An Octanuclear Molybdenum(VI) Complex Containing Coordinative ly 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 degrees 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 degrees C for 3 days. The molecular structure of 2 comprises a purely inorganic core, Mo4O8(mu(3)-OH)(2)(mu(2)-O)(2), attached to two peripheral oxobridged binuclear units, Mo2O4(mu(2)-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 degrees 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 9,10epoxystearate selectivity at 94% Ole conversion, reached within 24 h reaction). lodometric titrations revealed no measurable "non-productive" decomposition of TBHP.