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 °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
∼5 °C per additional carbon atom, leading to an overall decrease across the series of 19 °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
°C with the Zn(II) complex lying to higher temperature (Cu(tact1:1) = 121 °C, (Zn(tact1:1) = 148 °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 °C decrease in the cyclization temperature (Cu(tact1:2) = 112 °C), whereas the temperature for the Zn(II)
analogue increases by 15 °C (Zn(tact1:2) = 163 °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.