Urea-Functionalized Self-Assembled Molecular Prism for Heterogeneous Catalysis in Water

Reaction of a ditopic urea “strut” (L₁) with cis-(tmen)­Pd­(NO₃)₂ yielded a [3+3] self-assembled molecular triangle (T) [L₁ = 1,4-di­(4-pyridylureido)­benzene; tmen = N,N,N′,N′-tetramethylethane-1,2-diamine]. Replacing cis-(tmen)­Pd­(NO₃)₂ in the above reaction with an equimolar mixture of Pd­(NO₃)₂ and a clip-type donor (L₂) yielded a template-free multicomponent 3D trigonal prism (P) decorated with multiple urea moieties [L₂ = 3,3′-(1H-1,2,4-triazole-3,5-diyl)­dipyridine]. This prism (P) was characterized by NMR spectroscopy, and the structure was confirmed by X-ray crystallography. The P was employed as an effective hydrogen-bond-donor catalyst for Michael reactions of a series of water-insoluble nitro-olefins in an aqueous medium. The P showed better catalytic activity compared to the urea-based ligand L₁ and the triangle T. Moreover, the confined nanospace of P in addition to large product outlet windows makes this 3D architecture a perfect molecular vessel to catalyze Diels–Alder reactions of 9-hydroxymethylanthracene with N-substituted maleimide in the aqueous medium. The present results demonstrate new observations on catalytic aqueous Diels–Alder and Michael reactions in heterogeneous fashion employing a discrete 3D architecture of Pd­(II). The prism was recycled by simple filtration and reused several times without significant loss of activity.