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

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.

Heterobimetallic lanthanide helicates

Thomas Binderup Jensen

Based on the ligand LAB the new heterobitopic ligands LAB2, LAB3, LAB4 and LAB5 have been designed and synthesised. The five ligands were designed to selectively form heterobimetallic complexes of composition LnLn'(L)36 when reacted with a pair of lanthanide ions Ln(ClO4)3 and Ln'(ClO4)3 in solution. The benzimidazole-pyridine-benzimidazole (bpb) moiety codes for the larger and softer lanthanide ions, while the benzimidazole-pyridine-amide (bpa) moiety codes for the smaller and harder ions of the lanthanide series. When reacted with 1/3 equivalent La3+ and 1/3 equivalent Lu3+ LAB forms in excess of 90 % [LnLn'(L)3]6+, the percentage being lower for pairs of lanthanide ions closer in size. The substituents on the new ligands modify the hardness of the nitrogen atom of the pyridyl group. The electron donating NEt2 group in LAB2 increases the hardness of the bpa group, which should improve the selectivity of the ligand. A similar improvement was expected for LAB5 in which the electron withdrawing Cl substituent makes the bpb moiety of the ligand softer. The two other new ligands LAB3 and LAB4 were, based on similar arguments, not expected to exhibit improved selectivity and were synthesised for comparison. Due to the tendency of LAB2 and LAB5 to form high proportions of HHT (Head-Head-Tail) isomers their selectivity turned out to be lower than anticipated. The proportion of heterobimetallic complexes reaches only 65 and 92 %, respectively, for the LaLu couple of lanthanides. The ligands LAB3 and LAB4, as expected, also give lower yields (up to 87 and 79 %) of heterobimetallic complexes. To investigate the details of the Head-Head-Head/Head-Head-Tail equilibrium variable temperature NMR measurements were carried out and the H and S values were determined for 26 different complexes. It was found that the equilibria were characterised by a subtle interplay of enthalpic and entropic effects, with the two isomers differing only by a few kJ/mol. Structures of five complexes of LAB3 were determined by X-ray diffraction. The structures contain molecular ions of composition [LnLn'(L)3]6+ in which the three ligand strands are wrapped around the two lanthanide ions in a helical fashion. To obtain information about the structure of the complexes in solution analysis of the lanthanide induced paramagnetic shift has been carried out. The so-called one proton analysis showed that contributions from two paramagnetic lanthanide ions in the same complex are additive. Extraction of structural factors demonstrated that the complexes of all the ligands are isostructural in solution. Comparison with the X-ray data proved that the solid state structures are maintained in solution. Finally, a modified one proton analysis has been devised to separate contact and pseudo contact shifts while taking into account the variation of the crystal field parameter B20 along the lanthanide series.

EPFL2006

Multielectron redox chemistry of lanthanide Schiff-base complexes

Marinella Mazzanti

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.

2012

Molecular Control of Macroscopic Cubic, Columnar, and Lamellar Organizations in Luminescent Lanthanide-Containing Thermotropic Liquid Crystals

Abstract: The connection of lipophilic gallic acid derivatives at the 5,5¢- or 6,6¢-positions of the rigid 2,6-bis(1-ethyl-benzimidazol-2-yl)pyridine core provides two pro-mesogenic tridentate ligands L10 and L12, whose molecular shapes, anisometries, and directional intermolecular ð-stacking can be tuned. X-ray diffraction data in the crystalline state, combined with solution 1H NMR measurements, show that complexation with trivalent lanthanides, Ln(III), produces the neutral hemi-disklike complexes [Ln(Li)(NO3)3] i ) 10, 12), which dimerize to give the rodlike bimetallic complexes [Ln2(Li)2(NO3)6] at lower temperature. The relevant thermodynamic parameters for the latter process depend on the nature of the ligand, the size of the metal ion, and the strength of the intermolecular interactions involved in the condensed phase. These three-dimensional models obtained for the complexes in the crystals and in solution are eventually confronted with small-angle XRD profiles recorded in the intermediate thermotropic liquid crystalline phase, in which the rigidity of the packed polyaromatic cores is maintained, while the alkyl chains are molten. According to the specific geometries and nuclearities of the molecular complexes, three types of mesophases (lamellar, columnar, and cubic) can be induced, which provides a direct correlation between the microscopic arrangements and the macroscopic ordering in lanthanide-containing metallomesogens.

2005

Heterobimetallic lanthanide helicates

Thomas Binderup Jensen

EPFL2006