Charge-transfer interactions between piperidine as donor with different σ- and π-acceptors: Synthesis and spectroscopic characterization

2.1 Preparation of piperdine-acceptor charge-transfer complexes (acceptor = IBr, DDQ, DCQ and DBQ)

2.2 Instrumentation and physical measurements

The electronic spectra of the donors, acceptors and the resulted CT-complexes were recorded in the region of (200–800 nm) by using a Jenway 6405 Spectrophotometer with quartz cells, 1.0 cm path in length. Photometric titration were performed at 25 °C for the reactions of donors with acceptors in chloroform, as follow: the concentration of the donors in the reaction mixtures was kept fixed at 5.0 × 10⁻⁴ M, while the concentration of acceptors were changed over a wide range from X × 10⁻⁴ to Y × 10⁻⁴ M. These produced solutions with donor: acceptor molar ratios varying from 1:0.25 to 1:4.00. IR measurements (KBr discs) of the solid donors, acceptor and CT-complexes were carried out on a Bruker FT-IR spectrophotometer (400–4000 cm⁻¹).

3.1 Electronic spectra of Pip/IBr, Pip/DDQ, Pip/DCQ and Pip/DBQ systems

The electronic spectra of the reaction mixtures containing IBr, DDQ, DCQ and DBQ with Pip as donor in CHCl₃ and/or CH₃OH show absorption bands located at (258 and 373 nm) for Pip/IBr, at (255, 401 and 550 nm) for Pip/DDQ, at 411 nm for Pip/DCQ and 399 nm for Pip/DBQ (Fig. 1A–D). These definite absorption bands do not belong to any of the reactants and well known to be characteristic of the formation of new CT-complexes; [(Pip)(IBr)], [(Pip)(DDQ)], [(Pip)(DCQ)] and [(Pip)(DBQ)]. Photometric titrations between (Pip) and mentioned acceptors; (IBr and DDQ) in CHCl₃, (DCQ and DBQ) in CH₃OH based on the (258 and 373 nm) for Pip/IBr, (255 and 401 nm) for Pip/DDQ, (411 nm) for Pip/DCQ and (399 nm) for Pip/DBQ systems reveals that the stoichiometry of the reactions is 1:1 (Fig. 2A–D). This was concluded on the bases of the obtained elemental analysis data of the isolated solid CT-complexes as indicated in the Table 1, as well as from the complexes infrared spectra, which indicate the existence of the bands characteristic for both the Pip and the acceptors.

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Figure 1

Electronic absorption spectra of: (A) Pip–IBr reaction in CHCl₃, (B) Pip–DDQ reaction in CHCl₃, (C) Pip–DCQ reaction in CH₃OH and (D) Pip–DBQ reaction in CH₃OH. (a) = donor (1.0 × 10⁻⁴ M), (b) = acceptor (1.0 × 10⁻⁴ M) and (c) = CT-complex.

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Figure 2

Photometric titration curves for: (A) Pip–IBr system in CHCl₃ at 258 and 373 nm, (B) Pip–DDQ system in CHCl₃ at 255 and 401 nm, (C) Pip–DCQ system in CH₃OH at 411 nm and (D) Pip–DBQ system in CH₃OH at 399 nm.

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In the photometric titration measurements, the concentration of Pip was kept fixed at 1.00 × 10⁻⁴ M, while the concentration of the acceptors was varied over the range of 0.25 × 10⁻⁴–3.00 × 10⁻⁴ M. The stoichiometry of the CT-complexes was determined by applying molar ratio method (Skoog, 1985). The symmetric curves with maximum at 1.00 mol fraction indicate the formation of 1:1 CT-complexes (Fig. 2A–D). The association constant (K) values and molar extinction coefficients (ɛ) of the CT-complexes studies have been determined using the 1:1 modified Benesi–Hildebrand Eq. (1) Abu-Eittah and Al-Sugeir, 1976. $C_{\text{a}}^{\text{o}}\text{and}{C}_{\text{d}}^{\text{o}}$ are the initial concentrations of the represented acceptors (IBr, DDQ, DCQ and DBQ) and the donor Pip, respectively, while A is the absorbance of the CT-complexation bands around 258 and 373 nm for [(Pip)(IBr)], 255 and 401 nm for [(Pip)(DDQ)], 411 nm for [(Pip)(DCQ)] and 399 nm for [(Pip)(DBQ)] complexes. Plotting the values of the ( $C_{\text{a}}^{\text{o}}·C_{\text{d}}^{\text{o}}/A$ ) against ( $C_{\text{a}}^{\text{o}}+C_{\text{d}}^{\text{o}}$ ) values for each acceptor, a straight lines are obtained with a slope of 1/ɛ and intercept of 1/Kɛ as shown in Fig. 3(A–D), for the reaction mixtures of Pip with (IBr and DDQ) and (DCQ and DBQ) in CHCl₃ and CH₃OH, respectively. The values of both K and ɛ associated with these complexes; [(Pip)(IBr)], [(Pip)(DDQ)], [(Pip)(DCQ)] and [(Pip)(DBQ)] are given in Table 2. The oscillator strength (f) which is a dimensionless quantity used to express the transition probability of the CT band (Lever, 1985) and the transition dipole moment (μ) of the CT-complexes (Tsubomura and Lang, 1964), The high values of both the formation constant (K) and the extinction coefficients (ɛ) of the resulted CT-complexes back to the expected high stabilities of the formed CT-complexes as a result of the expected high donation of the Pip nucleus.

