Magnesium and zinc complexes of functionalised, saturated N-heterocyclic carbene ligands: carbene lability and functionalisation, and lactide polymerisation catalysis

The synthesis of magnesium and zinc complexes of bidentate anionic alkoxide ligands with saturated-backbone carbene groups is reported. Mono(ligand) and bis(ligand) complexes [M(L-R)N ''](2) and [M(L-R)(2)] (M = Mg, Zn, N '' = N(SiMe3)(2), L-R = [OCMe2CH2{CNCH2CH2NR}] R = Pr-i, Mes, Dipp) have been isolated, and some structurally characterised and compared with the new unsaturated carbene complex [Mg(L) 2]. Reactions with silyl halides show either addition across the metal carbene bond, or across the metal alkoxide bond, in accordance with the metals' electronegativity difference: the metal alkoxide bonds are stronger for MgII complexes, for which the carbene is silylated to form zwitterionic [MgI(Me3SiLR)N ''] (Me3SiLR = OCMe2CH2{Me3SiCNCH2CH2NR}) while the metal-bound alkoxide group is silylated in the Zn-II complexes forming [ZnI(Me3SiOLR)N ''] (Me3SiOLR = Me3SiOCMe2CH2{CNCH2CH2NR}). The proligand [HLR] is silylated at the alcohol group, forming the iodide salt [Me3SiOCMe2CH2{HCNCH2CH2NR}]I. Preliminary results on the use of these complexes as initiators for the polymerisation of rac-lactide are reported, and suggest different initiation mechanisms are occurring for the two metals, in agreement with the different silylation reactivity observed. The polymerisation reactions are facile at room temperature even without an initiator, and yield polymers of reasonable molecular weight and heterotacticity and with good PDI. These are the first magnesium NHC complexes demonstrated to effect lactide polymerisation. Also, an adduct instead of the anticipated potassium alkoxycarbene is generated from the reaction of the proligand [HLR] with potassium amide KN ''; this has been structurally characterised.