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(O2)(BPHA)2]
(1), [MoO(O2)(BOTHA)2] (2), [MoO(O2)(BMTHA)2] (3), [MoO(O2)(BPTHA)2] (4), and [Mo(O)2(CPHA)2] (5), respectively.
The O and O2 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(O2)(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 → epoxide conversion, it was necessary to synthesize a peroxo-rich
compound, namely, [MoO(O2)2BMTHA]- (7), but the attempted synthesis culminated in the isolation of
[MoO(O2)2(C6H5COO)]- (8), obviously, via the hydrolysis of coordinated BMTHA.