From gas-phase ionization energies to solution oxidation potentials: Dimolybdenum tetraformamidinate paddlewheel complexes

• Gas-phase ionization energies correlate directly with solution oxidation potentials.
• Remote substituents cause substantial shifts in the metal–metal δ bond ionizations.
• Shifts are not caused by direct inductive charge effects or overlap effects.
• Shifts are caused by the change in potential field at the metals.
• The shifts are moderated in solution by solvent stabilization of the positive ion.

The gas-phase ionization energies of a series of Mo2(DPhF)4 paddlewheel complexes (DPhF is the N,N′-diphenylformamidinate anion with p-CH3, p-Cl, m-Cl, p-CF3, or m-CF3 phenyl substituents) have been measured by ultraviolet photoelectron spectroscopy (UPS) and compared with the solution oxidation potentials measured by cyclic voltammetry (CV) reported by Ren and coworkers. A linear relationship was found between the gas-phase ionization energies and the solution oxidation potentials. Density functional theory (DFT) computations clarify the individual electronic and thermodynamic factors that contribute to the correlation. The metal–metal delta bond electron energy is the largest factor in determining the solution oxidation potential. The substituents shift the metal–metal orbital energies by changing the through-space field potential at the metals rather than by an inductive change in charge at the metals or orbital overlap effects. The cation solvation energies determine the extent that the potential shifts are attenuated in solution. The results show that substituent field effects and solvation have major roles in determining the dimetal redox chemistry even when the dimetal unit is protected from direct interaction with the substituent and the solvent.

Dimetal paddlewheel complexes display an extraordinarily wide range of electron energies and electrochemical potentials for a single class of molecules. The dimetal tetraformamidinate complexes are ideal for revealing the key electronic factors that determine the electrochemical potentials through a combination of photoelectron spectroscopy, cyclic voltammetry, and density function theory examination.Figure optionsDownload as PowerPoint slide