Shunhua Li

Stabilization of anti-aromatic and strained five-membered rings with a transition metal

Anti-aromatic compounds, as well as small cyclic alkynes or carbynes, are particularly challenging synthetic goals. The combination of their destabilizing features hinders attempts to prepare molecules such as pentalyne, an 8π-electron anti-aromatic bicycle with extremely high ring strain. Here we describe the facile synthesis of osmapentalyne derivatives that are thermally viable, despite containing the smallest angles observed so far at a carbyne carbon. The compounds are characterized using X-ray crystallography, and their computed energies and magnetic properties reveal aromatic character. Hence, the incorporation of the osmium centre not only reduces the ring strain of the parent pentalyne, but also converts its Hückel anti-aromaticity into Craig-type Möbius aromaticity in the metallapentalynes. The concept of aromaticity is thus extended to five-membered rings containing a metal-carbon triple bond. Moreover, these metal-aromatic compounds exhibit unusual optical effects such as near-infrared photoluminescence with particularly large Stokes shifts, long lifetimes and aggregation enhancement.

Nanometer fluorescent hybrid silica particle as ultrasensitive and photostable biological labels

Nanometer-sized fluorescent hybrid silica (NFHS) particles were prepared for use as sensitive and photostable fluorescent probes in biological staining and diagnostics. The first step of the synthesis involves the covalent modification of 3-aminopropyltrimethoxysilane with an organic fluorophore, such as fluorescein isothiocyanate, under N2 atmosphere for getting a fluorescent silica precursor. Then the NFHS particles, with a diameter of well below 40 nm, were prepared by controlled hydrolysis of the fluorescent silica precursor with tetramethoxysilane (TMOS) using the reverse micelle technique. The fluorophores are dispersed homogeneously in the silica network of the NFHS particles and well protected from the environmental oxygen. Furthermore, since the fluorophores are covalently bound to the silica network, there is no migration, aggregation and leakage of the fluorophores. In comparison with common single organic fluorophores, these particle probes are brighter, more stable against photobleaching and do not suffer from intermittent on/off light emission (blinking). We have used these newly developed NFHS particles as a fluorescent marker to label antibodies, using silica immobilization method, for the immunoassay of human alpha-fetoprotein (AFP). The detection limit of this method was down to 0.05 ng mL(-1) under our current experimental conditions. We think this material would attract much attention and be applied widely in biotechnology.

A cyclometalated palladium-azo complex as a differential chromogenic probe for amino acids in aqueous solution

Solutions of a cyclometalated palladium-azo complex exhibited differential UV-Vis absorption spectra in the presence of alpha-amino acids with different side chain groups.

pH-Switchable Inversion of the Metal-Centered Chirality of Metallabenzenes: Opposite Stereodynamics in Reactions of Ruthenabenzene with l- and d-Cysteine

We report herein the first study on the chemical interaction between metallabenzenes and bioactive molecules. Due to its unique stereoelectronic activities, a phenanthroline-derived ruthenabenzene [Ru{CHC(PPh(3))CHC(PPh(3))CH}Cl(C(12)H(8)N(2))(PPh(3))]Cl(2) (1) selectively binds cysteine in aqueous solution at physiological pH and then undergoes a dynamic inversion of configuration at the Ru center. The structure of the L-cysteine-binding product of 1 has been determined by means of X-ray diffraction. The replacement of the L-cysteine with the D form results in an inverted stereodynamic effect. Furthermore, the inversion process of the Ru-centered configuration could be conveniently controlled by a simple pH adjustment. This is attributed to the significant influence of a special intramolecular electrostatic interaction on the dynamic epimerization process of the cysteine-binding product.

Annulation of Metallabenzenes: From Osmabenzene to Osmabenzothiazole to Osmabenzoxazole**

