[SnI8 {Fe(CO)4 }4 ]2+ : Highly Coordinated Sn+II I8 Subunit with Fragile Carbonyl Clips.

[SnI8 {Fe(CO)4 }4 ][Al2 Cl7 ]2 contains the [SnI8 {Fe(CO)4 }4 ]2+ cation with an unprecedented highly coordinated, bicapped SnI8 prism. Given the eightfold coordination with the most voluminous stable halide, it is all the more surprising that this SnI8 arrangement is surrounded only by fragile Fe(CO)4 groups in a clip-like fashion. Inspite of a predominantly ionic bonding situation in [SnI8 {Fe(CO)4 }4 ]2+ , the I- ⋅⋅⋅I- distances are considerably shortened (down to 371 pm) and significantly less than the van der Waals distance (420 pm). The title compound is characterized by single-crystal structure analysis, spectroscopic methods (EDXS, FTIR, Raman, UV/Vis, Mössbauer), thermogravimetry, and density functional theory methods.

High coordination numbers (CNs) are preferably realized at high pressure and with small ligands.1 In the solid state, moreover, high CNs are preferred in densely packed structures (e.g. perovskites) and supported by high lattice energies. For cationic tin (Sn2+, Sn4+), flexible coordination is possible and most often refers to tetrahedral and octahedral arrangements.2 To date, higher CNs are known only for chelating ligands (e.g. crown ethers)3 or chelating oxoanions at high pressure (e.g. CN 10 in β‐SnB4O7 made at 7.5 GPa/1100 °C).4 In all these cases, multidentate ligands and/or high pressure are a prerequisite. For monodentate ligands, the CN of Sn2+/4+ is typically limited to six. The binary SnI2 shows (5+2) coordination with five shorter (303–326 pm) and two significantly longer Sn–I distances (373 pm).5 Even for fluorine, the smallest halide, only sixfold coordination was reported (e.g. [NH4]3[SnF6]F).6 Eightfold coordination with iodine, to the best of our knowledge, is also unknown for the significantly larger Pb2+ ion. With this background, [SnI8{Fe(CO)4}4][Al2Cl7]2 is surprising for several reasons: 1) eightfold halide coordination of Sn2+ is observed for the first time; 2) despite its size and repulsive I−I⋅⋅⋅I−I interaction, eightfold coordination occurs with the most voluminous stable halide; 3) the SnI8 building unit is surrounded only by fragile Fe(CO)4 groups in a clip‐like fashion.

[SnI8{Fe(CO)4}4][Al2Cl7]2 was prepared by heating SnI4 and Fe2(CO)9 in the ionic liquid [BMIm]Cl/AlCl3 (ratio 1:3; [BMIm]: 1‐butyl‐3‐methylimidazolium) to 130 °C in argon‐filled, sealed glass ampoules. This temperature was required to dissolve all starting materials. When the reaction mixture was slowly cooled to room temperature (1 K h−1), numerous large black crystals (up to 0.5 mm3) with a slight violet shimmer were obtained starting at about 50 °C (Figure 1; Supporting Information: Figures S1–S5). The reaction can be rationalized by reduction of Sn+IV to Sn+II and oxidation of Fe±0 to Fe+II [Eq. (1)]:

Figure 1

Decomposition of [SnI8{Fe(CO)4}4][Al2Cl7]2 single crystals after separation from the mother liquor (in perfluorinated polyalkylether under argon at 25 °C): a) Photos of a crystal taken over a period of 5 minutes (light microscopy); b) scheme of the decomposition with reaction products.

An excess of Fe2(CO)9 (i.e. SnI4/Fe2(CO)9=1:1) supports the crystal quality and leads to higher yields (≈50 %) of the title compound. Surprisingly, [SnI8{Fe(CO)4}4][Al2Cl7]2 also is the first compound in the Sn‐Fe‐I system with iodine interlinking iron and tin. This type of halide‐based linkage was previously reported only for chlorine‐bridged compounds (e.g. [Cp(CO)2FeClSnCl4]).7

X‐ray structure analysis based on single crystals revealed the title compound to crystallize in the monoclinic space group C2/c (Table S1, Figures S2 and S3) and to contain [SnI8{Fe(CO)4}4]2+ cations and [Al2Cl7]− anions.8 Beside crystal structure analysis, the chemical composition was validated by energy‐dispersive X‐ray spectroscopy (EDXS) with a Sn/I/Fe ratio of 1.3:8:4.3 (scaled on iodine as the heaviest element) which fits well with the expectation (1:8:4).

