Tuning the Oxidation Level, the Spin State, and the Degree of Electron Delocalization in Homo- and Heteroleptic Bis(α-diimine)iron Complexes

The four-coordinate heteroleptic complex [Fe-III((F)pda(2-))((F)dad(*-))] (1) and its homoleptic analogue [Fe-II((F)dad(*-))2] (2), where (F)pda(2-) is the closed-shell ligand N,N'-bis(pentafluorophenyl)-o-phenylenediamide(2-) and (F)dad(*-) is the singly reduced N,N'-bis(pentafluorophenyl)-2,3-dimethyl-1,4-diazabutadiene pi-radical anion, have been synthesized. X-ray crystallographic studies reveal a twisted tetrahedral geometry of the FeN4 coordination polyhedron in both 1 and 2. The electronic structures of 1 and 2 were probed by magnetic susceptibility measurements, Fe-57 Mossbauer and electronic spectroscopy, and density functional theory (DFT) calculations. In spite of their similar geometries and a common triplet ground state (S-t = 1), the electronic structures of 1, 2, and the previously reported homoleptic analogue [Fe-III((F)pda(2-))((F)pda(*-))] (3), where (F)pda(*-) is a one-electron-oxidized form of (F)pda(2-), differ. The electronic structure of 2 consists of two (F)dad(*-) radicals coupled antiferromagnetically to a high-spin Fe-II center, whereas in 3, only one (F)pda(*-)radical is coupled antiferromagnetically to an intermediate-spin Fe-III ion. This ligand mixed-valent species exhibits class-III behavior. Heteroleptic 1 contains a single (F)dad(*-) radical coupled antiferromagnetically to an intermediate-spin Fe-III center but behaves as a class-II ligand mixed-valent species. The observed diversity in the electronic structures of 1-3 is ascribed to the difference in the redox potentials of the ligands. Analysis of reduced orbital charges and spin densities obtained from DFT calculations also suggests that the electronic structures of 1-3 are best described as either a high-spin Fe-II ion coordinated to two radical monoanions (2) or as an intermediate-spin Fe-III ion coordinated to one radical monoanion and one closed-shell dianion (1, 3).