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Structural insight into halide-coordinated [Fe4S4XnY4−n]2− clusters (X, Y = Cl, Br, I) by XRD and Mössbauer spectroscopy

Andreas O. Schüren ab, Benjamin M. Ridgway b, Florencia Di Salvo bc, Luca M. Carella d, Verena K. Gramm a, Elisa Metzger e, Fabio Doctorovich bc, Eva Rentschler d, Volker Schünemann e, Uwe Ruschewitz a and Axel Klein *a
aUniversität zu Köln, Mathematisch-Naturwissenschaftliche Fakultät, Department für Chemie, Institut für Anorganische Chemie, Greinstraße 6, D-50939 Köln, Germany. E-mail: axel.klein@uni-koeln.de; Tel: +49-221-470-4006
bINQUIMAE-CONICET-Universidad de Buenos Aires, Intendente Güiraldes 2160, Pabellón 2, Piso 3, C1428EGA, Buenos Aires, Argentina
cUniversidad de Buenos Aires, Facultad de Ciencias Exactas y Naturales, Departamento de Química Inorgánica, Analítica y Química Física, Intendente Güiraldes 2160, Pabellón 2, Piso 3, C1428EGA, Buenos Aires, Argentina
dJohannes Gutenberg Universität Mainz, Department Chemie, Duesbergweg 10-14, 55128 Mainz, Germany
eTU Kaiserlautern Department of Physics, 67663 Kaiserlautern, Germany

Received 3rd October 2022 , Accepted 14th December 2022

First published on 14th December 2022


Abstract

Iron sulphur halide clusters [Fe4S4Br4]2− and [Fe4S4X2Y2]2− (X, Y = Cl, Br, I) were obtained in excellent yields (77 to 78%) and purity from [Fe(CO)5], elemental sulphur, I2 and benzyltrimethylammonium (BTMA+) iodide, bromide and chloride. Single crystals of (BTMA)2[Fe4S4Br4] (1), (BTMA)2[Fe4S4Br2Cl2] (2), (BTMA)2[Fe4S4Cl2I2] (3), and (BTMA)2[Fe4S4Br2I2] (4) were isostructural to the previously reported (BTMA)2[Fe4S4I4] (5) (monoclinic, Cc). Instead of the chloride cubane cluster [Fe4S4Cl4]2−, we found the prismane-shaped cluster (BTMA)3[Fe6S6Cl6] (6) (P[1 with combining macron]). 57Fe Mössbauer spectroscopy indicates complete delocalisation with Fe2.5+ oxidation states for all iron atoms. Magnetic measurements showed small χMT values at 298 K ranging from 1.12 to 1.54 cm3 K mol−1, indicating the dominant antiferromagnetic exchange interactions. With decreasing temperature, the χMT values decreased to reach a plateau at around 100 K. From about 20 K, the values drop significantly. Fitting the data in the Heisenberg–Dirac–van Vleck (HDvV) as well as the Heisenberg Double Exchange (HDE) formalism confirmed the delocalisation and antiferromagnetic coupling assumed from Mössbauer spectroscopy.


Introduction

Cubane-shaped [Fe4S4] clusters represent a very interesting class of cofactors in biology and are involved in various functions in a cell's life cycle.1–10 Due to their mixed valence character with formally two ferric (Fe3+) and two ferrous (Fe2+) iron atoms, [Fe4S4]2+ clusters show extraordinary electrochemical properties which are crucial for their biological functions in electron transfer and redox catalysis reactions.1–10 During the last 40 years, numerous [Fe4S4]2+ clusters with terminal thiolate ligands have been prepared in order to model the structural, electronic and electrochemical properties as well as the reactivity of the naturally occurring clusters.5–26 Moreover, the fascinating nature of [Fe4S4] clusters has triggered their use as precursors for electrocatalytic materials.27–33 Halide-based [Fe4S4X4]2− with X = Cl, Br, or I are known to be useful precursors because halides can be easily exchanged with thiolate or other ligands by salt metathesis reactions. Homoleptic halide clusters have thus been studied previously and a couple of preparation methods have been published (Scheme 1).12,14,15,32–48
image file: d2dt03203a-s1.tif
Scheme 1 Reported preparation pathways for the [Fe4S4X4]2− clusters. (1) Müller et al.,37 (2) Kanatzidis et al.,32 (3) Pohl et al.,38–40 (4) Holm et al.,36,41 Henderson et al. for X = Cl,43–45 Tuczek et al. for X = Cl or Br,46 (5) Holm et al. for X = Cl,41 and (6) Holm et al. for X = Cl, Br, I,36 Tuczek et al. for X = I.46

We have recently contributed to this very versatile method (3) starting from [Fe(CO)5] and (BTMA)X (BTMA = benzyltrimethylammonium), (Ph4P)X, and halogens or interhalogens obtaining excellent yields for the compounds (BTMA)2[Fe4S4I4] and (Ph4P)2[Fe4S4Br4] containing the homoleptic [Fe4S4X4]2− clusters.48 By the same method, we obtained the mixed-halide cluster compounds (BTMA)2[Fe4S4BrI3], (BTMA)2[Fe4S4ClI3], and (BTMA)2[Fe4S4Cl2I2]. Using [Fe(CO)5], sulphur, ferrous halides FeX2 and (BTMA)X and adopting method (2), we obtained (BTMA)2[Fe4S4Cl4], (BTMA)2[Fe4S4Br4], (BTMA)2[Fe4S4Cl2I2], (BTMA)2[Fe4S4Br2Cl2] and (BTMA)2[Fe4S4Br2I2]. By replacing the halide FeX2 with tetrahalido ferrates (Ph4P)[FeX4] or (Et4P)[FeX4], we obtained (Et4N)2[Fe4S4Cl4] and (Ph4P)2[Fe4S4Br4]. The compounds were obtained in the pure form as microcrystalline solids, while single crystals were not obtained in this study.48 A careful study of species in solution by UHR-ESI-MS(−) showed that the mixed halide clusters [Fe4S4X2Y2]2− and [Fe4S4XY3]2− scramble their halide ligands in a coordinative disproportionation in THF solution, yielding all possible species [Fe4S4X4−nYn]2− (X/Y = Cl, Br, I). The same was concluded from the UV-vis-NIR absorption spectroscopy results in MeCN solution. Powder X-ray diffraction (PXRD) indicated that the compounds (BTMA)2[Fe4S4Br4], (BTMA)2[Fe4S4Br2Cl2], (BTMA)2[Fe4S4Cl2I2] and (BTMA)2[Fe4S4Br2I2] were isostructural to the previously reported (BTMA)2[Fe4S4I4] (monoclinic, Cc).40

