Tricarbonylrhenium(I) Complexes of Dinucleating Redox-Active Pincer Ligands

Two homobimetallic tricarbonylrhenium­(I) complexes of new redox-active dinucleating pincer ligands have been prepared to assess the impact of spacer size on the first oxidation potentials with respect to mononucleating analogues and on intramolecular electronic communication. The new pincer ligands feature two tridentate <i>NNN</i>- sites each composed of two pyrazolyl flanking donors and a diarylamido anchor that are either directly linked (to form a central benzidene core, H₂(L1)) or linked via a <i>para</i>-phenylene group (to form a <i>para</i>-terphenyldiamine core, H₂(L2)). The bimetallic complexes of the deprotonated ligands, [<i>fac</i>-Re­(CO)₃]₂(μ-L1), <b>1</b>, and [<i>fac</i>-Re­(CO)₃]₂(μ-L2), <b>2</b>, were fully characterized in solution and the solid state including by single-crystal X-ray diffraction for <b>1</b>. The electrochemical properties of each depended strongly on solvent and electrolyte. Complex <b>1</b> exhibits two one-electron oxidations in all electrolyte-containing solutions but with separations between first and second oxidation potentials, Δ<i>E</i>_1/2, between 119 and 316 mV depending on conditions. On the other hand, cyclic voltammetry of <b>2</b> showed one two-electron oxidation in DMF with NBu₄PF₆ as an electrolyte but two one-electron oxidations with a maximal separation in Δ<i>E</i>_1/2 of 96 mV in CH₂Cl₂ with NBu₄B­(C₆F₅)₄ as an electrolyte. The oxidized complexes <b>1</b>^<b>n+</b> and <b>2</b>^<b>n+</b> (<i>n</i> = 1, 2) were prepared by chemical oxidation and were studied spectroscopically (UV–vis/NIR, EPR). The mono-oxidized complex <b>1</b>^<b>+</b> behaves as a Robin–Day Class III species, while <b>2</b>^<b>+</b> is a Robin–Day Class II species that shows thermal valence trapping at 77 K by EPR spectroscopy. As suggested from theoretical studies using DFT methods, the oxidized complexes maintain considerable ligand radical character, so their electronic structures can be formulated as (CO)₃Re^I(μ-L^<i>n</i>+)­Re^I(CO)₃ (<i>n</i> = 1 or 2)

Tricarbonylrhenium(I) Complexes of Dinucleating Redox-Active Pincer Ligands

Two homobimetallic tricarbonylrhenium­(I) complexes of new redox-active dinucleating pincer ligands have been prepared to assess the impact of spacer size on the first oxidation potentials with respect to mononucleating analogues and on intramolecular electronic communication. The new pincer ligands feature two tridentate <i>NNN</i>- sites each composed of two pyrazolyl flanking donors and a diarylamido anchor that are either directly linked (to form a central benzidene core, H₂(L1)) or linked via a <i>para</i>-phenylene group (to form a <i>para</i>-terphenyldiamine core, H₂(L2)). The bimetallic complexes of the deprotonated ligands, [<i>fac</i>-Re­(CO)₃]₂(μ-L1), <b>1</b>, and [<i>fac</i>-Re­(CO)₃]₂(μ-L2), <b>2</b>, were fully characterized in solution and the solid state including by single-crystal X-ray diffraction for <b>1</b>. The electrochemical properties of each depended strongly on solvent and electrolyte. Complex <b>1</b> exhibits two one-electron oxidations in all electrolyte-containing solutions but with separations between first and second oxidation potentials, Δ<i>E</i>_1/2, between 119 and 316 mV depending on conditions. On the other hand, cyclic voltammetry of <b>2</b> showed one two-electron oxidation in DMF with NBu₄PF₆ as an electrolyte but two one-electron oxidations with a maximal separation in Δ<i>E</i>_1/2 of 96 mV in CH₂Cl₂ with NBu₄B­(C₆F₅)₄ as an electrolyte. The oxidized complexes <b>1</b>^<b>n+</b> and <b>2</b>^<b>n+</b> (<i>n</i> = 1, 2) were prepared by chemical oxidation and were studied spectroscopically (UV–vis/NIR, EPR). The mono-oxidized complex <b>1</b>^<b>+</b> behaves as a Robin–Day Class III species, while <b>2</b>^<b>+</b> is a Robin–Day Class II species that shows thermal valence trapping at 77 K by EPR spectroscopy. As suggested from theoretical studies using DFT methods, the oxidized complexes maintain considerable ligand radical character, so their electronic structures can be formulated as (CO)₃Re^I(μ-L^<i>n</i>+)­Re^I(CO)₃ (<i>n</i> = 1 or 2)

