Multielectron redox chemistry of lanthanide Schiff-base complexes

Multielectron redox chemistry, which is essential in metal catalysed chemical transformations, is not easily accessible in lanthanide complexes. Here we explored the reductive chemistry of lanthanide complexes with tetradentate Schiff bases acting as redox-active ligands with the objective of identifying new pathways to lanthanide multielectron redox transfer. The chemical reduction with alkali metals of heteroleptic [Nd(salophen)X] (salophen = N,N′-bis(salicylidene)phenylenediamine, X = I, OTf) and of a series of homoleptic K[Ln(Rsalophen)2] complexes of trivalent lanthanides has resulted respectively in the synthesis of the new dinuclear Nd(iii) complex K2[Nd2(cyclo-salophen)(THF)2] and in the synthesis of a series of mononuclear lanthanide(iii) complexes of general formula K3[Ln(bis-Rsalophen)] (R = H, Me, tBu). Ligand reduction and C-C bond formation are supported by X-ray crystal structures. Proton NMR studies demonstrate that the K2[Nd 2(cyclo-salophen)(py)2] complex can transfer four electrons in the reaction with oxidizing agents such as AgOTf through the breaking of the two C-C bonds. Moreover the electrochemistry and reactivity of the mononuclear complexes K3[Ln(bis-Rsalophen)] show that they can act as formal two electron reductants and that their oxidation potential can be tuned by changing the substituents on the ligand. These results illustrate that Schiff bases provide a new way to introduce multielectron redox events at lanthanide centers and a new route to highly reactive mono- and polynuclear complexes of lanthanides. © The Royal Society of Chemistry 2012.

Enantiopure helical (supra)molecular arrays containing lanthanides

Marco Antonio Lama

The diastereoselective synthesis of chiral coordination compounds with d metals proved the effectiveness of the pinene-bipyridine approach in predetermining the chirality at the metal centre. The strong Lewis acidic character of lanthanide cations required the modification of this class of ligands in order to adapt their coordination properties to the binding affinity of Ln(III) toward hard donor atoms like oxygen. The pinene bipyridine carboxylic derivative (+)-L- (figure 1) has been designed and synthesized to form configurationally stable lanthanide complexes. This ligand proved its effectiveness as chiral building block for the synthesis of oligometallic supramolecular structures. Indeed a self-assembly process takes place with complete diastereoselectivity between the enantiomerically pure (-) or (+) L- and Ln(III) ions (Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er) in methanol, giving rise to trinuclear enantiopure structures with general formula Ln3(L)6(µ3-OH)(H2O)32 , possessing predetermined helical chirality. The ligands in this case display a new version of supramolecular helical chirality since they are wrapping around a trimetallic core instead of one single metal centre. The same components afford a second polynuclear structure if the reaction is carried out in CH3CN. In this medium a three-dimensional tetranuclear array self-assembles (with general formula Ln4(L)9(µ3-OH)2). The nine ligands are forming three distinct helical domains, two supramolecular as in the case of the trinuclear array and one classical around a single metal centre, with predetermined configuration. The two classes of compounds have been structurally characterized in solid state (X-ray and IR) and in solution (ES-MS, NMR). Their chiroptical properties were investigated by means of Circular Dichroism and Circularly Polarized Luminescence (for the emitting compounds). Trinuclear and tetranuclear arrays are shown to interconvert in CH3CN, in a process mediated by water. This process was rationalized taking into account the nature of the Ln(III) ions, the initial concentration of the components and the addition or removal of water in solution. The availability of both the enantiomers of the ligand prompted us to study the self-recognition capabilities of the two systems. 1H-NMR and CD studies reveal an almost complete chiral recognition in the self-assembly leading to the trinuclear complex. The process leading to tetranuclear species appears instead perturbed, probably due to the higher number of fundamental components involved (4 metals and 9 ligands instead of 3 metals and 6 ligands). Figure 1: Reaction conditions leading to the synthesis of trinuclear and tetranuclear Ln(III) complexes with ligand (+)-L-. The introduction of a carboxylic moiety onto the 6' posititon of the (-)-5,6- and (-)-4,5- pinene bipyridine lead to the tridentate ligands (-)-HL1 and (-)-HL2, respectively. Mononuclear neutral complexes are obtained upon reaction with Ln(ClO4)3. Different extents of diastereoselectivity are pointed out in the two systems by X-ray analysis and CD spectroscopy. In particular the ligand (-)-HL1, in which the pinene moiety is next to the binding site, seems to be more effective in predetermining the configuration at the metal centre. A low diastereoselectivity is instead expressed in the reactions with (-)-HL2 as revealed by the X-ray structure of [Pr{(-)-L2}3] (both Δ and Λ isomers detected) and by the CD spectrum void of signal.

