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Niobium In Oxalate Complexes: From Discrete Molecules to Heterometallic Supramolecular Structures

The oxalate ion, C2O42– is well known in the chemistry of niobium. Oxalic acid was among the first agents used for the dissolution of the melt obtained by the fusion of Nb2O5 and KHSO4. The first niobium(V) oxalate-containing species of the composition M3[NbO(C2O4)3]•nH2O (M = NH4+, Na+, K+, Rb+) were isolated at the beginning of the last century. As found later, niobium atom in these compounds is heptacoordinated by oxygen atoms forming a distorted pentagonal bipyramid. The same coordination polyhedron was found in the ammonium and caesium salts of the diaquabis(oxalato)oxoniobates(V) M[NbO(C2O4)2(H2O)2]•nH2O (M = NH4+, K+, Rb+, Cs+, ½Mg2+), being formed by two bidentate oxalate groups, two water molecules and one oxo-oxygen atom. Specific supramolecular motifs and solvatomorphism have been found within the series of compounds [M(bpy)3]2[NbO(C2O4)3]Cl•nH2O (M = Fe2+, Co2+, Ni2+, Cu2+, Zn2+ ; bpy = 2, 2'-bipyridine) crystallizing in the monoclinic (n = 11) and orthorhombic (n = 12) crystal systems. In the oxalate-bridged heterometallic complexes [{; ; ; M(bpy)2}; ; ; 2(µ-C2O4)][M(bpy)2(µ-C2O4)NbO(C2O4)2]2•0.5bpy•7H2O (M = Cu2+, Zn2+) and [{; ; ; Cu(phen)2}; ; ; 2(µ-C2O4)][Cu(phen)2(µ-C2O4)NbO(C2O4)2]2•8H2O (phen = 1, 10-phenanthroline) one-dimensional motifs in the structures are generated through the ligand stacking interactions. The square-antiprismatic coordination of niobium(IV) atoms was found in the compounds K4[Nb(C2O4)4]•3H2O, K4[Nb(C2O4)4]•4H2O•1/2H2C2O4 and K2(H3NCH2CH2NH3)[Nb(C2O4)4]∙4H2O.

niobium, oxalate, complexes, structures

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