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Figure 3

The plot of ( $C_{\text{d}}^{\text{o}}+C_{\text{a}}^{\text{o}}$ ) values against ( $C_{\text{d}}^{\text{o}}·C_{\text{a}}^{\text{o}}/A$ ) values for: (A) Pip–IBr system in CHCl₃ at 373 nm, (B) Pip–DDQ system in CHCl₃ at 401 nm, (C) Pip–DCQ system in CH₃OH at 411 nm and (D) Pip–DBQ system in CH₃OH at 399 nm.

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This phenomena is also strongly supported by determined the dissociation energy (W), which can be deduced from the corresponding CT energy, donor ionization potential (I_p) and electron affinity of the acceptor (DDQ, 1.95 eV; CHL, 1.37 eV; CLA, 1.10 eV) by using the following Eq. (1) McConnel et al., 1964:

The calculated value of (W) in the case of DDQ as acceptor is 4.50 eV. The energy of the n–π* interaction was calculated, where λ_CT is the wavelength of the CT band of the complexes; 3.33 eV (IBr), 3.10 eV (DDQ), 3.03 eV (DCQ) and 3.12 eV (DBQ).

The ionization potential (I_p) of the free donor of the highest filled molecular orbital on the donor was determined from the CT energies of the CT band of its complexes with DDQ and CHL using the following Eqs. (2) and (3) Aloisi and Pignataro, 1973; Foster et al., 1969:

The equilibrium constants are strongly dependent on the nature of the used acceptor including the type of electron withdrawing substituents to it, such as cyano and halo groups. For example, Table 2, the value of equilibrium constant for [(Pip)(IBr)] and [(Pip)(DCQ)] complexes in CHCl₃ and methanol, respectively, are almost the same. This value is about a twice times higher than the values of equilibrium constant for the complexes [(Pip)(DDQ)]. The [(Pip)(DBQ)] CT-complex has a five times higher than IBr/Pip and DCQ/Pip complex and so higher by nine times comparison with [(Pip)(DDQ)] charge-transfer complex.

3.2 Infrared spectra

The Infrared spectra of the 1:1 CT-complexes formed from the interaction of the donor and the respected acceptors with the general formula, [(Pip)(acceptor)], together with the corresponding free acceptor (DDQ, DCQ and DBQ) and donor Pip, are shown in Fig. 4(A–G). Full assignments concerning the all of infrared bands were located in the spectra are listed in Table 3.

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Figure 4

Infrared spectra of: (A) DDQ, (B) DCQ, (C) DBQ, (D) [(Pip)(IBr)], (E) [(Pip)(DDQ)], (F) [(Pip)(DCQ)] and (G) [(Pip)(DBQ)] complexes.

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Figure 4

Infrared spectra of: (A) DDQ, (B) DCQ, (C) DBQ, (D) [(Pip)(IBr)], (E) [(Pip)(DDQ)], (F) [(Pip)(DCQ)] and (G) [(Pip)(DBQ)] complexes.

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Figure 4

Infrared spectra of: (A) DDQ, (B) DCQ, (C) DBQ, (D) [(Pip)(IBr)], (E) [(Pip)(DDQ)], (F) [(Pip)(DCQ)] and (G) [(Pip)(DBQ)] complexes.

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Figure 4

Infrared spectra of: (A) DDQ, (B) DCQ, (C) DBQ, (D) [(Pip)(IBr)], (E) [(Pip)(DDQ)], (F) [(Pip)(DCQ)] and (G) [(Pip)(DBQ)] complexes.

