Introduction

In recent years, sustainable synthetic methods have gained more attention as they address many environmental problems [1, 2]. Among them, solvent-free methods [3, 4] are an attractive tool for the synthesis of biologically active frameworks. Multicomponent domino reactions are an interesting strategy due to the great synthetic efficiency in a one-pot operation [5,6,7,8,9,10,11]. These protocols have rendered many advantages including convergent routes, facile automation and reduction of workup stages, extraction and purification processes. Needless to say, the combination of these two protocols can be effective in the construction of novel heterocycles in the greener perspective [12].

Molecular frameworks comprising quinoline/pyridine bear potential for biological and medicinal applications [13, 14]. In particular, the phenanthroline core presents in many natural products such as meridine and ascididemin [15] and some synthetic phenanthrolines display significant antitumor activities [16, 17] (Fig. 1) besides serving as ligands [18, 19] and organic semiconductor materials [20]. Benzophenanthrolines also serve as protein tyrosine kinase inhibitors, which can be used in mammalian carcinomas [21].

Fig. 1
figure 1

Naturally occurring phenanthroline derivatives, meridine analogues and biologically active benzophenanthroline

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It is interesting to note that syntheses of benzophenanthrolines are relatively scarce in the literature, and one of such syntheses is the reaction between (E)-2-benzylidene-7-chloro-9-phenyl-3,4-dihydroacridin-1(2H)-one and malononitrile in the presence of NaH in refluxing ethanol–benzene mixture or in the presence of montmorillonite KSF in ethanol [22]. This method uses toxic solvents and offers a limited substrate scope. In this context, here we disclose the assembly of novel benzophenanthrolines viz 2,4-diaryl-5,6-dihydrobenzo[j][1,7]phenanthrolines, (Scheme 1). The final products bear structural resemblance to quinoline-based heterocycles. The present study is the part of our ongoing research program on the assembly of novel biologically relevant heterocycles through multicomponent/domino and green transformations [23,24,25,26,27].

Scheme 1
scheme 1

Outline of this work

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Results and discussion

The present study commenced with the optimization of a model three-component reaction between 2-[4-bromobenzylidene]-3,4-dihydro-1-(2H)-acridinone, 1-(2-oxo-2-phenylethyl)pyridin-1-ium bromide and ammonium acetate. This reaction was explored in different solvents, viz. methanol, ethanol, water, diethylene glycol, N,N-dimethylformamide, acetonitrile, tetrahydrofuran, and also under solvent-free conditions (Table 1).

Table 1 Solvent influence on the synthesis of 4m
figure b
Full size table

As can be seen from the data listed in Table 1, the best result was obtained by heating the reaction mixture under solvent-free conditions at 110 °C to furnish 4-(4-bromophenyl)-2-phenyl-5,6-dihydrobenzo[j][1,7]-phenanthroline (4m) in 81% yield (Table 1, entry 6). Protic solvents (ethanol, methanol) afforded higher yields than the other solvents tested.

Having established the optimal conditions for our reaction, the scope of the reaction was first examined focusing on the 2-[arylmethylidene]-3,4-dihydro-1(2H)-acridinone component. Acridinones bearing electron-withdrawing and electron-donating groups in the aryl part proceeded smoothly to give the respective products (Table 2). Similarly, for 1-(2-oxo-2-arylethyl)pyridin-1-ium bromides, the presence of electron-donating, neutral and electron-withdrawing groups in the aryl part is well tolerated, affording products in good yields (Table 2). A practical advantage of this protocol is the fact that no column purification is needed. After completion of the reaction (monitoring by TLC), the mixture was treated with ice water and the resulting solid 4 was filtered and washed with water. Single recrystallization of the product from ethanol afforded analytically pure samples. To the best of our knowledge, this is the first report of a multicomponent reaction employing pyridinium ylides generated in situ for the construction of 2,4-diaryl-5,6-dihydrobenzo[j][1,7]-phenanthrolines under solvent-free conditions.

Table 2 Synthesis of 2,4-diaryl-5,6-dihydrobenzo[j][1,7]phenanthrolines 4
figure c
Full size table