[SnI8{Fe(CO)4}4][Al2Cl7]2 turned out to be highly reactive and decomposes immediately in the presence of air and moisture. Whereas single crystals are stable under inert conditions in the ionic liquid, removal from the mother liquor even under argon causes fast decomposition (≤5 min) at room temperature (Figure 1), which was stopped only at about −50 °C. The decomposition of single crystals at room temperature can be monitored by light microscopy in perfluorinated polyalkylether (Figure 1 a). Like a glass of champagne the crystals start forming gas bubbles, which roll off after having reached a certain size. In addition to the evolution of CO, the crystals decolorize within 5 minutes from the surface to the interior (Figure 1). This decolorization from deep black to blotchy orange and white can be ascribed to the following reaction [Eq. (2)]:

The formation of AlCl3 and SnI2 is indicated by characteristic colorless needles (AlCl3) and orange crystals (SnI2). The lack of any C≡O vibrations in the FTIR spectra points to the absence of metal carbonyls (e.g., Fe(CO)5, Fe2(CO)9, [cis‐Fe(CO)4I2]) as decomposition products (Figure S1). In contrast to room‐temperature decomposition, the heating of the title compound leads to a three‐step decomposition between room temperature and 500 °C with evaporation of CO, AlCl3, I2, and SnCl2, leaving iron metal as a solid residue (Figure S6). The synthesis temperature of 40–50 °C (in the ionic liquid) and the rapid decomposition under inert conditions at 25 °C (and below) illustrate the metastability and fragile nature of the title compound.

The unusual [SnI8{Fe(CO)4}4]2+ cation is centered by Sn+II, which in turn is eightfold coordinated by iodide (Figure 2 a). The resulting SnI8 polyhedron can be described as a bicapped triangular prism (Figure 2 b,d). The Sn–I distances of 317.3(2)–330.6(3) pm in the prism (Figure 2 c) are comparable to those of SnI2 5 but significantly longer than in SnI4 (Table 1),6 which points to Sn+II and ionic bonding. The Sn–I distances of the two capping iodine atoms of the central SnI8 subunit (332.0(1), 341.2(3) pm) are slightly longer than the Sn–I distances within the prism (Figure 2 c). Even these longer Sn–I distances are nevertheless significantly shorter than the extended distances in SnI2 (373.4 pm)5 and the sum of the Sn–I van der Waals radii (411 pm).9 Taking the large volume and the repulsion of the negatively charged I−I neighbors into account, such short Sn–I distances in combination with eightfold coordination in the [SnI8{Fe(CO)4}4]2+ cation are unprecedented.

Figure 2

[SnI8{Fe(CO)4}4]2+ cation: a) Overview with crystallographic numbering of the atoms; b) side view of the bicapped triangular SnI8 prism; c) Sn–I distances (in pm); d) top view of the bicapped triangular SnI8 prism.

Table 1

Selected distances (d) in the [SnI8{Fe(CO)4}4]2+ cation according to single‐crystal structure analysis and selected references (Py: pyridine; L: (μ‐TePh)Fe(CO)2Cp).

Compound	d(Sn–I) [pm]	d(Fe–I) [pm]
[SnI8{Fe(CO)4}4]2+	317.3(2)–341.2(3)	262.6(2)–264.9(2)
SnI2 5	302.7–325.7 373.4	/
SnI4 10	269.7	/
FeI2 (solid)11	/	288.5
Fe2I4 (dimer, gas phase)12a [Fe(CO)3(Py)I2]12b	/ /	248.0, 268.3 265.5, 266.7
[Fe(CO)3(L)I2]12c	/	265.3

