Four- and five-coordinate magnesium compounds containing ketiminate ligands. Synthesis and characterization of L2Mg, L2Mg(LH), and L2Mg(Py), where L = MeC(O)CHC(NAr)Me

Magnesium complexes containing ketiminate ligands were synthesized and characterized. MgBu2 reacted readily in toluene with two equiv. of [MeC(O)CHC(NHAr)Me], where Ar = 2,6-diisopropylphenyl, to generate [MeC(O)CHC(NAr)Me]2Mg (1) in 43% yield. The four-coordinate magnesium compound 1 is very moisture sensitive and acts as a Lewis acid, accepting one equiv. of Lewis base to form five-coordinate magnesium compounds. Compound [MeC(O)CHC(NAr)Me]2Mg[MeC(O)CHC(NHAr)Me] (2) was obtained in 57% yield from the reaction in toluene of MgBu2 with three equiv. of [MeC(O)CHC(NHAr)Me]. Treatment of 1 with one equiv. of free ketimine ligands [MeC(O)CHC(NHAr)Me] also led to the formation of 2. The bulky η1-ketimine of 2 can be replaced with a less bulky Lewis base such as pyridine. Treatment of 1 with excess pyridine in toluene at ambient temperature led to the formation of compound [MeC(O)CHC(NAr)Me]2Mg[NC5H5] (3) as colorless crystalline solids in 51% yield. Compounds 1, 2, and 3 were characterized by NMR and X-ray crystallography. Compounds 2 and 3 showed no activity toward the polymerization of ε-caprolactone at 25 °C after 3 h. However, when the temperature was increased to 70 °C, compounds 2 and 3 efficiently catalyzed polymerization of ε-caprolactone to generate high molecular weight poly-ε-caprolactones. The polydispersity index (PDI) of these poly-ε-caprolactones is in the range 1.57–3.18.

Graphical abstractA series of magnesium complexes containing ketiminate ligands were synthesized. The four-coordinate magnesium compound [MeC(O)CHC(NAr)Me]2Mg (1) acts as a Lewis acid, accepting one equiv. of Lewis base such as [MeC(O)CHC(NHAr)Me] and pyridine to form five-coordinate magnesium compounds. The magnesium compounds act as initiators at 70 °C to polymerize ε-caprolactone and the polydispersity index (PDI) of these poly-ε-caprolactones is in the range 1.57–3.18.Figure optionsDownload full-size imageDownload as PowerPoint slide