Formation, acidity and charge reduction of the hydrates of doubly charged ions M 2+ (Be 2+, Mg 2+, Ca 2+, Zn 2+)

Abstract

There are two methods for producing in the gas phase doubly charged metal ion hydrates, M(H 2O) n 2+ (or other ion ligand L ML n 2+ complexes). In the clustering method, one starts with the naked ion M 2+, and in the presence of a third (bath) gas and water vapor, the ion hydrates form by ion-molecule clustering reactions. The second method is based on electrospray with which a spray of aqueous solutions containing the dissolved salts M 2+ + 2X −, leads to gas phase M(H 2O) n 2+ with a distribution around n ≈ 8. For M, which has a high second ionization energy, IE(M 2+), both methods can fail to produce a full range of hydrates with a given n, because of the interference of a charge reduction reaction which involves intramolecular proton transfer. This reaction becomes possible at n = 2; (M(H 2O) 2 2+)∗ = MOH + + H 3O +, and competes with the simple ligand loss: (M(H 2O) 2 2+)∗ = M(H 2O) 2+ + H 2O. The thermally excited (M(H 2O) 2 2+)∗ results in the clustering method by the exothermicity of the forward clustering reaction and in the electrospray method by the thermal declustering required to produce lower n ions. Ab initio calculations are presented for the energies of the above reactions and transition states for Mg 2+ and Ca 2+. These show that the transition state for the charge reduction reaction is much lower than that for the simple ligand loss at n = 2. However, as n increases, the two transition states move closer together and above a given n = r, simple ligand loss becomes dominant. The capabilities and limitations of the two methods to produce hydrates of a given n is discussed. Experimental results illustrate competing charge reduction and simple H 2O loss for Be(H 2O) n 2+ under thermal equilibrium conditions at n ≈ 9. Charge reduction reactions when occurring in the forward clustering direction can be viewed as proton transfer reactions to the incoming H 2O molecule. These can be generalized by examining the proton affinities of the MOH(H 2O) n + ions, which are obtained by ab initio calculations. Proton transfer from M(OH) 2) n 2+ can be induced not only by H 2O but also by other bases B. Experimental results for the deprotonation of Zn(OH 2) n 2+, n = 8 or 9, by NH 3 are presented. The charge reduction reactions by which a deprotonated ligand attached to M is formed, can have synthetic utility. Examples are given for the production of methylthiolate complexes which may be useful for modeling ion complexes in which one of the ligands is the deprotonated amino acid residue cysteine.