The lack of precise structural information from single crystal XRD was unfortunate since the crystal structures of the [Fe4S4X4]2− clusters are quite interesting due to the frequent observation that the symmetry of [Fe4S4L4]n clusters strongly depends on the packing in the solid state. Frequently, distortion of the idealised cluster symmetry Td towards D2d was observed (Table 1).22 For the mixed-halide [Fe4S4X2Y2]2− clusters belonging to the vast group of heteroleptic [Fe4S4L2L′2]m clusters, the TdC2v symmetry reduction is immanent in the molecular symmetry of the cluster as is the TdC3v symmetry reduction for [Fe4S4L3L′1]m clusters. At the same time, the packing in the crystal and thus the size and shape of the counter cation can cause lowering of the cluster symmetry and for all three types even lower symmetry caused by crystal packing was reported (Table 1).37–61 Importantly, with these changes in cluster symmetry, essential properties such as the redox potentials, spin states, magnetic properties, and ligand exchange kinetics vary markedly.1,6,8,13,14,20–23,37–40,47,50,56,62–84

Table 1 Selected structural features of [Fe4S4L4]n [Fe4S4L3L′]n, and [Fe4S4L2L′2]n clusters
Cation Ligand set Crystal system Space group Cluster symmetrya Ref.
a Approximate symmetry of the cluster. b Approaches C3v symmetry as one Cl ligand shows strong H bonding to nBu3NH+. c BTMA+ = benzyltrimethylammonium. d Et2Dtc = N,N-diethyldithiocarbamate. e Selenium clusters [Fe4Se4X4]2−. f Me2Tu = N,N-dimethylthiourea. g tBu2Tu = N,N-di-tert-butyl-thiourea. h R = 2,4,6-(iPr)3C6H2, Et2Tu = N,N-diethylthiourea.
Et4N+ Cl Monoclinic P21/c D 2d 36 and 42
nPr4N+ Cl Monoclinic P21/n C 1 41
Ph4P+ Cl Monoclinic C2/c D 2d 37
nBu3NH+ Cl Monoclinic P21/n C 3v 70
[K4[FeCl4](C12H24O6)4]2+ Cl Cubic F23 T d 52
[Fe(MeCN)2[(P(OMe)3]4]2+ Cl Monoclinic P21/c D 2d 85
Ph4P+ Br Monoclinic C2/c D 2d 37
Et4N+ Bre Monoclinic P21/c S 4 87
[(nBu4N)2[Fe(DMF)6]]4+ Br Monoclinic P21/n T d 86
Ph4P+ I Tetragonal I41/a S 4 40
BTMA+c I Monoclinic Cc C 1 40
nBu4N+ Ie Monoclinic P21/n C 3v 89
[(Et4N)6[Fe2S2I4]]4+ I Tetragonal P42bc T d 60
nBu4N+ SMe Orthorhombic Pba2 T d 83
HnBu3N+ SPh Triclinic P[1 with combining macron] C 2v 70
HEt3N+ SPh Monoclinic P21/n D 2d 70
BTMA+c StBu Monoclinic P21/c C 1 82
Et4N+ StBu Tetragonal I[4 with combining macron]2m D 2d 82
Ph4P+ 2 Br 2 Cl Monoclinic C2/c D 2d 37
Ph4P+ 2 Cl 2 SPh Orthorhombic Pbcn D 2d 88
Ph4P+ 2 Cl 2 OPh Orthorhombic Pbcn D 2d 89
Ph4P+ 2 SPh 2 OpTol Orthorhombic P212121 C 1 89
Ph4P+ 2 Cl 2 S Et2Dtcd Monoclinic C2/c C 2 91
Ph4P+ 2 SPh 2 S Et2Dtcd Monoclinic C2/c C 2 91
Ph4P+ 3 Cl 1 S Et2Dtcd Monoclinic P21/c C s 91
[FeIL3]+ 3 I 1 S Me2Tuf Triclinic P[1 with combining macron] C 3v 90
None 2 I 2 S tBu2Tug Monoclinic P21/n C 2v 90
None 2 SR 2 S Et2Tuh Triclinic P1 D 2d 57


Herein, we report the synthesis and detailed study of the materials (BTMA)2[Fe4S4Br4] (1), (BTMA)2[Fe4S4Br2Cl2] (2), (BTMA)2[Fe4S4Cl2I2] (3), (BTMA)2[Fe4S4Br2I2] (4), and (BTMA)2[Fe4S4I4] (5) containing cubane type Fe4S4 clusters alongside with (BTMA)3[Fe6S6Cl6] (6) containing the prismane [Fe6S6Cl6]3− structure. After preparation in solution, the compounds were isolated as single crystals and we can provide full structural information from single crystal and PXRD analyses. Additionally, resonance Raman, 57Fe Mössbauer and magnetic data were recorded for the first time on such mixed-halide clusters, with the exception of (Ph4P)2[Fe4S4Br2Cl2] for which the structure and Mössbauer data have previously been reported.37

Results and discussion

Preparations

The materials were prepared as recently described (for details and analyses, see the ESI). As in our previous study,48 UHR-ESI-MS(−) showed that the mixed halide clusters [Fe4S4X2Y2]2− scramble their halide ligands leading to coordinative disproportionation in solution yielding all possible species [Fe4S4X4−nYn]2− (X/Y = Cl, Br, and I) (see the Experimental data in the ESI). The elemental analyses of the crystallised products clearly show a 2[thin space (1/6-em)]:[thin space (1/6-em)]2 ratio for X[thin space (1/6-em)]:[thin space (1/6-em)]Y. Of course this does not rule out contributions from the 3[thin space (1/6-em)]:[thin space (1/6-em)]1 and 1[thin space (1/6-em)]:[thin space (1/6-em)]3 cluster species, but then both must contribute with the same amount. The same is true for the 4[thin space (1/6-em)]:[thin space (1/6-em)]0 and 0[thin space (1/6-em)]:[thin space (1/6-em)]4 species. Using methods such as resonance Raman, single crystal XRD, PXRD, magnetic measurements and Mössbauer spectroscopy we not only tried to study the local cluster symmetry but also tried to trace other species than the 2[thin space (1/6-em)]:[thin space (1/6-em)]2 complexes [Fe4S4X2Y2]2−.

