Oxo- and oxoperoxo-molybdenum(VI) complexes with aryl hydroxamates: Synthesis, structure, and catalytic uses in highly efficient, selective, and ecologically benign peroxidic epoxidation of olefins

A solution obtained by dissolving MoO3 in H2O2 reacts separately with secondary hydroxamic acids ( viz., N- benzoyl N- phenyl hydroxamic acid ( BPHAH), N- benzoyl N- ortho-, - meta-, - para- tolyl hydroxamic acids, ( BOTHAH, BMTHAH, and BPTHAH, respectively), and N-cinnamoyl N-phenyl hydroxamic acid (CPHAH) affording [ MoO(O-2)( BPHA) 2] (1), [MoO(O-2)( BOTHA)(2)] (2), [MoO(O-2)( BMTHA)(2)] ( 3), [ MoO(O-2)( BPTHA)(2)] (4), and [Mo(O)(2)(CPHA)(2)] (5), respectively. The O and O-2 are situated cis to each other in 2 - 4, but in each case, they are disordered and distributed over four sites. This disorder does not exist in the 6- coordinate cis dioxo complex 5, to which crude MoO(O-2)( CPHA)(2) ( 5') was converted during recrystallization. An aqueous molybdate solution readily reacts with all those hydroxamic acids producing [ Mo(O)(2)( hydroxamate) (2)] ( 6). While 2, 3, and 4 possess a very distorted pentagonal bipyramidal structure, 5 has a distorted octahedral geometry. In the solid state, as well as in solution, 5 exists as two apparently enantiomerically related molecules differing in the orientation of the pendant phenyl rings. To emphasize that the formation and structural uniqueness of 5 compared to 1 - 4 is caused by the influence of the cinnamoyl residue, one compound of the 6 series, namely, [ Mo(O)(2)( BPHA)(2)] (6A), was structurally characterized to prove directly that the special stereochemical properties of 5 rely on the special electronic structure of CPHA- ligand. Complexes 1 - 5, as well as 6, show high potential and selectivity as catalysts in the epoxidation of olefins at room temperature in the presence of NaHCO3 as a promoter and H2O2 as a terminal oxidant. A comparative epoxidation study has been performed to determine the relative efficiency of the catalysts. To make the epoxidation method cost effective, a study to optimize the use of H2O2 has also been performed. To obtain evidence in favor of our suggested mechanism to this homogeneous olefin f epoxide conversion, it was necessary to synthesize a peroxo- rich compound, namely, [ MoO(O-2)(2)BMTHA](-) (7), but the attempted synthesis culminated in the isolation of [ MoO(O-2)(2)( C6H5COO)](-) ( 8), obviously, via the hydrolysis of coordinated B M THA.