Stereoselective Association between Acetylacetonatobis(ethylenediamine)cobalt(III) and Malonate Ions as Studied by Nuclear Magnetic Resonance

Abstract

Abstract The effect of added malonate upon the chemical shift and the exchange rate of amine hydrogens of acetylacetonatobis(ethylenediamine)cobalt(III) ion, [Co(acac)(en)2]2+, have been investigated by 1H and 13C NMR spectroscopy. In the absence of malonate, the amine groups give rise to two signals in the 1H spectra and two rate constants of amine exchange due to trans and cis (to coordinated oxygen) amine groups. Upon addition of malonate, the trans NH2 undergoes a down-field shift, the cis NH2 resonance splitting into two peaks. In the presence of malonate, all of the four kinds of amine hydrogens of [Co(acac)(en)2]2+ exhibit different exchange rates. The following conclusion was obtained from the results coupled with 13C NMR data, (i) The malonate ion works in two ways in amine exchange, as a general base catalyst and to form an exchange-inert ion-pair, (ii) The two trans hydrogens that are directed approximately parallel to the two-fold axis of the complex ion hydrogen-bond simultaneously to the two carboxyl groups of malonate, leading to a retardation of amine exchange rates. (iii) The rates of the other trans hydrogens and cis hydrogens are determined by a compromise between acceleration due to general base catalysis and deceleration due to forming ion-associated species, (iv) The conformation of coordinated ethylenediamine is fixed as either (δδ) or (λλ) in the hydrogen-bonded species.