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  • The Contribution of Ligand Flexibility to Metal Center Geometry Modulated ThermalCyclization of Conjugated Pyridine and Quinoline Metalloenediynes of Copper(I) andCopper(II)
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  • We report the syntheses, reactivities, and structure evaluations of a series of Cu(I) and Cu(II) metalloenediynesof conjugated 1,6-bis(pyridine-3)hex-3-ene-1,5-diyne (PyED, 7) and 1,6-bis(quinoline-3)hex-3-ene-1,5-diyne(QnED, 8) enediyne ligands, as well as their benzoenediyne analogues. Differential scanning calorimetrydemonstrates that the [Cu(PyED)2](NO3)2 (11) exhibits a Bergman cyclization temperature (156 °C) which isdramatically reduced from that of the corresponding [Cu(PyED)2](PF6) (19) analogue (326 °C), indicating thatlarge differences in the reactivities of these metalloenediynes can be accessed by variations in metal oxidationstate. The distorted, 4-coordinate dichloride compound Cu(PyED)(Cl)2 (15) exhibits a cyclization temperature(265 °C) between those of 11 and 19, suggesting that variation in geometry of the copper center is responsiblefor the wide range of reactivities. Similar results are obtained for the benzoenediyne and quinoline analogues.The structures of the Cu(II) systems have also been evaluated by a combination of electronic absorption and EPRspectroscopies which reveal tetragonal, 6-coordinate structures for the bis(enediyne) complexes, and tetrahedrallydistorted 4-coordinate Cu(enediyne)Cl2 species. For the bis(quinoline) enediyne derivatives 12 and 14 the largerg-anisotropy (g∥ = 2.27−2.28; g⊥ = 2.06−2.07) indicates strong oxygen coordination from counterion. Molecularmechanics/dynamics calculations reveal that the geometries of these metal centers force the alkyne termini to awide range of distances (3.85−4.20 Å), thereby accounting for the variability in Bergman cyclization temperatures.Overall, the results show that ligand rigidity plays a prominent role in the conformational response of the enediyneto metal center geometry, which results in enhanced variations in the Bergman cyclization temperatures betweencomplexes of different geometries.
  • The syntheses, reactivities, and structure evaluations of a series of Cu(I) and Cu(II) metalloenediynes of conjugated 1,6-bis(pyridine-3)hex-3-ene-1,5-diyne (PyED, 7) and 1,6-bis(quinoline-3)hex-3-ene-1,5-diyne (QnED, 8) enediyne ligands, as well as their benzoenediyne analogues, are reported. Differential scanning calorimetry studies demonstrate that large differences in the reactivities of these metalloenediynes can be accessed by variations in ligand flexibility and metal center geometry. The results show that ligand rigidity plays a prominent role in the conformational response of the enediyne to metal center geometry, which results in enhanced variations in the Bergman cyclization temperatures between complexes of different geometries.
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