Substituent effects in dinuclear paddlewheel compounds: electrochemical and spectroscopic investigations

This review summarizes substituent effects in dinuclear compounds supported by diarylformamidinate (form) ligands, where aryl is phenyl substituted with various polar groups (X). These dinuclear compounds, M2(form)4(Lax)m with M as Mo, Ru, Rh, or Ni, Lax as either neutral or anionic axial ligand, and m=0-2, may undergo up to three (quasi)reversible one-electron, metal-centered redox processes. Linear correlations have been established between the electrode potentials (E1/2, Epc or Epa) and the substituent constants (σ) according to the following Hammett equation: ΔE=E(X)−E(H)=ρ(Σ σ), with the reaction constant ρ ranging from +60 to +100 mV. The existence of such relationships indicates that the energy levels of the frontier orbitals of dinuclear species are controlled precisely by the substituent within each series, and generally decrease with an increase in the electron withdrawing ability of the substituent. However, nearly identical UV-vis absorption spectra were observed within each homologous series, and one can infer a minimal substituent perturbation on the distribution of upper valence molecular orbitals. Axial ligation of organic π acids such as CO and phenylacetylide in both the diruthenium and dirhodium series also provides insight to the activation of these substrates by the dinuclear core, and the linear free energy relationship therein. A brief overview of other investigations of substituent effects in dinuclear compounds is provided also.