Zhenyang Lin

Ruthenium-catalyzed azide-alkyne cycloaddition: scope and mechanism.

The catalytic activity of a series of ruthenium(II) complexes in azide-alkyne cycloadditions has been evaluated. The [Cp*RuCl] complexes, such as Cp*RuCl(PPh 3) 2, Cp*RuCl(COD), and Cp*RuCl(NBD), were among the most effective catalysts. In the presence of catalytic Cp*RuCl(PPh 3) 2 or Cp*RuCl(COD), primary and secondary azides react with a broad range of terminal alkynes containing a range of functionalities selectively producing 1,5-disubstituted 1,2,3-triazoles; tertiary azides were significantly less reactive. Both complexes also promote the cycloaddition reactions of organic azides with internal alkynes, providing access to fully-substituted 1,2,3-triazoles. The ruthenium-catalyzed azide-alkyne cycloaddition (RuAAC) appears to proceed via oxidative coupling of the azide and alkyne reactants to give a six-membered ruthenacycle intermediate, in which the first new carbon-nitrogen bond is formed between the more electronegative carbon of the alkyne and the terminal, electrophilic nitrogen of the azide. This step is followed by reductive elimination, which forms the triazole product. DFT calculations support this mechanistic proposal and indicate that the reductive elimination step is rate-determining.

A Facile Route to Aryl Boronates: Room-Temperature, Copper-Catalyzed Borylation of Aryl Halides with Alkoxy Diboron Reagents

A simple but effective copper-catalyzed borylation of aryl halides, including electron-rich and sterically hindered aryl bromides, with alkoxy diboron reagents occurs under mild conditions (see scheme). Preliminary DFT studies of the mechanism suggest that sigma-bond metathesis between a copper-boryl intermediate and the aryl halide generates the aryl boronate product.

Metallophosphors of platinum with distinct main-group elements: a versatile approach towards color tuning and white-light emission with superior efficiency/color quality/brightness trade-offs

A new series of phosphorescent platinum(II) cyclometalated complexes with distinct electronic structures has been developed by simple tailoring of the phenyl ring of ppy (Hppy = 2-phenylpyridine) with various main-group moieties in [Pt(ppy-X)(acac)] (X = B(Mes)2, SiPh3, GePh3, NPh2, POPh2, OPh, SPh, SO2Ph substituted at the para position). Their distinctive electronic characters, resulting in improved hole-injection/hole-transporting or electron-injection/electron-transporting features, have confined/consumed the electrons in the emission layer of organic light-emitting diodes (OLEDs) to achieve good color purity and high efficiency of the devices. The maximum external quantum efficiency of 9.52%, luminance efficiency of 30.00 cd A−1 and power efficiency of 8.36 lm W−1 for the OLEDs with Pt-B (X = B(Mes)2) as the emitter, 8.50%, 29.74 cd A−1 and 19.73 lm W−1 for the device with Pt-N (X = NPh2), 7.92%, 22.06 cd A−1 and 13.64 lm W−1 for the device with Pt-PO (X = POPh2) as well as 8.35%, 19.59 cd A−1 and 7.83 lm W−1 for the device with Pt-SO2 (X = SO2Ph) can be obtained. By taking advantage of the unique electronic structures of the Pt-Ge (X = GePh3) and Pt-O (X = OPh) green emitters and the intrinsic property of blue-emitting hole-transport layer of 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB), single-dopant white OLEDs (WOLEDs) can be developed. These simple WOLEDs emit white light of very high quality (CIE at (0.354, 0.360), CRI of ca. 97 and CCT at 4719 K) even at high brightness (>15000 cd m−2) and the present work represents significant progress to address the bottle-neck problem of WOLEDs for the efficiency/color quality/brightness trade-off optimization that is necessary for pure white light of great commercial value.

