Synthesis of the first sulphur-containing platinum(II) alkenylarylalkynyl complexes by photoirradiation

Abstract

A new synthetic route has been developed to construct a new series of sulphur-containing trans-platinum(II) alkenylarylalkynyl complexes with general formula trans-[(PR₃)₂–Pt–{C≡C–Ar–CH=CH(SPh)}₂], (where R = ethyl and Ar = phenylene 2a, biphenylene 2b, 2,5-dimethylphenylene 2c, and 2,5-dimethoxyphenylene 2d), having one phenylthio moiety in each alkenyl backbone. Reaction of the diterminal alkynyl areneacetylide of the trans-platinum(II) complex with benzenethiol in chloroform, upon photoirradiation, readily affords the trans-platinum(II) alkenylarylacetylides in good to excellent yields with good regioselectivity. The absorption and photoluminescence spectra indicated that the emission was exhibited in the range 399–425 nm depending on the nature of the acetylide ligand bound to the central metal platinum.

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The research on molecular functional materials is undoubtedly one of the most promising active areas of topical interest owing to their potential capability for applications as useful devices. Researchers have started developing and searching for versatile building blocks for the construction of materials with desired properties to facilitate various applications. In the past several decades, there has been a growing interest in the design of conjugated ring systems which represent an interesting material exhibiting different material properties of the constituent molecules as well as intermolecular interactions. So, materials containing π-conjugated molecular systems is important as it provides an avenue to organic materials having desired optical, electronic, and optoelectronic properties.^1,2 Although most of the work in this field has focused on conjugated materials that are comprised of organic building blocks, there has been an increasing interest in the properties of organometallic conjugated materials such as organogelators,³ molecular wires,⁴ nonlinear optics.² This interest derives from the fact that incorporation of heavy metals into an organic conjugated framework can elicit large effects on the electronic and optical properties of the materials.⁵ There are many M–C≡C bond-forming reactions that are useful for the synthesis of a wide variety of transition metal–alkynyl conjugated systems.^6–8 Among them, the platinum(II) phosphine bis-alkynyl system, with its simple square planar geometry, has been widely explored.

Bearing this in mind, a plan was initiated to investigate the possibility of introducing phenylthio moiety into trans-platinum(II) alkynylarylacetylides to generate newly designed functionalized trans-platinum(II) alkenylarylacetylides having one phenylthio moiety in each alkenyl backbone. In view of the fact that acetylides are ideal moieties for the synthesis of rigid-rod organometallic species, it is expected that the introduction of phenylthio moiety containing functionalized acetylide derivatives would impact new properties to these systems.² In this communication, for the first time, we report a novel photochemical synthetic route for the synthesis of a new series of trans-platinum(II) alkenylarylalkynyl complexes, trans-[(PR₃)₂–Pt–{C≡C–Ar–CH=CH(SPh)}₂] (A) having one phenylthio moiety in each alkenyl backbone (where, Ar = phenylene, biphenylene, 2,5-dimethylphenylene, 2,5-dimethoxyphenylene, and R = ethyl).

When a mixture of benzenethiol and platinum(II) complex containing an extended alkynyl ligand trans-(Et₃P)₂Pt{C≡C–C₆H₄–C≡CH}₂ (1a) in chloroform was photoirradiated⁹ under tungsten lamp, photochemically a novel addition product (2a), a sulphur-containing platinum(II) alkenylarylalkynyl complex, was obtained in good yield (82%, isolated yield, Scheme 1). Its structure was elucidated by UV/Vis, IR, NMR spectroscopy and ESI-HR mass spectrometry as well as elemental analysis.

Scheme 1 Photochemical addition of benzenethiol to platinum(II) alkynylarylalkynyl complex.

To optimize reaction conditions, various solvents were examined under similar conditions (Table 1). Chloroform was found to be the best solvent (isolated yield 82%, entry 1, Table 1), although the addition reaction also proceeded smoothly in benzene (isolated yield 77%, entry 2, Table 1) and toluene (isolated yield 66%, entry 3, Table 1).

