Privileged Role of Thiolate as the Axial Ligand in Hydrogen Atom Transfer Reactions by Oxoiron(IV) Complexes in Shaping the Potential Energy Surface and Inducing Significant H‑Atom Tunneling

An H/D kinetic isotope effect (KIE) of 80 is found at −20 °C for the oxidation of 9,10-dihydro­anthracene by [Fe^IV(O)­(TMCS)]⁺, a complex supported by the tetramethylcyclam (TMC) macrocycle with a tethered thiolate. This KIE value approaches that previously predicted by DFT calculations. Other [Fe^IV(O)­(TMC)­(anion)] complexes exhibit values of 20, suggesting that the thiolate ligand of [Fe^IV(O)­(TMCS)]⁺ plays a unique role in facilitating tunneling. Calculations show that tunneling is most enhanced (a) when the bond asymmetry between C–H bond breaking and O–H bond formation in the transition state is minimized, and (b) when the electrostatic interactions in the O---H---C moiety are maximal. These two factorswhich peak for the best electron donor, the thiolate ligandafford a slim and narrow barrier through which the H-atom can tunnel most effectively

Characterization of the Fleeting Hydroxoiron(III) Complex of the Pentadentate TMC-py Ligand

Nonheme mononuclear hydroxoiron­(III) species are important intermediates in biological oxidations, but well-characterized examples of synthetic complexes are scarce due to their instability or tendency to form μ-oxodiiron­(III) complexes, which are the thermodynamic sink for such chemistry. Herein, we report the successful stabilization and characterization of a mononuclear hydroxoiron­(III) complex, [Fe^III(OH)­(TMC-py)]²⁺ (<b>3</b>; TMC-py = 1<i>-</i>(pyridyl-2′-methyl)-4,8,11-trimethyl-1,4,8,11-tetrazacyclotetradecane), which is directly generated from the reaction of [Fe^IV(O)­(TMC-py)]²⁺ (<b>2</b>) with 1,4-cyclohexadiene at −40 °C by H-atom abstraction. Complex <b>3</b> exhibits a UV spectrum with a λ_max at 335 nm (ε ≈ 3500 M^–1 cm^–1) and a molecular ion in its electrospray ionization mass spectrum at <i>m</i>/<i>z</i> 555 with an isotope distribution pattern consistent with its formulation. Electron paramagnetic resonance and Mössbauer spectroscopy show <b>3</b> to be a high-spin Fe­(III) center that is formed in 85% yield. Extended X-ray absorption fine structure analysis reveals an Fe–OH bond distance of 1.84 Å, which is also found in [(TMC-py)­Fe^III–O–Cr^III(OTf)₃]⁺ (<b>4</b>) obtained from the reaction of <b>2</b> with Cr­(OTf)₂. The <i>S</i> = 5/2 spin ground state and the 1.84 Å Fe–OH bond distance are supported computationally. Complex <b>3</b> reacts with 1-hydroxy-2,2,6,6-tetramethylpiperidine (TEMPOH) at −40 °C with a second-order rate constant of 7.1 M^–1 s^–1 and an OH/OD kinetic isotope effect value of 6. On the basis of density functional theory calculations, the reaction between <b>3</b> and TEMPOH is classified as a proton-coupled electron transfer as opposed to a hydrogen-atom transfer

