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Tuning facial-meridional isomerisation in monometallic nine-coordinate lanthanide complexes with unsymmetrical tridentate ligands
T. Le Borgne, P. Altmann, N. André, J.-C.G. Bünzli, G. Bernardinelli, , J. Weber and C. Piguet
Dalton Transactions, (5) (2004), p723-733
DOI:10.1039/b316035a | unige:3251 | Abstract | Article HTML | Article PDF
The unsymmetrical tridentate benzimidazole–pyridine–carboxamide units in ligands L1–L4 react with trivalent lanthanides, LnIII, to give the nine-co-ordinate triple-helical complexes [Ln(Li)3]3+ (i=1–4) existing as mixtures of C3-symmetrical facial and C1-symmetrical meridional isomers. Although the β13 formation constants are 3–4 orders of magnitude smaller for these complexes than those found for the D3-symmetrical analogues [Ln(Li)3]3+ (i=5–6) with symmetrical ligands, their formation at the millimolar scale is quantitative and the emission quantum yield of [Eu(L2)3]3+ is significantly larger. The fac-[Ln(Li)3]3+↔mer-[Ln(Li)3]3+ (i =1–4) isomerisation process in acetonitrile is slow enough for Ln=LuIII to be quantified by 1H NMR below room temperature. The separation of enthalpic and entropic contributions shows that the distribution of the facial and meridional isomers can be tuned by the judicious peripheral substitution of the ligands affecting the interstrand interactions. Molecular mechanics (MM) calculations suggest that one supplementary interstrand -stacking interaction stabilises the meridional isomers, while the facial isomers benefit from more favourable electrostatic contributions. As a result of the mixture of facial and meridional isomers in solution, we were unable to obtain single crystals of 13 complexes, but the X-ray crystal structures of their nine-co-ordinate precursors [Eu(L1)2(CF3SO3)2(H2O)](CF3SO3)(C3H5N)2(H2O) ( 6, C45H54EuF9N10O13S3, monoclinic, P21/c, Z=4) and [Eu(L4)2(CF3SO3)2(H2O)](CF3SO3)(C4H4O)1.5 ( 7, C51H66EuF9N8O15.5S3, triclinic, P, Z=2) provide crucial structural information on the binding mode of the unsymmetrical tridentate ligands.