The Influence of Alkali Metal Cation Size on the Formation and Dissociation of Crown Ether Fullerene Dimers in Electrospray Mass Spectrometry

Crown ether fullerenes, C₆₀H­(cr – H), form two different dimers in electrospray ionization mass spectrometry, a noncovalently bound, metal-bridged dimer [C₆₀H­(cr – H)]₂M⁺, and a singly bonded fullerene dimer of the type (cr – H)–C₆₀–C₆₀–(cr – H)­M_<i>n</i>^<i>n</i>+, with <i>n</i> = 1, 2. In this work, the dependence of both the formation and the dissociation of these dimers on the respective sizes of the crown ether moiety (12cr4, 15cr5, 18cr6) and of the alkali metal ion (M⁺ = Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺) is studied. The size ratio of crown ether cavity to metal cation size has a profound influence on the formation of the noncovalent dimers, [C₆₀H­(cr – H)]₂M⁺. These are only formed if the cation is larger than the crown ether cavity. Also, the doubly charged, covalent dimers [C₆₀(cr – H)]₂M₂²⁺ show a dependence on the size ratio. The proposed formation mechanism involves the recombination of two C₆₀(cr – H)­M^•+ radical cations, which can only occur if the repulsion between the two charges is sufficiently reduced by encapsulating the metal cations within the crown ether moiety. Only the formation of the singly charged covalent dimer [C₆₀(cr – H)]₂M⁺ was found to be independent of the size ratio, in line with a radical-based dimerization mechanism. The fragmentation behavior, however, is strongly dependent on the size ratio, revealing the influence of intramolecular crown–metal–crown complexes