Heterometallic Complexes Containing Lanthanides

Stephen Faulkner and Manuel Tropiano Chemistry Research Laboratory, University of Oxford, UK 

Lanthanide complexes containing more than one metal ion can have clearly defined properties that derive from their structure, and that can be controlled and exploited in a wide variety of ways. However, before addressing these, we must consider how the nature of the chemistry of the/-block elements defines approaches to the preparation of heterometallic systems. The study of the coordination chemistry of/-block ions in the solid phase and in solution has advanced enormously over the course of the last half-century, and the key principles that define its use have developed over the course of time — illustrating in microcosm the underpinning principles of coordination chemistry and supramolecular chemistry. 

Coordination compounds formed by hard and charged donors with lanthanides, like those of d- and s-block metal ions, are usually thermodynamically stable — especially if coordinated by chelating ligands. Table 9.1 shows a comparison of thermodynamic constants for some key polydentate ligand systems in common with lanthanides. 

The kinetic stability of the lanthanide complexes wifli polydentate aminocarboxylate ligands has been extensively studied mainly for gadoUunium complexes given their use as MRI contrast agents. In this case the literature clearly shows a relationship between the rigidity of the ligand and the kinetics of formation and dissociation of the metal complexes. Due to their flexible stmcture, the complexation of lanthanide ions by EDTA and DTPA is a fast process. In the case of DTPA and its derivatives, the kinetics of complexation is 

When considering the kinetic stability of a lanthanide chelate, dissociation and trans-metallation reactions are the main processes that must be taken into account. For complexes formed with EDTA, the proton-assisted dissociation dominates over direct attack by another metal ion. Indeed, the structure does not leave space for direct attack, because none of the acetate arms remains exposed [7]. This can be contrasted with the DTPA case where the two residual negative charges on Ln(DTPA) make the direct attack reaction more likely. A study by Briicher et al. [8] demonstrated that at physiological pH the transmetallation mechanism dominates and that the kinetics of the process largely depends on the attacking 

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Another type of lanthanide-containing magnetic materials are 4f-3d/4d heterometallic complexes, which have attracted much attention as part of the tremendous development of molecular magnetism [2, 3]. 

Most heterometallic cluster species containing both lanthanide and transition metal elements in the cluster core structure will not be included. Such cluster complexes are of tremendous fundamental and practical interests, and interested readers would be better served with review articles dedicated just to this particular family of lanthanide-containing materials (Benelli and Gatteschi, 2002). However, heterometallic clusters containing recognizable cluster motifs composed solely of lanthanide elements will be discussed. 

Natrajan LS, Villaraza AJL, Kenwright AM, Faulkner S (2009) Controlled preparation of a heterometallic lanthanide complex containing different lanthanides in symmetrical binding pockets. Chem Commun 6020-6022... 

At the simplest, an anionic lanthanide complex can interact with a cation to form a labile heterometallic complex in solution. Such complexes have been known for many years, though they have not always been recognised as such. For instance, Bryden et al. [14] showed that Na shifts were observed for sodium chloride solutions containing lanthanide complexes of DOTA. Fig. 9.2 shows the variation in observed Na chemical shift between... 

The solid phase lends itself to the preparation of lanthanide containing heterometallic materials. In this case the choice of lanthanide complexes is ample since it is not only limited to kinetically inert complexes. The solid state acts as kinetic trap so that building blocks formed by p-diketonates are widely applied for designing multimetallic architectures such as the one represented in Fig. 9.3 [17—19]. 

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