The structures of products 4 were deduced from one- and two-dimensional NMR spectroscopic data as detailed for 4n as a representative example (Fig. 2). In the 1H NMR spectrum of 4n, the H-3 appears as a singlet at 7.59 ppm, which shows (i) a C,H-COSY correlation with the signal at 120.0 ppm due to C-3 and (ii) HMBCs with C-2, C-1′, and C-4a, appearing at 154.1, 137.4, and 128.0 ppm, respectively. The 5-CH2 hydrogen atoms appear as a multiplet around 3.09–3.13 ppm. These hydrogen atoms show (i) H,H-COSY correlation with 6-CH2 hydrogen atoms, (ii) C,H-COSY correlations with C-5 at 24.7 ppm, and HMBCs with C-6 at 31.9 ppm, C-4a at 128.0 ppm, C-6a at 158.9 ppm and C-12b at 151.6 ppm. The multiplet at 3.26–3.30 ppm arises from the 6-CH2 hydrogens which show (i) H,H-COSY correlation with 5-CH2 hydrogens (ii) C,H-COSY correlation with C-6 at 31.9 ppm, and HMBCs with C-5, C-4a, and C-6a, appearing at 24.7, 128.0, and 158.9 ppm, respectively. The H-12 appears as a singlet at 9.23 ppm, which shows (i) C,H-COSY correlation with carbon signal at 132.7 ppm due to C-12, and HMBCs with C-11 at 128.8 ppm, C-6a at 158.9 ppm, C-7a at 148.0 ppm, and C-12b at 151.6 ppm. Finally, the structure of the compound was confirmed by single-crystal X-ray crystallography of 4g (Fig. 3).

Fig. 2
figure 2

1H and 13C NMR chemical shifts and selected HMBCs of 4n

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Fig. 3
figure 3

ORTEP representation of 4g

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A plausible mechanism for the formation of 2,4-diaryl-5,6-dihydrobenzo[j][1,7]phenanthrolines 4 is depicted in Scheme 2. The Michael addition of pyridinium ylide 5 (generated in situ from 2) to 1 presumably affords pyridinium enolate 6, which subsequently reacts with ammonia available from the dissociation of ammonium acetate to afford enamine 8 via 7. Intermediate 5 is ultimately transformed into product 4 via elimination–condensation reactions. Another plausible mechanism involves the hemiaminal formation and intramolecular attack onto the other carbonyl followed by elimination.

Scheme 2
scheme 2

Proposed mechanism for the formation of 4

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It is interesting that the other possible product 4′ via chemoselective intramolecular SN2 substitution [28,29,30] is not formed at all.

Conclusion

In summary, we have described a facile synthesis of novel dihydrobenzo[j][1,7]phenanthrolines through solvent-free Krohnke-type one-pot three-component domino reactions of 2-[arylmethylidene]-3,4-dihydro-1(2H)-acridinones and 1-(2-oxo-2-arylethyl)pyridin-1-ium bromide in the presence of excess ammonium acetate. This protocol avoids the use of expensive catalysts, toxic solvents and chromatographic separation.

Experimental part

General

All melting points reported in this work were measured in open capillaries and are uncorrected (Sigma, 71281). Silica gel-G plates (Merck) were used for TLC analysis with a mixture of petroleum ether of boiling range 60–80 °C and ethyl acetate as eluent. 1H and 13C NMR spectra were recorded at 300 and 75 MHz, respectively, on a Bruker 300 MHz (Avance) instrument using either CDCl3 or DMSO-d6 and tetramethylsilane as the internal standard. Chemical shifts are reported as δ values (ppm) (s = singlet; d = doublet; t = triplet; td = triplet of doublet; m = multiplet). All one- and two-dimensional NMR spectra were obtained using standard Bruker software throughout. Elemental analyses were performed on a PerkinElmer 2400 Series II Elemental CHN analyzer. Mass spectra were recorded on a LCQ Fleet mass spectrometer, Thermo Fisher Instruments Limited, USA. Electrospray ionization mass spectrometry (ESI–MS) analysis was performed in the positive ion and negative ion modes on a liquid chromatography ion trap.

Single-crystal X-ray diffraction studies

Single crystals of 4-(2-methoxyphenyl)-2-phenyl-5,6-dihydrobenzo[j][1,7]phenanthroline 4g were grown by slow evaporation solution growth method using ethanol as solvent at room temperature. Suitable crystals were selected for single-crystal X-ray diffraction studies. Single-crystal X-ray data set was collected on a Bruker AXS SMART APEX-2 diffractometer equipped with graphite monochromator. The structure was solved by direct methods and refined by full-matrix least-squares calculations using SHELXL-2014. Crystallographic data (excluding structure factors) for compound 4g in this manuscript have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication number 4g CCDC 968695. Copies of the data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [Fax: +44 (0)1223-336033 or e-mail:deposit@ccdc.cam.ac.uk].

General procedure for the synthesis of 2,4-diaryl-5,6-dihydrobenzo[j][1,7]phenanthrolines 4

A mixture of 2-[arylmethylidene]-3,4-dihydro-1(2H)-acridinones (1 mmol), 1-(2-oxo-2-arylethyl)pyridin-1-ium bromide (1 mmol), and ammonium acetate (1.5 mmol) was heated under neat conditions at 110 °C for 6–9 h. After completion of the reaction (TLC monitoring), the mixture was poured onto ice water and the resulting solid was filtered and washed with water. This solid was recrystallized from ethanol to afford pure product 4.

