Electronic Effects of Aminoindenyl ligands Coordinated to Manganese: Structures and Properties of a Mn(0) Metalloradical and Bimetallic Mn(-I)/Mn(I) Adduct

Studying the redox behavior of Earth-abundant metal complexes and understanding their reactivity under reducing conditions is of fundamental importance for activating small molecules. We present the electrochemical behavior and reactivity of amine-functionalized indenylmanganese(I) complexes that form the piano-stool complexes MnI(CO)3(IndR) (R = pyrrolidinyl, piperazinyl), which exhibit two cathodic redox waves via cyclic voltammetry (CV). Electrochemical, spectroscopic, and structural comparisons of these complexes with the undecorated indenyl species reveal that the pyrrolidine-substituted tricarbonylmanganese complex MnIndPyrr is the most electron rich due to favorable π donor effects from the amine into the ring system. These results are also strongly supported by DFT computations. Although CV studies show clean redox behavior, reaction outcomes in the presence of common chemical reductants are strongly dependent on the reagents used. Reduction of MnIndPyrr with KC8 in the presence of the encapsulating agent 2,2,2-Crypt forms a rare Mn(0) metalloradical, [K(2,2,2-Crypt)][MnIndPyrr] which was characterized by single crystal X-ray diffraction, EPR spectroscopy, IR spectroscopy, and DFT calculations. Attempts to isolate a doubly reduced Mn(-I) adduct in the presence of 18-crown-6 resulted in indenide anion loss to generate a mixed-valence Mn(-I)/Mn(I) adduct where the two metal centers bind to a single indenyl moiety. This study emphasizes the dramatic differences between observing redox-reversible behavior on the electroanalytical scale and performing preparative scale reduction reactions with alkali metals and encapsulating agent, as these reagents have a profound impact on the reaction outcome.