Anion Control of Lanthanoenediyne Cyclization

Abstract

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$$^–$, Cl$^–$, OTf$^–$) was prepared by utilizing an enediyne-containing [2 + 2] hexaaza-macrocycle (2). The solid-state Bergman cyclization temperatures, measured via DSC, decrease with the denticity of X (bidentate NO$_3$$^–$, T = 267–292 °C; monodentate Cl$^–$, T = 238–262 °C; noncoordinating OTf$^–$, T = 170–183 °C). 13C NMR characterization shows that the chemical shifts of the acetylenic carbon atoms also rely on the anion identity. The alkyne carbon closest to the metal binding site, C$_{\textrm{A}}$, exhibits a $Δδ$ > 3 ppm downfield shift, while the more distal alkyne carbon, C$_{\textrm{B}}$, 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$$^–$ < Cl$^–$ < OTf$^–$. This belies a greater degree of electronic rearrangement in the coordinated macrocycle as the denticity of X and its accompanying shielding of the metal’s Lewis acidity decrease. Computationally modeled structures of LnX$_3$ show a systematic increase in the lanthanide–2 coordination number (CN$_{\textrm{La-mc}}$ = 2 (NO$_3$$^–$), 4 (Cl$^–$), 5 (H$_2$O, model for OTf$^–$)) and a decrease in the mean Ln–N bond length (La–N$_{\textrm{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.