2,4-Di-p-tolyl-5,6-dihydrobenzo[j][1,7]phenanthroline (4a)

Isolated as off white solid. Yield: 83%, m.p. = 227–228 °C; 1H NMR (300 MHz, CDCl3) δH: 2.45 (s, 3H, –CH3), 2.46 (s, 3H, –CH3), 3.12–3.17 (m, 2H), 3.24–3.29 (m, 2H), 7.34–7.36 (m, 6H, Ar–H), 7.55 (td, 1H, J = 7.8, 1.2 Hz, Ar–H), 7.64 (s, 1H, Ar–H), 7.72 (td, 1H, J = 7.8, 1.2 Hz Ar–H), 8.00–8.08 (m, 2H, Ar–H), 8.10 (d, 2H, J = 8.1 Hz, Ar–H), 9.27 (s, 1H, Ar–H); 13C NMR (75 MHz, CDCl3) δC: 21.1, 21.2, 24.7, 31.9, 120.4, 125.9, 126.7, 127.8, 128.1, 128.3, 128.5, 128.6, 128.9, 129.2, 129.4, 129.7, 132.5, 135.9, 136.5, 138.0, 138.8, 147.7, 149.7, 151.0, 155.2, 159.2; ESI–MS m/z, calcd: 412.19; found: 413.31 [M + 1]; anal. calcd for C30H24N2: C, 87.35; H, 5.86; N, 6.79%. Found C, 87.49; H, 5.74; N, 6.89%.

2-(4-Methoxyphenyl)-4-p-tolyl-5,6-dihydrobenzo[j][1,7]phenanthroline (4b)

Isolated as off white solid. Yield: 84%, m.p. = 198–200 °C; 1H NMR (300 MHz, CDCl3) δH: 2.46 (s, 3H, –CH3), 3.11–3.16 (m, 2H), 3.24–3.29 (m, 2H), 3.90 (s, 3H, –OCH3), 7.06 (d, 2H, J = 8.7 Hz, Ar–H), 7.34 (s, 4H, Ar–H), 7.55 (t, 1H, J = 7.5 Hz, Ar–H), 7.61 (s, 1H, Ar–H), 7.72 (td, 1H, J = 7.8, 1.5 Hz, Ar–H), 8.01 (d, 1H, J = 7.8 Hz, Ar–H), 8.06 (d, 1H, J = 8.7 Hz, Ar–H), 8.17 (d, 2H, J = 8.7 Hz, Ar–H), 9.25 (s, 1H, Ar–H); 13C NMR (75 MHz, CDCl3) δC: 21.3, 24.7, 32.1, 55.4, 114.1, 120.0, 126.0, 127.5, 128.1, 128.4, 128.7, 128.9, 129.2, 129.8, 132.0, 132.5, 136.0, 138.1, 147.8, 149.7, 151.0, 154.9, 159.3, 160.5; anal. calcd for C30H24N2O: C, 84.08; H, 5.65; N, 6.54%. Found C, 84.00; H, 5.71; N, 6.58%.

2-Phenyl-4-p-tolyl-5,6-dihydrobenzo[j][1,7]phenanthroline (4c)

Isolated as off white solid. Yield: 81%, m.p. = 205–206 °C; 1H NMR (300 MHz, CDCl3) δH: 2.47 (s, 3H, –CH3), 3.13–3.17 (m, 2H), 3.25–3.30 (m, 2H), 7.34 (s, 4H, Ar–H), 7.46–7.48 (m, 1H, Ar–H), 7.52–7.57 (m, 3H, Ar–H), 7.67 (s, 1H, Ar–H), 7.72 (td, 1H, J = 7.8, 1.2 Hz, Ar–H), 8.01 (d, 1H, J = 8.1 Hz, Ar–H), 8.07 (d, 1H, J = 8.4 Hz, Ar–H), 8.21 (d, 2H, J = 7.2 Hz, Ar–H), 9.27 (s, 1H, Ar–H); 13C NMR (75 MHz, CDCl3) δC: 21.1, 24.7, 32.0, 120.7, 126.0, 126.8, 128.0, 128.1, 128.4, 128.6, 128.7, 128.8, 128.9, 129.2, 129.7, 132.5, 135.9, 138.1, 139.3, 147.9, 149.8, 151.2, 155.2, 159.2; ESI–MS m/z, calcd: 398.18; found: 399.29 [M + 1]; anal. calcd for C29H22N2: C, 87.41; H, 5.56; N, 7.03%. Found C, 87.54; H, 5.67; N, 6.94%.

