MnBr2/18-crown-6 coordination complexes showing high room temperature luminescence and quantum yield

Abstract

The reaction of manganese(II) bromide and the crown ether 18-crown-6 in the ionic liquid [(n-Bu)₃MeN][N(Tf)₂] under mild conditions (80–130 °C) resulted in the formation of three different coordination compounds: MnBr₂(18-crown-6) (1), Mn₃Br₆(18-crown-6)₂ (2) and Mn₃Br₆(18-crown-6) (3). In general, the local coordination and the crystal structure of all compounds are driven by the mismatch between the small radius of the Mn²⁺ cation (83 pm) and the ring opening of 18-crown-6 as a chelating ligand (about 300 pm). This improper situation leads to different types of coordination and bonding. MnBr₂(18-crown-6) represents a molecular compound with Mn²⁺ coordinated by two bromine atoms and only five oxygen atoms of 18-crown-6. Mn₃Br₆(18-crown-6)₂ falls into a [MnBr(18-crown-6)]⁺ cation – with Mn²⁺ coordinated by six oxygen atoms and Br – and a [MnBr(18-crown-6)MnBr₄]⁻ anion. In this anion, Mn²⁺ is coordinated by five oxygen atoms of the crown ether as well as by two bromine atoms, one of them bridging to an isolated (MnBr₄) tetrahedron. Mn₃Br₆(18-crown-6), finally, forms an infinite, non-charged ¹_∞[Mn₂(18-crown-6)(MnBr₆)] chain. Herein, 18-crown-6 is exocyclically coordinated by two Mn²⁺ cations. All compounds show intense luminescence in the yellow to red spectral range and exhibit remarkable quantum yields of 70% (Mn₃Br₆(18-crown-6)) and 98% (Mn₃Br₆(18-crown-6)₂). The excellent quantum yield of Mn₃Br₆(18-crown-6)₂ and its differentiation from MnBr₂(18-crown-6) and Mn₃Br₆(18-crown-6) can be directly correlated to the local coordination.