Influence of electronic and steric effects on stability constants and electrochemical reversibility of divalent ion complexes with glycine and sarcosine. A glass electrode potentiometric, sampled direct current polarographic, virtual potentiometric, and molecular modelling study.
Cd(II) complexes with glycine (gly) and sarcosine (sar) were studied by glass electrode potentiometry, direct current polarography, virtual potentiometry, and molecular modelling. The electrochemically reversible Cd(II)-glycine-OH labile system was best described by a model consisting of M(HL), ML, ML2, ML3, ML(OH) and ML2(OH) (M = Cd(II), L = gly) with the overall stability constants, as log beta, determined to be 10.30+/-0.05, 4.21+/-0.03, 7.30+/-0.05, 9.84+/-0.04, 8.9+/-0.1, and 10.75+/-0.10, respectively. In case of the electrochemically quasi-reversible Cd(II)-sarcosine-OH labile system, only ML, ML2 and ML3 (M = Cd(II), L = sar) were found and their stability constants, as log beta, were determined to be 3.80+/-0.03, 6.91+/-0.07, and 8.9+/-0.4, respectively. Stability constants for the ML complexes, the prime focus of this work, were thus established with an uncertainty smaller than 0.05 log units. The observed departure from electrochemical reversibility for the Cd-sarcosine-OH system was attributed mainly to the decrease in the transfer coefficient alpha. The MM2 force field, supplemented by additional parameters, reproduced the reported crystal structures of diaqua-bis(glycinato-O,N)nickel(II) and fac-tri(glycinato)-nickelate(II) very well. These parameters were used to predict structures of all possible isomers of (i) [Ni(H2O)4(gly)]+ and [Ni(H2O)4(sar)]+; and (ii) [Ni(H2O)3(IDA)] and [Ni(H2O)3(MIDA)] (IDA = iminodiacetic acid, MIDA = N-methyl iminodiacetic acid) by molecular mechanics/simulated annealing methods. The change in strain energy, deltaU(str), that accompanies the substitution of one ligand by another (ML + L' --> ML' + L), was computed and a strain energy deltaU(str) = +0.28 kcal mol(-1) for the reaction [Ni(H2O)4(gly)]+ + sar --> [Ni(H2O)4(sar)]+ + gly was found. This predicts the monoglycine complex to be marginally more stable. By contrast, for the reaction [Ni(H2O)3IDA] + MIDA --> [Ni(H2O)3MIDA] + IDA, deltaU(str) = -0.64 kcal mol(-1), and the monoMIDA complex is predicted to be more stable. This correlates well with (i) stability constants for Cd-gly and Cd-sar reported here; and (ii) known stability constants of ML complex for glycine, sarcosine, IDA, and MIDA.
Cukrowski
Ignacy
I
Department of Chemistry, University of Pretoria, NW-1 Building, Pretoria 0002, South Africa. ignacy.cukrowski@up.ac.za
Marques
Helder M
HM
Mkwizu
Tumaini S
TS
Magampa
Philemon P
PP
Serge
Claudette
C