OTTLE Cell Research Articles

Electrochemical disproportionation in new ruthenium (II) nitro complexes with 2,4,6-tris(2-pyridyl)-1,3,5-triazine detected by IR spectroelectrochemistry with an OTTLE cell

The syntheses and characterization of new nitro complexes of Ru(II) with 2,4,6-tris(2-pyridyl)-1,3,5-triazine (tptz) and different substituted bipyridines are reported. A disproportionation reaction can occur after applying an oxidation potential corresponding to the couple RuII/RuIII forming the nitrate and the nitrosyl complexes, which were detected by cyclic voltammetry. This reaction was confirmed by IR-spectroelectrochemistry with an OTTLE cell, for the first time. Additionally, for 2,2′-bipyridine we have obtained as ClO4− salt a new nitrosyl-complex which presents an elevated IR stretching frequency.

Manganese Tricarbonyl Complexes with Asymmetric 2-Iminopyridine Ligands: Toward Decoupling Steric and Electronic Factors in Electrocatalytic CO2 Reduction.

Manganese tricarbonyl bromide complexes incorporating IP (2-(phenylimino)pyridine) derivatives, [MnBr(CO)3(IP)], are demonstrated as a new group of catalysts for CO2 reduction, which represent the first example of utilization of (phenylimino)pyridine ligands on manganese centers for this purpose. The key feature is the asymmetric structure of the redox-noninnocent ligand that permits independent tuning of its steric and electronic properties. The α-diimine ligands and five new Mn(I) compounds have been synthesized, isolated in high yields, and fully characterized, including X-ray crystallography. Their electrochemical and electrocatalytic behavior was investigated using cyclic voltammetry and UV-vis-IR spectroelectrochemistry within an OTTLE cell. Mechanistic investigations under an inert atmosphere have revealed differences in the nature of the reduction products as a function of steric bulk of the ligand. The direct ECE (electrochemical-chemical-electrochemical) formation of a five-coordinate anion [Mn(CO)3(IP)]-, a product of two-electron reduction of the parent complex, is observed in the case of the bulky DIPIMP (2-[((2,6-diisopropylphenyl)imino)methyl]pyridine), TBIMP (2-[((2-tert-butylphenyl)imino)methyl]pyridine), and TBIEP (2-[((2-tert-butylphenyl)imino)ethyl]pyridine) derivatives. This process is replaced for the least sterically demanding IP ligand in [MnBr(CO)3(IMP)] (2-[(phenylimino)methyl]pyridine) by the stepwise formation of such a monoanion via an ECEC(E) mechanism involving also the intermediate Mn-Mn dimer [Mn(CO)3(IMP)]2. The complex [MnBr(CO)3(IPIMP)] (2-[((2-diisopropylphenyl)imino)methyl]pyridine), which carries a moderately electron donating, moderately bulky IP ligand, shows an intermediate behavior where both the five-coordinate anion and its dimeric precursor are jointly detected on the time scale of the spectroelectrochemical experiments. Under an atmosphere of CO2 the studied complexes, except for the DIPIMP derivative, rapidly coordinate CO2, forming stable bicarbonate intermediates, with no dimer being observed. Such behavior indicates that the CO2 binding is outcompeting another pathway: viz., the dimerization reaction between the five-coordinate anion and the neutral parent complex. The bicarbonate intermediate species undergo reduction at more negative potentials (ca. -2.2 V vs Fc/Fc+), recovering [Mn(CO)3(IP)]- and triggering the catalytic production of CO.

Functional Characterization of the FoxE Iron Oxidoreductase from the Photoferrotroph Rhodobacter ferrooxidans SW2

Photoferrotrophy is presumed to be an ancient type of photosynthetic metabolism in which bacteria use the reducing power of ferrous iron to drive carbon fixation. In this work the putative iron oxidoreductase of the photoferrotroph Rhodobacter ferrooxidans SW2 was cloned, purified, and characterized for the first time. This protein, FoxE, was characterized using spectroscopic, thermodynamic, and kinetic techniques. It is a c-type cytochrome that forms a trimer or tetramer in solution; the two hemes of each monomer are hexacoordinated by histidine and methionine. The hemes have positive reduction potentials that allow downhill electron transfer from many geochemically relevant ferrous iron forms to the photosynthetic reaction center. The reduction potentials of the hemes are different and are cross-assigned to fast and slow kinetic phases of ferrous iron oxidation in vitro. Lower reactivity was observed at high pH and may contribute to prevent ferric iron precipitation inside or at the surface of the cell. These results help fill in the molecular details of a metabolic process that likely contributed to the deposition of precambrian banded iron formations, globally important sedimentary rocks that are found on every continent today.

