Stable Ag(II) Coordination Complexes in the Gas Phase

Abstract

The preparation and study of gas-phase transition-metal complexes in their higher oxidation states, i.e., Cu(II), Cr(III), Fe(II), etc., presents a considerable technical challenge. Charge transfer prevents such species from being “grown” as cluster ions and techniques, such as electrospray, do not always produce the desired charge state or allow for experiments to be performed on a broad range of ligands. Discussed here are new results from a technique which promises to overcome some of these problems, and appears capable of producing complexes from a wide variety of metals and ligands. Data are presented for complexes based on silver(II) in association with a broad range of ligands, including pyridine, tetrahydrofuran, and benzene. For each [AgLn]2+system, two important quantities are identified: (i) the minimum number of ligands required to form a stable unit and (ii) the value of n for which the intensity distribution reaches a maximum. For nitrogen-containing ligands these numbers are 2 and 4, respectively, and for oxygen-containing ligands 4 and 5. A series of aromatic ligands all exhibit coordination numbers of 2. For several of the nitrogen-based ligands the most stable combinations correspond to those identified in the condensed phase, and [Ag(pyridine)4]2+ is a very good example of such behavior. In the case of the oxygen-containing ligands, there are no direct condensed-phase analogues, but some of the more stable combinations identified may offer prospects for future preparative work. Within the latter group, not only was the presence of stable silver(II)/CO2 complexes very unexpected, but with [Ag(CO2)4]2+ being the most stable combination, the pattern of behavior is markedly different from that of other oxygen-containing ligands. The composition and charge states of many of the stable complexes were confirmed via collisional activation, where both ligand loss and charge-transfer processes could be identified. Only one example of a chemical reaction could be clearly identified as being initiated by the presence of silver(II).