Unveiling controlled breaking of the mirror symmetry of Eu(fod)3 with α-/β-pinene and BINAP by circularly polarised luminescence (CPL), CPL excitation, and 19F-/31P{1H}-NMR spectra and Mulliken charges

Abstract

We investigated whether the ‘Pfeiffer effect’ of optically inactive Eu(fod)₃ (fod = 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionate) is detectable when it is dissolved in four chiral hydrocarbons, namely, (1S)-/(1R)-α-pinene, (1S)-/(1R)-β-pinene, (1S)-/(1R)-trans-pinane and (S)-/(R)-limonene. Among these solvents, α-pinene and β-pinene induced clear circularly polarised luminescence (CPL) signals in CPL-silent Eu(fod)₃ at the electric dipole (ED)-forbidden/magnetic dipole (MD)-allowed ⁵D₀ → ⁷F₁ transition at 593 nm and the ED-allowed/MD-forbidden ⁵D₀ → ⁷F₂ transition at 613 nm due to Eu(III). The CPL excitation (CPLE) spectra monitoring the ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transitions confirmed that these CPL bands are induced by the chirality of α-pinene. Eu(fod)₃ in the presence of non-charged chiral aromatic additives, including (S)-/(R)-2,2′-bis(diphenylphosphino)-1,1′-binaphthyl (BINAP), (S)-/(R)-[1,1′-binaphthalene]-2,2′-diylbis[1,1-diphenyl-1,1′-phosphine-oxide] (BINAPO) and (S)-/(R)-α-phenylethylamine (PEA), also showed mirror-image CPL spectra with different extents of dissymmetry factors in their CPL signals. The ¹⁹F-NMR spectra revealed that the CPL signals had two different origins, which can be categorized as the Pfeiffer-perturbed system at an outer-sphere and a strong coordinative interaction at an inner-sphere Eu(III). The former was exemplified by α-pinene, BINAP and dilute PEA, whereas the latter was observed with BINAPO and neat PEA. The strong coordinative interaction of Eu(fod)₃ with BINAPO was verified by comparing its ³¹P{¹H}-NMR spectrum with that in the presence of BINAP. The Mulliken charges of Eu(fod)₃ and the chiral compounds obtained with MP2(6-311G) calculations led us to hypothesize that the surplus charge neutralization can act as an invisible attractive force between several ligands in the Eu(III) complexes and CPL-inducible chiral compounds.