Photochemistry of Furyl- and Thienyldiazomethanes: Spectroscopic Characterization of Triplet 3-Thienylcarbene

Photolysis (λ > 543 nm) of 3-thienyldiazomethane (<b>1</b>), matrix isolated in Ar or N₂ at 10 K, yields triplet 3-thienylcarbene (<b>13</b>) and α-thial-methylenecyclopropene (<b>9</b>). Carbene <b>13</b> was characterized by IR, UV/vis, and EPR spectroscopy. The conformational isomers of 3-thienylcarbene (<i>s</i>-<i>E</i> and <i>s</i>-<i>Z</i>) exhibit an unusually large difference in zero-field splitting parameters in the triplet EPR spectrum (|<i>D</i>/<i>hc</i>| = 0.508 cm^–1, |<i>E</i>/<i>hc</i>| = 0.0554 cm^–1; |<i>D</i>/<i>hc</i>| = 0.579 cm^–1, |<i>E</i>/<i>hc</i>| = 0.0315 cm^–1). Natural Bond Orbital (NBO) calculations reveal substantially differing spin densities in the 3-thienyl ring at the positions adjacent to the carbene center, which is one factor contributing to the large difference in <i>D</i> values. NBO calculations also reveal a stabilizing interaction between the sp orbital of the carbene carbon in the <i>s</i>-<i>Z</i> rotamer of <b>13</b> and the antibonding σ orbital between sulfur and the neighboring carbonan interaction that is not observed in the <i>s</i>-<i>E</i> rotamer of <b>13</b>. In contrast to the EPR spectra, the electronic absorption spectra of the rotamers of triplet 3-thienylcarbene (<b>13</b>) are indistinguishable under our experimental conditions. The carbene exhibits a weak electronic absorption in the visible spectrum (λ_max = 467 nm) that is characteristic of triplet arylcarbenes. Although studies of 2-thienyldiazomethane (<b>2</b>), 3-furyldiazomethane (<b>3</b>), or 2-furyldiazomethane (<b>4</b>) provided further insight into the photochemical interconversions among C₅H₄S or C₅H₄O isomers, these studies did not lead to the spectroscopic detection of the corresponding triplet carbenes (2-thienylcarbene (<b>11</b>), 3-furylcarbene (<b>23</b>), or 2-furylcarbene (<b>22</b>), respectively)

Correlations between the Electronic Properties of <i>Shewanella oneidensis</i> Cytochrome <i>c</i> Nitrite Reductase (ccNiR) and Its Structure: Effects of Heme Oxidation State and Active Site Ligation

