Study of the coordination of quinuclidine to a chiral zinc phthalocyanine dimer

Abstract

<p> We attempted the calculation of an accurate equilibrium constant for the dimerization process of enantiomerically pure Zn-1 using UV-vis dilution experiments. At millimolar concentration Zn-1 is involved in a chemical exchange process between its monomeric and dimeric state that is slow on the 1H NMR timescale. We performed variable-temperature 1H NMR experiments in CDCl3 solution to determine the dimerization constant value at different temperatures and performed a van&rsquo;t Hoff plot to derive the thermodynamic parameters of the process. The calculated thermodynamic data revealed that the dimerization process is entropy-driven and enthalpically opposed. We also probed the coordination of quinuclidine, 1-azabicyclo[2.2.2]octane, 2, to the Zn-phthalocyanine using UV-vis and 1H NMR titrations in CDCl3 solution. At micromolar concentration the Zn-1 exclusively exists in solution as a monomer and forms a simple 1:1, 2&bull;1-Zn, complex with quinuclidine having a stability constant of K(2&bull;1-Zn) = 106 M-1. On the other hand, the 1H NMR titrations carried out at 298 K and at millimolar concentration showed that Zn-1 was present in solution as the dimer and formed 2:1, 2&bull;(1-Zn)2, and 2:2, 22&bull;(1-Zn)2, complexes by coordination to 2. In addition, the 1:1 complex, 2&bull;(1-Zn), showed a reduced dimerization constant compared to the uncoordinated parent monomer 1-Zn. At high quinuclidine concentration, the 1:1 complex, 2&bull;1-Zn, derived from the coordinated dimer dissociation was also detected. The 1H NMR spectra of the titrations displayed separate signals for some hydrogen atoms of the Zn-phthalocyanine in each one of the four species. Remarkably, the chemical exchange processes involving free and bound quinuclidine in the monomeric and dimeric complexes showed different kinetics on the NMR timescale.</p>