[(Ethylenediamine)Pt(uracilate)]4, a metal analogue of calix[4]arene.: Coordination and anion host-guest chemistry related to its conformational dynamics

The open "molecular box" [(en)Pt(UH-N-1,N-3)](4)(NO3)(4) (with en = 1,2-diaminoethane, UH = uracil monoanion) resembles calix[4]arenes in its structure and solution dynamics. It adopts a 1,3-alternate conformation in the solid-state (la), but in solution and after deprotonation to [(en)Pt(U-N-1,N-3)](4), a second major species (cone conformer (1b-4H(+))) occurs. 1b-4H(+) acts as an efficient ligand for additional metal ions through the ore-surface formed by the four O-2 exocyclic atoms of the uracil nucleobases. As shown here, 1b-4H(+) can incorporate a single metal ion only, giving rise to the formation of species of type {[(en)PtU](4)M}(X)(n) with M = Zn-II (2a), Be-II (3) (not isolated in the solid state), and Lam (4); X = NO3, SO4/2; n = 2, 3. In addition, both the protonated species of the cone conformer (Ib, pH 2-4) and compounds 2a (at pH 3-8) and 3 (at pH 3-5) act as hosts for organic anions in water, as deduced from H-1 NMR studies. It is proposed that the cone conformers act as anion hosts due to a combination of positive charge as well as apolarity and size of the cavity. Host-guest complexes of type {[(en)-PtU](4)Zn}(X)(NO3;SO4/2) with X = p-toluenesulfonate (2b) and 3-(trismethylsilyl)-1-propanesulfonate (2c) included in the cone cavity have been prepared and association constants have been determined by H-1 NMR spectroscopy. The fact that 4 does not act as a host may be due to a possible tetradentate coordination of La to 1b-4H(+) which may result in a flatter cone cavity than in compounds 2 and 3.