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

Abstract

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 approximately 3 A. 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 approximately 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 pi-stacked bonding interaction between the two disq- ligands.

How a Redox‐Innocent Metal Promotes the Formal Reductive Elimination of Biphenyl Using Redox‐Active Ligands

“…Similarly, the AF-coupled singlet is the ground state for the Ti and Hf analogues (see Table 1); consistent with weak coupling, the triplet is almost isoenergetic with the AF singlet. This finding is different from a similar Zr IV complex, for which the CS singlet was lower in energy, [10] due to geometrical differences. In the previous work the redox-active ligands were able to overlap their singly occupied molecular orbitals (SOMOs) by forming a p-p interaction.…”

“…Here, we employ B3LYP, [11] but when a non-hybrid functional is used, [10] the spin densities on the isq ligands decrease (see Supporting Information) and the closed-shell configuration becomes more prevalent-this is a serious, and widely recognized problem in broken-symmetry (BS) DFT. [12] In lieu of experimental data, higher level multireference methods, such as CASSCF, should be employed.…”

A Redox‐Active Ligand as a Reservoir for Protons and Electrons: O2 Reduction at Zirconium(IV)

“…Thus, in the reaction the ligands serve as an electron reservoir to help effect multielectron reactivity at a non-redox-active metal. Similar reactions with redox-active ligands have been elucidated for oxidative addition, [12][13][14] reductive elimination, [15] and nitrene trans- fer [16,17] reactivity at d 0 complexes of the group 4 and 5 metals. In this report, we extend this approach beyond the utilization of the ligands as just electron reservoirs to develop a redox-active ligand that serves as both an electron and proton reservoir for O 2 reduction.…”

Four‐Electron Oxidative Formation of Aryl Diazenes Using a Tantalum Redox‐Active Ligand Complex

“…An alternative strategy is to incorporate redox-active ligands into the metal coordination sphere to supply reducing or oxidizing equivalents during a multielectron transformation. [5] Herein, we report the use of a tridentate redox-active ligand, N,N-bis(3,5-di-tert-butyl-2-phenoxide)amide ([ONO red ] 3À ), [6] coordinated to tantalum, to effect the fourelectron oxidative formation of aryl diazenes. In its reduced form, [ONO red ] 3À is a planar, tridentate ligand that coordinates to transition metals in a meridional geometry.…”