Synthesis and Characterization of Group VI Metal Complexes for Non-Classical Oxygenation Reactions

Metal complexes supported by redox-active ligands have the ability to promote multielectron transfer reactions. Advances in multielectron transfer reactions have significant impacts in catalysis and renewable energy storage. The synthesis and characterization of group VI metal complexes using the DOPO ligand, 2,4,6,8-tetra-tert-butyl-1-oxo-1H-phenoxazin-9-olate, is described, and their reactivity in non-classical oxygenation reactions is examined. H(DOPO^Q) is prepared using a modified literature procedure, where 2,6-dihydroxy-3,5-di-tert-butylaniline and 3,5-di-tert-butylcatechol are reacted at room temperature in the presence of 10 mol% triethylamine under aerobic conditions. Cr(DOPO)₂ is synthesized by the reaction of H(DOPO^Q) with chromocene. Its crystal structure reveals C_2v symmetry, with one partially reduced ligand, DOPO^SQ, more tightly bonded to Cr than the other fully oxidized ligand, DOPO^Q. The magnetic susceptibility of Cr(DOPO)₂ is measured both as a solid and as a toluene solution, showing Curie-Weiss paramagnetism with some temperature independent paramagnetism in the solid. Pb(DOPO^Q)₂ is prepared by dissolving lead acetate trihydrate in a heated methanol solution with H(DOPO^Q). Its geometry differs from most homoleptic DOPO metal complexes, with the DOPO ligands forming a κ²-like coordination to Pb due to the stereochemically active lone pair of electrons in the 6s metal orbital. Pb(DOPO^Q)₂ is employed in the synthesis of M(DOPO)₂ (M = Mo, W) from M₂Br₄(CO)₈. The crystal structures of Mo(DOPO)₂ and W(DOPO)₂ are isomorphous, with the DOPO ligands coordinated in a meridional fashion to the corresponding metal centers. ¹H and ¹³C NMR spectroscopy along with metrical data from x-ray diffraction studies support a fully reduced catecholate form for both ligands. Reaction of the molybdenum and tungsten complexes, M(DOPO^Cat)₂, with excess Me₃NO gives MO₂(DOPO)₂, with both DOPO ligands in the fully oxidized quinonoid form, in addition to paramagnetic products. Reaction of M(DOPO^Cat)₂ with a weaker amine N-oxide such as NMO only gives traces of MO₂(DOPO)₂, with the major product being H(DOPO^Q) and paramagnetic products. The molybdenum dioxo complex, MoO₂(DOPO^Q)₂, has been prepared independently from MoO₂Cl₂(dmf)₂ and Pb(DOPO^Q)₂. Addition of 2 equivalents of PPh₃ to MoO₂(DOPO^Q)₂ results in regeneration of Mo(DOPO^Cat)₂ and formation of OPPh₃ via the intermediacy of a PPh₃- containing species observable by NMR. Less bulk and more basic PMe₂Ph achieves the same net deoxygenation more rapidly, but without formation of detectable amounts of an intermediate. Enterobactin is prepared using a modified procedure, and its reactivity with molybdenum(VI) is examined. A one-pot reaction of 2,3-bis(benzyloxy)benzoyl chloride with tris(N-trityl-L-serine) trilactone affords hexabenzylenterobactin directly. This procedure consolidates the removal of the N-trityl protecting groups and amide bond formation between the trilactone backbone and the side chains into one facile step. While MoO₂(acac)₂ reacts with enterobactin, factors such as the limited solubility of enterobactin and aggregation of enterobactin-Mo complexes into oligomers inhibit the formation of a stable C₃-symmetric enterobactin-Mo(VI) complex.