Crystal structure and kinetics of the acid-catalysed decomposition of (N,N′-bis(2-pyridylmethylene)-1,3-diamino-2,2-dicarboxyethylpropane)copper(II)perchlorate

The crystal structure of the complex CuL(OH)₂](ClO₄)₂·H₂O; L=N,N′-bis(2-pyridylmethylene)-1,3-diamino-2,2-dicarboxyethylpropane, determined for the first time, reveals a penta-coordinated copper(II) centre in a distorted square-pyramidal geometry with a weakly bonded axial water molecule. In order to assess possible routes to the putative free ligand N,N′-bis(2-pyridylmethylene)-1,3-diaminopropane-2-carboxylic acid the behaviour (stability) of the complex towards both acid and alkaline aqueous solution has been investigated. In 1.0 M acidic chloride media at 25 °C the complex undergoes slow decomposition in two stages giving 2-pyridylmethylammonium chloride, formaldehyde and diethylmalonate in addition to aqueous Cu²⁺. The initial fast stage has both H⁺-dependent (5.32±0.18×10⁻⁵ M⁻² s⁻¹) and independent (1.59±0.10×10⁻⁵ s⁻¹) paths, the latter believed to reflect rapid chloride equilibration at the labile axial site followed by rate determining isomerisation to place a CuNH bond at the axial site leading to CuN bond cleavage. The rate constant for the H⁺-dependent faster stage carries a H⁺]² dependence suggesting that here protonation of both secondary NH groups occurs within the rate-determining step prior to decomposition. Assistance from the axial site is also proposed. Absorbance changes monitored as a function of H⁺] for the ∼10× slower second stage suggest that it may involve intermediates resulting from the H⁺-independent initial step.