Fine tuning of the catalytic effect of a metal-free porphyrin on the homogeneous oxygen reduction

The catalytic effect of tetraphenylporphyrin on the oxygen reduction with ferrocene in 1,2-dichloroethane can be finely tuned by varying the molar ratio of the acid to the catalyst present in the solution. The mechanism involves binding of molecular oxygen to the protonated free porphyrin base, in competition with ion pairing between the protonated base and the acid anion present

Optical and infrared spectroelectrochemical studies of CN-substituted bipyridyl complexes of Ruthenium(II)

Ruthenium(II) polypyridyl complexes [Ru(CNMe-bpy)x(bpy)3−x]2+ (CN-Me-bpy = 4,4′-dicyano-5,5′-dimethyl2,2′-bipyridine, bpy = 2,2′-bipyridine, and x =1−3, abbreviated as 12+, 22+, and 32+) undergo four (12+) orfive (22+ and 32+) successive one-electron reduction steps between −1.3 and −2.75 V versus ferrocenium/ferrocene (Fc+/Fc) in tetrahydrofuran. The CN-Me-bpy ligands are reduced first, with successive one-electron reductions in 22+ and 32+ being separated by 150−210 mV; reduction of the unsubstituted bpy ligand in 12+ and 22+ occurs only when all CN-Me-bpy ligands have been converted to their radical anions. Absorption spectra of the first three reduction products of each complex were measured across the UV, visible, near-IR (NIR), and mid-IR regions and interpreted with the help of density functional theory calculations. Reduction of the CN-Mebpy ligand shifts the ν(CN) IR band by ca. −45 cm−1, enhances its intensity ∼35 times, and splits the symmetrical and antisymmetrical modes. Semireduced complexes containing two and three CN-derivatized ligands 2+, 3+, and 30 show distinct ν(C N) features due to the presence of both CN-Me-bpy and CN-Me-bpy•−, confirming that each reduction is localized on a single ligand. NIR spectra of 10, 1−, and 2− exhibit a prominent band attributable to the CN-Me-bpy•− moiety between 6000 and 7500 cm−1, whereas bpy•−-based absorption occurs between 4500 and 6000 cm−1; complexes 2+, 3+, and 30 also exhibit a band at ca. 3300 cm−1 due to a CN-Me-bpy•− → CN-Me-bpy interligand charge-transfer transition. In the UV−vis region, the decrease of π → π* intraligand bands of the neutral ligands and the emergence of the corresponding bands of the radical anions are most diagnostic. The first reduction product of 12+ is spectroscopically similar to the lowest triplet metal-to-ligand charge-transfer excited state, which shows pronounced NIR absorption, and its ν(CN) IR band is shifted by −38 cm−1 and 5−7-fold-enhanced relative to the ground state

Ruthenium stilbenyl and Diruthenium Distyrylethene Complexes : Aspects of Electron Delocalization and Electrocatalyzed Isomerization of the Z-Isomer

Regio- and stereoselective insertion of the terminal ethynyl functions of 4-ethynylstilbene, the E and Z isomers of 4,4′-bis(ethynylphenyl)ethene and a backbone-rigidified cyclohexenyl derivative of the Z isomer into the Ru–H bond of the complex RuClH(CO)(PiPr3)2 provides the corresponding vinyl ruthenium complexes, which have been characterized spectroscopically and by X-ray crystallography. Large red shifts of the UV/vis absorption bands evidence efficient incorporation of the vinyl metal subunit(s) into the conjugated π-system. All complexes oxidize at low potentials. The various oxidized forms of all complexes were generated and characterized by UV/vis/NIR, IR and EPR spectroscopies. These studies indicated electrocatalytic Z→E isomerization of the oxidized Z-distyrylethene complex Ru-Z2, which is prevented in its backbone-rigidified derivative Ru-Z2fix. The radical cations of the E and the configurationally stable cyclohexene-bridged Z-derivatives are spin-delocalized on the EPR time scale but charge-localized on the faster IR time scale. The degree of ground-state charge delocalization in the mixed-valent state has been quantified by the incremental shifts of the Ru–CO bands upon stepwise oxidation to the radical cations and the dications and was found to be remarkably large (19% and 9%) considering redox splittings ΔE1/2 of just 49 or 74 mV. Quantum chemical studies with various levels of sophistication reproduce our experimental results including the electronic spectra of the neutral complexes and the intrinsically localized nature of the radical cations of the dinuclear complexes

Singlet diradical complexes of ruthenium and osmium: geometrical and electronic structures and their unexpected changes on oxidation

