Fluorenyl-tethered N-heterocyclic carbene: An effective ancillary support for heteroleptic magnesium organometallics

Abstract

Judicious designing of ancillary ligands with appropriate stetrolelectronics is critical for the chemistries of electropositive alkaline earths that typically make labile coordination to conventional donor sites. We report here a series of magnesium complexes [(LH)Mg(A)Br(thf)] (A = nBu (1), N(SiMe3)2 (2)), [(L)Mg{N(SiMe3)2}] (3), and [(L)MgMe(thf)] (4) supported by a N-heterocyclic carbene LH ([(Flu)H-(CH2)2-NHCDipp]; Dipp = 2,6-iPr2-C6H3) with a fluorenyl sidearm and its monoanionic version L- ([(Flu)-(CH2)2-NHCDipp] -). Complexes 1, 2, and 4 are characterized by single crystal X-ray diffractometry. 3 and 4 are additionally analyzed by DFT calculations. The pseudo-bidentate L- appears to be quite an effective ancillary support as evident from the monomeric nature of 4 and the thermal stability of 3 and 4 towards the deleterious Schlenk equilibrium. This is also reflected in 4′s catalytic activity in intramolecular hydroamination/cyclization of two primary aminoalkenes as model substrates, for which the presence of a spectator ligand and its ability to thwart the Schlenk equilibrium are critical. It’s also the 1st NHC-based main group catalyst for intramolecular hydroamination. Complex 2 also catalyzes the hydroboration of model substrates of carbonyl, ester, amide, and nitrile functions.