Lanthanide Triple-Stranded Helicates: Controlling the Yield of the Heterobimetallic Species

Hexanuclear Ln6 L6 Complex Formation by Using an Unsymmetric Ligand

Multinuclear, self-assembled lanthanide complexes present clear opportunities as sensors and imaging agents. Despite the widely acknowledged potential of this class of supramolecule, synthetic and characterization challenges continue to limit systematic studies into their self-assembly restricting the number and variety of lanthanide architectures reported relative to their transition metal counterparts. Here we present the first study evaluating the effect of ligand backbone symmetry on multinuclear lanthanide complex self-assembly. Replacement of a symmetric ethylene linker with an unsymmetric amide at the center of a homoditopic ligand governs formation of an unusual Ln6 L6 complex with coordinatively unsaturated metal centers. The choice of triflate as a counterion, and the effect of ionic radii are shown to be critical for formation of the Ln6 L6 complex. The atypical Ln6 L6 architecture is characterized using a combination of mass spectrometry, luminescence, DOSY NMR and EPR spectroscopy measurements. Luminescence experiments support clear differences between comparable Eu6 L6 and Eu2 L3 complexes, with relatively short luminescent lifetimes and low quantum yields observed for the Eu6 L6 structure indicative of non-radiative decay processes. Synthesis of the Gd6 L6 analogue allows three distinct Gd⋯Gd distance measurements to be extracted using homo-RIDME EPR experiments.

Sorting Phenomena and Chirality Transfer in Fluoride-Bridged Macrocyclic Rare Earth Complexes

The reaction of fluoride anions with mononuclear lanthanide(III) and yttrium(III) hexaaza-macrocyclic complexes results in the formation of dinuclear fluoride-bridged complexes. As indicated by X-ray crystal structures, in these complexes two metal ions bound by the macrocycles are linked by two or three bridging fluoride anions, depending on the type of the macrocycle. In the case of the chiral hexaaza-macrocycle L1 derived from trans-1,2-diaminocyclohexane, the formation of these μ2-fluorido dinuclear complexes is accompanied by enantiomeric self-recognition of macrocyclic units. In contrast, this kind of recognition is not observed in the case of complexes of the chiral macrocycle L2 derived from 1,2-diphenylethylenediamine. The reaction of fluoride with a mixture of mononuclear complexes of L1 and L2, containing two different Ln(III) ions, results in narcissistic sorting of macrocyclic units. Conversely, a similar reaction involving mononuclear complexes of L1 and complexes of achiral macrocycle L3 based on ethylenediamine results in sociable sorting of macrocyclic units and preferable formation of heterodinuclear complexes. In addition, formation of these heterodinuclear complexes is accompanied by chirality transfer from the chiral macrocycle L1 to the achiral macrocycle L3 as indicated by CPL and CD spectra.

Multimetallic Lanthanide Complexes: Using Kinetic Control To Define Complex Multimetallic Arrays

Kinetically inert lanthanide complexes are proving to be highly effective building blocks for the preparation of complex heterometallic architectures, allowing complete control of metal ion domains, which cannot be achieved under thermodynamic control. Kinetic stability may render perceivable labile coordination bonds more durable than several types of covalent interactions. For complexes in clinical use, the significance of kinetic stability cannot be overstated, and this Account treats the topic accordingly. Kinetically inert complexes can be used as building blocks for elaborate synthesis. For instance, it is now possible to prepare heterometallic lanthanide complexes containing two or more different lanthanide ions by linking kinetically robust complexes together. This approach can yield bimetallic (f-f' or d-f) and trimetallic (f-f'-f″) lanthanide complexes. In this Account, we describe our studies exploiting the slow dissociation of lanthanide complexes derived from 1,4,7,10-tetraazadodecane-1,4,7,10-tetraacetic acid (DOTA) related ligands to link complexes together through synthetic manipulation of pendent groups on the ligand skeleton or through coordination of bridging donor groups to a d-block metal center. In the course of this work, we have developed a variety of such methods, ranging from peptide coupling and diazotization to Ugi and click chemistry and have also explored the use of alternative strategies that combine orthogonal protecting group chemistry with sequential complexation of different lanthanide ions or that use self-assembly to deliver well-defined multimetallic systems. These well-defined bimetallic systems also have considerable scope for exploitation. Since the earliest studies, it has been clear that there is potential for application in the burgeoning field of molecular imaging. Heterometallic lanthanide complexes can be used as single-molecule bimodal imaging agents through incorporation of MRI active and luminescent components. Alternatively, conventional luminescence methods can be exploited in conjunction with lanthanide luminescence. In the simplest cases, a single lanthanide can be used to achieve a switchable response in combination with a transition metal complex. Bimetallic f-f' complexes allow the full potential of the approach to be realized in systems in which one lanthanide responds to changes in the concentration of an analyte, while a second lanthanide center can be used to define the concentration of the probe itself. This offers a new solution to the old dichotomy of ratiometric imaging that can potentially be applied widely.

