Electrochemical oxidation and nucleophilic addition reactions of metallocenes in electrospray mass spectrometry

Metallocenes, substituted metallocenes, and organometallic salts containing metal-carbon bonds have been investigated by electrospray mass spectrometry (ESMS). Organometallic salts yielded stable cations in high abundances; metallocenes appeared in the oxidized form [i.e., as bis(cyclopentadienyl) metal cations, Cp2M+]. Identifications were confirmed by inspection of the natural isotope distribution patterns. ESMS response vs concentration (log-log) plots showed a linear range between 10(-9) and 10(-5) M for both CP2Fe and [Cp2Fe]+[PF6]-. For solutions of equivalent concentration, however, the salt form always yielded a higher abundance of Cp2Fe+. Neutral metallocenes are ionized by an electrochemical oxidation (electron-removal) mechanism at or near the electrospray (ES) capillary (needle). Increasing electrospray appearance potentials for Cp2M+ (M = metal) formation correlated with increasing haff-wave potentials for oxidation [E1/2(OX)] and increasing ionization energies (IE) in the series: decamethylferrocene, 1,1'-dimethylferrocene, ferrocene, formylferrocene, carboxylferrocene, acetylferrocene, 1,1'-discetylferrocene, osmocene, and ruthenocene. For osmocene [relatively high E1/2-(ox) and IE], ESMS conditions were established whereby protonation was competitive with oxidation, thus creating two pathways of ion formation. In methylene chloride solvent, chloride ion was incorporated into the inner sphere of certain metallocenes subjected to ESMS, forming Cp2RuCl+ and Cp2OsCl+ complexes via nucleophilic addition. These latter Ru(IV) and Os(IV) species also formed analogous gas-phase complexes with trifluoroacetate ion and hydroxide ion. Results have been rationalized on the basis of ion stability considerations.

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