Towards a selective synthetic route for cobalt amino acid complexes and their application in ring opening polymerization of rac-lactide

Abstract

Catalysts based on Co, amino acids, and 2,2-bipyridine present an attractive and economic alternative in ring opening polymerization, and possess advantageous ligand coordination properties combined with a variety of accessible oxidation states and coordination geometries.

Cobalt complexes with α-amino acid ligands catalyze the incorporation of CO2into cyclic carbonates

“…33 By optimizing the reaction conditions, solvent, and order of reagent addition, Co( iii ) complexes were isolated as chloride salts in higher yields and free of cis -[Co(bipy) 2 Cl 2 ] and [Co(bipy) 2 (aa)]Cl formation. 32 Diastereotopic isomers were present as expected, and their ratio was analyzed by 1 H NMR spectroscopy, but no effort was made to perform their separation (see the ESI† and Experimental). Herein, the desired cationic species were prepared by employing a 1 : 2 ratio of bipy : aa followed by the subsequent addition of an equivalent of Co( ii ) chloride in aqueous ethanol.…”

“…32 Initially, trinuclear Co( ii ) complexes formed readily that reacted further with CoCl 2 to form cis -[Co(bipy) 2 Cl 2 ] complex as the kinetically favored product even in the presence of excess free amino acid. 32 An earlier report on alanine, glycine, and aminoisobutyrate Co( iii ) complexes obtained as perchlorate salts via a photolytic reaction reported the formation of at least two diastereotopic isomers and low yields along with a mixture of other bipy/aa complexes. 33 Preparations in DMSO and CH 3 CN from CoCl 2 yielded up to 40% yields and mixtures of products.…”

“…The structural elucidations of 6 and 9 reveal a hexacoordinated Co( iii ) center adopting a slightly distorted octahedral geometry analogous to the previously reported cobalt glycinate and leucinate complexes. 32 The molecular structure of 6 is shown in Fig. 2.…”

“…33 By optimizing the reaction conditions, solvent, and order of reagent addition, Co(III) complexes were isolated as chloride salts in higher yields and free of cis-[Co(bipy) 2 Cl 2 ] and [Co(bipy) 2 (aa)] Cl formation. 32 Diastereotopic isomers were present as expected, and their ratio was analyzed by 1 H NMR spectroscopy, but no effort was made to perform their separation (see the ESI † and Experimental). Herein, the desired cationic species were prepared by employing a 1 : 2 ratio of bipy : aa followed by the subsequent addition of an equivalent of Co(II) The HRMS data show that the molecular ions for complexes 1-9 are consistent with ionization resulting in the loss of a chloride counterion, with values corresponding to the molecular cation [Co(aa) 2 (bipy)] + , and that the isotope distribution patterns perfectly match those of the simulated patterns (Fig.…”

“…The results suggest the thermal activation of a precursor species. 32 To better understand the structure-reactivity relationship of cobalt-amino acids in CO 2 activation, this work targets the cobalt amino acid complexes to prepare highly efficient CO 2 / epoxide coupling catalysts.…”