Swallow-Tailed Alkyl and Linear Alkoxy-Substituted Dibenzocoronene Tetracarboxdiimide Derivatives: Synthesis, Photophysical Properties, and Thermotropic Behaviors

A series of dibenzocoronene tetracarboxdiimide derivatives decorated with alkyl swallow-tail and alkoxy moieties were synthesized, and their structures were characterized. 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) as an effective oxidant was first used in the benzannulation of perylene diimides with the almost quantitative yield. The thermotropic behavior was investigated using differential scanning calorimetry (DSC) and polarization optical microscopy (POM). The introduction of alkyl swallow-tail and alkoxy substituents facilitates thermotropic liquid crystalline behavior. The branching site of alkyl swallow-tail units at the α position and the longer alkoxy chains played a similar role in lowering the mesophase transition as well as isotropization transition temperatures. The UV–vis absorption spectra of all compounds appeared as absorption in 425–600 nm region, and POM images of certain compounds exhibited characteristic columnar hexagonal (Col_h) packing and readily self-assembled into a homeotropic alignment toward the substrate

New Reactions of Terminal Hydrides on a Diiron Dithiolate

Mechanisms for biological and bioinspired dihydrogen activation and production often invoke the intermediacy of diiron dithiolato dihydrides. The first example of such a Fe₂(SR)₂H₂ species is provided by the complex [(<i>term</i>-H)­(μ-H)­Fe₂(pdt)­(CO)­(dppv)₂] ([H<b>1</b>H]⁰). Spectroscopic and computational studies indicate that [H<b>1</b>H]⁰ contains both a bridging hydride and a terminal hydride, which, notably, occupies a basal site. The synthesis begins with [(μ-H)­Fe₂(pdt)­(CO)₂(dppv)₂]⁺ ([H<b>1</b>(CO)]⁺), which undergoes substitution to afford [(μ-H)­Fe₂(pdt)­(CO)­(NCMe)­(dppv)₂]⁺ ([H<b>1</b>(NCMe)]⁺). Upon treatment of [H<b>1</b>(NCMe)]⁺ with borohydride salts, the MeCN ligand is displaced to afford [H<b>1</b>H]⁰. DNMR (EXSY, SST) experiments on this complex show that the terminal and bridging hydride ligands interchange intramolecularly at a rate of 1 s^–1 at −40 °C. The compound reacts with D₂ to afford [D<b>1</b>D]⁰, but not mixed isotopomers such as [H<b>1</b>D]⁰. The dihydride undergoes oxidation with Fc⁺ under CO to give [<b>1</b>(CO)]⁺ and H₂. Protonation in MeCN solution gives [H<b>1</b>(NCMe)]⁺ and H₂. Carbonylation converts [H<b>1</b>H]⁰ into [<b>1</b>(CO)]⁰

Unsensitized Photochemical Hydrogen Production Catalyzed by Diiron Hydrides

The diiron hydride [(μ-H)­Fe₂(pdt)­(CO)₄(dppv)]⁺ ([H<b>2</b>]⁺, dppv = <i>cis</i>-1,2-C₂H₂(PPh₂)₂) is shown to be an effective photocatalyst for the H₂ evolution reaction (HER). These experiments establish the role of hydrides in photocatalysis by biomimetic diiron complexes. Trends in redox potentials suggests that other unsymmetrically substituted diiron hydrides are promising catalysts. Unlike previous catalysts for photo-HER, [H<b>2</b>]⁺ functions without sensitizers: irradiation of [H<b>2</b>]⁺ in the presence of triflic acid (HOTf) efficiently affords H₂. Instead of sacrificial electron donors, ferrocenes can be used as recyclable electron donors for the photocatalyzed HER, resulting in 4 turnovers

Isolation of a Mixed Valence Diiron Hydride: Evidence for a Spectator Hydride in Hydrogen Evolution Catalysis

