Formation and dissociation mechanism of amide complexes III. Water substitution as the rate limiting factor for the interconversion of Cu²⁺ complexes with neutral and deprotonated amide groups

Abstract

Surnrnavy. The protonation and deprotonation rates of the coordinated amide group in the Cu2+ complexes with Na-(2-pyridyl-methyl)-glycinamide (I) and N"-(Z-pyridyl-methyl)-glycineethylamide (11) have been studied by stopped flow techniques. It is shown that the rate determining step of the formation of the complex with the deprotonated amide group is given by the rate of water dissociation from Cu2+. Weaker bases than OH .-or stronger acids than water can react by a different path, in which the proton transfer and/or the rotation from the 0-co-ordinated into the N-co-ordinsted form and vice vevsa is rate determining.In aqueous solution amides form two different types of complexes with heavy metal ions. The neutral amide group is bound through its carbonyl oxygen [ Z ] , whereas the deprotonated group co-ordinates via the nitrogen atom as in (1) [ 3 ] .-

Nickel(II) Complexes of Monofunctionalized Pyridine-Azamacrocycles: Synthesis, Structures, Pendant Arm “On-Off” Coordination Equilibria, and Peroxidase-like Activity

“…The λ max shift to longer wavelengths (550−600 nm) also suggests that “on” coordination of the arm leads to structural and electronic changes from a low-spin, square planar complex to a high-spin, octahedral complex . In this case, the deprotonated amide N, rather than the carbonyl O, coordinates to the nickel ion. , Calculated p K a for both complexes was found to be 11.3. This value is close to the reported protonation constant for Cu(II)-azamacrocyclic complex with an amide pendant arm (p K a = 10.98) …”

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

“…The kinetics of this interconversion have been studied in detail for oligopeptides 131 [4] and for a series of synthetic ligands having the amide group in terminal position [1] [5]. Whereas the first type of ligands is interesting because of its relation to biological systems, the second type is simpler to investigate on a molecular level,…”

“…Models of CU,L,~+ (4) and C U~L~H -,~+ (5) show that in both cases favorable structures exist with essentially no steric interactions. The two coordination squares around the Cu2+ do not lie in the same plane, but are displaced relatively to each other so that a two-steps stair results.…”

Transition Metal Ions and Amides, VI. Complexation of the neutral and the anionic forms of 3, 7‐diazanonanedioic acid diamide and 3, 7‐diazanonanedioic acid diethylamide with Cu2+ and Ni2+

“…A straightforward study of the interconversion of complexes with the neutral (I) and the deprotonated (11) amide group is difficult with oligopeptides and similar ligands like N, N'-diglycyl-ethylenediamine, since pre-equilibria of unknown influence must be taken into account [Z]. In two previous papers [2] [3] we have shown that ligands with terminal amide groups are especially suitable for that purpose. In the case of the terdentate ligands Na-(2-pyridylmethy1)-glycineamide and Na-(2-pyridylmethyl)-glycineethylamide, a stabilization by 3-4 orders of magnitude was observed when the carbonyl oxygen was coordinated to the metal ion forming a fivemembered chelate ring.…”

“…To test the generality of the results obtained for Cu2+ in [2] and [3] and to extend the studies to other metal ions, we have synthesized a series of amides with more than three potentially coordinating groups. Here, we report on the stabilities and absorption spectra of the Cu2f and Ni2+ complexes with 3'7-diazanonanedioic acid diamide (DANA = I11 a) and its diethyl derivative 3,7-diazanonanedioic acid diethylamide (DANEA = IIIb).…”