Chiral Pyrimidine Metallacalixarenes: Synthesis, Structure and Host–Guest Chemistry

Abstract

A set of enantiomerically pure cyclic multinuclear complexes with the formula cis-[a(2)PdL](n) (n+) [a(2)=(R,R)-1,2-diaminocyclohexane (R,R-dach), (S,S)-1,2-diaminocyclohexane (S,S-dach); n=4, 6; LH=2-hydroxypyrimidine (2-Hpymo), 4,6-dimethyl-2-hydroxypyrimidine (2-Hdmpymo) and 4-hydroxypyrimidine (4-Hpymo)] were obtained by reaction of cis-[a(2)Pd(H(2)O)(2)](2+) and LH in aqueous media. The polynuclear complexes were studied by (1)H NMR spectroscopy and X-ray crystallography. These studies revealed that the N1,N3-bridging mode exhibited by the pyrimidine moieties is ideally suited for formation of inorganic analogues of calixarenes (metallacalixarenes) in a self-assembly process. The most stable species are the tetranuclear metallacalix[4]arenes, which are obtained in all cases. Hexanuclear species, namely, [a(2)Pd(2-dmpymo)](6) (6+), were also isolated and fully characterised. (1)H NMR experiments show conversion of [a(2)Pd(2-dmpymo)](6) (6+) to [a(2)Pd(2-dmpymo)](4) (4+) on heating. Analogously to organic calixarenes, these systems are also capable of incorporating hard metal ions at the oxo surface. Additionally, investigations on the receptor properties of these metallacalixarenes towards mononucleotides showed that enantioselective recognition processes occur in aqueous media.

“Directed” Assembly of Metallacalix[n]arenes with Pyrimidine Nucleobase Ligands of Low Symmetry: Metallacalix[n]arene Derivatives ofcis‐[a2M(cytosine‐N3)2]2+(M=PtII, PdII;n=4 and 6)

“…The cation in 8 thus does not show the compressed feature of the corresponding (dach)Pd II hexanuclear species with 2-dmpymo ligands. [11] As in 7, the intramolecular hydrogen bonding between the N4H 2 and O2 groups of (Figure 9). The other five anions and the water molecules surround the cation, forming multiple hydrogen bonds.…”

“…By using the C s -symmetric pym nucleobase cytosine, we have demonstrated that the number of feasible isomers for n = 4 can be reduced to one by applying preformed building blocks such as cis-[a 2 M(cytosine- also been obtained. [11] Ring fragments have likewise been characterized, [10] as have species with mixed N,O coordination. [10] A complication can arise from the propensity of these metallacalix[n]arenes to coordinate additional transition-metal ions through exocyclic oxygen atoms (uracil, thymine, and cytosine) and/or deprotonated exocyclic amino groups (cytosine).…”

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

“…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. [8] Also, three-dimensional polymeric materials displaying similar structural motifs, for example, rings with four or six metal ions, have been reported, which show interesting adsorption phenomena.…”

“…Unlike related metallacalix[n]arenes with n = 3, [6] 4, [1,2,4] and 6, [7] compound 9 a with n = 8 no longer has the shape of a ring but rather adopts a strongly folded structure ( Figure 11). Compound 9 a crystallizes in space group P4 2 /n and has two crystallographically independent cations.…”

“…system and characterized by X-ray crystal structure analysis was (7). The cation of 7 is depicted in Figure 9 and salient structural data are listed in Table 4.…”