Effect of Redox Active Ligands on the Electrochemical Properties of Manganese Tricarbonyl Complexes

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Abstract
The synthesis, structural characterization, and electrochemical behavior of the neutral Mn(azpy)(CO)3(Br) 4 (azpy = 2-phenylazopyridine) complex is reported and compared with its structural analogue Mn(bipy)(CO)3(Br) 1 (bipy = 2,2′-bipyridine). 4 exhibits reversible two-electron reduction at a mild potential (−0.93 V vs Fc+/0 in acetonitrile) in contrast to 1, which exhibits two sequential one-electron reductions at −1.68 V and −1.89 V vs Fc+/0 in acetonitrile. The key electronic structure differences between 1 and 4 that lead to disparate electrochemical properties are investigated using a combination of Mn–K-edge X-ray absorption spectroscopy (XAS), Mn–Kβ X-ray emission spectroscopy (XES), and density functional theory (DFT) on 1, 4, their debrominated analogues, [Mn(L)(CO)3(CH3CN)][CF3SO3] (L = bipy 2, azpy 5), and two-electron reduced counterparts [Mn(bipy)(CO)3][K(18-crown-6)] 3 and [Mn(azpy)(CO)3][Cp2Co] 6. The results reveal differences in the distribution of electrons about the CO and bidentate ligands (bipy and azpy), particularly upon formation of the highly reduced, formally Mn(−1) species. The data show that the degree of ligand noninnocence and resulting redox-activity in Mn(L)(CO)3 type complexes impacts not only the reducing power of such systems, but the speciation of the reduced complexes via perturbation of the monomer–dimer equilibrium in the singly reduced Mn(0) state. This study highlights the role of redox-active ligands in tuning the reactivity of metal centers involved in electrocatalytic transformations.
Funding Information
  • Division of Chemistry (CHE-1213403)
  • Stanford Precourt Institute for Energy (2017-4-Waymouth)
  • National Institutes of Health
  • National Institute of General Medical Sciences (P41GM103393)

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