Haizhu Yu

Alternative Mechanistic Explanation for Ligand-Dependent Selectivities in Copper-Catalyzed N- and O-Arylation Reactions

The ligand-dependent selectivities in Ullmann-type reactions of amino alcohols with iodobenzene by β-diketone- and 1,10-phenanthroline-ligated Cu(I) complexes were recently explained by the single-electron transfer and iodine atom transfer mechanisms (Jones, G. O., Liu, P., Houk, K. N., and Buchwald, S. L. J. Am. Chem. Soc. 2010, 132, 6205.). The present study shows that an alternative, oxidative addition/reductive elimination mechanism may also explain the selectivities. Calculations indicate that a Cu(I) complex with a negatively charged β-diketone ligand is electronically neutral, so that oxidative addition of ArI to a β-diketone-ligated Cu(I) prefers to occur (and occur readily) in the absence of the amino alcohol. Thus, coordination of the amino alcohol in its neutral form can only occur at the Cu(III) stage where N-coordination is favored over O-coordination. The coordination step is the rate-limiting step and the outcome is that N-arylation is favored with the β-diketone ligand. On the other hand, a Cu(I) complex with a neutral 1,10-phenanthroline ligand is positively charged, so that oxidative addition of ArI to a 1,10-phenanthroline-ligated Cu(I) has to get assistance from a deprotonated amino alcohol substrate. This causes oxidative addition to become the rate-limiting step in the 1,10-phenanthroline-mediated reaction. The immediate product of the oxidative addition step is found to undergo facile reductive elimination to provide the arylation product. Because O-coordination of a deprotonated amino alcohol is favored over N-coordination in the oxidative addition transition state, O-arylation is favored with the 1,10-phenanthroline ligand.

Bimetallic Au2Cu6 Nanoclusters: Strong Luminescence Induced by the Aggregation of Copper(I) Complexes with Gold(0) Species

The concept of aggregation-induced emission (AIE) has been exploited to render non-luminescent Cu(I) SR complexes strongly luminescent. The Cu(I) SR complexes underwent controlled aggregation with Au(0) . Unlike previous AIE methods, our strategy does not require insoluble solutions or cations. X-ray crystallography validated the structure of this highly fluorescent nanocluster: Six thiolated Cu atoms are aggregated by two Au atoms (Au2 Cu6 nanoclusters). The quantum yield of this nanocluster is 11.7 %. DFT calculations imply that the fluorescence originates from ligand (aryl groups on the phosphine) to metal (Cu(I) ) charge transfer (LMCT). Furthermore, the aggregation is affected by the restriction of intramolecular rotation (RIR), and the high rigidity of the outer ligands enhances the fluorescence of the Au2 Cu6 nanoclusters. This study thus presents a novel strategy for enhancing the luminescence of metal nanoclusters (by the aggregation of active metal complexes with inert metal atoms), and also provides fundamental insights into the controllable synthesis of highly luminescent metal nanoclusters.

Crystallization-induced emission enhancement: A novel fluorescent Au-Ag bimetallic nanocluster with precise atomic structure

Crystallization-induced emission enhancement was achieved in metal nanoclusters for the first time. We report the first noble metal nanocluster with a formula of Au4Ag13(DPPM)3(SR)9 exhibiting crystallization-induced emission enhancement (CIEE), where DPPM denotes bis(diphenylphosphino)methane and HSR denotes 2,5-dimethylbenzenethiol. The precise atomic structure is determined by x-ray crystallography. The crystalline state of Au4Ag13 shows strong luminescence at 695 nm, in striking contrast to the weak emission of the amorphous state and hardly any emission in solution phase. The structural analysis and the density functional theory calculations imply that the compact C–H⋯π interactions significantly restrict the intramolecular rotations and vibrations and thus considerably enhance the radiative transitions in the crystalline state. Because the noncovalent interactions can be easily modulated via varying the chemical environments, the CIEE phenomenon might represent a general strategy to amplify the fluorescence from weakly (or even non-) emissive nanoclusters.

Shape-Controlled Synthesis of Trimetallic Nanoclusters: Structure Elucidation and Properties Investigation.

The shape-controlled synthesis of metal nanoclusters (NCs) with precise atomic arrangement is crucial for tailoring the properties. In this work, we successfully control the shape of alloy NCs by altering the dopants in the alloying processes. The shape of the spherical [Pt1 Ag24 (SPhMe2 )18 ] NC is maintained when [AuI SR] is used as dopant. By contrast, the shape of Pt1 Ag24 is changed to be rodlike by alloying with [AuI (PPh3 )Br]. The structures of the trimetallic NCs were determined by X-ray crystallography and further confirmed by both DFT and far-IR measurements. The shape-preserved [Pt1 Au6.4 Ag17.6 (SPhMe2 )18 ] NC is in a tristratified arrangement-[Pt(center)@Au/Ag(shell)@Ag(exterior)]-and is indeed the first X-ray crystal structure of thiolated trimetallic NCs. On the other hand, the resulting rodlike NC ([Pt2 Au10 Ag13 (PPh3 )10 Br7 ]) exhibits a high quantum yield (QY=14.7 %), which is in striking contrast to the weakly luminescent Pt1 Ag24 (QY=0.1 %, about 150-fold enhancement). In addition, the thermal stabilities of both trimetallic products are remarkably improved. This study presents a controllable strategy for synthesis of alloy NCs with different shapes (by alloying heteroatom complexes coordinated by different ligands), and may stimulate future work for a deeper understanding of the morphology (shape)-property correlation in NCs.

