On the unexpected stability of the dianion of perylene diimide in water--a computational study

It was recently reported (Shirman, J. Phys. Chem. B, 2008, 112, 8855) that the stable dianion of perylene diimide can be prepared in water. Herein, a computational study (using DFT at the M06-2X/6-31++G** level of theory) of this species is presented. It is shown that this dianion is aromatic and that its reaction with water is highly endergonic. The primary cause for this is the stabilization provided by the enhanced aromaticity of the dianion relative to its neutral counterpart. Comparison with other aromatic dianions is also presented.     

An oxidation induced by potassium metal. Studies on the anionic cyclodehydrogenation of 1,1'-binaphthyl to perylene

Oxidative cyclization of 1,1'-binaphthyl (1) to perylene (2) can be achieved in essentially quantitative yield by the action of three or more equivalents of potassium metal in hot tetrahydrofuran. An overall reaction mechanism is proposed that accounts for all of the experimental observations reported by previous investigators and those from the present studies. The trans-6a,6b-dihydroperylene dianion (6(2-)) is believed to be the pivotal intermediate from which H(2) is lost. A radical chain reaction involving free hydrogen atoms (H(?)) in the two-step propagation cycle is proposed to explain the formation of H(2) from 6(2-). Anionic cyclodehydrogenations of this sort are complementary to those performed under strongly acidic/oxidizing conditions, photochemically, or thermally (flash vacuum pyrolysis), and a better understanding of how they occur, together with the optimized synthetic protocol reported here, should encourage their wider use in organic synthesis.     

The cyclooctatriene-eta2-ynyl potassium zwitterionic radical: evidence for a potassium organometallic

Low-temperature (-120 degrees C) dehydrohalogenation of bromocyclooctatetraene (BrC8H7) with either sodium or potassium tert-butoxide followed by alkali metal reduction was used to generate the anion radical of [8]annulyne (C8H6*-) in tetrahydrofuran. EPR analysis at -120 degrees C reveals an extraordinarily large metal splitting when K or Cs (aK of 0.214 G and aCs of 3.26 G) serves as the reducing agent. The large aM is due to the metal cation interacting with the p-orbitals, within the alkyne moiety, that are in the plane of the ring system. The ionic radius of K+ is 1.33 A, which is larger than the B3LYP predicted distance between carbons 1 and 2 (1.23 A). However, the ionic radius of Na+ is only 0.95 A, and it is too small to simultaneously interact with both p-orbitals. Hence, no aM is observed when Na (ordinarily aNa > aK) or Li serves as the reducing agent. After the addition of 18-crown-6 to either the K or the Cs reduced system, two anion radicals are present. One is the system where the 18-crown-6 encapsulated metal complex is normally ion paired over the face of the ring system and aM = 0. The other is the cyclooctatriene-eta2-ynyl 18-crown-6 encapsulated metal zwitterion radical exhibiting a large aM. The ion pair to organometallic equilibrium constant is 1.6 +/- 0.1 and 3.5 +/- 0.1 for the K and Cs systems, respectively.     

Rubidium-mediated birch-type reduction of 1,2-diphenylbenzene in tetrahydrofuran

The reaction of 1,2-diphenylbenzene with rubidium metal in THF yields extremely sensitive and pyrophoric [η(5)-{1,2-diphenyl-2,5-cyclohexadienyl}rubidium](∞) (1). Compound 1 characterizes a possible intermediate in a Birch-type reaction and represents a very rare example of a fully characterized organorubidium complex as well as an open main-group metal pentadienide as part of a six-membered ring. In the solid state the rubidium atoms interact with the cyclohexadienyl moiety, whereas the coordination sphere of the soft cation is additionally stabilized exclusively by several metal π-arene interactions despite the presence of strongly coordinating donors. The bonding situation was elucidated by MP2/def2-TZVPP calculations including population analysis.     

Diindeno[cd:1m]perylene dianion. Local vs. peripheral contributions to the aromatic character

Diindeno[cd;lm]perylene ($\text{1}$) is shown to undergo a two electron reduction process to its dianion $\text{2}$. Both neutral and doubly charged systems exhibit an enhanced diamagnetic character which is believed to be acquired via two different mechanisms.     

