Oxidative ring-opening of aromatics: Thermochemistry of iron, copper and cobalt biphenyl carboxylates

Abstract

Thermochemistry of aromatic carboxylates has received little attention in the literature, despite their relevance as intermediates in the oxidative ring-opening of aromatic compounds. The thermal behavior of selected transition metal aromatic carboxylates was studied as a first step to determine the effectiveness of the metals as potential catalysts for decarboxylation. Iron, cobalt, and copper salts of biphenyl carboxylic acids were synthesized and their thermal behavior over the temperature range 25–600 °C was investigated. Characterization of these compounds indicated that, with exception of the copper (II) biphenyl-2-carboxylate, all metal carboxylates corresponded to homogeneous materials of different morphology. Structures in which metals might be present in more than one oxidation state or forming metal–metal bonds are possible. Infrared spectroscopy showed that all synthesized carboxylates existed in bridging configuration. In all cases, thermal analysis revealed a complicated behavior. Decomposition occurred as a multistage process and exhibited a rather complex mechanism. The decomposition of the iron and copper carboxylates resembled a cascade of reactions, i.e., the individual decomposition steps could not be resolved due to overlapping; whereas, the decomposition of the cobalt compounds followed a more stepwise type of process, i.e., the individual decompositon steps could be differentiated. Aromatic carboxylates of moderate thermal stability, with decomposition temperatures above 240 °C, were formed. Thermal stability depended on the metal and increased in the order Cu < Fe < Co. Production of benzoic acid during the decomposition of the copper carboxylates was observed. Production of fluorenone suggested that the Fe, Co and Cu carboxylates decomposed, at least partially, via the formation of an organic ketone and metal oxides. In the case of the copper carboxylates, the reduction of the copper by forming cuprous oxide (Cu2O) also took place.