[(Ethylenediamine)Pt(uracilate)]₄, a Metal Analogue of Calix[4]arene. Coordination and Anion Host−Guest Chemistry Related to Its Conformational Dynamics

Abstract

The open "molecular box" [(en)Pt(UH-N(1),N(3))](4)(NO(3))(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 (1a), 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 oxo-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 La(III) (4); X = NO(3), SO(4)/2; n = 2, 3. In addition, both the protonated species of the cone conformer (1b, 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 (1)H 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)(NO(3),SO(4)/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 (1)H 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.

Mixed‐Metal (Platinum, Palladium), Mixed‐Pyrimidine (Uracil, Cytosine) Self‐Assembling Metallacalix[n]arenes: Dynamic Combinatorial Chemistry with Nucleobases and Metal Species

“…The complexity of the compounds formed not only results from the possibility of the two different nucleobases in building block 1 engaging in different connectivities with the Pd entities, but also from the potential for the formation of oligomers of different sizes and different conformations; in the case of cyclic tetranuclear Pt 2 Pd 2 species, this can, in principle, lead to the various arrangements (cone, partial cone, 1,2-alternate, 1,3-alternate) known from calix [4]arene chemistry. A further complication arises from the fact that, depending on the mutual orientation of the exocyclic groups of the two nucleobases (O2 and O4 of uracil, O2 and N4 of cytosine), these sites can be engaged in additional chelation of [Pd NH 3 or amine; a 2 : diamine; M: Pt or Pd) and substituted pyrimidine ligands; more importantly, a structural and functional analogy between both types of compounds is evidenced by their capabilities to act as host compounds for guest molecules [3,4] and to permit decoration of the oxygen "rim" by additional metal ions, [5] as well as by the fact that they come in different ring sizes ranging from n = 3 [6] to n = 4 [1][2][3][4] and n = 6. [7] As demonstrated by Navarro and co-workers, there is also the possibility to combine the pyrimidine ligand with another ditopic ligand, such as 4,7-phenanthroline.…”

“…[9] A remarkable feature of the discrete metallacalix [n]arenes derived from the pyrimidine nucleobases uracil or thymine is their propensity to add additional square-planar cis-Ma 2 II (M: Pt, Pd) or cis-octahedral metal entities (Ni II , Cu II ) to yield higher nuclearity species. Octanuclear compounds have been derived from Pt-calix [4]arenes [5a, 10] and pentanuclear species from either Pt-calix [4]arenes [4, 5c] or Pt-calix [3]arenes. [11,12] These findings are in a way reminiscent of observations with other Pt complexes containing model nucleobases, such as 1-methyluracil, 1-methylthymine, 1-methylcytosine, or 9-alkylpurines, for which we have systematically studied the formation of multinuclear derivatives.…”

Cyclic Tetranuclear and Hexanuclear Palladium(II) Complexes and Their Host−guest Chemistry

“…As a result of the size of the DSS, only one DSS can be accommodated per host hexamer ( Figure 6B), creating the two different environments in the [Pd(en)(5′GMP)] 6 -DSS structure and resulting in the two equal intensity lines for the 5′-GMP H8, H1′, 31 P, and 13 C resonances in the host-guest spectra. Figure 7.…”

“…The cyclic polynuclear metal species are frequently cationic and, in some cases, have been found to behave as hosts for anions. 3,4,6,12,13 Among the ligands used to bridge between the metal centers in the cyclic complexes, the nucleobases, nucleosides, and nucleotides have been frequently investigated, especially with Pt(II) and Pd(II). 2,3,14 The preferred binding site of metal ions to guanine and inosine derivatives is the N7 position, but N1 also becomes a binding site upon deprotonation.…”

“…However, the crystal structures of several other cyclic nucleobase-containing Pt(II) complexes have been obtained, for example [Pt(en)(uracilate)] 4 (NO 3 ) 4 , which has been shown to weakly bind anions as guests in aqueous solution 13,20 and Pt(II) trimers containing bridging 1-methylcytosine 21,22 or 9-methyladenine. 23 The structure of the cyclic hexamer [(PMe 3 ) 2 Pt(9-methylguanine)] 6 (NO 3 ) 6 · 18H 2 O has been found to have S 3 symmetry with the 9-methylguanines located alternately above and below the plane of the six Pt(II).…”