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 (E-I(0) and E-2(0)), 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 E-1(0) >> E-1(0), 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 E-1(0) congruent to E-2(0), 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 source-in 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.