Unsymmetrically urea silatranes: Synthesis, characterization and a selective on–off fluorescence response to acetate anion

2.1 Materials and methods

2.2 Syntheses

2.3 Fluorescence titration

All titration experiments were performed on 2.5 × 10⁻⁵ M solutions of compounds 2, 3, 4 and 5 in CH₃CN. Typically, aliquots of freshly prepared Bu₄NX (X = F⁻, Cl⁻, Br⁻, and CH₃COO⁻) standard solutions (10⁻³ M in CH₃CN) were added and the fluorescence spectra of the samples were recorded.

3.1 Synthesis

A conventional method for the synthesis of substituted urea involves reaction of alkyl isocyanates with primary or secondary amines in organic solvents. The reactions of isocyanates with amines, having active hydrogen, are key reactions of isocyanate chemistry. A qualitative consideration of resonance hybrid structures of isocyanates indicates that the electron density is greatest on oxygen due to highest net negative charge and least on carbon with highest net positive charge, the nitrogen atom being intermediate with a net negative charge. The reactions of isocyanates with active hydrogen compounds involve attack by a nucleophilic center (nitrogen atom in case of primary/secondary amines and oxygen atom in alcohols) upon the electrophilic carbon of the isocyanates. Using conventional method, the unsymmetrically substituted urea functionalized silatranes 2–5 were synthesized by the rearrangement reaction of 3-isocyanatopropylsilatrane 1 and amines viz. pyrrolidine, piperidine, N-methylbenzenamine and 3-methylpyridine-2-amine as shown in Scheme 1 without any catalyst. The organosilicon isocyanates readily react with amines in a highly exothermic way. The reaction conditions are refluxing the mixture of 3-isocyanatopropylsilatrane with primary/secondary amines in chloroform at 80 °C for 4 h.

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Scheme 1 Synthesis of unsymmetrically substituted urea functionalized silatranes 2–5.

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3.2 Spectroscopic studies

FT-IR Spectra for all silatranes were recorded as neat spectra in the range of 4000–400 cm⁻¹. The presence of symmetric deformational vibration peak of the silatranyl skeleton possessing transannular N → Si bond is observed in the region 610–570 cm⁻¹ for all silatranes. The Si—O stretching vibration is assigned to the bands present in 1100–1075 cm⁻¹. The absorption band assigned in the range of 1550–1520 cm⁻¹ is indicated for HN—C(⚌O). IR spectra exhibit characteristic absorption bands of >C⚌O in the region of 1660–1625 cm⁻¹ for unsymmetrically substituted urea silatranes, which are detected at relatively low values due to weakening of C⚌O bond by resonance involved in NHC(⚌O)NH moiety. The stretching vibrations of methylene and methyl groups are appeared in the region 3000–2800 cm⁻¹. The asymmetric and symmetric NH stretching modes are assigned to the region 3400–3200 cm⁻¹.

The NMR spectroscopic (¹H and ¹³C) data, recorded at room temperature, for all compounds are consistent with the structure of the synthesized products. In ¹H NMR spectra, an upfield triplet appears for the methylene group attached to the silicon atom i.e. SiCH₂ for all silatranes due to increased electron density on silicon atom due to silatranyl skeleton. The CCH₂C proton of propyl chain resonances split up into a multiplet due to coupling with adjacent methylene protons and appeared in region of δ ≈ 1.45–1.60 ppm. The CCH₂N protons are confirmed by quartet due to coupling of adjacent methylene protons and NH group in the region of 3.02–3.60 ppm, and the exact value is seen to depend upon exocyclic group attached. The silatranes possessing Si(OCH₂CH₂)₃N silatranyl moiety consist of two intense triplets due to NCH₂ (δ ≈ 2.75–2.90 ppm) and OCH₂ protons (δ ≈ 3.65–3.80 ppm) as depicted. The NH proton attached to propyl chain is observed as a broad singlet peak in the region of 4.20–5.50 ppm. In ¹H NMR spectrum, methylene protons attached to nitrogen of pyrrolidine and piperidine appear as triplets in region of 2.94–3.05 ppm and other methylene protons appear as multiplets due to adjoining methylene carbon in the range of 1.19–1.23 ppm for compounds 2 and 3. The methyl group attached to secondary amine nitrogen and directly to aromatic ring appears at 3.15 and 2.75 ppm in case of compounds 4 and 5 respectively. The aromatic protons are observed in the range 7.19–7.39 ppm.

In ¹³C NMR spectra, the methylene carbon of propyl chain attached to silicon atom is appeared as the most shielded carbon atom which is observed around δ ≈ 12.50–14.20 ppm for all silatranes. The peak due to CCH₂C is observed in the region δ ≈ 24.00–25.50 ppm. The peak for CCH₂N is revealed with large variation in the region δ ≈ 42.00–50.13 ppm, certainly depending upon the exocyclic moiety. The silatranes possessing Si(OCH₂CH₂)₃N show two peaks in the region δ ≈ 51.00–55.50 and δ ≈ 56.50–58.50 ppm which are assigned to silatranyl NCH₂ and OCH₂ respectively. All unsymmetrically substituted urea and carbamate silatranes exhibit less shielded carbonyl carbon in the region 155.23–157.56 ppm.

