Enhancement of Metal−Metal Coupling at a Considerable Distance by Using 4-Pyridinealdazine as a Bridging Ligand in Polynuclear Complexes of Rhenium and Ruthenium

• Published in: Inorganic chemistry Vol. 45; no. 1; pp. 127 - 136
• Main Authors: Cattaneo, Mauricio, Fagalde, Florencia, Katz, Néstor E, Leiva, Ana María, Schmehl, Russell
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 09.01.2006
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic051312b

Novel polynuclear complexes of rhenium and ruthenium containing PCA (PCA = 4-pyridinecarboxaldehyde azine or 4-pyridinealdazine or 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene) as a bridging ligand have been synthesized as PF6 - salts and characterized by spectroscopic, electrochemical, and photophysical techniques. The precursor mononuclear complex, of formula [Re(Me2bpy)(CO)3(PCA)]+ (Me2bpy = 4,4‘-dimethyl-2,2‘-bipyridine), does not emit at room temperature in CH3CN, and the transient spectrum found by flash photolysis at λexc = 355 nm can be assigned to a MLCT (metal-to-ligand charge transfer) excited state [(Me2bpy)(CO)3ReII(PCA - )]+, with λmax = 460 nm and τ < 10 ns. The spectral properties of the related complexes [{Re(Me2bpy)(CO)3}2(PCA)]2+, [Re(CO)3(PCA)2Cl], and [Re(CO)3Cl]3(PCA)4 confirm the existence of this low-energy MLCT state. The dinuclear complex, of formula [(Me2bpy)(CO)3ReI(PCA)RuII(NH3)5]3+, presents an intense absorption in the visible spectrum that can be assigned to a MLCT dπ(Ru) → π*(PCA); in CH3CN, the value of λ max = 560 nm is intermediate between those determined for [Ru(NH3)5(PCA)]2+ (λmax = 536 nm) and [(NH3)5Ru(PCA)Ru(NH3)5]4+ (λmax = 574 nm), indicating a significant decrease in the energy of the π*-orbital of PCA. The mixed-valent species, of formula [(Me2bpy)(CO)3ReI(PCA)RuIII(NH3)5]4+, was obtained in CH3CN solution, by bromine oxidation or by controlled-potential electrolysis at 0.8 V in a OTTLE cell of the [ReI,RuII] precursor; the band at λmax = 560 nm disappears completely, and a new band appears at λmax = 483 nm, assignable to a MMCT band (metal-to-metal charge transfer) ReI → RuIII. By using the Marcus−Hush formalism, both the electronic coupling (H AB) and the reorganization energy (λ) for the metal-to-metal intramolecular electron transfer have been calculated. Despite the considerable distance between both metal centers (∼15.0 Å), there is a moderate coupling that, together with the comproportionation constant of the mixed-valent species [(NH3)5Ru(PCA)Ru(NH3)5]5+ (K c ∼ 102, in CH3CN), puts into evidence an unusual enhancement of the metal−metal coupling in the bridged PCA complexes. This effect can be accounted for by the large extent of “metal−ligand interface”, as shown by DFT calculations on free PCA. Moreover, λ is lower than the driving force −ΔG° for the recombination charge reaction [ReII,RuII] → [ReI,RuIII] that follows light excitation of the mixed-valent species. It is then predicted that this reverse reaction falls in the Marcus inverted region, making the heterodinuclear [ReI,RuIII] complex a promising model for controlling the efficiency of charge-separation processes.