New iso and heteropolyoxomolybdates: synthesis and molecular structure of the anions [Mo VI 8O 26(OH)] 5−, [HAs IIIAs VMo VMo VI 8O 34] 6− and [HAs IIIAs VMo VMo VI 8O 34{Co(C 5H 5N) 2(H 2O) 3}] 4−

Abstract

The hydrothermal reaction of Na 2MoO 4 with pyridine in water at pH 5 and 130 °C gives the octamolybdate anion [Mo VI 8O 26(OH)] 5− ( 1) which crystallises as the pyridinium salt; its molecular structure derives from that of the parent α-octamolybdate anion [Mo VI 8O 26] 4− by opening two molybdenumoxygen bond and adding a hydroxo bridge. The same reaction in the presence of NaAsO 2 yields the mixed-valence arsenatomolybdate [Has IIIAs VMo VMo VI 8O 34] 6− ( 2) which is also isolated as the pyridinium salt. Anion 2 has a lacunary structure like an open basket, which derives from the famous α-Keggin structure by removing three edge-sharing MoO 6 octahedra and by capping a trioxygen face of three remaning MoO 6 octahedra with an AsH group. Reaction of 2 with Co 2+ leads to the anion [HAs IIIAs VMo VMo VI 8O 34{Co(C 5H 5N) 2(H 2O) 3}] 4− ( 3) which crystallises as a double pyridinium salt together with anion 2. The structure of 3 derives from that of 2 by attaching a Co(C 5H 5N) 2(H 2O) 3 fragment to a terminal oxo ligand. Reaction of 2 with hydrogen peroxide produces the fully oxydised α-Keggin anion [As VMo VI 12O 40] 3− ( 4) which was found to crystallise as the tetrabutylammonium salt surprisingly with three independent molecules in the unit cell, two of them showing a remarkable disorder.