A Strongly Coupled Mixed Valence State Between Ru3 Clusters. Intramolecular Electron Transfer on the Infrared Vibrational Time Scale in a Pyrazine (pz) Bridged Dimer of Triruthenium Clusters, [{Ru3(μ3-O)(μ-CH3CO2)6(CO)(abco)}2(μ-pz)] (abco = 1-azabicyclo[2,2,2]octane)

Abstract

A pyrazine bridged dimer of triruthenium clusters [{RuIII2RuII(μ3-O)(μ-CH3CO2)6(CO)(abco)}2(μ-pz)] (abco =1-azabicyclo[2,2,2]octane, pz = pyrazine) gives an inter-cluster mixed valence state upon one electron reduction. The splitting of the reduction waves in cyclic voltammetry, ΔE, was 470 mV. This corresponds to a comproportionation constant for formation of the mixed valence state of 9.0 × 107. The IR spectrum of the mixed valence species in the ν(CO) region, measured by reflectance IR spectroelectrochemistry, showed a completely coalesced ν(CO) band. This represents delocalized redox behavior on the infrared vibrational timescale. The rate constant for intramolecular electron transfer was estimated to be 1 × 1012 s-1 at -18 °C by simulating the ν(CO) absorption bandshape. The mixed valence complex showed an intervalence charge transfer (ICT) band at 12500 cm-1. Hush analysis provides a spectroscopic estimate of HAB to be 2490 cm-1. Crystal structures of [{RuIII2RuII(μ3-O)(μ-CH3CO2)6(CO)(abco)}2(μ-pz)]·2CH2Cl2 and its precursor, [RuIII2RuII(μ3-O)(CH3CO2)6(CO)(abco)2]·CH2Cl2, were determined by X-ray crystallography. Trends in Ru-O(oxo) and Ru-O(acetate) distances in these compounds show that the divalent site is localized on the Ru center attached to the carbonyl ligand.

Electronic Structural Studies of the Ru3(III,II,II) Mixed-Valent State of Oxo-Centered Triruthenium Clusters

“…The introduction of the carbonyl ligand provides a convenient spectroscopic handle, while the structural integrity of the metal-carboxylate core provides well-behaved, reversible electrochemistry when measured by cyclic voltammetry. − Upon a one-electron reduction (Ru 3 (III,III,II) → Ru 3 (III,II,II)), the anionic triruthenium clusters, [1 CpyCpy ] − – [6 PyDmap ] − (Cpy, 4-cyanopyridine; Py, pyridine; Dmap, N , N -dimethyl-4-aminopyridine), display an intense absorbance (ε > 4000 M –1 cm –1 ) in the near-infrared (NIR) region, which appeared to be multiple overlapping features. An intense NIR absorbance is characteristic behavior indicative of a strongly coupled mixed-valent system. − The mixed-valent character in this state originates from an electronic coupling between Ru­(II) and Ru­(III) states of the two pyridine-ligated ruthenium atoms. While it has not yet been possible to characterize the Ru 3 (III,II,II) state by X-ray crystallography, from the proximity of the ruthenium atoms, we expect a significantly delocalized electron density between the two ruthenium centers.…”

Time-Resolved Electron Transfer in Porphyrin-Coordinated Ruthenium Dimers: From Mixed-Valence Dynamics to Hot Electron Transfer

“… a Potentials are versus FcCp* 2 , obtained from Henderson et al ref . b Ion pair energy obtained as Δ G IP 0 = e ( E D ox / D 0 – E A/A red 0 ) – ( e 2 /(4 πε 0 ε S R )), with ε butyronitrile = 20.7 and R = 6.4 Å. ,, c Δ G FET 0 = Δ G IP 0 – E 00 , with E 00 ( S 1 ) = 2.04 eV …”

“… b Ion pair energy obtained as Δ G IP 0 = e ( E D ox / D 0 – E A/A red 0 ) – ( e 2 /(4 πε 0 ε S R )), with ε butyronitrile = 20.7 and R = 6.4 Å. ,, …”

Electron Dynamics and IR Peak Coalescence in Bridged Mixed Valence Dimers Studied by Ultrafast 2D-IR Spectroscopy

“…A group of dimers of trinuclear ruthenium clusters (Figure ) has been studied extensively for their mixed valence properties. − In particular, infrared spectroscopic investigations have focused on peak coalescence of the carbonyl vibrations of the mixed valence species. The carbonyl ligand at each metal cluster site serves as a spectator ligand to electron dynamics.…”

“…In the mixed valence state of these systems, the degree of peak overlap has been tuned over a broad range of coalescence. − Spectra of some mixed valence dimers show virtually no coalescence, whereas others have only one carbonyl peak. No single sample can be tuned through the full range of coalescence; however, changes in temperature and solvent can lead to large changes in the spectra for a given dimer.…”