Cobalt and iron complexes of chiral C₁- and C₂-terpyridines: Synthesis, characterization and use in catalytic asymmetric cyclopropanation of styrenes

Optically pure C₁- and C₂-terpyridine ligands (L) form cobalt(II) and iron(II) complexes of formula [Co(L)Cl₂] and [Fe(L)Cl₂], respectively, and Iron(III) complexes of formulas [Fe(L)Cl₃]. Structures of three new chiral cobalt(II) and one iron(III) complexes were analysed using X-ray crystal structure analysis. These complexes were shown to be precursor of efficient catalyst for cyclopropanation. Reaction with AgOTf converted the complex to active catalyst, which gave enantioselectivities of up to 76% ee for the trans-isomers and 83% ee for the cis-isomers of styrene cyclopropanes with ethyl diazoacetate. Hammett studies showed the active species for both cobalt and iron complexes to have a non-linear relationship to σ_p constant.