A mitochondria-targeted ratiometric two-photon fluorescent probe for biological zinc ions detection

A mitochondria-targeted ratiometric two-photon fluorescent probe (Mito-MPVQ) for biological zinc ions detection was developed based on quinolone platform. Mito-MPVQ showed large red shifts (68 nm) and selective ratiometric signal upon Zn(2+) binding. The ratio of emission intensity (I488 nm/I420 nm) increases dramatically from 0.45 to 3.79 (ca. 8-fold). NMR titration and theoretical calculation confirmed the binding of Mito-MPVQ and Zn(2+). Mito-MPVQ also exhibited large two-photon absorption cross sections (150 GM) at nearly 720 nm and insensitivity to pH within the biologically relevant pH range. Cell imaging indicated that Mito-MPVQ could efficiently located in mitochondria and monitor mitochondrial Zn(2+) under two-photon excitation with low cytotoxicity.

Substrate-Assisted, Transition-Metal-Free Diboration of Alkynamides with Mixed Diboron: Regio- and Stereoselective Access to trans-1,2-Vinyldiboronates

A substrate-assisted diboration of alkynamides using the unsymmetrical pinacolato-1,8-diaminonaphthalenato diboron (pinBBdan) is described. The transition-metal-free reaction proceeds in a regio- and stereoselective fashion to exclusively afford trans-vinyldiboronates in good to excellent yields. Notably, Bdan and Bpin are installed on the α- and β-carbon atoms, respectively.

Mechanism of Boron-Catalyzed N-Alkylation of Amines with Carboxylic Acids

Mechanistic study has been carried out on the B(C6F5)3-catalyzed amine alkylation with carboxylic acid. The reaction includes acid-amine condensation and amide reduction steps. In condensation step, the catalyst-free mechanism is found to be more favorable than the B(C6F5)3-catalyzed mechanism, because the automatic formation of the stable B(C6F5)3-amine complex deactivates the catalyst in the latter case. Meanwhile, the catalyst-free condensation is constituted by nucleophilic attack and the indirect H2O-elimination (with acid acting as proton shuttle) steps. After that, the amide reduction undergoes a Lewis acid (B(C6F5)3)-catalyzed mechanism rather than a Brønsted acid (B(C6F5)3-coordinated HCOOH)-catalyzed one. The B(C6F5)3)-catalyzed reduction includes twice silyl-hydride transfer steps, while the first silyl transfer is the rate-determining step of the overall alkylation catalytic cycle. The above condensation-reduction mechanism is supported by control experiments (on both temperature and substrates). Meanwhile, the predicted chemoselectivity is consistent with the predominant formation of the alkylation product (over disilyl acetal product).

The coordination of amidoxime ligands with uranyl in the gas phase: a mass spectrometry and DFT study

Sequestering uranium from the ocean is a promising solution to fulfill the demand for nuclear energy. Motivated by this purpose, a series of amidoxime ligands and their analogs have been developed with high absorption capacity and selectivity. An in-depth understanding of the structural information of the uranyl-ligand complexes is essential to improve the performance of the ligands. Herein, we have studied the coordination of three amidoxime ligands (6-methoxyl-naphtha-2-amidoxime, NAO; glutarimidedioxime, GIO; and gluardiamidoxime, GDO) with uranyl in the gas phase by mass spectrometry. The identifications of the electrospray ionization (ESI) generated species, the fragmentation pathways upon dissociation, the relative binding affinities of the ligands, and the hydration reactions have been conducted and compared to reveal their structural information in the gas phase. The binding modes for all the complexes were suggested based on the experimental results and were further studied by density functional theory (DFT) calculations.

Mechanism of Nickel‐Catalyzed Suzuki–Miyaura Coupling of Amides

The Ni-catalyzed Suzuki-Miyaura coupling of N-tert-butoxycarbonyl (N-Boc)-protected amides provides a versatile strategy for the construction of C-C bonds. In this study, density functional theory (DFT) methods have been used to elucidate the mechanism of this reaction, with particular emphasis on the roles of N-Boc, K3 PO4 and H2 O. Our results corroborated those of previous reports, indicating that the overall catalytic cycle consists of three steps, including oxidative addition, transmetalation, and reductive elimination. Three of the possible transmetalation mechanisms were examined to interpret the effects of K3 PO4 and H2 O. According to the most feasible of these transmetalation mechanisms, K3 PO4 (acting as a Lewis base) would initially interact with the Lewis acid PhBpin to give a K3 PO4 -PhBpin complex, which would readily undergo a hydrogen transfer step with H2 O. The H transfer in the transmetalation step was determined to be the rate-determining step. Notably, the theoretical results showed good agreement with the experimental data.

Modulating photo-luminescence of Au2Cu6 nanoclusters via ligand-engineering

In this work, the luminescence of Au2Cu6 nanoclusters was controlled by tailoring the ligand to metal charge transfer via engineering the phosphine ligands with electron-donating or -withdrawing substituents. The fluorescence intensity was significantly enhanced from the Au2Cu6 nanocluster with P(Ph–F)3 ligands (quantum yield QY = 5.7%) to that with P(Ph–OMe)3 ligands (QY = 17.7%). In addition, the fluorescence of Au2Cu6 protected by P(Ph–OMe)3 slightly red-shifts compared to that of Au2Cu6 protected by P(Ph–F)3, which is similar to the trends of UV-vis spectra tendency.