Synthesis and crystal structure of an acetylenic ferrocenyl substituted phosphaalkene

Inorganica Chimica Acta 471 (2018) 741–745

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Research paper

Synthesis and crystal structure of an acetylenic ferrocenyl substituted phosphaalkene Dominique Miesel, Marcus Korb, Alexander Hildebrandt, Heinrich Lang ⇑ Technische Universität Chemnitz, Faculty of Natural Sciences, Institute of Chemistry, Inorganic Chemistry, D-09107 Chemnitz, Germany

a r t i c l e

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Article history: Received 24 October 2017 Received in revised form 4 December 2017 Accepted 6 December 2017 Available online 7 December 2017 Keywords: Phosphaalkene Solid-state structure Electrochemistry Ferrocene

a b s t r a c t (E)-(5,5-Dimethyl-1,4-diferrocenylhex-2-yn-1-ylidene)(2,4,6-tri-tert-butylphenyl) phosphine (3) was obtained in the reaction of 2,4,6-tri-tert-butylphenyl phosphine (1) with 1,4-diferrocenyl butadiyne (2) in minor yields. Compound 3 was structurally characterized by single crystal X-ray diffraction. The molecular structure of 3 consists of a linear butyne C4 backbone, which is in conjugation with a phosphene moiety resulting in the phosphapent-2-yn-4-ene motif with one ferrocenyl substituent bonded to a Csp3 and the other one to the Csp2 carbon atom. Both sandwich moieties are located at the same site of the C4 axis opposing the tert-butyl groups. This results in an alternated stacking within the crystal packing along [0 1 0] with weak T-shaped p interactions within the ferrocenyl containing layer. Electrochemical measurements revealed two reversible redox events for the two ferrocenyl groups at 20 mV and at 135 mV. Ó 2017 Elsevier B.V. All rights reserved.

1. Introduction The synthesis of phosphaacetylene in 1961 raises the interest for the syntheses of compounds with P,C double and triple bonds, of which, however, HC„P and H2C@PH are highly unstable at room temperature [1–4]. One possibility to stabilize molecules with P,C double or triple bonds is the kinetic stabilization by sterically demanding substituents, i.e. the 2,4,6-tri-tert-butylphenyl group [5–18]. Due to the steric hindrance, this substituent is well-suited for the stabilization of P@C double bonds [5,9,19–23]. In contrast, molecules containing a conjugated P@C double bond and a C„C triple bond to form acetylenic phosphaalkene-ynes are rarely described [24,25]. The incorporation of a phosphorus atom in a p conjugated system results in easily modifiable optoelectronic properties [24–30]. Furthermore, phosphaalkenes and –alkines were used in the synthesis of triazaphospholes, which are promising luminescent materials [31]. Herein, we describe an example of an en-yne P@CAC„C compound which was obtained in the synthesis of 2,5-diferrocenylsubstituted phospholes [32]. Such molecules are of interest, for example, to study electron transfer processes in the corresponding mixed-valent species between the ferrocenyl/ferrocenium units

⇑ Corresponding author. E-mail address: [email protected] (H. Lang). https://doi.org/10.1016/j.ica.2017.12.014 0020-1693/Ó 2017 Elsevier B.V. All rights reserved.

[33–48]. Phospholes bearing sterically demanding substituents can be used for a flattening of the phosphorus’ environment resulting in an increase of the delocalization [32,49–53]. For the synthesis of ferrocenyl phospholes, phosphines ArPH2 (Ar = phenyl, ferrocenyl, mesityl, 2,4,6-triphenylphenyl, 2,4,6-tri-tert-butylphenyl) are reacted with 1,4-diferrocenyl butadiyne [32]. Solely, in the case of Ar = 2,4,6-tri-tert-butylphenyl, next to the desired phosphole, the formation of (E)-(5,5-dimethyl-1,4-diferrocenylhex-2yn-1-ylidene)(2,4,6-tri-tert-butylphenyl) phosphine was observed. Herein, the properties, structural and electrochemical behavior of this molecule are described.

2. Materials and methods 2.1. General data All reactions were carried out under an atmosphere of argon using standard Schlenk techniques. Tetrahydrofuran was purified by distillation from sodium/benzophenone ketyl, toluene was obtained from a MBRAUN (MB-SPS 800) solvent drying and purification system (double column solvent filtration, working pressure 0.5 bar). For electrochemistry HPLC grade dichloromethane was purified by distillation from calcium hydride. For column chromatography silica with a particle size of 40–60 mm (230–400 mesh (ASTM), Fa. Macherey-Nagel) was used.

