## 1 Introduction
The former series mainly refers to soluble Pd salts or complexes having various ligands, including phosphine and N-heterocyclic carbenes [25], while the latter usually signifies the Pd nanoparticles, clusters, or single atoms immobilized over solid supports such as silica, polymers, or activated carbon [26]. No matter which kind of catalyst is used, the active Pd species usually shows a tendency to be reduced and subsequently agglomerated to inactive Pd\({}^{(0)}\) black during catalysis [26; 27]. In order to decrease this unfavorable inclination, ionic nitrogen compoundssuch as ammonium, amidinium, and pyridinium salts were ever introduced to stabilize the active Pd (Pd(\({}^{0}\)) or Pd\({}^{2+}\)) species, where the effects of stabilization, fixation, modulation of catalyst morphology, and reaction medium, as well as phase-transfer property on catalysis, were highly anticipated [26]. However, this endeavor was not completely effective, and quite a few ionic nitrogen compounds showed inhibitory rather than accelerating or stabilizing effects on the Pd-catalyzed reactions [26]. Obviously, there is still a lot of room to explore efficient methods to immobilize expensive Pd catalysts for recycling. Traditionally, immobilization of Pd salts or complexes into solid supports has created heterogeneous catalysts for the Suzuki-Miyaura coupling reaction, and sometimes solid supports can serve as base generated in situ in the catalytic reaction [26]. However, the immobilization present cannot avoid leaching and the degradation of the Pd catalyst, probably because the dispersion of solvent (water and organic molecules) into the pores of solid support, which destroys the chemical or physical linkage between the Pd component and support [26]. Therefore, in order to securely fix Pd, new support materials, instead of the common solid supports (silicate, aluminum oxide or other known solid acids), deserve further exploration. The catalysis carried out in ionic liquids (ILs) has experienced a tremendous growth over the past two decades [28]. There are numerous examples of versatile catalytic reactions that have been effectively accomplished in this kind of media [28]. Theoretically, room temperature ionic liquids (usually denoted as RTILs or ILs) are composed of ions with melting points lower than 100 \({}^{\circ}\)C, which are nonvolatile, nonflammable, chemically stable, easy to recover, and eco-friendly for many chemical reactions, arousing much interest in their usage as either a catalyst or solvent [28; 29]. Furthermore, the great enthusiasm for exploring ILs in catalysis has not only been propelled by the eco-friendly properties of ILs, but also owing to the unique polar or ionic nature of ILs, which the common molecular solvents do not show, subsequently immobilizing catalytic components tightly and lastingly [28; 30]. Therefore, the leaching of catalytically active components from ILs would become slower and minimal, product extraction and purification become more convenient, and the catalyst activity could be retained for a long time [31]. Meanwhile, certain catalytic reactions that cannot be performed in common organic solvents seem more possible in ILs [32]. Due to the demand for developing new \(1H\)-indazole-derived pharmaceuticals, the present low efficiency of C-3 functionalization of \(1H\)-indazole, as well as the high potential of Pd-catalyzed Suzuki-Miyaura coupling in ILs, this work aimed to develop the C-3 functionalization of \(1H\)-indazole reactions using Pd-catalyzed Suzuki-Miyaura coupling. In order to improve the recovery and recycling of the Pd catalyst, a series of imidazolium ILs BMImX (BMIm\({}^{+}\) = 1-\(n\)-butyl-3-methylimidazolium; \(X^{-}\) = PF\({}_{6}\)\({}^{-}\), BF\({}_{4}\)\({}^{-}\)) were introduced as a support material, which not only had controllable hydrophilicity, owing to variation of the anions, but also contained a nitrogen atom, showing coordination effects for stabilizing Pd ions.