2-(4-Fluorophenyl)-4-p-tolyl-5,6-dihydrobenzo[j][1,7]phenanthroline (4d)

Isolated as off white solid. Yield: 79%, m.p. = 218–220 °C; 1H NMR (300 MHz, CDCl3) δH: 2.46 (s, 3H, –CH3), 3.13–3.17 (m, 2H), 3.25–3.29 (m, 2H), 7.19–7.26 (m, 2H, Ar–H), 7.34 (s, 4H, Ar–H), 7.55 (t, 1H, J = 7.2 Hz, Ar–H), 7.62 (s, 1H, Ar–H), 7.73 (t, 1H, J = 7.8 Hz, Ar–H), 8.01 (d, 1H, J = 8.1 Hz, Ar–H), 8.06 (d, 1H, J = 8.1 Hz, Ar–H), 8.19 (dd, 2H, J = 8.7, 5.4 Hz, Ar–H), 9.24 (s, 1H, Ar–H); 13C NMR (75 MHz, CDCl3) δC: 21.3, 24.8, 32.1, 115.5, 115.8, 120.5, 126.2, 128.1, 128.2, 128.5, 128.6, 128.7, 129.3, 130.0, 132.6, 135.5, 135.8, 138.3, 147.9, 150.0, 151.3, 154.3, 159.3; anal. calcd for C29H21FN2: C, 83.63; H, 5.08; N, 6.73%. Found C, 83.70; H, 5.03; N, 6.69%.

2-(4-Nitrophenyl)-4-p-tolyl-5,6-dihydrobenzo[j][1,7]phenanthrolines (4e)

Isolated as off white solid. Yield: 76%, m.p. = 238–240 °C; 1H NMR (300 MHz, CDCl3) δH: 2.47 (s, 3H, –CH3), 3.16–3.20 (m, 2H), 3.26–3.20 (m, 2H), 7.35 (s, 4H, Ar–H), 7.57 (t, 1H, J = 7.2 Hz, Ar–H), 7.73 (s, 1H, Ar–H), 7.74–7.77 (m, 1H, Ar–H), 8.03 (d, 1H, J = 8.1 Hz, Ar–H), 8.07 (d, 1H, J = 8.4 Hz, Ar–H), 8.38–8.41 (m, 4H, Ar–H), 9.23 (s, 1H, Ar–H); 13C NMR (75 MHz, CDCl3) δC: 21.2, 24.8, 31.8, 121.2, 123.9, 126.2, 127.4, 127.9, 128.2, 128.5, 128.6, 129.3, 129.8, 130.0, 132.6, 135.3, 138.5, 145.1, 148.0, 148.1, 150.2, 151.8, 152.4, 158.9; ESI–MS m/z, calcd: 443.16; found: 444.27 [M + 1]; anal. calcd for C29H21N3O2: C, 78.54; H, 4.77; N, 9.47%. Found C, 78.42; H, 4.91; N, 9.42%.

4-(2-Methoxyphenyl)-2-(4-methoxyphenyl)-5,6-dihydrobenzo[j][1,7]phenanthroline (4f)

Isolated as off white solid. Yield: 77%, m.p. = 235–236 °C; 1H NMR (300 MHz, CDCl3) δH: 2.81–2.90 (m, 1H), 2.99–3.09 (m, 1H), 3.22–3.29 (m, 2H), 3.81 (s, 3H, –OCH3), 3.90 (s, 3H, –OCH3), 7.04–7.08 (m, 3H, Ar–H), 7.11 (td, 1H, J = 7.2, 0.9 Hz, Ar–H), 7.30 (dd, 1H, J = 7.5, 1.8 Hz, Ar–H), 7.46 (td, 1H, J = 7.8, 1.5 Hz, Ar–H), 7.54 (td, 1H, J = 7.5, 0.9 Hz, Ar–H), 7.58 (s, 1H, Ar–H), 7.71 (td, 1H, J = 7.5, 1.5 Hz, Ar–H), 8.01 (d, 1H, J = 7.2 Hz, Ar–H), 8.06 (d, 1H, J = 8.4 Hz, Ar–H), 8.16 (d, 2H, J = 9.0 Hz, Ar–H), 9.26 (s, 1H, Ar–H); 13C NMR (75 MHz, CDCl3) δC: 24.4, 31.9, 55.3, 110.8, 114.0, 120.6, 120.8, 126.0, 127.8, 128.1, 128.3, 128.6, 129.0, 129.7, 129.8, 130.4, 132.1, 132.3, 146.9, 147.7, 150.3, 154.8, 156.4, 159.6, 160.3; anal. calcd for C30H24N2O2: C, 81.06; H, 5.44; N, 6.30%. Found C, 80.93; H, 5.38; N, 6.34%.

