New Molybdenum(II) Complexes with α-Diimine Ligands: Synthesis, Structure, and Catalytic Activity in Olefin Epoxidation

Abstract

Three new complexes [Mo(η3-C3H5)Br(CO)2{iPrN=C(R)C5H4N}], where R = H (IMP = N-isopropyl 2-iminomethylpyridine), Me, and Ph, were synthesized and characterized, and were fluxional in solution. The most interesting feature was the presence, in the crystal structure of the IMP derivative, of the two main isomers (allyl and carbonyls exo), namely the equatorial isomer with the Br trans to the allyl and the equatorial with the Br trans to one carbonyl, the position trans to the allyl being occupied by the imine nitrogen atom. For the R = Me complex, the less common axial isomer was observed in the crystal. These complexes were immobilized in MCM-41 (MCM), following functionalization of the diimine ligands with Si(OEt)3, in order to study the catalytic activity in olefin epoxidation of similar complexes as homogeneous and heterogeneous catalysts. FTIR, 13C- and 29Si-NMR, elemental analysis, and adsorption isotherms showed that the complexes were covalently bound to the MCM walls. The epoxidation activity was very good in both catalysts for the cis-cyclooctene and cis-hex-3-en-1-ol, but modest for the other substrates tested, and no relevant differences were found between the complexes and the Mo-containing materials as catalysts.

Molybdenum (VI) Complexes Containing Pyridylimine Ligands: Effect of the Imine Nitrogen Substituent in the Epoxidation Reaction

“…The complexes were characterized by FTIR, elemental analysis, ESI, 1 H, 13 C NMR, and X‐ray crystallography. The FTIR spectra of all nine complexes (See experimental section, Table S1) are in agreement with the coordination of the pyridylimine ligand to the MoCl 2 O 2 core through the nitrogen atoms [21–23] . The ν C=N stretching frequencies are observed around 1647 cm −1 and 1597 cm −1 .…”

“…The FTIR spectra of all nine complexes (See experimental section, Table S1) are in agreement with the coordination of the pyridylimine ligand to the MoCl 2 O 2 core through the nitrogen atoms. [21][22][23] The ν C = N stretching frequencies are observed around 1647 cm À 1 and 1597 cm À 1 . The strong band at 903 cm À 1 (average) and medium band at 937 (average) were assigned to the asymmetric and symmetric stretching vibrations, respectively, of the cis-[MoO 2 ] unit.…”

“…However, the presence of different substituents (H, Me, Ph) at the imine-carbon did not significantly change the activity. [23] Despite the outcome, we consider that the selection of the appropriate substituent present at the nitrogen of the imine moiety could change the catalytic activity.…”