Table 1 Optimization of photochemical addition reaction of benzenethiol with trans-platinum(II) acetylides (1a) in various solvent^a

Entry	Pt-acetylide	Solvent	Duration (h)	Yield^b (%), 2a
a Reactions were carried out in 0.6 mL of deuterated solvent under photoirradiation by using 1a, trans-(Et3P)2Pt{C≡C–C6H4–C≡CH}2 (0.1 mmol), and benzenethiol (0.25 mmol).b Isolated yield.
1	1a	Chloroform-d	3	82
2	1a	Benzene-d6	3	77
3	1a	Toluene-d8	4	66

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Under the optimized reaction conditions, several platinum(II) acetylides, trans-(Et₃P)₂Pt(C≡C–Ar–C≡CH)₂, (1) (where, Ar = phenylene, biphenylene, 2,5-dimethylphenylene, and 2,5-dimethoxyphenylene) were also examined‡ (Scheme 2). The results are summarized in Table 2. The scope of this reaction is quite general. The yields of the isolated addition products range from good to excellent with good regioselectivity. This methodology tolerates trans-platinum(II) acetylide complexes, with extended alkynyl ligands of diterminal acetylides, containing different arene rings and substituted arene rings. Bearing methyl groups at 2- and 5-positions on the aryl ring of platinum(II) acetylides provided very good yield (85%, entry 3, Table 2) of addition product. Bearing methoxy group at 2- and 5-positions on the aryl ring of platinum(II) acetylides also provided very good yields, (84%, entry 4, Table 2). E/Z isomers were identified according to their coupling constants of the vinylic proton.⁹ E isomer is formed predominantly over Z isomer; probably owing to minimizing steric repulsion, major product is E isomer.

Scheme 2 Photochemical addition of benzenethiol to various platinum(II) alkynylarylalkynyl complexes.

Table 2 Optimization of photochemical addition reaction of benzenethiol with various platinum(II) acetylides^a

Entry	Pt-acetylide	Hetero atom reagent	Duration (h)	Isolated Yield^b (%), 2, (E/Z)
a Reactions were carried out in chloroform solvent under photoirradiation by using trans-(Et3P)2Pt{C≡C–Ar–C≡CH}2 (0.1 mmol) and benzenethiol (0.25 mmol).b Inseparable mixer of E/Z isomers were estimated by ^1H NMR.
1	1a	PhSH	3	82 (2a) (60/40)
2	1b	PhSH	4	90 (2b) (60/40)
3	1c	PhSH	3	85 (2c) (89/11)
4	1d	PhSH	3	84 (2d) (94/6)

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All these newly synthesized compounds were isolated as pale yellow to yellow solids with yields ranging from 82 to 90%. They display good solubility in common organic solvents but are insoluble in hexane. The characteristic features of these new synthesized trans-platinum(II) complexes were based on their IR, ¹H and ³¹P{H} NMR spectra and positive ESI-HR [M + Na]⁺ mass spectra.

