Enhancement of metal-metal coupling at a considerable distance by using 4-pyridinealdazine as a bridging ligand in polynuclear complexes of rhenium and ruthenium

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(Me(2)bpy)(CO)(3)(PCA)](+) (Me(2)bpy = 4,4'-dimethyl-2,2'-bipyridine), does not emit at room temperature in CH3CN, and the transient spectrum found by flash photolysis at lambda(exc) = 355 nm can be assigned to a MLCT (metal-to-ligand charge transfer) excited state [(Me(2)bpy)(CO)(3)Re-II(PCA(-))](+), with lambda(max) = 460 nm and tau < 10 ns. The spectral properties of the related complexes [{Re(Me(2)bpy)(CO)(3)}(2)(PCA)](2+), [Re(CO)(3)-(PCA)(2)Cl], and [Re(CO)(3)Cl](3)(PCA)(4) confirm the existence of this low-energy MLCT state. The dinuclear complex, of formula [(Me(2)bpy)(CO)(3)Re-I(PCA)Ru-II(NH3)(5)](3+), presents an intense absorption in the visible spectrum that can be assigned to a MLCT d(pi)(Ru) -> pi(*)(PCA); in CH3CN, the value of lambda(max) = 560 nm is intermediate between those determined for [Ru(NH3)(5)(PCA)](2+) (lambda(max) = 536 nm) and [(NH3)(5)Ru(PCA)Ru(NH3)(5)](4+) (lambda(max) = 574 nm), indicating a significant decrease in the energy of the pi(*)-orbital of PCA. The mixed-valent species, of formula [(Me(2)bpy)(CO)(3)Ru(PCA)Ru(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 [Re-I,Ru-II] precursor; the band at lambda(max) = 560 nm disappears completely, and a new band appears at lambda(max) = 483 nm, assignable to a MMCT band (metal-to-metal charge transfer) Re-I -> Ru-III. By using the Marcus-Hush formalism, both the electronic coupling (H-AB) and the reorganization energy (A) for the metal-to-metal intramolecular electron transfer have been calculated. Despite the considerable distance between both metal centers (similar to 15.0 angstrom), there is a moderate coupling that, together with the comproportionation constant of the mixed-valent species [(NH3)(5)Ru(PCA)Ru(NH3)(5)](5+) (K-c similar to 10(2), 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, A is lower than the driving force -Delta G degrees for the recombination charge reaction [Re-II,Ru-II] -> [Re-I,Ru-II] 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 [Re-I,Ru-III] complex a promising model for controlling the efficiency of charge-separation processes.