π^•−π^• Bonding Interactions Generated by Halogen Oxidation of Zirconium(IV) Redox-Active Ligand Complexes

The new complex, [Zr(pda)2]n (1, pda2- = N,N‘-bis(neo-pentyl)-ortho-phenylenediamide, n = 1 or 2), prepared by the reaction of 2 equiv of pdaLi2 with ZrCl4, reacts rapidly with halogen oxidants to afford the new product ZrX2(disq)2 (3, X = Cl, Br, I; disq- = N,N‘-bis(neo-pentyl)-ortho-diiminosemiquinonate) in which each redox-active ligand has been oxidized by one electron. The oxidation products 3a−c have been structurally characterized and display an unusual parallel stacked arrangement of the disq- ligands in the solid state, with a separation of ∼3 Å. Density functional calculations show a bonding-type interaction between the SOMOs of the disq- ligands to form a unique HOMO while the antibonding linear combination forms a unique LUMO. This orbital configuration leads to a closed-shell-singlet ground-state electron configuration (S = 0). Temperature-dependent magnetism measurements indicate a low-lying triplet excited state at ∼750 cm-1. In solution, 3a−c show strong disq--based absorption bands that are invariant across the halide series. Taken together these spectroscopic measurements provide experimental values for the one- and two-electron energies that characterize the π-stacked bonding interaction between the two disq- ligands.新型配合物[Zr(pda)₂]ₙ（化合物1，其中pda²⁻为N,N'-双(新戊基)-邻苯二胺根（N,N'-bis(neo-pentyl)-ortho-phenylenediamide），n=1或2），由2当量的pdaLi₂与四氯化锆（ZrCl₄）反应制备得到。该配合物可与卤氧化剂快速反应，得到产物ZrX₂(disq)₂（化合物3，X=Cl、Br、I；disq⁻为N,N'-双(新戊基)-邻亚胺半醌根（N,N'-bis(neo-pentyl)-ortho-diiminosemiquinonate）），其中每个氧化还原活性配体均被单电子氧化。氧化产物3a~3c已完成结构表征，在固态下呈现出罕见的disq⁻配体平行堆叠排布，配体间距约为3 Å。密度泛函计算（Density functional calculations）表明，disq⁻配体的单电子占据分子轨道（Singly Occupied Molecular Orbital, SOMO）之间存在成键型相互作用，进而形成独特的最高占据分子轨道（Highest Occupied Molecular Orbital, HOMO），而其反键线性组合则形成独特的最低未占据分子轨道（Lowest Unoccupied Molecular Orbital, LUMO）。该轨道构型对应闭壳层单重态基态电子组态（S=0）。变温磁学测量结果显示，体系存在约750 cm⁻¹的低能级三重激发态。在溶液中，3a~3c展现出强烈的基于disq⁻的吸收带，且该吸收带在整个卤族系列中保持恒定。综合上述光谱学测试结果，可获得表征两个disq⁻配体间π堆叠相互作用的单电子和双电子能量实验值。