Ran Lin

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.

Synthesis and Characterization of a Metallapyridyne Complex

Building bridges: The first m-metallapyridine and the first metallapyridyne were synthesized under mild reaction conditions. The two complexes are metal-bridged polycyclic metallabenzenoid aromatics, in which the transition-metal center is shared by both six-membered rings. The synthetic method permits the use of metallabenzene as a starting material to access higher π-electron metallaaromatics.

Synthesis, Characterization and Electrochemical Properties of Stable Osmabenzenes Containing PPh3 Substituents

Treatment of [OsCl(2)(PPh(3))(3)] with HC[triple bond]CCH(OH)C[triple bond]CH/PPh(3) produces the osmabenzene [Os{CHC(PPh(3))CHC(PPh(3))CH}Cl(2)(PPh(3))(2)][OH] (2), which is air stable in both solution and solid state. The key intermediate of the one-pot reaction, [OsCl(2){CH=C(PPh(3))CH(OH)C[triple bond]CH}(PPh(3))(2)] (3), and the related complex [Os(NCS)(2){CHC(PPh(3))CH(OH)C[triple bond]CH}(PPh(3))(2)] (7) have been isolated and characterized, further supporting the proposed mechanisms for the reaction. Reactions of 3 with PPh(3), NaI, and NaSCN give osmabenzene 2, iodo-substituted osmabenzene [Os{CHC(PPh(3))CHCICH}I(2)(PPh(3))(2)] (4), and thiocyanato-substituted osmabenzene [Os{CHC(PPh(3))CHC(SCN)CH}(NCS)(2)(PPh(3))(2)] (5) respectively. Similarly, reaction of [OsBr(2)(PPh(3))(3)] with HC[triple bond]CCH(OH)C[triple bond] CH in THF produces [OsBr(2){CH=C(PPh(3))CH(OH)C[triple bond]CH}(PPh(3))(2)] (9), which reacts with PPh(3)/Bu(4)NBr to give osmabenzene [Os{CHC(PPh(3))CHC(PPh(3))CH}Br(2)(PPh(3))(2)]Br (10). Ligand substitution reactions of 2 produce a series of new stable osmabenzenes 11-17. An electrochemical study shows that osmabenzenes 2, 12, and 14-17 have interesting different electrochemical properties due to the different co-ligand. The oxidation potentials of complexes 2, 12, 16, and 17 with Cl, NCS, and N(CN)(2) ligands gradually positively shift in the sequence of Cl<NCS<N(CN)(2). Among the six compounds, only 12 and 17 undergo a well-behaved, nearly reversible and a quasi-reversible reduction process, respectively, indicating that two NCS or N(CN)(2) ligands contribute to the stabilization of their reduced states.

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.

Nucleophilic Aromatic Addition Reactions of the Metallabenzenes and Metallapyridinium: Attacking Aromatic Metallacycles with Bis(diphenylphosphino)methane to Form Metallacyclohexadienes and Cyclic eta(2)-Allene-Coordinated Complexes

The reactions of phosphonium-substituted metallabenzenes and metallapyridinium with bis(diphenylphosphino)methane (DPPM) were investigated. Treatment of [Os{CHC(PPh(3))CHC(PPh(3))CH}Cl(2)(PPh(3))(2)]Cl with DPPM produced osmabenzenes [Os{CHC(PPh(3))CHC(PPh(3))CH}Cl(2){(PPh(2))CH(2)(PPh(2))}]Cl (2), [Os{CHC(PPh(3))CHC(PPh(3))CH}Cl{(PPh(2))CH(2)(PPh(2))}(2)]Cl(2) (3), and cyclic osmium eta(2)-allene complex [Os{CH=C(PPh(3))CH=(eta(2)-C=CH)}Cl(2){(PPh(2))CH(2)(PPh(2))}(2)]Cl (4). When the analogue complex of osmabenzene 1, ruthenabenzene [Ru{CHC(PPh(3))CHC(PPh(3))CH}Cl(2)(PPh(3))(2)]Cl, was used, the reaction produced ruthenacyclohexadiene [Ru{CH=C(PPh(3))CH=C(PPh(3))CH}Cl{(PPh(2))CH(2)(PPh(2))}(2)]Cl(2) (6), which could be viewed as a Jackson-Meisenheimer complex. Complex 6 is unstable in solution and can easily be convert to the cyclic ruthenium eta(2)-allene complexes [Ru{CH=C(PPh(3))CH=(eta(2)-C=CH)}Cl{(PPh(2))CH(2)(PPh(2))}(2)]Cl(2) (7) and [Ru{CH=C(PPh(3))CH=(eta(2)-C=CH)}Cl(2){(PPh(2))CH(2)(PPh(2))}(2)]Cl (8). The key intermediates of the reactions have been isolated and fully characterized, further supporting the proposed mechanism for the reactions. Similar reactions also occurred in phosphonium-substituted metallapyridinium [OsCl(2){NHC(CH(3))C(Ph)C(PPh(3))CH}(PPh(3))(2)]BF(4) to give the cyclic osmium eta(2)-allene-imine complex [OsCl(2){NH=C(CH(3))C(Ph)=(eta(2)-C=CH)}{(PPh(2))CH(2)(PPh(2))}(PPh(3))]BF(4) (11).