In IR spectra the ν(C≡C) stretching frequency is diagnostic of the characterization of the metal ethynyl complexes (MC≡C) and the absence of the terminal ≡C–H confirms the completion of the reaction. The platinum(II) alkenylarylalkynyl complexes display a single sharp ν(C≡C) absorption band in the range of 2090–2099 cm⁻¹, confirming the platinum-acetylenic carbon bond (PtC≡C) retained on the newly formed platinum(II) complexes 2. The IR spectra of each platinum(II) complexes 2 showed no bands in the range of 3200–3300 cm⁻¹, characteristic of ≡C–H stretching vibration, thus confirming that the terminal acetylenic groups selectively undergo the addition of benzenethiol. So, the IR spectra of platinum(II) complexes 2 provided important information, confirming that the addition reaction proceeded only at the terminal alkyne bond of platinum(II) alkynylarylalkynyl complexes 1. In the ¹H NMR spectra, the terminal acetylenic proton of each platinum(II) complexes 2 disappeared (about 3 ppm) and new peaks observed in each platinum(II) complexes 2 in the range of 6.42–7.03 ppm for the vinyl proton, and in all cases, signals arising from the organic spacer, phenylthio moiety and ethyl phosphine protons also display peaks in the expected region. The characteristic feature of the trans-platinum(II) alkenylarylalkynyl complexes, trans-[(R₃P)–Pt–{C≡C–Ar–CH=CH(SPh)}₂], (2) is a double doublet because of their alkenyl backbone [–HC=CH(SPh)] in the ¹H NMR spectra. A singlet in the ³¹P{H} NMR spectra was observed in each case, characteristic of a –Pt–(PR₃)₂– unit in a trans geometry. The ³¹P{H} NMR spectra of platinum(II) complexes 2 showed the expected signals consisting of three lines due to coupling with ¹⁹⁵Pt. The trans geometry around the platinum-diphosphine centres was confirmed by ³¹P{H} NMR spectroscopy based on the J_Pt–P coupling constant.¹⁰ The E/Z inseparable mixtures displayed singlet in ³¹P{H} NMR, because in both cases (E/Z), geometry is retained up to –(PR₃)₂Pt–{C≡C–Ar–CH=CH–}₂ and only changed in terminal alkyne, which converted to adduct of benzenethiol {–CH=CH(SPh)}. The J_Pt–P values obtained, i.e., 2366, 2367, 2380, and 2367 Hz for the platinum(II) complexes 2a, 2b, 2c, and 2d respectively, are in agreement with the values previously reported for other square planar platinum(II) complexes with trans geometry; cis coupling constants are generally much larger (about 3500 Hz).¹⁰ The molecular formulae for the complexes were also established by the intense molecular ion [M + Na]⁺ peaks in the positive ion ESI-HR mass spectra observed at m/z 924.2502 for 2a, at m/z 1076.2831 for 2b, at m/z 980.3151 for 2c, and at m/z 1044.2886 for 2d.

The UV/Vis spectroscopic characterizations were carried out on all four platinum(II) complexes. For all four platinum(II) complexes the lowest energy absorption bands in the UV/Vis spectra, in chloroform solution, at room temperature, occurred in the range 362–395 nm. The values of λ_max are reported in Table 3. The absorption bands are slightly influenced by the presence of different organic spacers. The lowest energy band, in each case, was tentatively assigned to a predominantly π(C≡C) → π*(C≡C) transition by comparison with related systems¹¹ but can be considered to have some LMCT character resulting from the possible admixture of a platinum (n + 1) p orbitals and a ligand π* orbital,¹² and their position moves to longer wavelengths due to coordination of acetylide ligands. As compared to the absorption band of platinum(II) complexes 1, we find that the position of the lowest energy absorption bands are red-shifted, in the platinum(II) complexes 2, after the inclusion of benzenethiol into the platinum(II) complexes 1. The highest redshift (27 nm) observed for our trans-platinum(II) complexes is recorded for complex 2c. The UV/Vis absorption maxima of complexes 1a, 1b, 1c, and 1d are observed at 345, 356, 335, and 377 nm respectively, and those of their corresponding thiolation adduct complexes 2a, 2b, 2c, and 2d are observed at 366, 364, 362 and 395 nm, respectively. In each case, a small redshift is observed, and the shifts are 21, 8, 27, and 18 nm for complexes 2a, 2b, 2c, and 2d, respectively as compared to complexes 1a, 1b, 1c, and 1d, respectively. This reveals that π-conjugation is preserved through the metal site by mixing of the frontier orbitals of metal and the ligand. K. S. Schanze et al. reported at 2004,¹³ butyl phosphine containing trans-platinum(II) alkenylarylalkynyl complex, trans-[(PBu₃)₂Pt{C≡C–C₆H₄–CH=CH(Ph)}₂], having one phenyl ring in each alkenyl backbone and UV/Vis absorption maxima was observed at 370 nm. As compared to trans-[(PBu₃)₂Pt{C≡C–C₆H₄–CH=CH(Ph)}₂] complex, sulphur-containing trans-platinum(II) complex 2d red-shifted (25 nm) but the other three complexes slightly blue-shifted. Preliminary measurements concerning the luminescence properties were also performed. The photoluminescence spectra recorded for the solutions of the trans-platinum(II) complexes under excitation at the wavelength of the absorption maximum (λ_max = 362–395 nm) showed emission, maxima in the region 399–425 nm (Table 3). The room temperature photoluminescence spectra, in chloroform, for trans-platinum(II) complexes 2a–d displayed emission bands in the blue region of the electromagnetic spectra. The feature is characteristic to emission from a singlet excited state (fluorescence) because of the small energy shift (stokes shift).¹⁴