Reaction of HL, HLa (2-[(2-N-phenylamino)phenylazo]pyridine), HLb (2-[{2-N-(4-methylphenyl)amino}phenylazo]pyridine), or HLc (2-[{2-N-(4-chlorophenyl)amino}phenylazo]pyridine), with KRuO4 or OsO4 and PPh3 under exhaustive deoxygenation (PPh3 → OPPh3) yields diamagnetic compounds ML2. Crystal structure determination for M(La)2 indicates the radical dianion state, L*2-, for the ligands as evident from the typical N-N bond length of about 1.33 Å for a one-electron reduced azo function. The resulting spin-coupled complexes, MIV(L*2-)2, can be oxidized in two reversible one-electron steps, as probed by cyclic voltammetry and UV-vis-NIR spectroelectrochemistry. The paramagnetic intermediates, [M(La)2]+, are distinguished by intense NIR absorption, largely metal-centered spin as revealed by EPR, and, in the case of [Os(La)2]I3, by crystallographically determined shortening of the N=N bond to about 1.30 Å, corresponding to a coordinated unreduced azo function. Thus, oxidation of the complex MIV(L*2-)2 involves partial reduction of the metal in [MIII(L-)2]+ because intramolecular double electron transfer is offsetting the external charge removal. Density-functional theory calculations were employed to confirm the structural features and to support the spectroscopic assignments

Singlet diradical complexes of chromium, molybdenum, and tungsten with azo anion radical ligands from M(CO)<SUB>6</SUB> precursors

The homoleptic diamagnetic complexes M(mer-L)2, M=Cr, Mo,W (1a,b, 2a,b, and 4a,b), were obtained by reacting the hexacarbonyls M(CO)6 with the tridentate ligands 2-[(2-N-arylamino)phenylazo]pyridine (HL = NH4C5N=NC6H4N(H)C6H4(H) (HLa) or NH4C5N=NC6H4N(H)C6H4(CH3) (HLb)) in refluxing n-octane. In the case of M=Mo, the dinuclear compounds [Mo(L)(pap)]2(&#956;-O) (3a,b) (pap=2-(phenylazo)pyridine), were obtained as second products in moist solvent. X-ray diffraction analysis for Cr(Lb)2 (1b), Mo(La)2 (2a), and W(La)2 (4a) reveals considerably distorted-octahedral structures with trans-positioned azo-N atoms and cis-positioned 2-pyridyl-N and anilido nitrogen atoms. Whereas the Nazo-M-Nazo angle is larger than 170&#176;, the other two trans angles are smaller, at about 155&#176; (M=Cr, 1b) or 146&#176; (M=Mo, W; 2a, 4a), due to the overarching bite of the mer-tridentate ligands. The bonds from M to the neutral 2-pyridyl-N atoms are distinctly longer by more than 0.08 &#197; than those to the anilido or azo nitrogen atoms, reflecting negative charge on the latter. The N-N bond distances vary between 1.339(2) &#197; for 1b and 1.373(3) &#197; for 4a, clearly indicating the azo radical anion oxidation state. Considering the additional negative charge on anilido-N, the mononuclear complexes are thus formulated as MIV(L2-)2. The diamagnetism of the complexes as shown by magnetic susceptibility and 1H NMR experiments is believed to result from spin-spin coupling between the trans-positioned azo radical functions, resulting in a singlet diradical situation. The experimental structures are well reproduced by density functional theory calculations, which also support the overall electronic structure indicated. The dinuclear 3a with N-N distances of 1.348(10) &#197; for La and 1.340(9) &#197; for pap is also formulated as an azo anion radical-containing molybdenum(IV) species, i.e., [MoIV(L2-)(pap-)]2(&#956;-O). All compounds can be reversibly reduced; the Cr complexes 1a,b are also reversibly oxidized in two steps. Electron paramagnetic resonance spectroscopy indicates metal-centered spin for 1a+ and 1a- and g &#8776; 2 signals for 2a-, 3a+, 3a-, and 4a-. Spectroelectrochemistry in the UV-vis-NIR region showed small changes for the reduction of 2a, 3a, and 4a but extensive spectral changes for the reduction and oxidation of 1a

An odd-electron complex [Ru<SUP>k</SUP>(NO<SUP>m</SUP>)(Q<SUP>n</SUP>)(terpy)]<SUP>2+</SUP> with two prototypical non-innocent ligands

Six combinations of oxidation states are conceivable for the paramagnetic title complex. Single-crystal X-ray diffraction, spectroscopic analysis (IR, EPR at conventional and high frequency), and DFT calculations establish that it is the iminosemiquinone radical structure that is formed: [Ruk(NOm)(Qn)(terpy)]2+ (k=2+, m=1+, n=1-)

Femtosecond Fluorescence and Intersystem Crossing in Rhenium(I) Carbonyl-Bipyridine Complexes

