Inorganic-organic hybrid chitosan-based Schiff base-Ni complex as a novel, highly efficient and recyclable heterogeneous catalyst for synthesis of pyrazolophthalazinediones

A Schiff base based on chitosan was synthesized through treatment of chitosan and p-dimethylaminobenzaldehyde in methanol solution. This biopolymeric Schiff base was used to prepare a new first row transition metal complex of Ni(II). The biopolymeric Schiff base and the synthesized tetra-coordinated complex were characterized using Fourier transform infrared, H-1 NMR and C-13 NMR techniques. Then, an efficient synthetic method for functionalized 1H-pyrazolo[1,2-b] phthalazine-5,10-diones was successfully developed using one-pot domino reaction of ninhydrin and malononitrile with 3-arylamino-5,5-dimethyl-2-cyclohexenones catalysed by the chitosan-based Schiff base complex of Ni(II) at room temperature. The advantages of this protocol are easy work-up, short reaction times and high yield of products and also the catalyst can be readily isolated from the reaction mixture and recycled without loss of catalytic activity.