4-(2-Methoxyphenyl)-2-phenyl-5,6-dihydrobenzo[j][1,7]phenanthroline (4g)

Isolated as off white solid. Yield: 82%, m.p. = 235–236 °C; 1H NMR (300 MHz, DMSO-d6) δH: 2.73–2.78 (m, 1H), 2.89–2.91 (m, 1H), 3.14 (m, 2H), 3.76 (s, 3H, –OCH3), 7.09–7.19 (m, 2H,Ar–H), 7.24 (d, 1H, J = 7.2 Hz, Ar–H), 7.46–7.60 (m, 5H, Ar–H), 7.71–7.76 (m, 2H,Ar–H), 7.97 (d, 1H, J = 8.1 Hz, Ar–H), 8.17 (d, 1H, J = 8.1 Hz, Ar–H), 8.29 (d, 2H, J = 7.2 Hz, Ar–H), 9.25 (s, 1H, Ar–H); 13C NMR (75 MHz, DMSO-d6) δC: 24.6, 31.6, 55.9, 111.8, 121.2, 121.5, 126.7, 127.2, 127.3, 128.0, 128.6, 128.7, 129.2, 129.3, 129.5, 130.3, 130.5, 130.6, 130.8, 132.2, 139.0, 147.6, 147.8, 150.4, 154.6, 156.5, 159.6; anal. calcd for C29H22N2O: C, 84.03; H, 5.35; N, 6.76%. Found C, 83.93; H, 5.47; N, 6.84%.

2-(4-Fluorophenyl)-4-(2-methoxyphenyl)-5,6-dihydrobenzo[j][1,7]phenanthroline (4 h)

Isolated as off white solid. Yield: 78%, m.p. = 245–247 °C; 1H NMR (300 MHz, CDCl3) δH: 2.85–2.92 (m, 1H), 3.00–3.11 (m, 1H), 3.23–3.30 (m, 2H), 3.82 (s, 3H, –OCH3), 7.05 (d, 1H, J = 8.1 Hz, Ar–H), 7.11 (t, 1H, J = 7.5 Hz, Ar–H), 7.18–7.24 (m, 2H, Ar–H), 7.29 (dd, 1H, J = 7.5, 1.5 Hz, Ar–H), 7.47 (td, 1H, J = 7.5, 1.8 Hz, Ar–H), 7.55 (t, 1H, J = 7.5 Hz, Ar–H), 7.59 (s, 1H, Ar–H), 7.72 (td, 1H, J = 7.5, 1.5 Hz, Ar–H), 8.01 (d, 1H, J = 8.1 Hz, Ar–H), 8.06 (d, 1H, J = 8.4 Hz, Ar–H), 8.17–8.21 (m, 2H, Ar–H), 9.25 (s, 1H, Ar–H); 13C NMR (75 MHz, CDCl3) δC: 24.6, 31.9, 55.4, 110.9, 115.4, 115.7, 120.9, 121.0, 126.0, 127.7, 128.1, 128.5, 128.6, 128.7, 128.9, 129.8, 129.9, 130.4, 132.4, 135.7, 147.2, 147.9, 150.7, 154.1, 156.4, 159.5, 163.5 (1JC–F = 246.5); ESI–MS m/z, calcd: 432.16; found: 433.25[M + 1]; anal. calcd for C29H21FN2O: C, 80.54; H, 4.89; N, 6.48%. Found C, 80.61; H, 5.03; N, 6.43%.

2-(4-Chlorophenyl)-4-(2-methoxyphenyl)-5,6-dihydrobenzo[j][1,7]phenanthroline (4i)

Isolated as off white solid. Yield: 85%, m.p. = 224–226 °C; 1H NMR (300 MHz, CDCl3) δH: 2.83–2.92 (m, 1H), 3.01–3.11 (m, 1H), 3.23–3.30 (m, 2H), 3.81 (s, 3H, –OCH3), 7.05 (d, 1H, J = 8.4 Hz, Ar–H), 7.12 (t, 1H, J = 7.5 Hz, Ar–H), 7.28 (t, 1H, J = 7.5 Hz, Ar–H), 7.44–7.52 (m, 1H, Ar–H), 7.49 (d, 2H, J = 8.7 Hz, Ar–H), 7.55 (t, 1H, J = 7.5 Hz, Ar–H), 7.61 (s, 1H, Ar–H), 7.72 (t, 1H, J = 7.8 Hz, Ar–H), 8.01 (d, 1H, J = 8.1 Hz, Ar–H), 8.06 (d, 1H, J = 8.4 Hz, Ar–H), 8.15 (d, 2H, J = 8.4 Hz, Ar–H), 9.24 (s, 1H, Ar–H); 13C NMR (75 MHz, CDCl3) δC: 24.6, 31.9, 55.4, 110.9, 111.0, 120.9, 121.1, 126.1, 127.7, 128.1, 128.5, 128.7, 128.8, 129.8, 130.0, 130.2, 130.4, 132.4, 134.9, 137.9, 147.2, 150.8, 153.9, 156.4, 159.4; ESI–MS m/z, calcd: 448.13; Found: 449.25 [M + 1]; anal. calcd for C29H21ClN2O: C, 77.58; H, 4.71; N, 6.24%. Found C, 77.44; H, 4.64; N, 6.29%.

