**3. Experimental**

*Nanocatalysts*

**Figure 1.**

**Figure 2.**

*SEM image of ZrO2 NPs.*

*XRD spectra of ZrO2 NPs.*

**62**

**Figure 3.**

*TEM image of ZrO2 NPs.*

particle size of ZrO2 NPs **(Figure 1)**.

zirconia (JCPDS card no. 79-1769). The broadening of peaks shows the smaller

Morphological studies were done with the help of SEM and TEM analysis of 600°C calcinated ZrO2 NPs sample that are shown in **Figures 2** and **3**, respectively. SEM analysis shows that NPs are non-homogenous and agglomerated and it also indicates the spherical nature and nano size (nm regime) of the ZrO2 nanoparticles

#### **3.1 Typical procedure for the synthesis of ZrO2 NPs**

0.075 M solution of ZrOCl2.8H2O was prepared and then precipitated with NH4OH (25%) with continuous stirring on a magnetic stirrer till the PH raises in the range of 10–10.5.This resulted in the formation of precipitate of zirconium hydroxide. The precipitate was filtered and washed with double distilled water until traces of chloride ion were completely removed from the filtrate. Complete removal of chloride ion from filtrate was checked by titrating it with AgNO3 solution using potassium chromate as indicator. Now, the precipitate was dried in oven at 80–90°C for 24 h and calcinated at 600°C for 3 h in order to formation of white nano zirconia powder.

#### **3.2 General procedure for the synthesis of substituted imidazoles 4a-s**

To a mixture of isatin derivatives **1a–g** (1 mmol), ammonium acetate **2** (5 mmol), substituted aromatic aldehydes **3a–f** (1 mmol), 15 mol% of ZrO2 NPs was added (**Scheme 1**). The mixture was heated and stirred at 110°C for 30 min. The progress of the reaction was monitored by thin layered chromatography (n-hexane:ethyl acetate, 1:1). After completion, 20 ml acetone was added to the reaction mixture; the catalyst was removed by filtration and washed with xylene and acetone. Then, 50 ml of double distilled water is added to the liquid portion. This resulted in the formation of precipitate of products **4a–s**. The precipitate was filtered, dried and recrystallized with ethanol.

#### *3.2.1 2-Phenyl-3,4-dihydroimidazo[4,5-b]indole (4a)*

Brownsolid, **IR (KBr) υ:** 3400, 3209, 3019, 2964, 1660, 1614, 1567, 1484, 1316, 1210, 1171, 1010, 877, 742, 653, 580 cm<sup>−</sup><sup>1</sup> . **1H NMR (300 MHz, DMSO) δ:** 7.80–8.86 (m, 9H, aromatic protons), 9.15 (s, 1H, NH), 9.66 (s, 1H, NH) ppm. **13C NMR (75.45 MHz, DMSO) δ:** 124.0, 126.7, 127.5, 130.2, 130.7, 132.0, 133.7, 135.5, 139.1, 148.2, 160.9 ppm. Anal. Calcd for C15H11N3: C, 77.24; H, 4.74; N, 18.01 Found C, 77.20; H, 4.76; N, 18.03.

#### *3.2.2 7-Chloro-2-phenyl-3,4-dihydroimidazo[4,5-b]indole (4b)*

Brownsolid, **IR (KBr) υ:** 3364, 3190, 2981, 2964, 1648, 1609, 1559, 1447, 1311, 1199, 1143, 1019, 872, 744, 651, 566 cm<sup>−</sup><sup>1</sup> . **1H NMR (300 MHz, CDCl3) δ:** 7.51–8.59 (m, 8H, aromatic protons), 9.14 (s, 1H, NH), 9.45 (s, 1H, NH) ppm. **13C NMR (75.45 MHz, CDCl3) δ:** 123.9, 125.7, 128.5, 128.6, 130.3, 130.8, 132.7, 135.0, 137.5, 149.2, 159.4 ppm. Anal. Calcd for C15H10ClN3: C, 67.28; H, 3.78; N, 15.72. Found C, 67.32; H, 3.76; N, 15.70.

## *3.2.3 2-(2-Nitrophenyl)-3,4-dihydroimidazo[4,5-b]indole (4c)*

Brownsolid, **IR (KBr) υ:** 3332, 3201, 2995, 2917, 1658, 1623, 1549, 1485, 1348, 1280, 1176, 1068, 864, 708, 667, 544 cm<sup>−</sup><sup>1</sup> . **1H NMR (300 MHz, DMSO) δ:** 7.61– 8.69 (m, 8H, aromatic protons), 9.67 (s, 1H, NH), 9.94 (s, 1H, NH) ppm. **13C NMR (75.45 MHz, DMSO) δ:** 123.8, 126.7, 128.8, 129.8, 130.1, 131.8, 135.4, 135.8, 135.9, 148.3, 160.7 ppm. Anal. Calcd for C15H10N4O2: C, 64.74; H, 3.62; N, 20.13 Found C, 64.69; H, 3.65; N, 20.14.
