[M(CO)4(2,2′-bipyridine)] (M = Cr, Mo, W) as efficient catalysts for electrochemical reduction of CO2 to CO at a gold electrode

Group 6 complexes of the type [M(CO)4(bpy)] (M=Cr, Mo, W) are capable of behaving as electrochemical catalysts for the reduction of CO2 at potentials less negative than those for the reduction of the radical anions [M(CO)4(bpy)].−. Cyclic voltammetric, chronoamperometric and UV/Vis/IR spectro-electrochemical data reveal that five-coordinate [M(CO)3(bpy)]2− are the active catalysts. The catalytic conversion is significantly more efficient in N-methyl-2-pyrrolidone (NMP) compared to tetrahydrofuran, which may reflect easier CO dissociation from 1e−-reduced [M(CO)4(bpy)].− in the former solvent, followed by second electron transfer. The catalytic cycle may also involve [M(CO)4(H-bpy)]− formed by protonation of [M(CO)3(bpy)]2−, especially in NMP. The strongly enhanced catalysis using an Au working electrode is remarkable, suggesting that surface interactions may play an important role, too

Spectro-electrochemical studies on [Ru(TAP)2(dppz)]2+ - insights into the mechanism of its photosensitized oxidation of oligonucleotides

[Ru(TAP)2(dppz)]2+ (TAP = 1,4,5,8-tetraazaphenanthrene; dppz = dipyrido[3,2-a:2′,3′-c]phenazine) is known to photo-oxidize guanine in DNA. Whether this oxidation proceeds by direct photoelectron transfer or by proton-coupled electron transfer is still unknown. To help distinguish between these mechanisms, spectro-electrochemical experiments have been carried out with [Ru(TAP)2(dppz)]2+ in acetonitrile. The UV–vis and mid-IR spectra obtained for the one-electron reduced product were compared to those obtained by picosecond transient absorption and time-resolved infrared experiments of [Ru(TAP)2(dppz)]2+ bound to guanine-containing DNA. An interesting feature of the singly reduced species is an electronic transition in the near-IR region (with λmax at 1970 and 2820 nm). Density functional and time-dependent density functional theory simulations of the vibrational and electronic spectra of [Ru(TAP)2(dppz)]2+, the reduced complex [Ru(TAP)2(dppz)]+, and four isomers of [Ru(TAP)(TAPH)(dppz)]2+ (a possible product of proton-coupled electron transfer) were performed. Significantly, these predict absorption bands at λ > 1900 nm (attributed to a ligand-to-metal charge-transfer transition) for [Ru(TAP)2(dppz)]+ but not for [Ru(TAP)(TAPH)(dppz)]2+. Both the UV–vis and mid-IR difference absorption spectra of the electrochemically generated singly reduced species [Ru(TAP)2(dppz)]+ agree well with the transient absorption and time-resolved infrared spectra previously determined for the transient species formed by photoexcitation of [Ru(TAP)2(dppz)]2+ intercalated in guanine-containing DNA. This suggests that the photochemical process in DNA proceeds by photoelectron transfer and not by a proton-coupled electron transfer process involving formation of [Ru(TAP)(TAPH)(dppz)]2+, as is proposed for the reaction with 5′-guanosine monophosphate. Additional infrared spectro-electrochemical measurements and density functional calculations have also been carried out on the free TAP ligand. These show that the TAP radical anion in acetonitrile also exhibits strong broad near-IR electronic absorption (λmax at 1750 and 2360 nm)

Manganese tricarbonyl complexes with asymmetric 2 2‑iminopyridine ligands: toward decoupling steric and electronic 3 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

Electrocatalytic reduction of carbon dioxide with a manganese(I)tricarbonyl complex containing a nonaromatic α‑diimine ligand

The 2e reduced anion [Mn(CO)3(iPr-DAB)]− (DAB = 1,4- diazabuta-1,3-diene, iPr = isopropyl) was shown to convert in the presence of CO2 and a small amount of water to the unstable complex [Mn(CO)3(iPr-DAB)(η1-OCO2H)] (OCO2H− = unidentate bicarbonate) that was further reductively transformed to give a stable catalytic intermediate denoted as X2, showing νs(OCO) 1672 and 1646 (sh) cm−1. The subsequent cathodic shift by ca. 650 mV in comparison to the single 2e cathodic wave of the parent [Mn(CO)3(iPr-DAB)Br] triggers the reduction of intermediate X2 and catalytic activity converting CO2 to CO. Infrared spectroelectrochemistry has revealed that the high excess of CO generated at the cathode leads to the conversion of [Mn(CO)3(iPr-DAB)]− to inactive [Mn(CO)5]−. In contrast, the five-coordinate anion [Mn(CO)3(pTol-DAB)]−(pTol = 4-tolyl) is completely inert toward both CO2 and H2O (solvolysis). This detailed spectroelectrochemical study is a further contribution to the development of sustainable electro- and photoelectrocatalysts of CO2 reduction based on abundant first-row transition metals, in particular manganese

Spectro-electrochemical Studies on [Ru(TAP)2 (dppz)]2+ - Insights into the Mechanism of its Photosensitized Oxidation of Oligonucleotides

