Poly(p-phenylenephosphaalkene): A π-Conjugated Macromolecule Containing PC Bonds in the Main Chain

Abstract

An unprecedented yellow polymer with low-coordinate phosphorus atoms in the backbone has been prepared. The material is soluble in polar organic solvents, and moderate molecular weights (Mn=2900–10 500 g mol−1) were estimated from 31P NMR spectroscopic end-group analysis. The possible π-conjugation was investigated by UV/Vis spectroscopy, which revealed a red shift in λmax for the polymer when compared with colorless molecular-model systems (see picture; left: model system, right: new polymer, in THF). Approximately twenty years ago, several examples of stable neutral compounds possessing acyclic (p–p) π bonds involving the heavier p-block elements were prepared.1 Subsequently, the synthesis, structures, and reactivity of numerous low-coordinate molecules has received extensive study and continues to attract considerable attention.2 Despite current interest in the preparation of organic macromolecules possessing π-conjugated backbones,3 to our knowledge, the incorporation of heavy-element multiple bonds into a π-conjugated polymer is unprecedented.4, 5 Furthermore, the incorporation of inorganic elements into the polymer backbone is synthetically challenging and often results in materials with unique properties.6 Therefore, the development of methods to prepare π-conjugated polymers containing heavier main-group (p–p) π bonds is of fundamental interest, and may ultimately lead to materials with novel properties.7 The poly(p-phenylenevinylene)s (PPVs)1 are an exciting class of luminescent organic macromolecules containing CC bonds which pose many synthetic challenges.3a,3c, 8 However, the possible incorporation of other stable multiple bonds, such as the well-established PC moiety,9 into the PPV structure has not been explored.10 Herein, we report the synthesis and characterization of a poly(p-phenylenephosphaalkene), a π-conjugated macromolecule containing phosphorus(III)–carbon double bonds in the polymer backbone. An elegant and general route to phosphorus(III)–carbon double bonds involves the rapid and thermodynamically favorable [1,3]-silatropic rearrangement of an acylphosphane to a phosphaalkene (Scheme 1).1a From a preparative standpoint, this method is probably the most convenient and versatile route to phosphaalkenes with minimal steric protection.11 We initiated our investigations by preparing model compounds 1 and 2 for the polymer 3, under conditions chosen to mimic a typical condensation polymerization. Therefore, phosphaalkene 1 was prepared in the absence of solvent by stirring mesitylene-2-carboxylic acid chloride and PhP(SiMe3)2 at 50 °C for several days. Analysis of the reaction mixture by 31P NMR spectroscopy showed only two signals (δ=149.2, 54 % and 134.0, 46 %), assigned to the E and Z isomers of 1, respectively. After distillation (110 °C; 0.1 mmHg), analytically pure 1 was isolated as a pale yellow liquid (yield, 75 %). The [1,3]-silatropic rearrangement of an acylphosphane to a phosphaalkene. Examples of molecules possessing two or more phosphaalkene moieties bridged by arylene spacers are uncommon;12 furthermore, there are only two previous reports of bis(phosphaalkene)s prepared through [1,3]-silatropic rearrangement.11b, 12b Thus, we set out to prepare 2 from a concentrated solution of PhP(SiMe3)2 (2 equiv) and 4 in THF and hexanes. After several days of heating and monitoring by 31P NMR spectroscopy, the PhP(SiMe3)2 was completely consumed, and pure 2 (yield, 42 %) was isolated as a colorless powder from a concentrated hexanes solution (−35 °C). Unexpectedly, the 31P NMR spectrum of 2 in CDCl3 shows eight resonances distributed over the regions expected for E- (44 %) and Z-phosphaalkene (56 %) isomers. In addition, there were six resolved signals for OSiMe3 groups in the 1H NMR spectrum. Four signals are expected for the three possible isomers (E,E; E,Z; Z,Z), thus, we postulate that the additional NMR signals arise from restricted rotation of the P=C groups about the central aryl plane in 2. In order to prepare the target poly(p-phenylenephosphaalkene), two bifunctional starting reagents (4 and 5) were required. The silylated phosphane 5 was prepared by treating 1,4-diphosphanobenzene13 with MeLi (4 equiv) in diethyl ether followed by addition of Me3SiCl (4 equiv).14 Analytically pure 5 was obtained as a colorless solid after vacuum sublimation at 100 °C. The thermolysis of 4 and 5 was conducted just above their melting temperature (85 °C) in a vacuum-sealed Pyrex tube. In a typical experiment, after about 24 h the initially colorless, free-flowing liquid was highly viscous and yellow.15 Poly(p-phenylenephosphaalkene) (3) was purified by precipitation of the polymer from a concentrated THF solution with cold hexanes (ca. −30 °C) and subsequent drying in vacuo. The brittle yellow solid (yield, 35 %) was dissolved in C6D6 and analyzed by 31P NMR spectroscopy, which showed broad overlapping signals for the E and Z isomers in 3 and for the polymer end groups (see Figure 1).16 The 29Si NMR (DEPT) spectrum exhibited three signals (δ=21 and 18, 3 (OSiMe3); 1.4 ppm (d), 3 P(SiMe3)2 end groups) with the signals arising from OSiMe3 groups in 3 showing similar chemical shifts to those in 1 (δ=21.3, 18.2 ppm). 31P NMR spectrum (C6D6) of 3 (trial 2) after precipitation with hexanes. An estimate of the molecular weight (Mn) of several samples was obtained from relative integration of the 31P NMR signals for P(SiMe3)2 end groups and PC units.17 The results are shown in Table 1; samples of 3 had moderate degrees of polymerization (X̄n, n in 3) between 5 and 21, not unusual for a step-growth reaction. Moreover, the elemental analyses, including chlorine analysis for two samples, were consistent with the molecular weights estimated from end-group analysis. The 13C NMR spectrum exhibited resonances consistent with the assigned structure and, importantly, broad signals for the CP moiety were detected at δ=212 and 198 ppm. The infrared spectra of films of 3 were remarkably similar to those for 1, 2, and other analogous phosphaalkenes.11a The thermal stability of 3 was assessed by thermogravimetric analysis (TGA) under dry helium. The polymer 3 was stable to weight loss up to 190 °C, whereupon approximately 40 % was lost, followed by an additional 20 % at 400 °C. After heating to 800 °C, 40 % of the mass remained as a black solid. Compound tpolym X̄ M UV/Vis Z/E [h] [g mol−1] λmax [nm] 1 328 310 0.85 2 550 314 1.27 3 (trial 1) 21 5 2 900 328 1.12 3 (trial 2) 27 21 10 500 338 1.14 3 (trial 3) 28 12 6 300 334 1.06 3 (trial 4) 34 12 6 300 334 1.05 The electronic structure of the new phosphaalkenes prepared was probed in THF solution (ca. 10−5 M) by using UV/Vis spectroscopy. Few detailed UV/Vis studies have been conducted on phosphaalkenes,12e, 18 although there are two possible chromophores; (n–π*) and (π–π*). Typical spectra for the polymer (3) and model compounds (1 and 2) are shown in Figure 2. Broad absorbances were observed for 1 (λmax=310 nm) and 2 (λmax=314 nm). Analysis of