Table 3 UV/Vis data for complexes 2a–d, in CHCl₃, at room temperature

Complexes^a	λmax (nm) absorption	ε (l mol^−1 cm^−1)	λmax (nm) emission
a The concentration is 1 × 10^−5 M.
2a	366	51 688	399
2b	364	79 751	411
2c	362	63 492	407
2d	395	46 124	425

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We have developed a new synthetic method for the simultaneous introduction of phenylothio moiety (benzenethiol) into terminal carbon–carbon triple bond of platinum(II) alkynylarylalkynyl complexes regioselectively, through photoirradiation to synthesize new platinum(II) alkenylarylalkynyl complexes containing one phenylthio moiety in each alkenyl backbone. Addition reaction proceeded only at the terminal alkyne bond of platinum(II) alkynylarylalkynyl complexes, because internal triple bond is sterically hindered. We are continuing to investigate various arene containing extended alkynes of platinum(II) diacetylide complexes as a reactants for the addition of substituted arylthiols and the optical properties of the products.

Notes and references

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15. Upon photoirradiation under tungsten lamp (500 W), cool water was passed on the sealed tube to maintain room temperature.
16. Crude product 2b was purified by column chromatography on silica gel eluting with hexane and dichloromethane.

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Footnotes

† Electronic supplementary information (ESI) available: ¹H and ³¹P{H} NMR spectra. See DOI: 10.1039/c4ra01410c

