Synthesis and characterization of π cyclopentadienyl complexes of manganese and iron with one to five fluorosilyl substituents. Crystal and molecular structures of [Mn(C₅Br{SiMe₂F}₄)(CO)₃] and [Mn(C₅{SiMe₂F}₅)(CO)₃]

Abstract

The reaction of AgSbF₆ with the cymantrenyl hydrosilanes [Mn{C₅Br_5−n(SiMe₂H)_n}(CO)₃] (n = 1–5) gives the corresponding fluorosilanes. With the ferrocenylhydrosilanes [Fe{C₅Br_5−n(SiMe₂H)_n}(C₅H₅)] fluorination and oxidation to give the ferricenium salts [Fe{C₅Br_5−n(SiMe₂F)_n}(C₅H₅)]⁺SbF₆⁻ (n = 1–5) occurs. Reaction of the latter with cobaltocene gives the corresponding neutral fluorosilanes. All compounds were characterized by a combination of NMR methods (¹H, ¹³C{¹H} and ¹⁹F; sometimes also ²⁹Si) and in part as well by IR and HR mass spectrometry. The reactivity of the cymantrenyl fluorosilanes towards carbanions and MeOH was studied. X-ray diffraction analysis of [Mn{C₅Br_5−n(SiMe₂F)_n}(CO)₃] (n = 4–5) shows paddle-wheel orientations of the SiMe₂F groups.

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Introduction

Among the halosilanes, the fluorosilanes are particularly interesting because of their usefulness for the preparation of highly coordinated silicon compounds.^1,2 This property allowed for their use as fluorescent fluoride sensors,^3–5 but also as catalysts for C–F activation.⁶ However, they are also used for such different applications like the ¹⁸F fluorination for cancer diagnostics,^7,8 electrolyte additives for lithium secondary batteries,⁹ or “organic electroluminescent materials and devices”.¹⁰ For their synthesis, several methods are available, starting either from alkoxy-,¹¹ chloro-,^12–15 or hydrosilanes.^16–22 Most known fluorosilyl-cyclopentadienyl complexes are derivatives of ferrocene. They have been prepared from the corresponding hydrosilanes via fluorination with BF₃·Et₂O,²³ from silaferrocenophanes via ring opening reactions,^24–26 from dinuclear doubly SiMe₂ bridged ferrocenes via protonolysis with HBF₄,²⁷ or from ferrocenylchlorosilanes with CuF₂.²⁸ Methods for functional group transformations including Si–X groups on group 4 metallocenes were summarized in 1999²⁹ and 2015.³⁰ Cyclopentadienyl complexes with more than two fluorosilyl substituents could not be obtained by any of these methods. As our group is interested in the per-functionalization of the cymantrenyl- and ferrocenyl systems, we decided to study the possibility of obtaining cyclopentadienyl complexes with five fluorosilyl substituents. The results of these studies are reported in the following.

Results and discussion

Synthesis

We decided to examine only the hydrosilane complexes [C₅Br_5−n(SiMe₂H)_n]ML (ML = Mn(CO)₃, 1a–e, FeCp, 2a–e, n = 1–5).^31,32 First of all, we tried the reaction of the Monosilyl-tetrabromocymantrene [Mn(C₅Br₄SiMe₂H)(CO)₃] (1a) with CuF₂·2H₂O in refluxing CCl₄. After 15 hours, only traces of the desired fluorosilane [Mn(C₅Br₄SiMe₂F)(CO)₃] (3a) besides unreacted 1a could be isolated.

While the reaction of the salts Ph₃C⁺(WCA)⁻ (WCA = weakly coordinating anion) with hydrosilanes can lead to the formation of silylium ions, when no easily accessible fluorides are part of the WCA,³³ trityl salts with anions like BF₄⁻ or SbF₆⁻ give only the corresponding fluorosilanes. The reaction of 1a–d with Ph₃CSbF₆ yielded the desired fluorosilanes 3a–3d always contaminated with Ph₃CH (in case of the reaction with 1d, also the partial hydrolysis product 9b was observed, vide infra). All attempts to separate the compounds met with failure (Scheme 1).

Scheme 1 Reaction of the cymantrenyl-hydrosilanes 1a–1d with Ph₃CSbF₆. For the structures of the fluorosilanes see Scheme 2.

Although the use of AgBF₄ as a fluorination reagent for an optically active hydrosilane was noted already in 1969,²² and the use of AgSbF₆ for the fluorination of a triorganotin hydride was mentioned in 1984,³⁴ both silver salts apparently have never been used for this purpose again. Still, as both are available commercially, we decided to look into their reactivity towards metallocenyl-hydrosilanes. First, we studied the cheaper AgBF₄. However, we found, that this reaction took a very long time, at least at room temperature, and after several days, still unreacted hydrosilanes could be found in the product. Particularly, in the reaction of pentakis(dimethylsilyl)ferrocene, 2e, with 5.5 equivalents of AgBF₄ for 24 h, and standard work-up (see Experimental part) single crystals could be obtained, that contained according to its mass spectra all members of the series [CpFe{C₅(SiMe₂H)_n(SiMe₂F)_5−n}] (n = 0–5).

Thus, we turned to AgSbF₆ as a fluorination reagent. With 1a in CH₂Cl₂, we could observe rapid formation of elementary silver. After work-up, a mixture of 3a and the dinuclear disiloxane [(OC)₃Mn(C₅Br₄SiMe₂)]₂O (9e) was obtained.

When the other silylated bromocymantrenes [Mn{C₅Br_5−n(SiMe₂H)_n}(CO)₃] (1b–e, n = 2–5) were treated with an excess (with respect to the number of hydrosilyl groups) of AgSbF₆ in CH₂Cl₂, the corresponding fluorosilylcymantrenes 3b–e could be obtained in medium to excellent yields (for structural drawings, see Scheme 2). For comparison, we treated also [Mn{C₅H(SiMe₂)₄}(CO)₃] (1f) with AgSbF₆ and obtained the expected product of fluorination [Mn{C₅H(SiMe₂F)₄}(CO)₃] (5d) in 51% yield. Careful examination of the NMR spectra showed, that sometimes the products of partial desilylation were also formed (e.g.5b in the reaction of 1c, or 5d in the reaction of 1e) (Scheme 3).

Scheme 2 Structural drawings of all compounds described in this publication.

Scheme 3 The reaction of 1a with AgSbF₆.

Next, we looked at the ferrocene system. Here some complications might be expected, due to the known ability of AgSbF₆ to act as a strong oxidant towards substituted ferrocenes to giveferricenium salts.³⁵ In a first experiment, we treated [Fe{C₅(SiMe₂H)₅}(C₅H₅)] (2e) in CH₂Cl₂ with 5.5 equivalents AgSbF₆. A bluish green solid was obtained, insoluble in hexane, nearly insoluble in CH₂Cl₂ and soluble in water with blue colour, which gradually disappeared. Extraction of the aqueous phase with hexane yielded, after evaporation in vacuo, only ferrocene. We concluded that the silver ion had oxidized the ferrocene to a ferricenium species, which was unstable in water. Therefore, we decided to reduce the primary oxidation product with cobaltocene. This time, we examined [Fe{C₅Br₃(SiMe₂H)₂}(C₅H₅)] (2b). After reaction with 2.5 eq. AgSbF₆ in CH₂Cl₂ a dark green oil was obtained, which was re-dissolved in CH₂Cl₂ and treated with a hexane solution of cobaltocene, which yielded nearly immediately a yellow solution. Evaporation of solvent gave [Fe{C₅Br₃(SiMe₂F)₂}(C₅H₅)] (4b) in approximately 40% yield as a yellow oil (Scheme 4). The observed contamination with 6a and 6b (and very small amounts of IIIb) was due both to impurities in the starting material 2b and partial desilylation reactions.

Scheme 4 Synthesis of 4b from 2b.

Since this approach was successful, we repeated it for all members of the series [Fe{C₅Br_5−n(SiMe₂H)_n}(C₅H₅)] (n = 1–5, 2a–2e) and obtained the corresponding fluorosilanes [Fe{C₅Br_5−n(SiMe₂F)_n}(C₅H₅)] (n = 1–5, 4a–e) (for structural formulae, see Scheme 2).

