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

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.