Resonance Raman spectroscopy

The resonance Raman spectra of (BTMA)2[Fe4S4Br4] (1), (BTMA)2[Fe4S4Br2Cl2] (2), (BTMA)2[Fe4S4Cl2I2] (3), and (BTMA)2[Fe4S4Br2I2] (4) were recorded at 458 and 514 nm excitation wavelengths (Fig S1, ESI). Four bands can be distinguished in the NIR range from 180 to 580 cm−1. Unfortunately, these bands are quite invariant on the first view which is expectable, as their main contributions come from the Fe–S bridging modes.46–48,92–94 This is in line with our previous FIR studies and confirms that the absorption bands observed around 500 nm (ε of about 2000 cm−1 M−1) in the UV-vis-NIR absorption spectra in solution have their origin essentially in the Fe4S4 cubane (S → Fe LMCT).48 Unfortunately, direct irradiation of the X → Fe LMCT band at around 700 nm (ε ∼ 1500 cm−1 M−1) was technically not feasible. Nevertheless, two trends were observed. The introduction of the light Cl led to a slight high-energy shift of all bands and decreased symmetry led to detectable wavelength dependence for all four bands. In homoleptic compound 1, only the bands at about 500 and 400 cm−1 vary with the excitation wavelength, while the bands at 283 and 219 cm−1 are completely invariant. The bands which were most sensitive to variation in the halides and wavelength were ranging from 285 to 276 cm−1 and were assigned to the Tb1 mode in Td symmetry while the resonances from 404 to 398 cm−1 probably fit to Tb2 mode in Td symmetry.46,47,93,94 These modes indicate already a distortion below D2d.46 The intense bands at 220 to 216 cm−1 have clearly their origin in the symmetry reduction lower than D2d for the homoleptic cluster in 1 or lower than C2v for the heteroleptic derivatives 2, 3, and 4.

X-ray crystal structures

Single crystals of high quality were obtained for 1 to 4 and (BTMA)3[Fe6S6Cl6] (6) directly from the synthesis procedure or were obtained through the slow diffusion of diethyl ether into saturated MeCN solutions. The crystal structures of 1 to 4 were solved and refined in the monoclinic space group Cc, while that of 5 was found to crystallise in the triclinic space group P[1 with combining macron] (details in Fig. S2 to S6 and Tables S1–S9 in the ESI). The Fe4S4 compounds are thus isostructural to (BTMA)2[Fe4S4I4] (5),40 regardless if they are pure clusters like 1 or mixed halide species like 2, 3, and 4.

Like in (BTMA)2[Fe4S4I4] (5),40 the iron sulphur clusters in (BTMA)2[Fe4S4Br4] (1) are arranged in planes at the [110] direction packed with the BTMA+ cation in between. The planes are rotated by 90° around the c axis direction (Fig. 1).


image file: d2dt03203a-f1.tif
Fig. 1 Crystal structure of (BTMA)2[Fe4S4Br4] (1) viewed along the [110] direction (a) and rotated by 90° around the crystallographic c axis to show segregated columns of anions and cations (b). The bottom inset shows atom colouring: H atoms are omitted for clarity.

The intermolecular C–H⋯S and C–H⋯Br hydrogen bonds with the CH3 and CH2 groups of the BTMA+ cations (Fig. 2) exceed 3 Å than the H⋯X distance and are thus weak.95 Br⋯Br interactions between the cluster ions form a chain-like infinite motif along the [110] direction (Fig. 1 and 2) occurring between Br3 and Br4 are 3.814(2) Å long and thus slightly exceed the sum of the van der Waals radii.96,97


image file: d2dt03203a-f2.tif
Fig. 2 Intermolecular C–H⋯S, C–H⋯Br, and Br⋯Br interactions in (BTMA)2[Fe4S4Br4] (1); distances are in Å and angles are in °.

The idealised Td symmetry of the cubane-shaped [Fe4S4Br4]2− dianion is significantly decreased. The Fe4 cluster core is compressed along the Fe3⋯Fe4 edge with an extraordinarily short distance of 2.717(2) Å (Tables S2, S5 and S6). This is 0.048 Å shorter than the average of the other Fe⋯Fe distances (2.765 Å). The Fe–S bond lengths vary from 2.249(2) to 2.305(2) Å showing four markedly decreased values, one for each Fe atom (Table S2). The analysis of the Br–Fe–Fe and S–Fe–Br angles revealing the asymmetrical distortion of the clusters is connected to the intermolecular Br⋯Br interactions between Br3 and Br4 (Fig. 2). The Br3–Fe3–Fe4 (138.53(10)°) and the Br4–Fe4–Fe3 (138.03(10)°) angles are smaller than all other Br–Fe–Fe angles (between 140.06(10)° and 151.03(10)°), whereas the opposite S4–Fe3–Br3 (119.66(12)°) and S2–Fe4–Br4 (120.03(12)°) angles are larger compared with 108.30(11) to 118.87(12)°.

The lengths of the other two terminal Fe–Br bonds are not affected by the compression and the values between 2.332(2) and 2.354(2) Å coincide with those observed for the Ph4P+ structure.37 We thus suspect that the Br⋯Br interactions are rather repulsive in the packing. In contrast to this, the I⋯I interactions in (BTMA)2[Fe4S4I4] (5) seem to be favorable as the I⋯I distance of 3.917 Å is below the sum of the van der Waals radii (4.3 Å).40 In contrast to this, in the PPh4+ derivative, the I⋯I distances are longer which contradicts the idea that the I⋯I interactions might govern the cluster distortion.40

For the mixed-cluster compounds, crystalline material was obtained from (BTMA)2[Fe4S4Br2Cl2] (2), (BTMA)2[Fe4S4Cl2I2] (3), and (BTMA)2[Fe4S4Br2I2] (4) and single crystal XRD data were collected at 293 K and 173 K. All three compounds crystallise isostructural (Cc) to the homoleptic 1 and 5[thin space (1/6-em)]40 and no phase transition was observed on going from 293 to 173 K. The heteroleptic clusters [Fe4S4X2Y2]2− show mixed occupancies of both halides on each position as reported before for (Ph4P)2[Fe4S4Br2Cl2],37 and we refined the data of the compounds 2, 3, and 4 using the SUMP command (details and data in the ESI).