The desired fluorosilanes were always contaminated by products of partial desilylation. These desilylation reactions were most likely due to a reaction of the generated SbF₅ with adventitious moisture present in the solvent, which led to formation of HF, or, in part, to corresponding reactions on the silicagel chromatographic columns. Unfortunately, due to unknown reasons, all products of the reduction step with cobaltocene, were also contaminated with small amounts of phthalate esters (mostly bis(isooctyl)phthalate). Attempts to separate these compounds by chromatography met with failure.

Reactivity

Hydrolysis and alcoholysis. The Si–F bond is one of the strongest known (ca. 638–697 kJ mol⁻¹, compared to a value of 512–570 kJ mol⁻¹ for the Si–O and 400–490 kJ mol⁻¹ for the Si–Cl bond).³⁶ From a thermodynamic view, the hydrolysis of fluorosilanes should be endothermic. However, one should keep in mind, that kinetically the Si–F bond is quite labile, mostly due to bond polarization, but also to the high tendency for formation of HF and/or its water adducts.³⁷ All the metallocenyl fluorosilanes prepared by us could be handled in air without apparent decomposition, which is in sharp contrast with our observations with the corresponding chlorosilanes.³⁸ Still, the mass spectra of the ferricenium ions 4c⁺, 4d⁺ and 4e⁺, which were the primary products of the reactions of the corresponding hydrosilanes with AgSbF₆, showed the presence of the disiloxanes 10a–d (for structural formulae, see Scheme 2). These are structural analogs of the hydrolysis products, which we observed in the alcoholysis reactions of the chlorosilanes [Mn{C₅(SiMe₂Cl)_nBr_5−n}(CO)₃], n = 3–5.³⁸ As NMR spectra of the isolated fluorosilanes 3a–e and 4a–e in C₆D₆ did not change during the course of one day, it can be assumed, that the hydrolysis occurred during the fluorination step, possibly induced by the presence of SbF_5. When compound 3a was treated with LiOH in Et₂O for 15 h, only the desilylation product IIIb and the disiloxane 9e could be isolated.

A quite unusual reaction was observed for compound 3e. Some of the single crystals, that had been obtained by recrystallization from hexane/MeOH solution and apparently contained some liquid MeOH in a specimen, were left standing in air for a couple of weeks. A ¹H-NMR spectrum, taken of the meanwhile “weathered” crystals, showed besides the original 3e also the presence of [Mn{C₅H(SiMe₂OMe)₄}(CO)₃] (11a) and [Mn{C₅H(SiMe₂OMe)₂(Si₂Me₄O)}(CO)₃] (11b). Apparently, the MeOH had induced partial desilylation, methanolysis of the Si–F bond and oxadisilole formation between neighbouring SiMe₂OMe groups.

Alkylation. Fluorosilanes have been reported to undergo Si–C bond formation reactions via treatment with lithium organyls and Grignard reagents.^{14,23,39–41} However, these reactions are rather slow in the absence of catalysts. In a first experiment, we treated 3c with MeMgCl, and obtained a mixture of [Mn{C₅Br₂(SiMe₃)₃}(CO)₃] (12a) and [Mn{C₅HBr₂(SiMe₃)₂}(CO)₃] (12b). Next, we treated 3d with MeMgCl, and obtained a mixture of [Mn{C₅Br(SiMe₂F)₂(SiMe₃)₂}(CO)₃] (12c) and [Mn{C₅H(SiMe₂F)₂(SiMe₃)₂}(CO)₃] (12d), while with compound 5d only 12d could be isolated. However, when 3e was treated with MeMgCl, the unchanged fluorosilane was recovered even after prolonged reaction time (Scheme 5).

Scheme 5 The reaction of the fluorosilanes 3c–e with MeMgCl.

We studied also the reactivity of 3c and 3d towards LiMe and of 3e towards AlMe₃. The reaction of 3c with LiMe yielded a mixture of 12a, 12b and [Mn{C₅H₃(SiMe₃)₂}(CO)₃] 12e, while the reaction of 3d with LiMe gave a mixture of several compounds, with [Mn{C₅H(SiMe₃)₄}(CO)₃] (12f) and 12d as major components. Compounds 12a,b and 12f had been prepared by us earlier on a different route.^42,43 But the pentasubstituted 3e did not even react with AlMe₃ to give any SiMe₃ compounds, and 3e could be recovered unchanged in 60% yield.

NMR spectroscopy

All compounds were characterized by ¹H, ¹⁹F and ¹³C{¹H} NMR spectroscopy, some also by ²⁹Si NMR spectroscopy. As the NMR resonances of the substituted cyclopentadienyl rings in the series of cymantrenyl- and ferrocenyl-fluorosilanes appeared quite similar, the corresponding compound pairs 3a/4a, 3b/4b, 3c/4c and so on, are discussed together.

The mono(fluorosilanes) 3a/4a. If the solution structure of these compounds were static, the methyl groups were not equivalent (proximal and distal position with respect to the metal), and two signals each in the ¹H NMR and ¹³C NMR spectra should be expected. However, for both compounds only one signal was observed in all NMR spectra, and therefore, a fast exchange of the proximal and distal methyl groups at room temperature can be assumed. Sharp doublets in the ¹H spectra (A part of A₆X spin system, J_H–F = 7.3 Hz (3a); 7.4 Hz (4a)), and the ¹³C{¹H} NMR spectra (AX spin system, J_C–F = 18.3 Hz (3a); 14.9 Hz (4a)) characterize the SiMe₂F groups. The ¹⁹F NMR spectrum of 3a shows only a broad signal with two satellites due to fluorine–silicon coupling (J_F–Si = 282 Hz), while the corresponding spectrum of 4a shows a well-resolved septet (X part of A₆X spin system, J_H–F = 7.8 Hz) also with two satellite signal groups (J_F–Si = 279 Hz). The ²⁹Si NMR spectra of both compounds show a doublet each, due to Si–F coupling (J_Si–F = 282 Hz for 3a and 280 Hz for 4a). It should be noted in this context, that the known ferrocene derivative [Fe(C₅H₄SiMe₂F)(C₅H₅)] was also fully characterized by NMR spectroscopy: ¹H NMR: doublet with J_H–F of 6.9 Hz; ¹³C{¹H} NMR: doublet with J_C–F of 17 Hz; ¹⁹F NMR: septet with J_F–H of 6.8 Hz; ²⁹Si NMR: doublet with J_Si–F of 272 Hz.²⁴

The bis(fluorosilanes) 3b/4b. In these compounds the methyl groups on silicon are diastereotopic. This was also noticed in the known compound [Fe{C₅H₃(SiMe₂F)₂}(C₅H₅)], which has the silyl substituents in relative 1,2 position, however.²⁷ In this compound, the ¹H NMR spectrum (at 270 MHz) showed a triplet for the methyl groups (“J_H–F” = 7.8 Hz), and the ¹³C{¹H} NMR spectrum showed two doublets of doublets, with ²J_C–F ≈ 17 Hz and ⁵J_C–F ≈ 1.0 Hz. No ¹⁹F or ²⁹Si NMR data were given. The ¹H NMR spectrum (at 400 MHz) of 3b shows two close (Δδ = 4.9 Hz) doublets with ³J_H–F = 7.3 and 7.3 Hz, while its ¹³C{¹H} NMR spectrum (at 101 MHz) shows two doublets with ²J_C–F = 13.8 and 15.3 Hz. For compound 4b, the ¹H NMR spectrum (at 270 MHz) shows an apparent doublet of doublets, with J_H–F = 7.4 Hz and 1.7 Hz, respectively, and the ¹³C{¹H} NMR spectrum (at 68 MHz) two close (Δδ = 10.6 Hz) doublets with ²J_C–F of 15.1 and 15.0 Hz. The C₅H₅ ligand in 4b is observed as a singlet in both ¹H and ¹³C{¹H} NMR spectra, with peak widths of 0.49 and 2.52 Hz, respectively. The ¹⁹F NMR spectra of both compounds appear as septets with ²⁹Si satellites (³J_F–H = 7.3 Hz (3b) and 7.8 Hz (4b), respectively; ¹J_F–Si = 281 Hz (3b) and 279 Hz (4b)).

The tris(fluorosilanes) 3c/4c. Although the ¹H NMR spectra of 3b and 4b are in principle higher order spectra (in a “static” model A₃A′₃B₃B′₃XX′ spin systems), they can be treated as first order spectra. However, a first look at the methyl regions of the ¹H NMR spectra of compounds 3c and 4c shows, that first order treatment is no longer possible (Fig. 1 and S3).

