Structure and dissociation characteristics of metal chloride anion clusters containing redox-active metal ions studied by laser desorption and electrospray ionization mass spectrometry and ab initio calculations

Copper chloride anion clusters with both copper oxidation states can be made by laser desorption of CuCl2 crystals. We have used this method to study the dissociation characteristics of such cluster ions. The stability and the structure of the observed complexes were probed by ab initio calculations. These calculations show that many of these complexes are bridged structures. Thus, for the Cu2Cl4 dimer anion, formally [ClCu-Cl-CuCl2]-, with putative mixed copper oxidation states, the two copper ions become equivalent through bridging. Such bridging does not occur when redox inactive metal ions are present as in [ClCu-Cl-CaCl2]-. By observing the dissociation characteristics of a variety of metal chloride cluster anions produced from binary mixtures, the following Cl- affinity order is obtained: FeCl3 > CuCl > CaCl2 > FeCl2 > AgCl ≈ CuCl2≈ ZnCl2 > LiCl. Ab initio calculations on the Cl- affinity of selected chlorides confirm this order as do Cl- affinity estimates from the experimentally known vertical electron detachment energies of the superhalogens CaCl3- and LiCl2-. An equimolar mixture of CuCl2 and FeCl3 produces an intense cluster ion, which, from 65Cu labeling experiments, is best described as FeCl4-⋯Cu+⋯-Cl4Fe, a Cu+ bound superhalogen FeCl4- dimer. The Cu+ ion can be replaced by the redox inactive alkali cations and by Ag+ but these metal ion bound FeCl4- dimers show an entirely different fragmentation behavior which is attributed to the absence of bridging. Electrospray ionization (ESI) of CuCl2 produces an extended series of (CuCl2)nCl- anions (n = 1-11) and so in ESI very limited reduction of Cu2+ takes place. The (CuCl2)nCl- anions show an abundant dissociation via loss of neutral Cu2Cl4 which according to our ab initio calculations is 9 kcal/mol more stable than two CuCl2.

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