Correlation of nitrogen chirality, R or S, with metal chelate chirality, Δ or Λ, in a series of reduced tripodal Schiff base complexes. A route to total spontaneous resolution

Abstract

Tripodal Schiff base ligands, Lox, were prepared in situ by the 3:1 condensation of 5-methyl-4-imidazole carboxaldehyde, 2-imidazole carboxaldehyde or 1-methyl-2-imidazole carboxaldehyde with tris(2-aminoethyl)amine, tren. The Schiff base ligands were reduced in situ by established procedures with sodium borohydride to give the amine form of the ligand, Lred. Complexes of manganese(II) or iron(II), [MLred] X2 (X=ClO4-, BF4- or PF6-) were prepared and characterized by IR, ESI MS, single crystal X-ray diffraction and Mössbauer spectroscopy for the iron(II) complex. The structures of the complexes are best described as mono capped trigonal antiprismatic with the center nitrogen atom of the tren capping the amine face. In all cases the three amine nitrogen atoms of the tripodal ligand are homochiral (RRR or SSS) within the same complex. Further, in the eight complexes examined the chirality of the nitrogen atom correlates with the metal chelate chirality. Specifically R correlates with Λ, and S correlates with Δ. Simple molecular mechanics calculations are consistent with this experimental observation.