Jean-Claude Bunzli - Tuning facial-meridional isomerisation in monometallic nine-co-ordinate lanthanide complexes with unsymmetrical tridentate ligands

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  Le Borgne,T., Altmann,P., Andre,N., Bunzli,J.C.G.,   Bernardinelli,G., Morgantini,P.Y., Weber,J., Piguet,C. Dalton   Transactions 2004 (5) 723-733

  Abstract: The unsymmetrical tridentate   benzimidazole-pyridine-carboxamide units in ligands L1-L4 react   with trivalent lanthanides, Ln(III), to give the nine-co-ordinate   triple-helical complexes [Ln(Li)(3)](3+) (i=1-4) existing as   mixtures of C-3-symmetrical facial and C-1-symmetrical meridional   isomers. Although the beta(13) formation constants are 3-4 orders   of magnitude smaller for these complexes than those found for the   D-3-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+)reversible   arrowmer-[Ln(Li)(3)](3+) (i=1-4) isomerisation process in   acetonitrile is slow enough for Ln=Lu-III to be quantified by H-1   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 pi-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 1:3 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, P2(1)/c, Z=4) and   [Eu(L4)(2)(CF3SO3)(2)(H2O)](CF3SO3)(C4H4O)(1.5) (7,   C51H66EuF9N8O15.5S3, triclinic, P (1) over bar, Z=2) provide   crucial structural information on the binding mode of the   unsymmetrical tridentate ligands

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