Ultrafast electronic-vibrational relaxation upon excitation of the singlet charge-transfer b1A' state of [Re(L)(CO)3(bpy)]n (L = Cl, Br, I, n = 0; L = 4-Et-pyridine, n = 1+) in acetonitrile was investigated using the femtosecond fluorescence up-conversion technique with polychromatic detection. In addn., energies, characters, and mol. structures of the emitting states were calcd. by TD-DFT. The luminescence is characterized by a broad fluorescence band at very short times, and evolves to the steady-state phosphorescence spectrum from the a3A" state at longer times. The anal. of the data allows us to identify three spectral components. The first two are characterized by decay times t1 = 85-150 fs and t2 = 340-1200 fs, depending on L, and are identified as fluorescence from the initially excited singlet state and phosphorescence from a higher triplet state (b3A"), resp. The third component corresponds to the long-lived phosphorescence from the lowest a3A" state. In addn., it is found that the fluorescence decay time (t1) corresponds to the intersystem crossing (ISC) time to the two emissive triplet states. t2 corresponds to internal conversion among triplet states. DFT results show that ISC involves electron exchange in orthogonal, largely Re-localized, MOs, whereby the total electron momentum is conserved. Surprisingly, the measured ISC rates scale inversely with the spin-orbit coupling const. of the ligand L, but the authors find a clear correlation between the ISC times and the vibrational periods of the Re-L mode, suggesting that the latter may mediate the ISC in a strongly nonadiabatic regime

Ultrafast Excited-State Dynamics ef [Re(L)(CO)(3)(bpy)](n) Complexes: Involvement of the Solvent

Ultrafast excited-state dynamics of [Re(L)(CO)(3)(bpy)](n) (L = Cl, Br, n = 0; L = 4-ethyl-pyridine (Etpy), n = 1+; bpy = 2,2'-bipyridine) have been investigated in dimethylformamide (DMF) solution by fluorescence up-conversion (FIUC) and UV - vis transient absorption (TA) with similar to 100 is time,resolution. TA was also measured in the [1-ethyl-3-methyl-imidazolium]BF4 ionic liquid. The complexes show a very broad fluorescence band at 540-550 nm at zero time delay, which decays with 100-140 Is (depending on L) by intersystem crossing (ISC) to pi pi* intraligand ((IL)-I-3) and a Re(L)(CO)(3) -> bpy charge-transfer ((CT)-C-3) excited states. A second emission decay component (1.1-1.7 ps). apparent in the red part of the spectrum, is attributed to (IL)-I-3 -> (CT)-C-3 conversion, leaving phosphorescence from the lowest (CT)-C-3 state as the only emission signal at longer time delays. The triplet conversion is slower in DMF? than acetonitrile, commensurate with solvation times. Full assignment of the excited-state absorption at long delay times is obtained by TD-DFT calculations on the lowest triplet state, showing,2 that the 373 nm band is the sole diagnostics of bpy reduction in the CT excited state. Bands in the visible are clue to Ligand-to-Metal-Charge-Transfer (LMCT) transitions. Time-resolved UV - vis absorption spectra exhibit a units-of-ps rise of all absorption features attributed to (IL)-I-3 -> (CT)-C-3 conversion as well as electronic and vibrational relaxation, and a similar to 15 ps rise of only the 373 nm pi pi*(bpy(center dot-)) band which slows down to similar to 1 ns in the ionic liquid solvent. It is proposed that this slow relaxation originates mainly from restructuring of solvent molecules that are found very close to the metal center, inserted between the ligands. The solvent thus plays a key role in controlling the intramolecular charge separation, and this effect may well be operative other classes of metal-based molecular complexes

Vinyl ruthenium-modified biphenyl and 2,2'-bipyridines

International audienceWe report here on ruthenium alkenyl complexes 2 and 3 derived from 2,2'-bipyridine and their Re(CO)3X adducts 4a,b and 5. Detailed electrochemical studies on these complexes and spectroscopic characterization of their oxidized forms by IR, UV/vis/NIR, and electron paramagnetic resonance spectroscopies as well as quantum chemical studies reveal sizable (bridging) ligand contributions to the redox orbitals. Engagement of the free bipy functions of complexes 2 and 3 in binding to the electron-withdrawing fac-Re(CO)3X (X = Br, Cl) moiety enhances the metal-to-ligand charge-transfer character of the optical excitations, causes sizable anodic shifts of the redox potentials, and decreases the number of observable anodic redox waves by one when compared to complexes 2 and 3. Despite the decreasing electron density at the terminal or bridging alkenyl bipyridine ligand, the anodic redox processes still maintain appreciable ligand character as is seen by the shifts of the Ru(CO) and Re(CO)3 stretching frequencies on oxidation. Binding of the fac-Re(CO)3X moiety also attenuates the degree of ground-state delocalization in the mixed-valent states.[on SciFinder (R)