Synthesis and Spectroscopic Properties of Ferrocenyl Derivative Containing Donor and Acceptor Groups

Much interest has been devoted to organometallic NLO materials. We have become interested in exploring the utility of ferrocenyl group as the conjugating bridge. Thus, we synthesized 1-{{[1,3-(5-methylbenzo)dithiol]-2-yli- dene}methyl}-1’-[2-( p-nitrophenyl)-( E)-ethenyl]ferrocene ( 1). This new ferrocenyl compound has a donor and an acceptor group in 1,1’-positions. Investigations of the solvatochromic property of the compound revealed that it has polarized structure in a polar solvent, such as DMF. SHG efficiency of the compound was estimated by an SHEW (second-harmonic generation with the evanescent wave) method.

• Nonlinear Optical Material
• Ferrocenyl Derivative
• Solvatochromic Property
• SHG Efficiency

1. Introduction

Large second-order hyperpolarizabilities are known to be associated with the structure of molecules that have large differences in dipole moments between ground- and excited-states. A typical NLO (nonlinear optical) material consists of an electron-accepting group, a bridge, and an electron-donating group. The bridge is conjugated, thus allowing for good communication between the donor and the acceptor moieties. This sort of materials is useful in optical frequency conversion and optical data processing, and is expected to play a central role in the next generation of optoelectronics [1] [2] .

Although investigations were initially focused on purely inorganic system, a wide variety of organic NLO materials have now been discovered and have been vigorously investigated [3] [4] . In addition, since the report in 1987 by Green et al. that a ferrocenyl derivative A (Scheme 1) had an excellent SHG efficiency (62 times of the urea standard) [5] , much interest has been devoted to organometallic compounds possessing this property [6] [7] .

It has been pointed out that organometallic structures are intriguing candidates for study as NLO chromophores by virtue of their low-energy, yet sometimes intense, electronic transitions. Especially, metallocene derivatives have been the most thoroughly investigated compounds to date. In these compounds, metallocenyl groups are utilized as an effective electron donor. Indeed, it was found that the dipole projections of the β tensors of ferrocene complexes are comparable to those of methoxyphenyl systems with similar acceptors [8] . We have become interested in exploring the utility of ferrocenyl group as the conjugating bridge because both aromatic cyclopentadienyl rings in ferrocene interact with iron atom through their π-electron systems [9] . Previously Marder and co-workers observed that replacement of a cyclopentadienyl ring in complex A with a more electron donating pentamethylcyclopentadienyl ring leads to increased optical nonlinearity [8] . On the basis of a similar line of reasoning, we envisioned that incorporating an electron-rich substituent at one of the cyclopentadienyl rings would enhance the NLO properties.

We report herein the facile synthesis of a novel ferrocenyl derivative 1 (Scheme 2) bearing conjugated electron donating and accepting substituent in 1,1’-positions starting from ferrocenyl dicarbaldehyde 2. Syntheses of such asymmetrical disubstituted ferrocenes have become the focus of some attention [10] .

Scheme 1. Two canonical representation of A.

Scheme 2. Target compound 1.

2.1. General Considerations

Unless otherwise noted, all reactions were carried out under anaerobic and anhydrous conditions using Ar or N₂ and conventional Schlenk techniques using the general methods. Solvents were dried and purified in the usual manner, and stored under an atmosphere of Ar. Commercially available chemicals were used as such without any further purification. Infrared spectra (IR) were determined on a Perkin-Elmer 1600 series spectrometer using KBr disks prepared under inert atmosphere. NMR spectra were measured using a JEOL JNMEX270 SPECTROMETER at 270 MHz (¹H) and 67.5 MHz (¹³C). ¹H- and ¹³C-NMR chemical shifts were referenced to trimethylsilane (TMS). UV-vis absorption spectra were recorded on a JASCO model V-550 spectrophotometer. The determination of the second harmonic intensity of powder samples was made relative to that of powdered mNA (meta-nitroaniline) using an SHEW (second-harmonic generation with the evanescent wave method). The details of the experimental arrangement were described elsewhere [11] . Literature methods were used to prepare ferrocene-1,1’-dicarbaldehyde 2 [12] and Bu₃P=CH-4-C₆H₄NO₂ (7) [13] .

