PubMed2006
Organotitanium fluorides (C5Me4R)TiF3 (R = H, Me, Et) sublimate with formation of crystalline dimers. From solution, we obtained crystals of dimers and tetramers. The tetramer [{(C5Me5)TiF3}4] irreversibly dissociates in the solid state to dimers (DeltaH = 8.33 kcal mol(-1)). The variable-temperature (1)H and (19)F NMR spectroscopy measurements of the toluene-d(8) solution of [{(C5Me5)TiF3}2] revealed at 202 K one monomeric, two dimeric (with C2h and Cs symmetry), two tetrameric (with D2 and C2v symmetry), and two trimeric (both C2 symmetry) molecules. With the increase in temperature and dilution of the solution, the composition of the solution shifts to the smaller molecules. The thermodynamic and activation parameters for the reversible dissociation of dimers to monomers in the solution are DeltaH = 9.2 kcal mol(-1), DeltaS = 24.2 cal mol(-1) K(-1), DeltaH(double dagger) = 12.2 kcal mol(-1), DeltaS(double dagger) = 9.7 cal mol(-1) K(-1). The dissociation path with a weakly double-bridged transition-state dimer was proposed. The thermodynamic parameters for the reversible dissociation of the C2v tetramer to the dimers in solution are DeltaH = 7.9 kcal mol(-1) and DeltaS = 26.8 cal mol(-1) K(-1). From both tetramers, the D2 molecule is 0.34(5) kcal mol(-1) lower in enthalpy and 6.5(5) cal mol(-1) K(-1) lower in entropy than the C2v molecule. The structures of both trimers were proposed. The low-temperature 19F NMR spectra of the CDCl3 solution of [{(C5Me5)TiF3}2] are consistent with equilibria of a monomer, two dimers (with C2h and Cs symmetry), and a trimer. The vapor pressure osmometric molecular mass determination of CDCl3 solution of [{(C5Me5)TiF3}2] at 302 K is consistent with the equilibrium of the dimer and the monomer.