Heterobimetallic lanthanide helicates

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.

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

Jean-Claude Bünzli, Thomas Binderup Jensen, Rosario Scopelliti

Abstract: The heterobitopic ligands LABX (X=1,2, 3,4 or 5),diff ering only by a Cl or NEt2 substituent,have been designed to complex with a pair of lanthanide ions to form triple-stranded bimetallic helicates of overall composition [Ln2ACHTUNGTRENUNG(LABX)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 LAB2 complexes to 93–96% for LAB4 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 DH and DS,whic h vary in opposite direction along the lanthanide series for complexes with the same ligand in a way that keeps the value of DG almost constant. The results are interpreted in terms of differences in interstrand stacking,ion–dip ole interactions and metal–metal repulsion.

2007

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

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

Enantiopure helical (supra)molecular arrays containing lanthanides

Marco Antonio Lama

EPFL2006