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• Facile synthesis of magnetic N -heterocyclic copper(II) complex. • Structural verification of the nanocatalyst by TEM, TG/DTG, FESEM, FTIR, XRD, EDS and VSM analyses. • Catalytic performances of Fe 3 O 4 /SiO 2 -Thiotet-Cu(II) in the 4-NP, CR, MB and Cr(VI) reduction in water. • Catalyst can be reused nine times without any notable decreasing in its catalytic performance. • Antibacterial activity of the Fe 3 O 4 /SiO 2 -Thiotet-Cu(II) nanocatalyst against E. coli and S. aureus. Development of more efficient, low cost and sustainable (nano)catalysts for reduction of various pollutants in water is highly desirable and is still a major challenge. Herein, we report the immobilization of Cu(II)-5-(methylthio)-1 H -tetrazole complex (Thiotet-Cu(II)) on Fe 3 O 4 /SiO 2 superparamagnetic nanoparticles (NPs) through a versatile process and the application of the nanoparticles prepared as highly efficient and recyclable nanospherical catalysts. The as-prepared Fe 3 O 4 /SiO 2 -Thiotet-Cu(II) is used as a recyclable and highly active nanocatalyst in the reduction of hexavalent chromium [Cr(VI)], 4-nitrophenol (4-NP), Congo Red (CR) and Methylene Blue (MB) in aqueous media. The surface nature of Fe 3 O 4 NPs changes after modification, which causes a notable increase in the catalytic prowess and antimicrobial performance. Moreover, the ensuing nanocatalyst remains high reduction efficiency of ~95% after nine cycles without meaningful loss of its efficiency and can be conveniently separated/decanted using an external magnet, demonstrates high recyclability, excellent chemical stability and high potential for practical environmental applications. The present protocol is clean and eco-friendly and does not use any toxic reductants. [ABSTRACT FROM AUTHOR]

• Efficient synthesis of magnetic NPs-supported N -heterocyclic palladium(II) nanocatalyst. • Reduction of Cr(VI), 4-NP, NS, and K 3 [Fe(CN) 6 ] in water using Fe 3 O 4 @SiO 2 -Thiotet-Pd(II). • Characterization of the nanocomplex using TG-DTG, FESEM, FT-IR, TEM, VSM, EDS and XRD. • Nanocatalyst can be reused eight times without any considerable changes in the catalytic activity. • Antimicrobial activity of the magnetic nanocomplex against E. coli. A novel magnetic palladium(II) complex bearing N -heterocyclic ligand has been successfully fabricated via directly anchoring and immobilizing the structurally defined 5-(methylthio)-1 H -tetrazole with a long tail ((3-chloropropyl)trimethoxysilane) onto the silica encapsulated Fe 3 O 4 nanomagnetic particles (Fe 3 O 4 @SiO 2 -Thiotet-Pd(II)) and used as a magnetically recoverable nanospherical catalyst. Fourier transforms infrared spectroscopy (FT-IR), thermogravimetric-differential thermal analysis (TG-DTA), transmission Electron Microscopy (TEM), field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), energy dispersive X-ray spectrometry (EDS) and vibrating sample magnetometer (VSM) analytical techniques have been applied to characterize the magnetic nanocatalyst prepared. The antimicrobial activity of Fe 3 O 4 @SiO 2 -Thiotet-Pd(II) was also evaluated by disk diffusion assay. The synthesized magnetically recoverable nanocatalyst exhibited enormously high catalytic activity in the rapid, efficient, and room temperature reduction of nigrosin (NS), 4-nitrophenol (4-NP), potassium hexacyanoferrate(III) (K 3 [Fe(CN) 6 ]), and hexavalent chromium [Cr(VI)] in aqueous media. Herein, we describe an unprecedented example of thiotetrazole grafted on Fe 3 O 4 @SiO 2 nanomagnetic, which can be separated by a magnet and recycled eight times with no appreciable decrease of catalytic activity and efficiency. Generally, NaBH 4 induced reduction of NS, 4-NP reduction to 4-aminophenol (4-AP), K 3 [Fe(CN) 6 ] reduction to K 4 [Fe(CN) 6 ] as well as HCOOH induced reduction of Cr(VI) to Cr(III) catalyzed by Fe 3 O 4 @SiO 2 -Thiotet-Pd(II) were achieved within an extremely short span of time. The results showed that Fe 3 O 4 @SiO 2 -Thiotet-Pd(II) was effective against Escherichia coli as a gram negative bacteria. [ABSTRACT FROM AUTHOR]