Switching the proton-coupled electron transfer mechanism for non-canonical tyrosine residues in a protein.

The proton-coupled electron transfer (PCET) reactions of tyrosine (Y) are instrumental to many redox reactions in nature. This study investigates how the local environment and the thermodynamic properties of Y influence its PCET characteristics. Herein, 2- and 4-mercaptophenol (MP) are placed in the well-folded αC protein (forming 2MP-αC and 4MP-αC) and oxidized by external light-generated [Ru(L)] complexes.

Alternating Metal-Ligand Coordination Improves Electrocatalytic CO Reduction by a Mononuclear Ru Catalyst.

Molecular electrocatalysts for CO -to-CO conversion often operate at large overpotentials, due to the large barrier for C-O bond cleavage. Illustrated with ruthenium polypyridyl catalysts, we herein propose a mechanistic route that involves one metal center that acts as both Lewis base and Lewis acid at different stages of the catalytic cycle, by density functional theory in corroboration with experimental FTIR. The nucleophilic character of the Ru center manifests itself in the initial attack on CO to form [Ru-CO ] , while its electrophilic character allows for the formation of a 5-membered metallacyclic intermediate, [Ru-CO CO ] , by addition of a second CO molecule and intramolecular cyclization.

A Comparative Study on the Thermodynamics of Halogen Bonding of Group 10 Pincer Fluoride Complexes.

The thermodynamics of halogen bonding of a series of isostructural Group 10 metal pincer fluoride complexes of the type [(3,5-R - POCOP )MF] (3,5-R - POCOP =κ -C HR -2,6-(OPtBu ) with R=H, tBu, COOMe; M=Ni, Pd, Pt) and iodopentafluorobenzene was investigated. Based on NMR experiments at different temperatures, all complexes 1-tBu (R=tBu, M=Ni), 2-H (R=H, M=Pd), 2-tBu (R=tBu, M=Pd), 2-COOMe (R=COOMe, M=Pd) and 3-tBu (R=tBu, M=Pt) form strong halogen bonds with Pd complexes showing significantly stronger binding to iodopentafluorobenzene. Structural and computational analysis of a model adduct of complex 2-tBu with 1,4-diiodotetrafluorobenzene as well as of structures of iodopentafluorobenzene in toluene solution shows that formation of a type I contact occurs.

Mechanistic insights on the non-innocent role of electron donors: reversible photocapture of CO by Ru-polypyridyl complexes.

The ability of [Ru(Butpy)(dmbpy)(MeCN)] (1-MeCN) to capture CO, with the assistance of triethanolamine (TEOA), has been assessed under photocatalytically-relevant conditions. The photolability of 1-MeCN has proven essential to generate a series of intermediates which only differ by the nature of their monodentate ligand. In DMF, ligand photoexchange of 1-MeCN to give [Ru(Butpy)(dmbpy)(DMF)] (1-DMF) proceeds smoothly with a quantum yield of 0.

Controlling Hydrogen Evolution during Photoreduction of CO to Formic Acid Using [Rh(R-bpy)(Cp*)Cl] Catalysts: A Structure-Activity Study.

The photochemical reduction of CO to formic acid catalyzed by a series of [Rh(4,4'-R-bpy)(Cp*)Cl] and [Rh(5,5'-COOH-bpy)(Cp*)Cl] complexes (Cp* = pentamethylcyclopentadienyl, bpy = 2,2'-bipyridine, and R = OCH, CH, H, COOCH, CF, NH, or COOH) was studied to assess how modifications in the electronic structure of the catalyst affect its selectivity, defined as the HCOOH:H product ratio. A direct molecular-level influence of the functional group on the initial reaction rate for CO versus proton reduction reactions was established. Density functional theory computations elucidated for the first time the respective role of the [RhH] and [Cp*H] tautomers, recognizing rhodium hydride as the key player for both reactions.

Judicious Ligand Design in Ruthenium Polypyridyl CO Reduction Catalysts to Enhance Reactivity by Steric and Electronic Effects.