Enantiomeric self-recognition in homo- and heterodinuclear macrocyclic lanthanide(III) complexes

The controlled formation of lanthanide(III) dinuclear μ-hydroxo-bridged [Ln(2)L(2)(μ-OH)(2)X(2)](n+) complexes (where X = H(2)O, NO(3)(-), or Cl(-)) of the enantiopure chiral macrocycle L is reported. The (1)H and (13)C NMR resonances of these complexes have been assigned on the basis of COSY, NOESY, TOCSY, and HMQC spectra. The observed NOE connectivities confirm that the dimeric solid-state structure is retained in solution. The enantiomeric nature of the obtained chiral complexes and binding of hydroxide anions are reflected in their CD spectra. The formation of the dimeric complexes is accompanied by a complete enantiomeric self-recognition of the chiral macrocyclic units. The reaction of NaOH with a mixture of two different mononuclear lanthanide(III) complexes, [Ln(1)L](3+) and [Ln(2)L](3+), results in formation of the heterodinuclear [Ln(1)Ln(2)L(2)(μ-OH)(2)X(2)](n+) complexes as well as the corresponding homodinuclear complexes. The formation of the heterodinuclear complex is directly confirmed by the NOESY spectra of [EuLuL(2)(μ-OH)(2)(H(2)O)(2)](4+), which reveal close contacts between the macrocyclic unit containing the Eu(III) ion and the macrocyclic unit containing the Lu(III) ion. While the relative amounts of homo- and heterodinuclear complexes are statistical for the two lanthanide(III) ions of similar radii, a clear preference for the formation of heterodinuclear species is observed when the two mononuclear complexes contain lanthanide(III) ions of markedly different sizes, e.g., La(III) and Yb(III). The formation of heterodinuclear complexes is accompanied by the self-sorting of the chiral macrocyclic units based on their chirality. The reactions of NaOH with a pair of homochiral or racemic mononuclear complexes, [Ln(1)L(RRRR)](3+)/[Ln(2)L(RRRR)](3+), [Ln(1)L(SSSS)](3+)/[Ln(2)L(SSSS)](3+), or [Ln(1)L(rac)](3+)/[Ln(2)L(rac)](3+), results in mixtures of homochiral, homodinuclear and homochiral, heterodinuclear complexes. On the contrary, no heterochiral, heterodinuclear complexes [Ln(1)L(RRRR)Ln(2)L(SSSS)(μ-OH)(2)X(2)](n+) are formed in the reactions of two different mononuclear complexes of opposite chirality.

A Polyoxyethylene-Substituted Bimetallic Europium Helicate for Luminescent Staining of Living Cells

The homoditopic ligand H2LC3 has been designed to form neutral triple-stranded bimetallic helicates of overall composition [Ln2(LC3)3]. The grafting of the polyoxyethylene fragments ensures water solubility and also favors cell penetration while being amenable to further derivatization. The ligand pKa values have been determined by spectrophotometric titration and range from 3.5 (sum of the first two) to 10.3. The thermodynamic stability of the helicates is large at physiological pH (logbeta23 in the range 22-23). The ligand triplet state has an adequate energy (0-phonon transition at approximately 20,800 cm(-1)) for sensitizing the luminescence of EuIII (Q=11%). Analysis of the EuIII emission spectrum points to an overall pseudo D3 symmetry for the metal environment. No significant effect of [Eu2(LC3)3] is observed on the viability of several cancerous cell lines (MCF-7, HeLa, Jurkat, and 5D10). The cell imaging properties of the EuIII helicate are demonstrated for the HeLa cell line by luminescence microscopy. Bright EuIII emission is seen for helicate concentration>50 microM and after 20-30 min loading time. The helicate stains the cytoplasm and the permeation mechanism is likely to be endocytosis.

Thermodynamic Parameters Governing the Self-Assembly of Head–Head–Head Lanthanide Bimetallic Helicates

The heterobitopic ligands L ABX (X=1, 2, 3, 4 or 5), differing only by a Cl or NEt(2) substituent, have been designed to complex with a pair of lanthanide ions to form triple-stranded bimetallic helicates of overall composition [Ln2(L ABX)3]6+. The percentage of HHH (head-head-head) isomer, in which each of the three ligand strands coordinates to the same lanthanide ion with the same coordination unit, is deciding the ability of the ligands to selectively form heterobimetallic complexes containing one luminescent and one magnetic or two different luminescent ions. It deviates significantly from the statistical value of 25 % and ranges from 6-20 % for L AB2 complexes to 93-96 % for L AB4 complexes. The equilibrium between HHT (head-head-tail) and HHH isomers has been investigated in detail for homobimetallic helicates (Ln=Y, La, Ce, Pr, Nd, Sm, Eu, Lu) by means of variable temperature NMR and thermodynamic parameters have been determined. The equilibrium is characterized by small values of DeltaH and DeltaS, which vary in opposite direction along the lanthanide series for complexes with the same ligand in a way that keeps the value of DeltaG almost constant. The results are interpreted in terms of differences in interstrand stacking, ion-dipole interactions and metal-metal repulsion.

Recent advances in the formation of luminescent lanthanide architectures and self-assemblies from structurally defined ligands

This article gives some highlights of the recent advances in the development of novel lanthanide based complexes, conjugates and self-assembly structures formed from the use of organic ligands and organo-metallic (transition metal) complexes, that are designed with the aim of capitalising on the high coordination requirement of the lanthanide ions. The examples shown, demonstrate the versatility of the lanthanide ions as luminescent probes and sensors that emit at long wavelength either in the visible or the near infrared (NIR) part of the electromagnetic spectrum.