DFT calculations of alternative structures in the allyl-nickel catalyzed polymerization of butadiene

• Published in: International journal of quantum chemistry Vol. 60; no. 7; pp. 1409 - 1417
• Main Authors: Boegel, H., Tobisch, S.
• Format: Journal Article
• Language: English
• Published: New York John Wiley & Sons, Inc 1996
• DOI: 10.1002/(SICI)1097-461X(1996)60:7<1409::AID-QUA23>3.0.CO;2-Y
• ISSN: 0020-7608

The π‐allyl insertion mechanism of the 1,4‐cis polymerization of butadiene by means of allyl‐nickel catalysts has been studied theoretically by density functional theory (DFT) for the ligand free cationic butenyl nickel(II) complexes [Ni(C3H5)(C4H6)]+, I, [Ni(C3H5)(C4H6)(C2H4)]+, II, and [Ni(C7H11)(C4H6)]+, III. DFT energy profiles have been determined for the insertion of s‐cis‐butadiene into the anti‐η3‐butenyl nickel(II) bond in the supine and prone orientation of the reacting ligands. One of the objectives of the study was to support the proposed π‐allyl insertion mechanism. With increasing size of the model compound there is better agreement between the calculated reaction and activation energies in relation to the known experimental behavior. Among the different arrangements (anti‐ or syn‐butenyl) of the allylic end of the growing polymer chain and the relative orientation (supine or prone) of cisoid butadiene at the catalytic site, the anti‐butenyl/prone reacts in compound III with a small activation barrier (8.2 kcal/mol) to the product (‐5.7 kcal/mol). The stabilizing effect of the coordination of an additional double bond of the growing polymer chain for the correct description of the geometrical and energetical aspects of the insertion reaction was demonstrated by II. The results support the s‐cis‐butadiene insertion into the anti‐η3‐butenyl nickel(II) bond according to the proposed π‐allyl mechanism. © 1996 John Wiley & Sons, Inc.