Organocatalytic Asymmetric Synthesis of 1,1-Diarylethanes by Transfer Hydrogenation

A new organocatalytic transfer hydrogenation strategy for the asymmetric synthesis of 1,1-diarylethanes is described. Under mild conditions, a range of 1,1-diarylethanes substituted with an o-hydroxyphenyl or indole unit could be obtained with excellent efficiency and enantioselectivity. We also extended the protocol to an unprecedented asymmetric hydroarylation of 1,1-diarylalkenes with indoles for the synthesis of a range of highly enantioenriched 1,1,1-triarylethanes bearing acyclic all-carbon quaternary stereocenters. These diaryl- and triarylethanes exhibit impressive cytotoxicity against a number of human cancer cell lines. Preliminary mechanistic studies combined with DFT calculations provided important insight into the reaction mechanism.

Phosphorescence color tuning by ligand, and substituent effects of multifunctional iridium(III) cyclometalates with 9-arylcarbazole moieties.

The synthesis, isomeric studies, and photophysical characterization of a series of multifunctional cyclometalated iridium(III) complexes containing a fluoro- or methyl-substituted 2-[3-(N-phenylcarbazolyl)]pyridine molecular framework are presented. All of the complexes are thermally stable solids and highly efficient electrophosphors. The optical, electrochemical, photo-, and electrophosphorescence traits of these iridium phosphors have been studied in terms of the electronic nature and coordinating site of the aryl or pyridyl ring substituents. The correlation between the functional properties of these phosphors and the results of density functional theory calculations was made. Arising from the propensity of the electron-rich carbazolyl group to facilitate hole injection/transport, the presence of such a moiety can increase the highest-occupied molecular orbital levels and improve the charge balance in the resulting complexes relative to the parent phosphor with 2-phenylpyridine ligands. Remarkably, the excited-state properties can be manipulated through ligand and substituent effects that allow the tuning of phosphorescence energies from bluish green to deep red. Electrophosphorescent organic light-emitting diodes (OLEDs) with outstanding device performance can be fabricated based on these materials, which show a maximum current efficiency of approximately 43.4 cd A(-1), corresponding to an external quantum efficiency of approximately 12.9 % ph/el (photons per electron) and a power efficiency of approximately 33.4 Lm W(-1) for the best device. The present work provides a new avenue for the rational design of multifunctional iridium-carbazolyl electrophosphors, by synthetically tailoring the carbazolyl pyridine ring that can reveal a superior device performance coupled with good color-tuning versatility, suitable for multicolor-display technology.

Structural and bonding characteristics in transition metal-silane complexes.

Transition metal-silane complexes containing metal-eta 2-H-Si coordination display different structural and bonding characteristics in comparison to other sigma-complexes, such as dihydrogen and alkane (agostic) complexes. The different characteristics can be related to the strong sigma*-accepting properties of the eta 2-silane ligand(s) because of the weaker H-Si sigma bond. Various examples of metal-silane complexes have been reviewed and their structural stabilities have been systematically discussed. Silyl-hydride complexes having substantial silyl-hydrido interactions have also been emphasized.

Robust Tris‐Cyclometalated Iridium(III) Phosphors with Ligands for Effective Charge Carrier Injection/Transport: Synthesis, Redox, Photophysical, and Electrophosphorescent Behavior