EPFL2006

Self-assembly of highly luminescent lanthanide complexes promoted by pyridine-tetrazolate ligands

Marinella Mazzanti

Two tridentate pyridine-tetrazolate ligands (H 2pytz and H 2pytzc), analogues of the well-known dipicolinate (H 2dpa) ligand, have been synthesized in a straightforward manner and used for lanthanide(iii) coordination. The structures of the resulting tris-ligand complexes were determined in solution ( 1H-NMR), where they remain undissociated, as well as in the solid state (X-ray diffraction). The solubility of these anionic complexes can be easily tuned by changing the countercation. The bis-tetrazolate-pyridine ligand H 2pytz sensitizes very efficiently both the visible and near-IR emission of the lanthanides, with unusually high luminescence quantum yields in solid state (61% and 65% for Eu and Tb, respectively, and 0.21% for Nd) and in water (63% for Tb and 18% for Eu). Furthermore, the absorption window of the complexes is significantly extended towards the visible region up to 330 nm. The results show that the bis-tetrazolate-pyridine ligand provides a very attractive alternative to the classic dipicolinate ligand. © 2012 The Royal Society of Chemistry.

2012

Low-dimensional supramolecular architectures at metal surfaces

Sebastian Stepanow

This thesis is concerned with supramolecular architectures assembled at metal surfaces. The investigations pursued two objectives. On the one hand, the focus was placed on the fabrication of low-dimensional surface-supported network structures by applying the concepts of conventional three-dimensional supramolecular chemistry at surfaces. Three main driving forces were explored for the construction of low-dimensional assemblies: hydrogen bonding, metal coordination and ionic interactions. The realized structures were characterized by scanning tunneling microscopy (STM) under ultra-high vacuum conditions. In particular, the interplay between the adsorbate-substrate coupling and lateral adsorbate interactions was addressed in the experiments. Further, the electronic and magnetic properties of the fabricated coordination structures were investigated. This characterization was done by x-ray absorption experiments at the synchrotron facility in Grenoble. Moreover, the response of the metal units and open cavity structures to the adsorption of large organic and small gas molecules was studied by temperature controlled STM experiments. The first part of the thesis deals with hydrogen bonded supramolecular assemblies. Simple aromatic carboxylic acids were employed to study the assembly principles and in particular effects arising from the substrate and molecular structure. The investigation of TPA (1,4-benzenedicarboxylic acid) adlayers on Cu(100) at different substrate temperatures revealed how the protonation status of the carboxyl moieties affects the topology of the assemblies. Further, the influence of the molecular backbone functionality and symmetry is addressed in comparative investigations on TPA and PDA (2,5-pyrazinedicarboxylic acid) as well as BDA (4,4'-biphenyldicarboxylic acid) and SDA (4',4" trans-ethene-l,2-diyl-bisbenzoic acid) adlayers on Cu(100). In the second part of the thesis, metal-ligand interactions were employed for the construction of supramolecular architectures at a Cu(100) surface. The concepts of conventional coordination chemistry were successfully applied at the surface. Iron atoms in conjunction with rod-like aromatic molecular linkers comprising carboxyl, pyridyl and hydroxyl functional groups assemble into a rich variety of open network structures with distinctly arranged mono- and diiron coordination centers and well-defined cavities. The results show that the adsorbate-substrate coupling considerably affects the local coordination environment of the iron units when the molecular backbone length is varied. The occurrence of structural isomerism in open network structures is addressed in a comparative study of heterofunctional and homofunctional linker molecules. It is shown that the former ligand allows only one topologically pure structure while the other linker forms two phases with different topology. The hydroxyl ligand has been used to construct distorted hexagonal coordination networks containing threefold coordinated single iron atoms. The comparison between the networks on Cu(100) and Ag(111) surfaces shows marked differences, which are attributed to adsorbate-substrate interactions. Finally, a novel design strategy for the fabrication of two-dimensional coordination structures at surfaces is introduced by using alkali metal ions in conjunction with carboxylate ligands. The interaction between the components is dominated by electrostatic forces and is of intermediate bond strength as in the case of hydrogen bonding and transition metal coordination. It is expected, that substrate symmetry and registry plays a larger role in the network formation, because the ionic interaction is not directional. The chemical as well as magnetic properties of the fabricated coordination networks are examined in the last part of the thesis. The adsorption of C60 molecules on open network structures reveals how cavity size and chemical functionality affect the bond strength to the guest molecules as well as the number of accommodated C60 molecules. Since transition metal clusters, and in particular diiron units, are the catalytic active centers in various proteins, the reactivity of various mono- and diiron coordination structures to molecular oxygen was investigated in temperature controlled STM experiments. In particular, the focus was placed on the structural relaxation upon gas adsorption and chemical reaction. At last, the magnetic properties of the metal coordination centers are addressed in XMCD experiments. The iron units are found to be paramagnetic at temperatures down to 5 K and show distinct magnetic anisotropy and magnetization loops. These experiments demonstrate that surface-supported metal coordination structures are a promising class of materials for applications in the field of heterogenous catalysis and surface magnetism.