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

Abstract

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.

Synthesis and Reactivity of Mn–CF3 Complexes

“…Again, ionic bonding in complex (6) could explain the longer M-C RF bond distance despite having more electron density on the metal as discussed by Hughes [4]. (3) 2020, 1910 Mn(CO) 3 (Phen)CF 3 (4) 2010, 1930 Mn(CO) 2 (Tpy)CF 3 (6) 2020, 1900, 1850 Mn(CO) 2 (NNS)CF 3 (7) 2020, 1910, 1900…”

“…The Mn-C CF3 bond distance in (3), however, is significantly shorter than that in (1). The reasoning for this observation may be due to more significant π-backbonding into the low-lying C-F σ*orbitals as the N-donor ligand adds more electron density to the metal center [3]. Research by Grushin and Macgregor, however, suggests that M-CF 3 bonding has little to no (<8%) π-backbonding character [35].…”

“…Research by Grushin and Macgregor, however, suggests that M-CF 3 bonding has little to no (<8%) π-backbonding character [35]. If significant ionic character is invoked for the Mn-C CF3 bond then the replacement of 2 CO ligands with 2 hard N donors would decrease the Lewis acidity at Mn, increasing its interaction with the partial positive charge of the CF 3 carbon [3,4]. In contrast, the Mn-C CF3 bond distance in (6) is significantly longer than that in (1) in spite of the additional N-donors and one less CO ligand competing for the metal's π-back-donation.…”

“…Organometallic compounds and especially metal alkyls (M-R) are immensely important players in catalysis [1,2]. Catalysis utilizing metal fluoroalkyl complexes, however, is less common due to the inherent stability of metal perfluoroalkyl bonds (M-R F ) [3][4][5]. Nonetheless, these compounds are useful to the increasingly important field of fluoro-organic synthesis [6][7][8][9][10][11][12][13][14].…”

“…Finally, research by Kiplinger, Hughes, and co-workers demonstrated that decomposition of the shock-sensitive Cp 2 TiF(CF 3 ) complex produced various insoluble oligomers in which both -(CF 2 CF 2 )-and -(CF 2 CFH)-units were identified. The precise mechanism for the formation of these fluoro-oligomers, including whether Ti nanoparticles were involved, was never determined [3].…”