In our previous study, we found that in solution and therefore also during synthesis, all possible cubane species [Fe4S4X4−xYx]2− (x = 0 to 4) are present with [Fe4S4X2Y2]2− as the dominating one.48 Since the here reported materials were obtained through slow crystallisation by ether diffusion at −18 °C in the presence of BTMA+ cations, slight deviations of x from the ideal value of 2 in mixed halide clusters [Fe4S4XxYy]2− might be expected.37 But only when we were freely refining the data recorded at 173 K for compound 3 (Cl[thin space (1/6-em)]:[thin space (1/6-em)]I), we obtained a halide ratio of 1.76[thin space (1/6-em)]:[thin space (1/6-em)]2.24 deviating from the ideal 2[thin space (1/6-em)]:[thin space (1/6-em)]2 (Table S10). Unfortunately, the quality of the data did not allow satisfying refinement. The same problems were reported for the structure solution and refinement of (Ph4P)2[Fe4S4Br2Cl2].37

In line with earlier assumptions, we believe that the cluster composition and thus the cluster symmetry have no effect on the packing and the clusters arrange randomly in the structures.

A comparison of the unit cell volumes of the mixed-ligand compounds (BTMA)2[Fe4S4X2Y2] (2 to 4) with those of the homoleptic derivatives 1 and 5 and the calculated cluster volumes using the so-called Solvent Excluded Volume,98 including the Cl4 cluster derivative (Fig. 3 and Table 2), showed that by exchanging smaller halides with larger ones, the cell volumes and the cluster volumes increase along the series Cl4 < Cl2Br2 < Br4 < Cl2I2 < Br2I2 < I4 in line with the increasing ionic radii99 (Cl: 1.81 Å, Br: 1.96 Å, I: 2.20 Å).


image file: d2dt03203a-f3.tif
Fig. 3 Comparison of unit cell and the calculated cluster volumes (A) of [Fe4S4X4−xYx]2− (x = 0 or 2). The calculated so-called solvent excluded volumes (VSE) (B).98 Note that the values for 1 to 4 are from data at 173 K, while 5 and “Cl4” are from 298 K data.
Table 2 Cell volumes and calculated cluster volumes of [Fe4S4X4−xYx]2− clusters (x = 0 or 2)a
  Cell volume 298 Kb Cell volume 173 Kb Cluster volume 298 Kc Cluster volume 173 Kc
a Values in Å3. b From single crystal structures of (BTMA) salts. c Calculated solvent excluded volume (VSE).98 d Data not available for BTMA salts; values for other salts are not comparable. e Derived from the (Et4N)2[Fe4S4Cl4] structure determined at 213 K.36,42 f From ref. 49, at 150 K. g BzEt3N+ salt at 150 K, from ref. 100. h From ref. 40. i BzEt3N+ salt at 173 K, from ref. 101.
Cl4 d d 221.71e 221.78f
Cl2Br2 3269.0(2) 3201.6(1) 242.51 232.65
Br4 3340.8(4) d 244.35 244.38g
Cl2I2 3389.6(1) 3323.3(2) 325.62 277.02
Br2I2 3440.5(2) 3342.6(1) 326.00 286.03
I4 3514.8(2)h d 328.66h 328.94i


This underlines that potential contributions from cluster species with 1[thin space (1/6-em)]:[thin space (1/6-em)]3 and 3[thin space (1/6-em)]:[thin space (1/6-em)]1 ratios cannot be detected if they are present in equal amounts or make only minor contributions. So, we cannot exclude mixed-clusters with 3[thin space (1/6-em)]:[thin space (1/6-em)]1 and 1[thin space (1/6-em)]:[thin space (1/6-em)]3 halide ratios to contribute to the mixed-halide cluster compounds, but we have also no evidence for them.

Due to this, the Fe–X bond distances of the mixed-halide clusters do not allow a straightforward comparison with literature data from (BTMA)2[Fe4S4I4] (5),40 (Ph4P)2[Fe4S4I4],40 (Ph4P)2[Fe4S4X4] (X = Br, Cl),37 (Et4N)2[Fe4S4Cl4],36,42 (nPr4N)2[Fe4S4Cl4]41 (Table S7).

The Fe4S4 cluster cores in the mixed-halide compounds 2 to 4 showed that the characteristic distortion of the [Fe4S4] core is lower than the D2d symmetry (Fig. S5 and S6). For all three clusters, a significant shortening of the Fe3⋯Fe4 distance (Tables S5 and S6) and distortion of the X3/4–Fe3/4–Fe4/3 angles and their opposite S–Fe3/4–Br3/4 angles (Table S2) were found. As for the homoleptic compound 1, the intermolecular halide⋯halide interactions (Table S5) were observed parallel to the compressions of the cluster core along the Fe3⋯Fe4 axis, while at the same time the Fe1⋯Fe2 axis is slightly elongated (Tables S5 and S6). For interactions with iodide contribution (i.e. I⋯I, Br⋯I and Cl⋯I), the observed distances are always below the sum of the van der Waal radii pointing to an attractive interaction whereas the Br⋯Br distances are larger as in the homoleptic 1. Br⋯Cl and Cl⋯Cl interactions are not observed.

