[TOP] Nature Chemistry: Realizing a Brand-new Aromatic System, Metallapentalyne. (2013-06-23)
Date:2018-06-23 22:15    Author:jiang   

Aromaticity, one of the most fundamental concepts in organic chemistry, provides intrinsic stabilisation for cyclic compounds as a result of the delocalisation of π electrons. Currently, aromatic compounds (e.g., benzene, benzene derivatives, porphyrins, fullerenes, carbon nanotubes and graphene) have been applied in almost every field, including chemical engineering, biomedicine, materials science, energy science and environmental science.
  On the contrary, antiaromatic compounds with 4n 
π electrons are difficult to prepare and isolate because of they are extremely unstable. In addition, the synthesis of small cyclic alkynes or carbynes is also challenging because of the high ring strain that results from their nonlinear triple bonds. Therefore, pentalyne has never been synthesized due to the dual destabilization from the antiaromaticity and extreme ring strain in a five-membered ring. Recently, we incorporated an osmium centre into the bicyclic rings and synthesized the first metallapentalyne. This research has been published on Nature Chemistry (2013, 5, 698-703) entitled “Stabilization of anti-aromatic and strained five-membered rings with a transition metal”.
  It is difficult to synthesize small cyclic alkynes and metal carbynes. So far the smallest isolated examples of this family are 1-zirconacyclopent-3-yne and osmabenzyne, reported by Suzuki's (Science 295, 660 (2002)) and Jia's group (Angew. Chem. Int. Ed. 40, 1951 (2001)), respectively. The corresponding bond angles at the sp-hybridized carbon atom are 156.2° and 155.9° in the former and 148.7° in the latter. Now, the record of such an angle is refreshed to 129.5°.
  More interestingly, this metallapentalyne is exceptionally stable (stored at room temperature for three months and persistent thermally at 120 C in air for three hours) although it contains the smallest angle at the carbyne carbon. Calculations done by the group of Jun Zhu at our college revealed that osmapentalyne is aromatic, indicated by the large “isomerization stabilization energy” (ISE) values and negative nucleus-independent chemical shift (NICS) values and supported by the experimental observations (planarity and delocalized structure of the metallabicycles, downfield 1H chemical shifts). According to molecular orbital (MO) calculations, there are four occupied π MOs that reflect the π delocalization along the perimeter of the bicyclic system. These four molecular orbitals can be derived principally from the orbital interactions between the p atomic orbitals of the C7H5 unit and two of the d orbitals of the Os atom. Accordingly, the eight-membered ring in osmapentalyne is regarded as a cyclic eight-center eight-electron (8c-8e) Craig-type Möbius aromatic system, which is suggested by Paul v. R. Schleyer at University of Georgia and is reinforced by the results of canonical molecular orbital NICS computations. Thus the concept of aromaticity has now been extended to such metallabicycles as osmapentalynes that contain 8c-8e effective dπ-p
π Craig-type conjugation/delocalization of Möbius aromaticity.
  Unique structures lead to unique properties. Specifically, the metal-carbon triple bond can be shifted to another five-membered ring. Promising application is indicated by their unusual optical properties such as near-infrared photoluminescence excited by the blue light with particularly large Stokes shifts, long lifetimes and aggregation-induced enhancement.
  Dr. Torsten Beweries and Prof. Uwe Rosenthal at the University of Rostock commented this significant progress as News and Views entitled “Breaking the rules” on Nature Chemistry with DOI:10.1038/nchem.1702. 'The paper is an excellent one -- it's quite amazing that the parent Os system, molecule 1, chooses to give the osmapentalyne,' says Roald Hoffmann, the 1981 Nobel laureate in chemistry, in a private communication.
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