Molybdenum(VI) Dioxo and Oxo-Imido Complexes of Fluorinated β-Ketiminato Ligands and Their Use in OAT Reactions

Substitution of a methyl by a trifluoromethyl moiety in well-known beta-ketimines afforded the ligands (Ar)NC(Me)CH2CO(CF3) (HLH, Ar = C6H5; HLMe, A r=2,6-Me2C6H3; HLiPr Ar = 2,6-(Pr2C6H3)-Pr-i). Subsequent complexation to the [MoO2](2+) core leads to the formation of novel complexes of general formula [MoO2(L-R)(2)] (R = H, 1; R = Me, 2; R = iPr, 3). For reasons of comparison the oxo-imido complex [MoO((NBu)-Bu-t)(L-Me)(2)] (4) has also been synthesized. Complexes 1-4 were investigated in oxygen atom transfer (OAT) reactions using the substrate trimethylphosphine. The respective products after OAT, the reduced Mo-IV complexes [MoO(PMe3)(L-R)(2)] (R = H, 5; R = Me, 6; R = iPr, 7) and [Mo((NBu)-Bu-t)(PMe3)(L-Me)(2)] (8), were isolated. All complexes have been characterized by NMR spectroscopy, and 1-4 also by cyclic voltammetry. A positive shift of the Mo-VI-Mo-V reduction wave upon fluorination was observed. Furthermore, molecular structures of complexes 2, 4, 5, and 8 have been determined via single crystal X-ray diffraction analysis. Complex 8 represents a rare example of a Mo-IV phosphino-imido complex. Kinetic measurements by UV-vis spectroscopy of the OAT reactions from complexes 1-4 to PMe3 showed them to be more efficient than previously reported nonfluorinated ones, with ligand L' = (Ar)NC(Me)CH2CO(CH3) [MoO2(L')(2)] (9) and [MoO((NBu)-Bu-t)-(L')(2)] (10), respectively. Thermodynamic activation parameters Delta H-double dagger and Delta S-double dagger of the OAT reactions for complexes 2 and 4 have been determined. The activation enthalpy for the reaction employing 2 is significantly smaller (12.3 kJ/mol) compared to the reaction with the nonfluorinated complex 9 (60.8 kJ/mol). The change of the entropic term Delta S-double dagger is small. The reaction of the oxo-imido complex 4 to 8 revealed a significant electron-donating contribution of the imido substituent.