Li24[MnN3]3N2 and Li5[(Li1-xMnx)N]3, two intermediates in the decomposition path of Li7[MnN4] to Li2[(Li1-xMnx)N]:: An experimental and theoretical study

The crystal structure of Li-7[(MnN4)-N-V] was re-determined. Isolated tetrahedral [(MnN4)-N-V](7-) ions are arranged with lithium cations to form a superstructure of the CaF2 anti-type (P43n, No. 218, a = 956.0(1) pin, Z = 8). According to measurements of the magnetic susceptibility, the manganese (tetrahedral coordination) is in a d(2) S = 1 state. Thermal treatment of Li-7[(MnN4)-N-V] under argon in the presence of elemental lithium at various temperatures leads to Li-24[(MnN3)-N-III](3)N-2, Li-5[(Li1-xMnx)N](3), and Li-2[(Li1-xMnx1)N], respectively. Li-24[(MnN3)-N-III](3)N-2 (P31c, No. 163, a = 582.58(6) pm, c = 1784.1(3) pm, Z = 4/3) crystallizes in a trigonal unit cell, containing slightly, but significantly nonplanar trigonal [MnN3](6-) units with C-3v Symmetry. Measurements of the magnetic susceptibility reveal a d(4) S = I spin-state for the manganese (trigonal coordination). Nonrelativistic spin-polarized DFT calculations with different molecular models lead to the conclusion that restrictions in the Li-N substructure are responsible for the distortion from planarity of the [(MnN3)-N-III](6-). Li-5[(Li1-xMnx)N](3) (x = 0.59(1), P62m, No. 189, a = 635.9(3) pm, c = 381.7(2) pm, Z = 1) is an isotype of Li-5[(Li1-xNix)N](3) with manganese in an average oxidation state of about +1.6. The crystal structure is a defect variant of the alpha -Li3N structure type with the transition metal in linear coordination by nitrogen. Li-2[(Li1-xMnx1)N] (x = 0.67(1), P6/mmm, No. 191, a = 371.25(4) pm, c = 382.12(6) pm, Z = 1) crystallizes in the alpha -Li3N = Li-2[LiN] structure with partial substitution of the linearly nitrogen-coordinated Li-species by manganese(I). Measurements of the magnetic susceptibility are consistent with manganese (linear coordination) in a low-spin d(6) S = I state.