2.2. Synthesis of 2-(Dimethoxyphosphoryl)-1,3-(5-Methylbenzo)Dithiol (5)

This compound was prepared starting from toluene-3,4-dithiol by using known procedures (58% overall yield) [14] , [15] , [16] . IR (KBr) δ (C-H) 1436, 1390, ν (P=O) 1251, ν (P-O) 1040 cm⁻¹; ¹H-NMR (CDCl₃) δ = 7.03 - 7.09 and 6.82 (m, aromatic), 4.87 (d, ²J_PH = 5.3 Hz, 1H, S₂CH), 3.79 (d, ³J_PH = 10.6 Hz, 6H, POCH₃), 2.26 (s, 3H, C₆H₃CH₃).

2.3. Synthesis of 1-{{[1,3-(5-Methylbenzo)Dithiol]-2-Ylidene}Methyl}-1’-Formylferrocene (6)

A solution of 5 (1.66 g, 6.01 mmol) in THF (80 mL) was treated with n-BuLi (4.2 mL, 1.6 M in hexane, 6.61 mmol) at −78˚C. Then a cooled (−78˚C) solution of 2 (7.26 g, 30.0 mmol) in 200 mL of THF was added dropwise. The reaction mixture was stirred at this temperature for 90 min before being allowed to warm slowly to room temperature and stirred overnight. The solvent was evaporated to dryness under reduced pressure. The residue was transferred to a silica gel column and chromatographed with dichlorometane-hexane (1:3) to give compound 6 (0.27 g, 12%). IR (KBr) ν (C-H, aromatic) 3420, ν (C-H, CHO) 2921, ν (C=O) 1681, ν (C=C) 1578 cm⁻¹. ¹H-NMR (C₆D₆) δ = 9.89 (s, 1H, C₅H₄CHO), 6.55 - 6.80 (m, 3H, aromatic), 5.89 (s, 1H, S₂CCHC₅H₄), 4.56 and 4.14 (s, 2H; s, 2H, C₅H₄CHO), 4.32 and 3.97 (s, 2H; s, 2H, S₂CCHC₅H₄), 1.88 (s, 3H, C₆H₃CH₃); ¹³C-NMR δ = 192.1 (C₅H₄CHO), 136.9 (S₂CCHC₅H₄), 136.0, 135.7, 131.0, and 121.0 - 127.1 (aromatic), 110.2 (S₂CCHC₅H₄), 85.1, 69.9, and 68.7 (S₂CCHC₅H₄), 80.8, 74.0, and 70.6 (C₅H₄CHO), 20.7 (C₆H₃CH₃).

2.4. Synthesis of 1-{{[1,3-(5-Methylbenzo)Dithiol]-2-Ylidene}Methyl}-1’-[2-(p-Nitrophenyl)-(E)-Ethenyl]Ferrocene (1)

A solution of 7 (0.39 g, 1.04 mmol) in toluene (20 mL) was treated with n-BuLi (0.73 mL, 1.6 M in hexane, 1.14 mmol) under Ar at −78˚C. The solution was stirred for 60 min at −78˚C, then allowed to warm slowly to room temperature. The reaction mixture was stirred at this temperature for 120 min. To a cooled (0˚C) solution of 0.27 g (0.69 mmol) of 6 in 20 mL of diethyl ether was added the above solution and the resulting mixture was allowed to warm slowly to room temperature, then stirred overnight. The solvent was removed under reduced pressure and purification using preparative TLC on silica gel with dichloromethane-hexane (1:1) yielded 1 (0.010 g, 2.8%) as a purple powder. IR (KBr) ν (C=C) 1626 and 1591, ν (NO₂) 1509 and 1339 cm⁻¹: ¹H-NMR (C₆D₆) δ = 7.76 (d, J = 8.9 Hz, 2H, CHCHNO₂), 7.08 (d, J = 8.9 Hz, 2H, CHCHNO₂), 6.77 - 6.82 and 6.55 - 6.66 (m, 3H, aromatic), 6.26 (d, J = 11.9 Hz, 1H, C₅H₄CH=CH), 6.20 (d, J = 11.9 Hz, 1H, C₅H₄CH=CH), 5.95 (s, 1H, S₂CCHC₅H₄), 4.35 and 4.06 (s, 2H; s, 2H, S₂CCHC₅H₄), 4.12 and 4.01 (s, 2H; s, 2H, CHCHC₅H₄), 1.96 (s, 3H, C₆H₃CH₃); ¹³C-NMR δ = 148.0 (CNO₂), 146.0 (CCHCHCNO₂), 138.0 (S₂CCHC₅H₄), 137.0 (CS₂CCH), 133.4 (H₃CCCH), 132.9 (C₅H₄CH=CH), 130.6 (CHCHCCNO₂), 128.4, 123.8 and 122.2 (aromatic CH, methylbenzodithiol ring), 127.0 (C₅H₄CH=CH), 124.7 (CHCNO₂), 112.6 (S₂CCHC₅H₄), 85.8 (S₂CCHC), 82.5 (CCH=CH), 72.2 (CHCHCCH=CH), 71.7 (S₂CCHCCHCH), 71.1 (CHCCH=CH), 70.3 (S₂CCHCCH), 22.0 (C₆H₃CH₃). Anal. Found: C, 62.61; H, 3.97; N, 2.74. Calcd for C₂₇H₂₁NO₂S₂Fe: C, 63.41; H, 4.14; N, 2.74.

