Density Functional Analysis of Ancillary Ligand Electronic Contributions to Metal-Mediated Enediyne Cyclization

Density functional theory (DFT) has been used to study electronic perturbations induced by ancillary halogen ligation within metalloenediyne constructs, and the subsequent affect upon thermal Bergman cyclization temperatures. To isolate electronic from geometric components of Bergman cyclization thermodynamics, model diamine- and diphosphine-enediynes (L = 1,6-diamino- or 1,6-diphosphino-cis-1,5-hexadiyne-3-ene) of Mn(II), Cu(II), Zn(II), and Pd(II) with ancillary chloride ligands have been examined computationally and compared to more complex ethylenediamine-based metalloenediyne frameworks of the form MLX2 (X = Cl, Br, 1; L = 1,4-dibenzyl-1,4-diaza-cyclododec-8-ene-6,10-diyne) with distorted square-planar (Cu(II)), T-d (Zn(II)), and D-4h (Pd(II)) geometries. In the latter systems, the ethylenediamine linkage restricts the conformation of the enediyne backbone, causing the alkyne termini separation to be nearly independent of metal geometry (3.75-3.82 angstrom). Within the Zn(II) family, steric effects are shown to induce conformational changes on the cyclization potential energy surface (PES) prior to the Bergman transition state, introducing distinct electron-electron repulsive interactions. Multiple metal and ligand conformations are also observed on the Cu(II) metalloenediyne cyclization PES. In contrast, square-planar Pd(II) compounds exhibit overlap between the out-of-plane halogen lone pairs and metal d orbitals, as well as the enediyne pi system, reminiscent of an organometallic push-pull reaction mechanism. These systems have significantly higher predicted activation barriers toward cycloaromatization due to enhanced electron repulsion.