Kekulé diradicaloids derived from a classical N-heterocyclic carbene

Two-electron reduction of bis(1,3-imidazolinium) salts 2 and 3 with KC8 gives rise to stable diradicaloids 4 and 5, respectively. Calculations reveal a very low singlet–triplet energy gap ΔES–T for 5 (10.7 kcal mol–1), while ΔES–T for 4 (29.1 kcal mol–1) is rather large.


Cyclic Voltammetry
Cyclic voltammetry (CV) experiments were carried out using a PGSTAT 101 electrochemical workstation (METROHM). All experiments were carried out under an atmosphere of argon in degassed and anhydrous acetonitrile solution containing t Bu4NPF6 (0.1 M) at a scan rate of 50 mV s -1 up to 500 mV s -1 . The setup consisted of a glassy carbon working electrode (surface area = 0.04 cm 2 ), a glassy carbon counter electrode, and a silver wire immersed in a saturated LiCl solution in EtOH and 0.1 M Bu t 4NPF6 solution in acetonitrile as the reference electrode. The recorded voltammograms were referenced to the internal standard Fc/Fc + (ferrocene/ferrocenium) couple.

EPR spectroscopy
The continuous wave (CW) EPR experiments were performed at room temperature (298 K) in a Bruker standard ST9402 resonator and with a Bruker ELEXSY E500 spectrometer. The microwave frequency was 9.63 GHz and the modulation amplitude was 0.3 mT.

X-Ray Diffraction Studies
Single crystals were examined on a Rigaku Supernova diffractometer using. Using Olex2 1 , the structure was solved with the ShelXT 2 structure solution program using Intrinsic Phasing and refined with the ShelXL 3 refinement package using Least Squares minimization. The asymmetric unit of compound 3 contains besides the half the molecule of 3 some methanol solvent molecules, one is fully occupied, another one (O2/C35) is partly (61%) occupied.

Computational Studies
Geometry optimizations were performed using the Gaussian 09 optimizer 4 together with TurboMole V6.5. 5 All geometry optimizations were computed using the functional B3LYP 6 and BH&HLYP 7 in combination with the def2-SVP basis set. 8 The stationary points were located with the Berny algorithm 9 using redundant internal coordinates. Analytical Hessians were computed to determine the nature of stationary points. The improvements in the electronic energies were carried out by computing single points on the B3LYP/def2-SVP geometries at the B3LYP/def2-TZVPP, BH&HLYP/def2-TZVPP, PBE0/def2-TZVPP 10 and M06-2X/def2-TZVPP 11 levels of theory.
Time-dependent density functional theory (TDDFT) was employed to calculate excitation energies as implemented in ORCA 4.0.1.2. 12 We used the functional B3LYP in combination the def2-SVP basis sets. The solvent THF was described in this case by the conductor-like polarizable continuum model,

CPCM. 13
The diradical character (yi) is defined by the weight of the doubly excited configuration in the multiconfigurational (MC)-SCF theory and is formally expressed in the spin-projected UFH (PUHF) theory 14 as: (1) Where Ti is the orbital overlap between the orbital pair and it ca be represented by the occupations numbers (ni) of the UHF natural orbitals (UNOs): The diradical character yi obtained from the UNO occupations number have a value between 0 and 1.
In purely closed-shell system nHOMO-i and nLUMO+i = 0, then y = 0. When the occupations of the two orbitals are equal the system is a pure diradical and y = 1. The diradical characters have been computed from the occupation number of the lowest unoccupied natural orbital (LUNO) at the UHF/6-31G(d,p) level of theory.
The CASSCF(2,2)+NEVPT2/def2-SVP calculations were performed on the model systems 4 Me and 5 Me using the ORCA 4.0.1.2 software. 12 The singlet diradical index d proposed by Neese and coworkers [14][15] was calculated, for an ab initio CI calculation with the canonical MOs, by the following equation: Where c0 2 is the weight of the closed-shell configuration in the CI wave function and cd 2 is weight of the double excitation computed.