AII-frans-[CIRuII(py)4(NC)RuII(py) 4(CN)RuII(py)4(N0)](PF6) 4: a redox-active 2-donor/1-acceptor system based on the electrophilic {RuNO}6 motif

Abstract

The new linear homotrinuclear compound frans-[CIRuII(py) 4(NC)RuII(py)4(CN)RuII(py) 4(NO)](PF6)4 was prepared by reaction between the nitro complex trans-[(NC)RuII(py)4(CN)Ru II(py)4(NO2)]+ and the solvento complex obtained by reaction between [CIRuII(py)4(NO)] 3+ and N3- in acetone. The trans-[CIRu II(py)4 (NC)RuII(py)4(CN)Ru II(py)4(NO)]4+ ion (I) has been characterized by 1H NMR and IR spectroscopy (vN0 = 1919 cm -1). This species displays intense electronic absorptions in the visible region which can be assigned to donor-acceptor charge-transfer transitions (DACT) involving {RuNO}6-centered acceptor orbitals and donor orbitals located on the two different neighboring metal centers at ca. 6.7 and 12.6 Å distance from the metal in the {RuNO}6 fragment. Addition of OH- to I generated the nitro complex with a second-order rate constant of (12.5 ± 0.2) × 103 M-1 s-1 (25 ̊C). Cyclic oltammetry experiments complemented by spectroelectrochemistry in the UV-vis-NIR region reveal that I can be reversibly reduced at 0.49 or 0.20 V vs AgCI/Ag for acetonitrile and water, respectively, and oxidized at 0.71 or 0.57 V vs AgCI/Ag. The spectroscopic and spectroelectrochemical information (UV-vis-NIR, X-band EPR) supplemented with electronic structure computation (DFT) reveals that the one-electron reduction is centered on the nitrosyl moiety to yield a {RuNO}7 species, while oxidation occurs on the chlororuthenium side of the molecule. Both processes yield significant changes of the electronic spectra which are discussed in parallel with the electronic structure picture as obtained by DFT.