RESUMO
Designing efficient Fe(ii) chromophores requires optimization of numerous, at times conflicting, properties. It has been suggested that replacement of polypyridine ligands with cyclometalated analogs will be effective at destabilizing the quintet state and therefore extending the lifetime of photoactive metal-to-ligand charge transfer states. However, cyclometalated Fe(ii) complexes are not oxidatively stable due to the strong electron-donating nature of this ligand, which limits their applicability. Here we use density functional theory calculations to show how simple addition of nitro and carboxylic acid groups to these cyclometalated complexes can engender a less oxidizable Fe(ii) center while maintaining, or even improving, the favorable ligand field strength.
RESUMO
One of the most compelling strategies for utilizing redox-active ligands is to perform redox events at the ligands to avoid accessing prohibitively high energy oxidation states at the metal center. This has been demonstrated experimentally in many systems, yet there is little understanding of the fundamental electronic structures involved with these transformations or how to control them. Here, the reductive elimination of biphenyl from [M(isq)2Ph2] (M = Ti, Zr, and Hf and isq = 2,4-di-tert-butyl-6-tert-butyliminosemiquinone) was studied computationally. It was found that the metal remains in the +IV oxidation state and all redox chemistry was mediated by the redox-active ligands. Two types of electron-transfer mechanisms were identified, an asymmetric unpaired electron transfer (UET) and a symmetric pairwise electron transfer (PET), the former always being lower in energy. The energetic differences between these two mechanisms were explained through simple molecular orbital theory arguments. Despite the metal's redox-inactivity, it still has a marked influence on the calculated energetics of the reaction, with the Ti systems being much more reactive than the Zr/Hf systems. This primarily originates from the shorter Ti-Ph bond, which leads to a stronger filled-filled interaction between these ligands at the reactant state. This greater reactant destabilization leads to the lower activation energies.
