Morphology Control of Au-Ni Hybrid Nanoparticles: Exploring Heterostructures and Optical Tuning.

Hybrid nanoparticles (NPs) have attracted considerable attention because of their ability to provide diverse properties by integrating the inherent properties of multiple components; however, synthetic strategies to control their morphology remain unexplored. In this study, a new method was used to control the morphology and optical properties of Au-Ni heterostructure (ANH) NPs. Unique morphological changes were observed by varying the Au/Ni precursor ratio from 2:1 to 1:4, exhibiting a shape transformation from dumbbell-like to quasi-spherical owing to the Ni NP size expansion, whereas the Au NP maintained their size. Moreover, increasing the Ni ratio induced plasmonic band broadening and wavelength redshift, resulting in color changes from red to navy and black. In terms of the structure, the atomic orientation of the crystallite showed that even a large lattice mismatch can result in heterojunctions at the NPs. In addition, the reaction aliquots uncovered heterogeneous nucleation and growth of ANH NPs in the colloidal system, demonstrating Ni reduction on the preformed Au NP owing to the reduction in potential gap. This study provides new insights into controlling the morphology of hybrid NPs using colloidal synthesis and the design of optimized materials for various applications.

Iridium-Catalyzed Hydrogenation of a Phenoxy Radical to the Phenol: Overcoming Catalyst Deactivation with Visible Light Irradiation.

Piano-stool iridium hydride complexes bearing phenylpyridine ligands are effective precatalysts for promoting the formation of element-hydrogen bonds using H2 as the stoichiometric H-atom source. Irradiation with blue light resulted in a profound enhancement of catalyst turnover for the iridium-catalyzed hydrogenation of the aryloxyl radical 2,4,6-tBu3-C6H2O• to the corresponding phenol. Monitoring the progress of the reaction revealed the formation of an iridium 3,3-dimethyl-2,3-dihydrobenzofuranyl compound arising from two C-H activation events following the proton-coupled electron transfer (PCET) step. Under thermal conditions, this compound was inactive for catalytic aryloxide hydrogenation, representing a deactivation pathway. Irradiation with blue light under H2 released the free heterocycle and regenerated the piano-stool iridium hydride precatalyst, establishing a pathway for catalyst recovery and overall enhanced turnover.

Iridium-Catalyzed Chemo-, Diastereo-, and Enantioselective Allyl-Allyl Coupling: Accessing All Four Stereoisomers of (E)-1-Boryl-Substituted 1,5-Dienes by Chirality Pairing.

Here, we report a highly chemo-, diastereo-, and enantioselective allyl-allyl coupling between branched allyl alcohols and α-silyl-substituted allylboronate esters, catalyzed by a chiral iridium complex. The α-silyl-substituted allylboronate esters can be chemoselectively coupled with allyl electrophiles, affording a diverse set of enantioenriched (E)-1-boryl-substituted 1,5-dienes in good yields, with excellent stereoselectivity. By permuting the chiral iridium catalysts and the substrates, we efficiently and selectively obtained all four stereoisomers bearing two consecutive chiral centers. Mechanistic studies via density functional theory calculations revealed the origins of the diastereo- and chemoselectivities, indicating the pivotal roles of the steric interaction, the ß-silicon effect, and a rapid desilylation process. Additional synthetic modifications for preparing a variety of enantioenriched compounds containing contiguous chiral centers are also included.

Synthesis, Electronic Structure, and Reactivity of a Planar Four-Coordinate, Cobalt-Imido Complex.

A four-coordinate cobalt-imido complex, (tBu mPNP)Co=NMes (tBu mPNP=modified PNP pincer ligand) has been synthesized from addition of 2,4,6-trimethylphenylazide (Mes-N3 ) to the corresponding dinitrogen complex. The solid-state structure determined by X-ray diffraction established a rare, idealized planar geometry with a Co=N bond distance of 1.716(2) Å. Magnetic measurements revealed an S=1 ground state with CAS-SCF calculations supporting radical character on the imide nitrogen. Thermolysis of the cobalt-imido compound induced selective insertion of the imido group into a Co-P bond and yielded a three-coordinate cobalt complex with a distorted T-shaped geometry. Transition state analysis conducted with DFT calculations established the thermodynamic stability of the P-N coupled product and provided insight into the exclusive selectivity.

Catalytic Hydrogenation of a Manganese(V) Nitride to Ammonia.

