Macrocyclic stibine-bridged [1.1.1] and [1.1.1.1]ferrocenophanes

Abstract

Our interest in the design of antimony-bridged ferrocenophanes has led us to revisit the reaction of 1,1′-dilithioferrocene with PhSbCl₂, leading to the isolation of the corresponding [1.1.1] and [1.1.1.1]ferrocenophanes, whose macrocyclic structure has been verified by X-ray diffraction. The [1.1.1]ferrocenophane has been used as a ligand for gold(I) halides, affording, in the case of the chloride derivative, a complex that exhibits carbophilic reactivity.

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Phosphorus-bridged [1]ferrocenophanes^1,2 have drawn significant attention, not only for the generation of polymers^3–9 but also as precursors for macrocycles,¹⁰ including trimeric ones such as I (Fig. 1),¹⁰ which has been used as a tridentate ligand for transition metals.¹⁰ Efforts directed toward heavier analogs of such systems have emerged, including in the case of arsenic.¹¹ Two recent publications have described the first example of such antimony-based systems.^12,13 The first one includes antimony-bridged [1]ferrocenophanes of type II (Fig. 1) which feature a bulky aryl group appended to the group 15 center.¹² The stabilization provided by the 2,6-dimesitylphenyl group in IIb has allowed for the isolation and characterization of the monomer, which can be subsequently polymerized. Parallel efforts from our group have investigated the less sterically demanding phenyl substituent at antimony.¹³ While we were not able to isolate the corresponding [1]ferrocenophane derivative, we succeeded in obtaining respectable yields of the distiba[1.1]ferrocenophanes of type III (Fig. 1). Keeping in mind that larger oligomers are known in the case of the phosphorus systems,¹⁰ we have now decided to take a more careful look at the products of the reaction leading to III.

In this communication, we report that this reaction also affords a trimeric [1.1.1]ferrocenophane and a tetrameric [1.1.1.1]ferrocenophane, the structures of which have been established. As part of our interest toward the development of antimony-based gold(I) systems as catalysts,^14–20 we also describe our efforts to use these antimony macrocycles as ligands for gold(I) centres and as a platforms for carbophilic reactivity.

Fig. 1 Selected examples of pnictogen-containing ferrocenophanes.

