Formation of alkyne-bridged ferrocenophanes using ring-closing alkyne metathesis on 1,1’-diacetylenic ferrocenes

Novel alkyne-bridged ferrocenophanes [fc{CO2(CH2)nC≡}2] (2a: n = 2; 2b: n = 3) were synthesized from the corresponding terminal diacetylenic ferrocenes [fc{CO2(CH2)nC≡CH}2] (1a: n = 2; 1b: n = 3) through ring-closing alkyne metathesis (RCAM) utilizing the highly effective molybdenum catalyst [MesC≡Mo{OC(CF3)2CH3}3] (MoF6; Mes = 2,4,6-trimethylphenyl). The metathesis reaction occurs in short time with high yields whilst giving full conversion of the terminal alkynes. Furthermore, the solvent-dependant reactivity of 2a towards Ag(SbF6) is investigated, leading to oxidation and formation of the ferrocenium hexafluoroantimonate 4 in dichloromethane, whereas the silver(I) coordination polymer 5 was isolated from THF solution.

S2 analyser (Thermo Fisher Scientific, Bremen, Germany) in direct infusion mode using a custom made microspray-device mounted on a Proxeon nanospray ion source. The microspray-device allows for the sample infusion through a stainless steel capillary (90 µm i. d.). Accurate mass measurements in the orbitrap were performed using the lock mass option of the instrument control software using the cation of tetradecyltrimethylammonium bromide (0.1 mg/mL; 256.29988 amu) as internal mass reference.

General Procedure for the synthesis of ferrocenyl esters 1a and 1b
To a solution of the alcohol (2.1 equiv, 13.5 mmol), NEt3 (2.1 equiv, 13.5 mmol), and DMAP (0.08 equiv, 0.5 mmol) in DCM (20 mL) at 0 °C a solution of ferrocenyl dichloride 3 (2 g, 6.4 mmol) in DCM (20 mL) is slowly added via a dropping funnel under inert gas atmosphere. The resulting orange suspension is stirred overnight at room temperature. The reaction mixture is then washed with water and sat. NaCl (aq), dried over Na2SO4 and evaporated under reduced pressure. The dark brown and oily crude product is purified using column chromatography (hexane/EtOAc 1:5). The products 1a and 1b are obtained as crystalline orange solids.

Catalytic ring-closing alkyne metathesis
Scheme S1. Catalytic RCAM of substrates 1a and 1b towards the monomeric ferrocenophanes 2a and 2b; in some cases the formation of the dimeric ferrocenophanes S1 and S2 were observed by means of NMR spectroscopy and mass spectrometry.

Preparation of ferrocenophane 2a
To a solution of 1a (0.25 mmol) and MS 5Å in toluene (56 mL, 4.5 mM) under inert Ar atmosphere the catalyst MoF6 (2 mol %, 0.005 mmol) is added as a solid. After stirring at room temperature for 2 hours the reaction mixture is filtered over silica and the patch is washed with Et2O and DCM. The orange filtrate is evaporated under reduced pressure and the crude product was purified by flash column chromatography on silica gel with 1:5 hexane/EtOAc yielding 2a as an orange crystalline powder. In a second attempt the reaction was repeated using 12 mL of toluene (21 mM). Crystals suitable for X-ray diffraction analysis could be obtained from a slowly cooled

Preparation of ferrocenophane 2b
To a solution of 1b (0.25 mmol) and MS 5 Å in toluene (56 mL, 4.5 mM) under inert Ar atmosphere the catalyst MoF6 (2 mol %, 0.005 mmol) is added as a solid. After stirring at room temperature for 4 hours the reaction mixture is filtered over silica and the patch is washed with Et2O and DCM. The orange filtrate is evaporated under reduced pressure and the crude product (42 mg, 89%) was purified by flash column chromatography on silica gel with 5:1 hexane/EtOAc yielding 25 mg (53%) of 2b as an orange crystalline powder. The dimeric compound S2 could only be eluted as a mixture with 2b in a ratio of roughly 1:0.75 (16 mg). However, single crystals of 2b suitable for X-ray analysis could be obtained from that mixture from a DCM solution layered with n-hexane.

Crystallographic Data
Numerical data for the coordination polymer 5 are collected in Tables S1 and S2. Crystallographic data for all structures are given in Tables S3-S5

Special features and exceptions:
1a: The crystal structure was refined as a 2-component inversion twin with a twin ratio of 38% for the minor component. The iron atom was found disordered over two positions with an occupancy of 74% for the major position. The two positions of the iron atoms would correspond to a b-glide plane. Since the occupancy of these two atoms is far from 50%, we decided to refine this structure in the noncentrosymmetric space group Pca21.

4:
A part of the main molecule is refined with a disorder model comprising two positions.

5:
Three disordered molecules of THF were refined over two positions each. The respective atoms were refined isotropically.   Table S1 and Table S2.