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.