Lanthanides complexation kinetics

It is important to note that the kinetics of lanthanide complexation reactions in general involve rapid association and dissociation reactions, except for structurally complex ligands like edta. Generally, lanthanide complexation kinetics in aqueous media can be considered sufficiently rapid as to have minimal effect on separations. Phase-transfer rates may be important in some systems, and should be considered in the optimization of an analytical separation procedure. The kinetics of lanthanide complexation reactions has been discussed in a previous report (Nash and Sulhvan 1991). There has been some consideration of kinetics-based separations for f-elements (Nash 1994, Merciny et al. 1986), but no useful analytical applications based solely on differences in lanthanide kinetics are known. 

Assays based on luminescent lanthanide ions were developed initially in the 1970s, when instrumentation became available which could distinguish long-lived luminescence from a shortlived background. Leif and co-workers reported the first attempts to use lanthanide complexes (in this case europium complexes with 1,10-phenanthroline and 7-diketonates, i.e., [Eu(phen)(diketo-nate)3]) as tags for antibodies.107 These proved kinetically unstable in the pH regime required... 

In this reaction, a rhodium atom complexed to a chiral diphosphine ligand ( P—P ) catalyzes the hydrogenation of a prochiral enamide, with essentially complete enan-tioselectivity and limiting kinetic rates exceeding hundreds of catalyst turnovers per second. While precious metals such as Ru, Rh, and Ir are notably effective for catalysis of hydrogenation reactions, many other transition-metal and lanthanide complexes exhibit similar potency. 

For the application of lanthanide complexes in medical diagnosis and therapy, a general requirement is that the ion Ln3+ and the ligand should remain associated while the complex is in the body, i. e. their dissociation should be minimal, since the free ligand and Ln3+ are toxic. For the dissociation to be negligible, the complexes must be kinetically inert under physiological conditions. Since the complexation properties of the lanthanide ions and Y3+ are quite similar, it is of interest to compare the results obtained as concerns the kinetic behavior of Gd3+ complexes with those known for the complexes of other lanthanides and Y3+. 

The use of aqueous chiral lanthanide complexes in the determination of the enantiomeric purity of chiral a-hydroxy acids has also been assessed by H NMR [21], Large lanthanide induced shifts, chemical shift non-equivalence and an apparent absence of kinetic resolution in complex formation is observed upon addition of racemic lactate to [Yb.3a]3+ (Figure 1). The lactate CH3 resonances are clearly resolved for the... 

The most thermodynamically stable and kinetically inert complexes of the trivalent lanthanides are those of the ligand DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate) (42, 43). Our search for lanthanide macrocyclic complexes that would remain intact for longer time periods led us to examine derivatives of DOTA. There are two potential difficulties with the use of DOTA complexes of the trivalent lanthanides for RNA cleavage. First, the overall negative charge on the complex is not conducive to anion binding for example, Gd(DOTA)-does not bind hydroxide well (44). Second, DOTA complexes of the middle lanthanides Eu(III) and Gd(III) have only one available coordination site for catalysis. The previous lanthanide complexes that we used, e.g., Eu(L1)3+, were good catalysts and had at least two available coordination sites. 

The dissociation kinetics of lanthanide complexes with the macrocycle, 1,7-diaza-4,10,13-trioxacyclopentadecane-A, A -diacetic acid (K21DA) were studied in acetic acid-acetate buffer with Cu2+ as the scavenger of the ligand [73]. The dissociation rates were... 

Kinetics of formation and dissociation of lanthanide complexes [La(III) = Pr, Eu, Tb, Ho, and Yb] with l-phenyl-3-methyl-4-benzoyl-5-pyrazolone (HPMBP) in toluene-water phase were studied by monitoring the fate of La3+— arsenazo III (AZ) complex (MAZ) in the aqueous phase with the hydrophilic separator. The structures of the ligands and AZ are given below ... 

Poole, R.A., Bobba, G, Cann, M.J., et al. (2005) Synthesis and characterisation of highly emissive and kinetically stable lanthanide complexes suitable for usage in cellulo . Organic and Biomolecular Chemistry, 3, 1013-1024. 

The lanthanide complexes lack distinct ultraviolet-visible spectra and, hence, kinetic information on their complex formation and dissociation reactions was obtained indirectly by the metallochromic indicator method [9]. These studies indicate that in the case of the Cyanex 272 complexes, the CPC efficiencies are mainly limited by the slow dissociation of the M(HL2)HL+ complex at the heptane-H20 interface. 

Edta-related lanthanide complexes are of course coordinatively unsaturated in the absence of other ligands. In water, they are hydrated and the water of hydration may be displaced by other species. The kinetics of such a process, the reaction of europium 1,2-diaminocyclohexane-tetraacetate, [Eu(dcta)aq] with iminodiacetate ion, ida , to give [Eu(dcta)(ida)] ", have been studied by the elegant method of selective tunable laser excitation of the transition in... 

In addition to a detailed discussion of hydration dynamics of the lanthanides, Lincoln s review also covers the kinetics of solvation in nonaqueous media and complexation kinetics. As our focus is on the aqueous chemistry of the lanthanides, we will not discuss recent developments in nonaqueous lanthanide solution chemistry. The intervening four years have seen the publication of a handful of studies of the kinetics of lanthanide chelate dissociation kinetics. In the following section we will discuss the best of these results. 

Hambright et al. (1988) have also studied the kinetics of displacement of the Gd " ion from the gadolinium(III) complex of TSPP by ethylenediaminetetraacetate (EDTA) giving Gd(EDTA) and H2(TSPP) as the main products. This represents the first example of metal removal from a metalloporphyrin by a chelating ligand. A mechanism has been proposed to account for the kinetic data. The water-soluble lanthanide complexes of TMPyP also undergo demetallation in the presence of EDTA (Haye and Hambright 1991). Similar to the acid solvolysis reactions, a linear relationship between log k and the ionic radius of the metal center can be established, and complexes with smaller metal center are more stable toward demetallation by EDTA. 

Polymerization of diallg l vinylphosphonate monomers was nevertheless efficiently carried out in the presence of lanthanide derivatives and especially cyclopentadienyl lanthanide complexes, used both as initiators and catalysts. Very recently, Shen et al performed the synthesis of poly (diethyl vinylphosphonate) using a lanthanide tris(borohydride) below 50 °C. The authors showed that the polymerization eould be controlled and proceeds under pseudo-first-order kinetics, giving rise to high molecular weight polymers, i.e. ranging from 20 to 40 kDa with molecular weight dispersity below 1.7. 

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