Research into transition-metal-containing metallaaromatics is attracting considerable current attention. Since the first example of metallabenzenes were reported by Roper et al. in 1982, a wide variety of monocyclic metallaaromatics have been successfully isolated and characterized. However, the chemistry of fused metallaaromatics is less developed. Most of the reported fused metallaaromatics were constructed by the cyclometalation of an unsaturated precursor. 7] Only one exceptional example, based on the metal insertion reaction of benzothiophene, has been reported. In principle, it may be another efficient approach to construct fused metallaaromatics from monocyclic metallaaromatics; however, such a possibility has not been realized to date. Herein, we report a novel annulation reaction leading to the first metallabenzothiazole 2 based on the intramolecular nucleophilic aromatic substitution (SNAr) reaction of metallabenzene 1. Furthermore, transformation of 2 to the first metallabenzoxazoles 4 and 5 is also presented (Scheme 1). It is now well-established that arenes, and especially heteroarenes, could undergo SNAr reaction if electron-withdrawing substituents are positioned ortho or para to the leaving group (typically a halogen) on the ring. Recently, we reported the synthesis of osmabenzene 1 bearing a phosphonium substituent and a reactive thiocyano group on the metallacycle. In this system, the metal center and the electron-withdrawing phosphonium group may exert significant influence on the electron density of the aromatic ring. In this regard, complex 1 might be expected to undergo intramolecular SNAr reactions to construct new metallaaromatic compounds. With this idea in mind, we studied the reaction of osmabenzene 1 with MeONa/MeOH, from which osmabenzothiazole 2 was isolated as a green solid (Scheme 1). This new product is air-stable and can be kept for several days without appreciable decomposition either in the solid state or in solution at room temperature. The structure of 2 has been confirmed by X-ray diffraction. As shown in Figure 1, it contains a perfectly planar metallabenzothiazole unit. The maximum deviation from the least-squares plane of the whole metallabenzothiazole system is 0.0096 . As expected, the lengths of the Os1 C1 (1.954(7) ) and Os1 C5 (1.953(8) ) and the C C bonds within the osmabenzene ring are within the range of those observed for other osmabenzenes. 9] The C N and C S bond lengths of the thiazole moiety are similar to those

New Highly Stable Metallabenzenes via Nucleophilic Aromatic Substitution Reaction

Treatment of the ruthenabenzene [Ru{CHC(PPh(3))CHC(PPh(3))CH}Cl(2)(PPh(3))(2)]Cl (1) with excess 8-hydroxyquinoline in the presence of CH(3)COONa under air atmosphere produced the S(N)Ar product [(C(9) H(6)NO)Ru{CHC(PPh(3))CHC(PPh(3))C}(C(9)H(6)NO)(PPh(3))]Cl(2) (3). Ruthenabenzene 3 could be stable in the solution of weak alkali or weak acid. However, reaction of 3 with NaOH afforded a 7:1 mixture of ruthenabenzenes [(C(9)H(6)NO)Ru{CHC(PPh(3))CHCHC}(C(9)H(6)NO)(PPh(3))]Cl (4) and [(C(9)H(6)NO)Ru{CHCHCHC(PPh(3))C}(C(9)H(6)NO)(PPh(3))]Cl (5), presumably involving a P-C bond cleavage of the metallacycle. Complex 3 was also reactive to HCl, which results in a transformation of 3 to ruthenabenzene [Ru{CHC(PPh(3))CHC(PPh(3))C}Cl(2)(C(9)H(6)NO)(PPh(3))]Cl (6) in high yield. Thermal stability tests showed that ruthenabenzenes 4, 5, and 6 have remarkable thermal stability both in solid state and in solution under air atmosphere. Ruthenabenzenes 4 and 5 were found to be fluorescent in common solvents and have spectral behaviors comparable to those organic multicyclic compounds containing large π-extended systems.

Turn-On Luminescent Sensing of Metal Cations via Quencher Displacement: Rational Design of a Highly Selective Chemosensor for Chromium(III)

A highly selective luminescent chemosensor for Cr(3+) in aqueous solution was assembled by a low-selectivity luminogenic receptor with Cu(2+) as a metal quencher. Three tetranitrogen chelating sites were integrated into the multichannel receptor with a tris(1,10-phenanthroline)ruthenium(II) luminophore at the core. This receptor (2) exhibits chelating affinity for many transition-metal cations, among which Cu(2+) efficiently quenches the emission. The further addition of Cr(3+) into the Cu(2+)-titrated solution of 2 results in a metal-exchange reaction and a sensitive turn-on luminescence response highly selective over other metal cations. The quencher displacement sensing strategy in this design can be a simple but efficient approach for OFF-ON luminescent sensing of metal cations that inherently lack selective ligands.

Switching the ligand-exchange reactivities of chloro-bridged cyclopalladated azobenzenes for the colorimetric sensing of thiocyanate

A dinuclear cyclopalladated complex of methyl orange shows a sensitive chromogenic response towards thiocyanate over a series of other anions in aqueous solution at physiological pH.

Specific ratiometric fluorescent sensing of Hg2+via the formation of mercury(II) barbiturate coordination polymers

A structurally simple and easily synthesized anthracene derivative bearing a barbituric acid moiety (AnB) was developed as a novel fluorescent chemosensor for Hg2+ in terms of specific ratiometric fluorescence response to Hg2+ in aqueous solution via the formation of Hg2+–AnB coordination polymers.

Off/On Fluorescent Chemosensors for Organotin Halides Based on Binuclear Ruthenium Complexes

National Basic Research Program of China [2011CB910403]; National Natural Science Foundation of China [21175113, 20925208, 20835005]; Fundamental Research Funds for the Central Universities [2011121015]