‡ General synthetic procedure: (2a) a mixture of trans-[(Et₃P)₂Pt{C≡C–C₆H₄–C≡CH}₂] (1a) (0.068 g, 0.1 mmol) and benzenethiol (0.027 g, 0.25 mmol) in chloroform-d (0.6 mL) was degassed using nitrogen atmosphere, and added to a sealed tube. The resulting mixture was photoirradiated under tungsten lamp for 3 h.¹⁵ The completion of the reaction was determined by TLC. The reaction product was evaporated to dryness. The crude product was purified by column chromatography on silica gel eluting with hexane and ethyl acetate and gave the title complex (2a) as a pale yellow solid in 82% yields (0.074 g), E/Z ratio: 60/40. IR (solid state, KBr): ν 2099 (C≡C) cm⁻¹; ¹H NMR (400 MHz, CDCl₃): (E/Z ratio: 60/40): δ 7.45–7.18 (m, 18H, Ar-H, SPh), 6.80 (d, 1.20H, J_H–H = 15.6 Hz), 6.71 (d, 1.20H, J_H–H = 15.6 Hz), 6.54 (d, 0.80H, J_H–H = 10.8 Hz), 6.42 (d, 0.80H, J_H–H = 10.8 Hz), 2.21–2.13 (m, 12H, CH₂) and 1.27–1.17 (m, 18H, CH₃); ¹³C NMR (100 MHz, CDCl₃): δ 8.23, 16.27, 109.73, 109.85, 121.66, 121.69, 124.57, 124.60, 125.68, 126.70, 127.05, 127.33, 128.27, 128.42, 129.06, 129.10, 129.44, 129.92, 130.69, 131.08, 132.41, 133.14, 135.59 and 136.37; ³¹P NMR (161.83 MHz, CDCl₃): δ 11.65 (J_Pt–P = 2366 Hz); ESI-HRMS [M + Na]⁺ m/z = 924.2502 (100%), calc. mass: 901.2633, anal. calc. for C₄₄H₅₂P₂PtS₂: C, 58.59; H, 5.81%. Found: C, 58.55; H, 5.88%. Similarly, others three complexes were synthesized. 2b: as a pale yellow solid in 90% yields¹⁶ (0.095 mg), E/Z ratio: 60/40. IR (solid state, KBr): ν 2098 (C≡C) cm⁻¹; ¹H NMR (400 MHz, CDCl₃): (E/Z ratio: 60/40): δ 7.63–7.24 (m, 26H, Ar-H, SPh), 6.91 (d, 1.20H, J_H–H = 15.2 Hz), 6.75 (d, 1.20H, J_H–H = 15.2 Hz), 6.61 (d, 0.80H, J_H–H = 10.8 Hz), 6.51 (d, 0.80H, J_H–H = 10.8 Hz), 2.21–2.18 (m, 12H, CH₂) and 1.28–1.20 (m, 18H, CH₃); ¹³C NMR (100 MHz, CDCl₃): δ 8.36, 16.33, 109.42, 123.15, 125.92, 126.35, 126.39, 126.46, 126.54, 126.81, 126.91, 127.20, 128.05, 129.15, 129.74, 130.06, 131.23, 131.43, 135.15, 135.24, 136.18, 136.88, 137.03, 139.58 and 140.13;³¹P NMR (161.83 MHz, CDCl₃): δ 11.70 (J_Pt–P = 2367 Hz); ESI-HRMS [M + Na]⁺ m/z = 1076.2831 (20%), calc. mass: 1053.3259, anal. calc. for C₅₆H₆₀P₂PtS₂: C, 63.80; H, 5.74%. Found: C, 63.89; H, 5.92%. 2c: as a pale yellow solid in 85% yields (0.081 g), E/Z ratio: 89/11. IR (solid state, KBr): ν 2090 (C≡C) cm⁻¹; ¹H NMR (400 MHz, CDCl₃): (E/Z ratio: 89/11): δ 7.44–7.08 (m, 14H, Ar-H, SPh), 6.95 (d, 1.77H, J_H–H = 14.8 Hz), 6.72 (d, 1.77H, J_H–H = 15.2 Hz), 6.66 (d, 0.23H, J_H–H = 10.8 Hz), 6.45 (d, 0.23H, J_H–H = 10.0 Hz), 2.43 (s, 0.69H, Ar-p-CH₃), 2.37 (s, 5.31H, Ar-p-CH₃), 2.26 (s, 6H, Ar-p-CH₃), 2.17–2.13 (m, 12H, CH₂) and 1.23–1.15 (m, 18H, CH₃); ¹³C NMR (100 MHz, CDCl₃): δ 8.29, 16.24, 19.24, 19.38, 20.81, 20.93, 108.61, 113.50, 122.08, 124.92, 125.76, 126.26, 126.51, 126.79, 128.19, 129.02, 129.10, 129.71, 130.73, 131.85, 131.89, 131.93, 132.73, 133.05, 135.64, 136.0, 136.23 and 136.57; ³¹P NMR (161.83 MHz, CDCl₃): δ 12.18 (J_Pt–P = 2380 Hz); ESI-HRMS [M + Na]⁺ m/z = 980.3151 (98%), calc. mass: 957.3259, anal. calc. for C₄₈H₆₀P₂PtS₂: C, 60.17; H, 6.31%. Found: C, 59.80; H, 6.36%. 2d: as a yellow solid in 84% yields (0.086 g), E/Z ratio: 94/6. IR (solid state, KBr): ν 2096 (C≡C) cm⁻¹; ¹H NMR (400 MHz, CDCl₃): (E/Z ratio: 94/6): δ 7.47–7.20 (m, 10H, SPh), 7.03 (d, 1.88H, J_H–H = 15.6 Hz), 6.89 (d, 1.88H, J_H–H = 15.6 Hz), 6.82 (d, 0.12H, J_H–H = 12.8 Hz), 6.80 (s, 2H, Ar-H), 6.78 (s, 2H, Ar-H), 6.44 (d, 0.12H, J_H–H = 10.4 Hz), 3.85–3.77 (12H, Ar-OCH₃) 2.29–2.21 (m, 12H, CH₂) and 1.26–1.18 (m, 18H, CH₃); ¹³C NMR (100 MHz, CDCl₃): δ 8.38, 16.09, 56.12, 105.41, 109.17, 115.91, 118.34, 122.32, 122.93, 126.44, 128.08, 128.98, 129.15, 129.91, 136.12, 150.47 and 154.39; ³¹P NMR (161.83 MHz, CDCl₃): δ 11.81 (J_Pt–P = 2367 Hz); ESI-HRMS [M + Na]⁺ m/z = 1044.2886 (92%), calc. mass: 1021.3056, anal. calc. for C₄₈H₆₀O₄P₂PtS₂: C, 56.40; H, 5.92%. Found: C, 56.33; H, 6.00%.

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