Structural identification of two differently coordinated heptamolybdate ligands in a hexamagnesium compound

Abstract

Dissolution of freshly prepared molybdenum trioxide in hot aqueous magnesium carbonate followed by crystallization results in the formation of an all-inorganic heptamolybdate compound viz. [Mg(H sub(2)O) sub(6)] sub(3)[Mg(H sub(2)O) sub(5)(Mo sub(7)O sub(24))][(H sub((2)O) sub(5) Mg (Mu sub(2)-Mo sub(7) O sub(24))Mg(H sub(2)O) sub(5)].6H sub(2)O (1). The crystal structure, spectral characteristics, thermal and electrochemical properties of 1 are reported. Compound 1, which crystallizes in the acentric polar space group Cc, contains six unique Mg(II) ions. The two crystallographically independent heptamolybdate anions function as a monodentate (n1) and Mu sub(2-) bridging bidentate ligand, respectively, forming the anionic [Mg(H sub(2)O) sub(5)(Mo sub(7) O sub(24))] sup(4?) and [(H sub(2)O) sub(5) Mg(Mu sub(2-) Mo sub(7) O sub(24))Mg(H sub(2) O) sub(5)] sup(2?) species, which are charge balanced by three unique hexaaquamagnesium(II) cations. The electrochemical and conductivity studies of 1 reveal the presence of [Mg(H sub(2)O) sub(6)] sup(2+) cations and uncoordinated (Mo sub(7) O sub(24)) sup(6?) anions in solution. Thermal decomposition of 1 leads to the formation of Mg sub(2) Mo sub(3) O sub(11) via Mg sub(6) Mo sub(14)O sub(48) and 3Mg sub(2) Mo sub(3) O sub(11).5MoO sub(3).