Molybdenum complexes with a μ-O{MoO2}2 core: their synthesis, crystal structure and application as catalysts for the oxidation of bicyclic alcohols using N-based additives

Abstract

Three binuclear μ-oxidobis{dioxidomolybdenum(VI)} complexes, [(μ-O){Mo^VIO₂(MeOH)}₂(L¹)] 1, [(μ-O){Mo^VIO₂(MeOH)}₂(L²)] 2, and [(μ-O){Mo^VIO₂(MeOH)}₂(L³)] 3 with potential dibasic tetradentate azine ligands [H₂L¹ = azine of salicylaldehyde, H₂L² = azine of 3-methoxysalicyaldehyde and H₂L³ = azine of 3,5-ditertiarybutylsalicylaldehyde] are reported. The complexes are characterized via elemental analysis, various spectroscopic techniques (FT-IR, UV-Vis, ¹H and ¹³C NMR), thermogravimetric analysis and single-crystal X-ray diffraction (2 and 3) studies. These complexes are binuclear having a μ-O{MoO₂}₂ core where ligands coordinate through a set of phenolic oxygen and azine nitrogen to each molybdenum, thus behaving as bis(bidentate). These complexes are investigated as catalysts for the oxidation of two bicyclic alcohols (fenchyl alcohol and isoborneol) using 30% H₂O₂ as the oxidant in the presence of NEt₃ as the additive. The conversion of bicyclic alcohols within the additives follows the order: NEt₃ > py > NH₃ > K₂CO₃ > KOH. In the presence of an N-based additive, fenchyl alcohol and isoborneol selectively give high yields of fenchone and camphor, respectively, while in the absence of the additive the conversion is relatively low.