Unusual Attractive Au–π Interactions in Small Diacetylene‐Modified Gold Clusters

It is well known that alkynes act as π‐acids in the formation of complexes with metals. We found unprecedented attractive Au–π interactions in diacetylene‐modified [core+exo]‐type [Au₈]⁴⁺ clusters. The 4‐phenyl‐1,3‐butadiynyl‐modified cluster has unusually short Au–C_α distances in the crystal structure, revealing the presence of attractive interactions between the coordinating C≡C moieties and the neighboring bitetrahedral Au₆ core, which is further supported by IR and NMR spectra. Such weak interactions are not found in mono‐acetylene‐modified clusters, which indicates that they are specific for diacetylenic ligands. The attractive Au–π interactions are likely associated with the low energy of the π* orbital in the diacetylenic moieties, into which the valence electrons of the gold core may be back donated. The [Au₈]⁴⁺ clusters show clear red‐shifts of >10 nm with respect to the corresponding mono‐acetylenic clusters in UV/Vis absorption bands, which indicates substantial electronic perturbation effects of the Au–π interactions.     

An Inherently Chiral Au24 Framework with Double‐Helical Hexagold Strands

2,3‐bis(diphenylphosphino)butane enantiomers (chiraphos, L) used as chiral auxiliaries results in the preferential formation of an unprecedented Au₂₄ framework with inherent chirality. The crystal structure of [Au₂₄L₆Cl₄]²⁺ ( 1 ) has a square antiprism‐like octagold core twinned by two helicene‐like hexagold motifs, where the inherent chirality is associated with the helical arrangement. The clusters carrying (R,R)‐ and (S,S)‐ diphosphines had right‐ and left‐handed strands, respectively. Circular dichroism spectra showed peaks in the visible to near‐IR region, some of which did not coincide with absorption bands, suggesting the enantiomeric Au₂₄ frameworks possess unique chiroptical properties. The Au₂₄ frameworks were thermally robust, which could be attributed to the superatomic concept (18 e^? system) and the steric constraint effects of the bridging ligand units.     

Unique [core+two] structure and optical property of a dodeca-ligated undecagold cluster: critical contribution of the exo gold atoms to the electronic structure

A novel dodeca-ligated undecagold cluster having a nonagold core plus two exo-attached gold atoms was synthesized. Unlike the conventional icosahedron-based "core-only" isomer, the [9+2] cluster showed an intense visible absorption band. Theoretical calculation showed the involvement of the exo gold atoms in the generation of the unique electronic structure.     

Ubiquitous 8 and 29 kDa gold:alkanethiolate cluster compounds: mass-spectrometric determination of molecular formulas and structural implications

The molecular formulas and charge state distributions of thus-far known ubiquitous alkanethiolate-protected gold clusters with core-masses of 8 and 29 kDa were assessed using electrospray ionization mass spectrometry. The 8 and 29 kDa clusters were determined to be composed of single species, [Au38(SCn)24]z and [Au144(SCn)59]z, respectively, with charge states of z >/= 0. Possible geometric structures for Au38(SCn)24 and Au144(SCn)59 are discussed, based on the structures of relevant systems that have been recently determined experimentally and theoretically: [Au25(SR)18]- and Au102(SR)44, in which the Au cores are protected by monomers [-SR-Au-SR-] and/or dimers [-SR-Au-SR-Au-SR-]. Their preferential formation and chemical robustness are proposed as being associated with high stability due to geometric factors, while the Au-thiolate interface takes on common motifs regardless of the underlying Au core.     

A photo‐degradable helix: Synthesis,structure, and photolysis of optically active poly[2,7‐bis(4‐t‐butylphenyl)‐9‐methylfluoren‐9‐yl acrylate]

2,7‐Bis(4‐t‐butylphenyl)‐9‐methylfluoren‐9‐yl acrylate ( BBPMFA ) was synthesized and polymerized using α,α′‐azobisisobutyronitrile or n‐Bu₃B‐air as a radical initiator and using the complex of 9‐fluorenyllithium with (S)‐(+)‐1‐(2‐pyrrolidinylmethyl)pyrrolidine as an optically active anionic initiator. Although the radical polymerization led to rather low‐molecular‐weight products at low yields, the anionic polymerization afforded polymers with higher molecular weights in higher yields. The poly( BBPMFA ) obtained by the anionic polymerization was slightly rich in isotacticity (meso diad 57%) and showed an intense circular dichroism (CD) spectrum and large dextrorotation. The intensity of the CD spectrum and magnitude of optical activity increased with an increase in M_n, suggesting that the polymer possesses a preferred‐handed helical conformation. The CD spectrum disappeared within 1 s on irradiation to the polymer in a CHCl₃ solution using a 500‐W Hg‐Xe lamp. This was ascribed to fast photolysis of the ester linkage leading to a loss of helical conformation of the entire chain. Photolysis products of poly( BBPMFA ) were poly(acrylic acid) and 2,7‐bis(4‐t‐butylphenyl)‐9‐methylenefluorene (2,7‐bis(4‐t‐butylphenyl)dibenzofulvene). The photolysis reaction seemed to proceed through the “unzipping” mechanism. The rate constant of photolysis of poly( BBPMFA ) under irradiation at monochromated 325 nm was around 0.01 s^?1 independent of molecular weight. Photolysis at 325 nm was approximately 2400 times faster than that for chemical ester solvolysis under a neutral condition in the dark. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Large-scale synthesis of thiolated Au25 clusters via ligand exchange reactions of phosphine-stabilized Au11 clusters

Phosphine-stabilized Au11 clusters in chloroform were reacted with glutathione (GSH) in water under a nitrogen atmosphere. The resulting Au:SG clusters exhibit an optical absorption spectrum similar to that of Au25(SG)18, which was isolated as one of the major products from chemically prepared Au:SG clusters (Negishi, Y. et al. J. Am. Chem. Soc. 2005, 127, 5261). Rigorous characterization by optical spectroscopy, electrospray ionization mass spectrometry, and polyacrylamide gel electrophoresis confirms that the Au25(SG)18 clusters were selectively obtained on the sub-100 mg scale by ligand exchange reaction under aerobic conditions. The ligand exchange strategy offers a practical and convenient method of synthesizing thiolated Au25 clusters on a large scale.