A Density Functional Study of Ethene Hydrogenation Reactions Catalyzed by Titanocene Complexes

Density functional calculations have been carried out to investigate the mechanisms of ethene hydrogenation reactions catalyzed by four different titanocene complexes, Cp 2 Ti(CO) 2, Cp 2 Ti(CH 3 ) 2, Cp 2 Ti(C 6 H 5 ) 2, and Cp 2 Ti( p -C 6 H 4 CH 3 ) 2 (Cp: cyclopentadienyl group = η 5 -C 5 H 5 ). The molecular geometries of the ground and transition states in these reactions have also been evaluated. The hybrid density functional method B3PW91 showed the best agreement with the experimental geometries of Cp 2 Ti(CO) 2 . B3PW91 computations of activation parameters for the thermal decomposition of Cp 2 TiMe 2 also showed good agreement with previous experimental data on a similar complex, (η 5 -C 5 Me 5 ) 2 TiMe 2 . Ethene hydrogenation by Cp 2 Ti(CO) 2 proceeds in the following order: first and second bond dissociations of Ti−C(CO) bonds followed by the formation of Cp 2 TiH 2 from (Cp 2 Ti + H 2 ) and then ethene hydrogenation by Cp 2 TiH 2 . B3PW91 computations indicated that continuous heating of the system is necessary until the activation barrier of ethene hydrogenation by Cp 2 TiH 2 is overcome. This is because the first three reactions occur nonspontaneously (at room temperature, Δ G = 18.4, 12.8, and 7.4 kcal/mol, respectively). This qualitative finding is supported by the corresponding experimental temperature (= 65 °C). For the Cp 2 TiR 2 catalysts (where R = methyl, phenyl, and tolyl groups), ethene hydrogenation is found to be of first and second σ bond metathesis for the Ti−C(R) bond and H−H bond to form Cp 2 TiH 2, followed by ethene hydrogenation by Cp 2 TiH 2 . Another ethene hydrogenation begins with the first σ bond metathesis for the Ti−C(R) bond and H−H bond, which is followed by the reductive elimination of RH to form Cp 2 Ti, the formation of Cp 2 TiH 2, and then ethene hydrogenation by Cp 2 TiH 2 . In both hydrogenation reactions for the Cp 2 TiR 2 catalysts, first σ bond metathesis reactions are found to be rate-determining and the Δ G ⧧ s are calculated to be very close in value ( Δ G ⧧ = 31.5 kcal/mol, with R = CH 3; 32.0 kcal/mol, with R = C 6 H 5; and 32.6 kcal/mol, with R = C 6 H 4 CH 3, at room temperature). Contrary to the case of Cp 2 Ti(CO) 2, all reactions by Cp 2 TiR 2 (except for the formation of Cp 2 TiH 2 ) are spontaneous, or Δ G s < 0. The Δ G s in the case of Cp 2 TiR 2 are found to be sufficient to overcome the activation Gibbs free energies for the subsequent reactions. Only the activation barrier for first σ bond metathesis by Cp 2 TiR 2 has to be overcome by a proper temperature control. Since the Δ S for first σ bond metathesis is negative, the hydrogenation by Cp 2 TiR 2 takes place below room temperature. These results are supported by the corresponding experimental temperature (= 0 °C). Alternative hydrogenation pathways through Ti−C(CO) bond dissociation of Cp 2 Ti(CO)(H 2 ) or through α-H abstraction of Cp 2 Ti(CH 3 ) 2 have also been discussed.