Multiply Charged Redox-Active Oligomers in the Gas Phase: Electrolytic Electrospray Ionization Mass Spectrometry of Metallocenes

The inherent constant-current electrolytic (CCE) nature of electrospray ionization mass spectrometry (ESI-MS) is used to study a variety of neutral metallocenes, including oligomeric metallocenes. ESI mass spectra that display the doubly charged ion of certain biferrocenes (M2+) can be readily obtained by careful manipulation of solution conditions and structure of the biferrocene. The intensity ratio of M2+ to the singly charged (M+) biferrocene, IM2+/IM+, is strongly affected by the formal potentials of the two ferrocene sites (E01 and E02), the distance between the two redox centers, the analyte infusion (flow) rate, and concentration of the neutral metallocene. For a biferrocene whose redox centers are well separated and E01 ≫ E02, lower analyte concentrations and flow rates yield a higher IM2+/IM+, but the maximum achievable value is only 0.04. However, if the redox centers are physically well separated and E01 ≅ E02, values of up to 0.3 for IM2+/IM+ are attainable; through use of probability theory and IM2+/IM+, the electrochemical potential at the electrospray needle can be calculated. Multiply charged ions for several oligo(ferrocenyldimethylsilanes) are observed when electrolyte-containing solutions of the oligo(ferrocenyldimethylsilanes) are directly infused into the ESI sourcein the case of the octa(ferrocenyldimethylsilane), the M4+ species is observed. To our knowledge, this is the first demonstration of ions with greater than two charges formed by electrolytic ESI-MS.