Ligand-Centered Redox Activity: Redox Properties of 3d Transition Metal Ions Ligated by the Weak-Field Tris(pyrrolyl)ethane Trianion

Abstract

First-row transition metal complexes of the tris(pyrrolyl)ethane (tpe) trianion have been prepared. The tpe ligand was found to coordinate in a uniform η1,η1,η1-coordination mode to the divalent metal series as revealed by X-ray diffraction studies. Magnetic and structural characterization for complexes of the type [(tpe)MII(py)][Li(THF)4] (M: Mn, Fe, Co, Ni) reveal each divalent ion to be high-spin and have a distorted trigonal-monopyramidal geometry in the solid state. The pyridine ligand binds significantly canted from the molecular C3 axis due to a stabilizing π-stacking interaction with a ligand mesityl substituent. Cyclic voltammetry on the [(tpe)MII(py)]- series reveals a common irreversible oxidation pathway that is entirely ligand-based, invariant to the divalent metal bound. This latter observation indicates that fully populated ligand-based orbitals from the tpe construct are energetically most accessible in the electrochemical experiments, akin to their dipyrromethane analogues. Chemical oxidation of [(tpe)FeII(py)]- yields a product in which the ligand has dissociated one pyrrole (following tpe oxidation and H-atom abstraction) and binds a second equivalent of pyridine to form the neutral, tetrahedral FeII species (κ2-tpe)Fe(py)2. Similarly, chemical oxidation of the Zn(II) analogue shows evidence for tpe oxidation by electron paramagnetic resonance spectroscopy (77 K, toluene glass) with an isotropic signal for the organic radical at g = 2.002. Density functional theory analysis on this family of complexes reveals that the highest lying molecular orbitals are completely ligand-based, corroborating our proposed electronic structure assignment.