The mixed-valence diiron hydrido complex (μ-H)­Fe₂(pdt)­(CO)₂(dppv)₂ ([H<b>1</b>]⁰, where pdt =1,3-propanedithiolate and dppv = <i>cis</i>-1,2-C₂H₂(PPh₂)₂), was generated by reduction of the differous hydride [H<b>1</b>]⁺ using decamethylcobaltocene. Crystallographic analysis shows that [H<b>1</b>]⁰ retains the stereochemistry of its precursor, where one dppv ligand spans two basal sites and the other spans apical and basal positions. The Fe---Fe bond elongates to 2.80 from 2.66 Å, but the Fe–P bonds only change subtly. Although the Fe–H distances are indistinguishable in the precursor, they differ by 0.2 Å in [H<b>1</b>]⁰. The X-band electron paramagnetic resonance (EPR) spectrum reveals the presence of two stereoisomers, the one characterized crystallographically and a contribution of about 10% from a second symmetrical (<i>sym</i>) isomer wherein both dppv ligands occupy apical–basal sites. The unsymmetrical (<i>unsym</i>) arrangement of the dppv ligands is reflected in the values of <i>A</i>(³¹P), which range from 31 MHz for the basal phosphines to 284 MHz for the apical phosphine. Density functional theory calculations were employed to rationalize the electronic structure of [H<b>1</b>]⁰ and to facilitate spectral simulation and assignment of EPR parameters including ¹H and ³¹P hyperfine couplings. The EPR spectra of [H<b>1</b>]⁰ and [D<b>1</b>]⁰ demonstrate that the singly occupied molecular orbital is primarily localized on the Fe center with the longer bond to H, that is, Fe^II–H···Fe^I. The coupling to the hydride is <i>A</i>(¹H) = 55 and 74 MHz for <i>unsym</i>- amd <i>sym</i>-[H<b>1</b>]⁰, respectively. Treatment of [H<b>1</b>]⁰ with H⁺ gives 0.5 equiv of H₂ and [H<b>1</b>]⁺. Reduction of D⁺ affords D₂, leaving the hydride ligand intact. These experiments demonstrate that the bridging hydride ligand in this complex is a spectator in the hydrogen evolution reaction

Computational Investigation of [FeFe]-Hydrogenase Models: Characterization of Singly and Doubly Protonated Intermediates and Mechanistic Insights

The [FeFe]-hydrogenase enzymes catalyze hydrogen oxidation and production efficiently with binuclear Fe metal centers. Recently the bioinspired H₂-producing model system Fe₂(adt)­(CO)₂(dppv)₂ (adt=azadithiolate and dppv=diphosphine) was synthesized and studied experimentally. In this system, the azadithiolate bridge facilitates the formation of a doubly protonated ammonium-hydride species through a proton relay. Herein computational methods are utilized to examine this system in the various oxidation states and protonation states along proposed mechanistic pathways for H₂ production. The calculated results agree well with the experimental data for the geometries, CO vibrational stretching frequencies, and reduction potentials. The calculations illustrate that the NH···HFe dihydrogen bonding distance in the doubly protonated species is highly sensitive to the effects of ion-pairing between the ammonium and BF₄^– counterions, which are present in the crystal structure, in that the inclusion of BF₄^– counterions leads to a significantly longer dihydrogen bond. The non-hydride Fe center was found to be the site of reduction for terminal hydride species and unsymmetric bridging hydride species, whereas the reduced symmetric bridging hydride species exhibited spin delocalization between the Fe centers. According to both experimental measurements and theoretical calculations of the relative p<i>K</i>_a values, the Fe_d center of the neutral species is more basic than the amine, and the bridging hydride species is more thermodynamically stable than the terminal hydride species. The calculations implicate a possible pathway for H₂ evolution that involves an intermediate with H₂ weakly bonded to one Fe, a short H₂ distance similar to the molecular bond length, the spin density delocalized over the two Fe centers, and a nearly symmetrically bridged CO ligand. Overall, this study illustrates the mechanistic roles of the ammonium-hydride interaction, flexibility of the bridging CO ligand, and intramolecular electron transfer between the Fe centers in the catalytic cycle. Such insights will assist in the design of more effective bioinspired catalysts for H₂ production

Iron-Catalyzed 1,2-Selective Hydroboration of <i>N</i>‑Heteroarenes

A N₂-bridged diiron complex [Cp*­(Ph₂PC₆­H₄S)­Fe]₂­(μ-N₂) (<b>1</b>) has been found to catalyze the hydroboration of <i>N</i>-heteroarenes with pinacolborane, giving <i>N</i>-borylated 1,2-reduced products with high regioselectivity. The catalysis is initiated by coordination of <i>N</i>-heteroarenes to the iron center, while the B–H bond cleavage is the rate-determining step

