Generation of Annelated Dicarbenes and Their Alkali-Metal Chelate Complexes in Solution: Equilibrium between Hetero- and Homoleptic NHC Lithium Complexes

Attempts to generate the free bis­(N-heterocyclic
carbene) vegiR (R = nPr, tBu; vegiR = 2,7-dihydro-2,7-dialkyldiimidazo­[1,5-b:5′,1′-f]­pyridazine-1,8-diylidene)
from its imidazolium salts 1 with alkali-metal bases
resulted exclusively in the formation of the respective lithium, sodium,
and potassium complexes 2–4 due to
the strongly chelating properties of the dicarbene. DFT calculations
reveal pronounced dispersion interactions in the case of N-tert-butyl substituents as the reason for the formation of the homoleptic
lithium species 2b-H in solution. The dynamic behavior
of the lithium complexes in equilibrium was studied by NMR techniques.
Attempts to liberate the free carbenes by addition of the respective
crown ethers failed for the lithium and sodium complexes. Deprotonation
of the imidazolium salts 1a,b with the strong
metal-free phosphazene base P4-tBu ({(Me2N)3PN}3PNtBu) generated successfully
the free dicarbenes vegiR (5) and monocarbene 6b in solution.