The electrochemical properties of <i>Shewanella oneidensis</i> cytochrome <i>c</i> nitrite reductase (ccNiR), a homodimer that contains five hemes per protomer, were investigated by UV–visible and electron paramagnetic resonance (EPR) spectropotentiometries. Global analysis of the UV–vis spectropotentiometric results yielded highly reproducible values for the heme midpoint potentials. These midpoint potential values were then assigned to specific hemes in each protomer (as defined in previous X-ray diffraction studies) by comparing the EPR and UV–vis spectropotentiometric results, taking advantage of the high sensitivity of EPR spectra to the structural microenvironment of paramagnetic centers. Addition of the strong-field ligand cyanide led to a 70 mV positive shift of the active site’s midpoint potential, as the cyanide bound to the initially five-coordinate high-spin heme and triggered a high-spin to low-spin transition. With cyanide present, three of the remaining hemes gave rise to distinctive and readily assignable EPR spectral changes upon reduction, while a fourth was EPR-silent. At high applied potentials, interpretation of the EPR spectra in the absence of cyanide was complicated by a magnetic interaction that appears to involve three of five hemes in each protomer. At lower applied potentials, the spectra recorded in the presence and absence of cyanide were similar, which aided global assignment of the signals. The midpoint potential of the EPR-silent heme could be assigned by default, but the assignment was also confirmed by UV–vis spectropotentiometric analysis of the H268M mutant of ccNiR, in which one of the EPR-silent heme’s histidine axial ligands was replaced with a methionine

Evidence Favoring Molybdenum-Carbon Bond Formation in Xanthine Oxidase Action: 17O- and 13C-ENDOR and Kinetic Studies

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Electronic Communication Across Diamagnetic Metal Bridges: A Homoleptic Gallium(III) Complex of a Redox-Active Diarylamido-Based Ligand and Its Oxidized Derivatives

Complexes with cations of the type [Ga­(L)₂]^<i>n</i>+ where L = bis­(4-methyl-2-(1H-pyrazol-1-yl)­phenyl)­amido and <i>n</i> = 1, 2, 3 have been prepared and structurally characterized. The electronic properties of each were probed by electrochemical and spectroscopic means and were interpreted with the aid of density functional theory (DFT) calculations. The dication, best described as [Ga­(L^–)­(L⁰)]²⁺, is a Robin-Day class II mixed-valence species. As such, a broad, weak, solvent-dependent intervalence charge transfer (IVCT) band was found in the NIR spectrum in the range 6390–6925 cm^–1, depending on the solvent. Band shape analyses and the use of Hush and Marcus relations revealed a modest electronic coupling, <i>H</i>_ab of about 200 cm^–1, and a large rate constant for electron transfer, <i>k</i>_et, on the order of 10¹⁰ s^–1 between redox active ligands. The dioxidized complex [Ga­(L⁰)₂]³⁺ shows a half-field Δ<i>M</i>_s = 2 transition in its solid-state X-band electron paramagnetic resonance (EPR) spectrum at 5 K, which indicates that the triplet state is thermally populated. DFT calculations (M06/Def2-SV­(P)) suggest that the singlet state is 21.7 cm^–1 lower in energy than the triplet state

Electronic Communication Across Diamagnetic Metal Bridges: A Homoleptic Gallium(III) Complex of a Redox-Active Diarylamido-Based Ligand and Its Oxidized Derivatives

Complexes with cations of the type [Ga­(L)₂]^<i>n</i>+ where L = bis­(4-methyl-2-(1H-pyrazol-1-yl)­phenyl)­amido and <i>n</i> = 1, 2, 3 have been prepared and structurally characterized. The electronic properties of each were probed by electrochemical and spectroscopic means and were interpreted with the aid of density functional theory (DFT) calculations. The dication, best described as [Ga­(L^–)­(L⁰)]²⁺, is a Robin-Day class II mixed-valence species. As such, a broad, weak, solvent-dependent intervalence charge transfer (IVCT) band was found in the NIR spectrum in the range 6390–6925 cm^–1, depending on the solvent. Band shape analyses and the use of Hush and Marcus relations revealed a modest electronic coupling, <i>H</i>_ab of about 200 cm^–1, and a large rate constant for electron transfer, <i>k</i>_et, on the order of 10¹⁰ s^–1 between redox active ligands. The dioxidized complex [Ga­(L⁰)₂]³⁺ shows a half-field Δ<i>M</i>_s = 2 transition in its solid-state X-band electron paramagnetic resonance (EPR) spectrum at 5 K, which indicates that the triplet state is thermally populated. DFT calculations (M06/Def2-SV­(P)) suggest that the singlet state is 21.7 cm^–1 lower in energy than the triplet state

