Gas-Phase Characterization of Redox-Active Manganese Complexes

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.