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Carbon is known as the "cornerstone" of life, and related research continues to promote the development of chemical theory and application. Although the chemical structure of carbon mostly follows the "eight-electron rule", there are always some exceptions. For example, triphenyl carb radicals (Ph3C•) and carbococation (Ph3C+), which have seven and six valence electrons, respectively. Another striking example is Carben (R2C:), a dicoordinated carbon species containing only six valence electrons that has been considered by many scientists as a "highly reactive and difficult-to-isolate intermediate" for more than a century. But in 1988, that notion was completely overturned. At that time, Professor Guy Bertrand of the Université Paul Sabatier University in France successfully isolated and characterized the first singlet carbene (J. Bertrand). Am. Chem. Soc., 1988, 110, 6463）。 Since then, after decades of exploration and research, carbene has not only proven to be separable, but also used in chemistry, materials science and medicine.
Scientists are always happy to challenge established rules. Although carbine has broken the eight-electron rule, one wonders: Can we further remove one or two non-bonded electrons from carbine, and thus get carbon species with less than six electrons in the valence layer?
Early spectroscopic studies have shown that diphenylcarbene cations (Ph2C•+) can be formed by single-electron oxidation of diphenyldiazomethane (Ph2CN2), but such cations disappear rapidly after formation. Although several research teams have attempted to oxidize nitrocyclic carbene (NHC) and cyclic (alkyl) (amino) carbene (CAAC) in recent years, the result is often the conjugated acid of carbene. This is because carbene cations are extremely unstable and can quickly extract hydrogen radicals from solvents. Discussion of carbine's dication (R2C2+) has not been seen in the literature. This may be because, due to the transient nature of carbene cations, it decomposes before it undergoes a second oxidation.
Recently, researchers such as Guy Bertrand and Ying Kai Loh of the University of California, San Diego, successfully reported the synthesis and structure of carbene dications with only four valence electrons, and published the results in the journal Nature. To avoid the formation of fleeting carbene cations, the team adopted a two-step synthesis strategy: first reacting carbene with a two-electron oxidant, then removing the oxidant and removing two electrons at the same time.
Figure 1. Synthesis of 1(TfO)2 (a) and theoretical computational analysis (b). Image source: Nature
The authors chose I2 as a dual electron oxidant to compare their recently reported carbene 1 (J. 1). Am. Chem. Soc. 2023, 145, 2064) reacted to form iodine-substituted carbodions, which were subsequently treated with NaOSiMe3 to obtain a neutral bis(imino)carbonyl compound 2 (Figure 1a). By reacting with trifluoromethanesulfonic anhydride (O2–grabbing reagent), they obtained the target compound 1 (OTf)2, the dication 12+ of carbene 1. NMR studies showed 119.8 ppm in 12+ carbene carbon signals. The 19F NMR spectrum showed only one signal at –77.9 ppm, which coincided with the uncoordinated trifluoromethanesulfonate anion.
Figure 2. Crystal structure of 1(TfO)2. Image source: Nature
As shown in Figure 2, the single crystal structure of Compound 1 (TfO)2 reveals its jagged chemical structure, which is similar to the [4] lichen configuration. The CAAC at the end exhibits a cis-bending pattern with only a slight deviation from the vertical, approximately 79.6(2)°. The carbon atom C1 in the center deviates slightly from the linear arrangement, and the average distance from N1 and N1' on either side is 1.197(2) Å, within the range of Csp–N double bonds. The distance between C2–N1 and C2'–N1' at the end, i.e. 1.349(3) Å, is exactly between the Csp2–N single and double bonds. The two peripheral trifluoromethanesulfonate ions stretch towards the central carbon atom C1, but are hindered by the surrounding large nitrogen barrier heterocyclic imime, resulting in an average C–O distance of 3.020(3) Å.
Carben double cation 12+ has equal electronity with [4]liene. Its molecular orbitals LUMO and LUMO+1 are mainly represented by empty p(π)-type orbitals perpendicular to each other of C1, which contain the contributions of C2 and C2' (Figure 1b). These orbitals have the helical properties of the Möbius type, which is similar to the [4] front-line orbitals of the liene. It is worth noting that LUMO+2 is very similar to the HOMO (solitary electron pair orbital) of carbene 1, which further confirms that 12+ is actually a two-electron oxidation product of carbene 1. From the theoretical analysis, we can see that C1 is the main electrophilic activation site of this molecule, and its affinity for fluoride ions and hydrogen anion is comparable to phosphorus (V) double cation (NHC=N) 3P2+.
In fact, when compound 12+ reacts with neutral Lewis base dimethylaminopyridine (DMAP) or pyridine, a single Lewis base stabilized dicationic compounds 32+ and 42+ are obtained (Figure 3), confirming that C1 is the major electrophilic site. Surprisingly, compound 12+ reacts with 1-isocyanadamantane to form (cyano) carbocation 5+ and 1-adamantane trifluoromethanesulfonate. This process can be seen as a transfer from cyanoanion from alkyl carboxycation to 12+, reflecting the latter's stronger electrophilicity. Since the SN2 reaction path is not allowed, the authors suspect that 1-isocyanadamantane first forms a highly electron-deficient cyanodication with 12+ and then decomposes into 5+ and 1-adamantane cations.
The authors then explored the anionic extraction capabilities of compound 12+ and found that it could grab methoxy anions from dimethoxyethane and chloride ions from trimethylchlorosilane, demonstrating that 12+ has excellent oxygen-loving and halogenophic properties. Compound 12+ reacts with H− and C6F5− to form neutral compound 8; The O2− transfer reagent NaOSiMe3 was used to obtain bis(imine) carbonyl compound 2, which further demonstrated the existence of two empty orbitals on the 12+ carbon center and its double electrophilic properties.
Figure 3. Reactivity studies of 1(TfO)2. Image source: Nature
In the field of transition metal catalysis, reversible redox of two electrons is commonplace, but it is extremely rare in the chemistry of main group elements. After confirming that carbene 1 can be formed into 12+ through the oxidation of two electrons, the researchers used a two-stage method to complete the reverse two-electron reduction: first using H− as a reducing agent, followed by deprotonation and release of two electrons. The specific step is to react 12+ with H3B•NEt3 to obtain protonated bis(imino) carbene; The next deprotonation step resulted in successful regeneration of 1. In this way, 1/12+ constructs a unique two-electron reversible redox system between carbene and its double cations.
So far, the author briefly describes Professor Bertrand's outstanding work on the synthesis of carbene double cations, a carbon species containing four valence electrons. Imagine more than thirty years ago, when Professor Bertrand first revealed singlet carbenes, it was hard to foresee the huge role carbines would play in chemistry and materials science. Nevertheless, Professor Bertrand admitted in an interview with C&EN [1] that he did not know the specific application of carbene double cations ("I don't know. It’s just a fundamental discovery.” ）。 But perhaps, this shift from "useless at first glance" to "possible wide application" reflects the infinite charm of basic research.
A crystalline doubly oxidized carbene
Ying Kai Loh, Mohand Melaimi, Milan Gembicky, Dominik Munz & Guy Bertrand
Nature, 2023, DOI: 10.1038/s41586-023-06539-x
Resources:
1. Crystalline dication defies the octet rule
https://cen.acs.org/physical-chemistry/chemical-bonding/Crystalline-dication-defies-octet-rule/101/web/2023/09
Mentor introduction
Guy Bertrand
https://www.x-mol.com/university/faculty/1095
(This article is contributed by LLL_SUSTech)