The Exploration of Neutral Azoligand-Based Grubbs Type Palladium(II) Complex as Potential Catalyst for the Copolymerization of Ethylene with Acrylonitrile:  A Theoretical Study Based on Density Functional Theory

• Published in: Organometallics 2005-03, Vol.24 (6), p.1242-1251
• Main Authors: Yang, Sheng-Yong, Szabo, Miklos J, Michalak, Artur, Weiss, Thomas, Piers, Warren E, Jordan, Richard F, Ziegler, Tom
• Format: Article
• Language: English
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om049015m

A combined quantum-mechanical (QM) and molecular-mechanical (MM) method has been used to explore the potential of neutral azoligand-based Grubbs type Pd(II) complexes as catalysts for ethylene−acrylonitrile copolymerization. The first part of the investigation includes complexation of the monomers to the catalyst, the first insertion step, and isomerization of the first insertion product 1. The results show that the π-complexation energies of acrylonitrile and ethylene are comparable, but the acrylonitrile insertion has a lower barrier over ethylene insertion by ca. 5 kcal/mol in the first step. The leading product 1 might conduct a series of isomerizations with a lower barrier to form different chelate structures. However, the most stable isomers have the CN group in the α-position. The second part explores the further role of the kinetic insertion product 1. It is shown that 1 readily can complex another ethylene or acrylonitrile monomer. Unfortunately, it is even more favorable for 1 to coordinate its Pd(II) center to a nitrogen atom on a α-cyano-alkyl group of another 1 complex, leading to inert oligomers of 1 that do not further react with ethylene or acrylonitrile. Trimers of 1 are preferred over dimers because they allow for an optimal linear Pd−NC bonding mode with an angle of ∼180°. The preference for palladium coordination to the nitrogen on a α-cyano-alkyl group over N-coordination to acrylonitrile stems from the fact that the acrylonitrile nitrogen is less electron rich due to donation to the π* orbital of the olefinic bond.