Small changes - huge influences : NMR chemical shifts of Ni(II) complexes with polar substrates

Abstract

Neutral Ni(II) complexes have been shown to be highly valuable as robust and versatile catalysts in olefin polymerization. But they show reduced reactivity when the polar monomers methyl acrylate and vinyl acetate are incorporated. To get further insight into this behavior, NMR chemical shift calculations were performed on the system [(N,O) Ni (H) (PMe3)] 1 (N,O = κ2-N,O-{2,6-(3,5-(F3C)2C6H3)2C6H3) - N = C(H)-3,5-I2-2-O-C6H2}). The chemical shifts show reasonable agreement with experiment but are also extremely influenced by geometrical features of the complex as well as the inserted substrate. The first prominent feature, the low-field shift of the Ccarbonyl in the incorporated monomer, can only be reproduced when it is in close proximity to the Ni and in this way hinders the attack of a new monomer. Second, the almost 100 ppm difference in the chemical shift of the carbon of the two substrates directly bound to Ni can be reasoned by the different directionality of polarization as disclosed by natural bond orbital (NBO) analysis.