PUB - Publikationen an der Universität Bielefeld

Complex formation in the gas phase between transition metal ions M+ = Fe+, Co+, Ni+, Cu+, Zn+, and Ag+ and dimethyl[2.n]paracyclophane-enes 1-4 (n = 3-6) has been studied by secondary ionization mass spectrometry (SIMS) and tandem mass spectrometry. With the exception of Zn+, complex formation was observed if dry mixtures of salts of a transition metal and a cyclophane were bombarded with a 30 keV primary Cs+ ion beam in a SIMS ion source. Using liquid SIMS, abundant signals of a complex [M(cyclophane)](+) were only obtained for Ag+ and NBA as liquid matrix. Besides a large signal of M+ and a significant signal of [M(cyclophane)](+) all SIMS mass spectra contained a distinct peak of the molecular ion [cyclophane](.+) and a series of small peaks of hydrocarbon fragment ions. Dimeric adduct ions [M(cyclophane)(2)](+), typical of gas phase complexation of transition metal ions and planar aromatic hydrocarbons, are only detected in the mass spectra of mixtures of 1 and salts of Co or Ag. It is concluded from the experimental results that complex formation occurs in the gas phase of the SIMS ion source by ion/molecule reaction between the sputtered metal ion M+ and the neutral cyclophane evaporating from the heated target holder in competition with charge transfer. By this model the efficiency of complex formation between a certain metal ion and cyclophane can be estimated from the intensity ratio r = [M(cyclophane)](+)/[cyclophane](.+). From this ratio and the results of separate experiments, in which two M+ compete in complex formation with cyclophane 3 or in which a mixture of cyclophanes is used for complexation of Ag+, it is shown that complex formation increases in the series Fe+ < Co+ approximate to Ni+ < Cu+ much less than Ag+ and in the series 1 < 2 < 3 approximate to 4. The selectivity in the series of cyclophanes as well as the lack of formation of dimeric complexes [M(cyclophane)(2)](+) concurs with the formation of "in" complexes (IC) or "side-on" complexes (SC), in which the metal ion is more or less buried in the cavity of the cyclophane ligand. This is corroborated by collision-induced dissociation experiments, which show-with the exception of Ag+ complexes-intense losses of small hydrocarbon fragments and/or extensive decomposition of the complex ions, but no major dissociation into the components, and by semiempirical AM1 analysis of the structures of the complexes. (C) 2001 Elsevier Science B.V.