Studies on the Reduction of [(C 5 Me 5 ) 2 Mo 2 O 5 ] in Methanol/Water/Acetate Solutions by On‐Line Electrochemical Flowcell and Electrospray Mass Spectrometry

The complex [Cp* 2 Mo 2 O 5 ] (Cp* = η 5 ‐C 5 Me 5 ) and its electrochemical reduction products in acetic acid/acetate‐buffered (pH = 4.0) water/methanol solutions were investigated by combined electrochemical (EC) flowcell and on‐line electrospray ionization mass spectrometry (ESI‐MS). Mono‐, di‐, tri‐, and tetranuclear organometallic molybdenum oxides were identified in the starting solution. The effect of the relevant ESI‐MS parameters (ionic mode, heated capillary voltage, and heated capillary temperature) and of the concentration on the observed distribution of ions in the mass spectrometer was studied in order to minimize side reactions in the ESI chamber. It was verified that reduction in the ESI‐MS is undetectable under open‐circuit conditions. The on‐line electrochemical study revealed the potential‐dependent formation of previously unknown mono‐, di‐, tri‐, and tetranuclear Mo V , Mo IV , and mixed‐valence complexes. The compounds were identified by their characteristic isotope patterns and their ion trap MS n fragmentations. The observed formation potentials reflect the higher stability of the multinuclear species relative to the mononuclear ones with the same oxidation state. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)