Electrochemical Reductive Deprotonation of an Imidazole Ligand in a Bipyridine Tricarbonyl Rhenium(I) Complex

AbstractThe reduction pathway of fac‐[ReI(imH)(CO)3(bpy)]+ was studied in situ by UV/Vis/NIR–IR spectroelectrochemistry with an OTTLE cell. The complex underwent 1e– reduction of the 2,2′‐bipyridine (bpy) ligand and intramolecular electron transfer, which resulted in the conversion of the axial imidazole (imH) ligand to 3‐imidazolate (3‐im–). This step was followed by two bpy‐based 1e– reductions to ultimately produce five‐coordinate [Re(CO)3(bpy)]– and free 3‐im–. The identity of the reduction product fac‐[Re(3‐im–)(CO)3(bpy)] has been proven by partial chemical deprotonation of the parent complex followed by IR spectroelectrochemistry. This is the first time that the reductive electrochemical conversion of metal‐coordinated imidazole to terminal 3‐imidazolate has been observed.

Coupling Between Plasmonic Resonances in Nanoparticles and Porphyrins Molecules

A driving force for the growing interest in nanoassemblies is the use of nanoparticles linked to organic molecules. The goal of our research is to investigate the coupling of gold and silver nanoparticles (GNP and AgNP) with porphyrins molecules. We prepared water-soluble GNP and AgNP by reducing the noble metal complex salt with sodium citrate. The exchange of the initial ligand (citrate ions) with porphyrins formed hybrid nanostructures and change the electronic properties of GNP and AgNP. We highlighted the importance of the pH value to obtain the coupling of nanoparticles and porphyrins. We have shown that the absorption spectra of AgNP have a strong overlap with that of porphyrins. The effect of dye molecules on the plasmon properties of nanoparticles was demonstrated by UV-Vis, fluorescence and electrochemical spectroscopy (OTTLE cell). These new hybrid materials will be helpful for the design of light harvesting cells and sensors.

Rare monomeric structure of CpRh(III) dithiolene complex: Combined ESR and electronic absorption studies on electrochemically generated Rh(II) species

Abstract The [CpRhIII(mnt)] (1) (Cp = η5-cyclopentadienyl, mnt = maleonitrile-1,2-dithiolate) was characterized as a rare monomeric structure. The CV of 1 showed a reversible reduction wave which attributed to Rh(II)/Rh(III) couple. UV–Vis spectral measurement during electrolysis using an OTTLE cell indicated that lifetime of electrochemically generated 1 − (Rh(II) species) was only a few seconds. Isotropic (giso = 2.045, Aiso = 1.15 mT) and anisotropic ESR (g1 = 2.173, a1 = 1.85 mT, g2 = 1.982, a2 = 0.98 mT) spectra of 1 − were observed while electrochemical reduction of 1. Both ESR spectra showed hyperfine splittings due to the central Rh (I = 1/2).

In-situ generation and analysis of charge transfer materials using an ottle cell and resonance Raman scattering

In-situ generation and analysis of charge transfer materials using an ottle cell and resonance Raman scattering

Syntheses, redox and UV–Vis spectroelectrochemical properties of mono- and dinuclear tris(pyrazolyl)borato-oxomolybdenum(IV) complexes with pyridine ligands

