Anion Control of Lanthanoenediyne Cyclization.

A suite of lanthanoenediyne complexes of the form Ln(macrocycle)X3 (Ln = La3+, Ce3+, Eu3+, Gd3+, Tb3+, Lu3+; X = NO3-, Cl-, OTf-) was prepared by utilizing an enediyne-containing [2 + 2] hexaaza-macrocycle (2). The solid-state Bergman cyclization temperatures, measured via DSC, decrease with the denticity of X (bidentate NO3-, T = 267-292 °C; monodentate Cl-, T = 238-262 °C; noncoordinating OTf-, T = 170-183 °C). 13C NMR characterization shows that the chemical shifts of the acetylenic carbon atoms also rely on the anion identity. The alkyne carbon closest to the metal binding site, CA, exhibits a Δδ > 3 ppm downfield shift, while the more distal alkyne carbon, CB, displays a concomitant Δδ ≤ 2.5 ppm upfield shift, reflecting a depolarization of the alkyne on metal inclusion. For all metals studied, the degree of perturbation follows the trend 2 < NO3- < Cl- < OTf-. This belies a greater degree of electronic rearrangement in the coordinated macrocycle as the denticity of X and its accompanying shielding of the metal's Lewis acidity decrease. Computationally modeled structures of LnX3 show a systematic increase in the lanthanide-2 coordination number (CNLa-mc = 2 (NO3-), 4 (Cl-), 5 (H2O, model for OTf-)) and a decrease in the mean Ln-N bond length (La-Naverage = 2.91 Å (NO3-), 2.78 Å (Cl-), 2.68 Å (H2O)), further suggesting that a decrease in the anion coordination number correlates with an increase in the metal-macrocycle interaction. Taken together, these data illustrate a Bergman cyclization landscape that is influenced by the bonding of metal to an enediyne ligand but whose reaction barrier is ultimately dominated by the coordinating ability of the accompanying anion.