n‐Dopants Based on Dimers of Benzimidazoline Radicals: Structures and Mechanism of Redox Reactions

Dimers of 2‐substituted N,N′‐dimethylbenzimidazoline radicals, (2‐Y‐DMBI)₂ (Y=cyclohexyl (Cyc), ferrocenyl (Fc), ruthenocenyl (Rc)), have recently been reported as n‐dopants for organic semiconductors. Here their structural and energetic characteristics are reported, along with the mechanisms by which they react with acceptors, A (PCBM, TIPS‐pentacene), in solution. X‐ray data and DFT calculations both indicate a longer C?C bond for (2‐Cyc‐DMBI)₂ than (2‐Fc‐DMBI)₂, yet DFT and ESR data show that the latter dissociates more readily due to stabilization of the radical by Fc. Depending on the energetics of dimer (D₂) dissociation and of D₂‐to‐A electron transfer, D₂ reacts with A to form D⁺ and A^? by either of two mechanisms, differing in whether the first step is endergonic dissociation or endergonic electron transfer. However, the D⁺/0.5 D₂ redox potentials—the effective reducing strengths of the dimers—vary little within the series (ca. ?1.9 V vs. FeCp₂^+/0) (Cp=cyclopentadienyl) due to cancelation of trends in the D^+/0 potential and D₂ dissociation energy. The implications of these findings for use of these dimers as n‐dopants, and for future dopant design, are discussed.     

Channel‐forming solvates of 6‐chloro‐2,5‐dihydroxypyridine and its solvent‐free tautomer 6‐chloro‐5‐hydroxy‐2‐pyridone

On crystallization from CHCl₃, CCl₄, CH₂ClCH₂Cl and CHCl₂CHCl₂, 6‐chloro‐5‐hydroxy‐2‐pyridone, C₅H₄ClNO₂, (I), undergoes a tautomeric rearrangement to 6‐chloro‐2,5‐dihydroxypyridine, (II). The resulting crystals, viz. 6‐chloro‐2,5‐dihydroxypyridine chloroform 0.125‐solvate, C₅H₄ClNO₂·0.125CHCl₃, (IIa), 6‐chloro‐2,5‐dihydroxypyridine carbon tetrachloride 0.125‐solvate, C₅H₄ClNO₂.·0.125CCl₄, (IIb), 6‐chloro‐2,5‐dihydroxypyridine 1,2‐dichloroethane solvate, C₅H₄ClNO₂·C₂H₄Cl₂, (IIc), and 6‐chloro‐2,5‐dihydroxypyridine 1,1,2,2‐tetrachloroethane solvate, C₅H₄ClNO₂·C₂H₂Cl₄, (IId), have I4₁/a symmetry, and incorporate extensively disordered solvent in channels that run the length of the c axis. Upon gentle heating to 378 K in vacuo, these crystals sublime to form solvent‐free crystals with P2₁/n symmetry that are exclusively the pyridone tautomer, (I). In these sublimed pyridone crystals, inversion‐related molecules form R²₂(8) dimers via pairs of N—H...O hydrogen bonds. The dimers are linked by O—H...O hydrogen bonds into R⁴₆(28) motifs, which join to form pleated sheets that stack along the a axis. In the channel‐containing pyridine solvate crystals, viz. (IIa)–(IId), two independent host molecules form an R²₂(8) dimer via a pair of O—H...N hydrogen bonds. One molecule is further linked by O—H...O hydrogen bonds to two 4₁ screw‐related equivalents to form a helical motif parallel to the c axis. The other independent molecule is O—H...O hydrogen bonded to two related equivalents to form tetrameric R⁴₄(28) rings. The dimers are π–π stacked with inversion‐related dimers, which in turn stack the R⁴₄(28) rings along c to form continuous solvent‐accessible channels. CHCl₃, CCl₄, CH₂ClCH₂Cl and CHCl₂CHCl₂ solvent molecules are able to occupy these channels but are disordered by virtue of the site symmetry within the channels.     

Syntheses, structures, and magnetic characterization of dicyanometalate(II) building blocks: [NEt4][(Tp*)MII(CN)2] [MII = Cr, Co, Ni; Tp* = hydridotris(3,5-dimethylpyrazol-1-yl)borate

The syntheses and structures of three dicyanometalate(II) complexes, [NEt(4)][(Tp*)M(II)(CN)(2)].nMeCN.(1/2)Et(2)O (M(II) = Cr, 1, n = (1/2); Co, 2, n = 1; Ni, 3, n = 1) are described; magnetic studies indicate that 3 is diamagnetic while 1 and 2 are paramagnetic high- and low-spin S = 2 and (1/2) complexes, respectively.     