Fig. 1 ¹H NMR spectrum (SiCH₃ region; measured at 270 MHz) for compound 4c.

The two diastereotopic pairs of methyl groups at the SiMe₂F groups in 1,2 positions appear as two broad “triplets”, which overlap with the doublet of the two enantiotopic methyl groups at the “isolated” SiMe₂F group at 4-position. The overlap can be removed by measuring the spectrum at 500 MHz (see Fig. S9), but not the “strange” appearance of the triplets. A first-order analysis (as indicated in the left half of Fig. 1) would yield an interpretation of the “triplet” as a “doublet of doublets” with ³J_H–F′ ≈ ⁶J_H–F′ = 3.7 Hz, which does not make sense in comparison with the coupling constants found in compounds 3a,b and 4a,b. Unfortunately, we neither have access to NMR simulation programs capable of simulating such an A₃A′₃B₃B′₃C₃C′₃XX′Y spin system nor computational capabilities to perform these calculations. Also, the ¹³C{¹H} NMR spectra of compounds 3c and 4c have to be treated as higher order spectra (Fig. 2 and S23). There are two apparent “triplets” for the two diastereotopic methyl carbons of the SiMe₂F groups in 1,2 position and one doublet for the enantiotopic methyl carbons of the SiMe₂F group in position 4.

Fig. 2 ¹³C{¹H} NMR spectrum (SiCH₃ region, measured at 126 MHz) of compound 4c.

First order analysis, as indicated in the (left) spectrum of 3c, would yield apparent coupling constants ²J_C–F ≈ ⁵J_C–F for the diastereotopic methyl groups of ca. 8.5 and 10.0 Hz, respectively, and ²J_C–F = 15.1 Hz for the isolated SiMe₂F group. Analogous interpretation of the spectrum of compound 4c would yield identical values. Comparison with the J_C–F values found for compounds 3a,b and 4a,b shows, that at least a first order interpretation of the “triplet” signals is not appropriate. Therefore, the observed spectra have to be interpreted as overlap of AXYZ, BXYZ and CXYZ spectra (due to the fact, that for statistical reasons only one of the methyl carbons can be a ¹³C isotop, the totally ¹³C enriched AA′BB′CXX′Y changes to three different isotopomers, and the XX′Y fluorine part changes to XYZ⁴⁴). Careful inspection of the spectrum of 4c shows also the presence of “satellites” due to ¹J_C–Si couplings of ca. 50 Hz for the “triplets” and 60 Hz for the “doublet”. The C₅H₅ ligand in 4c is observed in both ¹H and ¹³C{¹H} NMR spectra as a singlet, with peak widths of 0.84 and 3.25 Hz, respectively. The ¹⁹F NMR spectra of both compounds show two signals with relative intensities of 2 : 1. Both signals are broad with no resolvable fine structure for compound 3c. However, for compound 4c, the stronger signal is partially resolved, and the weaker signal appears as a well-resolved septet with ³J_F–H = 7.6 Hz (Fig. S14 and S19).

The tetrakis(fluorosilanes) 3d/4d/5d. As might be expected from the preceding section, the ¹H NMR spectra of the tetrakis(fluorosilanes) 3d and 4d look even more complicated and cannot be treated as first order (Fig. 3 and S4).

Fig. 3 ¹H NMR spectrum (SiCH₃ region, at 270 MHz) of compound 4d.

In addition to the complications by the higher order spin system (A₃A′₃B₃B′₃XX′)(C₃C′₃D₃D′₃YY′), further complications arise by dynamic effects due to hindered rotations of the individual SiMe₂F groups, as can be seen in the VT NMR spectra of 3d (vide infra).

The ¹³C{¹H} NMR spectrum of compound 3d shows 16 lines in the SiCH₃ region (Fig. S24; unfortunately, no expanded view is available), while for compound 4d twelve peaks can be resolved (Fig. 4). From theory, the spectrum in the SiCH₃ region should be an overlap of (AXYZW), (BXYZW), (CXYZW) and (DXYZW) spin systems. The C₅H₅ signal for compound 4d appears in both ¹H and ¹³C{¹H} as a singlet, with peak widths of 1.90 Hz and 3.06 Hz, respectively. In comparison with the ¹H NMR spectra of 4b and 4c, this is a significant line broadening, which hints to a reduction of the free rotation of the C₅H₅ ring around the Fe-centroid axis.

Fig. 4 ¹³C{¹H} NMR spectrum (SiCH₃ region, measured at 68 MHz) of compound 4d.

The ¹⁹F NMR spectra of 3d and 4d show two well separated (Δδ = 9.1 and 6.5 ppm, respectively) broad singlets (for compound 4d peak widths of 30 and 24 Hz, respectively) with relative intensity 1 : 1 (Fig. S15 and S20).

The ¹H and ¹³C{¹H} NMR room temperature spectra of the bromine-free compound 5d show similar broad signals as the bromo compound 3d (Fig. 5). Particularly, the ¹H NMR spectra look very similar, with just the relative positions of the “doublets” and “triplets” exchanged.

Fig. 5 Room temperature ¹H (left) and ¹³C{¹H} NMR spectra (SiCH₃ region, at 270 and 67.9 MHz, respectively) of compound 5d.

A formal first-order analysis might extract eight doublets each from both spectra, however, complications arise here again from dynamic line broadening effects, as can be seen in the VT NMR spectra (vide infra). The room temperature ¹⁹F NMR spectrum at 376 MHz shows one broad singlet (half width ca. 300 Hz) and a partially resolved multiplet with relative intensities 1 : 1 (Fig. S16).

The pentakis(fluorosilanes) 3e/4e. As soon as all five substituents are SiMe₂F, the molecules become part of the group of “multi-5-rotor molecular propellers”.⁴⁵ So far, the majority of these compounds contains the pentaarylcyclopentadienyl, the [C₅(ⁱPr)₅], and the [C₅(CH₂Ph)₅] ligands. A few examples with [C₅(SiMe₂H)₅] ligands exist also. In the context of the present study, most relevant are [CoCp(Cⁱ₅Pr₅)]⁺PF₆⁻,⁴⁶ [FeCp(C₅ⁱPr₅)],⁴⁷ [Fe(C₅ⁱPr₅)(CO)₂Br],⁴⁸ [Mo(C₅ⁱPr₅)(CO)₃Me],⁴⁹ as well as [Mn{C₅(SiMe₂H)₅}(CO)₃],³¹ [Fe{C₅(SiMe₂H)₅}Cp],³² and [Fe{C₅(SiMe₂H)₅}₂].⁵⁰ All of these compounds have in common, that the substituents ⁱPr or SiMe₂H are “gear-meshed” creating a metallocenic chirality. The most stable form has the C–H or Si–H bond in or close to the cyclopentadienyl ring plane and five methyl groups each on both sides of this plane. Therefore, the ¹H NMR spectra of all [C₅ⁱPr₅] complexes show one septet for the C–H protons and two doublets for the diastereotopic methyl protons, while the ¹³C{¹H} NMR spectra show one singlet each for the diastereotopic methyl carbons. The same observation is made for the dekakis(dimethylsilyl)ferrocene,⁵⁰ while for the two other complexes with the [C₅(SiMe₂H)₅] ligand only one doublet is observed in the ¹H NMR and one singlet in the ¹³C{¹H} NMR spectra.^31,32 Similar “simplified” spectra can be observed in some of the [C₅ⁱPr₅] complexes at higher temperatures.^46,48,49

At r.t., both ¹H and ¹³C{¹H} NMR spectra of compound 3e show two broad unresolved multiplets for the SiMe₂F groups, besides a singlet for the Mn(CO)₃ carbon atoms and a poorly resolved multiplet for the cyclopentadienyl carbon atoms (Fig. S5 and S25). The expected spin systems (A₃B₃X)(A′₃B′₃X′)₂(A′′₃B′′₃X′′)₂ for the SiMe₂F protons, overlap of a (AXX′₂X′′₂) with a (BXX′₂X′′₂) system for the SiMe₂F carbon atoms and a (MXX′₂X′′₂) for the cyclopentadienyl ring carbon atoms together with the effects of possibly hindered rotations make a detailed analysis of these spectra impossible. However, on cooling to −10 °C the ¹H NMR spectrum changes to a triplet (J = 7.3 Hz) and a doublet-of-doublets (J = 7.0 and 6.9 Hz)-structure (Fig. 6, further discussion in the VT section).

