Chemistry in Acetone Complexes of Metal Dications: A Remarkable Ethylene Production Pathway Jianhua Wu, Dan Liu, Jian-Ge Zhou, and Frank Hagelberg* Computational Center for Molecular Structure and Interactions, Department of Physics, Atmospheric Sciences, and Geoscience, Jackson State UniVersity, Jackson, Mississippi 39217 Sung Soo Park Computer-Aided Engineering Group, Samsung Electro-Mechanic Co. Ltd., Suwon, South Korea Alexandre A. Shvartsburg Biological Sciences DiVision, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352 ReceiVed: December 13, 2006; In Final Form: March 15, 2007 Electrospray ionization can generate microsolvated multiply charged metal ions for various metals and ligands, allowing exploration of chemistry within such clusters. The finite size of these systems permits comparing experimental results with accurate calculations, creating a natural laboratory to research ion solvation. Mass spectrometry has provided much insight into the stability and dissociation of ligated metal cations. While solvated singly charged ions tend to shrink by ligand evaporation, solvated polycations below a certain size exhibit charge reduction and/or ligand fragmentation due to organometallic reactions. Here we investigate the acetone complexes of representative divalent metals (Ca, Mn, Co, Ni, and Cu), comparing the results of collision-induced dissociation with the predictions of density functional theory. As for other solvated dications, channels involving proton or electron transfer compete with ligand loss and become dominant for smaller complexes. The heterolytic C-C bond cleavage is common, like in DMSO and acetonitrile complexes. Of primary interest is the unanticipated neutral ethylene loss, found for all metals studied except Cu and particularly intense for Ca and Mn. We focus on understanding that process in the context of competing dissociation pathways, as a function of metal identity and number of ligands. According to first-principles modeling, ethylene elimination proceeds along a complex path involving two intermediates. These results suggest that chemistry in microsolvated multiply charged ions may still hold major surprises. 1. Introduction Complexes of metal ions with organic and biological mol- ecules are a topic that combines problems central to many areas of science. Of interest to physical chemistry is the formation of solvation shells and order in solutions 1 and the structural and phase transitions in finite systems. 2 For inorganic chemistry, the issues are metal coordination in solid-state complexes 3,4 and fundamental organometallic reactivity including the metal catalysis of bond cleavages. 5,6 For biochemistry, ligated metal ions are useful models to understand biological and toxicological processes that involve metal binding, 7-9 for example with respect to hemes and metalloproteins. 7 From the analytical viewpoint, peptides and other organic molecules cationized by metals tend to fragment differently from protonated analogs. 10-13 This often provides more specific or complementary mass- spectrometric identifications, in particular aiding isomer separa- tions 12 and proteomic sequencing strategies. 10,11 The variable yet finite size of microsolvated metal ions makes them an ideal laboratory to develop and validate theoretical methods, such as the geometry optimization algorithms and model potentials needed to describe larger complexes, mesoscopic “droplets”, and solutions. 14,15 For smaller complexes, first-principles calculations 16-18 are crucial to interpret and guide experimental work. The field of microsolvated metal ions has started from singly charged species, which remain the subject of most studies to date. Those complexes are readily produced by many means, including sequential adsorption of vapor molecules on a bare metal cation. Condensation of neutrals on ions is exothermic, and such clusters grow to essentially any size, depending on the vapor temperature and pressure. Ligation of multiply charged metal ions in that manner is prevented by charge reduction. The second and higher ionization energies (IE) of nearly all metals exceed 12 eV (Table 1), while the first IE (IE1) of typical organic molecules range 19 from 8 to 12 eV (9.7 eV for acetone considered here). Hence the transfer of electron from a ligand (L) to metal ion (M) is normally exothermic and occurs on contact, followed by immediate dissociation driven by Coulomb repulsion. Even when the IE1 of L exceeds the second IE (IE2) of M, charge reduction precluding complex formation may still proceed by other routes. For example, an attempt to add water (IE1 ) 12.6 eV) to Ca 2+ results in an interligand H + transfer yielding 20 CaOH + and H 3 O + . However, polyvalent metal ions are stable in bulk solutions due to charge stabilization by many solvent molecules. Hence * To whom correspondence should be addressed. E-mail: frank.d.hagelberg@ccaix.jsums.edu. 4748 J. Phys. Chem. A 2007, 111, 4748-4758 10.1021/jp068574z CCC: $37.00 © 2007 American Chemical Society Published on Web 05/16/2007