Anion Control of Lanthanoenediyne Cyclization.

A suite of lanthanoenediyne complexes of the form Ln(macrocycle)X 3 (Ln = La 3+ , Ce 3+ , Eu 3+ , Gd 3+ , Tb 3+ , Lu 3+ ; X = NO 3 - , 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 NO 3 - , T = 267-292 °C; monodentate Cl - , T = 238-262 °C; noncoordinating OTf - , T = 170-183 °C). 13 C 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, C A , exhibits a Δδ > 3 ppm downfield shift, while the more distal alkyne carbon, C B , 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 < NO 3 - < 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 LnX 3 show a systematic increase in the lanthanide- 2 coordination number (CN La-mc = 2 (NO 3 - ), 4 (Cl - ), 5 (H 2 O, model for OTf - )) and a decrease in the mean Ln-N bond length (La-N average = 2.91 Å (NO 3 - ), 2.78 Å (Cl - ), 2.68 Å (H 2 O)), 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.