Title
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
Author
Abd Jalil Jalilah, Fumio Asanoma, Michiya Fujiki
Year
2018
Journal
Inorganic Chemistry Frontiers
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 invisible attractive forces between several ligands in the Eu(III) complexes and CPL-inducible chiral compounds.
Instrument
J-820