4-(4-Chlorophenyl)-2-(4-fluorophenyl)-5,6-dihydrobenzo[j][1,7]phenanthroline (4j)

Isolated as off white solid. Yield: 86%, m.p. = 213–214 °C; 1H NMR (300 MHz, CDCl3) δH: 3.08–3.13 (m, 2H), 3.25–3.30 (m, 2H), 7.19–7.27 (m, 2H, Ar–H), 7.37 (d, 2H, J = 8.4 Hz, Ar–H), 7.50 (d, 2H, J = 8.4 Hz, Ar–H), 7.55–7.58 (m, 1H, Ar–H), 7.58 (s, 1H, Ar–H), 7.73 (t, 1H, J = 7.2 Hz, Ar–H), 8.00 (d, 1H, J = 8.1 Hz, Ar–H), 8.06 (d, 1H, J = 8.1 Hz, Ar–H), 8.19 (dd, 2H, J = 8.7, 5.4 Hz, Ar–H), 9.23 (s, 1H, Ar–H); 13C NMR (75 MHz, CDCl3) δC: 24.6, 31.8, 115.5, 115.8, 120.0, 126.1, 127.8, 127.9, 128.4, 128.5, 128.6, 128.8, 130.0, 132.6, 134.5, 135.1, 137.0, 147.9, 148.6, 151.4, 154.3, 158.9, 163.6 (1JC–F = 247.0); anal. calcd for C28H18ClFN2: C, 76.97; H, 4.15; N, 6.41%. Found C, 77.09; H, 4.05; N, 6.49%.

4-(4-Bromophenyl)-2-p-tolyl-5,6-dihydrobenzo[j][1,7]phenanthroline (4k)

Isolated as off white solid. Yield: 87%, m.p. = 222–223 °C; 1H NMR (300 MHz, CDCl3) δH: 2.45 (s, 3H, –CH3), 3.08–3.12 (m, 2H), 3.25–3.30 (m, 2H), 7.30–7.36 (m, 4H, Ar–H), 7.57 (t, 1H, J = 7.2 Hz, Ar–H), 7.60 (s, 1H, Ar–H), 7.65 (d, 2H, J = 8.4 Hz, Ar–H), 7.73 (t, 1H, J = 7.2 Hz, Ar–H), 8.00–8.05 (m, 2H, Ar–H), 8.09 (d, 2H, J = 8.4 Hz, Ar–H), 9.26 (s, 1H, Ar–H); 13C NMR (75 MHz, CDCl3) δC: 21.3, 24.7, 32.0, 120.0, 122.6, 126.1, 126.7, 127.5, 128.0, 128.4, 128.6, 128.8, 129.5, 129.9, 130.4, 131.8, 132.7, 136.2, 137.7, 139.1, 147.9, 148.5, 151.3, 155.4, 159.0; ESI–MS m/z, calcd: 476.09; found: 477.22 [M + 1]; anal.calcd for C29H21BrN2: C, 72.96; H, 4.43; N, 5.87%. Found C, 73.02; H, 4.56; N, 5.91%.

4-(4-Bromophenyl)-2-(4-methoxyphenyl)-5,6-dihydrobenzo[j][1,7]phenanthroline (4l)

Isolated as off white solid. Yield: 83%, m.p. = 222–223 °C; 1H NMR (300 MHz, CDCl3) δH: 3.06–3.10 (m, 2H), 3.24–3.29 (m, 2H), 3.90 (s, 3H, –OCH3), 7.06 (d, 2H, J = 8.7 Hz, Ar–H), 7.31 (d, 2H, J = 8.1 Hz, Ar–H), 7.52–7.57 (m, 2H, Ar–H), 7.65 (d, 2H, J = 8.1 Hz, Ar–H), 7.72 (t, 1H, J = 7.2 Hz, Ar–H), 8.00 (d, 1H, J = 8.1 Hz, Ar–H), 8.06 (d, 1H, J = 8.4 Hz, Ar–H), 8.16 (d, 2H, J = 8.4 Hz, Ar–H), 9.24 (s, 1H, Ar–H); 13C NMR (75 MHz, CDCl3) δC: 24.6, 31.8, 55.4, 114.2, 119.4, 123.1, 123.7, 126.2, 126.9, 128.0, 128.2, 128.4, 128.5, 128.7, 129.6, 130.0, 131.4, 132.7, 134.7, 140.6, 148.0, 148.5, 151.6, 155.4, 158.8; anal. calcd for C29H21BrN2O: C, 70.59; H, 4.29; N, 5.68%. Found C, 70.50; H, 4.40; N, 5.61%.

