Electronic Perturbation in a Molecular Wire of [IrCl5(NO)]– Units

FLORENCIA DI SALVO; NATALIA ESCOLA; DAMIÁN A. SCHERLIS; DARÍO A. ESTRIN; CARLOS BONDÍA; DANIEL MURGIDA; JOSÉ M. RAMALLO-LÓPEZ; FÉLIX G. REQUEJO; LINDA SHIMON; FABIO DOCTOROVICH

CHEMISTRY-A EUROPEAN JOURNAL

Año: 2007 vol. 13 p. 8428 - 8436

0947-6539

The nitrosyl in [IrCl5(NO)]- is probably the most electrophilic known to date. This fact is reflected by its extremely high IR frequency in the solid state, electrochemical behavior, and remarkable reactivity in solution. PPh4[IrCl5(NO)] forms a crystal in which the [IrCl5(NO)]- anions are in a curious wire-like linear arrangement, in which the distance between the N-O moiety of one anion and the trans chloride of the upper one nearby is only 2.8 A. For the same complex [IrCl5(NO)] but with a different counterion, Na[IrCl5(NO)], the anions are stacked one over the other in a sideby-side arrangement. In this case the electronic distribution can be depicted as the closed-shell electronic structure IrIII
-NO+, as expected for any d6 third row transition metal complex. However in PPh4[IrCl5(NO)] an unprecedented electronic perturbation takes place, probably due to NO.–Cl
- acceptor– donor interactions among a large number of [IrCl5(NO)]
- units, favoring a different electronic distribution, namely the open-shell electronic structure IrIV-
NO. This conclusion is based on XANES experimental evidence, which demonstrates that the formal oxidation state for iridium in PPh4-[IrCl5(NO)] is +4, as compared with +3 in K[IrCl5(NO)]. In agreement, solid-state DFT calculations show that the ground state for [IrCl5(NO)]
- in the PPh4 salt comprises an open-shell singlet with an electronic structure which encompasses half of the spin density mainly localized on a metalcentered orbital, and the other half on an NO-based orbital. The electronic perturbation could be seen as an electron promotion from a metal–chloride to a metal–NO orbital, due to the small HOMO–LUMO gap in PPh4-[IrCl5(NO)]. This is probably induced by electrostatic interactions acting as a result of the closeness and wire-like spatial arrangement of the Ir metal centers, imposed by lattice forces due to pi–pi stacking interactions among the phenyl rings in PPh4+. Experimental and theoretical data indicate that in PPh4[IrCl5(NO)] the Ir
-N-
O moiety is partially bent and tilted.