A kinetically inert and optically active Cr~(III) partner in thermodynamically self-assembled heterodimetallic non-covalent d-f podates

摘要

Stoichiometric mixing of the segmental ligand 2-{6-[N,N-diethylcarboxamido]pyridin-2-yl}-1,1-dimethyl-5,5'-methylene-2'-(5-methylpyridin-2-yl)bis[1H-benzimidazole] (L) with Ln(CF_3SO_3)_3 (Ln = La-Lu) and Cr(CF_3SO_3)_2 under an inert atmosphere produces quantitatively the self-assembled triple-stranded non-covalent podates (HHH)-[LnCr~IIL_3]~(5+). Air oxidation of the low-spin Cr~II complexes gives selectively the head-to-head-to-head podates (HHH)-[LnCr~IIL_3]~(6+) into which inert Cr~(III) has been incorporated. The X-ray crystal structures of [LnCr~(III)(L)_3](CF_3SO_3)_6(CH_3CN)_4 (Ln = Eu, 7; Ln = Lu, 8) confirm the formatin 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 temperatures, 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)-[TbCr~(III)L_3]~(6+), the better spectral overlap between the emission spectrum of Tb~(III)(~5D_4->~7F_J) and the absorption spectrum of Cr~(III)(~4A_2-~4T_2) provides a quantitative Tb~(III)->Cr~(III) energy transfer (#eta#>= 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)-[Cr~(III)L_3]~(3+).