Azasilicon-Bridged Heterocyclic Arylamines : Syntheses , Structures and Photophysical Properties

The lithium κ1-enamides, Me2NSiMe2CHC(Ph)-{2,6-(R1)2-4-(R2)C6H2}NLi·3THF (R1 = iPr, R2 = H L1; R1 = Et, R2 = H L2; R1 = Me, R2 = H L3; R1 = R2 = Me L4; R1 = Et, R2 = Me L5), in the presence of titanium tetrachloride, undergo intermolecular rearrangement cyclization reactions resulting in 1,3-migration of the silicon groups and the elimination of dimethylamine affording five examples of bis-azasilicon-bridged heterocyclic arylamines, [{2,6-(R1)2-4-(R2)C6H2}N(Ph)CCSiMe2]2 (R1 = iPr, R2 = H D1; R1 = Et, R2 = H D2; R1 = Me, R2 = H D3; R1 = R2 = Me D4; R1 = Et, R2 = Me D5) in good yield, respectively. The molecular structures of D1 – D5 show the two fused N-Si-C-C-C rings to be co-planar indicative of extended π-conjugation, while their photophysical properties reveal them to be green/blue emitting with high luminescent quantum yields (ΦF range: 75 99%). Furthermore, compounds D serve as versatile reactants undergoing ring opening on hydrolysis to afford the saturated 1,4-diimines [2,6-(R1)2-4-(R2)C6H2}N(Ph)C-CH2]2 (R1 = iPr, R2 = H E1; R1 = Et, R2 = H E2; R1 = Me, R2 = H E3; R1 = R2 = Me E4; R1 = Et, R2 = Me E5). Alternatively, D can be employed in a redox-promoted cascade reaction to afford the conjugated 1,4-diimines, (E)-[{2,6-(R1)2-4-(R2)C6H2}N=C(Ph)CH]2 (R1 = iPr, R2 = H F1; R1 = Et, R2 = H F2; R1 = Me, R2 = H F3; R1 = R2 = Me F4; R1 = Et, R2 = Me F5). In addition to D1 – D5, E1 – E3, E5, F2 and F3 have been the subject of single crystal X-ray diffraction studies.


Introduction
2][3][4][5][6][7][8][9] These types of bridges not only stiffen the organic skeleton but also contribute to the electronic structure through orbital interactions which can result in fascinating properties.8][19] Elsewhere, the presence of both furan and silole moieties in the π-conjugated framework has led to extremely high fluorescence quantum yields and good thermal stability. 20,213][24][25] As representative examples, the bissilicon-bridged stilbenes A, 26 the tetrakissilicon-bridged derivative B 27a,28 and the alternating Si,C-bridged C 27b have been disclosed (Scheme 1).Notably, these types of compound have shown great potential as new materials for organic-based electronic devices and ambipolar carrier transporting.Please do not adjust margins Please do not adjust margins As silicon possesses a 3d orbital, it can undergo conjugation with adjacent sp 2 atoms, and when in the presence of a conjugated chain it can provide a further delocalization pathway.][31][32][33] However, there are in general very few reports of conjugated molecules that contain linked but inequivalent heteroatoms 33 and, to the knowledge of the authors, no azasilicon-bridged examples reported to date; an observation likely due to the limited synthetic strategies available.

Synthesis and Characterization
The lithium Me), using procedures previously reported (Scheme 2). 34action of L1 -L5 with TiCl4 in a 2:1 molar ratio in THF forms Me D5) in good yields (Scheme 2).These bis-azasilicon-bridged heterocycles D are formed as either yellow or green crystalline solids and are soluble in Et2O or THF but sparingly soluble in CH2Cl2.All compounds have been characterized by 1 H and 13 C NMR spectroscopy and microanalysis.In addition, their molecular structures have been confirmed by single crystal X-ray diffraction.
7][28] This would suggest, like these previously reported molecules, that the π-conjugation is effectively extended over the entire molecule.Furthermore, to the best of our knowledge, these structures are the first examples of bis-azasilicon-bridged heterocyclic arylamines.Please do not adjust margins