On the other hand, in [Fe4S4X4]2− (X = Br or I) derivatives containing R4N+ and Ph4P+ cations,9,10,29,30,40,56,69 both Br⋯Br and I⋯I interactions are longer than in the BTMA+ structures by least 0.24 Å (Br) and 0.45 Å (I), respectively (Tables S6 and S7). The halide⋯halide distances in the homoleptic compounds 1 and 5[thin space (1/6-em)]40 are also shorter compared with the published Ph4P+ structures.37,40 This is also in line with the higher density of the BTMA+ structures (Tables S5 and S6). As a consequence of these shorter halide⋯halide interactions, the BTMA+ structures show markedly higher Fe⋯Fe distortions for both the homoleptic and the mixed-halide clusters. When taking the difference between the largest and the smallest Fe⋯Fe distances, the BTMA+ structures lie at around 0.05 Å (Table S6), while the Ph4P+ derivatives show values around 0.02 Å (Table S7). Within the BTMA+ structures, the homoleptic Br4 and I4 show the largest deviations and both these two homoleptic structures and the three mixed-halide structures were found to have the same distortion pattern: two shortened and four extended Fe⋯Fe distances with overall very low cluster symmetry and very similar cluster shapes (Fig. S5 and S6).

Attempts to crystallise [Fe4S4Cl4]2− with BTMA+ by the same method led to the formation of a prismane structure (BTMA)3[Fe6S6Cl6] (6) which crystallised in the triclinic P[1 with combining macron] space group. The prismane cluster [Fe6S6Cl6]3− is positioned on the inversion centre and is surrounded by three BTMA+ cations (Fig. S2). Based on this structural feature, the asymmetric unit is represented by two halves of the clusters and three counter ions which can be combined to get two units of 6 per unit cell (Fig. S3). Pronounced intermolecular H bonding interactions are absent (Fig. S4). The prismane structure can be described as two alternating layered [Fe3S3] rings in a chair conformation. Its D3d symmetry is only slightly perturbed. The Fe–Cl and Fe⋯Fe distances are found to be very similar (Table S9) and comparable to bromide and chloride prismanes with Et4N+.87–91 The Cl–Fe–S angles ranging from 108.26(4)° to 115.96(5)° are quite close to an ideal tetrahedral angle. The bulk material of 6 is not completely phase pure but does definitely not contain the (BTMA)2[Fe4S4Cl4] phase isostructural to (BTMA)2[Fe4S4Br4] (1) (see Powder XRD). The [Fe4S4Cl4]2− cluster was previously crystallised with a number of cations, such as Et4N+,36,42nPr4N+,41 or Ph4P+.37 We were unable to detect even traces of (BTMA)2[Fe4S4Cl4] (see powder XRD, Mössbauer, or magnetic measurement). At the moment, we have no explanation other than a superior crystallisation of prismane 6.

Powder XRD studies

In previous investigations,48 we have already shown that the Fe4S4 cluster compounds (1 to 5) are obtained as homogeneous materials with no signs of phase separation. Very weak additional reflections point to some minor impurities of unknown identity. No significant differences between the crystal structures obtained at 170 K and RT are visible excluding the possible phase transitions upon cooling. The diffraction pattern of the Fe6S6 prismane (Fig. S7) shows that 6 is only a minority phase in the reaction product. It was speculated that the main product could be the still unknown chloride (BTMA)2[Fe4S4Cl4]. The simulated diffraction pattern of (BTMA)2[Fe4S4Br4] (1) is provided for comparison. Assuming that 1 and the unknown chloride crystallise in the isotypic crystal structures, the pattern of the chloride should be shifted to higher 2θ values thus completely excluding this assumption. However, it should be noted that the formation of the chloride (BTMA)2[Fe4S4Cl4] crystallising in another crystal structure cannot completely be ruled out at this point. In addition, another modification of (BTMA)3[Fe6S6Cl6] can possibly make up the majority of the studied material.

57Fe Mössbauer spectroscopy

For (BTMA)2[Fe4S4Br4] (1) a symmetric doublet with an isomer shift δ = 0.48 mm s−1 was recorded (Fig. S8A). Analysis by fitting using a Lorentzian line shape also reveals a quadrupole splitting ΔEQ of 1.77 mm s−1 and a line width at half maximum Γ of 0.42 mm s−1 (Table 3).
Table 3 Mössbauer parameters at 77 K of the complexes obtained from spectra fitting analysisa
  n δ ΔEQ Γ Fe
a Contribution n/%, isomer shift δ/mm s−1, quadrupole splitting ΔEQ/mm s−1, line width at half maximum Γ/mm s−1. Iron oxidation state Fe. b Two species simulated in a 50[thin space (1/6-em)]:[thin space (1/6-em)]50 ratio. c Minor species of 11% (see text and Fig. S8†).
1 100 0.48 ± 0.01 1.07 ± 0.02 0.42 ± 0.01 2.5+
2 100 0.48 ± 0.01 1.07 ± 0.03 0.35 ± 0.01 2.5+
3 A: 50 0.48 ± 0.02 1.16 ± 0.02 0.37 ± 0.02 2.5+
B: 50 0.46 ± 0.01 0.80 ± 0.01 0.47 ± 0.02 2.5+
4 100 0.48 ± 0.01 1.05 ± 0.02 0.35 ± 0.01 2.5+
5 89 0.48 ± 0.02 1.02 ± 0.02 0.35 ± 0.01 2.5+
6 A: 96 0.48 ± 0.02 0.88 ± 0.02 0.35 ± 0.01 2.5+


An isomer shift of 0.48 mm s−1 is in keeping with the previously reported [Fe4S4Br4]2− clusters with different counter ions.86,87 It is also in the range observed for [4Fe–4S]2+ centres occurring in 4Fe–4S proteins. These clusters exhibit complete delocalisation of one electron in the Fe2+–Fe3+ spin pairs. The ferromagnetic double exchange between the iron spins formally leads to Fe2.5+–Fe2.5+ pairs with the pair spin Sp = 9/2.6 In turn, antiparallel (antiferromagnetic) coupling of the pair spins leads to the clusters’ diamagnetic ground state.6,71,75,102,103 The Mössbauer spectrum of 5 was fitted alongside a singlet signal (11%) which can be assigned to monomeric (NEt4)[FeI4]89 (Fig. S8B). Besides this, the behaviour is the same as for 1. Correspondingly, all Fe atoms in the homoleptic 1 and 5 (Fig. S8) have an oxidation state of 2.5+.

