Binding of polar monomers in the complexes with organometallic ethylene polymerization catalysts — Natural orbitals for chemical valence and energy decomposition analysis

The binding mode of polar monomers in complexes with late-transition-metal catalysts for ethylene polymerization was studied by density functional theory (DFT) calculations. The Ziegler–Rauk energy decomposition scheme was used to characterize the geometry distortion and steric and orbital-interaction terms in the bonding energy, while Natural Orbitals for Chemical Valence (NOCV) were applied to describe the donation and back-bonding components of the bond between the monomer and the catalyst. The NOCV analysis allowed for comparison of the donor–acceptor properties of different monomers in the σ- and π-complexes. The complexes with the model, cationic Ni– or Pd–diimine catalysts, N^N–Ni(H)+ and N^N–Pd(H)+, and the neutral Ni–anilinotropone system, N^O–Ni(H), were investigated. The monomers studied included: simple olefins (Et and Pr); examples of oxygen- and nitrogen-containing polar monomers (methyl acrylate (MA), vinyl acetate (VAc), their fluorinated derivatives (FMA, FVAc), vinyl ether (VE), acrylonitrile (AN), and β-butenoic nitrile (BN); vinyl and allyl amines (VAm, PrAm); and a tertiary dimethyl vinyl amine (MVAm). The results demonstrate that the metal-based fragment has a significant influence on the relative stability of the σ- and π-complexes; the π-binding mode increases in the following order: N^N–Ni(H)+ < N^N–Pd(H)+ < N^O–Ni(H). The results of the Ziegler–Rauk bond-energy decomposition indicate that for some monomers (MA, FMA, VAc, AN, VAm, MVAm) the preference of the coordination mode has a steric (electrostatic and Pauli) origin. For other monomers (VE, FVAc, BN, PrAm) the changes in the orbital-interaction terms are important as well. The results of the NOCV analysis indicate that for both, σ- and π-coordination modes there exist components describing σ-donation and π-back-bonding. The sequences of σ-donor and π-acceptor properties of monomers in the π-complexes as well as σ-complexes are similar for the considered catalysts.