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

Abstract

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.