Reaction of [Mo VI(Tp Me,Me)(O) 2Cl] with a variety of pyridine-based ligands [pyridine (py), 4,4′-bipyridine (bpy), 4-phenylpyridine (phpy) and 1,2′-bis(4-pyridyl)ethene (bpe)] in toluene in the presence of Ph 3P affords the mononuclear oxo-Mo(IV) complexes [Mo(Tp Me,Me)(O)Cl(L)] (L=py, phpy or monodentate bpy; abbreviated as Mo(py), Mo(phpy) and Mo(bpy), respectively) and the dinuclear complexes [{Mo(Tp Me,Me)(O)Cl} 2(μ-L)] (L=bpy, bpe; abbreviated as Mo 2 (bpy), Mo 2 (bpe), respectively). The complex Mo 2 (bpy), together with the by-product [{Mo(Tp Me,Me)(O)Cl} 2(μ-O)], have been crystallographically characterised. Electrochemical studies on the oxo-Mo(IV) complexes reveal the presence of reversible Mo(IV)/Mo(V) couples at around −0.3 V versus ferrocene/ferrocenium in every case. For the dinuclear complexes Mo 2 (bpy) and Mo 2 (bpe) these redox processes are coincident, indicating that they are largely metal-centred and not significantly delocalised across the bridging ligand. In contrast, Mo 2 (bpe) alone shows two reversible reductions, separated by 320 mV; these could be described as ligand-centred reductions of the bpe bridge, or as Mo(IV)/Mo(III) couples which—because of their separation—are substantially delocalised onto the bridging ligand. UV–Vis spectroelectrochemical studies using an OTTLE cell at 243 K revealed that oxidation of the complexes results in spectral changes (collapse of the Mo(IV) d–d transitions, loss in intensity of the Mo→pyridine MLCT transition) consistent with the formation of a Mo(V) state following metal-centred oxidation, but that one-electron reduction of Mo 2 (bpe) results in appearance of numerous intense transitions more characteristic of a ligand radical following ligand-centred reduction.

Transition-Metal-Promoted C−N Bond-Formation Processes − Low-Spin FeIII, FeII, and NiII Complexes of 2-[(Arylamido)phenylazo]pyridine − X-ray Structure, Redox- and Spectroelectrochemistry

The metal-promoted amination of an aromatic ring of coordinated L1 [L1, pap = 2-(phenylazo)pyridine] is described. Whereas the labile cobalt complex [Co(L1)3]2+ prefers ortho-amination, yielding L2H {2-[2-(arylamino)phenylazo]pyridine}, the corresponding [Fe(L1)3]2+ complex produces the para-aminated product L3H {2-[4-(arylamino)phenylazo]pyridine}. Both L2H and L3H have been isolated in the pure state and were fully characterised. They have low pKa values, for example pKa(L2aH) = 8.5 ± 1 and pKa(L3aH) = 9.1 ± 1. Upon deprotonation, the ligand L2H behaves as a potential tridentate N,N,N donor. It reacts with anhydrous FeCl3 and NiCl2·6H2O to produce cationic [Fe(L2)2]+ (1+) and neutral [Ni(L2)2] (2), respectively. The cationic ferric complex has been isolated as its perchlorate salt, which is paramagnetic with one unpaired electron (1.66 μB). It shows a rhombic EPR spectrum with g1 = 2.11, g2 = 2.08, g3 = 1.93. The room-temperature magnetic moment of the nickel complex is 2.89 μB, which confirms the presence of two unpaired electrons. The representative X-ray structures of [1]ClO4 and 2 are reported. In both cases the azo nitrogen atoms of the coordinated ligand, [L2]− approach the metal centres more closely, and there is a significant degree of ligand backbone conjugation. The complexes display multiple redox responses. Chemical reduction of 1+ with dilute hydrazine affords the corresponding ferrous complex [Fe(L2)2] (1) in almost quantitative yield. The spectral changes upon electrolysis of the above couples have been recorded in an OTTLE cell.

Transition-Metal-Promoted C−N Bond-Formation Processes − Low-Spin FeIII, FeII, and NiII Complexes of 2-[(Arylamido)phenylazo]pyridine − X-ray Structure, Redox- and Spectroelectrochemistry

