Inclusion complex formation involving a new class of transition-metal host

Nickel(II) Cyclidenes with Appended Ethylpyridine Receptor Centers as Molecular Tweezers for Dicarboxylic Acids

A series of 14-, 15-, and 16-membered nickel(II) cyclidene macrocycles appended with 2-aminoethyl(2-pyridine) receptors I-III, respectively, were prepared and characterized by X-ray crystallography and NMR techniques. The 14- and 15-membered macrocycles I and II exist in a planar or extended Z-configuration, whereas the 16-membered macrocycle III was saddle shaped and had two asymmetric configurations in the unit cell (IIIa in a "capped" configuration and IIIb in an "open" configuration). Variable-temperature (1)H NMR studies of III in CD(3)CN were conducted (25-65 degrees C), and at room temperature, the interconversion between capping and uncapping is slow on the NMR time scale, resulting in a broad spectrum, whereas at 65 degrees C, interconversion was fast. (1)H NMR binding studies indicated I-III bind unsaturated dicarboxylic acids in a 1:1 stoichiometry with binding constants approaching 400 M(-)(1) in CD(3)CN, and the binding strength was dependent on the shape of the macrocyclic cyclidene platforms, whereas monocarboxylic acids were not bound. Generally, the planar 14-membered cyclidene I bound diacids the weakest and the 16-membered cyclidene III bound diacids the strongest. The presence of nuclear Overhauser effect spectrometry cross peaks in a 20 mM solution of 1:1 II-maleic acid indicates that the binding mode is ditopic with the guest being encapsulated by the aminoethylpyridine arms above the macrocyclic framework.

Synthesis and characterization of chiral tetraaza macrocyclic nickel(II) and palladium(II) complexes

Chiral tetraaza macrocyclic nickel(II) and palladium(II) complexes 2a-e, containing one or two (R,R)-(-)-1,2-cyclohexanediyl bridges, were synthesized by template condensation reactions and characterized by (1)H and (13)C NMR, IR, UV-vis, and mass spectrometry. The electrophilic reactivity of 2a was explored. Crystal structures of Ni complex 2b and metal-free ligand 5 were determined by single-crystal X-ray diffraction.

Ligand binding inside the cavities of lacunar and saddle-shaped cyclidene complexes: molecular mechanics and molecular dynamics studies

Cobalt(II) complexes with tetradentate macrocyclic cyclidene ligands are known to coordinate one additional axial base molecule, leaving the sixth vacant coordination site at the metal ion available for small ligand (e.g., O2) binding. Molecular mechanics and molecular dynamics simulations provide a microscopic view of 1-methylimidazole (MeIm) binding within the cavities of several lacunar (bridged) and saddle-shaped (unbridged) cyclidenes and uncover the roles of the bridges and the walls of the clefts in steric protection of the cobalt(II) coordination site. Short bridges (C3 and C6) prevent inside-the-cavity MeIm binding because of severe ligand distortions leading to high-energy penalties (58 and 25 kcal/mol, respectively), while long bridges (C8 and C12) flip away from the MeIm binding site, allowing for penalty-free MeIm inclusion. In the unbridged saddle-shaped complex, there is no energy difference between inside- and outside-the-cavity MeIm binding. The preferential existence of the coordinatively unsaturated, five-coordinate species Co(unbrCyc)(MeIm)2+ should therefore be explained by electronic, rather than steric, factors. Molecular dynamics and free energy simulations reveal the presence of a weak (ca. 4 kcal/mol in the gas phase and ca. 2 kcal/mol in methanol solution) noncovalent MeIm binding site at the entrance of the cleft of cobalt(II) unbridged cyclidene, at a distance of about 4 A from the metal ion. The macrocycle geometry remains undistorted at such large Co-N(MeIm) separations, while the cavity opens up by 0.9 A upon covalent MeIm binding (Co-N(MeIm) distance of 2 A). An increase in macrocycle strain energy upon MeIm inclusion is compensated by favorable nonbonded interactions between the incoming base and the walls of the unbridged cyclidene.