poly(p-phenylenephosphaalkene) (3) revealed a broad absorbance (λmax=328–338 nm) and a tail stretching into the visible region. We speculate that the bathochromic shift observed for poly(p-phenylenephosphaalkene) compared with 1 and 2 suggests some degree of π-conjugation through the phenylene and PC units. However, the red shift for 3 is less than that for trans-PPV compared with trans-stilbene (ca. 426 nm vs. 294/307 nm), which we attribute to conformational nonplanarity in the main chain, caused by the bulky C6Me4 groups in 3.19, 20 In addition, the breadth of the absorbance for 3 may be caused, in part, by the mixture of isomers present (Z/E≈1.1; compare cis-stilbene (276 nm) and trans-stilbene (294/307 nm)),20 and/or the polydispersity of the material. Further studies are necessary to confirm the extent of π-conjugation in 3. UV/Vis spectra of: 1 —; 2 - - - -; 3 (trial 3) —; 3 (trial 4) – – –. In summary, we have prepared and characterized the first π-conjugated polymer containing PC bonds in the main chain. Future studies will explore the scope of this synthetic methodology and attempt to develop routes to air- and moisture-stable poly(p-phenylenephosphaalkene)s. All manipulations were performed under a nitrogen atmosphere in a glove box or using standard Schlenk techniques. Assignment of NMR spectra were made with the aid of COSY, APT, HMQC, and HMBC experiments. The E and Z isomers of 1, 2, and 3 were assigned by comparison with analogous systems; the signals arising from the E isomer are observed downfield from those of the Z isomer in the 31P NMR spectrum.11, 21 1: Bis(trimethylsilyl)phenylphosphane (5.6 g, 22.0 mmol) and mesitylene-2-carboxylic acid chloride (4.0 g, 21.9 mmol) were stirred at 50 °C, and over several days quantitative conversion to 1 was observed by 31P NMR spectroscopy. Pure 1 (5.4 g, 75 %) was isolated as a pale yellow liquid after vacuum distillation (b.p. 110° C, 0.1 mmHg). 1: 31P NMR (121.5 MHz, C6D6): δ=149.2 (s, 54 %, E-1), 134.0 ppm (s, 46 %, Z-1); 1H NMR (400.1 MHz, CDCl3): E-1: δ=7.13–7.01 (m, 5 H; o, m, p-Ph), 6.73 (s, 2 H; m-Mes), 2.20 (s, 9 H; o, p-CH3), 0.42 ppm (s, 9 H; OSi(CH3)3), Z-1: δ=7.79 (m, 2 H; o-Ph), 7.35 (m, 3 H; m, p-Ph), 6.91 (s, 2 H; m-Mes), 2.48 (s, 6 H; o-CH3), 2.32 (s, 3 H; p-CH3), −0.05 ppm (s, 9 H; OSi(CH3)3); 13C NMR (CDCl3, 100.6 MHz): E-1: δ=197.3 (d, 1J (C,P)=49 Hz; CP), 138.5 (d, 1J (C,P)=39 Hz; i-Ph), 138.0 (d, 2J (C,P)=9 Hz; i-Mes), 137.4 (s; p-Mes), 134.2 (d, 3J (C,P)=5 Hz; o-Mes), 133.0 (d, 2J (C,P)=13 Hz; o-Ph), 128.0 (s; m-Mes), 127.7 (d, 3J (C,P)=6 Hz; m-Ph), 127.5 (s; p-Ph), 21.0 (s; p-CH3), 19.9 (s; o-CH3), 0.3–0.1 ppm (m; OSi(CH3)3), Z-1: δ=210.2 (d, 1J (C,P)=41 Hz; CP), 139.5 (d, 1J (C,P)=44 Hz; i-Ph), 138.1 (s; p-Mes), 136.8 (d, 2J (C,P)=28 Hz; i-Mes), 136.5 (d, 3J (C,P)=8 Hz; o-Mes), 133.3 (d, 2J (C,P)=13 Hz; o-Ph), 128.4 (s; m-Mes), 128.1 (s; m-Ph), 127.5 (s; p-Ph), 21.1 (s; p-CH3), 20.7 (s; o-CH3), 0.3–0.1 ppm (m; OSi(CH3)3); 29Si NMR (C6D6, 79.5 MHz): δ=21.3 (s), 18.2 ppm (s); UV/Vis (THF): λmax (ε)=310 nm (6000); IR (neat): =2921 (m), 2853 (m), 1601 (w), 1456 (s), 1377 (m), 1252 (vs), 1187 (vs), 847 cm−1 (s); MS (EI, 70 eV): m/z (%): 330 (3), 329 (10), 328 (44) [M+], 253 (1), 252 (4), 251 (23) [M+−C6H5], 148 (9), 147 (100) [C10H11O], 74 (5), 73 (72) [C3H9Si]; elemental analysis: C19H25OPSi: calcd C 69.48, H 7.67, found C 69.54, H 7.60. 