Fig. 6 ¹H spectrum of compound 3e (SiCH₃ region, at 270 MHz, −20 °C; S and W are the impurities silicon grease and water).

Unfortunately, the NMR samples studied for compound 4e were contaminated by 4d and 6d, and therefore a reliable exact analysis of its NMR spectra is not possible. However, some general features can be discussed. In the room temperature ¹H NMR spectrum (in C₆D₆, 270 MHz), the SiMe₂F methyl protons appear as a broad unresolved “singlet” (half width ca. 17 Hz) and the C₅H₅ protons as an extremely broad (half with 58 Hz) signal (Fig. S11). When the same compound is measured at −70 °C (in toluene-d⁸, 400 MHz), the methyl protons appear as two doublets of doublets. Measurement of the ¹³C{¹H} NMR spectrum at −70 °C (in toluene-d⁸, 101 MHz), gives for the methyl carbon atoms 8 lines, which can also be interpreted as two doublets of doublets (Fig. 7).

Fig. 7 Low temperature (−70 °C) ¹H (top) and ¹³C{¹H} (bottom) NMR spectra of compound 4e (SiCH₃ region, at 400 and 101 MHz, respectively).

The obvious similarity of the LT ¹H and ¹³C{¹H} NMR spectra in the SiMe₂F region is on first sight not compatible with the expected spin systems, as explained above for the spectra of compound 3e. The appearance of “doublets of doublets” with similar couplings (¹H: δ = 0.68 (dd, J_H–F = 8.4 and 6.9 Hz), 0.55 (dd, J_H–F = 8.3 and 5.4 Hz); ¹³C{¹H}: δ = 2.93 (dd, J_C–F = 16.0 and 10.0 Hz), 2.68 (dd, J_C–F = 16.5 and 14.2 Hz)) suggests, that each methyl group “sees” two fluorine atoms, and coupling occurs “through space”. Further long-range couplings are apparently too small to be resolved. This is reminiscent of the spectra obtained for [Mo{C₆(SiMe₂F)₆}(CO)₃], described by Sakurai et al. some 30 years ago.⁵¹ However, in that complex only triplets (¹H) or unresolved multiplets (¹³C) were obtained for the methyl groups at r.t., while for the free ligand at r.t. septets were observed. This difference was explained by the assumption, that a “merry-go-round” of the F atoms around the periphery of the free ligand, which makes all F atoms equivalent, was no longer possible in the coordinated ligand. It should be noted, however, that a “gear-meshed” rotation of the SiMe₂F groups is possible for six-membered rings, but not for five-membered ones (parity rule for gear trains⁴⁵).

VT NMR studies. The ¹H NMR spectra of the monosilyl compounds 3a and 4a showed at r.t. only one singlet, which is an indication of rapid rotation of the SiMe₂F group around the C_cp–Si bond, as no difference between distal and proximal methyl groups is observed. The ¹H NMR spectra of the disilyl compounds 3b and 4b show two doublets, which might be interpreted either as arising from the two diastereotopic methyl groups with no sign of long-distance couplings ⁷J_H–F, indicating a frozen or slow rotation, or as a doublet of doublets, arising from time-averaged methyl group signals, showing both ³J_H–F and ⁷J_H–F couplings. The general appearance of the spectra corresponds to the ones seen for [Fe(C₅H₃ⁱPr₂)₂] and [Mo(C₅H₃ⁱPr₂)(CO)₃Me],⁴⁹ and also [Co(C₅H₃ⁱPr₂)(PMe₃)₂],⁵² but in contrast to [Co(C₅H₃ⁱPr₂)(CO)₂],⁵³ which shows only one doublet for the isopropyl methyl groups. As the ¹⁹F NMR spectra taken at the same temperature show only septets, the former interpretation of hindered rotation is preferred. As the room temperature ¹H and ¹³C NMR spectra of the remaining compounds gave mostly only broad and badly resolved spectra, we decided to examine the VT-NMR spectra of compounds 3d,e and 5d as well as of compounds 4d,e and 6d.

When a toluene solution of 4d is gradually cooled down, the “triplet” feature broadens first and loses its multiplet appearance, then both signals become broader and unresolved, and finally only a broad singlet with an extremely broad shoulder can be observed at −70 °C (Fig. S32). On the other hand, if the solution is heated to +70 °C, both signals get better resolved. Unfortunately, instrument limitations did not allow for extension to both higher and lower temperatures. The observed behaviour is very unusual-opposite to what would be expected, and at present we have no explanation for it.

However, when a toluene solution of 3e is gradually cooled down from r.t. to −20 °C and the heated to +45 °C, a behaviour like expected can be observed (Fig. S33): on cooling the broad doublet feature observed at r.t. gets better resolved and a triplet (which actually might be a poorly resolved doublet of doublets) and a doublet of doublets appear at −20 °C. On heating the broad “doublet” coalesces to a singlet at 45 °C. Further heating to 70 °C or cooling to −78 °C does not change the appearance of the spectra (only slight “sharpening” of the “singlet” is observed at +70 °C). Again, due to instrumental limitations, it is not possible to say, if and towards which multiplicity the broad singlet might change at higher temperatures. We also had a look at the Cp region of the ¹³C{¹H} spectra at three temperatures (Fig. S34): quite unexpected, the appearance of the quaternary Cp signal did not change significantly both when going to high or to low temperature: two poorly resolved multiplets were observed at all temperatures, with the signals at high temperature showed less resolution than the ones at low temperature, as might be expected.

Cooling a toluene solution of 5d from r.t. to −20 °C resulted in the appearance of three broad doublets, which broadened even more on going to −70 °C yielding finally three broad singlets (Fig. S35). On the other hand, heating this solution to 60 °C resulted also in broadening, but the appearance as “doublet” and “triplet” remained. When looking at the ¹⁹F NMR spectra at r.t. and −80 °C, the appearance as two unresolved singlets did not change. However, the difference in chemical shifts between these singlets slightly increased on cooling down.

A VT ¹⁹F NMR study of the ferrocene derivative 4d (contaminated with 6d) showed two broad singlets at r.t. (half widths ca. 33 Hz), that collapsed at −70 °C to two extremely broad (half widths 777 and 300 Hz, respectively) signals with no fine structure, that nearly disappeared in the noise (Fig. S36).

Similarly, when a toluene solution of a 2 : 1 mixture of 4d and 4e was cooled from r.t. to −70 °C, the broad singlet for the C₅H₅ protons (half width 9.3 Hz) split up into two sharper singlets (half widths ca. 3.9 Hz). While the appearance of the SiMe₂F multiplets also changed upon cooling, due to the severe overlap of the multiplet signals no further discussion is possible (Fig. S37). It should be noted, that the assignment of the Cp signals to compounds 4d and 4e is based on comparison with purer samples of both species, and also to the corresponding ¹³C and mass spectra. Still, it is hard to imagine, why the signals are broadened at r.t. An exchange between compounds 4d and 4e seems chemically impossible, and a slowed rotation of a C₅H₅ ligand has never been observed at such temperatures even in the case of extremely sterically demanding C₅R₅ ligands in ferrocenes of the type [Fe(C₅R₅)(C₅H₅)]. However, for the sterically related compound [Mo(C₅^iPr₅)(CO)₃Me] it was reported, that a broad isopropyl resonance at r.t. split into two sets of signals at low temperature, and this was explained to the presence of two conformational isomers with respect to the relative orientation of the isopropyl groups.⁴⁹ A broad singlet for the cyclopentadienyl carbons in [Fe(C₅ⁱPr₅)(CO)₂Br] was reported for the r.t. ¹³C NMR spectrum, however, no low temperature data and also no ¹H data were given.⁴⁸ Although this is no “proof”, an interpretation of the broad Cp resonance at r.t. as a sign of the presence of two or more conformational isomers cannot be excluded. A ¹⁹F VT NMR study of the same sample showed a very broad resonance (half width 182 Hz) at r.t. and a significantly sharper (half width 33 Hz) singlet with no fine structure at −70 °C (Fig. S38). This behaviour is quite interesting in light of the complete opposite observation in the ¹⁹F NMR spectra of 4d (vide supra), however, it parallels the observations made for the ¹H NMR spectrum. Unfortunately, again instrumental limitations prevented measurements at lower or higher temperatures.

