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 β-ketimines afforded the ligands (Ar)­NC­(Me)­CH₂CO­(CF₃) (HL^H, Ar = C₆H₅; HL^Me, A r= 2,6-Me₂C₆H₃; HL^iPr, Ar = 2,6-ⁱPr₂C₆H₃). 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(N^tBu)­(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(N^tBu)­(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(N^tBu)­(L′)₂] (10), respectively. Thermodynamic activation parameters ΔH^‡ and ΔS^‡ 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 ΔS^‡ is small. The reaction of the oxo-imido complex 4 to 8 revealed a significant electron-donating contribution of the imido substituent.