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2.2. Instruments FT IR spectra were recorded with a Nicolet IR 200 spectrometer (Thermo Company). NMR spectra were recorded with a Bruker Avance III 500 spectrometer (500.3 MHz for 1H, 125.7 MHz for 13 C{1H} and 202.5 MHz for 31P{1H} spectra). Chemical shifts are reported in d (parts per million) downfield from tetramethylsilane with the solvent as reference signal (1H NMR: CDCl3, d = 7.26; 13C {1H} NMR: CDCl3, d = 77.16; 31P{1H} NMR: standard external rel. 85% H3PO4, d = 0.0; P(OMe)3, d = 139.0). The melting point was determined using a Gallenkamp MFB 595 010 M melting point apparatus. The high-resolution mass spectrum was recorded with a Bruker Daltonik micrOTOF-QII spectrometer.

2.3. Electrochemistry Electrochemical measurements of 3 (1.0 mmol
L 1 [NnBu4][B (C6F5)4] as supporting electrolyte) in anhydrous dichloromethane were performed in a dried, argon purged electrochemical cell at 25 °C with a Radiometer Voltalab PGZ 100 electrochemical workstation interfaced with a personal computer. For the measurements a three electrode cell containing a Pt auxiliary electrode, a glassy carbon working electrode and an Ag/Ag+ (0.01 mmol
L 1 [AgNO3]) reference electrode fixed on a Luggin capillary was used. The working electrode was pretreated by polishing on a Buehler microcloth first with a 1 mm and then with a ¼ micron diamond paste. The reference electrode was constructed from a silver wire inserted in a 0.01 mmol
L 1 [AgNO3] and a 0.1 mol
L 1 [NnBu4][B (C6F5)4] acetonitrile solution in a Luggin capillary with a VycorÒ tip. This Luggin capillary was inserted in a second Luggin capillary containing a 0.1 mol
L 1 [NnBu4][B(C6F5)4] dichloromethane solution and a VycorÒ tip. Experiments under the same conditions showed that all reduction and oxidation potentials were reproducible within ±5 mV. Experimental potentials were referenced against an Ag/Ag+ reference electrode but the presented results are referenced against ferrocene as an internal standard as required by IUPAC [54]. To achieve this, each experiment was repeated in the presence of 1 mmol
L 1 decamethylferrocene (Fc⁄). Data were processed on a Microsoft Excel worksheet to set the formal reduction potentials of the FcH/FcH+ couple to 0.0 V. Under our conditions the Fc⁄/Fc⁄+ redox couple was at 619 mV vs FcH/FcH+, DEp = 60 mV, while the FcH/FcH+ couple itself was at 220 mV vs Ag/Ag+, DEp = 61 mV [55].

2.4. Single crystal X-ray diffraction analysis Suitable single crystals of 3 for X-ray diffraction analysis were obtained by diffusion of n-hexane into a dichloromethane solution containing 3 at ambient temperature. Data were collected with an Oxford Gemini S diffractometer at 109.85(10) K with Mo Ka radiation (k = 0.71073 Å). The structure was solved by direct methods and refined by full-matrix least-squares procedures on F2 [56,57]. All non-hydrogen atoms were refined anisotropically and a riding model was employed in the treatment of the hydrogen atom positions.

2.5. Reagents All starting materials were obtained from commercial suppliers and were used without further purification. 2,4,6-Tri-tert-butylphenyl phosphine [58] and 1,4-diferrocenylbutadiyne [59] were prepared according to published procedures.