2: To a mixture of bis(trimethylsilyl)phenylphosphane (0.93 g, 3.7 mmol) and 4 (0.47 g, 1.8 mmol) was added hexanes:tetrahydrofuran (5 mL:2 mL) until dissolved. The solution was stirred at 85 °C in a closed vessel for a several days and 31P NMR spectroscopy showed quantitative formation of 2. The solvent was removed in vacuo giving a pale yellow oil, from which 2 was isolated (0.42 g, 42 %) as a colorless powder from hexanes at −35 °C. 2: 31P NMR (CDCl3, 121.5 MHz): δ=155.2 (s, 20 %), 154.9 (s, 4 %), 150.7 (s, 2 %), 149.5 (s, 18 %), 134.0 (s, 23 %), 131.8 (s, 4 %), 129.9 (s, 12 %), 129.6 ppm (s, 17 %); 1H NMR (CDCl3, 300.1 MHz): δ=7.8–6.9 (m, 10 H; Ph-H), 2.39, 2.38, 2.23, 2.19, 2.11, 2.05, 2.02 (s, 12 H; Ar-CH3), 0.38, 0.31, 0.30 (s; OSi(CH3)3, E isomers (44%)), −0.09, −0.10, −0.15 ppm (s; OSi(CH3)3, Z isomers (56%)); 13C NMR (CDCl3, 75.5 MHz,): δ=211.8 (d, 1J (C,P)=44 Hz; C=P, Z-2), 198.6 (d, 1J (C,P)=49 Hz; CP, E-2), 140–137 (m; i-Ph and i-Ar), 134–132 (m; o-Ph), 131–130 (m; o-Ar), 128–127 (m; m-Ph and p-Ph), 19–17 (m; Ar-CH3), 0.8–0.1 ppm (m; OSi(CH3)3); UV/Vis (THF): λmax (ε)=314 nm (28 000); IR (neat): =3052 (s), 2956 (vs), 2922 (sh), 1451 (sh), 1432 (s), 1251 (vs), 1192 (vs), 981 (s), 900 (sh), 854 cm−1 (vs); MS (EI, 70 eV): m/z (%): 553 (3), 552 (12), 551 (35), 550 (82) [M+], 475 (4), 474 (6), 473 (26) [M+−Ph], 443 (2), 442 (3), 441 (7) [M+−PHPh], 371 (4), 370 (12), 369 (51) [M+−P(Ph)SiMe3], 74 (9), 73 (100) [SiMe3]; elemental analysis: calcd C 65.42, H 7.32, found C 65.32, H 7.47. 3: The same procedure was followed for each trial (1–4). All glassware was rinsed with Me3SiCl and flame dried prior to use. Compounds 4 (0.601 g, 2.32 mmol) and 5 (1.00 g, 2.32 mmol) were mixed as finely ground powders, and flame sealed in vacuo in a thick-walled Pyrex tube. The sample was placed in a preheated (85 °C) oven, whereupon the solids melted forming a colorless, free-flowing liquid. After 6–8 h, the mixture showed an increase in viscosity and was yellow. The reaction was monitored until the liquid was almost immobile (ca. 24 h), and the yellow/orange material was removed from the oven. The tube was broken, Me3SiCl was removed in vacuo, and the residue dissolved in a minimum amount of THF (ca. 3 mL). The viscous solution was evenly distributed over the walls of the flask, and cold hexanes (ca. −30° C) were added rapidly to precipitate the polymer as a yellow solid. The hexanes-soluble fraction was removed and the polymer 3 remained (0.384 g, 35 %) as a bright yellow glassy solid after drying in vacuo. 3: 31P NMR (CDCl3; 121.5 MHz): δ=157–149 (br m; E-3), 138–124 (br m; Z-3), −137 ppm (br; P(SiMe3)2 end groups; see Table 1 for Z/E ratio, and degree of polymerization for each trial). All integrations for end-group analyses are reported with a relaxation delay of 2.0 s; however, spectra were obtained by using 20 s and 30 s delays, and integrals were identical. 29Si NMR (CDCl3, 79.5 MHz): δ=21.7–20.5 (br m), 18.4–17.0 (br m), 1.4 ppm (d; 1J(Si, P)=26 Hz, end groups); 1H NMR (CDCl3, 400.1 MHz): δ=7.8–6.6 (br m; C6H4), 2.5–2.1 (br m; C6(CH3)4), 0.5–−0.5 ppm (br m; Si(CH3)3); 13C NMR (CDCl3, 100.6 MHz): δ=211.9 (br; Z-CP), 197.9 (br; E-CP), 142.0 (br; i-C6Me4), 139.1 (br; i-C6H4), 132.4, 130.2 (br; o-C6H4, o-C6Me4), 18.6, 17.5 (br s; C6(CH3)4), 0.7, 0.2 ppm (br s; OSi(CH3)3); UV/Vis (see Table 1); IR (film): =2955 (m), 2921 (m), 2849 (m), 1252 (vs), 1187 (s), 846 cm−1 (vs); elemental analysis: [C24H34O2P2Si2]n + [C26H43O2P2Si3Cl]: trial 1 calcd (n=5) C 59.80, H 7.32, found C 59.89, H 7.26, trial 3 calcd (n=12) C 60.43, H 7.28, Cl 0.57, found C 60.27, H 7.39, Cl 0.62, trial 4 calcd (n=12) C 60.43, H 7.28, Cl 0.57, found C 59.64, H 7.39, Cl 1.10. 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