title: Tuning the oxidation level, the spin state, and the degree of electron delocalization in homo- and heteroleptic bis(alpha-diimine)iron complexes.
abstract: 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, 57Fe Mössbauer 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).
authors.affiliation: Max-Planck-Institut für Bioanorganische Chemie, Stiftstrasse 34-36, 45470 Mülheim an der Ruhr, Germany. marik@mpi-muelheim.mpg.de
authors.forename: Marat M
authors.initials: MM
authors.lastname: Khusniyarov
authors.forename: Thomas
authors.initials: T
authors.lastname: Weyhermüller
authors.forename: Eckhard
authors.initials: E
authors.lastname: Bill
authors.forename: Karl
authors.initials: K
authors.lastname: Wieghardt
mesh_terms: D018360:Crystallography, X-Ray; D004583:Electrons; D007097:Imines / Q000737:chemistry*; D007501:Iron / Q000737:chemistry*; D008024:Ligands; D008958:Models, Molecular; D015394:Molecular Structure; D010084:Oxidation-Reduction; D013237:Stereoisomerism; D013696:Temperature
chemical_list: D007097:Imines; D008024:Ligands; D007501:Iron