Fourfold Diamond Structure of the Dipotassium and Disodium Salts of 1,3,5,7‐Adamantanetetracarboxylic Acid

The (isomorphous) crystal structures of the dipotassium and disodium salts of 1,3,5,7‐adamantanetetracarboxylic acid are described. In the tetragonal, non‐polar crystals of space group P 4 2 / nbc hte tetrahedral dihydrogen tetracarboxylate anions join to form four hydrogen‐bonded superdiamond networks interpenentrating each other. The cations are arranged in chains (“metal‐over‐metal”] within fourfold helical channels (double antiparallel double‐helices). The short, negatively charged O‐H…O ⊖ hydrogen bonds are symmetric on average. However, in the dipotassium salt actually disordered, asymmetric hydrogen bonds are likely to be present (double‐minimum potential, O‐H…O ⊖ distance 2.52 Å). In analogy to potassium dihydrogen phosphate, KH 2 PO 4 , the present tetrahedral dipotassium dihydrogen tetracarboxylate may thus potentially form polar, ferroelectric crystals at low temperature with an ordered array of asymmetric hydrogen bonds. In the course of the crystallization of the disodium salt of adamantanetetracarboxylic acid, tetragonal crystals of a tetrahydrate of the tetrasodium salt were accidentally obtained (space group I 4 1 /α) and their structure determined. The arrangement of the hydrated anions may be characterized either as a hydrogen‐bonded, distorted triple‐diamond structure, or else aas a “semi‐clathrate”, consisting of non‐intersecting orthogonal water chains interpenetrating each other and tetracarboxylate tetraanions functioning as cross‐links between these chains by means of hydrogen bonds.