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 A. 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 A. The compounds were fully characterized by infrared vibrational spectroscopy, NMR spectroscopy, elemental analyses and X-ray diffraction studies.