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

Abstract

The molybdenum and tungsten g 3 -allyl dicarbonyl complexes bearing N-heterocyclic carbene (NHC) ligands [M(g 3 -C 3 H 5 )Cl(CO) 2 (bis-NHC Bz )] (M = Mo, W; bis-NHC Bz = 1,1 0 -dibenzyl-3,3 0 -methylenediimidazoline-2,2 0 -diylidene) have been prepared from the corresponding acetonitrile precursors [M(g 3 -C 3 H 5 )Cl(CO) 2 (NCMe) 2 ] by treatment with free carbene. Their catalytic performance in epoxidation of cis-cyclooctene using H 2 O 2 as oxidant has been studied. All complexes can be applied as catalysts precursors in olefin epoxidation displaying 100% selectivity for the formation of cyclooctene oxide. The tungsten acetonitrile precursor [W(g 3 -C 3 H 5 )Cl(CO) 2 (NCMe) 2 ] displayed the highest catalytic activity achieving quantitative conversion of epoxide in 30 min. The molybdenum NHCbased compound [Mo(g 3 -C 3 H 5 )Cl(CO) 2 (bis-NHC Bz )] displayed higher activity when the epoxidation reaction was performed using H 2 O 2 as oxidant compared to tert-butyl hydroperoxide.

Cyclopentadienyl molybdenum alkyl ester complexes as catalyst precursors for olefin epoxidation

“…1 Mononuclear complexes such as [MoO 2 X 2 L n ] (X = halide, alkyl, siloxy; L = mono-or bidentate ligand), [2][3][4][5][6] [η 5 -(C 5 R 5 )Mo(CO) 3 X] (R = H, CH 3 , CH 2 Ph; X = halide, Me, Et, ansa-bridge, CF 3 ) [7][8][9][10][11] and binuclear complexes of the type [(η 5 -(C 5 R 5 ) 2 M 2 O 5 ] (M = Mo, W) 12,13 have been used as epoxidation catalysts with tert-butyl hydroperoxide (TBHP) or H 2 O 2 as the oxidant. [13][14][15] Cyclopentadienyl (Cp) Mo complexes sometimes display activities similar to or even higher than that of the well-examined methyltrioxorhenium (MTO)-H 2 O 2 system. [16][17][18][19][20] During the past decade, monomeric CpMo tricarbonyl complexes have been established as suitable precursors for catalytically active molybdenum dioxo or oxo-peroxo complexes, which are formed in situ with organic hydroperoxides after oxidative decarbonylation.…”

N-heterocyclic carbene complexes of Group 6 metals

“…9 η 3 -allyl M(II)NHC complexes[151,152] η 3 -2-Methylallyl Mo(II)-NHC complexes Mo-55(a-e) are prepared through the rearrangement of coordinated imidazole to NHC ligands upon the treatment of a strong base followed by alkylation (Scheme 3.3) [153]. Complexes Mo-55(b,d,e) adopt pseudo-octahedral coordination geometries.…”

Catalytic Activity of Molybdenum(II) Complexes in Homogeneous and Heterogeneous Conditions

“…Substitution of Mo by W has dramatic consequences in the capability of the complexes C4 and C5 to oxidize all the substrates, except 1-oct, with yields close to 100% for trans-3, for instance, in both cases. Enhanced activity of W complexes relative to the Mo ones has been described, for instance, in the epoxidation of cis- cyclooctene catalyzed by N-heterocyclic carbene analogues of C1 . The material MCM-C4 displays much higher conversions than C4 for all the substrates, but the immobilization of C5 leads to almost complete loss of activity.…”