DFT studies on substituent effects in palladium-catalyzed olefin polymerization

Gradient-corrected density functional theory has been used to study substit uents effects on the cationic N<^>N-Pd(II) diimine catalyst in ethene and p ropene polymerization. Here N<^>N = -C(R)-N(Ar)-N(Ar)-C(R)- with R = H, -CH 3, -An and Ar = H, C6H5, -2,6-C6H3- (Me)(2), -2,6-C6H3(Pr-i)(2). Calculatio ns have been performed on the [N<^>N-Pd(II)-P](+) (P = n-propyl and isoprop yl) alkyl complexes (1) and the corresponding [N<^>N-Pd(II)-P(eta(2)-CH2CHR 0)](+) pi-complexes of ethene (R-0 = H) and propene (R-0 = CH3), as well as the ethene and propene (1,2- and 2,1-) insertion transition states. The re sults show that an increase in the size of the substituents on the Pd(II) c atalyst enhances the preference of 1 for the isomer with the branched isopr opyl alkyl group P, while for the olefin complexes [N<^>N-Pd(II)-P(eta(2)CH (2)CHR(0))](+) the isomer with the linear n-propyl group P becomes preferre d. Further, an increase in the size of the substituents affects the relativ e binding of ethene and propene. Thus, the electronic preference of propane complexes is overridden by steric factors in the case of the largest subst ituents. The regioselectivity of propene insertion is strongly affected as well: an increase in steric demand decreases the 2,1-:1,2-insertion ratio, with 1,2-insertion becoming favored for the sterically most congested catal yst [R = -CH3, Ar = -2,6-C6H3(Pr-i)(2)].