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

Amine-functionalized indenyl ligands were reacted with Mn(I) precursors to form the piano-stool complexes Mn(CO)3(IndR) (R= Pyrrolidinyl, piperazinyl) which exhibit two cathodic redox waves via cyclic voltammetry. 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. Chemical reduction of MnIndPyrr in the presence of the cation encapsulating cryptand 2,2,2-Crypt resulted in the formation of the rare monometallic 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 loss to generate a mixed-valence Mn(-I)/Mn(I) adduct where the two metal centers bind to a single indenyl moiety. The pyrrolidine-substituted indenide anion can be independently prepared in high yields by the facile direct re-duction of the protonated indene in the presence of 2,2,2-Crypt. The order of addition and the identity of the encapsulating agent are critical to the observed reaction outcomes, highlighting their decisive role under reducing conditions.