Asymmetric synthesis of 3,4-dihydrocoumarin motif with an all-carbon quaternary stereocenter via a Michael-acetalization sequence with bifunctional amine-thiourea organocatalysts

Asymmetric domino Michael-acetalization reactions of 2-hydroxynitrostyrene and 2-oxocyclohexanecarbaldehyde with a bifunctional thiourea-tertiary-amine organocatalyst, e.g., the Takemoto catalyst, followed by oxidation providing the 1',3-spiro-2'-oxocyclohexan-3,4-dihydrocoumarin having one all-carbon quaternary stereocenter with excellent diastereo- and enantioselectivities (up to >99% ee), are described. The structures and absolute configurations of the products were confirmed by X-ray analysis.     

Emissive iridium(III) diimine complexes formed by double cyclometalation of coordinated triphenylphosphite

We report on the synthesis of a new series of iridium(III) complexes functionalized with various diimine chromophores, together with a facially coordinated dicyclometalated phosphite chelate and a monodentate anionic ancillary. This conceptual design presents a novel strategy in obtaining a new class of iridium(III) diimine complexes without employment of traditional nitrogen-containing polyaromatic cyclometalates. Additionally, we discuss the basic charactersistics of the ground and lower-lying excited states involved, as documented by crystal structural, photophysical studies, and density functional theory calculations. Fabrication of the green-emitting organic light-emitting diodes with one such dopant, [Ir(dbbpy)(tpit)NCS] (2b), where dbbpy and tpit represent di-tert-butyl-2,2'-bipyridine and dicyclometalated triphenylphosphite, respectively, was successfully made, attaining a peak external quantum efficiency (η(ext)), a luminance efficiency (η(l)), and a power efficiency (η(p)) of 14.1%, 46.6 cd A(-1), and 39.9 lm W(-1), respectively.     

Bond characterization on a Cr-Cr quintuple bond: a combined experimental and theoretical study

A combined experimental and theoretical charge density study on a quintuply bonded dichromium complex, Cr(2)(dipp)(2) (dipp = (Ar)NC(H)N(Ar) and Ar = 2,6-i-Pr(2)-C(6)H(3)), is performed. Two dipp ligands are bridged between two Cr ions; each Cr atom is coordinated to two N atoms of the ligands in a linear fashion. The Cr atom is in a low oxidation state, Cr(I), and in low coordination number condition, which stabilizes a metal-metal multiple bond, in this case, a quintuple bond. Indeed, it gives an ultrashort Cr-Cr bond distance of 1.7492(1) ? in the complex. The bond characterization of such a quintuple bond is undertaken both experimentally by high-resolution single-crystal X-ray diffraction and theoretically by density functional calculation (DFT). Electron densities are depicted via deformation density and Laplacian distributions. Bond characterizations of the complex are presented in terms of topological properties, Fermi hole function, source function (SF), and natural bonding orbital (NBO) analysis. The electron density at the Cr-Cr bond critical point (BCP) is 1.70 e/?(3), quite a high value for metal-metal bonding and mainly contributed from the metal ion itself. The quintuple bond is confirmed with one σ, two π, and two δ interactions by NBO analysis and Fermi hole function. The molecular orbitals (MOs) illustrate that five bonding orbitals are predominantly contributed from the 3d orbitals of the Cr(I) ion. The effective bond order from NBO analysis is 4.60. The detail comparison between experiment and theory will be given. Additionally, three closely related complexes are calculated for systematic comparison.     

Organocatalyzed Michael-Henry reactions: enantioselective synthesis of cyclopentanecarbaldehydes via the dienamine organocatalysis of a succinaldehyde surrogate

Asymmetric formal [3+2] cycloadditions of 4-hydroxybut-2-enal, a succinaldehyde surrogate, and nitroalkenes with an organocatalyst provided cyclopentanecarbaldehydes containing four consecutive stereogenic centers with excellent enantioselectivities.     

Spin-canting magnetization in an unusual Co4 cluster-based layer compound from a 2,3-dihydroxyquinoxaline ligand

The self-assembly of Co(O(2)CPh)(2) with a 2,3-dihydroxyquinoxaline (H(2)dhq) linker has revealed a new two-dimensional cluster-based compound, [Co(4)(OMe)(2)(O(2)CPh)(2)(dhq)(2)(MeOH)(2)](n), which shows spin-canted magnetization and a definite magnetic hysteresis loop.     

