Structure Modulated Electronic Contributions to Metalloenediyne Reactivity: Synthesis and Thermal Bergman Cyclization of MLX₂ Compounds

The synthesis of novel metalloendiyne complexes MLRX2 (where L = 1,4-dibenzyl/diethyl-1,4-diaza-cyclododec-8-ene-6,10-diyne, X = halogen) are reported with their X-ray crystal structures and thermal Bergman cyclization temperatures. Two distinct types of constructs are obtained; the Zn(II) compounds are tetrahedral, while the Cu(II) and the Pd(II) compounds are all distorted- or square-planar. Each possesses structurally similar enediyne conformations and critical distances (3.75−3.88 Å). The tetragonal Cu(II) species all exhibit Bergman cyclization temperatures between 140 and 150 °C in the solid state, while the square-planar Pd(II) analogues possess similar critical distances but cyclize at significantly higher temperatures (205−220 °C). In contrast, the Zn(II) derivatives show a marked halogen dependence, with X = Cl having the highest Bergman cyclization temperature, which is comparable to the Pd(II) square-planar set, while the ZnLX2 compound with X = I shows the lowest Bergman cyclization temperature (144 °C), similar to the Cu(II) derivatives. Moreover, for the planar constructs, the R group has little influence on the cyclization temperatures; however, for the tetrahedral ZnLX2 compounds, the steric influence of the R group plays a more significant role in the cyclization reaction coordinate by influencing the stability of the precyclized intermediate. This complex set of results is best interpreted by a combination of steric contributions and electronic interactions between the halogen through space (in the case of Zn(II)) and through bonds (in the case of Pd(II)) and the π orbitals of the endiyne fragment. In contrast, for Cu(II) systems, the distorted square-planar geometry permits neither direct through space nor symmetry-allowed through bond communication between the orbital partners, and thus little variation in Bergman cyclization reactivity is observed.