Ligand-centered redox activity: Redox properties of 3d transition metal ions ligated by the weak-field tris(pyrrolyl)ethane trianion

Article abstract of DOI:10.1021/ic100028y

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 n¹,n¹,n-coordination mode to the divalent metal series as revealed by X-ray diffraction studies. Magnetic and structural characterization for complexes of the type [(tpe)M^II(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 -stacking interaction with a ligand mesityl substituent. Cyclic voltammetry on the [(tpe)M^II(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 orbitais from the tpe construct are energetically most accessible In the electrochemical experiments, akin to their dipyrromethane analogues. Chemical oxidation of [(tpe)Fe^II(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 Fe^II 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.