Synthesis, Crystal Structure, and Magnetic Properties of K 6 Mn 4 O 7 Featuring a Novel Two‐Dimensional Poly‐oxomanganate(II) Anion

K 6 Mn 4 O 7 was synthesized via the azide/nitrate route from a stoichiometric mixture of the precursors KN 3 , KNO 3 , and MnO, and alternatively from the binary constituents K 2 O and MnO, in an all‐solid state reaction. Its crystal structure [ P 1 , Z = 1, a = 603.46(3), b = 647.69(4), c = 891.36(5) pm, α = 90.477(2)°, β = 108.417(2)°, γ = 115.358(2)°] consists of cluster‐like Mn 4 O 10 units, each composed of four edge sharing MnO 4 tetrahedra. The Mn 4 O 10 building blocks, on their part, are linked by six vertices, forming a 2D arrangement, 2 ∞ [Mn 4 O 4/1 O 6/2 ] n 6– , with the potassium cations occupying the space in between. The temperature dependence of the magnetic susceptibility is dominated by antiferromagnetic coupling along a low‐dimensional magnetic exchange path. The heat capacity reveals no clear sign of long‐range magnetic ordering, supporting an interpretation of K 6 Mn 4 O 7 as a new low‐dimensional magnetic system.