Gas-Phase Characterization of Redox-Active Manganese Complexes

Abstract

Manganese complexes exhibit a rich redox chemistry, usually accompanied by structural reorganization during the redox processes often followed by ligand dissociation or association. The push-pull ligand 2,6-diguanidylpyridine (dgpy) stabilizes manganese in the oxidation states +II, +III, and + IV in the complexes [Mn(dgpy)₂]ⁿ⁺ (n = 2-4) without change in the coordination sphere in the condensed phase [Heinze et al., Inorganic Chemistry, 2022, 61, 14616]. In the condensed phase, the manganese(IV) complex is a very strong oxidant. In the present work, we investigate the stability and redox activity of the Mn^IV complex and its counterion (PF₆^-) adducts in the gas phase, using two modified 3D Paul ion trap mass spectrometers. Six different cationic species of the type [Mn^x(dgpy)₂(PF₆)_y]ⁿ⁺ (x = II, III, IV, y = 0-3, n = 1-3) could be observed for the three oxidation states Mn^IV, Mn^III, and Mn^II, of which one observed complex also contains a reduced dgpy ligand. Mn^II species showed the highest relative stability in collision induced dissociation and UV/vis photo dissociation experiments. The lowest stability is observed in the presence of one or more counterions, which correlates to a lower total charge n+. Gas phase UV/vis spectra show similar features as the condensed phase spectra only differing in relative band intensities. The strongly oxidizing Mn^IV complex reacts with triethylamine (NEt₃) in the gas phase to give Mn^III, while Mn^III species show little reactivity toward NEt₃.

Keywords: UV/vis; collisions; electrospray ionization; gas-phase; mass spectrometry; reactivity.