Self-assembled metallocycles with two interactive binding domains

Five metallocycles 1a-e have been self-assembled from S-shaped bispyridyl ligands 2a-e and a palladium complex, [Pd(dppp)(OTf)₂] (dppp = 1,3-bis(diphenylphosphanyl)propane), and have been characterized by elemental analysis and various spectroscopic methods including ¹H NMR spectroscopy and electrospray ionization (ESI) mass spectrometry. These metallocycles all are monocyclic compounds, but can fold to generate two binding domains bearing hydrogen-bonding sites based on pyridine-2,6-dicarboxamide units. The binding properties of the metallocycles with N,N,N′,N′- tetramethylterephthalamide (G) have been probed by means of ESI mass spectrometry and ¹H NMR spectroscopy. The results both in the gas phase and in solution are consistent with the fact that the metallocycles accommodate two molecules of the guest G. Thus, the ESI mass spectra clearly show fragments corresponding to the 1:2 complexes in all cases. ¹H NMR studies on 1a and G support the formation of a 1:2 complex in solution; the titration curves are nicely fitted to a 1:2 binding isotherm, but not to a 1:1 binding isotherm. In addition, a Job plot also suggests a 1:2 binding mode between 1a and G, showing maximum complexation at ∼0.33 mol fraction of the metallocycle 1a in CDCl₃. The binding constants K₁ and K₂ are calculated to be 1600 and 1400 M^-1 (± 10%), respectively, at 25 °C in CDCl₃, indicative of positively cooperative binding. This positive cooperativity was confirmed by the Hill equation, affording a Hill coefficient of n = 1.6. Owing to insufficient solubility in CDCl₃, for comparison purposes the binding properties of the metallocycles 1b-e were investigated in a more polar medium, 3% CD ₃CN/CDCl₃. ¹H NMR titrations revealed that the metallocycles all bind two molecules of the guest G with Hill coefficients ranging from 1.4 to 1.8. This positive cooperativity may be attributed to a structural reorganization of the second binding cavity when the first guest binds to either one of the subcavities present in the metallocycles.