2.6. Synthesis of (E)-(5,5-dimethyl-1,4-diferrocenylhex-2-yn-1ylidene)(2,4,6-tri-tert-butylphenyl) phosphine (3) To 0.25 g (0.9 mmol) of 2,4,6-tri-tert-butylphenyl phosphine (1) dissolved in a 1:1 mixture of tetrahydrofuran/toluene (40 mL, v/v) were added drop-wisely 0.4 mL (2.5 M, 0.9 mmol) of butyllithium at 0 °C. After stirring this solution for 30 min at 0 °C the solution was added drop-wisely to 0.43 g (1.0 mmol) of 1,4-diferrocenylbutadiyne (2) dissolved in 20 mL of tetrahydrofuran. The resulting reaction solution was stirred overnight at ambient temperature and afterwards, all volatiles were removed in vacuo. The remaining solid was purified by column chromatography (column size: 3  15 cm, silica) using a n-hexane/dichloromethane mixture (ratio 10:1; v/v) as the eluent. After evaporation of all volatiles, 3 was obtained as a red solid. Yield: 0.03 g (0.04 mmol, 5% based on 1). 2,5-Diferrocenyl-1-(2,4,6-tri-tert-butylphenyl)-1H-phosphole was obtained as main product (0.23 g, 0.3 mmol, 36%) [32]. IR data (KBr, m/cm 1): 3100 (w, CAH), 2968 (s, CAH), 2873 (m, CAH), 2188 (w, C„C). 1H NMR (500.3 MHz, CDCl3, d): 0.64 (s, 9H, CH3/ CHtBu), 1.33 (s, 9H, p-CH3/C6H2), 1.58 (s, 9H, o-CH3/C6H2), 1.63 (s, 9H, o-CH3/C6H2), 3.18 (d, 5JHP = 3.2 Hz, 1H, CHtBu), 4.03 (m, 1H, C5H4), 4.04 (m, 1H, C5H4), 4.08 (m, 1H, C5H4), 4.12 (s, 5H, C5H5), 4.18 (m, 1H, C5H4), 4.28 (s, 5H, C5H5), 4.41 (m, 1H, C5H4), 4.45 (m, 1H, C5H4), 4.92 (m, 1H, C5H4), 5.00 (m, 1H, C5H4), 7.45 (m, 2H, C6H2). 13C{1H} NMR (125.7 MHz, CDCl3, d): 28.1 (s, CH3/CHC (CH3)3), 31.6 (s, p-CH3/C6H2-C(CH3)3), 33.5 (d, 4JCP = 6.2 Hz, o-CH3/ C6H2-C(CH3)3), 33.7 (d, 4JCP = 6.6 Hz, o-CH3/C6H2-C(CH3)3), 35.2 (s, q C/C6H2-p-C(CH3)3), 36.1 (d, 5JCP = 2.7 Hz, qC/CHC(CH3)3), 38.4 (s, q C/C6H2-o-C(CH3)3), 38.5 (s, qC/C6H2-o-C(CH3)3), 46.6 (d, 4JCP = 1.6 Hz, CHtBu), 66.2 (d, JCP = 17.4 Hz, CH/C5H4), 67.0 (s, CH/C5H4), 67.1 (s, CH/C5H4), 69.1 (s, CH/C5H4), 69.2 (s, C5H5), 69.4 (s, CH/ C5H4), 69.5 (s, CH/C5H4), 69.6 (s, C5H5), 69.9 (s, CH/C5H4), 84.2 (d, JCP = 24.7 Hz, C„C), 86.6 (s, Ci/C5H4), 90.6 (d, 2JCP = 34.5 Hz, Ci/ C5H4), 105.8 (d, JCP = 11.1 Hz, C„C), 122.1 (d, 3JCP = 13.0 Hz, CH/ C6H2), 136.5 (d, 1JCP = 54.8 Hz, Ci/C1-C6H2), 149.7 (s, qC/C4-C6H2), 153.8 (s, qC/C2/C6-C6H2), 154.4 (s, qC/C2/C6-C6H2), 163.2 (d, 1JCP = 40.8 Hz, C@P). 31P{1H} NMR (202.5 MHz, CDCl3, d): 261.9 (s). HRMS (ESI-TOF, m/z): calcd for C46H57Fe2P: 752.2893, found: 752.2931 [M]+.

3. Results and discussion (E)-(5,5-Dimethyl-1,4-diferrocenylhex-2-yn-1-ylidene)(2,4,6tri-tert-butylphenyl) phosphine (3) was obtained by reacting 2,4,6tri-tert-butylphenyl phosphine (1) and 1,4-diferrocenylbutadiyne (2) according to a reaction procedure described by Märkl and Potthast [60–62] in the presence of n-butyllithium (Scheme 1). The respective Z-isomer was not isolated. As 3 was obtained in every reaction of 1 with 2 in minor yield by using n-butyllithium as lithiation reagent, one of the tBu groups of the 2,4,6-tri-tert-butylphenyl phosphine has to be transferred to the C4 chain of 2. The migration of tBu groups is widely known for supermesityl compounds in the presence of electrophiles, due to the good stabilization of the resulting carbocations [63–66]. However, an electrophilic mechanism is improbable. The formation of unexpected 3 is likely due to a nucleophilic attack of a vinyl anion at a phosphine tBu group. After deprotonation of the phosphine with nBuli [67], the attack at the butadiyne C1 carbon gives an allenic intermediate LiFcC@C@C@CFc(PHAr). In a consecutive step, this anion nucleophilically reacts with a tBu group of a further ArPHLi molecule giving Fc(tBu)C@C@C@CFc(PHAr). Either by 1,5sigmatropic rearrangement or by a deprotonation/protonation mechanism, the P-bonded proton is transferred to the C4 carbon atom resulting in butyne 3.

