Structural characterization of metalloporphyrin-oligomer multianions by mass spectrometry and ion mobility spectrometry—Observation of metastable species

Abstract Anions formed by electrospray ionization of solutions of Mn(III) meso-Tetra(4-sulfonatophenyl)porphyrin–hydrochloride [MnClC 44 H 24 N 4 (SO 3 H) 4 ] and Fe(III) meso-Tetra(4-sulfonatophenyl)porphyrin–hydrochloride [FeClC 44 H 24 N 4 (SO 3 H) 4 ] in water/methanol have been characterized by mass spectrometry. Extensive oligomer formation has been observed. The structures of the dimeric and trimeric species have been characterized by ion mobility spectrometry in combination with semi-empirical calculations. We find sandwich structures connected by (mostly electrostatic) bounds between the central metal atom and one of the oxygen atoms of the sulfonic groups. We observe highly charged (up to six-fold) dimeric species which according to the calculations are metastable with respect to dissociation into the respective (triply charged) monomers. Their dissociation is however prevented by a large Coulomb barrier. The findings are rationalized by a simple electrostatic model.