Crystalline Molecular Assemblies of Complexes Showing Eightfold Coordinated Niobium(IV) Dodecahedral Geometry in the...

The reactivity of 2,3-pyridine-dicarboxylic (known as quinolinic or H₂qui) acid and 2,5-pyridine-dicarboxylic (known as isocinchomeronic or H₂icc) acid has been investigated as a complexing agent toward the niobium(IV) tetrachloride precursor (NbCl₄·2THF) in different organic solvent mixtures. It resulted in the isolation of four crystalline assemblies of mononuclear coordination complexes 1-4 [Nb(HL)₄·solvent], where HL is the monoprotonated quinolinate (Hqui) ligand (complexes 1-3) or the monoprotonated isocinchomeronate ligand (complex 4). For each complex, the discrete niobium(IV) center is eightfold coordinated to four oxygen atoms from the deprotonated carboxylate arm and four nitrogen atoms from the pyridine part of the dicarboxyl ligand with a dodecahedral environment [NbO₄N₄]. The remaining carboxyl arm (either in 3 or in 5 position) remained under its protonated form, leading to neutral [Nb(HL)₄] moieties for compounds 1, 2, and 4, or the anionic [Nb(qui)(Hqui)₃]^- moiety for compound 3. The complexes are observed in various molecular arrangements, involving intercalated solvent molecules such as acetonitrile in compound 1 ([Nb(Hqui)₄·0.8(CH₃CN)], obtained at room temperature), a mixture of acetonitrile and pyridine in compound 2 ([Nb(Hqui)₄·0.7CH₃CN·2PYR], obtained via the solvothermal reaction at 80 °C), a mixture of pyridine and triethylamine, in addition with water and chloride species, in compound 3 ([Nb(qui)(Hqui)₃·Cl·HPYR·HTEA·1.5H₂O], obtained via solvothermal reaction at 80 °C), and N,N-dimethylformamide in compound 4 ([Nb(Hicc)₄·6DMF], obtained at room temperature). The d¹ configuration expected for the niobium(IV) centers has been analyzed by magnetic measurements, as well as by EPR and XPS. An anti-ferromagnetism transition has been observed at very low temperatures for complexes 1 (3.6 K) and 4 (3.3 K), for which the shortest Nb···Nb interatomic lengths occur.