Heterobimetallic lanthanide helicates: deciphering substituent effects by crystallography and NMR analysis

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)3](ClO4)6 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.

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