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

The syntheses of tetradentate enediyne macrocycles with 24 (tact 1:1)-, 26 (tact 1:2)-, and 28 (tact2:2)-membered rings are described, along with thei r thermal reactivities and those of the corresponding Cu(II) (Cu(tact1:1), Cu(tact1:2)) and Zn(H) (Zn(tact1:1), Zn(tact1:2)) complexes. These enediyne macrocyclic ligands are not benzannulated and thus exhibit thermal Bergman cyclization temperatures near 200 degreesC by differential scanning calori metry (DSC). Moreover, the synthetic route allows incorporation of addition al carbon atoms into the macrocycles which increases their conformational f lexibilities and lowers their Bergman cyclization temperatures. Specificall y, as the size of the macrocycle increases, the temperatures at which these compounds undergo Bergman cyclization decrease by similar to5 degreesC per additional carbon atom, leading to an overall decrease across the series o f 19 degreesC. Incorporation of Cu(II) and Zn(II) into these macrocycles fu rther reduces their cyclization temperatures relative to those of the free ligands. More uniquely, for Cu(tact1:1) and Zn(tact1:1), the observed cycli zation temperatures vary by 27 degreesC with the Zn(II) complex lying to hi gher temperature (Cu(tact 1: 1) = 121 degreesC), (Zn(tact 1: 1) = 148 degre esC). As the macrocycle size is increased, the decrease in the Bergman cycl ization temperatures observed for the free ligands does not systematically hold for the Cu(H) and Zn(II) derivatives. Rather, the Cu(H) complex exhibi ts the expected 9 degreesC decrease in the cyclization temperature (Cu(tact 1:2) = 112 degreesC), whereas the temperature for the Zn(H) analogue increa ses by 15 degreesC (Zn(tact1:2) = 163 degreesC). From the X-ray crystal str ucture of the free ligand and the geometric structural preferences of the e lectronic configurations of Cu(H) and Zn(ll), the higher cyclization temper atures for the Zn(H) complex with the larger ring size can be explained by a distortion of the macrocycle toward a more tetrahedral metal center geome try.