With new chemistry and advantageous configuration, the lithium-oxygen (Li-O₂) battery promises a much higher specific energy than traditional lithium-ion batteries. The limited understanding on the complicated battery reactions therein, however, has become a major bottleneck of its development for applications requiring a high energy efficiency and long cycle-life. Herein, in a confined potential window with negligible electrolyte degradation, we studied the rechargeability of Li-O₂ cathodes with pre-filled well-defined discharge products of Li₂O₂, Li₂CO₃, LiOH, or their combinations. Our results suggest Li₂CO₃ as the most difficult species to be electrochemically decomposed among the three lithium compounds, whereas the presence of LiOH notably increases the initial charge potential. The clearly visible difference in the charge behavior and cycling stability of these artificially “discharged” electrodes provides a guideline for the development of future high-performance Li-O₂ batteries.

A 2D coordination polymer prepared with bulky diethylformamide solvates exhibits channels which allow dipyridyl bridging ligands to diffuse into the crystal lattice. The absorbed dipyridyls thread through the pores of one layer and substitute the surface diethylformamide molecules on the neighboring layers to stitch alternate layers to form flexible interpenetrated metal–orgaic frameworks. The threading process also results in exchange of the bulky diethylformamide solvates for aqua to minimize congestion and, more strikingly, forces the slippage of two-dimensional layers, while still maintaining crystallinity.

N-heterocycle-based ligands are versatile building-blocks for coordination oligomer and polymer assemblies of first-row transition metals. Herein, we review recent developments in the design and synthesis of some nitrogen-rich azole-based ligands, their assembly modes in coordination oligomers and polymers, and the potential applications of the resulting metallosupramolecules.

Phosphine exchange of [Ru^IIBr(MeCOO)(PPh₃)₂(3-RBzTh)] (3-RBzTh=3-benzylbenzothiazol-2-ylidene) with a series of diphosphines (bis(diphenylphosphino)methane (dppm), 1,2-bis(diphenylphosphino)ethylene (dppv), 1,1′-bis(diphenylphosphino)ferrocene (dppf), 1,4-bis(diphenylphosphino)butane (dppb), and 1,3-(diphenylphosphino)propane (dppp)) gave mononuclear and neutral octahedral complexes [RuBr(MeCOO)(η²-P₂)(3-RBzTh)] (P₂=dppm (2), dppv (3), dppf (4), dppb (5), or dppp (6)), the coordination spheres of which contained four different ligands, namely, a chelating diphosphine, carboxylate, N,S-heterocyclic carbene (NSHC), and a bromide. Two geometric isomers of 6 (6a and 6 b) have been isolated. The structures of these products, which have been elucidated by single-crystal X-ray crystallography, show two structural types, I and II, depending on the relative dispositions of the ligands. Type I structures contain a carbenic carbon atom trans to the oxygen atom, whereas two phosphorus atoms are trans to bromine and oxygen atoms. The type II system comprises a carbene carbon atom trans to one of the phosphorus atoms, whereas the other phosphorus is trans to the oxygen atom, with the bromine trans to the remaining oxygen atom. Complexes 2, 3, 4, and 6a belong to type I, whereas 5 and 6 b are of type II. The kinetic product 6 b eventually converts into 6a upon standing. These complexes are active towards catalytic reduction of para-methyl acetophenone by 2-propanol at 82 °C under 1 % catalyst load giving the corresponding alcohols. The dppm complex 2 shows the good yields (91–97 %) towards selected ketones.

A series of mixed-ligand, mononuclear complexes trans-[PdX₂(NSHC)(azole)] [NSHC = N,S-heterocyclic carbene; azole = imidazole, 1-(2,4,6-trimethylphenyl)-1H-imidazole, benzimidazole, benzothiazole and benzoxazole] have been prepared and characterised by X-ray single-crystal diffraction. These complexes catalyse the sp²–sp³ cross-coupling of fluoroaryl halides with arylboronic acid to give diarylmethanes with good yields and high turnovers. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Five new tetradentate ligands [NNNN] with benzimidazolyl-imine or amine nitrogen donors have been synthesized in good yields under mild conditions from easily available substrates. trans-N,N′-bis(1-Ethyl-2-benzimidazolylmethylene)cyclohexane-1,2-diimine is the best accelerating ligand in this series that supports the Cu^I-catalyzed Ullmann N-arylation and the direct three-component azide–alkyne cycloaddition reaction to give the corresponding substituted imidazole, pyrazole, and triazole in high yields. Single-crystal X-ray diffraction analysis of its complex with CuI reveals a novel one-dimensional coordination polymer of the metal chain bridged alternately by the [NNNN] ligand and diiodides.

Three unsymmetrical benzimidazolium-pyrazole N-N ligands 2-(1-propylbenzimidazolylmethyl)-3,5-di-R-pyrazole (R=H, Me, t-Bu) have been conveniently prepared and structurally analyzed. The solid lattice packing of the R=Me compound at 223 K reveals one-dimensional “zig-zag” water chains stabilized by organic molecular channels through N⋅⋅⋅HO and OH⋅⋅⋅O bonding. These hybrid ligands add to palladium(II) to give high yields of air-stable complexes that are fully characterized by NMR, ESI, and X-ray single-crystal crystallography. They are active Suzuki catalysts at room temperature towards cross-coupling of unactivated aryl bromides and 5- or 6-membered heteroaryl bromides with arylboronic acids with turnover frequencies (TOF) reaching as high as 60,000 h⁻¹. Their catalytic efficiency is significantly better than that of the CN carbene-imidazole analogue. These catalysts are also active in Heck and Sonogashira cross-coupling reactions of aryl bromides giving the desired products in good yields. These results suggested that these Pd(II) complexes with N-based hybrid ligands are versatile and efficient catalysts for different types of cross-coupling reactions under aerobic conditions.

Mixed-ligand N,S-heterocyclic carbene (NSHC) complexes, trans-[PdBr₂(NSHC)(Py)] (NSHC=3-benzyl- or 3-propyl-benzothiazolin-2-ylidene), have been obtained from bridge-cleavage reactions of the dinuclear complex, [Pd(μ-Br)Br(NSHC)]₂, in pyridine at room temperature. Use of neutral N-bidentate donors (L=pyrazine, 1,2-bis(4-pyridyl)ethane, 4,4′-bipyridine and trans-1,2-bis(4-pyridyl)ethylene) yields the dinuclear spacer-bridged [Pd₂Br₄(NSHC)₂(μ-L)] complexes. The X-ray single-crystal structures of the pyridyl, bridging pyrazine and 1,2-bis(4-pyridyl)ethane complexes are reported. These air-stable complexes are active in the Suzuki–Miyaura coupling reactions of selected aryl bromides. The dinuclear complexes are generally more active than their mononuclear pyridyl analogues. The benzyl derivatives consistently outperform the n-propyl counterparts.