To begin, a dilute solution of 1,1′-dilithioferrocene·tmeda (tmeda = N,N,N′,N′-tetramethylethylenediamine) was treated with one equiv. of PhSbCl₂ to afford a mixture of products from which the novel trinuclear (1) and tetranuclear (2) macrocycles were isolated as yellow-orange solids in 4% and 3% yield respectively, along with compound III (6%), following silica gel column chromatography (Fig. 2a). While 1 is the antimony analog of I, tetrameric 2 is unprecedented in the chemistry of group 15 ferrocenophanes, even though such structures have been observed in the chemistry of silaferrocenophanes.²¹ It is notable that 1 and 2 were isolated directly from the reaction mixture at room temperature without light irradiation or heating of their isolated monomer, an approach often used for the generation of polyferrocene materials.^{21,22 1}H NMR spectroscopy indicates the presence of non-equivalent ferrocene units for both 1 and 2 in solution that are differentiated by their cyclopentadienyl (Cp) proton resonances. In the case of 1, the ¹H NMR spectrum can be interpreted based on 12 resonances with four of them accidentally overlapping in pairs. This spectrum indicates that all Cp rings are non-equivalent and that the molecule lacks any symmetry. This spectrum is consistent with the crystal structure of this derivative, which shows a trimeric motif in the asymmetric unit with no apparent symmetry element. The lack of symmetry in the structure of 1 is in contrast to the observed C₃ and C_s symmetry of the phosphorus analog.¹⁰ The lack of symmetry in 1 also manifests in the differing Sb–Sb distances of 4.875(6) Å (Sb1–Sb2), 5.691(6) Å (Sb2–Sb3) and 5.024(6) Å (Sb1–Sb3), as established by single-crystal X-ray diffraction analysis (Fig. 2b). A similar situation is encountered in the case 2, which is best interpreted on the basis of 16 individual resonances, with the accidental overlap of six of them leading to 13 observed massifs. Recording the ¹H NMR spectra of 1 and 2 in CDCl₃ at 55 °C does not induce notable spectral changes, highlighting the non-fluxional nature of these macrocycles under these conditions (Fig. S12 and S13). In contrast and somewhat surprisingly, 2 crystalizes as a rare S₄ tetramer as shown in Fig. 2c, with a unique Sb–Sb separation of 4.5097(12) Å. The antimony atoms adopt a trigonal pyramidal geometry with the C(Cp)–Sb–C(Cp) angles for 1 in the range of 93.8(2)°–97.8(2)°, close to the value of 93.3(5)° measured for 2.¹³ The UV-vis spectra of compounds 1 and 2 display weak d–d transitions at λ_max = 451 and λ_max = 454 nm, respectively, (Fig. S17) which is consistent with the spectral features of other ferrocenophanes.^23,24 The presence of multiple ferrocene units in these systems prompted us to explore their electrochemical behaviour. When scanned anodically at a rate of ν = 100 mV s⁻¹, the cyclic voltammogram of 1 displays three pseudo reversible oxidation waves at E¹_1/2 = 0.045 V, E²_1/2 = 0.195 V, and E³_1/2 = 0.327 V vs. Fc^+/0 (Fig. S18). These features are similar to those of a previously reported trisilaferrocenophane that shows three reversible oxidations.²¹ On the other hand, the tetrameric macrocycle 2 exhibits four pseudo reversible oxidation events at E¹_1/2 = 0.070 V, E²_1/2 = 0.103 V, E³_1/2 = 0.246 V, and E⁴_1/2 = 0.286 V vs. Fc^+/0 (Fig. S18). This electrochemical behaviour is again reminiscent of an analogous silicon-based ferrocenophane,²¹ with two sets of a pair of closely spaced oxidation waves, with the caveat that the associated cathodic waves of each pair of closely spaced peaks appear as a single two-electron reduction event.²¹

Fig. 2 (a) Synthesis of 1 and 2. Solid-state structures of (b) 1 and (c) 2. Thermal ellipsoids are drawn at the 50% probability level. The hydrogen atoms and interstitial solvent molecules are omitted for clarity. Selected atoms are labeled. Selected bond lengths (Å) and angles (°): 1: Sb1–C1 2.135(4), Sb1–C11 2.123(5), Sb1–C31 2.151(5), Sb2–C16 2.134(5), Sb2–C21 2.133(5); C1–Sb1–C11 97.83(18), C1–Sb1–C31 94.26(18): 2: Sb1–C1 2.152(10), Sb1–C11 2.160(15); C1–Sb1–C11 95.2(5), C1–Sb1–C1′ 93.3(5).

