Thallium(III) Complexes of a Chlorophyll‐ <i>a</i> Derivative Asymmetrically Coordinated With Single Axial Anionic Ligands

ABSTRACT Methyl pyropheophorbide‐ a , one of the chlorophyll‐ a derivatives, was reacted with thallium(III) trifluoroacetate in tetrahydrofuran at room temperature to yield the corresponding Tl(III) complex with trifluoroacetate as an axial ligand. The anionic trifluoroacetate ligand was readily exchanged for chloride by treatment with brine. The resulting five‐coordinated complex with Cl − was fully characterized by 1 H NMR and visible absorption spectroscopies as well as mass spectrometry. Notably, the meso ‐protons of trifluoroacetate and chloride complexes coupled strongly with the stable Tl isotopes (the nuclear spin quantum numbers = 1/2) through four bonds to give doublet peaks (the coupling constants = 40–58 Hz). Their 1 H NMR spectra in deuterated chloroform indicated that the Tl(III) chlorins with an axial ligand were epimeric mixtures around the chiral central metal. Molecular model calculation proposed their absolute configurations at the asymmetric Tl(III) atom. The α‐ligated complexes were estimated to be preferable over the β‐ligated epimers. The epimeric ratios varying from 1.6 to 2.0 were dependent on the axial ligands. The obtained Tl(III) complex in dichloromethane efficiently absorbed violet and red lights but hardly emitted red‐to‐far‐red light due to the heavy atom effect.