Complexes of dichloro(ethylenediamine)palladium(II) observed from aqueous solutions by electrospray mass spectrometry

Abstract

Aqueous solutions of dichloro(ethylenediamine)palladium(II) were investigated using electrospray mass spectrometry (ESMS). The most abundant peak (m/z 436.8) was attributed to the dimeric Pd(en)Cl 2 ·Pd(en)Cl ϩ ion. We conjecture that the structures of the observed ions arise from the clustering of the hydrolysis products of the parent compound. This hypothesis was tested experimentally by carrying out a series of collision-induced dissociation (CID) experiments and deuterium exchange reactions. It was also assessed by performing density functional theory (DFT) calculations, from which optimized structures and reaction energetics were obtained. These results were compared with our earlier ESMS study of an aqueous Pd(en)Br 2 solution. Calculations were also carried out on the Pd(en)Br 2 system to facilitate the comparisons. Conclusions are drawn regarding the species present in the two aqueous solutions. (J Am Soc Mass Spectrom 2007, 18, 769 -777) © 2007 American Society for Mass Spectrometry T he last decade has seen a resurgence of interest in the coordination chemistry of transition metals. This is due in large part to their therapeutic value as antitumor agents. Many of these agents are platinum(II) complexes, the four best known being cisplatin, carboplatin, oxaliplatin, and nedaplatin [1][2][3]. Palladium(II) complexes have also attracted attention for similar reasons [4][5][6][7].In vivo, hydrolysis of these compounds occurs, and it is these hydrolytic products that yield their medicinal activity [8,9]. The hydrolytic products are also strongly linked to issues of renal toxicity [10]. The mechanisms of action for these coordination complexes are closely tied to their hydrolysis products, and these products and mechanisms are poorly understood, partly due to the possibility of multiple equilibria with similar energetics [11,12]. It is for these reasons that coordination complexes of Ni(II), Pd(II), and Pt(II) in aqueous solutions have been the focus of a number of investigations [13][14][15][16][17].Electrospray mass spectrometry (ESMS) is particularly well suited for the study of metal-containing aqueous solutions, and a variety of such studies have been conducted [18,19]. We recently reported the results of one such study into aqueous solutions of dibromo(ethylenediamine)palladium(II) [20]. When the solution was electrosprayed into a quadrupole ion-trap mass spectrometer, a number of previously unreported species were detected, the principle one being a dimeric ion formed from the Pd(en)Br 2 parent compound and the Pd(en)Br ϩ ion. The dimeric ion was subjected to collision-induced dissociation (CID), and these MS n experiments revealed a series of ions formed through the sequential loss of HBr molecules. Complexes observed from the electrospray process are not necessarily a reflection of those complexes in solution, but they may instead simply represent species in solution from which the complexes are produced in the electrospray desolvation process. This has been observed previously in solutions containi...

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