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

Abstract

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.

Synthesis and catalytic epoxidation potentiality of oxodiperoxo molybdenum(VI) complexes with pyridine-2-carboxaldoxime and pyridine-2-carboxylate: the crystal structure of PMePh3[MoO(O2)2(PyCO)]

“…As part of our continued interest in using oxoperoxomolybdenum complexes as oxidation catalysts, we were intrigued by previous reports [26,27] at the failure of the Mo-complexes to activate H 2 O 2 and by using a molybdenum complex, PPh 4 [MoO(O 2 ) 2 (SaloxH)] [6] (SaloxH 2 = Salicylaldoxime) as catalyst along with NaH CO 3 as cocatalyst [28,29] we were able to activate H 2 O 2 and the integrated catalyst, co-catalyst and oxidant functioned as a very efficient peroxidic epoxidation system. Inspired by this result, we reported some other peroxo complexes, which showed higher to much higher catalytic efficiencies [6][7][8][9][10] for olefin epoxidation. In this paper, we report the synthesis, structural characterization, and catalytic epoxidation activities of two oxodiperoxo complexes, [MoO(O 2 ) 2 (PyCOXH) (H 2 O)] (1) (PyCOXH = Pyridine-2-carboxaldoxime) and PMePh 3 [MoO(O 2 ) 2 (PyCO)] (2) (PyCOH = Pyridine-2-carboxylic acid).…”

“…[6,8,9]) showed that the oximate ligands coordinated to MO(O 2 ) 2 (M = Mo or W) moiety produced complexes, which were more potential epoxidation-catalyst than the corresponding carboxylate ligands attached to the same moiety. In this work, we find a reverse situation by observing that 2 is a more potential catalyst than 1.…”

“…accommodates the bidentate ligands by forming oxomonoperoxo complexes of the type [MO(O 2 )(bidentate) 2 ] (the bold font indicates bulky ligand) [6][7][8]. Recently, it has been shown that the MoO(O 2 ) 2 cores with bidentate ligands are useful catalysts in the epoxidation of olefins [6][7][8][9][10]. Epoxidation of olefins and arenes is an important transformation in organic synthesis since the epoxy compounds are widely used, for example in the manufacture of polyurethanes, unsaturated resins, glycols, surfactants, etc.…”

N-Heterocyclic Carbene-Based Molybdenum and Tungsten Complexes as Efficient Epoxidation Catalysts with H2O2 and tert-Butyl Hydroperoxide

“…It is cheap, readily available and gives water as the only by-product [15]. Molybdenum and tungsten complexes have been widely investigated because of their demonstrated high activity for selective oxidation with TBHP [16][17][18][19][20] and H 2 O 2 [21][22][23][24][25][26][27][28]. However, to the best of our knowledge, the catalytic performance of Mo(II) and W(II) carbonyl compounds in olefin epoxidation using H 2 O 2 has never been reported.…”

Synthesis and catalytic epoxidation potential of oxodiperoxo molybdenum(VI) complexes with 2-hydroxybenzohydroxamate and 2-hydroxybenzoate: the crystal structure of PPh4[MoO(O2)2(HBA)]

“…Of the many ways to accomplish alkene epoxidation using transition metal complexes as catalysts [2][3][4][5][6][7], H 2 O 2 is probably the best ecologically sustainable ''green'' terminal oxidant [8], after dioxygen. Quite a few reports have already appeared in the area of homogeneous catalytic epoxidation with hydrogen peroxide as oxidant and a variety of transition metal complexes as catalysts [3,[9][10][11][12][13][14][15][16][17][18][19][20]. In the oxodiperoxo chemistry of molybdenum and tungsten, it has already been shown that the MO(O 2 ) 2 (M = Mo or W) cores, when bound to one bidentate or two unidentate ligands, afford complexes that are useful catalysts in the epoxidation of alkenes [12][13][14][15][16][17][18][19][20].…”