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The effect of proximal Zn halides on Ni-catalyzed ethylene polymerization is reported in this work. A series of (NON)NiLX (NON = 2,6-bis-((2,6-diisopropylphenyl)imino)methyl phenoxide; LX = methallyl or L = py, X = tolyl, 2–4) ethylene polymerization precatalysts have been synthesized, as well as a heterobimetallic Ni/Zn complex, (NON)Ni(C4H7)ZnBr2 (5). Each precatalyst could be activated (or promoted) by ZnX2 (X = Cl, Br, Et) to polymerize ethylene. In situ recruitment of ZnX2 by the free imine binding pocket of the NON complexes results in the generation of heterobimetallic active species that produce lower Mn polyethylene than monometallic controls. Room temperature ZnX2-promoted polymerizations with these catalysts resulted in bimodal Mn distributions that result from different catalyst speciation: “dangling” imine-ligated ZnX2 species yield higher Mn polymer while N,O-chelated ZnX2 species yield lower Mn polymer. Running polymerizations at higher temperature yields in only lower Mn polymer resulting from exclusive formation of the thermodynamically favored N,O chelated Ni/Zn heterobimetallic. DFT calculations indicate that this bridging bimetallic complex undergoes β-H elimination more facilely than monometallic Ni analogues, resulting in lower molecular weight polymers.

A series of heteroleptic bipyridine ruthenium complexes were prepared using known synthetic methods. Each compound incorporated one electron withdrawing 4,4'-dicarboxylic acid-2,2'-bipyridine and two bipyridines each of which had electron donating dialkylamine substituents in the 4 and 4' positions. The electronic absorption spectra exhibited absorptions that moved to lower energy as the donor ability of the amine substituent increased. Density functional calculations established that the HOMO was delocalized over the metal and two pyridine groups located trans to the pyridines of the dicarboxylic acid bipyridine. The LUMO was delocalized over the dicarboxylic acid bipyridine. Cyclic voltammetry of the deprotonated compounds exhibit one quasi-reversible oxidation and three reductions. Coupled with the emission data, the excited state reduction potentials were estimated to range from -0.93 to -1.03 V vs. NHE. Monodispersed 3.2 nm diameter nanocrystals (NCs) of zinc oxide were found to quench partially the excited state of the dyes via a static quenching electron transfer process involving the formation of a dyad of the complex and the NC. The magnitude of the partial quenching of complexed dyes was correlated to the distribution of band gaps for the NCs, which is an inverse function of diameter. Dyes attached to the NCs on the small end of the particle size distribution had electron transfer rates that were uncompetitive with radiative and nonradiative decay mechanisms.

The ability of various group 10 α-diimine and salicylaldimine polymerization catalysts to undergo chain transfer with main group metal alkyls during ethylene polymerization has been investigated in depth. The catalyst systems with the most efficient chain transfer were found to be cationic (α-diimine)Ni catalysts paired with dialkyl zinc chain-transfer reagents, in which all growing polymeryl chains were transferred to Zn on the basis of 13C NMR analysis. In these systems, chain transfer was found to be dependent on the sterics of both the catalyst and the chain-transfer reagent (CTR). When less sterically encumbered catalysts or CTRs were utilized, the relative rate of bimetallic chain transfer to chain propagation was increased; however, in cases where chain termination via β-H elimination was extremely rapid, chain transfer to Zn was kinetically not viable. Importantly, chain transfer from (α-diimine)Ni catalysts to Zn alkyls is also very sensitive to the strength of the Zn–C bond: ZnMe2 (186 kJ/mol) i...

Oleate-capped CdS nanocrystals (NCs) dispersed in dichloromethane were found to quench the excited-state fluorescence of the terthiophene derivative 3′,4′-dibutyl-5″-phenyl-[2,2′:5′,2″-terthiophene]-5-carboxylic acid (1-CO2H). Infrared and 1H NMR spectroscopies provided evidence that 1-CO2H substitutes for oleate on the surface of the CdS NCs. Upon binding, the fluorescence of 1-CO2H is quenched, and the 1H NMR lines from 1-CO2H are broadened. The importance of the carboxylate group in binding to the CdS NC was further established by examining the behavior of a similar fluorophore where the carboxylic acid group was replaced with a bromo substituent (1-Br). The CdS NCs had no influence on the fluorescence intensity or NMR line shapes of 1-Br. For 1-CO2H, Stern–Volmer plots indicated a nearly linear increase in I0/I as the CdS NCs’ concentration was increased, but as the dye/NC ratio reached ∼20/1, I0/I reached a maximum of ∼8 and began to decrease. By a dye/NC ratio of 2/1, the I0/I reached a steady value...

