Influence of the Electronic Configuration in the Properties of d⁶–d⁵ Mixed-Valence Complexes

We report here the spectroscopic properties of four very closely related mixed-valence cyanide-bridged bimetallic complexes, <i>trans</i>-[Ru­(T)­(bpy)­(μ-NC)­Ru­(L)₄(CN)]³⁺ (T = tris­(1-pyrazolyl)­methane (tpm, <b>a</b>) or 2,2′;6′,2″-terpyridine, (tpy, <b>b</b>), and L = pyridine (py, <b>1</b>) or 4-methoxypyridine (MeOpy, <b>2</b>)). In acetonitrile all the complexes present intervalence charge transfer (IVCT) transitions in the NIR region, but their intensities are widely different, with the intensity of the transition observed for <b>1a</b>–<b>b</b>³⁺ around four times larger than that observed for <b>2a</b>–<b>b</b>³⁺. This contrasting behavior can be traced to the different nature of the dπ acceptor orbitals involved in these transitions, as confirmed by (TD)­DFT calculations. The spectroscopy of <b>1a</b>–<b>b</b>³⁺ provides evidence that the spin density is delocalized over the two ruthenium ions, such as a narrowing of the IVCT bands that results in the resolution of the expected three bands, and a weak solvent dependence of the energy of these transitions. The spectroscopy of <b>2a</b>–<b>b</b>³⁺ instead indicates that the spin density is localized on one ruthenium ion. The IVCT in these systems is particularly weak due to the configuration of the Ru­(III), where the vacant orbital is perpendicular to the cyanide bridge

Electronic Energy Transduction from {Ru(py)₄} Chromophores to Cr(III) Luminophores

Despite the large body of work on {Ru­(bpy)₂} sensitizer fragments, the same attention has not been devoted to their {Ru­(py)₄} analogues. In this context, we explored the donor–acceptor <i>trans</i>-[Ru­(L)₄{(μ-NC)­Cr­(CN)₅}₂]^4–, where L = pyridine, 4-methoxypyridine, 4-dimethylaminopyridine. We report on the synthesis and the crystal structure as well as the electrochemical, spectroscopical, and photophysical properties of these trimetallic complexes, including transient absorption measurements. We observed emission from chromium-centered d–d states upon illuminating into either MLCT or MM′CT absorptions of {Ru­(L)₄} or {Ru-Cr}, respectively. The underlying energy transfer is as fast as 600 fs with quantum efficiencies ranging from 10% to 100%. These results document that {Ru­(py)₄} sensitizer fragments are as efficient as {Ru­(bpy)₂} in short-range energy transfer scenarios

Communication between Remote Moieties in Linear Ru–Ru–Ru Trimetallic Cyanide-Bridged Complexes

In this article, we report the structural, spectroscopic, and electrochemical properties of the cyanide-bridged complex salts <i>trans</i>-[(NC)­Ru^II(L)₄(μ-CN)­Ru^II(py)₄Cl]­PF₆ and <i>trans</i>-[Ru^II(L)₄{(μ-CN)­Ru^II(py)₄Cl}₂]­(PF₆)₂ (L = pyridine or 4-methoxypyridine). The mixed-valence forms of these compounds show a variety of metal-to-metal charge-transfer bands, including one arising from charge transfer between the remote ruthenium units. The latter is more intense when L = 4-methoxypyridine and points to the role of the bridging ruthenium unit in promoting mixing between the dπ orbitals of the terminal fragments

Electronic Structure of the Water Oxidation Catalyst <i>cis</i>,<i>cis</i>-[(bpy)₂(H₂O)Ru^IIIORu^III(OH₂)(bpy)₂]⁴⁺, The Blue Dimer

The first designed molecular catalyst for water oxidation is the “blue dimer”, <i>cis</i>,<i>cis</i>-[(bpy)₂(H₂O)­Ru^IIIORu^III(OH₂)­(bpy)₂]⁴⁺. Although there is experimental evidence for extensive electronic coupling across the μ-oxo bridge, results of earlier DFT and CASSCF calculations provide a model with magnetic interactions of weak to moderately coupled Ru^III ions across the μ-oxo bridge. We present the results of a comprehensive experimental investigation, combined with DFT calculations. The experiments demonstrate both that there is strong electronic coupling in the blue dimer and that its effects are profound. Experimental evidence has been obtained from molecular structures and key bond distances by XRD, electrochemically measured comproportionation constants for mixed-valence equilibria, temperature-dependent magnetism, chemical properties (solvent exchange, redox potentials, and p<i>K</i>_a values), XPS binding energies, analysis of excitation-dependent resonance Raman profiles, and DFT analysis of electronic absorption spectra. The spectrum can be assigned based on a singlet ground state with specific hydrogen-bonding interactions with solvent molecules included. The results are in good agreement with available experimental data. The DFT analysis provides assignments for characteristic absorption bands in the near-IR and visible regions. Bridge-based dπ → dπ* and interconfiguration transitions at Ru^III appear in the near-IR and MLCT and LMCT transitions in the visible. Reasonable values are also provided by DFT analysis for experimentally observed bond distances and redox potentials. The observed temperature-dependent magnetism of the blue dimer is consistent with a delocalized, diamagnetic singlet state (dπ₁*)² with a low-lying, paramagnetic triplet state (dπ₁*)¹(dπ₂*)¹. Systematic structural–magnetic–IR correlations are observed between ν_sym(RuORu) and ν_asym(RuORu) vibrational energies and magnetic properties in a series of ruthenium-based, μ-oxo-bridged complexes. Consistent with the DFT electronic structure model, bending along the Ru–O–Ru axis arises from a Jahn–Teller distortion with ∠Ru–O–Ru dictated by the distortion and electron–electron repulsion