The hydrogenation reactions of diphenylcarbene 1, fluorenylidene 2, and dibenzocycloheptadienylidene 3 were investigated in solid H2 and D2 matrices and in H2‐ and D2‐doped argon matrices at cryogenic temperatures. The reactivity of the carbenes towards H2 increases in the order 1<3<2. Whereas 1 is stable in solid H2, 2 and 3 react fast under the same conditions via quantum chemical tunneling. In D2 both 1 and 3 are stable, whereas 2 slowly reacts. The different reactivity of the three carbenes is rationalized in terms of differing carbene stabilization energies.
The reactions of the three triplet ground state arylcarbenes diphenylcarbene 1, fluorenylidene 2, and dibenzocycloheptadienylidene 3 with the Lewis acids H O, ICF , and BF were studied under the conditions of matrix isolation. H O was selected as typical hydrogen bond donor, ICF as halogen bond donor, and BF as strong Lewis acid. H O forms hydrogen-bonded complexes of the singlet carbenes with 1 and 2, but not with 3. This is rationalized by the larger singlet-triplet gap of 3, which does not allow to stabilize the singlet state below the triplet state by hydrogen bonding. With ICF , both 1 and 3 form halogen-bonded complexes of the singlet states of the carbenes. This indicates that halogen bonding stabilizes singlet carbenes more than hydrogen bonding. Carbene 2 reacts differently from 1 and 3 by forming an iodonium ylide, thus avoiding antiaromatic destabilization of the fluorenyl unit. With BF , all three carbenes form zwitterionic Lewis acid/base complexes.
This study reports, for the first time, the experimental study of the hydrogen-bonded complexes of HO and MeOH with 1,3-dimethylimidazol-2-ylidene, which is a dimethyl-substituted N-heterocyclic carbene, using matrix isolation infrared spectroscopy. The hydrogen bond was found to be established between the carbene carbon and the hydrogen in the O-H group of HO or MeOH. The hydrogen-bonded complexes of N-heterocyclic carbenes are significantly stronger than many conventional hydrogen-bonded systems, as is evidenced by the large red shifts observed in the infrared frequencies of complexed HO and MeOH. The experimental results were corroborated by computations performed at MP2 and M06-2X levels of theory, using 6-311++G(d,p) and aug-cc-pVDZ basis sets, which indicated large interaction energies (∼9 kcal mol) for these complexes. Single-point calculations at the CCSD level of theory were also performed. Atoms-in-molecules (AIM), NBO, and LMOEDA analyses were also performed to understand the nature of the intermolecular interactions in these complexes. The dominant interaction was the electron delocalization from the carbene carbon to the σ* orbital of O-H of HO or MeOH.
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