Comparison of chemical interactions with Li+ and catalytic reactivity of electrochemically generated [FeICl(L)]2− and [CoI(L)]− complexes (L = salen or salophen)

Abstract

The cyclic voltammetric behavior of [FeIIICl(salen)] complexes has been investigated in CH3CN and compared to that obtained with [CoII(salen)] analogues. Details of the mechanism associated with iron- and cobalt-salen complex reduction in the presence of the lithium cation have been elucidated by comparison of simulated and experimental voltammograms. Electrogenerated [FeICl(salen)]²⁻ and [FeICl(salophen)]²⁻ complexes catalyze the dehalogenation of halo-alkyl compounds as is the case with [CoI(salen)]⁻ complexes, even in the presence of the lithium cation which allows the reduction to occur at a less negative potential.

Electrochemical Conversion of CO2 to CO by a Competent FeI Intermediate Bearing a Schiff Base Ligand

“…The CV trace of the mononuclear FeCl(L) (0.8 mm in acetonitrile with 0.1 m tetrabutylammonium tetrafluoroborate, Figure 1) displayed a first reversible cathodic process at E 1/2 = À 0.69 V vs Fc + /Fc (ΔE = 84 mV) attributed to the Fe III /Fe II couple, followed by a second, quasi-reversible one at E 1/2 = À 2.00 V vs Fc + /Fc, and attributed to reduction of Fe II to Fe I . [45] For Fe 2 (μ-O)(L) 2 the first, quasireversible cathodic wave encountered in the forward scan is cathodically shifted (E 1/2 = À 1.32 V vs Fc + /Fc, ΔE = 100 mV) and is attributed to the Fe III to Fe II reduction of both iron centers of the FeÀ OÀ Fe μ-oxo bridge. [49] The more negative potential required to reduce Fe III to Fe II in the dinuclear species with respect to the mononuclear one is in agreement with previous literature, [49] and was observed also for iron porphyrin analogs.…”

“…Salophen ligand is easily synthesizable from commercially available reagents; the idea of investigating its iron complexes for electrocatalytic reduction of CO 2 originated from previous observations in the literature. The electrochemical properties of FeCl(L) have been the subject of seminal research in the field of electrocatalysis by Bond and co-workers, [45] that identified sequential reduction of the Fe III center to Fe II and finally to Fe I , with this latter species being electrocatalytically active towards reductive dehalogenation of benzyl halides. [45] Furthermore, the analog cobalt(II) salophen complex was reported as a CO 2 -to-CO reduction electrocatalyst by Isse et al, [46] involving an electrogenerated cobalt(I) active intermediate.…”

“…The electrochemical properties of FeCl(L) have been the subject of seminal research in the field of electrocatalysis by Bond and co-workers, [45] that identified sequential reduction of the Fe III center to Fe II and finally to Fe I , with this latter species being electrocatalytically active towards reductive dehalogenation of benzyl halides. [45] Furthermore, the analog cobalt(II) salophen complex was reported as a CO 2 -to-CO reduction electrocatalyst by Isse et al, [46] involving an electrogenerated cobalt(I) active intermediate. Indeed, several examples in the literature showed a common, privileged ligand choice for the design of cobalt and iron CO 2 reduction catalysts.…”

“…Mononuclear FeCl(L) and μ-oxo dinuclear Fe 2 (μ-O)(L) 2 complexes were synthesized in 60-65 % yield by a slight modification of literature procedures, that involve the addition of an iron precursor (FeCl 3 • 6H 2 O and Fe(NO 3 ) 3 • 9H 2 O for FeCl(L) and Fe 2 (μ-O)(L) 2 , respectively) to a solution of the salophen ligand in ethanol, in the presence of triethylamine as base additive, followed by precipitation and washing of the product. [45,49] Distinctive features between FeCl(L) and Fe 2 (μ-O)(L) 2 can be evidenced through electrospray ionization mass spectrometry (ESI-MS) and UV/Vis spectroscopy. In particular, ESI-MS spectra of both compounds exhibit an intense peak at m/z = 370, attributed to the [Fe(L)] + ion ( Figure S2), while peaks attributed to dinuclear ions and centered at m/z = 757 ([Fe 2 (L) 2 (μ-OH)] + ) and 785 were observed only in the case of Fe 2 O(L) 2 ( Figure S2).…”

“…Concerning UV/Vis spectroscopy, the two coordination compounds exhibit a notably different electronic absorption ( Figure S3), consistent with literature references. [45,49] A band peaking at 365 nm (ɛ = 1.3 • 10 4 L mol À 1 cm À 1 ) is observed for FeCl(L), while the dinuclear Fe 2 (μ-O)(L) 2 complex is characterized by a redshifted maximum at 405 nm (ɛ = 2.0 • 10 4 L mol À 1 cm À 1 ); these spectral features, absent for the free ligand and characterized by high intensity and molar extinction coefficients, could therefore be appropriately described as charge-transfer bands between the Fe ions and the salophen ligand.…”