Self-assembly of organometallic macrocycles and cages

Neutral metallamacrocycles with four or ten metal centers were obtained by reaction of [(R3P)PdCl2]2 with 2,3-dihydroxypyridine and derivatives or 2-hydroxynicotinic acid in the presence of base. In all these assemblies, the two O-donor atoms of the ligand bind to one metal center to form a chelate ring, while the N-donor atom binds to another metal center, linking the subunits together. The tetrameric complexes showed a low affinity for small aromatic diols. The procedure was adapted for the synthesis of water-soluble macrocycles: ligands which contain both the 2,3-dihydroxypyridine motive and a tertiary amine function that can be protonated were reacted in water with the acetato-bridged complex [(Et3P)Pd(CH3COO)2]2 to give tetrameric macrocycles in over 90% yields. Scrambling experiments showed that the Pd(II) macrocycles are highly inert. The reaction of ReBr3(CO)32 with 3-hydroxy-1,2,3-benzotriazine-4(3H)-one in the presence of base gave the ionic rhenium complex [ReBr(C7H4N3O2)(CO)3][NEt4], from which a neutral metallamacrocycle was obtained upon abstraction of (NEt4)Br. Its connectivity is comparable to that of the Pd(II) macrocycles. In coordinating solvents, this macrocycle disrupts and the monomeric solvent adducts are formed. In the presence of BF4-, it undergoes an anion-templated rearrangement to give a new, trimeric macrocycle which cocrystallizes with an unusual C3-symmetric [Ag(η2-benzene)3(OH2)][BF4] complex, the tetrafluoroborate anion being encapsulated in the cavity of the macrocycle. By reacting [(cymene)RuCl2]2 with 3,5-pyridinedicarboxylic acid, a trifunctional ligand in which the three donor atoms are unable to coordinate to the same metal center, an hexameric coordination cage was obtained. This complex has a trigonal antiprism geometry. Half of the carbonyl O-atoms of the bridging ligands are positioned in close proximity to each other, forming two binding sites on the surface. The cage can thus act as an exo-receptor for alkali metal cations (Na+, K+, Cs+). 1H and 13C NMR investigations as well as a single crystal X-ray analysis revealed that upon addition of alkali metal salts, the hexanuclear assembly partly rearranges to form a dodecameric coordination cage with icosahedral geometry. Although entropically disfavored, this process takes place because in this new complex all carbonyl O-atoms are part of a binding site (eight metal cations can be accommodated on the surface of the cage). Furthermore, due to the reduced surface curvature of the dodecanuclear assembly, these O-atoms are closer together, forming more suited binding sites for the alkali guest cations. To best of our knowledge, a coordination cage with an icosahedral geometry has not been reported so far.