For both heteroleptic compounds (BTMA)2[Fe4S4Br2Cl2] (2) and (BTMA)2[Fe4S4Br2I2] (4), symmetric Lorentzian-type doublets with δ values of 0.48 mm s−1 were recorded (Fig. S9). Thus, the average oxidation state of iron in both mixed-halide-clusters is 2.5+ with complete delocalisation.

In contrast, (BTMA)2[Fe4S4Cl2I2] (3) exhibited a slightly asymmetric Mössbauer doublet whose analysis reveals the equal contributions of the two species (Fig. 4).


image file: d2dt03203a-f4.tif
Fig. 4 57Fe Mössbauer spectrum of (BTMA)2[Fe4S4Cl2I2] (3) at 77 K (dots: collected data, parameters for the simulated species A and B in Table 3).

Species A causes a doublet with δ1 = 0.48 mm s−1 and ΔEQ1 = 1.16 mm s−1. Species B causes an isomer shift with δ2 = 0.46 mm s−1 which is within the experimental error identical to that of species B. Thus, 3 also has all irons in the 2.5+ oxidation state (Table 3). The different values of ΔEQ may be a result of the large differences in the EN of Cl and I.

It should be noted that in contrast to the analysis with two “nested” doublets presented above, the Mössbauer spectrum of 3 can alternatively be fitted with two “crossed” doublets (Fig. S11). This leads to δA = 0.54 and δB = 0.37 mm s−1 which may indicate electron localisation. However, high spin Fe3+ and Fe2+ sites have significantly different ΔEQ values, and the analysis with two “crossed” doublets shows almost identical ΔEQ values of 1.0 mm s−1 (Fig. S11) which contradicts electron localisation. Therefore, we conclude that indeed 3 has all irons in the 2.5+ state.

Such a situation has also been observed for [Fe4S4X2Y2]2− clusters with monodentate ligands X, Y = Cl, OPh and SPh show δ values between 0.46 and 0.50 mm s−1 for both coordination sites.56

For (Ph4P)2[Fe4S4Br4], δ = 0.488 mm s−1 and ΔEQ = 0.662 mm s−1 were reported at 77 K.37 A crude measurement of the mixed-halide compound (Ph4P)2[Fe4S4Br2Cl2] was reported with a shift of about 0.00 mm s−1 and significant line-broadening37 similar to that observed for 3.

The high similarity of all clusters in the BMTA+ containing materials in their Mössbauer spectra is in line with the very similar distortion of the clusters from Td to very low symmetry (see the XRD section).

For (BTMA)3[Fe6S6Cl6] (6), an isomer shift of 0.48 mm s−1 was recorded and the oxidation state of each iron inside the prismane core can be assigned to 2.5+. Like the previous prismane cluster, the core of 6 consists of three Fe2+–Fe3+ spin pairs coupling ferromagnetically and resulting in Fe+2.5 on average.86 The observed ΔEQ of 0.88 mm s−1 is smaller than those reported for other prismanes,12,38,85–90,102–108 but overall, the isomer shift and the quadrupole splitting for 6 are very similar to those of the (Et4N)3[Fe6S6Cl6] prismane with δ = 0.495 mm s−1 and ΔEQ = 1.085 mm s−1 at 125 K.86

For fitting the collected data, a second species with a contribution of 4% was necessary. This second doublet was fitted with δ = 1.40, ΔEQ = 1.01, and Γ = 0.40 mm s−1 and is due to the FeCl2 impurities from the starting material (Fig. S10). For (Ph4P)2[Fe4S4Cl4], a δ value of 0.49 mm s−1 was reported at 77 K with a ΔEQ of 0.67 mm s−1.56,86 Although PXRD showed that the crystal structure of (BTMA)3[Fe6S6Cl6] is only the minor phase in the isolated material, the Mössbauer results provide good evidence that the major component of the material contains the [Fe6S6Cl6]3− prismanes. At the same time, the Mössbauer results do not completely rule out the occurrence of the compound (BTMA)4[Fe4S4Cl4] as the reported data for (Ph4P)2[Fe4S4Cl4] is not very different from our data for 6.

Magnetic measurements

The molar magnetic susceptibilities of all four compounds are shown in Fig. 5 as χMT vs. T plots. All compounds show quite similar magnetic behaviour (Fig. 4 and Fig. S12–S14, ESI).
image file: d2dt03203a-f5.tif
Fig. 5 Temperature dependence of the molar magnetic susceptibility depicted as χMT vs. T of the homoleptic cluster (BTMA)2[Fe4S4Br4] (1, black) and the heteroleptic clusters (BTMA)2[Fe4S4Br2Cl2] (2, green), (BTMA)2[Fe4S4Cl2I2] (3, red), and (BTMA)2[Fe4S4Br2I2] (4, blue).

At 298 K, the observed χMT values are 1.17 cm3 K mol−1, 1.41 cm3 K mol−1, 1.54 cm3 K mol−1 and 1.12 cm3 K mol−1 for 1 to 4, respectively, far below the expected spin-only value of 14.75 cm3 K mol−1 for four uncoupled spins with S1 = S2 = 4/2 (ferrous) and S3 = S4 = 5/2 (ferric).

Upon lowering the temperature, a steady decrease in the χMT-products is observed for all compounds, indicating strong dominant antiferromagnetic interactions between the iron ions. Further cooling below 100 K initially leads to no significant change in the values. However, upon cooling only below 20 K, the χMT values decrease significantly.

For cluster systems with four antiferromagnetically coupled spin centres, an HDvV (Heisenberg–Dirac–van Vleck) spin Hamiltonian, image file: d2dt03203a-t1.tif, with one single exchange interaction parameter J leads to a highly degenerated spin ground state. However, this assumption is incompatible with the results of the Mössbauer investigations and especially with the observed temperature-dependent magnetic moments for the four compounds (Fig. 5).

The application of a full HDdV with two exchange interactions, which includes a stronger antiferromagnetic exchange interaction J1, within the two Fe2+–Fe3+ dimers, and an exchange interaction, J2, accounting for the inter-dimer interaction, allows quite satisfactory simulations for the curves for all four compounds (Fig. 5 and Table 4).

