Room-temperature Chemical Synthesis of C2: Evidence for Quadruple Bonding Character and Role in Nanocarbon Formation

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Diatomic carbon (C2) exists in carbon vapour, comets, the stellar atmosphere, and interstellar matter, but although it was discovered in 1857, it has proved frustratingly difficult to characterize (Figure 1), since C2 gas occurs/exists only at extremely high temperatures (above 3500°C). Since 1930, several experimental methods to generate C2 have been developed by using extremely high energy processes, such as electric carbon arc and multiple photon excitation, and the C2 species obtained were reported to exhibit singlet dicarbene (double bond) and/or triplet biradical (triple bond) behavior. In contrast, recent theoretical simulations suggest that C2 in the ground state should have a singlet biradical (quadruple bond) character. These various theoretical and experimental findings have sparked extensive debate on the molecular bond order and electronic state of C2 in the scientific literature, probably because of the lack of a method for the selective synthesis of ground state C2.

Fig 1 | Long-standing dispute between experimental and theoretical chemists concerning the nature of diatomic carbon (C2).

   Here, we present the first straightforward room-temperature/pressure synthesis of C2 in a flask. The key strategy underlying the present achievement is the use of hypervalent iodane chemistry, aiming to utilize the phenyl-λ3-iodanyl moiety as a hyper-leaving group (ca. 106 times greater leaving ability than triflate). We found that ground state Cgenerated in this way behaves exclusively as a singlet biradical with quadruple bonding, as predicted by theory. This settles a long-standing difference of opinion between experimental and theoretical chemists.

  We also discovered that the spontaneous, solvent-free reaction of in-situ-generated C2 under an argon atmosphere results in the formation of graphite, carbon nanotubes (CNTs), carboncones, amorphous carbon, and fullerene (C60) at room temperature (Figure 2). Today, nanocarbons such as graphene, CNTs, and fullerenes, in which sp2-carbon takes the form of a planar sheet, tube, ellipsoid, or hollow sphere, are at the heart of nanotechnology. But, in contrast with the rapid growth of their practical applications, the mechanisms of their formation remain unclear.

   Therefore, this is not only the first chemical synthesis of nanocarbons at ordinary temperature and pressure, but also provides experimental evidence that C2 may serve as a key intermediate of various sp2-carbon allotropes.

Fig 2 | Solvent-free reaction of in-situ-generated C2 in a mortar at room temperature leads to spontaneous formation of carbon allotropes.

Masanobu UCHIYAMA

Professor, Univ. of Tokyo

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