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

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(2v)) 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.

High‐Resolution Crystal Structure of a Silver(I)–RNA Hybrid Duplex Containing Watson–Crick‐like CSilver(I)C Metallo‐Base Pairs

“…[6] On the other hand, the geometry is completely different from ac omplex between Ag I and 1-methylcytosine where two Ag I ions bridge between two 1-methylcytosines in trans geometry through O2ÀAg I ÀN3 coordinate bonds (Supporting Information, Figure S3 b), [13] and also from acomplex between Pt II and cytosine where one Pt II ion makes square planer coordination with two cytosines and two ammonias (Supporting Information, Figure S3 c). [14] . .…”

Crystal structures of bis[2-(pyridin-2-yl)phenyl-κ2N,C1]rhodium(III) complexes containing an acetonitrile or monodentate thyminate(1−) ligand

“…These anions can act as suitable bridging ligands for the construction of functional polymetallic coordination compounds because they provide multiple donor atoms to metal atoms in a configurationally fixed fashion. For example, some tetra-and pentanuclear Pt II complexes bridged by thym 2have been described (Khutia et al, 2011;Rauterkus & Krebs, 2004). We have also reported some cyclic tetranuclear Cp*Rh III (Cp* = pentamethylcyclopentadienyl) complexes bridged by thym 2and incorporating an another metal cation in the central hydrophilic cavity of their metallacalix[4]arene motifs (Kashima et al, 2015;Sakate et al, 2016).…”

Supramolecular Architectures Based on Metal–Cytosine Systems

“…Neutral cytosine, although presenting different donor positions, shows a relatively predictable coordination mode through N3 . The other possible binding modes are through N1, and simultaneously, through N3 and O2 atoms, while in some cases, coordination solely through O2 is reported.…”

“…Although there are many examples of purine‐based systems, it still remains a challenge to provide similar compounds with pyrimidine nucleobases, since, as can be deduced from their lower number of heteroatoms, their coordination and hydrogen‐bonding interactions with other species are reduced . On the other hand, there exist interesting examples of cytosine derivatives linked to transition‐metal complexes, but the coordination of unsubstituted neutral cytosine to metal centers has been less studied , …”