We next investigated the coordination behavior of these new systems towards gold(I).^14,15 Treating 1 with one equiv. of AuI in CH₂Cl₂ afforded compound 3 in 73% yield (Fig. 3a). The formation of 3 was confirmed by X-ray diffraction and NMR spectroscopy. Complexation with gold iodide generates a more symmetrical structure with only three ¹H (in 1 : 1 : 2 ratio) and five ¹³C NMR signals due to the presence of three equivalent ferrocene units. As confirmed by the crystal structure of 3 (Fig. 3b), the three antimony donors are coordinated to the gold center, leading to Au–Sb bond distances of 2.628(3) Å, 2.611(3) Å, and 2.627(5) Å, which are comparable to those found in tri-stibine gold complexes.²⁵ The coordination sphere of the gold center also includes the iodide ligand, bound via an Au–I bond of 2.747(9) Å. The tetrahedral coordination geometry of the gold center shows only minor distortions as indicated by the Sb–Au–Sb angles of 105.5(8)°, 104.5(3)°, and 102.0(6)°. We found that 2 also reacted with AuI, as supported by ESI-MS, which showed a peak corresponding to [2-Au]⁺ at m/z value of 1728.7263. Unfortunately, attempts to crystallize the corresponding gold complex were unsuccessful. For this reason, additional investigation of this family of compounds focused on 1 and its gold complexes. Intrigued by the possibility of using such macrocyclic antimony ligands as platforms for gold-mediated carbophilic catalysis,²⁶ we decided to test 3 in the cycloisomerization of N-propargyl-4-fluorobenzamide (4) a substrate that we have often used to benchmark the catalytic activity of gold complexes.^27–30 Monitoring by ¹H NMR spectroscopy indicated negligible or no conversion to the cyclic product 5 (Fig. S19). In contrast, an active catalyst could be generated by the combination of 1 with one equiv. of (tht)AuCl (tht = tetrahydrothiophene) in CH₂Cl₂, as indicated by the rapid conversion of 4 into 5. The reaction reached a conversion of 94% after 90 minutes (Fig. 4 and Fig. S20). Although we were not able to isolate the gold chloride complex of 1, which we assume is responsible for the observed activity, its formation is supported by in situ ¹H NMR spectroscopy (Fig. S15) and by ESI-MS, which showed a peak corresponding to [1-Au]⁺. For comparative purposes, we also tested the catalytic activity of pure (tht)AuCl and compound 1 alone (Fig. S21 and S22). At the same time point and under analogous reaction conditions, the use of (tht)AuCl led to very low conversion (9%), while compound 1 was simply inactive.

Fig. 3 (a) Synthesis of 3 and (b) solid state structure of 3. Thermal ellipsoids are drawn at the 50% probability level. The hydrogen atoms and interstitial solvent molecules are omitted for clarity. Selected atoms are labeled. Selected bond lengths (Å) and angles (°): 3: Sb1–C1 2.121(3), Sb1–C11 2.127(3), Sb1–Au 2.628(3), Sb2–Au 2.611(3), Sb3–Au 2.627(5), Au–I 2.747(9); C1–Sb1–C11 97.9(13), C11–Sb1–Au 117.8(8), Sb1–Au–Sb2 105.5(8), Sb2–Au–Sb3 104.5(3), Sb1–Au–Sb3 102.0(6).

Fig. 4 Cycloisomerization of N-propargyl-4-fluorobenzamide, 4 as a function of time in the presence of different catalysts. See figure for conditions.

This work describes heretofore unknown macrocyclic tri- and tetra-stibines, featuring 1,1′-ferrocenediyl units as linkers. In addition to reporting the isolation and structural characterization of these species, our work shows that these compounds may serve as ligands for gold(I), as unambiguously established in the case of the tristibine system. Interestingly, combining the tridentate derivative with a gold(I) chloride precursor forms a catalytically active stibine-gold complex that readily cyclizes N-propargyl-4-fluorobenzamide in the absence of an added activator. The mechanism by which the catalysis occurs implies dissociation of the chloride ligand, a phenomenon that we have not yet been able to verify. Ongoing studies aim to shed light on this puzzling aspect.

A. T. has isolated and characterized compounds 1 and 2 and studied the catalytic activity of 1. S. B. synthesized and characterized compound 3. M. K finalized the crystallographic analysis of 1–3. F. P. G. directed the study and corrected the original draft. All co-authors participated in data curation and manuscript preparation.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article can be found in the supplementary information (SI). Supplementary information: synthetic details, characterization data, spectroscopic data, crystallographic details, and computational details. See DOI: https://doi.org/10.1039/d6cc02816k.

CCDC 2551732–2551734 contain the supplementary crystallographic data for this paper.^31a–c

Acknowledgements

This work was fully carried out at Texas A&M University with support from the Department of Energy and the Welch Foundation (A-1423).

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Footnote

† A. T and S. B. contributed equally to this study.

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