Synthesis, Crystal Structure, and Thermal Behavior of Organically Templated Three-Dimensional Tunnel Structures Based on α-Keggin Phosphododecamolybdate and Diazines

Three salts of diazonium cation (C 4 H 5 N 2 + ) encapsulated α-Keggin phosphododecamolybdate, (pyridazinium) 3 [PMo 12 O 40 ]· 1 / 2 H 2 O ( 1 ), (pyrimidinium) 3 [PMo 12 O 40 ]·3H 2 O ( 2 ), and (pyrazinium) 3 [PMo 12 O 40 ]·3H 2 O ( 3 ) have been synthesized at room temperature and atmospheric pressure in an organic−aqueous medium. Compounds 1 − 3 crystallize in the trigonal space group R 3̄. Crystal data for compound 1 is as follows: a = 17.704(6) Å, c = 22.918(3) Å, V = 6221(3) Å 3, Z = 6, R = 0.028 for 3708 observed reflections with I > 3σ( I ); for compound 2, a = 19.022(1) Å, c = 21.524(1) Å, V = 6745(1) Å 3, Z = 6, and R = 0.043 for 2143 observed reflections with I > 2σ( I ); for compound 3, a = 17.820(2) Å, c = 24.369(2) Å, V = 6701(1) Å 3, Z = 6, and R = 0.047 for 3699 observed reflections with I > 3σ( I ). These compounds contain the Keggin clusters arranged in a three-dimensional tunnel structure where the organic templates are sandwiched by two polyanions. Although the three salts are isostructural, the arrangement of organic cations and water molecules relative to the polyanion framework shows some significant differences that have been analyzed by means of ab initio calculations. Differences in the thermal behavior of the three compounds have also been explained on the basis of the different crystal arrangements.