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Scheme 1. Synthesis of phosphine 3 and the corresponding phosphole [32].

Compound 3 is stable towards air and moisture both in the solid state and in solution. It has been characterized by IR and NMR (1H, 13 C{1H}, 31P{1H}) spectroscopy, and ESI-TOF mass-spectrometry. The electrochemical behavior was investigated by cyclic voltammetry (CV) and square wave voltammetry (SWV). The structure of 3 in the solid state was determined by single crystal X-ray diffraction analysis. In the IR spectrum of 3 the absorption of the C„C triple bond is observed at 2188 cm 1, which is characteristic for such molecules [68]. In the 1H NMR spectrum of 3 four singlets for the protons of the tert-butyl moieties between 0.64 ppm and 1.63 ppm were found (Experimental Part). Each proton of the ferrocenyl C5H4 rings appears as separate multiplet between 4.03 and 5.00 ppm due to the planar chirality of the Fc backbone and the E configuration. The signals for the C5H5 units are observed as singlets at 4.12 and 4.28 ppm, respectively. The CHtBu proton next to the C,C triple bond resonates at 3.18 ppm, while the multiplet of the C6H2 protons was found at 7.45 ppm. In the 13C{1H} NMR spectrum of 3 the signals for the tert-butyl units appear between 28 and 39 ppm (Experimental Part). The occurrence of the o-tBu signals as two independent signals might be due to a hindered rotation caused by the steric demand of the tert-butyl groups. This is supported by the occurrence of the aromatic CH protons as doublets with roof effect (Figure SI1, Supporting Information). The Csp3 carbon atom next to the C„C unit is found as doublet at 46.6 ppm (4JCP @ 1.6 Hz). For the sp hybridized carbon atoms two doublets at 84.2 ppm (JCP @ 24.7 Hz) and 105.8 ppm (JCP = 11.1 Hz) were observed. The doublet of the C,P double bond appears at 163.2 ppm (1JCP = 40.6 Hz). In the 31P{1H} NMR spectrum one singlet at 261.9 ppm is observed, in a typical range for en-yne P@CAC„C compounds [25,30]. The molecular structure of 3 in the solid state has been determined by single-crystal X-ray diffraction analysis. Suitable crystals were obtained by diffusion of n-hexane into a dichloromethane solution containing 3 at ambient temperature. The ORTEP with selected bond lengths, bond angles, and torsion angles of 3 is shown in Fig. 1. Compound 3 crystallizes in the monoclinic space group P21/c with one crystallographic independent molecule in the asymmetric unit, which contains one 1,4-diferrocenyl-but-2-yne molecule with the ferrocenyls in a syn arrangement. The P@C (1.693(5) Å) and C„C (1.197(7) Å) bond lengths are in accordance to those of similar compounds [24–28,30,69]. The bond angles around the P@C carbon atom (115.6(4) °, 125.5(4) °, 118.9(4) °) are typical for sp2 hybridized carbons and resemble those found in Refs. [24– 28,30,69]. The Csp2-bonded ferrocenyl exhibits an eclipsed (1.6(4) °) and the Csp3-bonded ferrocenyl unit a staggered (24.2(4) °) conformation. They are rotated out of the central linear C4 moiety by 18.4(5) ° of the Fe1 labeled one to the P1—C21—C22 plane and by 27.2(4) ° of the Fe2 labeled substituent to the C11—C24—C23 plane, resulting in an almost parallel position of both sandwich building blocks.

Fig. 1. ORTEP (50% probability level) of the molecular structure of 3 with the atom numbering scheme. All hydrogen atoms except the one bonded to C24 and one molecule of dichloromethane have been omitted for clarity. Selected bond distances (Å), angles (°), and torsion angles: Fe1–D1 = 1.6620(7), Fe1–D2 = 1.6575(7), Fe2–D3 = 1.6485(7), Fe2–D4 = 1.6485(7), C21–C22 = 1.426(7), C22–C23 = 1.197(7), C23–C24 = 1.459(7), P1–C21 = 1.693(5), P1–C29 = 1.847(5), D1–Fe1–D2 = 179.44 (6), D3–Fe2–D4 = 179.81(6), C1–C21–P1 = 118.9(4), C22–C21–P1 = 125.5(4), C1–C21–C22 = 115.6(4), C21–P1–C29 = 104.3(2), C21–C22–C23 = 175.3(5), C22–C23–C24 = 177.9(5), C1–D1–D2–C6 = 1.6(4), C11–D3–D4–C16 = 24.2(4). D1 = centroid of C1–C5; D2 = centroid of C6–C10, D3 = centroid of C11–C15; D4 = centroid of C16–C20.

