Perfluorinated alkoxide iron(II) complexes: counter cation control of structural, electronic, and O2 reducing properties and undercoordinated perfluorinated chromium(II) complexes: synthesis, characterization and reduction of non-innocent imine ligands
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Abstract
The electronic structures and spectroscopic behavior of three high-spin FeII complexes supported by fluorinated O-donor ligands were studied in detail: square-planar {K(DME)2}2[Fe(pinF)2] (S, 2) and quasi square-planar {K(2,2,2-crypt)}2[Fe(pinF)2] (S’, 4) as well as trigonal-planar {K(18C6)}[Fe(OC4F9)3] (T) (pinF = perfluoropinacolate and OC4F9 = tris-perfluoro-t-butoxide), using field-dependent 57Fe Mössbauer and high-field and -frequency Electron Paramagnetic Resonance (HFEPR) spectroscopies. Resulting spectroscopic parameters indicate that while S/S' have a dz2 orbital ground state, T has a dyz ground state. Computational investigation suggests that the ground state electronic configuration and geometry of T’s Fe site are determined by the interaction of the [Fe(OC4F9)3]- complex anion with the {K(18C6)}+ cation, whereas the two distinct countercations of S/S' have a negligible influence on the electronic structures and geometries of their [Fe(pinF)2]2- moieties.Two more {K(L)}2[Fe(pinF)2] were synthesized to complete the series (where L = unencapsulated, 1, (DME)2, 2, 18C6, 3 and 2,2,2-crypt, 4). This series of four high-spin, square planar complexes rapidly react with O2 to form a potent intermediate proposed to be an Fe(III) superoxide, {Fe(O2)} n-O2, which hydroxylates alkane C-H bonds (BDFE > 99 kcal/mol). Oxidants 1-O2 – 4-O2 were shown to have different abilities to perform OAT to PPh3, affected by the degree of K+ encapsulation: 1-O2 and 2-O2 were unable to perform OAT, whereas 3-O2 and 4-O2 performed sub-stoichiometric conversions of PPh3 to OPPh3. Cation encapsulation effects on the degree of ionic character were quantified by solution conductivity. Also reported is the oxidized Fe-containing product {K(18C6)}2[FeIII(OH)(pinF)2], 5, isolated from reaction of 3 and excess O2.
A series of unusually undercoordinated CrII complexes bound to perfluorinated O-donor ligands (ORF) was prepared and thoroughly characterized. The coordination environment of these complexes is tuned by the equivalents of alkoxide ligand ORF (two, 7-8, three, 6, 9-10 and four, 11-13) and the degree of encapsulation of the alkali metal cations (unencapsulated, 9, with 18C8, 6, 10-11 and 2,2,2-cyrpt, 12), which, through control of the K…O/F interactions, is shown to impact the Cr(III)/Cr(II) redox potentials. Also reported are a {CrIV=O} (14) and {CrII(OPPh3)} (15) moieties, resulting from reactions with Me3NO and OPPh3, respectively. Their formations indicate differing oxygen atom accepting abilities of bidentate alkoxide-supported 13 and monodentate alkoxide-supported 6. Electrochemical investigation of 7-13 consistently demonstrated the thermondynamic barrier to the oxidation of square planar CrII to proposed tetrahedral CrIII.
These reducing CrII complexes, along with late transition metal complexes {K(18C6)}[M(OC4F9)3] (M = Mn, Fe, Co) were employed to generate a series of complexes of redox active N-donor ligands. Both terminally bound (pyridine, 17-20, 26-28, 2,2-bipyridine, 27, phenazine, 21) and bridging (pyrazine, 22-25) complexes were prepared. Pyrazine-bridged 22 exhibits antiferromagnetic (AFM) coupling through the pyrazine pi-system, whereas later metals (Mn, 23, Fe, 24, Co, 25) do not support communication between the metal centers. A stable 2,2-bipyridine radical-coordinated 27 was isolated, as well as the first structurally characterized metal-bound pyridyl radical, 28.
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2024