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

Abstract

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+ ≈ Ni+ < Cu+ ≪ Ag+ and in the series 1 < 2 < 3 ≈ 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.