Preparation and properties of dinuclear dioxomolybdenum(VI) complexes with ONO–ONO-type hexadentate Schiff base ligands

Preparation and properties of dinuclear cis-dioxomolybdenum(VI) complexes containing ONO–ONO-type hexadentate Schiff bases derived from 2,5-dihydroxyterephthalaldehyde and aminoalcohols in a 1:2 molar ratio are reported. When 2-amino-2-methyl-1,3-propanediol was used as the aminoalcohol, the 1H NMR spectrum of the complex showed two sets of resonances, indicating the presence of two isomers. The origin of the isomerism was elucidated by variable-temperature 1H NMR spectroscopy and by comparing the 1H NMR spectrum with those of the analogous dinuclear complexes derived from (1R,2S)- and (1S,2R)-norephedrine and with those of the corresponding mononuclear complexes. The Schiff base ligand derived from 2,5-dihydroxyterephthalaldehyde and 2-amino-2-methyl-1,3-propanediol contains two prochiral carbon atoms, which become chiral (R and S) upon coordination. The combination of these two chiral centers generates three stereoisomers, RR, SS, and RS (=SR), and the NMR characteristics were accounted for by these isomers.