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.