Gas-Phase Oxidation of Reactive Organometallic Ions

Analysis of highly reactive compounds at very low concentration in solution using electrospray ionization mass spectrometry requires the use of exhaustively purified solvents. It has generally been assumed that desolvation gas purity needs to be similarly high, and so most chemists working in this space have relied upon high purity gas. However, the increasingly competitiveness of nitrogen generators, which provide gas purity levels that vary inversely with flow rate, prompted an investigation of the effect of gas-phase oxygen on the speciation of ions. For moderately oxygen sensitive species such as phosphines, no gas-phase oxidation was observed. Even the most reactive species studied, the reduced titanium complex [Cp₂Ti(NCMe)₂]⁺[ZnCl₃]^– and the olefin polymerization precatalyst [Cp₂Zr(µ-Me)₂AlMe₂]⁺ [B(C₆F₅)₄]^–, only exhibited detectable oxidation when they were rendered coordinatively unsaturated through in-source fragmentation. Computational chemistry allowed us to find the most plausible pathways for the observed chemistry in the absence of observed intermediates. The results provide insight into the gas-phase oxidation of reactive species and should assure experimentalists that evidence of significant oxidation is likely a solution rather than a gas-phase process, even when relatively low-purity nitrogen is used for desolvation.