4-(4-Bromophenyl)-2-phenyl-5,6-dihydrobenzo[j][1,7]phenanthroline (4m)

Isolated as off white solid. Yield: 81%, m.p. = 227–228 °C; 1H NMR (300 MHz, CDCl3) δH: 3.05–3.07 (m, 2H), 3.22–3.24 (m, 2H), 7.27 (d, 2H, J = 7.8 Hz, Ar–H), 7.46–7.54 (m, 4H, Ar–H), 7.59 (s, 1H, Ar–H), 7.63 (d, 2H, J = 7.8 Hz, Ar–H), 7.70 (t, 1H, J = 7.5 Hz, Ar–H), 7.96 (d, 1H, J = 7.8 Hz, Ar–H), 8.06 (d, 1H, J = 8.1 Hz, Ar–H), 8.19 (d, 2H, J = 7.2 Hz, Ar–H), 9.22 (s, 1H, Ar–H);13C NMR (75 MHz, CDCl3) δC: 24.7, 31.9, 120.3, 122.6, 126.1, 126.9, 127.8, 128.0, 128.5, 128.8, 129.1, 129.9, 130.4, 131.8, 132.7, 137.7, 139.1, 148.0, 148.6, 151.5, 155.4, 159.0; ESI–MS m/z, calcd: 462.07; found: 463.27 [M + 1]; anal. calcd for C28H19BrN2: C, 72.58; H, 4.13; N, 6.05%. Found C, 72.72; H, 4.01; N, 6.00%.

4-(4-Bromophenyl)-2-(4-chlorophenyl)-5,6-dihydrobenzo[j][1,7]phenanthroline (4n)

Isolated as off white solid. Yield: 88%, m.p. = 204–206 °C; 1H NMR (300 MHz, CDCl3) δH: 3.09–3.13 (m, 2H), 3.26–3.30 (m, 2H), 7.31 (d, 2H, J = 8.4 Hz, Ar–H), 7.51 (d, 2H, J = 8.4 Hz, Ar–H), 7.56–7.59 (m, 1H, Ar–H), 7.59 (s, 1H, Ar–H), 7.66 (d, 2H, J = 8.4 Hz, Ar–H), 7.74 (t, 1H, J = 7.2 Hz, Ar–H), 8.01 (d, 1H, J = 8.1 Hz, Ar–H), 8.07 (d, 1H, J = 8.1 Hz, Ar–H), 8.14 (d, 2H, J = 8.4 Hz, Ar–H), 9.23 (s, 1H, Ar–H); 13C NMR (75 MHz, CDCl3) δC: 24.7, 31.9, 120.0, 122.8, 126.3, 128.0, 128.1, 128.4, 128.5, 128.8, 129.0, 130.1, 130.4, 131.9, 132.7, 135.3, 137.4, 137.5, 148.0, 148.7, 151.6, 154.1, 158.9; anal. calcd for C28H18BrClN2: C, 67.56; H, 3.64; N, 5.63%. Found C, 67.68; H, 3.59; N, 5.55%.

4-(3-Nitrophenyl)-2-p-tolyl-5,6-dihydrobenzo[j][1,7]phenanthroline (4o)

Isolated as off white solid. Yield: 76%, m.p. = 276–278 °C; 1H NMR (300 MHz, CDCl3) δH: 2.45 (s, 3H, –CH3), 3.07–3.11 (m, 2H), 3.27–3.31 (m, 2H), 7.36 (d, 2H, J = 8.1 Hz, Ar–H), 7.56 (t, 1H, J = 7.5 Hz, Ar–H), 7.63 (s, 1H, Ar–H), 7.68–7.79 (m, 3H, Ar–H), 8.01 (d, 1H, J = 7.8 Hz, Ar–H), 8.06 (d, 1H, J = 8.4 Hz, Ar–H), 8.11 (d, 2H, J = 8.1 Hz, Ar–H), 8.33–8.36 (m, 2H, Ar–H), 9.27 (s, 1H, Ar–H); 13C NMR (75 MHz, CDCl3) δC: 21.3, 24.6, 31.7, 119.9, 123.1, 123.7, 126.2, 126.7, 127.3, 128.0, 128.3, 128.4, 128.7, 129.5, 129.6, 130.0, 132.8, 134.7, 135.9, 139.3, 140.5, 147.1, 147.9, 148.3, 151.6, 155.7, 158.7; anal. calcd for C29H21N3O2: C, 78.54; H, 4.77; N, 9.47%. Found C, 78.47; H, 4.81; N, 9.42%.

