Molybdenum η³-Allyl Dicarbonyl Complexes as a New Class of Precursors for Highly Reactive Epoxidation Catalysts with tert-Butyl Hydroperoxide

Abstract

New Mo(II) diimine derivatives of [Mo(η3-allyl)X(CO)2(CH3CN)2] (allyl = C3H5 and C5H5O; X = Cl, Br) were prepared, and [Mo(η3-C3H5)Cl(CO)2(BIAN)] (BIAN = 1,4-(4-chloro)phenyl-2,3-naphthalenediazabutadiene) (7) was structurally characterized by single-crystal X-ray diffraction. This complex adopted an equatorial−axial arrangement of the bidentate ligand (axial isomer), in contrast with the precursors, found as the equatorial isomer in the solid and fluxional in solution. The new complexes of the type [Mo(η3-allyl)X(CO)2(N−N)] (N−N is a bidentate chelating dinitrogen ligand) were tested for the catalytic epoxidation of cyclooctene using tert-butyl hydroperoxide as oxidant. All catalytic systems were 100% selective toward epoxide formation. While their turnover frequencies paralleled those of related Mo(II) carbonyl compounds or Mo(VI) compounds bearing similar N-donor ligands, they exhibited similar olefin conversions in consecutive catalytic runs. The acetonitrile precursors were generally more active than the diimine complexes, and the chloro derivatives more active than the bromo ones. Combined vibrational and NMR spectroscopy and computational studies (DFT) were used to investigate the nature of the molybdenum species formed in the catalytic system with [Mo(η3-C3H5)Cl(CO)2{1,4-(2,6-dimethyl)phenyl-2,3-dimethyldiazabutadiene}] (4) and to propose that the resulting species may be dimeric bearing oxide bridges.

Immobilisation of η3‐Allyldicarbonyl Complexes of MoII with Bidentate Nitrogen Ligands within Aluminium‐Pillared Clays

“…The complexes of [MoX(CO) 2 (η 3 -C 3 H 5 )(CH 3 CN) 2 ] with adp and dab exhibit different structures in the solid state, corresponding to the two possible isomers, axial and equatorial (Scheme 2). While [MoX(CO) 2 (η 3 -C 3 H 5 )(dab)] remains as a single axial isomer in solution, as in the solid state, [9] the adp derivative occurs in the solid as the equatorial isomer ( Figure 1). …”

“…[23] The unsupported complexes give practically the same conversions in two consecutive 24-h runs, revealing fairly high stability under the applied oxidising conditions (Table 4), as has been reported for similar species recently. [9] Similarly, the composite materials give comparable conversions when recycled (exemplified in Figure 10 for PILC/1-adp, and PCH/1-adp), with the exception of PILC/2-adp, which leads to a drop in conversion by a factor of 0.3. For PILC/1-adp and PILC/2-adp materials, ICP-AES analysis of the solids recovered after two runs indicated a reduction in the Mo content of 8-11 %; in the cases of PILC/1 and PILC/2 it was less than 6 %.…”

“…1,4-Bis(2,6-dimethylphenyl)-2,3-dimethyl-1,4-diazabutadiene (dab) [24] and the complexes [MoBr(η 3 3 )C=N(2,6-Me-C 6 H 3 )}] (2-dab) were prepared as reported in the literature. [9,25] All experimental manipulations were carried out under nitrogen using Schlenk techniques. 29 Si MAS NMR: δ = -94.2 (Q 3 ) ppm.…”

“…[16,17] A similar mechanistic feature may be involved for the compounds described above, as postulated in our recent work. [9] In general, no induction periods are observed for the carbonyl precursor complexes, suggesting that the OD is relatively fast and does not limit the rate of epoxidation of cyclooctene. In order to investigate if the oxidising species are Mo species and/or peroxide radicals derived from TBHP, the reaction of cyclooctene in the presence of 2 and PILC/2 was performed using equimolar amounts of olefin and 2,6-ditert-butyl-4-methylphenol, used as a radical scavenger (RS).…”

“…Recently, we found that complexes of the type [MoX(CO) 2 (η 3 -allyl)(N-N)], with N-N = (CH 3 CN) 2 or diimine ligands, were quite active and very selective catalyst precursors for olefin epoxidation using tert-butyl hydroperoxide (TBHP) as oxygen donor, without co-solvent, at 328 K. [9] The results obtained in two consecutive catalytic runs were quite similar. Nevertheless, these catalytic systems can suffer from well known drawbacks of homogeneous catalytic processes.…”