Crystallographic studies

One common feature of metal complexes of the general type {C_n(EXMe₂)_n}ML (n = 5 or 6, E = C or Si, X = H or F), which all show a “gear-meshed” arrangement of the ring substituents, is that they all suffer from disorder problems. This is true for the already-mentioned pentakis(isopropyl)cyclopentadienyl complexes [FeL(CO)₂]₂,⁴⁸ [FeL(C₅H₅)]⁴⁷ as well as [FeLBr]₂,⁵⁴ [CrL(C₅H₅)],⁵⁵ [SnL₂]⁵⁶ and [SiL(C₅H₅)]⁵⁷ (L = C₅ⁱPr₅) and also the pentakis(dimethylsilyl)cyclopentadienyl complexes [MnL′(CO)₃],³¹ [FeL′₂]⁵⁰ and [FeL′(C₅H₅)]³² (L′ = C₅{SiMe₂H}₅). Although this disorder was not always discussed at all in the original papers, it can be found in the cif-files available at the Cambridge Crystallographic Data Base. In all cases two positions were found for the methine carbon or silicon positions (mostly with uneven site occupation factors), and attached H atoms were added geometrically. In most cases splitting of the corresponding Cp carbon atoms was neither observed nor refined. For the already mentioned cobalt complex [CoL(C₅H₅)]⁺PF₆⁻,^46,58 no disorder was reported. However, a closer look at the cif file shows some strange features, and an overlooked disorder seems probable here, too. The “Disorder in the Crystal Structures of Hexakis(dimethylsilyl)benzene and its Tricarbonyl Chromium, Molybdenum and Tungsten Complexes” was even the title of a paper by Kahr et al. from 1992.⁵⁹ Although the crystal structure of C₆(SiMe₂F)₆ was reported,⁵¹ no mention was made of the occurrence of disorder, which, however, is present according to the cif file available at the CCDC. In all these hexasilylbenzene structures (except for one crystalline modification) two positions for the Si atoms could be found, however, due to poor data quality, no further splitting of atom positions could be found. This led to the appearance of non-radial C_ar–Si bonds. When comparing these structures, one should keep two geometrical restrictions in mind: (1) exchanging silicon for carbon in the EXMe₂ substituents “increases the interatomic distances in the side chains” with the consequence, that “dimethyl silyl groups…should be less tightly geared than the isopropyl groups”.⁶⁰ (2) Increasing the ring size in C_nR_n ligands from n = 5 to n = 6, increases on one hand the distance of the R group from the center of the molecule, which is, however, more than compensated by the decrease of the angle between neighbouring ring centroid–substituent vectors from 72 to 60°, which leads to increased crowding of the substituents.⁴⁵ For example, the distance between the Si atoms in the C₆(SiMe₂H)₆ complexes lie between 3.33 and 3.41 Å, while in the C₅(SiMe₂H)₅ complexes they are between 3.58 and 3.67 Å and between the methine carbon atoms in the C₅ⁱPr₅ complexes between 3.18 and 3.25 Å.

Crystal and molecular structure of [Mn{C₅(SiMe₂F)₅}(CO)₃], 3e. Compound 3e crystallizes in the orthorhombic space group Pna2₁ with one molecule on a general position in the asymmetric unit. As was expected, the molecule is disordered between two enantiomers, in an approximate 3 : 1 ratio. Fig. 8 shows an ORTEP3 representation of the molecular structure, with a superposition of both isomers.

Fig. 8 Molecular structure of compound 3e, superposition of both isomers.

Due to the application of numerous restraints on refinement of the structure, which used restrictions on many bond distances, a deeper discussion of the bond parameters is obsolete. However, some structural features shall be presented here. The Cp ring is planar (the σ_pln parameter, as defined by the program PLATON, is only 0.021 Å). The distance from manganese to the Cp centroid is 1.802(4) Å for the major isomer, which is slightly longer than the 1.784 Å observed for the starting compound 1e ³¹ and the 1.775 Å found in the only other known cymantrene derivative with five silyl substituents, [Mn{C₅(Si₂Me₄O)₂SiMe₂OMe}(CO)₃].³⁸ The C_cp–Si and Si–C_Me distances average at 1.893(7) and 1.847(4) Å, respectively. For the two literature examples, the C_cp–Si distances average both at 1.87(1) Å, while the Si–C_Me average at 1.88(1) and 1.83(1) Å, respectively. A comparison with the C_aryl–Si bond lengths of [C₆(SiMe₂H)₆] and its M(CO)₃ complexes as well in [C₆(SiMe₂F)₆] does not make sense due to the fact, that in these compounds only the splitting of the Si positions was calculated, while only an averaged position of the attached benzene carbon atom was used. However, comparison of the Si–C_Me distances is possible. In free [C₆(SiMe₂H)₆] this parameter averages at 1.87(1) Å, in its W(CO)₃ and Mo(CO)₃ complexes at 1.874(7) and 1.88(1) Å, respectively, and in [C₆(SiMe₂F)₆] at 1.86(1) Å. All these values are slightly longer than the one found in 3e. The Si–F bonds in 3e average at 1.637(7) Å for the major “isomer” (1.63(2) Å for the minor), while in [C₆(SiMe₂F)₆] the average value is 1.68(1) Å. It should be mentioned, however, that in the latter no split of fluorine positions was found, while in our calculations, the related fluorine positions were 0.32(3)–0.54(3) Å apart.

Two Si atoms (Si3 and Si4) are approximately in the plane of the Cp ring (Δ = −0.07(1) and −0.03(1) Å), respectively, two (Si2 and Si5) are significantly shifted to the distal ring side (Δ = −0.30(1) and −0.39(1) Å), while the remaining Si atom is shifted by 0.15(1) Å to the proximal ring side. One F atom (F4) resides in the Cp ring plane (Δ = 0.02(2) Å), two (F1 and F3) are on the proximal side (Δ = 0.60(2) and 0.33(2) Å, respectively) and the remaining two on the distal ring side (−0.81(2) and −0.63(2) Å). Despite these differences, all the C_cp–Si–F angles are very similar, averaging at 100.1(4)°.

The parameter h, as introduced by Sakurai et al.,⁵¹ describes the distance of the Si atom from the plane defined by the three attached carbon atoms, measures between 0.407 and 0.424(3) Å for the major “isomer” of 3e (0.36–0.44(1) Å for the minor). According to Sakurai, h = 0 Å for truly tbp and h = 0.6 Å for truly tetrahedral environments of the Si atom. Thus, the situation found in 3e corresponds to a slightly distorted tetrahedral geometry.

The crystal structure shows weak intra- and intermolecular “non-classical” hydrogen bonds between methyl hydrogen atoms and fluorine as well as oxygen atoms. No other interactions (as F⋯O or π–π) are found. A packing diagram is shown in Fig. S48.

Crystal and molecular structure of [Mn{C₅(SiMe₂F)₄Br}(CO)₃], 3d. Compound 3d crystallizes in the orthorhombic space group Pnma, with half a molecule in the asymmetric unit, and shows the same kind of disorder as 3e. It should be mentioned here, that apparently the ferrocene derivative [Fe{C₅(SiMe₂H)₄Br}₂] shows no sign of disorder.⁵⁰ Due to the fact, that the molecule resides on a mirror plane, the disorder is forced to be of a 1 : 1 type. Fig. 9 shows an ORTEP3 representation of the molecule.

Fig. 9 Molecular structure of compound 3d (ORTEP3 representation, 50% probability ellipsoids, only one enantiomer shown).

Due to the fact, that numerous restraints were necessary for a convergent refinement, a thorough discussion of bond parameters is not possible. However, as in the case of the structure of 3e, still several geometrical features deserve mentioning. The distance between manganese and the Cp ring centroid is 1.774(5) Å, and the C_cp–Si bonds and Si–C_Me bonds average at 1.875(5) Å and 1.79(2) Å, respectively, which are all shorter than in 3e. The Cp ring is planar (rms = 0.0184, σ_pln = 0.031), with Br1 being 0.160(1) Å on the distal side. Three of the Si atoms are also on the distal side (Si2, Si3 and Si4 by 0.333(3), 0.106(3) and 0.250(3) Å, respectively), while Si1 resides on the proximal side by 0.162(2) Å. Two F atoms reside on the proximal side (F1 and F3 by 0.61(1) and 0.35(1) Å), while F2 and F4 are far on the distal side (0.865(5) and 0.598(6)). Despite the large deviations of the F atoms from the ring plane, the C_cp–Si–F angles vary only between 100.5(7) and 104.8(4)°. The h parameter, defined as above, varies between 0.32 and 0.33(2) Å, and is thus significantly smaller than in 3e, but still far from indicating a tbp geometry around silicon.

