Nickel(II) and nickel(III) bis(dipeptido) complexes of .alpha.-aminoisobutyric acid

Abstract

3809development of positive charge in the transition state for these ligand substitution reactions. The increasing positive charge at the metal center during the breaking of the ruthenium-oxygen bond is destabilized by electron-withdrawing substituents, which therefore inhibits the dissociation of the aquo ligand. Thus, if ligand dissociation is the rate-determining step of the reaction, a corresponding reduction in the rate constant for ligand substitution for these complexes should be observed.A plot of the E l j 2 values of the ruthenium(III/II) redox couples obtained for the aquo(phosphine)ruthenium(II) complexes in CH2C12 vs x u is also linear (r2 = 0.93) and gave a p value of +0.10. The positive sign of the p value is consistent with the expectation that electron-donating groups stabilize, and electron-withdrawing groups destabilize, ruthenium(III), relative to the analogous unsubstituted complexes. The magnitude of p for this correlation indicates that the substituents on the phosphine ligands have a moderate effect on the resulting E l l , values, compared to other systems. Larger values of p have been obtained for the plot of Ell2 against x u for III/II redox couples of ruthenium with acetylacetonate (p = 0.38-0.50)88389 and nitrosoarene ( p = 1.05)90 ligands. The p values obtained from a Hammett correlation with ElI2 values for triazene 1-oxide complexes of ruthenium ( p = 0.10-0.18)91 closely match those found in this study. Finally, our observed p values are larger than those found for metal-centered redox reactions of substituted tetraphenylporphyrin complexes ( p = 0.02-0.08),92 where the ligand substituents are located further from the metal center and thus have a smaller effect on the EIl2 values of the metal complexes. Trialkylphosphine ligands were utilized in this study in order to investigate the role of the steric influence of the phosphine ligand on the aquo substitution kinetics. A plot of In k vs ligand cone Patterson, G. S.; Holm, R. H. A.; Shimizu, K.; Sato, G. P. Chem. Lett. 1985, 581-584. Bowden, W. L.; Little, W. F.; Meyer, T. J.; Salmon, D. J. Am. Chem. Mukherjee, R.; Chakravorty, A. J. Chem. SOC., Dalton Trans. 1983, 955-959. (a) Kadish, K. M.; Morrison, M. M. Inorg. Chem. 1976, 15, 980. (b) Walker, F. A,; Beroiz, D.; Kadish, K. M. J. Am. Chem. SOC. 1976, 98, 3484. (c) Kadish, K. M.; Morrison, M. M.; Constant, L. A,; Dickens, L.; Davis, D. G. J . Am. Chem. SOC. 1976, 98, 8387. SOC. i975,97,6897-6898.angle is given in Figure 4 and illustrates the linear rate constant enhancement promoted by the increasing phosphine ligand size. An increase in cone angle from 132 to 170' for the change from PEt3 to PCy3 corresponds to an increase in rate constant for ligand substitution by a factor of 750. This change is 3 times as large as the maximum variation in rate constants found for the substituted triphenylphosphine ligands, which suggests that the steric bulk of the coordinated phosphines has a much greater overall effect on the rate constant for aquo substitution for these complexes than the...

Nickel Ion Complexes of Amino Acids and Peptides

“…Some more recent results, however, suggests that the insertion of bulky, non-natural residues into the peptide backbone can modify the coordination geometry of nickel(II)-peptide complexes. For example, the dipeptides of α-aminoisobutyric acid (Aib) formed low-spin, six-coordinated complexes, [Ni(H Ϫ1 Aib 2 ) 2 ] 2Ϫ , with nickel(II) which can be easily oxidized to a relatively stable nickel(III) species [80]. α,β-Dehydro amino acids (or ∆-amino acids) contain a double bond between the α-and β-carbon atoms and their insertion into the peptide sequence also has a signifi cant impact on the coordination mode in peptide complexes [81,82].…”

“…Oxidation of blue [bis(dipeptide)Ni(II)] yields violet-black solutions of Ni(III) complexes which are long-lived species in neutral solution. The acid-catalyzed conversion of the violet-black complex (A) to the yellow species (B) is followed by the acid-independent redox reaction to give diamagnetic products (C) (Scheme 4; reproduced by permission from [80]) [6]. The studies of the redox decomposition of Cu(III) and Ni(III) tripeptide complexes have shown that amino acid residues with methyl groups on the α-carbon tend to stabilize the trivalent oxidation state.…”

“…Oxidation of the orange low-spin [Ni II (H Ϫ1 Aib 2 ) 2 ] 2Ϫ complex gives the dark olive-green tetragonally compressed [Ni III (H Ϫ1 Aib 2 ) 2 ] Ϫ that is very stable in neutral and basic solutions [80]. In perchloric or dichloroacetic acid solution (pH 0.3-2.4), the tetragonally compressed [Ni III (H Ϫ1 Aib 2 ) 2 ] Ϫ complex (A) decomposes to give Ni(II) plus peptide and oxidation products (C) in a three-reaction sequence (Scheme 5; reproduced by permission from [80]).…”

“…In perchloric or dichloroacetic acid solution (pH 0.3-2.4), the tetragonally compressed [Ni III (H Ϫ1 Aib 2 ) 2 ] Ϫ complex (A) decomposes to give Ni(II) plus peptide and oxidation products (C) in a three-reaction sequence (Scheme 5; reproduced by permission from [80]). Acid reacts with [Ni III (H Ϫ1 Aib 2 ) 2 ] Ϫ according to A → BЈ → B → C to give two other Ni(III) complexes before decomposition to Ni(II) occurs.…”

“…Oxidation of the [Ni(II)(Aib 2 ) 2 ] complex gives an olive-green [Ni III (H Ϫ1 Aib 2 )] Ϫ complex, which is indefi nitely stable in neutral solution and decomposes only slowly, even in strong base [80]. The relative rates of the self-redox decomposition of the [bis(dipeptido)Ni(III)] complexes decrease in the series GG Ͼ AG ϳ AibG Ͼ GA Ͼ AA ϾϾ Aib 2 .…”