Dynamic chiral-at-metal stability of tetrakis(d/l-hfc)Ln(iii) complexes capped with an alkali metal cation in solution

Abstract

Chiral tetrakis(β-diketonate) Ln(III) complexes Δ-[NaLa(d-hfc)₄(CH₃CN)] (1) and Λ-[NaLa(l-hfc)₄ (CH₃CN)] (2) (d/l-hfc⁻ = 3-heptafluo-robutylryl-(+)/(−)-camphorate) are a pair of enantiomers and crystallize in the same Sohncke space group (P2₁2₁2₁) with dodecahedral (DD) geometry. Typically positive and negative exciton splitting patterns around 320 nm were observed in the solid-state circular dichroism (CD) spectra of complexes 1 and 2, which indicate that their shell configurational chiralities are Δ and Λ, respectively. The apparent bisignate couplets in the solid-state CD spectra of [CsLn(d-hfc)₄(H₂O)] [Ln = La (3), Yb (5)] and [CsLn(l-hfc)₄(H₂O)] [Ln = La (4), Yb (6)] show that they are a pair of enantiomers and their absolute configurations are denoted Δ and Λ, respectively. The crystallographic data of 5 reveals that its coordination polyhedron is the square antiprism (SAP) geometry and it undergoes a phase transition from triclinic (α phase, P1) to monoclinic (β phase, C2) upon cooling. The difference between the two phases is brought about by the temperature dependent behaviour of the coordination water molecules, but this did not affect the configurational chirality of the Δ-SAP-[Yb(d-hfc)₄]⁻ moiety. Furthermore, time-dependent CD, UV-vis and ¹⁹F NMR were applied to study the solution behavior of these complexes. It was found that the chiral-at-metal stability of the three pairs of complexes is different and affected by both the Ln³⁺ and M⁺ ion size. The results show that the Cs⁺ cation can retain the metal center chirality and stablize the structures of [Ln(d/l-hfc)₄]⁻ or the dissociated tris(d/l-hfc)Ln(III) species in solution for a longer time than that of the Na⁺ cation, and it is important that the Cs⁺ ion successfully lock the configurational chirality around the Yb³⁺ center of the complex species in solution. This is reasoned by the short Cs⁺⋯FC, Cs⁺⋯O–Yb and Cs⁺⋯Yb³⁺ interactions observed in the crystal structure of α-5 and further confirmed by the chiral self-assembly of 5 or 6 from [Yb(H₂O)(d/l-hfc)₃] induced by CsI in a CHCl₃ solution.