Comparison of Hole-Transfer Superexchange in Dinuclear Mixed-Valence Ruthenium Complexes

Abstract

Three dinuclear complexes [{mer-Ru(NH3)3(bpy)}2(μ-L)][ClO4]4, where L = 2,5-dimethyl- (Me2dicyd2-), 2,5-dichloro- (Cl2dicyd2-), and unsubstituted 1,4-dicyanamidobenzene dianion (dicyd2-), have been synthesized and characterized by magnetic resonance, electrochemical, and electronic absorption spectroscopic techniques. A crystal structure of [{mer-Ru(NH3)3(bpy)}2(μ-dicyd)][ClO4]4·3H2O showed dicyd2- to be approximately planar with the cyanamido groups in an anti configuration. Crystal structure data are space group = P1̄, with a, b, and c equal to 12.5613(1), 12.8738(1), and 16.3267(2) Å, respectively, α, β, and γ equal to 76.756(1), 83.893(1), and 69.053(2)°, respectively, V = 2399.28(4) Å3, and Z = 2. The structure was refined using 6112 independent reflections with I > 2.5σ(I) to a final R factor of 0.0568. The strongly coupled mixed-valence complexes [{mer-Ru(NH3)3(bpy)}2(μ-L)]3+ where L = Me2dicyd2-, dicyd2-, and Cl2dicyd2- had decreasing comproportionation constants of 1.3 × 107, 9.3 × 106, and 3.5 × 105, respectively, which are consistent with a hole transfer superexchange mechanism for metal−metal coupling. The mixed-valence properties of these complexes together with analogous systems were compared in the context of a transformation from a localized to a delocalized mixed-valence state.

Long‐Range Electronic Coupling in Various Oxidation States of a C4‐Linked Tris(β‐diketonato)ruthenium Dimer

“…With resonance signals at 2.34, 2.20, and 1.91 the g anisotropy is distinctly smaller than for 1 À which would be expected for diminished metal and enhanced ligand participation at the singly occupied MO in 1 + in accord with the two intermediate resonance formulations in Scheme 1. [13] In agreement with established concepts [1,11,13] this result points to a hole-transfer mechanism for the electronic coupling as would be expected for a dianionic ligand [13] and oxidized metal centers. Alternatively, a combination of a neutral ligand with Ru III;II 2 mixed-valent centers may be considered which would be expected to exhibit a similar g anisotropy.…”

“…The latter indicates dissymmetry and thus localized valences on the vibrational time scale of about 10 À12 s, that is, Class II behavior according to the Robin and Day classification. [9] The EPR spectra taken at 4 K in CH 2 [11] For the classical 1 b* with a distorted tetrahedral coordinate ring carbon atom computations at this level give two imaginary frequencies and an energy 8.9 kcal mol À1 higher than for 1 b. The chemical shift of the ring carbon atom computed at the level used for 1 b was d = 103.6 ppm.…”

“…A very different result is obtained on stepwise one-electron oxidation of the metal centers: At 10 3.8 the K c value of the Ru IV;III 2 intermediate 1 + is larger than for 1 À but still far from the typical K c value of about 10 6 for the Creutz±Taube ion [3a] and 1,4-dicyanamidobenzene-bridged species, [11] the K c value of approximately 10 10 for directly C 4 -bridged compounds, [6] or even the K c > 10 13 for tetrazine-bridged systems. [1,12] However, results from IR-and UV/Vis-NIR spectroelectrochemistry (Figures 2 and 3) clearly indicate strong electronic coupling across a distance of 1.3 nm.…”

X-ray Structure of a Ruthenium Dinuclear Complex Incorporating 4,4′-Bis(cyanamidophenylethynyl)benzene as a Ligand, a Conjugated Bridge Two Nanometers in Length

“…Interestingly, there are few reports of crystal structures containing such molecular shapes. Thus Crutchley has described the crystal structures of dinuclear complexes with dicyanamidobenzene as a bridging ligand and either [Ru(NH 3 ) 5 ] 3ϩ , [5] trans-[Ru(NH 3 ) 4 (py)] 3ϩ [12] or mer-[Ru(NH 3 ) 3 (bpy)] 3ϩ [13] as terminal units (metalϪmetal distances 13.1, 10.9, and 13.1 Å , respectively). More recently, he has also reported the structure of [Ru(bpy)(tpy)-µ(adcp)- …”

Weitreichende elektronische Kopplung in unterschiedlichen Oxidationszuständen eines C4‐verknüpften Tris(β‐diketonato)ruthenium‐Dimers

“…Die Einelektronenoxidation von 1 führt zu einem deutlich anderem Ergebnis: Mit K c =10 3.8 ist die Komproportionierungskonstante der Ru${{{\rm IV},{\rm III}\hfill \atop 2\hfill}}$ ‐Zwischenstufe 1 + größer als diejenige von 1 − ; allerdings ist sie immer noch deutlich kleiner als die mit etwa 10 6 für das Creutz‐Taube‐Ion3a oder für 1,4‐Dicyanamidobenzol‐verbrückte Komplexe,11 ganz zu schweigen von K c ≈10 10 für direkt C 4 ‐verbrückte Verbindungen6 oder K c >10 13 für Tetrazin‐verbrückte Systeme 1. 12 Die Ergebnisse aus IR‐ und UV/Vis‐NIR‐Spektroelektrochemie (Abbildungen 2 und 3) belegen jedoch eine starke elektronische Kopplung über die Distanz von 1.3 nm hinweg: Bei 1730 nm wird für die formal d 4 /d 5 ‐konfigurierte Zwischenstufe eine sehr intensive ( ε =25 000 M −1 cm −1 ) und breite (Δ$\tilde {\nu}$ 1/2 =3400 cm −1 ) IVCT‐Bande beobachtet (Abbildung 3 a).…”