The crystal structure shows several weak intra- and intermolecular hydrogen bonds between methyl hydrogen and F, Br and O atoms. A packing diagram is shown in Fig. S51.

Experimental

Materials and methods

The starting compounds 1a–f and 2a–e were prepared as described by us earlier.^31,32 Reagents Ph₃CSbF₆, AgSbF₆ and AgBF₄ as well as cobaltocene were obtained commercially and used as such. Solvents used for the reactions (CH₂Cl₂ and hexane) were purchased in the highest available purity and saturated either with N₂ or Ar, while solvents for chromatography were of analytical grade and were used as obtained. All reactions were performed under dry N₂ or Ar, using standard Schlenk techniques (Schlenk flasks/tubes were stored in a hot oven at 160 °C for at least 24 h and were assembled under a purge of inert gas while being still hot. The assembled flasks were then evacuated using standard oil pump vacuum (ca. 0.01 Torr), and then flushed with inert gas).

For standard column chromatography silica gel 100 C18 Reversed Phase (Fluka) was used, or standard silica gel (0.035–0.070 mm, 60A, Merck) was pre-treated with sufficient dry Et₂O and SiMe₃Cl with stirring for several days; then solvents were evaporated and the residue was kept in high vacuum for 8 h, and finally flashed with dry argon.

NMR spectra were measured on JEOL ECP-270 or EX-400 or Eclipse 500 instrument, using C₆D₆ as solvent. The chemical shifts were obtained relative to the residual solvent signals, as defined by the MestReNova software (version 14.1.1–24751) (δ_CHD5 = 7.160 and 128.06 ppm, respectively). Mass spectra were obtained on Finnigan MAT 90 and JEOL Mstation 700 instruments, in DEI or FAB mode.

Synthetic procedures

General remark: ¹H, ¹⁹F and ¹³C NMR data are also given in the SI, Tables S2–S4.

Synthesis of [Mn(C₅Br₄{SiMe₂F})(CO)₃] (3a). A solution of 1a (135 mg, 0.23 mmol) in CH₂Cl₂ (6 mL) was treated at r.t. with AgSbF₆ (121 mg, 0.35 mmol) with stirring for 3 h. After complete evaporation of the solvent in vacuo the residue was suspended in hexane (20 mL), filtered through silanized silica gel and brought to dryness in vacuo. A yellow solid (135 mg) was obtained, which consisted according to its ¹H NMR spectrum of a 2 : 1 mixture of the desired 3a with the bimolecular disiloxane [Mn₂(C₁₀Br₈{Si₂Me₄O})(CO)₆] (9e) together with traces of [Mn(C₅Br₄H)(CO)₃] (IIa). All attempts to separate these compounds met with failure.

¹H NMR (270 MHz): δ = 0.340 (d, J_HF = 7 Hz). ¹³C{¹H} NMR (101 MHz): δ = 221.9 (C̲O), 94.0s, 90.8s, 78.7 (d, J_CF = 17 H, C̲₅Br₄Si), 0.96 (d, J_CF = 18 Hz, SiC̲H₃). ¹⁹F NMR (84.3 MHz): δ = −151.4 (“s”, br, J_FSi = 282 Hz). ²⁹Si{¹H} NMR (22 MHz) δ = 21.8 (d, J_Si–F = 282 Hz). IR (Nujol) ν_CO = 2040 vs., 1971 vs.

Synthesis of [Mn(C₅Br₃{SiMe₂F}₂)(CO)₃] (3b). A solution of 1b (200 mg, 0.36 mmol) in CH₂Cl₂ (5 mL) was treated with AgSbF₆ (469 mg, 1.37 mmol) with stirring for 5 h at r.t. After complete evaporation of the solvent in vacuo the residue was suspended in hexane (20 mL), filtered through silanized silica gel and brought to dryness in vacuo. A yellow crystalline solid (150 mg, 0.25 mmol, 69%) was obtained.

¹H NMR (400 MHz): δ = 0.414 “d”, 0.401 “d”. ¹³C{¹H} NMR (101 MHz): δ = 221.8 (C̲O), 100.0s, 96.3s, 85.1 (d, J_CF = 15 Hz, C̲₅Br₃Si₂), 1.3 and 0.8 (2 d, J_CF = 14 and 15 Hz, SiC̲H₃). ¹⁹F NMR (84.3 MHz): δ = −152.1 (“h”, J_FSi = 281 Hz). ²⁹Si{¹H} NMR (22 MHz) δ = 21.5 (d, J_Si–F = 279 Hz). IR (Nujol) ν_CO = 2039 vs., 1964 vs. EA (calc./found, %): C 24.29/23.67; H 2.04/2.28π.

Synthesis of [Mn(C₅Br₂{SiMe₂F}₃)(CO)₃] (3c). A solution of 1c (325 mg, 0.61 mmol) in CH₂Cl₂ (6 mL) was treated with AgSbF₆ (720 mg, 2.10 mmol) with stirring for 15 h at r.t. After complete evaporation of the solvent in vacuo the residue was suspended in hexane (20 mL), filtered through silanized silica gel and brought to dryness in vacuo. A yellow oil (315 mg) was obtained, which consisted according to its ¹H NMR spectrum of a 17 : 1 mixture of the desired 3c with [Mn(C₅Br₂H{SiMe₂F}₂)(CO)₃] (5b). All attempts to separate these compounds met with failure.

¹H NMR (400 MHz): δ = 0.53–0.45 m, 0.43–0.38 m. ¹³C{¹H} NMR (101 MHz): δ = 222.7 (C̲O), 102.3s, 94.6 “dd”, 88.5 (d, J_CF = 15 Hz, C̲₅Br₂Si₃), 2.3 “dd”, 1.4 “d”, 1.3 “dd” (SiC̲H₃). ¹⁹F NMR (84.3 MHz): δ = −149.5 (“s”, br, 2F, J_FSi = 280 Hz), −151.7 (m, 1F, J_FSi = 281 Hz). ²⁹Si{¹H} NMR (22 MHz) δ = 23.2 (d, J_Si–F = 280 Hz), 21.4 (d, J_Si–F = 281 Hz). IR (Nujol) ν_CO = 2032 vs., 1955 vs.

Synthesis of [Mn(C₅Br{SiMe₂F}₄)(CO)₃] (3d). A solution of 1d (500 mg, 0.97 mmol) in CH₂Cl₂ (10 mL) was treated with AgSbF₆ (1500 mg, 4.37 mmol) with stirring for 15 h at r.t. After complete evaporation of the solvent in vacuo the residue was suspended in hexane (20 mL), filtered through silanized silica gel and brought to dryness in vacuo. A yellow crystalline solid (343 mg, 0.58 mmol, 60%) was obtained. Recrystallization from hexane at −20 °C gave some crystals suitable for X-ray diffraction.

¹H NMR (270 MHz): δ = 0.571 “td”, 0.455 “dt”. ¹³C{¹H} NMR (101 MHz): δ = 223.2 (C̲O), 106.6s, 105.9 (d, J_CF = 18 Hz), 96.7 (d, J_CF = 20 Hz, C̲₅BrSi₄), 2.4 and 2.1 (2 × “ddd”, SiC̲H₃). ¹⁹F NMR (377 MHz): δ = −139.6 and −148.7 (2 × “s”, br, J_FSi = 268 and 278 Hz). ²⁹Si{¹H} NMR (22 MHz, C₆D₆) δ = 23.2 (d, J_Si–F = 280 Hz), 21.4 (d, J_Si–F = 281 Hz). IR (Nujol) ν_CO = 2029 vs., 1968 vs., 1952 vs. m.p. 108–111 °C. EA: (calc./found, %) C 32.69/33.05; H 4.12/4.19.