Table 4 Fitting parameter obtained by applying HDvV and HDEa
  HDvV HDE
J 1/cm−1 J 2/cm−1 g J/cm−1 B/cm−1 g
a HDvV = Heisenberg–Dirac–van Vleck; HDE = Heisenberg double exchange.
1 −129 (3) −2.6 (3) 1.775 (16) −257 (10) 462.2 (5) 1.66 (5)
2 −135 (7) −3.2 (3) 2.09 (5) −256 (10) 462.5 (5) 1.91 (6)
3 −115 (2) −2.0 (2) 1.853 (11) −274 (13) 554.6 (9) 1.74 (8)
4 −132 (1) −2.8 (2) 1.759 (6) −291 (13) 552.1 (7) 1.65 (6)


However, in three of the four fits, a g value considerably below 2.0 was obtained. For the Fe4S4 cluster, this phenomenon is well-known6 and assigned to the presence of spin-dependent delocalisation which is related to the double exchange theory devolved by Anderson and Hasewaga.109 They have shown that resonance delocalisation in a mixed-valence dimer leads to additional splitting ±B(S + 1/2) of the energy-split spin ladder JS(S + 1) for the exchange coupled spin system.

To estimate the double exchange parameter B in the present Fe4S4 tetramers, we followed the approach recently reported by Henthorn, Cutsail et al.110 and applied the solved Bleaney–Bowers equation for dimers of high spin d5–d6 mixed-valence centres (Table 4, eqn (S1), (S2), and Fig. S14). Note that the data at low temperatures were not taken into account for fitting to E = −JS(S + 1) ± B(S + 1/2). This is in accordance with the literature and the interested reader is referred to the publication and the references therein for further information.110 The values obtained for B are consistent with the reported values6,111–113 in all cases and clearly show the presence of electron delocalisation, which is also evident from the Mössbauer data, showing only a single doublet assigned to a mixed valence state. Most importantly, the ratio |B/J| is relatively small, so the spin ground state of S = 1/2 can be taken as given for the dimers. This justifies the application of the HDdV formalism, which in analogy leads to an S = 1/2 ground state for the antiferromagnetically coupled Fe2+–Fe3+ dimers. Furthermore, with the HDvV-formalism, we are now able to take the inter-dimer interaction into account. Although the values for J2 are almost two orders of magnitude smaller than J1, the resulting final ground states for the Fe4S42+-clusters are diamagnetic with S = 0, which matches perfectly with the decrease in the χMT values at low temperature.

In the temperature-dependent magnetisation measurements for (Et4N)2[Fe4S4Cl4], the magnetic moment μ increased from 0.45 to 2.48 μB upon increasing the temperature from 50 to 338 K,36 which is in good agreement with the behaviour in our compounds.

Conclusions

The iron sulphur halide clusters [Fe4S4Br4]2− and [Fe4S4X2Y2]2− (X, Y = Cl, Br, I) were synthesised in excellent yields and purity on a gram scale using a recently refined method starting from [Fe(CO)5], elemental sulphur, I2 and benzyltrimethylammonium (BTMA+) iodide, bromide and chloride. Through crystallisation, we obtained materials showing a clear 1[thin space (1/6-em)]:[thin space (1/6-em)]1 ratio of X[thin space (1/6-em)]:[thin space (1/6-em)]Y for the mixed-halide cluster compounds. Single crystals of (BTMA)2[Fe4S4Br4] (1), (BTMA)2[Fe4S4Br2Cl2] (2), (BTMA)2[Fe4S4Cl2I2] (3) and (BTMA)2[Fe4S4Br2I2] (4) were isostructural to the previously reported (BTMA)2[Fe4S4I4] (5) (monoclinic, Cc). In the same way, we tried to synthesise the previously unknown chloride cubane cluster compound (BTMA)2[Fe4S4Cl4]. But we obtained the prismane-shaped cluster (BTMA)3[Fe6S6Cl6] (6) (P[1 with combining macron]) instead.

No phase transitions were observed for the mixed-halide cluster containing materials from powder XRD at 298 or 170 K or single crystal XRD at 293 and 173 K. Although in solution four different clusters [Fe4S4X4−xYx]2− (x = 0 to 4) might occur for the mixed-halide compounds, the structure refinements gave no indication for the X[thin space (1/6-em)]:[thin space (1/6-em)]Y ratio deviating from 2[thin space (1/6-em)]:[thin space (1/6-em)]2. Also, a closer look on the cluster volumes (solvent excluded volumes, the unit cell volumes and the cluster distortions) gave evidence for other species than X2Y2. On the other hand, our XRD data do not rule out X1Y3 species if they were present in the same amount as X3Y1 species and all cluster species arrange randomly in the structures.

However, Mössbauer spectroscopy rules out larger amounts of such X3Y1 species. For the homoleptic 1 and 5 and the mixed-halide clusters 2, 3, and 4, Lorentzian-type doublets were observed in 57Fe Mössbauer spectroscopy in line with a complete delocalisation with Fe2.5+ oxidation states for all iron atoms.

Magnetic measurements for 1 to 4 showed small χMT values at 298 K ranging from 1.12 to 1.54 cm3 K mol−1. From about 20 K, the values drop significantly. Fitting the data in the Heisenberg–Dirac–van Vleck (HDvV) formalism gave g values markedly below 2 and confirmed the complete delocalisation and antiferromagnetic coupling assumed from Mössbauer spectroscopy.

Thus, the herein described synthesis is reliable and robust and allows the production of mixed-halide clusters [Fe4S4X2Y2]2− (X, Y = Cl, Br, I) on a gram scale and with high purity. In particular, the [Fe4S4Cl2I2]2− derivative might be interesting for further studies as the very different halide ligands might allow the selective substitution of two halide ligands to form new mixed ligand [Fe4S4] clusters.

Experimental section

Materials

The compounds were prepared and crystallised following already published procedures48 by diffusion techniques from layered MeCN/diethyl ether mixtures at −18 °C over a period of 3 weeks. After filtration, washing with diethyl ether and drying under anaerobic and dry atmosphere (argon) using Schlenck techniques the opaque black rod shape crystals were stored and prepared for analyses in an argon-filled glove box at room temperature. The details and analytics are available in the ESI.

