Luminescence of the Mn2+ ion in non-Oh and Td coordination environments: the missing case of square pyramid.

The first observation of luminescence for Mn(ii) complexes with a square-pyramidal geometry (C4v) of the metal is reported. The complexes of such type, namely [Mn(L)2Hal]X, where L is an O,O'-chelating ligand Me2N-CH(Ph2P[double bond, length as m-dash]O)2, Hal = Cl or Br, and X = Br, I or [MnCl4], show at ambient temperature red photoluminescence (λ≈ 620 nm) with millisecond lifetimes (3.8-7.6 ms). This emission, stemming from spin-forbidden 4E(G) →6A1(S) transitions in square-pyramidally coordinated Mn2+, is found to be responsive to temperature. Upon cooling the complexes from 300 to 77 K, the red-shift of the emission band by ≈20 nm takes place, which is accompanied by the increase of its integral intensity and lifetime. The peculiarities of the discovered Mn2+(C4v) luminescence are discussed against the classical luminescence that occurs in octahedral (Oh) and tetrahedral (Td) surroundings.