Luminescent Manganese(II) Iminophosphorane Derivatives

The reaction between the iminophosphorane ligand <i>N</i>-phenyl-1,1,1-triphenylphosphanimine (NPh=PPh₃) and anhydrous manganese(II) halides allowed the isolation of complexes with the general formula [MnX₂(NPh=PPh₃)₂] (X = Cl, Br, I). The compounds showed luminescence in the green region attributed to the ⁴T₁(⁴G)→⁶A₁(⁶S) transition of the metal centre in the tetrahedral field, which was superimposed in the cases of X = Cl and X = Br on weak ligand-centred fluorescence. The emission and excitation spectra were compared with those of the free ligand and of the related zinc(II) bromo-complex. DFT calculations on the free ligand and on the manganese(II) bromo-complex helped to rationalise the experimental data. The protonation of NPh=PPh₃ led to the formation of the iminium cation [NHPh=PPh₃]⁺, which was used as a building block for the synthesis of organic-inorganic hybrids with the general formula [NHPh=PPh₃]₂[MnX₄] (X = Cl, Br, I). The crystal structure of [NHPh=PPh₃]₂[MnBr₄] was determined by means of X-ray diffraction. Green photoluminescence associated with the metal-centred transition was also observed for the organic-inorganic hybrids, with higher quantum yields with respect to the neutral [MnX₂(NPh=PPh₃)₂] complexes. In the case of X = I, luminescence from the cation was superimposed on that from the tetraiodomanganate anion upon excitation of the compound with near-UV light.