Formation and dissociation mechanism of amide complexes. V. Interconversion of dimeric and monomeric Cu²⁺‐complexes with 1,8‐diamino‐3,6‐diaza‐2,7‐octanedione: Comparison between the reactivity of terminal and internal amide groups

Abstract

SummaryThe protonation and deprotonation rates of the coordinated amide groups in the Cu2+-complexes of 1,8-diamin0-3,6-diaza-2,7-octanedione (DED = L) have been studied by stopped-flow techniques. Starting at low pH from Cu2+ and DED the dimeric C~2L,~+-complex, fully formed within the mixing time of the stopped-flow instrument, reacts in two consecutive steps to yield the final product CuLH-,. The rate constants of the forward and backward reactions have been determined and are given in Table 1. The intermediate was identified as Cu2L2H-?+ by measuring its V1S.-absorption spectrum.The rate constants for the interconversion of the amide groups from the 0-to the N-coordinated form in the Cu2+-complexes of DED, 2,10-diox0-1,4,8,11-tetraaza-undecane (DANA) and triglycine (TRIGLY) are compared with each other. It is shown that these rate constants are similar, no matter whether the amide group is terminal or internal as long as the rotation is easily possible as is the case in the dimeric species Cu2L2+ and Cu2L2H-?+. However, for CuLH-, the interconversion only takes place after opening of one of the chelate rings in a rapid protonation preequilibrium.