A series of Ru polypyridyl complexes of the structural design [Ru (R-tpy)(NN)(CH CN)] (R-tpy=2,2':6',2''-terpyridine (R=H) or 4,4',4''-tri-tert-butyl-2,2':6',2''-terpyridine (R=tBu); NN=2,2'-bipyridine with methyl substituents in various positions) have been synthesized and analyzed for their ability to function as electrocatalysts for the reduction of CO to CO. Detailed electrochemical analyses establish how substitutions at different ring positions of the bipyridine and terpyridine ligands can have profound electronic and, even more importantly, steric effects that determine the complexes' reactivities. Whereas electron-donating groups para to the heteroatoms exhibit the expected electronic effect, with an increase in turnover frequencies at increased overpotential, the introduction of a methyl group at the ortho position of NN imposes drastic steric effects.

Activating a Low Overpotential CO2 Reduction Mechanism by a Strategic Ligand Modification on a Ruthenium Polypyridyl Catalyst.

The introduction of a simple methyl substituent on the bipyridine ligand of [Ru(tBu3 tpy)(bpy)(NCCH3 )](2+) (tBu3 tpy=4,4',4''-tri-tert-butyl-2,2':6',2''-terpyridine; bpy=2,2'-bipyridine) gives rise to a highly active electrocatalyst for the reduction of CO2 to CO. The methyl group enables CO2 binding already at the one-electron reduced state of the complex to enter a previously not accessible catalytic cycle that operates at the potential of the first reduction. The complex turns over with a Faradaic efficiency close to unity and at an overpotential that is amongst the lowest ever reported for homogenous CO2 reduction catalysts.

Tunable Electrochemical and Catalytic Features of BIAN- and BIAO-Derived Ruthenium Complexes.

This article deals with a class of ruthenium-BIAN-derived complexes, [Ru(II)(tpm)(R-BIAN)Cl]ClO4 (tpm = tris(1-pyrazolyl)methane, R-BIAN = bis(arylimino)acenaphthene, R = 4-OMe ([1a]ClO4), 4-F ([1b]ClO4), 4-Cl ([1c]ClO4), 4-NO2 ([1d]ClO4)) and [Ru(II)(tpm)(OMe-BIAN)H2O](2+) ([3a](ClO4)2). The R-BIAN framework with R = H, however, leads to the selective formation of partially hydrolyzed BIAO ([N-(phenyl)imino]acenapthenone)-derived complex [Ru(II)(tpm)(BIAO)Cl]ClO4 ([2]ClO4). The redox-sensitive bond parameters involving -N═C-C═N- or -N═C-C═O of BIAN or BIAO in the crystals of representative [1a]ClO4, [3a](PF6)2, or [2]ClO4 establish its unreduced form.

Electronic structures and selective fluoride sensing features of Os(bpy)2(HL(2-)) and [{Os(bpy)2}2(μ-HL(2-))](2+) (H3L: 5-(1H-benzo[d]imidazol-2-yl)-1H-imidazole-4-carboxylic acid).

The article deals with the newly designed mononuclear and asymmetric dinuclear osmium(ii) complexes Os(II)(bpy)2(HL(2-)) (1) and [(bpy)2Os(II)(μ-HL(2-))Os(II)(bpy)2](Cl)2 ([2](Cl)2)/[(bpy)2Os(II)(μ-HL(2-))Os(II)(bpy)2](ClO4)2 ([2](ClO4)2), respectively, (H3L = 5-(1H-benzo[d]imidazol-2-yl)-1H-imidazole-4-carboxylic acid and bpy = 2,2'-bipyridine). The identity of 1 has been established by its single crystal X-ray structure. The ligand (HL(2-))-based primary oxidation process (E, 0.

Sensitivity of a strained C-C single bond to charge transfer: redox activity in mononuclear and dinuclear ruthenium complexes of bis(arylimino)acenaphthene (BIAN) ligands.

The new compounds [Ru(acac)2(BIAN)], BIAN = bis(arylimino)acenaphthene (aryl = Ph (1a), 4-MeC6H4 (2a), 4-OMeC6H4 (3a), 4-ClC6H4 (4a), 4-NO2C6H4 (5a)), were synthesized and structurally, electrochemically, spectroscopically, and computationally characterized. The α-diimine sections of the compounds exhibit intrachelate ring bond lengths 1.304 Å < d(CN) < 1.