<i>Shewanella oneidensis</i> Cytochrome <i>c</i> Nitrite Reductase (ccNiR) Does Not Disproportionate Hydroxylamine to Ammonia and Nitrite, Despite a Strongly Favorable Driving Force

Cytochrome <i>c</i> nitrite reductase (ccNiR) from <i>Shewanella oneidensis</i>, which catalyzes the six-electron reduction of nitrite to ammonia <i>in vivo</i>, was shown to oxidize hydroxylamine in the presence of large quantities of this substrate, yielding nitrite as the sole free nitrogenous product. UV–visible stopped-flow and rapid-freeze-quench electron paramagnetic resonance data, along with product analysis, showed that the equilibrium between hydroxylamine and nitrite is fairly rapidly established in the presence of high initial concentrations of hydroxylamine, despite said equilibrium lying far to the left. By contrast, reduction of hydroxylamine to ammonia did not occur, even though disproportionation of hydroxylamine to yield both nitrite and ammonia is strongly thermodynamically favored. This suggests a kinetic barrier to the ccNiR-catalyzed reduction of hydroxylamine to ammonia. A mechanism for hydroxylamine reduction is proposed in which the hydroxide group is first protonated and released as water, leaving what is formally an NH₂⁺ moiety bound at the heme active site. This species could be a metastable intermediate or a transition state but in either case would exist only if it were stabilized by the donation of electrons from the ccNiR heme pool into the empty nitrogen p orbital. In this scenario, ccNiR does not catalyze disproportionation because the electron-donating hydroxylamine does not poise the enzyme at a sufficiently low potential to stabilize the putative dehydrated hydroxylamine; presumably, a stronger reductant is required for this

Homoleptic Nickel(II) Complexes of Redox-Tunable Pincer-type Ligands

Different synthetic methods have been developed to prepare eight new redox-active pincer-type ligands, H­(X,Y), that have pyrazol-1-yl flanking donors attached to an <i>ortho</i>-position of each ring of a diarylamine anchor and that have different groups, X and Y, at the <i>para</i>-aryl positions. Together with four previously known H­(X,Y) ligands, a series of 12 Ni­(X,Y)₂ complexes were prepared in high yields by a simple one-pot reaction. Six of the 12 derivatives were characterized by single-crystal X-ray diffraction, which showed tetragonally distorted hexacoordinate nickel­(II) centers. The nickel­(II) complexes exhibit two quasi-reversible one-electron oxidation waves in their cyclic voltammograms, with half-wave potentials that varied over a remarkable 700 mV range with the average of the Hammett σ_p parameters of the <i>para</i>-aryl X, Y groups. The one- and two-electron oxidized derivatives [Ni­(Me,Me)₂]­(BF₄)_<i>n</i> (<i>n</i> = 1, 2) were prepared synthetically, were characterized by X-band EPR, electronic spectroscopy, and single-crystal X-ray diffraction (for <i>n</i> = 2), and were studied computationally by DFT methods. The dioxidized complex, [Ni­(Me,Me)₂]­(BF₄)₂, is an <i>S</i> = 2 species, with nickel­(II) bound to two ligand radicals. The mono-oxidized complex [Ni­(Me,Me)₂]­(BF₄), prepared by comproportionation, is best described as nickel­(II) with one ligand centered radical. Neither the mono- nor the dioxidized derivative shows any substantial electronic coupling between the metal and their bound ligand radicals because of the orthogonal nature of their magnetic orbitals. On the other hand, weak electronic communication occurs between ligands in the mono-oxidized complex as evident from the intervalence charge transfer (IVCT) transition found in the near-IR absorption spectrum. Band shape analysis of the IVCT transition allowed comparisons of the strength of the electronic interaction with that in the related, previously known, Robin–Day class II mixed valence complex, [Ga­(Me,Me)₂]²⁺