Zinc Guanidinate Complexes and Their Application in Ring‐Opening Polymerisation Catalysis

Abstract

Abstract The insertion reaction between 1 equiv. of carbodiimide, R′N=C=NR′, and the zinc bis(amide) [Zn{N(SiMe 3 ) 2 } 2 ] affords the homoleptic zinc( II ) guanidinate compounds [Zn{(Me 3 Si) 2 NC(NR′) 2 } 2 ] ( 3 , R′ = i Pr; 4 , R′ = Cy) in preference to the monosubstituted compounds. Compound 3 also results from the protonation reaction of 1 equiv. of the neutral guanidine (Me 3 Si) 2 NC(N i Pr)(NH i Pr) ( 2 ) with [Zn{N(SiMe 3 ) 2 } 2 ] or ZnMe 2 . In contrast, the N ‐dimethylguanidine analogue Me 2 NC(N i Pr)(NH i Pr) ( 1 ) reacts cleanly with 1 equiv. of ZnMe 2 to afford the dimeric species [Zn{Me 2 NC(N i Pr) 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)Me·{Me 2 NC(N i Pr)(NH i Pr)}] ( 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 (μ‐{Me 2 NC(N i Pr) 2 })(μ‐NMe 2 )] ( 7 ) as a result of the extrusion of NMe 2 − from the guanidine‐based ligand. The amine‐elimination reaction between [Zn{N(SiMe 3 ) 2 } 2 ] and 1 equiv. of 1,3,4,6,7,8‐hexahydro‐2 H ‐pyrimido[1,2‐ a ]pyrimidine (hppH) proceeds smoothly to afford the mixed (guanidinate)(amide) complex [Zn(hpp){N(SiMe 3 ) 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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004