Density functional study of a d(2)-C5H5Nb(butadiene)R+ ethene polymerization catalyst

We have studied the ethylene polymerization catalyst C5H5Nb(butadiene)Cl-2 + MAO using primarily density functional theory (DFT). The active species w as assumed to be C5H5Nb(butadiene)R+. Chain initiation and propagation as w ell as different termination processes were modeled. The ethene coordinatio n is very weak, and no free energy minimum was found. Insertion into the me tal-alkyl bond has an energy barrier of 4 kcal/mol for R = CH3 and 6 kcal/m ol for R = C2H5. The ethene insertion transition state is clearly stabilize d by agostic interaction, with metal-hydrogen distances of 2.07-2.16 Angstr om. However, in alkyl conformations these bonds are longer and correspond t o only weak agostic interaction. In the absence of strong agostic interacti on the resting state alkyl complexes are floppy and different conformations interconvert easily. Termination via hydrogen transfer to a coordinated et hene molecule ejecting a terminal alkene has a high energy barrier of 17 kc al/mol. An alternative termination process via beta -elimination and subseq uent alkene ejection is also very expensive, 43 kcal/mol. The propagation f ree energy barrier for the concerted reaction is 21 kcal/mol, which consist s mostly (80%) of ethene coordination. The termination free energy barrier via hydrogen transfer to coordinated alkene is 30 kcal/mol and that via bet a -elimination is 28 kcal/ mel. The free energies have been determined in a vacuum using the harmonic approximation. The key intermediates were also o ptimized using MP2 supplemented with single-point calculations using CCSD. These methods gave stronger complexation energies, resulting in lowering th e propagation barrier by approximately 3-4 kcal/mol and increasing the beta -elimination barrier by 6-7 kcal/mol. The BSSEs in MP2 and DFT complexatio n energies were estimated to be 15-20 and 1-3 kcal/mol, respectively, using DZ and DVZP bases.