New iron(II) spin crossover coordination polymers [Fe(μ-atrz)3]X2·2H2O (X = ClO4¯, BF4¯) and [Fe(μ-atrz)(μ-pyz)(NCS)2]·4H2O with an interesting solvent effect

A potential bridging triazole-based ligand, atrz (trans-4,4'-azo-1,2,4-triazole), is chosen to serve as building sticks and incorporated with a spin crossover metal center to form a metal organic framework. Coordination polymers of iron(II) with the formula [Fe(μ-atrz)(3)]X(2)·2H(2)O (where X = ClO(4)(-) (1·2H(2)O) and BF(4)(-) (2·2H(2)O)) in a 3D framework and [Fe(μ-atrz)(μ-pyz)(NCS)(2)]·4H(2)O (3·4H(2)O) in a 2D layer structure were synthesized and structurally characterized. The magnetic measurements of 1·2H(2)O and 2·2H(2)O reveal spin transitions near room temperature; that of 3 exhibits an abrupt spin transition at ~200 K with a wide thermal hysteresis, and the spin transition behavior of these polymers are apparently correlated with the water content of the sample. Crystal structures have been determined both at high spin and at low spin states for 1·2H(2)O, 2·2H(2)O, and 3·4H(2)O. Each iron(II) center in 1·2H(2)O and 2·2H(2)O is octahedrally coordinated with six μ-atrz ligands, which in turn links the other Fe center forming a strong three-dimensional (3D) network; counteranion and water molecules are located in the voids of the lattice. The FeN(6) octahedron of 3·4H(2)O is formed with two atrz, two pyrazine (pyz) ligands, and two NCS(-) ligands, where the ligands atrz and pyz are bridged between iron centers forming a 2D layer polymer. A zigzag chain of water molecules is found between the layers, and there is a distinct correlation between the thermal hysteresis with the amount of water molecules the exist in the crystal.     

Synthesis, structure and oxygen-sensing properties of iridium(III)-containing coordination polymers with different cations

Four iridium(III)-containing coordination polymers 1-4 using Ir(ppy)(2)(H(2)dcbpy)PF(6) (L-H(2), ppy = 2-phenylpyridine, H(2)dcbpy = 4,4'-dicarboxy-2,2'-bipyridine) as the bridging ligand, [ZnL(2)]·3DMF·5H(2)O (1), [CdL(2)(H(2)O)(2)]·3DMF·6H(2)O (2), [CoL(2)(H(2)O)(2)]·2DMF·8H(2)O (3) and [NiL(2)(H(2)O)(2)]·3DMF·6H(2)O (4), have been synthesized and structurally characterized. The emissions from 1-4 are ascribed to a metal-to-ligand charge transfer transition (MLCT). The absolute emission quantum yields for 1-4 in single crystals were measured in air to be 0.274, 0.193, 0.001 and 0.002, respectively. The noteworthy oxygen-sensing properties of 1-4 as well as L-H(2) in a single crystal were also evaluated. The Stern-Volmer quenching constant, K(SV) values, of 1-4 and L-H(2) can be deduced to be 0.834, 2.820, 1.328, 1.111 and 2.476, respectively. The results show promising K(SV) values (e.g.2) that are competitive or even larger than those of many known Ir-complexes. Moreover, the short response time (e.g. compound 2) and recovery times toward oxygen of 1-4 have been measured in their single crystal forms. The reversibility experiments for 1-4 were carried out for seven repeated cycles. As a result, >75% recovery of intensity for 1 and 2 on each cycle demonstrates a high degree of reproducibility during the sensing process. It should be noted that iridium(III)-containing coordination polymers with high emission intensity and notable oxygen sensing properties are obscure, especially in the single crystal form. This, in combination with its fine reversibility, leads to success in single crystal oxygen recognition based on photoluminescence imaging. The detection limit could be 0.50% for gaseous oxygen. Moreover, the temperature effect of compound 2 in a single crystal upon application as an oxygen sensor was expected.     

A remarkable ligand orientational effect in osmium-atom-induced blue phosphorescence

A new series of Os(II)-based carbonyl complexes cis(CO),trans(Npy,Npy),cis(Ntz,Ntz)-[Os(CO)2(bptz)2] (1), cis(CO),cis(Npy,Npy),trans(Ntz,Ntz)-[Os(bptz)2(CO)2] (2), and cis(CO),trans(Npy,Npy),cis(Ntz,Ntz)-[Os(CO)2(fptz)2] (3), where bptz and fptz denote 3-tert-butyl-5-(2-pyridyl)- and 3-trifluoromethyl-5-(2-pyridyl)-1,2,4-triazolate, respectively, have been designed and synthesized in an effort to achieve high efficiency, room-temperature blue phosphorescence. Although 1 and 2 are geometric isomers, remarkably different excited-state relaxation pathways were observed. Complex 1 exhibits strong phosphorescence in CH3CN (Phi(p) approximately 0.47) and as a single crystal at room temperature, whereas complex 2 is nearly nonemissive under similar conditions. The associated relaxation dynamics have been comprehensively investigated by spectroscopic and relaxation dynamics as well as by theoretical approaches. Our results lead us to the conclusion that for complex 2, the "loose bolt" effect of metal-ligand bonding interactions plays a crucial role in the fast radiationless deactivation of this type of geometrical isomer. Fine adjustment can also be achieved by functionalizing the ligands so that the electron-withdrawing nature of the CF3 group in 3 stabilizes the HOMO of the triazolate moiety, thus moving the emission further into the pure "blue" region; this results in highly efficient phosphorescence and renders 3 particularly attractive for application in blue OLED devices.