Chemoselective reduction of aldehydes with zinc borohydride in tetrahydrofuran

A variety of structurally different aldehydes undergo chemoselectire reduction over ketones with zinc borohydride in tetrahydrofuran at −10°C to the corresponding alcohols.     

Reactivity in the upper limits of the reduction potential in solution: arene dianion intermolecular carbolithiation of alkenes

Lithium salts of dianions derived from arenes of high reduction potential (biphenyl, naphthalene) can carbometallate terminal alkenes (propene, isobutene) in an intermolecular fashion, affording partially dearomatized alkylated aryl anions, which are susceptible to further functionalization by electrophilic capture. This form of reactivity, specific of the arene dianion, deviates from the typical alkali metal-like reactivity displayed by these complexes, affording in most cases regio- and stereocontrolled products. Simple semiempirical calculations (PM3) help predicting the regiochemical outcome of this reaction, where some of the most inexpensive organic starting materials are involved.     

An ambient stable core-substituted perylene bisimide dianion: isolation and single crystal structure analysis

Here we report the first example of an isolable, ambient stable perylene bisimide (PBI) dianion which was synthesized by catalytic reduction of a highly electron deficient PBI derivative. The remarkable stability of this unprecedented dianion in air for months facilitated its complete characterization by different methods, including single crystal X-ray analysis. Furthermore, solvent dependent cyclic and square wave voltammetry studies revealed that the formation of PBI dianions is preferred in more polar solvents, whereas the generation of PBI radical anions should be favoured in less polar solvents.     

Preparation and decomposition of potassium alkalide-lipophilic crown ether complexes in tetrahydrofuran

Cyclohexano-15-crown-5, cyclohexano-18-crown-6, dicyclohexano-15-crown-5, and dicyclohexano-18-crown-6, but not dicylohexano-16-crown-5, in THF dissolve potassium metal to form dark blue potassium alkalide solutions at ambient temperature. On standing, the potassium alkalide complexes decompose and the solutions turn colorless at differing rates. Identification of the products provides insight into the decomposition mechanism.     

Molten potassium pyrosulfate: The reactions of seven metal oxalates

The reactions of Li₂C₂O₄, Na₂C₂O₄, K₂C₂·H₂O, CaC₂·H₂O, ZnC₂O₄, La₂(C₂O₄)₃ and K₂TiO(C₂O₄)₂· 2H₂O with K₂S₂O₇ were investigated using thermal methods of analysis. Reaction products were analysed by various techniques. It was found that anhydrous oxalates reacted with K₂S₂O₇, evolving a mixture of CO₂ and CO with the formation of K₂SO₄ and the corresponding metal sulfates, which, in the reactions of ZnC₂O₄ and K₂TiO(C₂O₄)₂ 2H₂O, probably existed as K₂[Zn(SO₄)₂] and K₄[Ti(SO₄)₄], respectively. Water was found to be an additional product in the hydrated metal oxalate reactions. The stoichiometries of these reactions have been established from the thermogravimetric and acidimetric results.     

Spectroscopic evidence for the reduction of alkyl halides by metal hydrides via a single electron transfer mechanism

This report provides spectroscopic evidence to support a single electron transfer pathway to describe the reaction of metal hydrides with alkyl halides by direct EPR observation of the radical formed in the reaction.     

Rapid reduction of carboxylic acid salts with borane in tetrahydrofuran.

Carboxylic acid salts are rapidly reduced to the corresponding alcohols with two molar equivalents of borane in THF. A possible mechanism via acyloxyborane is presented.     

Metal-organic frameworks for electrochemical reduction of carbon dioxide: The role of metal centers

Direct electrochemical reduction of CO_2 into valuable chemicals and fuel is one of the most promising approaches to address the current energy crisis and lower CO_2 emission. Recently, numerous metal-organic framework(MOF) and their derived materials have extensively been developed as electrocatalysts for CO_2 reduction owing to their unique structure including porosity, large specific surface area, and tunable chemical structures. In this review, the recent progress of MOF-based electrocatalysts for CO_2 reduction was summarized and discussed. Detailed discussions mainly focus on the synthesis and mechanism of pristine MOFs and MOF-derived materials for electrocatalytic CO_2 reduction. These examples are expected to provide clues to rational design and synthesis of stable and high-performance MOFs-based electrocatalysts for CO_2 reduction.