Macrocyclic metalloenediynes of Cu(II) and Zn(II): a thermal reactivity comparison

The syntheses of tetradentate enediyne macrocycles with 24 (tact1:1)-, 26 (tact1:2)-, and 28 (tact2:2)-membered rings are described, along with their thermal reactivities and those of the corresponding Cu(II) (Cu(tact1:1), Cu(tact1:2)) and Zn(II) (Zn(tact1:1), Zn(tact1:2)) complexes. These enediyne macrocyclic ligands are not benzannulated and thus exhibit thermal Bergman cyclization temperatures near 200 degrees C by differential scanning calorimetry (DSC). Moreover, the synthetic route allows incorporation of additional carbon atoms into the macrocycles which increases their conformational flexibilities and lowers their Bergman cyclization temperatures. Specifically, as the size of the macrocycle increases, the temperatures at which these compounds undergo Bergman cyclization decrease by approximately 5 degrees C per additional carbon atom, leading to an overall decrease across the series of 19 degrees C. Incorporation of Cu(II) and Zn(II) into these macrocycles further reduces their cyclization temperatures relative to those of the free ligands. More uniquely, for Cu(tact1:1) and Zn(tact1:1), the observed cyclization temperatures vary by 27 degrees C with the Zn(II) complex lying to higher temperature (Cu(tact1:1) = 121 degrees C, (Zn(tact1:1) = 148 degrees C). As the macrocycle size is increased, the decrease in the Bergman cyclization temperatures observed for the free ligands does not systematically hold for the Cu(II) and Zn(II) derivatives. Rather, the Cu(II) complex exhibits the expected 9 degrees C decrease in the cyclization temperature (Cu(tact1:2) = 112 degrees C), whereas the temperature for the Zn(II) analogue increases by 15 degrees C (Zn(tact1:2) = 163 degrees C). From the X-ray crystal structure of the free ligand and the geometric structural preferences of the electronic configurations of Cu(II) and Zn(II), the higher cyclization temperatures for the Zn(II) complex with the larger ring size can be explained by a distortion of the macrocycle toward a more tetrahedral metal center geometry.