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

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posted on 02.04.2008, 00:00 by Nicole A. Ketterer, Hongjun Fan, Karen J. Blackmore, Xiaofan Yang, Joseph W. Ziller, Mu-Hyun Baik, Alan F. Heyduk
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 3ac 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, 3ac 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.