/

/

/

/

/

/

The I⋅⋅⋅I distances in the central SnI8 subunit cover a wide range and can be assigned to two groups. Pairs of iodine atoms are bridged by Fe(CO)4 groups in a clip‐like fashion and exhibit distances of 376.3–384.4 pm (Figure 3 a). The respective Fe–I distances (262.6(2)–264.9(2) pm) are shorter than in solid FeI2 (288.5 pm)11 and compare to those of bridging Fe–I in the Fe2I4 dimer and other carbonyldiiodoiron compounds (Table 1).12 A second group of I⋅⋅⋅I distances relates to non‐Fe(CO)4‐clipped pairs of iodine with shorter (370.6–423.8 pm, Figure 3 b) and longer distances (412.6–453.7 pm, Figure S4). Several I⋅⋅⋅I distances are significantly less than the twofold van der Waals distance of iodine (420 pm).13 Such close contacts typically indicate attractive bonding as they are known from iodine–iodide interactions (I±0⋅⋅⋅I−I) in polyiodides.13 In [SnI8{Fe(CO)4}4]2+, however, the close contacts occur between negatively charged iodide atoms (I−I⋅⋅⋅I−I). This finding can be attributed predominately to the Fe(CO)4 clips since increasing the I⋅⋅⋅I distance would cause a widening of the I‐Fe‐I angle, which, however, is more or less constrained to 90° (91.3(1)–93.4(1)°).

Figure 3

I⋅⋅⋅I distances (in pm) in the [SnI8{Fe(CO)4}4]2+ cation: a) Fe(CO)4‐clipped pairs of iodide with I‐Fe‐I angles (in °) including schematic illustration, b) non‐Fe(CO)4‐clipped pairs of iodide (for longer I⋅⋅⋅I distances see: Figure S4).

The [SnI8{Fe(CO)4}4]2+ cation could also be formally described as a Sn2+‐centered coordination complex with four uncharged [cis‐Fe(CO)4I2] ligands. However, it should be noted that no equilibrium of Sn2+ and [cis‐Fe(CO)4I2] was observed—neither for the synthesis nor for the decomposition of the title compound. [cis‐Fe(CO)4I2] as a compound has been barely studied generally and previously characterized predominately by vibrational spectroscopy,14 whereas single‐crystal structure analysis is still missing. With the viewpoint of a central Sn2+ coordinated by fragile [cis‐Fe(CO)4I2] ligands, the existence of a [SnI8{Fe(CO)4}4]2+ cation with close iodide contacts and eightfold coordination is still surprising.

The valence states of Sn+II and Fe+II, as already suggested by the chemical composition and distances, were further validated by Mössbauer spectroscopy and Fourier transform infrared spectroscopy (FTIR). 119Sn Mössbauer spectra clearly point to the presence of Sn+II (Table S2 and Figure S7). Unfortunately, 57Fe Mössbauer spectra could not be recorded due to the strong absorption of iodine (see the Supporting Information). With regards to iron, however, FTIR spectra are indicative and show two strong (2156, 2100 cm−1) and two weak (2115, 2135 cm−1) CO vibrations (Figure 4 a; Table S3 and Figure S8), which are well in agreement with references such as [cis‐Fe+II(CO)4I2] (2129, 2084, 2060 cm−1).14 In contrast, the CO vibrations of zero‐valent iron carbonyls like Fe(CO)5 (2000, 2025 cm−1)15 and Fe2(CO)9 (2084, 2034 cm−1)16 occur at lower wavenumbers. This observation can be ascribed to stronger σ‐bonding and weaker π‐backbonding, which confirms the lower electron density and the positive valence state of iron in [SnI8{Fe(CO)4}4]2+. These findings are confirmed by Raman spectroscopy (Figure 4 b; Figure S9). With the help of theoretical calculations, Raman bands close to 590, 390, 200, and 185 cm−1 can be attributed to ν(Fe‐C) and δ(Fe‐C‐O) as well as to ν sym(FeI2) and ν antisym(FeI2). Calculated Raman spectra of the [SnI8{Fe(CO)4}4]2+ cation are also in good agreement with the experimental data (Table S4 and Figure S10).

