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 CF3SO3- counterion forming the salt clusters [Va,b/CF3SO3](3+) and [Va,b/(CF3SO3)(2)](2+) with reduced charge states. Ion-pairing that yields [Va,b/CF3SO3](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)2Cl](+). The meta (a) and para (b) isomers and the positional isomers II2+ and III2+ display nearly identical ESI-MS and ESI-MS/MS spectra. ESI-MS/MS of I-Va,b(n+) shows that the Ru-py(P) is, intrinsically, the weakest bond since this bond breaks preferentially upon CID. Copyright (C) 2004 John Wiley Sons, Ltd.