Structure–chiroptical property relationship of kinetically labile camphor-derivative β-diketone Yb(iii) complexes: do the adducts coexist as diastereomers or not?

Abstract

The present work examines the relationship between the crystal structures and chiroptical properties of four chiral Yb(III) complexes with camphor-derivative β-diketone ligands by means of solid-state circular dichroism (CD) spectroscopy. For the seven-coordinate complexes, [Yb(H₂O)(d-hfc)₃] (I) and [Yb(H₂O)(l-hfc)₃] (II) (d/l-hfc⁻ = 3-heptafluorobutyryl-(+)/(−)-camphorate), the Λ- and Δ-diastereomers coexist in their crystals and no apparent bisignate couplets are observed in their solid-state CD spectra. A theoretical study indicates that the ground-state energy difference between the two diastereomers I and II is only 0.913 kcal mol⁻¹, which explains why they could coexist in a crystal environment with the ratio of 1 : 1. While, eight-coordinate complexes Δ-[Yb(TPPO)₂(d-hfc)₃]·CHCl₃·3C₆H₁₂ (III) and Λ-[Yb(TPPO)₂(l-hfc)₃]·CHCl₃·3C₆H₁₂ (IV) (TPPO = triphenylphosphine oxide) are enantiopure in the solid-state, and typically negative and positive exciton splitting patterns around 330 nm are observed in their solid-state CD spectra. The solid-state CD spectra of these four complexes are in accordance with their X-ray single-crystal analyses. Besides, their solution CD spectra show that no particular isomer predominates in solution.