Interaction of (dien)Pd(II) complexes with the amino group of cytidine: a kinetic and NMR study

Abstract

Interaction between cytidine and (dien)Pd(II) has been studied at high pH by spectrophotometry and by 1H, 13C and 15N NMR. With increasing pH interaction of (dien)Pd(II) with the exocyclic NH 2 group is observed, and this (dien)Pd-N4 complex is more stable at high pH than the (dien)Pd-N3 complex, which exists at neutral pH. Furthermore, at the higher concentrations used in the NMR spectroscopy (0.1-0.2 M) a binuclear complex implying both the N3 and N4 (exocyclic amino) sites is present, pH- jump experiments revealed two kinetically distinct reactions. The faster ‘Reaction I’ is catalysed by OH − and is attributed to the formation of the (dien)Pd- N4 complex. This reaction proceeds by a mechanism in which fixation of Pd(II) at the N3 site is required for the deprotonation of the NH 2 group by OH −. This process is followed by an intramolecular transfer of (dien)Pd(II) from N3 to the deprotonated N4. The slower ‘Reaction II’ is attributed to dissociation of both Pd-N3 and Pd-N4 complexes at high pH and leads to the thermodynamically stable (dien)Pd- (OH) +. Kinetic data for this process are in good agreement with those observed for the formation of (dien)Pd-N3.