We demonstrate a method for high-throughput catalyst screening using a parallel pressure reactor starting from the initial synthesis of a nickel α-diimine ethylene polymerization catalyst. Initial polymerizations with the catalyst lead to optimized reaction conditions, including catalyst concentration, ethylene pressure and reaction time. Using gas-uptake data for these reactions, a procedure to calculate the initial rate of propagation (kp) is presented. Using the optimized conditions, the ability of the nickel α-diimine polymerization catalyst to undergo chain transfer with diethylzinc (ZnEt2) during ethylene polymerization was investigated. A procedure to assess the ability of the catalyst to undergo chain transfer (from molecular weight and 13C NMR data), calculate the degree of chain transfer, and calculate chain transfer rates (ke) is presented.

Pyrroles are structurally important heterocycles. However, the synthesis of polysubstituted pyrroles is often challenging. Here, we report a multicomponent, Ti-catalysed formal [2+2+1] reaction of alkynes and diazenes for the oxidative synthesis of penta- and trisubstituted pyrroles: a nitrenoid analogue to classical Pauson-Khand-type syntheses of cyclopentenones. Given the scarcity of early transition-metal redox catalysis, preliminary mechanistic studies are presented. Initial stoichiometric and kinetic studies indicate that the mechanism of this reaction proceeds through a formally Ti(II)/Ti(IV) redox catalytic cycle, in which an azatitanacyclobutene intermediate, resulting from [2+2] alkyne + Ti imido coupling, undergoes a second alkyne insertion followed by reductive elimination to yield pyrrole and a Ti(II) species. The key component for catalytic turnover is the reoxidation of the Ti(II) species to a Ti(IV) imido via the disproportionation of an η(2)-diazene-Ti(II) complex.

Using a modification of a recently reported convenient synthesis of the isobutyl ester form of the bis(isothiocyanato) ruthenium dye (N3), we report the isolation and spectroscopic characterization of the minor isomer in which one NCS− is N-bound and the other is S-bound. The synthesis involves the reaction of Ru(iBu2dcbpy)2Cl2, where iBu2dcbpy is the diisobutyl ester of 4,4′-dicarboxy-2,2′-bipyridine, with a source of thiocyanate. The impact of the isocyanate salt, solvent, and temperature on the yield of the linkage isomers is presented. In addition to the two linkage isomers 1 and 2, the partially substituted chloroisothiocyanato complex 3, Ru(iBu2dcbpy)2Cl(NCS), was also isolated. Selective removal of the chloride using silver triflate provided a path to [Ru(iBu2dcbpy)2(pyridine)(NCS)]OTf, 4, in high yield. The complexes were characterized by 1H and 13C NMR, IR, and electronic absorption spectroscopies and mass spectrometry. At 80 °C in DMSO-d6, the isomerization of 2 to 1 is complete and exhibits first-order kinetics with a rate constant of 0.00014 s−1. Room temperature hydrolysis of the isobutyl ester groups of 2 using [nBu4 N]OH in acetonitrile produced a mixture of the two linkage isomers.

Indium phosphide nanocrystals (InP NCs) with diameters ranging from 2 to 5 nm were synthesized with a scalable, flow-through, nonthermal plasma process at a rate ranging from 10 to 40 mg/h. The NC size is controlled through the plasma operating parameters, with the residence time of the gas in the plasma region strongly influencing the NC size. The NC size distribution is narrow with the standard deviation being less than 20% of the mean NC size. Zinc sulfide (ZnS) shells were grown around the plasma-synthesized InP NCs in a liquid phase reaction. Photoluminescence with quantum yields as high as 15% were observed for the InP/ZnS core-shell NCs.

Inorganic quantum dots (QDs) have excellent optoelectronic properties. But, due in part to a lack of a suitable medium for dispersion, they have not been extensively used in optoelectronic devices. With the advent of organic semiconductors, the integration of quantum dots into optoelectronic devices has become possible. Such devices are termed as hybrid organic/inorganic quantum dot light emitting devices. In hybrid organic/inorganic quantum dot light emitting devices, the mechanisms of charge and/or energy transfer into the quantum dots include Forster energy transfer and direct charge injection. Forster energy transfer involves formation of excitons on organic semiconductors, followed by an energy transfer onto the inorganic quantum dots, where the exciton recombines resulting in emission of a photon. Direct charge injection is the mechanism in which the electrons and holes are directly injected into the quantum dots and they recombine on the quantum dots to result in a photon. Which mechanism is operat...