Ligand Substituent Effects in Manganese Pyridinophane Complexes: Implications for Oxygen-Evolving Catalysis

Abstract

A series of Mn(II) complexes of differently substituted pyridinophane ligands, (PyNR)MnCl (R = Pr, Cy) and [(PyNR)MnF](PF) (R = Pr, Cy,Bu) are synthesized and characterized. The electrochemical properties of these complexes are investigated by cyclic voltammetry, along with those of previously reported (PyNMe)MnCl and the Mn(III) complex [(PyNMe)MnF](PF). The electronic structure of this and other Mn(III) complexes is probed experimentally and theoretically, via high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy ab initio quantum chemical theory (QCT), respectively. These studies show that the complexes contain relatively typical six-coordinate Mn(III). The catalytic activity of these complexes toward both HO disproportionation and HO oxidation has also been investigated. The rate of HO disproportionation decreases with increasing substituent size. Some of these complexes are active for electrocatalytic HO oxidation; however this activity cannot be rationalized in terms of simple electronic or steric effects.

Functionalized pyridine in pyclen-based iron(iii) complexes: evaluation of fundamental properties

“…[3][4][5]7 Congeners of pyclen have been widely studied to tune their properties and reactivity. 8 For example, it was observed that 12and 13-membered cavities cannot accommodate planar coordination compared to 14-membered py-2,3-cyclam, which signicantly affects the electrochemical properties of the complex and leads to the stabilization of rather uncommon metal oxidation states. 9,10 Introduction of pendant arms, amine functionalization on the macrocycle, and differing numbers of coordinating atoms are all examples of other modications that have been studied to tune reactivity of pyridinophanes to achieve changes to geometric rigidity, electrochemical properties, and coordination environments, respectively.…”

“…24,25 Metal complexes derived from L2 and modications to the C-C linkers have been extensively explored, as noted above. 8,[25][26][27] The 4-OMe substituted pyridinophane was studied as a precursor to a scaffold used for iron-catalyzed oxidation chemistry. 26 The 4-Br substituted pyridinophane was developed as an intermediate for synthesis of molecules explored for biomedical imaging.…”

Valence tautomerism and spin crossover in pyridinophane–cobalt–dioxolene complexes: an experimental and computational study

“…15 High spin manganese(II) and manganese(III) complexes of L-N4R2 (R = Me, iPr and tBu) ligands are known and do not exhibit electronic lability. [16][17][18][19] The L-N4R2 family of ligands display remarkable conformational flexibility in their metal complexes. 20 Clearly, very subtle electronic and steric factors govern the redox states and charge distributions in these metal complexes.…”

Catalysis of a Single Transition Metal Site for Water Oxidation: From Mononuclear Molecules to Single Atoms

“…The TM centers are usually bound by two (the first and the second) concentric coordination spheres. In the first coordination sphere, corrole, porphyrin and pyridinophane containing pyrrole or pyridine rings are the most commonly used ligands. These ligands are directly attached to the high‐valent TM ions, forming stable four‐coordinated TMN 4 units ( Figure a) as water oxidation centers .…”