Characterization of Three Members of the Electron‐Transfer Series [Fe(pda)₂]ⁿ (n=2−, 1−, 0) by Spectroscopy and Density Functional Theoretical Calculations [pda=Redox Non‐innocent Derivatives of N,N′‐Bis(pentafluorophenyl)‐o‐phenylenediamide(2−, 1^.−, 0)]

Abstract

The four-coordinate iron complexes, [Fe(III)(pda(2-))(pda(.-))] (1) and [AsPh(4)](2)[Fe(II)(pda(2-))(2)] (2) were synthesized and fully characterized; pda(2-) is the closed-shell ligand N,N'-bis(pentafluorophenyl)-o-phenylenediamido(2-), and pda(.-) represents its one-electron-oxidized pi-radical anion. Single-crystal X-ray diffraction studies of 1 and 2 performed at 100(2) K reveal a distorted tetrahedral coordination environment at the iron centers, as a result of the intramolecular pi-pi interactions between C(6)F(5) rings. The electronic structures of 1 and 2 were unambiguously determined by a combination of (57)Fe Mössbauer and electronic spectroscopy, magnetic susceptibility measurements, X-ray crystallography, and DFT calculations. Compound 1 contains an intermediate-spin Fe(III) ion (S(Fe)=3/2) strongly antiferromagnetically coupled to a pi-ligand radical (S(R)=1/2) yielding an S(t)=1 ground state. Complex 2 possesses a high-spin Fe(II) center (S(Fe)=2) with two closed-shell dianionic ligands. Complexes 1 and 2 are members of the redox series [Fe(pda)(2)](n) with n=0 for 1 and n=2- for 2. The anion n=1- has been reported previously in the coordination salt [Fe(dad)(3)][Fe(pda)(2)] (3; dad=N,N'-bis(phenyl)-2,3-dimethyl-1,4-diaza-1,3-butadiene). A complicated temperature-dependent electronic structure has been observed for this salt. Here, DFT calculations performed on 3 confirm the previous assignments of spin- and oxidation-states. Thus, [Fe(pda)(2)](n) (n=0, 1-, 2-) constitutes an electron-transfer series, which has also been established by cyclic voltammetry; the mono- and dications (n=1+ and 2+) are also accessible in solution, but have not been further investigated. The (57)Fe Mössbauer spectra of [Fe(pda)(2)](n) species in 1 and 3 show extremely large quadrupole splitting constants due to addition of the valence and covalence contributions that have been confirmed by DFT calculations.

Radical ligand-containing single-molecule magnets

“…The more diffuse orbitals and lower electronegativity of the nitrogen donor atoms in o-iminobenzosemiquinone and odiiminobenzosemiquinone radical ligands have been found to provide better orbital overlap within the metal-nitrogen bond, leading to an increase in the strength of the exchange coupling [122][123][124][125][126][127][128][129][130][131]. Moreover, functionalization at the nitrogen atoms allows control of the steric hindrance at the metal center and ultimately the electronic environment of the ligand.…”

“…For instance, DFT calculations of the coupling constant in Fe( tBu PDA ISQ )( tBu PDA PDI ) resulted in values near J = −640 cm −1 , corresponding to an intermediate spin of S = 3/2 for the Fe III center that is strongly antiferromagnetically coupled to the S = 1/2 radical ligand [130]. The ligand-to-ligand intervalence charge-transfer band located at 1190 nm suggests that the complex can be assigned to a fully delocalized class III mixed-valence system, as a result of electron delocalization between one benzosemiquinone radical and one closed-shell dianion ligand, similar to related compounds of Ni II , Pd II , and Pt II [129,132].…”

Invariant Oxidation State of Copper but not of Ruthenium in Complexes with Noninnocent N‐(2‐Methyl‐5,8‐dioxo‐5,8‐dihydroquinolin‐7‐yl)acetamide: A Combined Structural, Electrochemical and Spectroelectrochemical Investigation

“…[6,9] After early studies on dithiolenes [10] and dioxolenes, [9] this field has seen a recent surge owing to the availability of advanced spectroscopic techniques and improvement in theoretical methods as well as interest in reactivity studies. [11][12][13][14][15][16][17] Such studies have also been performed on metal complexes of other ligands which show valence ambiguity. [18] Ruthenium complexes with quinone ligands present complicated chal-the UV/Vis/NIR, IR as well as EPR spectrum that are compatible with L-centered redox processes.…”

Alkali metal derivatives of an ortho-phenylene diamine

“…4 and Table 4 40 and is close to the sum of the covalent radii of cesium and nitrogen (3.03 Å). 25 As a result of the highly electropositive and polarisable nature of the large Cs + ion, complex 6 crystallises as a polymeric species, in which the coordination environment of Cs1 is supplemented by the presence of six short Cs⋯C(aryl) interactions 29 with the Ar-C centres of the sixmembered aromatic backbone of an additional PDA framework, resulting in an η 6 35 The fact that 6 crystallises as a polymeric species can be attributed to the larger radius of cesium compared to the preceding group 1 metals (Cs + 1.65; Li + 0.78, Na + 0.98, K + 1.33 Å). 33 It is possible that the large, electropositive cesium centre is too large to enable two cesium centres to be accommodated by the PDA ligand.…”