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

• The structure of five new manganese(II) and iron(II) reduced Schiff base complexes of a tripodal ligand are described.
• Complexes contain three chiral nitrogen atoms.
• In all cases the chirality of the nitrogen atom, R or S, is the same in all three arms.
• In all cases there is an observed correlation on the nitrogen chirality with the twist chirality of the metal complex.
• Results of molecular mechanics calculations are consistent with the above correlation.

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.

In reduced tripodal Schiff base complexes of manganese(II) and iron(II) all three amine nitrogen atoms are of the same chirality (RRR or SSS) and there is an observed correlation of nitrogen chirality, R or S, with metal twist chirality, Λ or Δ, respectively. The amine hydrogen atoms (pictured as spheres) of the ΔSSS stereoisomer (shown on right) are located in the sterically less constricted position while those of the ΔRRR stereoisomer (shown on left) are in a more crowded position.Figure optionsDownload as PowerPoint slide