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“Directed” Assembly of Metallacalix[ n ]arenes with Pyrimidine Nucleobase Ligands of Low Symmetry: Metallacalix[ n ]arene Derivatives of cis ‐[a 2 M(cytosine‐ N3 ) 2 ] 2+ (M=Pt II , Pd II ; n =4 and 6)

2011; Wiley; Volume: 17; Issue: 15 Linguagem: Inglês

10.1002/chem.201002722

1521-3765

Anupam Khutia, Pablo J. Sanz Miguel, Bernhard Lippert,

Metal-Organic Frameworks: Synthesis and Applications

Abstract Pyrimidine (pym) ligands with their two endocyclic N‐donor atoms provide 120° angles for molecular constructs, which, with the 90° angle metal fragments cis ‐a 2 M II (M=Pt, Pd; a=NH 3 or a 2 =diamine), form cyclic complexes known as metallacalix[ n ]arenes (with n =3, 4, 6, 8, …︁). The number of possible isomers of these species depends on the symmetry of the pym ligand. Although highly symmetrical ( C 2 v ) pym ligands form a single linkage isomer for any n and can adopt different conformations (e.g., cone, partial cone, 1,3‐alternate, and 1,2‐alternate in the case of n =4), low‐symmetry pym ligands ( C s ) can produce a higher number of linkage isomers (e.g., four in the case of n =4) and a large number of different conformers. In the absence of any self‐sorting bias, the number of possible species derived from a self‐assembly process between cis ‐a 2 M II and a C s ‐symmetrical pym ligand can thus be very high. 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‐ N3 ) 2 ] n + or cis ‐[a 2 M(cytosinate‐ N1 ) 2 ] (for the latter, see the accompanying paper: A. Khutia, P. J. Sanz Miguel, B. Lippert, Chem. Eur. J. 2011 , 17 , DOI: 10.1002/chem.2010002723) and treating them with additional cis ‐a 2 M II . Moreover, intramolecular hydrogen‐bonding interactions between the O2 and N4H 2 sites of the cytosine ligands reduce the number of possible rotamers to one. This approach of the “directed” assembly of a defined metallacalix[4]arene is demonstrated.