New polyoxomolybdenum coordination compounds: Synthesis and characterization of mixed-valent Mo6O13(Hsal)2(sal)2(acac)2 and homovalent Mo4O10(acac)4 (Hsal- = 2-HO–C6H4image, sal2- = 2-O–C6H4image)

Coordination complexes containing extended metal–oxygen frameworks have been studied for a number of applications, including catalysis and as optical materials; however, very few neutral complexes that contain extended metal–oxygen frameworks have been characterized. The new hexanuclear, mixed-valence MoV/MoVI coordination complex Mo6O13(Hsal)2(sal)2(acac)2 has been synthesized by direct reaction of salicylic acid (2-HOC6H4CO2H; H2sal) with Mo(O)2(acac)2. The mixed-valence compound is composed of two edge-sharing Mo4(μ-O)4 rings, in which four Mo atoms are MoVI and two are MoV. The tetranuclear complex Mo4O10(acac)4 was obtained from residual washings of the same reaction and is composed of a single Mo4(μ-O)4 ring having only MoVI atoms. XPS studies confirmed that the hexanuclear complex contains both MoV and MoVI. Magnetic measurements reveal that the hexanuclear complex is diamagnetic suggesting an electron–electron coupling between the two MoV centers through an oxo-bridge.