Hydride Transfer from Iron(II) Hydride Compounds to NAD(P)⁺ Analogues

Iron­(II) hydride complexes of the “piano-stool” type, Cp*­(P-P)­FeH (P-P = dppe (<b>1H</b>), dppbz (<b>2H</b>), dppm (<b>3H</b>), dcpe (<b>4H</b>)) were examined as hydride donors in the reduction of <i>N</i>-benzylpyridinium and acridinium salts. Two pathways of hydride transfer, “single-step H^–” transfer to pyridinium and a “two-step (e^–/H^•)” transfer for acridinium reduction, were observed. When 1-benzylnicotinamide cation (BNA⁺) was used as an H^– acceptor, kinetic studies suggested that <b>BNA</b>^<b>+</b> was reduced at the C6 position, affording 1,6-BNAH, which can be converted to the more thermally stable 1,4-product. The rate constant <i>k</i> of H^– transfer was very sensitive to the bite angle of P–Fe–P in Cp*­(P-P)­FeH and ranged from 3.23 × 10^–3 M^–1 s^–1 for dppe to 1.74 × 10^–1 M^–1 s^–1 for dppm. The results obtained from reduction of a range of <i>N</i>-benzylpyridinium derivatives suggest that H^– transfer is more likely to be charge controlled. In the reduction of 10-methylacridinium ion (<b>Acr</b>^<b>+</b>), the reaction was initiated by an e^– transfer (ET) process and then followed by rapid disproportionation reactions to produce <b>Acr</b>_<b>2</b> dimer and release of H₂. To achieve H^• transfer after ET from [Cp*­(P-P)­FeH]⁺ to acridine radicals, the bulkier acridinium salt 9-phenyl-10-methylacridinium (<b>PhAcr</b>^<b>+</b>) was selected as an acceptor. More evidence for this “two-step (e^–/H^•)” process was derived from the characterization of <b>PhAcr^•</b> and [<b>4H</b>]^<b>+</b> radicals by EPR spectra and by the crystallographic structure confirmation of [<b>4H</b>]^<b>+</b>. Our mechanistic understanding of fundamental H^– transfer from iron­(II) hydrides to NAD⁺ analogues provides insight into establishing attractive bio-organometallic transformation cycles driven by iron catalysis

Hydride Transfer from Iron(II) Hydride Compounds to NAD(P)⁺ Analogues

Iron­(II) hydride complexes of the “piano-stool” type, Cp*­(P-P)­FeH (P-P = dppe (<b>1H</b>), dppbz (<b>2H</b>), dppm (<b>3H</b>), dcpe (<b>4H</b>)) were examined as hydride donors in the reduction of <i>N</i>-benzylpyridinium and acridinium salts. Two pathways of hydride transfer, “single-step H^–” transfer to pyridinium and a “two-step (e^–/H^•)” transfer for acridinium reduction, were observed. When 1-benzylnicotinamide cation (BNA⁺) was used as an H^– acceptor, kinetic studies suggested that <b>BNA</b>^<b>+</b> was reduced at the C6 position, affording 1,6-BNAH, which can be converted to the more thermally stable 1,4-product. The rate constant <i>k</i> of H^– transfer was very sensitive to the bite angle of P–Fe–P in Cp*­(P-P)­FeH and ranged from 3.23 × 10^–3 M^–1 s^–1 for dppe to 1.74 × 10^–1 M^–1 s^–1 for dppm. The results obtained from reduction of a range of <i>N</i>-benzylpyridinium derivatives suggest that H^– transfer is more likely to be charge controlled. In the reduction of 10-methylacridinium ion (<b>Acr</b>^<b>+</b>), the reaction was initiated by an e^– transfer (ET) process and then followed by rapid disproportionation reactions to produce <b>Acr</b>_<b>2</b> dimer and release of H₂. To achieve H^• transfer after ET from [Cp*­(P-P)­FeH]⁺ to acridine radicals, the bulkier acridinium salt 9-phenyl-10-methylacridinium (<b>PhAcr</b>^<b>+</b>) was selected as an acceptor. More evidence for this “two-step (e^–/H^•)” process was derived from the characterization of <b>PhAcr^•</b> and [<b>4H</b>]^<b>+</b> radicals by EPR spectra and by the crystallographic structure confirmation of [<b>4H</b>]^<b>+</b>. Our mechanistic understanding of fundamental H^– transfer from iron­(II) hydrides to NAD⁺ analogues provides insight into establishing attractive bio-organometallic transformation cycles driven by iron catalysis

Classification of imaging results based on the WHO/IWG-E classification of cystic echinococcosis (n = 27).

<p>Classification of imaging results based on the WHO/IWG-E classification of cystic echinococcosis (n = 27).</p