Synthesis and Characterization of Monomeric Magnesium and Zinc complexes supported by 1,5,9-Trimesityldipyrromethene for use in polymerization studies

Abstract

Magnesium and zinc complexes supported by a bulky bidentate ancilliary ligand have been synthesized and characterized. The compounds (pyac)MgnBu(THF),1; (pyac)Zn(nBu),2; (pyac)Mg(CH2CH2Ph)(THF), 3; (pyac)ZnEt, 4; and (pyac)MgOBut(THF),5; where pyac = 1,5,9 – trimesityl dipyrromethene ligand , are all moisture and air sensitive. The n – butyl group in compounds 1, 2 and 3 showed unexpected α – 1H NMR pattern in solution that may be simulated as an AA’XX’ pattern. Single crystal X-ray diffraction studies carried out on 1 and 5 revealed a distorted tetrahedral structure about the metal centre. Compounds 1, 2 and 5 react with 13CO2 forming carboxylates.

Preliminary polymerization studies showed that all compounds initiate and sustain ring opening polymerization of lactides and ε-caprolactone. However, the zinc alkyls were comparatively very slow when compared to the magnesium alkyls. Compound 1 exhibited good stereocontrol in the polymerization of rac and L – Lactide resulting in heterotactic and isotactic polylactide respectively.

Reaction of compound 1 with benzophenone results in the formation of an alkoxide with the elimination of 1 - butene. In the case of Grignard reagents (depending on the R group), the same reaction gives tertiary alcohol, or a mixture of secondary and tertiary alcohol. The reactivity mechanism of compound 1 proceeds exclusively via β – hydrogen transfer. However, presence of a β – hydrogen in the R group of the Grignard reagents results in secondary alcohol as the majority product. If the R group is a methyl the tertiary alcohol is exclusively obtained.