Complex-formation reactions of Cu(ii) and Zn(ii) 2,2′-bipyridine and 1,10-phenanthroline complexes with bicarbonate. Identification of different carbonate coordination modes

Abstract

A series of carbonato complexes was isolated in the reaction of [ML2(H2O)]2+ (M = Cu(II), Zn(II), L = bpy (2,2′-bipyridyl) and phen (1,10-phenanthroline) with HCO3−. Two monomeric carbonato Cu(II) complexes with different bidentate distortion, [Cu(phen)2(CO3)]·7H2O and [Cu(phen)2(CO3)]·11H2O, and three binuclear carbonato complexes with four different bridging modes, {[Cu(phen)2]2(μ2-CO3)}(ClO4)2·4.25H2O containing two different carbonate coordination modes, {[Cu(phen)2]2(μ2-CO3)}(ClO4)2·DMF·H2O and [(bpy)2(H2O)Zn(μ2-CO3)Zn(bpy)2](NO3)2·7H2O, were characterized by X-ray crystallography. UV-Vis spectra of [CuL2(H2O)]2+ ions were recorded as a function of pH in the absence and presence of NaHCO3, and revealed evidence for a carbonation process in the pH range 6 to 10 and a hydrolysis process in the pH range 10 to 13. 13C NMR spectra of [(bpy)2(H2O)Zn(μ2-CO3)Zn(bpy)2]2+ revealed formation of polynuclear carbonato complexes in solution. Complex-formation kinetics of [CuL2(H2O)]2+ (L = bpy and phen) with HCO3− were studied by stopped-flow using a pH-jump technique. The results indicated rapid formation of [Cu(bpy)2OCO2H]+ and [Cu(phen)2OCO2H]+, followed by a slow step (induced by the pH jump) for which the observed first order rate constants were found to be identical for both complexes and independent of the complex and carbonate concentrations employed. The thermal and pressure activation parameters for this step were also determined. The reaction is assigned to a pH-jump induced ring-closure reaction of the unstable five-coordinate monodentate bicarbonato intermediates, [Cu(bpy)2OCO2H]+ and [Cu(phen)2OCO2H]+, to form the pseudo-octahedral bidentate carbonato complexes [Cu(bpy)2O2CO] and [Cu(phen)2O2CO], respectively. A possible mechanism for the carbonation of CuN4 complexes with an in-plane coordinated water molecule in a trigonal bipyramidal structure is proposed.