The Full d3-d5 Redox Series of Mononuclear Manganese Complexes: Geometries and Electronic Structures of [Mn(dgpy)2]n

Abstract

Although manganese ions exhibit a rich redox chemistry, redox processes are often accompanied by structural reorganization and a high propensity for ligand substitution, so that no complete structurally characterized manganese(II,III,IV) complex series without significant ligand sphere reorganization akin to the manganese(II,III,IV) oxides exists. We present here the series of pseudo-octahedral homoleptic manganese complexes [Mn(dgpy)₂]ⁿ⁺ (n = 2-4) with the adaptable tridentate push-pull ligand 2,6-diguanidylpyridine (dgpy). Mn-N bond lengths and N-Mn-N bond angles change characteristically from n = 2 to n = 4, while the overall [MnN₆] coordination sphere is preserved. The manganese(III) complex [Mn(dgpy)₂]³⁺ exhibits a Jahn-Teller elongated octahedron and a negative D = -3.84 cm^-1. Concomitantly with the consecutive oxidation of [Mn(dgpy)₂]²⁺ to [Mn(dgpy)₂]⁴⁺, the optical properties evolve with increasing ligand-to-metal charge transfer character of the absorption bands culminating in the panchromatic absorption of the purple-black manganese(IV) complex [Mn(dgpy)₂]⁴⁺.