Electrospray mass and tandem mass spectrometry of homologous and isomeric singly, doubly, triply and quadruply charged cationic ruthenated meso ‐(phenyl) m ‐( meta ‐ and para ‐pyridyl) n ( m + n = 4) macrocyclic porphyrin complexes

Ten homologous or isomeric singly, doubly, triply and quadruply charged cationic macrocyclic complexes I–Va, b n + ( n = 1–4) formed by the coordination of [Ru(bipy) 2 Cl] + to the pyridyl N‐atoms of a series of meso ‐(phenyl) m ‐( meta or para ‐pyridyl) n ‐porphyrins ( m + n = 4) were transferred to the gas phase and structurally characterized by electrospray ionization (ESI) mass (MS) and tandem mass (MS/MS) spectrometry. Previously known to be stable in solution and in the solid state, I–Va, b n + are found to constitute also a new class of stable, long‐lived multiply charged gas‐phase ions with spatially separated charge sites. Increasing intramolecular electrostatic repulsion from Ia, b + to IVa, b 3+ facilitates in‐source and tandem collision‐induced dissociation (CID). However, for the quadruply charged ions Va, b 4+ , electrostatic repulsion is alleviated mainly by ion pairing with the CF 3 SO 3 − counterion forming the salt clusters [ Va,b /CF 3 SO 3 ] 3+ and [ Va,b /(CF 3 SO 3 ) 2 ] 2+ with reduced charge states. Ion‐pairing that yields [ IVa,b /CF 3 SO 3 ] 2+ is also observed as a minor ESI process for the triply charged ions IVa, b 3+ . The gaseous ions I–Va, b n + ( n = 2, 3 or 4) dissociate by sequential ‘charge partitioning’ with the formation of two cationic fragments by the release of [Ru(bipy) 2 Cl] + . The meta ( a ) and para ( b ) isomers and the positional isomers II 2+ and III 2+ display nearly identical ESI‐MS and ESI‐MS/MS spectra. ESI‐MS/MS of I–Va, b n + shows that the Rupy(P) is, intrinsically, the weakest bond since this bond breaks preferentially upon CID. Copyright © 2004 John Wiley & Sons, Ltd.