The catalytic hydrogenation of a metal nitride to produce free ammonia using a rhodium hydride catalyst that promotes H2 activation and hydrogen-atom transfer is described. The phenylimine-substituted rhodium complex (η5-C5Me5)Rh(MePhI)H (MePhI = N-methyl-1-phenylethan-1-imine) exhibited higher thermal stability compared to the previously reported (η5-C5Me5)Rh(ppy)H (ppy = 2-phenylpyridine). DFT calculations established that the two rhodium complexes have comparable Rh-H bond dissociation free energies of 51.8 kcal mol-1 for (η5-C5Me5)Rh(MePhI)H and 51.1 kcal mol-1 for (η5-C5Me5)Rh(ppy)H. In the presence of 10 mol% of the phenylimine rhodium precatalyst and 4 atm of H2 in THF, the manganese nitride (tBuSalen)Mn≡N underwent hydrogenation to liberate free ammonia with up to 6 total turnovers of NH3 or 18 turnovers of H• transfer. The phenylpyridine analogue proved inactive for ammonia synthesis under identical conditions owing to competing deleterious hydride transfer chemistry. Subsequent studies showed that the use of a non-polar solvent such as benzene suppressed formation of the cationic rhodium product resulting from the hydride transfer and enabled catalytic ammonia synthesis by proton-coupled electron transfer.

Iridium-Catalyzed Enantioselective C(sp3)-H Amidation Controlled by Attractive Noncovalent Interactions.

While remarkable progress has been made over the past decade, new design strategies for chiral catalysts in enantioselective C(sp3)-H functionalization reactions are still highly desirable. In particular, the ability to use attractive noncovalent interactions for rate acceleration and enantiocontrol would significantly expand the current arsenal for asymmetric metal catalysis. Herein, we report the development of a highly enantioselective Ir(III)-catalyzed intramolecular C(sp3)-H amidation reaction of dioxazolone substrates for synthesis of optically enriched γ-lactams using a newly designed α-amino-acid-based chiral ligand. This Ir-catalyzed reaction proceeds with excellent efficiency and with outstanding enantioselectivity for both activated and unactivated alkyl C(sp3)-H bonds under very mild conditions. It offers the first general route for asymmetric synthesis of γ-alkyl γ-lactams. Water was found to be a unique cosolvent to achieve excellent enantioselectivity for γ-aryl lactam production. Mechanistic studies revealed that the ligands form a well-defined groove-type chiral pocket around the Ir center. The hydrophobic effect of this pocket allows facile stereocontrolled binding of substrates in polar or aqueous media. Instead of capitalizing on steric repulsions as in the conventional approaches, this new Ir catalyst operates through an unprecedented enantiocontrol mechanism for intramolecular nitrenoid C-H insertion featuring multiple attractive noncovalent interactions.

Harnessing Secondary Coordination Sphere Interactions That Enable the Selective Amidation of Benzylic C-H Bonds.

Engineering site-selectivity is highly desirable especially in C-H functionalization reactions. We report a new catalyst platform that is highly selective for the amidation of benzylic C-H bonds controlled by π-π interactions in the secondary coordination sphere. Mechanistic understanding of the previously developed iridium catalysts that showed poor regioselectivity gave rise to the recognition that the π-cloud of an aromatic fragment on the substrate can act as a formal directing group through an attractive noncovalent interaction with the bidentate ligand of the catalyst. On the basis of this mechanism-driven strategy, we developed a cationic (η5-C5H5)Ru(II) catalyst with a neutral polypyridyl ligand to obtain record-setting benzylic selectivity in an intramolecular C-H lactamization in the presence of tertiary C-H bonds at the same distance. Experimental and computational techniques were integrated to identify the origin of this unprecedented benzylic selectivity, and robust linear free energy relationship between solvent polarity index and the measured site-selectivity was found to clearly corroborate that the solvophobic effect drives the selectivity. Generality of the reaction scope and applicability toward versatile γ-lactam synthesis were demonstrated.

Transition Metal-Catalyzed C-H Amination: Scope, Mechanism, and Applications.

Catalytic transformation of ubiquitous C-H bonds into valuable C-N bonds offers an efficient synthetic approach to construct N-functionalized molecules. Over the last few decades, transition metal catalysis has been repeatedly proven to be a powerful tool for the direct conversion of cheap hydrocarbons to synthetically versatile amino-containing compounds. This Review comprehensively highlights recent advances in intra- and intermolecular C-H amination reactions utilizing late transition metal-based catalysts. Initial discovery, mechanistic study, and additional applications were categorized on the basis of the mechanistic scaffolds and types of reactions. Reactivity and selectivity of novel systems are discussed in three sections, with each being defined by a proposed working mode.

Trioctylphosphine-assisted morphology control of ZnO nanoparticles.

