화학공학소재연구정보센터
Inorganic Chemistry, Vol.57, No.10, 6065-6075, 2018
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 [((PDI)-P-Ph2PPr)Mn(CO)][Br]. Reducing [((PDI)-P-Ph2ppr)mn- (CO)][Br] with excess Na-Hg afforded a neutral paramagnetic complex, ((PDI)-P-Ph2PPr)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 mu(B)), EPR spectroscopy (S = 1/2), and density functional theory calculations. When [((PDI)-P-Ph2PPr)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 [(K4-Ph2PPrPDI)Mn(mu-eta(1),eta(1),eta(2)-CO)K(Et2O)](2) (nu(CO) = 1710 cm(-1), 1656 cm(-1)), while methyl tert-butyl ether treatment afforded dimeric [(K4-Ph2PPrPDI)Mn(mu-eta(1),eta(1)-CO)K(MTBE)](2) (nu(CO) = 1695 cm(-1), MTBE = methyl tert-butyl ether). Addition of 18-crown-6 to these products, or conducting the K-Hg reduction of [((PDI)-P-Ph2PPr)Mn(CO)][Br] in the presence of 18-crown-6, allowed for the isolation of a monomeric example, (K4-Ph2PPrPDI)Mn(mu-eta(1),eta(2)-CO)K(18-crown-6) (nu(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, (K4-Ph2PPrPDI)Mn(mu-eta(1),eta(2)-CO)K(18-crown-6) was found to react instantaneously with either HBF4 center dot OEt2 or HOTf to evolve H-2 and generate the corresponding Mn(I) complex, [((PDI)-P-ph2pr)Mn(CO)][BF4] or [((PDI)-P-Ph2PPr)Mn(CO)][OTf], respectively. These products are spectroscopically and electrochemically similar to previously reported [((PDI)-P-ph2ppr)Mn(CO)][Br]. It is believed that the mechanism of [((PDI)-P-ph2ppr)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 (PDI)-P-Ph2PPr.