Figure 4

Vibrational spectroscopy of [SnI8{Fe(CO)4}4][Al2Cl7]2: a) FTIR spectrum (detailed view of CO vibrations as inset, crystals of the title compound embedded in ionic liquid for stabilization (Figure S8); b) FT‐Raman spectrum (Figure S9).

The structure and bonding of the [SnI8{Fe(CO)4}4]2+ cation were finally verified by quantum chemical calculations based on density functional theory (DFT). Accordingly, a square antiprism of SnI8 with D 4 symmetry turned out to be a local minimum on the potential energy surface (Table S5 and Figure S11). Distortion of the D 4‐symmetric structure, however, resulted in D 2 symmetry being 32.5 kJ mol−1 more stable than D 4 symmetry. Due to two imaginary vibrational frequencies, however, this D 2 structure represents only a saddle point on the potential energy surface, which relaxes to a subgroup with S 4 symmetry (torsion angle I1‐Sn1‐I8‐I2: 15.1°) and a “real” local energy minimum being 1.9 kJ mol−1 even more stable (HOMO–LUMO gap: 1.66 eV). Although the potential energy surface is rather flat with regards to the torsion angle, DFT methods indicate that a bicapped trigonal SnI8 prism is more stable (−34.5 kJ mol−1) than square antiprismatic coordination, which excellently reflects the experimental observation (Table 2, Figure 5; Figure S11). In comparison to a calculated [cis‐Fe(CO)4I2] molecule, the larger I⋅⋅⋅I distance and I‐Fe‐I angle in [cis‐Fe(CO)4I2] are noticeable and point to the strong influence of tin in [SnI8{Fe(CO)4}4]2+.

Table 2

Selected distances (d) and angles (∡) in the [SnI8{Fe(CO)4}4]2+ cation according to DFT calculations and experimental data (calculated [cis‐Fe(CO)4I2] molecule as a reference).

Species	d(Sn–I) [pm]	d(Fe–I) [pm]	d(I⋅⋅⋅I) [pm]	∡(I‐Fe‐I) [°]
Calculated (DFT) [SnI8{Fe(CO)4}4]2+	333.9	267.9	367.6, 383.9	91.3
[cis‐Fe(CO)4I2]	/	267.4	397.1	95.9
Experimental data (single‐crystal structure analysis)
[SnI8{Fe(CO)4}4]2+	317.3(2)–341.2(3)	262.6(2)–264.9(2)	370.6(2)–384.4(2)	91.3(1)–93.4(1)

Figure 5

Bonding in [SnI8{Fe(CO)4}4]2+ with regard to the shortest I⋅⋅⋅I contacts: a) Calculated bonding MO (DFT) indicating a slight electron transfer from nonbonding iodide electron pairs into the formally vacant valence p atomic orbital of Sn2+ (isosurface values at ±0.04; *calculated I2⋅⋅⋅I8/I3⋅⋅⋅I6 are symmetry equivalent due to S 4 symmetry); b) Experimental data (single‐crystal structure analysis).

[°]

/

With regard to the I⋅⋅⋅I distances, DFT calculations confirm the variability of distances and the presence of short I⋅⋅⋅I contacts in the S 4‐symmetric [SnI8{Fe(CO)4}4]2+ cation for the Fe(CO)4‐clipped and non‐Fe(CO)4‐clipped pairs of iodine (Table 2). To verify whether the shortest I⋅⋅⋅I distances result from attractive interactions, Bader AIM analysis was performed (AIM: atoms in molecules; see the Supporting Information). Here, bond‐critical points (electron density ρ BCP: 0.012) were observed only for the I3⋅⋅⋅I6/I2⋅⋅⋅I8 couple (367.6 pm) (Figure 5 a, Figure S12). The electron density at this bond‐critical point, however, is four to five times lower than for the polyiodide I5 − (ρ BCP: 0.052, 0.038), which was calculated as a reference system (Figure S13). Moreover, this weak interaction does not result from direct I⋅⋅⋅I bonding but from slight electron transfer of nonbonding electron pairs on iodide to the formally vacant valence p atomic orbital of Sn2+ (Figure 5 a; Figure S14). All additional I⋅⋅⋅I contacts in the central SnI8 subunit did not result in bond‐critical points. In comparison to the calculated distances, finally, the respective I⋅⋅⋅I distances in the experimentally observed non‐S 4‐symmetric [SnI8{Fe(CO)4}4]2+ cation (I3⋅⋅⋅I6: 370.6, I2⋅⋅⋅I8: 379.4 pm) are in part significantly longer (Table 2, Figure 5 b). Taken together, the influence of attractive I⋅⋅⋅I interactions can be considered to be negligible.