Synthesis of rhenabenzenes from the reactions of rhenacyclobutadienes with ethoxyethyne.

Treatment of Na[Re(CO)5 ] with RCCCO2 Et (R=phenyl, naphthalen-1-yl, phenanthren-9-yl and pyren-1-yl) followed by reaction with acetyl chloride and ethanol afforded the rhenacyclobutadienes Re{-C(R)C(CO2 Et)C(OEt)}(CO)4 . Reactions of these rhenacyclobutadienes with HCCOEt produced rhenabenzenes Re{-C(R)C(CO2 Et)C(OEt)CHC(OEt)}(CO)4 . Except for R=Ph, new rhenacyclobutadienes with pendant alkenyl substituents Re{-C(R)C(C(OEt)CH(CO2 Et))C(OEt)}(CO)4 were also isolated from these reactions. The NMR spectroscopic and X-ray structural data, as well as the aromatic stabilization energy (ASE) values suggest that the rhenabenzenes are aromatic, with extensive delocalized π character.

Interconversion of Metallabenzenes and Cyclic η2-Allene-Coordinated Complexes

Treatment of the osmabenzene [Os{CHC(PPh(3))CHC(PPh(3))CH}Cl(2)(PPh(3))(2)]Cl (1) with excess 8-hydroxyquinoline produces monosubstituted osmabenzene [Os{CH C(PPh(3))CHC(PPh(3))CH}(C(9)H(6)NO)Cl(PPh(3))]Cl (2) or disubstituted osmabenzene [Os{CHC(PPh(3))CHC(PPh(3))CH}(C(9)H(6)NO)(2)]Cl (3) under different reaction conditions. Osmabenzene 2 evolves into cyclic η(2)-allene-coordinated complex [Os{CH=C(PPh(3))CH=(η(2)-C=CH(2))}(C(9)H(6)NO)(PPh(3))(2)]Cl (4) in the presence of excess PPh(3) and NaOH, presumably involving a P-C bond cleavage of the metallacycle. Reaction of 4 with excess 8-hydroxyquinoline under air affords the S(N) Ar product [(C(9)H(6)NO)Os{CHC(PPh(3))CHCHC}(C(9)H(6)NO)(PPh(3))]Cl (5). Complex 4 is fairly reactive to a nucleophile in the presence of acid, which could react with water to give carbonyl complex [Os{CH=C(PPh(3))CH=CH(2)}(C(9)H(6)NO)(CO)(PPh(3))(2)]Cl (6). Complex 4 also reacts with PPh(3) in the presence of acid and results in a transformation to [Os{CHC(PPh(3))CHCHC}(C(9)H(6)NO)Cl(PPh(3))(2)]Cl (7) and [Os{CH=C(PPh(3))CH=(η(2)-C=CH(PPh(3)))}(C(9)H(6)NO)Cl(PPh(3))]Cl (8). Further investigation shows that the ratio of 7 and 8 is highly dependent on the amount of the acid in the reaction.

Synthesis of a novel cyclodextrin-derived chiral stationary phase with multiple urea linkages and enantioseparation toward chiral osmabenzene complex

A novel chiral stationary phase was synthesized by immobilizing heptakis(6-azido-6-deoxy-2,3-di-O-p-chlorophenylcarbamoylated)-β-cyclodextrin onto silica gel surface via Staudinger reaction, and applied in enantiomeric separation of a pair of osmabenzene complexes, {Os[CHC(PPh3)CHC(PPh3)CH](C9H6NO)2}Cl (1), which is the first report for enantioseparation of the chiral-only-at-metal osmabenzene complex till now. The effects of separation conditions including salt additives, organic modifiers, pH values, and column temperature on the retention and resolution of the complex have been investigated in detail. Meanwhile, possible chiral recognition mechanism was presented. Chiral complex 1 was well resolved via semi-preparative chiral HPLC technique under optimization conditions and two pure enantiomers were further characterized by analytical HPLC, NMR spectra and solution circular dichroism (CD) spectra, respectively. Furthermore, absolute configuration of the enantiomer was confirmed by theoretical investigation of CD spectra.

Chemistry of Metallacyclobutadienes

Metallacyclobutadienes are analogues of cyclobutadienes in which one of the cyclobutadiene CR groups has been formally replaced by a transition-metal fragment. These metallacycles are interesting because they can play an important role in catalysis and can serve as starting materials for the syntheses of organometallic compounds such as metallabenzene, η5 -cyclopentadienyl, and η3 -cyclopropenyl complexes. Unlike cyclobutadienes, metallacyclobutadienes can be significantly more stable. A number of metallacyclobutadienes have now been isolated and thoroughly characterized, especially for those that contain transition metals of groups 5-9. Their properties have also been actively investigated. This article highlights the chemistry of metallacyclobutadienes with reference to their syntheses, reactivity, and structural properties.