Jean-Claude Bunzli - A kinetically inert and optically active Cr-III partner in thermodynamically self-assembled heterodimetallic non-covalent d-f podates

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  Cantuel,M., Bernardinelli,G., Imbert,D., Bunzli,J.C.G.,   Hopfgartner,G., Piguet,C. Journal of the Chemical   Society-Dalton Transactions 2002 (9)   1929-1940

  Abstract: Stoichiometric mixing of the segmental   ligand   2-{6-[N,N-diethylcarboxamido]pyridin-2-yl}-1,1'-dimethyl-5,5'-methylene-2'-(5-m ethylpyridin-2-yl)   bis[1H-benzimidazole] (L) with Ln(CF3SO3)(3) (Ln = La-Lu) and   Cr(CF3SO3)(2) under an inert atmosphere produces quantitatively   the self-assembled triple-stranded non-covalent podates   (HHH)-[LnCr(II)L(3)](5+). Air oxidation of the low-spin Cr-II   complexes gives selectively the head-to-head-to-head podates   (HHH)-[LnCr(III)L(3)](6+) into which inert Cr-III has been   incorporated. The X-ray crystal structures of   [LnCr(III)(L)(3)](CF3SO3)(6)(CH3CN)(4) (Ln = Eu, 7; Ln = Lu, 8)   confirm the formation of regular triple-helical cations   (HHH)-[LnCr(III)L(3)](6+) whose structure is maintained in   acetonitrile according to ESI-MS, spectrophotometry and NMR data.   Photophysical studies evidence efficient sensitization of both   Eu-III and Cr-III through ligand excitation at low temperature,   while a subsequent intramolecular Eu-III-->Cr-III energy   transfer (eta = 70%) limits Eu-centred luminescence and induces   directional light-conversion along the three-fold axis, resulting   in Cr-III emission. For (HHH)-[(TbCrL3)-L-III](6+), the better   spectral overlap between the emission spectrum of Tb-III (D-5(4)   --> F-7(J)) and the absorption spectrum of Cr-III   ((4)A(2)-->T-4(2)) provides a quantitative Tb-III Cr-III   energy transfer (eta greater than or equal to 99%) and long-range   intermetallic communication. De-complexation of Ln(III) with   water or EDTA(4-) gives the first inert and optically   active-Cr-III-containing triple-helical nonadentate receptor   (HHH)-[(CrL3)-L-III](3+)

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