Isolation of Mn(I) Compounds Featuring a Reduced Bis(imino)pyridine Chelate and Their Relevance to Electrocatalytic Hydrogen Production

We report the preparation and electronic structure determination of chelate-reduced Mn(I) compounds that are relevant to electrocatalytic proton reduction mediated by [(^Ph2PPrPDI)Mn(CO)][Br]. Reducing [(^Ph2PPrPDI)Mn(CO)][Br] with excess Na-Hg afforded a neutral paramagnetic complex, (^Ph2PPrPDI)Mn(CO). This compound was found to feature a low spin Mn(I) center and a PDI radical anion as determined by magnetic susceptibility measurement (1.97 μ_B), EPR spectroscopy (S = ¹/₂), and density functional theory calculations. When [(^Ph2PPrPDI)Mn(CO)][Br] was reduced with K-Hg, Mn(I) complexes with highly activated CO ligands were obtained. Recrystallization of the reduced product from diethyl ether solution allowed for the isolation of dimeric [(κ⁴-^Ph2PPrPDI)Mn(μ-n¹, n ¹, n ²-CO)K(Et₂O)]₂ (ν _CO = 1710 cm^-1, 1656 cm^-1), while methyl tert-butyl ether treatment afforded dimeric [(κ⁴-^Ph2PPrPDI)Mn(μ- n ¹, n ¹-CO)K(MTBE)₂]₂ (ν _CO = 1695 cm^-1, MTBE = methyl tert-butyl ether). Addition of 18-crown-6 to these products, or conducting the K-Hg reduction of [(^Ph2PPrPDI)Mn(CO)][Br] in the presence of 18-crown-6, allowed for the isolation of a monomeric example, (κ⁴-^Ph2PPrPDI)Mn(μ- n ¹, n ²-CO)K(18-crown-6) (ν _CO = 1697 cm^-1). All three complexes were found to be diamagnetic and were characterized thoroughly by multinuclear 1D and 2D NMR spectroscopy and single crystal X-ray diffraction. Detailed analysis of the metrical parameters and spectroscopic properties suggest that all three compounds possess a Mn(I) center that is supported by a PDI dianion. Importantly, (κ⁴-^Ph2PPrPDI)Mn(μ- n ¹, n ²-CO)K(18-crown-6) was found to react instantaneously with either HBF₄·OEt₂ or HOTf to evolve H₂ and generate the corresponding Mn(I) complex, [(^Ph2PPrPDI)Mn(CO)][BF₄] or [(^Ph2PPrPDI)Mn(CO)][OTf], respectively. These products are spectroscopically and electrochemically similar to previously reported [(^Ph2PPrPDI)Mn(CO)][Br]. It is believed that the mechanism of [(^Ph2PPrPDI)Mn(CO)][Br]-mediated proton reduction involves intermediates that are related to the compounds described herein and that their ambient temperature isolation is aided by the redox active nature of ^Ph2PPrPDI.