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 [FeIII(Fpda2−)(Fdad•−)] (1) and its homoleptic analogue [FeII(Fdad•−)2] (2), where Fpda2− is the closed-shell ligand N,N′-bis(pentafluorophenyl)-o-phenylenediamide(2−) and Fdad•− is the singly reduced N,N′-bis(pentafluorophenyl)-2,3-dimethyl-1,4-diazabutadiene π-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, 57Fe Mössbauer and electronic spectroscopy, and density functional theory (DFT) calculations. In spite of their similar geometries and a common triplet ground state (St = 1), the electronic structures of 1, 2, and the previously reported homoleptic analogue [FeIII(Fpda2−)(Fpda•−)] (3), where Fpda•− is a one-electron-oxidized form of Fpda2−, differ. The electronic structure of 2 consists of two Fdad•− radicals coupled antiferromagnetically to a high-spin FeII center, whereas in 3, only one Fpda•− radical is coupled antiferromagnetically to an intermediate-spin FeIII ion. This ligand mixed-valent species exhibits class-III behavior. Heteroleptic 1 contains a single Fdad•− radical coupled antiferromagnetically to an intermediate-spin FeIII 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 FeII ion coordinated to two radical monoanions (2) or as an intermediate-spin FeIII ion coordinated to one radical monoanion and one closed-shell dianion (1, 3).