STRUCTURE AND HIGHLY STEREOSELECTIVE FORMATION OF 1/2 METAL-TO-LIGAND COMPLEXES WITH THE TRIAMINE 2,6-BIS(PYRROLIDIN-2-YL)PYRIDINE - MESO-[2,6-BIS(PYRROLIDIN-2-YL)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 optically active (+)-(R,R)-(2,6-bis(pyrrolidin-2-yl)pyridine ((+)-bpp; (+)-I) were determined. Whereas the perchlorate 1 of the racemic complex contains the monomeric units with perfect C2 SYMMetry, the sulfate of the optically active compound crystallizes as a coordination polymer composed of sequences of three different rive-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 S4 and only 0.5 % of each enantiomer of the C2-symmetric optically active forms. The S4 symmetry of meso-[Ni{(+)-bpp}{(-)-bpp}](ClO4)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)(H2O)3]2+ and the square-planar [Ni(bpp)OH]+ in aqueous solution.