Molecular structure of methyl orange and its role in the process of [Pd(Azo)] compound and MOF formation

Abstract

In this work, we follow the process of metalorganic complex formation between Pd(II) ions and 4-[4-(Dimethylamino)phenylazo]benzenesulfonic acid sodium salt (methyl orange) in the area of its tautomeric equilibrium. Using UV–Vis spectrophotometry we followed intermediate stages related to inorganic complex association, ligand exchange, cis – trans complex isomerization, and bonding of metal to nitrogen and carbon in the phenyl ring. Obtained double kinetic curves registered at 470 and 273 nm suggest, that the process of complex formation and the cis – trans isomerization is first order in respect to methyl orange. For the process of metalorganic complex formation the activation parameters like energy, enthalpy, and entropy were determined (EA=45.69kJmol,ΔH≠=43.1±1.8kJmol,ΔS≠=−172.6J/(mol·K), respectively) using Arrhenius and Eyring dependencies. Moreover, based on obtained results coming from spectrophotometric study and DFT calculations, the mechanism and molecular structure of the intermediates and products during the process of metalorganic complex formation, were proposed. It was shown, that process takes place in four stages. The two first are related to N – Pd and Pd – C bond formation, and the next one is associated with H- shaped bridge formation. As the last one is aging leading to more complex structure building and finally to metal-organic frameworks formation.