Chelate-Size Effects on the Structures, Chemical Behavior, Properties, and Catalytic Activity of the New Palladium(II)−Allyl Complexes [Pd(η3-1-R1-C3H4){FcCH═N-CH2-(CH2)n-NMe2}][PF6] {Fc = (η5-C5H5)Fe(η5-C5H4), n = 2 or 1, and R1 = H or Ph}

Abstract

The synthesis, X-ray crystal structures, and the study of the solution behavior of the palladium(II) allyl complexes [Pd(η3-1R1-C3H4){FcCH═N-CH2-(CH2)n-NMe2}][PF6] {with Fc = (η5-C5H5)Fe(η5-C5H4), R1 = H, and n = 2 (4) or 1 (5) or R1 = Ph and n = 2 (6) or 1 (7)} are described. The structural studies of 4, 5, 6, and 7 showed that the allyl ligands were bound to palladium(II) in an η3-fashion and the ferrocenyl-Schiff bases behaved as neutral (N,N′) bidentate ligands. As observed for other Pd complexes of ligands 1 and 2, the imine conformation of the ligand in the complex strongly depends on the length of the hydrocarbon chain between the two nitrogen donors: compounds 4 and 6 exhibited an anti-(E) conformation, whereas the syn-(Z) form is observed in 5 and 7. 57Fe-Mössbauer spectra (in the solid state) and electrochemical studies based on cyclic voltammetry of 4, 5, 6, and 7 are reported. These studies reveal that the proclivity of the iron(II) toward oxidation is affected by the nature of the substituent R1 of the allyl group {H (in 4 and 5) or Ph (in 6 and 7)}. Compounds 4 and 5 have been tested in the palladium(II) allylic alkylation of (E)-3-phenyl-2-propenyl (cinnamyl) acetate using sodium diethyl 2-methylmalonate as the nucleophile. In all cases the formation of the trans-(E) isomer of the linear product (9) was strongly favored versus that of the branched derivative (10). The stoichiometric reactions of the palladium(II) allyl intermediates [Pd(η3-1-Ph-C3H4){FcCH═N-CH2-(CH2)n-NMe2}][PF6] {with n = 2 (6) or 1 (7)} with the nucleophile have been studied. Density functional theory (DFT) calculations of these systems have allowed us to rationalize the structural differences as well as the regioselectivity of the catalytic and stoichiometric reactions.