Hydrogen Bonding Switches the Spin State of Diphenylcarbene from Triplet to Singlet

Abstract

Spin specificity is one of the most important properties of carbenes in their reactions. Alcohols are typically used to probe the reactive spin states of carbenes: O-H insertions are assumed to be characteristic of singlet states, whereas C-H insertions are typical for the triplets. Surprisingly, the experiments presented here suggest that the spin ground state of diphenylcarbene 1 switches from triplet to singlet if the carbene is allowed to interact with methanol. Carbene 1 and methanol form a strongly hydrogen-bonded singlet ground state complex that was synthesized in low-temperature matrices and characterized by IR spectroscopy. This methanol complex is only metastable, and even at 3 K slowly rearranges to form the product of O-H insertion through quantum chemical tunneling. Thus, the ground state triplet (in the gas phase) carbene 1 forms exclusively the products expected from a singlet carbene. Whereas the assumption of spin specific reactions of carbenes is correct, the spin state itself can be changed by solvent interactions, and therefore widely accepted conclusions drawn from earlier experiments have to be revisited.

The Fluorenyl Cation

“…When D 2 O was used in these experiments, the spectrum of this new species looked virtually identical, and none of its bands showed significant isotopic shifts. Recently, we reported that triplet diphenylcarbene T-8 reacted with both single molecules of CH 3 OH [18] and H 2 O [14] to form strongly hydrogen-bonded complexes between the singlet carbene S-8 and the hydrogen bond donors. We therefore assign the new species formed in the reaction between T-6 and H 2 O to the singlet carbene complex S-6···H 2 O.…”

Theoretical study on interactions of N-heterocyclic carbene with the bare first-row transition metals

“…The low-lying states of 1 A 1 (σ 2 π 0 ) and 1 B 1 (σ 1 π 1 ) are 10.4 and 33.1 kcal/mol [45][46][47][48][49] above the ground state, respectively (Schemes 2, 3).…”

1‐Phenyl‐1,2,4,6‐cycloheptatetraene: the missing intermediate of the diphenylcarbene to fluorene rearrangement

“…Diphenylcarbene 5 has been also studied in cryogenic matrices [25][26][27][28] and shown to be rather photostable under these conditions. In particular, prolonged broad band UV-visible irradiation of matrix-isolated 5 using a high-pressure mercury arc lamp did not lead to the expected ring opening to cyclohepatetraene 8.…”