Thioether coordinated metalloenediynes: Syntheses, structures and thermal reactivity comparison

Two novel, thioether containing benzannulated enediyne ligands, 1,2-bis(3-(quinolin-8-ylsulfanyl)prop-1-ynyl)benzene (1), and 1,2-bis(3-(pyridin-2-ylmethylsulfanyl)prop-1-ynyl)benzene (2) have been synthesized and characterized along with their corresponding Cu(I), Ag(I), Cu(II), and Pd(II) metalloenediyne complexes (3–10). Single crystal X-ray structures of the d10 and d8 N2S2 complexes show thioether coordination leading to distorted tetrahedral and trans square planar geometries about the metal centers, respectively. Based on their X-ray structures, all complexes are expected to exhibit high Bergman cyclization temperatures (>200 °C), and indeed this is the case for the Cu(I) and Ag(I) complexes (211–218 °C). In contrast, the corresponding divalent, tetragonal and trans square planar complexes of Cu(II) and Pd(II) exhibit more facile reactivity and lower solid state Bergman cyclization temperatures (186–190 °C) which more closely resemble values expected for cis-complexes. The unusual order in the thermal reactivities of these constructs can be attributed to a previously evaluated mechanism by which weak thioether coordination permits an elevated temperature trans-to-cis geometric rearrangement prior to Bergman cyclization.

The d10 species of these novel thioether coordinated metalloenediynes have distorted but near tetrahedral geometries with weak thioether coordination, while the d9 and d8 species are likely trans tetragonal and square planar, respectively. Remarkably, the latter structures exhibit Bergman cyclization temperatures ∼25 °C lower than their tetrahedral counterparts, and on the order of 50–100 °C lower than that expected for complexes with trans geometries. This chemical behavior derives from the fluxionality of the thioether linkage and an unusual trans to pseudo cis conformational change during upon heating.Figure optionsDownload as PowerPoint slide