Effects of phenoxide ligation on iron-sulfur clusters. Preparation and properties of [Fe4S4(OAr)4]2- ions and the structure of bis(tetraethylammonium) tetrakis(phenolatosulfidoferrate) ([(C2H5)4N]2[Fe4S4(OC6H5)4])

Abstract

the intermediate formation of species II, because coordination of CO to the vacant coordination site in II would compete with coordination of PR3 at the site and the quantum yield would decrease. The quantum yield data in Table I show that the CC14 photoreaction becomes more efficient relative to the PR3 photoreaction as the irradiation wavelength decreases. This result is entirely reasonable in the context of our proposal of two reaction channels, because relatively more radicals should be formed from the more energetic excited states.The finding that the long-lived intermediate generated by flash photolysis reacts with CC147 coupled with our discovery that CC14 reacts with III to give only CpFe(CO)2Cl establish that it is not necessary to postulate 17-electron fragment intermediates as primary photoproducts based only on an observation that a chlorine abstraction product forms. In our scheme the chlorocarbon could react directly either with II or III (in the latter case by displacing phosphine) to give CpFe(CO)2Cl, thereby explaining why CpFe(CO)2Cl is the only product in the photolysis of Cp2Fe2(CO)4 with equimolar amounts of CC14 and PR3. However, the fact that 6 atm of CO does not markedly inhibit the reaction with CC14 suggests that the reaction of II with chlorocarbons is not very efficient compared to the radical pathway.The relatively long-lived photoproduct observed by Caspar and Meyer was proposed7 to be Cp2Fe2(CO)3, formed by Fe-CO bond dissociation. This particular photoproduct absorbs strongly in the visible, and it reacts with CO and other ligands. Because II should have a visible absorption band attributable to charge-transfer excitation from the coordinatively saturated iron to the 16-electron ("CpFeCO") center,20 the flash photolysis results7 can be accommodated equally well in terms of our mechanistic scheme.

N‐Coordinated [2Fe–2S] and [4Fe–4S] Clusters: Synthesis, Structures and Spectroscopic Characterisation

“…Selected interatomic distances and angles are listed in Table 1. To the best of the authors' knowledge, the N-coordinated cluster [Fe 4 S 4 (N(SiMe 3 ) 2 ) 4 ] (A) [9] and the O-coordinated cluster (NEt 4 ) 2 [Fe 4 S 4 (OPh) 4 ] (B) [13] are the only other examples of non-S-coordinated synthetic [4Fe-4S] clusters that have been structurally characterised, with the exception of several abiological clusters that bear ligands such as NO, CO or Cp. [14] 4+ cluster A has even larger Fe···Fe distances of 2.87 to 3.00 Å but is not really comparable due to its different oxidation state.…”

“…In general, the cluster-core geometry does not change much upon formal exchange of a thiolato by an indolato ligand, an observation that was already mentioned for the O-coordinated cluster. [13] Despite fast decomposition in dmf, a 1 H NMR spectrum could be recorded that shows broad signals at δ = 2.2 (methyl group), 5.6, 6.1, 7.0 and 8.1 ppm (one signal is either hidden beneath a solvent signal or too broad and thus not detectable). Likewise, ESI mass spectrometry confirmed cluster formation (see Supporting [13] The Mößbauer spectrum of 3 features a single doublet with δ = 0.44 mm/s and ∆E Q = 1.13 mm/s ( Figure 3, Table 2).…”

“…[13] Despite fast decomposition in dmf, a 1 H NMR spectrum could be recorded that shows broad signals at δ = 2.2 (methyl group), 5.6, 6.1, 7.0 and 8.1 ppm (one signal is either hidden beneath a solvent signal or too broad and thus not detectable). Likewise, ESI mass spectrometry confirmed cluster formation (see Supporting [13] The Mößbauer spectrum of 3 features a single doublet with δ = 0.44 mm/s and ∆E Q = 1.13 mm/s ( Figure 3, Table 2). Compared to the [2Fe-2S] analogue 2, the isomer shift is more positive, which is as expected for the lower oxidation number (2.5) of 3; similar values have also been observed for related complexes.…”

“…Compared to the [2Fe-2S] analogue 2, the isomer shift is more positive, which is as expected for the lower oxidation number (2.5) of 3; similar values have also been observed for related complexes. [7,13] The quadrupole splitting of 1.13 mm/s is relatively small compared to the Ocoordinated cluster B (∆E Q = 1.21 mm/s) [13] and various Scoordinated clusters; [20] unfortunately, no Mößbauer data are available for the amido-coordinated cluster A.…”

“…[13] All other known examples have to be considered as rather abiological clusters, since they have non-biomimetic terminal ligands like NO, CO, cyanide or Cp. [14] Although there is no natural [4Fe-4S] cluster known to date with coordination by more than one histidine, an all-N-ligated model system would further extend biomimetic Fe/S chemistry and might help to identify new unusual biological [4Fe-4S] clusters in the future.…”