This study investigates the morphological change in colloidal ZnO nanoparticles (NPs) synthesized with trioctylphosphine (TOP). The addition of TOP to the synthesis causes an evolution in the shape of ZnO NPs to tadpole-like particles from quasi-spherical particles at 300 °C. The total length of the tadpole-like ZnO NPs can be modified by controlling the molar ratio of TOP to oleylamine (OLAM). The tadpole-like particles are elongated as the concentration of TOP increased but decreased when the addition of TOP is excessive. These tadpole-like ZnO NPs transform to quasi-spherical NPs regardless of the amount of TOP at a reaction time of 3 h at 300 °C. At 200 °C, the effect of TOP on the ZnO NP synthesis differs from that at 300 °C. The ZnO NPs synthesized by controlling the molar ratios of surfactant ligands (TOP:OLAM = 2:100 and 70:100) at 200 °C share similar amorphous structures, while a crystalline ZnO phase is formed when the reaction time is 3 h. X-ray photoelectron spectroscopy analysis shows that TOP influences the oxidation of ZnO and suggests that a combination of OLAM and TOP plays a role in controlling the shape of ZnO NPs. These results provide critical insights to the utilization of TOP for a shape controlling ligand in ZnO NPs and suggest a new route to design oxide NPs.

Revisiting Arene C(sp2 )-H Amidation by Intramolecular Transfer of Iridium Nitrenoids: Evidence for a Spirocyclization Pathway.

Two mechanistic pathways, that is, electrocyclization and electrophilic aromatic substitution, are operative in most intramolecular C-H amination reactions proceeding by metal nitrenoid catalysis. Reported here is an alternative mechanistic scaffold leading to benzofused δ-lactams selectively. Integrated experimental and computational analysis revealed that the reaction proceeds by a key spirocyclization step followed by a skeletal rearrangement. Based on this mechanistic insight, a new synthetic route to spirolactams has been developed.

Mechanism of Rh-Catalyzed Oxidative Cyclizations: Closed versus Open Shell Pathways.

A conceptual theory for analyzing and understanding oxidative addition reactions that form the cornerstone of many transition metal mediated catalytic cycles that activate C-C and C-H bonds, for example, was developed. The cleavage of the σ- or π-bond in the organic substrate can be envisioned to follow a closed or an open shell formalism, which is matched by a corresponding electronic structure at the metal center of the catalyst. Whereas the assignment of one or the other mechanistic scenario appears formal and equivalent at first sight, they should be recognized as different classes of reactions, because they lead to different reaction optimization and control strategies. The closed-shell mechanism involves heterolytic bond cleavages, which give rise to highly localized charges to form at the transition state. In the open-shell pathway, bonds are broken homolytically avoiding localized charges to accumulate on molecular fragments at the transition states. As a result, functional groups with inductive effects may exert a substantial influence on the energies of the intermediate and transition states, whereas no such effect is expected if the mechanism proceeds through the open-shell mechanism. If these functional groups are placed in a way that opens an electronic communication pathway to the molecular sites where charges accumulate, for example, using hyperconjugation, electron donating groups may stabilize a positive charge at that site. An instructive example is discussed, where this stereoelectronic effect allowed for rendering the oxidative addition diastereoselective. No such control is possible, however, when the open-shell reaction pathway is followed, because the inductive effects of functional groups have little to no effect on the stabilities of radical-like substrate states that are encountered when the bonds are broken in a homolytic fashion. Whether the closed-shell or open-shell mechanism for oxidative addition is followed is determined by the ordering of the d-orbital dominated frontier orbitals. If the highest occupied molecular orbital (HOMO) is oriented in space in such a way that will give the organic substrate easy access to the valence electron pair, the closed-shell mechanism can be followed. If the shape and orientation of the HOMO is not appropriate, however, an alternative pathway involving singlet excited states of the metal that will invoke the matching radicaloid cleavage of the organic substrate will dominate the oxidative addition. This novel paradigm for formally analyzing and understanding oxidative additions provides a new way of systematically understanding and planning catalytic reactions, as demonstrated by the in silico design of room-temperature Pauson-Khand reactions.

Mechanism-Driven Approach To Develop a Mild and Versatile C-H Amidation through IrIII Catalysis.

Described herein is a mechanism-based approach to develop a versatile C-H amidation protocol under IrIII catalysis. Reaction kinetics of a key C-N coupling step with acyl azide and 1,4,2-dioxazol-5-one led us to conclude that dioxazolones are much more efficient in mediating the formation of a carbon-nitrogen bond from an iridacyclic intermediate. Computational analysis revealed that the origin of higher reactivity is asynchronous decarboxylation motion, which may facilitate the formation of Ir-imido species. Importantly, stoichiometric reactivity was successfully translated into catalytic activity with a broad range of substrates (18 different types), many of which are regarded as challenging to functionalize. Application of the new method enables late-stage functionalization of drug molecules.

