A kinetically inert and optically active CrIII partner in thermodynamically self-assembled heterodimetallic non-covalent d–f podates

Abstract

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(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^IIL₃]⁵⁺. Air oxidation of the low-spin Cr^II complexes gives selectively the head-to-head-to-head podates (HHH)-[LnCr^IIIL₃]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^IIIL₃]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 (η = 70%) limits Eu-centred luminescence and induces directional light-conversion along the three-fold axis, resulting in Cr^III emission. For (HHH)-[TbCr^IIIL₃]⁶⁺, the better spectral overlap between the emission spectrum of Tb^III (⁵D₄ → ⁷F_J) and the absorption spectrum of Cr^III (⁴A₂ → ⁴T₂) provides a quantitative Tb^III → Cr^III energy transfer (η ≥ 99%) and long-range intermetallic communication. De-complexation of Ln^III with water or EDTA⁴⁻ gives the first inert and optically active-Cr^III-containing triple-helical nonadentate receptor (HHH)-[Cr^IIIL₃]³⁺.