Spectroscopic and Electrochemical Characterization of an Aqua Ligand Exchange and Oxidatively Induced Deprotonation in Diiron Complexes

Abstract

Reaction of the unsymmetrical phenol ligand 2-((bis(2-pyridylmethyl)amino)methyl)-6-(((2-pyridylmethyl)benzylamino)methyl)-4-methylphenol (HL-Bn) or its 2,6-dichlorobenzyl analogue (HL-BnCl(2)) with Fe(H(2)O)(6)(ClO(4))(2) in the presence of disodium m-phenylenedipropionate (Na(2)(mpdp)) followed by exposure to atmosphere affords the diiron(II,III) complexes [Fe(2)(L-Bn)(mpdp)(H(2)O)](ClO(4))(2) and [Fe(2)(L-BnCl(2))(mpdp)(CH(3)OH)](ClO(4))(2), respectively. The latter complex has been characterized by X-ray crystallography. It crystallizes in the monoclinic system, space group P2(1)/n, with a = 13.3095(14) A, b = 20.1073(19) A, c = 19.4997(19) A, alpha = 90 degrees, beta = 94.471(2) degrees, gamma = 90 degrees, V = 5202.6(9) A(3), and Z = 4. The structure of the compound is very similar to that of [Fe(2)(L-Bn)(mpdp)(H(2)O)](BPh(4))(2) determined earlier, except for the replacement of a water by a methanol on the ferrous site. Magnetic measurements of [Fe(2)(L-Bn)(mpdp)(H(2)O)](BPh(4))(2) reveal that the two high-spin Fe ions are moderately antiferromagnetically coupled (J = -3.2(2) cm(-)(1)). Upon dissolution in acetonitrile the terminal ligand on the ferrous site is replaced by a solvent molecule. The acetonitrile-water exchange has been investigated by various spectroscopic techniques (UV-visible, NMR, Mössbauer) and electrochemistry. The substitution of acetonitrile by water is clearly evidenced by Mössbauer spectroscopy by a reduction of the quadrupole splitting value from 3.14 to 2.41 mm/s. In addition, it causes a 210 mV downshift of the oxidation potential of the ferrous site and a similar reduction of the stability domain of the mixed-valence state. Exhaustive electrolysis of a solution of [Fe(2)(L-Bn)(mpdp)(H(2)O)](2+) shows that the aqua diferric species is not stable and undergoes a chemical reaction which can be partly reversed by reduction to the mixed-valent state. This and other electrochemical observations suggest that upon oxidation of the diiron center to the diferric state the aqua ligand is deprotonated to a hydroxo. This hypothesis is supported by Mössbauer spectroscopy. Indeed, this species possesses a large quadrupole splitting value (DeltaE(Q) >or= 1.0 mm.s(-)(1)) similar to that of analogous complexes with a terminal phenolate ligand. This study illustrates the drastic effects of aqua ligand exchange and deprotonation on the electronic structure and redox potentials of diiron centers.

N2O reduction at a dissymmetric {Cu2S}-containing mixed-valent center

“…This behavior differs from the starting material, conrming ligand modications at the metal coordination spheres. 23 Finally, upon O 2 bubbling into either one of the above solutions, the resulting CV corresponds to that of [3$(m-OH)(OTf) 2 ] (Fig. OH À .…”

Multiple Aromatic Amination Mediated by a Diiron Complex

“…The ortho-benzylic proton appearing at d = 7.19 ppm in H(LH+NHTs) was absent in H(L-2H+2NHTs) and the two metabenzylic protons appearing at d = 7.02 and 7.53 ppm in H(LH+NHTs) had merged to d = 7.31 ppm in H(L-2H+2NHTs), both features supporting a bis ortho-benzylic substitution in H(L-2H+2NHTs). Insertion of iron in the two ligands was performed as usual [14] + (3'b). The two complexes had almost identical UV-Vis spectra but slight differences could be detected by NMR spectroscopy.…”

“…[14,15] When an acetonitrile solution of 1 a was titrated with ArINTs, its blue color (l max = 578 nm, e = 1300 m À1 cm À1 ) became progessively more intense, which corresponds to a two-fold increase in absorbance at about 585 nm and which was complete after the addition of 1.5(1) equivalents of ArINTs. Electrospray ionization mass spectrometry (ESI-MS) of the solution led to a spectrum that comprised two sets of three peaks ( Figure 1).…”

Deprotonation in Mixed-Valent Diiron(II,III) Complexes with Aniline or Benzimidazole Ligands

“…Complexes 2−7 were prepared by the usual procedures. 18,19 They were characterized by elemental analyses and ElectroSpray Ionization−Mass Spectroscopy (ESI-MS) and other spectroscopic techniques (see below).…”

“…By contrast, the larger spectral width observed for 5 is similar to that observed in 1 (ca. 550 ppm) 6 18,19 indicating localized iron valences. The spectrum of 5 is indeed strongly reminiscent of that recorded in the ≈65:35 cis/trans isomer mixture of 1 (see Supporting Information Figure S3).…”