Synthesis and structural characterization of tris(2-seleno-1-mesitylimidazolyl) hydroborato complexes: a new type of strongly electron donating tripodal selenium ligand

A new tripodal ligand that features three selenium donors, namely the tris(2-seleno-1-mesitylimidazolyl)hydroborato ligand, [Tse(Mes)], has been constructed via the reaction of KBH(4) with 1-mesitylimidazole-2-selone; comparison of the IR spectroscopic data of [Tse(Mes)]Re(CO)(3) with those of a variety of related LRe(CO)(3) complexes demonstrates that the [Tse(Mes)] ligand is more strongly electron donating than Cp, Cp*, [Tp], [Tp(Me(2))] and [Tm(Mes)] ligands.     

Synthesis and structural characterization of 1-mesityl-1,3-dihydro-imidazole-2-selone and bis(1-mesitylimidazol-2-yl)diselenide: experimental evidence that the selone is more stable than the selenol tautomer

1-Mesityl-1,3-dihydro-imidazole-2-selone, (seim(Mes))H, may be obtained from 1-mesitylimidazole via (i) deprotonation with Bu(n)Li, (ii) treatment with elemental selenium, and (iii) addition of HCl(aq). Structural characterization of (seim(Mes))H by X-ray diffraction demonstrates that the compound exists as the selone rather than selenol tautomer, a result that is in accord with DFT calculations. Solutions of (seim(Mes))H are oxidized by air to give bis(1-mesitylimidazol-2-yl)diselenide, (seim(Mes))(2). A corresponding investigation of (seim(Me))H demonstrates that, in contrast to a previous report, the selenium analogue of methimazole exists in the selone form with a structure analogous to that of methimazole. (1)H and (77)Se NMR studies demonstrate that the (seim(R)) groups of the selone (seim(R))H and diselenide (seim(R))(2) undergo facile exchange on the NMR time scale.     

Electrochemical investigations of the interconversions between catalytic and inhibited states of the [FeFe]-hydrogenase from Desulfovibrio desulfuricans

Studies of the catalytic properties of the [FeFe]-hydrogenase from Desulfovibrio desulfuricans by protein film voltammetry, under a H2 atmosphere, reveal and establish a variety of interesting properties not observed or measured quantitatively with other techniques. The catalytic bias (inherent ability to oxidize hydrogen vs reduce protons) is quantified over a wide pH range: the enzyme is proficient at both H2 oxidation (from pH > 6) and H2 production (pH < 6). Hydrogen production is inhibited by H2, but the effect is much smaller than observed for [NiFe]-hydrogenases from Allochromatium vinosum or Desulfovibrio fructosovorans. Under anaerobic conditions and positive potentials, the [FeFe]-hydrogenase is oxidized to an inactive form, inert toward reaction with CO and O2, that rapidly reactivates upon one-electron reduction under 1 bar of H2. The potential dependence of this interconversion shows that the oxidized inactive form exists in two pH-interconvertible states with pK(ox) = 5.9. Studies of the CO-inhibited enzyme under H2 reveals a strong enhancement of the rate of activation by white light at -109 mV (monitoring H2 oxidation) that is absent at low potential (-540 mV, monitoring H+ reduction), thus demonstrating photolability that is dependent upon the oxidation state.     

A mesoporous metal-organic framework with permanent porosity

A mesoporous metal-organic framework possessing permanent porosity has been synthesized and characterized for the first time.     

Oxidative addition of dihydrogen to (eta6-arene)Mo(PMe3)3 complexes: origin of the naphthalene and anthracene effects

In contrast to the benzene and naphthalene compounds (eta(6)-PhH)Mo(PMe(3))(3) and (eta(6)-NpH)Mo(PMe(3))(3), the anthracene complex (eta(6)-AnH)Mo(PMe(3))(3) reacts with H(2) to undergo a haptotropic shift and give the eta(4)-anthracene compound (eta(4)-AnH)Mo(PMe(3))(3)H(2). Density functional theory calculations indicate that the increased facility of naphthalene and anthracene to adopt eta(4)-coordination modes compared to that of benzene is a consequence of the fact that the Mo-(eta(4)-ArH) bonding interaction increases in the sequence benzene < naphthalene < anthracene, while the Mo-(eta(6)-ArH) bonding interaction follows the sequence benzene > naphthalene approximately anthracene.     

Flavin-based [2]rotaxanes

[reaction: see text] We report the synthesis of flavin-stoppered hydrogen bonded [2]rotaxanes 1 and 2. We also report the electrochemically controllable properties of these systems in solution, and for derivative 2, as an electropolymerized thin film.     

Tetracene derivatives as potential red emitters for organic LEDs

[structure: see text] As part of our program investigating the use of ethynylated acenes in organic electronics, we have prepared a series of functionalized tetracene derivatives in search of a material with red emission suitable for use in display technologies. A number of such compounds with functionalization on the alkyne and/or the acene ring were easily prepared in one or two steps from commercially available materials. Solution fluorescence quantum efficiencies were generally good for these derivatives, which possessed emission maxima spanning the range 540-637 nm. Simple light-emitting diodes fabricated from these compounds showed that one of the derivatives did exhibit red electroluminescence.