Figure 2.4: Molecular Structure of8.
—ORTEPplot, 50 % probability thermal ellipsoids; hydrogen atoms omitted for clarity.
2.3 Synthesis and Structural Characterisation of [ 2Fe–2S ] Clusters
The synthesis of[2Fe–2S]clusters with ligands3and6was possible on two different literature-known routes, namelyviathe well-established salt metathesis reaction using (NEt4)2[Fe2S2Cl4] (9)[140] (Scheme 2.4i) orvia the novel ligand-exchange route using (NEt4)2[Fe2S2(indolate)4](10)[151](Scheme 2.4ii). Although the first is applicable to all ligands, it usually gives low yields (11: 13–17 %) and separation of by-products can be laborious. The second route – which only works for ligands with a significantly higher acidity than indole to allow for ligand exchange on the cluster and production of free, i.e., protonated indole – gave acceptable product yields for 11 (52–63 %) and 12 (54–
72 %). Salt metathesis reactions were performed in THF/MeCN and the products were subsequently crystallised from MeCN/Et2O. The bulk material of all the complexes precipitated as a brown powder with only minor amounts of crystalline material. Us-ing the novel ligand-exchange pathway, reactions were carried out in MeCN for11or THF/MeCN for 12. In general, evaporation of the solvents in vacuo and subsequent washing with THF/Et2O to remove free indole gave sufficiently pure target complexes.
All five complexes are readily soluble in MeCN, DMF and DMSO while only the tert-butyl-derivative11cis sparingly soluble in THF and CH2Cl2.
Crystals suitable for X-ray diffraction were obtained for all complexes by diffusion of Et2O into a MeCN solution of the respective cluster. The O-coordinated clusters
Scheme 2.4: Synthesis of cluster compounds11and12.
11aand11ccrystallise as pseudo-C2h symmetric molecules with aC2axis through the iron atoms and the perpendicular mirror plane through the sulphide ions. In11b the general symmetry is preserved although theC2axis is missing due to the inequivalent phenyl rings (Figure 2.5). The S-coordinated cluster 12a crystallises in the same man-ner (Figure 2.6). Thesemeso-forms thus contain one axial-Rand one axial-S configured ligand, this isomer probably crystallising preferably. The other isomers are either pre-cipitated as powders or they racemise in solution. These findings are in accordance with other known biphenolate-coordinated[145, 151, 186] and dithiophenolate-coordinated[187] complexes.
However, 12b differs from that symmetry in its crystal structure as both ligands have the same configuration and the cluster is asymmetric (C1). This is accompanied by the loss of cluster core planarity (dihedral angle Fe–µ-S–Fe–µ-S: 9.781(64)°) that is also observed, yet to a lesser extent, in some of the other structures (11a: 5.865(40)°, 11b: 6.644(31)°).
2.3 Synthesis and Structural Characterisation of[2Fe–2S]Clusters
Figure 2.5: Molecular structures of O-coordinated complexes11: 11a (upper left), 11b (upper right),11c(bottom).
— ORTEP plots, 50 % probability thermal ellipsoids; counterions, solvent molecules and hydrogen atoms omitted for clarity; only crystallographically independent heteroatoms la-belled in case of11c.
A similar crystallisation behaviour has only been observed once in a crystal struc-ture of a related dithiophenolate-coordinated[2Fe–2S]cluster, which crystallised with two different counterions.[187]In contrast to the published structure where both enan-tiomers are present in the asymmetric unit, only one enantiomer of12bwas found in the crystal structure. While in the published example this is probably due to the coun-terions, the reason is unknown in case of12b, especially as the phenolate derivative11b crystallises like the other related complexes.
Figure 2.6: Molecular Structure of12a.
—ORTEPplot, 50 % probability thermal ellipsoids; counterions and hydrogen atoms omit-ted for clarity; only crystallographically independent heteroatoms labelled.
Fe Fe
S S S
S
S
S
Figure 2.7: Molecular Structure of12b.
—ORTEPplot, 50 % probability thermal ellipsoids; counterions, solvent molecules and hy-drogen atoms omitted for clarity.
2.3 Synthesis and Structural Characterisation of[2Fe–2S]Clusters
Table 2.1: Selected interatomic distancesdand anglesαof complexes11and12.
d(Fe· · ·Fe) d(Fe–µ-S) d(Fe–Xa)) α(Fe–µ-S–Fe) α(Xa)–Fe–Xa)) τ4b)
Relevant bond distances and angles (see Table 2.1) are in perfect agreement with the above-mentioned literature-known analogues. Fe· · ·Fe distances are unexceptional, ranging from 2.69 to 2.70 Å. In all complexes the iron atoms exhibit a slight devia-tion from tetrahedral coordinadevia-tion without a pronounced trend, as quantified by the τ4 value[188]. These observations reflect the minor influence that ligand substitution generally has on cluster core geometry.
significantly to higher field (to 2–3 ppm). The allyl groups also give broadened signals between 5 and 7 ppm. Although1H NMR signals can easily be assigned by comparison with other complexes, the remaining paramagnetism of the clusters causes broadening of the signals and prevents detection of any coupling.
The1H NMR spectrum of12bseems to contain a double set of signals as compared to the O-analogue11balthough it is difficult to exactly separate the signals due to the line broadening and the asymmetry of the phenyl rings (Figure 2.8). The same signal distribution is obtained at higher temperatures (70 °C), indicating no racemisation at this temperature. However, other properties of12bdo not differ significantly from the other complexes11and12a.
UV/vis spectra of clusters11and12are shown in Figure 2.9 and the absorption max-ima are collected in Table 2.2. They resemble those of related[2Fe–2S]clusters bearing biphenolate or bithiphenolate ligands. The O-coordinated clusters11can be compared to the parent compound bearing unsubstituted biphenol ligands[145]and a cluster bear-ing a tetrachlorobiphenol ligand[151]. Compared to both, the main visible band is red-shifted to 424 (11a,11b) and 443 nm (11c) as compared to 416 and 413 nm, respectively, due to the increase in electron density lowering the transition energies for ligand-to-metal charge transfer. Likewise, the main visible bands of the S-coordinated clusters12 are bathochromically shifted as compared to the unsubstituted parent dithiophenolate compound.[187]
Cyclic voltammograms were recorded for clusters11and12in acetonitrile to investi-gate general redox properties of the allyl-substituted clusters. Exemplarily, the voltam-mograms of clusters11a and12a are shown in Figure 2.10). While the O-coordinated clusters 11 show an irreversible reduction (11a: −1.40 V, 11b: −1.29 V halfwave po-tential vs. NHE), the S-coordinated complexes 12 are quasi-reversibly reducible (12a:
−1.05 V,12b:−1.07 V midpoint potentialvs. NHE; see Table 2.2 and Chapter 7.5 for