Structural Comparison of Iron Tetrapolyvanadate Fe(2)V(4)O(13) and Iron Polyvanadomolybdate Fe(2)V(3.16)Mo(0.84)O(13.42): A New Substitution Mechanism of Molybdenum(VI) for Vanadium(V).

Single crystals of the iron tetrapolyvanadate Fe(2)V(4)O(13) and the iron polyvanadomolybdate Fe(2)V(3.16)Mo(0.84)O(13.42) were grown from Fe(2)O(3)/V(2)O(5) and Fe(2)O(3)/V(2)O(5)/MoO(3) melts, respectively. Single-crystal X-ray diffraction revealed that the two structures are closely related. Both contained isolated Fe(2)O(10) octahedral dimers with similar orientations. The unusual U-shaped V(4)O(13)(6)(-) clusters in Fe(2)V(4)O(13) were ordered while the U-shaped [V(3.16)Mo(0.84)O(13.42)](6)(-) clusters in Fe(2)V(3.16)Mo(0.84)O(13.42) were disordered. The substitution of molybdenum(VI) for vanadium(V) revealed a new substitution mechanism, in which a corresponding stoichiometric amount of oxygen was brought into the structure for charge balance with no reduction of the V(5+) and Mo(6+) ions. Crystal data: for Fe(2)V(4)O(13), monoclinic, space group P2(1)/c (No. 14), with a = 8.300(2) Å, b =9.404(6) Å, c = 14.560(2) Å, beta = 102.08(1) degrees, and Z = 4; for Fe(2)V(3.16)Mo(0.84)O(13.42), monoclinic, space group P2(1)/c (No. 14), with a = 7.678(1) Å, b = 9.456(2) Å, c = 8.336(2) Å, beta = 109.50(2) degrees, and Z = 2.