A new Na/Mg inverse crown ether.

A 'missing' member of the inverse crown ether family, namely mu(4)-oxo-tetrakis(mu-2,2,6,6-tetramethylpiperidinido)dimagnesium(II)disodium(I), [Na(2)Mg(2)O(C(9)H(18)N)(4)], has been synthesized by blocking the alternative aromatic metallation route via the use of sterically hindered 1,3,5-mesitylene as a solvent. [Na(2)Mg(2)O(NR(2))(4)] (NR(2) is 2,2,6,6-tetramethylpiperidinide) is shown to form a cationic planar eight-membered ring with alternating metal and N atoms, which captures at its core an oxide guest that lies on an inversion centre [principal dimensions: Na-O = 2.2405 (11) A, Na-N = 2.445 (3) and 2.572 (3) A, Mg-O = 1.8673 (9) A, and Mg-N = 2.032 (2) and 2.063 (2) A].