Effect of the Position of Double Bonds of Unsaturated Carboxylic Acids on the Type of the Resulting Palladium(I) Coordination Polymers

A new class of palladium(I) polymers with unsaturated monocarboxylic acids was first isolated in coordination chemistry. These were obtained through the reaction of palladium(II) acetate Pd3(μ-MeCO2)6 with monocarboxylic acids such as 3-pentenoic acid and 4-pentenoic acid (with a non-conjugated double bond to the carboxyl group), cinnamic acid and crotonic acid (with a conjugated double bond to the carboxyl group), as well as the reaction with a dicarboxylic acid (itaconic acid) with a double bond conjugated to only one carboxyl group, similar to cinnamic and crotonic acids. Analysis of the composition of eight new synthesized compounds, along with their IR, UV-Vis, and ESR spectra, revealed that the resulting Pd(I) coordination polymers [Pd(RCOО)6]n with 3-pentenoic acid {[Pd(π-C5H7O2)]∙H2O}n and 4-pentenoic acid [Pd(π-C5H7O2)∙C5H8O2)]n are diamagnetic, while those with cinnamic acid [Pd(C9H7O2)]n, crotonic acid {[Pd(C4H5O2)]H2O}n, and itaconic acid {[Pd(C5H4O4)H2O]∙2H2O} are paramagnetic. The backbone of the obtained polymers is formed by bridging carboxylate groups and Pd–Pd bonds. The completion of the coordination polyhedron in the case of complexes with 3- or 4-pentenoic acids occurs via coordination of their double bonds with palladium. In the polymeric complexes with cinnamic and crotonic acids, an agostic bond forms, while in the complexes with itaconic acid, a water molecule is coordinated. According to EPR spectroscopy, the polymer with itaconic acid contains paramagnetic centers characterized by an unpaired electron spin density of 1018 spins/g, stable for up to a year, which makes it a promising precursor for the development of heterogeneous catalysts with enhanced catalytic activity.