Synthesis of [Mn(C₅H{SiMe₂F}₄)(CO)₃] (5d). A solution of 1f (350 mg, 0.80 mmol) in CH₂Cl₂ (10 mL) was treated with AgSbF₆ (1240 mg, 3.61 mmol) with stirring for 15 h at r.t. After complete evaporation of the solvent in vacuo the residue was suspended in hexane (20 mL), filtered through silanized silica gel and brought to dryness in vacuo. A yellow solid (210 mg, 0.41 mmol, 51%) was obtained.

¹H NMR (270 MHz): δ = 5.578 (s, C₅H), 0.488 “d”, 0.477 “d”, 0.360 “dd”, 0.327 “dd”. ¹³C{¹H} NMR (68 MHz): δ = 223.9 (C̲O), 106.8 “t”, 106.0 (“d”), 95.2 (“dd”, C̲₅HSi₄), 1.5 and 1.0 (2 × m, SiC̲H₃). ¹⁹F NMR (377 MHz): δ = −144.1 (“s”, br, J_FSi = 266 Hz), −151.2 (“q”, br, J_FSi = 275 Hz). ²⁹Si{¹H} NMR (53 MHz) δ = 26.0 (“d”, J_Si–F = 266 Hz), 22.5 (“dd” AXX′YY′, J_Si–F = 275 Hz and 4 Hz). IR (Nujol) ν_CO = 2021 vs., 1952 vs., 1938 vs. m.p. 43–45 °C. EA: (calc./found, %) C 37.76/37.89; H 4.96/5.13.

Synthesis of [Mn(C₅{SiMe₂F}₅)(CO)₃] (3e). A solution of 1e (500 mg, 1.01 mmol) in CH₂Cl₂ (10 mL) was treated with AgSbF₆ (1909 mg, 5.56 mmol) with stirring for 15 h at r.t. After complete evaporation of the solvent in vacuo the residue was suspended in hexane (20 mL), filtered through silanized silica gel and brought to dryness in vacuo. A yellow crystalline solid (252 mg, 0.43 mmol, 43%) was obtained. Recrystallization from hexane/methanol at −20 °C gave some crystals suitable for X-ray diffraction.

¹H NMR (270 MHz): δ = 0.80–0.45 m. ¹³C{¹H} NMR (68 MHz): δ = 223.8 (C̲O), 108.3 “d”, (C̲₅Si₅), 2.2 and 1.9 (2 × “t”, SiC̲H₃). ¹⁹F NMR (84.3 MHz): δ = −135.3 (“s”, br, J_FSi = 266 Hz). ²⁹Si{¹H} NMR (22 MHz) δ = 24.1 (“dd”, J_Si–F = 258 Hz and 12 Hz). IR (Nujol) ν_CO = 2024 vs., 1948 vs., 1910 sh. m.p. 133 °C. EA: (calc./found, %) C 36.95/36.79; H 5.17/5.10.

Synthesis of [Fe(C₅Br₄{SiMe₂F})(C₅H₅)] (4a). A solution of 2a (100 mg, 0.18 mmol) in CH₂Cl₂ (10 mL) was treated with AgSbF₆ (93 mg, 0.27 mmol) with stirring for 21 h. The suspension was filtered through glass wool, and the obtained greenish filtrate was treated with a solution of cobaltocene (25 mg, 0.13 mmol) in hexane (10 mL) with stirring for 30 min. The solvents were evaporated in vacuo and the residue extracted with hexane (10 mL). The extract was filtered through glass wool. Evaporation of the filtrate to dryness yielded a yellow oil (70 mg), which contained the desired product 4a as main organometallic component (79% purity; contamination with ca. 15% [Fe(C₅Br_5−nH_n)(C₅H₅)] (n = 0–2, Ib, IIb, IIIb), 3% apparently unreacted 2a and 3% [Fe(C₅Br₃H{SiMe₂F})(C₅H₅)]) (6a) together with an unspecified molar amount of phthalate esters.

¹H NMR (400 MHz): δ = 4.05 (s, C₅H₅), 0.440 (d, J_HF = 7 Hz, SiCH₃). ¹³C{¹H} NMR (101 MHz): δ = 78.6 (C̲₅H₅), 85.2s, 83.6s, 66.1 (d, J_CF = 15 Hz, C̲₅SiBr₄), 1.19 (d, J_CF = 15 Hz, SiC̲H₃). ¹⁹F NMR (254 MHz): δ = −151.3 (“h”, J_FSi = 279 Hz). ²⁹Si INEPT NMR (54 MHz): δ = 24.6 (d, J_Si–F = 280 Hz). HRMS (DEI⁺): m/z = 577.6660, calc. for C₁₂H₁₁⁷⁹Br₂⁸¹Br₂FSiFe 577.6658.

Synthesis of [Fe(C₅Br₃{SiMe₂F}₂)(C₅H₅)] (4b). A solution of 2b (240 mg, 0.38 mmol) in CH₂Cl₂ (10 mL) was treated with AgSbF₆ (384 mg, 1.12 mmol) with stirring for 20 h. The suspension was filtered through glass wool, and the obtained greenish filtrate was evaporated to dryness. The remaining dark green oil was washed with hexane and re-dissolved in CH₂Cl₂ (10 mL). A solution of cobaltocene (60 mg, 0.32 mmol) in hexane (10 mL) was added with stirring for 30 min. The resulting yellow solution was evaporated to dryness in vacuo. Extraction of the residue with two 10 mL portions of hexane, followed by evaporation in vacuo, left a yellow oil (90 mg), which contained the desired product 4b as main organometallic component (84% purity; contamination with ca. 12% [Fe(C₅Br₃H{SiMe₂F})(C₅H₅)]) (6a) and 4% [Fe(C₅Br₂H{SiMe₂F}₂) (C₅H₅)] (6b) and traces of IIIb, together with a small amount of phthalate esters.

¹H NMR (270 MHz): δ = 4.227 (s, C₅H₅), 0.508 and 0.502 (“dd”, SiCH₃). ¹³C{¹H} NMR (68 MHz): δ = 76.5 (C̲₅H₅), 87.8s, 87.3s, 70.9 (d, J_CF = 14 Hz, C̲₅Br₃Si₂), 1.38 (“dd”, SiC̲H₃). ¹⁹F NMR (254 MHz): δ = −151.5 (“h”, br, J_FSi = 279 Hz). HRMS (DEI⁺): m/z = 573.7704, calc. for C₁₄H₁₇⁷⁹Br₂⁸¹Br₁F₂Si₂Fe 573.7718.

Synthesis of [Fe(C₅Br₂{SiMe₂F}₃)(C₅H₅)] (4c). A solution of 2c (190 mg, 0.37 mmol) in CH₂Cl₂ (10 mL) was treated with AgSbF₆ (447 mg, 1.30 mmol) with stirring for 20 h. The suspension was filtered through glass wool, and the obtained greenish filtrate was evaporated to dryness (350 mg). The remaining dark green oil was re-dissolved in CH₂Cl₂ (10 mL), and a solution of cobaltocene (66 mg, 0.35 mmol) in hexane (10 mL) was added with stirring for 30 min. The resulting yellow solution was evaporated to dryness in vacuo. Extraction of the residue with two 10 mL portions of hexane, followed by evaporation in vacuo, left a yellow oil (130 mg), which contained the desired product 4c as main organometallic component (88% purity; contamination with ca. 12% [Fe(C₅Br₂H{SiMe₂F}₂)(C₅H₅)] (6b) and traces of IIIb, together with a small amount of phthalate esters).

¹H NMR (400 MHz): δ = 4.295 (s, C₅H₅), 0.59–0.56 m, 0.553 “d”, 0.50–0.47 m (SiCH₃). ¹³C{¹H} NMR (126 MHz): δ = 74.5 (C̲₅H₅), 90.0s, 77.7 “dd”, 74.3 (d, J_CF = 21 Hz, C̲₅Si₃Br₂), 2.70 m, 1.94 “t”, 1.64 “d”(SiC̲H₃). ¹⁹F NMR (254 MHz): δ = −151.3 and −149.0 (2 × “s”, br). HRMS (DEI⁺): m/z = 571.8765, calc. for C₁₆H₂₃⁷⁹Br₁⁸¹Br₁F₃Si₃Fe 571.8757.

