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

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Abstract
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 μB), 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-η1,η1,η2-CO)K(Et2O)]2 (νCO = 1710 cm–1, 1656 cm–1), while methyl tert-butyl ether treatment afforded dimeric [(κ4-Ph2PPrPDI)Mn(μ-η1,η1-CO)K(MTBE)2]2 (νCO = 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-η1,η2-CO)K(18-crown-6) (νCO = 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-η1,η2-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.
Funding Information
  • National Science Foundation (1651686)

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