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) (HLH, Ar = C6H5; HLMe, A r= 2,6-Me2C6H3; HLiPr, Ar = 2,6-iPr2C6H3). Subsequent complexation to the
[MoO2]2+ core leads to the formation of novel
complexes of general formula [MoO2(LR)2] (R = H, 1; R = Me, 2; R = iPr, 3). For reasons of comparison the oxo-imido complex [MoO­(NtBu)­(LMe)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
MoIV complexes [MoO­(PMe3)­(LR)2] (R = H, 5; R = Me, 6; R = iPr, 7) and [Mo­(NtBu)­(PMe3)­(LMe)2] (8), were isolated. All complexes have
been characterized by NMR spectroscopy, and 1–4 also by cyclic voltammetry. A positive shift of the MoVI–MoV 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 MoIV 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(NtBu)­(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.