Electronic Structure and Transformation of Dinitrosyl Iron Complexes (DNICs) Regulated by Redox Non-Innocent Imino-Substituted Phenoxide Ligand

The coupled NO-vibrational peaks [IR νNO 1775 s, 1716 vs, 1668 vs cm–1 (THF)] between two adjacent [Fe(NO)2] groups implicate the electron delocalization nature of the singly O-phenoxide-bridged dinuclear dinitrosyliron complex (DNIC) [Fe(NO)2(μ-ON2Me)Fe(NO)2] (1). Electronic interplay between [Fe(NO)2] units and [ON2Me]− ligand in DNIC 1 rationalizes that “hard” O-phenoxide moiety polarizes iron center(s) of [Fe(NO)2] unit(s) to enforce a “constrained” π-conjugation system acting as an electron reservoir to bestow the spin-frustrated {Fe(NO)2}9-{Fe(NO)2}9-[·ON2Me]2– electron configuration (Stotal = 1/2). This system plays a crucial role in facilitating the ligand-based redox interconversion, working in harmony to control the storage and redox-triggered transport of the [Fe(NO)2]10 unit, while preserving the {Fe(NO)2}9 core in DNICs {Fe(NO)2}9-[·ON2Me]2– [K-18-crown-6-ether)][(ON2Me)Fe(NO)2] (2) and {Fe(NO)2}9-[·ON2Me] [(ON2Me)Fe(NO)2][PF6] (3). Electrochemical studies suggest that the redox interconversion among [{Fe(NO)2}9-[·ON2Me]2–] DNIC 3 ↔ [{Fe(NO)2}9-[ON2Me]−] ↔ [{Fe(NO)2}9-[·ON2Me]] DNIC 2 are kinetically feasible, corroborated by the redox shuttle between O-bridged dimerized [(μ-ONMe)2Fe2(NO)4] (4) and [K-18-crown-6-ether)][(ONMe)Fe(NO)2] (5). In parallel with this finding, the electronic structures of [{Fe(NO)2}9-{Fe(NO)2}9-[·ON2Me]2–] DNIC 1, [{Fe(NO)2}9-[·ON2Me]2–] DNIC 2, [{Fe(NO)2}9-[·ON2Me]] DNIC 3, [{Fe(NO)2}9-[ONMe]−]2 DNIC 4, and [{Fe(NO)2}9-[·ONMe]2–] DNIC 5 are evidenced by EPR, SQUID, and Fe K-edge pre-edge analyses, respectively

Design and Synthesis of Cycloplatinated Polymer Dots as Photocatalysts for Visible-Light-Driven Hydrogen Evolution

By mimicking natural photosynthesis, generating hydrogen through visible-light-driven splitting of water would be an almost ideal process for converting abundant solar energy into a usable fuel in an environmentally friendly and high-energy-density manner. In a search for efficient photocatalysts that mimic such a function, here we describe a series of cycloplatinated polymer dots (Pdots), in which the platinum complex unit is presynthesized as a comonomer and then covalently linked to a conjugated polymer backbone through Suzuki–Miyaura cross-coupling polymerization. On the basis of our design strategy, the hydrogen evolution rate (HER) of the cycloplatinated Pdots can be enhanced by 12 times in comparison to that of pristine Pdots under otherwise identical conditions. In comparison to the Pt-complex-blended counterpart Pdots, the HER of cycloplatinated Pdots is over 2 times higher than that of physically blended Pdots. Furthermore, enhancement of the photocatalytic reaction time with high eventual hydrogen production and low efficiency rolloff are observed by utilizing the cycloplatinated Pdots as photocatalysts. On the basis of their performance, our cyclometallic Pdot systems appear to be alternative types of promising photocatalysts for visible-light-driven hydrogen evolution

{Fe(NO)₂}⁹ Dinitrosyl Iron Complex Acting as a Vehicle for the NO Radical

To carry and deliver nitric oxide with a controlled redox state and rate is crucial for its pharmaceutical/medicinal applications. In this study, the capability of cationic {Fe­(NO)₂}⁹ dinitrosyl iron complexes (DNICs) [(^RDDB)­Fe­(NO)₂]⁺ (R = Me, Et, Iso; ^RDDB = <i>N,N</i>′-bis­(2,6-dialkylphenyl)-1,4-diaza-2,3-dimethyl-1,3-butadiene) carrying nearly unperturbed nitric oxide radical to form [(^RDDB)­Fe­(NO)₂(^•NO)]⁺ was demonstrated and characterized by IR, UV–vis, EPR, NMR, and single-crystal X-ray diffractions. The unique triplet ground state of [(^RDDB)­Fe­(NO)₂(^•NO)]⁺ results from the ferromagnetic coupling between two strictly orthogonal orbitals, one from Fe d_<i>z</i>² and the other a π*_op orbital of a unique bent axial NO ligand, which is responsible for the growth of a half-field transition (Δ<i>M</i>_S = 2) from 70 to 4 K in variable-temperature EPR measurements. Consistent with the NO radical character of coordinated axial NO ligand in complex [(^MeDDB)­Fe­(NO)₂(^•NO)]⁺, the simple addition of MeCN/H₂O into CH₂Cl₂ solution of complexes [(^RDDB)­Fe­(NO)₂(^•NO)]⁺ at 25 °C released NO as a neutral radical, as demonstrated by the formation of [S₅Fe­(NO)₂]⁻ from [S₅Fe­(μ-S)₂FeS₅]^2–

Spectroscopic Definition of the Copper Active Sites in Mordenite: Selective Methane Oxidation

Two distinct [Cu–O–Cu]²⁺ sites with methane monooxygenase activity are identified in the zeolite Cu-MOR, emphasizing that this Cu–O–Cu active site geometry, having a ∠Cu–O–Cu ∼140°, is particularly formed and stabilized in zeolite topologies. Whereas in ZSM-5 a similar [Cu–O–Cu]²⁺ active site is located in the intersection of the two 10 membered rings, Cu-MOR provides two distinct local structures, situated in the 8 membered ring windows of the side pockets. Despite their structural similarity, as ascertained by electronic absorption and resonance Raman spectroscopy, the two Cu–O–Cu active sites in Cu-MOR clearly show different kinetic behaviors in selective methane oxidation. This difference in reactivity is too large to be ascribed to subtle differences in the ground states of the Cu–O–Cu sites, indicating the zeolite lattice tunes their reactivity through second-sphere effects. The MOR lattice is therefore functionally analogous to the active site pocket of a metalloenzyme, demonstrating that both the active site and its framework environment contribute to and direct reactivity in transition metal ion-zeolites

A Structurally Characterized Nonheme Cobalt–Hydroperoxo Complex Derived from Its Superoxo Intermediate via Hydrogen Atom Abstraction

Bubbling O₂ into a THF solution of Co^II(BDPP) (<b>1</b>) at −90 °C generates an O₂ adduct, Co­(BDPP)­(O₂) (<b>3</b>). The resonance Raman and EPR investigations reveal that <b>3</b> contains a low spin cobalt­(III) ion bound to a superoxo ligand. Significantly, at −90 °C, <b>3</b> can react with 2,2,6,6-tetramethyl-1-hydroxypiperidine (TEMPOH) to form a structurally characterized cobalt­(III)-hydroperoxo complex, Co^III(BDPP)­(OOH) (<b>4</b>) and TEMPO^•. Our findings show that cobalt­(III)-superoxo species are capable of performing hydrogen atom abstraction processes. Such a stepwise O₂-activating process helps to rationalize cobalt-catalyzed aerobic oxidations and sheds light on the possible mechanism of action for Co-bleomycin