Complexation of transition metal ions M+ (M = Fe, Co, Ni, Cu, Zn, Ag) by [2.n]paracyclophane-enes (n = 3, 4, 5, 6) in the gas phase: effect of the molecular cavity on the complexation capability

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 t andem mass spectrometry. With the exception of Zn+, complex formation was o bserved if dry mixtures of salts of a transition metal and a cyclophane wer e 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 obt ained 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 p eaks of hydrocarbon fragment ions. Dimeric adduct ions [M(cyclophane)(2)](), typical of gas phase complexation of transition metal ions and planar ar omatic 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/mol ecule reaction between the sputtered metal ion M+ and the neutral cyclophan e evaporating from the heated target holder in competition with charge tran sfer. By this model the efficiency of complex formation between a certain m etal ion and cyclophane can be estimated from the intensity ratio r = [M(cy clophane)](+)/[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+ approxima te 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 f ormation of dimeric complexes [M(cyclophane)(2)](+) concurs with the format ion 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 fragmen ts and/or extensive decomposition of the complex ions, but no major dissoci ation into the components, and by semiempirical AM1 analysis of the structu res of the complexes. (C) 2001 Elsevier Science B.V.