2-(4-Fluorophenyl)-4-(3-nitrophenyl)-5,6-dihydrobenzo[j][1,7]phenanthroline (4p)

Isolated as off white solid. Yield: 80%, m.p. = 245–247 °C; 1H NMR (300 MHz, CDCl3) δH: 3.08–3.13 (m, 2H), 3.27–3.32 (m, 2H), 7.25 (d, 2H, J = 8.4 Hz, Ar–H), 7.57 (t, 1H, J = 7.5 Hz, Ar–H), 7.61 (s, 1H, Ar–H), 7.70–7.79 (m, 3H, Ar–H), 8.02 (d, 1H, J = 8.1 Hz, Ar–H), 8.07 (d, 1H, J = 8.4 Hz, Ar–H), 8.20 (dd, 2H, J = 8.7, 5.4 Hz, Ar–H), 8.34–8.37 (m, 2H, Ar–H), 9.25 (s, 1H, Ar–H); 13C NMR (75 MHz, CDCl3) δC: 24.6, 31.7, 115.7, 119.8, 123.1, 123.6, 126.2, 127.6, 127.9, 128.1, 128.4, 128.5, 128.6, 128.7, 129.6, 130.1, 132.7, 134.6, 134.8, 140.3, 147.3, 148.0, 148.4, 151.8, 156.6, 163.7(1JC–F = 247.1); anal. calcd for C28H18FN3O2: C, 75.16; H, 4.05; N, 9.39%. Found C, 75.29; H, 3.96; N, 9.28%.

2-(4-Chlorophenyl)-4-(3-nitrophenyl)-5,6-dihydrobenzo[j][1,7]phenanthroline (4q)

Isolated as off white solid. Yield: 77%, m.p. = 244–246 °C; 1H NMR (300 MHz, CDCl3) δH: 3.09–3.14 (m, 2H), 3.28–3.33 (m, 2H), 7.52 (d, 2H, J = 8.7 Hz, Ar–H), 7.57 (t, 1H, J = 7.5 Hz, Ar–H), 7.63 (s, 1H, Ar–H), 7.71–7.80 (m, 3H, Ar–H), 8.03 (d, 1H, J = 8.1 Hz, Ar–H), 8.07 (d, 1H, J = 8.4 Hz, Ar–H), 8.16 (d, 2H, J = 8.4 Hz, Ar–H), 8.34–8.38 (m, 2H, Ar–H), 9.25 (s, 1H, Ar–H); 13C NMR (75 MHz, CDCl3 + DMSO-d6) δC: 24.1, 31.1, 119.6, 122.8, 123.1, 125.8, 127.4, 127.6, 127.7, 127.8, 128.3, 128.4, 129.4, 129.7, 132.3, 134.4, 134.7, 136.7, 139.6, 147.0, 147.4, 147.8, 153.8, 158.2; anal. calcd for C28H18ClN3O2: C, 72.49; H, 3.91; N, 9.06%. Found C, 72.62; H, 3.80; N, 9.15%.

4-(3-Nitrophenyl)-2-phenyl-5,6-dihydrobenzo[j][1,7]phenanthroline (4r)

Isolated as off white solid. Yield: 75%, m.p. = 231–232 °C; 1H NMR (300 MHz, CDCl3) δH: 3.09–3.13 (m, 2H), 3.28–3.33 (m, 2H), 7.49–7.59 (m, 4H, Ar–H), 7.66 (s, 1H, Ar–H), 7.70–7.80 (m, 3H, Ar–H), 8.02 (d, 1H, J = 8.1 Hz, Ar–H), 8.07 (d, 1H, J = 8.4 Hz, Ar–H), 8.21 (d, 2H, J = 7.2 Hz, Ar–H), 8.35–8.37 (m, 2H, Ar–H), 9.28 (s, 1H, Ar–H); 13C NMR (75 MHz, CDCl3) δC: 24.6, 31.7, 120.1, 123.1, 123.6, 126.1, 126.8, 127.6, 128.0, 128.3, 128.5, 128.6, 128.7, 129.2, 129.6, 130.0, 132.8, 134.6, 138.7, 140.5, 147.2, 148.0, 148.4, 151.7, 155.7, 158.6; anal. calcd for C28H19N3O2: C, 78.31; H, 4.46; N, 9.78%. Found C, 78.42; H, 4.52; N, 9.69%.