Tuning The Oxidation Level, The Spin State, And The Degree Ofelectron Delocalization In Homo- And Heterolepticrnbis(α-diimine)iron Complexes

摘要

The four-coordinate heteroleptic complex [Fe~(III)(~Fpda~(2-) )(~Fdad~(·-) )] (1) and its homoleptic analogue [Fe~(II)(~Fdad~(·-))_2] (2), where ~Fpda~(2-) is the closed-shell ligand N,N'-bis(pentafluorophenyl)-o-phenylenedia-mide(2-) and ~Fdad~(·-)_2 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 FeN_4 coordination polyhedron in both 1 and 2. The electronic structures of 1 and 2 were probed by magnetic susceptibility measurements, ~57Fe Moessbauer 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)(~Fpda~(2-))(~Fpda~(·-))] (3), where ~Fpda~(·-) is a one-electron-oxidized form of ~Fpda~(2-), differ. The electronic structure of 2 consists of two ~Fdad~(·-) radicals coupled antiferromagnetically to a high-spin Fe~(II) center, whereas in 3, only one ~Fpda~(·-) 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 ~Fdad~(·-) 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).