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In the 1 H NMR spectra for D1 -D5 the Si-Me protons are seen as a 12H singlet resonance in the range δ 0.53 to -0.03 due to their equivalent environments; similarly the 13 C NMR spectra gives a singlet for the corresponding carbons (δ 0.87 -1.98).Within the heterocyclic rings, only two carbon signals are evident in the 13 C NMR spectra with the C-Ph resonance in general seen more downfield (δ 144.47 -168.62)than the carbon atom belonging to the fused edge which is visible in a wider range (δ 65.93 -120.29).The microanalytical data obtained for D1 -D5 are in agreement with their elemental compositions.Though the mechanism of formation of D from L is not explored explicitly herein, TiCl4promoted cyclizations have some precedent. 35Nonetheless, salt elimination of lithium chloride, TiCl3(NMe2) formation and intermolecular nucleophilic attack by the resultant anionic ArRN unit on a positively charged RSiMe3 moiety with concomitant C-C bond formation seems likely.
To explore the water sensitivity of D, a tetrahydrofuran solution of each compound was treated with a mixture of water and THF affording, on work-up, the 1, ). Compounds E1 -E5 were isolated in high yields (85 -90%) as colorless solids and characterized by 1 H NMR spectroscopy.All examples of E are novel apart from E1 which has been recently reported albeit in low yield. 36n addition, E1, E2, E3 and E5 have been the subject of single crystal X-ray diffraction studies.
Crystals of E suitable for the single crystal X-ray determinations were grown from their diethyl ether solutions at -20 °C.A view of E1 is shown in Figure 6, while diagrams for E2, E3 and E5 are presented in Figures S1, S2 and S3 (see SI); selected bond distances and angles for all four structures are given in Table 2.The structures are all similar and contain an inversion center at the midpoint between C(1) and C(1 i ).In each case a n-butane chain is substituted at its 1 and 4-positions by both phenyl and N-aryl groups (aryl = 2,6i Pr2C6H3 (E1), 2,6-Et2C6H3 (E2), 2,6-Me2C6H3 (E3), 4-Me-2,6-Et2C6H2 (E5)).Inspection of the C(2)-N(1) distance is consistent with an imine double bond (range: 1.277(2) -1.2790(18) Å), while the N(1)-C(2)-C(1) bond angle is supportive of the expected sp 2 -hybridization (range: 123.41(13) -124.0(2)o ).By contrast at C(1), the angles of between 112.07 (17) and 113.99 (17) o for C(2)-C(1)-C(1 i ) confirm its sp 3 hybridization.The 1 H NMR data for E are further supportive of their structural type with a 4H singlet for the saturated N=CCH2CH2C=N protons at ca. δ 1.40 along with signals characteristic of the particular N-aryl substitution pattern.Clearly, the azasilicon compounds E are moisture sensitive and readily undergo C-Si bond cleavage resulting in the elimination of presumably Me2Si(OH)2 and protonation of the central carbon atoms.
We also explored the sensitivity of D towards redoxpromoted reactivity.Hence treatment of D with two molar equivalents of cuprous chloride, followed by the sequential addition of four equivalents of iodine and water gives the new 3).It is assumed the reaction proceeds by ring opening of D to give a ClSiMe2-containing intermediate which then undergoes iodination and then on hydrolysis affords F. 37 Compounds F1 -F5 have all been characterized by 1 H NMR spectroscopy and in the cases of F2 and F3 by single crystal X-ray diffraction.

