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.