Synthesis of [Fe(C₅Br{SiMe₂F}₄)(C₅H₅)] (4d). A solution of 2d (210 mg, 0.42 mmol) in CH₂Cl₂ (10 mL) was treated with AgSbF₆ (649 mg, 1.89 mmol) with stirring for 18 h. The suspension was filtered through glass wool, and the obtained greenish filtrate was evaporated to dryness. The remaining dark green oil was washed with hexane and re-dissolved in CH₂Cl₂ (10 mL). A solution of cobaltocene (75 mg, 0.40 mmol) in hexane (10 mL) was added with stirring for 30 min. The resulting yellow suspension was evaporated to dryness in vacuo. The residue was washed with hexane (10 mL). Extraction of the residue with two 10 mL portions of toluene, followed by evaporation in vacuo, left a dark solid together with an orange-coloured oil (160 mg), which contained the desired product 4d as main organometallic component (84% purity; contamination with ca. 12% [Fe(C₅Br₃H{SiMe₂F})(C₅H₅)] (6a) and 4% [Fe(C₅Br₂H{SiMe₂F}₂) (C₅H₅)] (6b) and traces of IIIb, together with a small amount of phthalate esters).

¹H NMR (270 MHz): δ = 4.297 (s, C₅H₅), 0.64–0.47 (m, SiCH₃). ¹³C{¹H} NMR (68 MHz): δ = 72.9 (C̲₅H₅), 93.1s, 84.0 (d, J_CF = 19 Hz), 81.4 (d, J_CF = 17 Hz, C̲₅BrSi₄), 2.93–1.81 m (SiC̲H₃). ¹⁹F NMR (254 MHz): δ = −141.5 and −148.0 (2 × “s”, br, J_FSi = 269 and 272 Hz). HRMS (DEI⁺): m/z = 569.9794, calc. for C₁₈H₂₉⁸¹BrF₄Si₄Fe 569.9798.

Synthesis of [Fe(C₅{SiMe₂F}₅)(C₅H₅)] (4e). (a) A solution of impure 2e (200 mg, <0.42 mmol, contains ca. 15% 2d) in CH₂Cl₂ (12 mL) was treated with AgSbF₆ (793 mg, 2.31 mmol) with stirring for 21 h. The suspension was filtered through glass wool, and the obtained greenish filtrate was evaporated to dryness. The remaining dark green oil was washed with hexane and re-dissolved in CH₂Cl₂ (10 mL). A solution of cobaltocene (80 mg, 0.42 mmol) in hexane (10 mL) was added with stirring for 30 min. The resulting yellow suspension was evaporated to dryness in vacuo. The residue was washed with hexane (10 mL). Extraction of the residue with two 10 mL portions of toluene, followed by evaporation in vacuo, left a greenish solid together with an orange-coloured oil (20 mg), which contained the desired product 4e besides 4d, [Fe(C₅H{SiMe₂F}₄)(C₅H₅)] (6d) and other unidentified substances.

¹H NMR (270 MHz): δ = 4.47 (s, br, C₅H₅), 0.47 (m, br, SiCH₃). ¹³C{¹H} NMR (101 MHz): δ = 70.8 (C̲₅H₅), 108.3 “d” (C̲₅Si₅), 3.85 and 3.69 (2 × “d“, SiC̲H₃). ¹⁹F NMR (377 MHz; −70 °C): δ = −135.7 (“s”, br, J_FSi = 288 Hz). HRMS (DEI⁺): m/z = 566.0867, calc. for C₂₀H₃₅F5i₅Fe 566.0855.

(b) A similar procedure was performed, using 150 mg of impure 2e (<0.30 mmol) and 330 mg AgBF₄ (1.65 mmol). After reduction with cobaltocene and standard work-up an orange-coloured oil was obtained (ca. 10 mg). Recrystallization from hexane at −25 °C gave a few yellow crystals. NMR spectroscopic examination of these crystals showed that they consisted of a mixture of all members of the series [Fe{C₅(SiMe₂H)_5−n(SiMe₂F)_n}(C₅H₅)], n = 0–5, in an approximate 1 : 0.8 : 0.7 : 0.25 : 0.5 : 2 ratios. Although the crystals could be measured on a diffractometer and an apparently reasonable solution could be found, it was not possible to resolve the different components. The general appearance was, however, similar to the picture of 3e (Fig. S52), showing the same kind of disorder between enantiomers.

Crystallography

Crystals were measured on a Bruker Kappa CCD (3e) or a SYNTEX P3 (3d) diffractometer. The obtained data sets were solved using SHELXT⁶¹ and refined using SHELXL 2018/3.⁶² Examination of the structure solutions was performed with the program PLATON as part of the WINGX program suite.⁶³ Graphics were prepared using either ORTEP3 for windows or MERCURY, both being part of the WINGX program suite.⁶⁴ For further general details of the crystal structure determinations see Table S1 of the SI.

Crystals of 3e were obtained from a hexane/MeOH mixture at −20 °C and mounted on a KappaCCD diffractometer. Structure solution by SHELXT yielded the complete molecule (Fig. S47, left), which showed some “strange” non-radial C_cp–Si bonds. A first difference Fourier analysis yielded five electron density maxima in the plane of the Cp ring close to the positions of the Si atoms (X1–X5 in Fig. S47, right). Atoms X1–X5 were interpreted as alternative Si positions, representing the enantiomer of the original solution. As the “strange” C–C–Si angles were present for both Si positions, it was decided, to set up also two series of Cp carbon atom positions with the necessary restraints to produce two C₅Si₅ systems with “proper” radial C–Si bonds. While this procedure turned out successful, there was still a great asymmetry in the Si–F and Si–CMe bonds left. It was therefore decided, to set up two sets of complete C₅(SiMe₂F)₅ ligands with two enantiomeric conformations and refining both the site occupation factors and several restraints (23 SADI restraints; SIMU and DELU for all C atoms), which lead satisfyingly to the final molecule. All non-hydrogen atoms could be refined anisotropically, except for the five Cp carbon atoms of the minor “isomer”.

Crystals of 3d were obtained from hexane solutions at −20 °C and mounted on a SYNTEX P3 four-circle diffractometer in omega-scan mode. Due to the instrumental limitations of a four-circle diffractometer, only a relatively small dataset could be obtained. Structure solution by SHELXT indicated, that the molecule resided on a mirror plane, with only half a molecule representing the asymmetric unit (Fig. S49a). Automatic application of the mirror operation generated the whole molecule; however, the occurrence of severe disorder problems became obvious (Fig. S49b). There were two positions for each substituent (leading to eight Si atoms and 2 Br atoms). As it was chemically not possible, that the Br atoms could bond to fluorine, a careful inspection of the situation allowed to select the atoms that were part of each “isomer” (Fig. S49c). Selecting one set of atoms (marked green in Fig. S49c) automatically generates the other set by application of the mirror operation. This necessitates a relative 1 : 1 occupation of both isomers. Starting from that point also a set of ring carbon positions was calculated to provide radial C–Si and C–Br bonds. Of course, many restraints (36 SADI restraints, +ISOR for all carbon atoms) were necessary to generate a stable “reasonable” refinement.

Conclusions

The synthesis and spectroscopic characterization of the complete series of fluorosilanes [M{C₅Br_5−n(SiMe₂F)_n}L] (ML = Mn(CO)₃; Fe(C₅H₅), n = 1–5) could be achieved. While the preparation of the cymantrene derivatives occurred without significant side reactions, the ferrocene derivatives were contaminated by several side products, derived from desilylation reactions. It might be advisable to isolate the ferricenium intermediates as salts with other fluorine-free anions and purify them by recrystallization instead of chromatography. After such a purification, the following reduction might hopefully yield cleaner products.

Author contributions

S. S. and S. B. designed and performed the experiments and he analytical characterization by IR, MS and NMR spectroscopy. K. S. supervised the experiments, performed the X-ray data solutions and refinements and wrote the final manuscripts.

Conflicts of interest

There are no conflicts to declare.

Data availability

Copies of NMR, IR and mass spectra. See DOI: https://doi.org/10.1039/d5dt01746g.

CCDC 2475453 and 2475454 contains the supplementary crystallographic data for this paper.^65a,b

Acknowledgements

We would like to thank Profs. Wolfgang Beck and Ingo Peter Lorenz for providing lab space and equipment as well as general support, and Dr Peter Mayer for performing the X-ray data collection of compound 3e.

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