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Please do not adjust margins Single crystals of F2 and F3 were grown from their diethyl ether solutions at -20 °C.Perspective views of F2 and F3 are given in Figures 7 and S4 (see SI), respectively; selected bond distances and angles for both are listed in Table 3.The structures are similar (albeit with F3 containing an inversion center) and are based on a E-configured 2-butene chain with the 1-and 4-positions substituted by phenyl and by N-Ar groups (Ar = 2,6-Et2C6H3 (F2), 2,6-Me2C6H3 (F3)).The presence of C=C double bonds in each is supported by the bond distances of 1.292(7) (Å) (F2) and 1.345(5) Å (F3) while the C(2)-C(1)-C(1 i ) angles of 127.0(6) o (F2) and 122.9(3) o (F3) support sp 2 -hybridization for C(1).Furthermore, the 1 H NMR data for F are supportive of their structural types with a 2H singlet for the unsaturated N=CCH=CHC=N protons at ca. δ 5.28.

Photophysical Properties of D1 -D5
The UV-vis and fluorescence spectra of D1 -D5 in diethyl ether solution are shown in Figures 8 and 9, respectively; the photophysical data are summarized in Table 4. Examination of the data reveals that the silicon-bridge shifts the absorption and emission maxima to a longer wavelength.Specifically, the absorption and emission maxima of D1 -D5 fall in the ranges 435-439 nm and 505-510 nm, respectively.[31][32][33] Overall, compounds D1 -D5 exhibit very similar absorption and emission spectra.In terms of the fluorescence quantum yields (ΦF), D1 -D5 display high values in the range from 0.75 to 0.99.Notably, the 2,6-diethylphenylcontaining D2 and the 4-methyl-2,6-diethylphenyl-containing D5 show exceptionally high quantum yields of 0.94 and 0.99, respectively; it is unclear as to the origin of these N-aryl group influenced differences.With regard to the color of the emission, D1 -D5 generate an intense greenish blue emission, highlighting the potential use of these molecules as new emitting materials.

Experimental
General considerations All manipulations were carried out under an atmosphere of argon using standard Schlenk techniques.Solvents were purchased from commercial sources.Deuterated solvents CDCl3 were dried over activated molecular sieves (4 Å) and vacuum transferred before use.Hexane was dried using sodium potassium alloy.Diethyl ether was dried and distilled from sodium/benzophenone and stored over a sodium mirror under argon.Dichloromethane was distilled over activated molecular sieves (4 Å) or CaH2.Toluene was heated to reflux in the presence of sodium/benzophenone and distilled under nitrogen prior to use.Glassware was oven-dried at 120 °C overnight.The NMR spectra were recorded on a Bruker DKX-600 spectrometer with TMS as the internal standard.Elemental analyses were performed with a Flash EA 1112 microanalyzer.UV/Vis absorption spectra and fluorescence spectra measurements were performed in diethyl ether at room temperature with a Shimadzu UV-757PC spectrometer and a Hitachi F-4600 fluorescence spectrophotometer, respectively.The lithium enamides, Me2NSiMe2CHC(Ph Me L4; R 1 = Et, R 2 = Me L5), were prepared as bright yellow solids in high yield (90 -98%) using procedures previously reported. [34]