Based on the results of DFT calculations, the bonding situation in [SnI8{Fe(CO)4}4]2+ is predominantly ionic in nature. This ionic bonding was further confirmed by Ahlrichs–Heinzmann population analysis (Table S6),17 which results in the “ionic” fragments Sn, I, and Fe(CO)4 with partial charges of +0.27, −0.07, and +0.57, respectively. Thus, the bonding situation in [SnI8{Fe(CO)4}4]2+ can be best described by a predominately ionic interaction of tin and iodine and a predominately covalent interaction of iodine and iron, where, of course, contributions of both bonding types are included in all cases. Further stabilization of [SnI8{Fe(CO)4}4][Al2Cl7]2 results from lattice energy and the long‐range cation–anion interaction.

In summary, [SnI8{Fe(CO)4}4][Al2Cl7]2 is a novel carbonyl compound that was synthesized in an ionic liquid. The title compound is highly sensitive and decomposes rapidly (≤5 min) after removal from the mother liquor under argon even at room temperature. The fact that its temperature of decomposition (≤25 °C) is below the temperature of synthesis (40–50 °C) points to the metastable nature of [SnI8{Fe(CO)4}4][Al2Cl7]2. Most remarkable is the [SnI8{Fe(CO)4}4]2+ cation with a central bicapped SnI8 prism and an unusually high eightfold coordination of Sn+II. Comparably short I⋅⋅⋅I contacts (down to 371 pm) are observed despite the negatively charged iodide and negligible attractive I⋅⋅⋅I interactions. The existence of this predominately ionic, Sn2+‐centered [SnI8{Fe(CO)4}4]2+ cation with fragile Fe(CO)4 clips, eightfold coordination of tin, and short iodide–iodide contacts is unprecedented. The synthesis in weakly coordinating ionic liquids near room temperature seems to be essential to obtain such a fragile compound at the border between formation and decomposition.

Experimental Section

Chemicals. All sample handling was strictly conducted under argon, applying standard Schlenk or glovebox techniques. Reactions were performed in argon‐filled and sealed glass ampoules, which were dried under reduced pressure (1×10−3 mbar) at 300 °C prior to use. Fe2(CO)9 (99.99 %, ABCR) and AlCl3 (99.99 %, ABCR) were used as supplied. SnI4 was synthesized by reacting Sn and I2 in toluene. [BMIm]Cl (99 %, IoLiTec) was dried under vacuum at 130 °C for 30 hours prior to use.

[SnI. SnI4 (80 mg, 0.13 mmol) and Fe2(CO)9 (45.5 mg, 0.13 mmol) were added to a 1:3 mixture of [BMIm]Cl (500 mg, 2.86 mmol) and AlCl3 (1145 mg, 8.59 mmol) as the ionic liquid. This mixture was heated in sealed glass ampoules at 130 °C for 4 days to guarantee the dissolution of all AlCl3 and all starting materials. When the reaction mixture was cooled to room temperature at a rate of 1 K h−1, the formation of black crystals with a weak violet shimmer was observed at a temperature of ≤50 °C along with tiny orange needles of SnI2. The crystals were separated from the ionic liquid by filtration through a glass frit and washed with the pure ionic liquid to remove unreacted starting materials and SnI2. Whereas the title compound was stable in the ionic liquid, its removal from the mother liquor even under inert conditions (argon) and even at room temperature results in rapid decomposition (≤5 min). The decomposition with release of CO was decelerated at lower temperature and stopped at about −50 °C.

Further details regarding synthesis, crystal structure analysis, spectroscopic characterization, and quantum chemical calculations can be obtained from the Supporting Information.

Conflict of interest

The authors declare no conflict of interest.

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Supplementary

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