2.5. Synthesis of 1-[2-(p-Nitrophenyl)-(E)-Ethenyl]-1’-Formylferrocene (8)

A solution of 7 (3.09 g, 8.27 mmol) in toluene (150 mL) was treated with n-BuLi (5.8 mL, 1.6 M in hexane, 9.1 mmol) under Ar at −78˚C. The solution was stirred for 60 min at −78˚C, then allowed to warm slowly to room temperature. The reaction mixture was stirred at this temperature for 120 min. To a cooled (0˚C) solution of 3.00 g (12.4 mmol) of 2 in 150 mL of diethyl ether was added the above solution and the resulting mixture was allowed to warm slowly to room temperature, then stirred overnight. The solvent was removed under reduced pressure and the residue was chromatographed on a silica gel eluted with toluene/hexane (10:1), affording 1.43 g (48%) of 8 as a purple powder. IR (KBr) ν (C=O) 1678, ν (C=C) 1631 and 1590, ν (NO₂) 1508 and 1336 cm⁻¹: ¹H NMR (C₆D₆) δ = 9.72 (s, 1H, C₅H₄CHO), 7.90 (d, J = 8.9 Hz, 2H, CHCHNO₂), 6.94 (d, J = 8.9 Hz, 2H, CHCHNO₂), 6.52 (d, J = 16.2 Hz, 1H, C₅H₄CH=CH), 6.37(d, J = 16.2 Hz, 1H, C₅H₄CH=CH), 4.47 and 4.03 (s, 2H; s, 2H, C₅H₄CHO), 4.19 and 3.97 (s, 2H; s, 2H, CHCHC₅H₄); ¹³C-NMR δ = 192.2 (C₅H₄CHO), 147.2 (CNO₂), 143.5 (CCHCHCNO₂), 130.4 (C₅H₄CH=CH), 126.4 (CHCHCCNO₂), 125.8 (C₅H₄CH=CH), 124.3 (CHCNO₂), 83.9 (CCH=CH), 81.0 (CHOCCH), 73.8 (CHOCCHCH), 71.1 (CH=CHCCHCH), 70.9 (CHOCCH), 68.7 (CH=CHCCH).

2.6. Reaction of 5 with 8

A solution of 5 (1.66 g, 6.01 mmol) in THF (100 mL) was treated with n-BuLi (4.2 mL, 1.6 M in hexane, 6.61 mmol) at −78˚C under Ar. The solution was stirred for 90 min at −78˚C. Then a cooled (−78˚C) solution of 8 (1.81 g, 5.01 mmol) in 100 mL of THF was added dropwise. The reaction mixture was stirred at this temperature for further 120 min before being allowed to warm slowly to room temperature and stirred overnight. The solvent was evaporated to dryness under reduced pressure. Then the resulting residue was transferred to a silica gel column and chromatographed with hexane-ethyl acetate. Compound 1 was further purified using preparative TLC (0.050 g, 2.0%).

3. Results and Discussion

We chose the sulfur-based substituent, which can be derived from toluene-3,4-dithiol, as the electron-donating group since Togni and co-workers had found that the analogous CH=CS₂C₆H₄ group could pour a significant amount of electron density on the ferrocene moiety [17] . They reported that analysis of the cyclic voltammograms exhibited this substituent makes the electron removal easier with respect to ferrocene by about 100 mV. As the electron-accepting group we chose nitrobenzene conjugating moiety, which enhances the molecular hyperpolarisabilities of the compound [5] .

The synthetic route commenced with known ferrocenyl dicarbaldehyde 2. This compound is readily available from the reaction of 1,1’-dilithioferrocene with DMF [12] . Compound 1 would be synthesized from the successive reactions of 2 with the appropriate Wittig- and Wittig-Horner reagents (Scheme 3).