The metal-promoted amination of an aromatic ring of coordinated L1 [L1, pap = 2-(phenylazo)pyridine] is described. Whereas the labile cobalt complex [Co(L1)3]2+ prefers ortho-amination, yielding L2H {2-[2-(arylamino)phenylazo]pyridine}, the corresponding [Fe(L1)3]2+ complex produces the para-aminated product L3H {2-[4-(arylamino)phenylazo]pyridine}. Both L2H and L3H have been isolated in the pure state and were fully characterised. They have low pKa values, for example pKa(L2aH) = 8.5 ± 1 and pKa(L3aH) = 9.1 ± 1. Upon deprotonation, the ligand L2H behaves as a potential tridentate N,N,N donor. It reacts with anhydrous FeCl3 and NiCl2·6H2O to produce cationic [Fe(L2)2]+ (1+) and neutral [Ni(L2)2] (2), respectively. The cationic ferric complex has been isolated as its perchlorate salt, which is paramagnetic with one unpaired electron (1.66 μB). It shows a rhombic EPR spectrum with g1 = 2.11, g2 = 2.08, g3 = 1.93. The room-temperature magnetic moment of the nickel complex is 2.89 μB, which confirms the presence of two unpaired electrons. The representative X-ray structures of [1]ClO4 and 2 are reported. In both cases the azo nitrogen atoms of the coordinated ligand, [L2]− approach the metal centres more closely, and there is a significant degree of ligand backbone conjugation. The complexes display multiple redox responses. Chemical reduction of 1+ with dilute hydrazine affords the corresponding ferrous complex [Fe(L2)2] (1) in almost quantitative yield. The spectral changes upon electrolysis of the above couples have been recorded in an OTTLE cell.

Infrared Spectroelectrochemical Investigation of Carbon Dioxide Reduction Mediated by the Anion [Ru(SnPh3)(Co)2(iPr-DAB)]- (iPr-DAB = N,N'-Diisopropyl-1,4-diaza-1,3-butadiene)

Carbon dioxide was found to react slowly with the coordinatively unsaturated anion [Ru(SnPh3)(CO)2(iPr-DAB)]- electrogenerated in an IR OTTLE cell via bielectronic reduction of [Ru(Cl)(SnPh3)(CO)2(iPr-DAB)]. This reaction produced free CO, CO32-, O2CH- and other two unidentified carboxylato species. The overall process is not catalytic in nature since the anion cannot be recovered after the attack of CO2 due to its conversion into a stable complex tentatively assigned as {Ru0(CO)2(iPr-DAB)}2. Although limited, the inherent reactivity of [Ru(SnPh3)(CO)2(iPr-DAB)]- towards CO2 demonstrated that the strong π-bonding within the (Sn)Ru(iPr-DAB) metallacycle, stabilizing the five-coordinate structure, is slightly less delocalized in comparison with the closely related, but totally inert anions [Mn(CO)3(α-diimine)]- (α-diimine = bpy, iPr-DAB).

The “Rieske” center in mitochondrial bc1 complex and in bacterial dioxygenases

Rieske type [2Fe-2S] clusters are present in the bc 1 complex of the respiratory chain as well as in bacterial dioxygenases. All these Rieske clusters are thought to have histidine ligands to the Fe II of the reduced complex as determined by ENDOR and ESEEM spectroscopy [1,2] but they differ in their redox potentials. We have compared the spectra and redox properties of the water soluble fragment of the Rieske center of bovine heart mitochondrial bc 1 complex (ISF; E m = + 3 0 0 mV) and of the ferredoxin of the benzene dioxygenase from Pseudomonas putida ML2 (Fdbed; E m = -150 mY). The redox potential of both proteins could be determined in solution by cyclic voltammetr~/ and by spectroelectrochemistry in an OTTLE cell. The redox potential of the ISF is pH dependent above pH = 7. From the fit of the pH dependence of the E m, two redox dependent pK a values of 7.6 and 9.2 on the oxidized protein were obtained [3]; the same pK values could also be deduced from the pH dependence of the CD spectrum [4]. These pK values are not observed in the dioxygenase Rieske clusters. The CD spectra of Rieske type clusters are significantly different from those of plant type ferredoxins. A strong negative CD band is present at 20 000 cm-' (500 rim) in all reduced Rieske clusters. Th s band could be assigned to the highest energy magnetically allowed d -~d transition (dz2 -~dxz) of the Fe II. This band is high!y indicative for the distortion of the tetrahedral symmetry of the Fe; in plant type ferredoxins having four cysteine ligands to the Fe, it occurs in the near infrared at 5800 cm-' [5].