Synthesis of [{2,6-(R 1 )2-4-(R 2 )C6H2}N(Ph)CCH2]2 (E)
General procedure.To a solution of D (1.0 mmol) in tetrahydrofuran (10 mL) at 0 o C was slowly added distilled water (4-6 mmol).The resulting light yellow solution was stirred overnight and then concentrated under reduced pressure.The residue was purified on a silica gel column using petroleum ether/ether (4:1) as the eluent affording E (0.85 -0.90 mmol) as colorless crystals in yields of between 85 -90%.R 1 = Et, R 2 = H (E2). 1 H NMR (300 MHz, CDCl3) δ 1.20-1.24(t, 12H, 4CH2CH3, 3 JHH = 6.6 Hz), 1.35 (s, 4H, CH2-CH2), 2.43-2.47(q, 8H, 4CH2CH3, 3 JHH = 6.9 Hz), 6.90-7.03,7.41-7.52,8.01-8.03(m, 16H, Ar and Ph). 13   General procedure.To a solution of D (2.0 mmol) in THF (25 mL) at -78 ºC, CuCl (4.0 mmol) was added to form a yellow-green solution which gradually turned to a black green.The mixture was stirred for 3 h at -78 o C then warmed to room temperature and left to stir at this temperature for 24 h.All volatiles were removed under reduced pressure and the residue taken up in CH2Cl2 (50 mL).The solution was filtered and all volatiles removed from the filtrate under reduced pressure.The resulting residue was cooled to -78 o C and four equivalents of I2 (8.0 mmol) in THF (30 mL) added resulting in a color change to brown.The reaction mixture was stirred for 3 h at -78 o C, then warmed to room temperature and left to stir at this temperature for 24 h.All volatiles were removed under reduced pressure and the resulting residue taken up in diethyl ether (30 mL) and filtered.The filtrate was cooled to 0 o C and distilled water (8-10 mmol) slowly added.The resulting solution was stirred overnight at room temperature.The brown solution was concentrated under reduced pressure and the residue purified on a silica gel column using petroleum ether/diethylether (4.5:1) as eluent to afford F in yields of between 42-56%; recrystallization from diethyl ether at 20 o C formed F as colorless crystals.
R 1 = i Pr, R 2 = H (F1). 1 H NMR (300 MHz, CDCl3) δ 1. 14 X-ray Crystallographic Studies.Single crystal X-ray diffraction data for D1 -D5, E1, E2, E3, E5, F2 and F3 were collected using Mo-Kα radiation (λ = 0.71073 Å) on a Bruker ApexII CCD diffractometer and Rigaku AFC 10 Saturn724 in the range 173(2) K to 293(2) K. Crystals were coated in oil and then directly mounted on the diffractometer under a stream of cold nitrogen gas.Cell parameters were obtained by global refinement of the positions of all collected reflections.Intensities were corrected for Lorentz and polarization effects and empirical absorption.The structures were solved by direct methods and refined by full-matrix least squares on F 2 .All hydrogen atoms were placed in calculated positions.Structure solution and refinement were performed by using the SHELXL-2014 package and Olex2 1.2 package. 38The remaining nonhydrogen atoms were then obtained from the successive difference Fourier maps.All non−H atoms were refined with anisotropic displacement parameters, while the H atoms were constrained to parent sites, using riding modes (SHELXTL). 39rystal data and processing parameters for D1 -D5 are summarized in Table 5, while those for E1, E2, E3, E5, F2 and F3 are presented in Table 6.

Figure 1 .
Figure 1.ORTEP representation of D1 with the thermal ellipsoids shown at the 30% probability level; all hydrogen atoms are omitted for clarity.

Figure 2 .
Figure 2. ORTEP representation of D2 with the thermal ellipsoids shown at the 30% probability level; all hydrogen atoms are omitted for clarity.

Figure 3 .
Figure 3. ORTEP representation of D3 with the thermal ellipsoids shown at the 30% probability level; all hydrogen atoms are omitted for clarity.

Figure 4 .
Figure 4. ORTEP representation of D4 with the thermal ellipsoids shown at the 30% probability level; all hydrogen atoms are omitted for clarity.

Figure 5 .
Figure 5. ORTEP representation of D5 with the thermal ellipsoids shown at the 30% probability level; all hydrogen atoms are omitted for clarity.

Table 3 .Figure 6 .
Figure 6.ORTEP representation of E1 with the thermal ellipsoids shown at the 30% probability level; all hydrogen atoms are omitted for clarity.

Figure 7 .
Figure 7. ORTEP representation of F2 with the thermal ellipsoids shown at the 30% probability level; all hydrogen atoms are omitted for clarity.
(2.0 × 10 -5 M). b Emission maxima upon excitation at the absorption maximum wavelengths.c Determined with Rhodamine B as a standard, unless otherwise stated.The ΦF is the average value of repeated measurements within ±5% error.d Determined with 9,10-diphenylanthracene as a standard.e Determined with anthracene as a standard.

Table 1 .
The structural features are similar and will be discussed

Table 5 .
Crystal data and data collection parameters for D1 -D5