Sensitivity of the valence structure in diruthenium complexes as a function of terminal and bridging ligands.

The compounds [(acac)2Ru(III)(μ-H2L(2-))Ru(III)(acac)2] (rac, 1, and meso, 1') and [(bpy)2Ru(II)(μ-H2L(•-))Ru(II)(bpy)2](ClO4)3 (meso, [2](ClO4)3) have been structurally, magnetically, spectroelectrochemically, and computationally characterized (acac(-) = acetylacetonate, bpy = 2,2'-bipyridine, and H4L = 1,4-diamino-9,10-anthraquinone). The N,O;N',O'-coordinated μ-H2L(n-) forms two β-ketiminato-type chelate rings, and 1 or 1' are connected via NH···O hydrogen bridges in the crystals. 1 exhibits a complex magnetic behavior, while [2](ClO4)3 is a radical species with mixed ligand/metal-based spin.

Bidirectional non-innocence of the β-diketonato ligand 9-oxidophenalenone (L-) in [Ru([9]aneS3)(L)(dmso)]n, [9]aneS3 = 1,4,7-trithiacyclononane.

The new compound [Ru(II)([9]aneS3)(L)(dmso)]ClO4 ([]ClO4) ([9]aneS3 = 1,4,7-trithiacyclononane, HL = 9-hydroxyphenalenone, dmso = dimethylsulfoxide) has been structurally characterised to reveal almost equal C-O bond distances of coordinated L(-), suggesting a delocalised bonding situation of the β-diketonato ligand. The dmso ligand is coordinated via the sulfur atom in the native (1+) and reduced states (1 and 1-) as has been revealed by X-ray crystallography and by DFT calculations. Cyclic voltammetry of 1+ exhibits two close-lying one-electron oxidation waves at 0.

Synthesis, spectral characterization, structures, and oxidation state distributions in [(corrolato)Fe(III)(NO)](n) (n = 0, +1, -1) complexes.

Two novel trans-A2B-corroles and three [(corrolato){FeNO}(6)] complexes have been prepared and characterized by various spectroscopic techniques. In the native state, all these [(corrolato){FeNO}(6)] species are diamagnetic and display "normal" chemical shifts in the (1)H NMR spectra. For two of the structurally characterized [(corrolato){FeNO}(6)] derivatives, the Fe-N-O bond angles are 175.

Four-center oxidation state combinations and near-infrared absorption in [Ru(pap)(Q)2]n (Q = 3,5-di-tert-butyl-N-aryl-1,2-benzoquinonemonoimine, pap = 2-phenylazopyridine).

The complex series [Ru(pap)(Q)2](n) ([1](n)-[4](n); n = +2, +1, 0, -1, -2) contains four redox non-innocent entities: one ruthenium ion, 2-phenylazopyridine (pap), and two o-iminoquinone moieties, Q = 3,5-di-tert-butyl-N-aryl-1,2-benzoquinonemonoimine (aryl = C6H5 (1(+)); m-(Cl)2C6H3 (2(+)); m-(OCH3)2C6H3 (3(+)); m-(tBu)2C6H3 (4(+))). A crystal structure determination of the representative compound, [1]ClO4, established the crystallization of the ctt-isomeric form, that is, cis and trans with respect to the mutual orientations of O and N donors of two Q ligands, and the coordinating azo N atom trans to the O donor of Q. The sensitive C-O (average: 1.

Electronic structure and catalytic aspects of [Ru(tpm)(bqdi)(Cl/H2O)]n, tpm = tris(1-pyrazolyl)methane and bqdi = o-benzoquinonediimine.

The diamagnetic complexes [Ru(tpm)(bqdi)(Cl)]ClO(4) ([1]ClO(4)) (tpm = tris(1-pyrazolyl)methane, bqdi = o-benzoquinonediimine) and [Ru(tpm)(bqdi)(H(2)O)](ClO(4))(2) ([2](ClO(4))(2)) have been synthesized. The valence state-sensitive bond distances of coordinated bqdi [C-N: 1.311(5)/1.