Peter C. Burgers: Tracing hydride anion transfer reactions in metal ion complex ions
Peter C. Burgers from the Department of Neurology, Erasmus Medical Centre, Rotterdam is the speaker of this seminar.
About the talk
We wish to explore the possible role of the acidity of divalent metal ions on the dissociation behavior of metal complex ions. We follow the reasoning of Yatsimirskii1.
Consider M to be one of the following period 4 divalent metal atoms: Ca, Mn, Fe, Co, Ni, Cu, and Zn. Next consider the acid/base reaction:
M2+ + H- --> MH+…………….(1)
where H¯ is the prototype Lewis base. The energy gain of reaction (1) is the Hydride Ion Affinity of M2+ (HIA(M2+)). As can be shown from thermochemical arguments, HIAs can be assessed from the following equation2:
HIA(M2+) = IEM(0->2) + PA(M) – 1385 kJ/mol………………(2)
where IEM(0->2) is the sum of the first and second ionization energy of the metal atom, PA(M) is the proton affinity of the metal atom and 1385 kJ/mol is the energy required for the process H¯ -> H+ + 2e, i.e. the sum of IE(H•) and EA(H•) where IE is the ionization energy and EA the electron affinity of H•. Since IEM(0->2) and PA(M) are known3, the HIAs can be calculated according to eq. (2) and they can be used as a measure of the Lewis acidity of the metal ion; thus the HIA for Ca2+ (a weak Lewis acid) is 1262 kJ/mol while that for the strong Lewis acid Zn2+ is much larger, 1863 kJ/mol.
HIAs can be used to rationalize a variety of fragmentation reactions observed for metal ion complex ions. For example, the complex ion HOCH2CH2OH•••Zn(OAc)+ (OAc is the acetate anion) fragments by loss of 60 Da and isotopic labeling experiments show that besides the not unexpected loss of CH3COOH, also loss of glycolaldehyde, HOCH2CH=O (also 60 Da) occurs4. From theoretical calculations, it is proposed that in this reaction a facile H¯ transfer from a CH2 group of HOCH2CH2OH to Zn2+ precedes an H+ transfer to ¯OOCCH3. Another example concerns the loss of acetic acid from polyamine•••Zn(OAc)+ complex ions, where experiments and theoretical calculations show that prior to the H+ shift, a spontaneous H¯ transfer occurs producing, after loss of acetic acid, complexes of unsaturated polyamines with ZnH+ as opposed to less stable complexes of deprotonated polyamines associated with Zn2+.
K. B. Yatsimirskii, Theoretical and Experimental Chemistry, 1981, 17, 75
P. J.A. Ruttink; T. M. Luider, P.C. Burgers, J. Mass Spectrom., 2011, 46, 1199
S.G. Lias, J.E. Bartmess, J.F. Liebman, J.L. Holmes, R.O. Levin, and W.G. Maillard. Gas-phase ion and neutral thermochemistry. J. Phys. Chem. Ref. Data, Suppl.1988, 1, 17 (Suppl 1), 104,
P.J.A. Ruttink, L. J.M. Dekker, T. M. Luider, P.C. Burgers, J. Mass Spectrom., 2012, 47, 869