N-Heterocyclic carbene stabilized adducts of alkyl magnesium amide, bisalkyl magnesium and Grignard reagents: trapping oligomeric organo s-block fragments with NHCs

Developing N-heterocyclic carbene (NHC) chemistry of simple organomagnesium compounds, this study reports the synthesis, X-ray crystallographic, and NMR spectroscopic characterization of three such new carbene complexes. The 1 : 1 alkyl magnesium amide : carbene complexes nBuMg(TMP)center dot IPr 1 and nBuMg(HMDS)center dot IPr 2 both exist as mononuclear complexes in the crystal but differ in solution as 2 remains intact whereas 1 undergoes a dynamic exchange involving partial decoordination of IPr [TMP is 2,2,6,6-tetramethylpiperidide; IPr is 1,3-bis-(2,6-diisopropylphenyl) imidazol-2-ylidene); HMDS is 1,1,1,3,3,3-hexamethyldisilazide]. Reaction of commercial nBu(2)Mg with IPr surprisingly produced the organoaluminium carbene complex nBu(3)Al center dot IPr, 3, which also forms a simple mononuclear structure in the crystal. The presence of the Al could be traced to the deliberate addition of a small quantity of Et3Al as a stabilizing agent in the commercial nBu(2)Mg reagent. Repeating this reaction with Al-free nBu2Mg afforded the hemisolvated carbene complex nBu(8)Mg(4)center dot 2IPr, 4, the stoichiometry of which is dictated by its structure rather than by that used in the initial reaction mixture. The molecular structure of 4 is tetranuclear with a linear chain of 4 Mg centres bridged by nBu ligands and capped at each end by terminal nBu and IPr ligands. Synthesized by treating the Grignard reagent nBuMgCl with IPr, nBuMgCl center dot IPr, 5, forms a cyclodimer structure with chloro bridges and terminal nBu and IPr ligands.