The mass spectra of all silatranes show general characteristics of silatrane fragmentation pattern. All silatranes demonstrate their respective molecular ion peaks along with peaks corresponding to addition of H, Na and K. Besides the molecular ion peak, a peak corresponding to 1-propylsilatranyl ion (m/z = 216) is observed upon the cleavage of γ-carbon of propyl chain. Further, it loses one of the OCH₂CH₂ arms to form bicyclic fragment (m/z = 172). Homolytic cleavage of Si-CH₂ results in a silatranyl fragment with direct Si—N covalent bond (m/z = 174). It is noteworthy that the molecular ion peak is more abundant as compared to silatranyl ion peak. This silatranyl fragment (m/z = 174) further loses a cyclic arm to form bicyclic moiety (m/z = 132). A very intense peak due to protonated triethanolamine (m/z = 150) is observed along with an abundant peak at m/z = 192 that is attributed to the formation of silatranyl adduct with NH₃ for all silatranes.

3.3 X-ray diffraction studies

Single crystal X-ray diffraction study on the three compounds showed that molecules 2 and 3 crystallize in a monoclinic crystal system (space group = P2₁/c) while molecule 4 in orthorhombic crystal system (space group = P2₁2₁2₁). The ORTEP plots of all three compounds are shown in (thermal ellipsoids drawn at 50% probability) Fig. 1. Details of data collection and structure refinement, and selected bond lengths and angles of all the three compounds are listed in Table 1 and supporting data Table 1 respectively. All the three compounds possess usual silatrane skeleton containing Si atom in distorted trigonal bipyramidal geometry where coordination of tripodal trianionic N(CH₂CH₂O—)₃ entities acts as tetradentate coordinating unit each to Si to form five membered rings, so that a trans Si—N bond is formed. Due to this bond, Si achieves a coordination number of five with trigonal bipyramidal geometry. The O atoms of the tripodal trianionic unit occupy equatorial positions and the N donor is present at apical site opposite to the urea-functionalized alkyl moiety. Among various variable parameters of the geometry around Si center, one of interest is the Si—N bond lengths, which are 2.143(1), 2.173(3) and 2.156(2) Å for 2, 3, and 4 respectively. Similarly, the bond angles N—Si—C_ax are found to be 178.64(7), 178.00(1) and 178.79(1)° for 2, 3 and 4 respectively. All the Si—N bonds and angles around Si are within the range of reported values (Singh et al., 2014a,b). Again the existing —CH₂—CH₂—CH₂— alkyl chain between the urea moiety and Si center decreases the possible effect of —NHCONH— on the Si—N bonds and overall geometry. All the complexes are monomeric in nature with a slightly distorted (τ 0.99–1.0) trigonal bipyramidal (TBP) geometry around the Si atom specially for 2 and 3. The distortion parameter τ = (β − α)/60 was calculated from the structural data where the value of τ should be 0 for perfect square-pyramidal geometry and 1 for a perfect TBP structure and is applicable to five-coordinate structures as an index of the degree of trigonality (Addison et al., 1984).

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Figure 1 ORTEP plots of 1, 2 and 3 (thermal ellipsoids drawn at 50% probability).

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It is clear that all the structures are differing by their urea attached moiety which actually play an important role for overall molecular interactions and lattice arrangement. Among the various types of molecular interactions, —NH⋯O type is most common for all the three structures with value of 2.900(2), 2.933(3) and 3.172(3) for molecules 2, 3 and 4 respectively (Table 2). From the values it is clear that the —NH⋯O type interaction is strongest in case of 2 and least in 4 (Fig. 2). There is a water molecule as lattice solvent in complex 2 where it is strongly H-bonded via —OH⋯O interactions and forces the two molecules to stay in a T-type fashion. A simplifier model for lattice arrangement is shown with each lattice structure (Fig. 3). It is clear that both molecules 2 and 3 have similar arrangement along bc-plane; however, there is compact paring alignment in 3 than in 2. In case of 4, the arrangement is totally different along bc-plane, where molecules prefer to lie on a head-to-head paring fashion in alternative line arrangement (Fig. 3).

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Figure 2 Molecular interactions via —NH⋯O for 2, 3 and 4.

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Figure 3 Lattice arrangement along bc-plane for compounds 2, 3 and 4.

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3.4 Fluorescence titration

The fluorescence emission spectra of 2, 3, 4 and 5 were observed at 375, 383, 414 and 467 nm credited to substituted urea group by excitation at 331, 350, 360 and 464 nm in CH₃CN. The anion sensing ability of the compounds 2, 3, 4 and 5 with different anions as tetrabutylammonium salts was investigated through fluorescence emission in CH₃CN. The fluorescence spectra of all the silatranes changed noticeably when small amount of AcO⁻ ions was added whereas none of the other anions (F⁻, Cl⁻ and Br⁻) produce any noticeable change in the emission spectra as shown in Fig. 4 for compound 5. These silatrane based receptors have shown strong binding to acetate over other ions such as F⁻, Cl⁻ and Br⁻. The titration is performed using 2.5 × 10⁻⁵ M solution of compounds 2–5 in CH₃CN with 15 equiv of AcO⁻. In Fig. 5, the emissions were switched off or quenched, due to the formation of anion receptor hydrogen bonded complexes for all silatranes upon addition of CH₃COO⁻ ions (Kondo et al., 2011a,b). It is noteworthy that the fluorescence intensity of substituted phenyl urea silatranes 4 and 5 is higher than that of substituted urea silatranes 2 and 3 and it may be due to phenyl ring conjugation. The fluorescence intensity of 4 and 5 was decreased much with a slight bathochromic shift upon addition of AcO⁻ ions in CH₃CN. The high fluorescence intensity and significant spectral changes of 4 and 5 provide simple and effective fluorescence anion sensors.

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Figure 4 Fluorescence emission spectra for compound 5 in the presence of tetrabutylammonium salt of CH₃COO⁻, F⁻, Cl⁻ and Br⁻.

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Figure 5 Fluorescence emission spectra for compounds 2, 3, 4 and 5 (2.5 × 10⁻⁵) upon addition of 15 equiv acetate ion in CH₃CN at room temperature.

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