometry of the oxygenated complex, [(CoL)2(O2)(OH)]3+ for L2 and L3 and [(CoL)2(O2)]4+ for L5, which has been elucidated by potentiometric titration and polarographic measurements. The results for the macrocyclic systems are compared with a linear tetra-amine system (L1). The cyclic nature of the macrocycles, while serving to raise the [CoL]2+ stability, markedly lowers the O2 affinity of the complexes. Rates for the reaction of Co2+, L, and O2 in acetate buffers (yielding the dioxygen adducts) are first order in [Co2+] and also in [L], but are independent of [O2], indicating that the formation of [CoL]2+ is the slowest step in the O2 uptake. The separate reaction of [CoL]2+ with O2 is first order in [CoL2+] and also in [O2], with the second-order rate constants being ca. 104 times faster than those for the formation of [CoL]2+ under comparable conditions. The presence of the macrocyclic ligand slows down (by ca. 102 times) the overall rate of O2 uptake compared with linear totra-amine systems.