The donating group was accessible in four steps from toluene-3,4-dithiol, which was initially converted into S,S’-methylene derivative 3 [14] . Treatment of 3 with trityl fluoroborate in CH₃CN afforded 1,3-dithiolylium tetrafluoroborate 4 [15] , which reacted with trimethyl phosphite in the presence of one molar amount of sodium iodide [16] . Resulting 2-methoxyphosphinyl-1,3-dithiol 5 was converted into the phosphonate ylide by n-BuLi, and treatment of the Wittig- Horner reagent with one molar amount of the ferrocenyl dicarbaldehyde 2 in THF gave mono- and di-substituted compounds respectively [17] .

Scheme 3. Synthesis of 6.

Thus under these conditions, the major product was found to be a disubstituted compound and the desired monosubstituted aldehyde 6 was obtained in only 2% yield on chromatographing the reaction mixture. Employment of substantial excess (5-fold) of 2 led to improved result (12%) but the major product was again the disubstituted compound. It seemed likely that the electron-do- nating group that was initially introduced into one of the Cp rings strongly activates the remaining formyl group. The reaction of the monosubstituted aldehyde 6 with the ylide Bu₃P=CH-4-C₆H₄-NO₂, which was prepared from 4-nitrobenzyl chloride [13] , afforded a complex mixture. Purification of the mixture via conventional chromatography gave compound 1, though in low yield (Scheme 4).

During the purification, we observed the formation of side-products. Hence, the oxidative decomposition of 1 in solvents under air might have taken place. It has been pointed out that ferrocenyl derivatives sometimes decompose in a dioxygen-containing solution to unidentified, insoluble oxidized products [18] . The resulting compound 1 displays reasonable air and thermal stabilities in the solid state. Complex 1 was fully characterized by spectroscopic methods as well as by elemental analyses (vide infra).

An alternative route, in which the electron-accepting group is introduced to 2 prior to the introduction of the electron-donating group, was also explored. Thus 2 reacted with the ylide Bu₃P=CH-4-C₆H₄-NO₂ (7) to give the corresponding derivative 8 (Scheme 5).

Preliminary experiments revealed that the relative E:Z ratios and the yields were found to depend upon the solvent used. The highest proportions of the trans-olefin were obtained in diethyl ether-toluene mixtures. Although the highest yield was obtained in DMF-toluene mixtures, the selectivity of the products was low in this solvent. In contrast to the above reaction between 2 and 5, the present reaction proceeded well and gave only a trace of the disubstituted compound. The two isomers (E and Z) are readily distinguished by ¹H-NMR

Scheme 4. Synthesis of 1.

Scheme 5. Synthesis of 8.

spectroscopy, where distinctive patterns appear for each olefin protons. The E-isomer always exhibits an AB quartet, with J_HH = 16 Hz, downfield to the pair of doublets (J_HH = 12 Hz) exhibited by the Z-isomer [19] [20] .

Treatment of the E-isomer of 8 with the phosphonate ylide 5 in THF led to a complex mixture. A crude separation was effected by a silica gel column and isolated 1 was further purified by preparative thin-layer chromatography (TLC) (Scheme 6).

¹H-NMR analysis of 1 revealed that it is a Z-isomer, as is judged from the ¹H-NMR coupling constant of the CH=CH ethylene bridge (J = 11.9 Hz) as mentioned above. We have not isolated the E-isomer from a product mixture, which suggests that E/Z isomerization of this ethylene bridge occurred during the reaction since the starting complex 8 was E-isomer. We believe that the isomerization reaction occurred in the purification step. In support of this, Green and co-workers observed that E-isomer of A was converted into Z-isomer by hydrochloric acid and they postulated that a Z/E equilibration of complex A catalyzed by acidic residues in a column [19] . Use of an alumina column was found to be somewhat less effective for purification of the product. Since such isomerization did not take place in the reaction between 2 and the ylide 7, it may be envisioned that the intermediate ion can be stabilized by the electron-donating group which is attached to one of the cyclopentadienyl ring in 1 (Scheme 7).

It has been noted that there is a correlation between the solvatochromism of a molecule and the magnitude of the molecular second-order polarizability (β) [21] . Thus UV-vis absorption spectra in cyclohexane, in dichloromethane, and in DMF were recorded for compound 1 (Table 1).

Scheme 6. Reaction of 8 with 5.

Table 1

Absorption spectral data for complex 1 and A a

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