Tetramethylammonium hexacyanotrimethylenecyclopropanide: Crystal structure of antiferromagnetic phase II and paramagnetic phase I.

C sub (16) H sub (12) N sub 7 - [(CH sub 3)sub 4 N]+ [C sub 3 { C(CN) sub 2 } sub 3] sup - - undergoes a structural and magnetic phase transition at 364K from triclinic phase II to monoclinic phase I as the temperature increases. Short N ... N contacts of 3.189(3) and 3.256(3)angstroms between radical hexacyanocyclopropanide anions, inclined respectively at 4. 4 and 0.4degrees to the anion plane, and pi-electron overlap between stacks of radicals separated by 3.24angstroms, form puckered sheets of strongly linked radical anions in phase II. Magnetic exchange interactions between radical anions result in quasi-two-dimensional antiferromagnetic spin coupling at low temperatures. Above the transition temperature, the pi-electron overlap decreases as the radical interstack contacts increase in length by about 0.1angstroms. The N ... N contacts close to the molecular plane remain unchanged in phase I, resulting in a quasi-one-dimensional array.