TY - JOUR
T1 - Isolation of Mn(I) Compounds Featuring a Reduced Bis(imino)pyridine Chelate and Their Relevance to Electrocatalytic Hydrogen Production
AU - Mukhopadhyay, Tufan K.
AU - Maclean, Nicholas L.
AU - Flores, Marco
AU - Groy, Thomas L.
AU - Trovitch, Ryan J.
N1 - Funding Information:
This material is based upon work supported by the National Science Foundation under Grant 1651686. R.J.T. and T.K.M would also like to thank the Donors of the American Chemical Society Petroleum Research Fund and the Research Corporation for Science Advancement for support.
PY - 2018/5/21
Y1 - 2018/5/21
N2 - 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-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.
AB - 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-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.
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U2 - 10.1021/acs.inorgchem.8b00588
DO - 10.1021/acs.inorgchem.8b00588
M3 - Article
C2 - 29738258
AN - SCOPUS:85047297279
VL - 57
SP - 6065
EP - 6075
JO - Inorganic Chemistry
JF - Inorganic Chemistry
SN - 0020-1669
IS - 10
ER -
