TTF Derivatives Containing Phosphonic Acid Group As Potent Component for Organic-Inorganic Hybrid

INTRODUCTION

Like ferrocene, tetrathiafulvalene (TTF) is a wellknown electron donor molecule, which exhibits two reversible one-electron redox cycles.1 The two-step redox processes are shown in Scheme 1. Utilizing this property, a lot of TTF derivatives have been investigated for the development of organic magnets, sensors, conducting materials, etc.1 Various functional compounds such as thiophene,2 ferrocene3 and azobenzene,4 containing the phosphonic acid group have been investigated for the preparation of organic-inorganic hybrid materials exhibiting catalytic activity, non-linear optical properties and photo-induced sensing. TTF amphiphiles with two phosphonic acid moieties were also investigated for the purpose of developing Langmuir-Blodgett magnetic thin films.5

In this study, two mono-phosphonic acid derivatives of TTF (compounds 1b and 2b in Scheme 2) are synthesized, via the corresponding ethyl phosphonate derivatives, and characterized spectroscopically. Their electrochemical and structural characterizations were also carried out. This search for a facile way of synthesizing monophosphonic acid derivatives was intended to be the first step in the preparation of zirconium phosphate-based two-dimensional magnetic materials.4

Scheme 1.Two one-electron redox processes of TTF.

Scheme 2.Synthesis of compounds 1 and 2.

 

EXPERIMENTAL

The starting materials (CET-BMTTTF5 and CET-EDTTTF6) shown in Scheme 2 were prepared according to the previously reported methods. Cesium hydroxide monohydrate, diethyl 2-bromoethyl phosphonate, triethylamine, bromotrimethylsilane (TMSBr) and HPLC-grade organic solvents were commercially purchased and used as received.

The infrared spectra were recorded by the KBr pellet method on a Perkin Elmer Spectrum 100 in the range of 4000~400 cm-1. The 1H NMR spectra were obtained on a Bruker Avance 500 or 300 using CDCl3 as a solvent. Electrochemical studies were carried out at room temperature with a CHI 620A Electrochemical Analyzer (CHI Instrument Inc.) in a CH2Cl2 solution containing 0.5 mM of the sample and 0.1 M n-Bu4NPF6 as the supporting electrolyte, using a Pt-button (r = 1 mm) working electrode, Ag/AgCl reference electrode and Pt-wire (φ = 1 mm) counter electrode at a scan rate of 100 mV s-1. All redox potentials were referenced against the standard Fc/Fc+ redox couple (E1/2 = +0.504 V vs. Ag/AgCl).

Single crystals of compound 2b suitable for X-ray structure analysis were grown by the slow-evaporation method in CDCl3. The X-ray crystallographic data was collected at 200(2) K on a SMART APEX CCD SYSTEM (Bruker) equipped with graphite-monochromated Mo Kα radiation (λ = 0.71073Å). The structure was solved by the direct method and refined by full-matrix least-squares analysis using anisotropic thermal parameters for non-hydrogen atoms with the SHELXTL program.7 The crystal data and structure refinement parameters for 2b·Et3N·H2O are listed in Table 1. The detailed crystallographic information is available from the author on request.

Table 1.Crystal data and structure refinement parameters for 2b·Et3N·H2O

Preparation of 1a To an anhydrous DMF solution (20 mL) of CET-BMTTTF (1.5 mmol, 572 mg) degassed for 30min was added dropwise to a minimum amount of a methanol solution of cesium hydroxide monohydrate (1.65 mmol, 277 mg) over a period of 20 min. During the course of this addition, the color of the solution mixture became dark (red-orange solution). After stirring this mixture for an additional 30 min, diethyl 2-bromoethyl phosphonate (1.4 mmol, 0.25 mL) was added. The resulting dark brown mixture was stirred for 3 h. The white precipitate was filtered off, and the solvent was removed in vacuo. The product was separated by column chromatography (SiO2, acetone: diethyl ether=3:2) as orange-brown oil. Yield: 62% (425 mg), 1H NMR (300 MHz, CDCl3, ppm) 6.44 (1H, CH, s), 4.15-4.06 (4H, CH2, m), 2.99-2.90 (2H, SCH2, m) 2.43 (6H, SCH3, s), 2.17-2.02 (2H, CH2P, m), 1.36-132 (6H, CH3, t), FT-IR (KBr, cm-1) 2982, 2921 (-CH3,-CH2CH2-), 1477, 1424, 1391, 1368 (-CH2S-), 1243 (P=O), 1054, 1027, 964 (P-OEt), 888 (asym S-C-S str), 784, 771 (P-C), 528 (R(RO)2P=O).

Preparation of 2a Compound 2a was prepared by the same method as that employed for compound 1a using DMF (25 mL), CET-EDTTTF (0.70 mmol, 265 mg), CsOH (0.77 mmol, 129 mg), and diethyl 2-bromoethyl phosphonate (0.77 mmol, 0.14 mL). Yield: 92% (318 mg), 1H NMR (300 MHz, CDCl3, ppm) 6.44 (1H, CH, s), 4.17-4.06 (4H, CH2, m), 3.26 (4H, CH2CH2, s) 2.98-2.93 (2H, SCH2, m), 2.13-2.02 (2H, CH2P, m), 1.36-131 (6H, CH3, t), FT-IR (KBr, cm-1) 2980, 2926, 2853 (-CH2CH2-), 1716 (C=C), 1463, 1442, 1411, 1391, 1367 (-CH2S-), 1241 (P=O), 1056, 1028, 961 (P-OEt), 886 (asym S-C-S str), 803, 772 (P-C).

