Extended insight into the catalytic activity of boron-doped graphitic carbon nitride for the synthesis of bis-pyrazolyl methanes and pyranopyrazoles

Abstract

In this study, boron-doped graphitic carbon nitride (BCN) was successfully prepared via thermal copolymerization of dicyandiamide and boric acid. The structural and morphological features of the as-prepared BCN were thoroughly characterized by various physicochemical techniques such as Fourier transform infrared spectroscopy (FT-IR), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-Vis Diffuse Reflectance Spectroscopy (UV-DRS), Brunauer-Emmett-Teller (BET) surface area analysis, and thermogravimetric analysis (TGA). The catalytic performance of BCN was then exploited in the heterogeneous multicomponent synthesis of bis(pyrazolyl)methanes and pyranopyrazoles. The superior catalytic activity of the catalyst stemmed from the B doping and N species located at the B-N-C sites, which impart acid-base dual functionality to the catalyst. These findings substantiate the pioneering utilization of BCN as a robust acid-base cooperative catalyst for the multicomponent synthesis of structurally diverse heterocycles. To the best of our knowledge, this is the first report demonstrating the use of BCN as a dual-function catalyst in MCR chemistry, opening new avenues for green and sustainable organic transformations.

Keywords: BCN; Bis-pyrazolyl methane; Boron doping; Carbon nitride; Dual function; Pyranopyrazoles.

Scheme 1

Schematic illustration representing the synthesis of BCN.

Fig. 1

FT-IR spectra of g-C₃N₄ and B_xCN samples where x is the weight ratio of B to CN precursors.

Fig. 2

Powder XRD patterns of g-C₃N₄ and B_xCN samples.

Fig. 3

XPS spectrum for B1s of B_0.1CN.

Fig. 4

SEM image (a), TEM image (b) of B_0.1CN with elemental mapping of Boron (c), Carbon (d), Nitrogen (e), and Oxygen (f); SEM (g) and TEM (h) images of CN.

Fig. 5

UV-vis DRS spectra of g-C₃N₄ and B_0.1CN samples (left) and the corresponding Tauc plots (right).

Fig. 6

TGA profile conducted in N₂ atmosphere of B_0.1CN.

Fig. 7

(a) CO₂-TPD and (b) NH₃-TPD profiles of g-C₃N₄ and BCN.

Fig. 8

Recyclability of B_0.1CN in the synthesis of 4a.

Fig. 9

(a) FT-IR spectra and (b) XRD patterns of fresh and six-times reused B_0.1CN; (c) SEM and (d) TEM image of reused B_0.1CN.

Fig. 10

Recyclability of B_0.1CN in the synthesis of 6a.

Fig. 11

Proposed mechanism for BCN-catalyzed synthesis of (a) bis-pyrazolyl methanes and (b) pyranopyrazoles.

Fig. 12

Molecular electrostatic potential surface of BCN.