Novel molybdenum(v) squarato complexes based on the dinuclear metal–metal bonded unit: Syntheses and structural characterization of dinuclear [Mo₂O₄(C₄O₄)(R-Py)₄] and tetranuclear [Mo₄O₈(C₄O₄)₄]⁴⁻

Reactions of mononuclear oxohalomolybdates(v) with squaric acid (3,4-dihydroxycyclobut-3-ene-1,2-dione) afforded two types of (Mo₂O₄)²⁺ containing species with the squarato ligand engaged in a μ₂-1,2-bis(monodentate) binding mode. In the presence of pyridine (Py) or 3,5-lutidine (3,5-Lut) neutral, dinuclear complexes [Mo₂O₄(C₄O₄)(Py)₄]·2MeOH·0.5Py 1a, [Mo₂O₄(C₄O₄)(Py)₄]·2Py 1b and [Mo₂O₄(C₄O₄)(3,5-Lut)₄]·2(3,5-Lut) 1c were obtained. A pair of nitrogen donor ligands and a squarate oxygen occupy the three coordination sites of each metal in the (Mo₂O₄)²⁺ unit in 1a, 1b and 1c. The squarate functions as a third, auxiliary bridge between a pair of molybdenum atoms at an average distance of 2.55 Å. Using 2,4,6-collidine (Col) afforded (ColMe)₄[Mo₄O₈(C₄O₄)₄]·2MeOH·2Col 2a (ColMe⁺ = N-methylcollidinium cation, NC₉H₁₄ ⁺) and (ColH)₄(PyEt)[Mo₄O₈(C₄O₄)₄]Br 2b (ColH⁺ = protonated 2,4,6-collidine, NC₈H₁₂ ⁺; PyEt⁺ = N-ethylpyridinium cation, NC₇H₁₀ ⁺). The tetranuclear anions of 2a and 2b consist of a rare, cube-like (Mo₄O₄(μ₃-O)₄)⁴⁺ core with four squarato ligands attached to its periphery. The squarato ligands in the [Mo₄O₈(C₄O₄)₄]⁴⁻ anions bridge pairs of molybdenum atoms from neighbouring (Mo₂O₄)²⁺ units, spanning an average distance of 3.43 Å. The compounds were fully characterized by infrared vibrational spectroscopy, NMR spectroscopy, elemental analyses and X-ray diffraction studies.