One- and two-dimensional coordination networks of the tetracyanoethylene anion-radicals with potassium counter-ions

Potassium-mirror reduction of tetracyanoethylene (TCNE) acceptor in tetrahydrofuran affords K(THF)₂ TCNE salt (1) showing double TCNE/K chains assembled via unusual μ₃-TCNE-bridging of potassium cations. These parallel ladder-type chains are further tethered by pairs of THF bridges between potassium centers and by intermolecular π-bonding in (TCNE)₂^2? dimers, and this results in formation of quasi-2-D coordination networks. In the presence of crown-ether ligand, the same potassium-mirror reduction lead to formation of [K(18-crown-6)(THF)₂]TCNE salt (2) in which monomeric tetracyanoethylene anion-radicals are positioned between bulky [K⁺(18-crown-6)(THF)₂] counter-ions. In comparison, crystallization of tetracyanoethylene anion-radicals with K⁺(18-crown-6) counter-ions in dichloromethane affords K(18-crown-6)TCNE salt (3) consisting of 1-D chains with 1,2-(N,N’)-TCNE bindings of potassium cations (nested in the crown-ether cavities). Temperature-dependent magnetic susceptibility study revealed essentially isolated tetracyanoethylene anion-radicals (S = 1/2) in this 1-D coordination polymer.     

Electron transfer versus nucleophilic pathways in the ion-pair annihilation of organoborate anions by carbonylmanganese(I) cations

Substituted carbonylmanganese cations [Mn(CO)₅L]⁺, where L=py, PPh₃ and PPh₂Me, readily react with various organoborate anions (tetramethylborate, methyltriphenylborate and tetraphenylborate) in THF solution to afford a mixture of dimanganese carbonyls, hydridomanganese carbonyls and alkylmanganese carbonyls. The formation of the dimanganese carbonyl dimers as well as the hydridomanganese carbonyls suggests the involvement of 19-electron carbonylmanganese radicals that stem from an initial electron transfer. On the other hand, the acetonitrile-substituted analogue [Mn(CO)₅(CH₃CN)]⁺ reacts with the same borate anions to afford the alkylated RMn(CO)₅, where R=CH₃ and C₆H₅, as the sole carbonylmanganese product. As such, this alkylative annihilation is best formulated as a direct attack on the carbonyl carbon by the borate nucleophile. The two different pathways can be understood in terms of the balance between the electrophilicity of the carbonyl ligand and the electron affinity of the carbonylmanganese cation.     

Ruthenium-catalyzed conversion of sp3 C-O bonds in ethers to C-C bonds using triarylboroxines

Catalytic conversion of unreactive sp(3) C-O bonds in alkyl ethers to C-C bonds is described. Alkyl ethers bearing 2- or 4-pyridyl groups were coupled with triarylboroxines in the presence of a ruthenium catalyst. Triarylboroxines bearing a variety of functional groups including electron-withdrawing and -donating groups can be used for the reaction. No additional base was required for the coupling with the organoboron reagents, and base-sensitive groups can be tolerated. The reaction is considered to proceed via dehydroalkoxylation followed by addition of triarylboroxines to form C-C bonds.     

Molecular aggregation of solid aromatic polymers. I. Small-angle x-ray scattering from aromatic polyimide film

Molecular aggregation in a commercial polyimide film, Du Pont Kapton, was investigated by small-angle x-ray scattering (SAXS). From the analysis of the desmeared SAXS curve, it is concluded that aggregation in the Kapton film can be elucidated in terms of a two-phase structure having electron density fluctuations within the phases. For comparison with the molecular aggregation in Kapton, molecular aggregation in polyimides synthesized in our laboratory was also investigated. It was found in this case that molecular aggregation is controlled by the initial imidization temperature. Molecular aggregation of polyamic acid and polyimide cyclized at a low temperature gives amorphous structures. On the other hand, molecular aggregation of polyimide cyclized at high temperatures gives two-phase structures like that of Kapton film. The SAXS curve for a polyimide having the two-phase structure shows a peak due to interference between ordered regions. The two-phase structure of the polyimide can be explained in terms of a one-dimensional model. The more ordered phase is produced at the higher initial imidization temperature. The relative density difference between two phases is only a few percent for polyimide films cyclized at high temperatures. This result shows that the two-phase structure of aromatic polyimide differs essentially from that of ordinary crystalline polymers.     

Halogen-bonded assembly of hybrid inorganic/organic 3D-networks from dibromocuprate salts and tetrabromomethane

The critical halogen-bonding motif (CBr...BrCu) is responsible for the successful synthesis of three-dimensional networks of unusual 4,5-connected dodecahedron cells (each containing the encapsulated cation) via the self-assembly of tetra-n-propylammonium dibromocuprate(I) and tetrabromomethane as prototypical donor/acceptor dyads.     

The question of aromaticity in open-shell cations and anions as ion-radical offsprings of polycyclic aromatic and antiaromatic hydrocarbons

Arene cation-radicals and anion-radicals result directly from the one-electron oxidation and reduction of many aromatic hydrocarbons, yet virtually nothing is known of their intrinsic (thermodynamic) stability and hence "aromatic character". Since such paramagnetic ion radicals lie intermediate between aromatic (Hückel) hydrocarbons with 4n + 2-electrons and antiaromatic analogues with 4n-electrons, we can now address the question of pi-delocalization in these odd-electron counterparts. Application of the structure-based "harmonic oscillator model of aromaticity" or the HOMA method leads to the surprising conclusion that the aromaticity of these rather reactive, kinetically unstable arene cation and anion radicals (as measured by the HOMA index) is actually higher than that of their (diamagnetic) parent-contrary to conventional expectations.     

Short synthesis of alkyl-substituted acenes using carbonyl-directed C-h and C-o functionalization

A convenient method for the synthesis of tetraalkylanthracenes and -pentacenes by means of ruthenium-catalyzed regioselective C-H alkylation of the corresponding acenequinones was developed. Dialkyldiarylpentacene was also synthesized using chemoselective tandem C-H alkylation/C-O arylation of dimethoxypentacenequinone. It was suggested that a tetraalkylpentacene is stable under air in the dark and possesses an appropriate HOMO level as active material for p-type organic field-effect transistors (OFETs).