Synthesis, Crystal Structure and Acid Catalysed Decomposition Kinetics of (N,N-Bis(2-pyridylmethylene-1,3-diamino-2,2-dicarboxyethyl)propane)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 degreesC 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 x 10(-5) M-2 s(-1)) and independent (1.59 +/- 0.10 x 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 similar to 10 x slower second stage suggest that it may involve intermediates resulting from the H+-independent initial step. (C) 2002 Elsevier Science B.V. All rights reserved.