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.
ABSTRACT
Heating a 1:1 mixture of (CO)5MnBr and the phosphine-substituted pyridine diimine ligand, (Ph2PPr)PDI, in THF at 65 °C for 24 h afforded the diamagnetic complex [((Ph2PPr)PDI)Mn(CO)][Br] (1). Higher temperatures and longer reaction times resulted in bromide displacement of the remaining carbonyl ligand and the formation of paramagnetic ((Ph2PPr)PDI)MnBr (2). The molecular structure of 1 was determined by single crystal X-ray diffraction, and density functional theory (DFT) calculations indicate that this complex is best described as low-spin Mn(I) bound to a neutral (Ph2PPr)PDI chelating ligand. The redox properties of 1 and 2 were investigated by cyclic voltammetry (CV), and each complex was tested for electrocatalytic activity in the presence of both CO2 and Brønsted acids. Although electrocatalytic response was not observed when CO2, H2O, or MeOH was added to 1 individually, the addition of H2O or MeOH to CO2-saturated acetonitrile solutions of 1 afforded voltammetric responses featuring increased current density as a function of proton source concentration (icat/ip up to 2.4 for H2O or 4.2 for MeOH at scan rates of 0.1 V/s). Bulk electrolysis using 5 mM 1 and 1.05 M MeOH in acetonitrile at -2.2 V vs Fc(+/0) over the course of 47 min gave H2 as the only detectable product with a Faradaic efficiency of 96.7%. Electrochemical experiments indicate that CO2 promotes 1-mediated H2 production by lowering apparent pH. While evaluating 2 for electrocatalytic activity, this complex was found to decompose rapidly in the presence of acid. Although modest H(+) reduction activity was realized, the experiments described herein indicate that care must be taken when evaluating Mn complexes for electrocatalytic CO2 reduction.
