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) (<b>1</b>; Cp = C₅H₅) in 60% isolated yield. Reaction of complex <b>1</b> 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 (<b>2</b>), Cl (<b>12</b>), Br (<b>13</b>), I (<b>14</b>), N₃ (<b>15</b>), and OSO₂Ph (<b>16</b>)) 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) (<b>1</b>) 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 (<b>1</b>), OTf (<b>2</b>), Cl (<b>12</b>), N₃ (<b>15</b>)) 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