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. Copyright © 2011 John Wiley & Sons, Ltd.