[Ru(TAP) 2 (dppz)] 2+ (TAP = 1,4,5,8-tetraazaphenanthrene; dppz = dipyrido[3,2-a:2′,3′-c]phenazine) is known to photo-oxidize guanine in DNA. Whether this oxidation proceeds by direct photoelectron transfer or by proton-coupled electron transfer is still unknown. To help distinguish between these mechanisms, spectro-electrochemical experiments have been carried out with [Ru(TAP) 2 (dppz)] 2+ in acetonitrile. The UV-vis and mid-IR spectra obtained for the one-electron reduced product were compared to those obtained by picosecond transient absorption and time-resolved infrared experiments of [Ru(TAP) 2 (dppz)] 2+ bound to guanine-containing DNA. An interesting feature of the singly reduced species is an electronic transition in the near-IR region (with λ max at 1970 and 2820 nm). Density functional and time-dependent density functional theory simulations of the vibrational and electronic spectra of [Ru(TAP) 2 (dppz)] 2+ , the reduced complex [Ru(TAP) 2 (dppz)] + , and four isomers of [Ru(TAP)(TAPH)(dppz)] 2+ (a possible product of proton-coupled electron transfer) were performed. Significantly, these predict absorption bands at λ > 1900 nm (attributed to a ligand-to-metal charge-transfer transition) for [Ru(TAP) 2 (dppz)] + but not for [Ru(TAP)(TAPH)(dppz)] 2+ . Both the UV-vis and mid-IR difference absorption spectra of the electrochemically generated singly reduced species [Ru(TAP) 2 (dppz)] + agree well with the transient absorption and time-resolved infrared spectra previously determined for the transient species formed by photoexcitation of [Ru(TAP) 2 (dppz)] 2+ intercalated in guanine-containing DNA. This suggests that the photochemical process in DNA proceeds by photoelectron transfer and not by a proton-coupled electron transfer process involving formation of [Ru(TAP)(TAPH)(dppz)] 2+ , as is proposed for the reaction with 5′-guanosine monophosphate. Additional infrared spectro-electrochemical measurements and density functional calculations have also been carried out on the free TAP ligand. These show that the TAP radical anion in acetonitrile also exhibits strong broad near-IR electronic absorption (λ max at 1750 and 2360 nm).Spectroelectrochemistry ReadingRoyal Irish Academy/Royal Society Exchange Programm

Solvent-Dependent Formation of Os(0) Complexes by Electrochemical Reduction of [Os(CO)(2,2′-bipyridine)(L)Cl₂]; L = Cl^–, PrCN

Cyclic voltammetry and ultraviolet–visible/infrared (UV–vis/IR) spectroelectrochemistry were used to study the cathodic electrochemical behavior of the osmium complexes <i>mer</i>-[Os^III(CO) (bpy)­Cl₃] (bpy = 2,2′-bipyridine) and <i>trans</i>(Cl)<i>-</i>[Os^II(CO) (PrCN)­(bpy)­Cl₂] at variable temperature in different solvents (tetrahydrofuran (THF), butyronitrile (PrCN), acetonitrile (MeCN)) and electrolytes (Bu₄NPF₆, Bu₄NCl). The precursors can be reduced to <i>mer</i>-[Os^II(CO) (bpy^•–)­Cl₃]^2– and <i>trans</i>(Cl)-[Os^II(CO)­(PrCN) (bpy^•–)­Cl₂]⁻, respectively, which react rapidly at room temperature, losing the chloride ligands and forming Os(0) species. <i>mer</i>-[Os^III(CO) (bpy)­Cl₃] is reduced in THF to give ultimately an Os–Os-bonded polymer, probably [Os⁰(CO) (THF)­(bpy)]_<i>n</i>, whereas in PrCN the well-soluble, probably mononuclear [Os⁰(CO) (PrCN)­(bpy)], species is formed. The same products were observed for the 2 electron reduction of <i>trans</i>(Cl)-[Os^II(CO)­(PrCN) (bpy)­Cl₂] in both solvents. In MeCN, similar to THF, the [Os⁰(CO) (MeCN)­(bpy)]_<i>n</i> polymer is produced. It is noteworthy that the bpy ligand in mononuclear [Os⁰(CO) (PrCN)­(bpy)] is reduced to the corresponding radical anion at a significantly less negative potential than it is in polymeric [Os⁰(CO) (THF)­(bpy)]_<i>n</i>: Δ<i>E</i>_1/2 = 0.67 V. Major differences also exist in the IR spectra of the Os(0) species: the polymer shows a broad ν­(CO) band at much smaller wavenumbers compared to the soluble Os(0) monomer that exhibits a characteristic ν­(<i>Pr</i>-CN) band below 2200 cm^–1 in addition to the intense and narrow ν­(CO) absorption band. For the first time, in this work the M⁰-bpy (M = Ru, Os) mono- and dicarbonyl species soluble in PrCN have been formulated as a mononuclear complex. Density functional theory (DFT) and time-dependent-DFT calculations confirm the Os(0) oxidation state and suggest that [Os⁰(CO) (PrCN)­(bpy)] is a square planar moiety. The reversible bpy-based reduction of [Os⁰(CO) (PrCN)­(bpy)] triggers catalytic reduction of CO₂ to CO and HCOO^–

Effects of Electrode–Molecule Binding and Junction Geometry on the Single-Molecule Conductance of bis-2,2′:6′,2″-Terpyridine-based Complexes

The single molecule conductances of a series of bis-2,2′:6′,2″-terpyridine complexes featuring Ru(II), Fe(II), and Co(II) metal ions and trimethylsilylethynyl (Me3SiC C−) or thiomethyl (MeS-) surface contact groups have been determined. In the absence of electrochemical gating, these complexes behave as tunneling barriers, with conductance properties determined more by the strength of the electrode−molecule contact and the structure of the “linker” than the nature of the metal-ion or redox properties of the complex

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-<i>tert</i>-butylphenyl)­imino)­methyl]­pyridine), and TBIEP (2-[((2-<i>tert</i>-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

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-<i>tert</i>-butylphenyl)­imino)­methyl]­pyridine), and TBIEP (2-[((2-<i>tert</i>-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