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 S4 Symmetry

✍ Scribed by Klaus Bernauer; François Gretillat; Helen Stoeckli-Evans; Ruth Warmuth

• Publisher: John Wiley and Sons
• Year: 1993
• Tongue: German
• Weight: 618 KB
• Volume: 76
• Category: Article
• ISSN: 0018-019X
• DOI: 10.1002/hlca.19930760136

✦ Synopsis

The X-Ray structures of the 1:l copper(I1) complexes with racemic and optically active (+)-(R,R)-(2,6bis(pyrro1idin-2-y1)pyridine ((+)-bpp; (+)-I) were determined. Whereas the perchlorate 1 of the racemic complex contains the monomeric units with perfect C, 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(I1) with (k)-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 C,-symmetric optically active forms. The S4 symmetry of meso-Ni{ (+)-bpp}((-)-bpp}, (3) was confirmed by X-ray analysis. The formation of binary complexes of cobalt(II), zinc(II), and cadmium(I1) with (*)-bpp shows an excess of 28,34, and 8%, respectively, of the meso-form over the statistical amount. CD and UVjVIS measurement show a pH-dependent high spin/low spin equilibrium between the octahedral [ N i ( b ~p ) ( H ~o ) ~] ~+ and the square-planar INi(bun)OHl+ in aaueous solution. 1 bpp N H H N 11 [M { ( R, R ) -bpp}l 1 1' ') Part XVI of 'Stereoselectivity in Reactions of Metal Complexes'. For part XV, see [I].

' ) From the equilibrium constants given in Table I , it can be calculated that the condition of complete formation of the binary species is not fulfilled under the conditions given in Fig. 2. Disregarding a possible influence of Tris buffer, the amount of 1 :1 complex in the equilibrium mixture is ca. 5% with the optically active ligand, whereas it is present at less than 0.2% with the racemic ligand.