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

Trifluoromethyltrimethylsilane, (CH 3 ) 3 SiCF 3, in the presence of CsF serves as an excellent CF 3 group-transfer reagent, and reaction with Cp 2 TiF 2 in THF gives the titanocene trifluoromethyl fluoride complex Cp 2 Ti(CF 3 )(F) ( 1; Cp = C 5 H 5 ) in 60% isolated yield. Reaction of complex 1 with the trimethylsilyl reagents, (CH 3 ) 3 SiX (X = OTf = OSO 2 CF 3, Cl, Br, I, N 3, and OSO 2 Ph), in a tetrahydrofuran or toluene solution affords the corresponding Ti–CF 3 derivatives Cp 2 Ti(CF 3 )(X) (X = OTf ( 2 ), Cl ( 12 ), Br ( 13 ), I ( 14 ), N 3 ( 15 ), and OSO 2 Ph ( 16 )) in good isolated yields of 67–84%. These compounds have been characterized by a combination of reactivity studies, IR and 1 H/ 13 C{ 1 H}/ 19 F NMR spectroscopies, and single-crystal X-ray diffraction. The Ti–CF 3 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–CF 2 ) 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 Cp 2 Ti(CF 3 )(F) ( 1 ) produces uniform spherical nanoparticles ranging from ∼70 to 120 nm in size and porous fluorinated oligomers and polymers containing both −(CF 2 –CF 2 )– and −(CF 2 –CFH)– units, as determined by a combination of SEM, XRD, XRF, XPS, and 19 F MAS NMR. Density functional theory results show good agreement with experimental structural data obtained for Cp 2 Ti(CF 3 )(X) (X = F ( 1 ), OTf ( 2 ), Cl ( 12 ), N 3 ( 15 )) and accurately predicts longer Ti–CF 3 distances than for each specific CH 3 analogue, and the trend extends to structurally related Zr and Hf analogues. Simpler model compounds from groups 4 and 8 (M(CH 3 ) 4, M(CH 3 ) 3 (CF 3 ), M(CH 3 ) 3 (CCl 3 ), and M(CH 3 ) 3 (CF 2 CF 2 CF 2 CF 3 ); 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–CH 3 relative to M–CF 3, whereas for the predominantly covalent group 8 analogues, π-back-bonding helps to shorten the predominantly covalent M–CF 3 relative to M–CH 3 . 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.