Crystalline Molecular Assemblies of Complexes Showing Eightfold Coordinated Niobium(IV) Dodecahedral Geometry in the Pyridine-Dicarboxylic Acid System

The reactivity of 2,3-pyridine-dicarboxylic (known as quinolinic or H 2qui) acid and 2,5-pyridine-dicarboxylic (known as isocinchomeronic or H 2icc) 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 d 1 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.