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A comparison of the relevant IR spectral bands of the free donor, Pip and acceptors (DDQ, DCQ and DBQ) with the corresponding appeared in the IR spectra of the isolated solid CT-complexes clearly indicated that the characteristic bands of Pip show some shift in the frequencies (Table 3), as well as some change in their bands intensities. This could be attributed to the expected symmetry and electronic structure changes upon the formation of the CT-complex. The infrared explanation will take separately for each CT-complex to give an idea about the position of complexation as follows.

3.2.1

3.2.1 In the case of [(Pip)(IBr)] CT-complex

The vibration frequency of the ν_(N–H) group for Pip observed at 3393 cm⁻¹ is shifted to 3426 cm⁻¹ in the IR spectrum of the CT-complex. The group of bands are exhibited at 2948, 2834, 2803 and 2731 cm⁻¹ in the [(Pip)(IBr)] complex assigned to ν_s(C–H) + ν_as(C–H) vibrations with small shift to lower wavenumbers compared with the free Pip. The spectrum of mono iodobromide/Pip complex includes a few medium strong absorption bands lying at 2620, 2507 and 2405 cm⁻¹ and could be assigned to hydrogen bonding (Bellamy, 1975). Importantly, in the spectrum of [(Pip)(IBr)] complex, the vibrations group of δ_(N–H), ν_(C–N), CNC deformation show clearly changes compared with those of free Pip, Table 3. This observation proved that the complexation of (Pip) with IBr takes place via the –NH group through forming the hydrogen bonding as shown in Scheme 1.

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Scheme 1

Structure of the [(Pip)(IBr)] CT-complex.

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3.2.2

3.2.2 In the case of [(Pip)(DDQ)] CT-complex

The IR spectra of the molecular complex of DDQ with Pip indicate the ν_(C≡N) and ν_(C–Cl) of the free acceptor are shifted to lower wavenumber values on complexation. Since DDQ is deprived from any acidic centers, thus we may conclude that the molecular complexes are formed through π–π* and/or n–π* charge migration from HOMO of the donor to the LUMO of the acceptor. Also, the shift of ν_(C⚌O) of DDQ from higher to lower value on complex formation. IR spectrum of the molecular complex of DDQ with Pip indicate that the single ν_(C≡N) of the free acceptor molecule which exhibited at (2250 and 2231 cm⁻¹) was shifted to a lower wavenumber value (2201 cm⁻¹) while the ν_(C⚌O) absorption band of the free DDQ at 1673 cm⁻¹ was shifted to lower value (1623 cm⁻¹). Careful interpretation of IR spectra strongly supported that the CT interaction in the case of Pip/DDQ complex occurs through n–π* transition deprotonation of –NH group of Pip to only one of the CN groups by forming intermolecular hydrogen bonding (Scheme 2). In addition, the characteristic bands of the hydrogen bonding are appearing in the IR spectrum of the studied complex at 2630, 2517, and 2417 cm⁻¹, this group of bands are not existed in both spectra of the free donor and acceptor.

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Scheme 2

Structure of the [(Pip)(DDQ)] CT-complex.

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3.2.3

3.2.3 In the case of [(Pip)(DCQ)] and [(Pip)(DBQ)] CT-complexes

The IR spectra of the CT-complexes of DCQ and DBQ are characterized by a group of bands within the 2338–2807 cm⁻¹ range which are not present in the spectra of the free reactants. These bands are due to the stretching mode of a hydrogen bonding (Bellamy, 1975). This fact resulted from the hydrogen bond interaction through the proton (–NH) of Piperidine donor and the oxygen atom of the carbonyl group of acceptor. This is further strongly supported by the clearly appearance of the peaks characteristic of the ν_(O–H) at the wavelength 3401 and 3424 cm⁻¹ for DCQ and DBQ CT-complexes, respectively. The ν_(C⚌O) group appearing at 1696, 1686, 1658, 1656 and 1634 cm⁻¹ are disappeared in the case of DCQ and shifted to 1620 cm⁻¹ for DBQ, this led us to predicted that the carbonyl group is involvement in the complexation. The shift of the infrared characteristic bands of the acceptor moiety to lower wavenumbers and the donor moiety to higher values reflects a donor to acceptor charge-transfer transition from π to π* type, donor (HOMO)→acceptor (LOMO) Foster et al., 1969 (see Scheme 3).

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Scheme 3

Structure of the [(Pip)(DCQ)] and [(Pip)(DBQ)] CT-complexes.

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