An Octanuclear Molybdenum(VI) Complex Containing Coordinatively Bound 4,4′-di-tert-Butyl-2,2′-Bipyridine, [Mo8O22(OH)4(di-tBu-bipy)4]: Synthesis, Structure, and Catalytic Epoxidation of Bio-Derived Olefins

The reaction of [MoO2Cl2(di-tBu-bipy)] (1) (di-tBu-bipy = 4,4′-di-tert-butyl-2,2′-bipyridine) with water at 100–120 °C in a Teflon-lined stainless steel autoclave, in an open reflux system, or in a microwave synthesis system gave the octanuclear complex [Mo8O22(OH)4(di-tBu-bipy)4] (2) as a microcrystalline powder in good yields. Single crystals of 2 suitable for X-ray diffraction were obtained by the reaction of MoO3 and di-tBu-bipy in water at 160 °C for 3 days. The molecular structure of 2 comprises a purely inorganic core, Mo4O8(μ3-OH)2(μ2-O)2, attached to two peripheral oxo-bridged binuclear units, Mo2O4(μ2-O)2(OH)(di-tBu-bipy)2. The inorganic core is composed of a unique assembly of four {MoO5} distorted square pyramids connected to each other via edge-sharing. Overall, the octanuclear complex adopts a highly distorted form strongly resembling an “S”-shaped molecular unit. Complex 2 was applied in the catalytic epoxidation of the biorenewable olefins DL-limonene (Lim) and methyl oleate (Ole), using tert-butylhydroperoxide (TBHP) as an oxygen donor, under mild reaction conditions (55 °C, air). The reactions of Lim and Ole gave the respective epoxide monomers in fairly high selectivities at high conversions (89% 1,2-epoxy-p-menth-8-ene selectivity at 96% Lim conversion; 99% methyl 9,10-epoxystearate selectivity at 94% Ole conversion, reached within 24 h reaction). Iodometric titrations revealed no measurable “non-productive” decomposition of TBHP.