Preparation of 1b and 2b Triethylamine (0.58 mL, 28 eq.) was added to a dichloromethane solution (4 mL) of 1b (0.15 mmol, 74 mg) or 2b (0.15 mmol, 73 mg), to which TMSBr (0.56 mL, 28 eq.) was slowly added from a syringe. During this addition, a white precipitate became evident in the flask. After stirring for 2 h, the solution was concentrated in vacuo and methanol (4 mL) was added to the resulting residue followed by additional stirring for 4 h. The product was concentrated in vacuo, dissolved in dichloromethane, and extracted by distilled de-ionized water. The extraction procedure using dichloromethane resulted in the product being obtained as orange brown oil. Single crystals of 2b were obtained in CDCl3.

1b: Yield: 54%, 1H NMR (500 MHz, CDCl3, ppm) 10.72 (2H ,OH, br), 6.26 (1H, CH, s), 3.11 (2H, SCH2, br) 2.44 (6H, SCH3, s), 2.02 (2H, CH2P, br), FT-IR (KBr, cm-1) 2972, 2919 (-CH3, -CH2CH2-), 2687 (P-OH), 1735 (C=C), 1456, 1428 (-CH2S-), 1291 (P=O), 1159 (P-O-H), 1047, 935 ((OH)2P=O), 804 (asym S-C-S str), 771 (P-C).

2b: Yield: 77%, 1H NMR (500 MHz, CDCl3, ppm) 11.84 (2H, OH, br), 6.36 (1H, CH, s), 3.29 (4H, CH2CH2, s), 3.02-2.99 (2H, SCH2, m), 1.98-1.91 (2H, CH2P, m), FT-IR (KBr, cm-1) 2977, 2928 (-CH2CH2-), 2678 (P-OH), 1488, 1452, 1422, 1392 (-CH2S-), 1287 (P=O), 1126 (P-O-H), 1073, 1061, 939, 920, 893 ((OH)2P=O), 835, 804 (asym S-C-S str), 771 (P-C).

 

RESULTS AND DISCUSSION

The TTF derivatives with ethylcyanide groups (CETBMTTTF and CET-EDTTTF) synthesized by the phosphite-based cross-coupling reaction6 were transformed into the corresponding diethyl phosphonate derivatives (1a and 2a, respectively). Subsequently, their treatment with TMSBr in the presence of triethylamine afforded the phosphonic acid derivatives (1b and 2b, respectively). The isolated compounds were identified with 1H NMR and FT-IR spectroscopies.

Fig. 1.Molecular structure of 2b with numbering scheme. The hydrogen atoms and solvated molecules are omitted for clarity. Selected bond lengths (Å) and angles (°): C1-C2 1.323(9), C3-C4 1.351(8), C5-C6 1.340(8), C7-C8 1.415(10), C9-C10 1.511(9), C10-P1 1.809(7), P1-O1 1.493(5), P1-O2 1.507(5), P1-O3 1.552(5), C4-C3-S2 123.3(5), C4-C3-S3 122.3(5), S2-C3-S3 114.4(4), C3-C4-S5 124.2(5), C3-C4-S4 121.9(5), S5-C4-S4 113.9(4), O1-P1-O2 115.5(3), O1-P1-O3 110.1(3), O2-P1-O3 109.9(3).

Single crystals of 2b were grown and its crystal structure was analyzed by the X-ray diffraction method. The molecular structure of 2b (Fig. 1) shows that the TTF moiety is slightly folded-up with the peripheral ethylene moiety folded downwards and the ethylphosphonic acid moiety is approximately vertical to the EDTTTF plane. The intramolecular bond lengths and angles fell in the range of their averaged values.8 Only one intermolecular interaction shorter than the sum of their van der Waals radii (3.70Å)8 is observed, viz. S4...S5* (3.583Å; *-x+1, -y+1, -z).

Table 2.aThe samples are dissolved in CH2Cl2 containing 0.1 M TBA⋅PF6 electrolyte and analyzed using a scan rate of 0.1 V/s, Pt-disk working electrode, Pt-wire counter electrode, and Ag/AgCl ref. electrode. All potential values are referenced to the Fc/Fc+ couple (E1/2 = +0.504 V). bE1/2=(Epa+Epc) / 2

Fig. 2.The CV (A) and its DPV (B) of 1b measured in CH2Cl2 (Fc/Fc+=0.504 V vs. Ag/AgCl). Compound 2b shows identical patterns in its CV and DPV to those of 1b.

Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) measurements of 1b and 2b were performed and their parameters are listed in Table 2. The two compounds show the same redox behavior, exhibiting two reversible redox-cycles (Fig. 2) at almost the same potentials. These cycles are undoubtedly attributable to the two redox-steps of the TTF moiety. The half-wave potentials of the EDTTTF moiety in 2b (E11/2=0.610 V and E21/2= 1.035 V) are slightly more anodic than those of BMTTTF (0.608 V and 1.000 V), possibly due to the additional peripheral 6-membered ring.

In summary, two TTF derivatives with the mono-phosphonic acid group, which can be utilized as a potent component for functional organic-inorganic hybrids such as zirconium phosphate, were successfully synthesized in a facile way of reacting the corresponding ethyl phosphonate with bromotrimethylsilane (TMSBr) and triethylamine, and characterized by 1H NMR and FT-IR spectroscopies. Their electrochemical properties also indicate the presence of the TTF moiety in the compounds. One of the compounds was also analyzed by the X-ray diffraction method.