π(radical)-π(radical) bonding interactions generated by halogen oxidation of zirconium(IV) redox-active ligand complexes

The new complex, [Zr(pda)(2)](n) (1, pda(2-) = N,N-'-bis(neo-pentyl)-ortho-phenylenediamide, n = 1 or 2), prepared by the reaction of 2 equiv of pdaLi(2) with ZrCl4, reacts rapidly with halogen oxidants to afford the new product ZrX2(disq)(2) (3, X = Cl, Br, I; disq(-) = NAr-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 similar to 3 angstrom. Density functional calculations show a bonding-type interaction between the SOMOs of the disq- ligands to form a unique HOMO while the antiboncling 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 similar to 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 disqligands.