MTO Schiff‐Base Complexes: Synthesis, Structures and Catalytic Applications in Olefin Epoxidation

Abstract

Several Schiff‐base ligands readily form complexes with methyltrioxorhenium(VII) (MTO) by undergoing a hydrogen transfer from a ligand‐bound OH group to a ligand N atom. The resulting complexes are stable at room temperature and can be handled and stored in air without problems. Due to the steric demands of the ligands they display distorted trigonal‐bipyramidal structures in the solid state, as shown by X‐ray crystallography, with the O− moiety binding to the Lewis acidic Re atom and the Re‐bound methyl group being located either in cis or trans position to the Schiff base. In solution, however, the steric differences seem not to be maintained, as can be deduced from 17O NMR spectroscopy. Furthermore, the Schiff‐base ligands exchange with donor ligands. Nevertheless, the catalytic behaviour is influenced significantly by the Schiff bases coordinated to the MTO moiety, which lead either to high selectivities and good activities or to catalyst decomposition. A large excess of ligand, in contrast to the observations with aromatic N‐donor ligands, is detrimental to the catalytic performance as it leads to catalyst decomposition.

A (salicylidene)aniline derived Schiff-base adduct of methyltrioxorhenium(VII)–Cis-and trans-coordination of the ligand

“…It is important to note that after complexation the imine group vibration shifts from 1617 to 1645 cm À1 and the phenolic OH group and coupled ring vibrations disappear, but a broad and weak NH stretching band can be seen at 2580 cm À1 . This is in accord with the previously assumed transfer of the phenolic-OH proton to the imine nitrogen atom under formation of a zwitter ion, the negatively charged oxygen coordinating as nucleophile to the electrophilic Re(VII) atom of MTO [12]. Proton and carbon NMR indicate only a weak interaction of the Re atom to the Schiff base, reflected only in a small chemical shift difference of the Re-CH 3 protons (D(d) = 0.05 ppm) and carbon (D(d) = 0.1 ppm) atom from non-coordinated to coordinated state of the MTO.…”

“…It was noted that cis and trans coordination is dependent on the ligand employed. In the case of N-donor ligands, cis coordination is very rare and has, to the best of our knowledge, been described only twice [7,12].…”

“…The weak coordination of the Lewis bases largely prevent a successful immobilization of the otherwise excellent catalytic system [10] and the attempt to transfer chirality by introducing it via the base ligand into the system also suffers from both the huge excess of (expensive) base ligand needed and the insufficient chirality transfer due to the weak ligand-metal interaction [11]. The application of Schiff-base ligands instead of N-donor ligands seemed to solve at least one problem: large ligand excesses were not necessary to achieve high activities and selectivities [12]. Nevertheless, Schiff bases are also weakly coordinated to the Re atom and the reasons behind the observation of a cis or a trans coordination (with respect to the Re-CH 3 bond) of the Schiff base to the MTO moiety remained unknown, although some influence of the substituents at the Schiff base was considered to be the driving force.…”

Synthesis of di‐nitrogen Schiff base complexes of methyltrioxorhenium(VII) and their application in epoxidation with aqueous hydrogen peroxide as oxidant

“…The differences of 20-27 and 60-80 cm −1 between ν sym (Re O) and ν asym (Re O) correspond to the octahedral and trigonal-bipyramidal coordination geometry, respectively. [25,30,32] For the free MTO, the difference is 33 cm −1 , which corresponds to a tetrahedral coordination geometry. [39] Therefore, it can be deduced that the structures of the title complexes are closer to an octahedral geometry, in which the Schiff bases act as bidentate ligands to coordinate to MTO through the two nitrogen atoms.…”

Zirconium phenylphosphonate-anchored methyltrioxorhenium as novel heterogeneous catalyst for epoxidation of cyclohexene

“…Meanwhile, the direct addition of excess pyridine [8], aromatic N-bases [ [12], [13], and certain MTO-Schiff base adducts or microencapsuled Lewis base adducts 4 (monodentate and bidentate, aromatic and aliphatic nitrogen containing ligands [14]- [16]) could decrease the diol products by neutralizing the acidity of MTO catalyst system.…”