Ruthenium Complexes of Thiaporphyrin and Dithiaporphyrin

Successful synthesis and characterization of the six-coordinated complex [Ru­(STTP)­(CO)­Cl] (<b>1</b>; STTP = 5,10,15,20-tetratolyl-21-thiaporphyrinato) allowed the development of the coordination chemistry of ruthenium–thiaporphyrin through dechlorination and metathesis reactions. Accordingly, [Ru^II(STTP)­(CO)­X] (X = NO₃^<b>–</b> (<b>2</b>), NO₂^<b>–</b> (<b>3</b>), and N₃^<b>–</b> (<b>4</b>)) was synthesized and analyzed by single-crystal X-ray structural determination and NMR, UV–vis, and FT-IR spectroscopic methods. An independent reaction of STPPH and [Ru­(COD)­Cl₂] led to [Ru^III(STTP)­Cl₂] (<b>5</b>), which possessed a higher-valent Ru­(III) center and exhibited good stability in the solution state. This stability allowed reversible redox processes in a cyclic voltammetric study. Reactions of [Ru­(S₂TTP)­Cl₂] (S₂TTP = 5,10,15,20-tetratolyl-21,23-dithiaporphyrinato) with AgNO₃ and NaSePh, also via the metathesis strategy, resulted in novel dithiaporphyrin complexes [Ru^II(S₂TTP)­(NO₃)₂] (<b>6</b>) and [Ru⁰(S₂TTP)­(PhSeCH₂SePh)₂] (<b>7</b>), respectively. The structures of <b>6</b> and <b>7</b> were corroborated by X-ray crystallographic analyses. Complex <b>7</b> is an unprecedented ruthenium(0)–dithiaporphyrin with two <i>bis</i>(phenylseleno)­methanes as axial ligands. A comparison of the analyses of the crude products from reactions of NaSePh and CH₂Cl₂ with or without [Ru­(S₂TTP)­Cl₂], further supported by UV–vis spectral changes under stoichiometric reactions between [Ru­(S₂TTP)­Cl₂] and NaSePh, suggested a reaction sequence in the order of (1) formation of a putative [Ru^II(S₂TTP)­(SePh)₂] intermediate, followed by (2) the concerted formation of PhSe–CH₂Cl and simultaneously a reduction of Ru­(II) to Ru(0) and finally (3) nucleophilic substitution of PhSeCH₂Cl by excess PhSe^–, resulting in PhSeCH₂SePh, which readily coordinated to the Ru(0) and completed the formation of <i>bis</i>(phenylseleno)­methane complex <b>7</b>

Ruthenium Complexes of Thiaporphyrin and Dithiaporphyrin

Successful synthesis and characterization of the six-coordinated complex [Ru­(STTP)­(CO)­Cl] (<b>1</b>; STTP = 5,10,15,20-tetratolyl-21-thiaporphyrinato) allowed the development of the coordination chemistry of ruthenium–thiaporphyrin through dechlorination and metathesis reactions. Accordingly, [Ru^II(STTP)­(CO)­X] (X = NO₃^<b>–</b> (<b>2</b>), NO₂^<b>–</b> (<b>3</b>), and N₃^<b>–</b> (<b>4</b>)) was synthesized and analyzed by single-crystal X-ray structural determination and NMR, UV–vis, and FT-IR spectroscopic methods. An independent reaction of STPPH and [Ru­(COD)­Cl₂] led to [Ru^III(STTP)­Cl₂] (<b>5</b>), which possessed a higher-valent Ru­(III) center and exhibited good stability in the solution state. This stability allowed reversible redox processes in a cyclic voltammetric study. Reactions of [Ru­(S₂TTP)­Cl₂] (S₂TTP = 5,10,15,20-tetratolyl-21,23-dithiaporphyrinato) with AgNO₃ and NaSePh, also via the metathesis strategy, resulted in novel dithiaporphyrin complexes [Ru^II(S₂TTP)­(NO₃)₂] (<b>6</b>) and [Ru⁰(S₂TTP)­(PhSeCH₂SePh)₂] (<b>7</b>), respectively. The structures of <b>6</b> and <b>7</b> were corroborated by X-ray crystallographic analyses. Complex <b>7</b> is an unprecedented ruthenium(0)–dithiaporphyrin with two <i>bis</i>(phenylseleno)­methanes as axial ligands. A comparison of the analyses of the crude products from reactions of NaSePh and CH₂Cl₂ with or without [Ru­(S₂TTP)­Cl₂], further supported by UV–vis spectral changes under stoichiometric reactions between [Ru­(S₂TTP)­Cl₂] and NaSePh, suggested a reaction sequence in the order of (1) formation of a putative [Ru^II(S₂TTP)­(SePh)₂] intermediate, followed by (2) the concerted formation of PhSe–CH₂Cl and simultaneously a reduction of Ru­(II) to Ru(0) and finally (3) nucleophilic substitution of PhSeCH₂Cl by excess PhSe^–, resulting in PhSeCH₂SePh, which readily coordinated to the Ru(0) and completed the formation of <i>bis</i>(phenylseleno)­methane complex <b>7</b>