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

Abstract

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 PF(6-) 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 CH(3)CN, 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)(NH(3))(5)](3+), presents an intense absorption in the visible spectrum that can be assigned to a MLCT d(pi)(Ru) --> pi(PCA); in CH(3)CN, the value of lambda (max) = 560 nm is intermediate between those determined for [Ru(NH(3))(5)(PCA)](2+) (lambda(max) = 536 nm) and [(NH(3))(5)Ru(PCA)Ru(NH(3))(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)Re(I)(PCA)Ru(III)(NH(3))(5)](4+), was obtained in CH(3)CN 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 (lambda) for the metal-to-metal intramolecular electron transfer have been calculated. Despite the considerable distance between both metal centers (approximately 15.0 Angstroms), there is a moderate coupling that, together with the comproportionation constant of the mixed-valent species [(NH(3))(5)Ru(PCA)Ru(NH(3))(5)](5+) (K(c) approximately 10(2), in CH(3)CN), 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, lambda is lower than the driving force -DeltaG degrees for the recombination charge reaction [Re(II),Ru(II)] --> [Re(I),Ru(III)] 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.

pH‐Induced Luminescence Changes of Chromophore‐Quencher Tricarbonylpolypyridylrhenium(I) Complexes with 4‐Pyridinealdazine

“…Characteristic spin-allowed intraligand πǞπ* transitions of the polypyridyl ligands give rise to intense UV absorptions between 200 and 330 nm. [6,7,9] The mononuclear complexes 1 and 5 present minimum-energy absorptions at λ max = 346 and 352 nm, respectively, which can be assigned, as observed previously in the analogous complexes [Re(X 2 bpy)(CO) 3 …”

“…The solubility of rhenium(I) complexes in water has a direct impact on their applications in radiopharmaceuticals. [7] The values of the carbonyl stretching frequencies (ν CO ) in the IR spectra of complexes 1 and 2 (2032 and 1918 cm -1 , respectively) are almost the same as those reported for related complexes of the type fac-[Re(4,4Ј-X 2 -bpy)(CO) 3 (L)]PF 6 , [6,7,11,12] whereas the values of the carbonyl stretching frequencies in the IR spectra of complexes 5 and 6 (2037 and 1927 cm -1 , respectively) are slightly higher than those of 1 and 2, thereby indicating a stronger π-backbonding effect from Re to the bipyridine rings substituted with moderately strong electron-withdrawing groups, as reported previously for complexes such as fac-[Re(4,4Ј-X 2 -bpy)(CO) 3 (L)]PF 6 {X = C(O)NEt 2 , CO 2 Et 2 and L = py-PTZ [10-(4-picolyl)phenothiazine]}. [13] All these values of ν CO are consistent with a facial configuration of the carbonyl groups of local C 3v symmetry.…”

“…[6,7,11,12] All the complexes are soluble in acetonitrile, dichloromethane, and acetone and their purity was confirmed by chemical analysis, CV, and IR, NMR, and UV/Vis spectroscopy. The ester complexes are soluble in water and are stable for several days in neutral solutions at low concentrations (Յ10 -4 ), whereas the phenyl complexes are not soluble in water, even with Cl -as the counterion.…”

“…The corresponding unsymmetrical dinuclear complexes [(4,4Ј-X 2 -bpy)-(CO) 3 bpy)(CO) 3 (PCA)] + and [(4,4Ј-X 2 -bpy)(CO) 3 Re(µ-PCA)-Re(CO) 3 (4,4Ј-X 2 -bpy)] 2+ (bpy = 2,2Ј-bipyridine, X = CH 3 or H, and PCA = 4-pyridinealdazine). [6,7] Emission from the lowest-energy metal-to-ligand charge-transfer excited state 3 MLCT [d π (Re)Ǟπ*(X 2 bpy)] is completely quenched in these complexes by the presence of PCA in the mononuclear species at room temperature in CH 3 CN. Quenching of luminescence has been observed before in tricarbonylpolypyridylrhenium(I) complexes with electron-acceptor ligands (L) such as N-methyl-4,4Ј-bipyridinium (MQ + ), N-benzyl-4,4Ј-bipyridinium (BzQ + ), or N-(4-pyridyl)-β-(N-methylpyridinium-3-yl)acrylamide (pyAm-Mepy + ).…”

“…+ (λ max = 330-380 nm), [6,7] to overlapping metal-to-ligand charge transfers (MLCT) of the type d π (Re)Ǟπ*(X 2 bpy) and d π (Re)Ǟπ*(PCA). These same bands are redshifted to λ max = 350 and 355 nm, respectively, in the homodinuclear Re I -Re I complexes 2 and 6, as reported previously for symmetric dinuclear complexes of the type [(X 2 bpy)(CO) 3 Re(L)Re(CO) 3 (X 2 bpy)] 2+ .…”