Instrumentation

The resonance Raman spectra were recorded either with 457 nm or 514 nm excitation (13 mW, Coherent Innova 70c) in backscattering geometry using a confocal microscope coupled to a single stage spectrograph (Jobin Yvon XY 800) equipped with a CCD detector.

For (BTMA)2[Fe4S4Br4] (1), (BTMA)2[Fe4S4Br2Cl2] (2), (BTMA)2[Fe4S4Br2I2] (4) and (BTMA)3[Fe6S6Cl6] (6), single crystal data collection at 293(2) K was performed using STOE-IPDS (I, II or IIT) diffractometers using graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The samples have been measured in capillaries sealed under an argon atmosphere (0.2 mm diameter). The structures were solved by direct methods using the WinGX114 package and/or Olex2.115 For some of them, first structure proposals were obtained with Sir2014.116 Model refinement was carried out with SHELXL2018/1117,118 by employing full-matrix least-squares methods on F2. All non-hydrogen atoms were treated anisotropic and the hydrogen atoms were included by using appropriate riding models. Numeric absorption correction was performed using X-Red119 and X-Shape120 packages. CCDC Deposition Numbers are 2048374 for (BTMA)2[Fe4S4Br4] (1), 2048953 for (BTMA)2[Fe4S4Br2Cl2] (2), 2055695 for (BTMA)2[Fe4S4Br2I2] (4), and 2048377 for (BTMA)3[Fe6S6Cl6] (6), respectively.

For (BTMA)2[Fe4S4Br2Cl2] (2), (BTMA)2[Fe4S4Cl2I2] (3) and (BTMA)2[Fe4S4Br2I2] (4), LT single crystal data were collected at 173(2) K and for (BTMA)2[Fe4S4Cl2I2] (3) at 293(2) K using an Oxford Diffraction Xcalibur Gemini Eos diffractometer using graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The crystals were transferred under argon to Paratone oil and mounted on a nylon loop. For cooling, cold dry air was provided by an Oxford Cryosystems Desktop Cooler. CrysAlis Pro,121 from Oxford Diffraction, was used to collect the initial frames for determination of the unit cell. Subsequently, the program was used to plan a data collection. In most cases, a full sphere of data were collected. After collection, data reduction was carried out with the Crysalis Pro suite and multiscan absorption correction was carried out (SCALE3 ABSPACK scaling algorithm).122 The space group selection was made using the inbuilt program, GRAL, which selected the monoclinic space group Cc for all structures. The output files were used for initial structure solution using SHELXle123 or Olex2.115 Subsequently, the structures were refined using SHELXL 2018/1.117,118 The mixed-halide clusters showed signs of disorder and this was modelled at first by allowing the occupancies for the halide ligands to refine freely. In the final refinement, the occupancies of the disordered pairs of halides, i.e., bromide and chloride, iodide and chloride, or bromide and iodide, were restrained using the SUMP command (SHELXL).117,118 In all cases, this strategy resulted to be more adequate to treat the data and obtain better quality refinement indicators and realistic Fe–X and X⋯Y distances. The CCDC Deposition Numbers are 2048376, 2048378, 2048375 (173 K) and 2048954 (293 K) for (BTMA)2[Fe4S4Br2Cl2] (2), (BTMA)2[Fe4S4Cl2I2] (3), and (BTMA)2[Fe4S4Br2I2] (4), respectively.

The selected bond lengths and angles of the compounds were calculated using the PLATON software.124 The graphical representations were done by Mercury125 and PyMOL,126 and the editing of CIF files was performed using the PublCif127 software.

Synchrotron X-ray powder diffraction data collection was performed at DESY (Hamburg, beamline P02.1, storage ring Petra III with λ = 0.207203 Å and a PerkinElmer XRD 1621 flat panel detector). The samples were measured in capillaries sealed under an argon atmosphere (0.5 mm diameter). The collected data was transformed with Fit2d128 and analysed with WinXPow.129

The 57Fe Mössbauer data were recorded in the transmission geometry using a constant acceleration spectrometer operated in conjunction with a 512-channel analyser in the time-scale mode (WissEl GmbH) using Wissoft 2003. The detector consisted of a proportional counter and the source contained 57Co diffused in Rh with an activity of 1.4 GBq. The spectrometer was calibrated against α-iron at room temperature. Sample cooling to 77 K was achieved by placing the samples in a continuous flow cryostat from Oxford Instruments. The spectral data were transferred from the multi-channel analyser to a PC for further analysis employing the public domain program Vinda130 running on the Microsoft Excel 2003® platform. The spectra were analysed by least-squares fits using Lorentzian line shapes.

The magnetic measurements were performed with a Quantum Design MPMS-XL-7 SQUID magnetometer using powdered microcrystalline samples. The variable temperature susceptibility data were collected in the temperature range of 2 to 300 K under an applied field of 0.1 Tesla. The experimental susceptibility data were corrected for the temperature-dependent magnetic contribution of the holder and the underlying diamagnetism using Pascal's constants.

Author contributions

AOS: investigation, data curation, visualisation, writing – original draft, and funding acquisition. BMR: investigation, data curation, and visualisation. FDS: data curation, visualisation, and supervision. LC: investigation, data curation, validation, and visualisation. VKG: investigation, data curation, and visualisation. FD: methodology, validation, project administration, supervision, and funding acquisition. ERE: methodology, validation, and supervision. VS: methodology, validation, and supervision. UR: methodology, validation, project administration, supervision, funding acquisition, and writing – review and editing. AK: methodology, validation, project administration, supervision, and writing – review and editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors are grateful to Peter Kliesen, Malgorzata Smolarek and Silke Kremer (Department of Chemistry, University of Cologne) for single crystal X-ray diffraction, IR spectroscopy and microanalysis data collection. We are indebted to HASYLAB at DESY (Hamburg, Germany) for the kind support with synchrotron radiation. AOS acknowledges the German Academic Exchange Programme (DAAD) for traveling grants (EuroBIC 10 Thessaloniki, Greece & EuroBIC 11 Granada, Spain) and CONICET for financial support (Código de Beca: 105 201301 00912 CO).

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Footnote

Electronic supplementary information (ESI) available. CCDC 2048374, 2048953, 2055695, 2048377, 2048376, 2048378, 2048375 and 2048954. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2dt03203a

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