Electronic Effects of Aminoindenyl Ligands Coordinated to Manganese: Structures and Properties of a Mn0 Metalloradical and Bimetallic Mn–I/MnI 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 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 to 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. The reduction of MnIndPyrr with KC8 in the presence of the encapsulating agent 2,2,2-Crypt led to the formation of a rare Mn0 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 that generated a mixed-valence Mn–I/MnI adduct, where the two metal centers were bound 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 agents, as these reagents have a profound impact on the reaction outcome.