Generation and characterization of a fairly stable triplet carbene

Abstract

Most molecules are held together by covalent bonds-electron pairs jointly shared by the two atoms that are linked by the bond. Free radicals, in contrast, have at least one unpaired electron. In the case of carbon-based radicals, the carbon atom at the radical centre no longer makes four bonds with other atoms as it would do in its normal, tetravalent state. The presence of unpaired electrons renders such radicals highly reactive, so they normally occur only as transient intermediates during chemical reactions. But the discovery by Gomberg in 1900 of triphenylmethyl, the first relatively stable free radical containing a central trivalent carbon atom, illustrated that radicals with suitable geometrical and electronic structures can be stable. Compounds containing a divalent carbon atom that uses only two of its four valence electrons for bonding are usually less stable than Gomberg-type radicals with trivalent carbon. Although the role of these so-called carbenes in chemical reactions has long been postulated, they were unambiguously identified only in the 1950s. More recently, stable carbenes have been prepared, but the singlet state of these molecules, with the two nonbonding valence electrons paired, means that they are not radicals. Carbenes in the second possible electronic state, the triplet state, are radicals: the two nonbonding electrons have parallel spins and occupy different orbitals. Here we report the preparation and characterization of a triplet carbene, whose half-life of 19 minutes at room temperature shows it to be significantly more stable than previously observed triplet carbenes.

Unconventional magnetism in all-carbon nanofoam

“…48 It has recently been shown by Tomioka et al 49 that even the very reactive divalent carbon radicals ("carbenes") can be stabilized to show half-lives of several minutes if the two spins can be separated onto distinct sites that are sterically protected in this way. We note that the magnetic relaxation behavior of our samples implies that there are spin centers which show differing degrees of reactivity and/or steric protection.…”

Triplet Carbenes: From Fleeting Existence to Attractive Molecular Units.

“…The lifetime of the triplet DPCs was greatly increased as chlorine (6), methyl (7), and bromine substituents were introduced at the ortho positions, and bis(2,6-dibrormo-4-tert-butylphenyl)carbene (8) H. TOMIOKA was found to have a half-life of approximately 20 s in degassed benzene at room temperature [2]. A carbene surviving for more than a few seconds can be regarded as very stable for a triplet carbene, but this lifetime is still ephemeral for a "real" molecule.…”

“…The half-life for 3 15 was estimated to be 19 min. This is the longest-lived triplet carbene thus far generated in our research group [6].…”

Fluorophenoxycarbene inside a Hemicarcerand: A Bottled Singlet Carbene

“…This stabilization enables the generation of the fleeting and voraciously electrophilic singlet methylene (CH 2 ) species, [2] through the long-lived triplet bis [9-(10-phenyl)anthryl]carbene [3] to such stable and isolable species as the phosphorusstabilized singlet carbene (iPr) 2 PCSiMe 3[4] and the aminestabilized Arduengo carbene 1,3-di-(1-adamantyl)imidazol-2-ylidene [5a] or (iPr) 2 NCN(iPr) 2 .[5b]Extrinsic stabilization through confinement inside a molecular container should provide additional stabilization to carbenes.[6] In fact, incarceration in the inner phase of a hemicarcerand is one of the most powerful approaches for the room-temperature stabilization of highly reactive intermediates.[7] The surrounding host protects the guest from dimerization or reaction with bulk-phase species that are too large to enter the inner phase. We report herein the first example of a normally nonisolable, fleeting singlet carbene that is rendered persistent by incarceration.…”