Why is the Ir(III)-Mediated Amido Transfer Much Faster Than the Rh(III)-Mediated Reaction? A Combined Experimental and Computational Study.

The mechanism of the Ir(III)- and Rh(III)-mediated C-N coupling reaction, which is the key step for catalytic C-H amidation, was investigated in an integrated experimental and computational study. Novel amidating agents containing a 1,4,2-dioxazole moiety allowed for designing a stoichiometric version of the catalytic C-N coupling reaction and giving access to reaction intermediates that reveal details about each step of the reaction. Both DFT and kinetic studies strongly point to a mechanism where the M(III)-complex engages the amidating agent via oxidative coupling to form a M(V)-imido intermediate, which then undergoes migratory insertion to afford the final C-N coupled product. For the first time, the stoichiometric versions of the Ir- and Rh-mediated amidation reaction were compared systematically to each other. Iridium reacts much faster than rhodium (∼1100 times at 6.7 °C) with the oxidative coupling being so fast that the activation of the initial Ir(III)-complex becomes rate-limiting. In the case of Rh, the Rh-imido formation step is rate-limiting. These qualitative differences stem from a unique bonding feature of the dioxazole moiety and the relativistic contraction of the Ir(V), which affords much more favorable energetics for the reaction. For the first time, a full molecular orbital analysis is presented to rationalize and explain the electronic features that govern this behavior.

Mechanistic studies on the Rh(III)-mediated amido transfer process leading to robust C-H amination with a new type of amidating reagent.

Mechanistic investigations on the Cp*Rh(III)-catalyzed direct C-H amination reaction led us to reveal the new utility of 1,4,2-dioxazol-5-one and its derivatives as highly efficient amino sources. Stepwise analysis on the C-N bond-forming process showed that competitive binding of rhodium metal center to amidating reagent or substrate is closely related to the reaction efficiency. In this line, 1,4,2-dioxazol-5-ones were observed to have a strong affinity to the cationic Rh(III) giving rise to dramatically improved amidation efficiency when compared to azides. Kinetics and computational studies suggested that the high amidating reactivity of 1,4,2-dioxazol-5-one can also be attributed to the low activation energy of an imido-insertion process in addition to the high coordination ability. While the characterization of a cationic Cp*Rh(III) complex bearing an amidating reagent was achieved, its facile conversion to an amido-inserted rhodacycle allowed for a clear picture on the C-H amidation process. The newly developed amidating reagent of 1,4,2-dioxazol-5-ones was applicable to a broad range of substrates with high functional group tolerance, releasing carbon dioxide as a single byproduct. Additional attractive features of this amino source, such as they are more convenient to prepare, store, and use when compared to the corresponding azides, take a step closer toward an ideal C-H amination protocol.

Regiodivergent access to five- and six-membered benzo-fused lactams: Ru-catalyzed olefin hydrocarbamoylation.

We report herein a new strategy of the Ru-catalyzed intramolecular olefin hydrocarbamoylation for the regiodivergent synthesis of five- and six-membered benzo-fused lactams starting from N-(2-alkenylphenyl)formamides. Using a combined catalyst of Ru3(CO)12/Bu4NI in DMSO/toluene cosolvent (catalytic system A), a 5-exo-type cyclization proceeds favorably to form indolin-2-ones as a major product in good to excellent yield. When the reaction was conducted in the absence of halide additives in DMA/PhCl (catalytic system B), 3,4-dihydroquinolin-2-ones were obtained in major in moderate to high yield via a 6-endo cyclization process. An excellent level of regioselectivity was observed with a variety of substrates to deliver 5-exo- or 6-endo-cyclized lactams. It was found that while the selective cyclization was controlled primarily by the choice of catalytic systems employed, it was also greatly influenced by the structural nature of substrates. A halide-bridged trinuclear complex [Ru3(CO)10(µ2-I)](-) is postulated to be an active species in the catalytic system A. Two reaction pathways are proposed, in which the Ru-catalyzed oxidative addition of formyl C-H or N-H bond initiates the subsequent cyclization processes.

Mechanistic studies of the rhodium-catalyzed direct C-H amination reaction using azides as the nitrogen source.

