Ligand Exchange and Spin State Equilibria of Fe^II(N4Py) and Related Complexes in Aqueous Media

Abstract

We report the characterization and solution chemistry of a series of Fe(II) complexes based on the pentadentate ligands N4Py (1,1-di(pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)methanamine), MeN4Py (1,1-di(pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)ethanamine), and the tetradentate ligand Bn-N3Py (N-benzyl-1,1-di(pyridin-2-yl)-N-(pyridin-2-ylmethyl)methanamine) ligands, i.e., [Fe(N4Py)(CH(3)CN)](ClO(4))(2) (1), [Fe(MeN4Py)(CH(3)CN)](ClO(4))(2) (2), and [Fe(Bn-N3Py)(CH(3)CN)(2)](ClO(4))(2) (3), respectively. Complexes 2 and 3 are characterized by X-ray crystallography, which indicates that they are low-spin Fe(II) complexes in the solid state. The solution properties of 1-3 are investigated using (1)H NMR, UV/vis absorption, and resonance Raman spectroscopies, cyclic voltammetry, and ESI-MS. These data confirm that in acetonitrile the complexes retain their solid-state structure, but in water immediate ligand exchange of the CH(3)CN ligand(s) for hydroxide or aqua ligands occurs with full dissociation of the polypyridyl ligand at low (<3) and high (>9) pH. pH jumping experiments confirm that over at least several minutes the ligand dissociation observed is fully reversible for complexes 1 and 2. In the pH range between 5 and 8, complexes 1 and 2 show an equilibrium between two different species. Furthermore, the aquated complexes show a spin equilibrium between low- and high-spin states with the equilibrium favoring the high-spin state for 1 but favoring the low-spin state for 2. Complex 3 forms only one species over the pH range 4-8, outside of which ligand dissociation occurs. The speciation analysis and the observation of an equilibrium between spin states in aqueous solution is proposed to be the origin of the effectiveness of complex 1 in cleaving DNA in water with (3)O(2) as terminal oxidant.

Identification and Spectroscopic Characterization of Nonheme Iron(III) Hypochlorite Intermediates

“…[11b] Addition of 0.5 equiv of the two-electron oxidant NaOCl (at pH 2.9) resulted in a near complete loss of absorption in the visible region of the spectrum, with an isosbestic point obtained at approximately λ =330 nm within 120 s at room temperature (Figure 1 a). No significant concomitant increase in absorption was detected at longer wavelengths (Figure 1 b).…”

“…These changes are consistent with an overall one-electron oxidation to form the corresponding Fe III -OH complex. [11b,c] Indeed, a weak absorption band at circa λ =480 nm remained and the characteristic EPR spectrum of a low-spin Fe III complex was detected (Figure 1 a, inset). This spectrum corresponds to the spectra of [Fe III (OH)(MeN4Py)] 2+ .…”

“…Fortuitously, at λ ex =473 nm the initial [Fe II (OH 2 )(MeN4Py)] 2+ ,[11b] the intermediate Fe III -OCl complex, and the Fe IV =O complex formed show resonance enhancement of their Raman scattering and allows the change in concentration of all three species to be followed concurrently by rR spectroscopy (Figure 4). The resonance-enhanced Raman bands of [Fe II (OH 2 )(MeN4Py)] 2+ disappear completely within approximately 1 min of addition of 2 equiv of NaOCl (Figure 4).…”

A Non‐Heme Iron Photocatalyst for Light‐Driven Aerobic Oxidation of Methanol

“…The addition of acetonitrile (to 2.5 vol%) after irradiation confirmed the integrity of the ligands by yielding the corresponding [(L)Fe II (CH 3 CN)] 2+ complex ( 1 ) quantitatively, as shown by comparison with the absorption spectrum of [(L)Fe II (CH 3 CN)] 2+ ( 1 ) in acetonitrile (Figure 2; see also Figure S14) 24, 28. The concomitant formation of 0.5 equivalents of formaldehyde (see the Supporting Information) confirmed that methanol was the source of electrons for the reduction.…”

Stabilisation of μ-peroxido-bridged Fe(iii) intermediates with non-symmetric bidentate N-donor ligands

“…S18, ESI †). 20 For the [Fe(NN) 3 ] 2+ complexes, 1 and 2, an intense MLCT band is observed in the electronic absorption spectrum at B500 nm that is typical of low-spin Fe(II) complexes. 11 The non-resonant Raman spectra of 1 and 2 in the solid state (Fig.…”