Synthesis and Characterization of Neutral and Cationic Magnesium Complexes Supported by NHC Ligands

A series of neutral N-heterocyclic carbene (NHC) magnesium alkyl complexes was synthesized by reaction of IMes (IMes = 1,3-bis­(2,4,6-trimethylphenyl)­imidazol-2-ylidene) and IPr (IPr = 1,3-bis­(2,6-diisopropylphenyl)­imidazol-2-ylidene) with Grignard reagents. In the presence of THF, the mononuclear species [Mg­(IMes)­(R)­(Br)­(THF)] (1, R = Me; 2, R = Et) and [Mg­(IPr)­(R)­(Br)­(THF)] (3, R = Me; 4, R = Et) were isolated. Complex 4 was further methylated to afford the Mg–THF adduct [Mg­(IPr)­(Me)2(THF)] (5), which crystallizes as the dimer [{(Mg­(IPr)­(Me)}2(μ-Me)2] (5′), thus evidencing the labile THF coordination in these systems. In contrast, in the presence of Et2O, the dinuclear Mg­(II) species [{(Mg­(NHC)­(Br)}2(μ-Me)2] (6, NHC = IMes; 7, NHC = IPr), [{(Mg­(IMes)­(Ph)}2(μ-Br)2] (8), [{(Mg­(IMes)­(CH2Ph)}2(μ-Cl)2] (9), and [{(Mg­(IPr)­(Ph)}2(μ-Br)2] (10) were isolated and characterized. The protonolysis reaction between the protio ligand [IMes-H]Cl and EtMgBr afforded the Mg­(II) dihalido dimer [{(Mg­(IMes)­(Br)}2(μ-Cl)2] (11). The solid-state molecular structures of species 5′ and 6–11 were established through X-ray crystallographic studies. THF adducts 1 and 3 may be readily ionized by a salt metathesis reaction with [BPh4]Na in the presence of THF to yield NHC-supported Mg­(II) alkyl cations [{Mg­(NHC)­(Me)­(THF)2}]+ (12, NHC = IMes; 13, NHC = IPr) as BPh4– salts. Reflecting the labile THF coordination to Mg­(II) in these cations, the mononuclear cation 12 crystallizes as the dication [{(Mg­(IMes)­(THF)}2(μ-Me)2]2+ (12′). Only partial ionization of the halido methyl dimer 6 was observed by reaction with [B­(C6F5)4]Li in PhBr to afford the tetranuclear NHC–Mg­(II) dicationic aggregate [{[Mg­(IMes)­(Me)]­[Mg­(IMes)]}­(μ-Me)2]2(μ-Br)2]2+ (14) as a B­(C6F5)4 salt. Computational studies were performed for selected neutral and cationic NHC–Mg­(II) derivatives, and all agree with the electronic structure and bonding at Mg­(II) being largely dominated by electrostatic interactions.