Unexpected Orange Photoluminescence from Tetrahedral Manganese(II) Halide Complexes with Bidentate Phosphanimines

Manganese(II) halide complexes with the general formula [MnX₂{(PhN=PPh₂)CH₂}], where X is bromine or iodine and (PhN=PPh₂)CH₂ is the bis-phosphanimine ligand 1,1'-methylenebis-(N,1,1-triphenylphosphanimine), were prepared and isolated. The structure of the two compounds was determined by single-crystal X-ray diffraction, revealing an approximately tetrahedral geometry at the metal centre. Unlike structurally comparable compounds containing phosphine oxides or related [O=P]-donors in the coordination sphere, which commonly show green emissions, solid samples of [MnBr₂{(PhN=PPh₂)CH₂}] and [MnI₂{(PhN=PPh₂)CH₂}] exhibited orange emissions upon irradiation with UV light. The emission spectra resulted excitation-independent. Superimposable steady-state luminescence spectra were collected for both compounds as powders and crystals suitable for X-ray diffraction. The excitation spectra and the ligand→metal antenna effect were affected by the coordinated halide, and only [MnBr₂{(PhN=PPh₂)CH₂}] showed bright luminescence under near-UV irradiation. Either ligand- or metal-centred transitions can account for the observed luminescence, and the luminescence decay curves were consistent with a multiplicity change from the excited to the ground state, with excited-state lifetimes in the range of hundreds of microseconds. Attempts to rationalize the unexpected luminescence were carried out based on DFT calculations.