Charge transfer of iron(III) monomeric and oligomeric aqua hydroxo complexes: Semiempirical investigation into photoactivity

Aqueous hydrolyses of iron(Ill) solutions were studied using electronic spectroscopy. Complete spectra from 200 to 800 nm were obtained for the four ferric aqua hydroxo complexes: Fe(H2O)(6)(3+), Fe(OH)(H2O)(5)(2+), Fe(OH)(2)-(H2O)(4)(+) , and the dimer Fe-2(mu-OH)(2)(H2O)(8)(4+). Semiempirical Zindo/s calculations were employed to assign which types of electronic transfers are involved so that the photoactivity as regards the photoreduction dissociation Fe-aq(III)-->Fe-hv(aq)II + OH. can be discussed. Fe3+ exhibits two LMCT from nonbonding p orbitals (nLp) located at 190 and 240 rim. Fe(OH)(2+) shows two major nLp(OH) --> d transitions at 205 and 295 nm. As regards its geometry, computed investigations using an Fe-OH distance of 2.05 Angstrom better fit than using a shorter distance (similar to1.8 Angstrom); the same conclusion remains constant for all hydroxo complexes. The dihydroxo form's spectrum was confronted to its common cis and trans expectable isomers plus an unusual pentacoordinate one. Even if the trans isomer is supposed to be the lowest Gibbs free energy species in solution, there is some evidence of the presence of the cis form; hence, both species must be close in energy. Other isolated nLp(OH) --> d transfer wavelengths are 205, 295, and 335 nm. As for the dimer, this study provides some clue in favor of the bis(mu-hydroxo)) description. Both water and hydroxo ligands are involved along the electronic transitions toward only d(1) metal-centered orbitals at 220 and 260 nm for H2O, 335 and 470 nm for OH-, and 205 nm for both. Charge transfers for the hydrogen oxide bridge form Fe-2(mu-H3O2)(H2O)(8)(5+) were also computed. Finally predictions about the two bis(mu-hydroxo) bridge trimer Fe-3(OH)(4)(H2O)(10)(5+) enable one to foresee a huge and broad charge transfer in the UV region (similar to240 nm) followed by a multi nLp(OH) --> d(1) transfer extending up to similar to650 nm.