The P@C double bond contains the 2,4,6-tri-tert-butylphenyl substituent (E) to the ferrocenyl with an almost planar torsion of 10.9(5) ° to the C4 arrangement and a perpendicular orientation of the C6-phenyl plane with 88.1(2) °. The phenyl ring is bended towards the molecule with an CAP—C angle of 104.3(2) °, providing space for the free electron pair, which is directed away from the rest of the molecule. The tert-butyl substituents are oriented in the same direction, contrary to the ferrocenyls. Whereas the Csp3-bonded tert-butyl moiety reveals a favourable gauche conformation (C28—C25—C24—C23, –175.6 (4) °), the torsion of the methyl groups at the phenyl bonded tert-butyl-substituents range between 1.9(5) ° (C31—C32—C43—C46) and 10.6(5) ° (C29—C34—C35—C38) in an almost coplanar orientation to the aromatic C6 plane. The crystal packing of 3 reveals weak intermolecular T-shaped p interactions [70] in a zick-zack pattern (Figure SI2, Supporting

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pair at the P atom. The bond lengths and angles of 3 are in accordance to similar compounds [24–28,30,69]. During electrochemical measurements two individual reversible redox processes at 20 mV and at 135 mV could be observed for the two unequal ferrocenyl moieties. The authors declare no competing financial interest. Acknowledgement We are grateful to the Fonds der Chemischen Industrie (FCI) for generous financial support. M.K. thanks the FCI for a PhD Chemiefonds Fellowship. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.ica.2017.12.014. References

Fig. 2. Cyclic voltammogram (solid line) and square wave voltammogram (dotted line) of 3; scan rate: 100 mV
s 1; in dichloromethane solutions (1.0 mmol
L 1) at 25 °C, supporting electrolyte 0.1 mol
L 1 [NnBu4][B(C6F5)4], working electrode: glassy carbon electrode.

Information) between the centroids of C1—C5 and C16—C20 labeled cyclopentadienyls with 4.9278(4) Å and plane intersections of 86 ° parallel to the c-axis [0 0 1]. The aliphatic and ferrocenylic substituents are directed to opposite directions, which results in an assembling in the packing (Figure SI3, Supporting Information). Thus, an alternation between layers of ferrocenyls and tert-butyl moieties along the b-axis [0 1 0] occurs. The phenyls assemble in a parallel displaced stacking with the shortest distance between the C32 labeled atoms (symmetry operation: 1–x, –y, –z) by 4.999(9) Å. These parallel planes are positioned on the (2 3 0) or (4 6 0) planes for the adjacent layer, parallel to the c-axis. The electrochemical properties of 3 were investigated by cyclic voltammetry and square wave voltammetry. An anhydrous dichloromethane solution containing 0.1 mol
L 1 of [NnBu4][B(C6F5)4] was used as supporting electrolyte [71]. The voltammetry measurements were performed at 25 °C. All potentials are referenced to the FcH/FcH+ redox couple [54]. The cyclic voltammogram of 3 is depicted in Fig. 2. Compound 3 shows, as typical for different redox-active moieties, two reversible redox events at 20 mV and 135 mV for the two unequal ferrocenyl groups. The redox processes exhibit differences of anodic and cathodic peak current of DEp = 79 mV to 86 mV. 4. Conclusion The reaction of 2,4,6-tri-tert-butylphenyl phosphine (1) and 1,4-diferrocenyl butadiyne (2) resulted in the formation of (E)(5,5-dimethyl-1,4-diferrocenylhex-2-yn-1-ylidene)(2,4,6-tri-tertbutylphenyl) phosphine (3). Compound 3 is characterized by a mC„C absorption in the IR spectra at 2188 cm 1. In the 13C{1H} NMR spectra the signals of the carbons of the C„C triple bond were found at 84.2 ppm and 105.8 ppm. The doublet of the carbon atom of the P@C group was observed at 163.2 ppm with 1JCP = 40.6 Hz, which is characteristic for this type of compounds [25,30]. Single crystal X-ray diffraction analysis of 3 showed a linear C4 unit in conjugation with a phosphine group and the two ferrocenyls syn at the C4 axis. The phenyl cycle on the opposite site is bended towards this C4 axis enabling increased space for the free electron

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