Structure and Highly Stereoselective Formation of 1:2 Metal‐to‐Ligand complexes with triamine 2,6‐bis(pyrolidin‐2‐yl)pyridine. meso ‐[2,6‐bis(pyrrolidin‐2yl)pyridine]nickel(II) ( meso ‐[Ni(bpp) 2 ] 2+ ), a Metal Complex with S 4 Symmetry

The X‐Ray structures of the 1:1 copper(II) complexes with racemic and opticall active (+)‐( R , R )‐(2,6‐bis(pyrrolidin‐2‐yl)pyridine ((+)‐bpp; (+)‐ I ) were determined. Whereas the perchlorate 1 of racemic complex contains the monomeric units with perfect C 2 symmetry, the sulfate of the optically active compound crystallizes as a coordination polymer composed of sequences of three different five‐coordinate units linked by sulfate bridges around a threefold screw axis (see 2 ). Formation of the 1:2 complexes of nickel( II ) with (±)‐bpp is shown to be highly stereoselective. Acidimetric titrations and CD measurements indicate that the equilibrium mixture contains 99% of the meso ‐complex 3 belonging to the point group S 4 and only 0.5% of each enantiomer of the C 2 ‐symmetric optically active forms. The S 4 symmetry of meso ‐[Ni{(+)‐bpp}{(−)‐bpp}]ClO 4 ) 2 ( 3 ) was confirmed by X‐ray analysis. The formation of binary complexes of cobalt(II), zinc(II), and cadmium(II) with (±)‐bpp shows an excess of 28, 34, and 8%, respectively, of the meso ‐form over the statistical amount. CD and UV/VIS measurement show a pH‐dependent high spin/low spin equilibrium between the octahedral [Ni(bpp)(H 2 O) 3 ] 2+ and the square‐planar [Ni(bpp)OH] + in aqueous solution.