Zinc Guanidinate Complexes and their Application in Ring-Opening Polymerisation Catalysis

The insertion reaction between 1 equiv. of carbodiimide, RN=C=NR, and the zinc bis(amide) [Zn{N(SiMe3)2}2] affords the homoleptic zinc(II) guanidinate compounds [Zn{(Me3Si)2NC(NR)2}2] (3, R = iPr; 4, R = Cy) in preference to the monosubstituted compounds. Compound 3 also results from the protonation reaction of 1 equiv. of the neutral guanidine (Me3Si)2NC(NiPr)(NHiPr) (2) with [Zn{N(SiMe3)2}2] or ZnMe2. In contrast, the N-dimethylguanidine analogue Me2NC(NiPr)(NHiPr) (1) reacts cleanly with 1 equiv. of ZnMe2 to afford the dimeric species [Zn{Me2NC(NiPr)2}Me]2 (5). Attempted protonation of the Zn-C bond in 5 with 2,6-di-tert-butylphenol resulted in preferential reaction at the Zn-N bond and formation of the mixed (aryloxo)methyl complex [Zn(OAr)M{Me2NC(NiPr)(NHiPr)}] (6), isolated as the guanidine adduct. Heating of 6, in an attempt to promote methane elimination, afforded a small amount of the dimetallic complex [{Zn(OAr)}2(-{Me2NC(NiPr)2})(-NMe2)] (7) as a result of the extrusion of NMe2- from the guanidine-based ligand. The amine-elimination reaction between [Zn{N(SiMe3)2}2] and 1 equiv. of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine (hppH) proceeds smoothly to afford the mixed (guanidinate)(amide) complex [Zn(hpp){N(SiMe3)2}]2 (8), which forms a dimer in the solid state. Crystal structures of compounds 3-8 are reported and a preliminary study of the activity of 8 in the ring-opening polymerisation of lactide is described