With the target to design and develop new functionalized green triplet light emitters that possess distinctive electronic properties for robust and highly efficient phosphorescent organic light-emitting diodes (PHOLEDs), a series of bluish-green to yellow-green phosphorescent tris-cyclometalated homoleptic iridium(III) complexes [Ir(ppy-X)(3)] (X=SiPh(3), GePh(3), NPh(2), POPh(2), OPh, SPh, SO(2)Ph, Hppy=2-phenylpyridine) have been synthesized and fully characterized by spectroscopic, redox, and photophysical methods. By chemically manipulating the lowest triplet-state character of Ir(ppy)(3) with some functional main-group 14-16 moieties on the phenyl ring of ppy, a new family of metallophosphors with high-emission quantum yields, short triplet-state lifetimes, and good hole-injection/hole-transporting or electron-injection/electron-transporting properties can be obtained. Remarkably, all of these Ir(III) complexes show outstanding electrophosphorescent performance in multilayer doped devices that surpass that of the state-of-the-art green-emitting dopant Ir(ppy)(3). The devices described herein can reach the maximum external quantum efficiency (eta(ext)) of 12.3 %, luminance efficiency (eta(L)) of 50.8 cd A(-1), power efficiency (eta(p)) of 36.9 Lm W(-1) for [Ir(ppy-SiPh(3))(3)], 13.9 %, 60.8 cd A(-1), 49.1 Lm W(-1) for [Ir(ppy-NPh(2))(3)], and 10.1 %, 37.6 cd A(-1), 26.1 Lm W(-1) for [Ir(ppy-SO(2)Ph)(3)]. These results provide a completely new and effective strategy for carrier injection into the electrophosphor to afford high-performance PHOLEDs suitable for various display applications.

Spectroscopic and Structural Characterization of the CyNHC Adduct of B2pin2 in Solution and in the Solid State

The Lewis base adduct of B(2)pin(2) and the NHC (1,3-bis(cyclohexyl)imidazol-2-ylidene), which was proposed to act as a source of nucleophilic boryl groups in the β-borylation of α,β-unsaturated ketones, has been isolated, and its solid state structure and solution behavior was studied. In solution, the binding is weak, and NMR spectroscopy reveals a rapid exchange of the NHC between the two boron centers. DFT calculations reveal that the exchange involves dissociation and reassociation of NHC rather than an intramolecular process.

Copper-mediated reduction of CO2 with pinB-SiMe2Ph via CO2 insertion into a copper-silicon bond

Reaction of [(IPr)Cu-OtBu] (1) with pinB-SiMe(2)Ph (2) leads to the Cu-silyl complex [(IPr)Cu-SiMe(2)Ph] (3). Insertion of CO(2) into the Cu-Si bond of 3 is followed by transformation of the resulting silanecarboxy complex [(IPr)Cu-O(2)CSiMe(2)Ph] (4) to the silanolate complex [(IPr)Cu-OSiMe(2)Ph] (5) via extrusion of CO. As 5 reacts readily with 2 to regenerate 3, a catalytic CO(2) reduction to CO is feasible. The individual steps were studied by in situ(13)C NMR spectroscopy of a series of stoichiometric reactions. Complexes 3, 4, and 5 were isolated and fully characterized, including single-crystal X-ray diffraction studies. Interestingly, the catalytic reduction of CO(2) using silylborane 2 as a stoichiometric reducing agent leads not only to CO and pinB-O-SiMe(2)Ph but also to PhMe(2)Si-CO(2)-SiMe(2)Ph as an additional reduction product.

Synthesis, Structure, and Reactivity of Anionic sp(2) -sp(3) Diboron Compounds: Readily Accessible Boryl Nucleophiles.

Lewis base adducts of tetra-alkoxy diboron compounds, in particular bis(pinacolato)diboron (B2 pin2 ), have been proposed as the active source of nucleophilic boryl species in metal-free borylation reactions. We report the isolation and detailed structural characterization (by solid-state and solution NMR spectroscopy and X-ray crystallography) of a series of anionic adducts of B2 pin2 with hard Lewis bases, such as alkoxides and fluoride. The study was extended to alternative Lewis bases, such as acetate, and other diboron reagents. The B(sp(2) )-B(sp(3) ) adducts exhibit two distinct boron environments in the solid-state and solution NMR spectra, except for [(4-tBuC6 H4 O)B2 pin2 ](-) , which shows rapid site exchange in solution. DFT calculations were performed to analyze the stability of the adducts with respect to dissociation. Stoichiometric reaction of the isolated adducts with two representative series of organic electrophiles-namely, aryl halides and diazonium salts-demonstrate the relative reactivities of the anionic diboron compounds as nucleophilic boryl anion sources.