Substituent effects on iron diimine complexes. I. Correlations with thermodynamic and spectral Properties in aqueous sulfuric acid

Abstract

Substituent effects on iron complexes FeL 3 2+, of aliphatic diimine ligands, H 3CNC(R)C(R′)Nz.sbnd;CH 3 (R,R′= H,H; H,CH 3; CH 3, CH 3; CH 3, C 2H 5;CH 2CH 2 CH 2CH 2; CH 2CH(CH 3)CH 2CH 2) or mixed diimine ligands, C 5H 4NC(R 1)N(R 2), (R 1,R 2=H,CH 3; H,C 2 H 5; H,n-C 3H 7; CH 3,CH 3; CH 3C 2H 5; CH 3,N-C 3H 7; C 6H 5CH 3) are reported. Formal electrode potentials were obtained in 4.0M H 2SO 4 and shown to correlate with Σσ* Taft polar parameters, except for substituents presenting large steric effects. Except for the n-propyl derivatives, it was possible to correlate all other compounds by considering that steric effects, E s, were also additive. The failure to include the n-propyl data shows that steric effects within the iron diimine grouping are larger than could be accounted for by the simple steric parameters used. Good linear free-energy relationships are also obtained for logε (ε, molar absorptivity) at the maximum wavelength of the inverse charge transfer bands of the ferrous complexes and Σσ*, or logε of the charge transfer bands for the ferric complexes and Σσ*, in the aliphatic series. In the mixed series such correlations were not obtained as a result of a more important role of steric effects. Based on these correlations, estimates of Σσ* and E s for the cyclohexanedione derivatives were also obtained.