Kinetic Studies on the Stability Enhancement of Mixed Ligand Copper(II) Complexs with 2,2′-Bipyridyl and Acetylacetonate

The rate constants for the formation and dissociation of the binary Cu2+-acetylacetonate(acac−) complex and the ternary 2,2′-bipyridyl(bpy)-Cu2+-acac− complex have been determined by the temperature-jump method. For the reactions Cu2++acac−1b-1b(acac)+, Cu(acac)++acac−2b-2b(acac)2, and Cu(bpy)2++acac−1t-1t-(bpy)(acac)+, the rate constants are k1b=9.0×108 M−1 s−1, k-1b=3.2 s−1, k2b=7.0×107 M−1 s−1, k-2b=13 s−1, k1t=1.1×109 M−1 s−1 and k-1t=1.9 s−1. A comparison of the reactions shows that the forward rate for the Cu(bpy)-(acac)+ formation is larger than that for Cu(acac)+ and, subsequently, the rate of the complex formation between Cu(bpy)2+ and acac− is enhanced by the coordinating bpy. On the other hand, the reverse rate for the ternary complexation reaction is smaller than for the binary. This illustrates that the dissociation of acac− of Cu(bpy)-(acac)+ becomes more difficult owing to the coordination of the bpy. The above two effects of the bpy may be driving forces producing high stability, i.e., a positive value of ΔlogK(K=[Cu(bpy)[L][Cu(bpy)][L]) for the ternary copper(II) complexes with bpy and other ligands containing O as donor atoms.