Titanium(IV) Trifluoromethyl Complexes: New Perspectives on Bonding from Organometallic Fluorocarbon Chemistry

Abstract

Trifluoromethyltrimethylsilane, (CH3)3SiCF3, in the presence of CsF serves as an excellent CF3 group-transfer reagent, and reaction with Cp2TiF2 in THF gives the titanocene trifluoromethyl fluoride complex Cp2Ti(CF3)(F) (1; Cp = C5H5) in 60% isolated yield. Reaction of complex 1 with the trimethylsilyl reagents, (CH3)3SiX (X = OTf = OSO2CF3, Cl, Br, I, N3, and OSO2Ph), in a tetrahydrofuran or toluene solution affords the corresponding Ti–CF3 derivatives Cp2Ti(CF3)(X) (X = OTf (2), Cl (12), Br (13), I (14), N3 (15), and OSO2Ph (16)) in good isolated yields of 67–84%. These compounds have been characterized by a combination of reactivity studies, IR and 1H/13C{1H}/19F NMR spectroscopies, and single-crystal X-ray diffraction. The Ti–CF3 linkage in these complexes is remarkably robust, and although the α-C–F bonds are elongated, there is no evidence of an α-fluoride (Ti···F–CF2) between the electrophilic Ti(IV) metal center and any of the C–F bonds in the trifluoromethyl group in the solid-state or in solution. In the solid-state, these complexes are shock-sensitive; energetic decomposition of Cp2Ti(CF3)(F) (1) produces uniform spherical nanoparticles ranging from ∼70 to 120 nm in size and porous fluorinated oligomers and polymers containing both −(CF2–CF2)– and −(CF2–CFH)– units, as determined by a combination of SEM, XRD, XRF, XPS, and 19F MAS NMR. Density functional theory results show good agreement with experimental structural data obtained for Cp2Ti(CF3)(X) (X = F (1), OTf (2), Cl (12), N3 (15)) and accurately predicts longer Ti–CF3 distances than for each specific CH3 analogue, and the trend extends to structurally related Zr and Hf analogues. Simpler model compounds from groups 4 and 8 (M(CH3)4, M(CH3)3(CF3), M(CH3)3(CCl3), and M(CH3)3(CF2CF2CF2CF3); M = Ti, Zr, Hf, Fe, Ru, Os)) were also examined and show that, for group 4 complexes, π-bonding is a significant factor in shortening the strongly ionic M–CH3 relative to M–CF3, whereas for the predominantly covalent group 8 analogues, π-back-bonding helps to shorten the predominantly covalent M–CF3 relative to M–CH3. The bonding analysis suggests that the significant elongation of C–F bonds α to metals is mainly a consequence of the electropositivity of the group 4 metal centers, with minor, if any, contributions from π-effects; the bond-lengthening effect is most pronounced at the α-position and decays rapidly on moving away from the metal.