Direct C-H amination of arenes offers a straightforward route to aniline compounds without necessitating aryl (pseudo)halides as the starting materials. The recent development in this area, in particular in the metal-mediated transformations, is significant with regard to substrate scope and reaction conditions. Described herein are the mechanistic details on the Rh-catalyzed direct C-H amination reaction using organic azides as the amino source. The most important two stages were investigated especially in detail: (i) the formation of metal nitrenoid species and its subsequent insertion into a rhodacycle intermediate, and (ii) the regeneration of catalyst with concomitant release of products. It was revealed that a stepwise pathway involving a key Rh(V)-nitrenoid species that subsequently undergoes amido insertion is favored over a concerted C-N bond formation pathway. DFT calculations and kinetic studies suggest that the rate-limiting step in the current C-H amination reaction is more closely related to the formation of Rh-nitrenoid intermediate rather than the presupposed C-H activation process. The present study provides mechanistic details of the direct C-H amination reaction, which bears both aspects of the inner- and outer-sphere paths within a catalytic cycle.

Rh(III)-catalyzed traceless coupling of quinoline N-oxides with internal diarylalkynes.

Quinoline N-oxides were found to undergo Cp*Rh(III)-catalyzed coupling with internal diarylalkynes to provide 8-functionalized quinolines through a cascade process that involves remote C-H bond activation, alkyne insertion, and intramolecular oxygen atom transfer. In this reaction, the N-oxide plays a dual role, acting as a traceless directing group as well as a source of oxygen atom, as confirmed by an (18)O-labeling experiment.

Calibration between color camera and 3D LIDAR instruments with a polygonal planar board.

Calibration between color camera and 3D Light Detection And Ranging (LIDAR) equipment is an essential process for data fusion. The goal of this paper is to improve the calibration accuracy between a camera and a 3D LIDAR. In particular, we are interested in calibrating a low resolution 3D LIDAR with a relatively small number of vertical sensors. Our goal is achieved by employing a new methodology for the calibration board, which exploits 2D-3D correspondences. The 3D corresponding points are estimated from the scanned laser points on the polygonal planar board with adjacent sides. Since the lengths of adjacent sides are known, we can estimate the vertices of the board as a meeting point of two projected sides of the polygonal board. The estimated vertices from the range data and those detected from the color image serve as the corresponding points for the calibration. Experiments using a low-resolution LIDAR with 32 sensors show robust results.

How Do Colloidal Nanoparticles Move in a Solution under an Electric Field?: In Situ Light Scattering Analysis.

Understanding the dynamics of colloidal nanoparticles (NPs) in a solution is the key to assembling them into solids through a solution process such as electrophoretic deposition. In this study, newly developed in situ analysis with light scattering is used to examine NP dynamics induced by a non-uniform electric field. We reveal that the symmetric directions of moving NP aggregates are due to dielectrophoresis between the cylindrical electrodes, while the actual NP deposition is based on the charge of NPs (electrophoresis). Over time, the symmetry of the dynamics becomes less evident, inducing feeble deposition as the less-ordered dynamics become stronger. Eventually, two separate deposition mechanisms emerge as the deposition rate decreases with the change in the NP dynamics. Furthermore, we identify the vortex-like NP motion between the electrodes. These in situ analyses provide insights into the electrophoretic deposition mechanism and NP behavior in a solution under an electric field for fine film construction.

Design of Eu(TTA)3phen-incorporated SiO2-coated transition metal oxide nanoparticles for efficient luminescence and magnetic performance.

The development of multifunctional nanoparticles (NPs) combining individual properties, such as magnetic, luminescence, and optical properties, has attracted significant research interest. In this study, europium (Eu)-incorporating iron oxide nanoparticles (IONPs) with Eu(TTA)3phen (ET-SIOPs) were successfully designed and shown to have luminescence and magnetic properties. The proposed synthetic method has three steps: (1) IONP synthesis, (2) SiO2 layer coating (1st coating), and (3) Eu-SiO2 layer coating (2nd coating). The morphology of the ET-SIOPs was well preserved after the 2nd coating was conducted. According to the photoluminescence (PL) spectra in the range of 500 to 700 nm, the Eu-incorporating SIOPs with Eu(TTA)3phen (ET-SIOPs) exhibited the highest emission intensity compared to the Eu-incorporating SIOPs synthesized with other Eu precursors. Furthermore, the ET-SIOPs exhibited long-term luminescence stability of 6 months. In addition, this method of double-layer coating can be applied to other materials synthesized with different compositions and shapes, such as MnO and SiO2 NPs. The findings of this study will not only provide new insights for the synthesis of luminescent-magnetic NPs with long-term luminescence stability and paramagnetic properties, but can also be applied for the design of various multifunctional NPs.